Development of highly reliable contacts on p-type CdTe for radiation detectors

2002 Fiscal Year Final Research Report Summary

• Project/Area Number: 13450287
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Structural/Functional materials
• Research Institution: KYOTO UNIVERSITY
• Principal Investigator: MURAKAMI Masanori Materials Science and Eng., Professor, 工学研究科, 教授 (70229970)
• Co-Investigator(Kenkyū-buntansha): MORIYAMA Miki Materials Science and Eng., Instructor, 工学研究科, 助手 (70303857)
• Project Period (FY): 2001 – 2002
• Keywords: radiation detectors / CdTe / contact materials

Research Abstract

Cadmium telluride (CdTe) is an attractive semiconductor material for X-ray and γ-ray radiation detectors. The advantages of CdTe are that it has a large bandgap energy (〜1.6eV) and a high average atomic number (Z_<cdTe>=50). However, due to the low mobility and short lifetime of holes in CdTe, a considerable amount of charge loss causes reduction of energy resolution in the detector. For a complete charge collection, very high bias voltage must be applied in the CdTe detectors. Thus, Schottky contacts, which provide very low leakage current with a high bias voltage, are required to achieve high resolution in CdTe detectors. Although the conventional approach to fabricate Schottky contacts for p-type CdTe involves the use of metals with low work function, such as indium (In), the time-dependent degradation of detection performance during device operation, known as "polarization effect", is often observed in the CdTe detectors with Schottky contacts. In order to apply CdTe to practical r adiation detector, highly stable Schottky contacts with low leakage current must be developed.
The purpose of the present study is twofold. The first is to develop of highly reliable Schottky In contacts for CdTe radiation detectors. The second is to understand the Schottky In contact formation mechanism by analyzing interfacial micro structure of the In films which were deposited at various substrate temperatures using X-ray diffraction (XRD) and transmission electron microscopy (TEM). These studies will provide us a guideline for developing Schottky contacts for p-type CdTe which would be used hi the radiation detectors.
The thermal treatment of the CdTe during the depositions of In remarkably decreased the leakage current and improved its stability. XRD analysis showed that In atoms reacted with CdTe forming In_4Te_3 or InTe during the depositions with T_S>200℃. The InTe contacts, which were prepared by the depositions with rather high T_s, drastically improved the stability of the detection performance. TEM observations revealed that the leakage current and the stability of the CdTe radiation detectors were strongly correlated with the micro structure of contact/CdTe interfaces.