Hydrogen and Nitrogen Ambient Effects on Epitaxial Lateral Overgrowth (ELO) of GaN VIA Metalorganic Vapor-Phase Epitaxy (Movpe)

Abstract

Ambient gas effect on the epitaxial lateral overgrowth (ELO) of GaN via metalorganic vapor-phase epitaxy (MOVPE) on a MOVPE-grown GaN (0001) / sapphire (0001) substrate with a SiO 2 stripe mask has been studied by means of field-emission scanning electron microscopy (SEM) and highresolution X-ray diffraction (XRD) analysis. Different ambient gases of nitrogen, hydrogen and their mixture (mixture ratio, hydrogen : nitrogen = 1 : 1) affect the lateral overgrowth rate, the surface morphology and the crystalline tilting of ELO-GaN layers. XRD revealed that the ELO-GaN layer on the SiO 2 mask aligned along the <1 -100> direction exhibited anisotropic crystalline tilting toward <11 -2 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 10 7 cm -2 , which is comparable to that in hydrogen ambient. IntroductionRecently, 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 [1,2]. 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 10 9 -10 10 cm -2 [3]. 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 [4].The SAG in MOVPE on GaN/sapphire and AlGaN/sapphire with SiO 2 stripe patterns was published for the first time in 1994 by Kato et al. [5], followed by that on GaN/sapphire [6]. Intensive studies on SAG have revealed that the ELO effectively reduces the dislocation density [4,[7][8][9][10][11]. The crystalline properties and the growth mechanism of ELO-GaN can be affected by mask patterns such as stripe direction [10][11][12][13] and growth conditions such as growth temperature [10], source gases [10], 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.

Epitaxial Lateral Overgrowth of GaN

“…The lateral overgrowth rates and the equilibrium facets developed in the process depend on both the crystallographic orientation of the stripe openings and the growth parameters, temperature, V/III ratio in the vapour phase, pressure, composition of the carrier gas (H 2 , N 2 ), fill factor (ratio of the stripe opening to the pattern period) [59][60][61][62][63]. It has been demonstrated that the lateral expansion is easier when the stripes are aligned along h1 1 100i GaN direction [44].…”

“…In short, from experiments carried out on h1 1 100i stripes on MOVPE GaN, Tadatomo et al [61] and Kawaguchi et al [68][69][70] concluded that H 2 as a carrier gas produces smooth surfaces, but a low lateral growth rate. Conversely, N 2 enhances the lateral growth rate, but the surface quality is poor.…”

Facet-controlled three-step growth of high-quality GaN on sapphire substrates by mass-production-type metalorganic vapor phase epitaxy

“…Hiramatsu et al reported that growth pressure control has a similar effect on facet formation to growth temperature in facet-controlled ELO (FACELO) technique [19]. Hydrogen and nitrogen ambient effects on facet formation have been reported by Tadatomo et al [20]. In this paper, facet control-based three-step growth was demonstrated using a mass-production-type 10 Â 2 00 MOVPE system.…”

“…It is reported that low growth pressure has a similar effect on facet formation to high growth temperature [19]. Additionally, the carrier gas mixture ratio also affects facet formation [20]. Hence, total optimization of growth process is important including t FC , T FC , P FC and R FC .…”

Low-cost high-quality GaN by one-step growth

“…Usui et al [2] and Nam et al [3] were the first to investigate GaN MOVPE growth by the ELO technique in 1997 and achieved low dislocation densities of the order of 10 7 cm À2 instead of 10 10 cm À2 for the best GaN/sapphire template. The control of the dislocation density in ELO samples is closely related to the initial ELO GaN structure, via several experimental parameters: the pattern orientation [3], the mask material [4], the mask fill factor [5,6], the growth temperature [7], the flow rate of gas sources [7], the ambient carrier gas [8], the impurity level [9], the template quality [10] and the reactor pressure [11]. For the HVPE process, the influence of the carrier-gas composition [12,13], the growth temperature [12] and the GaN template quality [14] were studied.…”