A. N. Smirnov

Supersensitive graphene-based gas sensor

Epitaxial graphene layers are produced with the aid of thermal destruction of the surface of a semi-insulating SiC substrate. Raman spectroscopy and atomic-force microscopy are employed in the study of the film homogeneity. A prototype of the gas sensor based on the films is fabricated. The device is sensitive to the NO2 molecules at a level of 5 ppb (five particles per billion). A possibility of the industrial application of the sensor is discussed.

Study of the crystal and electronic structure of graphene films grown on 6 H -SiC (0001)

The structural, chemical, and electronic properties of epitaxial graphene films grown by thermal decomposition of the Si-face of a semi-insulating 6H-SiC substrate in an argon environment are studied by Raman spectroscopy, atomic-force microscopy, the low-energy electron diffraction method, X-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy and X-ray absorption spectroscopy at the carbon K edge. It is shown that the results of a systematic integrated study make it possible to optimize the growth parameters and develop a reliable technology for the growth of high-quality single-layer graphene films with a small fraction of bilayer graphene inclusions.

Emission intensity of the λ = 1.54 μm line in ZnO films grown by magnetron sputtering, diffusion doped with Ce, Yb, Er

The effect of the Er3+-ion excitation type on the photoluminescence spectra of crystalline ZnO(ZnO〈Ce, Yb, Er〉) films is determined in the cases of resonant (λ = 532 nm, Er3+-ion transition from 4S3/2, 2H11/2 levels to 4I15/2) and non-resonant (λ = 325 nm, in the region near the ZnO band-edge emission) excitation. It is shown that resonant excitation gives rise to lines with various emission intensities, characteristic of the Er3+-ion intracenter 4f transition with λ = 1535 nm when doping crystalline ZnO films with three rare-earth ions (REIs, Ce, Yb, Er) or with two impurities (Ce, Er) or (Er, Yb), independently of the measurement temperature (T = 83 and 300 K). The doping of crystalline ZnO films with rare-earth impurities (Ce, Yb, Er) leads to the efficient transfer of energy to REIs, a consequence of which is the intense emission of an Er3+ ion in the IR spectral region at λmax = 1535 nm. The kick-out diffusion mechanism is used upon the sequential introduction of impurities into semiconductor matrices and during the postgrowth annealing of the ZnO films under study. The crystalline ZnO films doped with Ce, Yb, Er also exhibit intense emission in the visible spectral region at room temperature, which makes them promising materials for optoelectronics.

Synthesis of GaN nano- and microwire crystals induced by a titanium nanolayer

The possibilities of a new method of growing gallium nitride nano- and microwire crystals using continuous titanium films with a thickness of 10–30 nm during growth are described. It is shown that this method can provide growth of high-quality GaN nanowire crystals at an extremely high rate of about 10 μm/min.

X-ray diffraction study of short-period AlN/GaN superlattices

The structure of short-period hexagonal GaN/AlN superlattices (SLs) has been investigated by X-ray diffraction. The samples have been grown by metalorganic vapor-phase epitaxy (MOVPE) in a horizontal reactor at a temperature of 1050°C on (0001)Al2O3 substrates using GaN and AlN buffer layers. The SL period changes from 2 to 6 nm, and the thickness of the structure varies in a range from 0.3 to 1 μm. The complex of X-ray diffraction techniques includes a measurement of θ-2θ rocking curves of symmetric Bragg reflection, the construction of intensity maps for asymmetric reflections, a measurement and analysis of peak broadenings in different diffraction geometries, a precise measurement of lattice parameters, and the determination of radii of curvature. The thickness and strain of separate SL layers are determined by measuring the θ-2θ rocking curves subsequent simulation. It is shown that most SL samples are completely relaxed as a whole. At the same time, relaxation is absent between sublayers, which is why strains in the AlN and GaN sublayers (on the order of 1.2 × 10−2) have different signs. An analysis of diffraction peak half-widths allows us to determine the densities of individual sets of dislocations and observe their change from buffer layers to SLs.

Specific features of Raman spectra of III–V nanowhiskers

Raman spectra of GaAs nanowhiskers that are grown on different substrates and differ from one another by the content of the sphalerite and wurtzite phases have been investigated. Special attention has been focused on the manifestation of structural features in the scattering spectra of nanowhiskers. It has been established that the nanowhiskers are characterized both by random inclusions of wurtzite layers in the sphalerite structure and by the continuous growth in the wurtzite phase. The interpretation of the scattering spectrum agrees with the concept of summation of the dispersion curves of the sphalerite structure upon transition to the wurtzite structure, which leads to a transformation of zone-boundary modes at the L point of the Brillouin zone into zone-center modes of the wurtzite structure and, as a consequence, to the appearance of a number of new fundamental modes of different symmetries. An analysis of the Raman spectra has revealed the formation of the hexagonal 4H polytype in narrow layers of nanowhiskers due to a random packing of hexagonal layers. The coexistence of the sphalerite and wurtzite phases in GaAs nanowhiskers completely correlates with the photoluminescence spectra measured for the same samples.

Growth of diamond microcrystals by the oriented attachment mechanism at high pressure and high temperature

For the first time it has been experimentally shown that a powder of detonation nanodiamonds (DND) and a saturated acyclic hydrocarbon, mono- or dibasic alcohol, used as the reaction mixture after treatment at high pressures (5–8 GPa) and high temperatures (1300–1800°C) results in the formation of diamond single crystals up to 15 micron in size. The Raman spectrum indicates that the diamonds have a perfect of crystal structure. It has been suggested that the oriented attachment mechanism is responsible for growth of micrometer-size diamond single crystals out of DND particles with sizes of about 5 nm under these technological conditions.

Photoluminescence Spectra of thin Zno films grown by ALD technology

The photoluminescence of ZnO films grown by atomic layer deposition (ALD) on silicon substrates has been investigated. A new broad photoluminescence band has been revealed in the exciton region of the spectrum. The properties of the band in the spectra of the films with different crystallographic orientations of substrates have been studied in a wide temperature range at different excitation levels. A model describing the origin of the new band has been proposed.

Computer Simulation of the Structure and Raman Spectra of GaAs Polytypes

The structure, energy of formation, and Raman spectra of several polytypes (3C, 2H, 4H, and 8H) of gallium arsenide GaAs have been investigated using quantum mechanical calculations based on the local density functional theory. It has been found that the energy of the formation of hexagonal polytypes increases with an increase in the length of the periodicity and approaches the value corresponding to the ground state, i.e., to the structure of the 3C polytype. It has been shown that the calculated frequencies of normal vibrations of different polytypes are consistent, with good accuracy (±6 cm−1), with the scheme of folding of the phonon branches. In the calculated Raman spectra of the polytypes, there are new lines (forbidden in the spectrum of the 3C polytype) which can serve as characteristic lines of other polytypes. Similar lines can be found in the Raman spectra of GaAs nanowhiskers. This result has opened up new prospects for the application of Raman spectroscopy to the characterization of the structure of these nano-objects.

On the laser detachment of n-GaN films from substrates, based on the strong absorption of IR light by free charge carriers in n+-GaN substrates

The physical and technological basics of the method used to lift off lightly and moderately doped n-GaN films from heavily doped n+-GaN substrates are considered. The detachment method is based on the free-charge-carrier absorption of IR laser light, which is substantially higher in n+-GaN films.