J. S. Nelson

BOWING PARAMETERS FOR ZINC-BLENDE AL1-XGAXN AND GA1-XINXN

First‐principles calculations have been used to determine bowing parameters for disordered zinc‐blende Al1−xGaxN and Ga1−xInxN. The direct transition at Γ is found to bow downward for both materials with parameters +0.53 and +1.02 eV, respectively, while the Γ‐to‐X transition bows upward for Al1−xGaxN (parameter −0.10 eV) and downward for Ga1−xInxN (parameter +0.38 eV). The similarity of the calculated bulk zinc‐blende and wurtzite Γ‐point transitions also allows estimates to be made of the energy gap versus composition for wurtzite alloys.

Theoretical study of room temperature optical gain in GaN strained quantum wells

The determination of gain properties in group III nitride quantum wells is complicated by the incomplete knowledge of band structure properties, and the need for a consistent treatment of many‐body Coulomb effects. This letter describes an approach that involves a first‐principles band structure calculation, the results of which are incorporated into a microscopic laser theory where many‐body Coulomb effects are treated in a consistent manner. Using this approach, we investigate quantum well structures composed of alloys of GaN, AlN, and InN, in particular, GaN–AlInN, which has high confinement potentials in both strained and unstrained configurations.

First‐principles calculations for zinc‐blende AlInN alloys

First‐principles calculations have been performed for ordered and disordered zinc‐blende Al0.5In0.5N alloys including full relaxation of bond lengths and bond angles. The disordered alloy is predicted to have a mixing enthalpy of +39 meV/atom and a bowing parameter of +2.53 eV at the Γ‐point transition. The similarity of the bulk zinc‐blende and wurtzite Γ‐point transitions also allows an estimate to be made of the energy gap versus composition for wurtzite alloys. In particular, the wurtzite AlInN alloy lattice matched to GaN is predicted to have an energy gap of 5.0 eV.

VALENCE AND ATOMIC SIZE DEPENDENT EXCHANGE BARRIERS IN VACANCY-MEDIATED DOPANT DIFFUSION

First-principles pseudopotential calculations of dopant-vacancy exchange barriers indicate a strong dependency on dopant valence and atomic size, in contrast to current models of vacancy-mediated dopant diffusion. First-row elements (B, C, N) are found to have exchange barriers which are an order of magnitude larger than the assumed value of 0.3 eV (the Si vacancy migration energy).

Ab initio calculations of the energetics of the neutral Si vacancy defect

Ab initio plane-wave pseudopotential calculations for the neutral silicon vacancy indicate a formation energy of 3.6 eV, with the surrounding lattice undergoing a tetragonal distortion with the nearby atoms forming two dimers having bond lengths 2.91 A. Close in energy is a tetrahedrally distorted structure in which the nearby atoms relax towards the vacancy by 12.6% of the bulk bond length. Additional distortions with trigonal symmetry were also investigated, but no stable structures were found. The symmetry, energetics, and geometry are found to be a sensitive function of the computational basis-set and supercell used in the plane-wave calculations.

Fast through-bond diffusion of nitrogen in silicon

We report first-principles total energy calculations of interaction of nitrogen in silicon with silicon self-interstitials. Substitutional nitrogen captures a silicon interstitial with 3.5 eV binding energy forming a 〈100〉 split interstitial ground-state geometry, with the nitrogen forming three bonds. The low-energy migration path is through a bond bridge state having two bonds. Fast diffusion of nitrogen occurs through a pure interstitialcy mechanism: the nitrogen never has less than two bonds. Near-zero formation energy of the nitrogen interstitialcy with respect to the substitutional rationalizes the low solubility of substitutional nitrogen in silicon.

Leveraging scale effects to create next-generation photovoltaic systems through micro- and nanotechnologies

Current solar power systems using crystalline silicon wafers, thin film semiconductors (i.e., CdTe, amorphous Si, CIGS, etc.), or concentrated photovoltaics have yet to achieve the cost reductions needed to make solar power competitive with current grid power costs. To overcome this cost challenge, we are pursuing a new approach to solar power that utilizes micro-scale solar cells (5 to 20 μm thick and 100 to 500 μm across). These micro-scale PV cells allow beneficial scaling effects that are manifested at the cell, module, and system level. Examples of these benefits include improved cell performance, better thermal management, new module form-factors, improved robustness to partial shading, and many others. To create micro-scale PV cells we are using technologies from the MEMS, IC, LED, and other micro and nanosystem industries. To date, we have demonstrated fully back-contacted crystalline silicon (c-Si), GaAs, and InGaP microscale solar cells. We have demonstrated these cells individually (c-Si, GaAs), in dual junction arrangements (GaAs, InGaP), and in a triple junction cell (c-Si, GaAs, InGaP) using 3D integration techniques. We anticipate two key systems resulting from this work. The first system is a high-efficiency, flexible PV module that can achieve greater than 20% conversion efficiency and bend radii of a few millimeters (both parameters greatly exceeding what currently available flexible PV can achieve). The second system is a utility/commercial scale PV system that cost models indicate should be able to achieve energy costs of less than

Confined states and stability of GaAs–AlAs superlattices

0.10/kWh in most locations.

Demonstration of the effects of interface strain on band offsets in lattice-matched III-V semiconductor superlattices

Self‐consistent calculations performed on small superlattices provide evidence for confined states. Superlattices investigated contained n layers each of GaAs and AlAs in the [001] orientation. For n=4, an analysis of the self‐consistent charge distribution shows electron confinement in the AlAs region. The highest valence‐band state is always localized in the GaAs region giving a staggered band alignment for this superlattice. The calculated valence band offset is about 0.3 eV. The tendency towards nonstaggered band lineup is noted since Γc6(GaAs), which for n=2 superlattice lies above Xc6(AlAs) continues to shift down towards it when n is varied from 2 to 4. In large superlattices, both electrons and holes are confined in the GaAs region. We have also studied the stability of the ordered phases in epitaxially grown Ga1−xAlxAs alloys. Our total energy calculations suggest that the ordered structure (GaAs)1–(AlAs)1 is favored over the disordered one. However, the ordered structure has a negative heat of f...

Cost analysis of flat-plate concentrators employing microscale photovoltaic cells for high energy per unit area applications

A first principles total energy self‐consistent pseudopotential calculation is used to predict the band offset in the lattice‐matched superlattice InAs/Al0.8Ga0.2As0.14Sb0.86. We find that inclusion of interface strain changes the character of the band offset from nominally type II to strongly type II. The predicted band offset at the minimum energy configuration is in excellent agreement with the value determined from infrared photoluminescence measurements.