O. I. Vlasenko

High resolution CdTe X- and gamma-ray detectors with a laser-formed p-n junction

Room temperature In/CdTe/Au diode detectors have been developed with record energy resolution (0.7% FWHM at the 662 keV peak of 137Cs) and electrical and detection properties of the detectors have been investigated. The detectors were fabricated using laser-induced doping of a thin surface layer of semi-insulating CdTe crystals with In. High resistivity p-like CdTe(111) crystals pre-coated with an In dopant film were irradiated with nanosecond laser pulses in a water environment that made it possible to introduce and activate In dopant atoms with high concentration and form a steep p-n junction in the surface region of the crystals. Multiple laser irradiation of the samples from the In-coated side increased forward current of the diodes and decreased reverse one. The special surface processing was used, including chemical and thermal procedures, to modify the surface state of the CdTe crystals before deposition of an In dopant film and electrodes. Finally, the room temperature CdTe-based X- and γ-ray radiation detectors with a p-n junction have been obtained with extremely high energy resolution.

Laser-Induced Doping of CdTe Crystals in Different Environments

Different procedures of laser-induced doping of the surface region of semi-insulating CdTe semiconductor are discussed. CdTe crystals pre-coated with an In dopant film were subjected to irradiation with nanosecond laser pulses in different environments (vacuum, gas or water). The dopant self-compensation phenomenon was overcome under laser action and In impurity with high concentration was introduced in a thin surface layer of CdTe. In the case of a thick (300-400 nm) In dopant film, laser-induced shock wave action has been considered as the mechanism of solid-phase doping. Formed In/CdTe/Au diode structures showed high rectification depending on the fabrication procedure. Diodes with low leakage current were sensitive to high energy radiation.

Features of characteristics and stability of CdTe nuclear radiation detectors fabricated by laser doping technique

Native and impurity point defects, complexes and extended defects which are formed during CdTe crystal growth and fabrication of diode structure are crucial for CdTe-based X-ray and gamma-ray detectors, cause deterioration of parameters and limit widespread practical application. Therefore, control of defect formation in CdTe crystals and device structures is important to achieve excellent charge collection efficiency and high energy resolution. Photoelectric, electrical and spectral properties of M-p-n structured CdTe diodes fabricated by the optimized excimer laser doping technique have been studied at different conditions. To make the diodes, a relatively thick In film was deposited on the surface of CdTe crystal and then it was irradiated with a laser pulse. The film served as an n-type dopant source as well as an electrode after laser irradiation. A Schottky contact was deposited on the opposite side of CdTe crystals. The In/CdTe/Au detectors have showed promise for nuclear radiation devices. However, the variations of I-V and C-V characteristics, fluctuations of time dependences of leakage current and degradation of spectral characteristics were evidences that electrically active defects cause non-uniform carrier trapping and induce excessive noise, deteriorating the detector performance.

Surface processing of CdZnTe crystals

The Cd1-xZnxTe (x = 0.1) crystals from two different manufacturers were studied by photoconductivity (PC) measurements. The samples 1 and 2 were subjected to chemical etching and irradiation with nanosecond laser pulses. The IR transmittance spectra of the crystals before and after laser irradiation were also monitored. The PC spectrum of the sample 1 had a typical one-band shape while the spectrum of the sample 2 exhibited two bands roughly corresponding to the bandgaps of CdTe and Cd1-xZnxTe that could be attributed to inhomogeneities in the surface region of the crystal. The positions of the maximum and red edge of the PC spectra did not correspond to the component compositions x in the bulk of Cd0.9Zn0.1Te crystals, however chemical polishing etching of the samples in a brominemethanol solution or/and laser irradiation led to this correspondence. Moreover, depending of laser pulse energy density, irradiation of Cd1-xZnxTe crystals resulted in a short-wavelength shift of the PC spectra, transformation of two bands to one in the case of the sample 2, and an increase in the photosensitivity of the semiconductor. The laser processing provided equalization of parameters in the surface and bulk regions.

Characteristics and stability of diode type CdTe-based x-ray and gamma-ray detectors

The electrical and spectral properties of the M-p-n structured In/CdTe/Au detectors have been investigated in comparison with the characteristics of In/CdTe/Pt Schottky diode detectors. The high-resistivity (111) oriented Cl-compensated p-like CdTe single crystals were used in both the cases. The CdTe diode structures with a p-n junction were fabricated by the laser-induced doping procedure. The M-p-n and Schottky diodes were examined by measurements of I-V and C-V characteristics and spectra excited by a 57Co radioisotope at room temperature. From the obtained spectrum measurement data, FWHM, shift and peak-to-valley ratios concerning the 122 keV and 14.4 keV peaks were calculated and the time dependences of these parameters were analyzed. Both types of CdTe-based diode detectors showed high rectification properties that allowed to apply higher bias voltage to reach full charge carrier collection thanks to extension of the depleted layer and hence to obtain high energy resolution.

CdTe diode detectors with a p-n junction formed by laser-induced doping

The procedures of fabrication of X- and γ-ray CdTe-based diode detectors with a p-n junction formed by laser-induced doping of the surface region of high resistivity p-like crystals in different environments are considered. The use of a relatively thick In dopant film and laser irradiation of In/CdTe structure in liquid allowed to apply higher laser energy densities and multiple irradiation of an In film. Finally, the room temperature In/CdTe/Au diode detectors with extremely high energy resolution have been obtained.

The COCAE detector: An instrument for localization — Identification of radioactive sources

The COCAE project develops an instrument for localization and identification of radioactive sources. For the localization task it will exploit the Compton scattered photons within its detecting layers. It is based on pixilated Cd(Zn)Te matrices in a stacked configuration. Progress has been achieved in all the components necessary for this technology. CdZnTe crystals have been grown of up to 75mm in diameter. A wafer level pixelization process has been tested and its results will be presented. Very good energy resolution is necessary for the performance of the instrument. For this reason we have investigated planar M-p-n diodes fabricated by using laser-induced solid phase doping and Schottky diodes created by Arion etching of the Cd(Zn)Te surfaces before deposition of Ni electrodes. The energy resolution achieved is better than 1% FWHM @662KeV. The development of a pixel CMOS readout integrated circuit has been undertaken. It outputs in digital format address, energy and time information for the pixels which have collected charge above a given threshold. Simulation studies of the minimum detection limits, of the Compton sequencing algorithms, of the angular resolution and of the reconstruction of the radioactive source position have been performed. Experiments with a precursor setup using an existing hybrid detector are underway.

Modification of surface state of CdZnTe crystals and contact formation

Different surface treatments such as chemical etching in bromine-containing solutions and nanosecond laser annealing were applied to provide the appropriate surface states of the CdZnTe (CZT) crystals prior to formation of metal-semiconductor contacts and electrode deposition. The effects of chemical etching and laser irradiation with different energy densities on the characteristics of the surface region of CZT solid solutions were studied. Depending on number and energy density of laser pulses and irradiation ambient conditions, photoelectric, electrical and optical properties of CZT crystals are changed and can be modified. Laser processing of CZT samples with the certain energy densities resulted in an increase in photosensitivity and transformation of photoconductivity (PC) spectrum profile. This was attributed to an increase in homogeneity in the crystal surface area and equalization of structural characteristics in the subsurface region and bulk of the samples. Irradiation of CZT crystals pre-coated with an electrode metal film allowed us to modify the I-V characteristics that can be employed in the fabrication of CZT-based X-ray and γ-ray detectors.

Fabrication of high resolution X/γ-ray detectors using laser-induced doping of CdTe in liquid

The uncooled X/γ-ray detectors with high energy resolution (FWHM <; 1% at 662 KeV) have been obtained. The detectors were developed as M-p-n structured In/CdTe/Au diodes using solid-phase doping of the surface region of high resistivity p-like CdTe semiconductor crystals laser irradiation of an In dopant film predeposited on the CdTe(111)B surface. The key feature was that the In/CdTe samples were subjected to multiple irradiation with nanosecond laser pulses in a liquid medium. Using water as an environment under laser-induced doping allowed to increase laser pulse energy density and number of laser shots that resulted in the formation of a shallow and abrupt p-n junction. The created high barrier makes it possible to apply high bias voltage (up to 1000 V) without a significant increase in the leakage current and thus to extend the depletion region and improve the carrier charge collection in the M-p-n diodes. The fabrication technology has provided a relatively high yield of the X/γ-ray detectors with acceptable characteristics.

Formation of CdTe diode detectors by laser irradiation in water

The method of laser-induced solid phase doping of a thin surface layer of high resistivity CdTe crystals has been developed and M-p-n structured In/CdTe/Au diodes have been fabricated. Semi-insulating p-like CdTe crystals pre-coated with an In dopant film were irradiated with nanosecond laser pulses in water environment that made it possible to introduce and activate In dopant atoms with high concentration and form a steep p-n junction in the surface region of the crystal. Multiple laser irradiation of the samples from the In-coated side increased forward current of the diodes and decreased reverse one. The fabricated diode detectors demonstrate high energy resolution.