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.
The procedures of fabrication of electrical contacts and modification of their properties in high-resistivity CdTe:Cl crystals have been investigated to obtain diodes for nuclear radiation detectors. Excimer laser annealing was utilized to attain heavy In doping of the CdTe surface region. Using various preliminary treatments of the CdTe crystal surface and studying electrode vacuum deposition procedures and laser processing of CdTe crystals pre-coated with an In dopant film, the optimal conditions of fabrication of In/CdTe/Au diode structures were elaborated. It was considered that a Schottky barrier at the Au-CdTe contact was formed and a shallow built-in p-n junction at the In-CdTe interface arose as a result of both thermal In diffusion and effect of laser-induced shock waves under irradiation of the samples from the In-contact side. The In/CdTe/Au diodes had sharp I-V characteristics. Laser irradiation remarkably decreased the leakage current and increased the forward current. Moreover, the electrical properties of laser-formed p-n junctions were being improved after storage of the samples. The detectors showed a low leakage current (1-8 nA/cm2 at 100 V and 300 K), optimal electrical barrier (0.72 eV) and high energy resolution (3.1 keV FWHM at 122 keV at γ-ray peak of Co-57). Variations in leakage currents and radiation detection properties of fabricated CdTe wafers and pixels diced from the wafers were associated with laser stimulated defect formation in the bulk of CdTe and generation of extended defects during the pixels performance. The regimes of injection and recombination in fabricated CdTe diodes have been analyzed.
M-π -n detector and Schottky detector which were different barriers were evaluated by measuring spectrum as imaging detectors. These spectrum performances showed that the effect of polarization in these detectors was similarly occurred. The current-time characteristics and capacitance-time characteristics were measured for the detectors and it was found that these characteristics were due to the effect of polarization.
An X-ray color scanner that can distinguish between different photon energies has been developed using X-ray line sensors. This scanner consists of a 64-channel cadmium telluride (CdTe) radiation detection array, a 64-channel application specific integrated circuit (ASIC), and a signal processing circuit that controls both the ASIC and data communication. An X-ray or gamma ray photon absorbed by the CdTe detector is converted into an electric charge proportional to the energy of the incident photon and amplified to an electric pulse with an amplitude proportional to the photon's energy. The pulse's height is then analyzed by the ASIC using 5 different threshold voltages. The X-ray color scanner has the ability to count up to approximately 2 million photons per CdTe element per second. When used as a photon counting system, the X-ray color sensor has a very high signal to noise ratio, since it removes electric noise in the low energy spectrum during the analysis process. When appropriately selected energy thresholds are used, X-ray color scanners, used for X-ray imaging or X-ray CT, were shown to remove the effects of beam-scattering noise and beam-hardening.
Photon-counting mode CdTe two-dimension X-ray imaging device with energy distinction function was investigated. Two-dimension M-π-n CdTe detectors were fabricated on 0.5mm-thick single CdTe wafer by the excimer laser pattern doping technique. It is important that the imaging detector shows uniform performance in all pixels, so we measured energy spectra of the two-dimensional detector fabricated by the excimer laser pattern doping technique. The energy spectra about 4 keV of FWHM of 122 keV peak of Co-57 were obtained, and it was found that uniformity was high in all pixels. For investigating charge share problem, 2 x 2 structured patterned-dope detector with and without separation ditches were prepared and compared. Both sample showed almost same crosstalk carrier, about 3% calculated from Co-57 radioisotope energy spectrum above 50 keV regions. Only 0.05 % crosstalk counts was obtained from both samples by using narrow 6 KeV energy windows at 122 keV for estimation in energy distinction imaging with energy window. These results show that patterned n-type doping by metal mask and excimer laser technique is effective for pixel separation CdTe x-ray imaging device.
512-pixel CdTe super-liner imaging scanner was developed. This device was consist with 512 chips of M-π-n CdTe diode detector fabricated by excimer laser doping process, 8 chips of photon-counting mode 64ch ASIC with FPGA circuit, USB2.0 interface with 1-CPU. It has 5 discriminated levels and over 2Mcps count rate for X-ray penetration imaging. This imaging scanner has 512 discrete CdTe chips for detector arrays with the length of 2.0mm, width of 0.8mm and thickness of 0.5mm. These chips were mounted in four plover array rows for high-resolution imaging with 0.5mm-pitch, therefore the pixel pitch was over the pixel width. When images were taken with scanning system with this arrays, we could obtain over-resolution than pixel width. In this paper, this "over-resolution" imaging will be called "super resolution imaging". In high-resolution imaging device, the pixel devices on one substrate were formed by integrated process, or many discrete detector chips were installed on circuit board, usually. In the latter case, it is easer to make each detector chips than former case, and it are no need to consider charge sharing phenomena compare with one-chip pixel devices. However, a decrease in pixel pitch makes the mount to the detector chip to the ASIC board difficult because the handling will also be difficult The super-resolution technique in this scanner by pixel-shift method for X-ray imaging is shown in this paper
Energy discriminate type CdTe imaging detector was developed for hard X-ray imaging. The device has 4 x 128 structured 512 semi-linear M-π-n CdTe pixels with 0.5 mm pixel pitch and 256 mm length. Each pixel was 2mm x 0.8 mm size and connected to photon-counting type data processing circuit integrated as 64ch ASIC. The ASIC could be operated at high speed over 1M cps and it has 5 levels of energy discriminated thresholds and 15bit counter with each thresholds levels. The imaging detector was designed for energy discriminated hard X-ray imaging using X-ray tube source, since its high incident rate correspondence by high speed operating. The detector was consisted by 512-CdTe detector chips, 8-ASICs with control digital circuits, system control MPU, interface device and high-voltage source in the detector unit, and connected to conventional laptop personal computer thorough USB2.0 interface. In this study, we build energy-discriminated X-ray penetrating imaging system with this CdTe 512 pixels imaging detector unit, 90keV micro-focus X-ray source, mechanical scanning system and imaging software. The energy discriminate X-ray penetrating imaging was carried out by this system.
A 128-pixel gamma-ray imaging detector unit, which has high-energy resolution with room temperature operation, was fabricated by using diode-type CdTe detector. The diode structure was prepared by indium-doped n-type CdTe thin layer formed by excimer laser doping on one-side of high resistivity p-like single crystal CdTe wafer, and gold electrode as a Shottkey electrode evaporated on opposite side of the wafer. This diode-detectors showed good diode I-V characteristics with low leakage current. This CdTe detectors were pixelized in the 2mm × 2mm, and the 128 chips (32 × 4 chips) were mounted on the ceramic printed circuit boards at 3mm interval with 1mm gap. The printed circuit boards are directly connected with the MCSA-EX1 ASIC chip and 128 ch radiation spectrum analyzer systems. When using the Am-241 and the Co-57 as radioisotopes, the spectral response from all pixels had within 4.4 keV of FWHM at 122 keV peak of Co-57 for radiation performed at room temperature. The intensities of the peak from pixels were also uniform.
Selective operation of laser doping process and laser ablation process using KrF excimer laser irradiation (wavelength 248nm, pulse duration 20ns) was carried out that is used for fabricating an integrated gamma-ray imaging detector. At high vacuumed condition, laser irradiated surface region is heated up to ablated. At high-pressure condition, ablation is suppressed and impurity on the surface was melt and diffuse into the CdTe substrate.
We have fabricated an integrated imaging detector using above combination process. The detector showed low leakage current at room temperature and good gamma-ray detection property.
The fabrication methods and performances of CdTe radiation detectors in a p-i-n structure which helps to reduce the leakage current are reported. Two different types of doping methods were studied in order to attain heavily doped CdTe layers. One is the hydrogen plasma- radical-assisted metalorganic chemical vapor deposition (HPRA-MOCVD) and the other is the excimer laser processing, both of which are carried out at a low temperature. Using the HPRA-MOCVD, iodine doped n-type CdTe layers with carrier concentration around 1018 cm-3 and electron mobility of 100 cm2/V-s were grown epitaxially on the GaAs substrates at a low temperature of 150 degrees Celsius. As the other method, excimer laser was irradiated on the high resistivity CdTe crystals (resistivity: 109 (Omega) cm) by depositing a thin layer of dopant materials like Sb, Na2Te, or In on the crystals surfaces. For the Sb and Na2Te dopant, heavily doped p+-type thin layer was formed on the CdTe crystal, whereas In dopant exhibited n+-type layer. Carrier concentrations in those laser doped layers were in the order of 1019 cm-3. These low temperature processes can be adapted for the fabrication of CdTe and CdZnTe detectors.
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.
p-type doping of wide band gap II-VI semiconductors is a key technology to form the ohmic contact with metal electrodes in device fabrications. Using an alkaline metal compound such as K2S, Na2Se or Na2Te which contains dopant atoms, excimer laser doping experiments were carried out for ZnSe and CdTe. Influence of the electrical properties on this treatment was mainly measured by means of the Hall measurement. The resistivity of ZnSe drastically decreased from 105 to 10-2 Ohm cm and the value of hole carrier concentration increased up to 4.8 X 1019 cm-3. For CdTe also resistivity decreased from 105 to 10-1 Ohm cm. Formation of p-type ohmic contact in ZnSe p-n diode was also investigated.
With the use of polycrystalline cadmium telluride (CdTe) film as the photoconductive target a 1' x-ray imaging camera tube (vidicon) was developed. This vidicon yields an x ray signal current of 200 nA cm-2 for 2.58 mc kg-1 min-1 radiation. This is a higher value when compared with that of conventional x ray vidicon (45 nA cm-2) which uses PbO as the photoconductive target. At the target voltage of 35 V a dark current of 5 nA cm-2 is generated in the developed vidicon at room temperature. When this vidicon is used with a diode mode electron gun and a mixed field type deflection electrode, a resolution of more than 15 micrometers can be obtained. Rf sputtering method produces large and uniform films, therefore, 1.5' films could be easily fabricated. The vidicon developed with 1.5' CdTe film yields better shading characteristics, a wide field of vision which is twice as large, and a higher signal current compared to those of 1' type vidicon.
A new active mode technique for the measurement of 3D objects was developed using a CCD line sensor and a galvanomirror scanner. The CCD line sensor is placed in parallel to the laser light scanning plane. When a series of equally spaced laser light spots is projected on to a surface, the spot density on the surface is altered depending on the shape of the surface. By measuring the displacement of a light spot the distance between the camera and the object can be determined using trigonometry. During the measurements, laser light was vertically scanned and object was horizontally moved. The measurement error for an object of 1 m apart from the camera is 0.08 percent in scanning direction and 0.2 percent in object-height direction. A modified experiment also was performed for the purpose of shape recognition of a given object. The success of this experiment is demonstrated by using objects of rectangular, triangular and round shapes.
High quality amorphous silicon carbon (a-SiC:H) films with the wide bandgap of 2.4 eV are developed by using rf glow discharge deposition from highly hydrogen diluted C2H4 and SiH4. It has been shown that multi-layer photo-diodes using a-Si:H and a-SiC:H tend to have higher gain than 10, and special constructions of three multi-layers presented switching phenomena like a thyrister switch.
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