D. V. Kozlov

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.

Features of impurity-photoconductivity relaxation in boron-doped silicon

A series of studies of the impurity-photoconductivity relaxation in Si:B is carried out under pulse optical excitation by a narrow-band tunable radiation source in low and “heating” (10–500 V/cm) electric fields. It is shown that the dependence of the carrier-capture time in a band on the applied electric field is nonmonotonic and, in high fields (>75 V/cm), the capture time decreases with increasing field intensity, which is related to initiating the relaxation processes with optical-phonon emission within the band. The dependence of the relaxation rate for the carriers on the excitation-radiation wavelength is investigated, and a decrease in the carrier-capture time in the band is revealed in the vicinity of the Breit-Wigner-Fano resonances caused by direct capture at an impurity with optical-phonon emission.

The cyclotron resonance of holes in InGaAs/GaAs heterostructures with quantum wells in quantizing magnetic fields

The spectra of the cyclotron resonance of holes in the InGaAs/GaAs selectively doped heterostructures with quantum wells are studied in pulsed magnetic fields as high as 50 T at 4.2 K. The previously observed effect of the inverted (compared with the results of the single-particle calculation of the Landau levels) ratio of the spectral weight of two split components of the line of the cyclotron resonance, which is attributed to the effects of the exchange interaction of holes, is confirmed. It is found that the ratios of intensities of the components of the line of the cyclotron resonance profoundly differ on the ascending and descending branches of the magnetic field pulse, which may be associated with a long time of the spin relaxation of holes between the two lowest Landau levels, which constitute tens of milliseconds.

Effect of the direct capture of holes with the emission of optical phonons on impurity-photoconductivity relaxation in p -Si:B

A theoretical model developed for interpretation of the results of measurements of the impurity-photoconductivity relaxation in p-Si:B under pulsed optical excitation by a narrow-band tunable source of radiation in “heating” (10–500 V/cm) electric fields is presented. The model takes into account the capture of holes at the ground and lower excited states of boron with optical-phonon emission. It is shown that the dependence of the photoconductivity-relaxation time on the electric-field intensity can be unsteady taking into account these processes.

Impurity-induced photoconductivity of narrow-gap Cadmium–Mercury–Telluride structures

The photoconductivity (PC) spectra of CdHgTe (MCT) solid solutions with a Cd fraction of 17 and 19% are measured. A simple model for calculating the states of doubly charged acceptors in MCT solid solutions, which makes it possible to describe satisfactorily the observed photoconductivity spectra, is proposed. The found lines in the photoconductivity spectra of narrow-gap MCT structures are associated with transitions between the states of both charged and neutral acceptor centers.

Terahertz radiation associated with the impurity electron transition in quantum wells upon optical and electrical pumping

Radiation in the terahertz (THz) spectral range from structures with GaAs/AlGaAs doped quantum wells is investigated under conditions of the interband optical excitation of electron-hole pairs in n-type structures and impurity breakdown in a longitudinal electric field in p-type structures. The emission spectra are obtained. Emission is observed at low temperatures and shown to be determined by optical transitions between impurity states and transitions between the band and impurity states. Upon optical interband pumping, the impurity states are depopulated due to the recombination of electron-hole pairs with the involvement of impurities, while, in an electric field, the impurity states are depopulated due to impact ionization.

Relaxation kinetics of impurity photoconductivity in p-Si:B with various levels of doping and degrees of compensation in high electric fields

The relaxation of impurity photoconductivity in p-Si:B crystals subjected to pulsed optical excitation by a narrow-band continuously adjusted source of radiation in the range of “heating” (10–500 V/cm) electric fields is studied. A variation of dependence of the relaxation time on the electric field E at E > 75 V/cm due to the additional relaxation processes with the emission of an optical phonon is observed. The dependence of the rates of carrier relaxation on the intensity and wavelength of the excitation radiation indicates also that there is a long-lived excited state, which plays the role of a metastable trap level upon the relaxation of charge carriers.

Resonant states of shallow acceptors in uniaxially deformed germanium

The states of shallow acceptors in uniaxially deformed germanium are studied theoretically. A non-variational numerical computational method is developed for determining the energy and wave functions of localized states of holes in the acceptor field as well as the states of the continuous spectrum (including resonant impurity states). The dependence of the energy of the lower resonant state on strain is studied. It is found that this state is formed from the excited 4Γ8+ state with a binding energy of 1.3 meV (in the absence of deformation) and not from the ground state. The results presented in this work may be useful in the study of the conditions for the generation of far IR radiation in deformed p-Ge, which involves optical transitions between resonant and localized acceptor states.

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.