Microstructure and Interfaces of Ultra-Thin Epitaxial AlN Films Grown by Plasma-Enhanced Atomic Layer Deposition at Relatively Low Temperatures

Ramasis Goswami,Charles Eddy,Neeraj Nepal,Syed Qadri

doi:10.3390/coatings11040482

Abstract

We demonstrate the growth of ultra-thin AlN films on Si (111) and on a GaN/sapphire (0001) substrate using atomic layer epitaxy in the temperature range of 360 to 420 °C. Transmission electron microscopy and X-ray diffraction were used to characterize the interfaces, fine scale microstructure, and the crystalline quality of thin films. Films were deposited epitaxily on Si (111) with a hexagonal structure, while on the GaN/sapphire (0001) substrate, the AlN film is epitaxial and has been deposited in a metastable zinc-blende cubic phase. Transmission electron microscopy reveals that the interface is not sharp, containing an intermixing layer with cubic AlN. We show that the substrate, particularly the strain, plays a major role in dictating the crystal structure of AlN. The strain, estimated in the observed orientation relation, is significantly lower for cubic AlN on hexagonal GaN as compared to the hexagonal AlN on hexagonal GaN. On the Si (111) substrate, on the other hand, the strain in the observed orientation relation is 0.8% for hexagonal AlN, which is substantially lower than the strain estimated for the cubic AlN on Si(111).

Highlights

Aluminum nitride is a III-N compound semiconductor with a band gap in the range of 6.01–6.05 eV at room temperature [1]
To analyze the structure of the AlN film, we extracted the fast Fourier transform (FFT) from the film and the substrate, and we compared it with the Fast Fourier transforms (FFTs) of the substrate in the [] zone
We demonstrated the growth of ultra-thin AlN films on Si (111) and on GaN/(0001) sapphire substrate using atomic layer epitaxy at relatively low temperatures

Summary

Introduction

Aluminum nitride is a III-N compound semiconductor with a band gap in the range of 6.01–6.05 eV at room temperature [1]. This semiconducting material is a useful candidate in a number of potential applications in microelectronics because of its relatively high thermal conductivity, high electrical resistivity, low thermal expansion, chemical stability in air up to 1380 ◦C, and excellent thermal shock resistance. The epitaxial crystalline aluminum nitrides are used to manufacture surface acoustic wave sensors because of the piezoelectric properties of AlN [2,3] It exhibits both wurtzite and zinc-blende phases as do other III-N compounds. It has been reported that temperatures of 900 ◦C and higher are usually needed to obtain high-quality AlN films by these methods

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