Showing papers by "Paul H. Holloway published in 1991"

Laser‐target interactions during pulsed laser deposition of superconducting thin films

Abstract: The theoretical analysis of thermal effects induced by nanosecond laser irradiation on bulk YBa2Cu3O7 superconductor targets provides insight into the nature of the target’s ablation/evaporation characteristics during pulsed laser deposition of superconducting thin films. We have simulated the thermal history of YBa2Cu3O7 targets under intense nanosecond laser irradiation by numerically solving the one dimensional heat flow equation and taking into account the phase changes occurring at the near surface of the target. The numerical method is based on a higher‐order finite difference scheme with a smaller truncation error and is not restricted by any stability criterion, thereby allowing faster convergence to the exact solution. Temperature‐dependent optical and thermal properties of the irradiated material as well as the temporal variation in the laser intensity can be taken into account by this method. During planar surface evaporation of the target material, the subsurface temperatures were calculated t...

Optical constants of ZnSe in the far infrared.

Abstract: The absorption coefficient \ensuremath{\alpha} and refractive index n of bulk polycrystalline ZnSe have been derived from transmission measurements over wave-number ranges of 30--200 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and 250--600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Infrared multiphonon structures have been found and assigned. Variations of \ensuremath{\alpha} and n are satisfactorily described by a single-harmonic-oscillator model for the dielectric constant. Variations in \ensuremath{\alpha} caused by multiphonon processes suggest that anharmonic lattice dynamics are important above 250 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, especially for three-phonon processes, which include the fundamental TO(\ensuremath{\Gamma}) mode.

Mechanism of forming ohmic contacts to GaAs

Abstract: The distribution of Si dopant at the Au/GaAs interface after heat treatment has been studied using spatially resolved secondary‐ion mass spectrometry. Previously it has been shown that heat treatment changes as deposited Au thin film from Schottky contacts to ohmic contacts. The present study shows that the transition to an ohmic contact results from segregation of dopants in areas where GaAs is decomposed by reacting with the Au overlayer. Thus the ohmic contact is spatially very inhomogeneous at the metal/semiconductor interface. The mechanism leading to concentrations of the Si and nonuniform ohmic contacts is discussed, and segregation to the solid during decomposition of the GaAs is the most likely mechanism.

Regrowth of GaAs at the Au–Ge–Ga/GaAs interface under an As flux

Abstract: Isothermal regrowth of doped GaAs at the Au/GaAs interface was studied using scanning electron microscopy and Auger electron spectroscopy. Electrical properties of the interface were characterized using I–V and C–V measurement. Multilayer films of AuGe(1500 A) and Ga(200 A) were evaporated on chemically cleaned GaAs surfaces and annealed in N2 or an As flux. An n‐type, moderately doped, nonuniform regrown GaAs layer was formed at the interface of the substrate with little reaction between the metal films and the substrate even after annealing at 450 °C in an As flux. Current transport across the interface exhibited different behavior dependent upon whether regrowth had or had not regrown; (1) thermionic field emission(FET) through the grown GaAs/metal region, and (2) tunneling emission through the Au/original substrate interface area. Thicker Ga films result in nonuniform regrowth of GaAs both at the Au/air as well as the Au/GaAs interface.

Determination of the carrier concentration of doped ZnSe from infrared measurements

Abstract: Determination of the carrier concentration of n‐type ZnSe/GaAs heteroepitaxial films from absorption measurements is reported over the wave number range 30