Interface-engineering enhanced light emission from Si/Ge quantum dots

Abstract

Si quantum dots (QDs) have a significant improvement in luminous efficiency compared with bulk Si, achieved by alleviating the forbiddance of no-phonon Γ–Γ radiative transition determined by the law of momentum conservation. Two divergent mechanisms have been proposed to account for the breakdown of momentum conservation in Si QDs, one is due to the space-confinement-induced spread of k-space wave functions associated with Heisenberg uncertainty principle Δr · Δk > 1/2, and the other is due to the interface-effect-induced intervalley mixing between indirect and direct bandgap states. Both mechanisms could cause a small overlap of the electron and hole wave functions in k-space and make vertical transitions allowed, which leads to the zero-phonon light emission. In this work, we unravel the hierarchical relationship between these two primary mechanisms in the process of zero-phonon light emission from indirect bandgap QDs, by performing semiempirical pseudopotential calculation including many-body interaction on the room-temperature luminescent properties of a series of Si, Ge, and Ge/Si core/shell QDs. We show that the space confinement mechanism is dominant in both Si and Ge indirect bandgap QDs, and the interface-induced intervalley coupling mechanism plays a minor role. While in Ge/Si core/shell QDs, the interface-induced intervalley coupling mechanism has a more pronounced contribution to enhanced light emission, implying one can further enhance light emission via engineering interface based on the intervalley coupling mechanism. Given this, we further engineer the Ge QD interface by bringing four motifs of Si/Ge multiple layers from previously inverse designed Si/Ge superlattices and core/shell nanowires for light emitters. We show that two out of four motifs always give rise to two orders of magnitude enhancement in light emission relative to the Ge and Si QDs. We demonstrate that the interface engineering can enhance light emission in indirect bandgap QDs substantially and regulate the intervalley coupling mechanism as the primary factor over the space confinement mechanism in breaking the momentum conservation law.

Bandgap atomistic calculations on hydrogen-passivated GeSi nanocrystals

“…The passivation of NC surface is mandatory for excluding any surface localized states near the band edges as the calculations focus on the bandgap 23 , but also to ensure the computation convergence. The diameter dependences of the number of H atoms and the number of Ge and Si atoms in each NC (listed in Table S1 in SI) resulting from the model are shown in Fig.…”

“…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 matrix 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 structure 5 , 24 , and GeSi quantum dots 25 and also by infrared detection extended to longer wavelengths in NCs of direct bandgap GeSn alloys 26 .…”

A new direct band gap Si–Ge allotrope with advanced electronic and optical properties

“…The band gap of hexagonal Ge can be tuned by alloying with Si, according to studies by Fadaly et al, who reported a temperature-insensitive tunable emission in hexagonal SiGe alloy nanowire, obtained experimentally. 2 Besides hexagonal Ge, other ingenious approaches been proposed, such as straining, 3 nanostructuring [4][5][6] or amorphization. 7 In recent years, with the help of techniques like genome algorithms, there have been studies on new phases of IV-group compounds or allotropes with direct band gaps based on inverse design or a high-throughput structure search.…”