00> crystal axis with the W mask as well as the SiO
0> and <1 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 [Reference Amano, Kito, Hiramatsu and Akasaki1,Reference Nakamura and Fasol2]. These materials have also shown usefulness in electronic devices as an AlGaN/GaN heterostructure field effect transistors (HFETs) [Reference Wu, Keller, Keller, Nguyen, Le, Nguyen, Jenkins, Kehias, DenBaars and Mishra3,Reference Wu, Keller, Fini, Keller, Jenkins, Kehias, DenBaars and Mishra4]. 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 [Reference Bozler and Alley5]. 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 [Reference Ujiie and Nishinaga6,Reference Nakamura, Senoh, Nagahama, Iwasa, Yamada, Matsushita, Kiyoku, Kozaki, Sugimoto, Umemoto, Sano and Chocho7]. 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 [Reference Vetury, Marchand, Ibbetson, Fini, Keller, Speck, DenBaars and Mishra8]. Recently, Sakai et al.[Reference Sakai, Sunakawa and Usui9], Matsushima et al.[Reference Matsushima, Yamaguchi, Hiramatsu and Sawaki10] and Nam et al.[Reference Nam, Bremser, Zheleva and Davis11] demonstrated that the SAG/ELO technique of GaN gives us an embedded structure of amorphous SiO2 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 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 [Reference Kawaguchi, Nambu, Sone, Shibata, Matsushima, Yamaguchi, Miyake, Hiramatsu and Sawaki12]. In this study, we compare the SAG of GaN using W mask to that of SiO2 mask by MOVPE by means of scanning electron microscope (SEM) and cathodoluminescence (CL) measurements. Furthermore, the SAG/ELO of GaN using W masks by hydride vapor phase epitaxy (HVPE) are demonstrated for the first time.
Experimental Methods
The SAG of GaN using W masks was performed by atmospheric HVPE and MOVPE system on 3.0−4.5 μm thick (0001) GaN layer grown on a (0001) sapphire substrate with an low temperature (LT) buffer layer by MOVPE. A 120-nm-thick W film was deposited on the GaN surface by RF sputtering at R.T. Stripe windows of 10 μm wide with a periodicity of 20 μm was developed on the W film with conventional photolithography and wet chemical etching. The etching of W was performed with H2O2 at R.T. In the case of HVPE, GaCl and NH3 were used as the source gases, and N2 was used as the carrier gas. The flow rates of HCl and NH3 were 10 cc/min and 0.5 l/min, respectively. The growth temperature was 1090°C. In the case of MOVPE, TMG and NH3 were used as the source gases, and H2 was used as the carrier gas. The flow rates of TMG and NH3 were 18.7 μmol/min and 2.5 l/min, respectively. The growth temperature was 1060°C. More details of the preparation and growth processes by MOVPE were described in Ref. [Reference Kawaguchi, Nambu, Sone, Shibata, Matsushima, Yamaguchi, Miyake, Hiramatsu and Sawaki12].
Results and Discussion
The advantage of HVPE is at the high growth rate, which allows us to get a thick homogeneous layer and to obtain the GaN layer with good crystalline quality. According to previous results of the SAG/ELO using SiO2 mask by HVPE, the selectivity of the GaN growth on window regions was excellent and the thick ELO GaN layers had good crystalline quality [Reference Sakai, Sunakawa and Usui9,Reference Shibata, Sone, Yahashi, Yamaguchi, Hiramatsu, Sawaki and Itoh13,Reference Usui, Sunakawa, Sakai and Yamaguchi14]. Figures 1(a)-1(c) show typical SEM images of the SAG-GaN by HVPE using <1
Fig. 1 SEM images of the SAG-GaN by HVPE using the W mask along the <1
In the previous work, we succeeded in the SAG of GaN using W masks by MOVPE for the first time [Reference Kawaguchi, Nambu, Sone, Shibata, Matsushima, Yamaguchi, Miyake, Hiramatsu and Sawaki12]. In this work, we compare the W mask and the SiO2 mask in terms of shape SAG-GaN at the growth time of 120 min. No GaN polycrystals are observed on the W and SiO2 mask regions. Triangular voids were not observed on the W mask in comparison with ELO-GaN grown by HVPE (Figs. 1(b) and 1(c)). It is not clear at present why these differences occurs. There are several differences in growth conditions between MOVPE and HVPE such as growth atmosphere (H2 or N2), and source gases (TMG or Ga and HCl), which might cause the formation of voids on the W mask. The ELO of GaN is seen in the lateral direction. In the stripe pattern along the <11
Fig. 2 SEM images of the SAG-GaN by MOVPE with the growth time of 120 min. The SAG was carried out using the W mask along (a) the <11
To compare crystalline qualities of the SAG-GaN grown by MOVPE with the growth time of 120 min, we measured CL spectra at 133 K for the area of 10 μm × 10 μm on the surface, which schematically shows in Fig. 3. Figure 4 shows the CL spectra by the SAG-GaN using the W mask along (a) the <11
Fig. 3 Schematic diagrams of CL measurements. CL spectra measured from the GaN stripe region of 10 μm × 10 μm on the surface.
Fig. 4 CL spectra at 133 K from the GaN stripe obtained by the MOVPE-SAG using the W mask along (a) the <11
Conclusions
The SAG/ELO of GaN using W mask by MOVPE and HVPE was studied. In both growth methods, GaN polycrystals were observed neither on the W mask region nor on the SiO2 mask region. In the case of HVPE, the overgrown GaN region became wider, coalesced and finally formed continuous, flat and specular surface. Triangular voids were formed on the W masks due to reverse-mesa formation during the initial overgrowth stage. In the case of MOVPE, we obtained triangular cross-sectional shapes with {1
Acknowledgments
This work was partly supported by the Proposed-Based R&D Program of NEDO (97S02-015) and JSPS Research for the Future Program of Atomic Scale Surface and Interface Dynamics.
