Study of structure and optical properties of GaAs nanocrystalline thin films
TL;DR: In this article, thin films of GaAs were electrochemically prepared from acidic solutions of pure metallic Ga and As 2 O 3, and structural characterization of the nanoparticles were carried out by XRD technique which exhibits partial amorphization of the crystallites in the low electrolysis current regime.
Abstract: Nanoparticulate thin films of GaAs we electrochemically prepared from acidic solutions of pure metallic Ga and As 2 O 3 . Samples of different crystallite sizes were prepared by varying the electrolysis parameters. Structural characterization of the nanoparticles were carried out by XRD technique which exhibits partial amorphization of the crystallites in the low electrolysis current regime. Quantum confinement effect was prominently observed in the optical absorption spectra with blue-shift of absorption onsets with respect to the bulk band gap. Room temperature photoluminescence exhibit band edge luminescence as well as other surface related bands. Incorporation of transition element as impurity leads to enhanced luminescence intensity and generates deep traps.
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TL;DR: In this paper, the homogeneous and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35-50? diameter CdSe clusters are separated using transient photophysical hole burning.
Abstract: The homogeneous (single-cluster) and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35-50 ? diameter CdSe clusters are separated using transient photophysical hole burning. The clusters have the cubic bulk crystal structure, but their electronic states are strongly quantum confined. The inhomogeneous broadening of these features arises because the spectrum depends upon cluster size and shape, and the samples contain similar, but not identical, clusters. The homogeneous spectrum, which consists of a peak 140 cm -1 (17 meV) wide, with a phonon sideband and continuum absorption to higher energy, is compared to a simple molecular orbital model. Electron-vibration coupling, which is enhanced in small clusters, contributes to the substantial broadening of the homogeneous spectrum. The inhomogeneous width of the lowest allowed optical transition was found to be 940 cm -1 , or seven times the homogeneous width, in the most monodisperse sample.
266 citations
TL;DR: In this article, the effect of a transverse field on a cylindrical core/shell spin-1 Ising nanowire, within the effective field theory based on a probability distribution technique, is investigated.
Abstract: We investigate the effect of a transverse field on a cylindrical core/shell spin-1 Ising nanowire, within the effective-field theory based on a probability distribution technique, in order to clarify how the relevant thermodynamic quantities such a magnetizations, hysteresis loops, compensation behaviors, are influenced by a transverse field. From these studies, following interesting phenomena are found. (i) Beside a singly hysteresis loop, double, triple or even quadruple hysteresis loops occurs in the system. (ii) The P and N types of compensation behavior are obtained in addition to the Q-, R- and S-types. We also compare our results with some experimental and theoretical results and find in a qualitatively good agreement.
84 citations
TL;DR: In this paper, a two-step growth scheme was proposed to obtain a superior quality GaAs on silicon dioxide patterned Si(111) substrates by molecular beam epitaxy.
Abstract: High-quality and defect-free GaAs were successfully grown via molecular beam epitaxy on silicon dioxide patterned Si(111) substrates by a two-step growth technique. Compared with the one-step approach, the two-step growth scheme has been found to be a better pathway to obtain a superior-quality GaAs on Si. Taking advantages of low energy for both Si(111) surface and GaAs/Si(111) interface, the two-step grown GaAs of total ∼175 nm atop patterned Si(111) substrates exhibits atomically smooth surface morphology, single crystallininty and a remarkably low defect density. A low-temperature GaAs nucleation layer of the two-step growth helps relieve the misfit stress by accommodating the misfit dislocations at the very adjacent GaAs/Si interface. The excellent properties of the two-step grown GaAs were investigated and verified by field-emission scanning electron microscopy, atomic force microscopy, X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Finally we demonstrated a GaAs on Si ...
29 citations
TL;DR: In this paper, the influence of gamma irradiation on gallium arsenide based photovoltaic cells was investigated, where a synthetic radioactive isotope of cobalt Co-60 was used with an applied dose of up to 500kGy.
Abstract: The influence of gamma irradiation on gallium arsenide based photovoltaic cells was investigated. This type of solar cell is used for advanced space application where radiation degradation occurs in particular. A synthetic radioactive isotope of cobalt Co-60 was used with an applied dose of up to 500 kGy. These irradiation cause damage and degradation of the solar cell. A wide range of comparative characterisation methods was performed before and after irradiation. The effect of radiation on material morphology was described using electron and atomic force microscopy. Structural changes were investigated by Secondary Ion Mass Spectrometry (SIMS) with Time-of-Flight mass analysis (TOF) and Raman spectroscopy analysis. Also, the changes in noise fluctuations and current-voltage characteristics of the cell in the dark and under illumination were measured. Based on experimental measurements, the degradation was observed in the structural, optical and material cell properties as well as in electrical parameters.
29 citations
TL;DR: In this paper, the authors focused on the study of structure stability and electrical parameters of photovoltaic cells based on GaAs with Ge substrate, which are used especially in adverse environments such as space applications, so their working parameters should be stable even under extreme operating conditions.
Abstract: The work focuses on the study of structure stability and electrical parameters of photovoltaic cells based on GaAs with Ge substrate. Solar cells of this type are used especially in adverse environments such as space applications, so their working parameters should be stable even under extreme operating conditions. Changes of electrical characteristics of the cells were recorded in the form of noise measurements for examination of distinctions in the pn-junction. Current-voltage characteristics under the light illumination and in the dark environment for comparison of the cells performance were also measured. Infrared camera showed the thermal irradiation of the stressed and damaged parts and support to localize the defected areas. Atomic force microscope (AFM) was applied for observation of changes in three-dimensional topography with high resolution. Scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDS) showed morphology of the solar cells and provided the elemental analysis of the samples. Raman spectroscopy provided a structural fingerprint and helped to evaluate the influence of induced degradation methods. Variations of morphology and composition were compared, detected and well-observed. Furthermore, electrical measurements proved the solar cells to be stable under temperature stresses.
23 citations
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TL;DR: In this paper, the excited electronic states of semiconductor crystallites sufficiently small (∼50 A diam) that the electronic properties differ from those of bulk materials were modeled, and an approximate formula was given for the lowest excited electronic state energy.
Abstract: We model, in an elementary way, the excited electronic states of semiconductor crystallites sufficiently small (∼50 A diam) that the electronic properties differ from those of bulk materials. In this limit the excited states and ionization processes assume a molecular‐like character. However, diffraction of bonding electrons by the periodic lattice potential remains of paramount importance in the crystallite electronic structure. Schrodinger’s equation is solved at the same level of approximation as used in the analysis of bulk crystalline electron‐hole states (Wannier excitons). Kinetic energy is treated by the effective mass approximation, and the potential energy is due to high frequency dielectric solvation by atomic core electrons. An approximate formula is given for the lowest excited electronic state energy. This expression is dependent upon bulk electronic properties, and contains no adjustable parameters. The optical f number for absorption and emission is also considered. The same model is applied to the problem of two conduction band electrons in a small crystallite, in order to understand how the redox potential of excess electrons depends upon crystallite size.
4,322 citations
TL;DR: In this paper, the present status and new opportunities for research in this area of materials physical chemistry are reviewed, as well as a review of the present state and opportunities in this field.
Abstract: Semiconductor nanocrystals exhibit a wide range of size-dependent properties. Variations in fundamental characteristics ranging from phase transitions to electrical conductivity can be induced by controlling the size of the crystals. The present status and new opportunities for research in this area of materials physical chemistry are reviewed.
3,493 citations
TL;DR: Luminescent measurements show that the efficiency increases with decreasing size of the particles, as expected within the framework of an electron-hole localization theory, suggesting that doped nanocrystals are indeed a new class of materials heretofore unknown.
Abstract: We report for the first time that doped nanocrystals of semiconductor can yield both high luminescent efficiencies and lifetime shortening at the same time. Nanocrystals of Mn-doped ZnS with sizes varying from 3.5 to 7.5 nm were prepared by a room temperature chemical process. These nanosized particles have an external photoluminescent quantum efficiency as high as 18% at room temperature and a luminescent decay at least 5 orders of magnitude faster than the corresponding ${\mathrm{Mn}}^{2+}$ radiative transition in the bulk crystals. Luminescent measurements show that the efficiency increases with decreasing size of the particles, as expected within the framework of an electron-hole localization theory. These results suggest that doped nanocrystals are indeed a new class of materials heretofore unknown.
1,855 citations
TL;DR: In this article, the authors developed two theoretical models, including the effect of band nonparabolicity, that successfully explain the observed size dependence down to about 25 A. The electron-hole-in-a-box model with effective mass approximation cannot explain the observation size dependence.
Abstract: The transition of PbS from molecular to bulk form has been observed in polymer films. As the particle size decreases the band gap shifts to the blue and eventually approaches the transition energy of the first allowed excited state, X→A, of a PbS molecule. Discrete absorption bands also appear. The electron‐hole‐in‐a‐box model with effective mass approximation cannot explain the observed size dependence. We have developed two theoretical models, both including the effect of band nonparabolicity, that successfully explain the observed size dependence down to about 25 A.
787 citations
TL;DR: In this article, the homogeneous and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35-50 A diameter CdSe clusters are separated using transient photophysical hole burning.
Abstract: The homogeneous (single‐cluster) and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35–50 A diameter CdSe clusters are separated using transient photophysical hole burning. The clusters have the cubic bulk crystal structure, but their electronic states are strongly quantum confined. The inhomogeneous broadening of these features arises because the spectrum depends upon cluster size and shape, and the samples contain similar, but not identical, clusters. The homogeneous spectrum, which consists of a peak 140 cm−1 (17 meV) wide, with a phonon sideband and continuum absorption to higher energy, is compared to a simple molecular orbital model. Electron–vibration coupling, which is enhanced in small clusters, contributes to the substantial broadening of the homogeneous spectrum. The inhomogeneous width of the lowest allowed optical transition was found to be 940 cm−1, or seven times the homogeneous width, in the most monodisperse sample.
303 citations