Role of hole confinement in the recombination properties of InGaN quantum structures

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

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.

Electronic and excitonic properties of ultrathin (In,Ga)N layers: the role of alloy and monolayer width fluctuations

We present an atomistic theoretical analysis of the electronic and excitonic properties of ultrathin, monolayer thick wurtzite (In,Ga)N embedded in GaN. Our microscopic investigation reveals that (i) alloy fluctuations within the monolayer lead to carrier localization effects that dominate the electronic and optical properties of these ultrathin systems and that (ii) excitonic binding energies in these structures exceed the thermal energy at room temperature, enabling excitonic effects to persist even at elevated temperatures. Our theoretical findings are consistent with, and provide an explanation for, literature experimental observations of (i) broad photoluminescence linewidth and (ii) excitonic effects contributing to the radiative recombination process at elevated temperatures. When accounting for small structural inhomogeneities, such as local thickness fluctuations of one monolayer, "indirect" excitons may be found, with electrons and holes independently localized in different spatial positions. This result also provides further arguments for experimentally observed effects such as (i) non-exponential decay curves in time dependent photoluminescence spectra and (ii) the "S"-shape temperature dependence of the photoluminescence peak energies. Overall, our results provide fundamental understanding, on an atomistic level, of the electronic and optical properties of ultrathin, quasi 2D (In,Ga)N monolayers embedded in GaN, and offer guidance for the tailoring of their properties for potential future device applications.