In this paper, we report on the feasibility of oxidizing n-GaN by an electrochemical process in a mixture containing an aqueous solution of tartaric acid and propylene glycol. Photons generated by UV illumination were supplied at the electrolyte-GaN interface during the oxidation process. In the constant-voltage mode, X-ray photoelectron spectroscopy analysis revealed that relatively thick Ga oxide layer formed on the n-GaN surface. However, the oxide surface was very rough. In addition, we found metallic Ga components in the oxide layer or at the oxide-GaN interface for longer oxidation times. On the other hand, a thin Ga 2 O 3 layer with a smooth surface was grown by a constant-current process.
A normally-off AlGaN/GaN MOS heterojunction field-effect transistor (MOS-HFET) with a recessed gate structure formed by selective area regrowth is demonstrated. The fabricated MOS-HFET exhibits a threshold voltage of 1.7 V with an improved hysteresis of 0.5 V as compared with a device fabricated by a conventional dry etching process. An analysis of capacitance–voltage (C–V) characteristics reveals that the dry etching process increases interface state density and introduces an additional discrete trap. The use of the selective area regrowth technique effectively suppresses such degradation, avoiding the MOS interface from being exposed to dry etching. The results presented in this paper indicate that the selective area regrowth technique is promising for the fabrication of normally-off AlGaN/GaN MOS-HFETs.
Surface control of n-GaN was performed by applying a photoelectrochemical oxidation method in a glycol solution to improve the optical and electronic characteristics. The fundamental properties of the oxidation were investigated. The oxidation, chemical composition, and bonding states were analyzed by x-ray photoelectron spectroscopy and micro-Auger electron spectroscopy, in which confirmed the formation of gallium oxide on the surface. The oxide formation rate was about 8 nm/min under UV illumination of 4 mW/ cm 2 . After establishing the basic properties for control of n-GaN oxidation, the surface control technique was applied to achieve low-damage etching, enhancement of the photoluminescence intensity, and selective passivation of the air-exposed sidewalls in an AlGaN/GaN high electron mobility transistor wire structure. The capacitance-voltage measurement revealed the minimum interface-state density between GaN and anodic oxide to be about 5 ϫ 10 11 cm −2 eV −1 , which is rather low value for compound semiconductors.
Effects of surface states and surface passivation on photoluminescence ͑PL͒ properties of GaAs quantum wires ͑QWRs͒ are investigated. QWR samples were grown on ͑001͒ and ͑111͒B substrates by the selective molecular beam epitaxy ͑MBE͒ method. For surface passivation, an ultrathin ͑about 1 nm͒ Si interface control layer ͑Si ICL͒ was grown by MBE as an interlayer. In both of the selectively grown QWRs on ͑001͒ and ͑111͒B substrates, the PL intensity reduced exponentially with reduction of their wire-to-surface distance, being coexistent with a more gradual reduction due to carrier supply reduction. The exponential reduction was explained in terms of interaction between surface states and quantum confined states leading to tunneling assisted nonradiative recombination through surface states. Surface passivation by the Si-ICL method almost completely recovered PL intensities not only for QWRs on the ͑001͒ substrate, but also for QWRs on the ͑111͒B substrate.
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