Simplifying The Growth Of M-plane GaN
Growth of non-polar GaN on m-plane sapphire is realized without buffer layers and nitridation processes
Researchers at Kyung Hee University in Seoul, South Korea, have improved the reproducibility associated with the growth of non-polar GaN films on m-plane sapphire.
Deposition of these non-polar films, which can improve LED efficiency thanks to the absence of internal electric fields, has already been reported by a handful of groups. However, according to Chikyo Kim from Kyung Hee University, the pre-treatment conditions used by these groups differ substantially from one another, indicating that it is difficult to grow non-polar GaN films reproducibly with their approaches.
In comparison, Kim and his co-workers don’t use buffer layers or a nitridation step (exposure of a heated sapphire substrate to ammonia gas, which cracks on the surface of this wafer). “The main purpose of nitridation is to modify the top surface of a sapphire substrate by producing aluminium-nitrogen bonds, which are known to simplify subsequent growth of gallium nitride," explains Kim.
The Korean team’s study of its relatively simple growth process began by placing m-plane sapphire in a HVPE reactor and depositing GaN at growth temperatures of 950 °C to 1100 °C. Nitrogen was used as the carrier gas.
Samples were removed after 10 minutes of growth. None featured coalesced films, and X-ray diffraction analysis revealed domains with three different orientations: , and . Several growth conditions resulted in just the non-polar, m-plane orientation, including the combination of III-V ratios above 100 and growth temperatures of either 1000 °C or 1050 °C.
Scanning electron microscopy reveals the dimensions of the unmerged, m-plane GaN domains on sapphire.
The researchers scrutinized the samples with transmission electron microscopy, finding that the density of extended defects in interfacial region is higher than that for samples formed with nitridation. However, in the region away from the interface, their material that is grown with the simpler process is free from extended defects.
Increasing the growth time to 80 minutes and using flow rates of 6 sccm for HCl and 1600 sccm for ammonia led to the formation of completely merged, m-orientated films. However, in some instances, growth conditions led to films with a small proportion of domains that were not m-orientated. The surface of the coalesced films is flat, except from regions with pits that have a density of 1-2 pits µm-2.
One way to speed up the growth rate of these non-polar films is to increase the flow rate of HCl. “But an increase in the HCl flow rate could induce unwanted nucleation of other-oriented domains," warns Kim, who says that the growth technology requires further investigation.
Y. Seo et al. Appl. Phys. Express 5 121001 (2012)