Direct monitoring of the excited state population in biased SiGe valence band quantum wells by femtosecond resolved photocurrent experiments

Abstract

The authors report a direct measurement of the optical phonon intersubband hole relaxation time in a SiGe heterostructure and a quantitative determination of hole relaxation under electrically active conditions. The results were obtained by femtosecond resolved pump-pump photocurrent experiments using a free electron laser ͑wavelength 7.9 m͒. Additionally, the intensity dependence of the nonlinear photocurrent response was measured. Both types of experiments were simulated using a density matrix description. With one parameter set, a consistent modeling was achieved confirming the significance of the extracted heavy hole relaxation times. For an intersublevel spacing of 160 meV, a value of 550 fs was obtained. Driven by the strong need for cheap and integrable Sibased optoelectronic devices for a wide range of applications, considerable endeavors have been made to develop structures for light emission, modulation, and detection in this material system. While recent breakthroughs like the demonstration of a high-speed optical modulator in Si ͑Ref. 1͒ bring the concept of transition from electrical to optical interconnects closer to realization, the base for any silicon photonics, namely, a group IV laser source, still has to be developed. Up to now, the only lasing devices demonstrated in Si are a Raman laser 2 and Si-based impurity lasers, 3 which essentially lack the advantages associated with the silicon system by requiring an external pump laser source. For silicon as an indirect semiconductor the concept of infrared emitters based on quantum cascade ͑QC͒ heterostructures, which is very successfully applied to III-V material systems, constitutes a promising approach towards a SiGe infrared laser. But while infrared electroluminescence ͑EL͒ of various wavelengths has been demonstrated for p-type SiGe quantum cascade structures, [4][5][6] lasing has yet to be achieved. The buildup of population inversion in order to achieve lasing fundamentally depends on the relaxation lifetime of the excited energy level of the lasing transition. Therefore the measurement and optimization of the intersubband relaxation time constitutes a key issue for the realization of a Si cascade laser source. The intersubband hole relaxation time for transitions below the optical phonon energy ͑Si-Si: 58 meV, GeGe: 36 meV͒ is significantly larger than 10 ps and therefore is experimentally well accessible. 7 The lifetimes for energies above the optical phonon energies are smaller than 1 ps. Thus so far experiments suffered from a lack of time resolution 8 or from hole heating requiring subtraction of heating contributions in order to gain a value for the relaxation time. 9 In this work we report the first direct measurement of the excited heavy hole ͑HH͒ lifetime in the femtosecond regime for quantum well ͑QW͒ transition energies above the optical phonon energy. Time resolved measurements were performed using the free electron laser ͑FEL͒ FELIX at FOM Rijnhuizen. FELIX provides micropulses with full widths at half maximum down to 280 fs an...

SiGe quantum well infrared photodetectors on strained-silicon-on-insulator

“…The latter is a well-known spectral fingerprint region including the vibrational and rotational absorption lines used to unambiguously identify chemical compounds. Among the demonstrated group-IV building blocks for such a mid-infrared platform are quantum well infrared photodetectors (QWIPs) [16][17][18][19], quantum cascade (QC) emitters [20,21], silicon-on-insulator (SOI) based waveguides [22], Ge waveguides on SiN [23], SiGe waveguides [24], SiGe polarization rotators [25] and Ge-based multiplexers [26]. One of the fundamental challenges faced by SiGe-based platform components is the lattice mismatch between Si and Ge.…”

Gain recovery dynamics of a terahertz quantum cascade laser

“…A balanced twobeamsplitter arrangement 8 was used to split these into two collinearly propagating beams which could be delayed relative to each other using a mechanical delay line, but had the same intensity and polarization. This setup is shown schematically in Fig.…”

Long intersubband relaxation times in n-type germanium quantum wells

“…4,5 Electron-doped Ge-rich heterostructures have been identified to be more promising than n-type Si rich system because of the smaller confinement effective mass of electron at L point. 1,2 Furthermore, they permit to circumvent the difficulties of QCL designs based on p-type doped SiGe wells for which both the strong nonparabolicities of the different valence bands 6 and the carrier scattering among them limit the non-radiative intersubband relaxation times s Indeed, previous experimental determination of s in SiGe/Si heterostructures by time-resolved pumpprobe transmission experiments was restricted to p-type systems, 7-10 and very short s < 1 ps were reported 8,9 for ISBT energies above the longitudinal optical phonon energies x opt ¼ 37(63) meV for Ge(Si). At ISBT energies lower than x opt , the main scattering channel was instead due to alloy disorder in the SiGe wells, with values of s around 10 ps.…”