As-grown Films Research Articles

Monolithically grown CSPbBr3 by chemical vapor deposition for Self-Powered photodetector

The high-quality monolithic growth of perovskite films with larger grains on any substrate is a challenging task due to the instability of morphology, carrier mobility and chemical stability. In this work, we demonstrated the strategy to grow CsPbBr3 films with micron-sized grains along the (002) surface by chemical vapor deposition. The structural and optical characterizations have shown that the as-grown CsPbBr3 thin films are compact and packed without pin holes and have excellent optical quality for use in high-performance optoelectronic devices. The planar photodetector with the device configuration Ag/CsPbBr3/Ag shows a high photoresponsivity of 506.32 A/W and a specific detectivity of 8.29 × 1012 Jones at an incident light illumination of 22.3 µW/cm2. On the other hand, the vertically stacked device with ITO/SnO2/CsPbBr3/CuSCN/Ag configuration exhibits a photoresponsivity of 10.13 A/W at 0.05 V bias and 96 mA/W under 0 V bias. Specifically, the response time of the device in planar geometry is 241/205 ms, while the vertically stacked device has a response time of 20.51/21 ms at 0.05 V bias and 20.51/30 ms at 0 V bias. The results provide a new strategy for the growth of high-quality perovskite thin films and their effective application for high-performance optoelectronic devices.

Optimizing thermoelectric performance of BiSbPbTe thin films through controlled post-annealing: A comprehensive structural and electronic study

This study investigates the structural and electronic properties of BiSbPbTe thin films, focusing on the impact of post-annealing time and temperature. The X-ray diffraction (XRD) analysis reveals a stable rhombohedral phase formation in all samples, with no secondary phases detected within the detection limit. The high crystallinity of the films is confirmed, and post-annealing leads to an increase in peak intensity, indicating enhanced crystalline structure. Surface morphology analysis using scanning electron microscopy (SEM) demonstrates a transition from inhomogeneous features in as-grown films to homogeneous and dense formations in post-annealed films. The grain size increases with post-annealing time, attributed to diffusion, energy minimization, nucleation, growth, and thermal activation processes. The electronic transport properties, including carrier concentration and conductivity, are influenced by post-annealing temperature and duration. A stable carrier concentration is observed for films post-annealed for up to 2 h, while longer durations decrease due to reduced defect vacancies and grain boundaries. The increase in charge carrier mobility, confirmed by SEM results, contributes to enhanced electrical conductivity. The Seebeck coefficient shows an initial enhancement up to 2 h post-annealing, attributed to defect creation and increased grain boundaries. However, further increases in post-annealing duration result in a decrease in the Seebeck coefficient due to enhanced charge carrier mobility and reduced grain boundaries. The thermoelectric power factor (4.4 μW/cm1K2), determined by the square of the Seebeck coefficient and electrical conductivity, reaches its maximum value in the 2-h post-annealed sample, emphasizing the importance of optimizing post-annealing conditions for improved thermoelectric performance.

Investigation of the physico-chemical properties of In2S3 powder synthesized via solid-state reaction and In2S3 thin films prepared through thermal evaporation

In this study, we examined the physico-chemical properties of In2S3 powder produced using the solid-state reaction and of the In2S3 thin film obtained using the thermal evaporation technique. Thermally evaporated In2S3 thin films were subjected to vacuum annealing to study the impact of heating on the structural, morphological and optical properties of the samples. According to the X-ray diffraction analysis, the microstructure of as-grown thin films were an amorphous in nature which transformed into polycrystalline structure after annealing at T≥250 °C. This polycrystalline structure suggests the formation of the β-In2S3 phase with a tetragonal and/or cubic crystal structure. In addition, the energy dispersive spectroscopy analysis revealed that all samples show a slight excess of indium and a deficit of sulfur, compared to the stoichiometric ratios. Moreover, surface micrographs recorded by atomic force microscopy showed an increase in surface roughness after annealing. Optically, the transmittance spectra, T(λ), illustrated a high transmittance average value,∼70 %, while the reflectance spectra, R(λ), showed an average value of 25 % in the visible and NIR region. It appeared that heat treatment had no significant impact on the T and R spectra. The optical dispersion parameters of the treated samples were also studied. The direct and indirect allowed bandgap transitions values of all samples were found to ranging from 1.75 to 2.48 eV and from 1.80 to 2.38 eV, respectively. Such observation shows that the bandgap increased with annealing temperature. Electrically, n-type conductivity for In2S3 films annealed at 250 °C was recorded.

Phase-Centric MOCVD Enabled Synthetic Approaches for Wafer-Scale 2D Tin Selenides.

Following an initial nucleation stage at the flake level, atomically thin film growth of a van der Waals material is promoted by ultrafast lateral growth and prohibited vertical growth. To produce these highly anisotropic films, synthetic or post-synthetic modifications are required, or even a combination of both, to ensure large-area, pure-phase, and low-temperature deposition. A set of synthetic strategies is hereby presented to selectively produce wafer-scale tin selenides, SnSex (both x=1 and 2), in the 2D forms. The 2D-SnSe2 films with tuneable thicknesses are directly grown via metal-organic chemical vapor deposition (MOCVD) at 200°C, and they exhibit outstanding crystallinities and phase homogeneities and consistent film thickness across the entire wafer. This is enabled by excellent control of the volatile metal-organic precursors and decoupled dual-temperature regimes for high-temperature ligand cracking and low-temperature growth. In contrast, SnSe, which intrinsically inhibited from 2D growth, is indirectly prepared by a thermally driven phase transition of an as-grown 2D-SnSe2 film with all the benefits of the MOCVD technique. It is accompanied by the electronic n-type to p-type crossover at the wafer scale. These tailor-made synthetic routes will accelerate the low-thermal-budget production of multiphase 2D materials in a reliable and scalable fashion.

Effects of hydrogen doping on the phase structure and optoelectronic properties of p-type transparent SnO

Hydrogen (H) is a ubiquitous impurity, which can significantly affect the phase structure and the optoelectronic properties of oxide semiconductors. To date, a well-established understanding of the effects of H doping on the properties of p-type transparent SnO is still lacking. Here, we present a comparative study to reveal the role of H in SnO films by magnetron sputtering. We find that in as-grown undoped SnO films, in addition to the tetragonal SnO phase, secondary phases of SnO2 or Sn3O4 are present under certain growth temperatures. In contrast, a small fraction of metallic Sn phase is included in the as-grown H-doped SnO (SnO:H) films, attributed to the partial reduction of SnO by the H2 in the sputtering gas. Effects of post thermal annealing (PTA) on the properties of SnO films are also explored. Results strongly indicate that H doping or PTA facilitates the formation of Sn phase and Sn vacancies related defects, giving rise to the p-type conductivity observed in the undoped or H-doped SnO films. The present study provides a comprehensive understanding of the evolution of phase structures as well as defect properties of SnO films, which is crucial for their future studies and optoelectronic applications.

Effect of oxide layer state on the photoelectric properties of thermally sensitized PbS thin films

Lead sulfide (PbS) thin films were initially deposited through chemical bath deposition, followed by sensitization in a pure oxygen atmosphere at 600 °C. To investigate the influence of varying oxide layer states on the photoelectric properties of thermally sensitized PbS thin films, the surface oxide layer of the sensitized films was selectively removed using KOH etching. As the etching duration increased, a gradual decrease in surface oxygen content was observed, accompanied by an initial increase and subsequent decrease in detectivity. Notably, the optimum detectivity value (6.92 × 109 cm Hz1/2 W−1) was achieved after 10 s of KOH etching. Our investigation was further supplemented with field emission scanning electron microscopy and X-ray diffraction analyses to gain insights into the impact of the oxide layer state on the photoelectric properties of PbS thin films. Our findings led to the proposal of a dual conduction mechanism for carrier transport within PbS film. In the case of as-grown PbS films, carrier transport primarily occurs via crystal boundary transition. However, after oxygen sensitization, the formation of an oxygen-rich carrier channel at the crystal boundary shifts carrier transport dominance towards crystal boundary transport. The combined influence of both transmission mechanisms serves as a primary driver for the observed changes in the photoelectric properties of PbS films.

Nanocavity-Mediated Purcell Enhancement of Er in TiO2 Thin Films Grown via Atomic Layer Deposition.

The use of trivalent erbium (Er3+), typically embedded as an atomic defect in the solid-state, has widespread adoption as a dopant in telecommunication devices and shows promise as a spin-based quantum memory for quantum communication. In particular, its natural telecom C-band optical transition and spin-photon interface make it an ideal candidate for integration into existing optical fiber networks without the need for quantum frequency conversion. However, successful scaling requires a host material with few intrinsic nuclear spins, compatibility with semiconductor foundry processes, and straightforward integration with silicon photonics. Here, we present Er-doped titanium dioxide (TiO2) thin film growth on silicon substrates using a foundry-scalable atomic layer deposition process with a wide range of doping controls over the Er concentration. Even though the as-grown films are amorphous after oxygen annealing, they exhibit relatively large crystalline grains, and the embedded Er ions exhibit the characteristic optical emission spectrum from anatase TiO2. Critically, this growth and annealing process maintains the low surface roughness required for nanophotonic integration. Finally, we interface Er ensembles with high quality factor Si nanophotonic cavities via evanescent coupling and demonstrate a large Purcell enhancement (≈300) of their optical lifetime. Our findings demonstrate a low-temperature, nondestructive, and substrate-independent process for integrating Er-doped materials with silicon photonics. At high doping densities this platform can enable integrated photonic components such as on-chip amplifiers and lasers, while dilute concentrations can realize single ion quantum memories.

Phase transition mechanism of CsPbI2Br perovskite films induced by moisture

The humidity stability and phase transition mechanism of the all-inorganic perovskite CsPbI2Br based on an optimized dual-source co-evaporation preparation process are investigated at the film interface level. It is found that the CsPbI2Br films annealed at 300 °C for several minutes exhibit a best crystallinity and photoelectric properties. The as-grown CsPbI2Br film is confirmed to be a α phase with a dark brown cubic crystal structure and an average visible transparency of 35.9%. But it will be transformed into its δ phase with a transparent orthorhombic crystal structure and an average visible transparency of 80.3% after a certain amount of moisture exposure. Compared with the α phase film, the electronic structure of the δ phase has also changed significantly with a VBM shift of about 0.32 eV to high binding energy. The results of AR-XPS show that the water molecules in perovskite CsPbI2Br after a moisture exposure only adsorb on the surface rather than penetrate the interior of the lattice. When water molecules adsorb on the lattice surface, halide ions should migrate towards the lattice surface due to their high hydration enthalpy, resulting in halide vacancies within the lattice and causing the reduction of energy barrier for phase transition from α phase to δ phase. So the CsPbI2Br film will transform from its α phase to δ phase induced by water vapor, and this phase transition will be reversed to some extent after another annealing.

Physics based optical modeling of iron disulfide thin films

In this work, physics based optical modeling is carried out using iron disulfide thin films deposited by using a plasma-assisted, radio frequency-powered technique. Iron disulfide is a transition metal dichalcogenide material, exhibiting a variety of unique and excellent characteristics. Various characterization techniques are employed to examine the growth rate, film thickness, and behavior of as-grown iron disulfide thin films. Furthermore, the physics based optical modeling was performed using a combination of experimental techniques and computer modeling approaches. The analyzed thin films exhibit a bandgap of around 1.16 eV. The theoretically calculated values of absorbance, transmission, and reflectance show a good match with the experimental measurements. Moreover, a physics based optical model is developed based on the experimental data and is used to calculate the external quantum efficiency and the optically generated current density of the iron disulfide films to provide insight into its use as an absorber layer.

Kilovolt-class β-Ga2O3 MOSFETs on 1-in. bulk substrates

We report on the demonstration of β-Ga2O3 MOSFETs fabricated on 1-in. bulk substrates using metalorganic vapor phase epitaxy (MOVPE) with disilane (Si2H6) as the silicon precursor. Sheet charge uniformity of the as-grown films was measured via Hall and ranged from 5.9–6.7 × 1012 cm−2 with a uniform Hall mobility of 125–129 cm2/V s across the sample. MOSFET devices with a source-drain length of 5.1 μm were measured across the wafer and had a minimum on-resistance (RON) of 47.87 Ω mm with a maximum on-current (ION) of 165 mA/mm. For these same devices, the on-current (ION) and pinch-off voltage (VP) uniformity across the wafer were 137 ± 12 mA/mm and −27.3 ± 7.3 V, respectively. Devices showed low reverse leakage current until catastrophic breakdown occurred, with measured breakdown voltages (VBR) of up to 2.15 kV. This work provides valuable insights into understanding the growth, fabrication, and characterization processes for β-Ga2O3 FETs on full wafer-scale substrates. It also projects the promise of developing lateral β-Ga2O3 FETs with high current-carrying capabilities and breakdown voltages, especially on substrates of 1 in. or larger.

Effect of the roughness on the photoinduced growth of crystalline tellurium on MoTe2 matrix

In this work, a series of 5 nm thick films of molybdenum oxide (MoO3) were synthesized on SiO2/Si substrates by magnetron sputtering of molybdenum in low vacuum conditions. We used closed-space sublimation (CSS) to tellurize the Mo oxide and to produce pure-phase 1 T′-MoTe2 films by controlling the temperature inside the CSS reactor. Surface analyses showed that tellurium spreads over the surface of the as-grown films, forming localized crystalline precipitates whose distribution depends on the tellularization time. From Raman spectroscopy investigations, we observed that the exposure of these films to visible light leads to a growth of a highly-ordered trigonal phase of tellurium. The intensity evolution of the vibrational modes related to the crystalline phase of tellurium allows us to perform a quantitative analysis of the crystallization. Our results indicate that this process is consistent with the Kolmogorov-Johnson-Mehl-Avrami (KJMA) theory for the overall crystallization process. By the characteristic growth exponent, we introduce a more specific model which takes into account the diffusion and the dissociation of tellurium chains, which is used to probe how the effective growth rate is affected by the roughness of the samples.

Hybrid Molecular Beam Epitaxy for Single-Crystalline Oxide Membranes with Binary Oxide Sacrificial Layers.

The advancement in thin-film exfoliation for synthesizing oxide membranes has led to possibilities for creating artificially assembled heterostructures with structurally and chemically incompatible materials. The sacrificial layer method is a promising approach to exfoliate as-grown films from a compatible material system, allowing for their integration with dissimilar materials. Nonetheless, the conventional sacrificial layers often possess an intricate stoichiometry, thereby constraining their practicality and adaptability, particularly when considering techniques such as molecular beam epitaxy (MBE). This is where easy-to-grow binary alkaline-earth-metal oxides with a rock salt crystal structure are useful. These oxides, which include (Mg, Ca, Sr, Ba)O, can be used as a sacrificial layer covering a much broader range of lattice parameters compared to conventional sacrificial layers and are easily dissolvable in deionized water. In this study, we show the epitaxial growth of the single-crystalline perovskite SrTiO3 (STO) on sacrificial layers consisting of crystalline SrO, BaO, and Ba1-xCaxO films, employing a hybrid MBE method. Our results highlight the rapid (≤5 min) dissolution of the sacrificial layer when immersed in deionized water, facilitating the fabrication of millimeter-sized STO membranes. Using high-resolution X-ray diffraction, atomic-force microscopy, scanning transmission electron microscopy, impedance spectroscopy, and scattering-type near-field optical microscopy (SNOM), we demonstrate single-crystalline STO membranes with bulk-like intrinsic dielectric properties. The employment of alkaline earth metal oxides as sacrificial layers is likely to simplify membrane synthesis, particularly with MBE, thus expanding the research and application possibilities.

Generation of highly anisotropic physical properties in ferromagnetic thin films controlled by their differently oriented nano-sheets

We fabricated ferromagnetic nano-crystalline thin films of Co, Fe, Co–Fe and Co-rich and Fe-rich, Co–MT and Fe–MT (MT = transition metal), constituted by nano-sheets with a controlled slant. Visualization of these nano-sheets by Scanning Tunneling Microscopy and High-Resolution Transmission Electron Microscopy (HRTEM) showed typically tilt angles ≈56° with respect to the substrate plane, and nano-sheets ≈3.0–4.0 nm thick, ≈30–100 nm wide, and ≈200–300 nm long, with an inter-sheet distance of ≈0.9–1.2 nm, depending on their constitutive elements. Induced by this nano-morphology, these films exhibited large uniaxial magnetic anisotropy in the plane, the easy direction of magnetization being parallel to the longitudinal direction of the nano-sheets. In the as-grown films, typical values of the anisotropy field were between Hk ≈ 48 and 110 kA/m depending on composition. The changes in the nano-morphology caused by thermal treatments, and hence in the anisotropic properties, were also visualized by HRTEM, including chemical analysis at the nano-scale. Some films retained their nano-sheet morphology and increased their anisotropies by up to three times after being heated to at least 500 °C: for example, the thermal treatments produced crystallization processes and the growth of CoV and CoFe magnetic phases, maintaining the nano-sheet morphology. In contrast, other annealed films, Co, Fe, CoZn, CoCu… lost their nano-sheet morphology and hence their anisotropies. This work opens a path of study for these new magnetically anisotropic materials, particularly with respect to the nano-morphological and structural changes related to the increase in magnetic anisotropy.

Role of interdiffusion on the magnetism of ultrathin LaMnO3 films

We present a detailed study of the magnetic, spectroscopic and structural properties of ultrathin LaMnO3 films deposited on SrTiO3(001) substrates by oxide molecular beam epitaxy. We find that the as-grown LaMnO3 films are slightly reduced and present a significant magnetic moment, while annealing to 600 °C fully oxidizes and enhances its magnetic properties. From synchrotron x-ray photoemission spectromicroscopy, we find the presence of Sr, Ca and Si in the LaMnO3 film that diffuse from the SrTiO3 substrate; these impurities act as hole dopants, and can explain the presence of ferromagnetism in LaMnO3. This work highlights the importance of cation mobility at the elevated growth temperature in modifying the magnetic properties of ultrathin LaMnO3 films.

High harmonic generation in epitaxially grown zinc oxide films

High harmonic generation (HHG) in solids has been identified as a promising mechanism for light source generation and for spectroscopy of materials. HHG from bulk solids, however, often suffers from nonlinear propagation effects, resulting in a loss of spectral coherence and the skewing of spectroscopic measurements. Here, we study HHG in epitaxial ZnO thin films grown on Al2O3 substrates using atomic layer deposition. We find that the HHG emission consists of narrow spectral peaks, in contrast to those seen in bulk, and that the dependence of the harmonic yield on the film thickness differs for above-gap and below-gap harmonics, which can be understood from analytical models based on the linear and nonlinear response of the medium. The measured harmonic spectra depend qualitatively on the preparation of the films, with as-grown films generating even harmonic orders, which are absent in annealed films. The results are interpreted using transmission electron microscopy measurements, which indicate different morphologies for the as-grown and annealed films.

Porous NiMoO4 Nanosheet Films and a Device with Ultralarge Optical Modulation for Electrochromic Energy-Storage Applications.

Reducing building energy consumption, improving aesthetics, and improving occupant privacy as well as comfort by dynamically adjusting solar radiation are important application areas for electrochromic (EC) smart windows. However, the current transition metal oxides still cannot meet the requirements of neutral coloration and large optical modulation. We report NiMoO4 nanosheet films directly grown on fluorine-doped tin oxide glasses. The as-grown NiMoO4 film not only achieves neutral coloration from transparent to dark brown but also shows an ultralarge optical modulation (86.8% at 480 nm) and excellent cycling stability (99.4% retention of maximum optical modulation after 1500 cycles). Meanwhile, an EC device demonstrating good EC performance was constructed. These results will greatly promote the research and development of binary transition metal oxides for both EC and energy-storage applications, and NiMoO4 films may be an excellent candidate to replace NiO films as ion-storage layers in complementary EC devices with WO3 films as EC layers.

Gas Sensing Properties of PLD Grown 2D SnS Film: Effect of Film Thickness, Metal Nanoparticle Decoration, and In Situ KPFM Investigation.

This study employs novel growth methodologies and surface sensitization with metal nanoparticles to enhance and manipulate gas sensing behavior of two-dimensional (2D)SnS film. Growth of SnS films is optimized by varying substrate temperature and laser pulses during pulsed laser deposition (PLD). Thereafter, palladium (Pd), gold (Au), and silver (Ag) nanoparticles are decorated on as-grown film using gas-phase synthesis techniques. X-ray diffraction (XRD), Raman spectroscopy, and Field-emission scanning electron microscopy (FESEM) elucidate the growth evolution of SnS and the effect of nanoparticle decoration. X-ray photoelectron spectroscopy (XPS) analyses the chemical state and composition. Pristine SnS, Ag, and Au decorated SnS films are sensitive and selective toward NO2 at room temperature (RT). Ag nanoparticle increases the response of pristine SnS from 48 to 138% toward 2ppm NO2, which indicates electronic and chemical sensitization effect of Ag. Pd decoration on SnS tunes its selectivity toward H2 gas with a response of 55% toward 70ppm H2 and limit of detection (LOD) < 1ppm. In situ Kelvin probe force microscopy (KPFM) maps the work function changes, revealing catalytic effect of Ag toward NO2 in Ag-decorated SnS and direct charge transfer between Pd and SnS during H2 exposure in Pd-decorated SnS.

Hollow-cathode plasma deposited vanadium oxide films: Metal precursor influence on growth and material properties

Due to its different polymorphs, including vanadium pentoxide (V2O5) and vanadium dioxide (VO2), the vanadium oxide (VOX) compound is an immensely interesting material with many important applications. While atomic layer deposition (ALD) is among the possible VOX film synthesis methods, literature reports have majorly utilized thermal-ALD, which reveals as-grown amorphous VOX films. Further post-deposition annealing process is needed to crystallize these films. High-temperature crystallization indeed limits the use of low-temperature compatible materials, processes, and substrates. In this work, we report on the low-temperature crystalline VOX film growth in a hollow-cathode plasma-enhanced atomic layer deposition reactor using two different vanadium precursors, tetrakis(ethylmethylamino)vanadium and vanadium(V) oxytriisopropoxide. Oxygen plasmas were used as co-reactants at a substrate temperature of 150 °C. Along with the purpose of investing in the impact of metal precursors on VOX film growth, we also studied Ar-plasma in situ and thermal ex situ annealing to investigate possible structural enhancement and phase transformation. In situ Ar-plasma annealing was performed with 20 s, 20 SCCM Ar-plasma, while post-deposition ex situ annealing was carried out at 500 °C and 0.5 mTorr O2 pressure. In situ ellipsometry was performed to record instant film thickness variation and several ex situ characterizations were performed to extract the optical, structural, and electrical properties of the films. The outcomes of the study confirm that both metal precursors result in as-grown crystalline V2O5 films at 150 °C. On the other hand, post-deposition annealing converted the as-grown crystalline V2O5 film to VO2 film. Finally, we have also successfully confirmed the metal-to-insulator transition property of the annealed VO2 films via temperature-dependent structural and electrical measurements.

Direct localized growth of graphene on a substrate: a novel nickel-catalyzed CVD process assisted by H2 plasma.

An original metal catalyzed CVD methodology assisted by hydrogen plasma for the direct deposition of few-layer graphene on a substrate is presented. Graphene is grown at 900 °C directly on the surface of the substrate of technological interest by carbon diffusion through a nickel film by using methane (CH4) as the carbon precursor. Hydrogen atoms in the H2-plasma downstream are used to promote the solubilization of carbon atoms in Ni, thus favouring the growth of graphene at the Ni/substrate interface. Structural and transport properties of the as-grown multilayer graphene films on SiO2/Si and quartz substrates are provided. We demonstrate the peculiarity of this approach for controlling the thickness and transport properties of as-grown graphene films using process-step times. Finally, the potential of the proposed methodology for the bottom-up direct growth of patterned graphene is demonstrated.

Atomic-scale investigation of γ-Ga2O3 deposited on MgAl2O4 and its relationship with β-Ga2O3

Nominally phase-pure γ-Ga2O3 was deposited on (100) MgAl2O4 within a narrow temperature window centered at ∼470 °C using metal-organic chemical vapor deposition. The film deposited at 440 °C exhibited either poor crystallization or an amorphous structure; the film grown at 500 °C contained both β-Ga2O3 and γ-Ga2O3. A nominally phase-pure β-Ga2O3 film was obtained at 530 °C. Atomic-resolution scanning transmission electron microscopy (STEM) investigations of the γ-Ga2O3 film grown at 470 °C revealed a high density of antiphase boundaries. A planar defect model developed for γ-Al2O3 was extended to explain the stacking sequences of the Ga sublattice observed in the STEM images of γ-Ga2O3. The presence of the 180° rotational domains and 90° rotational domains of β-Ga2O3 inclusions within the γ-Ga2O3 matrix is discussed within the context of a comprehensive investigation of the epitaxial relationship between those two phases in the as-grown film at 470 °C and the same film annealed at 600 °C. The results led to the hypotheses that (i) incorporation of certain dopants, including Si, Ge, Sn, Mg, Al, and Sc, into β-Ga2O3 locally stabilizes the “γ-phase” and (ii) the site preference(s) for these dopants promotes the formation of “γ-phase” and/or γ-Ga2O3 solid solutions. However, in the absence of such dopants, pure γ-Ga2O3 remains the least stable Ga2O3 polymorph, as indicated by its very narrow growth window, lower growth temperatures relative to other Ga2O3 polymorphs, and the largest calculated difference in Helmholtz free energy per formula unit between γ-Ga2O3 and β-Ga2O3 than all other polymorphs.