Substitutional synthesis of sub-nanometer InGaN/GaN quantum wells with high indium content

I G Vasileiadis,E Dimakis,A Adikimenakis,G P Dimitrakopulos,L Lymperakis,C H Liebscher,V Devulapalli,A Gkotinakos,R Hübner,Th Karakostas,M Androulidaki,Ph Komninou,A Georgakilas

doi:10.1038/s41598-021-99989-0

Abstract

InGaN/GaN quantum wells (QWs) with sub-nanometer thickness can be employed in short-period superlattices for bandgap engineering of efficient optoelectronic devices, as well as for exploiting topological insulator behavior in III-nitride semiconductors. However, it had been argued that the highest indium content in such ultra-thin QWs is kinetically limited to a maximum of 33%, narrowing down the potential range of applications. Here, it is demonstrated that quasi two-dimensional (quasi-2D) QWs with thickness of one atomic monolayer can be deposited with indium contents far exceeding this limit, under certain growth conditions. Multi-QW heterostructures were grown by plasma-assisted molecular beam epitaxy, and their composition and strain were determined with monolayer-scale spatial resolution using quantitative scanning transmission electron microscopy in combination with atomistic calculations. Key findings such as the self-limited QW thickness and the non-monotonic dependence of the QW composition on the growth temperature under metal-rich growth conditions suggest the existence of a substitutional synthesis mechanism, involving the exchange between indium and gallium atoms at surface sites. The highest indium content in this work approached 50%, in agreement with photoluminescence measurements, surpassing by far the previously regarded compositional limit. The proposed synthesis mechanism can guide growth efforts towards binary InN/GaN quasi-2D QWs.

Highlights

III-nitride semiconductors have led to breakthroughs in optoelectronic devices due to their large range of direct band gaps, high carrier mobilities, and capacity for alloying
In series B, the quantum wells (QWs) as well as the first 1 nm of the barriers were deposited at Td = 470 °C, while the remaining 9 nm of the barriers were grown at a higher temperature (Tb = 550–650 °C) in order to obtain high crystal quality
An integrated framework of advanced HR(S)Transmission electron microscopy (TEM) methodologies permitted the quantitative determination of the indium content in these QWs either directly, through the atomic-scale quantification of Z-contrast mediated by image simulations, or indirectly by strain measurements with sub-nanometer spatial resolution

Summary

Introduction

III-nitride semiconductors have led to breakthroughs in optoelectronic devices due to their large range of direct band gaps, high carrier mobilities, and capacity for alloying. It was claimed that a dynamically mobile In/N adlayer can be ‘frozen’ in place through fast capping by the GaN barriers to avoid selective nitrogen desorption This stabilization was attributed to the nitrogen atoms directly below the indium atoms being strongly bonded to gallium atoms. The assertion that binary InN/GaN QWs can be deposited was disputed by other authors claiming that the indium content is kinetically limited to a maximum of xIn ≈ 25–33%20–22. Such studies employed almost stoichiometric or N-rich growth conditions and were correlated to I nxGa1−xN alloy o rdering[23]. We determined directly the indium content of QWs grown at 680 °C using quantitative high-resolution scanning transmission electron microscopy (HRSTEM)

Methods

Results

Discussion

Conclusion