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

Abstract

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 (Gamma) 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 Gamma valley, while the extrinsic electrons from n-type doping fill the L valleys to the level of the Gamma 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 7.6x10(19)/cm(3) by n-type doping, a net material gain of ~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 ~6kA cm(-2) for a typical edgeemitting double heterojunction structure. These results indicate that tensile strained n-type Ge is a good candidate for Si integrated lasers.

Lasing in direct-bandgap GeSn alloy grown on Si

“…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.…”

“…S6, SI) to a reduced carrier lifetime due to as yet unidentified extrinsic recombination centres along with the small energy separation between -and Lvalleys and valence interband absorption 22 . The operating temperature and lasing efficiency can be improved by introducing heterostructure layers comprising GeSnSi/GeSn 31 for carrier confinement, and by n-doping 19 .…”

Extraction of carrier lifetime in Ge waveguides using pump probe spectroscopy

“…This is estimated by using a Joint Density of States (JDOS) model to fit the absorption coefficient of Ge in the WG. 7 Transmission measurements were performed using a CW tunable laser source. Fig.…”

Electroluminescence from strained germanium membranes and implications for an efficient Si-compatible laser

“…3 shows that the emission energies from these three samples are in good agreement with the calculated band gap energies. When it comes to laser applications, strain effect on efficiency improvements will be much more significant than for LEDs because the optical net gain in a highly strained Ge is mainly contributed by the transition from the direct Γ valley to the light-hole band, and the smaller density of states (DOS) in the light-hole band facilitates population inversion, thus reducing the lasing threshold even further [16]. In the next section, therefore, we propose a VCSEL on a highly strained Ge membrane and discuss the extent to which strain reduces lasing threshold.…”

“…With an optical confinement factor of approximately 0.3 in both cases that is calculated from TMM, the threshold gain coefficients for two cases are 873cm -1 and 783cm -1 . Using a tight-binding model, and extrapolating the presumed absorption coefficients of [16] to higher strains, we obtained optical net gain spectra as a function of injection current density for both 0.2% and 1% strained Ge gain media cases as shown in Fig. 5(a).…”