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Indium-based micro-bump arrays, among other things, are used for the bonding of infrared photodetectors and focal plane arrays. In this paper, several aspects of the fabrication technology of micrometer-sized indium bumps with a smooth surface morphology were investigated. The thermal evaporation of indium has been optimized to achieve ~8 μm-thick layers with a small surface roughness of Ra = 11 nm, indicating a high packing density of atoms. This ensures bump uniformity across the sample, as well as prevents oxidation inside the In columns prior to the reflow. A series of experiments to optimize indium bump fabrication technology, including a shear test of single columns, is described. A reliable, repeatable, simple, and quick approach was developed with the pre-etching of indium columns in a 10% HCl solution preceded by annealing at 120 °C in N2.

A comprehensive investigation of the interfacial misfit (IMF) array formation has been carried out. The studies were based on the static phase diagram for GaAs (001) surface and As2 dimers on the surface. Prior to the initiation of the GaSb growth two attempts of the temperature decreasing were performed: before and after the GaAs termination. The GaAs was grown in the optimal conditions for GaSb material. The influence of the interruption time on GaSb/GaAs heterostructure parameters was examined. Two cases were investigated: with and without Sb-soaking of the GaAs surface. The periodic array of edge dislocations at GaSb/GaAs interface was confirmed using Burger’s circuit theory. Careful examination of misfit surroundings revealed one uncompleted Burger’s vector that indicated one dislocation of mixed type among eight of the edge type. The distance between lattice sites of dislocations was 5.51 nm on average. The crystal quality of 5.0 µm GaSb layer was characterized by FWHM2θ/ω = 42 arcsec, FWHMRC = 125 arcsec. The EPD = 4 × 106 cm− 2 was estimated after etching in FeCl3:HCl solution. The Δqz/Δqx ratio of 0.60 for 5.0 µm GaSb layer was higher than for 2.5 µm GaSb layer of 0.59. The probable reason was the thickness-dependent 60° dislocation density. The electrical parameters measured for 2.5 µm GaSb were: p = 4.0 × 1016cm−3 (2.0 × 1016cm−3) and µ = 599 cm2/V s (3420 cm2/V s) at 300 K (77 K).

We have developed a tunable semiconductor double-chirped mirror with high negative dispersion grown by molecular beam epitaxy. The simplified numerical plane-wave reflection transfer method was applied to design the dispersive mirror structure. The multilayer stack was grown of AlAs/GaAs materials and capped by an SiNx antireflective layer. The group delay dispersion as well as reflectivity characteristic were continuously tunable across the mirror surface from 1080 to 1000 nm. Within this range, the negative value of the dispersion changed from -3850±100 fs2 over the 6 nm band around 1080 nm to -2200±100 fs2 over 13 nm around 1000 nm with the reflectivity from the range of 99.0÷99.2%. The mirror performance was tested in a diode-pumped femtosecond Yb:KYW oscillator.

We report on the development of semiconductor double-chirped mirrors with the group delay dispersion of −3,800 ± 100 fs2 in the wavelength range between 1,058 ÷ 1,064 nm and reflectivity of 99.1 %. The simplified plane-wave reflection transfer method was used to design the mirror multilayer stack. The mirror contains an epitaxial AlAs/GaAs structure topped with a SiNx antireflective layer.

Genetic algorithm was applied for optimization of dispersion properties in semiconductor Bragg reflectors for applications in femtosecond lasers. Broadband, large negative group-delay dispersion was achieved in the optimized design: The group-delay dispersion (GDD) as large as $$-3500\,\hbox {fs}^{2}$$ was theoretically obtained over a 10-nm bandwidth. The designed structure was manufactured and tested, providing GDD $$-3320\,\hbox {fs}^{2}$$ over a 7-nm bandwidth. The mirror performance was verified in semiconductor structures grown with molecular beam epitaxy. The mirror was tested in a passively mode-locked Yb:KYW laser.

We have studied the influence of the beryllium doping on strain in the II-type InAs/GaSb superlattices (SLs) by means of high-resolution X-ray diffraction (HRXRD). Three analyzed superlattices were grown by molecular beam epitaxy. One of the examined superlattices was undoped. Two others structures, called doped SLs, composed of two superlattices: Be-doped and undoped which were grown one on the top of each other. The doping concentration was determined by secondary ion mass spectroscopy. The doping level was 1×1017 cm−3 and 2×1019 cm−3. For doped superlattices, the HRXRD measurements showed splitting of satellite (ST) peaks. Furthermore, the separation of ST peaks increase with doping level. In contrast, for undoped superlattice, the splitting of the ST peaks was not observed. Sometimes the separation of ST peaks can be caused by change of thickness period of superlattice or partial relaxation of the structure. However, we claim that in our experiment the splitting is caused by another mechanism: The presence of Be atoms in SL causes the change of average lattice constant of the superlattice. The influence of Be dopant on lattice parameter of superlattice was confirmed by theoretical simulations. Furthermore, the change of the lattice constant (Δa/a) of the GaSb:Be buffer was examined. The reduction of lattice parameter of GaSb was noticed. It was caused by the presence of Be doping and unintentionally incorporated As-atoms in the GaSb layer. It is very important to know that even very small Be concentration (1×1017 cm−3) causes the change of average lattice parameter of SL.

We investigate selected problems of the molecular beam epitaxy (MBE) technology of fabrication of semiconductor saturable absorbers, which are the main parts of broadband saturable absorber mirrors. These mirrors are designed for mode locking of solid-state lasers emitting in 808 nm band. We analyzed growth conditions during the MBE processes of fabrication of test structures containing the central part of the device with a quantum well (QW). Two main parameters were changed in the processes: the growth temperature of InGaAs/GaAs QW and the arsenic to metals flux ratio. Four types of growth conditions were applied during the test processes. Grown structures were tested by optical methods: photoreflectance and pump–probe spectroscopy (pump–probe measurements and saturation measurements). Obtained results enabled us to establish a correspondence between applied growth conditions and main optical parameters of the absorbers.

Partially relaxed InP/InAlAs/InGaAs heterostructures, grown by molecular beam epitaxy under tensile and compressive misfit stress, have been investigated by means of high-resolution X-ray diffractometry and spectroscopic ellipsometry. Additionally, chemical etching was used to determine polarity of the crystals and threading dislocation densities in the epitaxial layers. The measurements revealed a distinct anisotropy of the strain relaxation along two orthogonal 〈110〉 directions in the (001) plane of the structures, which resulted from an asymmetry in the formation of two types of misfit dislocations at the interface. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction Lattice-mismatched heterostructures of indium-based III–V compound semiconductors are of current interest owing to their applications in optoelectronic devices emitting in the infrared wavelength range. An important consequence of epitaxial growth of those heterostructures is a strain in the epitaxial layer that results from a difference in lattice parameters between the substrate and the layer. If the epitaxial layer is thin enough, the lattice misfit can be accommodated by elastic strain of the layer and the growth of the layer is pseudomorpfic. However, when the layer thickness exceeds a critical value the strain is relieved by the formation of misfit dislocations at the interface. The misfit dislocations are associated with threading dislocations which cross the epitaxial layer up to the surface. Both the strain and the dislocations in the epitaxial layers can often affect dramatically their optical and electronic properties. In semiconductor heterostructures with zinc-blende structure, epitaxially grown on (001) oriented substrates with a small lattice mismatch (less than about 1.5%), the dominant type of misfit dislocations are 60° dislocations [1]. Two types of 60° dislocations, referred to as α and β, lying along two orthogonal 〈110〉 crystallographic directions at the interface differ in their core structures. In the predominated glide set configuration the α and β dislocations consist of the extra half-planes terminated in a row of group V and group III atoms, respectively, in the dislocation core. At the interface of heterostructures grown under tensile stress the α and β misfit dislocations are oriented along the [110] and ] 10 1

At low temperatures, ionized impurities are the dominant scattering mechanism limiting carrier mobilities in most semiconductors. Calculations of ionized impurity scattering have usually been based on the assumption that the ions scatter independently of one another. The purpose of this paper is to investigate the method of mobility calculation in which the carrier spectrum is found by solving the neutrality equation numerically. A development of the method, which allows calculation of the temperature dependent mobility spectrum of all carriers used in the neutrality equation for the GaAs/GaAs sample is presented. In order to explain the transport properties the following topics are discussed: • Theoretical calculation of the n-type GaAs transport parameters. • Comparison between measured and calculated values of the conductivity and Hall concentration, resistivity, conductivity and Hall mobility. • The values of conductivity mobility for each carrier transport component. • Application of the method for characterization of n-GaAs on SI GaAs MBE epi-layers. The new method allows for determination of the carrier mobility spectrum and is applied to GaAs/SI-GaAs layers in which the Hall voltage is magnetic field dependent.

We discuss several problems of optimisation of the MBE growth of thick (3.5 ÷ 9 μm) InAs epilayers on GaAs(100) substrates. Three types of layers were grown: undoped, Si-doped, and Be-doped. The whole growth process has been divided into two parts: the initial stage and the main stage. For each stage, the optimum growth conditions comprising the substrate temperature, the growth rate, and the flux ratio have been found. The crystal quality and the transport properties of the layers have been analysed by the transmission electron microscopy, the double-crystal X-ray diffraction, and the Hall effect measurements. It has been demonstrated that epilayers grown under the optimum conditions have good structural and electrical properties. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The paper is a study of the microstructure of as grown semiconducting and semi-insulating GaAs crystals, as well as of fine structural changes induced in both materials by the implantation with a 5×1014 cm−2 dose of 3 MeV/amu nitrogen ions. X-ray measurements were carried out by means of a high-resolution diffractometer. The rocking curves of two as-grown GaAs samples studied are symmetrical. The rocking curve measured for the implanted semi-insulating crystal is broadened, while that for the implanted semiconducting crystal reveals a diffused subsidiary peak on the low angle side of the main maximum, indicating the presence of tensile stresses in the damaged layer at a depth of the mean ion range. The mathematical analysis of the reciprocal space maps gives a quantitative description of the experimental results.

Two micrometer thick In0.53Al0.47As and In0.50Al0.50As layers covered by 10 nm thick In0.53Ga0.47As cap layers were grown on the (0 0 1) oriented InP substrates by molecular beam epitaxy. The structures were investigated using high-resolution X-ray diffractometry, spectroscopic ellipsometry and atomic force microscopy (AFM). Strain release in the In0.53Al0.47As layer was almost symmetrical. Contrary to this the lateral strain in the In0.50Al0.50As layer was found to be different along the [1 1 0] and [ 1 1 0] crystallographic directions. The anisotropy of surface morphology for the both kinds of samples was observed in AFM image. The surface undulation was accompanied with the microcracks formation in the cap layer.

The magnetic field dependence of conductivity tensor components, magnetoresistance, and the Hall coefficient have been analyzed in an n-type Si-doped GaAs epilayer at temperatures from 11 to 295 K. Carriers from the conduction band and the impurity band take part in the electrical conduction. The conduction band is located in the epilayer and the impurity band is located in a narrow layer, less than 0.1 μm thick, between the GaAs buffer and GaAs semi-insulating substrate. At temperatures below 20 K the localization and magnetic freeze-out of the conduction band electrons have been taken into account as quantum corrections to the electrical conduction. The dependence of the mobility on energy has been considered in the analysis of the experimental data. A wide peak of partial conductions versus mobility appears in the mobility spectrum. From the analysis of the mobility spectrum of conduction band electrons it follows that at low temperatures the mobility of non-degenerated conduction band electrons is limited by scattering on screened charge centers. The mobility spectrum technique has been used as a tool for interpolation and extrapolation of the experimental data beyond the experimentally investigated magnetic field range.

At low temperatures In 0.53 Ga 0.47 As samples show an increase of carrier concentration, which can be explained in terms of a two carriers transport model. This type of problem exists since the beginning of the semiconductor era, dating back to monocrystalline germanium. We propose that in all the investigated layers, there are X atoms or charged dislocations in the region of the first monolayers, which are built in during epitaxial growth. The layers were intentionally undoped. They form an impurity band in which low mobility carriers dominate over the localised electron scattering due to the s–d exchange interaction. These carriers do not freeze out at liquid helium temperature and give rise to two transport media for electrons; a conduction band at higher temperatures and an impurity band at lower temperatures. The electron which fall down onto the previously ionised X atoms, then move by thermally activated hopping. We show that the two carriers model for In 0.53 Ga 0.47 As epitaxial layers are confirmed by the carrier concentration–temperature, carrier concentration–magnetic field, resistivity–magnetic field behaviour, and also by YKA theory also. The differences between the two transport models are so distinctive that observed phenomena may exist. This paper presents experimental results, which constitute comprehensive evidence for the complicated structure of the semiconductor epitaxial layers on the sample of n -type In 0.53 Ga 0.47 As/InP layer with n =2.2×10 15 /cm 3 .

The design of the vertical cavity surface emitting lasers (VCSELs) needs proper tuning of many different optical parameters of those structures. So, the optimisation of the VCSELs requires deep understanding of optical processes occurring in the active regions of such lasers. In a series of MBE processes, active regions of VCSELs as well as the whole VCSELs were grown. The active regions of the VCSEL structures were designed for λ=1000 nm and 980 nm emission. They consisted of a pair of distributed Bragg reflectors (DBRs) composed of AlAs and GaAs quarter wavelength layers and a cavity made of GaAs. The cavities contained one or several quantum wells (QWs) made of In 0.2 Ga 0.8 As. To optimise the optical characteristics of the active regions, several experimental methods have been applied. The Bragg reflectors and the whole microcavities were investigated by optical reflectivity. For selective excitation of a QW in a cavity active layer, a Ti–sapphire tuneable laser has been used. The fine tuning between the QW emission and the cavity Fabry–Perot resonance has been investigated by photoluminescence at varying temperatures of the sample. For monitoring the temporal evolution of the luminescence from the active region of the laser, time-resolved spectroscopy has been employed. The combination of many methods of optical investigations enabled a comprehensive characterisation and as a result an optimisation of the whole laser structure.

The paper presents high-resolution X-ray diffraction studies performed for Si single crystal: as-grown, implanted with a 5 x 10 1 4 ions cm - 2 dose of 3 MeV/n Ar ions, as well as implanted and annealed in a very high vacuum. The results are discussed on the basis of rocking curves and the mathematical analysis of the reciprocal space maps. It is shown that the lattice parameter is increased in an implanted part of the crystal, but long distance lattice curvature is not present. After annealing full relaxation of the crystal is stated.

The effect of treatment at up to 1270 K under hydrostatic argon pressure, up to 1.2 GPa, on strain relaxation of AlGaAs layers was investigated by X-ray diffraction and related methods. The 1.5 μm thick AlGaAs layers weregrown by molecular beam epitaxy method on 001 oriented semi-insulating GaAs substrate at 950 K. An increase in intensity of X-ray diffuse scattering, originating from hydrostatic pressure-induced misfit dislocations, was observed for all treated samples. For the samples treated at 920 K during 1 h under 0.6 GPa, the diffuse scattering was confined to the [110] crystallographic direction perpendicular to the direction of dislocations. For the samples treated at 1.2 GPa at the same temperature and time conditions as for 0.6G Pa, a different behaviour is observed, namely the diffuse scattering extends along all azimuthal directions, indicating that dislocations are created in both [110] and [110] directions. The change of strain after the treatment was most pronounced for the samples treated at 1.2 GPa for 1 h at 920 K.

Injection of spin polarized carriers from a diluted magnetic semiconductor (ZnMnTe) to non-magnetic semiconductor (GaInAs quantum well) has been demonstrated by observing 22% degree of circular polarization in magneto photoluminescence measurements. The degree of the circular polarization reflecting the degree of carriers spin alignment, shows that spin polarized carriers are able to diffuse under optical excitation from Zn0.97Mn0.03Te spin aligner to Ga0.8In0.2As QW via a 200A of GaAs spacer conserving their polarization despite a high (∼1010cm−2) density of dislocations present at the interface.

Molecular beam epitaxy has been used to grow epitaxial InAs onto (0 0 1) oriented SI–GaAs, at different substrate temperatures T s =(440–506 °C) and different V/III flux ratios. No influence of growth rate on the microstructure of the layers has been detected. Transport properties of the layers have been investigated by Hall effect measurements under a magnetic field of 0.6 T, at temperatures from 3.5 to 300 K. InAs samples exhibited a characteristic maximum of Hall coefficient (minimum of Hall concentration) at temperatures of about 60 K, which is not typical for semiconductors. We propose a model for interpreting this behaviour, which is extended to other nominally undoped samples.

The optimized nonuniform growth process was used to achieve spatially dependent reflectivity and dispersions characteristics in a highly dispersive semiconductor mirror. The mirror, together with a semiconductor saturable absorber mirror (SESAM), was used to demonstrate a tunable femtosecond Yb:KYW oscillator. In the passive modelocking regime the laser could be continuously tuned over 3.5 nm spectral band around 1032 nm with high resolution, maintaining the average output power above 140 mW.

The intra-pulse measurements of the beam steering in an AlGaAs/GaAs quantum cascade laser are presented in this paper. The experimental results are explained by a two-mode theoretical model. The near field of the laser radiation is calculated according to the effective index method and transposed to the far field numerically according to Huygens principle. The maximal observed value of deflection of the beam has been found to be \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$17^{\circ }$$\end{document}17∘. For supply currents in the range from 1.6 to 2.4 times the threshold the beam steering occurs only on one side of the resonator axis, and stays is the same for all current values. For higher supply current, it occurs alternately on both sides and exhibits a bistability. The time period of the beam direction change has been found to be about 40 ns for the lowest current and was decreasing with the current increase to about 20 ns.

The influence of the implantation with fast nitrogen ions on GaAs single crystal structure is studied by several methods. The energy of nitrogen ions was 2.85 MeV/n and the dose was 5×10 14 cm −2 . The scanning electron microscopy experiments have revealed that a damaged buried layer was created at a depth of 20 μm. The mathematical analysis of reciprocal space maps has shown that the N 2+ ion shot-through layers are only slightly deformed, whereas larger deformations, consisting in the existence of small areas of larger lattice parameter and local bending, are induced in the implanted part of the crystal.

We observed an effective injection of spin-polarized carriers from II–VI Zn 0.97 Mn 0.03 Te diluted magnetic semiconductor spin aligning layer into III–V-based, GaAs-based quantum well structure. By using circular polarized excitation and detection, we demonstrate that the injection of spin-polarized carriers indeed proceeds through the II–VI/III–V interface in spite of a huge lattice mismatch (∼7.8%) which is decorated by a great number of dislocations. This indicates that spins are quite robust and maintain their polarization memory even after passing through a dense array of misfit dislocations.

The aim of this paper is to present some problems in the materials engineering connected with miniaturization of geometric dimensions of electronic structures. The examples are taken from InAs epitaxial layers. Our results as well as various results from the literature on crystallographic and electronic properties of InAs epitaxial layers deposited on insulating GaAs 〈001〉 wafers by MBE method are described. A novel two-layer model derived from the one of Petritz is proposed. It allows us to calculate crystalline (FWHM) and electronic (concentration, mobility) properties of any one undoped InAs layer with any one thickness. A comparative analysis of our results and those from literature was performed. The electrochemical proof of the proposed model was presented.

We discuss some problems of molecular beam technology (MBE) technology of planar microcavities based on GaAs, AlGaAs and InGaAs compounds. This technology needs specific methods of in situ control and postgrowth characterization. One of the crucial problems is that of controlling the substrate temperature and the growth rate with very high accuracy. We demonstrate the usefulness of substrate temperature oscillations observed by infrared pyrometry for both the temperature and the growth rate control. For studying the perfection of layers and interfaces the cross-sectional transmission electron microscopy has been used. To optimize the optical characteristics of the microcavities, several experimental methods have been applied. The Bragg reflectors were investigated by optical reflectivity. For selective excitation of a quantum well (QW) in a cavity active layer, the Ti-sapphire tuneable laser have been used. The fine tuning between the QW emission and the cavity Fabry–Perot resonance has been investigated by photoluminescence for varying temperature of the sample.

Thin, undoped heterostructures are very interesting from the point of view of their transport properties. In such samples the contribution of the conduction band or valence band carriers to the conductivity can be small and effects associated with the interface can become dominant. In our previous paper we showed that in a thin, undoped In0.53Ga0.47As/ΙnΡ heterostructure nearly all the electrical conduction takes place in the impurity band at the interface [1]. The electrical conduction in the impurity band was about 95% of the total electrical conduction. It was suggested there that the impurity band originated from unintentionally introduced impurities and interfacial dislocations. The effect of dislocations on the electrical conduction in GaAs-based compounds has been widely investigated [2-12]. However, skew scattering has not been taken into account. It is well known that at the interface there is a high density of dislocations and we can expect a significant influence of dislocation on the electrical conduction in the layer in which the interface conduction is dominant. In the analysis of the electrical conduction, dislocations have been considered as

The performance of In x Ga 1~x As detectors operating in the 2}3.4 lm spectral range and temperature of 300 K has been analyzed theoretically as a function of wavelength, band gap and doping level with special emphasis on 2}2.5 lm and 3}3.5 lm atmospheric window devices. The calculations show that the dominant generation}recombination mechanism in p-type, intrinsic and in a lightly doped n-type InGaAs is the spin split-o! band Auger process (AS). Since the AS generation increases with the square of the hole concentration, the minimum thermal generation and the best performance can be obtained using moderately doped n-type material as the absorber region of a photovoltaic device. In principle, the ultimate performance can be achieved in the optimized homojunction devices with relatively thick n-type absorber region forming n}p junction with a thin p-type material. N-type doping of absorber region of InGaAs photodiodes at 300 K changes from 1]1014 to 5.2]1015 cm~3 for devices optimized for operation at 2 and 3.4 lm wavelength, respectively. ( 2000 Elsevier Science B.V. All rights reserved.

The electrical conduction in an n-type In0.53Ga0.47As/InP sample grown by molecular beam epitaxy has been analyzed in the magnetic field up to 1.5 T, at temperatures from 15 to 295 K. The electrical conduction has been ascribed to the impurity band (IB), located in the interface between the epilayer InGaAs and the substrate InP. The contribution of the conduction band electrons in bulk InGaAs layer to the electrical conduction was negligible. The IB conduction was almost metallic. We observed within the IB two conducting channels which give positive and negative contribution to the Hall voltage. The magnetoresistance in the IB at low temperatures was negative, and at high temperatures was positive. The negative magnetoresistance in the IB has been phenomenologicaly described by carriers with the constant concentration and the mobility increasing proportionally to the square of the applied magnetic field.

(a) Institute of Physics, Polish Academy of Sciences, Al. Lotniko´w 32/46,PL-02-668 Warsaw, Poland(b) Institute of Experimental Physics, Warsaw University, Ul. Hoz˙a 69,PL-00-681 Warsaw, Poland(c) Institute of Electron Technology, Al. Lotniko´w 32/46, PL-02-668 Warsaw, Poland(Received October 1, 1998)An analysis of extended defects generated during annealing by pulsed excimer laser radiation in sili-con crystals implanted with Ge ions is presented. The investigation was performed by means of twocomplementary methods: the interference-polarizing microscopy and the Lang X-ray transmissiontopography. The existence of extended defects was revealed. It has been stated that the distributionof these defects depends on the distribution of the power density in the laser beam cross-section.

A detailed study of low-temperature magnetoconductance in between $\ensuremath{\nu}=2$ and $\ensuremath{\nu}=3$ quantized Hall plateaus is presented. The data are obtained for disordered two-terminal submicron wires and rings defined in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterostructures modulation doped by Si. In the region between the plateaus, electron density of compressible liquid in the central current strip increases gradually when lowering the magnetic field. At low densities of the electrons in the strip, a sequence of peaks is detected in the dependence of conductance on the magnetic field. This unexpected effect is interpreted in terms of the Coulomb blockade associated with the charging of the current strip. In the same range of the magnetic fields, a temperature dependence of conductance, together with the absence of the Aharonov-Bohm oscillations for transport along the central strip in the ring, are taken as the evidence for an enhancement of the electron-phonon scattering rate at the percolation threshold. The frequency doubling of the Aharonov-Bohm oscillations in a ring with an additional potential barrier is discussed in terms of the phase concept. Slow time evolution of conductance is observed for higher concentrations of the electrons in the current strip. This surprising noise is attributed to glassy dynamics of localized electrons in the wire center, and to the corresponding time dependence of the impurity-assisted tunneling probability between the current carrying regions.

A detailed study of low-temperature magnetoconductance in between quantized Hall plateaux is presented. The data are obtained for disordered two-terminal submicron wires defined in GaAs/AlGaAs heterostructures modulation-doped by Si. Slow time evolution of conductance G is observed on the high-field side of the quantized plateaux – for filling factors ν

Al0.3 Ga0.7 As layers were grown by molecular beam epitaxy using substrate temperature 200-300°C, tetrameric As and two values of As/Ga+Al flux ratio i.e. 3 or 8. The post-growth annealing was performed in situ at 600°C for 20 min under As-overpressure. The samples were characterised by reflection high-energy electron diffraction, transmission electron microscope and room-temperature I— V measurements of n+/LT grown layer /n+ resistors. The resistivity and trap-filled limited voltage have been determined. The best layers exhibited p of the order of 109 Ω cm, were monocrystalline, uniformly precipitated and without dislocations.

Mechanism of the optical detection of cyclotron resonance via emission from 2D electron gas in modulation doped quantum wells and in high electron mobility structures of GaAs/AIGaAs is discussed based on the results of time-resolved optical detection of cyclotron resonance. An important role of impact ionization processes is demonstrated. We also show that microwave radiation destroys emission enhancement at the Fermi level and the relevant mechanism is proposed.

We have studied an enhancement of the oscillator strength for optical transitions near the Fermi energy in p-type modulation-doped quantum wells, which, so far, deserved much less attention than analogous n-type systems, because of the complicated valence band structure involved. The relatively wide (L = 150 A) quantum wells and high doping levels were used, containing more than one occupied subband. The enhancement in the photoluminescence intensity at the Fermi energy resulted from the strong correlation and multiple scattering of holes near the Fermi edge by the localized electrons.

The nature of a small “precursor” of resonance which appears in front of the main resonance peak in the current–voltage characteristic of resonant-tunnelling devices based on GaAs/AlGaAs double-barrier heterostructures is studied. The competition between the precursor and main-peak current is examined within the temperature range 2–400 K and under a magnetic field up to 13 T. The precursor is interpreted as the contribution to resonant tunnelling of ballistic electrons injected from a three-dimensional electron gas in the emitter region. Instead, the main resonance peak is due to the electron injection from a two-dimensional electron gas in a triangular well formed under bias in the emitter spacer layer.

We present the results of optical, steady-state and time-resolved studies of photoluminescence and photoluminescence excitation in high-quality Al 0.3 Ga 0.7 As/GaAs quantum wells in which the presence of large (larger than the exciton radius) atomically flat islands can be inferred, identical to the case of interrupted MBE growth. Migration of excitons towards lower-lying energy states induced by local potential fluctuations and/or progressive localisation has been revealed and the transition rate between quantum well regions 24 to 25 monolayers thick has been derived to be 290 ps −1 .

A new algorithm of the analysis of the electrical conduction in the layers forming a semiconductor structure has been described. In the algorithm, in contrast to earlier approaches, the mobility spectrum technique has been applied to the sheet conductivity tensor. The standard carriers parameters have been estimated from the tensor components in the high and low magnetic field limits. The proposed method has been illustrated by the analysis of results of the electrical conduction measurements versus magnetic field performed up to 1.6 T at 77 K in a molecular beam epitaxial (MBE) grown GaAs:Si structure. The sample was an MBE GaAs:Si layer grown on a buffer epilayer. As a result of the analysis, the mobility spectra and the standard parameters describing carriers in different layers have been found.

Low-temperature excitonic effects in a high-quality AlGaAs/GaAs single quantum well (25 monolayers wide) structure grown without growth interruptions at the interface were studied using steady-state, time-resolved and photoluminescence excitation spectroscopy. Heavy-hole free exciton and impurity (neutral donor) bound exciton emissions, associated with quantum well regions (islands) 24 and 25 monolayers wide, have been resolved and identified using excitation energy resonant with the heavy-hole exciton energy. Interisland exciton migration, until now reported only for growth interrupted structures, is observed, indicating that islands formation is not a specific property of growth interrupted structures. We estimate the rate of transfer of free excitons between regions of width 24 monolayers and 25 monolayers to be 290 . We show that the relative intensity of free-to-bound excitonic emissions strongly depends on interface morphology and that enhanced bound exciton recombination is observed under some conditions. The relevant mechanisms of such enhanced bound exciton recombination are proposed.

Photoreflectance spectroscopy has been applied to investigate MBE grown GaAs AlGaAs low dimension structures. The first is an HEMT type structure with buried 10 periods of 2.5 nm GaAs 2.5 nm AlGaAs superlattice. Oscillation-like signals associated with this SL have been observed and analysed in terms of the Franz-Keldysh oscillations (FKO) related to the presence of free carriers in the miniband structure. On the basis of these FKO we have evaluated the electric field in the SL region of the structure. This value is almost identical to the value obtained from FKO for the GaAs channel. The second structure, consisting of a sequence of 10 quantum wells, differs in the width of well from 3 up to 30 monolayers (ML) and with the same barrier thickness (96 ML). For the PR spectra at 300 K and 90 K, transitions in almost all wells have been observed. Energies of the observed transitions are in good agreement with the results of numerical calculations, based on the envelope function approximation for rectangular wells. Transitions associated with above barrier states of amplitude comparable to the transitions involving only bound states are also observed in our PR spectra.

Quantum cascade lasers (QCL’s) have proven their usefulness as light sources in many applications, like remote gas sensing, molecular spectroscopy or free-space communication. In most cases the high-quality low-divergence beam is desired. This work presents the theoretical analysis of QCL’s beam divergence. The electromagnetic field in the resonator is calculated according to effective index method. Theoretical results are compared with measurements.

The paper presents results of experimental investigations of spatial distribution of radiation emitted by quantum cascade lasers. Measurements have been performed by means of a unique goniometric profilometer specially de- signed for the large angle laser beams. The advantages and limitations of the set-up and the applied experimental method are discussed. The obtained results have enabled the analysis of dependence of geometry of the beam on the geometry of the laser structure and on the mount method of the laser chips. The angular divergence of the beams has also been tested as a function of laser power supply.

The results of photoluminescence, time-resolved photoluminescence, photoluminescence excitation and photoluminescence kinetics studies are presented for a Al0.3 Ga0.7Αs/GaΑs quantum well system grown without growth interruptions at the interfaces. The time-resolved photoluminescence measurements show drift of excitons towards lower energy states induced in a quantum well by potential fluctuations. We present also a first direct evidence for migration of free excitons from the 24 to 25 ML regions of the quantum well and interpret these results within a linear rate model, deriving the transition rate of 290 ps-1. Such inter-island migration processes have been observed till now only in growth interrupted structures. PACS numbers: 78.66.Fd, 78.47.+ρ, 73.20.Jc

Static phase diagrams for GaAs(001) and AlAs(001) surfaces have been determined using the reflection high-energy electron diffraction technique. The investigated structures have been grown by molecular beam epitaxy with a Riber 32P machine. Two types of observations of surface reconstructions were performed: without any arsenic flux and for variable arsenic flux. In the experiments both the substrate temperature and the As4 flux were measured directly (the absolute values of the As4 flux are given in the phase diagrams). On the GaAs(001) surface reconstructions (4 × 2), (3 × 1), (3 × 6), (2 × 4) and (2 × 2) were observed; on the AlAs(001) surface the reconstructions (3 × 2), (2 × 4), (2 × 2), and (2 × 3) were seen. The boundaries between different reconstructions have been determined precisely both for the GaAs(001) and the AlAs(001) surfaces. On the basis of the static phase diagrams the values of activation energies for phase transitions on the GaAs(001) and the AlAs(001) surfaces have been found. The obtained results have been compared with those already existing in the literature.

We report high-resolution, picosecond laser spectroscopy measurements of the relaxation of lowest-energy heavy-hole excitons in GaAs quantum wells grown with and without growth interruptions. In both cases excitons relax by losing potential energy in a diffusion motion driven by potential fluctuations in the quantum-well plane. In the quantum wells grown without growth interruptions the low-energy shift of exciton lines is comparable with the width of an inhomogeneously broadened line and shows two-step decay with a slow exponential component characteristic for exciton localization at well interfaces. In the growth-interrupted quantum wells in which the size of islands with constant well thickness is large compared with exciton diameter we observe splitting of the heavy-hole transitions into the multiplets of narrow lines corresponding to one monolayer difference in the well width. The energy shifts of each line in this case amount to only a fraction of the width of individual lines (or there is no shift at all) suggesting the interisland migration of excitons mediated by acoustic phonon scattering as being responsible for exciton relaxation. Again a two-step decay of the luminescence is observed at low temperatures (2 K). Temperature-dependent measurements show that at higher temperatures luminescence decay becomes governed by a single exponential as expected for delocalized excitons (no energy shift is observed during the exciton decay time). This allowed us to study directly intrinsic properties of excitons in quantum wells, i.e. to determine the lifetime of k∥ = 0 excitons, which is a fundamental parameter of the system and has been assessed by many theories. The experimentally determined, from the present work, values of the radiative lifetime are 24.4 ps for a 13 ML thick well and 21.8 ps for a 17 ML well. These values agree very well with theoretical estimates of Andreani et al. (Solid State Commun., 77 (1991) 641). The effective lifetimes measured as a time to decay to 1/e of the value of the maximum of PL intensity are considerably longer than the radiative lifetime, since in thermal equilibrium only a small fraction of excitons occupy the states with k∥ < k 0 which can decay radiatively.