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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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

We report that the oxidation of graphene-based highly transparent conductive layers to AlGaN/GaN/AlGaN ultra-violet (UV) light-emitting diodes (LEDs) was suppressed by the use of SiNX passivation layers. Although graphene is considered to be an ideal candidate as the transparent conductive layer to UV-LEDs, oxidation of these layers at high operating temperatures has been an issue. The oxidation is initiated at the un-saturated carbon atoms at the edges of the graphene and reduces the UV light intensity and degrades the current-voltage (I-V) characteristics. The oxidation also can occur at defects, including vacancies. However, GaN-based UV-LEDs deposited with SiNX by plasma-enhanced chemical vapor deposition showed minimal degradation of light output intensity and I-V characteristics because the graphene-based UV transparent conductive layers were shielded from the oxygen molecules. This is a simple and effective approach for maintaining the advantages of graphene conducting layers as electrodes on UV-LEDs. [ABSTRACT FROM AUTHOR]

The authors report on the direct comparison of the chemical etch characteristics on both Ga- and N-face gallium nitride (GaN) by phosphoric acid. First, Ga-face GaN was grown next to N-face GaN by a polarity inversion method in a metal-organic chemical vapor deposition reactor. Micro-photoluminescence, atomic force microscopy, scanning electron microscopy, and micro-Raman spectroscopy were used to analyze the etch characteristics of Ga- and N-face GaN before and after a H3PO4-based chemical etch. Ga-face was chemically stable in a phosphoric acid solution. However, the chemical etch continued proceeding on the N-face GaN due to the weak repulsive force to OH- ions. Dodecagonal nano-pyramids which dramatically enhanced the photoluminescence intensity were observed on N-face GaN after a H3PO4-based chemical etch. [ABSTRACT FROM AUTHOR]