The motivation for developing light-emitting devices on an indirect transition semiconductor such as silicon has been widely discussed for Si/SiO2 nanostructures. In this chapter, we report on the fabrication of Si/SiO2 quantum-confined amorphous nanostructured films and their optical properties. The Si/SiO2 nanostructures comprising amorphous Si, SiO2, and Si/SiO2 multilayers are grown using ultrahigh vacuum radio frequency magnetron sputtering. Optical absorption coefficients of the Si/SiO2 nanostructures are evaluated with regard to tentative integrated Si thicknesses. Optical energy band gaps of the Si/SiO2 multilayer films are in accordance with the effective mass theory and described as E0 = 1.61 + 0.75d−2 eV at the Si layer-integrated thicknesses ranging from 0.5 to 6 nm. Quantum confinement effects in the Si/SiO2 nanostructures are inferred from optical transmittance and reflectance spectra. The rapid-thermal-annealed Si/SiO2 multilayer films demonstrate the intensified photoluminescence at ~1.45 eV due to the formation of nanocrystalline silicon. The temperature dependence of the nanocrystalline luminescence intensity shows the nonmonotonous behavior which is interpreted invoking the Kapoor model.
- amorphous Si/SiO2
- quantum confinement
- optical properties
- absorption coefficient
- the Kapoor model
Silicon, the principal semiconducting material, inherits the indirect optical transitions from its band structure. The research efforts are put forth on realization of light emission effects in silicon-based Si/SiO2 nanostructured devices exploring hydrogenated amorphous Si [1, 2, 3, 4, 5, 6, 7, 8, 9], porous Si [10, 11, 12], Si quantum dots [13, 14, 15, 16, 17, 18, 19], amorphous Si quantum wells (QWs) [20, 21, 22, 23, 24], crystalline and nanocrystalline Si QWs [25, 26, 27, 28], and Er-doped QWs [29, 30, 31]. The fabrication of Si/SiO2 QWs has been an attractive area in process technology of Si-based light-emitting devices in last few decades. Various techniques are developed to synthesize the Si/SiO2 nanostructured films—molecular beam epitaxy (MBE) [32, 33, 34, 35, 36, 37], plasma-enhanced chemical vapor deposition (PECVD) [38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49], magnetron sputtering [50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65], electrochemical dissolution in electrolytes, ion implantation, and others. In this chapter, we investigate the growth of amorphous Si/SiO2 QWs employing an ultrahigh vacuum (UHV) radio frequency (RF) magnetron sputtering (MS) system. This method is simple and easy to use in a manual operation. The Si/SiO2 QW films are fabricated on sapphire and silicon wafers at room temperature to enable the atomic precision of the film growth via minimization of atomic movements during and after the depositions. Morphology, crystallinity, atomic bonding, and structures of the Si/SiO2 films are evaluated by means of focused-ion-beam scanning electron microscopy (FIB-SEM), X-ray diffraction (XRD), and high-resolution X-ray photoelectron spectroscopy (XPS). The Si/SiO2 films are distinguished with the layer number (the period), the Si thickness (the QW thickness), and the SiO2 thickness (the barrier thickness). The first identification of the quantum confinement effects is made speculating on the optical energy band gap determined from the optical absorption and reflectance measurements taking into account the energy band gaps of silicon and fused quartz as ~1.1 and ~7.8 eV, respectively. To minimize the experimental uncertainty, the Si/SiO2 films are deposited at room temperature. At the raised temperature conditions, the uncertainty remains the atomic diffusion, reactions, and oxidation at the Si/SiO2 interface. On the other hand, the deposited Si and SiO2 layers are expected as in amorphous conditions. At first, we restricted the Si/SiO2 nanostructured layer films to be amorphous. Photoluminescence (PL) is used to characterize the as-grown and annealed materials. The thermal annealing is expected to improve the photoluminescence characteristics.
2.1. Si/SiO2 layer films preparation
Si/SiO2 QWs films are synthesized in an ultrahigh vacuum (UHV; 3 × 10−8 Pa) RF MS system at a very small deposition rate (from 0.005 to 0.5 nm/s). The schematics of the UHV RF magnetron sputtering systems are shown in Figure 1. The ultrahigh vacuum chamber is equipped with two AJA A300 UHV RF magnetron sputtering guns connected to argon and oxygen gas lines, sputter ion guns, and 5 N Si and 5 N fused quartz SiO2 targets. Preparation temperatures are controlled at the substrate holder. Transparent substrates are used for optical measurements; crystalline and amorphous substrates are used to test the influence of substrate crystallinity on the film growth. All depositions are operated at room temperature on both transparent sapphire A and opaque Si (100) substrates. The polished sapphire substrates are etched in dilute HF and put into the UHV chamber. The base pressure of the chamber is 10−7 Pa, and the sputtering gun pressure during the plasma operation in argon is 2 × 10−1 Pa. The deposition process is a repetition of Si and SiO2 depositions separated by an interval time. The deposition parameters of the quantum-well structures are the well layer thickness, the barrier layer thickness, and the number of periodicity. The thickness of each layer is controlled by the deposition speed and the sputtering time. Basic deposition speeds are 0.05 nm/s for Si and 0.021 nm/s for SiO2 at an argon pressure of 0.2 Pa. Minimization of the atomic diffusion and the oxidation during the depositions are the main concerns. The Si/SiO2 nanostructure films are made of 1–50 periods consisting of 0.5–15 nm thick Si QWs and 0.5–6 nm thick SiO2 barriers. The 10-layer Si/SiO2 nanostructured layers are formed with the total thickness of 10–200 Å. Figure 2 is the SEM cross-sectional view of a 20-layer Si/SiO2 nanostructure comprising 2.0 nm Si and 2.1 nm SiO2. It shows 20 pairs of the white thin SiO2 layers and dark Si layers.
The XRD spectra (CuKα source) of 180-nm thick Si single layer and 150-nm thick SiO2 single layer prepared on the sapphire substrates at room temperature do not show the crystallinity of the samples. Both spectra reveal the noncrystalline characteristics of Si and SiO2 films.
2.1.2. Density-of-states structures
Atomic constitutions of each layer are evaluated with XPS on binding energy of Si2p and O1s electrons. The bulk Si2p core-level binding energy for Si(111) is ~99.3 eV and the bulk Si2p oxide binding energy value for SiO2 is ~103.7 eV referring to Keister . PHI 500 Versa Probe II scanning XPS microprobe is designed to take out a 10-degree signal, enable slow speed (0.01 nm/s SiO2) area etching, and equipped with a monochromatic AlKα X-ray source. The depth profiles are characterized by using a low-energy argon ion gun to avoid selective etching. The binding energy dependence of the core densities of states at each etched depth suggests periodic distributions of each atomic composition. This analytical technique has particular applicability to the evaluation of the density of states with atomic contributions. Figure 3(a) is the plot of the density of states from 97 to 107 eV as the parameters of depth. The profiles of two peaks (a) 98.9 eV Si 2p spectra (Si0) (element: un-oxidized) and the 103.2 eV Si 2p (oxidized) (Si4+) are shown. In Figure 3(b), 532.6 eV O1s single peaks are shown. The spectra (a) 103.2 eV and (b) 532.5 eV exhibit maxima at the same depth and spectrum (a) has a minimum at the bottom of (b). Figure 3(a), at the depth of 0.54 nm, 103.2 eV intensity peaks and 98.9 eV show a minimum. The density-of-states depth profiles explain the presence of the Si/SiO2-layered amorphous nanostructure fabricated using the UHV RF magnetron sputtering method at the atomic scale precision.
2.2. Optical properties of Si/SiO2 layer films
Optical transmittance spectra and reflectance spectra are measured with the help of JASCO V-670 visible and ultraviolet optical photometer at room temperature. Optical properties of an amorphous Si/SiO2 nanostructure film show the higher optical transmittance and wide optical window effects. Unique optical properties are a candidate for solar windows in solar cells or filters of ultraviolet light. The parameters characterizing the Si/SO2 film structures are the well layer thickness, the barrier layer thickness, and the number of periodicity. Figure 4 displays the optical reflectance and transmittance spectra of amorphous Si/SiO2-nanostructured layer films of various period numbers. The well thickness of the samples changes from 2 to 24 nm, while the barrier thickness is fixed at 4.8 nm. As the period’s number of layers increases, the optical reflection decreases and the optical transmittance increases markedly, although the onset energy of transmittance and the absorption edge wavelength show the constant values. The increasing period number enhances optical transmittance and decreases optical reflectance. The spectra are saturating at 8–12 barrier layers. Increasing the period number does not change the absorption edge energy. Nanostructure effects observed on the 12-layer Si/SiO2 film as the optical transmittance and reflectance effects are saturating. Figure 5 exhibits the optical reflectance (a) and transmittance spectra (b) of Si/SiO2 films as a function of the Si well thickness at the constant 12-period numbers and the constant barrier thickness of 4.8 nm. The increasing Si well thickness increases the reflectance and decreases the transmittance as expected. Also, the absorption edge energy shows the constant values. Figure 6 shows the barrier thicknesses dependence of optical reflectance (a) and optical transmittance (b) spectra for the constant 12-layer period and 2 nm well thicknesses. The increasing barrier thicknesses diminish the optical reflectance and enhance the optical transmittance.
2.2.1. Absorption coefficient
Absorption coefficients α (λ) are used as the index of intrinsic properties of thin film materials. Absorption coefficient α (λ) at a wave length λ is evaluated from Eq. (1) for the sample thickness d with the optical transmittance T(λ) and reflectance R(λ). On the Si/SiO2 nanostructure multilayer films, the integrated thickness of the Si layer, the reduced film thickness in Eq. (2) is used as the tentative thicknesses.
Figure 7 shows the dependence of the absorption coefficients on the Si well thickness of 12-layer Si/SiO2 films. The photon energy dependences of absorption coefficient show a sharp rise in the energy of absorption edges above 1000/cm. The dependence of the absorption edge energies on the Si well layer thickness is measured from 0.5 to 6 nm at the SiO2 barrier layer fixed at 2.4 nm. In Figure 8, (αhν)1/2 vs. photon energy is plotted. The absorption coefficients of amorphous films are related in Eq. (3) known as an amorphous plot to obtain the absorption edge energy.
2.2.2. Quantum confinement
In Figure 9, the absorption edge energy is plotted vs. the tentative well thicknesses in a Si/SiO2 multilayer structure and compared with the effective-mass theoretical estimations. Two types of the absorption edge energy evaluated from Figures 7 and 8 are indicated. The absorption edge energy becomes larger as the QW thickness gets smaller. The blue shifts of the absorption energy are impressive in Figure 9. The absorption edge energy values E0 are evaluated for each well thickness following the effective-mass theory, Eq. (4) . The Si layer thickness dependency of absorption edge energy is in accordance with the effective-mass theory for thicknesses 0.5 < d < 6 nm in Eq. (4). Therefore, a good agreement is obtained with the effective mass theory assuming infinite potential barriers . The thickness variation of the absorption edge energy shown in Figure 9 demonstrates a remarkable blue shift of the spectra as the Si layer thickness decreases. This shift can only be caused by the Si layer because the SiO2 barrier layer thickness is a constant value of 4.8 nm. This absorption edge energy is in accordance with E0 (eV) = 1.61 + 0.75 d−2 (eV).
Although quantum confinement is obtained from the optical absorption measurements, the recombination mechanism is still indistinct. To elucidate the latter, we investigate PL spectra of the Si/SiO2 multilayer nanostructures.
2.3. Thermal annealing of Si/SiO2 layer films
Since the as-deposited samples show very weak photoluminescence, two experimental efforts are made to improve the PL intensity. The first is the increasing the well number of Si/SiO2 films layered with 0.5–15 nm (Si) QWs from 10 to 50 periods. The second is the thermal annealing of Si/SiO2 films in nitrogen. In our work, RTA in nitrogen was performed at 700 and 1100°C for 30 min. Figure 10 shows the cross-sectional view of an RTA treated Si/SiO2 film. Apparently, the Si QW layers are changed revealing partially dark spots and eroded SiO2 barrier layers.
Figure 11 shows the XRD spectra of a 10-layer Si/SiO2 film on a sapphire substrate which is rapid-thermal annealed at 700 and 1100°C. The crystallization is clearly identified at 700°C by the splitting the (111), (220), and (311) Braggs peaks indicating that the amorphous Si layers are crystallized as the nanocrystal Si.
2.3.1. Photoluminescence of Si/SiO2 layer films
Photoluminescence spectra of as-deposited amorphous 10 layers Si/SiO2 films are excited at 325 nm by a He-Cd laser. The highest energy peaks at 2.35, 2.05, 1.81 eV, with subpeak at 1.45 eV are observed. The improved PL is observed upon crystallization of Si after subjecting the 50-layer Si/SiO2 multilayer nanostructures to RTA at 700 and 1100°C as shown in Figure 12. The spectra show a broadband peak and shoulders. The main peak energies are 1.62, 1.68, and 1.45 eV. In Figure 13, photoluminescence spectra of Si/SiO2 QWs annealed at 1100°C for 30 min in nitrogen are shown for the QW thickness ranging from 1.2 to 2.5 nm. The intensity becomes higher for the thinner QWs.
Figure 13 presents the well thickness dependence of PL spectra taken on the 50-layer Si/SiO2 structure upon RTA in nitrogen at 1100°C. The strongest PL is observed for the thinnest Si QW (1.2 nm), fading as the QW thickness increases. Figure 14 displays the temperature dependences of photoluminescence spectra. Among the three temperatures, the 80 K spectrum is the most intense, followed by the room temperature, and the 4 K photoluminescence. The Kapoor model, where two different recombination mechanisms are operative in different temperature ranges, can explain this nonmonotonous temperature dependence of PL. The Kapoor’s Eq. (5) consists of an Arrhenius-type term Tr and a Berthelot term TB. Rolver explained the effects as an interplay of thermal activation of excitons into optically active states and hopping occupation of dark states.
2.3.2. The Kapoor model
The temperature dependence of the photoluminescence intensity peaks observed at 80, 4, and 293 K are analyzed using the Kapoor empirical models [45, 67]. The simulation of the Si/SiO2 sample comprising 50 quantum wells (1.2 nm well width) annealed at 1100°C for 30 min in nitrogen is performed following Eq. (5). Figure 15 presents the result, which evidences a reasonable agreement between the experimental and simulated results using Tr = 70 K, TB = 80 K, and υ0 = 0.1.
Amorphous nanostructured Si/SiO2 films are smartly fabricated using a UHV RF magnetron sputtering system at room temperature. Absorption coefficients are evaluated considering the tentative well Si thickness and energy band gap energy of the Si/SiO2 layers. The photon energy dependence of absorption coefficient on the quantum well thickness is simulated taking into account the quantum-confined properties. The choice of the Si layer thicknesses interfacing the SiO2 barrier layer of the constant thickness (4.8 nm) mainly determines the blue shift of the absorption energy. Assuming the infinite potential SiO2 barriers, the effective-mass theory provides the fitted absorption coefficient edge energy in accordance with E (eV) = 1.61 + 0.75 d−2 (eV) for one-dimensionally confined amorphous Si (d: nm). The amorphous Si/SiO2 nanostructure films show the quantum confinement. Thermal annealing of the Si/SiO2 films affects the improvement of photoluminescence intensity. Anomalous temperature dependence of photoluminescence is attempted to be explained based on the Kapoor model. Future work is expected to resolve many more research questions.
The authors express deep thanks to Mr. Minoru Kondo, a graduate student of Waseda University, for corporate experimental works and to Dr. Atsushi Kawaharatsuka, Associate Professor of Waseda University, for many impressive discussions on quantum confinement. One of the authors (T. T.) expresses deep thanks to Dr. Tadashi Takahashi, Emeritus Professor at Tohoku University, for his encouragements and instructions.