Yu. B. Samsonenko

Photovoltaic Properties of p-Doped GaAs Nanowire Arrays Grown on n-Type GaAs(111)B Substrate

We report on the molecular beam epitaxy growth of Au-assisted GaAs p-type-doped NW arrays on the n-type GaAs(111)B substrate and their photovoltaic properties. The samples are grown at different substrate temperature within the range from 520 to 580 °C. It is shown that the dependence of conversion efficiency on the substrate temperature has a maximum at the substrate temperature of 550 °C. For the best sample, the conversion efficiency of 1.65% and the fill factor of 25% are obtained.

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.

Photoluminescence properties of InAs nanowires grown on GaAs and Si substrates.

We report the first photoluminescence (PL) characterization of InAs nanowires (NWs). The InAs NWs were grown on GaAs(111) B and Si(111) substrates using the Au-assisted molecular beam epitaxy (MBE) growth technique or metal-organic chemical vapor deposition (MOCVD). We compared the PL response of four samples grown under different conditions using MBE or MOCVD. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) patterns were utilized to determine the crystal structure and growth directions of the NWs to relate PL features to NW structural parameters. We observed mainly three PL peaks which were below, near and above InAs bandgaps, respectively. Temperature and excitation intensity dependence PL measurements were also performed to help elucidate the origins of the PL peaks of NWs. Of particular interest was a band-edge emission peak that was blue-shifted due to quantization effects of the InAs NWs, as confirmed by our calculation.

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 carrier transfer in tunnel injection structures

InGaAs tunnel injection nanostructures consisting of a single quantum well as injector and a quantum dot layer as emitter are studied by time-resolved photoluminescence spectroscopy. The quantum dot photoluminescence undergoes substantial changes when proceeding from direct quantum dot excitation to quantum well excitation, which causes an indirect population of the dot ground states. This results in a lowered effective carrier temperature within the dots. Results on the carrier transfer versus barrier thickness are discussed within the Wentzel–Kramers–Brillouin approximation. Deviations for barrier thicknesses <5nm are assigned to the formation of nanobridges that are actually detected by transmission electron microscopy.

Optical study of GaAs quantum dots embedded into AlGaAs nanowires

We report the photoluminescence characterization of GaAs quantum dots embedded in AlGaAs nanowires. Time-integrated and time-resolved photoluminescence was measured for both arrays and single quantum dot/nanowires. The optical spectroscopy data show the influence of growth temperature on the distribution of diameters and the presence of different crystalline phases in the AlGaAs nanowires. By means of scanning and transmission electron microscopy and photoluminescence we observed that the growth temperature has a strong influence on the homogeneity of the nanowires, in size and density. In photoluminescence spectra of a single quantum dot, spectral diffusion was observed in the exciton line. Formation of various crystalline phases in the AlGaAs nanowires leads to very long decay times for the nanowire luminescence, around 20 ns.

Terahertz generation by GaAs nanowires

The investigation of terahertz generation by the surface of n-InAs layers and by GaAs nanowires grown on n-GaAs (111) substrates is presented. It was observed that the terahertz radiation power efficiency is significantly higher in the second case.

STM and RHEED study of InAsGaAs quantum dots obtained by submonolayer epitaxial techniques

Formation of uniform arrays of InAs quantum dots on GaAs(100) singular and vicinal (3° towards [011] direction) surfaces crucially depends on the deposition mode chosen. For dots formed with continuous As flux impinging on the surface and simultaneous submonolayer In deposition cycles, intentional substrate misorientation significantly decreases the density of dots and stimulates their ordering along the [001] direction. On the contrary, growth using alternate In and As deposition cycles results in a reduced density of dots for singular surfaces and in strongly increased dot concentration for vicinal ones. Ordering of dots in chains along [001] and [010] directions is observed for the alternative deposition on singular substrates.

Study of processes of self-catalyzed growth of gaas crystal nanowires by molecular-beam epitaxy on modified Si (111) surfaces

The processes of growth of self-catalyzed GaAs crystal nanowires on Si (111) surfaces modified by three different methods are studied. For the technology of production of the GaAs nanowires, molecular-beam epitaxy is used. It is found that, in the range of substrate temperatures between 610 and 630°C, the surface density of nanowires and their diameter sharply increases, whereas the temperature dependence of the nanowire length exhibits a maximum at 610°C. An increase in the temperature to 640°C suppresses the formation of nanowires. The method that provides a means for the fabrication of purely cubic GaAs nanowires is described. A theoretical justification of the formation of the cubic phase in self-catalyzed GaAs nanowires is presented.

Formation of InGaAsGaAs quantum dots by submonolayer molecular beam epitaxy

Abstract We have performed a scanning tunneling microscopy study of the formation of (In,Ga)As/GaAs and InAs/GaAs quantum dot and quantum wire arrays on GaAs(100) and vicinal surfaces during submonolayer molecular beam epitaxy. During the initial stage of strained layer transformation (∼ 2 monolayers of InAs) the formation of well ordered quantum wire arrays along the [001] direction is observed. Further deposition results in dots placed in rows and then in an array of well separated dots. This effect is more pronounced in the case of vicinal surfaces where the dots are oriented along the [001] direction despite the surface is misoriented towards the [0 – 11] direction. Our study provides a new insight into the process of quantum dot and quantum wire arrays formation on GaAs(100) and vicinal surfaces.