2s zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA . __ B __ Nuclear Instruments and Methods in Physics Research A 380 (1996) 84-87 NUCLEAR INSTRUMENTS a METHOOS IN PHVSICS Material analysis and characterization on zone refined and zone leveled vertical zone melt GaAs for radiation spectrometers D.S. McGregor”‘*, A.J. Antolak”, E.S. Cross”, Z-Q. Fangf, M.S. Goorsky’, R.L. Henryd, R.B. James”, D.C. Lookf, M.G. Mier’, D.H. Morse”, P.E.R. Nordquistd, R. Olsen”, M. Schieber”, T.E. Schlesingerb, E. Soria”, J.E. Toneyh, J. Van Scyocb, H. YoonC zyxwvutsrqponm “k&a Nutional Laboratories, MS-671, Livermore, CA 94550, USA hDepartment zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of Electrical and Computer Engineering, Carnegie Mellon University. Pittsburgh, PA 15213, USA ‘Department of Materials Science and Engineering, UCLA. Los Angeles. CA. USA “Naval Research Laboratov, Washington. DC 20375T&Xl. USA ‘Solid State Electronics Directorate, Wright Patterson Air Force Base, Dayton. OH 45433. USA ‘University Research Center, Wright State University, Dayton OH 45435. USA zyxwvutsrqponmlkjihgfedcbaZY Abstract GaAs is a wide band gap ( 1.42 eV) semiconductor that has shown promise as a room temperature operated -y-ray detector. A practical y-ray detector would be large in volume, hence the resistivity of the material must be high to ensure large depletion volumes and low leakage currents. Commercially available semi-insulating (SI) bulk GaAs is compensated by a balance between native defect deep donors (EL2) and residual dopant impurities. The high concentrations of electrically active deep and shallow levels are believed to contribute to electric field distortions observed in y-ray detectors fabricated from SI bulk GaAs. Hence, the controlled reduction of native defects and contaminant impurities may yield improved bulk GaAs for -y-ray detectors. Custom grown vertical zone melt (VZM) bulk GaAs is presently under investigation as an alternative material for room temperature operated y-ray detectors. The VZM technique allows for zone refinement (ZR) and zone leveling (ZL) of the GaAs ingots. Custom growth of the material allows for controlled changes in the bulk crystal, including deliberate reductions in EL2 and impurity concentrations. Comparisons are made to commercial vertical gradient freeze (VGF) and liquid encapsulated Czochralski (LEC) bulk GaAs. 1. Introduction Bulk GaAs has been under investigation as an alter- native material for room temperature operated y-ray detec- tors since the early 1960s [ 1 J. Undoped bulk GaAs is made semi-insulating through a careful balance between native deep donor defect levels (labeled EL2) and residual shallow acceptor impurities [2]. It has been shown that reverse biased radiation detectors fabricated from SI undoped bulk GaAs have truncated electric field distribu- tions [ 1,3-61. The electric field is split into a high field region and a low field region, of which the high field region increases linearly at approximately I I*rn per applied volt. The truncated field effect ultimately reduces the maximum practical active region depth achievable in the devices [7]. It is presently believed that the compensat- ing native deep donor defect centers (EL2) are responsible to some degree for the perturbation in the observed electric fields [3,6-81. Assuming that the deep donor EL2 centers are respon- sible for the electric field perturbation, it makes practical sense to reduce the total density of EL2 centers. GaAs material remains semi-insulating provided that and N, >Nd, (2) where N,, is the deep donor concentration, N, is the shallow acceptor concentration, and N,, is the shallow donor concentration. Evidence indicates that the EL2 center is related to the arsenic antisite [9,lO], and GaAs is generally grown slightly arsenic rich to ensure that the concentration of EL2 centers is adequately large to produce semi-insulating material [l I]. The EL2 concen- tration can be reduced by lowering the arsenic concen- tration, however if semi-insulating material is to be produced, the total impurity concentration must be lowered in order meet the criteria set by Eq. (1) and Eq. (2). *Corresponding author. The vertical zone melt (VZM) growth technique has 0168-9002/96/$15.00 Copyright 01996 Elsevier Science B.V. All rights reserved PII SO168-9002(96)00337-3