A. I. Nikiforov

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.

Strained multilayer structures with pseudomorphic GeSiSn layers

The temperature and composition dependences of the critical thickness of the 2D–3D transition for a GeSiSn film on Si(100) have been studied. The regularities of the formation of multilayer structures with pseudomorphic GeSiSn layers directly on a Si substrate, without relaxed buffer layers, were investigated for the first time. The possibility of forming multilayer structures based on pseudomorphic GeSiSn layers has been shown and the lattice parameters have been determined using transmission electron microscopy. The grown structures demonstrate photoluminescence for Sn contents from 3.5 to 5% in GeSiSn layers.

On the fine structure of spectra of the inelastic-electron-scattering cross section and the Si surface parameter

Reflection electron-energy loss spectra are obtained for a series of Si samples with different crystallographic orientations, prepared under different technological conditions. Using the experimental spectra, the electron energy loss dependences of the product of the mean inelastic free path and differential inelastic electron scattering cross section are calculated. A new technique is suggested for analyzing the spectra of inelastic electron scattering cross section by simulating experimental spectra with the use of the three-parameter Tougaard universal cross section functions. The results of the simulation are used to determine the nature of loss peaks and to calculate the surface parameter.

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.

Splitting of frequencies of optical phonons in tensile-strained germanium layers

Tensile-strained germanium films in Ge/GeSn/Si/GeSnSi multilayer heterostructures grown by molecularbeam epitaxy on Si(001) substrates are investigated by Raman spectroscopy. Biaxial tensile strains in the films reach 1.5%, which exceeds values previously obtained for this system. Splitting of frequencies of long-wavelength optical phonons is experimentally observed; i.e., the shift of the frequency of the singlet induced by biaxial tensile strains is larger than the shift of the frequency of the doublet in agreement with calculations. The strain-induced shift of Raman scattering peaks from two-phonon scattering in germanium is also detected.

Formation of Ge/Si and Ge/GexSi1−x/Si nanoheterostructures by molecular beam epitaxy

A kinetic diagram of Ge growth on Si is constructed by methods of fast electron diffraction and scanning electron microscopy. Activation energies of morphological transitions from two-dimensional to three-dimensional growth and from hut-clusters to dome-type islands are determined. The curve of the 2D–3D transition has two segments that follow the Arrhenius law and refer to different mechanisms of two-dimensional growth: two-dimensional island mechanism in the temperature range of 300–525 °C with the activation energy of −0.11 eV and step motion in the temperature range of 525–700 °C with the activation energy of 0.15 eV. Transitions from hut-islands to dome-islands are also observed. The curve constructed for the hut-dome transition is approximated by two exponential segments that obey the Arrhenius law. The hut-dome transition activation energy is 0.11 eV in the temperature range of 350–550 °C and 0.24 eV in the temperature range of 550–700 °C. The maximum density of islands in the case of Ge growth on a GexSi1−x layer reaches 4 · 1011 cm−2. An increase in the composition leads to an increase in the density of Ge islands owing to a decrease in the length of migration of Ge adatoms on the GexSi1−x surface, as compared to the case of Ge growth on Si. The periodicity N, which is manifested as a (2 × N) superstructure, decreases during the reconstruction from 14 to 8 with increasing Ge content in the GexSi1−x layer. An increase in thickness or temperature leads to a decrease in periodicity and testifies to Ge segregation; in this case, stress relaxation occurs, which reduces the Ge diffisivity.

Photoluminescence enhancement in double Ge/Si quantum dot structures

The luminescence properties of double Ge/Si quantum dot structures are studied at liquid helium temperature depending on the Si spacer thickness d in QD molecules. A seven-fold increase in the integrated photoluminescence intensity is obtained for the structures with optimal thickness d = 2 nm. This enhancement is explained by increasing the overlap integral of electron and hole wavefunctions. Two main factors promote this increasing. The first one is that the electrons are localized at the QD base edges and their wavefunctions are the linear combinations of the states of in-plane Δ valleys, which are perpendicular in k-space to the growth direction [001]. This results in the increasing probability of electron penetration into Ge barriers. The second factor is the arrangement of Ge nanoclusters in closely spaced QD groups. The strong tunnel coupling of QDs within these groups increases the probability of hole finding at the QD base edge, that also promotes the increase in the radiative recombination probability.

Initial growth stages of Si–Ge–Sn ternary alloys grown on Si (100) by low-temperature molecular-beam epitaxy

Temperature dependence of the critical thickness of the transition from two-dimensional to threedimensional growth of the Ge1–5xSi4xSnx films grown on Si (100) by molecular-beam epitaxy in the temperature range 150–450°C has been experimentally determined. This dependence is nonmonotonic and is similar to that of the critical thickness for the transition from two-dimensional to three-dimensional growth in the case of the deposition of pure Ge on Si (100) and is caused by a change in the mechanism of two-dimensional growth. Data on the average size and the density of islands, and the ratio between the height of the islands and their lateral size are obtained by the methods of atomic force microscopy and scanning tunneling microscopy. As the growth temperature is increased from 200 to 400°C, the average size of the nanoislands increases from 4.7 to 23.6 nm.

Formation of a Stepped Si(100) Surface and Its Effect on the Growth of Ge Islands

The transition from a two-domain to one-domain surface on a Si(100) substrate is investigated. It is demonstrated using reflection high-energy electron diffraction that at a temperature of 600°C and a deposition rate of 0.652 Å/s onto a Si(100) substrate pre-heated to 1000°C and inclined at an angle of 0.35°C to the plane, a series of reflections from the 1 × 2 superstructure completely vanishes at a constant flow of Si. This is attributed to the transition of the surface from monoatomic to diatomic steps. At growth rates lower than 0.652 Å/s, the transition from a two-domain to one-domain surface is also observed; with a decrease in the growth rate, the intensity ratio I2 × 1/I1 × 2 decreases and the maximum of the dependences shifts toward lower temperatures. The complete vanishing of the series of superstructural reflections after preliminary annealing at a temperature of 700°C is not observed; this series only vanishes after annealing at 900 and 1000°C. The growth of Ge islands on a Si(100) surface preliminary annealed at a temperature of 800°C is studied. It is shown that the islands tend to nucleate at the step edges. A mechanism of Ge island ordering on the Si(100) surface is proposed.

Morphology, Structure, and Optical Properties of Semiconductor Films with GeSiSn Nanoislands and Strained Layers

The dependences of the two-dimensional to three-dimensional growth (2D-3D) critical transition thickness on the composition for GeSiSn films with a fixed Ge content and Sn content from 0 to 16% at the growth temperature of 150xa0°С have been obtained. The phase diagrams of the superstructure change during the epitaxial growth of Sn on Si and on Ge(100) have been built. Using the phase diagram data, it becomes possible to identify the Sn cover on the Si surface and to control the Sn segregation on the superstructure observed on the reflection high-energy electron diffraction (RHEED) pattern. The multilayer structures with the GeSiSn pseudomorphic layers and island array of a density up to 1.8xa0×xa01012xa0cm−2 have been grown with the considering of the Sn segregation suppression by the decrease of GeSiSn and Si growth temperature. The double-domain (10u2009×u20091) superstructure related to the presence of Sn on the surface was first observed in the multilayer periodic structures during Si growth on the GeSiSn layer. The periodical GeSiSn/Si structures demonstrated the photoluminescence in the range of 0.6–0.85xa0eV corresponding to the wavelength range of 1.45–2xa0μm. The calculation of the band diagram for the structure with the pseudomorphic Ge0.315Si0.65Sn0.035 layers allows assuming that photoluminescence peaks correspond to the interband transitions between the X valley in Si or the Δ4-valley in GeSiSn and the subband of heavy holes in the GeSiSn layer.