00> direction exhibited anisotropic crystalline tilting toward <11
0>. For ELO-GaN growth in nitrogen ambient, the growth rate of the (0001) facet decreases, the lateral overgrowth rate increases and the tilting of the ELO-GaN layer increases, while no smooth surface is obtained, in comparison with ELO-GaN growth in hydrogen ambient. For the mixture ambient, a smooth surface with a fast lateral overgrowth rate is achieved and the dislocation density is not more than 101. Introduction
Recently, high-performance optical devices such as light-emitting diodes (LEDs) and laser diodes (LDs) in green and ultraviolet regions have been developed using GaN, AlGaN and InGaN compound semiconductors [Reference Nakamura, Mukai and Senoh1, Reference Nakamura, Senoh, Nagahama, Iwasa, Yamada, Matsushita, Kiyoku and Sugimoto2]. These GaN-related devices are usually fabricated on a sapphire substrate, because there is no large-scale substrate for GaN heteroepitaxial growth. Due to the large differences in lattice constant, thermal expansion coefficient and chemical nature between GaN and sapphire, many threading dislocations are formed from the GaN or AlN buffer layer on the sapphire substrate, and the dislocation density has been reported to be of the order of 109 −1010 cm−2 [Reference Lester, Ponce, Craford and Steigerwald3]. Although high-brightness LEDs have been realized in spite of the high dislocation density, the reduction of the dislocation density is desired to improve the performance and reliability of these devices. The epitaxial lateral overgrowth (ELO) technique based on the selective area growth (SAG) has recently attracted considerable attention, since the fabrication of blue-violet laser diodes on the ELO-GaN layer using this technique resulted in the achievement of a long lifetime of more than 10,000 hours [Reference Nakamura, Senoh, Nagahata, Iwasa, Yamada, Matsushita, Kiyoku, Sugimoto, Kozaki, Umemoto, Sano and Chocho4].
The SAG in MOVPE on GaN/sapphire and AlGaN/sapphire with SiO2 stripe patterns was published for the first time in 1994 by Kato et al. [Reference Kato, Kitamura, Hiramatsu and Sawaki5], followed by that on GaN/sapphire [Reference Li, Jones, Roh, Turnbull, Reuter, Gu, Bishop and Coleman6]. Intensive studies on SAG have revealed that the ELO effectively reduces the dislocation density [Reference Nakamura, Senoh, Nagahata, Iwasa, Yamada, Matsushita, Kiyoku, Sugimoto, Kozaki, Umemoto, Sano and Chocho4, Reference Usui, Sunakawa, Sakai and Yamaguchi7-Reference Matsushima, Yamaguchi, Hiramatsu and Sawaki11]. The crystalline properties and the growth mechanism of ELO-GaN can be affected by mask patterns such as stripe direction [Reference Kapolnek, Keller, Vetury, Underwood, Kozodoy, DenBaars and Mishra10-Reference Beaumont, Gibart, Vaille, Haffouz, Nataf and Bouille13] and growth conditions such as growth temperature [Reference Kapolnek, Keller, Vetury, Underwood, Kozodoy, DenBaars and Mishra10], source gases [Reference Kapolnek, Keller, Vetury, Underwood, Kozodoy, DenBaars and Mishra10], growth pressure and ambient gas. However, the effects of ambient gas remain unclear. We investigate the effects of hydrogen gas, nitrogen gas and their mixture in an atmospheric MOVPE on ELO-GaN.
2. Experimental
A conventional atmospheric MOVPE apparatus with a horizontal reactor was used. A 3-μm-thick undoped MOVPE-grown GaN on sapphire using a low-temperature GaN buffer layer was used as the substrate. A SiO2 stripe pattern with a 4 μ m window width and a 4 μm mask width was aligned along the <11
The growth rates of the lateral face and the c-facet of ELO-GaN were estimated from the field emission scanning electron microscopy (SEM) images and the growth times.
In order to measure the dislocation density in the ELO-GaN layers, we observed the pits on an In0.2Ga0.8N layer (100 nm-thick) grown on an ELO-GaN layer. The growth pit density (GPD) on the InGaN layer is considered to correspond to the dislocation density of the underlying ELO-GaN layer [Reference Hiarmatsu, Matsushima, Shibata, Sawaki, Tadatomo, Okagawa, Ohuchi, Honda and Matsue14].
In order to investigate the crystallographic structure of the ELO-GaN layers, ω-scan X-ray diffraction (XRD) measurements were performed on the ELO-GaN (0004) plane as a function of ϕ (ϕ: the rotation angle of the sample about its surface normal), and reciprocal space mapping measurements were also carried out using a high-resolution X-ray diffractometer (Philips X’ Pert MRD).
3. Results and Discussions
Figures 1(a) and (b) show SEM images of GaN on the <1
Figure 1 SEM images of GaN on SiO2 stripe pattern along the <1
The fast growth rate of the (0001) facet results in the line structure of GaN with only the {1
Figures 2(a) and (b) show SEM images of the ELO-GaN layer on the <1
Figure 2 SEM images of GaN layer grown by ELO on SiO2 stripe pattern along the <1
In order to increase the lateral overgrowth rate similar to that like in nitrogen ambient and to improve surface morphology of the ELO-GaN layer similar to that in hydrogen ambient, their mixture ambient was employed for the growth. Figure 3 shows SEM images of GaN grown on the <1
Figure 3 SEM images of GaN layer grown by ELO on SiO2 line pattern along the <1
Table 1 Lateral growth rate and (0001) facet growth rate of ELO-GaN on the <1
| Lateral growth rate (μm/hr) | (0001) facet growth rate (μm/hr) | |
|---|---|---|
| H2 ambient | 2.6 | 5.5 |
| H2+N2 ambient | 3.0 | 4.0 |
| N2 ambient | 4.3 | 2.1 |
Figure 4 shows the distribution of pits on InGaN surface grown on the ELO-GaN in the mixture ambient, where the dots were obtained from the SEM image, and the bars indicate the positions of SiO2 masks under the ELO-GaN. The pit density can be seen in the coalescence regions on the masks and in the window regions. The density of dislocations above the masks decreased significantly, but dislocations were observed in the coalescence regions on the center of the masks. The dislocation densities obtained from the SEM image are less than 107 cm−2 above the masks and 4×108 cm−2 above the window regions, respectively, as shown in Table 2. These are of the same order as the dislocation density in hydrogen ambient [Reference Hiarmatsu, Matsushima, Shibata, Sawaki, Tadatomo, Okagawa, Ohuchi, Honda and Matsue14].
Figure 4 Distribution of pits on InGaN grown on GaN layer by ELO in the mixture ambient. Bars indicate the positions of SiO2 masks under the ELO-GaN.
Table 2 Dislocation density of ELO-GaN grown in the mixture.
| On mask | On window | |
| ELO region | Coalescence region | |
| < 107 cm−2 | 6 × 107 cm−2 | 4 × 108 cm−2 |
Figure 5 shows the full-widths at half maximum (FWHMs) of the XRD rocking curves for the (0004) plane as a function of ϕ. It is found that the FWHMs of the ELO-GaN layer are dependent on ϕ. The maximum FWHMs are observed at ϕ=90° and 270° (<11
Figure 5 The FWHMs of XRD rocking curves for the (0004) GaN plane as a function of ϕ. Ref. H2 indicates GaN on sapphire without ELO in hydrogen ambient.
4. Conclusion
We investigated the effects of hydrogen ambient, nitrogen ambient and their mixture ambient on ELO-GaN via atmospheric MOVPE. In hydrogen ambient, the ELO-GaN with a smooth surface was obtained using the <1
5. Acknowledgements
The authors would like to acknowledge the encouragement and support of Tadao Tachibana, Kouichi Suzuki and Keisuke Imai of Mitsubishi Cable Industries, Ltd.
