International Journal of Nanoscience

It is experimentally shown that, unlike evaporation drying, supercritical CO₂ drying makes it possible to obtain free-standing single-coil InGaAs/GaAs nanotubes and other nanoshells with wall thicknesses down to 1 nm without their deformation by capillary forces. The performed process optimization has allowed us to reduce tenfold the duration of the process for long nanotubes. For the first time, etching of sacrificial AlAs layers in supercritical media was performed.

Using a kinetic, Monte Carlo model influence of Schwoebel barriers on morphology of growing films was investigated. With the increase of the deposited dose, transition from 2D to 3D growth mode within Stranski–Krastanow (SK) region without changing interlayer exchange and the growth parameters, was observed. The influence of interlayer atomic exchange on 2D island density, average and maximum island sizes in different atomic layers of thin film were in question. The enlargement of island sizes with the increase of the atomic layer number was demonstrated. This effect is much more pronounced in heterosystems, where atomic hops in the upper layer dominate over hops to the lower layer. The mechanism for transition from two-dimensional to three-dimensional growth mode in one growth process was suggested.

Scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED) experiments were performed to study growth modes induced by hyperthermal Ge⁺ ion action during molecular beam epitaxy (MBE) of Ge on Si(100). The continuous and pulsed ion beams were used. These studies have shown that ion-beam bombardment during heteroepitaxy leads to decrease in critical film thickness for transition from two-dimensional (2D) to three-dimensional (3D) growth modes, enhancement of 3D island density and narrowing of island size distribution, as compared with conventional MBE experiments. The crystal perfection of Ge/Si structures with Ge islands embedded in Si was analyzed by Rutherford backscattering/channeling technique (RBS) and transmission electron microscopy (TEM). The results of Kinetic Monte Carlo (KMC) simulation have shown that two mechanisms of ion beam action can be responsible for stimulation of 2D–3D transition. They are: (1) surface defect generation by ion impacts and (2) enhancement of surface diffusion.

The structure of monoatomic steps on (001)-vicinal surface of Kossel crystal is investigated theoretically. We consider all step directions except for relatively small vicinities of <10>-directions. The structure of such steps at sufficiently low temperature is shown to be controlled only by interaction between second nearest neighbors. If this interaction is repulsive, steps have saw-like shape, while attractive interaction leads to straightening of steps. More general case of nonpairwise interactions is also considered.

Ternary diamond-like compounds in II–IV–V₂ semiconductor system heavily-doped with transition d-element Mn have been recently prepared. The materials grown in both forms — single crystal layers and polycrystalline bulks — exhibit well defined ferromagnetic hysteresis with a saturation behavior in the magnetization curve up to above room temperature. Curie temperatures are of T_C=310 K to 320 K for (Cd_1-xMn_x)GeP₂ and (Zn_1-xMn_x)GeP₂ compounds. The chemical states in the bulk of ZnGeP₂:Mn and interface of Mn-doped ferromagnetic layer on ZnGeP₂ (001) crystal, have been clarified by electron paramagnetic resonance and in situ photoemission spectroscopy. The as-prepared surface consists of Ge-rich, metallic Mn-compound. In and below the sub-surface region, dilute Mn²⁺ species as precursors of the DMS phase exist. Mn²⁺ ions are paramagnetic active on Zn²⁺ sites in the bulk and show five EPR sets of equidistant peaks. Theoretical band-gap calculation suggests a predominant antiferromagnetic order in stoichiometric (Cd, Mn)GeP₂ but the systems with vacancies as (Cd, V_C, Mn)GeP₂ or (Cd, Ge, Mn)GeP₂ are ferromagnetic and energetically stable. These materials are of great promise for room-temperature spintronics applications.

Magnetoresistance of a planar single crystalline Ni nanostructures has been measured at room and liquid helium temperatures. Bending GMR effect increases from 4% under diffusive to more than 200% under ballistic electron transport in cross-type nanostructures. Magnetoresistance measured for longitudinal and transverse magnetic fields are qualitatively different and explained by a nonsymmetrical magnetic domain structure in the central part of the cross.

The possibility of periodical mask fabrication with elements of minimal size by means of interference photolithography is considered. The method is based on application of striped interference picture of coherent light for photoresist exposition. The investigation of inorganic photoresist based on chalcogenide glass As₂S₃ reveals high resolution, sufficient contrast and vertical edge of the masks stripe. The influence of exposure dose and development parameters on photoresist profile are analyzed with the help of the method of computer simulation of the development process. The possibility of decreasing the effect of standing light wave by means of additional homogeneous light irradiation is demonstrated. The optimal conditions for minimum size elements fabrication are found, which enables fabrication of periodical grating with a 80-nm-wide line and two-dimensional periodical structures with pillars of 50 nm cross-section.

The methods are presented for fabrication of new nanoscale-organized planar inorganic nanostructures, ultrathin polymeric and nanocomposite films on solid substrates with incorporated nanosized functional and structural building blocks. The methods are based on interfacial synthesis and self-assembly, DNA templating and scaffolding. Ultimately thin monomolecular and multilayer ordered stable polymeric and nanocomposite films containing incorporated ligand-stabilized gold nanoclusters, interfacially in-film grown metallic (Au, Pd) nanoparticles and organized low-dimensional nanostructures were formed. N-alkylated derivatives of poly(4-vinilpyridine) were synthesized and used as water-insoluble amphiphilic polycations to form organized polymeric Langmuir monolayers and novel planar DNA/amphiphilic polycation complexes at the air–aqueous DNA solution interface. The extended net-like and quasi-circular toroidal condensed conformations of deposited planar DNA/amphiphilic polycation complexes were obtained in dependence on the amphiphilic polycation monolayer state during the DNA binding. Planar DNA/amphiphilic polycation complexes were used as nanotemplates for fabrication of organized planar bio-organic–inorganic hybrid nanostructures with ordered nanophase inorganic components (quasi-one-dimensional arrays of semiconductor (CdS) and iron oxide nanoparticles and nanorods) organized in planar matrix of deposited DNA/amphiphilic polycation complex film. The formed nanostructures were characterized by atomic force microscopy and transmission electron microscopy techniques.

Atomic force and reflection electron microscopes have been applied to investigate tip-induced local anodic oxidation of the titanium and silicon films. The number of parameters such as applied voltage, oxidation time, relative humidity and host material was under consideration. The special treatments were applied to reduce the roughness of the studied surfaces achieving the average roughness of the surface less than one angstrom. Some examples of nanostructures fabrication are demonstrated.

Photonic crystals are the very promising novel materials for micro- and nanophotonics for visible region. To produce photonic crystals for this region of light, artificial structures with characteristic sizes less than 1 μm have to be manufactured. Electrochemical deep anodic etching and plasma etching techniques is normally used to produce such structures in silicon wafers. However, standard way of deep anodic etching realization is not suitable for sub-micrometer porous silicon formation. In the present work combination of the deep anodic etching and focused ion beam techniques is used to produce ordered structure of macropores in silicon.

Nanometer-scale movements of domain walls in uniaxial garnet films have been studied by means of micromagnetization measurements using miniature gold and semiconductor Hall probes. At helium temperatures the domain walls are found to move by discrete jumps, which we attribute to pinning on isolated defects, and we were able to measure local hysteresis loops associated with pinning on individual pinning centers. The temperature dependence of the coercive field of a single pinning center allowed us to evaluate the characteristic energy and characteristic volume of the pinning center.

The local electronic structure of N atoms in Ga(In)AsN diluted alloys (N concentration of 3%) is determined by soft-X-ray emission and absorption spectroscopies as element specific probes. The experimental spectra reflect the local 2p orbital-projected density-of-states of N impurities, the main recombination centers in Ga(In)AsN, which appears to deviate dramatically from crystalline GaN in both valence and conduction bands. In particular, we observe a N local charge transfer from the valence band maximum to deeper valence states, which fundamentally limits the optical efficiency of Ga(In)AsN, unless different N local environments are formed. The incorporation of In in large concentrations forms In-rich N local environments such as In₄N, which become the main recombination centers in Ga(In)AsN due to a local decrease of the band gap. A k-conserving process of resonant inelastic X-ray scattering is discovered, which allows probing of the k-character of valence and conduction states despite the random alloy nature of Ga(In)AsN.

The experimental and calculated results of the investigation of electromagnetic field distribution including its polarization characteristics in the vicinity of the nanostructures are presented. The experimental investigation was realized by aperture type scanning near field optical microscopes (SNOMs) which operated in collection mode. Normal resolution which allows us to image down to 0.3 nm height surface steps was demonstrated for the shear force probe to surface gap control system of the SNOM. Theoretical computation of the electromagnetic field distribution was realized by finite-difference time-domain (FDTD) method. The experimental three-dimensional maps of intensity and polarization distribution as a result of light diffraction at nanoaperture in the metal screen, dielectric and metallized nanocylinders were obtained. The qualitative difference between the orthogonal polarized component distributions near nanoaperture was experimentally shown. The electromagnetic field concentration in the proximity of the dielectric nanocylinders was observed. This observation gives a good fit with the results of FDTD computations. A spiral type electromagnetic field distribution pattern was experimentally observed in the proximity of metallized nanocylinders, which is unexpected from both experimental and theoretical points of view.

We investigate the electronic and magnetic properties of ferromagnetic GaMnAs single layers as well as multilayer structures fabricated by low-temperature molecular beam epitaxy. The electrochemical capacitance–voltage profiling as well as Raman spectroscopy measurements in GaMnAs single layers reveal a vertical gradient in the carrier concentration and therefore a gradient in Curie temperature measured by superconducting quantum interference device. Detailed studies of carrier density and ferromagnetic transition before and after annealing of the layers are presented. We also report on an enhancement of the Curie temperature in GaMnAs/InGaMnAs superlattices, which is due to thin InGaMnAs or InGaAs films embedded into the GaMnAs layers. The pronounced increase of the Curie temperature is strongly correlated to the In concentration in the embedded layers. Curie temperatures up to 110 K are observed in such structures compared to 60 K in GaMnAs single layers grown under the same conditions. A further increase in T_C up to 130 K can be achieved using post-growth annealing at temperatures near the growth temperature.

We report on mesoscopic Hall sensors allowing accurate magnetization studies of submicron or nanometer-sized samples over a wide temperature range. Even at 300 K, the probes can reliably resolve local changes in dc field of ≈ 1 G with spatial resolution of ≈ 1 μm, which corresponds to a flux sensitivity of less than 0.1 ϕ₀ (ϕ₀=h/e is a flux quantum). The resolution increases 100 times at temperatures below 80 K. The capabilities of new micromagnetometers are demonstrated by measuring nm-scale movements of individual domain walls in a ferromagnet.

The electronic transport through bent single-wall carbon nanotubes is studied. Using a four-orbital per atom tight-binding model, the relaxed configurations of a (10, 0) semiconducting nanotube at different bending angles are first obtained. The optimized structures are then used in calculating conductance and current–voltage characteristics of the systems. These results are used to establish a correspondence between the mechanical deformation and transport properties, with potential applications in, e.g., nanoswitches. The source of the switching behavior is explained in terms of the localized states within the bent region.

The molecular nanoclusters proved to be very promising objects for applications in electronics not only because they have absolutely identical chemical structure and allow for bottom to top approach in constructing new electronic devices, but also for the possibility to design and create great variety of such clusters with specific properties. The formation and deposition of mixed Langmuir monolayers composed of inert amphiphile molecular matrix and guest ligand-stabilized metal-core nanoclusters are described. This approach allowed to obtain the ordered stable reproducible planar monolayer and multilayer nanocluster nanostructures on solid substrates. The use of novel polymeric Langmuir monolayers formed by amphiphilic polyelectrolytes and nanoclusters resulted in fabrication of ultimately thin monomolecular nanoscale-ordered stable planar polymeric nanocomposite films. The morphology and electron transport in fabricated nanostructures were studied experimentally using AFM and STM. The effects of single electron tunneling at room temperature through molecular cluster object containing finite number of localized states were theoretically investigated taking into account electron–electron Coulomb interaction. It is shown that tunnel current-bias voltage characteristic of such tunnel junction is characterized by a number of staircase steps equal to the number of cluster's eigenlevels, however the fronts of each steps are asymptotically linear with finite inclination. The analytically obtained current–voltage characteristics are in agreement with experimental results for electron tunneling through molecular nanoclusters at room temperatures.

Fe–Pt alloy thin and ultra thin films were deposited by the pulse laser evaporation method. Micronano Hall devices were fabricated from ultra thin films by an Ar plasma etching process using electron beam lithography procedures. The extraordinary Hall effect (EHE) in Fe–Pt alloy thin and ultra thin films and micronano Hall devices was investigated. The film thickness reduction is found to cause an increase in Hall voltage, which means an enhancement of field sensitivity of the Hall sensor. Maximum sensitivity S=260 Ω/T was measured in ultra thin film with thickness d=20 Å.

Peculiarities of the fabrication of field effect transistor (FET) at nanoscaled size on ultrathin silicon-on-insulator (SOI) was studied in details. Two types of FET transistor were successfully realized: in-plane-gate FET (IPGFET) with 40 nm minimum channel size and multichannel top-gate MOSFET on silicon-on-insulator. The deep submicron top-gate of Ti/Au embraces each of the conductive oxidized silicon wires placed with 400 nm pitch. The type and concentration of carries in a conductive channel of the ultrathin SOI was controlled by a bottom gate. The fabricated transistors demonstrated high transconductance and low threshold voltage. Some results of electron properties of the nano-FET transistors are presented.

Drift–diffusion model is applied for transport in a one-dimensional field effect transistor. A unified description is given for a semiconductor nanowire and a single wall nanotube basing on a self-consistent electrostatic calculations. General analytic expressions are found for basic device characteristic which differ from those for bulk transistors. We explain the difference in terms of weaker screening and specific charge density distribution in quasi-one-dimensional channel. The device characteristics are shown to be sensitive to the geometry of leads and are analyzed separately for bulk, planar and wire contacts.

Terahertz (THz) electroluminescence was produced by three different types of sources: intersubband transitions in silicon germanium quantum wells, resonant state transitions in boron-doped strained silicon germanium layers, and hydrogenic transitions from dopant atoms in silicon. The devices were grown by molecular beam epitaxy, fabricated by dry etching, and characterized by infrared spectroscopy. The absorption of THz was observed in silicon germanium quantum wells at energies corresponding to heavy hole and light hole intersubband transitions. These results suggest that SiGe nanotechnology is attractive for THz device applications.

We examine the static and high-frequency differential conductivity of semiconductor superlattices having various miniband dispersion laws. Our analysis shows that the anharmonicity of Bloch oscillations leads to the occurrence of negative high-frequency differential conductivity at frequency multiples of the Bloch frequency. This effect can arise even in regions of positive static differential conductivity. To achieve terahertz field amplification, we propose to employ structures having miniband dispersion law with decreasing effective electron mass as energy increases.

We have investigated light-current and spectral characteristics of 2-, 5- and 10-stack InAs/InGaAs/GaAs quantum dot (QD) ridge-waveguide lasers grown by MBE. Ultra-low threshold current of 1.43 mA was achieved for 2-stack QD laser. Simultaneous lasing at ground- and excited-states was observed. This effect is accounted for the finite time of carriers capture to the ground-state in QDs. Multi-stack QD structures enables to maintain continuous-wave (CW) ground-state lasing up to the current density of 100×J_th and to achieve the highest output power and efficiency ever recorded for any single-mode lasers of 1.3-μm-wavelength range.

The 17- and 25-period Ga_0.5In_0.5P/(Al_0.7Ga_0.3)_0.5In_0.5P quantum well structures were grown by metalorganic vapor phase epitaxy on a GaAs substrate misoriented by 10° from (001) to (111)A. A microcavity with dielectric oxide mirrors was fabricated on the basis of each structure. Lasing at 619 nm (632 nm) with 0.7 W (6 W) output power was achieved under scanning electron beam longitudinal pumping at room temperature using the 17-period (25-period) structure. The threshold current density at a 40 keV electron energy was 8 A/cm². It is shown that low threshold and high power lasing requires the position of the QWs to coincide with the antinodes of the cavity mode, at the maximum of the gain spectrum due to the QW ground state.

The specific features of the behavior of electron states in GaAs/AlGaAs double quantum well structure with strongly asymmetric barriers in an external electric field have been studied both theoretically and experimentally. This structure is the principal part of the active element of a quantum unipolar laser proposed by us earlier, since it contains the lower laser subband possessing the property of 2D–3D transformation. The drastic variations in the photoluminescence peak intensity, FWHM, and area with field demonstrate the possibility of controlling the dimensionality of this subband.

The first electron-beam pumped VCSEL was fabricated on CdSSe/CdS MQW structure grown by metalorganic vapor phase epitaxy on CdS substrate misoriented by 12–16° from (0001) towards (1010). Substrate misorientation results in more perfect surface morphology and intense cathodoluminesence. Lasing in the 535–590 nm spectral range was achieved at room temperature with different VCSELs. The laser power maximum of one VCSEL was as high as 2 W at RT, with electron energy 50 keV and current 1.7 mA. The threshold current was lower than that for the bulk CdSSe laser although the heterostructure used in the VCSEL has the type-II band alignment.