Bandgap atomistic calculations on hydrogen-passivated GeSi nanocrystals

Ovidiu Cojocaru,Toma Stoica,Magdalena Lidia Ciurea,Ana-Maria Lepadatu,George Alexandru Nemnes

doi:10.1038/s41598-021-92936-z

Abstract

We present a detailed study regarding the bandgap dependence on diameter and composition of spherical Ge-rich GexSi1−x nanocrystals (NCs). For this, we conducted a series of atomistic density functional theory (DFT) calculations on H-passivated NCs of Ge-rich GeSi random alloys, with Ge atomic concentration varied from 50 to 100% and diameters ranging from 1 to 4 nm. As a result of the dominant confinement effect in the DFT computations, a composition invariance of the line shape of the bandgap diameter dependence was found for the entire computation range, the curves being shifted for different Ge concentrations by ΔE(eV) = 0.651(1 − x). The shape of the dependence of NCs bandgap on the diameter is well described by a power function 4.58/d1.25 for 2–4 nm diameter range, while for smaller diameters, there is a tendency to limit the bandgap to a finite value. By H-passivation of the NC surface, the effect of surface states near the band edges is excluded aiming to accurately determine the NC bandgap. The number of H atoms necessary to fully passivate the spherical GexSi1−x NC surface reaches the total number atoms of the Ge + Si core for smallest NCs and still remains about 25% from total number of atoms for bigger NC diameters of 4 nm. The findings are in line with existing theoretical and experimental published data on pure Ge NCs and allow the evaluation of the GeSi NCs behavior required by desired optical sensor applications for which there is a lack of DFT simulation data in literature.

Highlights

We present a detailed study regarding the bandgap dependence on diameter and composition of spherical Ge-rich GexSi1−x nanocrystals (NCs)
GeSi NC is built in a similar way as Ge NC, but Ge atoms are randomly substituted by Si atoms to obtain the desired Ge concentration
Spherical H-passivated Ge-rich GexSi1−x NCs with Ge atomic concentration in the range of 50–100% and diameters from 1 to 4 nm were theoretically studied by atomistic density functional theory (DFT) calculations using the SIESTA software

Summary

Introduction

We present a detailed study regarding the bandgap dependence on diameter and composition of spherical Ge-rich GexSi1−x nanocrystals (NCs). Quantum confinement effect was evidenced in small Ge and GeSi N Cs16–18, enabling bandgap engineering along with composition19, shape[20] and strain[21,22] leading to tuning of optical and photoelectrical properties of NCs. It was shown that two mechanisms compete in achieving no-phonon radiative transitions in NCs of Si and Ge that are indirect bandgap semiconductors in bulk. One mechanism is related to the relaxation of momentum conservation law due to the spatial confinement and Heisenberg uncertainty principle, being dominant in both Si and Ge NCs, and the other mechanism is the inter-valley coupling between direct and indirect states induced by the interface of the NC with the embedding m atrix[23] Another way of bandgap tuning is by tailoring its level of directness, as recently demonstrated direct bandgap light emission in Ge and GeSi nanowires with hexagonal. By employing DFT computation it was shown that the local electronegativity of the defects is strongly dependent upon the nearest neighbor environment[33]

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Conclusion