Measurement and modeling of ultrafast carrier dynamics and transport in germanium/silicon-germanium quantum wells

Edward L Ginzton Laboratory, Stanford University, 348 Via Pueblo Mall, Stanford, CA 94305, USA.
Optics Express (Impact Factor: 3.53). 12/2010; 18(25):25596-607. DOI: 10.1364/OE.18.025596
Source: PubMed

ABSTRACT We measure the intervalley scattering time of electrons in the conduction band of Ge quantum wells from the direct Γ valley to the indirect L valley to be ~185 fs using a pump-probe setup at 1570 nm. We relate this to the width of the exciton peak seen in the absorption spectra of this material, and show that these quantum wells could be used as a fast saturable absorber with a saturation fluence between 0.11 and 0.27 pJ/μm. We also observe field screening by photogenerated carriers in the material on longer timescales. We model this field screening by incorporating carrier escape from the quantum wells, drift across the intrinsic region, and recovery of the applied voltage through diffusive conduction.

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report micron-sized Ge crystal arrays grown on deeply patterned Si substrates that yield a surge of the interband photoluminescence intensity by more than 2 orders of magnitude with respect to that typical for epitaxial layers directly grown on planar substrates. This finding is ascribed to the strongly modified internal quantum efficiency induced by controlling the nonradiative recombination at dislocations and to the improved light extraction offered by the array architecture. By spectrally resolving the interband and the dislocation-related luminescence, we address the parasitic activity of extended defects and its impact on the optical properties of the heterosystem. Such results are then exploited along with band gap engineering to design SiGe reflectors and Ge quantum wells that are effective in further amplifying the emission yield.
    05/2014; 1(4):044005. DOI:10.1103/PhysRevApplied.1.044005
  • [Show abstract] [Hide abstract]
    ABSTRACT: The epitaxial growth of Ge/Si0.15Ge0.85 multiple quantum wells (MQWs) on Si(111) substrates is demonstrated. A 3 μm thick reverse, double-step virtual substrate with a final composition of Si0.10Ge0.90 has been employed. High resolution XRD, TEM, AFM and defect etching analysis has been used for the study of the structural properties of the buffer and of the QWs. The QW stack is characterized by a threading dislocation density of about 3 × 107 cm−2 and an interdiffusion layer at the well/barrier interface of 2.1 nm. The quantum confined energy levels of this system have been calculated using the k·p and effective mass approximation methods. The Ge/Si0.15Ge0.85 MQWs have been characterized through absorption and photoluminescence measurements. The optical spectra have been compared with those of Ge/Si0.15Ge0.85 QWs grown on Si(001) through a thick graded virtual substrate.
    Journal of Applied Physics 07/2014; 116(4-4):-. DOI:10.1063/1.4891463 · 2.19 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report uniaxial tensile strains up to 5.7% along 〈100〉 in suspended germanium (Ge) wires on a silicon substrate, measured using Raman spectroscopy. This strain is sufficient to make Ge a direct bandgap semiconductor. Theoretical calculations show that a significant fraction of electrons remain in the indirect conduction valley despite the direct bandgap due to the much larger density of states; however, recombination can nevertheless be dominated by radiative direct bandgap transitions if defects are minimized. We then calculate the theoretical efficiency of direct bandgap Ge LEDs and lasers. These strained Ge wires represent a direct bandgap Group IV semiconductor integrated directly on a silicon platform.
    06/2014; 2(3). DOI:10.1364/PRJ.2.0000A8

Full-text (2 Sources)

Available from
May 17, 2014