Optical Pump–Terahertz Probe Study of HR GaAs:Cr and SI GaAs:EL2 Structures with Long Charge Carrier Lifetimes

The time dynamics of nonequilibrium charge carrier relaxation processes in SI GaAs:EL2 (semi-insulating gallium arsenide compensated with EL2 centers) and HR GaAs:Cr (high-resistive gallium arsenide compensated with chromium) were studied by the optical pump–terahertz probe technique. Charge carrier lifetimes and contributions from various recombination mechanisms were determined at different injection levels using the model, which takes into account the influence of surface and volume Shockley–Read–Hall (SRH) recombination, interband radiative transitions and interband and trap-assisted Auger recombination. It was found that, in most cases for HR GaAs:Cr and SI GaAs:EL2, Auger recombination mechanisms make the largest contribution to the recombination rate of nonequilibrium charge carriers at injection levels above ~(0.5–3)·1018 cm−3, typical of pump–probe experiments. At a lower photogenerated charge carrier concentration, the SRH recombination prevails. The derived charge carrier lifetimes, due to the SRH recombination, are approximately 1.5 and 25 ns in HR GaAs:Cr and SI GaAs:EL2, respectively. These values are closer to but still lower than the values determined by photoluminescence decay or charge collection efficiency measurements at low injection levels. The obtained results indicate the importance of a proper experimental data analysis when applying terahertz time-resolved spectroscopy to the determination of charge carrier lifetimes in semiconductor crystals intended for the fabrication of devices working at lower injection levels than those at measurements by the optical pump–terahertz probe technique. It was found that the charge carrier lifetime in HR GaAs:Cr is lower than that in SI GaAs:EL2 at injection levels > 1016 cm−3.

Investigation of Photoelectron Properties of Polymer Films with Silicon Nanoparticles

Hybrid samples consisting of polymer poly-3(hexylthiophene) (P3HT) and silicon nanoparticles were prepared. It was found that the obtained samples were polymer matrixes with conglomerates of silicon nanoparticles of different sizes (10–104 nm). It was found that, under illumination, the process of nonequilibrium charge carrier separation between the silicon nanoparticles and P3HT with subsequent localization of the hole in the polymer can be successfully detected using electron paramagnetic resonance (EPR) spectroscopy. It was established that the main type of paramagnetic centers in P3HT/silicon nanoparticles are positive polarons in P3HT. For comparison, samples consisting only of polymer and silicon nanoparticles were also investigated by the EPR technique. The polarons in the P3HT and Pb centers in the silicon nanoparticles were observed. The possibility of the conversion of solar energy into electric energy is shown using structures consisting of P3HT polymer and silicon nanoparticles prepared by different methods, including the electrochemical etching of a silicon single crystal in hydrofluoric acid solution and the laser ablation of single-crystal silicon in organic solvents. The results can be useful for solar cell development.

Influence of Divacancy-Oxygen Defects on Recombination Properties of n-Si Subjected to Irradiation and Subsequent Annealing

The variation of recombination properties in n-Si grown by the Czochralski method, doped to the free electron concentration n0 ∼ 10^14 ÷10^16 cm^−3, irradiated with 60Co y-quanta or 1-MeV electrons, and isochronously annealed for 20 min in the temperature interval 180–380∘C, in which divacancy-oxygen (V2O) complexes are formed and annealed, has been studied in detail. The nonequilibrium charge carrier lifetime т is found to significantly decrease after the annealing in a temperature interval from 180 to 280∘C, with the effect being stronger for low-resistive n-Si. It is shown that a change in т after the annealing at 180–380∘C is caused by divacancy defects, most probably V2O. By analyzing the experimental data with the help of the Shockley–Read–Hall statistics, it is found that the formation of V2O defects is characterized by an activation energy of 1.25±0.05 eV and a frequency factor of (1±0.5)×10^9 s^−1, and their annealing by an activation energy of 1.54±0.09 eV and a frequency factor of (2.1±1.4)×10^10 s^−1. The values of the hole capture cross-sections by singly and doubly charged acceptor states of V2O are obtained as: (5±2)×10^−13 and (8±4)×10^−12 cm^2, respectively.

Nonequilibrium Charge Carrier Lifetime Testing Equipment for Semiconductor Materials by a Contactless Microwave Phase Method

The accurate measurement of nonequilibrium charge carrier lifetime is of vital significance in research and manufacture of crystalline silicon solar cells. A testing equipment based on a contactless microwave phase method was implemented by being embedded with GPIB, FPGA and a lock-in analyzer. A friendly operation interface was developed, based on the graphic programming language LabVIEW. The virtue of the equipment is achieved by automatic data acquisition and processing, which improves the automatization, efficiency and accuracy of the measurement.

GaAs Pixel Detectors

ABSTRACTResults of investigation of X-ray sensors on the basis of GaAs compensated with chromium (HR GaAs) are presented in this work. HR GaAs material is shown to have the following physical parameters: the resistivity about 1GOhm*cm, the nonequilibrium charge carrier lifetime – hundreds of nanoseconds. Prototypes of microstrip and array HR GaAs sensors have been manufactured and tested. It is demonstrated that the sensors provide spatial resolution according to the pixel pitch and allow obtaining high quality X-ray images.