Y. Agata

Nagoya Institute of Technology, Nagoya, Aichi, Japan

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Publications (30)36.7 Total impact

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    ABSTRACT: Development of an electron-collecting-type pixel array by use of an epitaxially grown thick single-crystal CdTe layer on p +-Si substrate is discussed. To achieve such an array with an n-CdTe/p-like CdTe/p +-Si heterojunction diode structure, charge transport at the p-like CdTe/p +-Si heterointerface was studied. It was confirmed that ohmic conduction via holes occurs at this interface. A single-element detector was then fabricated by growth of 40 μm thick undoped p-like CdTe then 5 μm thick n-CdTe layers on the p +-Si substrate. Rectification by the diode detector was good, and its energy-resolving capability was demonstrated by detection of gamma peaks from the 241Am source, thus confirming the feasibility of using this structure for fabrication of an electron-collecting-type array.
    Journal of Electronic Materials 08/2014; 43(8). · 1.64 Impact Factor
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    ABSTRACT: We investigated MOVPE growth conditions to grow large-area and thick single crystal CdTe layers with uniform material properties directly on (211) Si substrates to develop nuclear radiation detectors. We found that group VI/II precursor flow-ratio as well as rapid thermal annealing performed by interrupting the growth at the initial stage has marked influence on the crystal quality. By using a VI/II precursor ratio of 3.0, and a 900 °C anneal performed in flowing hydrogen, we were able to achieve 1-sq inch sized thick single crystal CdTe that showed uniform material properties and high crystal quality throughout the wafer. We further demonstrated that the grown crystals were suitable for fabricating nuclear radiation detector. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 03/2014;
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    ABSTRACT: Annealing conditions of CdTe layers grown on Si substrates by metalorganic vapor-phase epitaxy were studied. Typically, 3-μm-thick n-type (211) CdTe layers were annealed for 60 s in flowing hydrogen at atmospheric pressure by covering their surfaces with bulk CdTe wafers. At annealing temperatures above 700°C, improvement of crystal quality was confirmed from full-width at half-maximum values of double-crystal rocking-curve measurements and x-ray diffraction measurements. Photoluminescence measurements revealed no deterioration of electrical properties in the annealed n-CdTe layers. Furthermore, annealing at 900°C improved the performance of radiation detectors with structure of p-like CdTe/n-CdTe/n +-Si substrate.
    Journal of Electronic Materials 11/2013; 42(11). · 1.64 Impact Factor
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    ABSTRACT: Charge transport properties of p-CdTe/n-CdTe/n+-Si diode-type nuclear radiation detectors, fabricated by growing p-and n-type CdTe epilayers on (211) n+-Si substrates using MOVPE, were studied by analyzing current-voltage characteristics measured at various temperatures. The diode fabricated shows good rectification properties, however, both forward and reverse biased currents deviate from their ideal behavior. The forward current exhibits typical feature of multi-step tunneling at lower biases, however, becomes space charge limited type when the bias is increased. On the other hand, the reverse current exhibits thermally activated tunneling-type current. It was found that trapping centers at the p-CdTe/n-CdTe junction, which were formed due to the growth induced defects, determine the currents of this diode, and hence limit the performance of the nuclear radiation detectors developed.
    Journal of Applied Physics 10/2013; 114(16):164510. · 2.21 Impact Factor
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    ABSTRACT: Details about the MOVPE growth of thick single crystal CdZnTe layers on (211)Si substrates are presented. The growth was carried out at a substrate temperature of 650°C, using dimethylcadmium, dimethylzinc, and diethyltellurium precursors. Control of Zn-concentration in the range from 0 to 0.2 was performed by controlling the precursors' flow-rates and ratio. Results from the XRD showed grown layers were single crystalline with no phase separation observed. The 4.2 K PL results show high intensity bound-exciton peaks which shifted to higher energies with increasing Zn-concentrations. A p-CdZnTe/p-CdTe/n-CdTe/n+-Si hetero-junction diode was fabricated and evaluated for its possible application in nuclear radiation detector development, which exhibited good rectification property.
    IEEE Transactions on Nuclear Science 01/2013; 60(4):2859-2863. · 1.22 Impact Factor
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    ABSTRACT: We present the design and fabrication of a 2D monolithic pixel type detector array using vapor-phase grown thick CdTe epitaxial layers on Si substrates. Each pixel in the array consists of a p-CdTe/n-CdTe/n+-Si heterojunction diode structure, and pixels were patterned by cutting deep vertical trenches using a dicing saw. We also developed a low-temperature epoxy based bonding technique to bond the array to the read out electronic chip. Preliminary evaluation shows fabricated array is capable for spectroscopic imaging application, and this fabrication technique is useful in developing larger imaging arrays.
    IEEE Transactions on Nuclear Science 12/2012; 59(6). · 1.22 Impact Factor
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    ABSTRACT: We present fabrication process of two-dimensional (2D) energy discriminating pixel-type imaging arrays. Each pixel in the array possesses a p-CdTe/n-CdTe/n+-Si heterojunction diode structure. The array is fabricated using metalorganic vapour-phase epitaxy grown thick layers of single crystal CdTe on (211)Si substrates. In order to avoid the charge sharing problem among the adjacent pixels, we used a mechanical saw based pixel pattering technique, which produced pixels that are isolated from each other by narrow vertical trenches. Preliminary evaluation with room-temperature reverse biased dark current values exhibited uniform response from the entire pixels in the array. This opens a new possibility for developing large-area imaging arrays in a simple and efficient way. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 08/2012; 9(8-9):1848-1851.
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    ABSTRACT: 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.
    IEEE Transactions on Electron Devices 01/2012; 59(12):3450-3455. · 2.06 Impact Factor
  • K. Yasuda, M. Niraula, Y. Agata
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    ABSTRACT: We present a summary of our work towards developing spectroscopic and nuclear imaging detectors using epitaxially grown thick single crystal CdTe layers on Si substrates. High crystalline quality thick single crystal CdTe layers (>260 mum) were obtained where the growth rates could be varied from 10-70 mum/h by adjusting the precursor's flow rates, ratios and the substrate temperatures. Both high resistivity p-like CdTe layers and highly conductive n+-CdTe layers with controlled electrical properties were obtained using iodine as a dopant, but using different growth conditions. Detectors were fabricated in a p-CdTe/n+-CdTe/n+-Si heterojunction diode structure, which demonstrated their energy discrimination capability by resolving energy peaks from a gamma source. Details on the growth characteristics and the fabrication process for a 2-D imaging array are presented.
    Proc SPIE 10/2010;
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    ABSTRACT: Electrical properties of halogen-doped CdTe layers grown on Si substrates using iodine and chlorine dopants are presented. No change in electrical properties of the layers was observed with chlorine as a dopant. However, doping with iodine resulted in highly conductive n-type layers or highly resistive p-type layers depending upon the growth conditions, even though a similar amount of dopant was introduced into the growth chamber. Layers grown at 560°C, with a vapor-phase Te/Cd precursor ratio of 3.0, were p-type. The resistivity of the layers remained unchanged for low dopant supply rates, but increased abruptly when the dopant supply rate was increased beyond a certain value. On the other hand, layers grown at 325°C with Te/Cd ratios from 0.1 to 0.25 were n-type. A maximum free electron concentration of 1.3×1017cm−3 was obtained at room temperature. The types and conductivities of the grown layers were strongly dependent on the growth conditions. KeywordsCdTe epilayers-halogen doping-MOVPE-Si substrates-radiation detectors
    Journal of Electronic Materials 01/2010; 39(7):1118-1123. · 1.64 Impact Factor
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    ABSTRACT: 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.
    IEEE Transactions on Nuclear Science 09/2009; · 1.22 Impact Factor
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    ABSTRACT: 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 600<sup>deg</sup>C. 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<sup>+</sup>-Si heterojunction diode structure, which exhibited clear rectifying behavior with a low value of room-temperature reverse bias leakage current, typically 0.11 muA/cm<sup>2</sup> at 100 V bias. The detector leakage current was reduced by three orders of magnitude from the room-temperature value at -30<sup>deg</sup>C. The detector clearly demonstrated its spectroscopy capability by resolving energy peaks from the <sup>241</sup>Am gamma isotope.
    IEEE Transactions on Nuclear Science 07/2009; · 1.22 Impact Factor
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    ABSTRACT: We report on the growth of very thick (>260μm) high-crystalline-quality single-crystal CdTe epitaxial films on (211) Si substrates in a metalorganic vapor-phase epitaxy reactor, and the development of gamma ray detectors and their radiation detection properties. Films were grown with a high growth rate varying from 40μm/h to 70μm/h. A heterojunction diode was fabricated by growing a 90-μm-thick CdTe layer on an n +-Si substrate, which exhibited good rectifying behavior and had a low reverse bias leakage current of 0.18μA/cm2 at 100V bias. The diode clearly demonstrated its gamma radiation detection capability by resolving energy peaks from the 241Am radioisotope during room-temperature measurements. By cooling the diode detector to −30°C, the leakage current could be reduced by three orders of magnitude from the room-temperature value. At this operating condition dramatic improvements in the pulse height spectrum were observed.
    Journal of Electronic Materials 08/2008; 37(9):1391-1395. · 1.64 Impact Factor
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    ABSTRACT: CdTe/n<sup>+</sup>-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<sup>+</sup>-Si substrate, followed by the growth of 100 mum thick undoped p-CdTe layer. The diode fabricated showed very good rectification property with a low value of the reverse bias leakage current, typically 1.26 times 10<sup>-7</sup> A/cm<sup>2</sup> for an applied reverse bias of 60 V. The diode clearly demonstrated the gamma radiation detection capability by resolving energy peaks from the <sup>241</sup>Am radioisotope during the radiation detection test performed at room
    IEEE Transactions on Nuclear Science 09/2007; · 1.22 Impact Factor
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    ABSTRACT: Deep isolation trenches with high aspect ratio were formed on CdTe crystals using the excimer laser etching technique. Vertical trenches 80-μm deep and 55-μm wide were formed in 5min by irradiating a laser on the CdTe crystal through a contact-type metal mask in vacuum. Surface chemistry of the laser-irradiated CdTe crystal was examined by Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and photoluminescence (PL) to determine the extent of laser-induced damage. It was found that the damaged layer remains confined to the thin surface layer in the submicron range, which could be easily removed by a light Br-methanol etch.
    Journal of Electronic Materials 07/2007; 36(8):837-840. · 1.64 Impact Factor
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    ABSTRACT: CdTe/n<sup>+</sup>-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<sup>+</sup>-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<sup>-7</sup> A/cm<sup>2</sup> for an applied reverse bias of 50V. The diode clearly demonstrated its gamma radiation detection capability by resolving energy peaks from the <sup>241</sup>Am radioisotope during the radiation detection test performed at room temperature.
    Nuclear Science Symposium Conference Record, 2006. IEEE; 12/2006
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    ABSTRACT: Room temperature nuclear radiation detectors with energy discrimination capability developed by growing thick cadmium telluride (CdTe) epitaxial layers directly on n<sup>+</sup>-Si substrates in a metal-organic vapor phase epitaxy system is reported for the first time. The CdTe/n<sup>+</sup>-Si heterojunction diode detector exhibited good rectification and charge collection properties. The reverse leakage currents were typically 1times10<sup>-7</sup> to 5times10<sup>-7 </sup> A/cm<sup>2</sup> at 50-V bias. The detector clearly demonstrated its energy discrimination capability by resolving gamma peak from the <sup>241</sup>Am radioisotope during radiation detection test at room temperature
    IEEE Electron Device Letters 12/2006; · 2.79 Impact Factor
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    ABSTRACT: Direct growth of high-quality, thick CdTe (211) epilayers, with thickness up to 100 µm, on Si (211) substrates in a vertical metalorganic vapor phase epitaxy system is reported. In order to obtain homo-orientation growth on Si substrates, pretreatment of the substrates was carried out in a separate chamber by annealing them together with pieces of GaAs at 800–900°C in a hydrogen environment. Grown epilayers had very good substrate adhesion. The full-width at half-maximum (FWHM) value of the x-ray double-crystal rocking curve from the CdTe (422) reflection decreased rapidly with increasing layer thickness and remained between 140–200 arcsec for layers >18 µm. Photoluminescence measurement at 4.2 K showed high-intensity, bound excitonic emission and very small defect-related deep emissions, indicating the high crystalline quality of the grown layers. Furthermore, a CdTe/n+-Si heterojunction diode was fabricated that exhibited clear rectifying behavior.
    Journal of Electronic Materials 01/2006; 35(6):1257-1261. · 1.64 Impact Factor
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    ABSTRACT: 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 μm. Radiation detectors were fabricated in p-CdTe/n-CdTe/n<sup>+</sup>-GaAs structure, where a 2-5 μm thick iodine-doped n-CdTe buffer layer was first grown on the n<sup>+</sup>-GaAs substrate, followed by about 100-μm-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 μA/cm<sup>2</sup> at 40-V bias, and clearly demonstrated energy discrimination capability by resolving the 59.5-keV gamma peak from the <sup>241</sup>Am radioisotope during the radiation detection test. Some results on direct growth of CdTe epilayers on Si substrates are also presented.
    IEEE Transactions on Nuclear Science 11/2005; · 1.22 Impact Factor
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    ABSTRACT: A CdTe/n+-GaAs heterojunction diode for a room-temperature nuclear radiation detector has been developed and demonstrated. The heterojunction diode was fabricated by growing a 2–5-µm-thick iodine-doped n-CdTe buffer layer on the n+-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/cm2 to 5 µA/cm2 at 40 V bias. The detector clearly demonstrated its energy resolution capability by resolving the 59.54-keV gamma peak from the 241Am radioisotope during the radiation detection test.
    Journal of Electronic Materials 05/2005; 34(6):815-819. · 1.64 Impact Factor