Group IV laser based on n-type and tensile-strained germanium – GRAAL

Growth of germanium by MBE , CVD or MOCVD
laser doping
Stress transfer by growth on relaxed buffers
Stress transfer using silicon nitride stressors
Electronic structure and near infrared emission modeling
Cw or pulsed Optical and electrical pumping

Germanium laser

The GRAAL project proposes to develop a completely new approach to demonstrate a laser source on silicon by using pure germanium, a group IV element and an indirect band gap material, as the gain medium. The interest of using germanium for integration is obvious due to its intrinsic compatibility with the silicon microelectronics environment. The main challenge is to overcome the limitation induced by the indirect band gap. We propose to lift this bolt by using a combination of n-type doping of germanium and application of a tensile strain. The n-type doping provides carriers to fill the L valley, and allows a more efficient carrier injection into the zone-center Gamma valley where the carriers can recombine efficiently like in III-V materials. The tensile strain reduces the energy splitting between the L and Gamma valleys, lifts the degeneracy in the valence band and can even lead to a direct band gap material. The combination of both effects is expected to lead to significant optical gains at room temperature and to the demonstration of a laser. We will develop in the project several approaches to apply a tensile strain on n-type germanium layers, in order to achieve a significant optical gain which will lead to the demonstration of a germanium-based laser under optical pumping in a first step and under electrical injection in a second step. This project aims to take advantage of several advances in the study of germanium and its recent recognition as a potential efficient material for light emission. Studies performed in particular by some partners of this consortium have already reported on the room temperature emission of Ge associated with the direct band gap which can be controlled by applying tensile strain. Numerical modeling based on multiband k.p formalisms has shown that a significant optical gain could be achieved with tensile-strained germanium. Control of germanium emission by imposing an external mechanical stress, the use of Si3N4 stressors, or growth on InGaAs buffer layers has been recently demonstrated by the Institut d'Electronique Fondamentale group in collaboration with Laboratoire de Photonique et de Nanostructures. These approaches have the capability to achieve large tensile strains.Triggered by these advances, the study of germanium-based lasers has become a hot topic in the community as it provides a new paradigm to integrate an optical source on silicon, an approach which was not recognized as pertinent until very recently. In the US, a strong effort was devoted to the study of germanium, in particular by the Massachusetts Institute of Technology (MIT) group of L. Kimerling. Cw optical gain in tensile-strained germanium was first reported in 2009 by the MIT group. In 2010, the demonstration of an optically-pumped germanium laser operating above room temperature in pulsed mode was reported by the same group. To demonstrate and to improve the performances of room temperature germanium lasers, a consortium between four academic labs has been set-up. Each partner is recognized as being among the top laboratories in their domain. The four laboratories involved in the GRAAL project are the following: Institut d'Electronique Fondamentale - Universite´ Paris-Sud – CNRS (IEF), Laboratoire de Photonique et de Nanostructures - CNRS (LPN), Centre Interdisciplinaire de NAnosciences de Marseille - CNRS (CINAM), Laboratoire de Physique des Interfaces et des Couches Minces – CNRS - Ecole polytechnique (LPICM).