Single Recombination Center Research Articles

Photorefractive effect of low-temperature-grown aluminum gallium arsenide

The ternary compound aluminum gallium arsenide is an important material that can be used in all-optical solid-state ultrafast diagnostic technology. The low-temperature-epitaxially-grown AlGaAs (LT-AlGaAs) not only has the characteristics of ultra-short carrier lifetime of low-temperature-grown gallium arsenide (LT-GaAs), but also possesses the advantage of adjustability of band gap, which will provide great flexibility for the design of ultra-fast diagnostic systems. We use low-temperature epitaxial growth technology to grow AlGaAs on a GaAs substrate. The low-temperature-grown AlGaAs can effectively absorb 400 nm pump light to generate excess carrier. Therefore, we use a femtosecond laser with a wavelength of 800 nm and a pulse width of 200 fs as a light source to generate 400-nm pump light after passing through the BBO crystal, and 800 nm light without frequency doubling as the probe light. Using such a light source, we build a pump probe experimental platform to test the LT-AlGaAs. We normalize the experimental results and deconvolute it with the normalized laser pulses to obtain the response function of the semiconductor to the pump light. Therefore, we know that the nonequilibrium carrier relaxation time is less than 300 fs, and the nonequilibrium carrier recombination time is 2.08 ps. Due to the special passivation process, the effect of surface recombination on the carrier decay process is greatly reduced. The As clusters introduced by low-temperature epitaxial growth form deep level defects are the main factor for accelerating carrier recombination. In order to understand the complex process of photogenerated nonequilibrium carriers in depth, we use the indirect recombination theory of single recombination center to calculate the carrier recombination process, and establish an LT-AlGaAs carrier evolution model. Thus we obtain the key physical parameter related to the recombination rate, which is the carrier trapping area. We also use a theoretical model of carrier-regulated refractive index to calculate the effect of carrier concentration on the amount of change in refractive index. Combining our AlGaAs carrier evolution model, we simulate the refractive index change process of LT-AlGaAs after being illuminated by pump light. The simulation results are in good agreement with the experimental results. The method can be used for the quantitative analysis of carrier evolution characteristics of semiconductor materials, and it can conduce to the optimization and improvement of ultra-fast response semiconductor materials.

Thermoluminescence study of X-ray and UV irradiated natural calcite and analysis of its trap and recombination level

Thermoluminescence (TL) of natural light-orange color calcite (CaCO3) mineral in micro-grain powder form was studied at room temperature X-ray and UV irradiation under various irradiation times. TL was recorded in linear heating rate (2K/s) from room temperature (300K) to 523K. Trapping parameters such as activation energy, order of kinetics, frequency factor have been evaluated by Computerized Glow Curve Deconvolution technique. Three electron trap centers had been estimated at depth 0.70, 1.30 and 1.49eV from the conduction band. Investigation of emission spectra recorded at various temperatures showed single recombination center at depth 2.74eV from the conduction band. Due to thermally assisted tunneling of electron and subsequent center-to-center recombination, a distinct peak of lower activation energy (0.60eV) was observed at relatively higher temperature (∼360K) for X-ray irradiated sample. In UV excitation, there was an indication of photo-transfer phenomenon, where low TL intensity might have been observed; but due to simultaneous excitation of electrons from valence band to the trap level, TL intensity was found to increase with UV irradiation time. The results obtained within temperature range 300–523K were explained by considering a band diagram.

Simulating the Radiation Response of GaAs Solar Cells Using a Defect-Based TCAD Model

Three-dimensional TCAD simulations are used for physics-based prediction of space radiation effects in III-V solar cells, and compared with experimentally measured characteristics of a p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> n GaAs solar cell with AlGaAs window. Dark and illuminated I-V curves as well as corresponding performance parameters are computed and compared with experimental data for 2 MeV proton irradiation at various fluences. We analyze the role of majority and minority carrier traps in the solar cell performance degradation. The traps/defects parameters used in the simulations, for n-type and p-type GaAs, are derived from Deep Level Transient Spectroscopy (DLTS) data. The physics-based models allow a good match between simulation results and experimental data. However, assuming the device performance is dominated by a single recombination center is not adequate to completely capture the degradation mechanisms controlling the photovoltaic performance.

On the expected order of kinetics in a series of thermoluminescence (TL) and thermally stimulated conductivity (TSC) peaks

As reported in the literature, both in experimental results and in simulated glow curves, in a series of TL peaks associated with a series of trapping states and a single recombination center, the peaks tend to be of first order. In the present work we show theoretically and demonstrate by examples of numerical simulations that the last peak in a series obtained by a model of a single recombination center and multiple traps may be of second order whereas the lower-temperature peaks are usually of first order. This is the case even when retrapping is significantly faster than recombination. In some cases, the last peak has a long tail, longer than that of second-order peaks, which has to do with a different mechanism that has been discussed in a recent paper. Similar simulations of a more complex and more realistic situation of a model with multiple trapping states and multiple recombination centers have been performed. The prevalence of first-order appearance of both the curves of free electrons, associated with thermally stimulated conductivity (TSC) and of TL, evaluated for randomly chosen sets of trapping parameters, is shown by histograms. The occurrence of a small number of very high values of the symmetry factor is also discussed.

Double Injection in Deep-Lying Impurity Semiconductors

The analysis of double injection for a semi-insulator model with a single recombination center extends previous treatments by varying the ratio of occupied and unoccupied centers. The voltage-current characteristic of the model consists of an Ohm's-law region followed by a square-law region that terminates into a vertically rising current (threshold of double injection). Expressions for the Ohm's-law-square-law intersection voltage and the threshold (breakdown) voltages represent experimental results better than expressions derived from earlier models. Together with experimental voltages from Au-doped Ge devices at low temperature, these expressions give the electron- and hole-capture-cross section of the recombination center.

Electron-Bombardment Induced Recombination Centers in Germanium

The rate of change of minority carrier lifetime in germanium crystals bombarded by 1-Mev electrons has been studied experimentally as a function of the initial resistivity of the material. Analysis of the data leads to the conclusion that a single recombination center controls lifetime in both n- and p-type material, that this level is located at either 0.21±0.005 ev from the conduction band or else at 0.26±0.005 ev from the valence band, and that the ratio of the cross section for capture of minority holes is 18 times the cross section for capture of minority electrons. The absolute value of the hole capture cross section is 4.5×10−15 cm2 if one uses the best available data for the probability of producing a Frenkel defect by a 1-Mev electron. This large value is compared to the findings of other authors in similar experiments.