Optically-Pumped Ge-on-Si Gain Media: Lasing and Broader Impact

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1 Optically-Pumped Ge-on-Si Gain Media: Lasing and Broader Impact J. Liu 1, R. Camacho 2, X. Sun 2, J. Bessette 2, Y. Cai 2, X. X. Wang 1, L. C. Kimerling 2 and J. Michel 2 1 Thayer School, Dartmouth College; 2 Microphotonics Monolithic Ge active photonic devices on Si! Unusual light emission behavior of band-engineered Ge-on-Si! Optical gain and Lasing: steady-state vs. ultrafast pumping! Mimic electrical injection by optical pumping! Broader impact

2 Enhance Direct Gap Transition of Ge by Tensile Strain and N-type doping Liu et al, Opt. Express. 15, (2007) Tensile strain drives Ge towards a direct gap material: Better photodetectors and electroabsorption modulators For efficient emission at nm: % tensile strain plus n- type doping to compensate energy difference between Γ and L valleys.

3 Enhance Direct Gap Transition of Ge by Tensile Strain and N-type doping Liu et al, Opt. Express. 15, (2007) Tensile strain drives Ge towards a direct gap material: Better photodetectors and electroabsorption modulators For efficient emission at nm: % tensile strain plus n- type doping to compensate energy difference between Γ and L valleys.

4 Unusual Features of Ge-on-Si Light Emission (1): PL Enhanced by N-Doping 70x increase Sun et al, Appl. Phys. Lett. 95, (2009) Active n-doping concentration (cm -3 ) PL increases with n-type doping due to band filling of the indirect L valleys, confirming the theoretical model Exactly opposite to the behavior of III-V semiconductors!

5 Recent Progress in PL Enhancement from Increased N-type Doping Bessette et al GFP 2011 Active n-type doping improved to ~3x10 19 cm -3 to further enhance PL intensity. Red shift of emission peaks observed n>2x10 19 cm -3. Amount of shift much less than both theoretical and experimental results (from absorption data) of previous literature, indicating better material quality. Mechanism of red-shift under investigation.

6 Unusual Features of Ge-on-Si Light Emission (2): Thermally Enhanced Emission n=1x10 19 cm -3 Γ L Thermal excitation pump λ=1064 nm Integrated PL intensity keeps increasing up to 100 C due to thermal excitation of electrons from indirect L to direct Γ valley. Again opposite to thermal-quenching of III-V materials Even at 160 C the Integrated PL intensity decrease by only ~20%. Such good thermal stability is beneficial for on-chip light sources.

7 Optical Gain and Lasing under Optical Pumping Lasing Liu et al, Opt. Lett. 34, 1738 (2009) Liu et al, Opt. Lett. 35, 679 (2010) Selectively grown structures to provide Ge lateral confinement of carriers ~50 cm -1 optical gain observed under steady-state optical pumping, leading to optically-pumped Ge-on-Si lasers at room temperature

8 Ultrafast Pump-Probe Spectroscopy to Study Carrier Dynamics in Ge Gain Media Carrier dynamics obtained setting different Δt between pump and probe pulses. Especially, directly measuring Auger recombination rate will help to select the best materials for high efficiency lasers.

9 1000 cm -1 Transient Gain under Ultrafast Pumping Near Direct Band Edge Δt<65 fs Pump λ= nm Transmittance through 1µm Ge-on-Si is 10% larger than that of Si substrate alone! Large transient gain of ~1000 cm -1, >> CW gain of 50 cm -1.. Most likely because the probe is launched before non-radiative process (Auger) Implication: ultrashort electrical pulse pumping to demonstrate the first electrically-pumped lasers?

10 Significant Effect of Pumping Photon Energy on Bleaching/Gain Spectrum (a) Gain (b) Bleaching dominated Free carrier absorption dominated When pumping photon energy >> the direct gap, the gain/bleaching spectrum is significantly modified. Excess energy of hot electrons may induce other transition paths that compromises gain. Bleaching gives way to free carrier absorption with Δt but the carrier density does not seem to change up to 70 ps: Indication of slower Γ L scattering with n-type doping?

11 Mimic Electrical Injection by Optical Pumping ΔE excess =ΔE c Barrier layer Ge ΔE excess Γ hv L ΔE c Γ L hv probe < E g (barrier) Γ L Heavy hole Ge Light hole Barrier Optical pumping Electrical injection Mimicking band-offset by optical pumping hv pump diffusion (completely absorbed by barrier layer) diffusion Mimicking carrier injection through heterojunctions by optical pumping The fact that the excess energy of hot electrons can affect gain/bleaching spectrum indicates that heterojunciton band offset needs to be optimized. Can be mimicked by carrier dynamics vs. pumping photon energy to find optimal band-offset.

12 Broader Impact (1): Engineer InGaP System for Green Lasers InGaP <50 mev Efficient green/yellow lasers poses a significant challenge since it falls into the gap of two material systems. InGaP system could be engineered for green lasers using our Ge laser approach with even more flexibility in band-engineering!

13 Broader Impact (2): Anti-droop and Anti-thermal-quenching LEDs for Solid State Lighting Anti-droop LEDs Sun et al, Opt. Lett 34, 1198 EL Current LEDs Working point I LEDs currently used for solid state lighting suffers from significant efficiency droop at high injection levels as well as thermal quenching. Band-engineered Ge-on-Si shows Anti-droop and Anti-thermalequeching behavior, which can be extended to LEDs for lighting.

14 ( 2009 ) Lett., 34, 1198 X. Sun, et al, Opt. Conclusions Band-engineering by tensile strain and n-type doping achieved Ge-on-Si gain media and lasers Carrier dynamics studies by ultrafast pump-probe spectroscopy can assist the development of electricallypumped lasers The band-engineering methods and unusual light emission behavior can be extended to other systems for superior device performance.