Ge1−xSnx alloys: Consequences of band mixing effects for the evolution of the band gap Γ-character with Sn concentration

Abstract

In this work we study the nature of the band gap in GeSn alloys for use in silicon-based lasers. Special attention is paid to Sn-induced band mixing effects. We demonstrate from both experiment and ab-initio theory that the (direct) Γ-character of the GeSn band gap changes continuously with alloy composition and has significant Γ-character even at low (6%) Sn concentrations. The evolution of the Γ-character is due to Sn-induced conduction band mixing effects, in contrast to the sharp indirect-to-direct band gap transition obtained in conventional alloys such as Al1−xGaxAs. Understanding the band mixing effects is critical not only from a fundamental and basic properties viewpoint but also for designing photonic devices with enhanced capabilities utilizing GeSn and related material systems.

Continuous-Wave Magneto-Optical Determination of the Carrier Lifetime in Coherent Ge1−xSnx/Ge Heterostructures

“…This value is consistent with the energy expected at low temperature for the optical transition through the fundamental indirect band gap of bulk Ge [30], thereby suggesting the prominence of the indirect-band-gap recombination in all the observed spectra of our coherent Ge 1−x Sn x films. The emergence of the indirect PL in Ge 1−x Sn x is also in line with the -L hybridization of the conduction-band edge pointed out by recent calculations [31,32].…”

“…The measured carrier lifetime τ corresponds to an effective lifetime for photogenerated carriers and encapsulates contributions from radiative (τ R ) and nonradiative (τ NR ) recombination processes: τ −1 = τ −1 R + τ −1 NR [44]. Recent analyses [31,32,45] have shown that the evolution of the Ge 1−x Sn x band gap is driven by Sn-induced hybridization between the Ge -point and L-point conduction-band-edge states. A direct band gap then emerges continuously with increasing Sn content, as the alloy conduction-band edge acquires increasing direct ( ) character.…”

“…2(b), decreases with increasing Sn content, reaching a minimum value of 19.1 ns for x = 0.05. While this estimate ignores the impact of shortrange substitutional alloy disorder on the band structure [32] and considers only zone-center transitions [46], these effects are expected to produce only minor quantitative changes to the computed τ R . This theoretical estimate does not consider phonon-assisted recombination.…”

Stretching the Equilibrium Limit of Sn in Ge1–xSnx Nanowires: Implications for Field Effect Transistors

“… 6 , 7 The drive to incorporate high Sn concentrations ( x > 0.1) in Ge 1– x Sn x alloy nanowires can be partly attributed to the presence of this band-mixing at lower Sn content Ge 1– x Sn x alloys. 7 − 11 Recent theoretical calculations have also supported that the indirect-gap to direct-gap transition proceeds via the continuous transition–with increasing x . 11 , 12 The optical and optoelectronic properties of an alloy can be influenced by the alloy composition, as demonstrated both theoretically 13 and experimentally.…”

“… 7 − 11 Recent theoretical calculations have also supported that the indirect-gap to direct-gap transition proceeds via the continuous transition–with increasing x . 11 , 12 The optical and optoelectronic properties of an alloy can be influenced by the alloy composition, as demonstrated both theoretically 13 and experimentally. 14 − 16 A definitive transition to a direct bandgap is required, with large enough Sn incorporation, for the use of Ge 1– x Sn x in efficient optoelectronic devices, such as photodiodes and photodetectors and photonic devices without the need for any external force such as induced strain.…”