Novel growth technique for the reducing dislocation density in GaN on sapphire substrate

Abstract

The selective lateral growth of GaN with extremely low dislocation density has been achieved for the first time directly on a masked sapphire substrate without a GaN epitaxial underlayer via metalorganic vapor phase epitaxy. Both the mask material for selective growth and the growth conditions have been optimized for this complete selective growth. Dislocation densities less than 5 × 10 6 cm -2 have been obtained when SiN was used as a mask and GaN was used as a low-temperature buffer layer. This reduction in threading dislocation density results from bending the threading dislocations toward the facet surfaces, which has been confirmed by cross-sectional transmission electron microscopy observations. This growth technique is very promising for practical device production, such as that for blue laser diodes, because a complete device structure with extremely low dislocation density can be grown by a single growth process.1 Introduction GaN-based materials are useful wide-gap semiconductors for optical and electronic devices, such as short-wavelength laser diodes (LDs), light emitting diodes (LEDs), and high-power field effect transistors (FETs). Some optical devices have already successfully commercialized, and it is expected that more GaN-related devices will be developed in the future. Since GaN substrates are not widely used, most of GaN-based devices are fabricated on sapphire substrates. There is a large mismatch of the lattice constant between the epitaxial GaN layer and the sapphire substrate, which results in generation of considerable threading dislocations in epitaxial GaN layers grown on sapphire substrates. These dislocations reduce the lifetime, in particular for LDs.Reducing the threading dislocation density is therefore an important issue for fabricating GaN-based devices on sapphire substrates. As a method for reducing threading dislocations, the epitaxial lateral overgrowth (ELO) [1,2] and Pendeo methods [3] are well known. These methods have helped to improve device characteristics such as the lifetime of LDs and the output power of ultraviolet LEDs. However, these methods require at least two crystal growth processes. In the first process, a GaN epitaxial underlayer is grown on the sapphire substrate. In the second process, selective lateral growth is conducted to reduce the dislocation density. Thus, in order to perform selective lateral growth, a thick GaN epitaxial underlayer (about 3 µm) is necessary on the sapphire substrate. This leads to an increase in the total thickness of the grown layer. Since the bending of the wafer that is caused by the increase in the total thickness of the grown layer lowers the accuracy in the photolithography processes, the yield is reduced.