Molecular Beam Homoepitaxy of N-polar AlN on bulk AlN substrates

TL;DR

This study demonstrates successful N-polar AlN homoepitaxy on bulk AlN substrates using plasma-assisted molecular beam epitaxy, achieving smooth surfaces, confirmed polarity, and low impurity levels, advancing the growth of high-quality N-polar AlN layers.

Contribution

It introduces a method combining in situ surface cleaning and high-temperature growth to achieve high-quality N-polar AlN homoepitaxy on bulk substrates.

Findings

Achieved smooth N-polar AlN surfaces with atomic steps.

Confirmed N-polarity via KOH etching.

Impurity levels of Si and H below noise, with low O and C concentrations.

Abstract

N-polar AlN epilayers were grown on the N-face of single crystal bulk AlN substrates by plasma assisted molecular beam epitaxy (PA-MBE). A combination of in situ thermal deoxidation and Al-assisted thermal desorption at high temperature helped in removing native surface oxides and impurities from the N-polar surface of the substrate enabling successful homoepitaxy. Subsequent epitaxial growth of AlN layer on the in situ cleaned substrates, grown in sufficiently high Al droplet regime, exhibited smooth surface morphologies with clean and wide atomic steps. KOH etch studies confirmed the N-polarity of the homoepitaxial films. Secondary ion mass spectrometry profiles show Si and H impurity concentrations below the noise levels, whereas O and C impurities concentrations of ~ 8x10^{17} atoms/cm^3 and ~ 2x10^{17} atoms/cm^3 are observed respectively. Though the structural defect densities are low, they interestingly appear as inversion domains of different dimensionalities.