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Babalola, Stephen
Ph.D. in Interdisciplinary Materials Science, December 2009

Research Information

Ph.D. Dissertation
Surface and Bulk Defects in Cadmium Zinc Telluride and Cadmium Manganese Telluride Crystals

Ph.D. Advisor
Leonard C. Feldman, Physics

Ph.D. Committee
Timothy Hanusa, Chemistry
Bridget Rogers, Chemical and Biomolecular Engineering
James Wittig, Electrical Engineering


Abstract. Cadmium Zinc Telluride (CZT) has emerged as the leading candidate material for room-temperature solid state X-ray and gamma-ray detector devices that find applications in nuclear nonproliferation and global security; medical and industrial imaging systems due to its attractive properties such as high average atomic number, optimal band-gap energy and high resistivity. Cadmium Manganese Telluride (CMT) is a potential alternative to CZT in nuclear detector applications with the possibility of obtaining larger, single grain crystals of more uniform composition than CZT. However, a major limiting factor is the sub-optimal quality of CZT and CMT crystals due to the many defects within the crystals. This dissertation reports studies of the surface and bulk defects in CZT and CMT crystals and how the defects affect the detector performance. Employing the microscale x-ray mapping with the National Synchrotron Light Source, we showed the detector performance to correlate with surface conditions, and using various techniques such as AFM and IR reflection microscopy coupled with different chemical etchants, we optimized the surface processing step of detector fabrication.

We have studied, amongst other things, the distribution of internal electric field within CZT and CMT detectors. Uniform distribution of internal electric field is desired in semiconductor nuclear detectors, and is usually assumed in the evaluation of CZT and CMT detector parameters. However, the electric field becomes distorted by the presence of defects that acts as traps and recombination centers. We have used a non-linear electro-optic (Pockels) effect to study the internal electric field of CZT and CMT detectors. We also developed an algorithm, using Interactive Data Language, or IDL, to calculate and visualize the internal electric field within the crystals. We have identified non-uniformities in the internal electric field, which are directly attributed to observed defects. The identification, quantification and analysis of these defects have enabled us to fabricate CMT nuclear detector and obtain, for the first time, a high energy (662KeV) gamma spectrum from a Cs source. This is a giant step towards realizing high energy room temperature nuclear detectors from CMT crystals. The findings associated with this thesis have contributed to the wider research efforts aimed at understanding the defects limiting the performance of CZT and CMT crystals as room temperature nuclear detectors. Some of these findings will be presented.

Selected Publications

Extended Defects in CdZnTe Radiation Detectors. Bolotnikov, AE; Babalola, SO; Camarda, GS; Chen, H; Awadalla, S; Cui, YG; Egarievwe, SU; Fochuk, PM; Hawrami, R; Hossain, A; James, JR; Nakonechnyj, IJ; Mackenzie, J; Yang, G; Xu, C; James, RB, IEEE TRANSACTIONS ON NUCLEAR SCIENCE, 56, 1775-1783 , (2009)

Study of Te inclusions in CdMnTe crystals for nuclear detector applications. Babalola, OS; Bolotnikov, AE; Groza, M; Hossain, A; Egarievwe, S; James, RB; Burger, A, JOURNAL OF CRYSTAL GROWTH, 311, 3702-3707 , (2009)

Effects of Te Inclusions on the Performance of CdZnTe Radiation Detectors. Bolotnikov, AE; Abdul-Jabbar, NM; Babalola, OS; Camarda, GS; Cui, Y; Hossain, AM; Jackson, EM; Jackson, HC; James, JA; Kohman, KT; Luryi, AL; James, RB, IEEE TRANSACTIONS ON NUCLEAR SCIENCE, 55, 2757-2764 , (2008)

Uniform Cr2+ doping of physical vapor transport grown CdSxSe1-x crystals. Roy, UN; Babalola, OS; Jones, J; Cui, Y; Mounts, T; Zavalin, A; Morgan, S; Burger, A, JOURNAL OF ELECTRONIC MATERIALS, 34, 19-22 , (2005)

Vapor growth and characterization of Cr-doped CdS0.8Se0.2 single crystals. Roy, UN; Babalola, OS; Cui, Y; Groza, M; Mounts, T; Zavalin, A; Morgan, S; Burger, A, JOURNAL OF CRYSTAL GROWTH, 265, 453-458 , (2004)

Vanderbilt University