Comparative study of vertical gradient freeze grown CdTe with variable Sn concentration
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V. Babentsov Institute for Semiconductor Physics, Kiev 03028, Ukraine
A. Fauler and M. Fiederle Materialforschungszentrum, Freiburg D-79104, Germany (Received 6 October 2005; accepted 17 January 2006)
Four CdTe ingots with gradually increased concentration of the Sn impurity have been grown by the vertical gradient freeze method and were characterized with glow discharge mass spectroscopy, photoinduced current transient spectroscopy, resistivity, photoconductivity, and photoluminescence techniques. It was shown that the Sn impurity strongly influences resistivity and photoconductivity of the material. Concentration of Sn must be higher than the total concentration of residual acceptors to reach strong compensation. The middle-gap donor level pins the Fermi-level. Photoconductive high resistivity material can be prepared with Sn concentrations in the melt in the range 1018–1019 cm−3. In total, 6 electron traps and 3 hole traps were identified in the band gap by several complementary techniques.
I. INTRODUCTION
Semi-insulating CdTe is a promising material for many applications. Among them, room temperature spectroscopy of x-rays and gamma radiation is of specific interest.1,2 Normally, CdTe grown from the Te side of a phase diagram is a low resistivity material of the p-type conductivity. High resistivity is obtained by compensation of the residual acceptors with shallow or deeplevel donors (D).3,4 Among shallow donors, ClTe and InCd are the most known.5,6 They compensate acceptors, e.g., a cadmium vacancy VCd, creating the A-center complex (VCd–D).7 Stabilization of the compensation condition against variation of the donor/acceptor concentration can be reached with a mid-gap native, or impurity donor level.8–18 In this work, we studied compensation of CdTe with a favorable Sn impurity, which has a donor level in the mid-gap, and whose atomic number (50) is between that of Cd (48) and Te (52).10 We present the results obtained on several CdTe ingots grown with various Sn concentration in the melt. The purpose of this study is to increase understanding of defect structure of CdTe:Sn with special attention paid to deep levels substantially influencing
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0117 J. Mater. Res., Vol. 21, No. 4, Apr 2006
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the mobility-lifetime product and response time of the material, which can be considered as a promising one for fabrication of x-ray detectors. II. EXPERIMENTAL
The CdTe:Sn ingots, further labeled as VG1, VG2, VG3, and VG4, were grown at the Institute of Physics, Charles University (Prague, Czech Republic) by the vertical gradient freeze (VGF) method with various concentration of Sn in the melt. The CdTe was synthesised from 6N purity Cd and Te in the same ampoule. The optimal Cd pressure for the CdTe growth was found to be of 1.2 atm at the axial temperature gradient of 5 °C/cm and cooling rate of 0.5 °C/h. The ingot VG1 was nominally undoped CdTe, while the next
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