Tensile-strained, n-type Ge as a gain medium for monolithic laser integration on Si

We analyze the optical gain of tensile-strained, n-type Ge material for Si-compatible laser applications. The band structure of unstrained Ge exhibits indirect conduction band valleys ( L) lower than the direct valley ( G) by 136 meV. Adequate strain and n-type doping engineering can effectively provide population inversion in the direct bandgap of Ge. The tensile strain decreases the difference between the L valleys and the G valley, while the extrinsic electrons from n-type doping fill the L valleys to the level of the G valley to compensate for the remaining energy difference. Our modeling shows that with a combination of 0.25% tensile strain and an extrinsic electron density of 7.6 x 10(19)/cm3 by n-type doping, a net material gain of similar to 400 cm(-1) can be obtained from the direct gap transition of Ge despite of the free carrier absorption loss. The threshold current density for lasing is estimated to be similar to 6kA cm(-2) for a typical edge-emitting double heterojunction structure. These results indicate that tensile strained n-type Ge is a good candidate for Si integrated lasers. (c) 2007 Optical Society of America.