A. I. Yakimov

Electronic structure of Ge/Si quantum dots

We have investigated theoretically the strain distribution in pyramid-shaped Ge/Si quantum dots (QDs) and their environment, using the atomistic approach and the Green function technique. Taking into account the results of strain calculations, we have studied the hole discrete spectrum by the tight-binding method. Energy levels, their dependence on dot size and wavefunction density distributions have been obtained. We have proposed a method for calculation of the Lande factor for localized states in QDs and calculated the value of the g-factor for the ground state in the Ge/Si dot. We have developed a theoretical model of spatially indirect excitons and excitonic complexes, localized on the QD. The binding energy and optical transition energy have been calculated for excitonic complexes with different numbers of electrons and holes.

In situ RHEED control of self-organized Ge quantum dots

Abstract In situ registration of high-energy electron diffraction patterns was used for constructing the diagram of structural and morphological states of the Ge film on the Si(100) surface. The following regions identified in the diagram: two-dimensional (2D)-growth, ‘hut’- and ‘dome’-clusters, ‘dome’-clusters with misfit dislocations at the interface. Variations in the lattice constants of the Ge film during the MBE growth on the Si(100) surface were determined. An increase in the lattice constant at the (100) surface was attributed to the elastic deformation at the stage of 2D growth and formation of ‘hut’-clusters and to the plastic relaxation for the ‘dome’-clusters. As a result, epitaxial silicon structures with germanium quantum dots of 15 nm base size at the density of 3×1011 cm−2 were synthesized. The total electron structure of the hole spectrum of Ge quantum dots in Si was established.

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.

Formation of zero-dimensional hole states in Ge/Si heterostructures probed with capacitance spectroscopy

Abstract Hole energy spectrum in Ge/Si(001) heterostructures grown by molecular-beam epitaxy are studied using capacitance spectroscopy at a temperature range of 4.2–300 K. We find that the formation of Ge islands as the effective film thickness exceeds six monolayers leads to the appearance of the zero-dimensional hole states associated with Ge quantum dots. Analysis of the capacitance–voltage characteristics of structures containing the quantum-dot ‘atoms’ and the quantum-dot ‘molecules’ reveals the Coulomb charging effect.

Ge/Si photodiodes with embedded arrays of Ge quantum dots forthe near infrared (1.3–1.5 µm) region

A method has been devised for MBE fabrication of p-i-n photodiodes for the spectral range of 1.3–1.5 µm, based on multilayer Ge/Si heterostructures with Ge quantum dots (QDs) on a Si substrate. The sheet density of QDs is 1.2×1012 cm−2, and their lateral size is ∼8 nm. The lowest room-temperature dark current reported hitherto for Ge/Si photodetectors is achieved (2×10−5 A/cm2 at 1 V reverse bias). A quantum efficiency of 3% at 1.3 µm wavelength is obtained.

Phononless hopping conduction in two-dimensional layers of quantum dots

Regularities are studied in charge transport due to the hopping conduction of holes along two-dimensional layers of Ge quantum dots in Si. It is shown that the temperature dependence of the conductivity obeys the Efros-Shklovskii law. It is found that the effective localization radius of charge carriers in quantum dots varies nonmonotonically upon filling quantum dots with holes, which is explained by the successive filling of electron shells. The preexponential factor of the hopping conductivity ceases to depend on temperature at low temperatures (T<10 K) and oscillates as the degree of filling quantum dots with holes varies, assuming values divisible by the conductance quantum e2/h. The results obtained indicate that a transition from phonon-assisted hopping conduction to phononless charge transfer occurs as the temperature decreases. The Coulomb interaction of localized charge carriers has a dominant role in these phononless processes.

Influence of delta-doping on the performance of Ge/Si quantum-dot mid-infrared photodetectors

The effect of delta-doping on the performance of ten-period Ge/Si quantum-dot (QD) mid-infrared photodetectors (λmax≃3.4 μm) was investigated. Ge QDs fabricated by molecular-beam epitaxy at 500 °C are overgrown with Si at 600 °C. Each Si barrier contains a boron delta-doping layer located near the QD plane to provide holes to the dots. Within the sample set, we examined devices with different positions of the δ-doping layer with respect to the QD plane, different distances between the δ-doping layer and the QD plane, and different doping densities. All detectors show pronounced photovoltaic behavior implying the presence of an internal inversion asymmetry. We observed a reversal of the voltage dependence of responsivity with respect to zero bias when the δ-doping plane is carried from the bottom to the top of the dot layer. This result indicates that the main reason for the asymmetric photoresponse is the existence of a built-in electric field due to the placing dopants in the barriers. Devices with a low...

Effect of overgrowth temperature on the mid-infrared response of Ge/Si(001) quantum dots

Ge/Si quantum dots fabricated by molecular-beam epitaxy at 500 °C are overgrown with Si at different temperatures Tcap, and their mid-infrared photoresponse is investigated. The photocurrent maximum shifts from 2.3 to 3.9 μm with increasing Tcap from 300 to 750 °C. The best performance is achieved for the detector with Tcap = 600 °C in a photovoltaic mode. At a sample temperature of 90 K and no applied bias, a responsivity of 0.43 mA/W and detectivity of 6.2 × 1010 cmHz1/2/W at λ = 3 μm were measured under normal incidence infrared radiation. The device exhibits very low dark current (Idark = 2 nA/cm2 at T = 90 K and U = −0.2 V) and operates until 200 K.

Ge'Si quantum-dot metal-oxide-semiconductor field-effect transistor

We report on the operation of Si metal–oxide–semiconductor field-effect transistor with an array of ∼103 10 nm diameter Ge self-assembled quantum dots embedded into the active channel. The drain current versus gate voltage characteristics show oscillations caused by Coulomb interaction of holes in the fourfold-degenerate excited state of the dots at T⩽200 K. A dot charging energy of ∼43 meV (i.e., >kT=26 meV at T=300 K) and disorder energy of ∼20 meV are determined from the oscillation period and the temperature dependence study of current maxima, respectively.

Midinfrared photoresponse of Ge quantum dots on a strained Si0.65Ge0.35 layer

We report on intraband photocurrent spectroscopy of Ge self-assembled quantum dots placed on a strained Si0.65Ge0.35 quantum well, which, in turn, is incorporated in a Si matrix. The p-type devices show broad spectral response ranging from 2 to 12 ?m. By a comparison between photocurrent measurements and the hole energy level scheme, as deduced from six-band k p calculations, the two main contributions to the photoresponse are identified. The absorption band between 2 and 4 ?m is attributed to the bound-to-continuum transitions between the bound states of the quantum dots and the continuum states in the Si barrier. The photoresponse at longer wavelength (4?12 ?m) is associated with hole transitions from the dots to the nearby Si0.65Ge0.35 layer.