Intentional polarity conversion of AlN epitaxial layers by oxygen

N Stolyarchuk,K March,J Zúñiga-Pérez,A Courville,T Markurt,M Albrecht,P Vennéguès

doi:10.1038/s41598-018-32489-w

Abstract

Nitride materials (AlN, GaN, InN and their alloys) are commonly used in optoelectronics, high-power and high-frequency electronics. Polarity is the essential characteristic of these materials: when grown along c-direction, the films may exhibit either N- or metal-polar surface, which strongly influences their physical properties. The possibility to manipulate the polarity during growth allows to establish unique polarity in nitride thin films and nanowires for existing applications but also opens up new opportunities for device applications, e.g., in non-linear optics. In this work, we show that the polarity of an AlN film can intentionally be inverted by applying an oxygen plasma. We anneal an initially mixed-polar AlN film, grown on sapphire substrate by metal-organic vapor phase epitaxy (MOVPE), with an oxygen plasma in a molecular beam epitaxy (MBE) chamber; then, back in MOVPE, we deposit a 200 nm thick AlN film on top of the oxygen-treated surface. Analysis by high-resolution probe-corrected scanning transmission electron microscopy (STEM) imaging and electron energy-loss spectroscopy (EELS) evidences a switch of the N-polar domains to metal polarity. The polarity inversion is mediated through the formation of a thin AlxOyNz layer on the surface of the initial mixed polar film, induced by the oxygen annealing.

Highlights

Group III-nitride materials (AlN, GaN, InN and their alloys) lack inversion symmetry along the c-direction, i.e., the films exhibit either an N-polar or a metal (Al, Ga, In) polar surface
scanning transmission electron microscopy (STEM) and energy-loss spectroscopy (EELS) experiments showed that oxygen atoms partially substitute nitrogen atoms in the N-polar wurtzite lattice, bond with Al-atoms, and form an AlxOyNz inversion domain boundary, which inverts the polarity to Al
Summarizing, we have demonstrated a way to intentionally invert N-polar AlN layers to Al-polar by introducing annealing in an oxygen-rich environment into the metalorganic vapor phase deposition (MOVPE) growth process

Summary

Introduction

Group III-nitride materials (AlN, GaN, InN and their alloys) lack inversion symmetry along the c-direction, i.e., the films exhibit either an N-polar or a metal (Al, Ga, In) polar surface. A conventional approach to achieve metal-polar films in metalorganic vapor phase deposition includes nitridation of the sapphire substrate and deposition of a low-temperature buffer layer; without a buffer layer the film exhibits mixed polarity[5,6,7,8] In the latter case, lateral overgrowth of either N- or metal-polar domains can be promoted to obtain a single polarity, by applying appropriate growth conditions[9,10,11]. Other research groups already demonstrated intentional conversion from metal- to N-polarity by Mg exposure of the growth surface during both metalorganic vapor phase deposition (MOVPE) and molecular beam epitaxy (MBE) In this case, a zig-zag shaped inversion domain boundary, formed of Mg3N2, promotes the polarity conversion in GaN18–21. The atomic structure of the AlxOyNz is in excellent agreement with the one that forms during nitridation of sapphire (0001) substrates in an MOVPE reactor[22]

Methods

Results

Discussion

Conclusion