Publications

Heterostructures based on ultrawide-bandgap (UWBG) semiconductors (bandgap>4.0eV), such as BN and diamond, hold significant importance for the development of high-power electronics in the next generation. However, achieving in situ heteroepitaxy of BN/diamond or vice versa remains exceptionally challenging due to the complex growth kinetics involved. In this work, we grew BN thin film on (100) single-crystal diamonds using pulsed laser deposition and investigated its structural, magnetic, optical, and thermal properties. The structural analyses confirmed the growth of BN films, which exhibited diamagnetic behavior at room temperature. Notably, the film demonstrated anisotropic refractive index characteristics within the visible-to-near-infrared wavelength range. The room-temperature cross-plane thermal conductivity of BN is 1.53 ± 0.77 W/mK, while the thermal conductance of the BN/diamond interface is 20±2MW/m2K. These findings have significant implications for a range of device applications based on UWBG BN/diamond heterostructures.

We extensively investigate the degradation of gallium nitride (GaN)-based high periodicity indium GaN (InGaN)-GaN multiple quantum well (MQW) solar cells submitted to optical stress at high excitation intensity and high temperature. The original results reported in this article indicate the presence of a thermally activated diffusion of impurities from the p -side of the devices toward the active region (AR), which favors the increase in the Shockley–Read–Hall (SRH) recombination within the MQWs. Appropriate fitting of the degradation kinetics according to Fick’s second law allowed the extrapolation of the diffusion coefficient of the defect involved in the degradation and of the related activation energy. The obtained values suggest that degradation originates from the diffusion of hydrogen. The proposed analytical methodology and the related results provide insight on MQW solar cells degradation and can be used to increase cell performance and reliability in novel applications and harsh environments.

Herein, a device study using technology computer-aided design simulation to theoretically analyze the electrical performance of ultrawide-bandgap boron nitride (BN)-based vertical junction devices is performed, including h-BN Schottky diode, h-BN pn diode, and h-BN/AlN pn diode; this is also the first demonstration of the BN power devices in simulation. The material properties of BN are defined with recently reported data, and the physical mechanisms of the device performance are systematically investigated. The h-BN junctions in this simulation shows excellent performance, especially for breakdown behaviors. Schottky diode shows a turn-on voltage of 0.6 V for Pt Schottky contact and breakdown voltages over 450 V for 5 μm, 6 × 1015 cm−3 p-type-doped drift layer; The h-BN pn diode shows a turn-on voltage of 6 V and breakdown voltages over 3 kV with a critical electric field of 13.6 MV cm−1 for 2.5 μm, 2 × 1016 cm−3 p-type-doped drift layer. The h-BN/AlN heterojunction pn diode shows a turn-on voltage of 5.8 V and breakdown voltage over 2 kV for 2.5 μm, 2 × 1016 cm−3 n-type-doped AlN drift layer. Herein, an understanding of the device principles of vertical BN junctions is provided, which can serve as a reference for the future development of robust BN power electronics.

Ultra-wide bandgap (UWBG) materials are poised to play an important role in the future of power electronics. Devices made from UWBG materials are expected to operate at higher voltages, frequencies, and temperatures than current silicon and silicon-carbide-based devices and can even lead to significant miniaturization of such devices. In the UWBG field, aluminum nitride and boron nitride have attracted great interest; however, the BxAl1−xN alloys are much less studied. In this work, using first-principles simulations combining density-functional theory and the cluster expansion method, the crystal structure of BxAl1−xN alloys is predicted. Seventeen ground state structures of BxAl1−xN with formation energies between 0.11 and 0.25 eV atom-1 are found. All of these structures are found to be dynamically stable. The BxAl1−xN structures are found to have predominantly a tetrahedral bonding environment; however, some structures exhibit sp2 bonds similar to hexagonal BN. This work expands knowledge of the structures, energies, and bonding in BxAl1−xN which aids their synthesis, the innovation of lateral or vertical devices, and discovery of compatible dielectric and Ohmic contact materials.

We develop an open-source python workflow package, pyGWBSE to perform automated first-principles calculations within the GW-BSE (Bethe-Salpeter) framework. GW-BSE is a many body perturbation theory based approach to explore the quasiparticle (QP) and excitonic properties of materials. GW approximation accurately predicts bandgaps of materials by overcoming the bandgap underestimation issue of the more widely used density functional theory (DFT). BSE formalism produces absorption spectra directly comparable with experimental observations. pyGWBSE package achieves complete automation of the entire multi-step GW-BSE computation, including the convergence tests of several parameters that are crucial for the accuracy of these calculations. pyGWBSE is integrated with Wannier90, to generate QP bandstructures, interpolated using the maximally-localized wannier functions. pyGWBSE also enables the automated creation of databases of metadata and data, including QP and excitonic properties, which can be extremely useful for future material discovery studies in the field of ultra-wide bandgap semiconductors, electronics, photovoltaics, and photocatalysis.

Phonon scattering at grain boundaries (GBs) is significant in controlling the nanoscale device thermal conductivity. However, GBs could also act as waveguides for selected modes. To measure localized GB phonon modes, milli-electron volt (meV) energy resolution is needed with subnanometer spatial resolution. Using monochromated electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) we have mapped the 60 meV optic mode across GBs in silicon at atomic resolution and compared it to calculated phonon densities of states (DOS). The intensity is strongly reduced at GBs characterized by the presence of 5- and 7-fold rings where bond angles differ from the bulk. The excellent agreement between theory and experiment strongly supports the existence of localized phonon modes and thus of GBs acting as waveguides.

Thick diamond films are needed for high power device applications. One approach is to grow via the step-flow growth mode on miscut substrates. In this paper we correlate variations of surface morphology and optical properties of a 250 μm-thick undoped diamond layer, grown at a rate of 10.4 μm/h, on a (001) diamond substrate with a miscut angle of 10 degrees towards a <010> direction. The epilayer surface comprises regions with island growth on (001) planes, and regions parallel to the miscut substrate surface exhibiting step-bunching and faceting and resulting in various degrees of surface roughness. The optical properties exhibit a combination of sharp and broad luminescence peaks, which are found to be specific to growth morphology and to the nitrogen content in the near-surface region.

Ultra-wide bandgap (UWBG) materials such as AlN and BN hold great promise for future power electronics due to their exceptional properties. They exhibit large band gaps, high breakdown fields, high thermal conductivity, and high mechanical strengths. AlN and BN have been extensively researched, however, their alloys, BxAl1−xN, are much less studied despite their ability to offer tunable properties by adjusting x. In this article, we predict the electronic properties of 17 recently predicted ground states of BxAl1−xN in the x=0−1 range using first-principles density functional theory and many-body perturbation theory within GW approximation. All the BxAl1−xN structures are found to be UWBG materials and have band gaps that vary linearly from that of wurtzite-phase (\emph{w}) AlN (6.19 eV) to that of \emph{w}-BN (7.47 eV). The bandstructures of BxAl1−xN show that a direct-to-indirect bandgap crossover occurs near x=0.25. Furthermore, we find that BxAl1−xN alloys have much larger dielectric constants than the constituent bulk materials (AlN=9.3 ε0 or BN=7.3 ε0), with values reaching as high as 12.1 ε0. These alloys are found to exhibit large dielectric breakdown fields in the range 9--35 MV/cm with a linear dependence on x. This work provides the much needed advancement in the understanding of the properties of BxAl1−xN to aid their application in next-generation devices.

Cubic boron nitride (c-BN), with a small 1.4% lattice mismatch with diamond, presents a heterostructure with multiple opportunities for electronic device applications. However, the formation of c-BN/diamond heterostructures has been limited by the tendency to form hexagonal BN at the interface. In this study, c-BN has been deposited on free standing polycrystalline and single crystal boron-doped diamond substrates via electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD), employing fluorine chemistry. In situ x-ray photoelectron spectroscopy (XPS) is used to characterize the nucleation and growth of boron nitride (BN) films as a function of hydrogen gas flow rates during deposition. The PECVD growth rate of BN was found to increase with increased hydrogen gas flow. In the absence of hydrogen gas flow, the BN layer was reduced in thickness or etched. The XPS results show that an excess of hydrogen gas significantly increases the percent of sp2 bonding, characteristic of hexagonal BN (h-BN), particularly during initial layer growth. Reducing the hydrogen flow, such that hydrogen gas is the limiting reactant, minimizes the sp2 bonding during the nucleation of BN. TEM results indicate the partial coverage of the diamond with thin epitaxial islands of c-BN. The limited hydrogen reaction is found to be a favorable growth environment for c-BN on boron-doped diamond.

The thermal conductivity Λ of wide bandgap semiconductor thin films, such as AlN, affects the performance of high-frequency devices, power devices, and optoelectronics. However, accurate measurements of Λ in thin films with sub-micrometer thicknesses and Λ > 100 W/(m-K) is challenging. Widely used pump/probe metrologies, such as time–domain thermoreflectance (TDTR) and frequency–domain thermoreflectance, lack the spatiotemporal resolution necessary to accurately quantify thermal properties of sub-micrometer thin films with high Λ. In this work, we use a combination of magneto-optic thermometry and TiN interfacial layers to significantly enhance the spatiotemporal resolution of pump/probe thermal transport measurements. We use our approach to measure Λ of 100, 400, and 1000 nm AlN thin films. We coat AlN thin films with a ferromagnetic thin-film transducer with the geometry of (1 nm-Pt/0.4 nm-Co)x3/(2 nm-TiN). This PtCo/TiN transducer has a fast thermal response time of <50 ps, which allows us to differentiate between the thermal response of the transducer, AlN thin film, and substrate. For the 100, 400, and 1000 nm thick AlN films, we determine Λ to be 200 ± 80, 165 ± 35, and 300 ± 70 W m−1 K−1, respectively. We conclude with an uncertainty analysis that quantifies the errors associated with pump/probe measurements of thermal conductivity, as a function of transducer type, thin-film thermal conductivity, and thin-film thickness. Time-resolved magneto-optic Kerr effect experiments can measure films that are three to five times thinner than is possible with standard pump/probe metrologies, such as TDTR. This advance in metrology will enable better characterization of nanoscale heat transfer in high thermal conductivity material systems like wide bandgap semiconductor heterostructures and devices.

We report on the low-frequency electronic noise in β-(AlxGa1−x)2O3 Schottky barrier diodes. The noise spectral density reveals 1/f dependence, characteristic of the flicker noise, with superimposed Lorentzian bulges at the intermediate current levels (f is the frequency). The normalized noise spectral density in such diodes was determined to be on the order of 10−12 cm2/Hz (f = 10 Hz) at 1 A/cm2 current density. At the intermediate current regime, we observed the random telegraph signal noise, correlated with the appearance of Lorentzian bulges in the noise spectrum. The random telegraph signal noise was attributed to the defects near the Schottky barrier. The defects can affect the local electric field and the potential barrier and, correspondingly, impact the electric current. The obtained results help in understanding the noise in Schottky barrier diodes made of ultra-wide bandgap semiconductors and can be used for the material and device quality assessment.

Wide bandgap semiconductors such as gallium oxide (Ga2O3) have attracted much attention for their use in next-generation high-power electronics. Although single-crystal Ga2O3 substrates can be routinely grown from melt along various orientations, the influence of such orientations has been seldom reported. Further, making rectifying p–n diodes from Ga2O3 has been difficult due to lack of p-type doping. In this study, we fabricated and optimized 2D/3D vertical diodes on β-Ga2O3 by varying the following three factors: substrate planar orientation, choice of 2D material and metal contacts. The quality of our devices was validated using high-temperature dependent measurements, atomic-force microscopy (AFM) techniques and technology computer-aided design (TCAD) simulations. Our findings suggest that 2D/3D β-Ga2O3 vertical heterojunctions are optimized by substrate planar orientation (−201), combined with 2D WS2 exfoliated layers and Ti contacts, and show record rectification ratios (>106) concurrently with ON-Current density (>103 A cm−2) for application in power rectifiers.

Ultrawide bandgap β-(AlxGa1−x)2O3 vertical Schottky barrier diodes on (010) β-Ga2O3 substrates are demonstrated. The β-(AlxGa1−x)2O3 epilayer has an Al composition of 21% and a nominal Si doping of 2 × 1017 cm−3 grown by molecular beam epitaxy. Pt/Ti/Au has been employed as the top Schottky contact, whereas Ti/Au has been utilized as the bottom Ohmic contact. The fabricated devices show excellent rectification with a high on/off ratio of ∼109, a turn-on voltage of 1.5 V, and an on-resistance of 3.4 mΩ cm2. Temperature-dependent forward current-voltage characteristics show effective Schottky barrier height varied from 0.91 to 1.18 eV while the ideality factor from 1.8 to 1.1 with increasing temperatures, which is ascribed to the inhomogeneity of the metal/semiconductor interface. The Schottky barrier height was considered a Gaussian distribution of potential, where the extracted mean barrier height and a standard deviation at zero bias were 1.81 and 0.18 eV, respectively. A comprehensive analysis of the device leakage was performed to identify possible leakage mechanisms by studying temperature-dependent reverse current-voltage characteristics. At reverse bias, due to the large Schottky barrier height, the contributions from thermionic emission and thermionic field emission are negligible. By fitting reverse leakage currents at different temperatures, it was identified that Poole–Frenkel emission and trap-assisted tunneling are the main leakage mechanisms at high- and low-temperature regimes, respectively. Electrons can tunnel through the Schottky barrier assisted by traps at low temperatures, while they can escape these traps at high temperatures and be transported under high electric fields. This work can serve as an important reference for the future development of ultrawide bandgap β-(AlxGa1−x)2O3 power electronics, RF electronics, and ultraviolet photonics.

Several different T and Π -gate architectures of GaN/AlGaN high electron mobility transistor (HEMT) are compared using full band cellular Monte Carlo (CMC) device simulator for device performance (dc and RF) while accounting for self-heating modeled through energy balance equation (EBE) for acoustic and optical phonons which self consistently couple charge and heat transport. Efficacy of Π -gate over T –gate is assessed for suppression of hot electron under device self-heating obtained through solution of electron energy distribution function (EDF) from complete nonlinear Boltzmann transport equation (BTE). The Π -gate suppresses 19.75% more hot electrons corresponding to a dc power of 2.493 W/mm for Vgs = − 0.6 V (max transconductance). For the dc performance, Ids is nearly the same for each device configuration over the entire bias range, whereas, for the RF performance the current gain was evaluated over a frequency range 20–120 GHz in each device for both thermal (including self-heating) and isothermal (without self-heating). The evaluated cut off frequency is around 7% lower for the thermal case than the isothermal case, and for the experimentally characterized T -gate device, and for the simulated cutoff frequency closely follows the experimental measurement.

High Al mole fraction AlGaN is an ultrawide bandgap semiconductor with potential applications in power electronics and deep UV detectors. Although n-type material is achievable with Si-doping, the role of Si is controversial, particularly for AlxGa1−xN with x > 0.8. For this paper, AlGaN films were grown by plasma-assisted molecular beam epitaxy onto bulk AlN substrates and doped with 1018–1020 cm−3 Si. We examine electron transport in heavily Si-doped AlxGa1−xN with x ≥ 0.65 using magnetic resonance, which allows us to probe the neutral donors directly rather than the free carriers and avoids complications due to electrical contacts. Transport was studied through the temperature-dependent linewidth of the electron paramagnetic resonance (EPR) signature for the neutral donor. Analysis shows evidence of hopping conductivity in the most lightly doped samples and impurity band formation in the most heavily doped ones. The EPR results, which are consistent with Hall measurements performed on the same samples, are promising for the development of highly conducting high Al content AlGaN.

We report on the growth of monoclinic β- and orthorhombic κ-phase Ga2O3 thin films using liquid-injection metal-organic chemical vapor deposition on highly thermally conductive 4H-SiC substrates using gallium (III) acetylacetonate or tris(2,2,6,6-tetramethyl-3,5-heptanedionato) gallium (III). Both gallium precursors produced the β phase, while only the use of the latter led to growth of κ-Ga2O3. Regardless of the used precursor, best results for β-Ga2O3 were achieved at a growth temperature of 700 °C and O2 flows in the range of 600–800 SCCM. A relatively narrow growth window was found for κ-Ga2O3, and best results were achieved for growth temperatures of 600 °C and the O2 flow of 800 SCCM. While phase-pure β-Ga2O3 was prepared, κ-Ga2O3 showed various degrees of parasitic β phase inclusions. X-ray diffraction and transmission electron microscopy confirmed a highly textured structure of β- and κ-Ga2O3 layers resulting from the presence of multiple in-plane domain orientations. Thermal conductivities of 53 nm-thick β-Ga2O3 (2.13 + 0.29/−0.51 W/m K) and 45 nm-thick κ-Ga2O3 (1.23 + 0.22/−0.26 W/m K) were determined by transient thermoreflectance and implications for device applications were assessed. Presented results suggest great potential of heterointegration of Ga2O3 and SiC for improved thermal management and reliability of future Ga2O3-based high power devices.

The sub-threshold mobility in a lateral Ga2O3 transistor is extracted by considering a distributed impedance model of the sub-threshold region. The model explains the occurrence of an observed sharp peak in the AC conductance, which in this case cannot be explained by interface trap states. The trap-free model discussed here reveals two different behaviours of mobility, attributed to scattering regimes where Coulomb scattering is either screened or unscreened. The minimum value of sub-threshold mobility extracted in the unscreened regime corresponds to 0.49 cm2/V-s.

Heteroepitaxial growth of β-Ga2O3 on (001) diamond by metal–organic chemical vapor deposition (MOCVD) is reported. A detailed study was performed with Transmission Electron Microscopy (TEM) elucidating the epitaxial relation of (−201) β-Ga2O3||(001) diamond and [010]/[−13–2] β-Ga2O3 ||[110]/[1–10] diamond, with the presence of different crystallographically related epitaxial variants apparent from selected area diffraction patterns. A model explaining the arrangement of atoms along ⟨110⟩ diamond is demonstrated with a lattice mismatch of 1.03–3.66% in the perpendicular direction. Dark field imaging showed evidence of arrays of discrete defects at the boundaries between different grains. Strategies to reduce the density of defects are discussed.

We report an investigation of the bulk optical, bulk acoustic, and surface acoustic phonons in thin films of turbostratic boron nitride (t-BN) and cubic boron nitride (c-BN) grown on B-doped polycrystalline and single-crystalline diamond (001) and (111) substrates. The characteristics of different types of phonons were studied using Raman and Brillouin-Mandelstam light scattering spectroscopies. The atomic structure of the films was determined using high-resolution transmission electron microscopy (HRTEM) and correlated with the Raman and Brillouin-Mandelstam spectroscopy data. The HRTEM analysis revealed that the cubic boron nitride thin films consisted of a mixture of c-BN and t-BN phases, with c-BN being the dominant phase. It was found that while visible Raman spectroscopy provided information for characterizing the t-BN phase, it faced challenges in differentiating the c-BN phase either due to the presence of high-density defects or the overlapping of the Raman features with those from the B-doped diamond substrates. In contrast, Brillouin-Mandelstam spectroscopy clearly distinguishes the bulk longitudinal and surface acoustic phonons of the c-BN thin films grown on diamond substrates. Additionally, the angle-dependent surface Brillouin-Mandelstam scattering data show the peaks associated with the Rayleigh surface acoustic waves, which have higher phase velocities in c-BN films on diamond (111) substrates. These findings provide valuable insights into the phonon characteristics of the c-BN and diamond interfaces and have important implications for the thermal management of electronic devices based on ultra-wide-band-gap materials.

Temperature dependence of the low-frequency electronic noise in NiOx/β-Ga2O3 p–n heterojunction diodes is reported. The noise spectral density is of the 1/f-type near room temperature but shows signatures of Lorentzian components at elevated temperatures and at higher current levels (f is the frequency). It is observed that there is an intriguing non-monotonic dependence of the noise on temperature near T = 380 K. The Raman spectroscopy of the device structure suggests material changes, which results in reduced noise above this temperature. The normalized noise spectral density in such diodes is determined to be on the order of 10−14 cm2 Hz−1 (f = 10 Hz) at 0.1 A cm−2 current density. In terms of the noise level, NiOx/β-Ga2O3 p–n diodes perform excellently for new technology and occupy an intermediate position among devices of various designs implemented with different ultra-wide-bandgap semiconductors. The obtained results are important for understanding the electronic properties of NiOx/β-Ga2O3 heterojunctions and contribute to the development of noise spectroscopy as the quality assessment tool for new electronic materials and device technologies.

We demonstrate threshold switching behaviors with working temperatures up to 500 ∘ C based on GaN vertical p-n diodes, and these devices survived a passive test in a simulated Venus environment (460 ∘ C, 94 bar, CO 2 gas flow) for ten days. This is realized via interface engineering through an etch-then-regrow process combination with a Ga 2 O 3 interlayer. It is hypothesized the traps in the interfacial layer can form/rupture a conductive path by trapping/detrapping electrons/holes, which are responsible for the observed threshold switching behaviors. To the best of our knowledge, this is the first demonstration of two-terminal threshold-switching memory devices under such high temperatures. These results can serve as a critical reference for the future development of GaN-based memory devices for harsh environment applications.

III-nitride InGaN material is an ideal candidate for the fabrication of high performance photovoltaic (PV) solar cells, especially for high-temperature applications. Over the past decade, significant efforts have been made to improve the PV performance of InGaN-based solar cells. In this paper, we perform a comprehensive review of the recent developments in InGaN-based solar cells. The topics of discussion include theoretical modeling, material epitaxy, device engineering, and high-temperature measurement. Particularly, we highlight subjects such as substrate technology, and properties that are unique to InGaN materials such as polarization control and their positive thermal coefficient. To date, outstanding high-temperature InGaN-based solar cells with quantum efficiency approaching 80% at 450 °C have been demonstrated. Future innovations in epitaxy science, device engineering, and integration methods are required to further advance the efficiency and expand the applications of InGaN-based solar cells.

This work demonstrated a comprehensive design and modeling of enhancement-mode (E-mode) beta-phase gallium oxide ( β -Ga2O3) current-aperture vertical electron transistors (CAVETs) using TCAD simulation. A new β -Ga2O3 CAVET [i.e., high electron mobility transistors (HEMT)-CAVETs] was proposed by introducing delta-doped β -(AlxGa 1−x )2O3/Ga2O3 ( x=0.2 ) heterostructure into the conventional CAVETs. The HEMT-CAVETs showed significant improvements in electron modulation, ON-state resistance (RON), and leakage due to the confined two-dimensional electron gas (2DEG) channel. The design space of doping, channel length ( Lch ), and aperture length ( Lap ) on the device threshold voltages ( VTH ), RON, breakdown voltage (BV), and OFF-state leakage were systematically investigated. With increasing delta-doping concentration, the HEMT-CAVETs showed smaller RON compared with the conventional CAVETs. The OFF-state leakage from the aperture and the current blocking layers (CBLs) in the β -Ga2O3 CAVETs was studied for the first time. Larger Lch prevented the OFF-state leakage current from the aperture and increased BV, but it also increased RON due to larger channel resistance. Small Lap could dramatically increase device RON due to the encroachment of the aperture by the depletion regions from the CBLs. The CBL breakdown played a key role in the device BV. The BV of the CBL increased with increasing CBL thickness and acceptor doping concentration, where kV-level BV and high peak electric fields of ∼8 MV/cm can be obtained with optimized CBLs. These results can serve as a critical reference for the future development of kV-class low RON β -Ga2O3 CAVETs for high-power, high-voltage, and high-frequency applications.

NiOx/β-Ga2O3p-n heterojunctions fabricated on  and β-Ga2O3 substrates show distinctly anisotropic electrical properties. All three devices exhibited excellent rectification ≥109, and turn-on voltages >2.0 V. The  device showed very different turn-on voltage, specific on-resistance, and reverse recovery time compared with  and  devices. Moreover, it is calculated that the interface trap state densities for  and  plane devices are 4.3 × 1010, 7.4 × 1010, and 1.6 × 1011 eV–1cm–2, respectively. These differences in the NiOx/β-Ga2O3 heterojunctions are attributed to the different atomic configurations, the density of dangling bonds, and interface trap state densities.

Two atomic layer etching (ALE) methods were studied for crystalline GaN, based on oxidation, fluorination, and ligand exchange. Etching was performed on unintentionally doped GaN grown by hydride vapor phase epitaxy. For the first step, the GaN surfaces were oxidized using either water vapor or remote O2-plasma exposure to produce a thin oxide layer. Removal of the surface oxide was addressed using alternating exposures of hydrogen fluoride (HF) and trimethylgallium (TMG) via fluorination and ligand exchange, respectively. Several HF and TMG super cycles were implemented to remove the surface oxide. Each ALE process was monitored in situ using multiwavelength ellipsometry. X-ray photoelectron spectroscopy was employed for the characterization of surface composition and impurity states. Additionally, the thermal and plasma-enhanced ALE methods were performed on patterned wafers and transmission electron microscopy (TEM) was used to measure the surface change. The x-ray photoelectron spectroscopy measurements indicated that F and O impurities remained on etched surfaces for both ALE processes. Ellipsometry indicated a slight reduction in thickness. TEM indicated a removal rate that was less than predicted. We suggest that the etch rates were reduced due to the ordered structure of the oxide formed on crystalline GaN surfaces.

This Letter reports the performance of vertical GaN-on-GaN p–n diodes with etch-then-regrown p-GaN after exposure to a simulated Venus environment (460 °C, ∼94 bar, containing CO2/N2/SO2 etc., atmosphere) for over 10 days, and compared them to the performance of GaN p–n diodes without the etch-then-regrow process. After the above-mentioned Venus test, temperature-dependent I–V and microscopy investigation were conducted to study the robustness of etch-then-regrow p-GaN and vertical GaN p–n diodes under harsh environments and operation up to 500 °C. p-electrode degradation is found to be the main issue of the device's performance. This is the highest temperature at which such characterization has been conducted for vertical GaN p–n diodes, therefore establishing a critical reference for the development of p-GaN regrown and vertical GaN-based electronics for extreme environments.

In this work, we demonstrate the high performance of β-Ga2O3 metal–insulator–semiconductor (MIS) diodes. An ultrathin boron nitride (BN) interlayer is directly grown on the Ga2O3 substrate by pulsed laser deposition. X-ray photoelectron spectroscopy, Raman spectroscopy, and high-resolution transmission electron microscopy confirm the existence of a 2.8 nm BN interlayer. Remarkably, with the insertion of the ultrathin BN layer, the breakdown voltage is improved from 732 V for Ga2O3 Schottky barrier diodes to 1035 V for Ga2O3 MIS diodes owing to the passivated surface-related defects and reduced reverse leakage currents. Our approach shows a promising way to improve the breakdown performance of Ga2O3-based devices for next-generation high-power electronics.

Temperature dependence of the low-frequency electronic noise in NiOx/β-Ga2O3 p–n heterojunction diodes is reported. The noise spectral density is of the 1/f-type near room temperature but shows signatures of Lorentzian components at elevated temperatures and at higher current levels (f is the frequency). It is observed that there is an intriguing non-monotonic dependence of the noise on temperature near T = 380 K. The Raman spectroscopy of the device structure suggests material changes, which results in reduced noise above this temperature. The normalized noise spectral density in such diodes is determined to be on the order of 10−14 cm2 Hz−1 (f = 10 Hz) at 0.1 A cm−2 current density. In terms of the noise level, NiOx/β-Ga2O3 p–n diodes perform excellently for new technology and occupy an intermediate position among devices of various designs implemented with different ultra-wide-bandgap semiconductors. The obtained results are important for understanding the electronic properties of NiOx/β-Ga2O3 heterojunctions and contribute to the development of noise spectroscopy as the quality assessment tool for new electronic materials and device technologies.

The room temperature growth of two-dimensional van der Waals (2D-vdW) materials is indispensable for state-of-the-art nanotechnology. Low temperature growth supersedes the requirement of elevated growth temperatures accompanied with high thermal budgets. Moreover, for electronic applications, low or room temperature growth reduces the possibility of intrinsic film-substrate interfacial thermal diffusion related deterioration of the functional properties and the consequent deterioration of the device performance. Here, we demonstrated the growth of ultrawide-bandgap boron nitride (BN) at room temperature by using the pulsed laser deposition (PLD) process, which exhibited various functional properties for potential applications. Comprehensive chemical, spectroscopic and microscopic characterizations confirmed the growth of ordered nanosheet-like hexagonal BN (h-BN). Functionally, the nanosheets show hydrophobicity, high lubricity (low coefficient of friction), and a low refractive index within the visible to near-infrared wavelength range, and room temperature single-photon quantum emission. Our work unveils an important step that brings a plethora of potential applications for these room temperature grown h-BN nanosheets as the synthesis can be feasible on any given substrate, thus creating a scenario for “h-BN on demand” under a frugal thermal budget.

Understanding the emergent electronic structure in twisted atomically thin layers has led to the exciting field of twistronics. However, practical applications of such systems are challenging since the specific angular correlations between the layers must be precisely controlled and the layers have to be single crystalline with uniform atomic ordering. Here, an alternative, simple, and scalable approach is suggested, where nanocrystallinetwo-dimensional (2D) film on 3D substrates yields twisted-interface-dependent properties. Ultrawide-bandgap hexagonal boron nitride (h-BN) thin films are directly grown on high in-plane lattice mismatched wide-bandgap silicon carbide (4H-SiC) substrates to explore the twist-dependent structure-property correlations. Concurrently, nanocrystalline h-BN thin film shows strong non-linear second-harmonic generation and ultra-low cross-plane thermal conductivity at room temperature, which are attributed to the twisted domain edges between van der Waals stacked nanocrystals with random in-plane orientations. First-principles calculations based on time-dependent density functional theory manifest strong even-order optical nonlinearity in twisted h-BN layers. This work unveils that directly deposited 2D nanocrystalline thin film on 3D substrates could provide easily accessible twist-interfaces, therefore enabling a simple and scalable approach to utilize the 2D-twistronics integrated in 3D material devices for next-generation nanotechnology.

Gallium nitride (GaN) multiple quantum well (MQW) solar cells proved to have very good performance in high-temperature conditions and under intense excitation. However, the long-term reliability under harsh conditions has not been investigated in the literature. The aim of this article is to fill this gap, by investigating the degradation mechanisms of GaN solar cells, submitted to forward current stress. The cells were characterized by means of dark and illuminated current–voltage ( I – V ) measurements, capacitance–voltage ( C – V ) measurements, and steady-state photocapacitance (SSPC). The current step-stress experiment showed an initial decrease in the main parameters of the devices (open-circuit voltage, external quantum efficiency (EQE), and optical-to-electrical power conversion efficiency). C – V and SSPC showed a correlation between the charge inside the active region of the device and the concentration of trap states. Also, a relation was found between the decrease in power conversion efficiency and the amount of charge in the active region of the devices. Degradation was ascribed to a redistribution of the charge in the active region, related to an increase in the density of midgap states ( EC−1.6 eV), resulting in the lowering of the efficiency of the devices.