Reliable operation of Cr2_2O3_3:Mg/ β\beta-Ga2_2O3_3 p-n heterojunction diodes at 600∘^\circC

$\beta$-Ga$_2$O$_3$-based semiconductor heterojunctions have recently demonstrated improved performance at high voltages and elevated temperatures and are thus promising for applications in power electronic devices and harsh-environment sensors. However, the long-term reliability of these ultra-wide band gap (UWBG) semiconductor devices remains barely addressed and may be strongly influenced by chemical reactions at the p-n heterojunction interface. Here, we experimentally demonstrate operation and evaluate the reliability of Cr$_2$O$_3$:Mg/ $\beta$-Ga$_2$O$_3$ p-n heterojunction diodes at during extended operation at 600$^\circ$C, as well as after 30 repeated cycles between 25-550$^\circ$C. The calculated pO2-temperature phase stability diagram of the Ga-Cr-O material system predicts that Ga$_2$O$_3$ and Cr$_2$O$_3$ should remain thermodynamically stable in contact with each other over a wide range of oxygen pressures and operating temperatures. The fabricated Cr$_2$O$_3$:Mg / $\beta$-Ga$_2$O$_3$ p-n heterojunction diodes show room-temperature on/off ratios >10$^4$ at $\pm$5V and a breakdown voltage (V$_{Br}$) of -390V. The leakage current increases with increasing temperature up to 600$^\circ$C, which is attributed to Poole-Frenkel emission with a trap barrier height of 0.19 eV. Over the course of a 140-hour thermal soak at 600$^\circ$C, both the device turn-on voltage and on-state resistance increase from 1.08V and 5.34 m$\Omega$-cm$^2$ to 1.59V and 7.1 m$\Omega$-cm$^2$ respectively. This increase is attributed to the accumulation of Mg and MgO at the Cr$_2$O$_3$/Ga$_2$O$_3$ interface as observed from TOF-SIMS analysis. These findings inform future design strategies of UWBG semiconductor devices for harsh environment operation and underscore the need for further reliability assessments for $\beta$-Ga$_2$O$_3$ based devices.Comment: 17 pages, 4 figure

Defect physics of BaCuChF (Ch=S, Se, Te) p-type transparent conductors

Native point defects, defect complexes, and oxygen impurities in BaCuChF were studied using density functional theory calculations, self-consistent thermodynamic simulations, and various experimental techniques. Unintentional p-type conductivity in BaCuChF is explained by the presence of copper vacancies with transition levels in the valence band. These acceptor-like defects are partially compensated by donor-like chalcogen vacancies with transition levels deep in the gap. Chalcogen vacancies also cause the experimentally observed subgap photoluminescence, optical absorption, and persistent photoconductivity in BaCuSF and BaCuSeF. In thermodynamic equilibrium, both copper and chalcogen vacancies have low formation enthalpies and are likely to form defect complexes among themselves and with fluorine interstitials. The calculated Fermi level pinning range in BaCuChF is narrow and located close to the valence band maximum. It makes BaCuChF a suitable transparent p-type contact layer for optoelectronic applications but hinders attempts to fabricate transparent thin-film transistors using this material. Oxygen-related defects do not affect bulk BaCuChF properties but surface oxidation decreases the mean free path of free holes by almost an order of magnitude

Ultrathin Stable Ohmic Contacts for High-Temperature Operation of β\beta-Ga2_2O3_3 Devices

Beta gallium oxide ($\beta$-Ga$_2$O$_3$) shows significant promise in the high-temperature, high-power, and sensing electronics applications. However, long-term stable metallization layers for Ohmic contacts at high temperature present unique thermodynamic challenges. The current most common Ohmic contact design based on 20 nm of Ti has been repeatedly demonstrated to fail at even moderately elevated temperatures (300-400$^{\circ}$C) due to a combination of non-stoichiometric Ti/Ga$_2$O$_3$ interfacial reactions and kinetically favored Ti diffusion processes. Here we demonstrate stable Ohmic contacts for Ga$_2$O$_3$ devices operating up to 500-600$^{\circ}$C using ultrathin Ti layers with a self-limiting interfacial reaction. The ultrathin Ti layer in the 5nm Ti / 100nm Au contact stack is designed to fully oxidize while forming an Ohmic contact, thereby limiting both thermodynamic and kinetic instability. This novel contact design strategy results in an epitaxial conductive anatase titanium oxide interface layer that enables low-resistance Ohmic contacts that are stable both under long-term continuous operation (>500 hours) at 600$^{\circ}$C in vacuum ($\leq$ 10$^{-4}$ Torr), as well as after repeated thermal cycling (15 times) between room temperature and 550$^{\circ}$C in flowing N$_2$. This stable Ohmic contact design will accelerate the development of high-temperature devices by enabling research focus to shift towards rectifying contacts and other interfacial layers.Comment: 25 Pages, 7 Figure

Bulk and film synthesis pathways to ternary magnesium tungsten nitrides

Bulk solid state synthesis of nitride materials usually leads to thermodynamically stable, cation-ordered crystal structures, whereas thin film synthesis tends to favor disordered, metastable phases. This dichotomy is inconvenient both for basic materials discovery, where non-equilibrium thin film synthesis methods can be useful to overcome reaction kinetic barriers, and for practical technology applications where stable ground state structures are sometimes required. Here, we explore the uncharted Mg-W-N chemical phase space, using rapid thermal annealing to reconcile the differences between thin film and bulk powder syntheses. Combinatorial co-sputtering synthesis from Mg and W targets in a N$_2$ environment yielded cation-disordered Mg-W-N phases in the rocksalt (0.1< Mg/(Mg+W) <0.9), and hexagonal boron nitride (0.7< Mg/(Mg+W) <0.9) structure types. In contrast, bulk synthesis produced a cation-ordered polymorph of MgWN$_2$ that consists of alternating layers of rocksalt-like [MgN$_6$] octahedra and nickeline-like [WN$_6$] trigonal prisms (denoted "rocksaline"). Thermodynamic calculations corroborate these observations, showing rocksaline MgWN$_2$ is stable while other polymorphs are metastable. We also show that rapid thermal annealing can convert disordered rocksalt films to this cation-ordered polymorph near the MgWN$_2$ stoichiometry. Electronic structure calculations suggest that this rocksalt-to-rocksaline structural transformation should also drive a metallic-to-semiconductor transformation. In addition to revealing three new phases (rocksalt MgWN$_2$ and Mg$_3$WN$_4$, hexagonal boron nitride Mg$_3$WN$_4$, and rocksaline MgWN$_2$), these findings highlight how rapid thermal annealing can control polymorphic transformations, adding a new strategy for exploration of thermodynamic stability in uncharted phase spaces

NiGa2_{2}O4_{4} interfacial layers in NiO/Ga2_{2}O3_{3} heterojunction diodes at high temperature

NiO/Ga$_{2}$O$_{3}$ heterojunction diodes have attracted attention for high-power applications, but their high-temperature performance and reliability remain underexplored. Here we report on the time evolution of the static electrical properties in the widely studied p-NiO/n-Ga$_{2}$O$_{3}$heterojunction diodes and the formation of NiGa$_{2}$O$_{4}$ interfacial layers when operated at $550^{\circ}$C. Results of our thermal cycling experiment show an initial leakage current increase which stabilizes after sustained thermal load, due to reactions at the NiO-Ga$_{2}$O$_{3}$ interface. High-resolution TEM microstructure analysis of the devices after thermal cycling indicates that the NiO-Ga$_{2}$O$_{3}$ interface forms ternary compounds at high temperatures, and thermodynamic calculations suggest the formation of the spinel NiGa$_{2}$O$_{4}$ layer between NiO and Ga$_{2}$O$_{3}$. First-principles defect calculations find that NiGa$_{2}$O$_{4}$ shows low p-type intrinsic doping, and hence can also serve to limit electric field crowding at the interface. Vertical NiO/Ga$_{2}$O$_{3}$ diodes with intentionally grown 5 nm thin spinel-type NiGa$_{2}$O$_{4}$ interfacial layers show excellent device ON/OFF ratio of > 10$^{10}$($\pm$3 V), V$_{ON}$ of ~1.9 V, and breakdown voltage of ~ 1.2 kV for an initial unoptimized 300-micron diameter device. These p-n heterojunction diodes are promising for high-voltage, high-temperature applications.Comment: 16 pages, 5 figure

Electronic structure and excitonic absorption in BaCuChF (Ch=S, Se, and Te)

Double excitonic absorption peaks are observed in textured BaCuSF and BaCuSeF thin films. The excitonic doublet separation increases with increasing fraction of heavy chalcogen in the thin-film solid solutions, in good agreement with the spin-orbit splitting of the valence bands calculated by density-functional theory. In BaCuSF and BaCuSeF, the excitons have large binding energies (95 and 65 meV, respectively) and can be observed at room temperature. A three-dimensional Wannier-Mott excitonic absorption model gives good agreement between the experimental and theoretical optical properties. Band gaps of BaCuSF and BaCuSeF calculated using the GW approximation agree with experiment. In BaCuTeF, transitions across the lowest direct energy gap and excitonic absorption are suppressed, extending its transparent range

Prediction and realisation of high mobility and degenerate p type conductivity in CaCuP thin films

Phosphides are interesting candidates for hole transport materials and p type transparent conducting applications, capable of achieving greater valence band dispersion than their oxide counterparts due to the higher lying energy and increased size of the P 3p orbital. After computational identification of the indirect gap semiconductor CaCuP as a promising candidate, we now report reactive sputter deposition of phase pure p type CaCuP thin films. Their intrinsic hole concentration and hole mobility exceed 1 1020 cm amp; 8722;3 and 35 cm2 V amp; 8722;1 s amp; 8722;1 at room temperature, respectively. Transport calculations indicate potential for even higher mobilities. Copper vacancies are identified as the main source of conductivity, displaying markedly different behaviour compared to typical p type transparent conductors, leading to improved electronic properties. The optical transparency of CaCuP films is lower than expected from first principles calculations of phonon mediated indirect transitions. This discrepancy could be partly attributed to crystalline imperfections within the films, increasing the strength of indirect transitions. We determine the transparent conductor figure of merit of CaCuP films as a function of composition, revealing links between stoichiometry, crystalline quality, and opto electronic properties. These findings provide a promising initial assessment of the viability of CaCuP as a p type transparent contac

Boron Phosphide Films by Reactive Sputtering Searching for a P Type Transparent Conductor

With an indirect band gap in the visible and a direct band gap at a much higher energy, boron phosphide BP holds promise as an unconventional p type transparent conductor. This work reports on reactive sputtering of amorphous BP films, their partial crystallization in a P containing annealing atmosphere, and extrinsic doping by C and Si. The highest hole concentration to date for p type BP 5 1020 cm amp; 8722;3 is achieved using C doping under B rich conditions. Furthermore, bipolar doping is confirmed to be feasible in BP. An anneal temperature of at least 1000 C is necessary for crystallization and dopant activation. Hole mobilities are low and indirect optical transitions are stronger than that predicted by theory. Low crystalline quality probably plays a role in both cases. High figures of merit for transparent conductors might be achievable in extrinsically doped BP films with improved crystalline qualit