Decreasing Dot Size Research Articles

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.

Self-organization processes of InGaAs/GaAs quantum dots grown by metalorganic chemical vapor deposition

In0.5Ga0.5As/GaAs (001) quantum dots (QDs) were grown by metalorganic chemical vapor deposition (MOCVD) on exactly (001) oriented substrates using the Stranski–Krastanow growth mode. The dot density and their relative geometrical arrangement are found to be strongly dependent on the substrate temperature. The dots have identical square shaped bases oriented along 〈100〉. For high densities a preferential relative alignment of the dots along the 〈110〉 directions is found. These dots tend to be arranged in a chainlike pattern with decreasing dot size towards the ends of the chains. From these observations the dot formation process for In0.5Ga0.5As quantum dots is suggested to be driven by energetics whereas the relative orientation is governed by kinetic effects.

Exciton-Neutral-Donor Complexes in Semiconductor Quantum Dots

The quantum states of exciton-neutral-donor complexes in semiconductor quantum dots are computed using a numerical matrix-diagonalization scheme. The results predict an increasing binding energy of exciton to neutral donor with decreasing dot size.

Deep etched InGaAs/InP quantum dots with strong lateral confinement effects

We have developed InGaAs/InP quantum dots with diameters down to 20 nm by high resolution electron beam lithography and wet chemical etching. Due to the damage free wet etching process, the structures are free of significant optically inactive (“dead”) layers. The quantum efficiency of the smallest dots investigated is on the order of a few percent of the efficiency of the two dimensional layers. With decreasing dot size the lateral quantization results in a significant shift of the luminescence line to higher energies. Furthermore the emission shows high energetic features which occur due to transitions between higher lateral quantized states in the dots.

Path-integral study of excitons and biexcitons in semiconductor quantum dots

Computational path integral methods have been used to study the excitonic properties of an effective mass quantum dot model. The results are compared to those of variational calculations. The variational method, using a free particle basis, accurately gives the ground and low lying excited state properties for small radii dots where the kinetic energy dominates over the Coulomb interaction. The path integral approach, which is not restricted to small radii dots, gives a thermal average over states. One thermal effect noted for this model is an inversion of the surface charge layer with temperature due to the competition between thermal and Coulomb energies. At high temperature, where Coulomb effects are secondary, the thermal deBroglie wavelength sets the scale for the density variation near a surface and thus the heavier holes approach the surface more closely than the electrons. As the temperature is lowered the now more important Coulomb binding causes the holes to be cushioned away from the surface by the electron cloud. This inversion temperature decreases as the dot radius increases. Both calculations predict increasing biexciton binding energy with decreasing dot size.

Theory of optically excited intrinsic semiconductor quantum dots.

The influence of the Coulomb interaction on one and two electron-hole-pair excitations in semiconductor quantum dots is analyzed. Using a numerical matrix-diagonalization scheme, the energy eigenvalues and the eigenfunctions of the relevant states are computed. Significant deviations from the strong-confinement approximation are observed. It is shown that the biexciton binding energy increases with decreasing dot size. This result is verified using third-order perturbation theory for small quantum dots. The optical properties of the quantum dots are computed, and it is shown that the Coulomb interaction significantly influences the allowed dipole transitions, causing increasing two-pair absorption on the high-energy side of the decreasing one-pair absorption. Surface-polarization effects are studied for quantum dots embedded in another dielectric medium.

Biexcitons in semiconductor quantum dots.

Theoretical and experimental results are reported which provide the first evidence for biexciton states in semiconductor quantum dots. The theory predicts an increasing biexciton binding energy with decreasing dot size. Unlike bulk semiconductors, quantum dots have excited biexciton states which are stable. These biexciton states are observed as pronounced induced absorption features on the high-energy side of the bleached exciton resonances in femtosecond and nanosecond pump-probe experiments of quantum dots in glass matrices.