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

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

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

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

Theory of antiferromagnetism in the electron-doped cuprate superconductors

On the basis of the Hubbard model, we present the formulation of antiferromagnetism in electron-doped cuprates using the fluctuation-exchange approach. Taking into account the spin fluctuations in combination with the impurity scattering effect due to the randomly distributed dopant-atoms, we investigate the magnetic properties of the system. It is shown that the antiferromagnetic transition temperature, the onset temperature of the pseudogap formation, the single particle spectral density, and the staggered magnetization obtained by the present approach are in very good agreement with the experimental results. The distribution function in momentum space at very low temperature is observed to differ significantly from that of the Fermi liquid. Also, we find zero-energy peak in the density of states (DOS) of the antiferromagnetic phase. This DOS peak is sharp in the low doping regime, and disappears near the optimal doping where the AF order becomes weak.Comment: 12 pages, 19 figure

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

Fermi surface evolution in the antiferromagnetic state for the electron-doped t-t'-t''-J model

By use of the slave-boson mean-field approach, we have studied the electron-doped t-t'-t''-J model in the antiferromagnetic (AF) state. It is found that at low doping the Fermi surface (FS) pockets appear around $(\pm\pi,0)$ and $(0,\pm\pi)$, and upon increasing doping the other ones will form around $(\pm{\pi\over 2},\pm{\pi\over 2})$. The evolution of the FS with doping as well as the calculated spectral weight are consistent with the experimental results.Comment: Fig. 4 is updated, to appear in Phys. Rev.

The temperature dependence of the local tunnelling conductance in cuprate superconductors with competing AF order

Based on the $t-t'-U-V$ model with proper chosen parameters for describing the cuprate superconductors, it is found that near the optimal doping at low temperature ($T$), only the pure d-wave superconductivity ($d$SC) prevails and the antiferromagnetic (AF) order is completely suppressed. At higher $T$, the AF order with stripe modulation and the accompanying charge order may emerge, and they could exist above the $d$SC transition temperature. We calculate the local differential tunnelling conductance (LDTC) from the local density of states (LDOS) and show that their energy variations are rather different from each other as $T$ increases. Although the calculated modulation periodicity in the LDTC/LDOS and bias energy dependence of the Fourier amplitude of LDTC in the "pseudogap" region are in good agreement with the recent STM experiment [Vershinin $et al.$, Science {\bf 303}, 1995 (2004)], we point out that some of the energy dependent features in the LDTC do not represent the intrinsic characteristics of the sample