Two-dimensional hole gas formation at the κ-Ga2O3 /AlN heterojunction interface

Polyakov A.Y., Nikolaev V.I., Pechnikov A.I., Stepanov S.I., Yakimov E.B., Scheglov M.P., Shchemerov I.V., Vasilev A.A., Kochkova A.A., Chernykh A.V., Chikiryaka A.V., Pearton S.J.

Thick (23 µm) films of κ-Ga2O3 were grown by Halide Vapor Phase Epitaxy (HVPE) on GaN/sapphire templates at 630 °C. X-ray analysis confirmed the formation of single-phase κ-Ga2O3 with half-widths of the high-resolution x-ray diffraction (004), (006), and (008) symmetric reflections of 4.5 arc min and asymmetric (027) reflection of 14 arc min. Orthorhombic κ-Ga2O3 polymorph formation was confirmed from analysis of the Kikuchi diffraction pattern in electron backscattering diffraction. Secondary electron imaging indicated a reasonably flat surface morphology with a few (area density ∼103 cm−2) approximately circular (diameter ∼50–100 µm) uncoalesced regions, containing κ-Ga2O3 columns with in-plane dimensions and a height of about 10 µm. Micro-cathodoluminescence (MCL) spectra showed a wide 2–3.5 eV band that could be deconvoluted into narrower bands peaked at 2.59, 2.66, 2.86, and 3.12 eV. Ni Schottky diodes prepared on the films showed good rectification but a high series resistance. The films had a thin near-surface region dominated by Ec − 0.7 eV deep centers and a deeper region (∼2 µm from the surface) dominated by shallow donors with concentrations of ≤1016 cm−3. Photocurrent and photocapacitance spectra showed the presence of deep compensating acceptors with optical ionization energies of ∼1.35 and 2.3 eV, the latter being close to the energy of one of the MCL bands. Deep level transient spectroscopy revealed deep traps with energies near 0.3, 0.6, 0.7, 0.8, and 1 eV from the conduction band edge. The results show the potential of HVPE to grow very thick κ-Ga2O3 on GaN/sapphire templates.

Zeman C.J., Kielar S.M., Jones L.O., Mosquera M.A., Schatz G.C.

We have systematically investigated the effects of all possible combinations of vacancies and silicon substitutions on the electronic structure of the β and κ phases of Ga 2 O 3 using plane-wave density functional theory (DFT) methods. It was found that V Ga defects are associated with a sufficient shift of the Fermi level to lower energy to induce p-type behavior, with formation energies in the range of 9.0 ± 0.2 eV. Calculations with single atom substitutions in the κ phase, including nitrogen, phosphorous, and silicon, did not show p-type character, although N O substitutions may lead to shallow acceptor states. In the pursuit of elucidating how MOCVD growth of Ga 2 O 3 can result in p-type behavior, as indicated by experimental results in the literature, we examined the role of combining hydrogen and silicon substitutions. The results showed that p-type behavior is observable when gallium atoms are substituted for hydrogen within the coordination sphere of Si O substitutions. This shows that silicon can act as an amphoteric dopant for p-type Ga 2 O 3 semiconducting materials when hydrogen is included with formation energies

Polyakov A.Y., Nikolaev V.I., Tarelkin S.A., Pechnikov A.I., Stepanov S.I., Nikolaev A.E., Shchemerov I.V., Yakimov E.B., Luparev N.V., Kuznetsov M.S., Vasilev A.A., Kochkova A.I., Voronova M.I., Scheglov M.P., Kim J., et. al.

Films of Ga2O3 were grown by Halide Vapor Phase Epitaxy (HVPE) on bulk heavily B-doped (001)-oriented diamond substrates using thin interlayers of Al2O3 deposited by HVPE or AlN/AlGaN deposited by metalorganic chemical vapor deposition. The growth with AlN/AlGaN was dominated by the formation of a highly conducting ɛ-phase with poor crystalline quality. For these samples, excessive leakage of Schottky diodes and of the Ga2O3/diamond heterojunction prevented meaningful electrical characterization. The film grown with the Al2O3 interlayer was mainly composed of (−201) β-Ga2O3 with an admixture of the ɛ-phase. The film had a low density of residual shallow donors, 5 × 1015 cm−3, with deep electron traps spectra consisting of the well documented centers for β-Ga2O3 near Ec 0.27, Ec 0.7, and Ec 1 eV, all of which are often ascribed to native defects or their complexes. The electrical properties of heterojunctions were mostly determined by the properties of the Ga2O3 films. Both Schottky diodes and heterojunctions showed measurable photosensitivity for 259 nm wavelength excitation, but very low photocurrent for near-UV (365 nm wavelength excitation).

Kaneko K., Masuda Y., Kan S., Takahashi I., Kato Y., Shinohe T., Fujita S.

Ultra-wide bandgap p-type α-(Ir,Ga)2O3 films with bandgaps of up to 4.3 eV have been obtained by unintentional doping or Mg doping. For Mg-doped films, Hall-effect measurements revealed a hole concentration of 9.9 × 1018 to 8.1 × 1019 cm−3 and a mobility of 0.13 − 0.92 cm2/V s, respectively. A preliminary test of a pn junction diode composed of p-type α-(Ir,Ga)2O3 and n-type α-Ga2O3 did not show catastrophic breakdown in the reverse direction until 100 V and the current on/off ratio at +3 V/−3V was 5 × 105. Since α-(Ir,Ga)2O3 and α-Ga2O3 take the same crystal structure and are well lattice-matched (with a lattice mismatch of <0.3%), the formation of a high-quality pn heterojunction is encouraged; this is one of the advantages of the corundum material system.

Ranga P., Cho S.B., Mishra R., Krishnamoorthy S.

We report on the modeling of polarization-induced two-dimensional electron gas (2DEG) formation at ${\epsilon}$-AlGaO3 / ${\epsilon}$-Ga2O3 heterointerface and the effect of spontaneous polarization (Psp) reversal on 2DEG density in ${\epsilon}$-Ga2O3 /${\epsilon}$-AlGaO3 / ${\epsilon}$-Ga2O3 double heterostructures. Density-functional theory (DFT) is utilized to calculate the material properties of ${\epsilon}$-Ga2O3 and ${\epsilon}$-AlGaO3 alloys. Using Schrodinger-Poisson solver along with DFT calculated parameters, the 2DEG density is calculated as a function of barrier type and thickness. By optimizing the layer thicknesses of ${\epsilon}$-Ga2O3/${\epsilon}$-AlGaO3/${\epsilon}$-Ga2O3 heterostructures, charge contrast ratios exceeding 1600 are obtained. This computational study indicates the high potential for ${\epsilon}$-Ga2O3-based heterostructure devices for non-volatile memories and neuromorphic applications.

Liu Z., Luo T.

In this chapter, an overview of the current research progress on the thermal properties of beta-gallium oxide (β-Ga2O3) is provided. Thermal properties of β-Ga2O3 are of great significance to the device reliability and performance in its potential applications. Previous research through both computational and experimental studies on β-Ga2O3 using various methods is reviewed. The most notable findings are the relatively low and highly anisotropicAnisotropic thermal conductivityThermal conductivity . At room temperature, the [010] direction has the highest thermal conductivityThermal conductivity of around 25 W/mK, while that in the [100] direction is measured to be the lowest, which is around 13 W/mK. We also make comparison between β-Ga2O3 and GaNGaN , another widely used semiconductor for power electronicsPower electronics . The relatively low thermal conductivity of β-Ga2O3 compared to GaNGaN may present a major challenge for its potential applications. Another important thermal property, heat capacityHeat capacity , of β-Ga2O3 at room temperature is measured to be 18.7 J/mol K. On the other hand, the effective thermal conductivityThermal conductivity in β-Ga2O3 thin filmThin film is shown to be larger than other gate oxides, providing a possibility of using it as gate dielectrics in GaNGaN device contacts. The thermal properties discussed in this chapter might be useful for thermal management and design of β-Ga2O3 devices.

Wu C., Guo D.Y., Zhang L.Y., Li P.G., Zhang F.B., Tan C.K., Wang S.L., Liu A.P., Wu F.M., Tang W.H.

β-Ga2O3 has attracted much attention due to its ultrawide-bandgap (∼4.9 eV) with a high breakdown field (8 MV/cm) and good thermal/chemical stability. In order for β-Ga2O3 to be used in electronic and optoelectronic devices, epitaxial growth technology of thin films should be given priority. However, challenges are associated with the trade-off growth rate with crystallization and surface roughness in conventional epitaxy. Herein, plasma enhanced chemical vapor deposition was used to grow the β-Ga2O3 epilayer, and the growth kinetics process has been systematically investigated. A high growth rate of ∼0.58 μm/h and a single 2¯01 plane orientation with a full width at half maximum value of 0.86° were obtained when grown on the c-plane sapphire substrate at the growth temperature of 820 °C. Then, a proposed model for the mechanism of nucleation and growth of β-Ga2O3 epitaxial films is established to understand the precursor transport and gas phase reaction process. This work provides a cheap, green, and efficient epitaxial growth method, which is indispensable for device applications of β-Ga2O3.

Nikolaev V.I., Stepanov S.I., Pechnikov A.I., Shapenkov S.V., Scheglov M.P., Chikiryaka A.V., Vyvenko O.F.

In this study we compare the growth of gallium oxide films by halide vapor phase epitaxy (HVPE) on various substrates under the same growth conditions. Gallium oxide films were deposited at 500 °C–600 °C on basal plane (0001) planar and patterned sapphire substrates, (0001) 2H-GaN, 4H-SiC, and 2 ¯ 01 bulk β-Ga2O3 substrates. The layers were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and cathodoluminescence (CL) techniques. Most of the films exhibit growth features of hexagonal symmetry. Sn-doped Ga2O3 films exhibit n-type electrical conductivity. Heterojunctions composed of n-type hexagonal Ga2O3:Sn and p-type GaN:Mg demonstrate diode-like I-V characteristics and emit light under forward bias.

Polyakov A.Y., Nikolaev V.I., Stepanov S.I., Pechnikov A.I., Yakimov E.B., Smirnov N.B., Shchemerov I.V., Vasilev A.A., Kochkova A.I., Chernykh A.V., Pearton S.J.

Films of α-Ga2O3 doped with Sn were grown by halide vapor phase epitaxy (HVPE) on planar and patterned sapphire substrates. For planar substrates, with the same high Sn flow, the total concentration of donors was varying from 1017 cm−3 to high 1018 cm−3. The donor centers were shallow states with activation energies 35–60 meV, centers with levels near Ec–(0.1–0.14) eV (E1), and centers with levels near Ec–(0.35–0.4) eV (E2). Deeper electron traps with levels near Ec−0.6 eV (A), near Ec−0.8 eV (B), Ec−1 eV (C) were detected in capacitance or current transient spectroscopy measurements. Annealing of heavily compensated films in molecular hydrogen flow at 500 °C for 0.5 h strongly increased the concentration of the E1 states and increased the density of the E2 and A traps. For films grown on patterned substrates the growth started by the formation of the orthorhombic α-phase in the valleys of the sapphire pattern that was overgrown by the regions of laterally propagating α-phase. No improvement of the crystalline quality of the layers when using patterned substrates was detected. The electric properties, the deep traps spectra, and the effects of hydrogen treatment were similar to the case of planar samples.

Cora I., Fogarassy Z., Fornari R., Bosi M., Rečnik A., Pécz B.

The temperature-driven phase transformation of metastable κ-Ga2O3 layers deposited on sapphire was studied by high resolution TEM. Annealing experiments up to 1000 °C were performed either in situ in vacuum within the TEM or ex situ in ambient air. This allowed for the detection of the atomistic mechanisms at the basis of κ to β phase transition. In the case of in situ TEM observations we could even record in real time the atomic rearrangement. We provide in this paper the relevant crystallographic relations between original κ and new β lattice. Surprisingly, the ex situ experiments demonstrated the additional formation of a γ-Ga2O3 intermediate phase at 820 °C. The remarkably different behavior between in situ and ex situ annealing experiments is explained in terms of ambient (ambient air or high vacuum) and heating rate. An extensive investigation of γ-Ga2O3, also a metastable phase, showed that it has a cubic defect spinel structure ( F d 3 ¯ m ) with disordered vacancies. Repeated observations of the metastable γ-Ga2O3 after two months show that the vacancies tend to order, and that the vacancies are fully ordered after one year.

Ahmadi E., Oshima Y.

Ga2O3 is an ultrawide bandgap semiconductor with a bandgap energy of 4.5–5.3 eV (depending on its crystal structure), which is much greater than those of conventional wide bandgap semiconductors such as SiC and GaN (3.3 eV and 3.4 eV, respectively). Therefore, Ga2O3 is promising for future power device applications, and further high-performance is expected compared to those of SiC or GaN power devices, which are currently in the development stage for commercial use. Ga2O3 crystallizes into various structures. Among them, promising results have already been reported for the most stable β-Ga2O3, and for α-Ga2O3, which has the largest bandgap energy of 5.3 eV. In this article, we overview state-of-the-art technologies of β-Ga2O3 and α-Ga2O3 for future power device applications. We will give a perspective on the advantages and disadvantages of these two phases in the context of comparing the two most promising polymorphs, concerning material properties, bulk crystal growth, epitaxial growth, device fabrication, and resulting device performance.

Xu J., Zheng W., Huang F.

This review introduces the developments in β-Ga2O3 materials growth and solar blind UV photodetectors in the last decade, summarizes their advantages and potential for improvement, and puts forward some suggestions for actual application requirements.

Polyakov A.Y., Smirnov N.B., Shchemerov I.V., Yakimov E.B., Nikolaev V.I., Stepanov S.I., Pechnikov A.I., Chernykh A.V., Shcherbachev K.D., Shikoh A.S., Kochkova A., Vasilev A.A., Pearton S.J.

Epitaxial layers of α-Ga2O3 with different Sn doping levels were grown by halide vapor phase epitaxy on sapphire. The films had shallow donor concentrations ranging from 1017 to 4.8 × 1019 cm−3. Deep level transient spectroscopy of the lowest doped samples revealed dominant A traps with level Ec − 0.6 eV and B traps near Ec − 1.1 eV. With increasing shallow donor concentration, the density of the A traps increased, and new traps C (Ec − 0.85 eV) and D (Ec − 0.23 eV) emerged. Photocapacitance spectra showed the presence of deep traps with optical ionization energy of ∼2 and 2.7 eV and prominent persistent photocapacitance at low temperature, surviving heating to temperatures above room temperature. The diffusion length of nonequilibrium charge carriers was 0.15 µm, and microcathodoluminescence spectra showed peaks in the range 339–540 nm, but no band-edge emission.

Parisini A., Bosio A., Montedoro V., Gorreri A., Lamperti A., Bosi M., Garulli G., Vantaggio S., Fornari R.

Low resistivity n-type ε-Ga2O3 epilayers were obtained for the first time either by adding silane to the gas phase during the metal organic vapour phase epitaxy deposition or by diffusing Sn in nominally undoped layers after the growth. The highest doping concentrations were few 1018 cm−3 and about 1017 cm−3 for Si and Sn doping, with corresponding resistivity below 1 and 10 Ω cm, respectively. Temperature dependent transport investigation in the range of 10-600 K shows a resistivity behavior consistent with the Mott law, suggesting that conduction through localized states dominates the electrical properties of Si- and Sn-doped samples. For both types of dopants, two different mechanisms of conduction through impurity band states seem to be present, each of them determining the transport behavior at the lower and higher temperatures of the measurement range.

Pearton S.J., Ren F., Tadjer M., Kim J.

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics with capabilities beyond existing technologies due to its large bandgap, controllable doping, and the availability of large diameter, relatively inexpensive substrates. These applications include power conditioning systems, including pulsed power for avionics and electric ships, solid-state drivers for heavy electric motors, and advanced power management and control electronics. Wide bandgap (WBG) power devices offer potential savings in both energy and cost. However, converters powered by WBG devices require innovation at all levels, entailing changes to system design, circuit architecture, qualification metrics, and even market models. The performance of high voltage rectifiers and enhancement-mode metal-oxide field effect transistors benefits from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. Reverse breakdown voltages of over 2 kV for β-Ga2O3 have been reported, either with or without edge termination and over 3 kV for a lateral field-plated Ga2O3 Schottky diode on sapphire. The metal-oxide-semiconductor field-effect transistors fabricated on Ga2O3 to date have predominantly been depletion (d-mode) devices, with a few demonstrations of enhancement (e-mode) operation. While these results are promising, what are the limitations of this technology and what needs to occur for it to play a role alongside the more mature SiC and GaN power device technologies? The low thermal conductivity might be mitigated by transferring devices to another substrate or thinning down the substrate and using a heatsink as well as top-side heat extraction. We give a perspective on the materials’ properties and physics of transport, thermal conduction, doping capabilities, and device design that summarizes the current limitations and future areas of development. A key requirement is continued interest from military electronics development agencies. The history of the power electronics device field has shown that new technologies appear roughly every 10-12 years, with a cycle of performance evolution and optimization. The older technologies, however, survive long into the marketplace, for various reasons. Ga2O3 may supplement SiC and GaN, but is not expected to replace them.

Pearton S.J., Ren F., Polyakov A.Y., Yakimov E.B., Chernyak L., Haque A.

Gallium oxide (Ga2O3) exists in different polymorphic forms, including the trigonal (α), monoclinic (β), cubic (γ), and orthorhombic (κ) phases, each exhibiting distinct structural and electronic properties. Among these, β-Ga2O3 is the most thermodynamically stable and widely studied for high-power electronics applications due to its ability to be grown as high-quality bulk crystals. However, metastable phases such as α-, γ-, and κ-Ga2O3 offer unique properties, including wider bandgap or strong polarization and ferroelectric characteristics, making them attractive for specialized applications. This paper summarizes the radiation hardness of these polymorphs by analyzing the reported changes in minority carrier diffusion length (LD) and carrier removal rates under various irradiation conditions, including protons, neutrons, alpha particles, and gamma rays. β-Ga2O3 demonstrates high radiation tolerance with LD reductions correlated to the introduction of electron traps (E2*, E3, and E4) and gallium–oxygen vacancy complexes (VGa–VO). α-Ga2O3 exhibits slightly better radiation hardness similar to κ-Ga2O3, which also shows minimal LD changes postirradiation, likely due to suppressed defect migration. γ-Ga2O3 is the least thermodynamically stable, but surprisingly is not susceptible to radiation-induced damage, and is stabilized under Ga-deficient conditions. The study highlights the role of polymorph-specific defect dynamics, doping concentrations, and nonuniform electrical properties in determining radiation hardness. We also discuss the effect of radiation exposure on the use of NiO/Ga2O3 heterojunction rectifiers that provide superior electrical performance relative to Schottky rectifiers. The presence of NiO does change some aspects of the response to radiation. Alloying with Al2O3 further modulates the bandgap of Ga2O3 and defect behavior, offering potentially tunable radiation tolerance. These findings provide critical insights into the radiation response of Ga2O3 polymorphs, with implications for their use in aerospace and radiation-hardened power electronics. Future research should focus on direct comparisons of polymorphs under identical irradiation conditions, defect identification, and annealing strategies to enhance radiation tolerance.

Seguret A., Okuno H., Roussel H., Rouvière J., Bujak A., Ferrandis P., Bellet-Amalric E., Consonni V., Monroy E.

Polyakov A.Y., Yakimov E.B., Shchemerov I.V., Vasilev A., Kochkova A.I., Nikolaev V., Pearton S.J.

Abstract The material system of ultra-wide bandgap Ga2O3 has already shown great promise in the field of solar-blind photodetectors with high photoresponsivity, high photoresponsivity gain and low dark current. These promising results have been achieved on Ga2O3 films of different polymorphs and by different methods, often not with particularly high crystalline quality. In fact, it would often seem the case that the lower the crystalline quality of the films, the higher the photosensitivity and its gain. This, however, is in most cases accompanied by unusually long photocurrent build-up and decay times. We show that the experimental results can be explained by models in which the high photosensitivity gain is related to the effects of holes being trapped by deep states, which, in Schottky diodes, results in a decrease of the Schottky barrier height with a consequent increase in the electron current, and in metal–semiconductor–metal (MSM) structures additionally gives rise to the usual gain increase due to the increased concentration and lifetime of electrons. We present and discuss models describing the effects in Ga2O3 Schottky diodes, MSM structures, and unipolar and bipolar heterojunctions, and we propose possible candidates for the role of the hole traps in different Ga2O3 polymorphs. We also discuss the existing results for the photocurrent build-up and decay times and offer possible explanations for the observed temperature dependences of the characteristic times where such data are present.

Ardenghi A., Bierwagen O., Lähnemann J., Luna E., Kler J., Falkenstein A., Martin M., Sacchi A., Mazzolini P.

Its large intrinsic polarization makes the metastable κ-Ga2O3 polymorph appealing for multiple applications, and the In-incorporation into both κ and β-Ga2O3 allows us to engineer their bandgap on the low-end side. In this work, we provide practical guidelines to grow thin films of single phase κ-, β-Ga2O3 as well as their (InxGa1−x)2O3 alloys up to x = 0.14 and x = 0.17, respectively, using In-mediated metal exchange catalysis in plasma-assisted molecular beam epitaxy (MEXCAT-MBE). The role of substrate temperature, oxidizing power, growth rate, and choice of substrate on phase formation and In-incorporation is investigated. As a result, the κ phase can be stabilized in a narrow deposition window irrespective of the choice of substrate [(i) α-Al2O3 (0001), (ii) 20 nm of (2̄01) β-Ga2O3 on α-Al2O3 (0001), and (iii) (2̄01) β-Ga2O3 single crystal]. Low growth rates/metal fluxes as well as growth temperatures above 700 °C tend to stabilize the β-phase independently. Lower growth temperatures and/or O-richer deposition atmospheres allow to increase the In-incorporation in both polymorphs. Finally, we also demonstrate the possibility to grow (2̄01) β-Ga2O3 on top of α-Al2O3 (0001) at temperatures at least 100 °C above those achievable with conventional non-catalyzed MBE, opening the road for better crystal quality in heteroepitaxy.

Zhang J., Deng L., Xia S., Guo C., Liu K., Chen L., Liu W., Xiao H., Yang Z., Guo W., Ye J.

Abstract Solid-state ultraviolet (UV) photodetectors (PDs) have received significant attention due to their advantages of small size, absence of external cooling, high selectivity and the ability to utilize the energy band structure semiconductor materials to achieve detection across various wavelengths. III-nitride thin films, as typical wide bandgap semiconductors with mature n-type and p-type doping capabilities, are ideal candidates for solid-state UV-PDs. However, a combination of III-nitride and other wide bandgap materials can either enrich the functionality of devices such as spectrum-selective and broadband UV detectionor offer opportunities to enhance device performance, including high photoresponsivity, high external quantum efficiency, low dark current and fast response time. This topical review focuses on giving a thorough review of the III-nitride-based hybrid-type UV PDs, their recent progress and future prospects. We highlight the different optical and electrical properties of various materials including GaN, Ga2O3, ZnO, perovskite, etc. By carefully choosing the materials on both sides of the heterojunction and modulating the thickness and Fermi levels and corresponding layers, p–i–n, Schottky or metal–semiconductor–metal-type PDs were successfully fabricated. They displayed outstanding device performance and novel spectral-selective properties. The advantages for future development of these hybrid-type PDs will be discussed, such as inherently formed p–n junction with large depletion regions at the interface of two different materials and capability of bandgap engineering to tune the band offset between the conduction and valence bands, thus enabling large barrier height for one type of carrier without influencing the other. The drawbacks of hybrid-type UV-PD due to poor interface quality and challenges in forming electrical contact in nanostructured hybrid UV-PD will also be discussed.

Demir B., Peterson R.L.

Kappa-phase gallium oxide (κ-Ga2O3) is a metastable gallium oxide polymorph and an ultrawide bandgap semiconductor with potential applications in various electronic and optoelectronic devices. In this study, we present the effect of mist flow stabilization during the initial stages of mist chemical vapor deposition process on the crystal quality of heteroepitaxial thin films of κ-Ga2O3 grown on c-plane sapphire. Two κ-Ga2O3 films were grown using the same growth parameters with the only differing factor being the stabilization of the initial mist flow. Structural and optical properties of the films were characterized. Both processes resulted in epitaxial growth; however, controlling the initial mist flow leads to an improved Ga2O3/Al2O3 interface and superior crystal quality. The film grown with regulated mist flow exhibited an optical bandgap of 4.93 eV, a smooth surface with a root mean square roughness of 1.56 nm and a rocking curve full-width half-maximum value of 0.17°. Also, Ti/Au deposition on κ-Ga2O3 resulted in an interface resembling that of β-Ga2O3 and Ti/Au, explaining previous reports of ohmic behavior between κ-Ga2O3 and Ti/Au contacts.

Chikoidze E., Leach J., Chi Z., von Bardeleben J., Ballesteros B., Gonçalves A., Tchelidze T., Dumont Y., Pérez-Tomás A.

Although two-dimensional electron gases have been realized in a number of semiconductor surfaces, examples of two-dimensional hole gases (2DHG) - the counterpart to 2DEG - are still very limited. Besides, owing to the deep energy level nature of potential dopants, achieving acceptor p-type β-Ga2O3 is a well-known challenge so far. In this work, we report what appears to be an exceptional p-type 2DHG surface on a Si-doped monoclinic (010) β-Ga2O3 crystal which otherwise is n-type in the bulk. The majority of the free carries at the surface have been determined to be holes with a sheet concentration of p ∼ 8.7 × 1013 cm−2 and a puzzlingly high mobility value of µh ∼ 80 cm2/(V·s) at room T.

Vyvenko O.F., Shapenkov S.V., Ubyivovk E.V., Bondarenko A.S., Pechnikov A.I., Nikolaev V.I., Stepanov S.I.

Raghuvansy S., McCandless J.P., Schowalter M., Karg A., Alonso-Orts M., Williams M.S., Tessarek C., Figge S., Nomoto K., Xing H.G., Schlom D.G., Rosenauer A., Jena D., Eickhoff M., Vogt P.

The heteroepitaxial growth and phase formation of Ga2O3 on Al-polar AlN(0001) templates by molecular-beam epitaxy (MBE) are studied. Three different MBE approaches are employed: (i) conventional MBE, (ii) suboxide MBE (S-MBE), and (iii) metal-oxide-catalyzed epitaxy (MOCATAXY). We grow phase-pure β-Ga2O3(2̄01) and phase-pure ϵ/κ-Ga2O3(001) with smooth surfaces by S-MBE and MOCATAXY. Thin film analysis shows that the crystallographic and surface features of the β-Ga2O3(2̄01)/AlN(0001) and ϵ/κ-Ga2O3(001)/AlN(0001) epilayers are of high crystalline quality. Growth and phase diagrams are developed to synthesize Ga2O3 on AlN by MBE and MOCATAXY and to provide guidance to grow Ga2O3 on several non-oxide surfaces, e.g., AlN, GaN, and SiC, by MBE, S-MBE, and MOCATAXY.

Polyakov A.Y., Yakimov E.B., Nikolaev V.I., Pechnikov A.I., Miakonkikh A.V., Azarov A., Lee I., Vasilev A.A., Kochkova A.I., Shchemerov I.V., Kuznetsov A., Pearton S.J.

In this study, the results of hydrogen plasma treatments of β-Ga2O3, α-Ga2O3, κ-Ga2O3 and γ-Ga2O3 polymorphs are analyzed. For all polymorphs, the results strongly suggest an interplay between donor-like hydrogen configurations and acceptor complexes formed by hydrogen with gallium vacancies. A strong anisotropy of hydrogen plasma effects in the most thermodynamically stable β-Ga2O3 are explained by its low-symmetry monoclinic crystal structure. For the metastable, α-, κ- and γ-polymorphs, it is shown that the net result of hydrogenation is often a strong increase in the density of centers supplying electrons in the near-surface regions. These centers are responsible for prominent, persistent photocapacitance and photocurrent effects.

Polyakov A., Lee I., Nikolaev V., Pechnikov A., Miakonkikh A., Scheglov M., Yakimov E., Chikiryaka A., Vasilev A., Kochkova A., Shchemerov I., Chernykh A., Pearton S.

AbstractThe structural and electrical properties of undoped and Sn doped κ‐Ga2O3 layers grown by epitaxial lateral overgrowth on TiO2/sapphire substrates using stripe and point masks show that the crystalline structure of the films can be greatly improved relative to conventional planar growth. The undoped films are semi‐insulating, with the Fermi level pinned near EC‐0.7 eV, and deep electron traps at EC‐0.5 eV and EC‐0.3 eV are detectable in thermally stimulated current and photoinduced current transient spectra measurements. Low concentration Sn doping results in net donor concentrations of ≈ 1013 cm−3, and deep trap spectra determined by electron traps at EC‐0.5 eV, and deep acceptors with an optical ionization threshold near 2 and 3.1 eV. Treatment of the samples in hydrogen plasma at 330 °C increases the donor density near the surface to ≈ 1019 cm−3. Such samples show strong persistent photocapacitance and photoconductivity, indicating the possible DX‐like character of the centers involved. For thin (5 µm) κ‐Ga2O3 films grown on GaN/sapphire templates, p‐type‐like behavior is unexpectedly observed in electrical properties and  we  discuss the possible formation of a 2D hole gas at the κ‐Ga2O3/GaN interface.

Polyakov A.Y., Nikolaev V.I., Pechnikov A.I., Yakimov E.B., Lagov P.B., Shchemerov I.V., Vasilev A.A., Kochkova A.I., Chernykh A.V., Lee I., Pearton S.J.

Changes induced by irradiation with 1.1 MeV protons in the transport properties and deep trap spectra of thick (>80 μm) undoped κ-Ga2O3 layers grown on sapphire are reported. Prior to irradiation, the films had a donor concentration of ∼1015 cm−3, with the two dominant donors having ionization energies of 0.25 and 0.15 eV, respectively. The main electron traps were located at Ec−0.7 eV. Deep acceptor spectra measured by capacitance-voltage profiling under illumination showed optical ionization thresholds near 2, 2.8, and 3.4 eV. The diffusion length of nonequilibrium charge carriers for ɛ-Ga2O3 was 70 ± 5 nm prior to irradiation. After irradiation with 1.1 MeV protons to a fluence of 1014 cm−2, there was total depletion of mobile charge carriers in the top 4.5 μm of the film, close to the estimated proton range. The carrier removal rate was 10–20 cm−1, a factor of 5–10 lower than in β-Ga2O3, while the concentration of deep acceptors in the lower half of the bandgap and the diffusion length showed no significant change.

Yakimov E.B., Nikolaev V.I., Pechnikov A.V., Polyakov A.Y., Shchemerov I.V., Vasilev A.A., Kulanchikov Y., Vergeles P.S., Yakimov E.E., Pearton S.J.

The κ-Ga2O3 polytype is attracting attention because of its high spontaneous electric polarization, which exceeds that of III-Nitrides. However, little is known of its transport and photoconductive properties. The electron beam induced current gain effect in Schottky barriers prepared on thick films of κ-Ga2O3 has been studied. It is shown that the gain originates in the depletion region of the Schottky barrier. It is demonstrated that the induced current gain takes place only in some local regions, several which increases with applied bias. Such unusual behavior can be explained by an inhomogeneous distribution of hole traps or by a formation of conductive channels under applied bias.

Yakimov E.B., Polyakov A.Y., Nikolaev V.I., Pechnikov A.I., Scheglov M.P., Yakimov E.E., Pearton S.J.

In this study, the structural and electrical properties of orthorhombic κ-Ga2O3 films prepared using Halide Vapor Phase Epitaxy (HVPE) on AlN/Si and GaN/sapphire templates were studied. For κ-Ga2O3/AlN/Si structures, the formation of two-dimensional hole layers in the Ga2O3 was studied and, based on theoretical calculations, was explained by the impact of the difference in the spontaneous polarizations of κ-Ga2O3 and AlN. Structural studies indicated that in the thickest κ-Ga2O3/GaN/sapphire layer used, the formation of rotational nanodomains was suppressed. For thick (23 μm and 86 μm) κ-Ga2O3 films grown on GaN/sapphire, the good rectifying characteristics of Ni Schottky diodes were observed. In addition, deep trap spectra and electron beam-induced current measurements were performed for the first time in this polytype. These experiments show that the uppermost 2 µm layer of the grown films contains a high density of rather deep electron traps near Ec − 0.3 eV and Ec − 0.7 eV, whose presence results in the relatively high series resistance of the structures. The diffusion length of the excess charge carriers was measured for the first time in κ-Ga2O3. The film with the greatest thickness of 86 μm was irradiated with protons and the carrier removal rate was about 10 cm−1, which is considerably lower than that for β-Ga2O3.