Growth and Characterization of InGaN Films by Plasma-assisted Molecular Beam Epitaxy

Abstract

There has been a great deal of interest in developing relaxed InGaN pseudo-substrates for the fabrication of efficient red/amber/green light emitting diodes (LEDs) and laser diodes (LDs) for augmented/virtual reality applications. Additionally, relaxed InGaN pseudo-substrates can be potentially advantageous to further increase the operating frequency of GaN-based high electron mobility transistors (HEMTs) that are being used for high-power RF applications. However, it has been widely believed that achieving full or even partial relaxation of an InGaN layer via abrupt growth of InGaN film on GaN is not feasible due to the formation of a high density of defects.

My Ph.D. thesis was focused on the development of relaxed InGaN pseudo-substrates by using novel approaches such as growth on alternative substrates (e.g. ZnO or compliant porousified GaN). For this purpose, plasma-assisted molecular beam epitaxy was utilized for the epitaxial growth of InGaN films. I used several characterization techniques, including X-ray diffraction (XRD), atom probe tomography (APT), transmission electron microscopy (TEM), and atomic force microscopy (AFM).

In the process of developing InGaN pseudo-substrates, I discovered certain growth conditions that lead to the spontaneous formation of superlattice (SL) structures, composed of InGaN layers with different compositions. I then further investigated the impact of GaN polarity, In flux, growth temperature, and strain on the formation of self-assembled super lattice (SASL) structures. I also showed that the formation of such SASL structures allowed the growth of InGaN films beyond their critical thickness coherently strained to the underlying GaN. This discovery can have potential applications for the fabrication of various optoelectronic devices such as photovoltaic cells, photo-detectors, LEDs, and LDs.

Therefore, to achieve relaxed InGaN films, a unique approach, an epitaxial growth on O-face ZnO substrates was also studied, as ZnO is lattice matched to In0.2Ga0.8N. For this purpose, atomically smooth GaN films, showing step edges, were first developed at low temperatures to suppress the interfacial reaction between nitrides and the ZnO substrate at elevated temperatures using metal-enhanced epitaxy. InGaN films as thick as 1 µm were grown. Although using the low-temperature GaN interlayer was shown to improve the InGaN structural quality, the dislocation density remained high.

To achieve relaxed-InGaN with high structural quality, growth on porosified GaN was then explored as an alternative approach. The impact of InGaN thickness and different compositionally graded structures on InGaN relaxation grown on tiled porous-GaN pseudo-substrates (PSs) was studied. In addition, the impact of the degree of porosification on the In incorporation and relaxation of InGaN was examined. 82% relaxed 1µm thick In0.18Ga0.82N, which is equivalent to a fully relaxed In-composition of 15%, on porous GaN PS, was obtained. Additionally, multi-quantum wells (MQWs) grown on the MBE InGaN-on-porous GaN base layers by MOCVD showed ~85 nm redshift in comparison with MQWs grown on planar GaN.

There can be several practical applications for the spontaneous formation of SASL in solar cells and optoelectronics (LEDs, lasers, photodetectors, and etc.). Additionally, next-generation electronic devices, including hot electron transistors (HETs) [1], InGaN-channel HEMTs could potentially benefit from relatively high temperature (690 °C) growth of high-quality InGaN and spontaneous formation of InGaN/GaN SASL. Moreover, high-quality relaxed InGaN with high In content can be utilized to get efficient red LEDs, especially micro-LED displays. Porous GaN can be used as a pseudo-substrate for both (Al,Ga)N and (In,Ga)N related alloys to cover whole visible spectrum.