From Event: SPIE Optical Engineering + Applications, 2017
Thallium bromide is a promising semiconductor for room temperature radiation detection. A primary remaining challenge of this soft material is defects associated with impurities, strain, reactivity, or electromigration. Defect states near mid-gap promote rapid loss of carriers at high densities by Shockley-Read-Hall bimolecular recombination. We describe measurement and 3-D mapping of nonlinear photoconductive response using femtosecond pulse trains at 836 nm (1.48 eV), somewhat above mid-gap of TlBr. A confocal two-photon microscopy setup with photocurrent detection affords spatial resolution perpendicular to the surface. We report analysis of the irradiance dependence of the induced current as a function of depth in the sample. There are three separate contributions to the second-order current response. There is intrinsic two-photon absorption of non-defective TlBr. A finding of this work is that at relatively low irradiance, there is additional significantly enhanced two-photon absorption concentrated within about 1 mm of the surface and attributed to resonant enhancement by real intermediate states near mid-gap. Those same mid-gap states potentially contribute an electron-hole recombination term proportional to the square of carrier density (for intrinsic production as in a radiation detector), and this is in fact confirmed by analysis of the measurements. The spatial distribution of the defects can be displayed and the cross section for bimolecular carrier quenching can be extracted.
Drew R. Onken, Sergii Gridin, K. Burak Ucer, Richard T. Williams, Amlan Datta, and Shariar Motakef, "Three-dimensional mapping and analysis of mid-gap defect distributions in TlBr by a two-photon photocurrent microscope," Proc. SPIE 10392, Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XIX, 1039212 (Presented at SPIE Optical Engineering + Applications: August 09, 2017; Published: 29 August 2017); https://doi.org/10.1117/12.2276866.
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