Ti- and Fe-related charge transition levels in β − Ga 2 O 3

McGlone J.F., Xia Z., Joishi C., Lodha S., Rajan S., Ringel S., Arehart A.R.

Two buffer traps at EC-0.7 eV and EC-0.8 eV have been individually identified as causing threshold voltage and on-resistance instabilities in β-Ga2O3 Si ∂-doped transistors grown by plasma-assisted molecular beam epitaxy (PAMBE) on semi-insulating Fe doped β-Ga2O3 substrates. The instabilities are characterized using double-pulsed current-voltage and isothermal constant drain current deep level transient spectroscopy. The defect spectra are compared between transistors grown using two different unintentionally doped buffer layer thicknesses of 100 nm and 600 nm. The EC-0.8 eV trap was not seen using the thicker buffer and is shown to correlate with the presence of residual Fe in thePAMBE buffer layer. The EC-0.7 eV trap was unchanged in concentration and is revealed as the dominating source of the threshold voltage instability. This trap is consistent with the characteristics of a previously reported intrinsic point defect [Ingebrigtsen et al., APL Mater. 7, 022510 (2019)]. The EC-0.7 eV trap is responsible for ∼70% of the total threshold voltage shift in the 100 nm thick buffer transistor and 100% in the 600 nm thick buffer transistor, which indicates growth optimization is needed to improve β-Ga2O3 transistor stability.Two buffer traps at EC-0.7 eV and EC-0.8 eV have been individually identified as causing threshold voltage and on-resistance instabilities in β-Ga2O3 Si ∂-doped transistors grown by plasma-assisted molecular beam epitaxy (PAMBE) on semi-insulating Fe doped β-Ga2O3 substrates. The instabilities are characterized using double-pulsed current-voltage and isothermal constant drain current deep level transient spectroscopy. The defect spectra are compared between transistors grown using two different unintentionally doped buffer layer thicknesses of 100 nm and 600 nm. The EC-0.8 eV trap was not seen using the thicker buffer and is shown to correlate with the presence of residual Fe in thePAMBE buffer layer. The EC-0.7 eV trap was unchanged in concentration and is revealed as the dominating source of the threshold voltage instability. This trap is consistent with the characteristics of a previously reported intrinsic point defect [Ingebrigtsen et al., APL Mater. 7, 022510 (2019)]. The EC-0.7 eV trap is respons...

Lee M., Peterson R.L.

Lee M., Peterson R.L.

Here we investigated interfacial reactions and interdiffusion of titanium/gold ohmic contacts with a tin-doped single-crystal β-Ga2O3 (010) substrate. After annealing at 470 °C for 1 min in N2 to form an ohmic contact, we studied the interface via scanning transmission electron microscopy and transmission electron microscopy with energy dispersive X-ray spectroscopy as well as electron energy loss spectroscopy. At the interface, annealing causes Ti to diffuse and oxidize, reducing Ga2O3 at the interface. This forms a defective β-Ga2O3 layer of 3-5 nm that has a relatively high Ti concentration. Above this is a 3-5 nm layer of Ti-TiOx that is partially lattice matched to the β-Ga2O3 substrate. The thermodynamic favorability of these redox reactions was explained by calculating Gibbs free energies of the reactions. In addition, the anneal causes interdiffusion of Ti and Au, until Au is in contact with the thin Ti-TiOx layer. A layer of Ti-rich nanocrystals, around 5 nm in diameter, is formed within the Au-Ti intermixed matrix, about 3 nm above the Ti-TiOx layer. Based on these observations, the ohmic properties are tentatively attributed to the interdiffusion of Ti and Au and the resulting thin Ti-TiOx layer, which helps band alignment. In addition, lattice matching of the defective Ga2O3 and Ti-TiOx layers to β-Ga2O3 facilitates the transport of carriers. A physical understanding of Ti/Au metallization can provide insights into future materials selection for thermally stable contacts in β-Ga2O3 power devices.

Ingebrigtsen M.E., Kuznetsov A.Y., Svensson B.G., Alfieri G., Mihaila A., Badstübner U., Perron A., Vines L., Varley J.B.

Single crystalline bulk and epitaxially grown gallium oxide (β–Ga2O3) was irradiated by 0.6 and 1.9 MeV protons to doses ranging from 5 × 109 to 6 × 1014 cm−2 in order to study the impact on charge carrier concentration and electrically active defects. Samples irradiated to doses at or above 2 × 1013 cm−2 showed a complete removal of free charge carriers in their as-irradiated state, whereas little or no influence was observed below doses of 6 × 1012 cm−2. From measurements at elevated temperatures, a thermally activated recovery process is seen for the charge carriers, where the activation energy for recovery follow a second-order kinetics with an activation energy of ∼1.2 eV. Combining the experimental results with hybrid functional calculations, we propose that the charge carrier removal can be explained by Fermi-level pinning far from the conduction band minimum (CBM) due to gallium interstitials (Gai), vacancies (VGa), and antisites (GaO), while migration and subsequent passivation of VGa via hydrogen-derived or VO defects may be responsible for the recovery. Following the recovery, deep level transient spectroscopy (DLTS) reveals generation of two deep levels, with energy positions around 0.75 and 1.4 eV below the CBM. Of these two levels, the latter is observed to disappear after the initial DLTS measurements, while the concentration of the former increases. We discuss candidate possibilities and suggest that the origins of these levels are more likely due to a defect complex than an isolated point defect.

Wickramaratne D., Dreyer C.E., Monserrat B., Shen J., Lyons J.L., Alkauskas A., Van de Walle C.G.

Deep level transient spectroscopy (DLTS) is used extensively to study defects in semiconductors. We demonstrate that great care should be exercised in interpreting activation energies extracted from DLTS as ionization energies. We show how first-principles calculations of thermodynamic transition levels, temperature effects of ionization energies, and nonradiative capture coefficients can be used to accurately determine actual activation energies that can be directly compared with DLTS. Our analysis is illustrated with hybrid-functional calculations for two important defects in GaN, which have similar thermodynamic transition levels and shows that the activation energy extracted from DLTS includes a capture barrier that is temperature dependent, unique to each defect, and, in some cases, large in comparison to the ionization energy. By calculating quantities that can be directly compared with the experiment, first-principles calculations thus offer powerful leverage in identifying the microscopic origin of defects detected in DLTS.

Galazka Z.

Abstract β-Ga2O3 is an emerging, ultra-wide bandgap (energy gap of 4.85 eV) transparent semiconducting oxide, which attracted recently much scientific and technological attention. Unique properties of that compound combined with its advanced development in growth and characterization place β-Ga2O3 in the frontline of future applications in electronics (Schottky barrier diodes, field-effect transistors), optoelectronics (solar- and visible-blind photodetectors, flame detectors, light emitting diodes), and sensing systems (gas sensors, nuclear radiation detectors). A capability of growing large bulk single crystals directly from the melt and epi-layers by a diversity of epitaxial techniques, as well as explored material properties and underlying physics, define a solid background for a device fabrication, which, indeed, has been boosted in recent years. This required, however, enormous efforts in different areas of science and technology that constitutes a chain linking together engineering, metrology and theory. The present review includes material preparation (bulk crystals, epi-layers, surfaces), an exploration of optical, electrical, thermal and mechanical properties, as well as device design/fabrication with resulted functionality suitable for different fields of applications. The review summarizes all of these aspects of β-Ga2O3 at the research level that spans from the material preparation through characterization to final devices.

Polyakov A.Y., Smirnov N.B., Shchemerov I.V., Pearton S.J., Ren F., Chernykh A.V., Lagov P.B., Kulevoy T.V.

Hole traps in hydride vapor phase epitaxy β-Ga2O3 films were studied by deep level transient spectroscopy with electrical and optical excitation (DLTS and ODLTS) and by photocapacitance and temperature dependence measurements. Irradiation with 20 MeV protons creates deep electron and hole traps, a strong increase in photocapacitance, and prominent persistent photocapacitance that partly persists above room temperature. Three hole-trap-like signals H1 [self-trapped holes (STH)], H2 [electron capture barrier (ECB)], and H3, with activation energies 0.2 eV, 0.4 eV, 1.3 eV, respectively, were detected in ODLTS. The H1 (STH) feature is suggested to correspond to the transition of polaronic states of STH to mobile holes in the valence band. The broad H2 (ECB) feature is due to overcoming of the ECB of the centers responsible for persistent photocapacitance for temperatures below 250 K. The H3 peak is produced by detrapping of holes from Ev + 1.3 eV hole traps believed to be related to gallium vacancy acceptors. One more deep acceptor with optical ionization threshold near 2.3 eV is likely responsible for high temperature persistent photocapacitance surviving up to temperatures higher than 400 K. The latter traps show a significant barrier for capture of electrons.

Polyakov A.Y., Smirnov N.B., Shchemerov I.V., Yakimov E.B., Pearton S.J., Fares C., Yang J., Ren F., Kim J., Lagov P.B., Stolbunov V.S., Kochkova A.

Carrier removal rates and electron and hole trap densities in β-Ga2O3 films grown by hydride vapor phase epitaxy (HVPE) and irradiated with 18 MeV α-particles and 20 MeV protons were measured and compared to the results of modeling. The electron removal rates for proton and α-radiation were found to be close to the theoretical production rates of vacancies, whereas the concentrations of major electron and hole traps were much lower, suggesting that the main process responsible for carrier removal is the formation of neutral complexes between vacancies and shallow donors. There is a concurrent decrease in the diffusion length of nonequilibrium charge carriers after irradiation, which correlates with the increase in density of the main electron traps E2* at Ec − (0.75–0.78) eV, E3 at Ec − (0.95–1.05) eV, and E4 at Ec − 1.2 eV. The introduction rates of these traps are similar for the 18 MeV α-particles and 20 MeV protons and are much lower than the carrier removal rates.

Mcglone J.F., Xia Z., Zhang Y., Joishi C., Lodha S., Rajan S., Ringel S.A., Arehart A.R.

Threshold voltage instability was observed on β-Ga 2 O 3 transistors using double-pulsed current-voltage and constant drain current deep level transient spectroscopy (DLTS) measurements. A total instability of 0.78 V was attributed to two distinct trap levels, at E C -0.70 and E C -0.77 eV, which need to be mitigated for future applications. The traps are likely located near the gate-drain edge and below the delta-doped layer, which is determined through the DLTS technique and an understanding of the fill and empty biasing conditions. The trap modulation was consistent with a gate leakage-based trap filling mechanism, which was demonstrated. It is likely that Fe is playing a role in the observed dispersion due to the close proximity of the Fe substrate.

Polyakov A.Y., Smirnov N.B., Shchemerov I.V., Gogova D., Tarelkin S.A., Pearton S.J.

The electrical properties of epitaxial β-Ga2O3 doped with Sn (1016–9 × 1018 cm−3) and grown by metalorganic chemical vapor deposition on semi-insulating β-Ga2O3 substrates are reported. Shallow donors attributable to Sn were observed only in a narrow region near the film/substrate interface and with a much lower concentration than the total Sn density. For heavily Sn doped films (Sn concentration, 9 × 1018 cm−3), the electrical properties in the top portion of the layer were determined by deep centers with a level at Ec-0.21 eV not described previously. In more lightly doped layers, the Ec-0.21 eV centers and deeper traps at Ec-0.8 eV were present, with the latter pinning the Fermi level. Low temperature photocapacitance and capacitance voltage measurements of illuminated samples indicated the presence of high densities (1017–1018 cm−3) of deep acceptors with an optical ionization threshold of 2.3 eV. Optical deep level transient spectroscopy (ODLTS) and photoinduced current transient spectroscopy (PICTS) detected electron traps at Ec-0.8 eV and Ec-1.1 eV. For lightly doped layers, the compensation of film conductivity was mostly provided by the Ec-2.3 eV acceptors. For heavily Sn doped films, deep acceptor centers possibly related to Ga vacancies were significant. The photocapacitance and the photocurrent caused by illumination at low temperatures were persistent, with an optical threshold of 1.9 eV and vanished only at temperatures of ∼400 K. The capture barrier for electrons causing the persistent photocapacitance effect was estimated from ODLTS and PICTS to be 0.25–0.35 eV.

Farzana E., Ahmadi E., Speck J.S., Arehart A.R., Ringel S.A.

Deep level defects were characterized in Ge-doped (010) β-Ga2O3 layers grown by plasma-assisted molecular beam epitaxy (PAMBE) using deep level optical spectroscopy (DLOS) and deep level transient (thermal) spectroscopy (DLTS) applied to Ni/β-Ga2O3:Ge (010) Schottky diodes that displayed Schottky barrier heights of 1.50 eV. DLOS revealed states at EC − 2.00 eV, EC − 3.25 eV, and EC − 4.37 eV with concentrations on the order of 1016 cm−3, and a lower concentration level at EC − 1.27 eV. In contrast to these states within the middle and lower parts of the bandgap probed by DLOS, DLTS measurements revealed much lower concentrations of states within the upper bandgap region at EC − 0.1 – 0.2 eV and EC − 0.98 eV. There was no evidence of the commonly observed trap state at ∼EC − 0.82 eV that has been reported to dominate the DLTS spectrum in substrate materials synthesized by melt-based growth methods such as edge defined film fed growth (EFG) and Czochralski methods [Zhang et al., Appl. Phys. Lett. 108, 052105 (2016) and Irmscher et al., J. Appl. Phys. 110, 063720 (2011)]. This strong sensitivity of defect incorporation on crystal growth method and conditions is unsurprising, which for PAMBE-grown β-Ga2O3:Ge manifests as a relatively “clean” upper part of the bandgap. However, the states at ∼EC − 0.98 eV, EC − 2.00 eV, and EC − 4.37 eV are reminiscent of similar findings from these earlier results on EFG-grown materials, suggesting that possible common sources might also be present irrespective of growth method.

Ingebrigtsen M.E., Varley J.B., Kuznetsov A.Y., Svensson B.G., Alfieri G., Mihaila A., Badstübner U., Vines L.

Using a combination of deep level transient spectroscopy, secondary ion mass spectrometry, proton irradiation, and hybrid functional calculations, we identify two similar deep levels that are associated with Fe impurities and intrinsic defects in bulk crystals and molecular beam epitaxy and hydride vapor phase epitaxi-grown epilayers of β-Ga2O3. First, our results indicate that FeGa, and not an intrinsic defect, acts as the deep acceptor responsible for the often dominating E2 level at ∼0.78 eV below the conduction band minimum. Second, by provoking additional intrinsic defect generation via proton irradiation, we identified the emergence of a new level, labeled as E2*, having the ionization energy very close to that of E2, but exhibiting an order of magnitude larger capture cross section. Importantly, the properties of E2* are found to be consistent with its intrinsic origin. As such, contradictory opinions of a long standing literature debate on either extrinsic or intrinsic origin of the deep acceptor in question converge accounting for possible contributions from E2 and E2* in different experimental conditions.

Polyakov A.Y., Smirnov N.B., Shchemerov I.V., Yakimov E.B., Yang J., Ren F., Yang G., Kim J., Kuramata A., Pearton S.J.

Deep electron and hole traps in 10 MeV proton irradiated high-quality β-Ga2O3 films grown by Hydride Vapor Phase Epitaxy (HVPE) on bulk β-Ga2O3 substrates were measured by deep level transient spectroscopy with electrical and optical injection, capacitance-voltage profiling in the dark and under monochromatic irradiation, and also electron beam induced current. Proton irradiation caused the diffusion length of charge carriers to decrease from 350–380 μm in unirradiated samples to 190 μm for a fluence of 1014 cm−2, and this was correlated with an increase in density of hole traps with optical ionization threshold energy near 2.3 eV. These defects most likely determine the recombination lifetime in HVPE β-Ga2O3 epilayers. Electron traps at Ec-0.75 eV and Ec-1.2 eV present in as-grown samples increase in the concentration after irradiation and suggest that these centers involve native point defects.

Pearton S.J., Yang J., Cary P.H., Ren F., Kim J., Tadjer M.J., Mastro M.A.

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (ε) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

Ingebrigtsen M.E., Vines L., Alfieri G., Mihaila A., Badstübner U., Svensson B.G., Kuznetsov A.

Electrical properties of Schottky contacts of high work-function metals (Pd, Au, and Ni) on (010) and (201) oriented β-Ga2O3 were investigated. Current-voltage characteristics reveal that all the contacts exhibit high rectifying behavior with ideality factors as low as 1.04. However, the reverse leakage currents were lower in the (010) samples compared to the (201) ones. Thermal admittance spectroscopy confirms a main charge carrier level to be at ~0.15 eV below the conduction band edge (Ec). Secondary ion mass spectrometry indicates that Si may be responsible for this donor level. Deep level transient spectroscopy reveals four levels (E1-E4) in the upper part of the band gap, with the corresponding energy level positions at 0.56, 0.76, 1.01, and 1.48 eV below Ec.

Huang J., Peng B., Sun K., Dong L., Yuan L., Yuan H., Zhang Y., Jia R.

Abstract Transition metals are not only residual impurities in β- Ga 2 O 3 single crystals, but also hold significant potential in doped substrates and epitaxial films for power electronics, optoelectronics, and memory devices. Current research primarily focuses on iron and titanium, with limited comprehensive studies on manganese. Using first-principles calculations with hybrid functionals, we investigated the formation energy, thermodynamic transition levels, and nonradiative capture coefficients of the Mn dopant in β- Ga 2 O 3 . To enhance accuracy, we ensured that both intrinsic and Mn-doped supercells satisfy the generalized Koopmans’ theorem, employing advanced methods such as the correct identification of ground-state defect configurations and image charge corrections independent of empirical dielectric constants. We report the relationship between the formation energy of substitutional Mn impurities and intrinsic defects with the Fermi-level position, revealing that Mn at the octahedral Ga sites is more stable under oxygen-rich conditions. Our findings indicate that Mn dopants at tetrahedral sites introduce a (0/–) transition level 0.68 eV below the conduction band minimum, which is shallower than previously reported. Compared to valence band holes, neutral Mn impurities more easily capture conduction band electrons nonradiatively, with a capture cross-section of approximately 10−11 cm2 at room temperature, which is larger than that of other common transition metal impurities, such as Fe and Ti. This suggests Mn’s potential as a fast electron recombination center in optoelectronic and electronic devices. We also briefly discuss the charge state changes associated with electron capture by Mn ions, which are relevant for resistive memory and spintronic device applications.

Kruszewski P., Fiedler A., Galazka Z.

In this Letter, we demonstrate the application of Deep Level Transient Spectroscopy (DLTS) and Laplace DLTS (L-DLTS) techniques to unintentionally doped β-Ga2O3 crystals grown by the Czochralski method. It is clearly shown that the capacitance signal associated with the electron emission from a trap level previously identified in the literature as E14 and characterized by an activation energy of 0.18 eV is found to be a superposition of electron emissions from two closely spaced energy levels. Furthermore, we noted that the corresponding L-DLTS signal splits into two well separated components with activation energies of 196 and 209 meV, and the splitting occurs as the electric field in the space charge region of a Schottky diode exceeds 2 × 107 V/m (0.2 MV/cm). Additionally, a strong dependency of DLTS and L-DLTS signals on the electric field strength and resulting enhancement of the electron emission from these two trap states agree well with the 1D Poole–Frenkel (PF) model, suggesting donor-like behavior of both states. Finally, we found that the barrier height for thermal emission of the electrons is significantly reduced in our samples by 121 meV due to the PF effect for experimental conditions corresponding to an electric field of 3.5 × 107 V/m (0.35 MV/cm).

Polyakov A.Y., Saranin D.S., Shchemerov I.V., Vasilev A.A., Romanov A.A., Kochkova A.I., Gostischev P., Chernykh A.V., Alexanyan L.A., Matros N.R., Lagov P.B., Doroshkevich A.S., Isayev R.S., Pavlov Y.S., Kislyuk A.M., et. al.

p-NiO/n-Ga2O3 heterojunction (HJ) diodes exhibit much larger changes in their properties upon 1.1 MeV proton irradiation than Schottky diodes (SDs) prepared on the same material. In p-NiO/Ga2O3 HJ diodes, the narrow region adjacent to the HJ boundary is found to contain a high density of relatively deep centers with levels near EC-0.17 eV and a depleted region in the immediate vicinity of the HJ boundary. The series resistance of the HJ diodes is slightly higher than for the Schottky diodes and shows a temperature dependence with activation energy ~ 0.12 eV, like the temperature dependence of the NiO film resistivity. Irradiation with 1.1 MeV protons leads to a decrease of the hole concentration in the NiO, with a high carrier removal rate of ~ 1.3 × 105 cm−1 and a strong compensation of the interfacial region where the concentration of the EC-0.17 eV centers decreases with a high rate of ~ 7 × 103 cm−1. The combined action of these two effects gives rise to the much stronger increase of the series resistance of the HJ diodes compared to Schottky diodes. The observed differences between the radiation response of the HJs and SDs cannot be credibly attributed to the changes of the density of any of the deep electron and hole traps detected in our experiments.

Fregolent M., Piva F., Buffolo M., De Santi C., Cester A., Higashiwaki M., Meneghesso G., Zanoni E., Meneghini M.

Abstract The study of deep-level defects in semiconductors has always played a strategic role in the development of electronic and optoelectronic devices. Deep levels have a strong impact on many of the device properties, including efficiency, stability, and reliability, because they can drive several physical processes. Despite the advancements in crystal growth, wide- and ultrawide-bandgap semiconductors (such as gallium nitride and gallium oxide) are still strongly affected by the formation of defects that, in general, can act as carrier traps or generation-recombination centers (G-R). Conventional techniques used for deep-level analysis in silicon need to be adapted for identifying and characterizing defects in wide-bandgap materials. This topical review paper presents an overview of reviews of the theory of deep levels in semiconductors; in addition, we present a review and original results on the application, limits, and perspectives of two widely adopted common deep-level detection techniques, namely capacitance deep-level transient spectroscopy and deep-level optical spectroscopy, with specific focus on wide-bandgap semiconductors. Finally, the most common traps of GaN and β-Ga2O3 are reviewed.

Dawe C.A., Markevich V.P., Halsall M.P., Hawkins I.D., Peaker A.R., Nandi A., Sanyal I., Kuball M.

In this work, conventional deep-level transient spectroscopy (DLTS) and high-resolution Laplace-DLTS (L-DLTS) have been used to characterize deep-level traps in (010) β-Ga2O3 epilayers grown by metal organic chemical vapor deposition on native Sn-doped substrates. Two types of epilayers have been studied, one doped with silicon during growth to about 1.5 × 1017 cm−3 and the other type was unintentionally doped (UID). Electrical measurements were conducted on Au and Pt Schottky barrier diodes. In the Si-doped samples, only one electron trap with emission activation energy of 0.42 eV (E0.42) and concentration of (6–8) × 1013 cm−3 has been detected. In the UID samples, in addition to the E0.42 trap, two other traps with activation energies for electron emission of 0.10 eV (E0.10) and 0.53 eV (E0.53) have been observed. Dependencies of electron emission rate (eem) on the electric field (E) as well as concentration-depth profiles {NT(W)} have been measured and analyzed for the E0.10 and E0.42 traps. The eem(E) dependence for the E0.10 trap is characteristic for a donor energy level, while that for the E0.42 trap indicates an acceptor level. The NT(W) dependencies show non-uniform spatial distributions of both the E0.10 and E0.42 traps in the UID samples, with the concentration of the E0.10 trap dropping from about 1 × 1015 cm−3 at 1.5 μm from the surface to about 2 × 1013 cm−3 at 0.5 μm, which indicates out-diffusion from the substrate or interface into the epilayer as a likely source. The results obtained are compared with the literature, and possible origins of the detected traps are discussed.

Vasilev A.A., Kochkova A.I., Polyakov A.Y., Romanov A.A., Matros N.R., Alexanyan L.A., Shchemerov I.V., Pearton S.J.

Direct observation of the capture cross section is challenging due to the need for extremely short filling pulses in the two-gate Deep-Level Transient Spectroscopy (DLTS). Simple estimation of the cross section can be done from DLTS and admittance spectroscopy data but it is not feasible to distinguish temperature dependence of pre-exponential and exponential parts of the emission rate equation with sufficient precision conducting a single experiment. This paper presents experimental data of deep levels in β-Ga2O3 that has been gathered by our group since 2017. Based on the gathered data, we propose a derivation of apparent activation energy (Eam) and capture cross section (σnm) assuming the temperature dependent capture via the multiphonon emission model, which resulted in a strong correlation between Eam and σnm according to the Meyer–Neldel rule, which allowed us to estimate low- and high-temperature capture coefficients C0 and C1 as well as capture barrier Eb. It also has been shown that without considering the temperature dependence of capture cross section, the experimental values of σn are overestimated by 1–3 orders of magnitude. A careful consideration of the data also allows to be more certain identifying deep levels by their “fingerprints” (Ea and σn) considering two additional parameters (EMN and σ00) and to verify the density functional theory computation of deep-level recombination properties.

Huang Y., Wang R., Yang D., Pi X.

4H Silicon carbide (SiC) is widely recognized as one of the most advanced wide bandgap semiconductors used in the production of high-efficiency power electronic devices. Impurities play a crucial role in achieving the desired electrical properties in 4H-SiC, yet the understanding of impurities in this material remains limited. In this study, first-principles formation-energy calculations were employed to establish a comprehensive database of formation-energy diagrams for impurities in 4H-SiC. This database includes valuable information on site preference, lattice distortion, solubility, and charge transition levels (CTLs) of the impurities. The site preference for each impurity is closely related to factors such as the Fermi energy, chemical potential, and the impurity species itself. To assess the lattice distortion caused by each impurity, a comparison was made between the volume changes before and after doping. Moreover, the solubility of each impurity was determined using the detailed balance theory, thereby enabling a direct measure of the maximum impurity concentration achievable in the material. Based on the CTLs, the impurities in 4H-SiC were classified into four categories: n-type impurities, p-type impurities, amphoteric impurities, and non-electroactive impurities. This comprehensive property database for impurities in 4H-SiC provides valuable insights for tailoring the material properties through controlled doping, thereby ultimately leading to enhanced performance of power electronic devices.

Langørgen A., Vines L., Kalmann Frodason Y.

The ultra-wide bandgap of gallium oxide provides a rich plethora of electrically active defects. Understanding and controlling such defects is of crucial importance in mature device processing. Deep-level transient spectroscopy is one of the most sensitive techniques for measuring electrically active defects in semiconductors and, hence, a key technique for progress toward gallium oxide-based components, including Schottky barrier diodes and field-effect transistors. However, deep-level transient spectroscopy does not provide chemical or configurational information about the defect signature and must, therefore, be combined with other experimental techniques or theoretical modeling to gain a deeper understanding of the defect physics. Here, we discuss the current status regarding the identification of electrically active defects in beta-phase gallium oxide, as observed by deep-level transient spectroscopy and supported by first-principles defect calculations based on the density functional theory. We also discuss the coordinated use of the experiment and theory as a powerful approach for studying electrically active defects and highlight some of the interesting but challenging issues related to the characterization and control of defects in this fascinating material.

Huang Y., Xu X., Yang J., Yu X., Wei Y., Ying T., Liu Z., Jing Y., Li W., Li X.

Lyons J.L.

Recently, LiGa5O8 was claimed to be a p-type dopable ultrawide-bandgap oxide, based on measurements of undoped material. Here, the electronic properties of potential acceptor dopant impurities in LiGa5O8 are calculated using hybrid density functional theory to evaluate their potential for causing p-type conductivity. As with the related compound LiGaO2, the heavy oxygen-derived valence bands lead to stable self-trapped holes in LiGa5O8. Acceptor defects and dopants also bind trapped holes (or small polarons), which lead to large acceptor ionization energies. The calculations here indicate that neither native acceptor defects (such as cation vacancies or antisites) nor impurity dopants can give rise to p-type conductivity in LiGa5O8. Optical transitions associated with these defects are also calculated, in order to allow for possible experimental verification of their behavior.

Wang Z.P., Sun N., Yu X.X., Gong H.H., Ji X.L., Ren F.-., Gu S.L., Zheng Y.D., Zhang R., Kuznetsov A.Y., Ye J.D.

Impacts of spatial charge inhomogeneities on carrier transport fluctuations and premature breakdown were investigated in Schottky ampere-class Ga2O3 power diodes. Three prominent electron traps were detected in Ga2O3 epilayers by a combination of the depth-resolved capacitance spectroscopy profiling and gradual dry etching. The near-surface trap occurring at 1.06 eV below the conduction band minimum (EC), named E3, was found to be confined within a 180 nm surface region of the Ga2O3 epilayers. Two bulk traps at EC − 0.75 eV (E2*) and at EC − 0.82 eV (E2) were identified and interconnected with the VGa- and FeGa-type defects, respectively. In the framework of the impact ionization model, employing the experimental trap parameters, the TCAD simulated breakdown characteristics matched the experimental breakdown properties well, consistently with inverse proportionality to the total trap densities. In particular, the shallowest distributed E3 trap with the deepest level is responsible for higher leakage and premature breakdown. In contrast, Ga2O3 Schottky diodes without E3 trap exhibit enhanced breakdown voltages, and the leakage mechanism evolves from variable range hopping at medium reverse voltages, to the space-charge-limited conduction at high reverse biases. This work bridges the fundamental gap between spatial charge inhomogeneities and diode breakdown features, paving the way for more reliable defect engineering in high-performance Ga2O3 power devices.

Chen J., Qu H., Sui J., Lu X., Zou X.

The study of interface states and bulk traps and their connection to device instability is highly demanded to achieve reliable β-Ga2O3 metal-oxide-semiconductor (MOS) devices. However, a comprehensive analysis of the capture/emission behavior of interface states and bulk traps can be challenging due to widespread time constant distribution. In this study, using capacitance transient measurement tools, trap states of the ZrO2/β-Ga2O3 MOS gate stack were explicitly investigated, particularly its bias- and temperature-dependent relaxation kinetics. As forward bias is enlarged, it is observed that the interface state density (Dit) increases by 12.6%. Two bulk traps with discrete levels identified as 0.43 eV (E1) and 0.74 eV (E2) below the conduction band minimum were extracted by deep-level transient spectroscopy. It is further revealed that the emission processes of E1 and E2 are thermally enhanced, while the capture processes remain insensitive to temperature. The electric-field dependence of E1 indicates that the dominant mechanism follows the rule of Poole–Frenkel emission. The capacitance–voltage (C–V) hysteresis deteriorated at a higher forward bias due to the higher trap density and increased population of trapped charges. These findings provide an important framework for future device optimization to improve the reliability and performance of β-Ga2O3 MOS devices.

Stehr J.E., Jansson M., Pearton S.J., McCloy J.S., Jesenovec J., Dutton B.L., McCluskey M.D., Chen W.M., Buyanova I.A.

Transition metal (TM) ions incorporated into a host from a wide bandgap semiconductor are recognized as a promising system for quantum technologies with enormous potential. In this work, we report on a TM color center in β-Ga2O3 with physical properties attractive for quantum information applications. The center is found to emit at 1.316 μm and exhibits weak coupling to phonons, with optically addressable higher-lying excited states, beneficial for single-photon emission within the telecom range (O-band). Using magneto-photoluminescence (PL) complemented by time-resolved PL measurements, we identify the monitored emission to be internal 1E→3A2 spin-forbidden transitions of a 3d8 TM ion with a spin-triplet ground state—a possible candidate for a spin qubit. We tentatively attribute this color center to a complex involving a sixfold coordinated Cu3+ ion.

Nikolaev V.I., Polyakov A.Y., Krymov V.M., Shapenkov S., Butenko P., Yakimov E.B., Vasilev A.A., Shchemerov I.V., Chernykh A.V., Matros N.R., Alexanyan L., Kochkova A.I., Pearton S.J.

Deep trap spectra and carrier diffusion lengths were measured for unintentionally doped β-Ga2O3 bulk crystals with (100) orientation. The 20-mm diameter, 15-mm length boule was pulled by the Czochralski method from gallium oxide in (010) direction. It is found that the net density of shallow donors in (100) plates cleaved from the crystal was 2.6 × 1017 cm−3, with ionization energies of 0.05 eV measured from admittance spectra. Three deep electron traps with respective ionization energies of 0.6 eV (concentration 1.1 × 1014 cm−3), 0.8 eV (concentration 3.9 × 1016 cm−3) and 1.1 eV (concentration 8.9 × 1015 cm−3) were detected by Deep Level Transient Spectroscopy. The dominant 0.8 eV trap is associated with the E2 centers due to Fe acceptors, the two other traps are the well documented E1 and E3 centers. The major deep acceptors in the lower half of the bandgap have optical ionization threshold of 2.3 eV and concentration of 4 × 1015 cm−3 and are believed to be due to the split Ga vacancies acceptors. The diffusion length of non-equilibrium charge carriers was 90 nm. The electrical properties of these (100) oriented crystals grown by Czochralski are quite similar to those synthesized by the undoped Edge-defined Film-Fed Growth technique.

Seyidov P., Varley J.B., Frodason Y.K., Klimm D., Vines L., Galazka Z., Chou T., Popp A., Irmscher K., Fiedler A.

AbstractThe thermal stability of different Schottky contacts (Au, Pt, and Ni) on (100) β‐Ga2O3 single crystals grown by the Czochralski method is investigated. Besides the examination of the Schottky barrier parameters, contact‐dependent defect levels are investigated by deep‐level transient spectroscopy (DLTS) in a 100–650 K (ramp‐up) and 650–100 K (ramp‐down) temperature cycle. Several defect levels are detected below the conduction band minimum at 0.41, 0.60, 0.77, 0.96, and 1.17 eV. In the temperature ramp‐down DLTS, the 1.17 eV level disappears, and the 0.60 eV level appears for all Schottky contacts. DFT calculations suggest that rearrangement and dissociation of a single hydrogen from a doubly‐hydrogenated Ga─O divacancy complex occurs during the temperature sweep under bias. The trap level at 0.96 eV only appears after the thermal load for the Ni contact, in contrast to Au and Pt, where it is present without a thermal budget. Temperature‐dependent leakage current (at −4 V) measurements indicate oxidation of Ni, and further thermodynamic analysis suggests alloying of Au‐Ga atoms at the Au/β‐Ga2O3 interface. These studies provide insight into the behavior induced by these common Schottky contacts and the alteration associated with temperature cycling.