Stephen K. O'Leary

Optical dispersion relationships in amorphous silicon grown by molecular beam epitaxy

We have produced a novel form of amorphous silicon (a-Si) using ultra-high-vacuum molecular beam epitaxy (MBE). From measurements of the specular reflectance spectrum at near normal incidence and the regular transmittance spectrum at normal incidence we have determined the spectral dependence of the refractive index, the extinction coefficient, the optical absorption coefficient, and the real and complex components of the dielectric function. These optical dispersion relationships are contrasted with those corresponding to other forms of a-Si.

Optical absorption in an amorphous silicon superlattice grown by molecular beam epitaxy

Abstract We have fabricated amorphous silicon (a-Si) superlattices, comprised of thin layers of a-Si separated by even thinner layers of SiO2 through ultra-high-vacuum molecular beam epitaxy and an ultraviolet ozone process. From measurements of the specular reflection spectrum at near normal incidence, and the regular transmittance spectrum at normal incidence, we have determined the spectral dependence of the optical absorption coefficient corresponding to the a-Si layers within such a superlattice deposited on sapphire. We contrast these results with those corresponding to thin films of a-Si deposited through ultra-high-vacuum molecular beam epitaxy and find that the optical absorption edge of the a-Si layers within the a-Si/SiO2 superlattices is sharper and occurs at higher energies as compared with the thin films of a-Si. We conjecture that both quantum confinement and impurities may be responsible for this effect.

X-ray induced effects in stabilized a-Se X-ray photoconductors

Abstract We report on X-ray induced changes in the electrical properties of stabilized amorphous selenium typical of the material used in direct conversion X-ray imaging devices. Carrier mobility and deep-trapping lifetime were measured using time-of-flight and interrupted-field time-of-flight (IFTOF) measurements. The hole and electron drift mobility is not affected by up to 1 R of exposure to 50 kV p X-rays. The hole lifetime decreases from 50 to 27 μs after exposure to 0.48 R. The hole lifetime slowly recovers to its initial value after circa 3000 min. The electron lifetime does not change after exposure to 1 R. The results are explained by an accumulation of excess hole traps due to capture of electrons in deep localized states. We also observe small changes in the dark current after exposure to X-rays.

An enhancement in the low-field electron mobility associated with a ZnMgO/ZnO heterostructure: The role of a two-dimensional electron gas

We determine the role that a two-dimensional electron gas, formed at a ZnMgO/ZnO heterojunction, plays in shaping the corresponding temperature dependence of the low-field electron Hall mobility. This analysis is cast within the framework of the model of Shur et al. [M. Shur et al., J. Electron. Mater. 25, 777 (1996)], and the contributions to the mobility related to the ionized impurity, polar optical phonon, piezoelectric, and acoustic deformation potential scattering processes are considered, the overall mobility being determined through the application of Mathiessens rule. The best fit to the ZnMgO/ZnO experimental results of Makino et al. [T. Makino et al., Appl. Phys. Lett. 87, 022101 (2005)] is obtained by setting the free electron concentration to 3×1018 cm−3 and the ionized impurity concentration to 1017 cm−3, i.e., within the two-dimensional electron gas formed at the heterojunction, the free electron gas concentration is a factor of 30 times the corresponding ionized impurity concentration. Ho...

Dislocation line charge screening within n-type gallium nitride

A revised electrostatic theory for the charged dislocation lines within n-type GaN is formulated, this formalism allowing for the screening of the charge trapped along the dislocation lines, by both free carriers and a partial ionization of the impurities within the space-charge region surrounding the dislocation lines. This goes beyond the abrupt space-charge region assumption of the Read model [W. T. Read, Jr., Philos. Mag. 45, 775 (1954)], where the only screening mechanism considered is a complete ionization of bulk donor atoms within the Read radius. In addition to determining the spatial distribution of the charge enveloping charged dislocation lines, this procedure also provides a solution to the electrostatic potential surrounding the dislocation lines. An iterative, self-consistent numerical approach to the solution of this problem is developed for the purposes of this analysis. A special limit for which the results of this model reduce to that of Read is indicated. The results obtained from our ...

A transition in the nature of the occupancy of the dislocation lines within n-type wurtzite gallium nitride

Within the framework of the model of Read [Philos. Mag. 45, 775 (1954)], we examine the occupancy of the dislocation lines within n-type wurtzite gallium nitride. In particular, we examine the transition that occurs as the bulk doping concentration is increased, from the depletion limit to the non-depletion limit. We note that an abrupt transitional bulk doping concentration can be defined. The dependence of this transitional bulk doping concentration on the dislocation line density is then determined. We note that existing theoretical results on the occupation statistics of the threading dislocation lines within wurtzite gallium nitride also exhibit such a transition. Since these theoretical results assume different structures for the core of the threading dislocation lines, we conclude that this transition between the depletion and non-depletion domains should be a universal feature, holding true irrespective of the particular structure being assumed for the core of the threading dislocation lines. Cond...