Semiconductor Microcrystals Research Articles

Optical measurements on semiconductor microcrystal beams generated by gas evaporation

We have developed a simple measurement method for measuring the optical spectra of free microcrystals, by combining the time-resolved and space-resolved spectroscopies with the gas evaporation method. The structure of the obtained spectra and their time- and space-dependencies indicate well the quantum-confinement effects of carriers and excitons in free semiconductors microcrystals.

Direct spectroscopic investigation of phase formation in glasses containing semiconductor microcrystals in correlation to their luminescence properties

AbstractThe formation of CdSe microcrystals in a glass matrix is studied by UV‐Vis in situ spectroscopy. Hereby the temperature range is found where nucleation takes place. The size distribution of the CdSe microcrystals depend on the growth rate, this means on growth temperature. The electronic properties which depend on the size of the CdSe quantum dots are investigated by photoluminescence and pump‐probe‐spectroscopy. The luminescence spectra give hints that more than one ‘family’ of CdSe microcrystals was grown during heat treatment.

OPTICAL SPECTRA OF FREE SEMICONDUCTOR MICROCRYSTALS PRODUCED BY THERMAL EVAPORATION IN NOBLE-GAS STREAM

A simple apparatus for measuring the optical spectra of free semiconductor microcrystals has developed. Well-directional microcrystal beams were produced and their extinction spectra show the quantum confinement of carriers or excitons in free semiconductor microcrystals. Experimental details and optical spectra of free and quasifree Ge, CuBr, ZnSe, and PbS microcrystals are presented.

Size quantization of the exciton in quasi-zero-dimensional structures: Theory

A theory of the exciton size quantization in a small semiconductor microcrystal is developed with regard to electron penetration from the microcrystal into the surrounding dielectric matrix. For the first time the effective mass of an exciton as a function of the parameters of the problem is determined from comparing the theory and experimental data.

Nanometric surface design of size‐quantized semiconductor microcrystals

Semiconductor microcrystals (Q‐particles) with diameters less that 100 Å, modified with viologen have recently been produced. One potential application for such particles would be for light energy conversion devices such as photocatalysts and photosenstizers in wet solar cells. The viologen moieties have the ability to function as electron mediators between the semiconductor particles and acceptors in solution. The preparation and potential applications of these ineresting materials are discussed.

特集 光機能材料 ｒｆ‐スパッタリング法による半導体微粒子ドープガラス薄膜の作製とその光学的性質

SiO2 glass thin films doped with semiconductor microcrystals such as CdSe, CdTe and GaAs were prepared by the rf-sputtering technique. The concentration and mean diameter of the microcrystals were dependent on the relative surface area of the semiconductor chips on SiO2 glass target plate, and the size distribution was strongly influenced by the sputtering condition. The suitable condition for each semiconductor has to be individually determined. The quantum size effect was found from the blue shift of the optical absorption edge. The shift for CdSe-doped SiO2 glass thin films clearly deviated from the simple theoretical predictions based on the confined effects. Thus, introduction of Coulomb interaction and the influence of the dielectric constant of matrix were found to be necessary to interpret the experimental data.

Persistent spectral hole burning in semiconductor nanocrystals.

A persistent spectral hole-burning phenomenon in nanometer-size semiconductor microcrystals (nanocrystals), namely, CdSe, ${\mathrm{CdS}}_{0.59}$${\mathrm{Se}}_{0.41}$, and CuCl, embedded in a crystal or glass was observed. When the spectrally narrow laser excites the inhomogeneously broadened absorption band of CdSe and CuCl nanocrystals, a narrow bleaching hole and induced absorption arise in the absorption spectra at 2 K and are present for more than a few hours after the laser irradiation. Long-lived bleaching was observed also in ${\mathrm{CdS}}_{0.59}$${\mathrm{Se}}_{0.41}$ nanocrystals together with photodarkening. A photophysical model explains the long-lived bleaching ascribed to the spectral antihole fairly well, but some part of the persistent spectral hole burning comes from the photochemical mechanism. Nanocrystals consisting of ${10}^{3}$--${10}^{4}$ atoms behave like molecules or ions in a matrix to give the persistent spectral hole-burning phenomenon. This means that the total system consisting of even one semiconductor nanocrystal and the matrix has more than one ground-state configuration. Not only the size distribution but also these ground-state configurations give inhomogeneous broadening to semiconductor nanocrystals.

Preparation and optical properties of semiconductor microcrystal-doped SiO 2 glass thin films by rf-sputtering

Semiconductor microcrystals were successfully doped in SiO 2 glass thin films by an rf-sputtering. Various types of semiconductor can be doped in the films and a quantum size effect can be seen in them. For the theoretical interpretation of the effect, the influence of the matrix and the electron transition process should be taken into account. χ (3) of the CdSe microcrystal-doped SiO 2 glass film is in the neighborhood of 10 −8 esu, comparable to that of the commercial bulk filter glasses.

Persistent spectral hole burning in semiconductor microcrystals

The persistent hole-burning phenomenon in nanometer-sized semiconductor microcrystals CdSe and CuCl was observed for the first time. The differential absorption spectra at 2 K consist of the narrow bleaching hole and the induced absorption which are conserved for more than 10 min after laser irradiation. The long-lived induced absorption is ascribed to the spectral antihole produced by a photophysical mechanism. It is clearly shown that the generally accepted model of inhomogeneous broadening of quantum dots cannot be used for explanation of the observed phenomenon.

Photothermal deflection and photoluminescence studies of CdS and CdSe quantum dots

We report on the observation of the quantum confinement effect by using photothermal deflection spectroscopy (PDS) experiment and the time dependence of optically induced degradation in CdS and CdSe semiconductor-doped glasses. The observed absorption peaks in the PDS experiment, together with a simple model, were used to evaluate the average radius of semiconductor microcrystals. It is found that the estimated average radii of quantum dots are consistent with that obtained from other methods. This result demonstrates that the PDS technique provides an alternative tool for the study of the optical properties of semiconductor microscrystals. The time dependence of the luminescence degradation of the impurity band, which is attributed to the process of Auger ionization, follows a stretched-exponential function. The inconsistency with the previously proposed exponential relaxation may be due to the size distribution of CdS and CdSe microcrystals.

Quantum size effect in semiconductor microcrystals

A growth technique of the semiconductor microcrystals in a glassy dielectric matrix has been developed. This technique permits to vary the size of the grown microcrystals in a controlled manner from some tens to thousands of angstroms. The size dependence of absorption spectra of a number of I–VII and II–VI compounds grown by this technique have been studied. The size of the microcrystals being decreased, a considerable short-wave-length shift of the exciton lines and the fundamental absorption edge has been observed. This phenomenon is due to the size quantization of the free carrier and exciton energy spectra in the microcrystals.

Microcavity effects in the photoluminescence of GaAs microcrystals

We have fabricated submicron GaAs crystals by pulverization of bulk material. Size selected crystals exhibit modified photoluminescence spectra with blue shifts of up to 10 meV. The observed behavior is explained by the enhancement and the inhibition of spontaneous emission in a three-dimensional optical microcavity formed by a semiconductor microcrystal.

Band-edge absorption and luminescence of nonspherical nanometer-size crystals.

The effect of nonspherical shape on the optical properties of semiconductor microcrystals with a degenerate valence band has been theoretically studied. The asymmetry of microcrystals leads to a splitting of the fourfold-degenerate ground hole state into two twofold-degenerate ones. The value of this splitting is proportional to the deviation from sphericity and inversely proportional to the square of the microcrystal size, and is the function of the ratio of light- to heavy-hole effective masses (\ensuremath{\beta}). This splitting could lead to the formation of long-lived electron-hole pairs in the prolate microcrystals of semiconductors with small \ensuremath{\beta} and in the oblate ones with \ensuremath{\beta}>0.14.

Femtosecond dephasing times in semiconductor microcrystals measured with incoherent light

Incoherent laser pulses are used to measure the dephasing time of photogenerated excitons in semiconductor-doped glasses. The coherent polarization dephasing time T2 is determined for several commercial glasses as well as for a semiconductor thin film. It is found that T2 varies systematically from less than 5 fs to longer than 70 fs, depending on the degree of excess excitation beyond the band edge. The temperature dependence of T2 is also measured. A systematic behavior of smaller T2 at higher temperature is observed in all samples. These T2 values are believed to result from the loss of orientational coherence of the excitons generated by laser excitation.

Refractive Index Changes in CdSxSe1 − xdoped Glasses Through the Photodeflection Method

Abstract Using the photo-deflection method in quasi c.w. regime refractive index variations of CdSx Se1 − x doped glasses have been observed consisting of two independent contributions Δn = Δnt + Δne. The photothermal term Δnt = (dn/dT) ΔT (ΔT is the temperature increase) is positive and slow in time, the electronic term Δne is negative and fast. The electronic term appears at first due to a reversible contribution (during a time scale of 1/ω, where ω is the chopper frequency) and after a time of a few tenths of seconds disappears irreversibly. It is attributed to a redistribution in space and energy levels of electrons in the semiconductor microcrystals. This redistribution is large enough when the electron relaxation is not too fast. The consequent refractive index change decreases in time and disappears under prolonged laser irradiation. A model is presented in which the presence of traps is taken into account.

Strong optical nonlinearities and laser emission of semiconductor microcrystals

Changes in the transmission of quasi-zero-dimensional CdSe microcrystals (quantum dots) induced by ultrashort laser pulses were investigated with picosecond time resolution. The transitions between the energy levels of electrons and holes spatially confined within microcrystals were observed as bleaching bands in nonlinear transmission spectra. Gain registered at the transitions between the lowest levels of size quantization in microcrystals allowed to achieve the regime of laser generation in Fabry-Perot resonator made of glass containing semiconductor microcrystals.

Low-temperature sensors based on telluride microcrystals

Abstract Cryogenic thermoresistive and magnetoresistive sensors based on semiconductor microcrystals of Te, TeSe solid solutions, PbTe and PbSnTe have been developed. The galvanomagnetic properties of the sensors are studied in a wide range of cryogenic temperatures. The sensors have very small outline dimensions and their performances in many cases may be described by sufficiently simple formulae.

Microcavity Effects in The Luminescence of GaAs Microcrystals

ABSTRACTExperimental photoluminescence spectra of GaAs microcrystals show pronounced variations compared to the luminescence of bulk GaAs. The observed spectra are explained by spectral enhancement and inhibition of spontaneous emission in a three-dimensional optical resonator formed by a dielectrically confined semiconductor microcrystal. The crystals were produced by pulverization of bulk GaAs, size-separated by sedimentation techniques, and characterized by transmission electron microscopy, electron diffraction and x-ray diffraction.

Correlation between the structural and optical properties of polydispersed II–VI quantum dots in glass matrix

We report the results of a systematic investigation of the structural and optical properties of polydispersed semiconductor microcrystals in glass. Transmission electron microscope experiments reveal the existence of microcrystals of different radii and size dispersion around the average radius. A simple method is proposed to extract the structural parameters by the Gaussian deconvolution of the size histogram. The presence of a polydispersed dot distribution accounts very well for the observed absorption spectra which exhibit different excitonic features corresponding to the estimated microcrystal sizes. The linewidth of the low temperature photoluminescence spectra is satisfactorily explained by a simple emission-broadening model based on the effective mass approximation which takes into account the dispersion of the quantum dot radius obtained by the transmission electron microscope experiments.

Anomalous change of extinction spectra of CuCl microcrystals

Optical transmission spectra of CuCl microcrystals embedded in NaCl crystals were studied around the Z3 exciton resonance by varying the annealing time. With the increase in the annealing time, extinction spectra of the Z3 exciton change anomalously from the absorption-type spectrum to the emission-type spectrum via the dispersion-type spectrum. The Mie theory [Ann. Phys. 25, 377 (1908)] successfully explains the spectral change due to the growth of microcrystals. We report here for the first time an anomalous change of the exciton spectra due to the growth of semiconductor microcrystals.