Applied Physics Letters, volume 121, issue 19, pages 191601

Formation of γ-Ga2O3 by ion implantation: Polymorphic phase transformation of β-Ga2O3

Publication typeJournal Article
Publication date2022-11-07
scimago Q1
wos Q2
SJR0.896
CiteScore6.1
Impact factor3.6
ISSN00036951, 10773118
Physics and Astronomy (miscellaneous)
Abstract

Ion implantation induced phase transformation and the crystal structure of a series of ion implanted β-Ga2O3 samples were studied using electron diffraction, high resolution transmission electron microscopy, and scanning transmission electron microscopy. In contrast to previous reports suggesting an ion implantation induced transformation to the orthorhombic κ-phase, we show that for 28Si+, 58Ni+, and stoichiometric 69Ga+/16O+-implantations, the monoclinic β-phase transforms to the cubic γ-phase. The γ-phase was confirmed for implantations over a range of fluences from 1014 to 1016 ions/cm2, indicating that the transformation is a general phenomenon for β-Ga2O3 due to strain accumulation and/or γ-Ga2O3 being energetically preferred over highly defective β-Ga2O3.

Yoo T., Xia X., Ren F., Jacobs A., Tadjer M.J., Pearton S., Kim H.
Applied Physics Letters scimago Q1 wos Q2  
2022-08-15 Abstract  
β-Ga2O3 is an emerging ultra-wide bandgap semiconductor, holding a tremendous potential for power-switching devices for next-generation high power electronics. The performance of such devices strongly relies on the precise control of electrical properties of β-Ga2O3, which can be achieved by implantation of dopant ions. However, a detailed understanding of the impact of ion implantation on the structure of β-Ga2O3 remains elusive. Here, using aberration-corrected scanning transmission electron microscopy, we investigate the nature of structural damage in ion-implanted β-Ga2O3 and its recovery upon heat treatment with the atomic-scale spatial resolution. We reveal that upon Sn ion implantation, Ga2O3 films undergo a phase transformation from the monoclinic β-phase to the defective cubic spinel [Formula: see text]-phase, which contains high-density antiphase boundaries. Using the planar defect models proposed for the [Formula: see text]-Al2O3, which has the same space group as β-Ga2O3, and atomic-resolution microscopy images, we identify that the observed antiphase boundaries are the {100}1/4 ⟨110⟩ type in cubic structure. We show that post-implantation annealing at 1100 °C under the N2 atmosphere effectively recovers the β-phase; however, nano-sized voids retained within the β-phase structure and a [Formula: see text]-phase surface layer are identified as remanent damage. Our results offer an atomic-scale insight into the structural evolution of β-Ga2O3 under ion implantation and high-temperature annealing, which is key to the optimization of semiconductor processing conditions for relevant device design and the theoretical understanding of defect formation and phase stability.
Kjeldby S.B., Azarov A., Nguyen P.D., Venkatachalapathy V., Mikšová R., Macková A., Kuznetsov A., Prytz Ø., Vines L.
Journal of Applied Physics scimago Q2 wos Q3  
2022-03-22 Abstract  
Defect accumulation and annealing phenomena in Si-implanted monoclinic gallium oxide (β-Ga2O3) wafers, having [Formula: see text], (010), and (001) orientations, were studied by Rutherford backscattering spectrometry in channeling mode (RBS/c), x-ray diffraction (XRD), and (scanning) transmission electron microscopy [(S)TEM]. Initially, the samples with different surface orientations were implanted with 300 keV 28Si+-ions, applying fluences in the range of 1 × 1014–2 × 1016 Si/cm2, unveiling interesting disorder accumulation kinetics. In particular, the RBS/c, XRD, and (S)TEM combined data suggested that the radiation disorder buildup in Si-implanted β-Ga2O3 is accompanied by significant strain accumulation, assisting crystalline-to-crystalline phase transitions instead of amorphization. Selected samples having [Formula: see text] orientation were subjected to isochronal (30 min) anneals in the range of 300–1300 °C in air. Systematic RBS/c and XRD characterization of these samples suggested complex structural transformations, which occurred as a function of the fluence and the temperature. Moreover, a detailed (S)TEM analysis of the sample implanted with 2 × 1016 Si/cm2 and annealed at 1100 °C was enhanced by applying dispersive x-ray and electron energy-loss spectroscopies. The analysis revealed silicon agglomerations in the form of silicon dioxide particles. Signal from silicon was also detected outside of the agglomerates, likely occurring as substitutional Si on Ga sites.
Azarov A., Bazioti C., Venkatachalapathy V., Vajeeston P., Monakhov E., Kuznetsov A.
Physical Review Letters scimago Q1 wos Q1 Open Access  
2022-01-06 Abstract  
Polymorphs are common in nature and can be stabilized by applying external pressure in materials. The pressure and strain can also be induced by the gradually accumulated radiation disorder. However, in semiconductors, the radiation disorder accumulation typically results in the amorphization instead of engaging polymorphism. By studying these phenomena in gallium oxide we found that the amorphization may be prominently suppressed by the monoclinic to orthorhombic phase transition. Utilizing this discovery, a highly oriented single-phase orthorhombic film on the top of the monoclinic gallium oxide substrate was fabricated. Exploring this system, a novel mode of the lateral polymorphic regrowth, not previously observed in solids, was detected. In combination, these data envisage a new direction of research on polymorphs in ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ and, potentially, for similar polymorphic families in other materials.
Jeon H.M., Leedy K.D., Look D.C., Chang C.S., Muller D.A., Badescu S.C., Vasilyev V., Brown J.L., Green A.J., Chabak K.D.
APL Materials scimago Q1 wos Q2 Open Access  
2021-10-01 PDF Abstract  
Conductive homoepitaxial Si-doped β-Ga2O3 films were fabricated by pulsed laser deposition with an as-deposited 2323 S cm−1 conductivity (resistivity = 4.3 × 10−4 Ω-cm, carrier concentration = 2.24 × 1020 cm−3, mobility = 64.5 cm2 V−1 s−1, and electrical activation efficiency = 77%). High quality homoepitaxial films deposited on commercial (010) Fe-compensated β-Ga2O substrates were determined by high-resolution transmission electron microscopy and x-ray diffraction. The β-Ga2O3 films have ∼70% transparency from 3.7 eV (335 nm) to 0.56 eV (2214 nm). The combination of high conductivity and transparency offers promise for numerous ultrawide bandgap electronics and optoelectronic applications.
Aarseth B.L., Granerød C.S., Galeckas A., Azarov A., Nguyen P.D., Prytz Ø., Vines L.
Nanotechnology scimago Q2 wos Q2  
2021-09-29 Abstract  
Semiconductor nanocrystals are often proposed as a viable route to improve solar energy conversion in photovoltaics and photoelectrochemical systems. Embedding the nanocrystals in, e.g. a transparent and conducting electrode of a solar cell will promote the photon absorption and subsequent transfer of the generated charge carriers from the nanocrystal, and thereby enhance the function of the electrode. This can be accomplished by embedding a semiconducting nanocrystal with a small bandgap in a transparent conducting oxide (TCO), which is commonly utilized as electrode in new generation solar cells. Here, we demonstrate the incorporation, formation, and functionalization of germanium (Ge) nanocrystals in zinc oxide utilizing ion implantation, where post implantation annealing at 800 °C results in diamond cubic Ge nanocrystals with sizes between 2 and 20 nm. Photoluminecence spectra show a distinct emission around 0.7 eV arising from the Ge nanocrystals, and with additional emission features up to 1.15 eV due to quantum confinement, demonstrating a novel functionalization and tunability of the TCO electrode.
Nikolskaya A., Okulich E., Korolev D., Stepanov A., Nikolichev D., Mikhaylov A., Tetelbaum D., Almaev A., Bolzan C.A., Buaczik A., Giulian R., Grande P.L., Kumar A., Kumar M., Gogova D.
Gallium oxide, and in particular its thermodynamically stable β-Ga2O3 phase, is within the most exciting materials in research and technology nowadays due to its unique properties. The very high breakdown electric field and the figure of merit rivaled only by diamond have tremendous potential for the next generation “green” electronics enabling efficient distribution, use, and conversion of electrical energy. Ion implantation is a traditional technological method used in these fields, and its well-known advantages can contribute greatly to the rapid development of physics and technology of Ga2O3-based materials and devices. Here, the status of ion implantation in β-Ga2O3 nowadays is reviewed. Attention is mainly paid to the results of experimental study of damage under ion irradiation and the properties of Ga2O3 layers doped by ion implantation. The results of ab initio theoretical calculations of the impurities and defect parameters are briefly presented, and the physical principles of a number of analytical methods used to study implanted gallium oxide layers are highlighted. The use of ion implantation in the development of Ga2O3-based devices, such as metal oxide field-effect transistors, Schottky barrier diodes, and solar-blind UV detectors, is described together with systematical analysis of the achieved values of their characteristics. Finally, the most important challenges to be overcome in this field of science and technology are discussed.
Singh R., Lenka T.R., Panda D.K., Velpula R.T., Jain B., Bui H.Q., Nguyen H.P.
Recently, there is a growing interest in Gallium Oxide (Ga2O3) as a promising semiconductor material for intended applications in RF, power electronics, and sensors with high capabilities over existing technologies due to its excellent material characteristics like large bandgap, well-controlled doping, and availability of large size inexpensive substrates. Bulk crystals of monoclinic β-Ga2O3 can be grown using melt growth techniques, which ensures large, uniform substrates with relatively low-cost per wafer as compared to GaN and SiC substrates which are usually grown using vapor growth techniques. A large critical field of β-Ga2O3 is beneficial for improving the DC performance of high voltage rectifiers and metal oxide semiconductor field-effect transistors (MOSFETs) and facilitates further lateral scaling of FETs for improved RF performance. Band structure of β-Ga2O3 indicates difficulty in p-type conductivity, so previously reported most of the β-Ga2O3 MOSFETs have been depletion mode, although enhancement mode operations were also demonstrated using recess-gate and charge-trapping gate stack structure. The β-Ga2O3 heterostructures have been widely reported using a high-quality epitaxial layer of β-(AlxGa1−x)2O3 after alloying Al with Ga2O3. The β-Ga2O3 modulation-doped FETs (MODFETs) have shown two-dimensional electron gas (2DEG) density of ~1012 cm−2 that form a good quality channel at the interface. Despite low room temperature electron mobility of around 180 cm2 V−1s−1, peak mobility of around 2800 cm2 V−1s−1 at 50 K was measured in the latest reported experimental work of β-Ga2O3 MODFET. III-nitride based GaN high electron mobility transistors (HEMTs) have been widely used in high power electronics and have shown 2DEG density ~ 1013 cm−2 and channel mobility of 2000 cm2 V−1s−1. This paper gives a perspective of Ga2O3 material towards making high electron mobility transistors (HEMTs) for a certain class of RF applications. Due to low in-plane lattice mismatch, a high-quality epitaxial layer of GaN and AlN have been grown on β-Ga2O3. Furthermore, due to the inherent polarization property of III-nitrides and large bandgap, higher 2DEG density ~1013 cm−2 and large conduction band offset >1.5 eV can be expected in AlN/β-Ga2O3 heterostructure. The various defects in WBG devices and their effects on the reliability aspects are also addressed.
Anber E.A., Foley D., Lang A.C., Nathaniel J., Hart J.L., Tadjer M.J., Hobart K.D., Pearton S., Taheri M.L.
Applied Physics Letters scimago Q1 wos Q2  
2020-10-12 Abstract  
Ion implantation-induced effects were studied in Ge implanted β-Ga2O3 with the fluence and energy of 3 × 1013 cm−2/60 keV, 5 × 1013 cm−2/100 keV, and 7 × 1013 cm−2/200 keV using analytical electron microscopy via scanning/transmission electron microscopy, electron energy loss spectroscopy, and precession electron diffraction via TopSpin. Imaging shows an isolated band of damage after Ge implantation, which extends ∼130 nm from the sample surface and corresponds to the projected range of the ions. Electron diffraction demonstrates that the entirety of the damage band is the κ phase, indicating an implantation-induced phase transition from β to κ-Ga2O3. Post-implantation annealing at 1150 °C for 60 s under the O2 atmosphere led to a back transformation of κ to β; however, an ∼17 nm damage zone remained at the sample surface. Despite the back transformation from κ to β with annealing, O K-edge spectra show changes in the fine structure between the pristine, implanted, and implanted-annealed samples, and topspin strain analysis shows a change in strain between the two samples. These data indicate differences in the electronic/chemical structure, where the change of the oxygen environment extended beyond the implantation zone (∼130 nm) due to the diffusion of Ge into the bulk material, which, in turn, causes a tensile strain of 0.5%. This work provides a foundation for understanding of the effects of ion implantation on defect/phase evolution in β-Ga2O3 and the related recovery mechanism, opening a window toward building a reliable device for targeted applications.
Castro-Fernández P., Blanco M.V., Verel R., Willinger E., Fedorov A., Abdala P.M., Müller C.R.
2020-08-17 Abstract  
Metastable γ-Ga2O3 nanocrystals have gained growing interest for a broad range of technological applications. However, a precise description of their atomic structure and changes thereof during the...
Cora I., Fogarassy Z., Fornari R., Bosi M., Rečnik A., Pécz B.
Acta Materialia scimago Q1 wos Q1  
2020-01-01 Abstract  
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.
Kalita P., Saini S., Rajput P., Jha S.N., Bhattacharyya D., Ojha S., Avasthi D.K., Bhattacharya S., Ghosh S.
2019-09-12 Abstract  
We report the formation of cubic phase, under ambient conditions, in thin films of Zirconia synthesized by electron beam evaporation technique. The stabilization of the cubic phase was achieved without the use of chemical stabilizers and/or concurrent ion beam bombardment. Films of two different thickness (660 nm, 140 nm) were deposited. The 660 nm and 140 nm films were found to be stoichiometric (ZrO2) and off-stoichiometric (ZrO1.7) respectively by Resonant Rutherford back-scattering spectroscopy. While the 660 nm as-deposited films were in the cubic phase, as indicated by X-ray diffraction and Raman spectroscopy measurements, the 140 nm as-deposited films were amorphous and the transformation to cubic phase was obtained after thermal annealing. Extended X-ray absorption fine structure measurements revealed the existence of Oxygen vacancies in the local structure surrounding Zirconium for all films. However, the amount of these Oxygen vacancies was found to be significantly higher for the amorphous films as compared to the films in the cubic phase (both 660 nm as-deposited and 140 nm annealed films). The cubic phase stabilization is explained on the basis of suppression of the soft X2- mode of vibration of the Oxygen sub-lattice due to the presence of the Oxygen vacancies. Our first-principles modeling under the framework of density functional theory shows that the cubic structure with Oxygen vacancies is indeed more stable at ambient conditions than its pristine (without vacancies) counterpart. The requirement of a critical amount of these vacancies for the stabilization of the cubic phase is also discussed.
Moloney J., Tesh O., Singh M., Roberts J.W., Jarman J.C., Lee L.C., Huq T.N., Brister J., Karboyan S., Kuball M., Chalker P.R., Oliver R.A., Massabuau F.C.
2019-09-06 Abstract  
Low temperature atomic layer deposition was used to deposit α-Ga2O3 films, which were subsequently annealed at various temperatures and atmospheres. The α-Ga2O3 phase is stable up to 400 °C, which is also the temperature that yields the most intense and sharpest reflection by x-ray diffraction. Upon annealing at 450 °C and above, the material gradually turns into the more thermodynamically stable e or β phase. The suitability of the materials for solar-blind photodetector applications has been demonstrated with the best responsivity achieved being 1.2 A W−1 under 240 nm illumination and 10 V bias, for the sample annealed at 400 °C in argon. It is worth noting however that the device performance strongly depends on the annealing conditions, with the device annealed in forming gas behaving poorly. Given that the tested devices have similar microstructure, the discrepancies in device performance are attributed to hydrogen impurities.
Hsieh Y., Li C., Lin C., Wang N., Li J.V., Houng M.
Thin Solid Films scimago Q2 wos Q3  
2019-09-01 Abstract  
We report a metal-insulator-semiconductor (MIS) diode with an α-Ga 2 O 3 thin-film insulator layer grown by liquid-phase deposition (LPD). α-Ga 2 O 3 exhibits a high energy bandgap of 4.9–5.3 eV, which can effectively reduce the leakage current density and improve the breakdown voltage of the diode. The α-Ga 2 O 3 thin films are synthesized from GaOOH with LPD. The α-GaOOH crystal is simply obtained by the dissociation of the Ga(OH) 3 precursor solution. GaOOH can be transformed into α-Ga 2 O 3 crystal and form a uniform thin film following post-growth annealing. When the α-Ga 2 O 3 thin film is inserted in between Ni and Si to form a Ni/α-Ga 2 O 3 /Si MIS diode, the barrier height of the diode increases by 0.4 eV and the on/off ratio by 100-fold from those of the Ni/Si Schottky diode. The Ni/α-Ga 2 O 3 /Si MIS diode exhibits a leakage current density of 1.07 × 10 −5 A/cm 2 under −2 V bias. The breakdown voltage of the diode reaches −166 V without the guard ring and other insulation structures. Our results demonstrate that LPD-grown α-Ga 2 O 3 thin films can obtain uniform and dense structure under short deposition time and at an annealing temperature of 400 °C. The uniform insulating layer of α-Ga 2 O 3 has a high potential in enhancing the electrical characteristic of diodes and other power electronic devices.
Vasanthi V., Kottaisamy M., Ramakrishnan V.
Ceramics International scimago Q1 wos Q1  
2019-02-01 Abstract  
Optical properties of Zn doped γ-Ga2O3 nanoparticles are investigated for the generation of white light, using near UV LED chip for phosphor-converted WLED (pc-WLED) application. Sol-gel combustion technique is adopted for the synthesis of Zn doped γ-Ga2O3 nanoparticles. The synthesized Ga2O3 nanoparticles are formed as metastable γ-phase with the face-centred cubic crystal structure. The structural stability is maintained up to 15 mol% doping of Zn ions. Formation of aggregated ultrafine particles of undoped and Zn doped γ-Ga2O3 with size less than 4 nm is found from TEM results. Doping of Zn ions has altered the optical band gap of γ-Ga2O3, and it varies from 4.13 to 3.97 eV. Broad visible emission in the wavelength range of 400–600 nm is observed and broadening of emission band increases after Zn doping. Formation of high defect concentration and smaller size of 10 mol% Zn doped γ-Ga2O3 nanoparticles resulted in higher emission intensity. Wide photoluminescence excitation band from 330 to 475 nm suggests that wide visible emission can be obtained from Zn doped γ-Ga2O3 nanoparticles with the excitation of both near UV and blue light. The bi-exponential behavior of PL decay curves of emission bands at the blue and yellow region indicated the complicate luminescence process of Zn doped γ-Ga2O3. The estimated lifetime of blue emission varies from 30 to 37 ns and of yellow emission varies from 54 to 62 ns. Under the excitation of 375 nm LED chip, warm white light emission with CIE color coordinates of (0.42, 0.33), color rendering index of ~ 87 and CCT of 2365 K is obtained for 10 mol% Zn doped γ-Ga2O3 nanoparticles which exemplifying its potential application in pc-WLED fabrication.
Xu Y., Park J., Yao Z., Wolverton C., Razeghi M., Wu J., Dravid V.P.
2019-01-10 Abstract  
It is well known that metastable and transient structures in bulk can be stabilized in thin films via epitaxial strain (heteroepitaxy) and appropriate growth conditions that are often far from equilibrium. However, the mechanism of heteroepitaxy, particularly how the nominally unstable or metastable phase gets stabilized, remains largely unclear. This is especially intriguing for thin-film Ga2O3, where multiple crystal phases may exist under varied growth conditions with spatial and dimensional constraints. Herein, the development and distribution of epitaxial strain at the Ga2O3/Al2O3 film-substrate interfaces is revealed down to the atomic resolution along different orientations, with an aberration-corrected scanning transmission electron microscope. Just a few layers of metastable α-Ga2O3 structure were found to accommodate the misfit strain in direct contact with the substrate. Following an epitaxial α-Ga2O3 structure of about couple unit cells, several layers (4-5) of transient phase appear as the intermediate structure to release the misfit strain. Subsequent to this transient crystal phase, the nominally unstable κ-Ga2O3 phase is stabilized as the major thin-film phase form. We show that the epitaxial strain is gracefully accommodated by rearrangement of the oxygen polyhedra. When the structure is under large compressive strain, Ga3+ ions occupy only the oxygen octahedral sites to form a dense structure. With gradual release of the compressive strain, more and more Ga3+ ions occupy the oxygen tetrahedral sites, leading to volumetric expansion and the phase transformation. The structure of the transition phase is identified by high-resolution electron microscopy observation, complemented by the density functional theory calculations. This study provides insights from the atomic scale and their implications for the design of functional thin-film materials using epitaxial engineering.
Kim C., Choi Y., Kim D., Kang H.Y., Jung J., Lee G.R., Kum H.S., Lee T.H., Chung R.B.
2025-07-02
Yu J., Luo S., Splith D., Selle S., Thieme K., Gierth S., Schultz T., Schlupp P., Sturm C., von Wenckstern H., Lorenz M., Koch N., Höche T., Grundmann M.
Journal of Applied Physics scimago Q2 wos Q3  
2025-05-02 Abstract  
Epitaxial growth of phase-pure and high-quality spinel γ-Ga2O3-based semiconductor thin films has been a big challenge for fundamental research on metastable defective inverse spinel γ-Ga2O3 semiconductors in view of potential device application. We report experimental results on epitaxial growth, microstructural, and electrical transport properties of (001)-oriented nominal γ-(Ga0.8Ge0.2)2O3 alloy semiconductor single crystal thin films with a coherent interface on cubic spinel (001) MgAl2O4 substrates by pulsed laser deposition using a Ge-rich target. Pristine films are found to be composed of about 2 nm thick insulating Ge-rich surface layers and the high-quality epitaxial n-type semiconductor film layers consisting of partially subvalent Ge2+ and Ga1+ cations as well as major components of normal Ge4+ and Ga3+ cations. Epitaxial films exhibit a direct bandgap of about 5.2 ± 0.1 eV and a valence band maximum of about 3.3 ± 0.1 eV below the Fermi level at room temperature. We further report a demonstration of γ-(Ga0.8Ge0.2)2O3 thin film-based metal-semiconductor field-effect transistor (MESFET) with the PtOx/Pt Schottky gate contact realized upon the surface pretreatment by Ar/O2 plasma etching. The MESFET device exhibits a clear field-effect with drain current modulation of about 105 orders of magnitude. This work not only significantly advances the fundamental and application-oriented research on epitaxial spinel γ-Ga2O3-based semiconductor films for practical device application but also offers new insight into microstructural characteristics of ultrawide bandgap spinel oxide semiconductor epitaxial thin films.
Nikolskaya A., Korolev D., Yunin P., Tatarskiy D., Trushin V., Matyunina K., Savushkina M., Mikhaylov A., Drozdov M., Nazarov A., Kudrin A., Revin A., Konakov A., Stepanov A., Tetelbaum D.
Vacuum scimago Q1 wos Q2  
2025-05-01
Zhao J., Fernández J.G., Azarov A., He R., Prytz Ø., Nordlund K., Hua M., Djurabekova F., Kuznetsov A.
Physical Review Letters scimago Q1 wos Q1 Open Access  
2025-03-25
Polyakov A.Y., Schemerov I.V., Vasilev A.A., Romanov A.A., Lagov P.B., Miakonkikh A.V., Chernykh A.V., Romanteeva E.P., Chernykh S.V., Rabinovich O.I., Pearton S.J.
The electrical properties and deep trap spectra of semi-insulating Ga2O3(Fe) implanted with Si ions and subsequently annealed at 1000 °C were investigated. A significant discrepancy was observed between the measured shallow donor concentration profile and the profile predicted by Stopping Power and Range of Ions in Matter simulations, indicating substantial compensation. Deep level transient spectroscopy revealed the presence of deep acceptors at Ec −0.5 eV with a concentration of ∼10¹⁷ cm−³, insufficient to fully account for the observed compensation. Photocapacitance spectroscopy identified additional deep acceptors with optical ionization thresholds near 2 and 2.8–3.1 eV, tentatively attributed to gallium vacancy-related defects. However, the combined concentration of these deep acceptors still fell short of explaining the observed donor deactivation, suggesting the formation of electrically neutral Si-vacancy complexes. Furthermore, the properties of Ga2O3 (Fe) implanted with Si and subjected to hydrogen plasma treatment at 330 °C were also examined. This material exhibited high resistivity with the Fermi level pinned near Ec –0.3 eV, similar to common radiation defects in proton-implanted Ga2O3. A prominent deep center near Ec −0.6 eV, consistent with the known E1 electron trap attributed to Si-H complexes, was also observed. These results highlight the challenges associated with Si implantation and activation in Ga2O3 and suggest that hydrogen plasma treatment, while effective for Ga-implanted Ga2O3 is less suitable for Si-implanted material due to the formation of compensating Si-H complexes.
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.
García-Fernández J., Kjeldby S.B., Zeng L.J., Olsson E., Vines L., Prytz Ø.
Applied Physics Letters scimago Q1 wos Q2  
2025-02-03 Abstract  
The influence of germanium (Ge) implantation on β-Ga2O3 followed by ex situ annealing through polymorph conversion in air at various temperatures (600–1100 °C) is studied. Atomic resolution aberration-corrected scanning transmission electron microscopy is employed to examine the structural and microstructural changes induced by the annealing process. The results show that the thermal annealing process leads to the formation of Ge nanocrystals, which subsequently disappear, leaving nano-voids inside the Ge-doped Ga2O3 matrix. In addition, the microstructure displays distinct crystallographic relationships in annealed β-Ga2O3 forming layers with well-defined interfaces. This work reveals the unique effects of Ge-implantation, demonstrating the possible functionalization of Ga2O3 with Ge nanocrystals.
Ratajczak R., Sarwar M., Kalita D., Jozwik P., Mieszczynski C., Matulewicz J., Wilczopolska M., Wozniak W., Kentsch U., Heller R., Guziewicz E.
Scientific Reports scimago Q1 wos Q1 Open Access  
2024-10-22 PDF Abstract  
AbstractRE-doped β-Ga2O3 seems attractive for future high-power LEDs operating in high irradiation environments. In this work, we pay special attention to the issue of radiation-induced defect anisotropy in β-Ga2O3, which is crucial for device manufacturing. Using the RBS/c technique, we have carefully studied the structural changes caused by implantation and post-implantation annealing in two of the most commonly used crystallographic orientations of β-Ga2O3, namely the (-201) and (010). The analysis was supported by advanced computer simulations using the McChasy code. Our studies reveal a strong dependence of the structural damage induced by Yb-ion implantation on the crystal orientation, with a significantly higher level of extended defects observed in the (-201) direction than for the (010). In contrast, the concentration and behavior of simple defects seem similar for both oriented crystals, although their evolution suggests the co-existence of two different types of defects in the implanted zone with their different sensitivity to both, radiation and annealing. It has also been found that Yb ions mostly occupy the interstitial positions in β-Ga2O3 crystals that remain unchanged after annealing. The location is independent of the crystal orientations. We believe that these studies noticeably extend the knowledge of the radiation-induced defect structure, because they dispel doubts about the differences in the damage level depending on crystal orientation, and are important for further practical applications.
Balog A.R., Lee C., Duarte-Ruiz D., Gayathri Ayyagari S.V., Jesenovec J., Chmielewski A.E., Miao L., Dutton B.L., McCloy J., Cocchi C., Ertekin E., Alem N.
2024-09-19
He R., Zhao J., Byggmästar J., He H., Djurabekova F.
Physical Review Materials scimago Q1 wos Q2  
2024-08-28
Zhao P., Cheng Y., Li L., Jia S., Guan X., Huang T., Li L., Zheng H., Wang J.
Advanced Functional Materials scimago Q1 wos Q1  
2024-07-06 Abstract  
AbstractTransmission electron microscopy (TEM) offers unprecedent atomic resolution imaging and diverse characterizations capabilities, which has been proved to be effective in correlating the atomic structures and compositions with the physical/chemical properties of semiconductor nanomaterials. This review aims to provide an overview of the latest advancements regarding the atomic structure/property relationship in semiconductor nanomaterials. First, by employing off‐axis electron holography, a comprehensive overview of the quantitative investigations into the atomic‐electronic structure relationship of semiconductors is presented. Second, by integrating in situ TEM technique with micro/nanoelectromechanical systems (M/NEMS), this review summarizes the recent advancements achieved in elucidating the intricate relationship between structure and properties of nanomaterials subjected to diverse stimuli such as stress, thermal, and electric fields. Moreover, the impact of electron beam irradiation on the microstructure of semiconductor nanomaterials is discussed. Lastly, current challenges and future research opportunities are proposed along with their potential applications.
Sarwar M., Ratajczak R., Mieszczynski C., Wierzbicka A., Gieraltowska S., Heller R., Eisenwinder S., Wozniak W., Guziewicz E.
Acta Materialia scimago Q1 wos Q1  
2024-04-01 Abstract  
Radiation-induced crystal lattice damage and its recovery in wide bandgap oxides, in particular beta-gallium oxide (β-Ga2O3), is a complex process. This paper presents the detailed study of defect accumulation in the β-Ga2O3 single crystal implanted with Ytterbium (Yb) ions and the impact of Rapid Thermal Annealing (RTA) on the defects formed. The (2¯01)oriented β-Ga2O3 single crystals were implanted with eleven fluences of Yb ions ranging from 1 × 1012 to 5 × 1015 at/cm2. Channeling Rutherford Backscattering Spectrometry (RBS/c) was used to study the crystal lattice damage induced by ion implantation and the level of structure recovery after annealing. The quantitative and qualitative analyses of collected spectra were performed by computer simulations. As a result, we present the first defect accumulation curve of β-Ga2O3 implanted with rare earth ion that reveals a two-step damage process. In the first stage, the damage of the β-Ga2O3 is inconspicuous, but begins to grow rapidly from the fluence of 1 × 1013 at/cm2, reaching the saturation at the random level for the Yb ion fluence of 1 × 1014 at/cm2. Further irradiation causes the damage peak to become bimodal, indicating that at least two new defect forms develop for the higher ion fluence. These two damage zones differently react to annealing, suggesting that they could origin from two phases, the amorphization phase and the new crystalline phase of Ga2O3. High-resolution x-ray diffraction (HRXRD) demonstrates the presence of strain and the γ phase of Ga2O3 after implantation, which disappear after annealing.
García-Fernández J., Kjeldby S.B., Zeng L., Azarov A., Pokle A., Nguyen P.D., Olsson E., Vines L., Kuznetsov A., Prytz Ø.
Materials Advances scimago Q1 wos Q2 Open Access  
2024-03-11 PDF Abstract  
In situ TEM heating studies of double γ/β-Ga2O3 polymorph structures revealed γ-to-β polymorph transition via the formation of β-Ga2O3 domains.
Gann K.R., Pieczulewski N., Gorsak C.A., Heinselman K., Asel T.J., Noesges B.A., Smith K.T., Dryden D.M., Xing H.G., Nair H.P., Muller D.A., Thompson M.O.
Journal of Applied Physics scimago Q2 wos Q3  
2024-01-05 Abstract  
Optimizing thermal anneals of Si-implanted β-Ga2O3 is critical for low resistance contacts and selective area doping. We report the impact of annealing ambient, temperature, and time on the activation of room temperature ion-implanted Si in β-Ga2O3 at concentrations from 5 × 1018 to 1 × 1020 cm−3, demonstrating full activation (>80% activation, mobilities >70 cm2/V s) with contact resistances below 0.29 Ω mm. Homoepitaxial β-Ga2O3 films, grown by plasma-assisted molecular beam epitaxy on Fe-doped (010) substrates, were implanted at multiple energies to yield 100 nm box profiles of 5 × 1018, 5 × 1019, and 1 × 1020 cm−3. Anneals were performed in an ultra-high vacuum-compatible quartz furnace at 1 bar with well-controlled gas compositions. To maintain β-Ga2O3 stability, pO2 must be greater than 10−9 bar. Anneals up to pO2 = 1 bar achieve full activation at 5 × 1018 cm−3, while 5 × 1019 cm−3 must be annealed with pO2 ≤ 10−4 bar, and 1 × 1020 cm−3 requires pO2 < 10−6 bar. Water vapor prevents activation and must be maintained below 10−8 bar. Activation is achieved for anneal temperatures as low as 850 °C with mobility increasing with anneal temperatures up to 1050 °C, though Si diffusion has been reported above 950 °C. At 950 °C, activation is maximized between 5 and 20 min with longer times resulting in decreased carrier activation (over-annealing). This over-annealing is significant for concentrations above 5 × 1019 cm−3 and occurs rapidly at 1 × 1020 cm−3. Rutherford backscattering spectrometry (channeling) suggests that damage recovery is seeded from remnant aligned β-Ga2O3 that remains after implantation; this conclusion is also supported by scanning transmission electron microscopy showing retention of the β-phase with inclusions that resemble the γ-phase.

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