Substitutional Synthesis of sub-nm Thick InGaN QWs
The synthesis of InGaN digital alloys in the form of short period InGaN/GaN superlattices is investigated by combining ab-initio and empirical potential calculations with PAMBE growth, Photoluminescence Spectroscopy, and HR(S)TEM characterization.
A major bottleneck towards achieving highly-efficient and chromatically-tunable optoelectronic devices operating across the optical spectrum relates to the possibility of bandgap engineering by tuning the indium content in InGaN QWs embedded in GaN, problem that is commonly referred to as the Green Gap, the dramatic decrease of the internal quantum efficiency of in the green region of the optical spectrum.
![Left: Chemical potential μInNps of InN epilayers pseudomorphic on GaN, i.e., biaxially strained to GaN, as function of the epilayers' thickness n for different surface terminations/reconstructions (see inset). μInNps is referred to the chemical potential of relaxed InN, μInN0. Due to the host matrix strengthening effect, the first InN ML is strongly bound to the GaN surface and is more stable at higher temperatures compared to thicker InN epilayers. Nevertheless, the chemical potential of a single InN ML on GaN is still larger than that of relaxed InN. Hence, a ML of InN on GaN decomposes at lower temperature than relaxed InN. Therefore,the host matrix strengthening cannot support the stabilization of a binary InN ML on GaN at temperatures above ≈500 °C. Right: Representative cross-sectional HRSTEM image of MQW heterostructures recorded along the [11-20] zone axis.](/4681445/original-1642605837.jpg?t=eyJ3aWR0aCI6ODQ4LCJmaWxlX2V4dGVuc2lvbiI6ImpwZyIsIm9ial9pZCI6NDY4MTQ0NX0%3D--6c77ba5a97325ed166aaa03916ecd2ba3e8ab58c "Left: Chemical potential μInNps of InN epilayers pseudomorphic on GaN, i.e., biaxially strained to GaN, as function of the epilayers' thickness n for different surface terminations/reconstructions (see inset). μInNps is referred to the chemical potential of relaxed InN, μInN0. Due to the host matrix strengthening effect, the first InN ML is strongly bound to the GaN surface and is more stable at higher temperatures compared to thicker InN epilayers. Nevertheless, the chemical potential of a single InN ML on GaN is still larger than that of relaxed InN. Hence, a ML of InN on GaN decomposes at lower temperature than relaxed InN. Therefore,the host matrix strengthening cannot support the stabilization of a binary InN ML on GaN at temperatures above ≈500 °C. Right: Representative cross-sectional HRSTEM image of MQW heterostructures recorded along the [11-20] zone axis.")
![Left: Chemical potential μInNps of InN epilayers pseudomorphic on GaN, i.e., biaxially strained to GaN, as function of the epilayers' thickness n for different surface terminations/reconstructions (see inset). μInNps is referred to the chemical potential of relaxed InN, μInN0. Due to the host matrix strengthening effect, the first InN ML is strongly bound to the GaN surface and is more stable at higher temperatures compared to thicker InN epilayers. Nevertheless, the chemical potential of a single InN ML on GaN is still larger than that of relaxed InN. Hence, a ML of InN on GaN decomposes at lower temperature than relaxed InN. Therefore,the host matrix strengthening cannot support the stabilization of a binary InN ML on GaN at temperatures above ≈500 °C. Right: Representative cross-sectional HRSTEM image of MQW heterostructures recorded along the [11-20] zone axis.](/4681445/original-1642605837.jpg?t=eyJ3aWR0aCI6MjQ2LCJvYmpfaWQiOjQ2ODE0NDV9--1650ded47d2b6d3c8d45aaf129fc36f68d76c9ac)
Right: Representative cross-sectional HRSTEM image of MQW heterostructures recorded along the [11-20] zone axis.
Digital InGaN alloys in the form of InGaN/GaN short period superlattices are investigated as a promising route to successfully address the green gap. In this work we combine atomistic calculations with PAMBE growth and HR(S)TEM characterization to investigate the growth of sub-nm thick InGaN QWs embedded in GaN host matrix. In order to mitigate the Gallium vs Indium competition (the Ga-N bonds are stronger than the In-N bonds) a series of growth circles consisting of modulation of the Indium and Ga fluxes are applied: InN deposition on a dry GaN surface, i.e., no excess Gallium is present on the surface, is followed by a GaN deposition and annealing and nitridation steps.
It has been proposed that a single ML of InN on GaN can be stabilized at considerably higher temperatures than fully relaxed thick InN epilayers, due to a host matrix strengthening effect. Our DFT calculations demonstrate that the first InN ML is indeed strongly bound to the GaN surface and is more stable at higher temperatures compared to thick InN epilayers. However, the single ML InN on GaN still decomposes at lower temperature than relaxed InN (see Figure above).
Despite the fact that a single ML of InN cannot be grown at high temperature (higher than ≈500 °C), there is a strong driving force for In to substitute for Ga at the GaN surface. The maximum In content is limited by the decomposition temperature of InxGa1-xN. However, the actual In content is defined by kinetics i.e., the substitutional kinetic barrier, the temperature, and the time the surface is exposed to indium flux and/or indium adlayer(s). In the next stage of GaN deposition, there are two mutually compensating active mechanisms: As in the first stage, there is a strong tendency for Ga to substitute for In and hence reduce the In content in the QW. However, the growth of GaN kinetically stabilizes the incorporated In.
In the present project single ML InGaN QWs with In content above 40% have been demonstrated. The substitutional synthesis of InGaN MQWs is controlled by both thermodynamics which define the maximum In content and by kinetics which define the actual In content. The substitutional barriers are the key atomistic parameters while temperatures and deposition times can be employed to optimize the synthesis of these alloys.
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Mr. I. G. Vasileiadis Mr. A. Gkotinakos Prof. George Dimitrakopulos Prof. Ph. Komninou Prof. Th. Karakostas |
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Ms. M. Androulidaki Prof. A. Georgakilas |
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Dr. R. Hübner Dr. E. Dimakis |