Normand Arthur Modine

Laser Gain and Threshold Properties in Compressive-Strained and Lattice-Matched GaInNAs/GaAs Quantum Wells

The optical gain spectra for compressive-strained and lattice-matched GaInNAs/GaAs quantum wells are computed using a microscopic laser theory. From these spectra, the peak gain and carrier radiative decay rate as functions of carrier density are determined. These dependences allow the study of lasing threshold current density for different GaInNAs/GaAs laser structures.

HARES: an efficient method for first-principles electronic structure calculations of complex systems

The authors discuss their new implementation of the Adaptive Coordinate Real-space Electronic Structure (ACRES) method for studying the atomic and electronic structure of infinite periodic as well as finite systems, based on density functional theory. This improved version aims at making the method widely applicable and efficient, using high performance Fortran on parallel architectures. The scaling of various parts of an ACRES calculation is analyzed and compared to that of plane-wave based methods. The new developments that lead to enhanced performance, and their parallel implementation, are presented in detail. They illustrate the application of ACRES to the study of elemental crystalline solids, molecules and complex crystalline materials, such as blue bronze and zeolites.

Optical Properties of InGaAsN: A New 1eV Bandgap Material System

InGaAsN is a new semiconductor alloy system with the remarkable property that the inclusion of only 2% nitrogen reduces the bandgap by more than 30%. In order to help understand the physical origin of this extreme deviation from the typically observed nearly linear dependence of alloy properties on concentration, we have investigated the pressure dependence of the excited state energies using both experimental and theoretical methods. We report measurements of the low temperature photoluminescence energy of the material for pressures between ambient and 110 kbar. We describe a simple, density-functional-theory-based approach to calculating the pressure dependence of low lying excitation energies for low concentration alloys. The theoretically predicted pressure dependence of the bandgap is in excellent agreement with the experimental data. Based on the results of our calculations, we suggest an explanation for the strongly non-linear pressure dependence of the bandgap that, surprisingly, does not involve a nitrogen impurity band. Additionally, conduction-band mass measurements, measured by three different techniques, will be described and finally, the magnetoluminescence determined pressure coefficient for the conduction-band mass is measured.

Magnetoluminescence properties of GaAsSbN/GaAs quantum well structures

We report a measurement of the variation of the diamagnetic shift of a heavy-hole exciton in a single coherently strained GaAs0.685Sb0.3N0.015/GaAs quantum well as a function of magnetic field up to 32 T at 1.3 K using photoluminescence spectroscopy. The excitons are known to be localized in this alloy system. This localization is simulated by assuming that the hole is completely immobilized, i.e., its mass is infinite. Using this model we have calculated the variation of the diamagnetic shift with magnetic field in this quantum well structure following a variational approach. We find that the observed variation of the diamagnetic shift with magnetic field agrees quite well with that calculated when the mass of the conduction electron in the well is assumed to be 0.09 m0, about 50% larger than in GaAs0.7Sb0.3, an increase similar to that found in GaAsN for the same nitrogen composition.

Atomic Size Mismatch Strain Induced Surface Reconstructions

The effects of lattice mismatch strain and atomic size mismatch strain on surface reconstructions are analyzed using density functional theory. These calculations demonstrate the importance of an explicit treatment of alloying when calculating the energies of alloyed surface reconstructions. Lattice mismatch strain has little impact on surface dimer ordering for the α2(2×4) reconstruction of GaAs alloyed with In. However, atomic size mismatch strain induces the surface In atoms to preferentially alternate position, which, in turn, induces an alternating configuration of the surface anion dimers. These results agree well with experimental data for α2(2×4) domains in InGaAs∕GaAs surfaces.

Magneto-optical properties of GaAsSb/GaAs quantum wells

We have measured the diamagnetic shift of a heavy-hole exciton in a single 60 A wide GaAs0.7Sb0.3/GaAs quantum well as a function of magnetic field up to 32 T at 1.3 K using photoluminescence spectroscopy. The sample was grown on (001)-oriented GaAs substrate using solid-source molecular beam epitaxy. We have calculated the variation of the diamagnetic shift as a function of magnetic field using a variational approach and a free exciton model. We assumed a weak type-I conduction-band lineup in our calculations. We found that the values thus obtained are more than twice as large as the observed values. A similar calculation assuming a complete localization of the heavy hole leads to the values of the diamagnetic shift which agree very well with the experimental data. Our study suggests that the excitons are strongly localized in GaAs0.7Sb0.3/GaAs quantum well structures at low temperatures, and that this heterostructure has a weak type-I conduction-band lineup.

Highly nonlinear defect-induced carrier recombination rates in semiconductors

Defects in semiconductors can induce recombination of carriers and thus can strongly influence the efficiency and performance of solid-state devices. In the analysis of device performance, defect-induced recombination is often assumed to depend linearly on the carrier concentration or to be given by a sum of Shockley-Read-Hall expressions taken independently for each known defect level. Under these assumptions, defect-induced recombination increases with carrier concentration more slowly than both band-to-band radiative recombination and Auger recombination and becomes relatively less important at higher carrier concentrations. However, we show that defects with multiple defect levels can induce recombination with a highly nonlinear dependence on carrier concentration. For such defects, the usual assumptions about the relative importance of different recombination mechanisms at different carrier concentrations may fail. In order to demonstrate the potential impact of this phenomenon on realistic devices, ...

Migration processes of the As interstitial in GaAs

Thermal migration processes of the As interstitial in GaAs were investigated using density-functional theory and the local-density approximation for exchange and correlation. The lowest-energy processes were found to involve the −1, 0, and +1 charge states, and to produce migration along ⟨110⟩-type directions. In the −1 and 0 charge states, migration proceeds via hops between split-interstitial stable configurations at bulk As sites through bridging saddle-point configurations in which the interstitial atom is equidistant from two adjacent bulk As sites. In the +1 charge state, the roles of these two configurations are approximately reversed and migration proceeds via hops between bridging stable configurations through higher-energy split-interstitial stable configurations bounded by a pair of distorted split-interstitial saddle-point configurations. The predicted activation energies for migration in the 0 and +1 charge states agree well with measurements in semi-insulating and p-type material, respective...

Kinetics-Controlled Degradation Reactions at Crystalline LiPON/Li x CoO2 and Crystalline LiPON/Li-Metal Interfaces

Detailed understanding of solid-solid interface structure-function relationships is critical for the improvement and wide deployment of all-solid-state batteries. The interfaces between lithium phosphorous oxynitride (LiPON) solid electrolyte material and lithium metal anode, and between LiPON and Lix CoO2 cathode, have been reported to generate solid-electrolyte interphase (SEI)-like products and/or disordered regions. Using electronic structure calculations and crystalline LiPON models, we predict that LiPON models with purely P-N-P backbones are kinetically inert towards lithium at room temperature. In contrast, transfer of oxygen atoms from low-energy Lix CoO2 (104) surfaces to LiPON is much faster under ambient conditions. The mechanisms of the primary reaction steps, LiPON structural motifs that readily reacts with lithium metal, experimental results on amorphous LiPON to partially corroborate these predictions, and possible mitigation strategies to reduce degradations are discussed. LiPON interfaces are found to be useful case studies for highlighting the importance of kinetics-controlled processes during battery assembly at moderate processing temperatures.

Recombination by band-to-defect tunneling near semiconductor heterojunctions: A theoretical model

Carrier transport and recombination are modeled for a heterojunction diode containing irradiation defects. Detailed attention is given to the role of band-to-trap tunneling and how it is affected by band offsets at the junction. Tunneled states are characterized by numerical solution of the one-band effective-mass envelope equation. The interaction with traps is treated assuming capture by the multi-phonon-emission mechanism. It is shown that tunneling can increase carrier recombination at defects by orders of magnitude in the presence of large band offsets. This explains why Npn InGaP/GaAs/GaAs heterojunction bipolar transistors with displacement damage from energetic-particle irradiation are observed to have high carrier recombination in the emitter-base depletion region.