AlGaN/GaN High-electron-mobility transistor (HEMT) structures on silicon were grown by Metalorganic chemical vapor deposition (MOCVD) with various growth pressures during the growth of 1.5 mm thick GaN. The grown samples were characterized by X-ray diffraction, secondary ion mass spectroscopy, and photoluminescence analysis which revealed increased dislocation density, high C impurity compensation of free carriers and yellow luminescence for low pressure samples. For GaN grown at 200 Torr pressure, the formation of highly resistive buffer with C concentration as high as 3:8 Â 10 17 cm À3 leads to reduced buffer leakage over one order of magnitude and an enhanced breakdown voltage of 425 V for a HEMT with gate-drain spacing of 4 mm. However, unlike atmospheric pressure grown samples, the presence of unintentional C in the semi-insulating GaN degraded the channel conduction and resulted in severe current collapse.
Nitrogen (N)-doped reduced graphene oxide (nRGO) is systematically incorporated into a TiO(2) -CdS photoelectrochemical (PEC) cell and its role is examined in the three main components of the cell: 1) the CdS-sensitized TiO(2) photoanode, 2) the cathode, and 3) the S(2-)/S(.-) aqueous redox electrolyte. The nRGO layer is sandwiched between TiO(2) nanorods (deposited by using a solvothermal method) and CdS (deposited by using the successive ionic-layer-adsorption and -reaction method). Scanning electron microscopy with energy dispersive X-ray analysis (EDS) reveals the spatial distribution of CdS and nRGO, whereas nRGO formation is evident from Mott Schottky analysis. Chronoamperometry and PEC analysis indicate that upon incorporation of nRGO, a photocurrent density that is at least 27 times higher than that of pristine TiO(2) is achieved; this increase is attributable to the ability of the nRGO to efficiently separate and transport charges. Stability analysis performed by continuous photoillumination over ∼3 h indicates a 26% and 42 % reduction in the photocurrent in the presence and absence of the nRGO respectively. Formation of SO(4)(2-) is identified as the cause for this photocurrent reduction by using X-ray photoelectron spectroscopy. It is also shown that nRGO-coated glass is as effective as a Pt counter electrode in the PEC cell. Unlike the benefits offered by nRGO at the anode and cathode, introducing it in the redox electrolyte is detrimental. Systematic and complementary electrolyte and film-based studies on this aspect reveal evidence of the capacitive behavior of nRGO. Competition between the nRGO and the oxidized electrolyte is identified, based on linear-sweep voltammetry analysis, as the limiting step to efficient charge transport in the electrolyte.
This work delineates the results of a systematic analysis of the photoelectrochemical (PEC) and optical responses of reduced graphene oxide (RGO) combined with a representative multi-metal oxide – bismuth titanate (BTO) on films. We investigated the role of RGO as a promoter of charge separation and transport, by suppressing recombination loss of carriers in BTO. The micro-structure of the BTO/RGO composite observed using transmission, scanning electron, and atomic force microscopy, revealed a well-integrated, and highly porous morphology compared to pristine BTO film. The integration of BTO with RGO resulted in a ∼7 fold increase in photocurrent density with an optimal RGO loading of 2 wt%. The performance of the BTO/RGO interface was further examined to obtain critical insights on: (i) how the electrolyte influences the stability of the BTO/RGO composite and (ii) what interactions prevail at the BTO/RGO interface under long-term illumination that impact RGO stability. This work assumes immediate significance because implementation of RGO as a charge transport agent in oxidative systems is currently heavily researched. Unless we can determine a mechanism to protect RGO, its long-term stability, especially under oxidative conditions of any form, must be thoroughly investigated on a case-by-case basis to ensure composite longevity.
InAlN/GaN epilayer on AlN/Sapphire template was grown by metalorganic chemical vapor deposition (MOCVD) with a very high electron sheet carrier density n s ¼ 2:6 Â 10 13 cm À2 and a Hall mobility as high as Hall ¼ 1170 cm 2 V À1 s À1 at room temperature. The electrical characteristics of the fabricated high-electron-mobility transistors (HEMTs) having 2 mm gate length and 15 mm gate width were demonstrated. Maximal-drain-current I Dmax ¼ 1299 mA/mm, and maximal transconductance g mmax ¼ 280 mS/mm were achieved for the InAlN barrier layer thickness of 10 nm. Reduced current collapse and as well as breakdown voltage as high as 400 V were observed.
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