Direct-bandgap light-emitting germanium in tensilely strained nanomembranes

Abstract

Silicon, germanium, and related alloys, which provide the leading materials platform of electronics, are extremely inefficient light emitters because of the indirect nature of their fundamental energy bandgap. This basic materials property has so far hindered the development of group-IV photonic active devices, including diode lasers, thereby significantly limiting our ability to integrate electronic and photonic functionalities at the chip level. Here we show that Ge nanomembranes (i.e., single-crystal sheets no more than a few tens of nanometers thick) can be used to overcome this materials limitation. Theoretical studies have predicted that tensile strain in Ge lowers the direct energy bandgap relative to the indirect one. We demonstrate that mechanically stressed nanomembranes allow for the introduction of sufficient biaxial tensile strain to transform Ge into a direct-bandgap material with strongly enhanced light-emission efficiency, capable of supporting population inversion as required for providing optical gain.

Lasing in direct-bandgap GeSn alloy grown on Si

“…However, pump-probe measurements of similarly doped and strained material did not show evidence for net gain 22 , and in spite of numerous attempts, researchers failed to substantiate above results up to today. Other investigated concepts concern the engineering of the Ge band structure towards a direct bandgap semiconductor using micromechanicallystressed Ge nanomembranes 9 or silicon nitride (Si 3 N 4 ) stressor layers 23 . Very recently, Süess et al 10 presented a stressor-free technique which enables the introduction of more than 5.7 % 24 uniaxial tensile strain in Ge µ-bridges via selective wet under-etching of a pre-stressed…”

“…In order to overcome this drawback, several routes have been followed, such as the all-optical Si Raman laser 2 or the heterogeneous integration of direct bandgap III-V lasers on Si [3][4][5][6][7] . Here, we report on lasing in a direct bandgap group IV system created by alloying Ge with Sn 8 without mechanically introducing strain 9,10 .…”

Optically Pumped GeSn Microdisk Lasers on Si

“…Great efforts have been made to modify these materials, e.g. by applying tensile strain and/or by forming alloys, [16][17][18][19] in order to obtain a direct band gap. Here, Ge plays an essential role, since the energy difference between the conduction band -valley at the center of the Brillouin zone and the L-valleys, the energetically lowest conduction bands, is only 140 meV.…”

High-k Gate Stacks on Low Bandgap Tensile Strained Ge and GeSn Alloys for Field-Effect Transistors

“…Low bandgap semiconductors with a direct gap are particularly desired for novel low power devices such as tunnel-field effect transistors (Tunnel-FET). 7,8 In this context, the electronic band structure of Ge can be tuned by applying biaxial tensile strain 9 or by Sn alloying 10 towards a fundamental direct bandgap which enhances the tunneling probability and thus the ON-current of Tunnel-FETs 8,11,12 . Vertical Tunnel-FET structures using strained Ge channels were recently proposed based on InGaAs 13 and GeSn 11 buffer substrates.…”