N. P. Stepina

Excitons in charged Ge/Si type-II quantum dots

Using electron-filling modulation absorption spectroscopy, we study the effect of quantum dot (QD) charging on the interband excitonic transitions in type-II Ge/Si heterostructures containing pyramidal Ge nanocrystals. In contrast to type-I systems, the ground-state absorption is found to be blueshifted when exciton-hole and exciton-exciton complexes are formed. We argue that this is the consequence of dominance of the hole-hole and electron-electron interactions compared to the electron-hole interaction due to the spatial separation of the electron and hole. The large oscillator strength (0.5) and the exciton binding energy (25 meV) are estimated from the experimental data. The results are explained by effects of the electron and hole localization and by electron wavefunction leakage in the dots. The electronic structure of spatially indirect excitons is calculated self-consistently in the effective-mass approximation for pyramidal-shaped Ge/Si QDs. The inhomogeneous strain distribution in the QD layer has been taken into account through modification of the confining potential. The calculations show that the electron of an indirect exciton resides in the Si near to the Ge pyramid apex due to maximum strain in this region, while the hole is confined close to the pyramid base. The electron-hole overlap is determined to be 15%. A satisfying agreement is found between all theoretical and experimental data.

Photoconduction in tunnel-coupled Ge/Si quantum dot arrays

The photoconduction in a tunnel-coupled Ge/Si quantum dot (QD) array has been studied. The photoconductance (PC) sign can be either positive or negative, depending on the initial filling of QDs with holes. The PC kinetics has a long-term character (102−104 s at T = 4.2 K) and is accompanied by persistent photoconduction (PPC), whereby the PC value is not restored on the initial level even after relaxation for several hours. These phenomena are observed upon illumination by light with photon energies both greater and smaller than the silicon bandgap. A threshold light wavelength corresponding to a long-term PC kinetics depends on the QD filling with holes. A model describing the observed PC kinetics is proposed, according to which the main contribution to the PC is related to the degree of QD filling with holes. By applying the proposed model to the analysis of PC kinetics at various excitation levels, it is possible to determine the dependence of the hopping conductance on the number of holes per QD. The rate of the charge carrier density relaxation exponentially depends on the carrier density.

Spin relaxation in inhomogeneous quantum dot arrays studied by electron spin resonance

Electron states in an inhomogeneous Ge/Si quantum dot array with groups of closely spaced quantum dots were studied by the conventional continuous-wave electron spin resonance and spin-echo techniques. We have found that the existence of quantum dot groups allows increasing the spin relaxation time in the system. The created structures permit us to change the effective localization radius of electrons by an external magnetic field. With the localization radius being close to the size of a quantum dot group, we obtain a fourfold increase in the spin relaxation time T1 as compared to conventional homogeneous quantum dot arrays. This effect is attributed to an averaging of the local magnetic fields produced by 29 Si nuclear spins and a stabilization of the Sz polarization during the electron back-and-forth motion within a quantum dot group.

Spatially indirect excitons in self-assembled Ge/Si quantum dots

We study the effect of the interactions of electrons and holes on the optical properties of Ge/Si type-II quantum dots (QDs). In contrast to type-I systems, the excitonic absorption is found to be blueshifted when exciton-hole and exciton-exciton complexes are formed. For a positively charged dot, we argue that this is the consequence of dominance of the hole-hole interaction compared with the electron-hole interaction due to the spatial separation of the electron and hole. When two excitons are excited in the dot, the electrons are found to be spatially separated and have different quantization energies. This is the reason why the biexciton absorption is blueshifted as compared with a single exciton. The spatial separation of electrons makes it possible for a dot to trap more electrons than there are holes. As a result, the conductivity of stacked arrays of Ge/n-Si QDs is found to decrease under interband optical excitation. The negative photoeffect is explained by the trapping of mobile electrons in the quantum well created by the Hartree potential of holes photoexcited in the dots.

Interlevel optical transitions and many-body effects in a dense array of Ge/Si quantum dots

Abstract We have investigated experimentally the mid-infrared normal-incidence response of holes confined in array of Ge/Si self-assembled quantum dots. The dots have a lateral size of approximately 15 nm and a density of 3×1011 cm−2. An in-plane polarized absorption in the 70–90 meV energy range is observed and attributed to the transition between the first two states in the dots. As the hole concentration in the dot ground state is increased, the absorption peak shifts to higher energies, its linewidth is reduced, and the lineshape is changed from an asymmetric to symmetric one. We attribute all features to a depolarization-type effect caused by collective interlevel excitations.

Selective oxidation of poly-Si with embedded Ge nanocrystals in Si/SiO2/Ge(NCs)/poly-Si structure for memory device fabrication

Selective oxidation of poly-Si in Si/SiO2/Ge(NCs)/poly-Si structure was proposed as the method of tunable synthesis of a control insulator in a memory device with Ge nanocrystals (NCs). Different behavior of oxidation resulting in partial and total oxidation of poly-Si was investigated using spectral ellipsometry and capacitance?voltage (CV) techniques. The ~2.1 V memory window corresponding to the electron and hole charging/discharging processes in NCs was obtained in Si/SiO2/Ge(NCs)/SiO2 structure. To compare the experimental and calculated CV characteristics the energies of the electron and hole ground states were determined.

Contribution of the electron-electron interaction to the optical properties of dense arrays of Ge/Si quantum dots

We present the results of an investigation of the light absorption due to interband and interlevel transitions and the photoconductivity in dense arrays of Ge quantum dots (QDs) in Si formed using the effect of self-organization during molecular-beam heteroepitaxy. It was found that the formation of charged exciton complexes composed of two holes and one electron, as well as of the be-exciton complexes in QDs of type II, leads to an increase in the energy of indirect (in real space) exciton transition, which is explained by the spatial separation of electron and hole. Self-consistent calculations of the wavefunctions for electrons and holes in exciton and in the exciton complexes showed that an electron in a single exciton is localized in the region of maximum stress for Si in the vicinity of the Ge pyramid apex, while a hole is localized near the pyramid base. In a be-exciton complex, electrons exhibit repulsion leading to their spatial separation. As a result, the second electron is bound at the boundary between Si and a continuous Ge layer in which the pyramid bases reside. The experimental data show that an increase in the charge carrier concentration in the ground state of QDs leads to a shortwave shift of the interband resonance and to the narrowing and shape change of the light absorption band, which is explained by depolarization of the external electromagnetic wave due to interaction with the collective charge density oscillations in the lateral direction of the array of Ge nanoclusters. It is established that the hole injection into an excited state of QDs leads to a longwave shift of the photoconductivity peak as a result of decay of the collective excitations and suppression of the depolarization effect.

Hall effect in hopping conduction in an ensemble of quantum dots

The Hall effect in heterostructures with a two-dimensional array of tunneling-coupled Ge quantum dots grown by molecular-beam epitaxy on Si is investigated. The conductivity of these structures in zero magnetic field at 4.2 K varies in the range of 10−12−10−4 Ω−1, which includes both the diffusive transport under weak localization conditions and hopping conduction. It is shown that the Hall effect can be discerned against the magnetoresistance-related background in both high- and low-conductivity structures. The Hall coefficient in the hopping regime exhibits a nonmonotonic dependence on the occupancy of quantum dots by holes. This behavior correlates with that of the localization length of the hole wavefunctions.

Effect Of Light Illumination On The Conductivity Of Tunnel‐coupled Ge/Si Quantum Dots

Lateral photoconductivity (PC) via dense array of Ge/Si quantum dots (QDs) has been studied at temperatures when hole hopping between dots is a dominant mechanism of charge transport. Photoconductivity with sign, depending on QDs occupancies and slow kinetics, was observed under illumination with photon energy more and less than silicon band‐gap. It was found out that the photoconductivity wavelength threshold λth depends on the filling factor η of dots with holes: the greater the η, the shorter the λth. The model based on the crucial influence of hole number in dots on the photoconductivity was proposed. Using this model to analyze PC kinetics at different light intensity we restored the dependence of hopping conductivity on QDs occupancies. It was shown that the rate of hole concentration recovering depends exponentially on concentration itself.

Hopping photoconduction and its long-time kinetics in a heterosystem with Ge quantum dots in Si

The effect of interband-transition-inducing illumination on the hole hopping conduction along a two-dimensional array of Ge quantum dots in Si was studied. It is found that the photoconductance has either positive or negative sign depending on the initial filling of quantum dots with holes. In the course of illumination and after switching off the light, long-time photoconduction kinetics was observed (102−104s at T=4.2 K). The results are discussed in terms of a model based on the spatial separation of nonequilibrium electrons and holes in a potential relief formed by positively charged dots. The effect of equalization of potential barrier heights as a result of photohole capture by the charged quantum dots during the process of illumination and relaxation is suggested as an additional factor for explaining the phenomenon of persistent conduction.