M. Niraula

Surface processing of CdTe compound semiconductor by excimer laser doping

Abstract Laser processing technique has been developed to achieve heavy impurity doping in II–VI compound semiconductor such as CdTe which is excellent material for application in high energy flux detector because of its high absorption coefficient and energy resolution due to large atomic number and high carrier mobility. At first, transient temperature increment in CdTe due to excimer laser radiation will be discussed using computer simulation data. Using KrF excimer laser with a pulse width of 20 ns and a wavelength of 248 nm, one shot of laser of energy 80 mJ/cm 2 increases the surface temperature of CdTe more than 1000°C, the melting point of CdTe. As the laser pulse is very short and the penetration depth of UV laser light is very short, the depth of processed and modified layer is limited to few hundred angstroms. Secondly, we have experimentally studied the laser doping process for the application to detectors of high energy flux. Resistivities of CdTe surface processed by laser doping drastically decreased from 10 5 to 10 −1 Ω cm. p–i–n diode was thus fabricated on a high resistivity single crystal CdTe using excimer laser doping on one side for p-layer and n-type CdTe grown epitaxially on the other side. The p–i–n diodes resulted showed promising characteristics and high energy sensitivity. This technique is also promising to form ohmic contacts in other II–VI semiconductor devices. Because we can adapt this process at the final stages during device fabrication and also the laser effect is limited to a very thin layer, hence there will be no influence in the bulk characteristics of the device.

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.

Radical assisted metalorganic chemical vapor deposition of CdTe on GaAs and carrier transport mechanism in CdTe/n-GaAs heterojunction

temperature range of 150‐300 °C. The grown films have high resistivity in the order of 10 7 V cm for the entire growth range. Applicability of this heteroepitaxial CdTe layer on n-GaAs as an x-ray detector was then investigated. The carrier transport mechanism of the CdTe/ n-GaAs heterojunction was studied by means of current‐voltage measurements at different temperatures. The forward current was characterized by multitunneling capture-emission current and space charge limited current. The reverse current was considered as the generation current from the heterojunction interface states through the analysis of capacitance‐voltage measurement. It is found that for devices using minority carriers, this heterojunction alone is not useful because of the high concentration of interface states. A suitable modification, like an isotype heterojunction between GaAs and CdTe before forming the p-n junction, seems to be necessary.

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

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The authors have developed M-/spl pi/-n (metal high-resistivity p-type crystal, highly n-type epilayer) CdTe detectors in a new design that are suitable for X-ray and /spl gamma/-ray spectrometry in the range of a few tens to several hundred kilo-electron volts. Using high-resistivity single crystal CdTe substrates (resistivity /spl sim/10/sup 9/ /spl Omega//spl middot/cm), an iodine-doped n-CdTe layer was grown homoepitaxially on one face of each wafer at a low substrate temperature of 150/spl deg/C using the hydrogen plasma radical assisted metalorganic chemical vapor deposition technique. An indium electrode was deposited on the n-CdTe side as an ohmic contact by evaporation without heating the crystals, while a gold electrode was deposited on the opposite side for metallic contact. The leakage current was decreased to around 0.5 nA for a 2/spl times/2 mm/sup 2/ detector of thickness 1 mm at room-temperature (18/spl deg/C) and around 10 pA at -15/spl deg/C for an applied negative bias of 350 V. Low leakage currents in the detector enabled the authors to apply higher bias voltages resulting in better charge collection efficiency and improved spectral responses for different radioisotopes.

-Si Substrates

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.

Development of high-resolution CdTe radiation detectors in a new M-/spl pi/-n design

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

Diode type CdTe strip detectors were fabricated in a new and simple technique for imaging and spectroscopy applications. On a high resistivity p-like single crystal CdTe wafer, n-type strips were formed by evaporating a thin indium layer on the crystal through a metallic shadow mask and then diffusing it inside the crystal by irradiating with an excimer laser. On the opposite side of the crystal a gold electrode was evaporated on the whole surface. The current-voltage characteristics of the detectors exhibited diode-like property with a low value of reverse bias leakage current from the individual strips. The spectral response from all strips was very good and uniform with typical energy resolutions of 2.5 keV and 3.6 keV FWHM at 59.5 keV peak of Am-241 and 122 keV peak of Co-57 radioisotopes, respectively for radiation detection tests performed at room temperature. Details about the detector fabrication and the spectral results that demonstrate the imaging and spectroscopy capabilities of these detectors are presented.