Development of Nuclear Radiation Detectors Based on Epitaxially Grown Thick CdTe Layers on n^sup +^-GaAs Substrates

Journal of Electronic Materials, Jun 2005 by Niraula, M, Yasuda, K, Takagi, K, Kusama, H, Et al

A CdTe/n^sup ^-GaAs heterojunction diode for a room-temperature nuclear radiation detector has been developed and demonstrated. The heterojunction diode was fabricated by growing a 2-5µm-thick iodine-doped n-CdTe buffer layer on the n^sup ^-GaAs substrates, followed by about 100-µm-thick undoped p-like single crystalline CdTe layer using metalorganic vapor-phase epitaxy. The n-type buffer layer was found to be essential to improve the junction property of the diode detector. The diode detectors exhibited good rectification property and had the reverse leakage currents typically from 1 µA/cm^sup 2^ to 5 µA/cm^sup 2^ at 40 V bias. The detector clearly demonstrated its energy resolution capability by resolving the 59.54-keV gamma peak from the ^sup 241^Am radioisotope during the radiation detection test.

Key words: CdTe, epitaxial layer, heterojunction diode, radiation detector, spectroscopic detectors

INTRODUCTION

Development of CdTe and CdZnTe room-temperature nuclear radiation detectors for x-ray, gamma-ray spectroscopy as well as imaging in a wide variety of applications has been a major research topic over many years. Intensive research, which mainly focuses on the melt-grown bulk crystals, has produced high-quality, small-area mono-crystal wafers that can be readily used in high-resolution spectroscopy and simpler monitoring applications.1-3 In spite of the continued effort made in the material development, the growth of a large-area and uniform mono-crystal has still been a challenging job. Present large-area bulk crystals consist of a large number of electrically active defects causing an inhomogeneous charge collection in the detector.4-7 This has significantly hampered further development of these detectors in large field-of-view imaging and high-sensitive, high-resolution spectroscopy applications as required in many medical and industrial applications.

The epitaxial growth of thick CdTe or CdZnTe layers on large-area substrates, such as GaAs or Si, is a promising way to achieve the required large-area growth. There are previous reports on the vaporphase epitaxial growth of CdTe or CdZnTe layers on GaAs or Si substrates; however, the grown epilayer thickness is limited to a few tens of micrometers or even less.8-12 A major problem for the detector development using vapor-phase grown crystals is the thickness of the grown crystals. Depending upon the photon energy, it is necessary to optimize the epilayer thickness accordingly in order to achieve reasonable detection efficiency. In this regard, metalorganic vapor-phase epitaxy (MOVPE) growth is most suitable because thick layer growth with good homogeneity over a large area can be obtained. In addition, this technique also offers flexibility in the device design by allowing the conduction types and carrier concentration control of the epilayers with impurity dopings. A detector for medical imaging working in low-energy photon (up to 100 keV) requires a detector thickness from 100 µm to 500 µm, which can be obtained from this growth technique. We have previously reported on the successful growth of thick (up to 200 µm) and highquality CdTe epilayers on the GaAs substrates, and presented our efforts on detector development, which were based on CdTe/n -GaAs heterojunction diodes.13,14 In spite of achieving a good diode property, the charge transport property of those diodes was rather poor, and it was not possible to obtain energy information of incident nuclear radiation. It was considered that the interface defect states at the junction as well as defect states in the CdTe layers possibly introduced due to the Ga diffusion were responsible for the poor transport property.14 In order to overcome this problem, we introduced a low-temperature-grown iodine-doped buffer layer. It was found that this buffer layer is essential to improving the heterojunction property. In this work, we report on the detector fabrication and show its ability to resolve the energy of the incident nuclear radiation. We further discuss the effect of the buffer layer on improving the detector property.

DETECTOR FABRICATION

The detectors were fabricated by growing undoped p-like thick single crystalline CdTe epitaxial layers of thickness about 100 µm on the n^sup ^-GaAs substrates at 415-560°C in an atmospheric pressure MOVPE system. The details about the CdTe growth on the GaAs substrates and their characterization are reported elsewhere.13-15 In order to improve the heterojunction property, a 2-5^m-thick iodinedoped n-CdTe buffer layer was first grown at a low substrate temperature of 350°C, and was followed by the undoped p-like thick layer growth. The buffer layer has a room-temperature electron concentration of 1016-1017 cm 3. Gold electrodes were then evaporated on both sides of the detectors as an ohmic contact. Finally, the detectors were cut into small (1 mm × 2 mm) pieces, mounted on a chip carrier, wire bonded, and then encased in an aluminum enclosure. The diode detectors were evaluated by the current-voltage carried, the time-of-flight (TOF) measurements, measurement with a N^sub 2^-laser (λ = 337 nm) excitation to determine the carrier transport properties, and nuclear radiation detection tests. The cross-sectional diagram of the detector fabricated is shown in Fig. 1.