Papers by Veselin Todorov Donchev
Physical Review B, 2003
Microphotoluminescence ͑PL͒ spectra of a single InAs/GaAs self-assembled quantum dot ͑QD͒ are stu... more Microphotoluminescence ͑PL͒ spectra of a single InAs/GaAs self-assembled quantum dot ͑QD͒ are studied under the main excitation of electron-hole pairs in the wetting layer ͑WL͒ and an additional infrared ͑IR͒ laser illumination. It is demonstrated that the IR laser with fixed photon energy well below the QD ground state induces striking changes in the spectra for a range of excitation energies and powers of the two lasers. For the main excitation above a threshold energy, defined as the onset of transitions between shallow acceptors and the conduction band in GaAs, the addition of the IR laser will induce a considerable increase in the QD emission intensity. This is explained in terms of additional generation of extra electrons and holes into the QD by the two lasers. For excitation below the threshold energy, the carrier capture efficiency from the WL into the QD is suggested to be essentially determined by the internal electric-field-driven carrier transport in the plane of the WL. The extra holes, generated in the GaAs by the IR laser, are supposed to effectively screen the built-in field, which results in a considerable reduction of the carrier collection efficiency into the QD and, consequently, a decrease of the QD PL intensity. A model is presented which allows estimating the magnitude of the built-in field as well as the dependence of the observed increase of the QD PL intensity on the powers of the two lasers. The use of an additional IR laser is considered to be helpful to effectively manipulate the emission efficiency of the quantum dot, which could be used in practice in quantum-dot-based optical switches.
Thin Solid Films, 2012
The infra-red (IR) transmission spectra of SiO x (x ≤ 2) layers containing crystalline or amorpho... more The infra-red (IR) transmission spectra of SiO x (x ≤ 2) layers containing crystalline or amorphous Si nanoparticles deposited on p-Si substrates is simulated in the range 300-1500 cm − 1. To that purpose the average dielectric function of the nanocomposites is calculated by means of the Bruggeman effective medium approximation. The IR spectra of the system (film and substrate) are computed. The results are compared with experimental IR spectra measured on Si-SiO x nanocomposite layers with identical composition, fabricated by thermal evaporation of SiO in vacuum followed by thermal annealing at 700°C or 1030°C. A good correspondence between theory and experiment is found from where valuable information about important characteristics of the investigated nanocomposites is obtained, such as matrix density, homogeneity and composition of the layers.
Semiconductor Science and Technology, 2002
ABSTRACT We analyse in detail the effect of the electric field on the thermionic emission in GaAs... more ABSTRACT We analyse in detail the effect of the electric field on the thermionic emission in GaAs/AlGaAs multiple quantum wells. The activation energy is found to decrease with the field. In order to explain this behaviour a model including a Poole-Frenkel barrier lowering is developed which agrees well with the experimental results.
physica status solidi (c), 2007
We report room temperature surface photovoltage (SPV) spectroscopy studies of both as-grown and i... more We report room temperature surface photovoltage (SPV) spectroscopy studies of both as-grown and interdiffused InAs quantum dash (QD)-in-InAlGaAs quantum well (QW) structures grown by molecular beam epitaxy on (100) InP substrates. The interdiffusion is achieved by means of rapid thermal annealing at 800 ºC for 30 s. The SPV spectra reveal step-like structures related to the optical absorption in the QDs, QWs, InGaAlAs separate carrier confinement layer, InAlAs lower cladding layer and InP substrate. The annealing results in a high-energy shift of these spectral structures. The shift is attributed to the group-III atoms intermixing from the thermal induced interdiffusion across the heterointerfaces. The effect of interdiffusion is larger for the elements with larger surface-to-volume ratio (QDs and QWs). The blue shift of the QD transition is observed also in the photoluminescence spectra. The results contribute to the optimization of the technological procedure for QD bandgap tuning via the interdiffusion technique.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2004
We describe the effect of 1 MeV electron irradiation on the dark current in GaAs/AlGaAs multiple ... more We describe the effect of 1 MeV electron irradiation on the dark current in GaAs/AlGaAs multiple quantum wells. A linear increase of the activation energy and a linear decrease of the preexponential current term with fluence are observed. They are accounted for by irradiation introduced electron traps, which cause a negative space charge in the barriers.
Nanotechnology, 2011
We performed a detailed investigation of the structural and optical properties of multi-layers of... more We performed a detailed investigation of the structural and optical properties of multi-layers of InP/GaAs quantum dots, which present a type II interface arrangement. Transmission electronic microscopy analysis has revealed relatively large dots that coalesce forming so-called quantum posts when the GaAs layer between the InP layers is thin. We observed that the structural properties and morphology affect the resulting radiative lifetime of the carriers in our systems. The carrier lifetimes are relatively long, as expected for type II systems, as compared to those observed for single layer InP/GaAs quantum dots. The interface intermixing effect has been pointed out as a limiting factor for obtaining an effective spatial separation of electrons and holes in the case of single layer InP/GaAs quantum-dot samples. In the present case this effect seems to be less critical due to the particular carrier wavefunction distribution along the structures.
Nano Letters, 2005
Individual quantum dots have been studied by means of microphotoluminescence with dual-laser exci... more Individual quantum dots have been studied by means of microphotoluminescence with dual-laser excitation. The additional infrared laser influences the dot charge configuration and increases the dot luminescence intensity. This is explained in terms of separate generation of excess electrons and holes into the dot from the two lasers. With increasing dot density and/or sample temperature, the increase of the luminescence intensity vanishes progressively, while the possibility to control the dot charge remains.
Journal of Physics D: Applied Physics, 2009
... The application of the vector model to the present SPV amplitude and phase spectra has allowe... more ... The application of the vector model to the present SPV amplitude and phase spectra has allowed us to understand and explain their complicated and seemingly contradictory behaviour. This ... 3. Vector model for the SPV signal As ...
Journal of Physics: Conference Series, 2010
The optical properties of multi-layer InAs/InP quantum wires (QWRs) with two different spacer thi... more The optical properties of multi-layer InAs/InP quantum wires (QWRs) with two different spacer thicknesses have been investigated by means of room temperature surface photovoltage (SPV) and photoluminescence (PL) spectroscopies, combined with empirical tight binding electronic structure calculations and structural data. The SPV and PL spectra reveal several features, which energy positions are in good agreement. They have been ascribed to excitonic transitions, which take place in the QWR families with heights differing by an integer number of monolayers. Comparing the experimental results with the theoretical ones, we have estimated the QWR family heights and the average atomic concentration of phosphorus in the QWRs. From the simultaneous analysis of the SPV amplitude and phase spectra, based on our vector model for SPV signal representation, a deeper understanding of the SPV results and of the mechanisms of carrier separation in the sample is obtained.
Journal of Physics: Conference Series, 2008
Room temperature surface photovoltage (SPV) spectroscopy is used to study the interband optical t... more Room temperature surface photovoltage (SPV) spectroscopy is used to study the interband optical transitions and intermixing processes in InAs quantum-dash-in-InAlGaAs quantum-well structures grown on InP substrates. The intermixing is performed by nitrogen ion implantation followed by rapid thermal annealing at 700°C in nitrogen ambient. The effect of group-III intermixing to the interband optical transition energies in the structures is revealed by SPV spectroscopy and the results are confirmed by photoluminescence measurements. A differential bandgap blueshift as large as 93 meV (176 nm) is observed in the intermixed sample compared to the as grown one. The SPV investigation confirms that this intermixing technique is a powerful tool for achieving the required wavelength of 1.55 µm for telecommunication applications.
Journal of Physics: Conference Series, 2012
A detailed study of InAs/InGaAs quantum dots in quantum well (DWELL) structures grown on GaAs sub... more A detailed study of InAs/InGaAs quantum dots in quantum well (DWELL) structures grown on GaAs substrates for infrared photodetectors was performed using surface photovoltage (SPV) spectroscopy. Three types of samples were investigated: as-grown, and annealed with dielectric coating SiO 2 or SiN. The annealing resulted in intermixing of the material components. The amplitude and phase SPV spectra were measured at room temperature under various experimental conditions. The comparison of the SPV with the photoluminescence (PL) spectra allows one to conclude that the spectral features are due to optical transitions in the DWELL structure. The blueshift observed of these features in the intermixed samples implies that the energy levels responsible for the transitions change correspondingly due to the intermixing process. The interface band-bending in the samples and the mechanisms of the carrier dynamics were determined by a comparative analysis of the SPV amplitude and phase spectra, using our vector model for representation of the SPV signal.
Journal of Physics: Conference Series, 2010
The optical properties of InAs/InP multi-layer quantum wire (QWR) structures of various spacer th... more The optical properties of InAs/InP multi-layer quantum wire (QWR) structures of various spacer thicknesses have been investigated by means of room temperature surface photovoltage and photoluminescence spectroscopy. Combined with empirical tight binding calculations, the spectra have revealed transitions assigned to QWR families with heights equal to integer number of 5, 6 and 7 monolayers. From the comparison of the experimental and theoretical results the atomic concentration of phosphorus in the wires has been estimated.
Journal of Applied Physics, 2011
We present a low-temperature (73 K) study of the optical properties of multi-layer type II InP/ G... more We present a low-temperature (73 K) study of the optical properties of multi-layer type II InP/ GaAs self-assembled quantum dots by means of surface photovoltage (SPV) spectroscopy, taking advantage of its high sensitivity and contactless nature. The samples contain 10 periods of InP quantum dot planes separated by 5 nm GaAs spacers. The SPV amplitude spectra reveal two major broad peaks, situated at low and high energies, respectively. These features are analyzed taking into account the type II character of the structure, the quantum coupling effects, the spectral behavior of the SPV phase, and the photoluminescence spectra. As a result they have been attributed to optical transitions in the quantum dots and the wetting layers, respectively. The main mechanism for carrier separation in the SPV generation process is clarified via the analysis of the SPV phase spectra. The influence of the substrate absorption on the SPV spectra is discussed in details. V
Journal of Applied Physics, 2007
... Lett. 4, 461 (1987). JACavailles, DAMiller, JECunningham, PLKWa, and A.Miller, IEEE J. Quantu... more ... Lett. 4, 461 (1987). JACavailles, DAMiller, JECunningham, PLKWa, and A.Miller, IEEE J. Quantum Electron. 28, 2486 (1992). EE ... Phys. 91, 6476 (2002). FAX: +(359-2)9625-276; electronic mail: [email protected]. Typical ...
Applied Physics Letters, 2004
ABSTRACT We report a micro-photoluminescence study on the influence of single and multi-quantum d... more ABSTRACT We report a micro-photoluminescence study on the influence of single and multi-quantum dots (QDs) on the exposure by a low-energy laser, in addition to the principal exciting laser. At low temperatures, the presence of the low-energy laser effectively quenches the single QD luminescence. This can be explained in terms of an induced screening of a built-in electric field, which plays an important role as a carrier capture mechanism. The influence of the low-energy laser is successively decreasing when the capture efficiency is increased either by elevated crystal temperature or by increased QD densities, full consistent with the proposed model. © 2004 American Institute of Physics.
Applied Physics Letters, 2012
ABSTRACT Rolled-up semiconductor tubes of various diameters made of alternating In0.215Ga0.785As/... more ABSTRACT Rolled-up semiconductor tubes of various diameters made of alternating In0.215Ga0.785As/GaAs layers have been investigated by means of Raman scattering. The optical and acoustic phonon modes of individual tubes have been studied and compared with the characteristics of the surrounding material. After tube formation, the frequency of the phonon modes shifts with respect to the as-grown material and disorder activated modes are observed. The frequency shifts are related to the residual strain in the tubes through the deformation potential approximation. Good agreement with atomistic valence force field simulations and x-ray micro-diffraction measurements is found. By comparison with x-ray data, a Raman strain constant K = 0.65 is proposed for In0.215Ga0.785As. In the low frequency range, acoustic mode doublets are observed on the tubes that are absent in the surrounding material. They show clear evidence of the formation of periodic superlattices after the rolling-up process, and give insight into the quality of their interfaces.
Applied Physics B: Lasers and Optics, 1999
We have developed a new approach (the LQR method) for calculating the reflectivity and transmissi... more We have developed a new approach (the LQR method) for calculating the reflectivity and transmission spectra of a multilayer optical material with N interfaces, as an alternative to the matrix method. The approach allows the inclusion of the effects of interface rou~@ness by introducing a "rough" element between adjacent layers. For this purpose we have developed an empirical model, which describes the effect of interface roughness on an optical beam passing through or being reflected from an interface. An assessment of' the interface roughness of a multilayer structure was carried out by fitting the experimental reflectivity spectrum of GaAs/AIGaAs multiple quantum well samples with and without oxidation of the barrier layers. The refractive index and the thickness of the oxidised layers were also obtained horn the fit.
Journal of Applied Physics, 2007
We report the study of interband optical transitions in the interdiffused InAs quantum dash ͑QD͒ ... more We report the study of interband optical transitions in the interdiffused InAs quantum dash ͑QD͒ in InAlGaAs quantum well ͑QW͒ structures using room temperature surface photovoltage ͑SPV͒ spectroscopy. SPV signals have been detected from all relevant portions of both the as-grown and interdiffused structures including the QD, QW, and cladding layer. The effect of group-III intermixing on the interband optical transition energies in the interdiffused structures has also been revealed by the SPV spectroscopy, and the results have been confirmed by photoluminescence measurements. The SPV investigation shows that the compositional intermixing occurs not only between the dash and the surrounding well but also between the well and the surrounding barrier. The results demonstrate the potential of the SPV spectroscopy as a nondestructive, contactless method to characterize optical transitions in complex semiconductor nanostructures at room temperature.
An alternative way to obtain quantum wells ŽQWs. with equidistant energy levels is proposed. It c... more An alternative way to obtain quantum wells ŽQWs. with equidistant energy levels is proposed. It consists of interface grading of an
initially rectangular QW induced by material interdiffusion. The latter can be realised for example by post-annealing. The electronic states
in a large number of interdiffused GaAsrAlGaAs QWs are studied by varying the well width and diffusion length. The calculations are
made by means of the envelope function approximation. It is shown that a nearly equidistant energy spectrum can be obtained in diffused
QWs ŽDQWs. by choosing an appropriate value of the initial QW width and by adjusting the diffusion length. The annealing times and temperatures of the sample resulting in the needed diffusion lengths are calculated and reasonable values are obtained.
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Papers by Veselin Todorov Donchev
initially rectangular QW induced by material interdiffusion. The latter can be realised for example by post-annealing. The electronic states
in a large number of interdiffused GaAsrAlGaAs QWs are studied by varying the well width and diffusion length. The calculations are
made by means of the envelope function approximation. It is shown that a nearly equidistant energy spectrum can be obtained in diffused
QWs ŽDQWs. by choosing an appropriate value of the initial QW width and by adjusting the diffusion length. The annealing times and temperatures of the sample resulting in the needed diffusion lengths are calculated and reasonable values are obtained.
initially rectangular QW induced by material interdiffusion. The latter can be realised for example by post-annealing. The electronic states
in a large number of interdiffused GaAsrAlGaAs QWs are studied by varying the well width and diffusion length. The calculations are
made by means of the envelope function approximation. It is shown that a nearly equidistant energy spectrum can be obtained in diffused
QWs ŽDQWs. by choosing an appropriate value of the initial QW width and by adjusting the diffusion length. The annealing times and temperatures of the sample resulting in the needed diffusion lengths are calculated and reasonable values are obtained.