International Journal of Nanoscience

The resistivity (ρ) of low mobility dilute 2D electron gas in an n-InGaAs/GaAs double quantum well (DQW) exhibits the monotonic "insulating-like" temperature dependence (dρ/dT < 0) at T = 1.8–70 K in zero magnetic field. This temperature interval corresponds to a ballistic regime (k_BT_τ/ħ > 0.1–3.5) for our samples, and the electron density is on an "insulating" side of the so-called B = 0 2D metal–insulator transition. We show that the observed features of localization and Landau quantization in a vicinity of the low magnetic-field-induced insulator–quantum Hall liquid transition is due to the σ_xy(T) anomalous T-dependence.

One-dimensional periodic structures with deep vertical grooves were obtained by wet anisotropic etching of (110)-oriented silicon-on-insulator (SOI) structure. Grooves of various widths (4–16 μm) were filled with a nematic liquid crystal (LC) E7. Inter-digital configuration of silicon electrodes was used to apply a voltage which altered the refractive index of the liquid crystal. Reproducible Freedericksz effect with a threshold voltage from 2 to 3 V was registered visually in the polarized light of a microscope and studied by means of micro-Raman spectroscopy. The electro-optical effect in the structures can be used to tune the band gaps of 1D photonic crystals, the spectral response of microcavities and to steer light beams in high contrast planar waveguides and waveguide arrays.

The model of weak localization in 2D semiconductor structures in the whole range of classically weak magnetic fields in the presence of the Elliot–Yafet spin relaxation has been developed. It was shown that the spin–orbit interaction influences the value of magnetoresistance in small magnetic fields (within diffusion approximation) and when diffusion approximation is no longer valid.

The method of photoyield spectroscopy with s- and p-polarized light excitation is developed for studies of 2D collective modes. The formation of nanoclusters has been studied for the Cs/GaAs(100)Ga-rich interface in a wide coverage range up to ~ 2 ML. Two adsorption phases with the strong Cs–Ga bonding and weak Cs–Cs bonding are found by using both stepwise and dynamic Cs deposition regimes. In the first adsorption phase, local interaction of Cs adatoms with Ga dimers occurs. The transition from the first to the second phase is found to take place at ~ 0.7 ML that corresponds to the saturation of all the Ga-dangling bonds. In the second phase, collective modes in quasi-2D Cs clusters of several sizes depending on the coverage and surface plasmon are revealed.

A two-dimensional photonic crystal with hexagonal lattice of air-holes is patterned into an active planar waveguide containing InAs/InGaAs quantum dots. Variable-angle reflectivity spectroscopy is used to map out the photonic band structure. Fano-type resonances observed in the measured reflectivity spectra in TE (TM) polarizations along the Γ–K (Γ–M) lattice direction are attributed to resonance coupling of the optically active photonic bands to external light. Angle-resolved photoluminescence measurements are shown to trace the band structure of the leaky mode. The revealed three-fold emission intensity enhancement of photonic crystals is ascribed to both Purcell and Bragg scattering effects.

Intersubband quantum-box (IQB) lasers, that is, devices consisting of 2D arrays of single-stage intersubband QB emitters, are proposed, as an alternative to 30-stage quantum-cascade (QC) devices, as sources for efficient room-temperature (RT) emission in the mid- and far-IR (3–5 and 8–12 μm) wavelength ranges. Preliminary results include: (1) the realization of the first mid-IR (λ = 4.7 μm) single-stage emitters operating at RT; (2) etch-and-regrowth at the nanoscale level by employing in situ gas etching and MOCVD regrowth; (3) the formation of 30 nm-diameter SiO₂ posts on 80 nm centers, thus forming the mask for the fabrication of IQBs via in situ etch and regrowth.

The tunneling and the lateral conductance of 2DEG formed in the GaAs with the δ-doped layer were measured simultaneously at hydrostatic pressures at helium temperatures. The resistivity of the δ-doped layer sharply increases by > 3 orders of magnitude at about 2 GPa, and its temperature coefficient changes from ~ - 0.04 to ~ - 4 K^-1. The tunneling resistance shows only slight change of exponential behavior at about 1.5 GPa, while a drastic change of the Zero Bias Anomaly (ZBA) takes place just at the metal–insulator transition. Such a behavior is interpreted in terms of pressure dependence of the GaAs band structure and the DX-level position.

Ultrasoft X-ray emission spectra (USXES) and X-ray absorption near-edge structure (XANES) spectra with the use of synchrotron radiation in the range of P L_2,3-edges were obtained for the first time for nanostructures with InP quantum dots grown on GaAs 〈100〉 substrates by vapor-phase epitaxy from metal–organic compounds. These spectra represent local partial density of states in the valence and conduction bands. The additional XANES peak is detected; its intensity depends on the number of monolayers forming quantum dots. Assumptions are made on the band-to-band origin of luminescence spectra in the studied nanostructures.

A possibility of the formation of light emitting devices, containing GaAs:Mn layers, by the MOCVD epitaxy process was demonstrated. It was shown that produced GaAs:Mn layers exhibit ferromagnetic properties at room temperature. Luminescent and electrical properties of the In(Ga)As/GaAs quantum-size heterostructures with incorporated GaAs:Mn layers were studied.

The growth mechanisms of GaAs nanowhiskers (NWs) during molecular beam epitaxy (MBE) are studied theoretically and experimentally. A kinetic model of the diffusion-induced NW growth is presented that allows one to predict the dependence of NW length on the drop radius and on the technologically controlled MBE growth conditions. The results of scanning electron microscopy studies of GaAs NWs grown at different conditions on the GaAs(111)B surface activated by Au are presented and analyzed. It is shown that the length of NWs increases with decreasing the drop radius and with decreasing the deposition rate of GaAs, while its temperature dependence has a certain maximum. The aspect ratio of MBE-grown GaAs NWs is higher than 100. The maximum length of NWs is several times larger than the effective thickness of the deposited GaAs. The obtained results demonstrate that the NW growth is controlled by the adatom diffusion toward their tip rather than by the adsorption-induced vapor–liquid–solid mechanism. The growth conditions' influence on the NW morphology may be used for the controlled fabrication of NWs by MBE for different applications.

The scattering of linearly polarized monochromatic light by a small protrusion on a metal surface is analyzed within the framework of perturbation theory. It is shown that even at normal incidence of light slight deviations of the protrusion shape from a circularly symmetric one can lead to formation of optical vortexes in the near-field region. The effect is due to resonant excitation of cylindrical surface plasmon waves. This agrees with the fact that the in-plane near-field intensity distribution experimentally observed by scanning near-field optical microscopy has distinct spiral patterns near metallized nanostructured polymer substrates.

Adatom incorporation into the "faceted" steps bordering the 2D nanoislands is analyzed. The step permeability and incorporation coefficients are derived for some typical growth situations. It is shown that the step consisting of equivalent straight segments can be permeable even in the case of fast edge migration if there exist factors delaying creation of new kinks. The step consisting of alternating rough and straight segments may be permeable if there is no adatom transport between neighboring segments through the corner diffusion.

Optical phenomena in quantum dot and quantum well nanostructures aimed at the development of mid-infrared lasers based on intraband electron transitions are investigated in the conditions of interband optical pumping. Evolution of photoluminescence spectra and interband light absorption with intensity of interband optical excitation is investigated in InAs/GaAs quantum dot structures. Optically induced intersubband mid-infrared light absorption is studied in undoped tunnel-coupled GaAs/AlGaAs quantum wells. The stabilization of the electron concentration at the ground level of tunnel-coupled quantum wells under increasing pumping level is established.

In this work we performed measurements of photoluminescence (PL), scanning tunneling microscopy (STM), and Rutherford backscattering (RBS) at grazing angles of incidence in a set of samples grown by molecular beam epitaxy, in which a Ge layer was deposited on a Si(001) substrate covered with a thin SiO₂ layer. Three different thicknesses for either layer were deposited: 0.5, 0.75 or 1 monolayer (ML) of SiO₂, and 0.3, 0.6 or 0.9 nm of Ge. The PL measurements for the samples with thicker layers show a broad band at ~ 0.85 eV superimposed on a dislocation related band at ~ 0.81 eV. The attribution of the high energy band to Ge islands in this sample is supported by STM and RBS measurements, as well as by PL measurements after hydrogen passivation of the sample surface. For the samples with thinner SiO₂ and Ge layers, no evidence for the formation of Ge islands was found.

The influence of hydrostatic pressure in the range 0–12 kbar on exciton photoluminescence spectrum of InAs quantum dot molecules was studied. The molecular terms related luminescence with the increase of excitation density was obtained. For all components baric coefficients were found. Anomalies in baric dependences for p₊- and d₊- excited molecule states are discussed.

The impurity breakdown was simulated in numerical calculations. The distribution function for an electron in the electric field was calculated using the Monte-Carlo method. The electron concentration in the impurity ground state and in the first subband was determined by solving the rate equations. It was found out that a population inversion between the 1s-level and the bottom of the first subband is likely to arise. The requirements for the population inversion to occur were determined.

Effect of external magnetic field on the optical orientation of electron spins in negatively charged InP quantum dots is studied experimentally using the PL pump–probe method.

Effect of the magnetic field on the rate of phase breaking is studied. It is shown that the magnetic field resulting in the decrease of phase relaxation rate makes the negative magnetoresistance due to suppression of the electron interference to be smoother in shape and lower in magnitude than that found with constant -value. Nevertheless our analysis shows that experimental magnetoconductance curves can be well fitted by the Hikami–Larkin–Nagaoka expression.¹ The fitting procedure gives the value of τ/τ_ϕ, where τ is the quasi-momentum relaxation time, which is close to the value of τ/τ_ϕ(B = 0) with an accuracy of 25% or better when the temperature varies within the range from 0.4 to 10 K. The value of the prefactor α found from this procedure lies within the interval 0.9–1.2.

We report on a theoretical study of the fine structure of excited excitonic levels in semiconductor quantum disks. A particular attention is paid to the effect of electron–hole long-range exchange interaction. We demonstrate that even in the axisymmetric quantum disks, the exciton P-shell is split into three sublevels. The analytical results are obtained in the limiting cases of strong and weak confinement. A possibility of exciton spin relaxation due to the resonant LO-phonon-assisted coupling between the P and S shells is discussed.

Dilute nitride GaInNAs and GaInNAsSb alloys grown on GaAs have quickly become excellent candidates for a variety of lower cost 1.2–1.6 μm lasers, optical amplifiers, and high power Raman pump lasers that will be required in the networks to provide high speed communications to the desktop. Because these quantum well active regions can be grown on GaAs, the distributed mirror technology for vertical cavity surface emitting lasers coupling into waveguides and fibers and photonic crystal structures can be readily combined with GaInNAsSb active regions to produce a variety of advanced photonic devices that will be crucial for advanced photonic integrated circuits. GaInNAs(Sb) provides several new challenges compared to earlier III–V alloys because of the limited solubility of N, phase segregation, nonradiative defects caused by the low growth temperature, and ion damage from the N plasma source. This paper describes progress in overcoming some of the material challenges and progress in realizing record setting edge emitting lasers, the first VCSELs operating at 1.5 μm based on GaInNAsSb and integrated photonic crystal and nanoaperture lasers.

Kinetics of polarized photoluminescence of the negatively charged InP quantum dots in weak magnetic field is studied experimentally. Effect of both the nuclear spin fluctuations and the dynamical nuclear polarization on the electron spin orientation is observed.

Transient characteristics of SiGe-QW laser structures were studied. The excitation of stimulated THz emission is shown to be the result of carrier injection through contacts. The mechanism of intra-center population inversion caused by carrier injection is suggested.

The optical properties of InAs/(Al)GaAs quantum dots (QDs) overgrowth by thin AlAs/InAlAs layers are studied as a function of temperature from 10 to 500 K. The QDs emit at 1.27 μm at room temperature. It is shown that the QD energetic spectrum can be tuned by overgrowth of AlAs/InAlAs to provide high temperature stability of the QDs optical properties. Transport of carriers between neighboring QDs is absent, and the carrier distribution remains nonthermal up to room temperature. It is shown that suppression of the thermal escaping of the carriers from QDs is conditioned by high energy separation between ground and excited states, absence of wetting layer level, and increase of carrier localization energy in QDs in case of the Al_0.3Ga_0.7As matrix.

The kinetics of spin orientated interwell excitons gas has been investigated in GaAs/AlGaAs coupled quantum wells (n–i–n heterostructures). In these structures, the electrons and the holes are spatially separated between neighboring quantum wells by a narrow AlAs barrier. The time evolution and kinetics of photoluminescence spectra of interwell excitons have been measured under the pulse resonance excitation of intrawell 1sHH excitons using a pulsed tunable laser. The temporal formation of collective excitons phase and the temperature dependence of its spin relaxation rate have been studied. It was observed that spin relaxation rate of interwell excitons in the collective phase is strongly reduced. The observed effect is an evidence of the coherence of indirect excitons' collective phase at the temperatures below the critical one.

The growth and characterization of multi-layer 1.3 μm InAs-GaAs quantum dot lasers is reported. It is demonstrated that the growth of the GaAs spacer layer, placed between the InAs quantum dot layers, must be carefully optimized to prevent defect formation. With optimized growth very low room temperature threshold current density (J_th) devices are obtained. Typical cw values are 32.5 and 17 A cm^-2 for as-grown and HR coated facet devices, respectively. Operation above 100°C is possible. Mechanisms contributing to the temperature sensitivity of J_th above room temperature are discussed. By combining the optimized growth with p-type modulation doping of the QDs both low J_th and high temperature stability of J_th are achieved at room temperature.

Experimental data are presented on variations of the in-plane lattice constant of Ge and Si films in the course of the MBE film growth on the silicon (100) surface. The in-plane lattice constant of the silicon film is shown to alter as the film grows; the changes reflect the process of relaxation of elastic strains that result from the misfit of the germanium and silicon lattice constants. Due to the presence of germanium islands, a considerably thicker silicon film is required to provide the strain relaxation. The dependence of distortion penetration depth to the silicon film on the effective germanium film thickness is obtained. TEM studies indicate the vertical ordering of the germanium island layers when the thickness of the Si layer in between Ge layers is not sufficient to provide the full strain relaxation.

The Rashba effect peculiarities in gated accumulation layers on the zero-gap HgCdTe are studied theoretically and experimentally. It is shown that the kinetic binding is strongly affected by spin–orbit interaction. Although the spin–orbit splitting is smaller in accumulation layers as compared with inversion ones, the "Rashba polarization", Δn/n, can achieve 100% in the kinetic confinement regime.