Home – Home – Jornals – Avtometriya 2013 number 5

A new approach to the creation of circularly ordered Ge nanoislands by epitaxy on the surface of a heterophase structure consisting of a Si(100) substrate with premade seeds in the form of SiGe nanodisks or SiGe nanorings is developed. It is shown that the spatial configuration of islands in the group is due to the nucleation of the islands in the area of local minima of the elastic energy density on the surface of a circular seed. On the basis of this approach, a number of multilayer structures with vertically aligned ring groups of quantum dots were grown. The elemental composition and luminescent properties of the ordered structures are studied.

The possibility of using AlN/Al₂O₃ substrates to grow AlGaN/GaN hetero-epitaxial structures with a two-dimensional electron gas is studied. A method of calibrating the temperature of the substrates by measuring the thermal radiation spectrum is proposed. Differences between AlN/Al₂O₃ substrates that lead to differences in the electrophysical parameters of the grown structures are determined. AlN/Al₂O₃ substrates were used to grow AlGaN/GaN samples with a two-dimensional electron gas mobility in excess of 1300 cm²/(V · s) at an electron concentration in the channel higher than 10¹³ cm⁻².

The formation of Si quantum dots on Ge is studied. Fundamental differences were found between the nucleation and growth of quantum dots on substrates with different orientations, related to both the formation of a SiGe solid solution layer and the shape of quantum dots. The temperature interval of formation of Si nanocrystals on Ge (111) is determined. It is shown that a change in the epitaxy temperature from 480 to 400 ºC increases the silicon content in quantum dots from 58 to 75 %. The effect of surfactants (in particular, hydrogen) on the nucleation and growth of silicon nanoislands is analyzed, showing that the use of hydrogen as a surfactant leads to a decrease in the size of quantum dots and a substantial increase in their density. The observed effects are attributed to the suppression of surface diffusion of atoms in the presence of hydrogen.

We consider the two-dimensional distribution of elastic strain in semiconductor heterostructures — quantum wires characterized by anisotropy of the elastic properties. The deformation is caused by the mismatch in lattice parameters between the material of the quantum wire and its environment (matrix).Such deformations affect the position of the energy bands, so that they should be taken into account in the calculation of the electronic states. It is shown that the strain distribution in an anisotropic medium is a linear combination of two distributions relating to transversely stretched modifications of the original quantum wire.

The problem of the applicability of the six-band kp -model to heterostructures with sharp boundaries is studied by calculating the energy spectrum of holes in the Ge/Si system with quantum dots. The boundary conditions which satisfy the conditions of particle flux conservation and the wave function continuity on the heterojunction are formulated at the level of differential equations and are characterized by a single parameter µ, which depends on the heterojunction properties. It is shown that a certain choice of µ leads to nonphysical interface states that fill the entire band gap. Conditions (range of µ) for the absence of such nonphysical states are determined by considering the simplest cases – a single heterojunction and a quantum well.

A new method of incorporating the elastic strain into the atomistic simulation of heteroepitaxial growth is proposed. The idea of this method is to include random displacements (thermal oscillations) of atoms within the lattice site into the calculation algorithm in addition to elementary events that change the atomic configuration (deposition of new atoms and jumps of atoms from one crystal lattice site to another). It appears to be possible to assign probabilities of elementary events in order to guarantee minimization of the free energy of the unperturbed system. The simulation based on this method reproduces the main heteroepitaxial growth effects, such as the formation of threedimensional islands on the strained wetting layer (Stranski–Krastanov growth mode) and the vertical alignment of nanoislands in the course of growth of multilayer heterostructures.

This paper presents a review of studies of the photovoltaic characteristics of Ge/SiGe/Si heterostructures containing layers of Ge quantum dots in Si and SiGe matrices. In the experiments, the elemental composition of the SiGe films and its profile, the doping level and profiles, the position of the doped layers of Si and SiGe relative to the plane of quantum dots, and the number of layers of quantum dots were varied. The results of these studies were used to implement infrared photodetector elements functioning at normal light incidence in atmosphere transmission windows of 3 ÷ 5 and 8 ÷ 12 µm. The photodetectors have a high (up to 10³) photoelectric gain, a high detection ability in the photovoltaic and photoconductive modes (up to 0.8 · 10¹¹ cm · Hz^1/2/W at a wavelength of about 4 m), and a current sensitivity of up to 1 mA/W, reach the background limited infrared performance already at a temperature of 110 K, and can be built in monolithic focal plane arrays on silicon substrates.

A bilayer cadmium–mercury–tellurium (CMT) heterostructure was designed consisting of photosensitive layers of compositions x_CdTe = 0.29–0.32 and x_CdTe = 0.220–0.230, sensitive in the spectral ranges of 3–5 and 8–12 µm, a barrier layer between them, and wide-band variable-gap layers on the heterojunction and the surface grown on a GaAs substrate with ZnTe and CdTe buffer layers. The molecular beam epitaxial (MBE) growth of the heteroepitaxial structure (HES) was controlled by real time ellipsometry. After the growth, the composition distribution throughout the thickness was measured by reflection spectra with layer-by-layer chemical etching. There is good agreement between the results of composition measurements using ellipsometry and reflection spectra. P-type conductivity of bilayer MBE CMT HESs was obtained after thermal annealing at 220–240 ºC in an inert gas (helium) for 24 h. The concentration of holes in the photosensitive layers is (4–10) ·10¹⁵ cm⁻³ and (8–20) · 10¹⁵ cm⁻³ at 78 K.

This paper gives the parameters of 320 × 256 element long-wavelength infrared photodetectors fabricated by a new improved technology based on heteroepitaxial mercury–cadmium–tellur structures. In these photodetectors, the variation of the photodiode bias voltage over the area of the array is minimized; inefficient photodiode regions related to both hybridization and spike-shaped growth defects of epitaxial films are eliminated; the current-voltage characteristics of the diodes in the resulting photodetectors are homogeneous and are limited by the diffusion current component up to −400 mV. The dark current is 0.25–0.45 nA, and R₀A = (0.6–3) · 10² Om · cm². The voltage sensitivity, the threshold irradiance, and the average NETD at the maximum sensitivity are 11.8 · 10⁸ V/W, 3.7 · 10⁻⁸ W/cm², and 26.8 mK, respectively. The percentage of defective elements is 1.5%.

Results of experimental studies of Pb_1−xSn_x Te:In films grown by molecular beam epitaxy with the tin concentration close to band inversion are presented. An optimal concentration of indium is determined, and films with x > 0.3 are obtained, where the so-called metal–insulator transition is observed. Photoconductor prototypes are fabricated, and the photocurrent induced by free electron laser radiation in the range of 140–205 µm is measured. A possibility of using photoconductors for recording the own radiation of a body heated to a temperature of 300 K in a passive mode, including systems of image registration in the terahertz spectral range, is estimated.

Ensembles of InAs quantum dots with a very low density (~10⁶ cm⁻²) are grown by molecular beam epitaxy, which allows the spectral characteristics of emission of single quantum dots to be studied by the method of cryogenic microphotoluminescence. With increasing quantum dot size, the splitting of exciton states is demonstrated to increase steadily to ~10² µeV. In the exciton energy range of 1.3–1.4 eV, the magnitude of this splitting is comparable with the natural width of the exciton lines. This result is important for the development of emitters of entangled photon pairs based on InAs quantum dots.

Surface-enhanced Raman scattering by optical and surface phonons in CdS, GaN, and CuS nanocrystals, and AlN nanowires is detected and studied. It is found that the presence of metal (Ag, Au, and Pt) nanoclusters noticeably modifies the Raman spectra of the nanostructures and results in a resonant increase in the intensity of optical phonon modes in CdS and CuS nanocrystals or in the emergence of surface modes in GaN nanocrystals and AlN nanowires. It is shown that the frequencies of the surface optical phonon modes of the examined nanostructures are in good agreement with the theoretical values calculated within the framework of the dielectric continuum model.

The time of spin relaxation of excitons in (In,Al)As/AlAs quantum dots with an indirect bandgap and type-I band alignment is determined by measuring the dynamics of photoluminescence circular polarization induced by a magnetic field B. The spin relaxation time τ_S increases with decreasing magnetic field in proportion to B⁻⁵; its value is ~40 µs in a magnetic field of 6 T at a temperature of 1.8 K. As the temperature T increases in a magnetic field of 7T, the value of τ_S decreases as T^−1.1. The character of the dependences of τ_S on the magnetic field and temperature evidences that spin relaxation of excitons is provided by a process with participation of one acoustic phonon.

Nanowire (NW) detection is one of the fast and highly sensitive methods. An NW biosensor based on silicon-on-insulator (SOI) structures are used in the reported study for real-time label-free biospecific detection of the NFATc1 (D-NFATc1) cancer marker. For this purpose, the SOI NWs are functionalized with NFATc1 aptamers used as macromolecular probes. It is demonstrated that such a biosensor can ensure a detection limits up to 10⁻¹⁵ M, which is comparable with the sensitivity ensured by an NW biosensor with immobilized antibodies used as macromolecular probes. The results of this study demonstrate that such approaches to the development of sensor elements for highly sensitive diagnostics of diseases are really promising.