Interface roughness effects on transport in tunnel structures

Direct simulations of interface roughness effects on transport properties of tunnel structures are performed using the planar supercell stack method. The method allows for the inclusion of realistic three-dimensional rough interfacial geometries in transport calculations. For double barrier resonant tunneling structures, we used our method to analyze the effect of roughness at each of the four interfaces, and to test for sensitivity of transport properties to island size and height. Our simulations yields the following conclusions: (1) We find that scattering of off-resonance states into on-resonance states provides the dominant contribution to interface roughness assisted tunneling. Analyses of scattering strength sensitivity to interface layer configurations reveals preferential scattering into Delta k parallel to approximate to 2 pi/lambda states, where lambda is the island size. (2) We find that roughness at interfaces adjacent to the quantum well can cause lateral localization of wave functions, which increases with island size and depth. Lateral localization can result in the broadening and shifting of transmission resonances, and the introduction of preferential transmission paths. In structures with wide and tall islands, it is possible to find localization over "islands" as well as localization over "oceans." (3) The leading rough interface is the strongest off-resonance scatterer, while rough interfaces adjacent to quantum well are the strongest on-resonance scatterers. The trailing interface is the weakest scatterer

Modeling Light-Extraction Characteristics of Packaged Light-Emitting Diodes

We employ a Monte Carlo ray-tracing technique to model light-extraction characteristics of light-emitting diodes. By relaxing restrictive assumptions on photon traversal history, our method improves upon available analytical models for estimating light-extraction efficiencies from bare LED chips, and enhances modeling capabilities by realistically treating the various processes which photons can encounter in a packaged LED. Our method is not only capable of calculating extraction efficiencies, but can also provide extensive statistical information on photon extraction processes, and predict LED spatial emission characteristics

Numerical spurious solutions in the effective mass approximation

We have characterized a class of spurious solutions that appears when using the finite difference method to solve the effective mass approximation equations. We find that the behavior of these solutions as predicted by our model shows excellent agreement with numerical results. Using this interpretation we find a set of analytical expressions for conditions that the Luttinger parameters must satisfy to avoid spurious solutions. Finally, we use these conditions to check commonly used sets of parameters for their potential for generating this class of spurious solutions

Description of bulk inversion asymmetry in the effective-bond-orbital model

We have extended the effective-bond-orbital model (EBOM) method [Y. C. Chang, Phys. Rev. B 37, 8215 (1988)] to include the effects of the bulk inversion asymmetry (BIA) present in zinc blendes. This is accomplished without adding to the number of basis states or extending the range of interaction. We have also investigated a variant form of the EBOM proposed in the original formulation that offers improved zone-center behavior, but may also generate spurious solutions in heterostructure calculations due to poor description of bulk zone-boundary band structure. We offer suggestions for avoiding this problem so that this variant form of EBOM may be used safely. In general, we find that the addition of BIA effects in EBOM results in improved descriptions of zone-center band structure, but also in a loss of accuracy far from the Brillouin-zone center. We illustrate the use of the BIA extension with band-structure calculations for bulk GaSb. We show that the spin splitting predicted by the extended EBOM method for an AlSb/GaSb superlattice is in good agreement with k·p calculations that include BIA effects

Fluctuations in the transmission properties of a quantum dot with interface roughness and impurities

We examine statistical fluctuations in the transmission properties of quantum dots with interface roughness and neutral impurities. For this purpose we employ a supercell model of quantum transport capable of simulating potential variations in three dimensions. We find that sample to sample variations in interface roughness in a quantum dot waveguide can lead to substantial fluctuations in the n=1 transmission resonance position, width and maximum. We also find that a strongly attractive impurity near the centre of a quantum dot can reduce these fluctuations. Nevertheless, the presence of more than a single impurity can give rise to a complex resonance structure that varies with impurity configuration

Interface Roughness Effects in Ultra-Thin Tunneling Oxides

Advanced MOSFET for ULSI and novel silicon-based devices require the use of ultrathin tunneling oxides where non-uniformity is often present. We report on our theoretical study of how tunneling properties of ultra-thin oxides are affected by roughness at the silicon/oxide interface. The effect of rough interfacial topography is accounted for by using the Planar Supercell Stack Method (PSSM) which can accurately and efficiently compute scattering properties of 3D supercell structures. Our results indicate that while interface roughness effects can be substantial in the direct tunneling regime, they are less important in the Fowler-Nordheim regime

Experimental and theoretical study of ultra-thin oxides

We report on an experimental and theoretical study of transport through thin oxides. The experimental study was carried out on the tunnel switch diode (TSD) which consists of an MOS junction on top of a pn junction. The properties of the TSD depends critically on the properties of the tunnel oxide layer. Our results indicate that these devices can exhibit two different modes of behaviour depending on the stress history of the oxide. An unstressed device exhibits a thyristor-like I-V characteristic with fairly low current density. As the oxide is stressed, however, the I-V characteristic discontinuously shifts into a higher-current thyristor-like mode in which current transport appears to be highly non-uniform and depends strongly on stress history. This suggests a possible structural change in the oxide layer which is not completely destructive in that the device continues to function. We present a possible theoretical model of such a structural change in which microscopic filaments are generated in the oxide. Calculations of J-V curves for such structures with varying filament heights qualitatively match stressed MOS I-V curves found in the literature and qualitatively explain the dual-mode behaviour of the TSD

Intersubband absorption in Si1–xGex/Si superlattices for long wavelength infrared detectors

We have calculated the absorption strengths for intersubband transitions in n-type Si1–xGex/Si superlattices. These transitions can be used for the detection of long-wavelength infrared radiation. A significant advantage in Si1–xGex/Si superlattice detectors is the ability to detect normally incident light; in Ga1–xAlxAs/GaAs superlattices intersubband absorption is possible only if the incident light contains a polarization component in the growth direction of the superlattice. We present detailed calculations of absorption coefficients, and peak absorption wavelengths for [100], [111], and [110] Si1–xGex/Si superlattices. Peak absorption strengths of about 2000–6000 cm^–1 were obtained for typical sheet doping concentrations ([approximately-equal-to]10^12 cm^–2). Absorption comparable to that in Ga1–xAlxAs/GaAs superlattice detectors, compatibility with existing Si technology, and the ability to detect normally incident light make these devices promising for future applications