Ultrafast Time-resolved Characterization of (Cd,Mn)Te Semiconductors as Radiation Detectors PDF Download

Author: Allen S. Cross
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Languages : en
Pages : 222

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"This thesis is devoted to the optical characterization of Cd1-xMnxTe [(Cd,Mn)Te] semiconductor based radiation detectors. In it I present my research on the study of (Cd,Mn)Te single crystal as both a material for detection of high-energy x rays as well as for sub-picosecond voltage sensing. The work includes the practical implementation of (Cd,Mn)Te, its characterization using ultrafast spectroscopy, and comparative measurements to other competitive materials. Characterization of the voltage sensitivity of (Cd,Mn)Te is accomplished using time-resolved spectroscopy and sub-picosecond electro-magnetic pulses. My research shows that the electro-optic (EO) effect of (Cd,Mn)Te is nearly an order of magnitude greater than previously believed, and accomplishes voltage sensitivities for THz signals greater than traditional EO transducers currently available. For measurements of low-frequency signals, the results agree with the unexceptionally low textbook value; however, through various experiments studying the EO dependence on temperature, fabrication technique, and frequency, I show that the culprit for the low sensitivity at low frequency is free-carrier screening. Furthermore, testing the wavelength dependence shows that the sensitivity is enhanced for optical probe energies approaching the crystal bandgap, which in of itself, is tunable in the visible to near infrared spectrum by changing the manganese concentration. Comparative measurements to LiTaO3 are made showing the same sub-picosecond temporal resolution, but significantly greater voltage sensitivity absent of dielectric loading. I further report on the implementation of (Cd,Mn)Te photoconductive devices (PCDs) as the diagnostics x-ray detection tool used for conducting inertial confinement fusion (ICF) experiments in the OMEGA Facility at the University of Rochester's Laboratory for Laser Energetics, and compare the results to those of standard diamond PCDs. I present measurements of the (Cd,Mn)Te PCDs, intended to characterize both the temporal and spectral dependence of x-ray emissions from laser-illuminated targets during ICF experiments. The Cd0.95Mn0.05Te single crystals, doped with vanadium and annealed in Cd vapor, exhibit resistivity of [...characters removed]. A 1-mm-long and 2.3-mm-long detector is placed in the same housing as a 1-mm-long diamond PCD. Each device is preceded by a Be x-ray filter with 37% x-ray transmission at the 1-keV cutoff energy. The energy of the incident OMEGA laser pulses vary from 2.3 kJ to 28 kJ. Using targets of empty plastic shells, we observe two x-ray emission events separated by 1.24 ns: the first event is caused by heating of the shell that created a hot corona, while the second event is an x-ray emission from the fully compressed target. ICF experiments of targets with steel cores enable the analysis of time-resolved relaxation dynamics of photo-excited carriers in the (Cd,Mn)Te crystals. According to my calculations, the (Cd,Mn)Te material can effectively absorb x rays with energies of up to 200 keV. Thus, the (Cd,Mn)Te PCDs are likely to complement the diamond detectors currently used in the laser-confinement fusion experiments."--Leaves v-vi.