Manfred Dür

High-field transport and electroluminescence in ZnS phosphor layers

A full-band Monte Carlo simulation of the high-field electron transport in the ZnSphosphor layer of an alternating-current thin-film electroluminescent device is performed. The simulation includes a nonlocal empirical pseudopotential band structure for ZnS and the relevant scattering mechanisms for electrons in the first four conduction bands, including band-to-band impact ionization and impact excitation of Mn 2+ luminescent centers. The steady-state electron energy distribution in the ZnS layer is computed for phosphor fields from 1 to 2 MV/cm. The simulation reveals a substantial fraction of electrons with energies in excess of the Mn 2+ impact excitation threshold. The computed impact excitation yield for carriers transiting the phosphor layer exhibits an approximately linear increase with increasing phosphor field above threshold. The onset of Mn 2+ impact excitation coincides with the onset of band-to-band impact ionization of electron-hole pairs which prevents electron runaway at high electric fields.

Impact ionization rate and high-field transport in ZnS with nonlocal band structure

The impact ionization rate in ZnS is calculated using a nonlocal empirical pseudopotential band structure and compared to previous results using a local calculation. The two resulting rates are then compared and simple fit formulas are presented. These are included in an ensemble Monte Carlo simulation of electron transport in bulk ZnS. The calculated impact ionization rate is then compared to experimental impact ionization coefficient data; reasonable agreement between the experimental data and the calculated impact ionization rate is obtained with an appropriate choice of optical deformation potentials.

Hole initiated impact ionization in wide band gap semiconductors

Band-to-band impact ionization by hot electrons and holes is an important process in high-field transport in semiconductors, leading to carrier multiplication and avalanche breakdown. Here we perform first principles calculations for the respective microscopic scattering rates of both electrons and holes in various wide band gap semiconductors. The impact ionization rates themselves are calculated directly from the electronic band structure derived from empirical pseudopotential calculations for cubic GaN, ZnS, and SrS. In comparison with the electron rates, a cutoff in the hole rate is found due to the relatively narrow valence bandwidths in these wide band gap semiconductors, which correspondingly reduces hole initiated carrier multiplication.

First principles modeling of high field transport in wide-band-gap materials

Abstract In the present work, we have theoretically investigated the electronic and transport properties of three wide-band-gap materials, ZnS, SrS, and GaN, using full-band ensemble Monte Carlo (EMC) simulation. We show a suppression of the hole impact ionization rate for ZnS and SrS in particular, and GaN to a lesser extent, due to the narrowness of the upper valence bands. The resulting impact ionization coefficient for electrons in ZnS simulated using the EMC with microscopically calculated phonon scattering rates is in good agreement with the re-interpreted data of Thompson and Allen (J. Phys. C: Solid State Phys. 20 (1987) L499).

The electron-phonon scattering rate of zinc blende GaN

Abstract We have performed calculations of the electron–phonon scattering rate of zinc blende GaN using the rigid psuedo-ion model. We find that the calculated rates are relatively insensitive to assumptions made regarding the underlying band structure and phonon dispersion and, to a very good approximation, follow the electronic density of states. The latter result allows an optical deformation scattering constant to be fitted.

Effect of intercarrier scattering on intersubband transitions in GaAs/AlGaAs quantum well systems

In the present work, we theoretically investigate the intersubband relaxation of electrons in quantum well systems during photoexcitation using an ensemble Monte Carlo approach. In particular, we compare with recent experimental results by Hartig et al. (Phys. Rev. B (1999)), in which time-resolved photoluminescence measurements are made of the second subband carrier population after band-to-band excitation in a wide coupled quantum well system. In these experiments, the first excited subband energy is less than the polar optical phonon energy. We find excellent agreement between the simulated decay time of the n = 2 subband, and the experimental photoluminescence decay, although it is difficult to distinguish the pure decay time due to electron-electron scattering versus that due to polar optical phonons caused by carrier heating effects

Acoustic phonon scattering in silicon quantum dots

Intravalley acoustic phonon scattering of electrons in fully quantized systems based on n-type inversion layers on a (100) surface of p-type Si is studied theoretically. The confining potential normal to the Si/SiO2 interface is modelled by a triangular quantum well. For the confinement in the lateral directions we assume a parabolic potential. The calculations reveal that the anisotropic electron-acoustic-phonon interaction strongly affects the scattering rate and the average scattering angle. The calculated transition rate of electrons from the first excited state to the ground state shows a strong dependence on spatial confinement and lattice temperature, with the longitudinal phonon mode giving the main contribution to the total rate.

Electron energy relaxation in silicon quantum dots by acoustic and optical phonon scattering

In the present work, we theoretically investigate the energy relaxation of electrons due to acoustic and optical phonon scattering in quantum-dot systems embedded in a Si metal-oxide-semiconductor structure with (100) surface orientation. The confinement potential normal to the Si/SiO2 interface is modeled by an infinite triangular quantum well. For the spatial confinement in the lateral directions due to depletion gates we assume a parabolic potential. The calculated transition rates for electron scattering between discrete energy levels in the dot are included in a simple transport model using Monte Carlo techniques to simulate the relaxation of electrons from higher levels back to the ground level. We find that the electron decay shows a non-exponential behavior. The simulated relaxation time strongly depends on the confinement in the lateral directions and may vary by several orders of magnitude.

Electron relaxation in silicon quantum dots by acoustic phonon scattering

Acoustic phonon scattering of electrons in fully quantized systems based on n-type inversion layers on a [100] surface of p-type Si is studied theoretically. The confining potential normal to the Si/SiO/sub 2/ interface is modeled by a triangular quantum well. For the confinement in the lateral directions we assume a parabolic potential. The calculations reveal that the anisotropic electron-phonon interaction strongly affects the scattering rate. The calculated transition rate of electrons from the first excited to the ground state shows a strong dependence on spatial confinement and lattice temperature.

Monte Carlo Simulations of High Field Transport in Electroluminescent Devices

High field transport in phosphor materials is an essential element of thin film electroluminescent device performance. Due to the high accelerating fields in these structures (1–3 MV/cm), a complete description of transport under high field conditions utilizing information on the full band structure of the material is critical to understand the light emission process due to impact excitation of luminescent impurities. Here we investigate the role of band structure for ZnS, GaN, and SrS based on empirical pseudopotential calculations to study its effect on the high field energy distribution of conduction band electrons.