β-Ga2O3 Thin Films Research Articles

Photo-conductivity as a transmission phenomenon: Application to the study of β−Ga2O3 thin film

The electrical conduction in semiconductors can be regarded as the flow of particles through an active medium. Starting from the conduction approach as a particle transmission problem, simple and closed expressions for transport coefficients are obtained in a bipolar model of a homogeneous semiconductor at non-steady-state conditions. These coefficients simplify the calculation of the conductivity as a time function and the evaluation of transport phenomena resulting from multiple processes. Noteworthily, they are independent of the dimensions of the sample, and thereby, suitable for the macroscopic case. The formulation described here can be used for the characterization of photoconductive samples from transport measurements and for the verification of band structure models. As an example, we apply this method to analyze the photo-response of a β-Ga2O3 thin film, obtaining estimates for electron (200 cm2/V⋅s) and hole (20 cm2/V⋅s) mobilities, recombination (2.53 × 10−24 cm2) and capture (3.91 × 10−20 cm2) cross-sections, concentrations of defects (deep defects: 3.068 × 1013 1/cm3, shallow defects: 7.556 × 1012 1/cm3) and the density of states of the bands.

Effect of impurities on the Raman spectra of spray-coated β-Ga2O3 thin films

Here, the incorporation of impurities into doped thin β-Ga2O3 films was studied by Raman spectroscopy, and a simple spring model was employed to estimate the impurity concentration from the impurity-modified frequencies of first-order phonon modes. β-Ga2O3 thin film samples were prepared using the spray-coating technique. As impurities, we used rare earth atoms (Er, Sm, and Gd) as well as Mg, Al, and Zn, with the nominal impurity concentrations varying from 0.5% up to 5.0%. As the impurities are expected to predominantly occupy Ga sites in the β-Ga2O3 lattice, heavier and lighter atoms than Ga should have a pronounced influence on Ga-related lattice vibrations. Therefore, in the Raman spectra of the thin films measured using 325-nm excitation, the impurity-induced shifts of the frequencies of vibrations involving Ga and O atoms were employed to estimate the impurity concentration. In addition, a high-impurity concentration can cause the formation of impurity-related oxides, as it is clearly visible for Zn. Besides, the Raman spectra with Mg as the impurity show that Mg most probably occupies interstitial rather than substitutional sites as the Raman modes do not shift with respect to the impurity concentration.

Sub-bandgap refractive indexes and optical properties of Si-doped β-Ga2O3 semiconductor thin films

In this article, we present a theoretical study on the sub-bandgap refractive indexes and optical properties of Si-doped β-Ga2O3 thin films based on newly developed models. The measured sub-bandgap refractive indexes of β-Ga2O3 thin film are explained well with the new model, leading to the determination of an explicit analytical dispersion of refractive indexes for photon energy below an effective optical bandgap energy of 4.952 eV for the β-Ga2O3 thin film. Then, the oscillatory structures in long wavelength regions in experimental transmission spectra of Si-doped β-Ga2O3 thin films with different Si doping concentrations are quantitively interpreted utilizing the determined sub-bandgap refractive index dispersion. Meanwhile, effective optical bandgap values of Si-doped β-Ga2O3 thin films are further determined and are found to decrease with increasing the Si doping concentration as expectedly. In addition, the sub-bandgap absorption coefficients of Si-doped β-Ga2O3 thin film are calculated under the frame of the Franz–Keldysh mechanism due to the electric field effect of ionized Si impurities. The theoretical absorption coefficients agree with the available experimental data. These key parameters obtained in the present study may enrich the present understanding of the sub-bandgap refractive indexes and optical properties of impurity-doped β-Ga2O3 thin films.

Electrical Conductivity of Pure and Cr3+-Doped β-Ga2O3 Thin Films

Electrical Conductivity of Pure and Cr3+-Doped β-Ga2O3 Thin Films

Machine learning supported analysis of MOVPE grown β-Ga2O3 thin films on sapphire

In this work, we demonstrate a machine learning approach, Random Forest, for the β-Ga2O3 growth rate prediction in the metal–organic vapor phase epitaxy (MOVPE) by analyzing the growth process of β-Ga2O3 on sapphire optically. The proposed model can assess the complex non-linear dependencies among the growth parameters and optimize them for the optimal growth rate. The model based on the process parameters (e.g., precursor concentration, chamber pressure, and push gas) provides high predictive power, reaching the coefficient of determination (R2) of 0.95 and 0.92 for the training and testing sets. The outcome of the model is applicable to both homoepitaxial and heteroepitaxial processes and on different substrate orientations.

Enhanced selective ammonia detection of spray deposited Cd-doped β-Ga2O3 thin films with low hysteresis effect

The Ga2O3 thin films at different concentrations (at %) of Cd dopant were deposited using spray pyrolysis technique. The X-ray diffraction patterns revealed Cd-doped β-Ga2O3 thin film are in monoclinic crystal structure with (2 0 1) orientation. Atomic force micrographs depict the change in morphology upon Cd dopant. At ambient temperature (∼30 °C), the results of ammonia sensing measurements of 4 at % Cd-doped Ga2O3 thin films point out the low-level detection (0.5 ppm) of ammonia with good sensing response (233 %), higher sensitivity (26 Ω/m), fast response-recovery time (3 s and 14 s), better selectivity, low hysteresis, and higher stability.

Sapphire substrate induced effects on β-Ga2O3 thin films

Sapphire substrate induced effects on β-Ga2O3 thin films

Schottky diode characteristics on high-growth rate LPCVD β -Ga2O3 films on (010) and (001) Ga2O3 substrates

High crystalline quality thick β-Ga2O3 drift layers are essential for multi-kV vertical power devices. Low-pressure chemical vapor deposition (LPCVD) is suitable for achieving high growth rates. This paper presents a systematic study of the Schottky barrier diodes fabricated on four different Si-doped homoepitaxial β-Ga2O3 thin films grown on Sn-doped (010) and (001) β-Ga2O3 substrates by LPCVD with a fast growth rate varying from 13 to 21 μm/h. A higher temperature growth results in the highest reported growth rate to date. Room temperature current density–voltage data for different Schottky diodes are presented, and diode characteristics, such as ideality factor, barrier height, specific on-resistance, and breakdown voltage are studied. Temperature dependence (25–250 °C) of the ideality factor, barrier height, and specific on-resistance is also analyzed from the J–V–T characteristics of the fabricated Schottky diodes.

Effect of oxygen vacancies in heteroepitaxial β-Ga2O3 thin film solar blind photodetectors

In this work, β-Ga2O3 thin films were deposited on GaN template and sapphire substrates by metal-organic chemical vapor deposition (MOCVD), respectively. Corresponding β-Ga2O3 thin film metal-semiconductor-metal (MSM) photodetectors (PDs) were prepared. Comparing the performance between these two heteroepitaxial β-Ga2O3 thin film PDs, oxygen vacancies were found to determine the discrepancy. The responsivity of β-Ga2O3 PDs on GaN increased with interdigital spacing, while the behavior of β-Ga2O3 PDs on sapphire was opposite. A photoconductive model of MSM structure was announced, showing the key role of oxygen vacancies in above observation. Meanwhile, the capture of photogenerated holes by oxygen vacancies not only enhanced the responsivity but also delayed the response time. This work paved the way for further optimization of heteroepitaxial β-Ga2O3 thin film PDs.

Growth and crystal phase transformation of ε-Ga2O3 grown on 4H–SiC by MOCVD

Ga2O3 thin films were hetero-epitaxially grown on 4H–SiC substrates by metal-organic chemical vapor deposition (MOCVD) under various growth conditions. The growth process and crystal quality of the ε-Ga2O3 thin films grown on C-face and Si-face of 4H–SiC substrates were compared. The ε-Ga2O3 film grown on a Si-face substrate shows better performance in improving crystal quality compared to the case of growth on a C-face. A ε-Ga2O3 thin film grown at 665 °C with an H2O flow rate of 200 sccm was thermally annealed at 900 °C for 10 min in an oxygen atmosphere for a crystal phase transformation to a β-Ga2O3 thin film. The RMS roughness of the transformed β-Ga2O3 thin film was 9.023 nm while the RMS value of directly grown was 129.1 nm. The estimated screw and edge dislocation density of the ε-Ga2O3 thin film were 1.41 × 1013 cm−2 and 4.92 × 1012 cm−2 respectively. Transformation of the crystal phase from ε-Ga2O3 to β-Ga2O3 can be considered as an effective method in hetero-epitaxial growth of Ga2O3 on SiC substrate.

Electronic structure modification in Fe-substituted β-Ga2O3 from resonant photoemission and soft x-ray absorption spectroscopies

Oriented thin films of β-(Ga1−x Fe x )2O3 were deposited by radio frequency magnetron sputtering on c-Al2O3 and GaN substrates. The itinerant character of the Fe 3d states forming the top of the valence band (VB) of the Fe-substituted β-Ga2O3 thin films has been determined from resonant photoelectron spectroscopy. Further, the admixture of the itinerant and localized characters of these Fe 3d states has been obtained for larger binding energies; i.e. deeper in the VB. The bottom of the conduction band (CB) for β-(Ga1−x Fe x )2O3 has been also found to have strongly hybridized states involving Fe 3d and O 2p states compared to that of Ga 4s in pristine β-Ga2O3. This suggests that β-Ga2O3 transforms from a band-like system to a charge-transfer system with Fe substitution. Furthermore, the bandgap red shifts with Fe composition, which has been found to be primarily related to the shift of the CB edge.

Crystalline anisotropy of β-Ga2O3 thin films on a c-plane GaN template and a sapphire substrate

In this work, crystalline anisotropy of heteroepitaxial () β-Ga2O3 films on a c-plane sapphire substrate and a GaN template was investigated using x-ray diffraction. The () ω-scan broadening of β-Ga2O3 on GaN exhibited six-fold rotational symmetric anisotropy along different azimuths, with maxima along the [010] direction and minima along the [102] direction, respectively. However, in β-Ga2O3 on sapphire, it was nearly isotropic. Smaller lattice mismatch between β-Ga2O3 and GaN were taken into account to explain the discrepancy, which also explained the better quality of β-Ga2O3 deposited on GaN. Our results present a new viewpoint of the crystallographic anisotropy of () β-Ga2O3 thin films.

Effects of Post Annealing on Properties of Β-Ga2o3 Thin Film Prepared by Rf Magnetron Sputtering

Effects of Post Annealing on Properties of Β-Ga2o3 Thin Film Prepared by Rf Magnetron Sputtering

X-ray diffraction analysis of gallium oxide thin films synthesised by a simple and cost-effective method

Wide energy gap beta type gallium oxide (Ga2O3) semiconductor material has attracted many researchers' interests due to its thermal and chemical stability. For synthesising Ga2O3 thin films, sol-gel spin coating is a simple and cost-efficient method, especially for spin coating on a cheap substrate such as silicon (Si) substrate. However, little is known about the spin coating growth of the Ga2O3 thin films on Si substrate. In this paper, special attention was first paid to the pre-treatment of the Si substrate and the coated layer. Subsequently, the effects of the annealing temperature on the structural, surface morphology and topography properties of the deposited films were investigated. In-depth XRD analysis using Williamson-Hall (W-H) method was performed to evaluate the crystallite size and micro strain of the deposited films. The results revealed the polycrystalline β-Ga2O3 thin films were successfully synthesised by sol-gel spin coating followed by annealing at different temperatures. Also, the pre-treatment processes have greatly improved the uniformity of the film's thickness. As the annealing temperature increases, the average particle size and the sample surface roughness increase dramatically. The XRD analysis results using the W-H and SSP methods revealed that the crystallite size increased when the annealing increased from 900 to 1000°C, but for the sample annealed at 1100°C, the results were affected by the formation of crystalline SiO2. The analysis results also showed that the micro strain is not the broadening key factor for the Bragg peaks.

Thermodynamics of Ion-Cutting of β-Ga2O3 and Wafer-Scale Heterogeneous Integration of a β-Ga2O3 Thin Film onto a Highly Thermal Conductive SiC Substrate

Thermodynamics of Ion-Cutting of β-Ga₂O₃ and Wafer-Scale Heterogeneous Integration of a β-Ga₂O₃ Thin Film onto a Highly Thermal Conductive SiC Substrate

Thermal transport in beta-gallium oxide thin-films using non-gray Boltzmann transport equation

Phonon transport in β-Ga2O3 thin films and metal-oxide field effect transistors (MESFETs) are investigated using non-gray Boltzmann transport equations (BTEs) to decipher the effect of ballistic-diffusive phonon transport. The effects of domain size, and energy dissipation to various phonon modes and subsequent phonon–phonon energy exchange on the thermal transport and temperature distribution is investigated using non-gray BTE. Our analysis deciphered that domain size plays a major role in thermal transport in β-Ga2O3 but energy dissipation to various phonon modes and subsequent phonon–phonon energy exchange does not affect the temperature field significantly. Phonon transport in β-Ga2O3 MESFETs on diamond substrate is investigated using coupled non-gray BTE and Fourier model. It is established that the ballistic effects need to be considered for devices with β-Ga2O3 layer thickness less than 1 μm. A non-gray phonon BTE model should be used near hotspot in the thin β-Ga2O3 layer as the Fourier model may not give accurate temperature distribution. The results from this work will help in understanding the mechanism of phonon transport in the β-Ga2O3 thin films and energy efficient design of its FETs.

Quasi-Epitaxial Growth of β-Ga2O3-Coated Wide Band Gap Semiconductor Tape for Flexible UV Photodetectors.

The epitaxial growth of technically important β-Ga2O3 semiconductor thin films has not been realized on flexible substrates due to the limitations of high-temperature crystallization conditions and lattice-matching requirements. We demonstrate the epitaxial growth of β-Ga2O3(-201) thin films on flexible CeO2(001)-buffered Hastelloy tape. The results indicate that CeO2(001) has a small bi-axial lattice mismatch with β-Ga2O3(-201), inducing simultaneous double-domain epitaxial growth. Flexible photodetectors are fabricated on the epitaxial β-Ga2O3-coated tape. Measurements reveal that the photodetectors have a responsivity of 4 × 104 mA/W, with an on/off ratio reaching 1000 under 254 nm incident light and 5 V bias voltage. Such a photoelectrical performance is within the mainstream level of β-Ga2O3-based photodetectors using conventional rigid single-crystal substrates. More importantly, it remained robust against more than 20,000 bending test cycles. Moreover, the technique paves the way for the direct in situ epitaxial growth of other flexible oxide semiconductor devices in the future.

Toward Precise n-Type Doping Control in MOVPE-Grown β-Ga2O3 Thin Films by Deep-Learning Approach

In this work, we train a hybrid deep-learning model (fDNN, Forest Deep Neural Network) to predict the doping level measured from the Hall Effect measurement at room temperature and to investigate the doping behavior of Si dopant in both (100) and (010) β-Ga2O3 thin film grown by the metalorganic vapor phase epitaxy (MOVPE). The model reveals that a hidden parameter, the Si supplied per nm (mol/nm), has a dominant influence on the doping process compared with other process parameters. An empirical relation is concluded from this model to estimate the doping level of the grown film with the Si supplied per nm (mol/nm) as the primary variable for both (100) and (010) β-Ga2O3 thin film. The outcome of the work indicates the similarity between the doping behavior of (100) and (010) β-Ga2O3 thin film via MOVPE and the generality of the results to different deposition systems.

Silicon-integrated monocrystalline oxide–nitride heterostructures for deep-ultraviolet optoelectronics

New opportunities for high-performance CMOS-compatible optoelectronic devices have accelerated the interest in vertically configured device topologies that enable next-generation photonic technologies. Lately, TiN has been identified as a promising refractory metal–ceramic for the hybrid integration of emerging semiconductor materials on a variety of substrates, including Si, MgO, and sapphire. Among these, Si is the least expensive and most commonly used element and substrate material in the semiconductor device industry. Following these examples, a hybrid oxide–nitride–Si stack is proposed and thoroughly investigated herein for its potential use in DUV optoelectronic device applications. The stack comprises β-Ga2O3 thin films grown heteroepitaxially on TiN/Si platforms, wherein the TiN interlayers were heteroepitaxially grown on bulk (100)-oriented Si and act as lattice-mismatched templates and bottom device electrodes. Albeit the relatively large lattice mismatch between Si and TiN, a low in-plane rotation of 3 ∘ revealed that the TiN layers continued to grow as a bulk crystal, paving the way for heteroepitaxial β-Ga2O3 thin films being grown without exhibiting amorphous and metastable phases. DUV photodetectors based on this optoelectronic heterostructure exhibited average peak spectral responsivity and external quantum efficiency levels as high as 249 A/W and 1.23 × 105%, respectively, in the ultraviolet-C regime at an illuminating power density of around 12 µW/cm2.

Electronic states of gallium oxide epitaxial thin films and related atomic arrangement

Amorphous, crystallized, and Mg-doped β-Ga2O3 thin films are investigated to understand the evolution of electronic states and the related atomic arrangement. In the process from the amorphous to crystallized phase of Ga2O3 films, disordered atoms transform into six-fold GaO6 octahedra. The denser structure creates more electron density. The crystallized phase widens the band gap, because of the conduction band changes attributed to the Ga 4s states hybridized with O 2s states. The Mg dopants create four-fold GaO4 tetrahedron deficiency. Moreover, the distances between Ga atoms become shorter for the doped film; however, the Ga–O distance is independent of the dopant. Mg dopants affect the oxygen states in valence band and Ga–O interaction at conduction band, which results in a slightly smaller band gap. The electronic evolution of the amorphous, crystallized and doped films is confirmed by the calculated density of states results. These results will be beneficial for the design of more effective semiconductive films.