ZnTe Quantum Dots Research Articles

Manifestation of interaction effects in Raman spectra of CdTe quantum-dot layers with ZnTe narrow barriers

The Raman spectra of superlattices consisting of layers of CdTe self-assembled quantum dots separated by ZnTe narrow barriers with thicknesses of 10 and 5 monolayers are investigated. It is found that, apart from the bands previously observed at frequencies of ∼120 and ∼140 cm−1 for samples with thicker barriers (25 and 12 monolayers), the Raman spectra exhibit a band at ∼147 cm−1 in the frequency range of CdTe vibrational modes. This band is attributed to a symmetric vibrational mode of a pair of quantum dots with oppositely directed oscillations of the dipole moments. It is this type of vibrational mode in the material surrounding the ZnTe quantum dot that accounts for the shift of the band at ∼200 cm−1 near the LO mode of ZnTe vibrations toward lower frequencies.

Probing the excited state distributions of CdTe∕ZnTe self-assembled quantum dots using resonant Raman scattering

Resonant Raman scattering is a sensitive means to probe electronic states in semiconductor materials even when such states are not accessible either through photoluminescence or transport techniques. We show that resonant Raman scattering can be used to probe the excited state distribution of CdTe∕ZnTe quantum dots at room temperature. In particular we apply this technique to study the changes of such quantum dots after rapid thermal annealing.

Effect of thermal annealing on the interband transitions and activation energies of CdTe∕ZnTe quantum dots

The effect of rapid thermal annealing on CdTe∕ZnTe quantum dots (QDs) was analyzed to investigate the interband transitions and the electron activation energy. The full width at half maximum of the photoluminescence (PL) peak corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band (E1‐HH1) in the CdTe∕ZnTe QDs annealed at 330°C decreased, and their integrated PL intensity of the E1‐HH1 transition peak significantly increased. The activation energy of electrons confined in CdTe∕ZnTe QDs annealed at 330°C increased as high as 77meV, which was the highest value among the as-grown and annealed samples. These results indicate that the crystallinity of the CdTe∕ZnTe QDs is improved by annealing, and the present results can help improve the understanding of the thermal annealing effect on the optical properties of CdTe∕ZnTe QDs.

Optical properties of self-assembled ZnTe quantum dots grown by molecular-beam epitaxy

The morphology and the size-dependent photoluminescence (PL) spectra of the type-II ZnTe quantum dots (QDs) grown in a ZnSe matrix were obtained. The coverage of ZnTe varied from 2.5 to 3.5 monolayers (MLs). The PL peak energy decreased as the dot size increased. Excitation power and temperature-dependent PL spectra are used to characterize the optical properties of the ZnTe quantum dots. For 2.5- and 3.0-ML samples, the PL peak energy decreased monotonically as the temperature increased. However, for the 3.5-ML sample, the PL peak energy was initially blueshifted and then redshifted as the temperature increased above 40K. Carrier thermalization and carrier transfer between QDs are used to explain the experimental data. A model of temperature-dependent linewidth broadening is employed to fit the high-temperature data. The activation energy, which was found by the simple PL intensity quenching model, of the 2.5, 3.0, and 3.5 MLs were determined to be 6.35, 9.40, and 18.87meV, respectively.

Correlated photon emission from a single II–VI quantum dot

We report correlation and cross-correlation measurements of photons emitted under continuous wave excitation by a single II–VI quantum dot (QD) grown by molecular-beam epitaxy. A standard technique of microphotoluminescence combined with an ultrafast photon correlation setup allowed us to see an antibunching effect on photons emitted by excitons recombining in a single CdTe∕ZnTe QD, as well as cross correlation within the biexciton (X2)-exciton (X) radiative cascade from the same dot. Fast microchannel plate photomultipliers and a time-correlated single photon module gave us an overall temporal resolution of 140ps better than the typical exciton lifetime in II–VI QDs of about 250ps.

Dimensional structural transition in CdTe∕CdxZn1−xTe nanostructures

CdTe nanostructures were grown on CdxZn1−xTe buffer layers by using molecular-beam epitaxy and atomic-layer epitaxy. The atomic force microscopy image showed that uniform CdTe quantum dots were formed on ZnTe buffer layer. Photoluminescence measurements showed that the excitonic peak corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band in the CdTe∕CdxZn1−xTe nanostructure shifted to a higher energy with increasing Cd mole fraction. The activation energy of the electrons confined in the CdTe∕ZnTe quantum dots was higher than those of electrons in CdTe∕CdxZn1−xTe nanostructures. These results can help improve understanding of the dimensional structural transition in CdTe∕CdxZn1−xTe nanostructures.

Successful growth of two different quantum dots on one substrate

We report the successful growth of ZnSe and ZnTe quantum dots (QDs) embedded in ZnS on GaAs substrate. These QDs have good optical properties and show quantum confinement effect. High-resolution electron scanning microscope studies show that these QDs are grown in Volmer–Weber mode. It is found that the size of the QDs is controlled by the growth duration. When the growth time is short, high density of QDs could be fabricated, but with a long growth time the small QDs get together to form a large cluster. We also show that with this growth method it is possible to grow both ZnSe quantum and ZnTe QDs on one substrate at the same time. For this dual QDs system, two peaks corresponding to the emission from the ZnSe dots ( 3.0 eV , blue–violet) and ZnTe dots ( 2.6 eV , green–blue) could be observed at the same time in the photoluminescence measurement.

Analysis of heavily tailed size distributions of ZnTe/ZnSe quantum dot structures by using the bootstrap methodology

We have used the bootstrap methodology to analyze dot size distributions of ZnTe quantum dot (QD) structures. The photoluminescence (PL) spectrum indicates that the ZnTe QD structure belongs to a type-II band alignment. The broadness with small fluctuations in the PL represents the spatial inhomogeneity of the QD sizes. The Schrödinger equation together with the first-order perturbation correction was numerically solved to correlate the dot size and the photon energy. Using the bootstrap “loess” curve fitting method, the PL spectrum was determined to be a normal distribution with a high significance level of 46% tested by a null hypothesis H0. By examining the slope of the complementary cumulative distribution function, we found that the size distribution is heavy tailed.

Formation of self-assembled ZnTe quantum dots on ZnSe buffer layer grown on GaAs substrate by molecular beam epitaxy

Self-assembled ZnTe quantum dot structures were grown by molecular beam epitaxy on GaAs substrates with a 200 nm ZnSe buffer layer. Surface morphology was studied by atomic force microscopy. A three-dimensional Volmer–Weber growth mode was identified. Two types of dots were observed. Strong photoluminescence observed at 1.9–2.2 eV was attributed to emission from the large type II ZnTe quantum dots. Emission from the smaller ZnTe quantum dots was observed at an energy of around 2.26 eV. The density of the larger and smaller dots was approximately 10 8/cm 2 and 10 9/cm 2, respectively.

Growth and photoluminescence study of ZnTe quantum dots

ZnTe quantum dots embedded in ZnS were grown successfully by controlling the flow duration in a metalorganic chemical vapor deposition system. Blueshift as large as 250 meV was observed in photoluminescence measurement, and the emission persists up to room temperature. The amount of blueshift decreases with increasing quantum dot size and for large quantum dots, no photoluminescence could be detected. From studying the temperature-dependent integrated intensity of the emission spectra, it is found that the activation energy for the quenching of photoluminescence increases with decreasing quantum dot size, and is identified as the binding energy of exciton in ZnTe quantum dot.