The temporal and spatial instability of photoelectric response of the CdZnTe crystals
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TRONIC AND OPTICAL PROPERTIES OF SEMICONDUCTORS
The Temporal and Spatial Instability of Photoelectric Response of the CdZnTe Crystals V. P. Mygal^ and A. S. Phomin^^ Zhukovsky National Aerospace University (KhAI), ul. Chkalova 17, Kharkov, 61070 Ukraine ^e-mail: [email protected] ^^e-mail: [email protected] Submitted August 15, 2005; accepted for publication September 14, 2005
Abstract—The spectral, temporal, and spatial instabilities of the photoelectric response I of CdZnTe crystals with a variety of structural inhomogeneities are revealed. It is shown that the character and interrelation of these instabilities are most pronounced in the diagrams dI(t)/dt = f(I(t)) and dI(ν)/dν = f(I(ν)), where ν is the frequency and t is time in which the processes accompanying the stability loss are clearly seen. PACS numbers: 72.40.+w, 61.72.Hh, 61.66.Fh DOI: 10.1134/S1063782606040063
The main problem of II–VI materials and the sensors based on them is the instability of characteristics, which is mainly caused by the presence of a variety of structural inhomogeneities in these materials. It has been shown that the reconstruction of the fields generated in CdZnTe crystals by two-dimensional structural defects, namely, twins, mosaicity blocks, slip bands, etc., significantly affects the photosensitivity spectrum and in some cases leads to instability in certain spectral ranges [1, 2]. Therefore, it is reasonable to assume that this reconstruction is also accompanied by temporal and spatial instability. The present study is mainly aimed at verifying this assumption. We studied both the time-independent photocurrent as a function of the location of a monochromatic optical probe and the dynamic photoresponse to the pulsed Π-like photoexcitation for Cd1 – xZnxTe crystals (x = 0.1–0.2) grown by vertical crystallization from the melt under various conditions [3]. Gold or indium–gallium contacts were deposited onto the opposite largest faces of the samples that had resistivity ρ ~ (1010–1011) Ω cm and were shaped as rectangular parallelepipeds 5 × 5 × 2 mm or 11 × 11 × 2 mm in size. The repetition rate of the monochromatic optical pulses varied in the range 0.01–1 Hz. The steady-state photocurrent spectra I = f(ν), where ν is the radiation frequency, and the dynamic photoresponse of the samples were measured in a field with the strength E = 10–104 V/cm using an electrometric transformer based on an AD795 operational amplifier. The signal was digitized, and the data obtained were processed using a computer. We examined the crystal quality of the samples by etching, infrared microscopy, and modified shadow method. The crystal surface was scanned by a monochromatic optical probe 50 µm wide using automated movement of
the sample with the rates from 0.5 to 10 mm/min. To reveal the stable (attractors) and unstable photoresponse cycles under Π-like photoexcitation, we studied the totalities of phase diagrams dI(t)/dt = f(I(t)), where t is time, which were obtained by monotonic heating and cooling of the samples according to [4]. Exam
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