Abstract
Oxide molecular beam epitaxy has emerged as an effective technique to fabricate complex oxide thin films and novel superlattices with atomic‐level precision. In this chapter, we first briefly introduce the oxide molecular beam epitaxy technique and then show how to use this technique to achieve high‐quality thin films with good stoichiometry. Moreover, we exhibit that the combination of oxide molecular beam epitaxy and in situ angle‐resolved photoemission spectroscopy is indeed a versatile toolkit to tailor and characterize properties of novel quantum materials.
Keywords
- oxide molecular beam epitaxy (OMBE)
- correlated materials
- angle‐resolved photoemission spectroscopy (ARPES)
1. Introduction
In transition metal oxides, the subtle interplay among charge, orbital, lattice and spin degrees of freedom gives rise to a spectrum of fascinating physical phenomena, including high‐temperature superconductivity [1], metal‐insulator transition [2], colossal magnetoresistance [3], and so on. Remarkably, in thin film interfaces and ultrathin films of correlated oxides, emergent physics, which does not exist in bulk crystals, occurs [4, 5]. As a well‐known example, two‐dimensional electron gas with high mobility amazingly emerges at the interface of two‐band insulators LaAlO3 and SrTiO3 [6]. This emergent electron gas was even found to be superconducting [7]. Another example is that strong ferroelectricity and ferromagnetism were found in EuTiO3/DyScO3 superlattices [8]. As Nobel laureate Herbert Kroemer said that ‘the interface is the device’ [9], these emergent physics may potentially revolutionize our modern technologies.
In order to access these thin film‐based physics, the first and most important step is to grow these oxide thin film structures with high quality. This needs exquisite control of growth, and usually is very challenging. In the past several decades, fortunately, reactive oxide molecular beam epitaxy (OMBE) has been proved to be an effective technique in the growth of some oxides with high quality, though being not easy [10–12]. Recently, the
In this chapter, we first present basics of OMBE technique. Then, we show how to grow high‐quality films with good stoichiometry, and the power of the integrated OMBE‐ARPES in studying and designing many‐body interactions in complex oxides.
2. Basics of OMBE technique
MBE is a vacuum deposition technique in which well‐defined thermal beams of atoms or molecules react at a crystalline surface to produce an epitaxial film. Originally, it was developed to fabricate GaAs and (Al, Ga)As films [18], and soon successfully expanded to other semiconductors as well as metals and insulators. In addition to molecular beams coming from individual heated element source, gas molecular may also be introduced into MBE. Including gas oxidants (e.g., oxygen or ozone) can make an OMBE, which is now applied to grow oxides [11, 12].
In 1985, Betts and Pitt began to use this technique to grow LiNbO3 films [19]. Later, motivated by the discovery of high‐temperature superconductivity, OMBE was used to grow complex cuprate thin films. Up to now, it has been broadly employed to fabricate a pool of oxides, including oxide superconductors (e.g., (Ba, K)BiO3, (La, Sr)2CuO4, Bi2Sr2Ca
While conventional MBE growth occurs in an ultra‐high vacuum, in OMBE growth the induction of active gas oxidants can pose new challenges in the instrumentation as well as the film growth [11, 17]. The presence of oxidant species requires the hardware to be necessarily compatible with an oxidizing environment, thus high‐temperature components (e.g., heater filaments, effusion cells and substrate holders, etc.) need to be made of highly oxidant‐resistive materials. Moreover, adequate pumping is needed to deal with the oxidant gas load. Furthermore, oxygen acts as another variable which needs to be optimized in the growth, and oxygen inside films is tricky to study and manipulate. In addition, the oxidants can oxidize the cell materials such that one cannot get well‐controlled fluxes as planned during growth. These challenges make the use of OMBE in the growth of oxides less mature than the use of MBE in semiconductor growth [11].
Figure 1 shows the schematic of a typical OMBE system. Single‐element evaporators are used to generate atomic beams for OMBE growth. Knudsen cells and crucibles are chosen for elements with desired fluxes below 2000°C, while electron beam evaporators are adopted for refractory elements (e.g., tungsten, ruthenium and iridium) which require higher temperature to provide the fluxes necessary for the growth. The atomic beams impinge upon the substrate unless they are blocked by shutters which are positioned at the output end of each cell and remotely controlled by a computer. The utilization of shutters enables the elemental fluxes to be supplied to in a continuous or a sequential way. The fluxes can be adjusted by changing the cell temperature, and are
Figure 2 displays the photo of DCA R450 OMBE system in Shanghai Institute of Microsystem and Information Technology (SIMIT). It is equipped with 10 changeable effusion cells, and one four‐seat e‐beam evaporator for at most four refractory elements, which can cover all transition metals of interests. It also has
3. Fabricating oxide thin films with good stoichiometry by OMBE
In this section, we show generally how to grow high‐quality oxide thin film with good stoichiometry by OMBE. Starting with choosing the proper substrate, we mainly talk about two growth methods commonly used—absorption‐controlled growth and shutter growth—to achieve high‐quality films.
Epitaxial thin films cannot be obtained without using proper substrates. The substrate not only guides the growth of thin film with the right crystalline structure but also provides the knob of strain which can essentially tune the electronic structure of the material [20]. Figure 3 displays lattice constants of single‐crystalline perovskite substrates which are commercially available and commonly used to grow perovskite or layered perovskite oxide thin films. If growing LaNiO3 films with a pseudo‐cubic lattice constant of 3.84 Å on LaAlO3 (3.75 Å) substrates, an in‐plane compressive strain was applied; if using SrTiO3 (3.905 Å) as the substrate, an in‐plane tensile strain was applied. In addition to the strain, in some cases, the choice of the substrate is vital to obtain high‐quality films. For example, Proffit et al. reported that (1 1 0) orthorhombic CaRuO3 films grown on orthorhombic (1 1 0) NdGaO3 substrates (symmetry matched) exhibit atomically smooth surfaces, whereas films on cubic lanthanum aluminate-strontium aluminium tantalate (LSAT) substrates (symmetry mismatched) show rather rough surfaces [21]. Another example is the film growth of LaNiO3 with polar orientations. As polar discontinuity is suggested to induce surface reconstructions which further lead to bad quality of films [22, 23], metallic Nb‐doped SrTiO3 and iso‐polarity LaAlO3 substrates were shown to be more suitable than the common SrTiO3 in the growth of LaNiO3 films [24].
As illustrated above, the remotely controlled shutters in OMBE allow elemental fluxes to be supplied to in a continuous or a sequential way. Take the growth of perovskite ABO3 (can be viewed as alternate stacking of AO and BO2 layers along the (0 0 1) direction) as an example. As schematically shown in Figure 4, both shutters of A cell and B cell keeping open in the whole growth make a co‐deposition growth. If the shutters of A cell and B cell alternately turn open (finishing the growth of one AO layer, and then starting the growth of one BO2 layer), we can call this the shutter growth. In either growth, stoichiometry is the most important goal which needs to be achieved.
A well‐known growth method to achieve stoichiometric films is the adsorption‐controlled growth, which was previously used to grow GaAs and recently has been used to fabricate several oxides such as PbTiO3 [25], BiFeO3 [26] and BiMnO3 [27]. It is used in the growth of some compounds containing volatile species which can re‐evaporate during growth while the others are less volatile. If always making this volatile species excess during the growth as well as optimizing the substrate temperature and oxygen partial pressure, stoichiometric growth can be conveniently achieved. Lee et al. reported the adsorption‐controlled growth of BiMnO3 in which the bismuth oxides are volatile [27]. Figure 5 displays the calculated Ellingham diagram and obtained RHEED patterns [27]. The Bi:Mn flux ratio was fixed to be 3:1. Besides, the substrate temperature and oxygen partial pressure were fully explored to finally expose the growth window (see shadow region II in Figure 5) for phase‐pure stoichiometric BiMnO3 films which were verified by the shiny diffraction spots in the RHEED pattern.
For most oxides, adsorption‐controlled growth unfortunately cannot be applied. To achieve good stoichiometry, generally, one has to adjust the deposition amount based on cycles of combined studies of QCM, RHEED patterns and oscillations, X‐ray diffraction (XRD) pattern fitting, Rutherford backscattering spectroscopy, and so on. In the homo‐epitaxial growth of SrTiO3, Schlom's group reported the empirical method of optimizing shuttered RHEED oscillations to successfully achieve stoichiometric SrTiO3 film within 1% composition deviation [28, 29]. In the shutter growth of SrTiO3, the intensity of diffraction spot would exhibit periodic oscillations at the pace of mechanically closing/opening shutters: in Ti doses, the intensity will decrease monotonically while in Sr doses the intensity will increase. It was shown that if oscillations exhibited smooth sinoidal shape with similar amplitude (Figure 6(a)), the resulted film was investigated to be stoichiometric [29]. If Sr is 10% excess, the combined feature of cusp and shoulder would show up (Figure 6(b)); if Sr is 10% deficient, the amplitude of individual oscillations would oscillate (Figure 6(c)). Thus, in growth, the real‐time performance of RHEED oscillations would infer what to do next to achieve the stoichiometry [29].
Compared to adsorption‐controlled growth, shutter growth is a more straightforward way to control the film thickness and grow complex oxide structures such as Ruddlesden‐Popper (RP) series A
4. The unique in situ combo of OMBE and ARPES
ARPES can directly visualize electronic band structures of solids, and therefore has emerged as an essential experimental technique to study various novel quantum materials such as superconductors and topological quantum materials [13, 14, 32]. It can be viewed as the ‘k‐space’ microscope, and can provide the essential information about how electrons move inside the material. Based on the well‐known photoelectric effect, an electron inside the solid can absorb an incident photon with a high enough energy and then emit out of the solid. If the kinetic and momentum of the photoelectrons are detected, the band structure of the material (as a function of binding energy and momentum) can be reconstructed in the context of conversation laws and some reasonable assumptions, as shown in Figure 8. Generally being a surface‐sensitive probe, ARPES demands clean and well‐ordered sample surface, which is usually obtained by cleaving single crystals.
Recently, there has been increasing awareness that the
Figure 9 shows the photo of such an
Below, we first present studies on ultrathin perovskite LaNiO3 films (
Bulk LaNiO3, though being strongly correlated with
Perovskite SrRuO3, a prototypical conductive ferromagnetic oxide, exhibits a kink in its band dispersion signalling the unusual electron dynamics therein [34]. The kink could originate from electron‐magnon coupling or electron‐phonon coupling. Uncovering the origin of this kink would hint on the studies of kinks’ origins in many other intriguing systems [37–39] including the cuprate superconductor family [40]. Yang et al. reported the systematic thickness‐dependent electronic structure studies on SrRuO3 films with well‐controlled thicknesses by using the OMBE and ARPES system [35]. Figure 11(a) shows the evolution of band dispersions of SrRuO3 films with reducing the film thickness. Evidently, in all these spectra, the slope of the dispersion near
Perovskite SrIrO3, due to the heavy element of Ir, is expected to have strong SOC which is the key ingredient in building topological quantum materials [43, 44]. Therefore, novel topological phases were proposed in artificial SrIrO3‐based structures [45]. In particular, SrIrO3 was proposed to be an exotic semimetal induced by the delicate interplay between SOC and electron correlations, in which a Dirac nodal ring near the
With the powerful capability of the OMBE‐ARPES combo, one can fabricate artificial superlattices which do not exist in nature and study their intriguing emergent physics. Monkman et al. reported the comprehensive investigations on the interfacial electronic structure of (LaMnO3)2
These examples reflect the powerful capability of the integrated OMBE‐ARPES system in studying many‐body interactions and resulted novel physics in complex oxides.
5. Conclusion and outlook
In this chapter, we presented the brief inductions to OMBE technique, growth methods and the
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