Fabrication of GaN with Buried Tungsten (W) Structures Using Epitaxial Lateral Overgrowth (ELO) via LP-MOVPE
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F99W2.3
In the ELO of GaAs, the W metal mask was employed and the W-buried structure was obtained to fabricate a permeable base transistor (PBT) or static induction transistor (SIT) [6]. The fabrication of the electronic devices such as PBTs or SITs via the ELO technique in GaN is very attractive for the use as high power and/or high frequency devices under crucial environments such as at high temperature or at highly radioactive environment. Recently, Kawaguchi et al. performed the selective area growth (SAG) of GaN using the W mask by metalorganic vapor phase epitaxy (MOVPE) and achieved high selectivity[7]. In this work, we demonstrate the ELO of GaN using the W mask by MOVPE and obtain the W-buried structure with GaN. The characteristics of ELO-GaN were also investigated by means of atomic force microscopy (AFM) and x-ray rocking curve (XRC). EXPERIMENTAL PROCEDURE MOVPE of GaN using a W mask was performed on two types of samples. The sample is a 4.0 µm-thick (0001) GaN epilayer, which had been grown on sapphire (0001) substrate using an GaN low temperature buffer layer by MOVPE. Another sample is a 40-100 nm thick AlGaN epilayer on the 4.0 µm-thick (0001) GaN epilayer. The 30 - 50 nm thick W mask was deposited by electron beam evaporation method. Stripe windows of 2 or 5 µm width with a periodicity of 4 or 10 µm, respectively, were developed in the direction of the GaN by conventional photolithographic method and then wet etching with a hydrogen peroxide (H2O2) and an ammonia solution. The source gases were trimethylgallium (TMG) and ammonia (NH3), respectively. Hydrogen gas was used as a carrier gas during the growth process. ELO of GaN was performed by LP (low pressure) - MOVPE system with a horizontal reactor. Two steps ELO technique was carried out to bury W mask completely. In the first step, a low temperature (950oC) growth was performed for 30 min to prevent the underlying GaN from being decomposed. In the second step, a high temperature (1050oC) growth was done for 90 min to bury W mask. - facets are grown selectively in Here, it is expected in both steps that the GaN with {1122} the first step and that the W mask is easily covered with GaN because of the fast ELO rate at a higher growth temperature in the second step, based on the results of ELO-GaN using SiO2 mask[8].
F99W2.3
RESULTS AND DISCUSSION In order to investigate effects of the W mask which affects the underlying GaN and AlGaN layers in hydrogen ambient at a high temperature, we attempted thermal annealing at 500 to 700 oC of GaN and AlGaN with a striped W mask. Consequently, the surface of GaN window regions was roughened, including Ga droplets and GaN whiskers. This phenomenon occurred above 500oC. It is reported that an underlying GaN layer with a striped SiO2 mask is decomposed higher than 900 oC in hydrogen ambient and there is no damage less than 900 oC[9]. Therefore, the W mask works as a catalyst to enhance decomposition of GaN. It is thought that W may produce radical hydrogen that can decompose GaN easily. On the other hand, the
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