Selective Area Growth (SAG) and Epitaxial Lateral Overgrowth (ELO) of GaN Using Tungsten Mask
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Selective area growth (SAG) and epitaxial lateral overgrowth (ELO) of GaN using tungsten (W) mask by metalorganic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE) have been studied. The selectivity of the GaN growth on the W mask as well as the Si0 2 mask is excellent for both MOVPE and HVPE. The ELO-GaN layers are successfully obtained by HVPE on the stripe patterns along the < 1100 > crystal axis with the W mask as well as the Si0 2 mask. There are no voids between the SiO 2 mask and the overgrown GaN layer, while there are triangular voids between the W mask and the overgrown layer. The surface of the ELO-GaN layer is quite uniform for both mask materials. In the case of MOVPE, the structures of ELO layers on the W mask are the same as those on the Si0 2 mask for the < 11 20 > and < 1Too > stripe patterns. No voids are observed between the W or Si0 2 mask and the overgrown GaN layer by using MOVPE. INTRODUCTION Wide band gap GaN and related nitrides have shown potential use in light emitting diodes (LEDs) and laser diodes (LDs) in green to blue light regions [1,2]. These materials have also shown usefulness in electronic devices as an AIGaN/GaN heterostructure field effect transistors (HFETs) [3,4]. The HFET structures have received interest because of the high performance with a high output power in microwave frequencies. The static induction transistors (SITs) also have a possibility for the power device at microwaves and have been expected to show ultimate performance at high temperatures because of no saturation of the drain current and the negative temperature coefficient of the leakage current [5]. For the past decades, various efforts have been done to realize power devices at high frequencies. One of the main difficulties have been on the formation of high quality metal-semiconductor contact with an embedded structure. Selective area growth (SAG) and epitaxial lateral overgrowth (ELO) of GaN has attracted much attention in the fabrication of optical and electrical devices with high performance [6,7]. Nishinaga et al. developed the idea of the SAG/ELO technique further and named this technique micro-channel epitaxy (MCE), that the lateral overgrown region on the mask will be free from dislocations which might be originated at the hetero-interface [8]. Recently, Sakai et al.[9], Matsushima et al.[10] and Nam et al.[l I] demonstrated that the SAG/ELO technique of GaN gives us an embedded structure of amorphous Si0 2 stripes in a high crystalline quality epitaxial layer. This technique will provide us embedded metal electrodes in an epitaxial GaN layer. If we could realize an embedded metal gate electrode in the epitaxial G 4.1 Mat. Res. Soc. Symp. Proc. Vol. 537 01999 Materials Research Society
GaN layer, we might realize SITs made from GaN, which will show high performance at high temperature, high frequency and high power operations. In a previous study, we attempted the SAG of GaN using tungsten (W) masks by metalorganic vapor phase epitaxy (MOVPE) for the first time [12]. In this stu
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