Der-San Chuu

Studies of grain size effects in rf sputtered CdS thin films

CdS thin films with various grain sizes were produced by rf sputtering and in situ postannealing processes. For a series of annealing time intervals, the average grain sizes of an as‐deposited thin film with thickness ∼4000 A were found to vary from ∼280 to ∼1500 A. Three bands located in the green, yellow, and red regions were observed in photoluminescence (PL) spectra of these as‐deposited films. The temperature dependence of (PL) spectra of CdS microcrystals was investigated. The variations of the (PL) spectra with average grain size and thickness of CdS thin films were also studied. It was found that the intensity of the yellow band was increased as increasing the film thickness while the intensity of the red band was decreased as the grain size of the film became larger. The peak positions of the yellow and red bands were found to be blue‐shifted as the films became thinner.

Raman investigations of the surface modes of the crystallites in CdS thin films grown by pulsed laser and thermal evaporation

CdS films have been grown on Corning glass by pulsed laser evaporation (PLE) and thermal evaporation (TE) techniques at a substrate temperature between room temperature and 250 °C. The quality of these films is investigated by resonance Raman scattering, x‐ray diffraction and optical transmittance. Our results reveal that the Raman shifts of the surface phonon mode are observed with 300 and 297 cm−1 by PLE and TE techniques, respectively, and as many as four overtones are obtained by PLE method. The difference of Raman shift between these two techniques are caused by the discrepancy of crystallite sizes which is larger for PLE technique. The crystallite sizes are in the range of 200–500 A in diameter. Highly oriented films have been grown by both of the techniques even when the substrate is at room temperature.

Growth of highly oriented tin oxide thin films by laser evaporation deposition

Conducting and transparent thin films of tin oxide were prepared by the laser evaporation of an undoped powder‐pressed polycrystalline tin oxide target onto unheated substrates. After characterizing these films, the results reveal that the films are highly oriented and with a grain size ∼0.2 μm. The nearly stoichiometric deposition of tin oxide films with deposition rates exceeding 24 A per pulse was obtained by this method. The lowest resistivity obtained is 3.0×10−3 Ω cm. The visible transmittance (between 4000 and 7000 A) is above 75%.

Nanomechanical properties of lead zirconate titanate thin films by nanoindentation

The nanomechanical properties of lead zirconate titanate (PZT) thin films were subjected to nanoindentation evaluation. A PZT thin film was created on a silicon substrate by radio frequency magnetron sputtering. The structure and surface morphology were analysed by x-ray diffraction and atomic force microscopy. Results show that PZT thin films were well ordered with a high (110) orientation and presented a pure perovskite-type structure and that the average roughness was reduced as the annealing temperature was increased. The Youngs modulus and hardness increased as the rapid annealing temperature increased from 600 to 800 °C, with the best results being obtained at 800 °C.

Quasistationary states of a relativistic field-emission-limited diode employing a high-transparency mesh anode

A relativistic field-emission-limited diode employing a high-transparency mesh anode is investigated via a self-consistent approach. The field emission process is described quantum mechanically by the Fowler–Nordheim equation. The cathode plasma and surface properties are considered within the framework of the effective work function approximation. Space-charge effects are described by Poisson’s equation including relativistic effects. Ionization effects at the high-transparency mesh anode are ignored. The numerical calculations are carried out on a time scale much shorter than the emergence of the gap closure. The quasistationary state of the diode exhibits a cutoff voltage. The electric field on the cathode surface is found to be saturated in the high-voltage regime and determined by the effective work function only.

Donor states in a multi-layered quantum dot

The ground and excited state energies of a hydrogenic impurity located at the centre of a multi-layered quantum dot (MLQD) are calculated in the framework of the effective-mass approximation. The MLQD consists of a spherical core (e.g. GaAs) and a coated spherical shell (e.g. Ga1-xAlxAs). The whole dot is then embedded inside a bulk material (e.g. Ga1-yAlyAs). We solve the Schrodinger equation exactly. The eigenfunctions of the impurity are expressed in terms of Whittaker function and Coulomb wavefunction. The state energies are expressed in terms of the shell thickness, core radius, total dot radius and the potential heights. Our calculation shows that, as the dot radius approaches infinity, the state energies of an impurity located at the centre of a multi-layered or a single-layered QD approach -1/n2 Ry, where n is the principal quantum number, Ry = µe4/2e22, µ and e are the electronic effective mass and the dielectric constant of GaAs material. Thus it behaves like a three-dimensional free hydrogen atom. For very small dot radius, however, the state energy of the hydrogenic impurity of a MLQD behaves very differently from that of a single-layered QD. For a multi-layered QD with finite shell and bulk potential barrier heights, the state energies of the impurity are found to be dependent on the difference of the shell potential (V2) and the bulk potential (V3).

Spontaneous emission of quantum dot excitons into surface plasmons in a nanowire

The spontaneous emission (SE) of quantum dot (QD) excitons into surface plasmons in a cylindrical nanowire is investigated theoretically. Maxwells equations with appropriate boundary conditions are solved numerically to obtain the dispersion relations of surface plasmons. The SE rate of QD excitons is found to be greatly enhanced at certain values of the exciton bandgap. Application in generation of remote entangled states via superradiance is also pointed out and may be observable with current technology.

Current Detection of Superradiance and Induced Entanglement of Double Quantum Dot Excitons

We propose to measure the superradiance effect by observing the current through a semiconductor double-dot system. An electron and a hole are injected separately into one of the quantum dots to form an exciton and then recombine radiatively. We find that the stationary current shows oscillatory behavior as one varies the interdot distance. The amplitude of oscillation can be increased by incorporating the system into a microcavity. Furthermore, the current is suppressed if the dot distance is small compared to the wavelength of the emitted photon. This photon trapping phenomenon generates the entangled state and may be used to control the emission of single photons at predetermined times.

Nanotribology and fractal analysis of ZnO thin films using scanning probe microscopy

Characteristics of crystalline structure, roughness and nanotribology of ZnO thin films deposited under various power conditions were achieved by means of x-ray diffraction and scanning probe microscopy (SPM). The ZnO thin films were deposited on the silicon (100) substrates by a radio frequency magnetron sputtering system. Fractal analysis was derived from SPM images, to calculate the fractal dimension complexity of the surface geometry, via a substituting structure function. The results show that the roughness decreases and nanowear rate increases as the sputtering power increases. In addition, the fractal dimensions of the ZnO thin films are also presented.

Quantum confinement effect on the impurity states of a spherical confining system

Whittaker function and scattering Coulomb wave function are used to calculate the eigen-energies of an impurity in a spherical confining system. Variation of the state energies with the radii of the confining system is obtained.