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The indium gallium nitride (InGaN) ternary alloy system offers an ideal semiconductor basis as a light absorber for driving chemical reactions such as the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). It is a direct bandgap semiconductor whose energy can be tuned for optimal solar absorption and overcoming kinetic overpotential losses, it can be grown epitaxially as a single crystal for high material quality, and it exhibits relatively high corrosion resistance in aqueous solutions for a III-V material. In this work, we report on the design, synthesis, and photoelectrochemical studies of InGaN in an anode configuration with a goal of using this material as a light absorber basis for driving solar fuels reactions such as the OER and the HER. Studies are shown reporting on how reactive vapor synthesis of GaN and InN with different structuring affects the light conversion properties and band diagram electrostatics in an electrochemical junction with ferrocene and cobaltocene redox pairs. This structure is extended to thin films of GaN and InGaN grown by molecular beam epitaxy for high efficiency light conversion, with the resulting electrochemical junction also made with ferrocene and cobaltocene. The effects of ternary alloying, doping, and the two different c-plane polarities (III-polar and V-polar) on the junction are investigated using the same electrochemical junction to evaluate the light conversion properties in relation to the electrostatics; specifically, tuning the bandgap energy for optimal light absorption, determining the c-plane polarity configuration for the electric fields to have the correct band alignment, and incorporating silicon doping to create a conductive substrate. The combination of these engineering techniques extends the depletion region of the active layer for high efficiency light absorption as similarly demonstrated for epitaxial phosphide films [1], and is utilized toward the goal of high performance solar energy conversion. [1] V. Parameshwaran, X. Xu, and B. Clemens, Journal of the Electrochemical Society, vol. 163, no. 8, pg. H714-H721 (2016).

High sensitivity ultraviolet (UV) avalanche photodiodes (APDs) are useful for various applications, including chemical and biological identification, optical wireless communications, and UV sensing systems. State-of-the-art SiC APDs exhibit a high peak quantum efficiency (QE) of 60% at 268 nm, but the response of these devices diminishes at longer wavelengths due to weak absorption and at shorter wavelengths due to increasing photo-generation of carriers in the top doped layer of this device, the short diffusion length of minority carriers in this doped region and the presence of a high density of surface states [1]. Recently, we have demonstrated that heterostructure III-Nitride/SiC diodes have promise for improving the efficiency of SiC based detectors throughout the UV spectrum. However, these devices require growth techniques for initiating two-dimensional growth on lattice mismatch substrates as well as suppressing impurity migration at the hetero-interface. p- SiC In this paper we explore the impact of in situ substrate preparation and migration enhanced epitaxy (MEE) on the nucleation and impurity concentration of thin AlN films grown by plasma-assisted molecular beam epitaxy on 4H-SiC. A series of 30nm AlN films were deposited on i-p epitaxial SiC structures grown on 4 degree miscut 4H-SiC. The in-situ preparation techniques explored include annealing at 740 °C while depositing and desorbing Ga with and without N plasma active and the shutter closed. The AlN films were grown at temperatures of 600 °C and 740 °C using a standard approach without growth interruption under N limited conditions or employ an MEE, or interrupted growth approach. The surface morphology of the samples were examined by atomic force microscopy and the composition of the films were studies by depth profiling x-ray photoemission spectroscopy (XPS). Figure 1 compares the morphology and the composition profile of the AlN film grown by MEE (sample A) and the standard process (sample B) with the N plasma off during the Ga preparation. Sample A exhibits an atomically smooth, 2D surface, and a nearly stoichiometric Al to N composition in the AlN layer that drops off at the heterointerface. In contrast, sample B has a higher roughness on a 2x2 mm scale with an absence of discernable steps on the substrate. XPS results indicate a slightly larger Al to N composition in the film (no visible droplets) and a detectable presence of Al in the SiC layer indicating migration. These results suggest that MEE approach can both promote the nucleation of AlN growth on SiC in a 2D mode as well as suppress the migration of Al into SiC. Figure 2 demonstrates the impact of an active N plasma source during the Ga preparation process despite closing the source shutter on the morphology and interface composition of the AlN films. Sample C was prepared with the N plasma on and was grown at 600 °C. The film exhibits a rough surface morphology despite the use of MEE, consistent with the observations of Okumura et al. [4]. Sample D, grown under similar conditions at a higher substrate temperature of 740 °C exhibits a 2D morphology characterized by micro-steps and possible step-bunching. Sample E was grown at 600 °C similar to sample D, but with the N plasma off during Ga deposition, exhibits an atomically smooth surface morphology consistent with that of the substrate. XPS studies of sample D indicate the presence of GaN as well as metallic Ga at the hetero-interface in contrast to that of sample E. This suggests that active N that leaks around the closed shutter leads to GaN island formation that prevents the nucleation of AlN in a 2D growth mode at lower substrate temperatures. The potential partial nitridation of the SiC surface during this process does not impede the nucleation of AlN in a 2D growth mode based upon studies of films grown on substrates annealed at 740 °C with the N plasma active but the shutter closed (data not shown). Figure 1

High sensitivity deep ultraviolet (DUV) photodetectors sensitive at wavelengths shorter than 280 nm are useful for various applications, including chemical and biological identification, optical wireless communications, and UV sensing systems. State-of-the-art SiC APDs, employing a recessed top window, exhibit a high peak quantum efficiency (QE) of 60% at 268 nm, but the response of these devices diminishes at shorter wavelength due to increasing photo-generation of carriers in the top doped layer of this device, the short diffusion length of minority carriers in this doped region and the presence of a high density of surface states [1]. In this paper we explore two approaches for increasing the DUV photoresponse of 4H-SiC photodiodes by increasing the collection of DUV- generated carriers near the surface through drift within the high E-field region. As a result, the photo-generated carriers are more efficiently collected and the impact of surface state recombination is reduced. These approaches include replacing the illuminated top doped SiC layer of the diode with (1) a transparent metal contact or (2) a doped wider band gap III-Nitride semiconductor structure that is transparent in the spectral range of interest. Figure 1 shows the measured unity gain spectral response of a p-n--metal 4H-SiC diode reverse biased at 30V and consisting of a 2 µm-thick p +-SiC layer doped 2x1018 cm-3, a 350 nm-thick n- -SiC layer lightly doped 5x1015 cm-3, and employing a thin 5 nm transparent Ni metal top contact (broken red line). The response for a typical p-n--n+-SiC diode consisting of a 2 µm thick p +-SiC layer doped 2x1018 cm-3, a 480 nm thick n- -SiC layer lightly doped 5x1015 cm-3, and a 100 nm thick n +-SiC layer doped 4.5x1018 cm-3 is provided for comparison (solid-black line). A nearly flat QE between 40-45% is observed for the p-n--metal detector between 200- 270 nm that exceeds the performance of the conventional p-n--n+ diode for wavelengths shorter than 235 nm despite absorption losses within the metal contact across the spectral range of ~40-50%. This enhancement is attributed to improved collection of photogenerated carriers through drift within the high E-field region despite the presence of surface states. This conclusion is supported by modeling the expected photocurrent in the p-n- -metal (red open circles) and conventional p-n--n+ (black open squares) diodes. An alternative approach that can reduce absorption losses in the spectral range of interest is to employ a transparent III-Nitride semiconductor n-type doped layer. Figure 2 shows the measured spectral response of a photodiode having a 2 µm-thick p +-SiC layer doped 2x1018 cm-3, a 350 nm-thick n- -SiC layer lightly doped 5x1015 cm-3, a 10 nm-thick AlN layer, and a 100 nm-thick Al0.85Ga0.15N layer. The devices exhibit a peak unity gain QE of 55-58% between 265-250 nm that diminishes at shorter wavelength due to absorption in the top AlGaN layer. Increasing reverse bias above 40V results in an increase in response peaking around ~ 250 nm that is associated with inhomogeneous gain at wavelengths shorter than 255 nm (Figure 2- top). This is attributed to increased absorption associated with valence to higher conduction band transitions in SiC that ensures that these photons are absorbed near the interface between SiC and AlN, where photo-generated holes will have maximum transit distance through the depletion region as well as the larger impact ionization rate for holes over electrons for SiC. Calculations of the expected gain vs. wavelength within a p-i-n SiC diode illuminated from the n- region for varying magnitudes of E-field in the depletion region show similar trends consistent with this interpretation (Figure 2 inset). [1] Liu H., Mcintosh, D., Bai, X., Pan, H, Liu, M., Campbell J. C., and Cha H, IEEE Photonic Technol. Lett. 20, 1551 (2008).