Search

Excitonic emission from single InAs/InGaAlAs/InP quantum dashes has been investigated in terms of controlling the polarization anisotropy by altering the shape of the processed sub-micrometer mesa structures. Photoluminescence has been measured from exemplary single quantum dashes emitting around 1.3 and 1.55 µm and placed inside rectangular mesas of various orientation, asymmetry, and sizes. The detected degree of linear polarization of bright exciton emission ranges from -0.1 to ca. 0.55, compared to 0.25 for dashes in unaltered or isotropic in-plane dielectric surrounding. These results are interpreted by numerical simulations using an emitter coupled with a single optical mode in such a mesa and outgoing in the direction normal to the sample surface. [ABSTRACT FROM AUTHOR]

We present a characterization of doped InAs layers in interband cascade lasers exploiting the plasmon-enhanced waveguiding. Fast differential reflectance was employed in order to identify the plasma-edge frequency via the Berreman effect and shown as an advantageous method when compared to other types of measurements. The carrier concentration was then derived and compared with the nominal doping densities. The emission properties of the investigated structures were studied by means of photoluminescence (PL). Its full-width at half-maximum and integrated intensity were extracted from PL spectra and analyzed in the function of the doping density (carrier concentration). The PL linewidth was found to be independent of the carrier concentration indicating an insignificant contribution of doping to the structural properties deterioration. The PL intensity decay with the carrier concentration suggests being dominated by Auger recombination losses. [ABSTRACT FROM AUTHOR]

Unstrained Ge1-xSnx layers of various Sn concentration (1.5%, 3%, 6% Sn) and Ge0.97Sn0.03 layers with built-in compressive (ε=-0.5%) and tensile (ε=0.3%) strain are grown by molecular beam epitaxy and studied by electromodulation spectroscopy (i.e., contactless electroreflectance and photo-reflectance (PR)). In order to obtain unstrained GeSn layers and layers with different built-in in-plane strains, virtual InGaAs substrates of different compositions are grown prior to the deposition of GeSn layers. For unstrained Ge1-xSnx layers, the pressure coefficient for the direct band gap transition is determined from PR measurements at various hydrostatic pressures to be 12.2±0.2meV/kbar, which is very close to the pressure coefficient for the direct band gap transition in Ge (12.9 meV/kbar). This suggests that the hydrostatic deformation potentials typical of Ge can be applied to describe the pressure-induced changes in the electronic band structure of Ge1-xSnx alloys with low Sn concentrations. The same conclusion is derived for the uniaxial deformation potential, which describes the splitting between heavy-hole (HH) and light-hole (LH) bands as well as the strain-related shift of the spin-orbit (SO) split-off band. It is observed that the HH, LH, and SO related transitions shift due to compressive and tensile strain according to the Bir-Pikus theory. The dispersions of HH, LH, and SO bands are calculated for compressive and tensile strained Ge0.97Sn0.03 with the 8-band kp Hamiltonian including strain effects, and the mixing of HH and LH bands is discussed. In addition, the dispersion of the electronic band structure is calculated for unstrained Ge1-xSnx layers (3% and 6% Sn) at high hydrostatic pressure with the 8-band kp Hamiltonian, and the pressure-induced changes in the electronic band structure are discussed. [ABSTRACT FROM AUTHOR]

In this work, silicon-rich silicon oxide films containing terbium were prepared by means of plasma enhanced chemical vapor deposition. The influence of hydrogen passivation on defects-mediated non-radiative recombination of excited Tb3+ ions was investigated by photoluminescence, photoluminescence excitation, and photoluminescence decay measurements. Passivation was found to have no effect on shape and spectral position of the excitation spectra. In contrast, a gradual increase in photoluminescence intensity and photoluminescence decay time was observed upon passivation for the main 5D4-7F5 transition of Tb3+ ions. This observation was attributed to passivation of non-radiative recombination defects centers with hydrogen. It was found that the number of emitted photons increases upon passivation as a result of two effects: (1) longer Tb3+ lifetime in the 5D4 excited state and (2) optical activation of new Tb3+ emitters. The obtained results were discussed and compared with other experimental reports. [ABSTRACT FROM AUTHOR]

We report theoretical studies of spin polarization dynamics in dilute nitride semiconductors. We develop a commonly used rate equation model [Lagarde et al., Phys. Status Solidi A 204, 208 (2007) and Kunold et al. Phys. Rev. B 83, 165202 (2011)] to take into account the influence of shallow localizing states on the temperature dependence of spin polarization dynamics and a spin filtering effect. Presented investigations show that the experimentally observed temperature dependence of a spin polarization lifetime in dilute nitrides can be related to the electron capture process by shallow localizing states without paramagnetic properties. This process reduces the efficiency of spin filtering effect by deep paramagnetic centers, especially at low temperatures. [ABSTRACT FROM AUTHOR]

The temperature dependence of carrier dynamics in GaNxPyAs1-y-x alloys has been investigated by time resolved photoluminescence. This investigation has shown that the decay time constant does not change significantly up to 100 K, and then starts to decrease rapidly above this temperature. Additionally, the decay times at the high-energy side of the spectrum decrease faster than those at the low-energy side. The effects have been explained by the interplay between carrier capture by radiative and nonradiative recombination centers. Detailed simulations show that the effect of carrier localization in the investigated materials is better described by double-scale potential fluctuations that are related to (i) distribution of localized states energy and (ii) bandgap fluctuations. In addition, it was observed that the increase in nitrogen concentration leads to a shorter decay time at room temperature, which is attributed to a larger concentration of non-radiative recombination centers. Furthermore, a post-growth annealing step leads to a longer decay time at room temperature, which is attributed to a reduction in non-radiative recombination centers. At low temperatures, the role of non-radiative centers is suppressed, and therefore the decay time does not differ significantly for samples with either different nitrogen concentrations or in both the as-grown and annealed samples. [ABSTRACT FROM AUTHOR]

Type II InAs/GaInSb quantum wells (QWs) grown on GaSb or InAs substrates and designed to be integrated in the active region of interband cascade lasers (ICLs) emitting in the mid infrared have been investigated. Optical spectroscopy, combined with band structure calculations, has been used to probe their electronic properties. A design with multiple InAs QWs has been compared with the more common double W-shaped QW and it has been demonstrated that it allows red shifting the emission wavelength and enhancing the transition oscillator strength. This can be beneficial for the improvements of the ICLs performances, especially when considering their long-wavelength operation. [ABSTRACT FROM AUTHOR]

Band gap alignment in GaAs1-xBix/GaAs quantum wells (QWs) was studied experimentally by photoreflectance (PR) and theoretically, ab initio, within the density functional theory in which the supercell based calculations are combined with the alchemical mixing approximation applied to a single atom in a supercell. In PR spectra, the optical transitions related to the excited states in the QW (i.e., the transition between the second heavy-hole and the second electron subband) were clearly observed in addition to the ground state QW transition and the GaAs barrier transition. This observation is clear experimental evidence that this is a type I QW with a deep quantum confinement in the conduction and valence bands. From the comparison of PR data with calculations of optical transitions in GaAs1-xBix/GaAs QW performed for various band gap alignments, the best agreement between experimental data and theoretical calculations has been found for the valence band offset of 5265%. A very similar valence band offset was obtained from ab initio calculations. These calculations show that the incorporation of Bi atoms into GaAs host modifies both the conduction and the valence band. For GaAs1-xBixBix with 01-xBix and GaAs in the range of ~60%-40% (~40%-60%), which is in good agreement with our conclusion derived from PR measurements. [ABSTRACT FROM AUTHOR]

In this work, we investigate the importance of lateral electronic coupling in a dense ensemble of anisotropic epitaxial nanostructures called quantum dashes. The respective confined state energy levels and the related tunneling times between two neighboring nanostructures are calculated using a simplified approach with parabolic effective masses in a single band k∙p approximation, and assuming infinite size of the quantum dashes in the elongation direction. There has been studied the influence of the cross-sectional dimensions of the dashes, their lateral separation and the barrier material. Eventually, the impact of the inhomogeneity within the ensemble of nanostructures has been discussed. For the presented calculations the InAs dashes on InP substrate have been chosen as a model system because of the high areal density and strong in-plane anisotropy obtained typically in the self-assembled growth by molecular beam epitaxy. [ABSTRACT FROM AUTHOR]

In this work, we will discuss the excitation and emission properties of Tb ions in a Silicon Rich Silicon Oxide (SRSO) matrix obtained at different technological conditions. By means of electron cyclotron resonance plasma-enhanced chemical vapour deposition, undoped and doped SRSO films have been obtained with different Si content (33, 35, 39, 50 at. %) and were annealed at different temperatures (600, 900, 1100 °C). The samples were characterized optically and structurally using photoluminescence (PL), PL excitation, time resolved PL, absorption, cathodoluminescence, temperature dependent PL, Rutherford backscattering spectrometry, Fourier transform infrared spectroscopy and positron annihilation lifetime spectroscopy. Based on the obtained results, we discuss how the matrix modifications influence excitation and emission properties of Tb ions. [ABSTRACT FROM AUTHOR]

Position of Fermi level on Al0.2Ga0.8N surface and distribution of electric field in Al0.2Ga0.8N/GaN transistor heterostructures without and with AlN layer were studied experimentally using contactless electroreflectance and theoretically solving Schrodinger-Poisson equation with various surface boundary conditions. It has been observed that the thin AlN layer changes very strongly the distribution of electric field in this heterostructure but the Fermi level position on Al0.2Ga0.8N surface does not change significantly. Its position is the same within experimental uncertainly (i.e., ∼0.5 eV below conduction band) for both bulk Al0.2Ga0.8N and Al0.2Ga0.8N/GaN heterostructures. [ABSTRACT FROM AUTHOR]

Band structure properties of the type-II W-design AlSb/InAs/GaIn(As)Sb/InAs/AlSb quantum wells have been investigated theoretically in a systematic manner and with respect to their use in the active region of interband cascade laser for a broad range of emission in mid infrared between below 3 to beyond 10 μm. Eight-band k·p approach has been utilized to calculate the electronic subbands. The fundamental optical transition energy and the corresponding oscillator strength have been determined in function of the thickness of InAs and GaIn(As)Sb layers and the composition of the latter. There have been considered active structures on two types of relevant substrates, GaSb and InAs, introducing slightly modified strain conditions. Additionally, the effect of external electric field has been taken into account to simulate the conditions occurring in the operational devices. The results show that introducing arsenic as fourth element into the valence band well of the type-II W-design system, and then altering its composition, can efficiently enhance the transition oscillator strength and allow additionally increasing the emission wavelength, which makes this solution prospective for improved performance and long wavelength interband cascade lasers. [ABSTRACT FROM AUTHOR]

Optical properties of molecular beam epitaxially grown type II 'W' shaped GaSb/AlSb/InAs/GaIn(As)Sb/InAs/AlSb/GaSb quantum wells (QWs) designed for the active region of interband cascade lasers have been investigated. Temperature dependence of Fourier-transformed photoluminescence and photoreflectance was employed to probe the effects of addition of arsenic into the original ternary valence band well of GaInSb. It is revealed that adding arsenic provides an additional degree of freedom in terms of band alignment and strain tailoring and allows enhancing the oscillator strength of the active type II transition. On the other hand, however, arsenic incorporation apparently also affects the structural and optical material quality via generating carrier trapping states at the interfaces, which can deteriorate the radiative efficiency. These have been evidenced in several spectroscopic features and are also confirmed by cross-sectional transmission electron microscopy images. While arsenic incorporation into type II QWs is a powerful heterostructure engineering tool for optoelectronic devices, a compromise has to be found between ideal band structure properties and high quality morphological properties. [ABSTRACT FROM AUTHOR]

Optical transitions in GaAs1-xNx/GaAs quantum wells (QWs) have been probed by two complementary techniques, modulation spectroscopy in a form of photoreflectance and surface photovoltage spectroscopy. Transition energies in QWs of various widths and N contents have been compared with the results of band structure calculations based on the 10-band k·p Hamiltonian. Due to the observation of higher order transitions in the measured spectra, the band gap discontinuities at the GaAsN/GaAs interface and the electron effective masses could be determined, both treated as semi-free parameters to get the best matching between the theoretical and experimental energies. We have obtained the chemical conduction band offset values of 86% for x = 1.2% and 83% for x = 2.2%, respectively. For these determined band offsets, the electron effective masses equal to about 0.09 mo in QWs with 1.2% N and 0.15 mo for the case of larger N content of 2.2%. [ABSTRACT FROM AUTHOR]

The band structure and optical gain have been calculated for GaInNAs/GaAs quantum wells (QWs) with various nitrogen concentrations within the 10-band and 8-band kp models. Two approaches to calculate optical properties of GaInNAs/GaAs QWs have been compared and discussed in the context of available material parameters for dilute nitrides and the conduction band nonparabolicity due to the band anti-crossing (BAC) interaction between the N-related resonant level and the conduction band of a host material. It has been clearly shown that this nonparabolicity can be neglected in optical gain calculations since the dispersion of conduction band up to the Femi level is very close to parabolic for carrier concentrations typical for laser operation, i.e., 5 × 1018 cm-3. This means that the 8-band kp model when used to calculate the optical gain is very realistic and much easier to apply in QWs containing new dilute nitrides for which the BAC parameters are unknown. In such an approach, the energy gap and electron effective mass for N-containing materials are needed, instead of BAC parameters. These parameters are available experimentally much easier than BAC parameters. [ABSTRACT FROM AUTHOR]

Contactless electroreflectance (CER) has been applied to study the Fermi-level position on GaSb surface in n-type and p-type GaSb Van Hoof structures. CER resonances, followed by strong Franz-Keldysh oscillation of various periods, were clearly observed for two series of structures. This period was much wider (i.e., the built-in electric field was much larger) for n-type structures, indicating that the GaSb surface Fermi level pinning position is closer to the valence-band than the conduction-band. From analysis of the built-in electric fields in undoped GaSb layers, it was concluded that on GaSb surface the Fermi-level is located ∼0.2 eV above the valence band. [ABSTRACT FROM AUTHOR]

The temperature dependence of the energy gap (E0) and the spin-orbit split (E0+ΔSO) transitions has been studied by photoreflectance for In0.53Ga0.47BixAs1-x layers with 0 < x ≤ 0.044. It has been observed that at 15 K the E0 transition shifts to red and significantly broadens with increasing Bi concentration, while the E0 + ΔSO transition is almost unaffected. The temperature-induced shifts of the E0 and E0 + ΔSO transitions in the temperature range of 15-295 K have been found to be ∼50-60 meV and ∼80-90 meV, respectively, which is very similar to the energy shift in the In0.53Ga0.47As host material over the same temperature range. Obtained results (energies and broadenings of E0 and E0+ΔSO transitions) have been analyzed using the Varshni and Bose-Einstein formulas. The Varshni and Bose-Einstein parameters have been found to be close to the parameters of conventional narrow bandgap III-V semiconductors. [ABSTRACT FROM AUTHOR]

We have investigated theoretically two kinds of dilute-nitride-based quantum well structures, InGaAsN/GaAs and InGaAsN/GaAsN/GaAs, both able to emit at 1.3 μm. The ground state transition energy and its oscillator strength have been probed as a function of the material composition in the single particle as well as the excitonic approximations. The modification of the bandgap energy due to nitrogen incorporation has been taken into account by using a two-level repulsion model. We have shown that in spite of a decrease of the electron-hole wave functions overlap with the mole fraction of nitrogen, the overall transition intensity of the excitonic transition can increase significantly due to the strongly composition dependent mass of the exciton. The latter makes dilute nitride quantum wells good candidates for the polaritonic physics and Bose-Einstein condensation of exciton polaritons at telecommunication wavelengths. We have also demonstrated that the exact values of the band offsets are necessary to be known as they have a critical impact on the actual transition oscillator strengths in these quantum wells. [ABSTRACT FROM AUTHOR]

The influence of non-radiative recombination on the photoluminescence decay time (τPL) has been studied for GaInNAs/GaAs quantum wells with Ga- and In-rich environments of N atoms. At low temperatures, this influence is suppressed, due to the carrier localization phenomenon, which leads to a spectral dispersion of τPL. For investigated samples, this dispersion has been found to be in the range of 0.2-2.0 ns. With the temperature increase, the free exciton emission starts to dominate instead of the localized exciton emission and the dispersion of τPL disappears. The dynamic of free exciton recombination is strongly influenced by the non-radiative recombination, which varies between samples, due to different concentration of non-radiative centers. The study of influence of non-radiative recombination on τPL has been performed at 180 K, since this temperature is high enough to eliminate the localized emission and activate non-radiative recombination and low enough to observe excitonic emission without strong contribution of free carrier recombination when the sample is excited with low power. It was observed that, for as-grown samples, the τPL increases from 0.14 to 0.25 ns with the change in As/III beam equivalent pressure ratio from 3.8 to 12.1 (in this case, it corresponds to the change in nitrogen nearest-neighbor environment from Ga- to In-rich), whereas, after annealing (i.e., also the change from Ga-rich to In-rich environment of N atoms), this time increases 2-4 times, depending on the As/III ratio. It has been concluded that the τPL is influenced by point defects rather than the nitrogen nearest-neighbor environment, but their concentration is correlated with the type of nitrogen environment. [ABSTRACT FROM AUTHOR]

In this work, we present both experimental data and simulations of multiexcitonic emission spectra of single self-assembled elongated In0.3Ga0.7As/GaAs quantum dots. The emission spectra reveal an unusual evolution with the increased excitation power density. First, a biexciton line appears simultaneously with its low energy sideband, the origin of which has already been postulated previously and related to the interaction of a quantum dot biexciton with excitons generated in the surrounding wetting layer. A further increase of the excitation causes a disappearance of the exciton line accompanied with a transformation of the biexciton sharp line and its sideband into a redshifting broad emission band. The latter recalls a typical feature of the transition from excitonic emission into electron-hole plasma called Mott transition, which is possible to occur in wire-like structures under the conditions of very high carrier densities. However, we propose an alternative explanation and show that this behavior can be well explained based on a multilevel rate equation model, indicating that such a dependence of the emission spectra is a fingerprint of a formation of multiexcitonic states. Further, we discuss the importance of various quantum system parameters as the radiative lifetimes or spectral linewidths. [ABSTRACT FROM AUTHOR]

We report on optically pumped lasing at 500 nm on InGaN laser structures grown by plasma assisted molecular beam epitaxy. The InGaN laser structures were grown under group III-rich conditions on bulk (0001) GaN substrates. The influence of the nitrogen flux and growth temperature on the indium content of InGaN layers was studied. We demonstrate that at elevated growth temperatures, where appreciable dissociation rate for In-N bonds is observed, the indium content of InGaN layers increases with increasing nitrogen flux. We show that growth of InGaN at higher temperatures improves optical quality of InGaN quantum wells, which is crucial for green emitters. The influence of piezoelectric fields on the lasing wavelength is also discussed. In particular, the controversial issue of partial versus complete screening of built-in electric field at lasing conditions is examined, supporting the former case. [ABSTRACT FROM AUTHOR]

Room temperature contactless electroreflectance (CER) has been applied to study optical transitions and the distribution of the built-in electric field in AlGaN/GaN heterostructures grown on c-, a-, m-, and (20.1)-plane GaN substrates obtained by the ammonothermal method. It has been clearly shown that polarization effects in the AlGaN/GaN heterostructures grown on the c-plane lead to a strong built-in electric field in the AlGaN layer. The aforementioned field was determined to be ∼0.43 MV/cm from the period of Franz-Keldysh oscillations (FKOs). In addition, polarization effects lead to the formation of a two dimensional electron gas at the AlGaN/GaN interface, which screens the band bending modulation in the GaN buffer layer, and, therefore, GaN-related excitonic transitions are not observed for this heterostructure. Such features/effects are also not observed in the AlGaN/GaN heterostructures grown on nonpolar and semipolar GaN substrates because any strong polarization effects are not expected in this case. For these heterostructures, very strong and sharp GaN excitonic resonances are clearly visible in CER spectra. The resonances are very similar to the excitonic transitions observed for the GaN epilayers deposited on nonpolar and semipolar substrates. Moreover, there is a very weak AlGaN-resonance without FKO for nonpolar and semipolar heterostructures instead of the strong AlGaN-related FKO, which is typical of polar AlGaN/GaN heterostructures. [ABSTRACT FROM AUTHOR]

We report on the microstructure and photoluminescence (PL) properties of Nd-doped SiO2 thin films containing silicon nanoparticles (Si-np) as a function of the annealing temperature and the Nd concentration. The thin films, which were grown on Si substrates by reactive magnetron co-sputtering, contain the same Si excess. Fourier transform infrared (FTIR) spectra show that a phase separation occurs during the annealing due to the agglomeration of the Si excess resulting in the formation of Si-np. Besides, after annealing, the films exhibit PL from excitonic states confined in Si-np. We showed that the intensity of the PL of Nd3+ ions that occurs at ∼0.92, 1.06, and 1.4 μm is maximal at low Nd concentration and while well-passivated Si-np are formed. FTIR and x-ray measurements showed that the increase in the Nd incorporation has detrimental effects on the PL of Nd3+ because of the formation of Nd2O3 nanocrystals and inherent disorder in the SiO2 host matrix. PL excitation measurements demonstrate that the PL of Nd3+ ions is nonresonant and follows the excitation of Si-np giving new evidence of the energy transfer from Si-np toward the rare earth ions. [ABSTRACT FROM AUTHOR]

Gallium oxide and more particularly β-Ga2O3 matrix is an excellent material for new generation of devices electrically or optically driven as it is known as the widest band gap transparent conductive oxide. In this paper, the optical properties of neodymium doped gallium oxide films grown by magnetron sputtering have been analyzed. The influence of the Nd ions concentration on the excitation/emission mechanisms of Nd ions and the role of gallium oxide matrix have been investigated. The grain size reduction into gallium oxide films have been observed when concentration of Nd increases. It has been found for all samples that the charge transfer is the main excitation mechanism for Nd ions where defect states play an important role as intermediate states. As a consequence Nd emission efficiency increases with temperature giving rise to most intensive emission at 1087 nm at room temperature. [ABSTRACT FROM AUTHOR]

Hereby, we present a few level rate equation model in a context of the interpretation of excitation power dependent exciton and biexciton emission intensity from single quantum-dot-like structures. We emphasize that it not only allows identifying the excitonic and biexcitonic emission from one quasi-zero-dimensional object, but gives also an insight into the kinetics of the carriers confined in the system (both the internal dynamics of the exciton within its fine structure and the relative exciton to biexciton lifetimes ratio), the regime of the confinement itself and the importance of the higher energy levels occupation. Eventually, there are presented and discussed examples of the rate equation model application for an analysis of the experimental data for several kinds of epitaxial nanostructures. [ABSTRACT FROM AUTHOR]

Contactless electroreflectance (CER) supported by photoluminescence (PL) has been applied to study (i) the surface band bending, (ii) the band gap bowing, and (iii) the Stokes shift for InGaN layers grown by molecular beam epitaxy with 0.14≤In≤0.36. The type of surface band bending has been investigated on the basis of the shape of CER resonance. It has been found that the surface band bending changes from n-type for layers with low indium content (In<27%) to flatband (or weak p-type band) for layers with In∼35%. The band gap bowing has been determined to be 1.4±0.2 and 2.1±0.3 eV for CER data with and without strain corrections, respectively. From this analysis it has been concluded that the reliable value of the bowing parameter for unstrained InGaN should be between 1.4 and 2.1 eV. Comparing CER with PL data it has been found that the Stokes shift rises from 20 to 120 meV when the indium concentration increased from 14% to 36%. In addition, it has been observed that the intensity of PL from InGaN layers decreased exponentially with the increase in the indium content. The last two findings are attributed to an easier formation of native point defects and stronger indium segregation in InGaN alloys with higher indium concentrations. [ABSTRACT FROM AUTHOR]

The temperature dependencies of optical transitions in as-grown Ga0.64In0.36N0.046As0.954 multiple quantum wells (QWs) of various widths (ranging from 3.9 to 8.1 nm) grown at the low temperature of 375 °C were studied by photoreflectance (PR). In addition to the 11H transition, which is the fundamental transition for this QW, the optical transitions between excited states (22H and 33H transitions, where the notation klH denotes the transition between the kth heavy-hole valence subband and the lth conduction subband) were clearly observed in the PR spectra. The temperature dependencies of the QW transition energies were analyzed using Varshni and Bose–Einstein expressions. It was found that with increasing temperature, both the ground-state and the excited-state transitions shift to the red without showing S-shape-type behaviors, which are typical for photoluminescence from this system. This shift does not depend on the QW width and amounts to ∼80 meV for the 11H transition and ∼100 meV for the 22H and 33H transitions in the temperature range of 10–300 K. These shifts are comparable to those of N-free QWs. [ABSTRACT FROM AUTHOR]

The carrier localization phenomenon has been investigated for GaBiAs by photomodulated transmittance (PT) and photoluminescence (PL). In the case of PT measurements, a decrease in the energy-gap related PT signal has been clearly observed below 180 K. In PL spectra a broad emission band very sensitive to the excitation power has been found. In comparison to the energy-gap related transition, this band is shifted to red. The recombination time for this band at low temperature decreases from 0.7 to 0.35 ns with the increase in the emission energy. All the findings are clear evidences for strong carrier localization in this alloy. [ABSTRACT FROM AUTHOR]

Photoreflectance has been applied to study the exciton energies and linewidths for hetero- and homoepitaxial GaN layers (i.e., GaN layers grown on sapphire and truly bulk GaN crystal obtained by ammonothermal method). In order to modulate the built-in electric field inside the samples and eliminate photoluminescence signal from photoreflectance spectra, the surface band bending was modulated by the laser light with the photon energy smaller than the energy gap of GaN, i.e., a 532 nm laser line. The Varshni-type and Bose–Einstein-type parameters that describe the temperature dependence of the exciton transition energies have been evaluated and compared for the two GaN epilayers. It has been concluded that the residual strain in the heteroepitaxial layer influences the exciton transition energy but does not influence the Varshni and Bose–Einstein parameters. It has been found that this strain significantly influences the exciton linewidth. The broadening parameter, which is associated with the temperature-independent mechanisms (Γ0 parameter corresponding to the exciton linewidth at 0 K), has been found to be ∼1 meV for the homoepitaxial layer. For the heteroepitaxial layer this parameter is twice higher (this work) or few times higher (previous papers). [ABSTRACT FROM AUTHOR]

Here, an optical study is presented on cuboidal single pillar microresonators with a quantum-dot-like emitter formed from local fluctuations in the InGaAs/GaAs quantum well potential. By means of microphotoluminescence, emission spectra as a function of the excitation power density were recorded. In the low excitation spectra a number of sharp lines corresponding to the single quantum dots photoluminescence was observed. With increasing excitation power the spectra become dominated by the microcavity modes, on the background of which an intensive and narrow line appears when a certain threshold excitation power is exceeded. A threshold power corresponding to the onset of the superlinear emission intensity power dependence was determined, which is accompanied by a strong decrease in the emission mode linewidth, where both are the distinctive features of the lasinglike behavior. The threshold power density and the exponent of the superlinear part of the input-output characteristic increase with the pillar lateral size (d) and the quality factor (Q), however, they both decrease when plotted as a function of Q/d2, which is the actual figure of merit of the spontaneous emission coupling factor (β). It shows the dominant influence of the volume change effect over the cavity Q (finesse). Thus, larger β values are assigned to the smaller micropillars, in spite of their lower Q values. A quantum dot character of the lasing has been confirmed in a temperature dependent experiment, which showed a number of emission intensity oscillations instead of the expected monotonic decay with the temperature increase, which is a fingerprint of the spectral tuning of the quantum dot emission spectrum through the optical cavity mode. [ABSTRACT FROM AUTHOR]

Modified van Hoof structures containing GaInNAs(Sb) quantum wells (QWs) with indium content varying from 8% to 32% have been investigated using contactless electroreflectance (CER) spectroscopy. In CER spectra, both the QW transitions and GaAs-related Franz–Keldysh oscillations (FKOs) have been clearly observed. The band gap discontinuity at the GaInNAs(Sb)/GaAs interface has been determined by analyzing the QW transitions. The built-in electric field in the GaAs cap layer has been extracted from the FKO periodicity. The Fermi level position in the GaInNAs(Sb) QW has been determined through knowledge of the electric field in the GaAs cap layer and band gap discontinuity in the GaInNAs(Sb)/GaAs QW. It has been found that the Fermi level is pinned for all samples at the same energy, ∼4.7 eV below the vacuum level. The Fermi level is located very close to the Fermi level stabilization energy, ∼4.9 eV below the vacuum level. A high concentration of native point defects in the as-grown material is the reason for the Fermi level pinning at this energy. [ABSTRACT FROM AUTHOR]

Photoreflectance (PR) and photoluminescence (PL) have been applied to study the band structure and PL thermal quenching for Ga0.24In0.76Sb/GaSb quantum wells (QWs) of the widths varying from 10 to 21 nm. In the case of PR spectra, a strong GaSb-related resonance followed by Franz–Keldysh oscillations and PR resonances associated with optical transitions between the QW ground and the excited states have been clearly observed. The QW transitions have been identified on the basis of theoretical calculations which were performed in the framework of the effective mass model. Satisfactory agreement between theoretical calculations and experimental data has been found for the conduction band offset of ∼80%–85% that is consistent with the theoretical predictions, which were obtained within the “model-solid” theory. In the case of PL measurements, a very efficient QW emission without any localization features has been observed in the temperature range of 10–280 K. It has been concluded that the thermal quenching of PL can be described by a standard expression with two characteristic activation energies, which can be attributed to an electron escape and a heavy-hole escape to the conduction and valence continua in GaSb barriers, respectively. The obtained activation energies are in the range of 20–30 and 80–100 meV for the first and the second quenching mechanism, respectively. The Varshni parameters for the QW ground state transition have been determined to be α=0.36–0.39 meV/K and β=155–175 K. [ABSTRACT FROM AUTHOR]

A fruitful approach to study the Fermi level position in GaInNAs/GaAs quantum wells (QWs) has been proposed in this paper. This approach utilizes contactless electroreflectance (CER) spectroscopy and a very simple design of semiconductor structures. The idea of this design is to insert a GaInNAs quantum well (QW) into a region of undoped GaAs layer grown on n-type GaAs substrate. The possible pinning of the Fermi level in the GaInNAs QW region modifies band bending in this system. In CER spectra both QW transitions and GaAs-related Franz-Keldysh oscillations (FKOs) are clearly observed. The analysis of QW transitions allows one to determine the band gap discontinuity at GaInNAs/GaAs interface whereas the analysis of FKOs allows one to determine the built-in electric field in the GaAs cap layer, and, finally, one is able to find the Fermi level pinning in GaInNAs QW region. [ABSTRACT FROM AUTHOR]

Silicon nanocrystals embedded in a silicon-rich silicon-oxide matrix have been fabricated at different silicon contents (38%, 40%, and 49%) using plasma-enhanced chemical vapor deposition and annealing at different temperatures in the range from 900 °C to 1100 °C. Their optical properties have been investigated by photoluminescence and transmittance measurements. Strong, room-temperature emission bands at ∼1.6 eV have been observed for all samples, with intensities dependent on the annealing temperature and Si content of the samples. From transmittance measurements, a redshift of the absorption edge has been detected when increasing the annealing temperature or Si content. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) from InAsN quantum dots embedded in a GaInNAs/GaAs quantum well (QW) has been investigated at low excitation conditions in the temperature range of 15–305 K. A very efficient emission at 1.3 μm with a small spectral broadening (30 meV) has been observed at room temperature for this system. The emission intensity decreases by only two decades in the whole investigated temperature range. Carrier escape into the states of the surrounding QW has been recognized as the main PL thermal quenching mechanism and the possible quenching via defect states has appeared to be negligible in this regime of excitation. [ABSTRACT FROM AUTHOR]

Photoreflectance (PR) modulation spectroscopy, supported by photoluminescence (PL) and atomic force microscopy, was applied to the study of the optical properties of InAs/GaAs structures at the transition from the typical two-dimensional epitaxial growth to three-dimensional Stranski-Krastanov mode of InAs self-assembled quantum dot (QD) formation. Room temperature photoreflectance was measured on several molecular-beam epitaxy (MBE) grown structures, with growth conditions optimized for the 1.3 μm emission (an important window for telecommunication applications), differing in the nominal thickness of InAs layer from 1 to 2.5 ML (monolayer). The evolution of optical transitions connected with energy levels confined in a very thin InAs/GaAs quantum well was observed. For a small InAs nominal layer thickness (up to the critical thickness for the formation of three-dimensional islands), the heavy (light)-hole level to electron level transitions shift towards lower energy, indicating the increase in the quantum well width. When the nominal InAs layer thickness exceeds the critical value, the transition energies remain constant. It implies the formation of the so-called wetting layer, whose thickness is independent of the amount of deposited InAs material (fully driven by the strain). Its energy level structure was calculated (exploiting the effective mass approximation, with strain effects) in order to determine the actual wetting layer thickness, which was found to be approximately 1.6 ML. The features connected with the transitions between levels confined in QDs appear in PR and PL spectra for the amount of InAs material exceeding this number. The energies of the QD transitions shift to the red when the InAs layer nominal thickness is increased from 1.7 to 2 ML (indicating the increase in the average dot sizes) but remain constant for thicker layers, which is the evidence that the additional InAs material increases the density of QDs, but not their sizes. It points out at the existence of size limitation effect in the MBE growth of self-assembled QDs. [ABSTRACT FROM AUTHOR]

The authors present a modulated reflectivity study of the wetting layer (WL) states in molecular beam epitaxy grown InAs/GaAs quantum dot (QD) structures designed to emit light in the 1.3–1.5 μm range. A high sensitivity of the technique has allowed the observation of all optical transitions in the QD system, including low oscillator strength transitions related to QD ground and excited states, and the ones connected with the WL quantum well (QW). The support of WL content profiles, determined by transmission electron microscopy, has made it possible to analyze in detail the real WL QW confinement potential which was then used for calculating the optical transition energies. We could conclude that in spite of a very effective WL QW intermixing, mainly due to the Ga–In exchange process (causing the reduction of the maximum indium content in the WL layer to about 35% from nominally deposited InAs), the transition energies remain almost unaffected. The latter effect could be explained in effective mass envelope function calculations taking into account the intermixing of the QW interfaces described within the diffusion model. We have followed the WL-related transitions of two closely spaced QD layers grown at different temperatures, as a function of the In content in the capping layer. We have shown that changing the capping layer from pure GaAs to In0.236Ga0.764As has no significant influence on the composition profile of the WL itself and the WL QW transitions can be usually interpreted properly when based on the cap-induced modification of the confinement potential within a squarelike QW shape approximation. However, some of the observed features could be explained only after taking into consideration the effects of intermixing and InGaAs cap layer decomposition. [ABSTRACT FROM AUTHOR]

Interband transitions in GaInNAsSb/GaAs single quantum wells (SQWs) with nominally identical nitrogen and antimony concentrations (2.5% N and 7% Sb) and varying indium concentrations (from 8% to 32%) have been investigated by contactless electroreflectance (CER). CER features related to optical transitions between the ground and excited states have been clearly observed. Energies of the QW transitions extracted from CER measurements have been matched with those obtained from theoretical calculations performed within the effective mass approximation for various conduction-band offsets (QC) and various electron effective masses. It has been found that the QC increases from 40% to 80% with the rise of the indium content from 8% to 32% and the electron effective mass is close to 0.09m0. The results show that the band gap discontinuity in GaInNAsSb/GaAs SQWs can be broadly tuned with a change in the indium concentration. [ABSTRACT FROM AUTHOR]

Contactless electroreflectance (CER) spectroscopy has been applied to study optical transitions in InAs/In0.53Ga0.23Al0.24As quantum dashes (QDashes) grown on an InP substrate by molecular beam epitaxy. CER resonances related to optical transitions in all relevant parts of the structure, i.e., InAs coverage, In0.53Ga0.23Al0.24As barriers, and the cap layer, have been clearly observed at room temperature. The signal, which is associated with light absorption in the InAs coverage, has been carefully analyzed, and the optical transitions in the wetting layer (WL) quantum well (QW) and QDashes have been identified in CER spectra. It has been shown that measurements of WL transitions and analysis of their energies allow us to determine the band gap discontinuity for the QDash/QDash-barrier interface. It has been found that the conduction band offset for the InAs/In0.53Ga0.23Al0.24As interface is close to ∼70%. Moreover, it has been observed that the intensity of the WL transition varies with the cap layer which was used to terminate the QDash structure. The conditions for the observation of WL transitions are discussed in this work. [ABSTRACT FROM AUTHOR]

We have investigated a set of InxGa1-xAs/GaAs quantum dot structures grown by solid source molecular beam epitaxy for a wide range of In content ranging from 30% to pure InAs/GaAs dots. It is well known that in a self-assembled growth mode, the dots are formed on a thin layer of the InxGa1-xAs material, which is called wetting layer (WL). The WL thickness is driven by the strain, i.e., lattice mismatch between the layer and substrate materials. Usually, the WL quantum well is not optically active in emission type of experiments (the whole radiative recombination goes through the dot states) and even if so, the heavy hole ground state transition is probed only. In order to detect all the possible transitions, e.g., transitions related to light hole and possible higher order heavy hole states (including those transitions, which are nominally parity forbidden), we have used modulation spectroscopy in a form of photomodulated reflectivity measurements. This is an absorptionlike method, which has been proven to be highly sensitive to even very low intensity transitions in low-dimensional semiconductor structures. The aim of this work is to determine in a contactless optical manner the thickness of the wetting layer as a function of indium content starting with very low content and low-strain structures (at the limit of self-assembled dot creation) up to typical InAs dots on gallium arsenide with the effective wetting layer as thin as 1.5 ML. The observed optical transitions have been identified based on energy level calculations for thin rectangular wells and using effective mass approximation in an envelope function approach in which the well width has been treated as a fitting parameter to the experimental data. [ABSTRACT FROM AUTHOR]

We have used photoreflectance (PR) and high-resolution x-ray diffraction (HRXRD) measurements to assess the composition of InxGa1-xAs1-yNy thin films (x∼20%, y∼3%) grown by molecular beam epitaxy on GaAs substrates with different surface orientations. The aim of our work is to investigate the effect of substrate orientation on the incorporation of N and In into the films. Whereas in principle the composition of the InxGa1-xAs1-yNy films cannot be characterized by HRXRD alone because lattice spacings depend on both x and y, we show that a combined analysis of the PR and HRXRD data allows us to determine the sample composition. Our data analysis suggests that the incorporation of N in (In, Ga)(As, N) exhibits some dependence on substrate orientation, although not as strong as previously observed in Ga(As, N). We determine shear deformation potentials for our samples that are sizably different than those obtained by linearly interpolating from the values of the pure binary compounds, which has already been observed in Ga(As, N) and contradicts the currently accepted idea that only the conduction band of dilute nitrides is perturbed by N. [ABSTRACT FROM AUTHOR]

We present the investigation of the modulation mechanisms in photoreflectance (PR) spectroscopy of an ensemble of self-assembled semiconductor quantum dots (QDs). In order to distinguish between possible factors contributing to the total modulation efficiency of QD transitions, a photoreflectance excitation experiment has been performed on an InAs/GaAs quantum dot structure grown by solid-source molecular beam epitaxy. It has been observed that the intensity of PR features related to QDs changes in a function of the wavelength of the pumping laser, tuned from above-GaAs band gap down to below wetting layer ground state transition. Based on this dependence we have shown that most of the QD PR signal intensity originates from the modulation of the built-in electric field caused by carriers photogenerated in GaAs layers. We also conclude that the modulation of QD transitions related to a possible modification of the dot properties due to filling them with carriers is negligible in PR experiment on an ensemble of dots. An additional confirmation of the PR results has been obtained by using contactless electroreflectance (CER), demonstrating that the line shape of PR and CER QD resonances is almost identical in both spectra. Thus, the QD transitions can be analyzed by using the standard low field line shape functional form applicable in any electromodulation spectroscopy. [ABSTRACT FROM AUTHOR]

Photoreflectance (PR) and contactless electroreflectance (CER) spectroscopies have been applied to study optical transitions in undoped and Si-doped AlGaN/GaN heterostructures at room temperature. Spectral features related to excitonic and band-to-band absorptions in GaN layer and band-to-band absorption in AlGaN layer have been resolved and analyzed. In addition, a broad spectral feature related to two-dimensional electron gas has been observed for the Si-doped heterostructure. It has been found that some of the mentioned optical transitions are not observed in CER spectra whereas they are very strong in PR spectra. This phenomenon is associated with different mechanisms of the modulation of built-in electric field in the investigated structure. A combination of PR and CER gives the possibility of a richer interpretation of both PR and CER spectra. [ABSTRACT FROM AUTHOR]

InxGa1-xAs/GaAs quantum dots (QDs) were grown by solid source molecular beam epitaxy for indium contents of around 30%, which assures the QD growth in the very low strain limit. The structures were fabricated for a constant nominal InxGa1-xAs layer thickness but varying content (strain) from below to far above the critical thickness conditions, which has allowed to detect the onset of three-dimensional island formation and their evolution with the increasing material amount (for higher In contents the critical thickness for island formation is smaller and hence a larger fraction of the InxGa1-xAs layer is spent on dot formation). In order to investigate the properties of such an uncommon QD system, photoreflectance and photoluminescence, combined with scanning electron microscopy, have been used. Optical transitions connected with the ternary layer have been observed and followed from the lowest content quantum well case through the transformation into three-dimensional islands on the wetting layer (WL) and a coexistence of the QD-related and WL-related transitions. Due to the observation of both heavy hole and light hole related transitions in photoreflectance spectra, the thickness of the wetting layer versus changed indium content could be determined, comparing the experimental data with the results of the effective mass envelope function calculations. [ABSTRACT FROM AUTHOR]

We have investigated the effects of interdiffusion and its technological parameters on the subband structure in compressively strained InGaAsP quantum wells (QWs) using photoreflectance and photoluminescence techniques. p-i-n laser structures with three QWs were grown by gas source molecular beam epitaxy and capped with dielectric films deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition and annealed using a rapid thermal annealing process. A numerical real-time wave-packet propagation method including static electric field, strain in the wells and barriers, and error function interface diffusion modeling is used to calculate the transition energies for the diffused QWs. It has been shown that the shift of the energy levels due to the interdiffusion related changes of the well confinement potential profile is a consequence of two competing processes: a change of the well width and an effective increase of the band gap energy resulting in a net blueshift of all optical transitions. Moreover, it has been found that quantum well intermixing does not significantly influence the built-in electric fields distribution. [ABSTRACT FROM AUTHOR]

We have investigated characteristics of photoluminescence (PL) spectra obtained from as-grown and annealed GaN0.027As0.863Sb0.11/GaAs single quantum wells (SQWs). For the as-grown SQW at low temperature (<150 K), a broadband emission at ∼0.9 eV has been observed in addition to the band-gap-related recombination at ∼0.95 eV. After annealing, this broad emission disappears and the band-gap-related PL peak blueshifts ∼55 meV. The Stokes shift for this peak is 11 meV at 10 K. Thus, the low-temperature PL peak is mainly associated with the recombination of localized excitons. A rise in temperature leads to a continuous change in this peak; the contribution associated with localized excitons decreases while the contribution associated with free-excitons increases. Above 150 K the line shape gradually loses the Gaussian profile and a high-energy tail appears due to a thermal band filling effect. In this temperature range (>150 K), the PL peak is attributed to free-carrier recombination. [ABSTRACT FROM AUTHOR]

The energy level structure of a step-like GaInNAsSb/GaNAs/GaAs quantum well (QW) has been investigated by photoreflectance (PR) spectroscopy and was analyzed by theoretical calculations. In the active region of this structure, i.e., GaInNAsSb/GaNAs QW, we have observed PR resonances related higher order QW transitions in addition to the ground state transition. Based on calculations from experimental data, we have found that the electron effective mass in the active QW is 0.12m0 and the conduction band offset for GaInNAsSb/GaAs interface is about 0.85. The emission observed from this structure at 10 K has a small Stokes shift (i.e., 6 meV and <2 meV for as-grown and annealed structures, respectively) and is without the exponential-like tail at the low-energy side. Hence, we conclude that the incorporation of Sb atoms into GaInNAs alloy helps to achieve QW structures which emit light at longer wavelength and have quite narrow emission line and small Stokes shift. [ABSTRACT FROM AUTHOR]

Dilute Ga1-xInxNyAs1-y/GaAs quantum wells with high In-content, which are under compressive strain, have been shown previously to exhibit multiple band gaps, likely due to the presence of different nitrogen nearest-neighbor environments, i.e., N-Ga4-mInm (0<=m<=4) short-range-order clusters. Here, photoreflectance (PR) measurements on lattice-matched dilute GaInNAs-on-GaAs layers with low indium and nitrogen content are reported, which give evidence that these layers also exhibit several distinct band gaps. These distinct band gaps, which were found to coexist, are associated with different nitrogen bonding configurations, as revealed by Raman spectroscopy. Thus, the metastable nature of GaInNAs seems to be a persistent intrinsic property, irrespective of strain and indium content. The annealing-induced blueshift of GaInNAs band gap energy, which is usually observed in this system, has been associated with the change in the intensity of PR resonances related to different N-Ga4-mInm configurations. [ABSTRACT FROM AUTHOR]

We report photoreflectance investigations of the energy-fine structure of GaInNAs/GaAs multiple quantum wells (MQWs) grown at different temperatures and postgrown treated by rapid thermal annealing (RTA). A “splitting” of the ground and excited QW transitious due to the presence of different nitrogen nearest-neighbor environments, i.e., N-Ga4-mInm(0<=m<=4) short-range-order clusters, has been observed. The RTA induces a nitrogen redistribution between the five possible N-Ga4-mInm configurations and thus leads to a blueshift of QW transitions. The magnitude of the blueshift and its dependence on the growth temperature and annealing temperature are investigated in this paper. [ABSTRACT FROM AUTHOR]