103 research outputs found
Reliable operation of CrO:Mg/ -GaO p-n heterojunction diodes at 600C
-GaO-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 CrO:Mg/ -GaO p-n heterojunction diodes at
during extended operation at 600C, as well as after 30 repeated cycles
between 25-550C. The calculated pO2-temperature phase stability diagram
of the Ga-Cr-O material system predicts that GaO and CrO should
remain thermodynamically stable in contact with each other over a wide range of
oxygen pressures and operating temperatures. The fabricated CrO:Mg /
-GaO p-n heterojunction diodes show room-temperature on/off
ratios >10 at 5V and a breakdown voltage (V) of -390V. The
leakage current increases with increasing temperature up to 600C, 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 600C, both the device
turn-on voltage and on-state resistance increase from 1.08V and 5.34
m-cm to 1.59V and 7.1 m-cm respectively. This increase
is attributed to the accumulation of Mg and MgO at the CrO/GaO
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
-GaO based devices.Comment: 17 pages, 4 figure
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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 -GaO Devices
Beta gallium oxide (-GaO) 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 (300-400C) due to a combination of
non-stoichiometric Ti/GaO interfacial reactions and kinetically favored
Ti diffusion processes. Here we demonstrate stable Ohmic contacts for
GaO devices operating up to 500-600C 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
600C in vacuum ( 10 Torr), as well as after repeated
thermal cycling (15 times) between room temperature and 550C in
flowing N. 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 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 that consists of alternating layers of rocksalt-like
[MgN] octahedra and nickeline-like [WN] trigonal prisms (denoted
"rocksaline"). Thermodynamic calculations corroborate these observations,
showing rocksaline MgWN 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 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 and MgWN,
hexagonal boron nitride MgWN, and rocksaline MgWN), these findings
highlight how rapid thermal annealing can control polymorphic transformations,
adding a new strategy for exploration of thermodynamic stability in uncharted
phase spaces
NiGaO interfacial layers in NiO/GaO heterojunction diodes at high temperature
NiO/GaO 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-GaOheterojunction diodes and the formation of
NiGaO interfacial layers when operated at 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-GaO interface. High-resolution TEM microstructure analysis of
the devices after thermal cycling indicates that the NiO-GaO
interface forms ternary compounds at high temperatures, and thermodynamic
calculations suggest the formation of the spinel NiGaO layer
between NiO and GaO. First-principles defect calculations find that
NiGaO shows low p-type intrinsic doping, and hence can also serve
to limit electric field crowding at the interface. Vertical NiO/GaO
diodes with intentionally grown 5 nm thin spinel-type NiGaO
interfacial layers show excellent device ON/OFF ratio of > 10(3 V),
V 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
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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
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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
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