Showing papers by "Thomas Fromherz published in 2005"

Intersubband absorption of strain-compensated Si1−xGex valence-band quantum wells with 0.7⩽x⩽0.85

Abstract: Strain-compensated, p-type SiGe quantum wells with a high Ge concentration of up to 85% have been grown on commercially available Si0.5Ge0.5 pseudosubstrates by molecular-beam epitaxy. Structural investigations by transmission electron microscopy and high-resolution x-ray reflection and diffraction showed that at a growth temperature around T=300°C, samples in excellent compliance with the design parameters, comparatively sharp interfaces, and negligible increase of growth-induced surface roughness can be grown. Comparison of polarization-dependent intersubband absorption measurements with simulated intersubband absorption spectra shows that for the quantum wells investigated in this work, the hole eigenstates, their in-plane dispersion, and the polarization-dependent intersubband transition matrix elements are accurately described by a strain-dependent, six-band k∙p Luttinger-Kohn Hamiltonian in which only one fitting parameter—the intersubband transition linewidth—is used.

Intra-valence-band mixing in strain-compensated SiGe quantum wells

Abstract: We explore the midinfrared absorption of strain-compensated $p\text{\ensuremath{-}}{\mathrm{Si}}_{0.2}{\mathrm{Ge}}_{0.8}∕\mathrm{Si}$ quantum wells for various well thicknesses and temperatures. Owing to the large band offset due to the large bi-axial strain contrast between the wells and barriers, the intersubband transitions energies from the ground state to the excited heavy hole (HH), light hole (LH), and split-off (SO) hole states up to $\ensuremath{\sim}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ are resolved. When HH2 is within $\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ of LH1 or SO1 a partial transfer of the HH1-HH2 oscillator strength to the HH1-LH1 or HH1-SO1 transitions is observed, which is otherwise forbidden for light polarized perpendicular to the plane of the wells. This is a clear sign of mixing between the HH and LH or SO states. A large temperature induced broadening of HH1-HH2 transition peak is observed for narrow wells indicating nonparabolic dispersion of the HH2 states due to the mixing with the LH/SO continuum. We found that the observations are in good agreement with the six-band $\mathbf{k}∙\mathbf{p}$ theory. A possible role of many-body effects in the temperature-induced negative peak shift is discussed.

Ge∕Si islands in a three-dimensional island crystal studied by x-ray diffraction

Abstract: Coplanar high-resolution x-ray diffraction has been used for the characterization of size, chemical composition, and strain of Ge∕Si (001) islands in a three-dimensional island crystal grown using self-assembly on a prepatterned (001) Si substrate. The measured diffusely scattered intensity is simulated using the kinematical approximation and the parameters of model islands are fitted. These simulations require calculations of the strain fields within the islands and the spacer layers. For this purpose, an analytical approach to solve the continuum elasticity equations has been extended to a full three-dimensional calculation. The Ge content in the islands is found to be on the average 40%, and the island shape does not change dramatically during capping.

Pump-probe measurement of lifetime engineering in SiGe quantum wells below the optical phonon energy

Abstract: We have directly determined with pump/probe spectroscopy the light hole (LH1) excited state lifetime for the lowest heavy hole to light hole intrawell subband transition (HH1–LH1) for three prototype samples of Si/SiGe strain-symmetrized multi-quantum well structures, designed to have the final LH1 state increasingly unconfined. The transition energy is below the optical phonon energy. We find that a decay time of 20 ps for sample 1 with a well width of 5.0 nm lengthens to 40 ps for sample 3 with a well width of 3.0 nm, in good agreement with the design. In addition, we have measured the lifetime for holes excited out of the well, from which we determine the lifetime for diagonal transitions (back into the well) to be of approx. several hundred picoseconds.

Highly nonlinear transport phenomena in fullerene based diodes

Abstract: Voltage‐current measurements of fullerene based diodes in the temperature range between 295 – 15 K are presented. At temperatures below 95 K and at high current densities the diodes exhibit a voltage instability with a voltage hysteresis for opposite current sweep directions. This observation is interpreted with the formation of highly conductive current filaments in the fullerene film.

High Mobility Si/SiGe Heterostructures for Spintronics Applications

Abstract: We investigate techniques for using Silicon-Germanium as material for solid-state quantum computing. Two topics are presented: First, the use of quantum well structures with different Ge content which can be used to shift the g-factor of conduction band electrons via voltages applied to gates above and below the structure. The change in g-factor allows then to put the electrons in and out of resonance in an Electron Spin Resonance experiment. Second, measurement of spin relaxation times of electrons confined in the strain field above Ge dots in a SiGe heterostructure. Photoluminescence (PL) shows indirect transitions from the VB of the Ge dots to the confined Si CB states. A single line is seen in ESR with g ≈ 2.0000 during illumination.