Table of contents

A brief review is given of electronic and transport properties of carbon nanotubes mainly from a theoretical point of view. The topics include a giant Aharonov-Bohm effect on the bandgap and a Landau-level formation in magnetic fields, absence of backward scattering except for scatterers with a potential range smaller than the lattice constant, a conductance quantization in the presence of short-range and strong scatterers such as lattice vacancies and transport across junctions between nanotubes with different diameters.

The phenomenon of light confinement in an isolated quantum dot, originating from the electromagnetic wave diffraction at the dot boundary, is discussed. It has been shown that at a certain condition the quantum dot behaves as a microcavity whose eigenmodes manifest themselves as additional, geometrical, resonances in the quantum dot electromagnetic response. The effect of induced magnetization of the quantum dot is predicted and illustrated by the example of magnetic resonances in spherical quantum dots. The radiative lifetime for a spherical quantum dot has been evaluated and a correlation has been discussed between radiative lifetimes in quantum dots and quantum wells.

We report measurements of photoluminescence, photoluminescence excitation spectroscopy and photoluminescence time decay on three MOVPE-grown InGaN/GaN multiple quantum well structures with 13% In in the wells and well widths L_z = 1.25, 2.5 and 5.0 nm. The PL spectra are dominated by single emission peaks, together with phonon sidebands spaced by a GaN LO phonon energy (92 meV). The peak energies are red-shifted with respect to energies calculated for exciton recombination in square quantum wells and the wide well sample also shows a significant Stokes shift between emission and absorption. Recombination lifetimes measured at 6 K are energy dependent, increasing as the photon energy is scanned downwards through the emission line. They also depend strongly on well width. On the low energy side of the 5 nm well emission line we measure lifetimes as long as 100 ns. Raising the temperature from 6 to 300 K results in a strong reduction of emission intensity for all samples and reduction of the lifetimes, though by a much smaller factor. The peak positions shift slightly to lower energy but by far less than the shift in the band edge. We consider three different theoretical models in an attempt to interpret this data, an exponential tail state model, a model of localization due to In/Ga segregation within the wells and the quantum confined Stark effect model. The QCSE model appears able to explain most of the data reasonably well, though there is evidence to suggest that, in addition, some degree of localization occurs.

Antiferromagnetic MnTe (sub)monolayers inserted into nonmagnetic CdTe/CdMgTe quantum wells are investigated by magneto-optical spectroscopy. In particular, the Zeeman splitting is determined from polarization dependent photoluminescence excitation experiments. Rapid thermal annealing (RTA) is applied to diffuse Mn out of the (sub)monolayer barriers into the quantum well. The resulting Mn diffusion leads to a substantial increase of the Zeeman splitting. Furthermore, the annealing induced broadening of the MnTe (sub)monolayer barrier in the well is accompanied by a decrease of the barrier height, leading to a blue shift or a red shift of the e₁-hh₁ transition, depending on the detailed sample structure. A comparison of the observed energy shifts with model calculations allows us to estimate the thickness of the MnTe barriers. These are found to be very close to their nominal values before annealing, regardless of the chosen MnTe band offset and effective masses.

Infrared photoluminescence (PL) has been studied in Ho-doped single-crystal silicon structures fabricated by solid-phase epitaxy and in crystalline holmium oxide in the temperature range from 4.2 to 300 K. PL arises from the internal 4f-shell transitions in Ho³⁺ ions. The maximum intensity of the PL associated with the transitions from the first excited state (⁵I₇) to the ground state (⁵I₈) in Ho-implanted silicon is observed at ~1.96 µm. Oxygen co-implantation enhances the intensity by an order of magnitude at 77 K. The luminescence intensity in Si:Ho:O structures increases by an order of magnitude upon 15 min annealing at 900 °C. The temperature quenching of the Ho-related PL is similar to that for Er-related PL in single-crystal Si: the intensity decreases by a factor of 25 on raising the temperature from 4.2 to 150 K, and above 130 K the activation energy is ~160 meV. Significant differences are observed between the PL spectra from Ho-doped silicon and holmium-oxide: (i) in addition to the PL lines related to the ⁵I₇→⁵I₈ transitions, there are lines due to transitions from the second excited state of Ho³⁺ ions (⁵I₆) to the ground state (⁵I₈) in Ho₂O₃; (ii)  for the ⁵I₇→⁵I₈ transitions the positions of the PL lines with a maximum intensity differ for Ho₂O₃ and Ho-doped silicon; and (iii) the PL intensity in Si:Ho:O is lower only by a factor of ~3, as compared with that in Ho₂O₃, despite the Ho concentration being ~4 orders of magnitude lower in the former. The second distinction is due to the crystal field splitting of the Ho³⁺ 4f-states.

The electronic structure of heterojunctions between II-VI semiconductors (CdS, CdTe) and layered transition metal dichalcogenide semiconductors (MoTe₂, WSe₂) were investigated by soft x-ray photoelectron spectroscopy. The interfaces were formed sequentially by molecular beam epitaxy (MBE) of the II-VI semiconductors on van der Waals (0001)-surfaces of layered compound single crystals. The II-VI semiconductors grow with their polar hexagonal axis ([111] for CdTe, [0001] for CdS) parallel to the non-polar substrate [0001]-axis, without evidence for lattice distortions as prooved by TEM and LEED. Band offsets were determined from soft x-ray photoelectron spectra. Large substrate independent interface dipoles of 1.2 eV for CdS and 0.8 eV for CdTe were determined. The deviation of experimental band offsets from the electron affinity rule prediction is described by a simple model which takes the polarity of the grown interfaces into account.

The generation-recombination processes in high-resistivity, undoped ZnSe single crystals as well as in ZnSe:Mg and ZnSe:Cu crystals were studied on the basis of an investigation of the photoconductivity kinetics on excitation with nanosecond laser pulses of an under-bandgap photon energy. The parameters of the recombination centres are estimated. It was established that the laser-stimulated increase in the steady-state photoconductivity of undoped ZnSe crystals with a low accidental impurity concentration was mainly due to formation of the slow recombination centres. The effect of accumulation of laser-induced point defects and its role in the observed photosensitization of highly pure ZnSe single crystals are discussed. The present results can be used in the design and analysis of various optoelectronic and optical devices using this compound.

Studies on magnetoresistance in the quantum Hall regime show that defects induced by proton irradiation strongly change the transport properties of an AlGaAs/GaAs two-dimensional electron gas. In particular, it is shown that the overshoot of the quantized value of the Hall resistance at odd filling factors can be introduced by these defects. A model, that explains the observation and assumes that the defects which have spins are essential for mediating the electron scattering in the two-dimensional electron gas, is presented.

Optical absorption spectra of GaSe and GaSe:Gd single crystals were investigated in the excitonic resonance energy region and just below. Free exciton (FE) transitions associated with the direct gap of GaSe and GaSe:Gd have been measured as a function of temperature in the range of 10-340 K. The parameters describing the temperature variation of both the spectral position and the broadening function of the excitonic resonance confirm the dominating role of the A^'(1)₁ homopolar phonon mode at 134.6 cm^-1. The Gaussian lineshape was used to fit the excitonic structures. The decreased absorption intensity and broadened lineshape of the excitonic resonances for GaSe:Gd crystals were attributed to the Gd dopant atoms. The exponentially increasing absorption tails were explained as Urbach-Martienssen (U-M) tails for both GaSe and GaSe:Gd samples in the 10-340 K temperature range. The characteristic tail width, Urbach's energy E_U, was obtained as a function of temperature. The temperature dependence of E_U was interpreted based on the general models on this rule. The Urbach energy increased as a function of temperature in the investigated temperature region for the Gd-doped sample. Such an increase of the Urbach energy can be explained as being due to the enhancement of electronic distortion caused by the structural disorder associated with the Gd atoms in the crystal lattice of GaSe.

Thin films of tetragonal lead monoxide have been prepared from spray pyrolytic decomposition of an aqueous solution of lead nitrate. The spray pyrolysis set-up developed for this study is also described briefly. From the structural investigations it has been found that the films generally grow with some admixture of monoclinic PbO_1.57 phase. The absorption coefficients of the films have been determined from transmittance measurements in the 400-1200 nm region. From an analysis of the room temperature optical absorption data, a direct transition band gap of 1.9 eV has been obtained for α-PbO.

Electroluminescence from SiO₂ has been studied both experimentally and theoretically. Numerical simulations have been carried out to investigate electroluminescence and impact ionization processes. A firm connection between electroluminescence and impact ionization has been shown.

The thermal conductivity and the thermoelectric power of two dimensional hole and electron gases in B and Sb delta-doped Si structures have been measured for the first time. The temperature range was 1.4 K to 30 K, and carrier sheet densities were in the range 0.3 to 1.0×10¹⁴ cm^-2. Kohn anomalies have been observed in all samples. The thermopower is dominated by the phonon drag contribution in each case. Calculations of the temperature dependence of the thermopower agree with experimental data when unscreened phonon scattering potentials are used for Si:B and Si:Sb.

We investigate the dependence of the threshold current of broad-area lasers on the stripe width. A comparison of differently deep-etched devices fabricated from the same wafer reveals that the stripe width dependence of the threshold current is caused by the current spreading effect. We propose a simple method to obtain a transparency current density characterizing a particular epitaxial structure without being influenced by current spreading.

A dependence of energy gap on composition x for Cd_1-xZn_xTe (x = 0-0.06) was found combining x-ray diffraction and low temperature photoluminescence measurements (free exciton energy) on Cd_1-xZn_xTe single crystals. The use of this dependence for evaluation of Zn content (x) in the crystals shows very good agreement with the near infrared absorption method. Published dependences E_g(x) are reviewed and compared with the dependence E_g(x,T = 4 K) = 1.606 + 0.520x + 0.254x² derived in this work.

The band offsets for strained Si_1-x-yGe_xC_y layers grown on Si(001) substrate and for strained Si_1-xGe_x layers grown on fully relaxed Si_1-zGe_z virtual substrates are estimated. The hydrostatic strain, the uniaxial strain and the intrinsic chemical effect of Ge and C are considered separately. Unknown material parameters relative to the latter effect are chosen to give the best agreement with the available experimental results for Si_1-xGe_x and Si_1-yC_y layers on Si. As a general trend concerning carrier confinement opportunities, it is found that a compressive strain is required to obtain a sizeable valence band offset, while a tensile strain is needed to obtain a conduction band discontinuity. In most cases the strain is responsible for a bandgap narrowing with respect to that of the substrate. The obtained results are in very good agreement with available experimental determinations of band offsets and bandgap changes for ternary alloys on Si(001).

A velocity-field study of several Si_0.8Ge_0.2/Si p-channel MOSFETs with self-aligned poly-Si gates, thick gate oxides and effective channel lengths ranging from 1.5 to 8.5 µm, was carried out at room temperature. Comprehensive two-dimensional simulations of devices using drift-diffusion (DD), and bulk Monte Carlo calibrated hydrodynamic (HD) and energy transport (ET) models have revealed enhanced high-field hole transport in strained-channel MOSFETs. A close agreement is obtained between higher-level (HD/ET) models and DD model with calibrated high-field mobility parameters. It is found that the relatively low value of extracted saturation velocity in long-channel Si_0.8Ge_0.2 p-MOSFETs increases considerably as the gate length is decreased. The increase in short-channel samples is attributed to non-equilibrium transport effects in the region near the source, resulting from higher mobility and longer relaxation times of holes in the strained SiGe layer. Our results not only confirm the expected advantage of strained SiGe p-MOSFETs in low-field transport, but also indicate that this is accompanied by an early onset of velocity overshoot, which may be beneficial in aggressively scaled devices.

A dramatic modification of the ionization energy of Si donors has been observed experimentally in a GaAs/AlGaAs double-well quantum-scale structure, caused by relocation of the electronic wave function from a δ-doped to an undoped quantum well in an external electric field. With the bias applied to the structure changing by less than 1 V, the impurity ionization energy falls from 15.5 meV down to zero. The observed anomalously high intensities of the photoluminescence transitions involving the first excited electron and heavy-hole subbands are explained by a distortion of the potential relief of the structure by the built-in field.

Ohmic contacts have been fabricated on Si-doped GaN using Ti/Al by low temperature annealing at 500 °C. A contact resistivity of 8.6×10^-6 ohm cm² was obtained for n-GaN samples doped to 3.67×10¹⁸ cm^-3. Secondary ion mass spectrometry (SIMS) analysis showed that Al diffused through the Ti layer after annealing. Furthermore, energy dispersive x-ray spectroscopy (EDS) analysis showed that reaction products of Al, Ti, Ga and N were present at the interface. Hence, a complex ternary or quaternary nitride as a possible low barrier height material to n-GaN may have resulted in good ohmic contact. Photoluminescence (PL) showed that there was no degradation in the epilayer quality of the film after annealing at 500 °C for 25 minutes.

We report on cyclotron resonance detected by far-infrared photoconductivity of the two-dimensional electron gas formed in undoped, top-gated GaAs/Al_0.3Ga_0.7As heterostructures. The photoconductivity method demonstrated here is readily extendable to quantum wires. The top-gated device architecture avoids the disorder inherent in conventional modulation-doped devices and allows precise in situ tuning of carrier density over two orders of magnitude. We observe very sharp resonances (6 mT at 1.5 K) indicating a very high mobility, which is attributed to the low level of impurities. The variation of the linewidth at small filling factor is also consistent with a low concentration of impurities. These results suggest that the filling-factor-dependent oscillations observed in linewidth are not due to the screening of ionized impurities. Filling-factor-dependent oscillations in photoconductivity intensity are also observed, with maxima occurring at even filling factors.

We investigate current filamentation in n-GaAs in the regime of low-temperature impurity breakdown for different sample and contact geometries. Computer simulations based on a dynamic microscopic model are compared with spatially resolved measurements in thin epitaxial layers. By varying the applied bias, load resistance and magnetic field, one can effectively control the shape and the size of the filaments in rectangular samples with two point contacts. Multistability and hysteresis due to the successive symmetry-breaking formation of multiple filaments are found in Corbino discs upon sweep-up and sweep-down of the voltage and explained by our model.

Annealing of InGaAs quantum dots (QDs) fabricated by metal-organic chemical vapour deposition and covered with a very thin GaAs cap layer completely eliminates large dislocated InGaAs clusters and remarkably improves the optical properties of the structures. A modal gain of ~4 cm^-1 is achieved in the 1.35 µm range. The elimination of defects allows the stacking of QDs emitting at 1.3 µm without deterioration of their optical and structural properties and reduces the QD density in the upper sheets.

We have developed a Monte Carlo simulator of the electromigration process in polycrystalline metal stripes. Stripes with different average grain size can be generated with Voronoi tesselation, and mapped onto a network of resistors. The proposed model includes the major role played by grain boundaries and by the current density redistribution within the stripe following void formation. Simulations of stripes with different grain sizes and different widths are shown, and a few expressions for the failure probability are compared on the basis of their capability of reproducing the experimental results. In addition, electromigration noise has been computed, recovering the characteristic 1/f^γ (γ≈2) behaviour. The substantial qualitative agreement between our calculations and the experimental results is a convincing test of the capability of the model proposed to include the relevant physics.

N-type hydrogenated nanocrystalline silicon (nc-Si:H) on p-type crystalline silicon heterojunctions were fabricated and electrically characterized. The nc-Si:H layer was deposited by plasma enhanced chemical vapour deposition. The C-V results confirm an abrupt heterojunction. The energy band diagram of the heterojunction was given on the basis of the C-V measurements. For the first time the electron affinity of nc-Si:H was estimated as 3.93 eV. J-V characteristics show good temperature stability and good rectifying properties. The temperature dependence of the J-V characteristics indicates that the forward conduction is determined by the recombination current on the nc-Si:H side of the depletion region, with an activation energy E_ac = 0.68 eV. The reverse conduction is dominated by currents generated in the depletion region with an activation energy E_ar = 0.65 eV, which is consistent with the activation energy for the recombination current. Combining this with the energy band diagram we quantitatively explain the reason for the good temperature stability of the heterojunctions.

Using Raman spectroscopy we have analysed the strain status of GaN films grown on sapphire substrates by NH₃ source molecular beam epitaxy (MBE). In addition to the expected compressive biaxial strain, in some cases GaN films grown on c-face sapphire substrates suffer from serious tensile biaxial strain. This anomalous behaviour has been well interpreted in terms of interstitial hydrogen-dependent lattice dilation. The hydrogen concentration in the films is measured by nuclear reaction analysis (NRA). With increasing hydrogen incorporation, the residual compressive biaxial strain is first further relaxed, and then turns into tensile strain when the hydrogen contaminant exceeds a critical concentration. The hydrogen incorporation during the growth process is found to be growth-rate dependent, and is supposed to be strain driven. We believe that the strain-induced interstitial incorporation is another way for strain relaxation during heteroepitaxy, besides the two currently well known mechanisms: formation of dislocations and growth front roughening.

We have already shown (1995 J. Appl. Phys.77 1531) that a high temperature annealing of Sb-doped GaAs layers, grown by liquid phase epitaxy (LPE), produces an electron trap with properties similar to the Sb-related double donor observed in the material grown by other techniques. In this work we have performed detailed temperature-dependent Hall and transient photocapitance data analysis of the trap in LPE-grown, annealed GaAs_1-xSb_x (x⩽0.02) layers to determine the first and second charge state energies of the trap. The results further confirm our earlier assignment of the trap to Sb_Ga defects which are produced during annealing.

Photosensitive structures have been prepared by heat treatment of p- and n-CuInSe₂ polycrystalline substrates at 500 °C in vacuum or air and subsequent deposition of CdS and In₂O₃ thin films on the CuInSe₂ surface. The photosensitivity spectra of the structures have been measured in natural and linearly polarized radiation. The photosensitivity is analysed in relation to the structure preparation conditions. Induced polarization photosensitivity was observed, and peculiarities of this phenomenon caused by the antireflection effect in thin films of the prepared structures are discussed. A conclusion is made that the polarization photoelectric spectroscopy can be used for diagnostics of photovoltaic solar cells.

This paper presents a 2D model and numerical simulation of a submicron resonant cavity enhanced metal-semiconductor-metal photodetector (RCE MSM-PD). Nonstationary effects caused by electron intervalley transfer are taken into account in calculating the RCE MSM-PD response. The analysis of the quantum efficiency and the 3 dB bandwidth shows that there is an optimum channel thickness, for which a maximum bandwidth-quantum efficiency product can be reached.

Properties of exactly compensated semiconductors have been studied under exciton-stimulated modulation of the charge on deep impurities. The electron and hole lifetimes are shown to depend on deep impurity concentration in a nonmonotonic manner. Their maximum values are determined by the exciton concentration and the probability of exciton modulation of trap charges. The results obtained are interpreted in view of the fact that, in a exactly compensated semiconductor, the equilibrium concentration of majority carriers decreases sharply, by several orders of magnitude. Simultaneously, the resistivity grows abruptly. The dependences of the Hall coefficient and Hall mobility on trap concentrations are also nonmonotonic. The exciton density and the probability of exciton modulation of the charge of deep traps are estimated for CdTe crystals at T = 300 K and shallow donor concentration of 10¹⁷ cm^-3. At high exciton densities, the rate of exciton-stimulated emission of trapped carriers becomes comparable with the rate of their capture, which decreases with increasing deep centre concentration. As a result, the lifetimes become higher. The exciton-stimulated modulation of deep trap occupancy through charge exchange dominates over that associated with energy exchange.

A newly designed high-barrier-gate Ga_0.51In_0.49P/In_xGa_1-xAs/GaAs pseudomorphic transistor with a step-compositioned channel (SC²) and bottomside delta-doped sheet (BD²S) structure has been fabricated successfully and studied. For a 1×100 µm² studied device, a high gate-to-drain breakdown voltage over 30 V is found. In addition, an available output current density up to 826 mA mm^-1 at a high gate voltage of 2.5 V, a maximum transconductance of 201 mS mm^-1 with a very broad transconductance operation regime of 3 V of gate bias (565 mA mm^-1 of drain current density) and a high dc gain ratio of 575 are obtained, simultaneously. Meanwhile, the maximum values of the unity current-gain cut-off frequency f_T and oscillation frequency f_max are 16 and 34 GHz, respectively.