Fano resonance in the impurity photoconductivity spectrum of InP doped with shallow donors

The Fano resonances in the impurity photoconductivity spectra of n-InP are investigated theoretically and experimentally. It is shown that the calculations describe the experimental data with an accuracy of up to 20%.     

Fano resonances in the impurity photoexcitation spectra of semiconductors doped with shallow donors

A theory describing the photoexcited current peaks in the spectral region corresponding to the energies of longitudinal optical phonons in semiconductors doped with shallow donors is developed. The experimental data available for n-GaAs are in good agreement with the results obtained using the proposed theory.     

Polarization dependence of intersubband absorption and photoconductivity in p-type SiGe quantum wells

We present a detailed study of the polarization dependence of subband absorption and photoconductivity in Si/SiGe quantum wells. For samples with a hole concentration ofp_s2.8×10¹² cm², bothp- ands-polarized absorptions have been observed and transitions to several excited states are clearly identified by comparison with self-consistent Luttinger-Kohn type calculations. The photoconductivity is surprisingly insensitive to the polarization, which indicates the importance of the subsequent transport process on the photocurrent responsitivity.     

Fano resonances in the impurity photocurrent spectra of GaAs samples and an InGaAs/GaAsP quantum-well heterostructure doped with shallow acceptors

The photocurrent spectra of p-GaAs samples and a p-InGaAs/GaAsP quantum-well heterostructure doped with shallow acceptors (C, Be, Zn) are investigated. Fano resonances associated with the ground and excited acceptor states are revealed and analyzed. It is found that sharp change in dielectric constant in the GaAs reststrahlen region strongly affects the lineshape of the Fano resonance associated with the acceptor ground state.     

Polarization dependence of two-photon transitions in quantum wells

Summary We give explicitly the polarization dependence of two-photon subband-subband transitions in semiconductor quantum wells. We consider transitions from heavy-hole subbands as well as from light-hole subbands. We study the polarization dependence in the case of absorption of one photon having an energy of the order of the band gap and one having an energy of the order of the subband separation. We show that the absorption structure depends on the polarization of the low-energy photon. We also give, in the case of equal photons with in-plane linear polarizations, the dependence of the transition rate on the angle between the polarizations.     

Fano resonances in stubbed quantum waveguides with impurities

We consider T–shaped, two–dimensional quantum waveguides containing attractive or repulsive impurities with a smooth, realistic shape, and study how the resonance behavior of the total conductance depends upon the strength of the defect potential and the geometry of the device. The resonance parameters are determined locating the relevant S–matrix poles in the Riemann energy surface. The total scattering operator is obtained from the S–matrices of the various constituent segments of the device through the –product composition rule. This allows for a numerically stable evaluation of the scattering matrix and of the resonance parameters.     

Far-infrared photoconductivity from the shallow donors in n-InP

The wavenumber of the 1s → 2p transition of the shallow donor in n-type InP is found to be 46.3 ± 0.4 cm⁻¹ in zero magnetic field, giving a conduction band effective mass of 0.085 ± 0.005 m₀ and a donor binding energy of 0.00765 eV. The Zeeman splitting of this transition is observed and yields an effective mass of 0.0810 ± 0.0005 m₀. Sharp structure is found in the spectral response of the photoconductivity close to the frequency of the LO phonons. This structure may be related to ‘total negative photoconductivity’.     

Impurity absorption of light involving resonant states of shallow donors in quantum wells

The energy spectrum of localized and resonant states of shallow donors in heterostructures GaAs/Al_xGa_1?xAs with quantum wells is calculated. The widths of the resonant states belonging to the second size quantization subband are determined. It is shown that the width of a resonance level is mainly determined by the interaction with optical phonons. The spectrum of impurity absorption of light due to electron transitions from the ground state of the donor to the resonant states belonging to the second size quantization subband is calculated.     

Screening effect on polarizabilities of shallow donors and acceptors in infinite-barrier quantum wells

Polarizabilities of shallow donors and acceptors in infinite-barrier GaAs/Ga_1−xAl_xAs quantum wells have been calculated using the Hasse variational method within the effective mass approximation. The effect of spatially dependent screening on polarizabilities is taken into account with anr-dependent dielectric response. The effects of electric and magnetic fields are also presented.     

Temperature dependence of electron mobility in GaAs/GaAlAs modulation doped quantum wells

We report the temperature dependence of the drift mobility in GaAsGa_1−x Al_x As modulation doped quantum wells. A theoretical model is formulated which includes scattering by (i) residual impurities located in the well, in the barrier, and at the “inverted” interface, (ii) Si donors and (iii) accoustic phonons. Contributions from the diffused profile of doped Si impurities and the background impurities piled up at the interface, are separated through S.I.M.S. experiments and temperature dependence of the scattering rate. The density of interface scattering centers increases with alloy composition x.     

Density of impurity states of shallow donors in a quantum well under intense laser field

The density of donor impurity states in a square GaAs–AlGaAs quantum well under an intense laser field is calculated taking into account the laser dressing effects on both the Coulomb potential and the confining potential. Using the effective-mass approximation within a variational scheme, the donor binding energy is obtained as a function of the laser dressing parameter, and the impurity position. Our results point out that a proper consideration of the density of impurity states may be of relevance in the interpretation of the optical phenomena related to shallow impurities in quantum wells, where the effects of an intense laser field compete with the quantum confinement.     

Doping and magnetic field effects on the dielectric constant of shallow donors in quantum wells

We investigate the effects of magnetic field, compensation and impurity concentration on the dielectric constant of a GaAlAs/GaAs quantum well (QW) due to intraimpurity transitions.     

Fano resonances as a probe of phase coherence in quantum dots

In the presence of direct trajectories connecting source and drain contacts, the conductance of a quantum dot may exhibit resonances of the Fano type. Since Fano resonances result from the interference of two transmission pathways, their line shape (as described by the Fano parameter q) is sensitive to dephasing in the quantum dot. We show that under certain circumstances the dephasing time can be extracted from a measurement of q for a single resonance. We also show that q fluctuates from level to level, and we calculate its probability distribution for a chaotic quantum dot. Our results are relevant to recent experiments by G?res et al. [Phys. Rev. B 62, 2188 (2000)].     

High resolution photoconductivity studies of residual shallow donors in ultrapure Ge

High resolution photoconductivity studies of residual shallow donors in ultrapure (n = 2 x 10¹⁰cm^-3) germanium have revealed absorption corresponding to transitions to higher lying levels not previously observed because of spectrometer limitations and/or line broadening due to impurity-impurity interactions. In this paper we list the results of our studies of four residual donors in high purity material, compare these results with a theoretical calculation for higher lying energy level spacings, and examine the temperature dependence of the two step photothermal photoconductivity process.     

Ising quantum Hall ferromagnet in magnetically doped quantum wells

We report on the observation of the Ising quantum Hall ferromagnet with Curie temperature T(C) as high as 2 K in a modulation-doped (Cd,Mn)Te heterostructure. In this system field-induced crossing of Landau levels occurs due to the giant spin-splitting effect. Magnetoresistance data, collected over a wide range of temperatures, magnetic fields, tilt angles, and electron densities, are discussed taking into account both Coulomb electron-electron interactions and s-d coupling to Mn spin fluctuations. The critical behavior of the resistance "spikes" at T-->T(C) corroborates theoretical suggestions that the ferromagnet is destroyed by domain excitations.