O. A. Kuznetsov

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.

Si1−xGex/Si(001) relaxed buffer layers grown by chemical vapor deposition at atmospheric pressure

Relaxed step-graded buffer layers of Si1−xGex/Si(001) heterostructures with a low density of threading dislocations are grown through chemical vapor deposition at atmospheric pressure. The surface of the Si1−xGex/Si(001) (x ∼ 25%) buffer layers is subjected to chemical mechanical polishing. As a result, the surface roughness of the layers is decreased to values comparable to the surface roughness of the Si(001) initial substrates. It is demonstrated that Si1−xGex/Si(001) buffer layers with a low density of threading dislocations and a small surface roughness can be used as artificial substrates for growing SiGe/Si heterostructures of different types through molecular-beam epitaxy.

Photoluminescence line width of self-assembled Ge(Si) islands arranged between strained Si layers

The effect of variations in the strained Si layer thicknesses, measurement temperature, and optical excitation power on the width of the photoluminescence line produced by self-assembled Ge(Si) nanoislands, which are grown on relaxed SiGe/Si(001) buffer layers and arranged between strained Si layers, is studied. It is shown that the width of the photoluminescence line related to the Ge(Si) islands can be decreased or increased by varying the thickness of strained Si layers lying above and under the islands. A decrease in the width of the photoluminescence line of the Ge(Si) islands to widths comparable with the width of the photoluminescence line of quantum dot (QD) structures based on direct-gap InAs/GaAs semiconductors is attained with consideration of diffusive smearing of the strained Si layer lying above the islands.

Shallow acceptors in Ge/GeSi strained multilayer heterostructures with quantum wells

The impurity photoconductivity spectra of Ge/Ge1−xSix strained heterostructures with quantum wells are investigated. It is established that the built-in deformation in quantum-size Ge layers substantially changes the spectrum of shallow acceptors, shifting it into the long-wavelength region of the far-IR range. In strong magnetic fields the photoconductivity lines are observed to split and shift as a function of the field. This makes it possible to carry out a classification of the transitions.

Probing the p-Ge1-xSix/Ge/p-Ge1-xSix quantum well by means of the quantum Hall effect

We have measured the temperature (0.1?T?15?K) and magnetic field (0?B?32?T) dependences of longitudinal and Hall resistivities for the p-Ge0.93Si0.07/Ge multilayers with different Ge layer widths 10?dw?38?nm and hole densities ps = (1-5)?1015?m-2. An extremely high sensitivity of the experimental data (the structure of magnetoresistance traces, relative values of the inter-Landau-level (LL) gaps deduced from the activation magnetotransport etc) to the quantum well (QW) characteristics has been revealed in the cases when the Fermi level reached the second confinement subband. The background density of states (5-10)?1014?m-2?meV-1 deduced from the activation behaviour of the magnetoresistance was too high to be attributed to the LL tails, but may be accounted for within a smooth random potential model. The hole gas in the Ge QW was found to separate into two sublayers for dw>~35?nm and ps?5?1015?m-2. Concomitantly the positive magnetoresistance emerged in the weakest fields, from which different mobilities in the sublayers were deduced. A model is suggested to explain the existence of the plateaux close to the fundamental values in a system of two parallel layers with different mobilities.

Special features of the formation of Ge(Si) islands on the relaxed Si1−xGex/Si(001) buffer layers

The results of studying the growth of self-assembled Ge(Si) islands on relaxed Si1−xGex/Si(001) buffer layers (x≈25%), with a low surface roughness are reported. It is shown that the growth of self-assembled islands on the buffer SiGe layers is qualitatively similar to the growth of islands on the Si (001) surface. It is found that a variation in the surface morphology (the transition from dome-to hut-shaped islands) in the case of island growth on the relaxed SiGe buffer layers occurs at a higher temperature than for the Ge(Si)/Si(001) islands. This effect can be caused by both a lesser mismatch between the crystal lattices of an island and the buffer layer and a somewhat higher surface density of islands, when they are grown on an SiGe buffer layer.

Nonmonotonic temperature dependence of the resistivity of p-Ge/Ge1−xSix in the region of the metal–insulator transition

In a two-dimensional (2D) hole system (multilayer p-Ge/Ge1−xSix) heterostructure with conductivity σ≈e2/h at low temperatures (T≈1.5 K) a transition from the insulator phase (dσ/dT>0) to a “metallic” phase (dσ/dT<0) is observed as the temperature is lowered, behavior that is in qualitative agreement with the predictions of the Finkelstein theory. In a magnetic field B perpendicular to the plane of the 2D layer one observes positive magnetoresistance depending only on the ratio B/T. We attribute the positive magnetoresistance effect to the suppression of the triplet channel of Fermi-liquid electron–electron interaction by the magnetic field owing to the strong Zeeman splitting of the hole energy levels.

Parabolic negative magnetoresistance in p-Ge/Ge1-xSix heterostructures

Quantum corrections for the conductivity due to the weak localization (WL) and the disorder-modified electron-electron interaction (EEI) are investigated for the high-mobility multilayer p-Ge/Ge1−xSex heterostructures at T=(0.1–20) K in magnetic field B up to 1.5 T. Negative magnetoresistance with logarithmic dependence on T and linear in B2 is observed for B⩾0.1 T. Such a behavior is attributed to the connection between the classical cyclotron motion and the EEI effect. The Hartree part of the interaction constant is estimated (Fσ=0.44) and the WL and EEI contributions to the total quantum correction Δσ at B=0 are separated (ΔσWL≈0.3Δσ; Δσee≈0.7Δσ).Quantum corrections to the conductivity due to the weak localization (WL) and the disorder-modified electron-electron interaction (EEI) are investigated for the high-mobility multilayer p-Ge/Ge1-xSix heterostructures at T = (0.1 - 20.0)K in magnetic field B up to 1.5T. Negative magnetoresistance with logarithmic dependence on T and linear in B^2 is observed for B>= 0.1T. Such a behavior is attributed to the interplay of the classical cyclotron motion and the EEI effect. The Hartree part of the interaction constant is estimated (F_/sigma = 0.44) and the WL and EEI contributions to the total quantum correction /Delta /sigma at B = 0 are separated (/Delta /sigma_{WL} ~ 0.3/Delta /sigma; /Delta /sigma_{EEI} ~ 0.7/Delta /sigma).

Electrically active centers in Si:Er light-emitting layers grown by sublimation molecular-beam epitaxy

Electrically active centers in light-emitting Si:Er layers grown by sublimation molecular-beam epitaxy (SMBE) on single-crystal Si substrates have been investigated by admittance spectroscopy with temperature scanning and by DLTS. The total density of electrically active centers is defined by shallow donor centers with ionization energies of 0.016–0.045 eV. The effect of growth conditions and post-growth annealing on the composition and density of electrically active centers has been studied. Significant differences in composition of the electrically active centers with deep levels and in channels of energy transfer from the electron subsystem of a crystal to Er3+ ions between Si:Er layers grown by SMBE and ion implantation have been revealed.