Ohmic contacts to N-face p-GaN using Ni/Au for the fabrication of polarization inverted light-emitting diodes.

The electrical properties of Ni-based ohmic contacts to N-face p-type GaN were investigated. The specific contact resistance of N-face p-GaN exhibits a linear decrease from 1.01 omega cm2 to 9.05 x 10(-3) omega cm2 for the as-deposited and the annealed Ni/Au contacts, respectively, with increasing annealing temperature. However, the specific contact resistance could be decreased down to 1.03 x 10(-4) omega cm2 by means of surface treatment using an alcohol-based (NH4)2S solution. The depth profile data measured from the intensity of O1s peak in the X-ray photoemission spectra showed that the alcohol-based (NH4)2S treatment was effective in removing the surface oxide layer of GaN.

Broadband omnidirectional antireflection coatings optimized by genetic algorithm.

An optimized graded-refractive-index (GRIN) antireflection (AR) coating with broadband and omnidirectional characteristics--as desired for solar cell applications--designed by a genetic algorithm is presented. The optimized three-layer GRIN AR coating consists of a dense TiO2 and two nanoporous SiO2 layers fabricated using oblique-angle deposition. The normal incidence reflectance of the three-layer GRIN AR coating averaged between 400 and 700 nm is 3.9%, which is 37% lower than that of a conventional single-layer Si3N4 coating. Furthermore, measured reflection over the 410-740 nm range and wide incident angles 40 degrees -80 degrees is reduced by 73% in comparison with the single-layer Si3N4 coating, clearly showing enhanced omnidirectionality and broadband characteristics of the optimized three-layer GRIN AR coating.

Design of optical path for wide-angle gradient-index antireflection coatings.

What we believe to be a new principle is introduced for the design and selection of gradient-index antireflection profiles that are effective over a wide range of incident angles as well as wavelengths at a given physical film thickness. It is shown that at oblique incidence the smoothness of the optical path of incident light inside a gradient-index film has a crucial effect on the overall reflection. Thus the smoothness of variations in refractive angle (rather than that of the index profile itself) needs to be maximized for wide-angle operation. As an example, the performance of Gaussian and Quintic profiles at large incident angles are considered in light of this point of view.

Silica nanorod-array films with very low refractive indices.

The refractive-index contrast is an important figure of merit for dielectric multilayer structures, optical resonators, and photonic crystals. This represents a strong driving force for novel materials that have refractive indices lower than those of conventional optically transparent materials. Silica nanorod-array dielectric films with unprecedented low refractive indices of 1.08 are demonstrated and shown to have viable optical properties including enhanced reflectivity of a single-pair distributed Bragg reflector.

Omnidirectional reflector using nanoporous SiO2 as a low-refractive-index material.

Triple-layer omnidirectional reflectors (ODRs) consisting of a semiconductor, a quarter-wavelength transparent dielectric layer, and a metal have high reflectivities for all angles of incidence. Internal ODRs (ambient material's refractive index n >> 1.0) are demonstrated that incorporate nanoporous SiO2, a low-refractive-index material (n = 1.23), as well as dense SiO2 (n = 1.46). GaP and Ag serve as the semiconductor and the metal layer, respectively. Reflectivity measurements, including angular dependence, are presented. Calculated angle-integrated TE and TM reflectivities for ODRs employing nanoporous SiO2 are R(int)/TE = 99.9% and R(int)/TM = 98.9%, respectively, indicating the high potential of the ODRs for low-loss waveguide structures.

Highly efficient light-emitting diodes with microcavities.

One-dimensional microcavities are optical resonators with coplanar reflectors separated by a distance on the order of the optical wavelength. Such structures quantize the energy of photons propagating along the optical axis of the cavity and thereby strongly modify the spontaneous emission properties of a photon-emitting medium inside a microcavity. This report concerns semiconductor light-emitting diodes with the photon-emitting active region of the light-emitting diodes placed inside a microcavity. These devices are shown to have strongly modified emission properties including experimental emission efficiencies that are higher by more than a factor of 5 and theoretical emission efficiencies that are higher by more than a factor of 10 than the emission efficiencies in conventional light-emitting diodes.

Resonance ionization mass spectrometry of AI(x)Ga(1-x)As: depth resolution, sensitivity, and matrix effects.

Resonance ionization mass spectrometry (RIMS) of neutral atoms sputtered from III-V compound semiconductors such as Al(x)Ga(1-x)As provides information that is complementary to secondary ion mass spectrometry with the added advantages of rejecting mass interferences, retaining good sensitivity, and reducing matrix effects. A GaAs sample, delta doped with Be, is used to measure depth resolution and Be secondary ion and atom yield. Because of the coupling of the pulsed RIMS lasers and continuous sputtering beam, duty cycle factors are used to determine the atom yield. A 3-D model of the geometrical overlap of laser and sputtered atoms is developed to ascertain the same utilization efficiency in RIMS. About 30% of the atoms sputtered in 1 micros are calculated to be in the laser beam. The atom yield was found to be near unity. The time-gated RIMS useful yield is ~2%. RIMS is used to minimize matrix effects in a depth profile of a Be-implanted AlAs/A1(0.2)Ga(0.8)As heterostructure and shows that Be diffuses from higher Al-containing layers at concentrations near 10(19) cm(-3). The atomization of As is shown to be affected by the Al content in a GaAs/Al(0.5)Ga(0.5)As structure.