Heaviest spacecrafts (excluding space stations and manned orbiters).
\r\n\tsustainability, financial and social investigations, and disruptive technologies. This book also covers urban resilience by considering different factors: health and wellbeing; economy and society; infrastructure and environment; leadership and strategy.
\r\n\r\n\tAs a self-contained collection of scholarly papers, the book will target an audience of practicing researchers, academics, PhD students and other scientists. Since it will be published as an Open Access publication, it will allow unrestricted online access to chapters with no reading or subscription fees.
\r\n\t
Zinc oxide (ZnO) has been important for the development of practical devices such as thin film transistors, magnetic semiconductors, transparent electrodes, and so on. ZnO has a large exciton energy of 60 meV, which raises the interesting possibility of utilizing excitonic effects at temperatures higher than 300 K (Thomas, 1960). Optically pumped UV stimulated emissions from ZnO layers have been demonstrated (Yu et al., 1997). Furthermore, MgxZn1-xO alloys are attracting a great deal of interest since they possess a higher band gap than ZnO (Sharma et al., 1999) and have been utilized for MgxZn1-xO/ZnO multiple and single-quantum wells (Chen et al., 2000, Makino et al., 2000). These structures can form low-dimensional systems and produce interesting quantum phenomena such as an increased excitonic binding energy (Coli & Bajaj, 20001) and two-dimensional (2-D) electron transport aspects that contribute to both basic science and practical applications. (Tsukazaki et al., 2007).
\n\t\t\tA variety of nanostructures in semiconductor materials have been made and investigated. The number of papers concerning nanostructures in ZnO is increasing yearly. Self-organized techniques provide advantages for nanoscale engineering and have yielded many impressive results. Therefore, surface nanostructures in Si and GaAs have been fabricated using various growth mechanisms. Stranski-Krastanov (S-K) growth on lattice mismatched systems induces three-dimensional (3-D) nanodots on the 2-D wetting layers. Lateral surface nanowires have been fabricated by a step-faceting mode on vicinal surfaces (Schönher et al., 2001). These surface nanostructures have been developed for zero-dimensional (0-D) quantum dots and one-dimensional (1-D) quantum wires, respectively (Wang & Voliotis, 2006). Low-dimensional properties are currently receiving attention as advantages for optoelectronics with ZnO.
\n\t\t\tIn epitaxial growth, lattice mismatch between an epilayer and a substrate plays a crucial role in epitaxy. Growth studies concerning ZnO epitaxy have been carried out using c- and a-sapphires (Vispute et al., 1997, Fons et al., 2000). Heteroepitaxial layers have a high dislocation density of 109 – 1010 cm-2 due to large mismatches in the lattice structure and thermal expansion (Viguè et al., 2001). The use of a ZnO substrate not only allows a reduction of the number of lattice defects involved in the epilayers, but also permits the selection of various growth directions without any lattice mismatch, which results in a direct understanding of growth dynamics. The growth polarity in ZnO is a primary factor. Zn (0001) and O (000-1) polarities have isotropic atom arrangements and possess spontaneous polarization along growth directions. On the other hand, the M-nonpolar (10-10) surface has an anisotropic atom structure, and the spontaneous polarization occurs parallel to a surface plane (Parker et al., 1998). For example, Zn-polar growth produces atomically flat surfaces due to a layer-by-layer mode (Kato et al., 2003, Matsui et al., 2004), whereas M-nonpolar ZnO layers result in anisotropic morphologies with a nanowires structure based on a step-edge barrier effect (Matsui & Tabata, 2005). Thus, the difference in growth directions influences the surface state, as well as optical and electrical properties in ZnO layers, which are more conspicuous through quantum structures (Matsui & Tabata, 2008). Quantum structures on various surface morphologies exhibit novel electronic and optical properties because quantized energy levels can be tailored by varying the geometric dimensions.
\n\t\t\tThis chapter is organized as follows. In Section 2, we first give a description of polar and nonpolar growth on ZnO layers and outline a difference of surface nanowires between ZnO and GaAs systems. The surface nanowires on the M-nonpolar ZnO (10-10) layer surface are largely different from those on high-index GaAs layer surfaces, indicating that the growth origin of surface nanowires on ZnO results from a new bottom-up process. In Section 3, we introduce layer growth using a pulse laser ablation technique. In Section 4, we discuss the growth origin of surface nanowires on ZnO layer surfaces from various viewpoints. Sections 4 and 5 are devoted to reports of anisotropic optical and electrical properties that are remarkably modulated by the anisotropic surface morphology. Some concluding remarks and future research directions in this field are given in Section 5.
\n\t\tZnO has a hexagonal wurtzite structure (a = 0.325 nm, c = 0.5201 nm) in which each Zn2+ ion bonds by a tetrahedron of four O2- ions, representing a structure that can be described as a number of alternating planes of Zn and O ions stacked along the c-axis [Fig. 1a]. Various surface-sensitive methods have been well used to investigate the polar surfaces in ZnO from fundamental and applied points of view. For example, the surface morphology was quite different for opposite polar surfaces when ZnO crystals were chemically etched (Mariano & Hanneman, 1963). Thus, epitaxy in ZnO with varying polarity should show different
\n\t\t\t\ta) Schematic structure of ZnO with a stacking sequence of Zn and O layers. (b) Structural models showing the bulk-terminated Zn-polar (0001) surfaces of ZnO. The surface unit cells are indicated.
kinetics and material characteristics. Therefore, it is important to understand the uppermost surface structure and morphology in a Zn-polar surface. Figure 1(b) shows a structural model of the Zn-polar (0001) surfaces of ZnO. All O atoms on the borders have three nearest neighbours, i.e., only one bond is broken. The Zn-polar surface is unstable due to the existence of a non-zero dipole moment perpendicular to the surface, which raises a fundamental question regarding stabilization mechanisms.
\n\t\t\t\t\n\t\t\t\t\tFigure 2 shows the surface morphologies of Zn-polar, O-polar and M-nonpolar ZnO layers. The Zn-polar ZnO layer showed a very flat surface with a roughness of 0.21 nm [Fig. 2(a)]. The O-polar ZnO layer displayed a rough surface with a roughness of 4.2 nm and formed hexagonal islands [Fig. 2(b)]. The difference in surface morphology between Zn- and O-polar ZnO layers is similar to that for Ga- and N-polar GaN (0001) and Zn- and Se-polar ZnSe (111), which can be explained for in terms of the difference in the number of dangling bonds (Sumiya et al., 2000, Ohtake et al., 1998). In the case of Zn-polarity, the Zn atoms of ZnO molecules generated from the laser ablation are likely to be incorporated with less migration due to three dangling bonds. This suggests that Zn-polar growth should be dominated by a two-dimensional mode, resulting in very smooth surfaces. On the other hand, an O-polar surface has longer surface migration due to the single dangling bond and is adhered to the sites of the step edges with two dangling bonds. These result in hexagonal islands that originated from a spiral growth mode. The growth kinetics of O-polar growth has been suggested using molecular-dynamics crystal-growth simulations (Kubo et al., 2000).
\n\t\t\t\tAFM images of Zn-polar (a), O-polar (b) and M-nonpolar (c) ZnO layer surfaces. (d) Structural models showing the M-nonpolar (10-10) surfaces of ZnO.
On the other hand, the M-nonpolar ZnO layer surface showed a highly anisotropic surface morphology with self-organized surface nanowires elongated along the [0001] direction [Fig. 2(c)] (Matsui & Tabata, 2006). The stoichiometric ZnO (10-10) surface is auto-compensated since it contains an equal number of Zn and O ions per unit area. Zn and O atoms of the surface form dimer rows running along the [-12-10] direction, as shown in Fig. 2(d), which produces two types of A and B step edges consisting of stable low-index (-12-10) and (0001) planes, respectively (Dulub et al., 2002). The [-12-10] direction represents an auto-compensated nonpolar surface, while the [0001] direction consists of a polar surface with either Zn or O termination. This type of anisotropic surface structure has been utilized in scientific studies of heterogeneous catalytic processes involving the absorption of molecular and metallic atoms on nonpolar surfaces (Cassarin et al., 1999).
\n\t\t\tSimilar surface nanostructures have been formed by a step-faceting growth mode on vicinal GaAs (775) B and (553) B substrates (Ohno et al., 2000, Yan et al., 2001). In GaAs, high-index or non-singular, planes are energetically unstable and tend to break up into low-index facets at normal epitaxial growth temperatures to minimize their surface energies. This process could produce periodic corrugations composed of nanometer-sized microscopic facets on originally flat surfaces. These ordered microscopic step arrays have been employed as the template for quantum wires. The most investigated high-index surfaces include (311) A, (331) A and (775) B. As an example, we present an AFM image of the GaAs (331) A layer surface in Fig. 3, which shows a highly anisotropic nanowire structure. The GaAs (331) A surface is thermally decomposed to a stable (111) A step and (110) terrace during homoepitaxial growth (Hong et al., 1988). This growth behaviour in GaAs indicates that the growth mechanism of the surface nanowires formed on non-vicinal ZnO (10-10) substrates differs from that of vicinal GaAs substrates. Thus, the surface nanowires on the low-index ZnO (10-10) surfaces are not fully related to the step-faceting process that was applied on the high-index GaAs surfaces.
\n\t\t\ta) AFM image of the GaAs (331)A layer surface. (b) Schematic atom arrangement on cross-section with the [1-10] azimuth of the GaAs (331) A surface.
ZnO layers and Mg0.12Zn0.88O/ZnO quantum wells (QWs) were grown at 400 - 600oC on M-plane ZnO (10-10) substrates (Crystec GmbH, Germany) using laser molecular beam epitaxy (laser-MBE). Figure 4 shows a schematic representation of the laser-MBE apparatus. The ZnO substrates were annealed ex situ at 1100 C in an oxygen atmosphere conducive to the formation of atomically flat surfaces, and then preannealed in situ at 600oC with an oxygen flow of 10-5 mbar for 20 min prior to laser-MBE growth. ArF excimer laser pulses (Compex 103: λ = 193 nm) were focused on ZnO and MgZnO targets located 4.5 cm from the substrates in an oxygen flow of 10-3 mbar. The number of Mg atoms in the MgZnO layer was
\n\t\t\tA schematic figure of the laser-MBE apparatus.
estimated using an electron probe micro-analyzer (EPMA). The growth process was monitored using reflection high electron energy diffraction (RHEED).
\n\t\t\tAtomic force microscopy (AFM: Seiko SPI-3800) was used for observations of surface morphologies. Local structure analyses were conducted by means of high-resolution transmittance electron microscopy. Structural properties were characterized by high-resolution x-ray diffraction (HR-XRD: Philips X’pert) using a double-crystal monochromator. Micro-PL [(μ)-PL] spectroscopy was carried out at room temperature using a fourth harmonic generation of yttrium aluminium garnet laser (Nd3+: YAG laser, 266 nm) excitation and a 0.85-m double monochromator (SPEX 1403) equipped with a nitrogen charge-coupled device camera. A reflective-type objective lens was used for this measurement to focus the laser to a diameter of 1 μm on the sample surface (Matsui et al., 2005). For polarized PL measurement, the sample was excited by a He-Cd laser (λ = 325 nm) in temperatures from 10 to 300 K. The luminescence was directed toward a Glan-Taylor prism to pick up polarization, and then passed through a depolarizer located behind the prism to eliminate the polarization. The spectrum was recorded using a 0.5-m single monochromator (SPEX 500M) equipped with a 1200 grooves/cm grating blazed at 500 nm. Electrical properties were measured using a four-probe Hall bar configuration with the perpendicular arms of the Hall bar aligned carefully in the [0001] and [11-20] directions. The Hall bars were fabricated by Ar ion milling of the samples through a photolithography-defined resist mask. The ex situ annealed ZnO substrate was treated as a semi-insulating substrate showing electrical resistivity in the order of 106 – 107 Ω cm.
\n\t\tWe describe the growth process and morphological evolution of the surface nanowires on the basis of RHEED and AFM investigations. The ZnO layers were grown at 550oC. At the very beginning of layer growth up to 8 nm in thickness, a 2D streak pattern appeared in place of sharp patterns of the ZnO substrates [Fig. 5(a) and 5(b)]. This is related to 2D nucleation at the initial growth stage, as evidenced by the smooth layer surface [Fig. 5(f)]. Continued growth of ZnO changed to a mixed pattern, which relates to the onset of the transition from 2D to 3D modes. This resulted from the appearance of a self-assembly of
\n\t\t\t\tRHEED patterns with the [0001] azimuth of the treated ZnO substrate (a) and ZnO layers with a thickness of (b) 8, (c) 20 and (d) 240 nm. AFM top view (2 x 2 μm2) of the treated ZnO substrate (e) and ZnO layers with different thicknesses [(f)-(h)]. Layer thicknesses are (f) 8, (g) 20 and (h) 250 nm
anisotropic 3D islands [Fig. 5(c) and 5(g)]. Finally, the RHEED pattern showed 3D spots due to an island growth mode that originated from the formation of surface nanowires [Fig. 5(d) and 5(h)]. Surface nanowires with high density (105 cm-1) that formed on the ZnO layers were homogeneously elongated along the [0001] direction above 5 μm with a few branches.
\n\t\t\t\tDue to lattice strains at the heterointerface of a layer/substrate, S-K growth naturally induces 3D islands that are surrounded by high-index facets on 2D wetting layers. This has been observed in InGaAs/GaAs heteroepitaxy (Guha et al., 1990, Matsui et al., 2006). In an effort to examine the crystallinity in greater detail, plan-view and X-TEM observations were conducted to investigate the structural quality of the layer. Figure 6 (a) shows a low-
\n\t\t\t\ta) Low- and high-resolution X-TEM images of the ZnO layer taken with the [11-20] zone axis. Inset shows the RSD obtained by FFT analysis. (c) A bright field plan-view TEM image of the ZnO layer with g = [11-20] excitation under two-beam conditions.
resolution X-TEM image with the [11-20] zone axis. Threading dislocations induced by lattice relaxation between the layer and substrate were not observed. The high-resolution X- TEM image in Fig. 6(b) reveals a lattice arrangement between a smoothly connected layer and substrate. A 3 x 3 nm2 space area selected from the layer region was utilized for a fast Fourier transform (FFT) analysis to examine local lattice parameters, and yielded a reciprocal space diffractogram (RSD) pattern [inset of Fig. 3(b)]. From the RSD pattern, the estimated strains (ε\n\t\t\t\t\tyy and ε\n\t\t\t\t\tzz) at the interface were approximately 0.10% and 0.18% with x, y and z being parallel to the [0001], [10-10] and [11-20] directions, respectively. Figure 6(c) shows a bright field plan-view TEM image with g = [11-20] excitation under two-beam conditions. Out-of-plane dislocations, marked by white open circles, were observed with a density of 3.2 x 107 cm-2, and originated from threading dislocations running perpendicular to the layer surface. On the other hand, there were no in-plane dislocations propagated along the [0002] and [11-20] directions for different g vector excitations. These results indicate that the homoepitaxial interface was almost strain free. Thus, the elongated 3D islands that appeared on the 2D layers were formed under coherent homoepitaxy and had no correlation with S-K growth.
\n\t\t\t\n\t\t\t\t\tFigure 7(a) and 7(b) show low-and high-resolution X-TEM images with the [0001] zone axis, respectively. A cross section of the surface nanowires displayed a triangular configuration with a periodicity of 84 nm. A high-resolution X-TEM image, marked by a white circle, revealed that the side facets did not consist of high-index facets, but instead had a step-like structure with a height of 0.27 nm that corresponded to half a unit of the m- axis. Side facets of the surface nanowires possessed uniform step spacing ranging from 0.1 to 0.2 nm, and were not surrouded by the high-index facets. A large number of surface nanowires showed flat tops with a (10-10) face and were separated laterally by deep grooves, as illustrated schematically in Fig. 7(e). A similar structure was also seen in the anisotropic 3D islands on the 20 nm-thick layers, which indicated that the surface nanowires resulted from a coarsening of anisotropic 3-D islands formed at the initial growth stage.
\n\t\t\t\ta) Low-and (b) high-resolution X-TEM images of the ZnO layer with a thickness of 240 nm. (c) Low-and (d) high-resolution X-TEM images of a 20 nm-thick ZnO layer. Insets (a) and (c) represent AFM images of the ZnO layers used for X-TEM observations. (e) Schematic representation of surface nanowires identified from X-TEM images.
The dependence of lateral periodicity of arrays on the thickness of the ZnO layers at a growth temperature (T\n\t\t\t\t\tg) of 420oC was initially investigated, as shown in Fig. 8(a). The ZnO layers with layer thickness below 10 nm possessed flat surfaces. With increasing a layer thickness up to 15 nm, anisotropically small islands formed along the [0001] direction formed on the layer surface, and then developed to the surface nanowires. The lateral periodicity was 28 nm for the 15 nm-thick ZnO layer, and was almost saturated at 43 nm for ZnO layers with a thickness between 50 and 380 nm. The critical layer thickness, which completely developed to the surface nanowires, was approximately 0.1 μm. The dependence of the saturated lateral periodicity on growth temperature was also investigated. The lateral periodicity increased monotonically from 42 nm at T\n\t\t\t\t\tg = 420oC to T\n\t\t\t\t\tg = 600oC [Fig. 8(b)].
\n\t\t\t\tThe saturation of the lateral periodicity with the layer thickness suggested that the surface migration of Zn-related ablation species, such as ZnO and Zn, supplied from the ablation targets is limited by the terrace width of the side facets (Ohtomo, et al., 1998). This was in agreement with the notion that the increase in periodicity with increasing growth temperature was due to prolonged surface diffusion of ablated species at high temperatures. The importance of surface diffusion was demonstrated using MgxZn1-xO alloys. An inhomogeneity in nanowire length with increasing Mg content was found in surface nanowires on layer surfaces of MgxZn1-xO (10-10) layers, although the lateral periodicity remained unchanged [Figs. 8(c) and 8(d)]. This may have been due to differences in surface migration and sticking probabilities of Zn- and Mg-related species. Thus, surface diffusion plays an important role in determining the size of nanowires, depending on the growth conditions and surface compositions. Moreover, highly anisotropic morphologies must be related because surface diffusion is much faster along the [0001] direction.
\n\t\t\ta) Dependence of lateral periodicity on layer thickness of ZnO layers at T\n\t\t\t\t\t\tg = 420oC. (b) Dependence of saturated lateral periodicity on growth temperature of ZnO layers. (c) Dependence of lateral periodicity on Mg content of MgxZn1-xO layers at T\n\t\t\t\t\t\tg = 420oC. (d) Dependence of nanowire length on Mg content (x) of MgxZn1-xO layers at T\n\t\t\t\t\t\t\n\t\t\t\t\t\t\tg\n\t\t\t\t\t\t = 420oC.
A multilayer morphology is determined not only by the transport of atoms within an atom layer (intralayer transport), but also by the transport of atoms between different atomic layers (interlayer transport). Thus, evolution of mound shapes is understood in terms of activation of atomic processes along the step edge. Therefore, a sequence of multilayer growth is governed by activation of atomic processes which enable exchange and hopping of atoms between different layers [Fig. 9(a)]. Schwoebel and Shipsey introduced the schematic potential energy landscape near a step that become the signature of what is often referred to as the Ehrlich-Schwoebel barrier (ESB) with a barrier energy of ΔE (Schwoebel, 1966). The mass transport of atoms between different layers is inhibited by a strong ESB effect, resulting in mound formation. This induces a nucleation of islands on the original surface together with inhibited interlayer transport. Once the islands are formed, atoms arriving on top of the islands will form second layer nuclei, and on top of this layer, a third layer will nucleate. This repetition leads to an increase in surface roughness with increasing layer thickness (Θ), resulting in the formation of mound shapes.
\n\t\t\t\ta) Upper part of the figure shows the descent of adatoms from an island by hopping and exchange. The lower part illustrates the energy landscape for hopping and the definition of ΔEs. (b) Structural models showing the M-nonpolar (10-10) surfaces of ZnO. The surface unit cells are indicated.
Mound formation is often observed on various systems such as semiconductors, metals, and organic materials. A mound structure possesses a small flat plateau at the top and a side facet with constant step spacing, and is separated from other mounds by deep grooves. This structure has been observed on dislocation-free metal homoepitaxial surfaces such as Pt/Pt (111) and Ag/Ag (100) systems, and is often referred to as a wedding cake (Michely & Krug, 2004). Here, mound formation emerging under reduced interlayer transport is described using the coarsening λ ~ Θ\n\t\t\t\t\tn, and the surface width w ~ Θ\n\t\t\t\t\t\n\t\t\t\t\t\tβ\n\t\t\t\t\t. λ and w values are the height-height correlation between the nanowires and the surface roughening, respectively. As seen in Figs. 10(a) and 10(b) , a coarsening exponent was indicated by n = 0.23, which was close to the n value of mound formation appearing during homoepitaxy. Moreover, the high β value of 0.60 was suitable for mound growth based on the ESB. This indicates that the surface nanowires formed during M-nonpolar ZnO homoepitaxy are due to the growth process originating at the ESB (Yu & Liu, 2008). The ESB was also seen for layer growth of O-polar ZnO with a hexagonal island surface. The appearance of an anisotropic morphology is related closely to a difference in surface diffusion and sticking probability as an important parameter. In M-nonpolar ZnO, the stoichiometric surface is auto-compensated since it contains an equal number of Zn and O ions per unit area. Surface Zn and O atoms form dimer rows running along the [1-210] direction, as shown in Fig. 6(b). This produces two types of A and B step edges consisting of stable low-index (1-210) and (0001) planes, respectively. The [1-210] direction represents an auto-compensated nonpolar surface, while the [0001] direction consists of a polar surface with either Zn or O termination. Thus, the origin of the surface nanowires is based not only on an ESB effect, but a difference in surface diffusion and sticking coefficient of atoms between the two types of step edges.
\n\t\t\t\ta) Height-height correlation (λ) and surface roughening (w) as a function of layer thickness.
AFM images (1 μm x 1μm) and cross-section profiles of ZnO layers on vicinal ZnO (10-10) substrates with off-angle of 0o-, 5o-, 10o-, and 15o from the (10-10)-oriented surface toward the (21-10)-oriented surface, indicated by (a), (b), (c), and (d), respectively. All nanowire arrays were elongated along the [0001] direction.
Anisotropic island growth on GaAs (001) based on the ESB effect is suppressed using vicinal GaAs (001) substrates with an off angle above a certain value (Johnson et al., 1994). AFM images of the ZnO layers on the vicinal substrates are shown in Fig. 11. With an increasing off angle, the cross-section profile of the AFM images gradually changed from symmetric to asymmetric shapes following the increase in lateral periodicity. The ZnO layer yielded a smooth surface with a roughness below 2 nm when the off angle of the substrate reached 15o. The off angle of 15o was close to that of the inclination of the surface nanowires. Figures 10 and 11 indicate that the surface nanowires originated from the ESB mechanism.
\n\t\t\tThe discovery of tenability of a band gap energy based on ZnO has made the alloy system a promising material for use in the development of optoelectronic devices. Characterization of alloys such as (Mg,Zn)O or (Cd,Zn)O is important from the viewpoint of band gap engineering and the p-n junction. It was found that a MgxZn1-xO alloy was a suitable material for the barrier layers of ZnO/(Mg, Zn)O super-lattices due to its wider band gap. Since the ionic radius of Mg (0.56Å) is very close to that of Zn2+ (0.60Å), Mg-rich (Mg, Zn)O alloys with a wurtzite phase have been stably converted even when a rock salt-structured MgO is alloyed. A Mg content doped into a ZnO layer usually depends on the surface polarity, a growth technique, and type of substrate. Figure 12(a) shows the Mg content in MgxZn1-xO layers as a function of the target Mg content. Under growth conditions in this work, the Mg content in Zn-polar layers was always 1.6 times higher than the content in the ablation targets. This can be attributed to the low vapor pressure of Mg-related species compared to that of Zn. The incorporation efficiency of Mg atoms into the layers is more enhanced for O-polarity. On the other hand, the incorporation efficiency of Mg atoms in the case of a M-nonpolar surface is located between Zn- and O-polarities because the M-nonpolar surface has two dangling bonds, while Zn- and O-polarities have three and one-dangling bond, respectively. With increasing Mg content, the band gap systematically increased at 300 K as shown by the results of reflectance spectroscopy [Fig. 12(b)].
\n\t\t\tA micro (μ)-PL technique is a powerful technique and can be employed to examine phase separation problems in MgZnO alloy layers. Figure 13(a) shows the variation in non-polarized μ-PL spectra at room temperature for MgxZn1-xO layers (x = 0 – 0.34). Band-edge emissions of all layers were systematically shifted to high energies with the Mg content alloys with different Mg contents at micro-scale. A driving force for the phase separation is closely related to lattice relaxation based on observations of Zn- and O-polar MgxZn1-xO homoepitaxial layers (Matsui et al., 2006). Therefore, a single phase of M-nonpolar MgxZn1-xO layers is only obtained below x = 0.12. 7-period Mg0.12Zn0.88O/ZnO MQWs were grown
\n\t\t\t\ta) Mg contents in Zn-polar, O-polar and M-nonpolar MgxZn1-xO layers as a function of the target contents. (b) Dependence of band gap energy on Mg content for M-nonpolar MgxZn1-xO layers.
on ZnO layers with surface nanowires at T\n\t\t\t\t\tg = 550oC and p(O2) = 10-3 mbar. The barrier thickness was set to 7 nm, and the well width was controlled from 1.4 to 4 nm. Figure 13(b) shows an AFM image of MQWs with a L\n\t\t\t\t\tW of 2.8 nm. Surface nanowires elongated along the [0001] direction were homogeneously retained even after growing the MQWs. Figure 9(c) shows an X-TEM image of MQWs with the [0001] zone axis. The layer with a bright contrast represents MgZnO barriers, while the dark layers represent ZnO wells, which indicate that the MgZnO layers repeat the surface structure of the underlying ZnO layers.
\n\t\t\ta) Non-polarized μ-PL spectra of MgxZn1-xO layers (x = 0, 0.07, 0.12, 0.19 and 0.34). The emission peak at 3.2 eV originates from the ZnO substrates. (b) Surface morphology and (c) X-TEM image of Mg0.12Zn0.88O/ZnO QWs with a L\n\t\t\t\t\tW = 2.8 nm.
ZnO has attracted great interest for new fields of optical applications. Interesting characteristics of wurzite structure include the presence of polarization-induced electric fields along the c-axis. However, the optical quality of a quantum-well structure grown along the c-axis suffers from undesirable spontaneous and piezoelectric polarizations in well layers, which lower quantum efficiency. The use of nonpolar ZnO avoids this problem due to an equal number of cations and anions in the layer surface. Nonpolar ZnO surfaces have in-plane anisotropy of structural, optical, acoustic, and electric properties, which is useful for novel device applications. In this session, we discuss polarized PL of M-nonpolar ZnO layers and Mg0.12Zn0.88O/ZnO QWs.
\n\t\t\t\t\n\t\t\t\t\tFigure 14 shows splitting of the valence band (VB) in ZnO under the influence of crystal-field splitting and spin-orbit coupling. The VB of ZnO is composed of p-like orbitals. Spin-orbit coupling leads to a partial lifting of the VB degeneracy, and the former six-fold degenerate VB is split into a four-fold (j = 3/2) and two-fold (j = 1/2) band. The spin-orbit coupling is negative. The j = 1/2 band is at a higher energy than the j = 3/2 band. On the other hand, the crystal field in ZnO results in further lifting of the VB degeneracy due to the lower symmetry of wurtzite compared to zinc blende. The crystal field causes a splitting of p states into Γ5 and Γ1 states. Crystal-field splitting Δ\n\t\t\t\t\t\tcf\n\t\t\t\t\t and spin-orbit coupling Δso together give rise to three types of two-fold degenerate valence bands, which are denoted as A (Γ9-symmetry), B (Γ7) and C (Γ7) (Reynolds et al., 1999). These energies are formulated as follows:
\n\t\t\t\tSchematic energy level of band splitting by the crystal-field (Δ\n\t\t\t\t\t\t\t\tcf\n\t\t\t\t\t\t\t) and spin-orbit (Δ\n\t\t\t\t\t\t\t\tso\n\t\t\t\t\t\t\t) interactions in a wurtzite structure. F\n\t\t\t\t\t\t\tA(X) and F\n\t\t\t\t\t\t\tC(X) correspond to A and C-excitons, respectively, which are indicated in the middle. In the electronic energy levels proposed by Park et al. and Reynolds et al., the uppermost Γ9 and Γ7 levels are interchanged.
For ZnO, the experiment gave E\n\t\t\t\t\tA-E\n\t\t\t\t\tB = 0.0024 eV and E\n\t\t\t\t\tC = 0.0404 eV (Fan et al., 2006). Solving the above two equations, we obtain Δcf (0.0391 eV) and Δso (-0.0035 eV). A- and B-excitons are referred to as heavy (HH) and light hole (LH) bands, respectively, and the crystal-field split-off hole (CH) was related to the C-exciton. The detection of E⊥c and E//c points to A-exciton (X\n\t\t\t\t\tA) and C-exciton (X\n\t\t\t\t\tc), respectively, where E represents the electric field vector (\n\t\t\t\t\t\tReynolds et al., 1999\n\t\t\t\t\t).
\n\t\t\t\t\n\t\t\t\t\tFigure 15(a) shows the E⊥c and E//c components of the normalized PL spectra of strain-free ZnO layers (Matsui &Tabata, 2009). Polarization direction is referenced in Fig. 16 (a). The peak energies of X\n\t\t\t\t\tA and X\n\t\t\t\t\tC were located at 3.377 and 3.419 eV, respectively. These energies coincided with the X\n\t\t\t\t\tA (3.377 eV) and X\n\t\t\t\t\tC (3.4215 eV) peaks in ZnO crystals, respectively. The dependence of peak intensities on temperature could be fitted using the Bose-Einstein relation with a characteristic temperature of 315 and 324 K from the X\n\t\t\t\t\tA and X\n\t\t\t\t\tC peaks, respectively. The bound exciton (D\n\t\t\t\t\t0\n\t\t\t\t\tX) peak disappeared at 120 K due to the activation energy of 16 meV. The polarization degree (P) is defined as (I⊥\n\t\t\t\t\t- I\n\t\t\t\t\t\n\t\t\t\t\t\t//\n\t\t\t\t\t\n\t\t\t\t\t) / (I⊥\n\t\t\t\t\t+ I\n\t\t\t\t\t\n\t\t\t\t\t\t//\n\t\t\t\t\t), where I⊥ and I\n\t\t\t\t\t\n\t\t\t\t\t\t//\n\t\t\t\t\t are the peak intensities for E⊥c and E//c, respectively. Figure 15 (b) shows the polarization-dependent PL spectra at 300 K. The layer strongly emitted polarized light. The P value was calculated as 0.49. Significant spectral shifts in PL were detected when altering the polarization angle. This is attributed to a difference in carrier distribution in the VB between the HH and CH levels at 300 K. Figure 15(c) shows the dependence of polarization angle on PL intensity. Experimental data (triangle dots) were in agreement with the cos (θ)2 fit line (solid) obeyed by Malus’ law.
\n\t\t\tThe polarization PL character in M-nonpolar MQWs is now discussed. ZnO wells are strain-free in the case of pseudomorphically grown MgZnO/ZnO MQWs. The PL spectra for E\n\t\t\t\t\t⊥\n\t\t\t\t\tc and E//c in MQWs with a well thickness (L\n\t\t\t\t\tW) of 2.8 nm are shown in Fig. 16(a).
\n\t\t\t\ta) Temperature dependence of PL spectra on strain-free ZnO layers for E⊥c (solid lines) and E//c (dotted lines). (b) Polarization-dependent PL spectra at 300 K taken in steps of Δθ = 15o. (c) PL intensity as a function of polarization angle θ. Inset shows a schematic representation of the measurement geometry and sample orientation
The emission peaks around 3.6 eV correspond to 7 nm-thick Mg0.12Zn0.88O barriers. At 300 K, an energy separation (ΔE) of 37 meV was found between the MQWs emissions of 3.372 eV (E⊥c) and 3.409 eV (E//c). The emission peak for E⊥c appeared under conditions of E//c below 120 K since the thermal distribution of carriers in the high-energy level for E//c is negligible at 10 K. A polarization degree close to unity was found with a high P of 0.92 at 10 K [Fig. 16(c)]. In contrast, excited carriers at 300 K were sufficiently distributed in the high-energy level, resulting in a low P of 0.43. Furthermore, ΔE between the emission peaks for E⊥c and E//c was retained at around 40 meV even at 60 K [Fig. 16(d)]. This ΔE was close to the theoretical ΔE between the X\n\t\t\t\t\tA and X\n\t\t\t\t\tC states (Mang et al., 1999). For unstrained bulk ZnO, a polarization magnitude of zero and unity in the C-exciton is detected along the normal direction and along the c-axis, respectively [Fig. 15(a)]. However, the confinement of M-nonpolar MQWs takes place perpendicular to the quantization of the [10-10] direction. This result generates weak mutual mixing of the different p orbitals. Therefore, a π polarization component is expected for the A-excitonic state in these MQWs. In the case of M-nonpolar MQWs, it is predicted that a 10% p\n\t\t\t\t\tz orbital component is involved with the A-excitonic states (Niwa et al., 1996), which is in agreement with the experimentally obtained P value of 0.92. MQWs with a L\n\t\t\t\t\tW of 1.4 nm showed that the polarized PL spectra of E⊥c and E//c were separated by a small ΔE of 27 meV at 300 K [Fig. 16(b)]. ΔE decreased with temperature, and then completely disappeared at 60 K. The P value also dropped for all of the temperature regions. These behaviours are due to a large admixture of p\n\t\t\t\t\tx to p\n\t\t\t\t\tz orbitals for E//c, originating from an inhomogeneous roughening between the well and barrier layers. The interface roughness increased a potential fluctuation of quantized levels in the MQWs, being reflected by the broadened PL lines (Waag et al., 1991).
\n\t\t\tPL spectra under E⊥c (solid lines) and E//c (dotted lines) on MQWs with L\n\t\t\t\t\tW = 2.8 nm (a) and 1.4 nm (b). (c) and (d) show the relationship of temperature with polarization degree (P) and energy separation (ΔE) on MQWs with different L\n\t\t\t\t\tW.
Transport properties were determined using a double Hall bar configuration with the [0001] and [1-210] directions [Fig. 17(a) and 17(b)]. Figure 17 (b) shows the temperature-dependent Hall mobility parallel (μ\n\t\t\t\tH[0001]) and perpendicular (μ\n\t\t\t\tH[1-210]) to the nanowires. μ\n\t\t\t\tH[0001] gradually increased with decreasing temperature and was almost retained below 150 K due to a suppression of ionized impurity scattering. The electron concentration of MQWs also saturated below 70 K, suggesting that the whole electric current flows as 2D-like transport through the ZnO wells. On the other hand, μ\n\t\t\t\tH[1-210] was much lower and resulted in large anisotropy of electron transport. The nature of the large transport anisotropy was evaluated by examining the temperature dependence of the conductivity (σ) for both directions [Fig. 17 (c)]. σ\n\t\t\t\t[0001] parallel to the nanowire arrays was almost independent of temperature below 70 K. Furthermore, the carrier concentration (n\n\t\t\t\te) was also constant below 70 K, suggesting that σ\n\t\t\t\t[0001] possessed metallic conductivity. In contrast, σ\n\t\t\t\t[1-210] perpendicular to the nanowire increased exponentially with T\n\t\t\t\t-1 in the region from 70 to 10 K. The activation energy E\n\t\t\t\ta was 12 meV, indicating that an effective potential barrier height of 12 meV is formed for electron motion perpendicular to the nanowire arrays.
\n\t\t\ta) Hall bar patterned used to investigate anisotropic transport. (b) An AFM image of the Mg0.12Zn0.88O/ZnO MQWs. (c) Temperature dependence of μ\n\t\t\t\t\t\tH[0001] and μ\n\t\t\t\t\t\tH[1-210] for MQWs with a L\n\t\t\t\t\t\tW of 2.8 nm. (d) Temperature dependence of σ\n\t\t\t\t\t\t[0001] and σ\n\t\t\t\t\t\t[1-210] for MQWs with a L\n\t\t\t\t\t\tW of 2.8 nm.
\n\t\t\t\tFigure 18 (a) shows the ratio of μ\n\t\t\t\tH[0001] and μ\n\t\t\t\tH[1-210] as a function of temperature. The curves correpond to different L\n\t\t\t\tW of 2.2, 2.8 and 4 nm. For MQWs with a L\n\t\t\t\tW of 4 nm, we observed no anisotropic behavior. However, the anisotropy of the Hall mobiliyu increased to 52 for MQWs with a L\n\t\t\t\tW of 2.8 nm at low temperatures. The transport properties indicate that an electron can move almost freely along the nanowires, but are blocked from moving perpendicular to the nanowires. We discuss a possible mechansim for this type of activation barrier. The large anisotropy of electron transport disappeared when a flat surface was realzied using Zn-polar MQWs, as shown in Fig. 18(b). Zn-polar MQWs shows an isotropic surface morphology and has sharp MgZnO/ZnO heterointerfaces, as confirmed using high-resolution X-TEM image [inset of Fig. 18(b)]. Interface-roughness scattering dominates low-temperature mobility in MQWs (Sakai et al., 1987). A slight roughness of the heterointerfaces induces a large fluctuation in quantization energy of confined electrons. This acts as a scattering potential barrier for electron motion and reduces mobility. Therefore, electrons may readily undergo frequent scattering in a direction perpendicular to the nanowires by potential barriers produced between nanowires, and, consequently, may become extremely immobile.
\n\t\t\tOn the other hand, parallel conductance along the nanowires involves a lower scattering probality than perpendicular transport due to a weak heterointerface modulation. However, the P value for MQWs with a L\n\t\t\t\tW of 2.2 nm decreased with a decrease in μ\n\t\t\t\tH[0001]. Inspection of polarized PL spectra showed that the energy fluctuations in the quantum well gradually increaed with decreasing L\n\t\t\t\tW [Fig. 18]. A decreased Hall moblity with a narrowing of LW has been observed on very thin InAs/GaSb MQWs since energy fluctuations in a quantum well are caused by an increase in interface roughness (Tsujino et al., 2004, Szmulowicz et al., 2007). It is concluded, therefore, that the large transport anisotropy was obtained through both a quantum size effect and small energy fluctuations in the quantum well, i.e., when L\n\t\t\t\tW was in the vicinity of 3 nm.
\n\t\t\ta) Temperature dependence of anisotropy of mobility (P) for M-nonpolar Mg0.12Zn0.88O/ZnO MQWs with different well thicknesses (L\n\t\t\t\t\t\tW) of 2.2, 2.8 and 4 nm. (b) Temperature depednence of Hall mobiliy (μ\n\t\t\t\t\t\tH[11-20]) and (μ\n\t\t\t\t\t\tH[10-10]) for Zn-polar Mg0.27Zn0.73O/ZnO MQWs with a L\n\t\t\t\t\t\tW of 2.4 nm.
Finally, the whole μ\n\t\t\t\tH[0001] for MQWs was higher than that for M-nonpolar ZnO single layers as a result of the MQWs structure. However, it was lower than the mobility of ZnO single crystals, which is associated with thermal diffusions of extrinsic impurities from the hydrothermal ZnO substrates as a mobility-limited mechanism. Al atoms of the order of 1016 cm-3 were incorporated thermally into the samples from the substrates, as confirmed by secondary ion mass spectroscopy. It is noted that the presence of a background impurity results in a decrease of the whole Hall mobility for the MQWs.
\n\t\tWe introduced a growth process for the anisotropic morphology that formed naturally on M-nonpolar ZnO (10-10) layer surfaces. Surface nanowires of high density elongated over 2 μm in length were observed during homoepitaxial growth. The surface nanowires gradually developed from elongated 3D mounds that originated from an ESB effect. Fabrication of M-nonpolar MQWs allowed examination of the relationship between electron transport and surface morphology. The observed transport anisotropy correlated strongly with the surface morphology, and was dependent on the crystal quality of the MQWs as determined by low-temperature polarized PL spectroscopy. The MQWs showed strong polarization light at 300 K with a P value of 0.43 for E⊥c. Polarization anisotropy was based on the selection rule and is attributed to emissions from A- and C-excitonic states. Deviation of the P value from unity at 10 K in MQWs was associated with the confinement-induced admixture of the P\n\t\t\t\tz orbit to the A-excitonic states, being a characteristic of the M-plane quantum well. As a possible mechanism of the large conductance anisotropy, we proposed that electron motion perpendicular to the nanowire arrays was restricted by the potential barrier caused by an interface roughness between the surface nanowires.
\n\t\t\tIn this chapter, we reported the formation of electrical channels with a 1-D conductance as in quantum wires using M-nonpolar MQWs on the lateral surface of a nanowires structrue. Semiconductor quantum wires have been the subject of extensive theoretical and experimental studies over the past two to three decades. These are motivated by various unique quantum effects predicted in 1-D electronic systems, such as strong Coulomb correlation, suppression of electron scattering and an increase of quantum confinement. Q uantum wires have many potential applications to optoelectronic devices, such as high-mobility field-effect transistors. These surface nanowire structures contribute additional degrees of freedom for future studies of electron transport in field-effect transistors and magnetoelectric devices.
\n\t\tThis work was supported in part by a Grant-in-Aid for Young Scientists (No. 18760231) from the Japan Society for the Promotion of Science, and a research grant from the Iketani Science and Technology Foundation (No. 081085-A)
\n\t\tAt the beginning of the space age, all satellites were “small.” Sputnik 1 was the first artificial Earth satellite (Figure 1a) [1]. It was launched by the Soviet Union from Baikonur Cosmodrome on October 4, 1957, into an elliptical low Earth orbit (LEO) with an inclination of 65°. Sputnik 1 was a 58-cm-diameter metal sphere, weighing approximately 84 kg, with four radio antennas transmitting at 20.005 and 40.002 MHz. Tracking and studying Sputnik 1 signals from Earth provided valuable information on upper atmosphere density, and the propagation of radio signals provided information on the ionosphere. Sputnik did not have solar panels, so the mission ended after 3 weeks when batteries died.
Pictures of (a) sputnik 1 [4], (b) explorer 1 [5], and (c) vanguard 1 [6].
Explorer 1 was the first US satellite (Figure 1b) [2], and the third one after Sputnik 1 and 2. It was launched from Cape Canaveral, Florida, on January 31, 1958. Explorer 1 was 205 cm tall and 15 cm in diameter, weighing nearly 14 kg. It was the first spacecraft to detect the Van Allen radiation belts. Explorer 1 did not have solar panels either, so after 4 months the mission ended when batteries were exhausted.
Vanguard 1 was the fourth artificial Earth satellite (Figure 1c) [3]. It was launched by the USA from Cape Canaveral on March 17, 1958, into a 654 by 3969 km elliptical orbit with an inclination of 34.25°. Vanguard 1 was a 16.5-cm-diameter aluminum sphere, weighing just 1.47 kg, and it was the first satellite with six solar cells powering two beacons at 108 and 108.03 MHz, which were used to measure the total electron content.
During the first two decades of the space age, each satellite had its own design. They were the art pieces of the space craftsmen. Standard spacecraft busses were practically unknown until the end of the 1970s. In the early 1980s, microsatellites emerged and adopted a radically different design approach to reduce costs, focusing on available and existing technologies and using properly qualified commercial off-the-shelf (COTS) components.
For many years, satellite mass increased as illustrated in Table 1. However, except for some military, astronomy, and specific communication applications, it seems that the era of massive satellites is over.
Spacecraft | Agency application | Mass | Duration |
---|---|---|---|
KH-11 Kennen (a.k.a. CRYSTAL, EECS, 1010) [7] | US NRO/optical imaging | 19,600 kg | 1976–present |
Proton [8] | USSR/astronomy | 17,000 kg | 1965–1969 |
Compton Gamma Ray Obs. [9] | US NASA/astronomy | 16,329 kg | 1991–2000 |
Lacrosse [10] | US NRO/SAR | 14,500–16,000 kg | 1988–2005 |
Hubble Space Telescope [11] | US NASA/astronomy | 11,110 kg | 1990–present |
ENVISAT [12] | ESA/Earth observation | 8211 kg | 2002–2012 |
Telstar 19 V [13] | Canada/communications | 7075 kg | 2018–present |
Heaviest spacecrafts (excluding space stations and manned orbiters).
The “small satellite mission philosophy” represents a design-to-cost approach, with strict cost and schedule constraints, often combined with a single mission objective in order to reduce complexity. Figure 2 from [14] summarizes the standardized definition of satellites according to their weight: picosatellites (0.1–1 kg), nanosatellites (1–10 kg), microsatellites (10–100 kg), and mini-satellites or small/medium satellites (100–1000 kg).
Satellite classification [14].
In the field of Earth observation (EO), this has led to smaller target-focused missions which, with reduced spacecraft and launch costs (shared rides), are enabling massive (>100) satellite constellations of nano- and microsatellites with reduced revisit times, unthinkable just a few years ago.
In the field of satellite communications, there are plans as well to deploy massive constellations of LEO satellites to provide worldwide Internet coverage, IoT services, and machine-to-machine (M2M) communications.
It is anticipated that enhanced inter-satellite communication capabilities (LEO-ground, LEO-LEO, LEO-MEO, and LEO-GEO) will also improve the performance of EO systems [15]. All this is leading to the evolution of the space segment from monolithic to distributed and federated satellite systems [16], aiming at establishing win-win collaborations between satellites to improve their mission performance by using the unused onboard resources.
The so-called CubeSat standard was conceived in 1999 by Profs. Jordi Puig-Suari of California Polytechnic State University (CalPoly) and Bob Twiggs of Stanford University to allow graduate students to conceive, design, implement, test, and operate in space a complete spacecraft in a “reasonable” amount of time (i.e., the duration of their studies). CubeSats are small satellite multiples of 1 U (1 U: 10 cm × 10 cm × 11.35 cm, weighing less than 1.33 kg), including all the basic subsystems as large satellites but using COTS components. The CubeSat “standard” only defines the mechanical external interfaces, i.e., those referring to the orbital deployer. Originally, it was never meant to be a standard, however, because of its simplicity, it soon became a “de facto” standard. As Prof. Twiggs said in an interview to Spaceflight Now in 2014: “It all started as a university education program satellite. It was kind of funny. I didn’t think that people would criticize it as much as they did, but we got a lot of feedback (…). Another thing that was kind of funny we had no interest from NASA or any of the military organizations. It just wasn’t anything they were interested in, so it was all funded without any funding from those aerospace organizations.” The first six CubeSats were launched on a Russian Eurockot on June 30th, 2003. Then, after more than a decade in which the concept silently matured in university labs, space agencies got interested and showed that CubeSat-based mission reliability could be improved by proper engineering. In 2013, it all took off on the commercial Earth Observation sector with the first launches from two companies that are now running 100+ CubeSats constellations for optical imaging or weather prediction, with very low revisit times. Today, many of the initial CubeSat limitations (most notably size, available power, and down-link bandwidth) are being overcome, and the same revolution is starting to take place in the fields of telecommunications, and astronomical scientific missions.
The current CubeSat Design Specification defines the envelopes for 1 U, 1.5 U, 2 U, 3 U and 3 U+, and 6 U form factors (see, e.g., CubeSat Design Specification Rev. 13 or 6 U CubeSat Design Specification in [17], Figure 3), and the standardization of 12 U and 16 U is in progress, although some companies have produced standards up to 27 U [18]. On the other side, smaller picosatellites, the so-called PocketQubes, about 1/8 the size of a CubeSat, have also been standardized [19].
CubeSat form factors from 1 U to 12 U [20].
Probably, what has had the most significant impact in the popularization of the CubeSat standard has been the capability to separate the interface between the spacecraft and the poly-picosatellite orbital deployer (P-POD) and between the dispenser and the rocket itself. There are two different classes of PODs. The first type is the classical one with four rails in the corners [17], and the second one is with tables [18]. Note however that modern deployers from ISIS and NanoRacks allow larger deployables, wider solar panels, and thinner rails as compared to original P-POD, e.g., increased extruded height up to 9 mm and up to 2 kg per 1 U.
As of June 2019, 64 countries have launched nanosatellites or CubeSats. The total number of nanosatellites launched is 1186, from which 1088 are CubeSats. Most of them (273) have been launched from the International Space Station at ~400 km orbital height with an inclination of 51.6° and the rest at low Earth orbits (LEO) typically at 500 km sun-synchronous orbit (SSO) with an inclination of 97.5° (217 CubeSats) and 580 km height with 97.8° inclination (80 CubeSats). So far, only two (MarCO-1 and MarCO-2) have performed interplanetary missions.
Figure 4 shows the number of nanosatellites launched per year (a) and organization, either companies, universities, space agencies, etc., or (b) form factor from picosats, 0.25 up to 16 U.
The number of nanosatellites launched per year and (a) organization or (b) form factor [21].
As it can be appreciated, until 2013 most CubeSats were launched by universities and research institutes, and most of them were 1 U or 2 U. However, in 2013 the first 3 U CubeSats from the Planet Labs Inc. [22] and Spire Global Inc. [23] were launched. That was the beginning of today’s revolution in EO, and—as of June 10, 2019—these two companies had launched the largest commercial constellations ever with 355 and 103 CubeSats, respectively. The following ones have launched at most seven CubeSats. Therefore, 3 U CubeSats are dominating the scene, and they will over the next decade, followed by far by the 1 U, 2 U, and 6 U form factors (Figure 5). However, it is expected that the next wave of growth will be based on 6 U and 12 U CubeSats, which offer the right balance between very capable payloads and limited manufacturing and launch costs.
The number of CubeSats by form factor [21].
Table 2 (extracted from the database in [21]) shows the main companies that have launched CubeSats, the number of launched and planned CubeSats, the year of the first launch, the form factor, the application field, and some technical details. The rows marked in light blue correspond to EO optical imaging, in light green to EO passive microwaves applications, in dark green to EO active microwaves applications, and in light orange typically to IoT and M2M communications. In the next sections, we will focus on the EO applications but keeping in mind that future advances in satellite communication networks will also improve the performance of EO systems and enable new ones as well as distributed ones (e.g., large synthetic apertures).
Organization | Launched/planned size | First launch | Form factor | Field | Technical and comments |
---|---|---|---|---|---|
Planet Labs | 355/150 | 2013 | 3 U | Earth observation | 29 MP sensor taking images with 3.7 m ground resolution and swath of 24.6 km × 16.4 km from 475 km altitude |
Spire | 103/150 | 2013 | 3 U | Weather, AIS, ADS-B, earthquake | Measure change in GPS signal after passing atmosphere to calculate precise profiles for temperature, pressure, and humidity. Investigating earthquake (ELF) detection |
AprizeSat | 12/12 | 2002 | Microsat | IoT/M2M | Low-cost satellite data services for monitoring the fuel level and oil and gas pipelines and mobile tracking of shipping containers, railcars, and trailers |
GeoOptics | 7/N/A | 2017 | 6 U | Weather | Using GPS radio occultation for weather data |
Swarm Technologies | 7/150 | 2018 | 0.25 U, 1 U | IoT/M2M | World’s smallest two-way communication satellites |
Commsat | 7/72 | 2018 | Microsat, 6 U, 3 U | IoT/M2M, AIS | Ladybeetle 1 is 100 kg and 3 CubeSats of 6 U and 3 of 3 U. Plans 4 more in 2019 and complete 72 satellites in 2022 |
Astro Digital | 6/25 | 2014 | 6 U, 16 U | Earth observation | 6 U has 22 m resolution in RGB and NIR. 16 U has 2.5 m resolution in RGB, red edge, and NIR with 70 MP sensor |
Fleet Space | 4/100 | 2018 | 3 U, 12 U, 1.5 U | IoT/M2M | Main constellation potentially with 12 U CubeSats |
Sky and Space Global | 3/200 | 2017 | 8 U, 6 U, 3 U | IoT/M2M | Communication service (voice, data, and M2M). Plans to use inter-satellite links |
NanoAvionics | 2/72 | 2017 | 6 U, 12 U | IoT/M2M | Global IoT constellation-as-a-service system aimed at IoT/M2M communication providers |
Helios Wire | 2/30 | 2017 | 6 U, 16 U | IoT/M2M | Uses 30 MHz of S-band spectrum to receive tiny data packages from billions of sensors |
Kepler Communications | 2/140 | 2018 | 3 U, 6 U | IoT/M2M, Internet | IoT/M2M data communication network. Monthly fee based on the data amount. Hope to achieve rates of 1–40 Mbps |
Analytical Space | 1/N/A | 2018 | 6 U | IoT/M2M, orbital data relay, optical comms. | In-orbit relays receiving radio and downlink to ground with laser communication enabling more data downlink from satellites |
Hiber | 2/48 | 2018 | 6 U | IoT/M2M | Sends small packets of data (140 characters, accompanied by time stamp, identifier, and location) |
Guodian Gaoke | 2/38 | 2018 | 6 U | IoT/M2M | Reliable and economical satellite IoT services and industry solutions for our customers |
Astrocast | 2/80 | 2018 | 3 U | IoT/M2M | Targeting L-band. Inter-satellite links. NanoSpace propulsion. Further 80 satellites in orbit by 2022 |
AISTech | 2/150 | 2018 | 2 U, 6 U | IoT/M2M, ADS-B, AIS, IR imaging | Two-way comms., thermal imaging to detect forest fires, aviation tracking (ADS-B) |
ICEYE | 2/18 | 2018 | Microsat | SAR | 21-launch agreement with Vector Space Systems. 10-platform agreement with York Space Systems |
Harris Corp. | 1/12 | 2018 | 6 U | Weather | Immediate access to 3D wind data sets from Harris-owned HyperCubes |
SIRION | 1/N/A | 2018 | CubeSat | IoT/M2M | IoT/M2M constellation. Partnered closely with Helios Wire. Sharing spectrum and satellites |
Reaktor Space | 1/36 | 2018 | 6 U, 2 U | Earth observation, hyperspectral | Hyperspectral constellation for smart agriculture with 100’s of spectral bands and 20 m resolution |
Myriota | 1/50 | 2018 | CubeSat | IoT/M2M | Run unique, patented software which provides reliable, direct-to-satellite Internet of Things (IoT) connectivity |
LaserFleet | 1/192 | 2018 | CubeSat | Internet, optical comms. | Provide reliable 1 Gbps communication rates to aircraft at altitude. Higher effective data rate at a lower cost than the best-in-class Ku/Ka/V |
ADASpace | 1/192 | 2018 | Microsat CubeSat | Earth observation | Establish a global, minute-level updated Earth image data network consisting of 192 satellites |
Orbital Micro Systems | 1/40 | 2019 | 3 U | Weather | Weather constellation utilizes microwave technology to capture temperature and moisture measurements, refreshed and delivered every 15 minutes |
Lacuna Space | 1/32 | 2019 | 3 U, 6 U | IoT/M2M | IoT/M2M constellation. Selected Open Cosmos to build 3 U demonstrator |
The main existing and planned CubeSats and microsat commercial constellations.
Blue for constellations for optical EO, light green for passive microwave EO, dark green for active microwave EO, and orange for IoT and M2M communications.
The interested reader is encouraged to consult [21] for the most updated information as these numbers can change rapidly. Note that the number of CubeSats that can be launched in a single rocket can be very high. The current record is held by the Indian rocket PSLV-C37 that, on February 15, 2017, launched Cartosat-2D and 103 CubeSats, from which 88 are from the Planet Labs Inc. and 8 are from the Spire Global Inc. The interested reader is invited to see the deployment of these satellites from the onboard camera at [24].
As illustrated in Table 2, by 2010 the maturity achieved by CubeSats and dispensers/launchers, on one side, and by some EO technologies (high-resolution multispectral imagery and GNSS-RO), on the other side, made possible that a number of companies developed applications based on commercial constellations. Today, thanks to an intense technology R&D, the situation is completely different.
The reasons for this have been threefold. On one side, due to their small size, it has been difficult to include deployable solar panels so as to increase the electrical power generated, and, on the other side, it has been difficult to include large antenna reflectors and to transmit enough RF power so as to have a satisfactory space-to-Earth link budget. The third reason was the poor pointing accuracy that now has significantly improved thanks to miniaturized star trackers and reaction wheels. So far, these reasons have kept active optical (LIDAR) and active microwave sensors (RADAR) away from CubeSats, although it has to be stated that synthetic aperture radars (SAR) have been recently boarded in microsatellite platforms successfully (ICEYE, Table 2).
For spaceborne EO applications, frequency bands are restricted to those in which the atmosphere exhibits a high transmissivity, that is, the microwave and millimeter-wave parts of the radio spectrum and the long-wave infrared (LWIR), near infrared (NIR), and visible (VIS) parts of the spectrum, as illustrated in Figure 6.
Electromagnetic spectrum with different bands indicated [25].
For astronomical observations, ground-based observations are also limited to Earth’s atmospheric windows in the radio and optical parts of the spectrum (Figure 6). Therefore, to explore the remaining parts of the EM spectrum, space-based observatories are required.
In any case, either for EO or astronomical observations, the lower cost of individual CubeSat-based missions allows having more units, which reduces the revisit time at a given cost. This offers a number of new science opportunities [26]:
Earth science:
Multipoint high temporal resolution of Earth processes
Mitigation of data gaps
Continuous monitoring
Astrophysics:
Space telescopes allow access to energies across the whole electromagnetic spectrum avoiding large gaps in the radio, far IR, and the entire high-energy range (UV to γ-rays).
Feasibility to conduct time domain programs, which are very challenging with flagship missions such as the Hubble Space Telescope and James Webb Space Telescope.
Heliophysics, e.g., measurement of plasma processes in the magnetosphere-ionosphere system.
Planetary science: in situ investigation of planetary surfaces or atmospheres.
Astronomy and astrophysics: low-frequency radio science and the search for extrasolar planets.
Biological and physical sciences, e.g., survival and adaptation of organisms to space
Since the CubeSat standard was proposed in 1999, it took about a decade for NASA to start the Educational Launch of Nanosatellites (ELaNa) initiative in 2010. Partnerships were established with universities in the USA to design and launch CubeSats through NASA’s CubeSat Launch Initiative (CSLI). Since then, 85 CubeSat missions have flown on 25 ELaNa calls, and 34 more CubeSats are manifested in 4 more calls. While it provides NASA with valuable opportunities to test emerging technologies that may be useful in future space missions, university students get involved in all phases of the mission, from the instrument and satellite design to its launch and monitoring.
As early as 2012, NASA’s Science Mission Directorate (SMD) technology programs began to accommodate the use of CubeSats for validation of new science instruments and strategically promoted the use of small spacecraft to advance its science portfolio.
On one side, the Earth Science Technology Office (ESTO), which is responsible for identifying and developing technologies in support of future Earth Science Division missions, manages three major observation technology programs that solicit new awards on a 2–3-year selection cycle, as shown in Table 3 [27].
Earth science program | Approx. funding | Description |
---|---|---|
Instrument Incubator (IIP) | $28 M/year | Nurtures the development and assessment of innovative remote sensing concepts in ground, aircraft, or engineering model demonstrations (early to mid-stage development) |
Advanced Components (ACT) | $5 M/year | Enables the research, development, and demonstration of component- and subsystem-level technologies to reduce the risk, cost, size, mass, and development time of missions and infrastructure |
In-Space Validation of Earth Science Technologies (InVEST) | $5 M/year | Advances the readiness of existing Earth science-related technology and reduces risks to future missions through space flight validation using CubeSats |
Earth science technology programs relevant to small satellites [27].
And on the other side, following the outcomes of [28] in 2014, the Planetary Science Division (PSD) has also made significant strides toward accommodating small satellites for exploration of the solar system and for astrophysics research. Table 4 [27] summarizes the three main planetary science technology programs.
Planetary science program | Approx. funding | Description |
---|---|---|
Planetary Instrument Concepts for the Advancement of Solar System Observations (PICASSO) | $4 M/year | Supports the development of spacecraft-based instrument components and systems that show promise for future planetary missions. The program supports early-stage technologies |
Maturation of Instruments for Solar System Exploration (MatISSE) | $6 M/year | Supports the advanced development of spacecraft-based instruments that may be proposed for future planetary missions that are at the middle stages of technology readiness |
Development and Advancement of Lunar Instruments (DALI) | $5 M/year | Supports the development of science instruments for small lunar landers and orbital assets that are at the middle stages of technology readiness |
Planetary science technology programs relevant to small satellites [27].
The result of these continued investments is summarized in Table 5, where a number of EO techniques that were infeasible in 2012 [29] were all feasible 5 years later [30], many of them demonstrated by CubeSat missions, some of them commercial, and some even operational constellations. CubeSat-based astronomy missions will be discussed later.
Technology | 2012 technology review by Selva and Krejci | 2017 technology review by Freeman et al. | Justification |
---|---|---|---|
Atmospheric chemistry instruments | Problematic | Feasible | PICASSO, IR sounders |
Atmospheric temperature and humidity sounders | Feasible | Feasible | — |
Cloud profile and rain radars | Infeasible | Feasible | JPL RainCube demo |
Earth radiation budget radiometers | Feasible | Feasible | SERB, RAVAN |
Gravity instruments | Feasible | Feasible | No demo mission |
Hi-res optical imagers | Infeasible | Feasible | Planet Labs. |
Imaging microwave radars | Infeasible | Problematic | Ka-Band 12 U design |
Imaging multispectral radiometers (Vis/IR) | Problematic | Feasible | AstroDigital |
Imaging multispectral radiometers (μW) | Problematic | Feasible | TEMPEST |
Lidars | Infeasible | Problematic | DIAL laser occultation |
Lightning imagers | Feasible | Feasible | — |
Magnetic field | Feasible | Feasible | InSPIRE |
Multiple direction/polarization radiometers | Problematic | Feasible | HARP Polarimeter |
Ocean color instruments | Feasible | Feasible | SeaHawk |
Precision orbit | Feasible | Feasible | CanX-4 and CanX-5 |
Radar altimeters | Infeasible | Feasible | Bistatic LEO-GEO/MEO |
Scatterometers | Infeasible | Feasible | CYGNSS (GNSS-R) |
Figure 7 illustrates some of these NASA CubeSat-based EO missions. They follow the 3 U or 6 U form factor and include deployable solar panels for higher electrical power generation capabilities. RainCube (Figure 7c) also includes a 0.5-m-diameter deployable Ka band that stows in 1.5 U. This antenna has a gain of 42.6 dBi, and it was optimized for the radar frequency of 35.75 GHz. References are provided for more information on the cited missions.
Artist’s view of (a) TEMPEST [31], (b) RAVAN [32], and (c) RainCube missions [33].
On the educational side, the ESA launched in February 2008 the first Call for CubeSat Proposals to universities in ESA member and cooperating states. Seven student-built CubeSats were launched onboard the Vega maiden flight on February 13, 2012. Since then, 12 more CubeSats have been enrolled in the first and second editions of the “Fly Your Satellite!” program.
Since 2013, the ESA has invested more than 16 M€ as part of the General Support Technology Program (GSTP) FLY Element [34], in 12 CubeSat IOD missions [35, 36]. As part of ESA’s Systems Department Project Office of the Systems Department, Directorate of Technical and Engineering Quality, in April 2019, the CubeSat Systems Unit was created.
Artist’s view of (a) GOMX-3 [35] and (b) GOMX-4 [36] nanosatellites in space (credits GomSpace) and (c) FSSCat mission [38, 39].
In addition to the work conducted by this unit, there are a number of other CubeSat-related initiatives in ESA:
The Directorate of Telecommunications and Integrated Applications is developing a pioneer series of CubeSat missions, to test novel telecommunication technologies.
The Directorate of Operations has OPS-SAT [37] ready to fly, an IOD test-bed for innovative mission control software.
The Directorate of Human and Robotic Exploration is considering a CubeSat mission to test out a key capability for Mars sample return optical detection and navigation to a sample container from the orbit.
The Science Directorate is also adapting some CubeSat technologies for operation in the deep space environment as well as studying the potential use of CubeSats in support of planetary science missions.
The Directorate of Earth Observation will fly FSSCat [38, 39], a double 6 U CubeSat mission for tandem observation of the polar regions and for soil moisture mapping using the FMPL-3 (UPC, ES), a combined L-band microwave radiometer and GNSS-Reflectometer using a software-defined radio, and HyperScout-2 (Cosine, NL), a VNIR and TIR hyperspectral imager enhanced with artificial intelligence for cloud detection (PhiSat-1).
The first ESA CubeSat projects are listed in Table 6. In addition to these missions, numerous studies have focused on the applications of CubeSat missions and miniaturized payloads, including remote sensing with cooperative nanosatellites, asteroid impact missions, lunar CubeSats, astrobiology/astrochemistry experiment CubeSats, asteroid observer missions, etc.
Organization | Mission | Launch | Form factor | Field | Technical and comments |
---|---|---|---|---|---|
GomSpace (DK) | GOMX-3 | 2015 | 3 U | Tech demo | ADS-B, GEO Satcom signal monitoring, X-band transmitter (Figure 8a) |
GomSpace (DK) | GOMX-4B | 2018 | 2 × 6 U | Tech demo Earth observation | Inter-satellite link and propulsion while in tandem with GOMX-4A (GomSpace, Ministry of Defense, DK), star tracker HyperScout compact hyperspectral VNIR imager (Cosine, NL) (Figure 8b) |
VKI (BE) | Qarman | 2019 | 3 U | Tech demo | Demonstrates reentry technologies, novel heatshield materials, new passive aerodynamic drag stabilization system, and telemetry transmission during reentry via data relay satellites in low Earth orbit |
RMI (BE) KU Leuven (BE) | SIMBA | 2019 | 3 U | Earth observation | Total solar irradiance and Earth radiation budget |
BIRA-IASB (BE) VTT (FI) Clyde Space (UK) | PICASSO | 2019 | 3 U | Atmosphere and ionosphere | Stratospheric ozone distribution, mesospheric temperature profile, and ionospheric electron density |
C3S and MTA EK (HU) ICL (UK) Astronika (PO) | RadCube | 2019 | 3 U | Tech demo Space weather | 3 U platform In situ space radiation and magnetic field in LEO |
RUAG (AU) TU Graz (AU) Seibersdorf Labor GmbH (AU) | PRETTY | — | 3 U | Earth observation | GNSS-R at low grazing angles, radiation dosimeter |
ESA | OPS-SAT | 2019 | 3 U | Tech demo | Experimentation with onboard and ground software by offering a safe and reconfigurable environment |
UPC (ES) Golbriak (EE) Deimos Eng (PT) Tyvak Intl. (IT) Cosine (NL) | FSSCat | 2019 | 2 × 6 U | Tech demo Earth observation | RF and O-ISL, federated satellite experiment 3Cat-5/A: Microwave radiometer and GNSS-R (UPC, ES) 3Cat-5/B: HyperScout-2 VNIR + TIR hyperspectral imager (Cosine, NL) (Figure 8c) |
The first ESA CubeSat-based missions.
In blue from the CubeSat Systems Unit, Directorate of Technical and Engineering Quality; in orange from the Directorate of Operations; and in green from the Directorate of Earth Observation (2017 ESA S^3 Challenge, Copernicus Masters Competition).
As highlighted in Sections 1.3 and 2.1, the majority of the CubeSats orbiting today are devoted to Earth observation, notably from two commercial companies, followed by communications. In the coming years, these two categories will still dominate. Although the largest increase will occur in communication satellites, the growth in scientific (non-EO) missions will not be negligible (from 10 to 20%, Figure 9) considering that the predicted number of satellites to be launched is going to multiply by more than a factor of 3 (see Figure 4).
Satellite application trends (1–50 kg): (a) 2014–2018 and (b) 2019–2023 (adapted from [14]).
In particular, until 2017 there were only 5 astronomy missions, and in the field interplanetary missions, until 2018 only 14 nano−/microsatellites had been launched to destinations outside the LEO. Beyond-the-Earth orbit is the domain of civil agencies who, for the sake of reliability, have been historically reluctant to invest in small satellites. However, things may be changing, since only in 2018 four nano−/microsatellites made their way beyond the Earth orbit, which is more than those in the previous 5 years all together, and 35 more are expected to be launched in the coming 5 years. Naturally, most of them target the moon, but a non-negligible fraction will be devoted to interplanetary missions (Figure 10).
CubeSats launched beyond the earth orbit: 14 from 2003 to 2018 and 35 planed from 2019 to 2023 (adapted from [14]).
As in other fields, at the beginning all the astronomy or heliophysics missions were conducted by universities, and it was not until 2017 that the first NASA JPL mission (ASTERIA) was launched. Achieving state-of-the-art astronomy with CubeSats has become possible due to advances in precision pointing, communications technology, and deployables, among others (Tables 5.1 and 5.2 of [40]). Table 7, distilled from [21], shows the main astronomy and beyond-the-Earth past and planned missions. It also shows that the majority of these missions are based on the 6 U form factor, which is the smallest one capable to accommodate all the advanced attitude determination and control systems (ADCS), larger deployable solar panels and antennas, as well as telescope optics. It is also remarkable that so far there are no funded CubeSat missions in the far IR because the thermal stability and detector cooling require cryo-coolers for CubeSats that have yet to be developed for astrophysics due to power and space limitations [41].
Organization | Mission | Launch | Form factor | Technical and comments |
---|---|---|---|---|
Morehead State University Kentucky Space | CXBN | 2012 | 2 U |
|
University of Colorado | CSSWE | 2012 | 3 U |
|
Austria Canada Poland | BRITE | 2013 2014 | 8 U (2 × 2 × 2) |
|
University of Colorado at Boulder | MinXSS | 2015 | 3 U |
|
JPL (USA) MIT (USA) | ASTERIA | 2017 | 6 U |
|
ERC, CNRS, ESEP Lab, PSL Université Paris, Fondation MERAC, CNES, CCERES and Obs. de Paris – LESIA | PicSat | 2018 | 3 U |
|
University of Iowa | HaloSat | 2018 | 6 U |
|
Spacety (China) | Tongchuan-1 | 2018 | 6 U |
|
University of Colorado Boulder | MinXSS-2 | 2018 | 3 U |
|
University of Colorado | CSIM | 2018 | 6 U |
|
DARPA | SHFT-1 | 2018 | 3 U |
|
NASA | MarCO-1/MarCO-2 | 2018 | 6 U |
|
University of Hawaii at Manoa | NEUTRON-1 | 2019* | 3 U |
|
Boston University | CuPID | 2019* | 6 U |
|
University of Colorado Boulder | CUTE | 2020 | 6 U |
|
Isaware (FI) | XFM Cube | 2020 | 3 U |
|
Lockheed Martin | LunIR | 2020 | 6 U |
|
Arizona State University | LunaH-Map | 2020 | 6 U |
|
NASA JPL | Lunar Flashlight | 2020 | 6 U |
|
Morehead State University | Lunar IceCube | 2020 | 6 U |
|
Arizona State University (USA) | SPARCS | 2021 | 6 U |
|
NASA’s Goddard Space Flight Center | BurstCube | 2021 | 6 U |
|
ESA Luxembourg Space Agency (LU) GomSpace (DK) | M-ARGO | 2023 | 12 U |
|
ESA | HERA CUBESAT | N/A | 2x6 U |
|
Non-comprehensive list of astronomy and beyond-the-earth CubeSat-based missions.
It is worth noting that the large number of CubeSats to be launched to the Moon in 2020 corresponds to the Artemis-1 mission (Figure 11), formerly known as Exploration Mission-1. The first mission for NASA’s Orion rocket and the European Service Module will send the spacecraft beyond the moon and back. Thirteen low-cost CubeSat missions were competitively selected as secondary payloads on the Artemis-1 test flight, all of them having the 6 U form factor. The selected CubeSats are Lunar Flashlight, Lunar South Pole, Near-Earth Asteroid Scout, BioSentinel (carrying the first living creatures into deep space since 1972), SkyFire, Lunar IceCube, CubeSat for Solar Particles (CuSP), Lunar Polar Hydrogen Mapper (LunaH-Map), EQUULEUS, OMOTENASHI, ArgoMoon, Cislunar Explorers, Earth Escape Explorer (CU-E3), and Team Miles.
Overview of the mission plan for Artemis-1: CubeSats will be deployed at steps A, B, C, and D [https://www.nasa.gov/image-feature/artemis-1-map].
Talking about interplanetary missions, on May 5, 2018, NASA launched a stationary lander called InSight to Mars. InSight landed on Mars on November 26, 2018. Riding along with InSight were two CubeSats—the first of this kind of spacecraft ever to fly to deep space [42]. Both MarCO-A and MarCO-B succeeded in a flyby of Mars, relaying data to Earth from InSight as it landed on Mars. Figure 12 shows an artist view of the MarCOs with the reflectarray used for communication purposes.
Artist view of MarCO-A and MarCO-B [42].
In addition to the “classical” astronomy, lunar and Martian missions cited above, CubeSats are nowadays finding their way to other bodies of the solar system, and there are proposals [43] to send them to Venus (CUVE mission), Deimos and Phobos asteroids (PRISM and PROME missions), comets (PrOVE mission), or Jupiter (ExCSITE mission, [44]). Figure 13 from [44] illustrates the LEO and beyond-LEO CubeSat exploration initiatives.
Solar system exploration with CubeSats and nanosats [44].
Since its conception in 1999, CubeSats have produced a “disruptive innovation”: from simple applications at the bottom of a market (mostly educational), they have relentlessly moved up, eventually displacing established medium-size competing satellites. However, CubeSats cannot displace all the large space missions as physics laws cannot be changed, i.e., large apertures and focal lengths are required to collect faint signals and achieve large angular resolution. However, CubeSats are finding their own niche in many Earth observation, astronomical, and communications applications where short revisit times or even continuous monitoring is required.
Early CubeSats typically had short lifetimes once in orbit (a few months), but with increased ground testing and added redundancies, lifetimes have grown significantly, up to 4–5 years in some cases.
Despite all these outstanding improvements, in order to exploit the full potential of CubeSats, many technologies still need to be developed. Table 8 summarizes the enabling technologies required for different science applications, indicating in red the most challenging technologies and applications, notably increased communications performance, reliability, thermal stability, and calibration accuracy, to form constellations or formation flying satellite topologies to create large interferometers and distributed apertures.
Science discipline | Enabling technology | Example application |
---|---|---|
Solar and space physics | Propulsion | |
Sub-arcsecond attitude control | High-resolution solar imaging | |
Missions beyond low Earth orbit | ||
Miniature field and plasma sensors | In situ measurements of upper atmosphere plasmas | |
Earth science | Propulsion | |
Miniaturized sensors | Stable, repeatable, and | |
Planetary science | Propulsion | Orbit insertion |
Direct/indirect to Earth communications | ||
Radiation-tolerant electronics | ||
Deployables | Deployable solar panel enhanced power generation | |
Deployable mirrors and antennas | ||
Astronomy and astrophysics | Propulsion | |
Sub-arcsecond attitude control | High-resolution imaging | |
High data rate | ||
Deployables | Increased aperture and thermal control | |
Miniaturized sensors | UV and X-ray imaging | |
Physical and biological | Thermal control |
CubeSat-enabling technologies and potential applications for each science discipline (adapted from [40]).
In the field of Earth observation, future developments in nanosat sensors will likely occur:
In the field of passive microwave sensors:
Miniature microwave and millimeter-wave radiometers for weather applications, such as the MiniRad which is onboard the Global Environmental Monitoring System (GEMS) constellation from Orbital Micro Systems [45], or
GNSS-R instruments with real-time processing for target detection/identification [46] or—as larger downlink bandwidths are available—with raw data acquisition and on-ground processing to optimize the processing according to the target, as planned in FMPL-3, the evolution of the FMPL-2 on board FSSCat [38, 39].
In the field of passive VNIR/TIR hyperspectral imagers, imagers will include a larger number of bands but will include advanced image compression algorithms to minimize the amount of information to be downloaded and will incorporate artificial intelligence to download only the information extracted instead of the raw data.
Also, both their calibration will have to be refined so as to improve the quality of the scientific data.
Due to power and antenna size requirements, active microwave sensors (e.g., radar altimeters and SARs) will likely remain in domain of mini- and microsats (< 100 kg, e.g., ICEYE constellation [47]), and it is unlikely that active optical sensor technology (i.e., lidars) develops in small satellites in the midterm.
In the field of astronomy, and in particular heliophysics, NASA has also been taking the lead. In 2017 NASA selected nine proposals under its Heliophysics Small Explorers Program [48]: (1) the Mechanisms of Energetic Mass Ejection Explorer (MEME-X), (2) the Focusing Optics X-ray Solar Imager (FOXSI), (3) the Multi-Slit Solar Explorer (MUSE), (4) the Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS), (5) the Polarimeter to Unify the Corona and Heliosphere (PUNCH), (6) the Atmospheric Waves Experiment (AWE), (7) the US Contributions to the THOR mission (THOR-US), (8) the Coronal Spectrographic Imager in the Extreme ultraviolet (COSIE), and (9) the Sun Radio Interferometer Space Experiment (SunRISE) mission concept, which is a space-based sparse array, composed of formation flying of six SmallSats designed to localize the radio emission associated with coronal mass ejections (CMEs) from the sun [49].
More recently, in August 2019, NASA selected two proposals to demonstrate SmallSat technologies to study interplanetary space [50]: (1) Science-Enabling Technologies for Heliophysics (SETH) will demonstrate two technologies, an optical communications technology and experiment to detect solar energetic neutral atoms as well as an array of waves and other particles that erupt from our sun, and (2) Solar Cruiser, which will deploy a nearly 18,000 square foot solar sail and a coronagraph instrument that would enable simultaneous measurements of the sun’s magnetic field structure and velocity of coronal mass ejections or CMEs.
As a final thought, quoting Prof. Puig-Suari, “Before cubesats, we were so conservative nobody was willing to try anything out of the ordinary. When we did, we discovered some of the things everybody said would not work, did work. The fundamental change was that there was a mechanism to go try to those things. Some will work and some will not, but it allows us to try them and that was very infrequent before cubesats arrived. That was really important. That was the big change.” And this is just the beginning of a new way to do Earth observation, astronomy, and satellite communications much more, in a different and more efficient way than it was done in the past decades. What will the future bring? Nobody knows, but certainly the future is being shaped today with these novel technologies, and only our imagination will set the limits.
This work has been supported by an ICREA Acadèmia award from the Generalitat de Catalunya to Prof. A. Camps. The author wants to express his gratitude to Profs. A. Golkar (Skoltech, Moscow, Russia) and H. Park (Universitat Politècnica de Catalunya, Barcelona, Spain) for the revision of this chapter and useful comments.
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