A.I. Ekimov

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.

Auger ionization of semiconductor quantum drops in a glass matrix

Abstract A microscopic model of degradation of both the nonlinear optical and luminescent properties of semiconductor-doped glasses is proposed. In the framework of the model the degradation is due to the process of Auger ionization of microcrystals when some electron-hole pairs are excited. The theoretical dependences of Auger ionization rate on the microcrystal size and energy band parameters of the glass/semiconductor heterostructures are obtained. The luminescence degradation kinetics have been studied in CdS microcrystals doped glasses with different average size at various pumping light intensities. The experimental dependences of the ionization rate on the microcrystal size and intensity are found to be in good agreement with the theoretical calculations. It is shown that an abrupt heterointerface leads to a strong increase of the Auger process rate. Oscillations of the Auger ionization probability and their dependences on the microcrystal size are predicted.

Spectra and decay kinetics of radiative recombination in CdS microcrystals

Abstract Low-temperature luminescence spectra of CdS microcrystals grown in a transparent silicate glass matrix have been studied. The microcrystal size could be varied during growth from a new tens to a few thousands of an angstrom. The spectra of fairly large microcrystals (≈400A) reveal lines originating from free and bound exciton annihilation. The exciton luminescence intensity drops with decreasing microcrystal size, the emission spectra of microcrystals of size less than a few hundred angstrom being dominated by transitions involving impurities and lattice defects. Effects associated with surface band bending in large microcrystals have been revealed and studied. A study has been made of the position and kinetics of the interimpurity luminescence bands as a function of microcrystal size. A theory has been developed to describe the energy spectrum of shallow donor levels and of the interimpurity recombination kinetics in semiconductor microcrystals.

Donor-like exciton in zero-dimension semiconductor structures

Abstract The absorption spectra of CuBr and CdS microcrystal have been investigated. The donor-like exciton with the hole location within a small region near the centre of microcrystal is found. The localization of the hole is due to the potential of electron localized by the boundaries of microcrystal. The quantum levels of hole produce the fine structure in the absorption spectra which has been observed.

Giant internal magnetic fields in Mn doped nanocrystal quantum dots

Abstract We observed a giant splitting of exciton spin sublevels in CdS nanocrystals, each doped on the average by one Mn ion. The splitting, which exists in zero external magnetic field, is caused by the gigantic internal magnetic field of the Mn ion and results from the enhancement of the short range spin–spin interactions in nanocrystal quantum dots. The splitting is seen in the strong magnetic circular dichroism of the CdS band edge transitions. The magnitude of the observed band edge splitting is in good agreement with a theoretical calculation of the effective magnetic field.

RESONANCE RAMAN SPECTROSCOPY OF ELECTRON-HOLE PAIRS : POLAR PHONON COUPLING IN SEMICONDUCTOR QUANTUM MICROCRYSTALS

Abstract We report on a novel type of interaction between surface phonons and confined electron-hole pairs revealed in resonance Raman scattering of GaP nanocrystals. Such nanocrystals are prepared for the first time in a glassy matrix with the process of recondensation growth. Experimental and theoretical results prove that phonons interacting with electron-hole pairs have orbital momentum l = 2 and distinguish from the well-known Frohlich surface phonons with l = 1. These selection rules are due to the symmetry of the quantum size states of holes and electrons taking part in resonance Raman scattering.

Enhancement of Exciton Exchange Interaction by Quantum Confinement in CdSe Nanocrystals

We have investigated the degree of linear polarization and time behaviour of photoluminescence of very small size CdSe nanocrystals under size-selective excitation. At low temperature we observe a luminescence line Stokes-shifted by a few meV and its phonon replica. The degree of linear polarization is positive at short times then becomes negative and remains constant and equal to -15%. The following model is proposed. The exciton state is split by electron-hole exchange interaction into two states with the optically forbidden state at lower energy. Laser light creates excitons in the upper energy state. The luminescence line arises from the lower energy state where excitons have relaxed after a few tens of ps. Recombination is made possible through a phonon assisted virtual transition to the confined B-exciton state. As a result the polarization degree is negative at long times. The value of the exchange energy obtained from the luminescence Stokes shift (8 meV in 40A-diameter nanocrystals) is much larger than the bulk value (0.1 meV). The enhancement of exchange interaction by quantum confinement and the value of the degree of polarization are in good agreement with theoretical estimates.

Interface effects on the properties of confined excitons in CuCl microcrystals

Abstract Optical properties of confined excitons were studied in nanometer-size CuCl microcrystals prepared by three different methods; by annealing in crystalline NaCl and in glass matrices, and by gas evaporation. Interface effects concerning the shape of microcrystals, the effect of finite barrier height and the photo-generated exciton trapping centers were discussed.

Subpicosecond dynamics of confined excitons and optical nonlinearities of CuCl quantum dots

Abstract We have investigated the dynamics of confined excitons and their optical nonlinearities in CuCl quantum dots embedded in NaCl and glass matrices using subpicosecond band-to-band excitation. The quantum dots in the glass matrix exhibit a smaller blue shift of the exciton bands and much weaker optical gain due to biexcitons.

Subpicosecond dynamics of confined excitons in CuCl nanocrystals

We performed subpicosecond pump-and-probe measurements of CuCl nanocrystals at 77 K using a UV pump light of moderate intensity and high repetition rate and also a UV white probe light. For CuCl nanocrystals embedded in a glass matrix and in an NaCl matrix, we compare the temporal behavior of the non-linear change in the confined exciton absorption band induced by the existence of one exciton per nanocrystal. The decay time of the non-linear change is found to be much faster in the glass matrix than in NaCl, reflecting shortening of the exciton lifetime due to the photofatigue effect.