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Optically detected cyclotroii resonance is used for the identification of recombination transitions of two-dimensional electron gas in A1GaAs/GaAs heterostructures. Two photoluminescence emissions are attributed to the recombination of the two-dimensional electron gas. These are the so-called H-band and the Fermi level singularity photoluminescence. Optical detection of cyclotron resonance is related to the change of the band bending across the GaAs active layer and the AlGaAs barrier, which is caused by impact ionization of shallow donors in the barrier region. Influence of a long range carrier scattering on ionized impurities on a mobility of the two-dimensional carriers is studied. PACS numbers: 78.66.Fd, 73.40.Kpa, 73.20.Dx, 71.25.Jd

The results of picosecond photoluminescence kinetics of four dicherent CdTe/CdMnTe multiquantum well structures grown by MBE on GaAs sub- strates are presented. The experimental results show that excitons in CdTe quantum wells are strongly localized by potential fluctuations. Photolumi- nescence decay times of the localized excitons are considerably shorter (about 120 ps) than those reported for free or quasi-free excitons. An influence of Mn in the barriers on exciton properties is demonstrated. For narrow quan- tum wells as well as for the multiquantum well structure with the highest Mn mole fractioii the excitons migrate during their decay to the states with a lower potential energy. Longer decay times are observed for quasi-localized excitons. We show also that for strongly localized excitons the energy transfer between localized and donor bound excitons is less efficient.

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.

Exciton properties in growth interrupted quantum wells of GaAs/AlGaAs are compared with those observed for structures grown without growth interruption during the molecular beam epitaxy process. We report observation of quasi-localized excitons in quantum well structures grown without growth interruptions. Quasi-localized excitons drift towards the states of a lower potential energy in the quantum well. For growth interrupted MBE structures islands with a constant quantum well thickness become large compared to the exciton radius. Free or lightly localized excitons are observed in that case. PACS numbers: 71.35.+-z, 73.20.Fz, 78.55.Cr

The thermal and chemical stabilities of group-III sesquioxides (Al2O3, Ga2O3, and In2O3) were comparatively investigated at an atmospheric pressure at heat treatment temperatures ranging from 250 to 1450 °C in a flow of either N2 or H2. In a flow of N2, the thermal decomposition of α-Al2O3 was not observed at the temperatures investigated, while the decompositions of β-Ga2O3 and c-In2O3 occurred above 1150 and 1000 °C, respectively, with no generation of group-III metal droplets on the surfaces. In contrast, the chemical reactions of α-Al2O3, β-Ga2O3, and c-In2O3 began at low temperatures of 1150, 550, and 300 °C in a flow of H2. Thus, the presence of H2 in the gas flow significantly promotes the decomposition of group-III sesquioxides. The order of thermal and chemical stabilities (α-Al2O3 ≫ β-Ga2O3 > c-In2O3) obtained experimentally was verified by thermodynamic analysis, which also clarified dominant decomposition reactions of group-III sesquioxides.

Photoluminescence (PL) studies of SF6/O2 plasma‐induced defect formation in n‐type silicon samples are reported. Ion bombardment of the silicon surface during the SF6 reactive‐ion etching (RIE) is shown to introduce defects giving rise to a broad PL band in the 0.70–1.00 eV spectral range and to the carbon‐related C and G lines. The role of oxygen during SF6/O2 RIE on the photoluminescence observed is analyzed. It is argued that oxygen contamination enhances the formation of PL centers via the creation of extended defects, such as oxygen precipitates. A lattice contraction nearby these extended defects is suggested to be responsible for the observed splitting of the C and G lines as well as the shift of the phosphorous bound exciton line detected after SF6/O2 RIE.

In this work, the first-ever growth of cubic-In2O3 at 1000 °C by halide vapor phase epitaxy (HVPE) was achieved, using gaseous InCl and O2 as precursors in a N2 flow. The growth rates of In2O3 layers on (001) β-Ga2O3 and (0001) sapphire substrates were 4.1 and 5.1 µm/h, respectively, even at the high growth temperature applied, which are approximately half the growth rate of β-Ga2O3 homoepitaxially grown by HVPE at 1000 °C. Theoretical thermodynamic analyses were also conducted, and results confirmed the growth of In2O3 at temperatures above 1000 °C by HVPE. The as-grown In2O3 layers were light yellow-green in color. The In2O3 layers grown on the (001) β-Ga2O3 and (0001) sapphire substrates exhibited an optical absorption edge at about 370 nm, in addition to n-type conductivity with electron concentrations of 2.7 × 1018 and 1.7 × 1018 cm−3, and electron mobilities of 16.2 and 22.7 cm2 V−1 s−1, respectively.

The lattice mismatch (strain) effects on the infrared absorption in p‐type In1−xGaxAs/InP quantum wells are investigated systematically for both tensile (x≳0.47) and compressive (x

The role of interface roughness in bringing about localization of the free exciton in narrow GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells is well known. In this paper we investigate the interaction of the localization potential due to interface roughness with that due to the presence of a shallow acceptor impurity. Using a selective excitation technique we are able to accurately measure the bound exciton binding energy in a narrow quantum well, thereby determining the effect of interface localization on the exciton to impurity binding energy. The binding energy decreases with increasing localization strength (due to interface roughness), a localization energy of 3 meV with respect to the free-exciton energy results in a decrease by 1.2 meV in the local binding energy of the bound exciton. The rapid diffusion of weakly localized free excitons is also demonstrated and contrasted with the absence of a similar migration process for bound excitons.

We employ the novel Fourier transform photoluminescence excitation spectroscopy to investigate deep defects in Si. The electronic structure of deep bound exciton systems in Si:Ag, Si:Au and Si:Fe is revealed, and is shown to be well-described by a pseudodonor model of isoelectronic centers.

We investigated the temperature-dependent electrical properties of Pt/Ga2O3 Schottky barrier diodes (SBDs) fabricated on n–-Ga2O3 drift layers grown on single-crystal n+-Ga2O3 (001) substrates by halide vapor phase epitaxy. In an operating temperature range from 21 °C to 200 °C, the Pt/Ga2O3 (001) Schottky contact exhibited a zero-bias barrier height of 1.09–1.15 eV with a constant near-unity ideality factor. The current–voltage characteristics of the SBDs were well-modeled by thermionic emission in the forward regime and thermionic field emission in the reverse regime over the entire temperature range.

The CdZnSe/ZnSe multiquantum well (MQW) and superlattice stuctures are currently intensively studied due to their possible application in blue light emitting and semiconductor laser diodes [1]. In this communication we present the results of photoluminescence (PL) transient and optically detected cyclotron resonance (ODCR) experiments. The aim of the present study is to identify recombination transitions from the QWs and to describe their properties. We show that bound exciton spectra contribute to the low temperature PL emission from the QWs.

Shake-up transitions involving QW hole subbands have been observed as satellites in selective photoluminescence spectra of undoped GaAs/AlGaAs QWs. These shake-up transitions are explained in terms of an interaction between localized exciton and valence-band hole states attached to the QW subbands, in which holes are shaken up from the n=1 heavy hole subband to higher subbands, either the n=1 light hole subband or the n=2 heavy hole subband. The required localization is due to the interface roughness; thus these new transitions are of intrinsic origin. From the observation of the intersubband shake-up processes we derive direct information about the hole inter-subband energies. Furthermore, the satellite intensity is strikingly enhanced in the presence of a magnetic field due to an increasing exciton localization related to the compression of its wave function in the field. The exciton wave function compression continues until its radius in the plane of the well is comparable with the radius of the "flat island" characterized by constant QW width. Accordingly, from the magnetic field dependence of the shake-up satellite intensity we can roughly estimate the size of the "flat islands" and consequently probe the interface roughness.

We study longitudinal electron transport in InGaN/GaN multiple quantum wells (MQWs) at moderate magnetic fields. We observe a stepwise behavior of both the Hall coefficient and magnetoresistivity. The peculiarities are explained by a magnetic-field-induced localization of electrons in a two-dimensional (2D) potential relief of the InGaN MQW due to composition fluctuations. We extend the model for a magnetic localization of electrons, treating every QW like a quasi-2D system with a cylindrical potential relief. The calculated values of the decrease of the sheet electron concentrations in a magnetic field based on such an assumption for 2D density of states in a InGaN MQW system are in good accordance with the experimentally obtained values.

Magneto-photoluminescence (PL) has been studied in type II Zn1−xMnxSe/ZnSySe1−y multiple quantum wells (QWs) with the Mn content x between 0.03 and 0.15. Two PL lines are observed in the samples with x equal to 0.05 and 0.15. The lines are attributed to the excitons of two different types, coexisting in the QWs. At least one of them is associated with the formation of an exciton magnetic polaron.

We have studied GaN films grown on a-plane sapphire by hydride vapor phase epitaxy and metalorganic vapor phase epitaxy. The in-plane lattice parameter was determined from sets of equivalent interplanar distances measured for six different directions in order to examine the effect of strain anisotropy. It is found that, in both types of films, the obtained six values of the in-plane lattice parameter can be grouped around two values. The strain anisotropy is estimated to have different value in the films grown by the two techniques and possible explanations are suggested.

The dependence of resonance energy transfer from Wannier-Mott excitons to an organic overlayer on exciton dimensionality is studied experimentally and by means of supporting simulations. The variation of temperature effectively tunes the balance between localized and free excitons, and allows to investigate the effect of the excitonic potential disorder on resonance energy transfer. Our theoretical calculations give insight into the experimentally observed temperature dependence of resonance energy transfer, and allow us to quantify the contribution from localized and free excitons. It is shown that free excitons can undergo resonance energy transfer at a rate that is an order of magnitude higher compared to localized excitons. In planar geometries nonradiative resonance energy transfer is dominating over radiative energy transfer and hence we propose hybrid inorganic-organic LEDs which are optimized for resonance energy transfer to an organic or QD-based color converter.

Efficient light-emitting diodes and photovoltaic energy-harvesting devices are expected to play an important role in the continued efforts towards sustainable global power consumption. Semiconductor nanowires are promising candidates as the active components of both light-emitting diodes1, 2, 3, 4, 5, 6 and photovoltaic cells7, 8, 9, 10, primarily due to the added freedom in device design offered by the nanowire geometry. However, for nanowire-based components to move past the proof-of-concept stage and be implemented in production-grade devices, it is necessary to precisely quantify and control fundamental material properties such as doping and carrier mobility. Unfortunately, the nanoscale geometry that makes nanowires interesting for applications also makes them inherently difficult to characterize. Here, we report a method to carry out Hall measurements on single core–shell nanowires. Our technique allows spatially resolved and quantitative determination of the carrier concentration and mobility of the nanowire shell. As Hall measurements have previously been completely unavailable for nanowires, the experimental platform presented here should facilitate the implementation of nanowires in advanced practical devices.

Todays energy saving solutions for general illumination rely on efficient white light emitting diodes (LEDs). However, the output efficiency droop experienced in InGaN based LEDs with increasing current injection is a serious limitation factor for future development of bright white LEDs. We show using cathodoluminescence (CL) spatial mapping at different electron beam currents that threading dislocations are active as nonradiative recombination centers only at high injection conditions. At low current, the dislocations are inactive in carrier recombination due to local potentials, but these potentials are screened by carriers at higher injection levels. In CL images, this corresponds to the increase of the dark contrast around dislocations with the injection (excitation) density and can be linked with droop related to the threading dislocations. Our data indicate that reduction of droop in the future efficient white LED can be achieved via a drastic reduction of the dislocation density by using, for example, bulk native substrates. (C) 2015 AIP Publishing LLC. Funding Agencies|Swedish Research Council (VR); Swedish Energy Agency

The infrared-absorption spectra of accepters confined in the center of GaAs/A1(x)Ga(1-x)As quantum wells in the presence of an external magnetic field have been calculated. The calculations are based on a four-band effective-mass theory in which the valence-band mixing as well as the mismatch of the band parameters and the dielectric constants between well and barrier materials have been taken into account. The dipole transition rule is assumed for the infrared absorption. The oscillator strengths of allowed transitions between the acceptor ground states and excited states, corresponding to the transitions G, D, and C in bulk GaAs, are calculated for different well widths in the range 50-200 Angstrom with an external magnetic field up to 10 T. The oscillator strength of these infrared acceptor transitions exhibits a strong polarization dependence.

A magneto-optical study of accepters in varying degrees of confinement in GaAs/Al-x Ga-l-x, As quantum wells (QW's) is presented. A model for the splitting of the acceptor bound exciton (BE) and the allowed BE transitions in the presence of a magnetic field is presented. Our experimental results for the acceptor BE emission agree with calculated g values for bound holes, e.g., g(3/2) = 0. 61 and g(1/2) = 0.35 for a 150-Angstrom-wide QW. Furthermore, the energy separation between the acceptor m(j) = +/-3/2 1S(T-6) ground state and the excited m(j) = +/- 3/2 2S(T-6) state as a function of applied magnetic field is derived from selective photoluminescence and resonant-Raman-scattering measurements. Finally, these experimental results are compared with theoretical predictions for the confined acceptor states.

The transition from a two-dimensional to a quasi-three-dimensional electron system has been studied in multiple quantum wells, by varying the barrier thickness ${\mathit{L}}_{\mathit{b}}$ for a constant 100-\AA{} well width. Dramatic differences in electron-hole correlation are observed due to the different extension of electron and hole wave functions. The 1S-2S energy separation of the heavy-hole exciton decreases from 8.3 meV for ${\mathit{L}}_{\mathit{b}}$=150 \AA{} to 3.1 meV for ${\mathit{L}}_{\mathit{b}}$=10 \AA{}. The acceptor-bound-exciton binding energy is constant, while the bound-exciton decay time increases dramatically with decreasing ${\mathit{L}}_{\mathit{b}}$.

Energy levels of ground and excited shallow acceptor states in the presence of an external magnetic field have been calculated for center-doped GaAs/AlxGa1-xAs quantum wells (QW's). The impurity states are calculated within a four-band effective-mass theory, in which the valence-band mixing as well as the mismatch of the band parameters and the dielectric constants between well and barrier materials have been taken into account. The g factors of the shallow acceptor 1s3/2 ground states and the 2P3/2 excited states are obtained for QW's with different well widths. The infrared optical transitions corresponding to the G, D, and C lines of acceptors in bulk GaAs have also been calculated versus magnetic fields up to 16 T for the case of acceptors confined in QW's.

In silver-doped silicon, a photoluminescence band is observed with zero-phonon lines at 778.92, 779.85, and 784.31 meV, which is concluded to be associated with the spin-triplet and spin-singlet states of a bound exciton. The photoluminescence excitation spectrum of this defect, with the transitions to s-like electronic excited states dominating, is measured using Fourier-transform techniques. The electronic structure of this defect, which is known in the literature as the deep ${\mathit{E}}_{\mathit{v}}$+0.34 eV silver donor, is well described by the pseudodonor model.

The exciton bound to the shallow Si-donor confined in a 100A wide GaAs quantum well has been studied in selective photoluminescence (SPL) and photoluminescence excitation (PLE) spectroscopy. The transition from the ground state, ls(Γ6), to the first excited state, 2s(Γ6), of the confined Si donor has been observed via two-electron transitions (TETs) of the donor bound exciton observed in SPL for the first time to the best of our knowledge. The interpretation of the TET peaks is confirmed by PLE measurements. Further, from Zeeman measurements, the magnetic field dependence of the donor ls(Γ6)-2s(Γ6) transition energy has been determined.

We present a combined experimental and theoretical investigation of shallow two-dimensional acceptor states in the presence of an external magnetic field for center-doped GaAs/AlxGa1-xAs quantum wells (QW's). The calculated 1S3/2(GAMMA6)-2S3/2(GAMMA6) transition energies of the acceptor state are in excellent agreement with our resonant Raman-scattering data. The g factors obtained from our calculations for varying well width of QW's are also in good agreement with available experimental data. These results confirm the value of the Luttinger parameter kappa = 1.2+/-0.05 for bulk GaAs.

The technique of optically detected cyclotron resonance (ODCR) has been introduced to increase the sensitivity of the conventional cyclotron resonance investigations. Here a new application of the ODCR is described. It is shown that the ODCR can be used for a high spectral resolution photoluminescence (PL) experiments and can allow the identification of recombination processes in both bulk and in low-dimensional systems.

The magnetization of Cd 1 − x Mn x Te can be reduced by the Mn 2+ magnetic resonance. The decrease of the magnetization affects all the “edge‘ emissions of the material studied, which shift towards higher energies. It also affects the relative efficiencies of the free exciton (FE) and the acceptor bound exciton (ABE) emissions. The latter decays mostly nonradiatively, which is due to a competing Auger transition. The large Auger recombination rate of the ABE is directly confirmed by the present optically detected magnetic resonance (ODMR) investigations.

At low temperatures a neutral impurity is able to capture a free cxciton to form a bound exciton state. The capture process is understood in bulk material as a phonon mediated relaxation of the free cxciton through successive excited states to the ground state of the bound exciton. The equivalent mechanism in a confined system has not been specifically studied to date. In this paper we present an investigation of the cxciton capture process in GaAs/Al0.3Ga0.7 As quantum wells using a picosecond time-resolved photolumincscence technique. We demonstrate that there are significant differences in the capture mechanism for narrow quantum wells in comparison to the bulk. In particular the capture efficiency is shown at first to increase with temperature. This behaviour is understood in terms of the role of localization of the free exciton in the potentials caused by the interface roughness. Higher temperatures destroy this localization process which otherwise limits the total capture rate for the exciton to the impurity. We also conclude that the distinct difference in the near bandgap low temperature excitonic spectra between bulk and confined material at low and moderate doping levels can be understood in terms of the corresponding differences in impurity capture efficiency.

Selenium-doped silicon quenched after heat treatment at 800 \ifmmode^\circ\else\textdegree\fi{}C exhibits deep luminescence similar to that previously reported in sulfur-doped silicon. The corresponding Se-related complex gives rise to two deep bound-exciton (BE) photoluminescence (PL) emissions, which at low temperature have no-phonon (NP) lines at 955.5 meV (${\mathrm{Se}}_{\mathit{A}}^{0}$) and 772.2 meV (${\mathrm{Se}}_{\mathit{B}}^{0}$), respectively. A series of equidistant phonon replicas is observed with the same phonon energy as in the sulfur case. By increasing the temperature in the PL experiments associated BE excited-state lines are detected higher in energy with a separation of 10.5 and 12.4 meV, respectively, from the lowest NP lines. The metastability of both the S- and Se-related defects is investigated and discussed as well as the influence of the excitation photon energy on the intensity of the PL emission. The metastability is also confirmed by optically detected magnetic-resonance results, which also show that the (${\mathrm{S}}_{\mathit{A}}^{0}$) and (${\mathrm{Se}}_{\mathit{A}}^{0}$) lines correspond to an excited spin-triplet state. Both defects are found to have a low symmetry. When they are in the A configuration the symmetry is monoclinic I with one axis tilted approximately 20\ifmmode^\circ\else\textdegree\fi{} from the [11\ifmmode\bar\else\textasciimacron\fi{}1] axis in the (110) plane.

SiC has in comparison with Si superior basic properties for applications in high-power, high-frequency and high-temperature electronics. The potential applications of SiC were known decades ago, but the poor quality of the material produced at that time has delayed the device development. However, during the last years the crystal growth process of SiC has been improved considerably. We will present some important properties of SiC, describe the two most common growth processes and discuss fundamental materials problems that remain to be solved. A further aspect, which we will discuss, is the polytypism of SiC, which may allow us to obtain generic knowledge of, for instance, defects in semiconductors.

The scenario for exciton binding by rara earth (RE) ions is described. The RE intra-shell emission can be induced by an energy transfer from RE bound excitons to th RE “core” states. Carrier binding can also deactivate the RE emission. Auger-type recombination channels are discussed based on the results of photoluminescence and optically detected cyclotron resonance (OCDR) studies of Yb InP and RE doped wide band gap II–VI compounds.

Novel applications of the technique of optically detected cyclotron resonance (ODCR) are discussed. This method is an extension of the conventional cyclotron resonance investigations and shows important advantages when applied to characterization of semiconductor materials. These advantages are due to a higher sensitivity and a longer momentum relaxation time caused by photoneutralization of ionized impurities. This in turn enables experiments at lower magnetic fields and lower microwave radiation frequency. Photoexcitation used in ODCR often results in a simultaneous observation of electron and hole cyclotron resonances in the same sample, which is a rare case in a conventional CR study. High magnetic field far infrared ODCR experiments utilize all these advantages of the method. For the most common X-band (10 GHz) microwave setups, the ODCR resolution often is too low to allow accurate CR determination of the band structure parameters of the material studied. In that case, ODCR may be used for ...

Magneto-optical spectroscopy in combination with tunable laser excitation is employed to study exciton spin alignment and injection in ZnMnSe/ZnCdSe quantum structures. This approach enables us to selectively create preferred spin orientation and to separately monitor subsequent spin injection from individual spin states, thus shedding light on a possible source of spin loss. It is shown that the limited spin polarization in a nonmagnetic quantum well due to spin injection from a ZnMnSe-based diluted magnetic semiconductor (DMS) is not caused by a limited degree of spin alignment in the DMS, which is in fact complete, but rather occurs during subsequent processes.

The deformation potentials of the E-1(TO) mode in AlN are experimentally determined by combining infrared reflection spectroscopy and x-ray diffraction measurements and using a reported value of th ...

The nucleation and microstructure of large-scale columnar domains present in hydride vapor phase epitaxial (HPVE)-GaN layers grown directly on sapphire have been studied using cathodoluminescence and transmission electron microscopy. The domains are distributed in a quasicontinuous layer close to the GaN/sapphire interface. The domain boundaries are found to be associated with stacking mismatch defects. They are initiated at steps on the sapphire surface and are formed between nucleation islands growing on adjacent terraces. The formation of these domains in the initial stages of HVPE-GaN heteroepitaxial growth is proposed to play an important role in the strain relaxation mechanism.

The influence of Si doping on exciton localization in modulation-doped GaN/Al0.07Ga0.93N multiple-quantum-well structures has been studied by means of photoluminescence (PL) and time-resolved PL measurements. Surprisingly, the PL decay time is constant in the range 320–420 ps for all doping levels at 2 K in these samples, due to a strong localization of the holes. The temperature dependence of radiative as well as nonradiative lifetime has also been evaluated between 2 K and room temperature for different Si doping.