Tuning gaps and phases of a two-subband system in a quantizing magnetic field

In this work we study the properties of a two-subband quasi-two-dimensional electron system in a strong magnetic field when the electron filling factor is equal to four. When the cyclotron energy is close to the intersubband splitting the system can be mapped onto a four-level electron system with an effective filling factor of two. The ground state is either a ferromagnetic state or a spin-singlet state, depending on the values of the inter-level splitting and Zeeman energy. The boundaries between these phases are strongly influenced by the inter-electron interaction. A significant exchange-mediated enhancement of the excitation gap results in the suppression of the electron-phonon interaction. The rate of absorption of non-equilibrium phonons is calculated as a function of Zeeman energy and inter-subband splitting. The phonon absorption rate has two peaks as a function of intersubband splitting and has a step-like structure as a function of Zeeman energy

Donor states in modulation-doped Si/SiGe heterostructures

We present a unified approach for calculating the properties of shallow donors inside or outside heterostructure quantum wells. The method allows us to obtain not only the binding energies of all localized states of any symmetry, but also the energy width of the resonant states which may appear when a localized state becomes degenerate with the continuous quantum well subbands. The approach is non-variational, and we are therefore also able to evaluate the wave functions. This is used to calculate the optical absorption spectrum, which is strongly non-isotropic due to the selection rules. The results obtained from calculations for Si/Si$_{1-x}$Ge$_x$ quantum wells allow us to present the general behavior of the impurity states, as the donor position is varied from the center of the well to deep inside the barrier. The influence on the donor ground state from both the central-cell effect and the strain arising from the lattice mismatch is carefully considered.Comment: 17 pages, 10 figure

Carbon dioxide laser saturation spectroscopy at kHz linewidths

Consideration is given to factors limiting the resolution in laser saturation experiments. At linewidths in the range 100 kHz to 1 MHz the most important factors arise from the residence time of the absorbing molecule within the radiation field and from saturation broadening. These problems are circumvented by using beams of large diameters and low powers. Two experiments yielding spectra at linewidths around 20 kHz (full-width half-maximum) are described in outline, each employing a CO<sub>2</sub> laser beam of diameter several centimetres. In one experiment 10 µm saturation spectra are obtained directly using an etalon technique; in the other radio-frequency transitions are obtained in a double-resonance experiment

Magneto-optics and magneto-capacitance studies of voltage-tuneable GaAs/AlGaAs quantum dots

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Far-infrared cyclotron resonance study of the effect of strain and localisation in Si/SiGe two dimensional electron gases

Far infrared cyclotron resonance measurements have been used to investigate the effective mass (m*) in the strained silicon channel of modulation-doped, two dimensional electron gases grown on relaxed Si1−xGex. Samples with germanium fractions from 24% to 31% were measured to investigate the influence of strain on m*. Little variation as a function of strain was observed, but for one sample, the resonance position was shifted up in frequency due to localisation effects. This persisted up to higher Landau level filling factors than has been observed previously in other materials systems and was accompanied by a large enhancement in the quantum lifetime

EBIC of strained Si/SiGe 2DEGs showing lateral electron confinement

Modulation-doped SiGe structures containing a two dimensional electron gas (2DEG) in a strained Si channel have been examined by far infrared (FIR), electron beam induced current (EBIC) and transmission electron microscopy (TEM). An apparent correlation between lateral confinement effects and inhomogeneities in the material beneath the Schottky barrier has been identified. These effects are not due to defects nucleating in the Si channel, but rather, may be associated with the defect structure in the underlying relaxed graded-composition buffer layer

Cyclotron resonance measurements of Si/SiGe two-dimensional electron gases with differing strain

Far-infrared cyclotron resonance measurements have been used to investigate the effective mass in the strained silicon channels of modulation-doped, two-dimensional electron gases grown on relaxed Si1−xGex. By using a range of Ge fractions x, the effect of strain was investigated. Consistent results were obtained when the resonance positions were fitted to a model for zero-dimensional confinement, yielding m*≈0.196 me for most samples. The use of this formula was justified by invoking electron localization due to a disorder potential. The observed confinement effect was strongest in two samples where the Si channel was partially relaxed, suggesting this to be a possible mechanism. Qualitatively different results were obtained for a sample with a high background concentration of donor impurities, indicating that the type of disorder present can affect the nature of the resonances

Intersubband electroluminescence from Si/SiGe cascade emitters at terahertz frequencies

The quantum cascade laser provides one possible method of realizing high efficiency light emitters in indirect band gap materials such as silicon. Electroluminescence results from Si/SiGe quantum cascade emitters are presented demonstrating edge emission from heavy-hole to heavy-hole transitions and light-hole to heavy-hole transitions. In surface-normal emission, only light-hole to heavy-hole electroluminescence is observed as predicted by theory. Intersubband emission is demonstrated at 2.9 THz (103 μm wavelength), 8.9 THz (33.7 μm), and 16.2 THz (18.5 μm) from the Si/SiGe quantum cascade heterostructures

Interwell intersubband electroluminescence from Si/SiGe quantum cascade emitters

The quantum cascade laser provides one potential method for the efficient generation of light from indirect materials such as silicon. While to date electroluminescence results from THz Si/SiGe quantum cascade emitters have shown higher output powers than equivalent III–V emitters, the absence of population inversion within these structures has undermined their potential use for the creation of a laser. Electroluminescence results from Si/SiGe quantum cascade emitters are presented demonstrating intersubband emission from heavy to light holes interwell (diagonal) transitions between 1.2 THz (250 μm) and 1.9 THz (156 μm). Theoretical modeling of the transitions suggests the existence of population inversion within the system