T. Tiedje

Limiting efficiency of silicon solar cells

The detailed balance method for calculating the radiative recombination limit to the performance of solar cells has been extended to include free carrier absorption and Auger recombination in addition to radiative losses. This method has been applied to crystalline silicon solar cells where the limiting efficiency is found to be 29.8 percent under AM1.5, based on the measured optical absorption spectrum and published values of the Auger and free carrier absorption coefficients. The silicon is assumed to be textured for maximum benefit from light-trapping effects.

Molecular beam epitaxy growth of GaAs1−xBix

GaAs1−xBix epilayers with bismuth concentrations up to x=3.1% were grown on GaAs by molecular beam epitaxy. The Bi content in the films was measured by Rutherford backscattering spectroscopy. X-ray diffraction shows that GaAsBi is pseudomorphically strained to GaAs but that some structural disorder is present in the thick films. The extrapolation of the lattice constant of GaAsBi to the hypothetical zincblende GaBi alloy gives 6.33±0.06 A. Room-temperature photoluminescence of the GaAsBi epilayers is obtained and a significant redshift in the emission of GaAsBi of ∼84 meV per percent Bi is observed.

Band gap of GaAs1−xBix, 0<x<3.6%

The band gap of GaAsBi epitaxial layers as a function of bismuth concentration up to 3.6% is determined. The optical transitions were measured by modulated electroreflectance. The energy of the band gap decreases at a linearized rate of 88 meV/% Bi, or 83 meV/% Bi for the heavy hole to conduction band transition for GaAsBi strained to GaAs. The valence-band splitting increases faster than that of GaAs under similar compressive strain whereas the temperature dependence of the observed GaAsBi transitions is similar to that of GaAs.

Composition dependence of photoluminescence of GaAs1-xBix alloys

Room temperature photoluminescence (PL) spectra have been measured for GaAs1−xBix alloys with Bi concentrations in the 0.2%–10.6% range. The decrease in the PL peak energy with increasing Bi concentration follows the reduction in bandgap computed from density functional theory. The PL peak energy is found to increase with PL pump intensity, which we attribute to the presence of shallow localized states associated with Bi clusters near the top of the valence band. The PL intensity is found to increase with Bi concentration at low Bi concentrations, peaking at 4.5% Bi.

Observation of leaky slab modes in an air-bridged semiconductor waveguide with a two-dimensional photonic lattice

An air-bridged, 120-nm-thick semiconductor slab with a two-dimensional (2D) square array of through holes on a 480 nm pitch (Λ) was fabricated using selective wet etching techniques. The second order photonic resonances of the structure were studied by comparing broadband optical scattering data with numerical solutions of Maxwell’s equations. Features observed in these spectra over a 1200 cm−1 range, near 9500 cm−1, indicate that the 2D texture splits the energy degeneracy of slab modes with propagation constants {±2π/Λ,0} and {0,±2π/Λ} by as much as 14%.

Temperature dependence of the Urbach edge in GaAs

The temperature dependence of the optical‐absorption edge (Urbach edge) of GaAs is measured in semi‐insulating and n‐type GaAs (n=2×1018 cm−3) over the temperature range from room temperature to 700 °C. Both the optical absorption and the temperature are measured using a diffuse reflectance technique. The characteristic energy of the exponential absorption edge is found to increase linearly with temperature, from 7.5 meV at room temperature to 12.4 meV at 700 °C, for semi‐insulating GaAs. The temperature dependent part of the width of the Urbach edge for semi‐insulating GaAs is six times smaller than predicted by the standard theory where the edge width is proportional to the phonon population.

Effect of molecular beam epitaxy growth conditions on the Bi content of GaAs1−xBix

We describe how the Bi content of GaAs1−xBix epilayers grown on GaAs can be controlled by the growth conditions in molecular beam epitaxy. Nonstandard growth conditions are required because of the strong tendency for Bi to surface segregate under usual growth conditions for GaAs. A maximum Bi content of 10% is achieved at low substrate temperature and low arsenic pressure, as inferred from x-ray diffraction measurements. A model for bismuth incorporation is proposed that fits a large body of experimental data on Bi content for a wide range of growth conditions. Low growth rates are found to facilitate the growth of bismide alloys with a low density of Bi droplets.

The electronic band structure of GaBiAs/GaAs layers: Influence of strain and band anti-crossing

The GaBixAs1−x bismide III-V semiconductor system remains a relatively underexplored alloy particularly with regards to its detailed electronic band structure. Of particular importance to understanding the physics of this system is how the bandgap energy Eg and spin-orbit splitting energy Δo vary relative to one another as a function of Bi content, since in this alloy it becomes possible for Δo to exceed Eg for higher Bi fractions, which occurrence would have important implications for minimising non-radiative Auger recombination losses in such structures. However, this situation had not so far been realised in this system. Here, we study a set of epitaxial layers of GaBixAs1−x (2.3% ≤ x ≤ 10.4%), of thickness 30–40 nm, grown compressively strained onto GaAs (100) substrates. Using room temperature photomodulated reflectance, we observe a reduction in Eg, together with an increase in Δo, with increasing Bi content. In these strained samples, it is found that the transition energy between the conduction an...

Optical absorption edge of semi-insulating GaAs and InP at high temperatures

The temperature dependences of the optical absorption edges of semi-insulating GaAs and InP have been measured from room temperature to 905 °C and 748 °C, respectively, with accuracies of ±1 °C at room temperature and ±5 °C at 900 °C. The temperature dependence of the optical absorption edge is adequately reproduced by an Einstein model although the Varshni model gives an improved fit to the band gap. Finally, the widths of the absorption edges are correlated with ionicity.

Clustering effects in Ga(AsBi)

The photoluminescence from a Ga(AsBi) sample is investigated as a function of pump power and lattice temperature. The disorder-related features are analyzed using a Monte Carlo simulation technique. A two-scale approach is introduced to separately account for cluster localization and alloy disorder effects. The corresponding characteristic energy scales of 11 and 45 meV are deduced from the detailed comparison between experiment and simulation.