Large inherent optical gain from the direct gap transition of Ge thin films

Abstract

The recent demonstration of Ge-on-Si diode lasers renews the interest in the unique carrier dynamics of Ge involving both direct (Γ) and indirect (L) valleys. Here, we report a large inherent direct gap optical gain ≥1300 cm−1 at room temperature from both tensile-strained n+ Ge-on-Si films and intrinsic Ge-on-insulator using femtosecond transmittance spectroscopy captured before direct-to-indirect valley scattering. This inherent direct gap gain is comparable to III-V semiconductors. For n+ Ge, this transient gain is ∼25× larger than its steady state gain, suggesting that reducing Γ→L or enhancing L→Γ intervalley scattering may significantly increase the optical gain of Ge lasers.

Ultralarge transient optical gain from tensile-strained, n-doped germanium on silicon by spin-on dopant diffusion

“…The measured peak gain value is thus consistent with the absorption coefficient, provided that the high injection into the direct valley allows almost 100% population inversion. The peak gain is a factor of >4 higher that that reported previously in a moderately doped GOS, 17) and the gain spectrum is much broader. We attribute these results to the much higher doping concentration.…”

“…The measurement apparatus was the same as reported in Ref. 17. The incidence pulse was a broadband fs light source covering the wavelength range of 1.4-1.75 µm, obtained from an optical parametric amplifier (OPA, Spectra-Physics OPA-800C) pumped by a high-energy Ti-sapphire fs pulse laser (Spectra-Physics Spitfire-Pro).…”

“…A comparison study 15,17) indicated that the optical gain under continuous-wave (CW) pumping is about 4% of the transient gain under fs pulse pumping in 1 × 10 19 cm −3 doped GOS because only a small fraction of injected electrons occupy the direct valley. At a higher doping level, this fraction should be larger and more electrons will be involved in the direct band-gap transition.…”

Spin and energy relaxation in germanium studied by spin-polarized direct-gap photoluminescence

“…Finally, the recently discovered lasing action in Ge on Si heterostructures, 9,67 albeit debated, [68][69][70][71] holds the promise of laser sources monolithically integrated onto the mainstream CMOS platform, 10 thus filling the gap for the development of the active devices needed to ground Si-photonics. At present, however, direct-gap electroluminescence [72][73][74][75] and lasing 67 in Ge-based heterostructures have been achieved only under high current densities and shown to be not efficient yet.…”

Redshift of the absorption edge in tensile-strained germanium layers

“…Thus, the authors of [6] reported that their photoconductivity measurements provide exper imental evidence of a reduction of the band gap in ten sile strained silicon to 0.59 eV. The superlinear growth of the emission power as a function of the pump power (i.e., lasing) in tensile strained n type Ge films was reported in [7].…”