SiGe Intersubband Lifetimes

The key to obtaining population inversion for Si/SiGe QCLs is a knowledge of the hole lifetimes in a subband state. We collaborate with Carl Pidgeon at Heriot Watt University and Ben Murdin at Surrey University to use pump-probe spectroscopy using the free electron laser, FELIX, at Utrecht in the Netherlands. Basically the free electron laser excites all carriers from the ground state to the excited subband state being investigated before a time dependent absorption measurement is used to measure the non-radiative lifetime.

In III-V materials such as GaAs, the intersubband lifetimes are limited by polar optical phonon scattering above roughly 60 K where the lifetime very quickly collapses to << 500 ps. Group IV materials such as Si, Ge and SiGe have no polar optical phonon scattering and so below the optical phonon energy, there can be no optical phonon scattering. Our work has demonstrated that the limiting scattering mechanism is alloy scattering at high temperatures. On the right is data from an intra-quantum well LH1 to HH1 lifetime measurement where the lifetimes hardly reduce at high temperatures unlike III-V materials.

A summary of all the LH1 to HH1 intrawell lifetimes at 150 K is shown on the left. Below the Ge-Ge optical phonon energy, all lifetimes are relatively long being > 10 ps. Above ~ 30meV, the lifetimes are all < 2 ps which is the width of the free electron pulse. Finally a interwell or diagonal transition between the HH1 states in adjacent quantum wells with a 1.8 nm stained-Si barrier between the wells. Again the temperature dependence is very weak with only a ~ x2 reduction in lifetime between 4 and 225 K. This suggests that Si/SiGe QCLs should operate at higher temperatures than III-Vs.

Significant publications:

M. Califano, N.Q. Vinh, P.J. Phillips, Z. Ikonic, R.W. Kelsall, P. Harrison, C.R. Pidgeon, B.N. Murdin, D.J. Paul, P. Townsend, J. Zhang, I.M. Ross and A.G. Cullis, "Interwell relaxation times in p-Si/SiGe asymmetric quantum well structures: the role of interface roughness" Physical Review B 75, 045339 (2007)

C.R. Pidgeon, P.J. Phillips, D. Carder, B.N. Murdin, T. Fromherz, D.J. Paul, W.-X. Ni and M. Zhao, "Pump-probe measurement of lifetime engineering in SiGe quantum wells below the optical phonon energy" Semiconductor Science and Technology 20, ppL50-L52 (2005)

R.W. Kelsall, Z. Ikonic, C.R. Pidgeon, P.J. Philips, P. Harrison, S.A. Lynch, P. Townsend, D.J. Paul, S.L. Liew, D.J. Norris and A.G. Cullis, "Intersubband lifetimes in p-Si/SiGe terahertz quantum cascade heterostructures" Physical Review B 71, 115326 (2005)

C.R. Pidgeon, P. Murzyn, J-P.R. Wells, I.V. Bradley, Z. Ikonic, R.W. Kelsall, P. Harrison, S.A. Lynch, D.J. Paul, D.D. Arnone, D.J. Robbins, D.J. Norris and A.G. Cullis, "Picosecond intersubband dynamics in p-Si/SiGe quantum well emitters" Applied Physics Letters 80, pp 1456-1458 (2002)