A Self-Assembled Double-Island Buffer Structure for Improving GaN-Based Materials Grown on Si Substrates

Abstract

This paper reports a new method to reduce threading dislocation density in GaN epilayers grown on (111) Si substrates by metalorganic chemical vapor deposition. This method utilizes an in-situ SiN x mask to produce a self-assembled double-island structure, which effectively reduces threading dislocation density from 9.6 × 10 9 cm −2 to 2.6 × 10 9 cm −2 without using any lithographic and regrowth processes. InGaN light-emitting diodes fabricated on the double-island buffer layer exhibit nearly 20% improvement on the optical output power as well as less energy localization effect, compared to those without the double-island buffer layer. Growing GaN on silicon (111) substrates has been regarded as a promising approach to achieving low-cost GaN-based light-emitting diodes and power devices. To grow a high quality GaN epitaxial layer on silicon substrates, it is necessary to overcome the cracking of GaN layer, which originates from the large difference in the thermal expansion coefficient between GaN and silicon.1 The other key issue to resolve is the high dislocation density of up to 10 10 cmin GaN due to the their 17% lattice constant mismatch. So far, there have been several methods proposed to tackle the cracking and high dislocation density issues, such as multi-epitaxial-lateral-overgrowth (multi-ELO), 2 graded AlGaN insertion layer, 3 AlGaN/GaN superlattice, and patterned substrate. 4 In situ deposition of a natural SiN x nano mask for lateral growth has been utilized to reduce dislocation density in GaN on sapphire. 5,6 It is reported that using a SiN x interlayer in GaN grown on Si (111) substrates cannot only reduce dislocations but also decrease thermal stress on GaN.7 Similar to the mechanism of epitaxial lateral overgrowth, the in-situ SiN x nanomask results in nucleation of nano size GaN islands with the {1-101} facets, which bend the threading dislocations from the <0001> directions toward the <1-100> directions.8 However, new dislocations are generated in the regions where islands coalesce. 9In this study, we develop a new growth method, which is a selfassembled island growth method based on two layers of in-situ SiN x mask, to lower the dislocation density in GaN without any regrowth processes. InGaN/GaN light-emitting diodes (LEDs) with improved performance are demonstrated on Si substrates using the double-island structure. Sample PreparationAll the samples in this study were grown on 2" Si (111) substrates using metal-organic chemical vapor deposition (MOCVD). Before the epitaxial growth, the native oxide of silicon substrates was chemically removed in hydrofluoric acid. A reference sample without any island structure was grown on a conventional buffer layer, consisting of 60 nm AlN, 100 nm Al 0.2 Ga 0.8 N, and 500 nm un-doped GaN. The samples with a single-or a double-island structure were formed by depositing an in-situ SiN x mask on the conventional buffer layer between the Al 0.2 Ga 0.8 N and GaN layers. The double-island structure was formed by repeating the growth sequence once more after t...