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The Silicon (Si) CMOS technology has long been the backbone of electronic devices. In these components, data communication is currently limited by the metal interconnects. An alternative solution based on optical transmission demands an additional light source due to the intrinsic indirect gap of Si, which prevents it from efficiently emitting light.
The requirement of a monolithic, Si-compatible light source has made another column IV semiconductor – Germanium (Ge) - a potential candidate. Ge has, as Si, an indirect gap, but with a smaller separation between Γ and L energy levels. A direct gap Ge-based material can be obtained by alloying it with Tin (Sn) – a column IV semi-metal. Recent studies have demonstrated lasing in optical cavities made with Ge1-xSnx alloy: So far, lasing in Fabry-Perot cavity [1, 2] and in micro-disk cavity [3, 4] were reported.
In this article, we study lasing in Ge1-xSnx - based photonic crystals, with 16% Sn concentration. Studies were conducted on Hn defect cavities (with n denoting the number of hexagonal ring of air holes removed from the center), and slow mode membranes – which are photonic crystals with low group velocity optical mode appeared in the photonic band structure, thus enhancing the interaction between emitted light and the gain medium.
A step-graded epitaxy technique was used to fabricate the samples. It consists in inserting a thick buffer with discrete Sn content increases between the Ge virtual substrate underneath and the main Ge0.84Sn0.16 layer on top. With this method, misfit dislocations are distributed more evenly in the grading (instead of propagating towards the surface), which preserves the crystalline quality of the partly relaxed Ge0.84Sn0.16 optical layer. Both Hn cavities and slow mode membranes were under-etched to make the active layer fully relaxed. The photonic crystals are slightly curved due to strong relaxation of the active layer. The structures were optically pumped with a 1064 nm pulsed laser. Fourier transform infrared (FTIR) spectroscopy was used to study the photoluminescence of these samples.
We demonstrate lasing on the H4 cavities and the slow mode membranes. The dimensions of both structures were calculated to pin the resonance mode (or slow mode under the light cone) around the emitted wavelength of relaxed Ge0.84Sn0.16. Based on the spectra and the Lin-Lout curve, we estimate the current lowest lasing threshold around 180 kW/cm2 for slow mode membrane at low temperature (15K), with emitted wavelength of 2880 nm. The lasing threshold obtained here is of the same order of magnitude of those reported for micro-disk and Fabry-Perot cavities [1, 2, 3, 4], suggesting that the intrinsic properties of Ge1-xSnx are the main limit factor to the performance. Surface passivation and carrier confinement with SiGeSn alloys and/or multi quantum well structures should be used to enhance the Ge1-xSnx radiative recombination efficiency.
[1] S. Wirths, R. Geiger, N. von den Driesch, G. Mussler, T. Stoica, S. Mantl, Z. Ikonic, M. Luysberg, S. Chiussi, J. M. Hartmann et al, Nature Photonics 9,88 (2015)
[2] S. Al-Kabi, S. A. Ghetmiri, J. Margetis, T. Pham, Y. Zhou, W. Dou, B. Collier, R. Quinde, W. Du, A. Mosleh et al, Appl. Phys. Lett. 109, 171105 (2016)
[3] D. Stange, S. Wirths, R. Geiger, C. Schulte-Braucks, B. Marzban, N. von den Driesch, G. Mussler, T. Zabel, T. Stoica, J-M Hartmann et al, ACS Photonics 3, 1279 (2016)
[4] V. Reboud, A. Gassenq, N. Pauc, J. Aubin, L. Milord, Q.M. Thai, M. Bertrand, K. Guilloy, D. Rouchon, J.Rothman et al, Appl. Phys. Lett. 111, 092101 (2017)
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