Lap Van Dao

Silicon Quantum Dots in a Dielectric Matrix for All-Silicon Tandem Solar Cells

We report work progress on the growth of Si quantum dots in different matrices for future photovoltaic applications. The work reported here seeks to engineer a wide-bandgap silicon-based thin-film material by using quantum confinement in silicon quantum dots and to utilize this in complete thin-film silicon-based tandem cell, without the constraints of lattice matching, but which nonetheless gives an enhanced efficiency through the increased spectral collection efficiency. Coherent-sized quantum dots, dispersed in a matrix of silicon carbide, nitride, or oxide, were fabricated by precipitation of Si-rich material deposited by reactive sputtering or PECVD. Bandgap opening of Si QDs in nitride is more blue-shifted than that of Si QD in oxide, while clear evidence of quantum confinement in Si quantum dots in carbide was hard to obtain, probably due to many surface and defect states. The PL decay shows that the lifetimes vary from 10 to 70 microseconds for diameter of 3.4 nm dot with increasing detection wavelength.

Temperature dependence of photoluminescence in silicon quantum dots

The optical properties of silicon quantum dots (QDs) embedded in a SiO2 matrix are investigated at various temperatures using photoluminescence (PL) and time-resolved photoluminescence. Two broad luminescence bands, the S-band located at 600–850 nm and the F-band located at 450–600 nm, are observed. In the S-band a stretched exponential time evolution is observed and the short wavelengths have significantly shorter lifetimes than the long wavelengths. In the low temperature regime, the process of carrier delocalization from the defect states and capture into the QDs is dominant, which results in a decrease in PL intensity from the F-band and an increase from the S-band. In the high temperature regime, the carriers captured into the QDs decrease due to competition between the defect states, which results in a PL intensity decrease for both bands. The PL intensity on the high energy side of the S-band decreases more strongly than that on the low energy side due to the state filling effect, which results in a 30 nm red shift. The S-band is attributed mainly to zero-phonon electron–hole recombination due to enhancement of the quantum confinement effect. The F-band has a single exponential evolution with a much shorter lifetime of nanoseconds and is attributed to defect states of silicon oxide.

Suppression of interdiffusion in GaAs/AlGaAs quantum-well structure capped with dielectric films by deposition of gallium oxide

J. Wong-Leung, P. N. K. Deenapanray, and H. H. Tan acknowledge the fellowships awarded by the Australian Research Council.

Generation of high flux, highly coherent extreme ultraviolet radiation in a gas cell

We report the generation of extreme ultraviolet radiation with high photon flux (1010–1012 photon/cm2 s), high spatial coherence (up to 0.95), and good spatial beam profile by high-order harmonic generation in various noble gases (argon, neon, and helium) in a gas cell. The photon flux was determined using an extreme ultraviolet spectrometer equipped with a charge-coupled device camera and the spatial coherence was determined from Young double-slit interference fringes. The high-order harmonic emission is confined to just a few orders because of the small phase mismatch in the cut-off region that allows macroscopic phase matching to be satisfied for just a few harmonics in this region. The efficiency and spatial beam profile are studied as a function of gas pressure and geometrical configuration.

Spectrally resolved femtosecond two-color three-pulse photon echoes: Study of ground and excited state dynamics in molecules

We report the use of spectrally resolved femtosecond two-color three-pulse photon echoes as a potentially powerful multidimensional technique for studying vibrational and electronic dynamics in complex molecules. The wavelengths of the pump and probe laser pulses are found to have a dramatic effect on the spectrum of the photon echo signal and can be chosen to select different sets of energy levels in the vibrational manifold, allowing a study of the dynamics and vibrational splitting in either the ground or the excited state. The technique is applied to studies of the dynamics of vibrational electronic states in the dye molecule Rhodamine 101 in methanol.

Anodic-oxide-induced interdiffusion in GaAs/AlGaAs quantum wells

Enhancement of interdiffusion in GaAs/AlGaAs quantum wells due to anodic oxides was studied. Photoluminescence, transmission electron microscopy, and quantum well modeling were used to understand the effects of intermixing on the quantum well shape. Residual water in the oxide was found to increase the intermixing, though it was not the prime cause for intermixing. Injection of defects such as group III vacancies or interstitials was considered to be a driving force for the intermixing. Different current densities used in the experimental range to create anodic oxides had little effect on the intermixing.

Three-dimensional electronic spectroscopy of excitons in asymmetric double quantum wells

We demonstrate three-dimensional (3D) electronic spectroscopy of excitons in a double quantum well system using a three-dimensional phase retrieval algorithm to obtain the phase information that is lost in the measurement of intensities. By extending the analysis of two-dimensional spectroscopy to three dimensions, contributions from different quantum mechanical pathways can be further separated allowing greater insight into the mechanisms responsible for the observed peaks. By examining different slices of the complete three-dimensional spectrum, not only can the relative amplitudes be determined, but the peak shapes can also be analysed to reveal further details of the interactions with the environment and inhomogeneous broadening. We apply this technique to study the coupling between two coupled quantum wells, 5.7 nm and 8 nm wide, separated by a 4 nm barrier. Coupling between the heavy-hole excitons of each well results in a circular cross-peak indicating no correlation of the inhomogeneous broadening. An additional cross-peak is isolated in the 3D spectrum which is elongated in the diagonal direction indicating correlated inhomogeneous broadening. This is attributed to coupling of the excitons involving the two delocalised light-hole states and the electron state localised on the wide well. The attribution of this peak and the analysis of the peak shapes is supported by numerical simulations of the electron and hole wavefunctions and the three-dimensional spectrum based on a density matrix approach. An additional benefit of extending the phase retrieval algorithm from two to three dimensions is that it becomes substantially more reliable and less susceptible to noise as a result of the more extensive use of a priori information.

Improved carrier collection in intermixed InGaAs/GaAs quantum wells

We have used photoluminescence up conversion to study the carrier capture times into intermixed InGaAs/GaAs quantum wells. We have found that the capture into the intermixed wells is markedly faster than capture into the reference (unintermixed) quantum wells. The reasons for the significant reduction in the capture time is related to the shape of the intermixed quantum well. Such a reduction in the capture time is beneficial both in terms of the quantum efficiency and the frequency response of intermixed optoelectronic devices.

Suppression of the internal electric field effects in ZnO/Zn0.7Mg0.3O quantum wells by ion-implantation induced intermixing

Strong suppression of the effects caused by the internal electric field in ZnO/ZnMgO quantum wells following ion-implantation and rapid thermal annealing, is revealed by photoluminescence, time-resolved photoluminescence, and band structure calculations. The implantation and annealing induces Zn/Mg intermixing, resulting in graded quantum well interfaces. This reduces the quantum-confined Stark shift and increases electron-hole wavefunction overlap, which significantly reduces the exciton lifetime and increases the oscillator strength.

Time-resolved and time-integrated photoluminescence analysis of state filling and quantum confinement of silicon quantum dots

In this paper we report studies of the optical properties of silicon quantum dot structures. From time-resolved and time-integrated photoluminescence measurements we investigate the state-filling effect and carrier lifetime, and discuss the parabolic confinement of quantum dot structures and the large energy splitting between quantum dot levels. The photoluminescence intensities for different quantum dot levels decay with a stretched exponential function and the decay times are in the range 2–100μs depending on the observation wavelength and the dot size.