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

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

Band alignment at the BaCuSeF/ZnTe interface

In situ photoemission spectroscopy experiments are used to characterize the interface between ZnTe and the wide band gap p-type semiconductor BaCuSeF. The contact is characterized by a null valence-band offset, a large conduction-band offset, and a chemically graded interface. By applying the transitivity rule for band offset and on the basis of similarities in chemical composition, BaCuSeF contact to chalcogenide photovoltaic absorber materials would be expected to have similar properties. By extension, BaCuChF (Ch=S,Se,Te) materials are suitable as p-layers in p-i-n double-heterojunction solar cells fabricated with CdTe, Cu(InGa)Se₂, and Cu₂ZnSnS₄ absorbers.This is the publisher’s final pdf. The published article is copyrighted by the American Institute of Physics and can be found at: http://scitation.aip.org/content/aip/journal/apl.Article Copyright (2010) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics

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

Origin of p-type conduction in single-crystal CuAlO₂

We report measurements of the structural, optical, transport, and magnetic properties of single crystals of the anisotropic p-type transparent semiconductor CuAlO₂. The indirect and direct band gaps are 2.97 and 3.47 eV, respectively. Temperature-dependent Hall measurements yield a positive Hall coefficient in the measured range and an activated carrier temperature dependence. The resistivity is anisotropic, with the ab-plane resistivity about 25 times smaller than the c-axis resistivity at room temperature. Both are activated with similar activation energies. The room-temperature ab-plane mobility is relatively large at 3 cm² V⁻¹ s⁻¹, and we infer a c-axis mobility of 0.12 cm² V⁻¹ s⁻¹. The Seebeck coefficient is positive at all measured temperatures, and has a T⁻¹ dependence over most of the measured range. The low-temperature paramagnetic moment is consistent with a spin-1/2 defect with a density of 3.4 X 10²⁰ cm⁻³. These results suggest that the conduction mechanism for p-type carriers in CuAlO₂ is charge transport in the valence band and that the holes are thermally activated from copper-vacancy acceptor states located about 700 meV above the valence-band maximum