I. V. Erofeeva

Shallow acceptors in strained Ge/Ge1−xSix heterostructures with quantum wells

The energies of localized acceptor states in quantum wells (strained Ge layers in Ge/Ge1−xSix heterostructures) were analyzed theoretically in relation to the quantum well width and the impurity position in the well. The impurity absorption spectrum in the far IR range is calculated. Comparison of the results of the calculation with experimental photoconductivity spectra allows an estimation of the acceptor distribution in the quantum well to be made. In particular, it was concluded that acceptors may largely concentrate near the heterointerfaces. The absorption spectrum is calculated taking into account the resonance impurity states. This allows the features observed in the short-wavelength region of the spectrum to be interpreted as being due to transitions into the resonance energy levels “linked” to the upper size-quantization subbands.

Resonant acceptor states in Ge/Ge1-xSix MQW heterostructures

A new nonvariational theoretical approach allowing us to calculate both the localized and the continuum states of charge carriers in quantum well heterostructures in the presence of the Coulomb potential has been developed. The method has been used to calculate the energy spectra of shallow acceptors in strained Ge/GeSi multiple-quantum-well heterostructures. Optical transitions from the acceptor ground states to the resonant states have been revealed in the measured far-infrared photoconductivity spectra of the heterostructures.

Kinetics of terahertz photoconductivity in p-Ge under impurity breakdown conditions

Relaxation times of impurity photoconductivity in p-Ge samples excited by a nanosecond narrow-band source of terahertz radiation are studied at various bias voltages. It is shown that the relaxation time in prebreakdown fields increases with the applied electric field and decreases as the impurity breakdown field is exceeded. Nonmonotonic photoconductivity kinetics is observed in the studied samples differing by acceptor concentrations and degrees of compensation when approaching the impurity breakdown field.

Cyclotron resonance quantum Hall effect detector

The far-infrared photoresponse (20-80 cm-1) of a GaAs/AlGaAs detector operating at cyclotron resonance under quantum Hall effect conditions has been investigated. The response has been shown to arise near the magnetic fields corresponding to even values of the Landau level filling factor. A possibility of detector band tuning by a simultaneous increase of the magnetic field and the two-dimensional electron concentration (the latter due to the persistent photoconductivity after bandgap illumination) is demonstrated. The time constant of the response has been shown to increase dramatically with the rise of the magnetic field. This paper was presented at the 8th International Symposium on Nanostructures: Physics and Technology (St Petersburg, 2000).

Relaxation of the impurity photoconductivity in p-Ge/Ge1−xSix quantum well heterostructures

In this paper we report a study on relaxation of the impurity photoresponse in strained p-Ge/GeSi heterostructures excited by pulsed THz radiation. The relaxation time is found to increase with the applied dc electric field, which is interpreted within the model of cascade carrier capture by the impurity centers. A second time scale of the impurity photoconductivity relaxation is observed near the impurity breakdown field, and in post-breakdown electric fields the relaxation time (decreasing with the field) is shown to be governed by the impact ionization rather than by the recombination.

Shallow acceptors in Ge/GeSi multi-quantum well heterostructures

Abstract The experimental spectra of residual shallow acceptors in strained quantum well Ge/GeSi heterostructures are interpreted on the basis of a new theoretical approach taking into account both confinement and strain effects. It is shown that the main lines in the spectra of undoped samples with narrow quantum wells result from the photoexcitation of the on-edge acceptors that have binding energy two times less than on-center acceptors.

Intersubband cyclotron resonance of holes in strained Ge/GeSi(111) heterostructures with germanium wide quantum wells and cyclotron resonance of 1L electrons in GeSi layers

The submillimeter (ℏω=0.5–5 meV) magnetoabsorption spectra of strained Ge/Ge1−xSix(111) multilayer heterostructures with thick Ge layers (dGe=300–850 Å, dGeSi≈200 Å, x≈0.1) are investigated at T=4.2 K upon band-gap optical excitation. It is revealed that the absorption spectra contain cyclotron resonance lines of 1L electrons localized in GeSi solid solution layers (unlike the previously studied structures with thin Ge layers as quantum wells for 3L electrons). The absorption spectra of the samples with thick Ge layers (dGe=800–850 Å) exhibit cyclotron resonance lines of holes due to transitions from the lower Landau levels in the first quantum-well subband to the Landau levels belonging to the third and fifth higher subbands.

Shallow-impurity-assisted transitions in the course of submillimeter magnetoabsorption of strained Ge/GeSi(111) quantum-well heterostructures

The submillimeter (f=130–1250 GHz) magnetoabsorption spectra of strained Ge/GeSi(111) multilayer heterostructures with quantum wells are investigated at T=4.2 K upon band-gap optical excitation. It is found that the magnetoabsorption spectra contain lines associated with the excitation of residual shallow acceptors. The resonance absorption observed can be initiated by optical transitions between the impurity states belonging to two pairs of Landau levels of holes in germanium quantum-well layers.

Time constant of the far-IR response of a quantum Hall device

Abstract : The characteristic time of far IR response of quantum Hall effect detector in GaA/AlGaAs has been investigated versus the magnetic field. The response time is shown to increase dramatically with the field due to the spatial separation of photoexcited electrons and holes captured by localized states formed by the disordered potential.

Far-Infrared Spectroscopy of Shallow Acceptors in Strained Ge/GeSi Quantum Well Heterostructures

Far-infrared photoconductivity spectra of strained multi-quantum well Ge/Ge1—xSix (≈ 0.1) heterostructures resulting from the excitation of shallow acceptors were investigated. The spectra are shown to be shifted to the long-wavelength end of the far-infrared range if compared with acceptors in bulk p-Ge due to the splitting of light and heavy holes subbands in Ge layers owing to the “built-in” deformation and size quantization. The shallow acceptor spectra in bulk germanium under uniaxial tension, that is “equivalent” to the deformation of Ge layers in the heterostructures, were calculated by the variational technique; the results were used for the interpretation of the experimental data. Ge/GeSi heterostructures are shown to be a sensitive photoelectric detector for the long wavelength end of FIR range.