Lasing in direct-bandgap GeSn alloy grown on Si

Abstract

Direct bandgap group IV materials may thus represent a pathway towards the monolithic integration of Si-photonic circuitry and CMOS technology.Although a group IV direct bandgap material has not been demonstrated yet, silicon photonics using CMOS-compatible processes has made great progress through the development of Si-based waveguides 12 , photodetectors 13 and modulators 14 . The thus emerging technology is rapidly expanding the landscape of photonics applications towards tele-and data communication as well as sensing from the infrared to the mid infrared wavelength range 15-17 . Today's light sources of such systems are lasers made from direct bandgap group III-V materials operated off-or on-chip which requires fibre coupling or heterogeneous integration, for example by wafer bonding 3 , contact printing 4,5 or direct growth 6,7 , respectively. Hence, a laser source made of a direct bandgap group IV material would further boost lab-on-a-chip and trace gas sensing 15 as well as optical interconnects 18 by enabling monolithic integration. In this context, Ge plays a prominent role since the conduction band minimum at the -point of the Brillouin-zone (referred to as -valley) is 3 located only approx. 140 meV above the fourfold degenerate indirect L-valley. To compensate for this energy difference and thus form a laser gain medium, heavy n-type doping of slightly tensile strained Ge has been proposed 19 . Later, laser action has been reported for optically 20 and electrically pumped Ge 21 doped to approx. 1 and 4×10 19 cm -3 , respectively. 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-stressedlayer. An alternative technique in order to achieve direct bandgap material is to incorporate Sn atoms into a Ge lattice, which primarily reduces the gap at the -point. At a sufficiently high fraction of Sn, the energy of the -valley decreases below that of the L-valley. This indirect-to-direct transition for relaxed GeSn binaries has been predicted to occur at about 20 % Sn by Jenkins et al. 25 , but more recent calculations indicate much lower required Sn concentrations in the range of 6.5-11.0 % 26,27 . A major challenge for the realization of such GeSn alloys is the low (< 1 %) equilibrium solubility of Sn in Ge 28 and the large lattice mismatch of about 15 % between Ge and -Sn. For GeSn grown on Ge substrates, this mismatch induces biaxial compressive strain causing a shift of the  and L-valley crossover towards higher Sn concentrations ...

Synthesis of Ge1−xSnx alloys by ion implantation and pulsed laser melting: Towards a group IV direct bandgap material

“…The achievable Sn concentration in this study is 6:2 at: %, fairly comparable with other techniques such as MBE and CVD that can achieve from 7 À 12 at: %. 15,42 The maximum Sn concentration we have achieved is also quite close to that required for a direct bandgap transition. 8 Future work building off this study will need to develop new fabrication steps to suppress the porosity for doses greater than 2:1 Â 10 16 ion cm À2 .…”

“…[10][11][12][13][14] However, only recently has Wirths et al demonstrated a direct bandgap alloy conclusively, with strong photoluminescence and a lasing effect at a Sn content of %11 at: %. 15 The aforementioned properties, together with the possibility of full compatibility with current Si technology, make Ge 1Àx Sn x an attractive material. However, achieving high quality crystalline Ge 1Àx Sn x with above 6:5 at: %Sn is challenging as the material is unstable at high Sn concentration because the solid solubility of Sn in Ge at ambient temperature is about 0:5 at: % .…”

“…16 For this reason, a non-equilibrium technique is essential for growing the alloy, such as molecular beam epitaxy (MBE), 10,11,17 sputter deposition, 18 or chemical vapour deposition (CVD). 15,19,20 Alternatively, ion-beam synthesis combined with nanosecond pulsed laser melting (PLM) can also provide a condition far from equilibrium 21 for achieving supersaturated Sn concentrations in Ge. For example, ion-beam synthesis and PLM have achieved dopant concentrations in Si several orders of magnitude higher than the equilibrium limit, such as In (2 Â 10 2 ) or Bi (5 Â 10 2 ).…”

Imaging Structure and Composition Homogeneity of 300 mm SiGe Virtual Substrates for Advanced CMOS Applications by Scanning X-ray Diffraction Microscopy

“…3 To address the vision of a monolithically integrated light source module in a Si CMOS environment, intense research on the integration of III−V materials as well as that on band gap engineering of group IV semiconductors is applied to achieve direct band gap materials either in the form of strained Ge or SiGeSn alloy systems. 4,5 For these purposes, Germanium virtual substrates (VS) on large diameter Si wafers are considered as a central materials platform to enable these advanced technologies. Germaniumbased VS are usually obtained by exploiting the compositional grading of Si 1−x Ge x strain-relaxed buffer (SRB) layers to reduce the misfit dislocation density by gradually lowering the lattice mismatch of the heterostructure with respect to the final functional layer.…”

The thermal stability of epitaxial GeSn layers

“…[2][3][4] Nonetheless, in order to move toward real-market applications, several issues still have to be addressed such as the low laser operating temperature. In particular, an increase of the Sn content in the active material beyond ∼12 at.…”