Daisuke Mochizuki

Improved spectrometric performance of CdTe radiation detectors in a p-i-n design

CdTe radiation detectors were fabricated using a p-i-n design and a significant improvement in the spectral properties was obtained during room temperature operation. An iodine doped n-CdTe layer was grown on the Te faces of the (111) oriented high resistivity CdTe crystals at the low substrate temperature of 150 °C. An aluminum electrode was evaporated on the n-CdTe side for the n-type contact, while a gold electrode on the opposite side acted as the p-type contact. Very low leakage currents, typically 60 pA/mm2, were attained at room temperature (25 °C) for an applied reverse bias of 250 V. Detectors exhibited excellent spectral responses with an energy resolution of 1.42, 1.7, and 4.2 keV FWHM at 59.5, 122, and 662 keV γ peaks, respectively.

Pattern doping on CdTe by excimer laser irradiation

An impurity diffusion technique using an excimer laser was studied for the p-type doping of CdTe. A thin layer of Na 2 Te was evaporated on the CdTe crystal, and laser irradiation decomposed the Na 2 Te and diffused Na atoms into the Cd site. The resistivity of the top layer of the crystal decreased to below 1 Ωcm and showed p-type conductivity. This technique is applied for fine-patterned p-type doping on the crystal by laser irradiation through a shadow mask. Furthermore, p-i-n CdTe diodes were fabricated for an X-ray imaging device by growing an iodine-doped n-CdTe layer on one side of the crystal and fine-patterned p-type doping on the other side. This device showed good diode properties with a well-saturated reverse bias current and good imaging properties.

Fabrication of CdTe detectors in a new p-i-n design for gamma-ray spectroscopy

CdTe and related compound semiconductors are useful for high energy flux detection at room temperature. We propose a new fabrication technique for CdTe detectors in p-i-n design that is suitable for hard x-ray and gamma-ray spectroscopy. Using a high resistivity single crystal CdTe substrate, an iodine doped n-CdTe layer is grown homoepitaxially on one face of the crystal using the hydrogen plasma-radicals- assisted metalorganic chemical vapor deposition method working at a low substrate temperature of 150 degrees C. An indium electrode is evaporated on the n-CdTe side for an ohmic contact, while a gold electrode is placed on the opposite side which acts as a p-type contact. The p-i-n detector thus fabricated exhibited low leakage current at room temperature operation, below 0.5 nA at an applied bias of 350 V for a 2 X 2 mm2 detector of thickness of 1 mm. Leakage current further decreased to 16 pA at 350 V while cooling the detector down to -15 degrees C. Spectral responses of the detector showed improved energy resolution for different radioisotopes of energies in the range form few tens of keV to several hundred keV and stable operation when operated at high applied biases or slightly cooling the detector. Performance of the different detectors thus fabricated will be presented.