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The low thermal conductivity of β-Ga2O3 is a significant concern for maximizing the potential of this ultra-wide bandgap semiconductor as a power switching device technology. Here, we report on the use of nanocrystalline diamond (NCD) deposited via microwave plasma enhanced chemical vapor deposition (MP-CVD) as a top-side, device-level thermal management solution on a lateral β-Ga2O3 transistor. NCD was grown via MP-CVD on β-(AlxGa1−x)2O3/β-Ga2O3 heterostructures prior to the gate formation of the field-effect transistor. A reduced growth temperature of 400 °C and a SiNx barrier layer were used to protect the oxide semiconductors from etching in the MP-CVD H2 plasma environment. Raman spectroscopy showed a highly sp3-bonded NCD film was obtained at 400 °C, with grain size of about 50–100 nm imaged via atomic force microscopy. The incorporation of the NCD heat-spreading layer resulted in a β-(AlxGa1−x)2O3/β-Ga2O3 heterostructure field-effect transistor showing a decrease in the total thermal resistance at the gate by 42%. The fabrication process, including the NCD etch in the gate region, will need to be improved to minimize the impact of these processes on important device characteristics (i.e., drain current, threshold voltage, and leakage current). [ABSTRACT FROM AUTHOR]

The effect of heteroepitaxial β-(AlxGa1−x)2O3 film thickness and Al content on surface morphology was characterized to experimentally determine the critical thickness limitations of the (010) β-(AlxGa1−x)2O3/Ga2O3 heterostructure. High-resolution x-ray diffraction was used to assess the strain state of the films; reciprocal space mapping (RSM) revealed that even cracked films were still fully strained. In cracked films, diffuse scattering was observed in RSMs, indicating lattice tilting. Cracking of the films was investigated using atomic force microscopy (AFM), x-ray topography (XRT), bright-field scanning transmission electron microscopy (BF-STEM), and high-resolution transmission electron microscopy. Using both AFM and XRT, the [001] direction was observed to be the most prevalent crack direction; however, cracks were also observed in the [100] direction. In uncracked regions of the films, XRT revealed the alignment of threading dislocations along the [001] direction. Cross-sectional imaging of the crack geometry and propagation was performed using BF-STEM, and it was observed that the cracks in the [001] direction extended through the thickness of the β-(AlxGa1−x)2O3 film (∼205 nm) and a further ∼100–200 nm into the β-Ga2O3 substrate. Experimental data for critical film thickness showed good agreement with previous theoretical calculations based on the Griffith criterion for crack propagation. [ABSTRACT FROM AUTHOR]

Codoping of gallium nitride for improved acceptor ionization has long been theorized; however, reduction to practice proves difficult via growth. Herein, implementation of codoping via ion implantation and symmetric multicycle rapid thermal annealing utilizing magnesium codoped with silicon or oxygen is demonstrated. Results show enhanced photoluminescence with both donor species but with an order of magnitude greater increase with concurrent p‐type hall for codoping with oxygen. Furthermore, the addition of nitrogen to balance stoichiometry suppresses defect photoluminescence signals. The incorporation of the donor and nitrogen demonstrates defect reduction beyond magnesium, only implants despite the additional implant dose and resultant damage with coimplantation. The enhanced hole concentrations evident with oxygen incorporation reveal important considerations for device design given unintentional doping during growth and future incorporation of ion implantation capabilities. [ABSTRACT FROM AUTHOR]

Selective area doping via ion implantation is crucial to the implementation of most modern devices and the provision of reasonable device design latitude for optimization. Herein, we report highly effective silicon ion implant activation in GaN via Symmetrical Multicycle Rapid Thermal Annealing (SMRTA) at peak temperatures of 1450 to 1530 °C, producing a mobility of up to 137 cm2/Vs at 300K with a 57% activation efficiency for a 300 nm thick 1 × 1019 cm−3 box implant profile. Doping activation efficiency and mobility improved alongside peak annealing temperature, while the deleterious degradation of the as-grown material electrical properties was only evident at the highest temperatures. This demonstrates efficient dopant activation while simultaneously maintaining low levels of unintentional doping and thus a high blocking voltage potential of the drift layers for high-voltage, high-power devices. Furthermore, efficient activation with high mobility has been achieved with GaN on sapphire, which is known for having relatively high defect densities but also for offering significant commercial potential due to the availability of cheap, large-area, and robust substrates for devices. [ABSTRACT FROM AUTHOR]

This paper reviews some of the basic issues in homoepitaxial growth of III-nitrides to enable a vertical device technology. It focuses on the use of metal organic chemical vapor deposition (MOCVD) to grow GaN and explores the effects of the native substrate characteristics on material quality, interface composition, and device performance. A review of theoretical work understanding dopants in the ultra-wide III-nitride semiconductors, AlN and BN, is also included for future efforts expanding the technology into those materials. [ABSTRACT FROM AUTHOR]

Plasma-enhanced atomic layer deposition (PEALD) enables the epitaxial growth of ultrathin indium nitride (InN) films at significantly reduced process temperatures and with greater control of layer thickness compared to other growth methods. However, the reliance on plasma-surface interactions increases the complexity of the growth process. A detailed understanding of the relationship between the plasma properties and the growth kinetics is therefore required to guide the tuning of growth parameters. We present an in situ investigation of the early-stage PEALD growth kinetics of epitaxial InN within three different plasma regimes using grazing incidence small-angle x-ray scattering (GISAXS). The GISAXS data are supported by diagnostic studies of the plasma species generation in the inductively coupled plasma source as a function of the relative concentrations of the nitrogen/argon gas mixture used in the growth process. The growth mode is found to be correlated to the production of nitrogen species in the plasma, with high concentrations of the atomic N species promoting Volmer–Weber growth (i.e., island growth) and low concentrations promoting Stranski–Krastanov growth (i.e., layer-plus-island growth). The critical thickness for island formation, island center-to-center distance, and island radius are found to increase with ion flux. Furthermore, the island center-to-center distance and areal density are observed to change only during plasma exposure and to continue changing with exposure even after the methylindium adlayer is believed to have fully reacted with the plasma. Our results demonstrate the potential to control the growth kinetics during PEALD of epitaxial films by intentionally accessing specific regimes of plasma species generation. [ABSTRACT FROM AUTHOR]

Metal-assisted chemical etching is a plasma-free open-circuit anisotropic etching method that produces high aspect ratio structures in various semiconductors. Here, for the first time, we demonstrate the formation of ordered micropillar arrays of homoepitaxial GaN, using photo-enhanced MacEtch with patterned platinum films as the catalyst. The GaN etching rate and morphology as a function of etching chemistry, growth method, and doping conditions are investigated, and the etch mechanism is analyzed. Etch rates and surface smoothness are found to increase with the Si-doping level in GaN, approaching those achieved by reactive ion etching and photoelectrochemical etching. Spatially resolved photoluminescence shows no degradation in near band edge emission and no newly generated defect peaks, as expected due to the high energy ion free nature. This approach can also potentially be applied to InGaN and AlGaN by tuning the etch chemistry and illumination wavelength, enabling a facile and scalable processing of 3D III-nitride based electronic and optoelectronic devices such as μLEDs and finFETs. [ABSTRACT FROM AUTHOR]

In light of the importance of selective area doping in GaN to enable planar process technology, and to avoid the complications from the etch/regrowth process, ion implantation is the recognizable alternative. Annealing to activate dopant species and repair the damage to a crystal poses a challenge for GaN since the material will decompose to Ga + N2 at atmospheric pressure and relevant temperatures. In this research, in situ high- and low-temperature epitaxial and ex situ sputtered AlN caps were examined in different stacking arrangements to study the optimum conditions for Mg ion implantation and activation. Concurrently, a matrix of different implantation doses was also investigated to better understand the dose-dependent activation. Each sample has a unique cap stack and four different implant doses, including an unimplanted reference quadrant. The results show that poorer quality cap films enable nitrogen to leave the crystal during annealing and leave nitrogen vacancies behind. Furthermore, a high dose is needed at the surface to facilitate ohmic contact formation. The results suggest that in situ epitaxial-grown AlN caps are more suitable for GaN activation annealing, and high-temperature thin caps provide the best barrier to prevent crystal disintegration. We reveal a timely strategy for preserving the quality of GaN crystal structure during the electrical activation of the ion-implanted Mg atoms. This work provides valuable information that bridges the gap between device processing and electrical characterization of GaN devices, presenting a clear path towards achieving an electrical activation of implanted Mg while maintaining the integrity of the crystalline structure of GaN. [ABSTRACT FROM AUTHOR]

Measuring the thermal conductivity of sub-surface buried substrates is of significant practical interests. However, this remains challenging with traditional pump–probe spectroscopies due to their limited thermal penetration depths. Here, we experimentally and numerically investigate the TPD of the recently developed optical pump–probe technique steady-state thermoreflectance (SSTR) and explore its capability for measuring the thermal properties of buried substrates. The conventional definition of the TPD (i.e., the depth at which temperature drops to 1/e value of the maximum surface temperature) does not truly represent the upper limit of how far beneath the surface SSTR can probe. For estimating the uncertainty of SSTR measurements of a buried substrate a priori, sensitivity calculations provide the best means. Thus, detailed sensitivity calculations are provided to guide future measurements. Due to the steady-state nature of SSTR, it can measure the thermal conductivity of buried substrates that are traditionally challenging by transient pump–probe techniques, exemplified by measuring three control samples. We also discuss the required criteria for SSTR to isolate the thermal properties of a buried film. Our study establishes SSTR as a suitable technique for thermal characterizations of sub-surface buried substrates in typical device geometries. [ABSTRACT FROM AUTHOR]

Present GaN technology consists primarily of heteroepitaxial, lateral high electron mobility transistors; however, high-power devices would be much more efficiently manufactured with a vertical geometry due to better blocking voltage scaling. This technology has yet to be realized due to the inconsistency of GaN wafer properties. These inconsistencies can be detected with several long-range, non-destructive techniques including Raman spectroscopy, optical profilometry, and photoluminescence mapping. In particular, Raman spectroscopy is an effective tool for determining the carrier concentration of GaN substrates and whether the wafers are uniform or inhomogeneous. In this work, vertical p-i-n GaN diodes are fabricated using both uniform and inhomogeneous wafers determined by using the A1 Raman peak position to monitor carrier concentration. The inhomogeneous samples have regular patterns of varying conductivity as a result of electron-donating defects brought about by changes in crystal stress. By avoiding these defects, we have found that diode performance improves by increasing the +/− 10 V rectification ratio by up to a factor of 5X and reducing the 200 V reverse bias leakage current by several orders of magnitude. Using the higher quality substrates with uniform electron carrier concentrations can improve the rectification ratio by another order of magnitude. [ABSTRACT FROM AUTHOR]

The first step of a multi-cycle rapid thermal annealing process was systematically studied. The surface, structure, and optical properties of Mg implanted GaN thin films annealed at temperatures ranging from 900 to 1200 °C were investigated by Raman spectroscopy, photoluminescence, UV-visible spectroscopy, atomic force microscopy, and Nomarski microscopy. The GaN thin films are capped with two layers of in-situ metal organic chemical vapor deposition -grown AlN and annealed in 24 bar of N2 overpressure to avoid GaN decomposition. The crystal quality of the GaN improves with increasing annealing temperature as confirmed by UV-visible spectroscopy and the full widths at half maximums of the E2 and A1 (LO) Raman modes. The crystal quality of films annealed above 1100 °C exceeds the quality of the as-grown films. At 1200 °C, Mg is optically activated, which is determined by photoluminescence measurements. However, at 1200 °C, the GaN begins to decompose as evidenced by pit formation on the surface of the samples. Therefore, it was determined that the optimal temperature for the first step in a multi-cycle rapid thermal anneal process should be conducted at 1150 °C due to crystal quality and surface morphology considerations. [ABSTRACT FROM AUTHOR]

The ultra-wide bandgap semiconductor gallium oxide (Ga2O3) offers substantial promise to significantly advance power electronic devices as a result of its high breakdown electric field and maturing substrate technology. A key remaining challenge is the ability to grow electronic-grade epitaxial layers at rates consistent with 20–40 μm thick drift regions needed for 20 kV and above technologies. This work reports on extensive characterization of epitaxial layers grown in a novel metalorganic chemical vapor deposition tool that permits growth rates of 1.0–4.0 μm h−1. Specifically, optical, structural and electrical properties of epilayers grown at ∼1 μm h−1 are reported, including employment in an operating MOSFET. The films demonstrate relatively smooth surfaces with a high degree of structural order, limited point defectivity (Nd − Na ≈ 5 × 1015 cm−3) and an optical bandgap of 4.50 eV. Further, when employed in a MOSFET test structure with an n+ doped channel, a record high mobility for a transistor structure with a doped channel of 170 cm2 V−1 s−1 was measured via the Hall technique at room temperature. This work reports for the first time a β-Ga2O3 MOSFET grown using Agnitron Technology's high growth rate MOCVD homoepitaxial process. These results clearly establish a significant improvement in epilayer quality at growth rates that can support future high voltage power device technologies. [ABSTRACT FROM AUTHOR]

Recent breakthroughs in bulk crystal growth of the thermodynamically stable beta phase of gallium oxide (β-Ga2O3) have led to the commercialization of large-area β-Ga2O3 substrates with subsequent epitaxy on (010) substrates producing high-quality films. Still, metalorganic chemical vapor deposition, molecular beam epitaxy, and processing of the (010) β-Ga2O3 surface are known to form subnanometer-scale facets along the [001] direction as well as larger ridges with features perpendicular to the [001] direction. A density function theory calculation of the (010) surface shows an ordering of the surface as a subnanometer-scale feature along the [001] direction. Additionally, the general crystal structure of β-Ga2O3 is presented, and recommendations are presented for standardizing (010) substrates to account for and control the larger-scale ridge formation. [ABSTRACT FROM AUTHOR]

Ion implantation of magnesium for p‐type GaN presents many opportunities; however, activation has proven difficult due to the decomposition of GaN at relevant annealing temperatures. Herein, testing the efficacy of multiple in situ and ex situ caps based on aluminum nitride and silicon nitride for GaN protection during annealing is presented. Photoluminescence shows better activation for in situ metal organic chemical vapor deposition (MOCVD)‐grown aluminum nitride caps compared with ex situ sputtered aluminum nitride and the best performance by ex situ plasma‐enhanced chemical vapor deposition (PECVD) silicon nitride. Furthermore, only samples annealed at the highest temperatures tested show preferential growth of UV luminescence to yellow‐green luminescence reinforcing the need for better capping solutions and higher temperature annealing. [ABSTRACT FROM AUTHOR]

The evolution of wide bandgap semiconductor materials has led to dramatic improvements for electronic applications at high powers and temperatures. However, the propensity of extended defects provides significant challenges for implementing these materials in commercial electronic and optical applications. While a range of spectroscopic and microscopic tools have been developed for identifying and characterizing these defects, such techniques typically offer either technique exclusively, and/or may be destructive. Scattering‐type scanning near‐field optical microscopy (s‐SNOM) is a nondestructive method capable of simultaneously collecting topographic and spectroscopic information with frequency‐independent nanoscale spatial precision (≈20 nm). Here, how extended defects within 4H‐SiC manifest in the infrared phonon response using s‐SNOM is investigated and the response with UV‐photoluminescence, secondary electron and electron channeling contrast imaging, and transmission electron microscopy is correlated. The s‐SNOM technique identifies evidence of step‐bunching, recombination‐induced stacking faults, and threading screw dislocations, and demonstrates interaction of surface phonon polaritons with extended defects. The results demonstrate that phonon‐enhanced infrared nanospectroscopy and spatial mapping via s‐SNOM provide a complementary, nondestructive technique offering significant insights into extended defects within emerging semiconductor materials and devices and thus serves as an important diagnostic tool to help advance material growth efforts for electronic, photonic, phononic, and quantum optical applications. [ABSTRACT FROM AUTHOR]

We report selective area n-type doping using ion implantation of Si into semi-insulating, C-doped GaN samples activated using both conventional rapid thermal annealing (RTA) and 30 atm N2 overpressure annealing. Implanted regions were tested for Si activation using Circular Transmission Line Measurements (CTLM), while linear and circular photoconductive switches (PCSS) in the unimplanted regions were used as a test vehicle to separate implanted Si dopant activation from leakage paths generated by N vacancy formation due to damage and decomposition during annealing. We observed that at an optimal temperature around 1060 °C, a low contact resistivity of $1\times 10^{-6}\,\,\Omega $ -cm2 was obtained while preserving the breakdown of the unimplanted regions. [ABSTRACT FROM AUTHOR]

Gallium nitride (GaN) is a wide-bandgap semiconductor that is useful for optoelectronics and high speed and high power electronics. Fabrication of GaN devices requires etching for many processing steps. Gas phase thermal atomic-layer-controlled etching is desirable for damage-free isotropic etching. In this letter, the thermal atomic layer etching (ALE) of crystalline GaN was demonstrated using sequential exposures of XeF2 and BCl3. GaN ALE was achieved with an etch rate of 0.55 Å/cycle at 195 °C using XeF2 exposures for 20 s at 40 mTorr and BCl3 exposures for 0.5 s at 50 mTorr. At the same reactant exposures, GaN etch rates varied with temperature from 0.18 Å/cycle at 170 °C to 0.72 Å/cycle at 300 °C. The GaN etch rates increased slowly with increasing XeF2 exposure. In addition, the GaN etch rate was self-limiting with respect to both increasing BCl3 pressures and BCl3 exposure times. This self-limiting behavior for BCl3 is consistent with a ligand-exchange mechanism for GaN ALE. Alternative fluorination reactants were also investigated including HF, SF4, and NF3 plasma. Sequential exposures of NF3 plasma and BCl3 yielded GaN etch rates of 2.5–2.9 Å/cycle at 250 °C. In contrast, the HF and SF4 fluorination reactants could not etch crystalline GaN. [ABSTRACT FROM AUTHOR]

Wide bandgap semiconducting nitrides have found wide-spread application as light emitting and laser diodes and are under investigation for further application in optoelectronics, photovoltaics, and efficient power switching technologies. Alloys of the binary semiconductors allow adjustments of the band gap, an important semiconductor material characteristic, which is 6.2 eV for aluminum nitride (AlN), 3.4 eV for gallium nitride, and 0.7 eV for (InN). Currently, the highest quality III-nitride films are deposited by metalorganic chemical vapor deposition and molecular beam epitaxy. Temperatures of 900 °C and higher are required to deposit high quality AlN. Research into depositing III-nitrides with atomic layer epitaxy (ALEp) is ongoing because it is a fabrication friendly technique allowing lower growth temperatures. Because it is a relatively new technique, there is insufficient understanding of the ALEp growth mechanism which will be essential to development of the process. Here, grazing incidence small angle x-ray scattering is employed to observe the evolving behavior of the surface morphology during growth of AlN by ALEp at temperatures from 360 to 480 °C. Increased temperatures of AlN resulted in lower impurities and relatively fewer features with short range correlations. [ABSTRACT FROM AUTHOR]

The activation of ion implanted p-type dopants in GaN is notoriously difficult as the extremely high temperatures required to activate implanted Mg also damage the GaN crystal. In this paper, we present refinements to our novel annealing process (symmetric multicycle rapid thermal annealing) to reduce surface damage and contamination responsible for elevated leakage currents and non-ideal diode behavior. Furthermore, we apply the technique to Mg-implanted bulk GaN substrates to enable vertical power device structures, demonstrating rectifying p-i-n junctions. In addition, the technique was applied for edge termination in both p-i-n and Schottky barrier diodes, realizing floating guard ring and junction termination extension structures. The processes demonstrated here represents a key enabling step for future GaN-based power devices. [ABSTRACT FROM AUTHOR]

Planar thin film β-Ga2O3 photodetectors were irradiated with 5 MeV protons at doses from 1013 to 1015cm-2, and the resulting effects on photocurrent, responsivity, quantum efficiency, and photo-to-dark current ratio at 254 nm wavelength were measured at both 25 and 150°C. The photocurrent increased with dose due to the introduction of damage from nonionizing energy loss by the protons. The total calculated vacancy concentration increased from 5×1015 to 5×1017 cm-3 over the dose range investigated. The dark current increased in proportion with the implant dose, leading to a decrease in the ratio of photocurrent to dark current. The photocurrent induced by 254 nm illumination increased with dose, from~0.3×10-7 A at 25°C for a dose of 1013cm-2 to ~10-6 A at a dose of 1015cm-2 at a fixed light intensity of 760 μW/cm². The phototo- dark current ratio decreased from ~60 in the control samples to ~9 after proton doses of 1015cm-2, with corresponding external quantum efficiencies of ~10³% in control samples, ~2×10³% for a dose of 1013cm-2, and 104% for a dose of 1015cm-2. The mechanism for the increase in photocurrent is the introduction of gap states, since the dark current of the photodetectors was increased by illuminating with sub-bandgap (red or green laser light) for the proton irradiated samples. [ABSTRACT FROM AUTHOR]

The temperature dependent photoresponse of planar photodetectors fabricated on β-Ga2O3 films grown on Al2O3 by metalorganic chemical vapor deposition to 254 nm wavelength, and blindnessto 365 nm light, are reported over the range of 25-350 °C. Ohmic contacts were formed by Siimplantationand annealing at 900 °C, followed by deposition of Ti/Au metallization. The photocurrentinduced by 254 nm illumination increased monotonically with temperature, from 2.5 ~10-7A at 25°C to ~2.2 ~10-6 A at 350 °C at a fixed 254 nm light intensity of 760 μW/cm². The photosensitivitydecreases at high temperatures in many photoconductors (thermal quenching), in sharpcontrast to the photosensitivity increase with high temperatures in this study. This is ascribed to thepresence of states in the gap of Ga²O³, whose presence was proven by exposure to below band-gapenergy. In this case, the current still increased due to the presence of defect levels in the band gapand the generation of photocurrent is due to a transition between the valence or conduction bandand impurity or defect levels within the band gap. The temperature dependent photo-to-dark currentratio for this wavelength was 328 at room temperature and decreased to ~9 at 350 °C. The responsivityincreased from 5 to 36 A/W over this temperature range, with corresponding external quantumefficiencies of 2.5 ~ 10³% at 25°C and 1.75 ~ 104% at 350 °C. Similarly large numbersreported for Ga2O3 photodetectors have previously been ascribed to carrier multiplication effects. [ABSTRACT FROM AUTHOR]

Top-side integration of nanocrystalline diamond films in the fabrication sequence of AlGaN/GaN high electron mobility transistors is demonstrated. Reliable oxygen plasma etching of the diamond capping layer, required for a diamond-before-gate process, was implemented by using a sacrificial SiN 'dummy' gate. Hall characterization showed minimal (∼6%) reduction in sheet carrier density and commensurate increase in sheet resistance, while maintaining mobility and on-state drain current density. Off-state drain current and threshold voltage were increased, likely by fluorination of the AlGaN surface after removal of the sacrificial gate, even though a 20 nm thick Al2O3 layer was used as a SF6-plasma etch stop. Pulsed IDS and on-resistance were improved, indicating that a 10 nm SiN/500 nm NCD could offer improved AlGaN surface passivation compared to a more conventional 100 nm thick PECVD SiN film. [ABSTRACT FROM AUTHOR]

The surface preparation for depositing Al2O3 for fabricating Au/Ni/Al2O3/n-GaN (0001) metal oxide semiconductor (MOS) capacitors was optimized as a step toward realization of high performance GaN MOSFETs. The GaN surface treatments studied included cleaning with piranha (H2O2:H2SO4=1:5), (NH4)2S, and 30% HF etches. By several metrics, the MOS capacitor with the piranha-etched GaN had the best characteristics. It had the lowest capacitance-voltage hysteresis, the smoothest Al2O3 surface as determined by atomic force microscopy (0.2nm surface roughness), the lowest carbon concentration (∼0.78%) at the Al2O3/n-GaN interface (from x-ray photoelectron spectroscopy), and the lowest oxide-trap charge (QT = 1.6×1011cm-2eV-1). Its interface trap density (Dit=3.7×1012cm-2eV-1), as measured with photon-assisted capacitance-voltage method, was the lowest from conduction band-edge to midgap. [ABSTRACT FROM AUTHOR]

Periodically oriented GaN surfaces have been investigated using scanning probe microscopy techniques to compare Ga- and N-polar regions on a common surface. An epitaxial layer comprising 16 μm-wide stripes of alternating surface polarity was grown by utilizing a patterned AlN layer on an N-polar GaN template. The regions of different polarity are easily distinguishable in atomic force microscopy (AFM) images, with the Ga-polar stripes, which nucleate on the patterned AlN regions being approximately 300 nm higher than the adjacent N-polar stripes. In addition, local surface potential measurements using scanning Kelvin probemicroscopy (SKPM) indicate that the N-polar regions are 0.2 to 0.5 eV higher in potential compared to Ga-polar ones, with lower surface potential contrast occurring after an HCl-based surface treatment. Using conductive AFM, electrons were injected into the surface to probe the surface charging behavior in dark. Only the N-polar regions demonstrated significant localized charging, where changes of over 1.5 eV were seen in SKPM images taken immediately after charging. This behavior was appreciably decreased by an HCl treatment, suggesting that a surface oxide in the N-polar regions plays a significant role in the charging behavior. In addition, the local surface photovoltage (SPV) was measured using above-bandgap illumination. The N-polar regions demonstrate a higher steady-state SPV value and a significantly slower restoration behavior as compared to the Ga-polar regions. The authors therefore find that several characterization techniques can readily distinguish the Ga- versus N-polar regions on this periodically oriented surface. [ABSTRACT FROM AUTHOR]

Wet chemical etching using hot KOH and H3PO4 solutions was performed on semipolar (112̅2) and nonpolar (112̅0) GaN films grown on sapphire substrates. An alternating KOH/H3PO4/KOH etch process was developed to control the orientation of the facets on the thin-film surface. The initial etch step in KOH produced c- and m-plane facets on the surface of both semipolar (112̅2) and nonpolar (112̅0) GaN thin-films. A second etch step in H3PO4 solution additionally exposed a (1̅1̅22̅) plane, which is chemically stable in H3PO4 solution. By repeating the chemical etch with KOH solution, the m-plane facets as seen in the original KOH etch step were recovered. The etching methods developed in our work can be used to control the surface morphologies of nonpolar and semipolar GaN-based optoelectronic devices such as light-emitting diodes and solar cells. [ABSTRACT FROM AUTHOR]

In preparing periodically-oriented gallium nitride (PO GaN) samples for optical applications, smooth surfaces are essential. Although post-growth processing of these surfaces is possible, it is challenged by significantly different reactivity of the two polar surfaces. To this end, it would be highly desirable to grow PO GaN structures with smooth surfaces. In this paper, the influence of growth parameters on the morphology of PO GaN structures is investigated. The three primary parameters investigated include the V/III ratio of the gas phase precursors, the nature of the carrier gas, and the geometry of the structure. The V/III ratio was varied from 1100 to 5500. Substitution of N2 for the standard H2 carrier gas is investigated. Finally, the effects of changing the structure periodicity from 2 to 100 mm are studied. Smoother structures (resulting from more equal growth rates of the two polarities) are achieved with increases in V/III ratio, substitution of H2 with N2, and by reducing the stripe width. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

An all-epitaxial approach was demonstrated to create coaxial plasmon laser structures composed of an aluminum plasmonic metal/SiNx dielectric/InGaN quantum well shell surrounding a p-GaN nanowire core. Strong UV luminescence was observed from as-grown vertically-aligned arrays as well as horizontally-aligned nanowires transferred to a transparent carrier wafer. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

This article presents the use of flexible carbon substrates for the growth of III-nitride nanowire light emitters. A dense packing of gallium nitride nanowires were grown on a carbon paper substrate. The nanowires grew predominantly along the a-plane direction, normal to the local surface of the carbon paper. Strong photo- and electro-luminescence was observed from InGaN quantum well light emitting diode nanowires. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The family of III-V nitride semiconductors has garnered significant research attention over the last 20-25 years, and these efforts have led to many highly successful technologies, especially in the area of light emitting devices such as light emitting diodes for solid state white lighting and lasers for high density optical read/write memories. These applications have taken advantage of a key material property of the III-N materials, namely a direct, tunable (0.7-6.2 eV, λ ∼ 200 nm to 1.7 μm) bandgap and have been accomplished despite a relatively poor level of material quality. But a direct, tunable bandgap is only one of many interesting properties of III-N materials of interest to potential future technologies. A considerable list of first and second order properties make this family of semiconductors even more attractive-namely, electric polarization, piezoelectricity, high breakdown field, pyroelectricity, electro-optic and photo-elastic effects, etc. The first few of these have found much utility in the development of high power transistors that promise significant commercial success in both communications and power switching applications. As these areas begin to flourish, it is reasonable to begin to explore what might be next for this versatile family of semiconductors. Here are highlighted three areas of significant potential for future III-N research-atomic layer epitaxy of complex heterostructures, variable polarity homo- and hetero-structures of arbitrary geometries, and nanowire heterostructures. Early results, key technical challenges, and the ultimate potential for future technologies are highlighted for each research path. [ABSTRACT FROM AUTHOR]

An undoped AlGaN/GaN nanowire (NW) demonstrated p-type conductivity solely based on the formation of hole carriers in response to the negative polarization field at the (000-1) AlGaN/GaN facet. A transistor based on this NW displayed a low-voltage transition from ohmic to space-charge-limited conduction. A numerical simulation showed that a highly asymmetric strain exists across the triangular cross section, which creates a doublet peak in the piezoelectric-induced polarization sheet charge at the (000-1) facet. Additionally, there is a strong interplay between the charge at the (000-1) AlGaN/GaN interface with depletion from the three surfaces, as well as an interaction with the opposing polarization fields at two semipolar {-110-1} facets. The charge distribution and resultant conduction regime is highly interdependent on the configuration of the multilayer structure, and it is not amenable to an analytical model. [ABSTRACT FROM AUTHOR]

An approach to fabricate a set of simultaneously operating dual-UV-wavelength detectors is described. The fabrication flow relies on the confined-epitaxy growth method. The confined epitaxial Alα Ga1--α N-layer stacking approach is used to establish simultaneous multiple UV-wavelength detection. The chosen stoichiometries of specific epitaxial layers set the wavelength sensitivity at approximately 355 nm for pixel A and 320 nm for pixel B. Spectral responsivity plots of the detectors clearly show the dual-UV-color sensitivity of the pair. The detectors have signal-to-noise ratios of 15 and 17 and spectral responsivity values of 0.12 A/W and 0.05 A/W for pixel A and pixel B, respectively. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high-electron mobility transistor (HEMT) structures were grown by metal–organic chemical vapor deposition on SiC, hydride vapor phase epitaxy (HVPE) GaN, and ammonothermal GaN substrates to achieve HEMTs with over five orders of magnitude variation in extended defect density. This enables a direct comparison of the effect of extended defects on device performance to achieve the best possible reliability. As-grown material was characterized by atomic force microscopy, electron channeling contrast imaging, and Raman spectroscopy. Devices were characterized by Hall, dc $I$ – $V$ , and pulsed $I$ – $V$ behavior. Reduced threading dislocation density provides an increased 2-D electron gas mobility, but inhibits ohmic contact formation resulting in high contact resistance. Transistor characteristics were nominally identical, with higher OFF-state leakage in the HEMTs on ammonothermal GaN. The pulsed $I$ – $V$ response indicated significantly reduced current collapse in the HEMT on HVPE GaN due to reduced buffer trapping. [ABSTRACT FROM PUBLISHER]

AlGaN/GaN high electron mobility transistors grown on Si, SiC, and sapphire substrates were exposed to 2-MeV proton irradiation in incremental fluences up to \(6 \times 10^{14}\) cm \(^{-2}\) . The devices were characterized initially and after each irradiation by Hall and dc \(I\) – \(V\) measurements to probe the mechanisms associated with radiation-induced degradation and failure. It was determined that defects created at the AlGaN/GaN interface introduce scattering centers near the two-dimensional electron gas (2DEG), which result in degraded mobility. Additionally, charged traps in the structure serve to screen the 2DEG resulting in reduced sheet carrier density. These two effects are responsible for degraded \(I\) – \(V\) behavior, including reduced saturation current and transconductance, increased ON-resistance, and positive threshold voltage shift. Interestingly, the sample with the most pre-existing defects was the most tolerant of radiation-induced damage. [ABSTRACT FROM PUBLISHER]

Boron-doped p^+ nanocrystalline diamond (NCD) films are implemented as heat spreading gate contacts to AlGaN/GaN high-electron-mobility transistors. This device demonstrates a reduced ON-resistance, reduced gate leakage, and significantly increased ON-state current density compared with the reference Ni/Au-gated devices from the same wafer. The NCD gate electrode is thermally stable, chemically stable, optically transparent, and places a heat spreading film in direct contact with the gate edge, which is the hottest part of the device. [ABSTRACT FROM AUTHOR]

We report on the development of a large-area few-layer graphene (FLG)-based transparent conductive electrode as a current spreading layer for GaN-based ultraviolet (UV) light-emitting diodes (LEDs). Large-area FLG was deposited on Cu using the chemical vapor deposition (CVD) method and subsequently transferred to the surface of the UV LED. UV light at a peak of 372 nm was emitted through the FLG-based transparent conductive electrode. The current spreading effects of FLG were clearly evident in both the optical images of electroluminescence (EL) and current-voltage (I-V) characteristics. Degradation of the FLG-based transparent conductive electrode could be induced by high power operation. Our results indicate that a large-area FLG-based electrode on GaN offers excellent current spreading and ultra-violet transparency properties when compared to the standard optoelectronic indium tin oxide (ITO) contact layer. [ABSTRACT FROM AUTHOR]

A novel device structure incorporating an ultrathin AIGaN barrier layer capped by an AIN layer in the source-drain access regions has been implemented to reliably control threshold voltage in AIGaN/GaN high-electron-mobility transistors. A recessed-gate structure has been used to decrease 2-D electron gas (2DEG) density under the gate, thus controlling threshold voltage while maintaining low on-resistance and high current density. The structure presented in this letter implements an ultrathin AIGaN structure grown by metal-organic chemical vapor deposition capped with AIN to maintain a high 2DEG density in the access regions. A selective wet etch using heated photoresist developer is used to selectively etch the AIN layer in the gate region to the AIGaN barrier. We have demonstrated a repeatable threshold voltage of +0.21 V with 4-nim AIGaN barrier layer thickness. [ABSTRACT FROM AUTHOR]

This paper reports pH-selective potentiometric responses of epitaxial nitrogen-polar (0001) GaN films on sapphire substrates. The potential of a nitrogen-polar GaN substrate increased with decreasing solution pH from 10 to 2 in a Nernstian manner, whereas it did not significantly change upon increasing the concentrations of the cationic and anionic interfering species. In particular, the negligible responses to anions marked a sharp contrast with previously reported results on gallium-polar GaN that exhibited Nernstian responses to anions. The potentiometric response to pH probably originates from the adsorption of H+ onto oxide-coated nitrogen-polar GaN substrates having polarization-induced negative surface charge. [ABSTRACT FROM AUTHOR]