Vol 52, No 16 (2018)

In this work, the DC and RF performance of a 20 nm gate length novel metal oxide semiconductor high electron mobility transistor (MOSHEMT) on Silicon substrate is studied using Sentaurus TCAD tool. The proposed MOSHEMT device features a novel T-gate structure, heavily doped In_0.52Ga_0.48As source/drain regions, delta doped planes on both sides of the In_0.75Ga_0.25As/InAs/In_0.75Ga_0.25As composite channel, a multi layer cap and a very thin layer of HfO₂ as gate dielectric. The TCAD simulation results obtained at room temperature using hydrodynamic carrier transport model indicates that the 20 nm gate length proposed MOSHEMT device is capable of providing a peak drain current of 2450 mA/mm at V_DS = 0.6 V and the peak transconductance obtained for the proposed device is 2900 mS/mm. The f_T and f_max obtained for the L_g = 20 nm proposed MOSHEMT on Silicon substrate are 372 and 480 GHz respectively. At 300 K, the measured Hall mobility of the electrons in the quantum well channel is 9600 cm²/(V s). The on state resistance (R_ON) obtained for the L_g = 20 nm proposed device is 116 Ohm μm. These devices are undoubtedly, the most suitable candidates for future sub millimetre wave applications.

The influence of electric field and current flow on the current–voltage (I–V) characteristics of TlGaSe₂ layered semiconductor was investigated by using a two-point probe measurement system. Threshold-type switching in I–V characteristics associated with current–controlled memory effect was observed in all investigated samples. Observed switching characteristic is close to the most experimentally realizable memristive systems. Experimental findings were analyzed by using a model of metal–insulator–semiconductor–insulator–metal (MISIM) structure having memristive behavior.

We experimentally investigate and analyze the electrical and optical characteristics of InGaAs/GaAs conventional quantum well laser diode and the quantum well laser diode with slightly-doped tunnel junction. It was found that the laser with slightly-doped tunnel junction has a nonlinear S-shape current-voltage characteristic. The internal quantum efficiencies of the laser with slightly-doped tunnel junction and the conventional laser are 21 and 87.3%, respectively. This suggests that the slightly-doped tunnel junction increased the barrier width and free carrier absorption, thus could reduce the electron tunneling probability and increase the internal loss. Furthermore, compared with the conventional laser, it was found that we could achieve 15 nm broadband spectrum from the laser with slightly-doped tunnel junction, due to the lasing dynamics reflecting the current dynamics. The results show that the slightly-doped tunnel junction plays a crucial role in the laser diode performances, which may lead to the realization of more applications.

Near-infrared InGaN alloys were grown on a strain-relaxed high-In-composition InGaN buffer layer based on GaN/sapphire substrate by plasma assisted molecular beam epitaxy. The In compositions of the InGaN light emitting layer and buffer layer determined by X-ray diffraction analysis are 56.7 and 61.1%, respectively. Transmission electron microscopy result shows that no dislocations newly generated at the interface of the InGaN structure layer and the underlying InGaN buffer layer can be observed obviously. The cathodoluminescence spectra exhibit two strong emission peaks at the near-infrared wavelengths of 1090 and 1200 nm, which can be attributed to intrinsic transition and recombination processes of the fabricated InGaN alloys according to the calculation using the energy versus composition equation with a reasonable bowing parameter.

Thin films of AlN:Tm are deposited on a Si(111) and Si(100) substrates and optical fiber by rf magnetron sputtering method. 200–400 nm thick films are deposited at various temperatures to observe the effect of temperature on their structure and morphology. Liquid nitrogen is used to deposit films at low temperature of –196°C. An electric heater is used to grow films at 250–700°C range. The deposited films are analyzed for their structure and thermal stability using X-ray diffraction and thermogravimetric analysis. Films deposited at liquid nitrogen temperature are amorphous and those deposited at high temperature are crystalline. Average grain size, strain and interatomic distance of the crystalline films are calculated. The grain size and inter-atomic distance increase from 36.54 to 45.12 nm and from 0.2368 to 0.25 nm respectively. Photoluminescence of the films excited by 783 nm crystal laser shows an intense infrared emission from Tm⁺³ at 802 nm as a result of ³H₄ → ³H₆ transition.

This work was devoted to the study of stable and unstable optical modes in segment shaped resonators. The size of the studied resonators was much larger than the wavelength of the emitted light, that allowed to use the approaches of geometric optics. It was observed, that with an increase in the δ/R deviation parameter (where δ is the deviation from the center of the half-disk) only a small number of stable spatial modes remain. The possibility of the existence of “sealed” (non-radiating modes) and “open” modes (radiating modes) in resonators of this type was also shown.

Anisotropic polymer films containing aligned Au nanorods and semiconductor nanoparticles of various shape were fabricated. The photoluminescence of semiconductor nanoparticles in these films is partially polarized. The value of the photoluminescence polarization degree of quantum dots embedded in an anisotropic PVA-film after 4.5-fold stretching P = 0.27 has been obtained.

An approach to solving the problem of the in situ bandgap determination in the extremely thin and chemically active nitride nanolayers fabricated in high vacuum on the n-GaAs surface has been suggested. The approach is based on measuring the interband transitions involving the quantum well states by the method of electron energy loss spectroscopy. The bandgap of the nitride layer formed on the GaAs surface by low-energy \({\text{N}}_{2}^{ + }\) ion implantation was determined to be 0.2 eV less than that of GaAs, which evidenced for creation of the GaAsN dilute alloy on the GaAs surface.

“Retroreflected light with strong linear polarization coinciding with that of the incident beams is detected from strongly absorbing nanoporous III–V semiconductors. Because of high polarization of retroreflected waves we assume that coherent backscattering is the underlying physical mechanism of this phenomenon”.

We present in this report the route to produce highly-textured Au₃Fe(111)/Fe(110) hybrid nanocrystals on an amorphous surface of SiO₂/Si by molecular beam epitaxy. By controlling the quantity of Au atoms deposited onto the SiO₂/Si it is possible to tune the average lateral size of resultant Au–Fe hybrid nanocrystals from 10–20 nm up to 100–150 nm at the same Fe nominal thickness deposited. This process is sensitive to the initial density and size of Au islands. Examination of Au–Fe hybrid nanocrystals obtained was carried out using X-ray diffraction, transmission and scanning electron microscopy, reflection high energy electron diffraction, and Kerr effect methods.

Metastable germanium monoxide (GeO) thin-insulating films have been investigated as a new promising material for atomic force microscope (AFM) oxidation lithography. The kinetics of GeO layer oxidation performed in the tapping mode of AFM was found to have a logarithmic relationship to oxide height versus pulse duration. Effect of humidity on oxidation of germanium monoxide thin films was studied at relative humidity 40, 60, 80%. When local anodic oxidation of GeO layer was carried out by AFM operating in the contact mode in high voltage (≥9 V) regime and at high relative humidity (RH = 80%), the size and shape of fabricated oxide was changed drastically.

In this paper we demonstrate the results on selective area growth of GaP nanowires via self-catalyzed growth method using molecular beam epitaxy (MBE) technique on patterned Si(111) substrates. The pattern fabrication method on a base of the photolithography process over an array of microspherical lenses has been studied theoretically and then optimized in order to obtain the nanostructures with controlled morphology. It was found that the positive resist thickness corresponding to the best achievable resolution in the subwavelength region is 250 nm in case of 1.5 μm silica spheres and excitation with 365 nm LED. The silica growth mask for selective epitaxy was fabricated. The ordered array of GaP nanowires was synthesized with MBE. Large scale ordering and selectivity of the growth technique is demonstrated.

InGaN/GaN nanorod structure is presented for the fabrication of light-emitting diodes by means of plasma-chemical etching. Processes of the etching of nanorods and the defect passivation in a planar InGaN/GaN quantum dot heterostructure are studied. The obtained results allow creating the light-emitting diode nanostructure with a greater efficiency compared to the classical planar technology.

New hybrid organic–inorganic complexes of perylene with nanodiamonds were developed and fabricated. The fact of complex formation was confirmed by the analysis of the infra-red (IR) absorption spectra and their spectral-luminescent properties. It was shown as a result of nanodiamond (ND) template synthesis that in new complexes based on NDs occur appearance of strong structured excitation bands in the ultra violet (UV) region of spectrum, within 230–300 nm. The formation of new UV excitation centers in complexes leads to a significant enhancement of luminescence in the green region of the spectrum.

It has been found that “hot” high-fluence Zn⁺ implantation leads to the formation of the extended layer with zinc-based nanoclusters of size up to 10 nm in SiO₂ film. The precipitate’s crystalline nature is confirmed by HRTEM studies. Post-implantation annealing at 700°C for 60 min leads to Zn atoms redistribution in implanted region, formation of the larger crystallites (up to 25 nm) and the small Zn nanoparticles surrounded by the oxide shells (core (Zn)/shell(ZnO) structure) as well as the voids formation at the interface “Zn nanocluster/SiO₂ matrix”. The number of the core (Zn)/shell (ZnO) structures and the voids increases after annealing at 700°C for 120 min.

Two different approaches for the etching of the gallium nitride epitaxial layers grown on Si(111) substrates by plasma-assisted molecular beam epitaxy have been elaborated. It is demonstrated that anisotropic etch profiles can be achieved by both photoenhanced wet chemical etching and reactive plasma-chemical etching methods. Moreover, it is shown, that the photoenhanced wet chemical etching allows to remove GaN layer without damaging silicon substrate.

The growth of ternary nanowires via the vapor-liquid-solid mechanism is one of the possible ways of band gap engineering. However, the dependence of ternary nanowire composition on growth parameters is still unclear. We compare two theoretical approaches to ternary nanowire composition determined either by the formation of the critical island or by the step-flow growth of the monolayer. For typical parameters of gold-catalyzed growth, these two models give similar results. In the case of self-catalyzed growth, the step-determined model predicts higher In content in the NW than the island-determined.

Within this work we model the spatial dependences of protein concentration, nucleation probability and the radius of lysozyme crystal in the experiments on capillary counter-diffusion crystallization. Within the model approach, we evaluate the conditions for the formation of lysozyme crystals with the target separation of 100 µm between centers of protein crystals. The predicted and observed number density, sizes and positions of protein crystals in capillary are in excellent agreement.

Silicon nanocrystals (SiNCs) with photoluminescence in the visible spectral range were synthesized by laser irradiation of silicon wafers in solutions. Laser irradiation leads to the formation of well-dispersed Si–NCs with an average diameter of about 5 nm, which upon excitation with 300−450 nm light exhibited room temperature luminescence tunable in the range from 410 to 650 nm. The results showed that photoluminescence (PL) is affected by both sizes and surface states of the Si–NCs. The results obtained allow to conclude that the synthesized nanoparticles (NPs) can be suitable materials for applications in fluorescence imaging along with photovoltaics.

A simple aqueous based synthesis technique at room temperature was performed for preparation of pure and manganese (Mn²⁺) doped ZnS quantum dots in the presence of glutathione as the capping agent. Under UV-excitation quantum dots show photoluminescence bands at 2.13 and 3.0 eV corresponded to Mn²⁺ and intrinsic ZnS emission respectively. ZnS quantum dots were used as the photocatalyst to remove organic dyes in aqueous solution, with methylene blue as a model dye. The influence of the Mn²⁺ concentration on the dye degradation efficiency was studied. The highest photocatalytic degradation rate of methylene blue was obtained for ZnS quantum dots in glutathione shell doped with 0.125 at % of Mn²⁺.

Plasmonic enhancement of the photoluminescence in hybrid structures with SiGe quantum dots and Ag nanoislands was found. Ag nanoislands grown on the top of the multilayer structures with SiGe quantum dots (QDs) support a surface plasmon resonance that can be tuned to the QD emission wavelength by changing of Ag nanoparticle parameters. Photoluminescence measurements of the hybrid metal-semiconductor nanostructures revealed a fourfold increase of the integral intensity of SiGe QD emission in the spectral range from 0.8 to 1 eV.