Y. Agata

Thermal Stress Relaxation in GaAs Layer on New Thin Si Layer over Porous Si Substrate Grown by Metalorganic Chemical Vapor Deposition

We have proposed and introduced a thin Si layer over porous Si (SPS) substrate instead of the conventionally used Si substrate to overcome the residual thermal stress in GaAs layer on Si substrate (GaAs/Si). From the results of X-ray diffraction, low-temperature photoluminescence and Raman scattering, it was found that a significant reduction of the residual thermal tensile stress has been achieved. Our data clearly show that the SPS substrate is a promising substrate for overcoming the problems in GaAs/Si.

Development of Heterojunction Diode-Type Gamma Ray Detectors Based on Epitaxially Grown Thick CdTe on

Room temperature nuclear radiation detectors with energy discrimination capability developed by growing thick cadmium telluride (CdTe) epitaxial layers directly on n+-Si substrates in a metal-organic vapor phase epitaxy system is reported for the first time. The CdTe/n+-Si heterojunction diode detector exhibited good rectification and charge collection properties. The reverse leakage currents were typically 1times10-7 to 5times10-7 A/cm2 at 50-V bias. The detector clearly demonstrated its energy discrimination capability by resolving gamma peak from the 241Am radioisotope during radiation detection test at room temperature

hboxn^+

CdTe-n/sup +/-GaAs heterojunction diodes for room-temperature nuclear radiation detectors have been developed and demonstrated. The heterojunction diode was fabricated by growing a thin n-type CdTe buffer layer followed by the undoped p-like CdTe layer of about a 100 /spl mu/m thickness on the n/sup +/-GaAs substrates using metal-organic vapor phase epitaxy. The diode detectors exhibited good rectification property and had the reverse leakage currents of a few /spl mu/A/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.

-Si Substrates

We report on the development of nuclear radiation detectors based on epitaxially grown thick single crystalline CdTe layers. The optimization of the CdTe growth on the GaAs substrates in a metalorganic vapor phase epitaxy resulted high-structural quality and thick CdTe layers of thickness up to 200 /spl mu/m. Radiation detectors were fabricated in p-CdTe/n-CdTe/n/sup +/-GaAs structure, where a 2-5 /spl mu/m thick iodine-doped n-CdTe buffer layer was first grown on the n/sup +/-GaAs substrate, followed by about 100-/spl mu/m-thick undoped p-like CdTe layer. The detectors exhibited good rectification property and good charge transport property. They showed reverse bias leakage currents typically from 1 to 5 /spl mu/A/cm/sup 2/ at 40-V bias, and clearly demonstrated energy discrimination capability by resolving the 59.5-keV gamma peak from the /sup 241/Am radioisotope during the radiation detection test. Some results on direct growth of CdTe epilayers on Si substrates are also presented.

Development of nuclear radiation detectors with energy resolution capability based on CdTe-n/sup +/-GaAs heterojunction diodes

High crystalline quality thick films of single crystal CdTe were grown directly on (211) Si substrates using MOVPE growth technique for gamma ray detector fabrication. A highest growth-rate of 65 mum/h was achieved at a substrate temperature of 600degC. Films were monocrystalline as confirmed from the X-ray diffraction pattern. Results from the 4.2 K photoluminescence measurement showed films were of good crystalline quality. The gamma detector was fabricated in a p-CdTe/n-CdTe/n+-Si heterojunction diode structure, which exhibited clear rectifying behavior with a low value of room-temperature reverse bias leakage current, typically 0.11 muA/cm2 at 100 V bias. The detector leakage current was reduced by three orders of magnitude from the room-temperature value at -30degC. The detector clearly demonstrated its spectroscopy capability by resolving energy peaks from the 241Am gamma isotope.

Development of nuclear radiation detectors with energy discrimination capabilities based on thick CdTe layers grown by metalorganic vapor phase epitaxy

CdTe/n+-Si heterojunction diodes were fabricated and characterized for the development of gamma ray detectors. With the careful control of the growth parameters thick single crystal CdTe epilayers of high-crystalline quality were grown directly on the (211) Si substrates in a metalorganic vapor phase epitaxy. The heterojunction diode was fabricated by growing a 5 mum thick n-type CdTe buffer layer on the n+-Si substrate, followed by the 100 mum thick undoped p-CdTe layer growth. The diode fabricated showed very good rectification property with a low value of the reverse bias leakage current, typically 1.2 times 10-7 A/cm2 for an applied reverse bias of 50V. The diode clearly demonstrated its gamma radiation detection capability by resolving energy peaks from the 241Am radioisotope during the radiation detection test performed at room temperature.

MOVPE Growth of CdTe on Si Substrates for Gamma Ray Detector Fabrication

The growth characteristics and the crystalline quality of thick (~ 100 µm) (100) CdTe epitaxial layers grown on (100) GaAs and (100) GaAs/Si substrates in a metalorganic vapor phase epitaxy system for X-ray, gamma-ray detectors applications were investigated. The growth rate was constant up to the layer thickness of 30 µm, however it decreased afterwards with increasing layer thickness, which was attributed due to the decrease of the temperature at the growth surface. The full width at half-maximum values of the X-ray double crystal rocking curve decreased rapidly with increasing layers thickness, and remained around 50–70 arcsec for the layers thicker than 30 µm on both types of substrates, indicating their high crystalline quality. However, a small amount of residual strain was still observed on these thick layers.

Characterization of CdTe/n+-Si Heterojunction Diodes for Nuclear Radiation Imaging Detectors

We present the design and fabrication details of X-ray spectroscopic imaging arrays using metalorganic-vapor-phase-epitaxy-grown thick single-crystal CdTe layers on an n+-Si substrate. Each pixel in the array consists of a p-CdTe/n-CdTe/n+-Si heterojunction diode structure, which was fabricated by subsequently growing n- and p-CdTe layers on the n+-Si substrate. A mechanical dicing process using a diamond blade was used to make deep cuts on the p-CdTe side to define the pixels in an (8 × 8) array. We further developed a low-temperature conductive-epoxy-based bonding technique to bond the array to the readout electronic circuit via an interface board. Preliminary evaluation shows that the array fabrication technique and the bonding technique work good, and the array is capable of discriminating energies of the incident photon and can be applied for the energy-discriminating imaging purpose.

Growth and Characterization of Thick (100) CdTe Layers on (100) GaAs and (100) GaAs/Si Substrates by Metalorganic Vapor Phase Epitaxy

We studied the effect of iodine doping of CdTe films on Si substrates grown by MOVPE at different growth conditions. A high resistivity film was obtained by adjusting the growth temperature, Te/Cd precursor flow ratio and the dopant flow-rate. Our results show the film resistivity does not change linearly with the dopant flow-rate. The resistivity remains low and similar to that of undoped value for low dopant flow-rate, but increases abruptly when the dopant flow-rate is increased beyond certain value. Photoluminescence measurements of the films confirmed the dopant incorporation in the crystal. We explained the result of resistivity change due to the compensation of excess shallow acceptors by deep intrinsic donors.

Fabrication and Characterization of X-Ray Spectroscopic Imaging Arrays Based on Thick Single-Crystal CdTe Epitaxial Layers

We studied CdTe nuclear radiation detectors with multi-electrodes structure. The detector consists of a common cathode, while the anode has been separated into three electrodes: the central collecting anode, a guard ring electrode, and a guiding electrode. The detector was biased in such a way that the central anode collects electrons while the guiding electrode repel electrons and guide them towards the central anode. Application of the multi-electrode structure, an improvement in the detector performance both in terms of leakage current and the spectroscopy performance was obtained in comparison to the simple planar type detectors.