A. M. de Paula

PROBING OF THE QUANTUM DOT SIZE DISTRIBUTION IN CDTE-DOPED GLASSES BY PHOTOLUMINESCENCE EXCITATION SPECTROSCOPY

We studied confinement effects in CdTe quantum dots by means of photoluminescence excitation spectroscopy. We show that by changing the detection energy we can resolve the spectrum of quantum dots of different sizes inside their much broader size distribution in CdTe‐doped glass. The spectra obtained show several well‐resolved lines. There is excellent agreement between the photoluminescence excitation spectra peak energies and calculations of the confined energy transitions based on a modified multiband envelope function model.

Size effects on the phonon spectra of quantum dots in CdTe‐doped glasses

We studied the confinement effects on the phonon spectra of CdTe quantum dots by means of resonant Raman scattering measurements. The spectra show clearly longitudinal optical phonons, surface phonons and some of their overtone combinations. We show that the scattering due to surface phonons increases as the quantum dot size decreases. The results are obtained by tuning the laser excitation energy to resonance for quantum dots of different sizes inside the broad size distribution in CdTe‐doped glasses.

CdTe quantum dots by melt heat treatment in borosilicate glasses

CdTe quantu m dots in borosilicate glasses were produced and their growth kinetics, the effects of quantum confinement for electrons and phonons and the time resolved optical transmission were studied. Quantum dots in the range of 10 to 25 with a 5% size dispersion were grown. The growth kinetics were studied by small angle X-ray scattering using synchrotron light. A spherical k .p model for the electron confinement explains absorption data and selection rules, and a dielectric continuum model for the phonon confinement could explain the resonant longitudinal and surface optical phonons, as well as their overtone combinations. The time response of absorption was studied by a pump-probe measurement on the femtosecond time scale. For samples showing a large amount of surface traps the recovery time can be as fast as 300 fs, while for an almost surface trap free sample it remains on the 1-3 ps time scale.

Effect of size dispersion on the optical absorption of an ensemble of semiconductor quantum dots

The linear optical absorption of an ensemble of semiconductor quantum dots randomly positioned in an insulating matrix is studied theoretically and experimentally for the CdTe/glass system. The calculation of the effective dielectric function of the system, whose imaginary part determines the absorption, is based on a modified Maxwell-Garnett formalism using a diagram technique to calculate the average renormalized polarizability of spherical quantum dots. This approach takes into account both the fluctuations of the polarization interaction due to the random positions of the spheres and their size dispersion. A comparison of the theoretical and experimental spectra permits determination of the mean quantum dot size and concentration. The dependence of these parameters on the postgrowth annealing time of the samples is not consistent with the existing theories on the spinodal decomposition of solid solutions.

PHOTOINDUCED INTERSUBBAND TRANSITION IN UNDOPED HGCDTE MULTIPLE QUANTUM WELLS

We present photoinduced intersubband absorption measurements in HgCdTe undoped quantum wells. The transition energies and the linewidths are well described by a full 8×8 k⋅p Kane model calculation. Also, based on this model we show that different in‐plane effective masses for the first and second electron subbands should be considered in order to properly fit the low energy side of the experimental spectra. The experimental results can be explained using the calculated intersubband oscillator strength with no exciton enhancement.