A. A. Tonkikh

Room-temperature light emission from a highly strained Si/Ge superlattice

We discuss the formation of a Si/Ge-superlattice (SL) generated by molecular beam epitaxy. Specific growth parameter were chosen to optimize the periodic structure of vertically stacked Ge islands. Optimized SLs show a strong photoluminescence at a wavelength in the region of 1.55 μm up to room temperature. The luminescence is explained by a recombination of electrons in a miniband and holes localized in the Ge islands. The morphology and the crystal structure of the SL, which are influenced by the growth parameters, were analyzed by transmission electron microscopy techniques. It is demonstrated that doping of the SL structure by antimony improves both structural and optical properties.

Atomic structure of MBE-grown GaAs nanowhiskers

The structural properties of MBE-grown GaAs and Al0.3Ga0.7 As nanowhiskers were studied. The formation of wurtzite and 4H-polytype hexagonal structures with characteristic sizes of 100 nm or larger in these materials was demonstrated. It is concluded that the Au-Ga activation alloy symmetry influences the formation of the hexagonal structure.

The Diffusion Mechanism in the Formation of GaAs and AlGaAs Nanowhiskers during the Process of Molecular-Beam Epitaxy

The formation of GaAs and AlGaAs nanowhiskers using molecular-beam epitaxy on GaAs (111)B surfaces activated with Au is theoretically and experimentally studied. It is experimentally shown that nanowhiskers whose length exceeds the effective thickness of the deposited GaAs by an order of magnitude can be grown. It is found that the experimental dependences of the nanowhisker length L on its diameter D can differ radically from those observed in the case of a vapor-liquid-solid growth mechanism. The L(D) dependences obtained in this study are decreasing functions of D. The above effects are related to the existence of the diffusion transport of atoms from the surface towards the tips of the whiskers, which leads to a considerable increase in the growth rate of thin whiskers. A theoretical model of the formation of nanowhiskers in the process of molecular-beam epitaxy is developed. The model provides a unified description of the vapor-liquid-solid and diffusion growth mechanisms and qualitatively explains the experimental results obtained.

Transient spectroscopy of InAs quantum dot molecules

Coupled pairs of InAs quantum dots are grown by molecular-beam epitaxy. Structural and optical characterization is done by means of transmission electron microscopy and photoluminescence, respectively. Photoluminescence spectra consist at least of three well-separated optical transitions that are assigned to molecular energy terms and a substantial exciton lifetime increase is observed. Detailed spectral analysis of the transient luminescence behavior indicates “intraterm” transitions that could be favorably used for the creation of midinfrared light sources.

Si/Ge Nanostructures for Optoelectronics Applications

The optical and structural properties of multilayer Si/Ge structures with precritical, as well as close-to-critical, germanium inclusions in a silicon matrix, for which the transition from the two-dimensional to island growth occurs, were studied. The possibility of obtaining intense photoluminescence at room temperature in both cases under optimally chosen growth parameters is demonstrated. The proposed approaches to producing an active region appear promising for applications in silicon-based optoelectronics.

Structural and optical properties of InAs quantum dots in AlGaAs matrix

Structural and optical properties of InAs quantum dots (QDs) grown in a wide-bandgap Al0.3Ga0.7As matrix is studied. It is shown that a high temperature stability of optical properties can be achieved owing to deep localization of carriers in a matrix whose band gap is wider than that in GaAs. Specific features of QD formation were studied for different amounts of deposited InAs. A steady red shift of the QD emission peak as far as ∼1.18 µm with the effective thickness of InAs in Al0.3Ga0.7As increasing was observed at room temperature. This made it possible to achieve a much higher energy of exciton localization than for QDs in a GaAs matrix. To obtain the maximum localization energy, the QD sheet was overgrown with an InGaAs layer. The possibility of reaching the emission wavelength of ~1.3 µm is demonstrated.

Electron diffraction on GaAs nanowhiskers grown on Si(100) and Si(111) substrates by molecular-beam epitaxy

The crystal structure of GaAs nanowhiskers grown by molecular-beam epitaxy on Si(111) and Si(100) substrates is investigated using reflection high-energy electron diffraction (RHEED). It is revealed that, in both cases, the electron diffraction images contain a combination (superposition) of systems of reflections characteristic of the hexagonal (wurtzite and/or 4H polytype) and cubic (sphalerite) phases of the GaAs compound. The growth on the Si(111) substrates leads to the formation of nanowhiskers with hexagonal (wurtzite and/or 4H polytype) and cubic (sphalerite) structures with one and two orientations, respectively. In the case of the Si(100) substrates, the grown array contains GaAs nanowhiskers that have a cubic structure with five different orientations and a hexagonal structure with eight orientations in the (110) planes of the substrate. The formation of the two-phase crystal structure in nanowhiskers is explained by the wurtzite—sphalerite phase transitions and/or twinning of crystallites.

Threshold Behavior of the Formation of Nanometer Islands in a Ge/Si(100) System in the Presence of Sb

Atomic-force microscopy is used to study the behavior of an array of Ge islands formed by molecular-beam epitaxy on an Si (100) surface in the presence of an antimony flux incident on the surface. It is shown that, as the Sb flux increases to a certain critical level, the surface density of the islands increases; however, if this critical level is exceeded, nucleation of the islands is suppressed and mesoscopic small-height clusters are observed on the surface. This effect is explained qualitatively in the context of a kinetic model of the islands’ formation in heteroepitaxial systems mismatched with respect to their lattice parameters.

Suppression of Dome-Shaped Clusters During Molecular Beam Epitaxy of Ge on Si(100)

Morphological properties of Ge nanoscale island arrays formed on the Si(100) surface during molecular beam epitaxy are studied using reflection high-energy electron diffraction and atomic-force microscopy. It is shown that codeposition of Sb and Ge significantly increases the density of final island arrays and suppresses the formation of dome-shaped clusters at substrate temperatures of 550–600°C. The results are discussed in the context of the kinetic model of formation of elastically strained islands in heteroepitaxial systems with a lattice mismatch.

1.3-1.4 µm photoluminescence emission from InAs/GaAs quantum dot multilayer structures grown on GaAs singular and vicinal substrates

Optical and structural properties of multilayer structures with InAs/GaAs quantum dots are investigated. It is shown that under optimal growth conditions, 1.3-1.4 µm emission can be achieved. Possible scenarios of quantum dot behaviour evaluation are discussed in a frame of elastic theory to explain differences in optical properties of the grown structures.