Th. Englert

Determination of existing stress in silicon films on sapphire substrate using Raman spectroscopy

Abstract Raman spectroscopy is used to determine built-in stresses in silicon on sapphire (SOS) devices. The method is direct, nondestructive and can be applied at various temperatures. For epitaxial silicon films on sapphire substrates a built-in stress of 7.0±0.3 kbar at room temperature and 8.7±0.3 kbar at liquid nitrogen temperature was measured.

Observation of oscillatory linewidth in the cyclotron-resonance of GaAs-AlxGa1-xAs heterostructures

Abstract The linewidth of the far infrared cyclotron resonance in the 2-D electron gas in GaAs-AlxGa1−xAs heterojunctions show oscillations between 0.070 and 0.32 T as a function of the resonance magnetic field at 5 K. With increasing temperature the maximum linewidth decreases, whereas the minimum increases. This oscillatory behavior is shown to be correlated with the filling factor of the Landau levels.

Observation of cyclotron resonance in the photoconductivity of two‐dimensional electrons

We have observed cyclotron resonance in the photoresponse of the two‐dimensional electron gas at the interface of a GaAs/AlGaAs heterojunction. A 0.5% change in resistivity was obtained under resonance conditions for a far‐infrared intensity of about 10−4 W/cm2.

Analysis of ρxx minima in surface quantum oscillations on (100) n-type silicon inversion layers

Abstract The analysis of resistance minima ρ xx min in surface quantum oscillations of n-type silicon inversion layers shows that for samples of high mobility ρ xx min depends on the energy gap in the density of states between the broadened Landau levels. For a constant magnetic field, the energy gap is determined by the level width, the Lande g -factor, and the valley splitting. From our analysis of the temperature dependence of ρ xx min and from measurements in a tilted magnetic Held, values for the level broadening and the g -factor could be deduced. For the first time experimental g-values were determined at Fermi energies where no enhancement due to many body effects is expected.

Magnetophonon resonances of the two-dimensional electron gas in GaAs/AlGaAs heterostructures

Abstract We have studied magnetophonon resonances of the two-dimensional (2D) electron gas at the GaAs/AlGaAs interface in a single interface heterostructure and a superlattice. In magnetic fields up to 30T, one set of oscillations is observed, corresponding to the coupling of 2D electrons with LO phonons of GaAs. The effective mass obtained directly from the magnetic field position of the fundamental resonance, where the Landau splitting equals the bulk phonon energy, and of the next two harmonics is m ∗ = (0.071 ± 0.0015)m 0 . A comparison with cyclotron resonance measurements on the same system and with bulk GaAs data gives an upper limit of about 2% for the mass corrections due to polaron effects and due to the confinement of the electron gas.

A study of intersubband scattering in GaAs-AlxGa1-xAs heterostructures by means of a parallel magnetic field

Abstract We demonstrate the influence of intersubband scattering in GaAs/AlGaAs heterostructures by means of a parallel magnetic field. For samples with a carrier density such that the second subband is lightly populated, the mobility can be increased by about 50% due to the suppression of intersubband scattering, when the subband separation is increased by a strong magnetic field parallel to the interface. This gives rise to a strong negative magnetoresistance. The onset of this negative magnetoresistance can be shifted by the application of a back-side gate voltage which varies the Fermi energy.

Transport measurements on InP inversion metal‐oxide semiconductor transistors

Using recently developed InP metal‐oxide‐semiconductor field‐effect transistors with SiO2 gate insulation, the transport properties of electrons in (100) inversion layers are investigated for surface carrier densities up to 7×1012 cm−2. Conductivity and magnetoconductivity measurements are analyzed in magnetic fields up to 19.7 T at 4.2 K. The depletion charge is varied in the range 0.56–1.66×1012 cm−2. The carriers in the surface channel behave like a quasi two‐dimensional electron gas. From the surface quantum oscillations, it is possible to determine the density of free carriers in the surface layer and the threshold voltage. A second electric subband is occupied at surface carrier concentrations above Ninv =4.5×1012 cm−2. The effective mobility is dominated by Coulomb and surface roughness scattering. The characteristic parameters for the two scattering mechanisms are evaluated.

Comments on the hole mass in silicon inversion layers

Abstract Published experimental data for hole masses mc in silicon inversion layers on (110), (111) and (100) surfaces are critically discussed. New results are presented for the (100) orientation. It is shown that self-consistent calculations of mc agree with those experimental data, which are compatible with cyclotron resonance results. It is demonstrated that a theoretical treatment by Falicov and Garcia, which predicts too large hole masses, is not tenable.

An investigation of the valley splitting in n-channel silicon 〈100〉 inversion layers

Abstract Measurements are reported of the depletion charge dependence of the valley splitting (ΔEv) in n-channel 〈100〉 silicon MOSFETs by the analysis of Shubnikov-de Haas oscillations in high magnetic fields. ΔEv was determined by observation of the condition where one Landau level shows four equally resolved peaks in σxx, which occurs when ΔEv is equal to one half of the spin splitting. Simulations show that this condition is insensitive to the lineshape. The depletion charge dependence was determined by tilting the magnetic field and is given by the relation ΔEv(Ndepl.) = ΔEv(0) + 0.9Ndepl. (meV/1012cm−2).

Quantum transport in GaAs doping superlattices

The resistance of the n channels in GaAs doping superlattices shows clear oscillations as a function of the magnetic field, and varies with the voltage applied between the n and the p layers and with the orientation of the magnetic field with respect to the doping layer plane. These experimental results are analyzed in terms of the two‐dimensional conductivity of the system, and the subband separation is derived from a comparison between the measurements and model calculations.