The Theoretical Optical Gain of Ge1-xSnx Nanowires

The electronic structures of Ge1-xSnx nanowires at the direct Gamma-valley and indirect L-valley is calculated using k center dot p effective-mass theory, and the results demonstrate that Ge1-xSnx nanowires with large diameter and Sn content can easily be engineered to be the direct-band-gap semiconductor. Furthermore, the optical gain of Ge1-xSnx nanowires as functions of the injected electron concentration and diameter are obtained. Compared with pure Ge nanowires, a remarkable peak gain can appear in Ge1-xSnx nanowires even though the injected electron concentration decreases. This is because incorporating Sn into Ge can reduce even reverse the energy difference of minimum bandgap between the direct Gamma-valley and indirect L-valley. Therefore, considering the free-carrier absorption loss, one can achieve a positive net peak gain in Ge1-xSnx nanowires, which indicates that Ge1-xSnx nanowires can be used as an ideal laser diode candidate in the field of Si-photonics.