Marc Cuzin, CEA
Waldes Dusi, Istituto TESRE/CNR
Ralph James, Sandia National Laboratories
Glenn Knoll, Univ of Michigan
Michael O'Connell, U.S. Department of Energy
Tuviah Schlesinger, Carnegie Mellon Univ
Paul Siffert, Laboratoire PHASE/CNRS
Mike Squillante, RMD
- Bicron Corp.
- eV Products
- Johnson Matthey Electronics
- Moxtek Inc.
- Noranda Advanced Materials
- R.M.D. Inc.
- Sandia National Laboratories
- Spire Corporation
1997 Fall Exhibitor
Proceedings published as Volume 487
of the Materials Research Society
Symposium Proceedings Series.
* Invited paper
SESSION I1: CADMIUM ZINC TELLURIDE GROWTH
Chair: E. E. Eissler
Monday Morning, December 1, 1997
Salon H/I (M)
8:30 AM *I1.1
STRUCTURE AND PROPERTIES OF VERTICAL HIGH PRESSURE BRIDGMAN CdZnTe. F.P. Doty, D.J. Knuteson and C. Isaacson, Digirad Corporation, San Diego, CA.
Ingots of CdZnTe grown by the high pressure Bridgman (HPB ) method contain extended defects which can affect performance and yield of nuclear detectors. Thermal stresses and wall effects in the growth process cause plastic strain in the bulk, and the typical long cycle times used to produce large ingots result in an annealed microstructure characterized by a mosaic of subgrains. Loss of interface stability due to high pulling speeds or thermal instability in the furnace can result in inclusions and voids in the ingot which extend for millimeters or even centimeters under extreme conditions. These defects provide sites for heterogeneous nucleation of precipitates upon cooling, thus many features are decorated with tellurium under typical melt stoichiometry, and can be viewed in IR transmission. This paper presents new data on the structure of CdZnTe ingots up to 10 kg in mass grown in Digirad's HPB furnaces. Defects have been mapped using IR transmission micro- and macro imaging. Correlation of defects with electrical properties of the material and performance of subsequently fabricated devices will be presented in detail.
9:00 AM I1.2
INCREASING SUBSTRATE YIELD DURING CRYSTAL GROWTH OF CADMIUM ZINC TELLURIDE: A THEORETICAL STUDY. Jeffrey J. Derby, Krisanne Edwards, Andrew Yeckel, Department of Chemical Engineering and Materials Science and Army HPC Research Center, University of Minnesota, Minneapolis, MN; Simon Brandon, Department of Chemical Engineering, The Technion, Haifa, ISRAEL.
Large, high-quality, single crystals of Cadmium Zinc Telluride (CZT) are required for substrate material in the fabrication of a variety of radiation detectors. Specifically needed is increased size, quality, and availability of bulk, single-crystal CZT material at a lower cost than current production techniques allow. The work presented here represents one component of an aggressive program aimed at meeting the above manufacturing needs through advanced materials purification, crystal growth system development, large-scale numerical process modeling, novel sensor development, advanced process control, and state-of-the-art characterization techniques. We focus on crystal growth issues which will directly impact the extremely low yield of current growth procedures. We analyze several growth strategies and design features of the vertical Bridgman process developed by Johnson Matthey Electronics, Inc., and the horizontal Bridgman process developed by Texas Instruments through the use of steady-state and time-dependent, finite element models. A variety of techniques to improve yield are analyzed, including the use of growth and pause strategies, thermal engineering for convex interfaces, and shelf growth morphologies.
9:15 AM I1.3
REDUCTION IN VERTICAL BRIDGMAN CRYSTAL GROWTH OF CdZnTc. Y. Tao and S. Kou, Department of Materials Science and Engineering, University of Wisconsin, Madison, WI.
An attempt was made to reduce segregation of Zn in the vertical Bridgman growth of Cd1-xZnxTe. In order to keep the composition of the growing crystal constant, a replenishing melt was supplied from a second crucible immersed in the melt from which the crystal grew. The replenishing crucible had a long small-diameter passageway between the two melts to suppress diffusion. To prevent evaporation the melts were encapsulated with B2O3 under pressurized Ar. Effective segregation reduction was obtained in Cd_0.96)%%Zn0.04Te crystals.
9:30 AM I1.4
MODELLING OF TRANSPORT AND SEGREGATION IN VAPOUR GROWTH OF Cd(Te,Se) CRYSTALS. T. Kunz, M. Laasch, K.W. Benz, Kristallographisches Institut, Universität Freiburg, Freiburg, GERMANY.
The growth of solid solution bulk crystals of defined lattice spacing is important for substrates. Additionally recent results have shown that Cd(Te,Se) and (Cd,Zn)Te crystals are promising for X-ray and g-ray detectors [1, 2]. CdTe and Cd(Te,Se) crystals have been grown from the vapour phase in a semi-closed arrangement by a Modified Markov Method. As it has already been shown, the partial pressures can be controlled by a suitable temperature profile  which is a particular demand for the growth of homogeneous solid solution crystals. Hitherto segregation in semi-closed vapour growth systems has rarely been studied systematically . Basing on one dimensional vapour transport calculations, segregation of Se and dopants such as V, Ga is derived and compared with experimental results obtained by EDX and Atomic Absorption Spectroscopy. It is shown that the compactness of the source material is one crucial parameter for the segregation. In the case of mixed crystals, segregation coefficients are derived from a solid solution model. In order to take into account effects of gravitational buoyancy additional two dimensional calculations using Finite Elements are performed. Preliminary results will be presented.
9:45 AM I1.5
GAMMA RAY SPECTROMETERS FABRICATED FROM MODIFIED VERTICAL BRIDGMAN/ANNEALED CZT CRYSTALS MATERIAL. K. Chattopadhgyay, H. Chen, K.T. Chen and Arnold Burger, Center for Photonic Materials and Devices, Fisk University, Nashville, TN; J.P. Flint and H.L. Glass, Johnson Matthey Electronics, Spokane, WA; R.B. James, Advanced Electronics Manufacturing Technologies Department, Sandia National Laboratories, Livermore, CA.
The CZT boule was grown by a modified vertical Bridgman process using in-situ compounding, Cd over-pressure and pyrolytic baron nitride crucible within a fused quartz ampoule. During growth, the Cd vapor pressure was near 1 atmosphere. These growth conditions tend to give high purity, good stoichiometry, few precipitates, and dislocation densities in the low to mid 10-4 cm -2 range. The starting materials were Johnson Matthey XTAL-Grade Cd, Zn and Te in a ratio to give Cd0.8Zn0.2Te. The boule was 1.3 kg with a 55 mm diameter. After growth, the oule was sliced perpendicular to the growth axis. No attempt was made to select any particular crystal orientation. The slices, after polishing, were annealed in a nearly saturated Cd, Zn atmosphere to fill residual Cd-site vacancies and achieve high resistivity. Single crystal samples were diced into 1 cm squares for evaluation as gamma ray detectors. The best detector results (4.5% resolution at 60 KeV) were achieved for a 2 hour anneal at 180C.
SESSION I2: CADMIUM ZINC TELLURIDE CHARACTERIZATION I - OPTICAL PROPERTIES
Chairs: Mark S. Goorsky and Pier F. Manfredi
Monday Morning, December 1, 1997
Salon H/I (M)
10:30 AM I2.1
OPTICAL STUDIES OF THE INTERNAL ELECTRIC FIELD DISTRIBUTIONS OF CdZnTe DETECTORS UNDER BIAS CONDITIONS. H. Yao, University of Nebraska, Lincoln, NE; R.J. Anderson, R.B. James, Sandia National Laboratories, Livermore, CA.
Polarized transmission optical profiles were used to characterize the CdZnTe (CZT) room-temperature radiation detectors. The internal electric field distributions of the CZT detectors under bias were probed by a 952 nm illumination between two crossed Glan-Taylor polarizers. A 16-bit digital charge coupled device (CCD) was employed as an image sensor. The 2-dimensional (2D) images mapping the internal electrical field intensity changes were obtained utilizing the Pockels electro-optic effect. Single-crystal and polycrystal CZT detectors were investigated under different operating bias voltages, respectively. Uniform and non-uniform internal electric field distributions throughout the detector volumes were observed and analyzed. The effects light illumination from different photon energies (inside and across the CZT energy band gap) were also be observed, compared and discussed. A theoretical model of the semiconductor energy band structure under the bias was established and used to understand the measurement results. CZT crystal defects images and internal electric field intensity distributions under different bias voltages will be presented and analyzed.
10:45 AM I2.2
OPTICAL AND ELECTRICAL PROPERTIES OF COPPER- AND CHLORINE-DOPED CADMIUM ZINC TELLURIDE. James E. Toney, Bruce A. Brunett, T.E. Schlesinger, Carnegie Mellon University, Integrated Microsystems Labs, Pittsburgh, PA; Eilene Cross, Ralph B. James, Sandia National Labs, Livermore, CA.
We have studied the optical and electrical properties of copper- and chlorine-doped Cd1-xZnxTe, with x varying from 0.1 to 0.5, by a variety of techniques, including low-temperature photoluminescence, photoinduced current transient spectroscopy, photocurrent mapping and alpha particle response. The PL spectra for Cu-doped material show donor-acceptor and bound exciton bands attributable to copper, and the positions of these bands are consistent with the published ionization energy of Cu in CdTe. There are also bands attributable to gold, providing evidence of Au diffusion into the material after the application of electroless contacts. The PL spectra for the Cl-doped material showed several defect bands between 0.9 and 1.5 eV in addition to the standard A-center band. There was evidence of decreasing crystal quality with increasing x, as measured by the intensity of the excitonic luminescence relative to donor-acceptor and defect bands. The PICTS spectra show a variety of electron and hole traps with activation energies ranging from 150 meV to 1 eV. Reliable determination of some trap levels by the standard Arrhenius plot method is made difficult by alloy broadening, and we have applied more sophisticated analysis techniques to determine these levels. We have also used alpha particle response to study the transport properties of the material. The connection between detector performance, mobility-lifetime products and levels observed in PICTS and PL spectra will be discussed.
11:00 AM I2.3
PHOTOLUMINESCENCE INVESTIGATION OF SURFACE OXIDATION OF Cd0.9Zn0.1Te DETECTORS. H. Chen1,2, K. Chattopadhaya3, A. Burger2, J. Heffelfinger3 and R.B. James3, 2 Center for Photonic Materials and Devices, Department of Physics, Fisk University, Nashville, TN; 3 Advanced Electronics Manufacturing Technologies Department, Sandia National Laboratories, Livermore, CA.
The effect of surface oxidation of Cd1-xZnxTe(x=0.1)crystal surfaces by chemical etching in hydrogen peroxide solution at diffierent concentrations and etching times was investigated by low temperature photoluminescence (PL). Peak shifts in the PL spectrum were observed due to the oxide layer, and the present of oxygen was foumd to cause an enhancement in the band attributed to acceptor sites. Scanning Electron Microscopy (SEM) and Tunneling Electron Microscopy (TEM) were employed to examine the oxide layers and oxide-CdZnTe interface. The significance of this surface oxidation on device passivation and the subsequent improvements in the detector performance are also discussed.
11:15 AM I2.4
PL AND EPR SPECTROSCOPY OF POINT DEFECTS IN DETECTOR-GRADE Cd1-xZnxTe. C.I. Rablau, S.D. Setzler, L.E. Halliburton and N.C. Giles, Dept of Physics, West Virginia Univ, Morgantown, WV; F. Patrick Doty, Digirad, San Diego, CA.
Cd1-xZnxTe (CZT) is an emerging material for room-temperature x-ray and -ray detectors. The identification and control of point defects and charge compensators are currently important issues. We have used liquid-helium temperature photoluminescence (PL), photoluminescence excitation (PLE) and electron paramagnetic resonance (EPR) spectroscopies to characterize point defects in a series of bulk CZT crystals grown by the high-pressure Bridgman technique. PL data is used to obtain qualitative comparisons of point defect concentrations, while the EPR technique gives quantitative defect information for the CZT samples. Luminescence due to shallow donors, shallow acceptors and deeper levels such as VCd-DCd (D=shallow donor) was monitored for a series of specially selected samples. The results of the PL and PLE studies were correlated with a detailed study of EPR and photo-induced EPR. An isotropic EPR signal attributed to shallow hydrogenic donors is observed under illumination using a tunable Ti:Sapphire laser. The intensity of the photo-EPR signal undergoes a dramatic enhancement by tuning the illumination photon energy, and the optimum tuning range for each sample is revealed by its PL spectrum. By resonant tuning, EPR spectroscopy is able to detect donor concentrations as low as 1013 cm-3 in bulk CZT crystals. Because the donors are ionized (nonparamagnetic) in the light-off condition, a comparable concentration of acceptors are present. The nature of the defect centers (impurities, vacancies, complexes), as well as the correlation between the defect concentrtions and the performance of the devices will be discussed. This work was supported by NASA SBIR contract No. NAS5-32803.
11:30 AM I2.5
ANALYSES OF COMPENSATION RELATED DEFECTS IN II-VI COMPOUNDS. A. Castaldini, A. Cavallini, B. Fraboni, INFM - Dept of Physics, Bologna, ITALY; P. Fernandez, J. Piqueras, Dpto Fisica de Materiales, Complutense, Madrid, SPAIN.
Deep levels in II-VI compounds have been investigated by means of cathodoluminescence (CL) and junction spectroscopy methods such as DLTS (deep level transient spectroscopy), PICTS (photo-induced current transient spectroscopy) and PDLTS (photo-DLTS). We have studied the role played by the deep traps in the material compensation process by comparing the results obtained from both semiconducting and semi-insulating samples, namely undoped CdTe, CdTe:Cl and Cd0.8Zn0.2Te. We have focussed our attention on the so-called center A and on two deep levels located near midgap, labelled H and E, which may intervene in the pinning of the Fermi level and in the resulting high resistivity of the material. Level H results to be common to all investigated samples while level E is present only in CdTe:Cl. The electron or hole nature of the trap is also deduced from a comparison of the complementary results obtained with PICTS and PDLTS methods. Moreover, it has been possible to attribute a donor or acceptor character to some deep levels. In particular a donor character could be attributed to level E which thus results to be a good candidate for the deep donor needed to explain the compensation process of semi-insulating CdTe:Cl.
11:45 AM I2.6
A STUDY OF THE COMPOSITION UNIFORMITY, ELECTRICAL AND SPECTROSCOPIC PROPERTIES OF CdZnTe DETECTORS. M-A. Jantunen, S. Nenonen, T. Gagliardi, Metorex International Oy, Espoo, FINLAND; M. Bavdaz, Space Science Department of the European Space Agency, Noordwiijk, NETHERLANDS; T. Tuomi, K.T. Hjelt, M. Juvonen, Helsinki University of Technology, Espoo, FINLAND.
The electrical and charge collection properties of a semiconductor detector play an important role in a spectrometer's final performance. However, the studies of these properties often concentrate on only a few samples. In this work 50 CdZnTe detectors from 12 different growth boules were characterized. The composition uniformity was evaluated with photoluminence (PL) measurements. From the current-voltage measurements the differences in the CdZnTe sample resistivities were investigated. With the spectroscopic measurements charge collection efficiencies, -products, and energy resolutions were characterized with an alpha and isotopic sources (5.9-122 keV). A wide selection of test results are presented indicating the variety of CdZnTe material.
SESSION I3: MERCURIC IODIDE
Chairs: X. J. Bao and J. Iwanczyk
Monday Afternoon, December 1, 1997
Salon H/I (M)
1:30 PM *I3.1
HgI2 TWO-DIMENSIONAL ARRAYS BASED ON RESISTIVE CHARGE DIVISION READOUT. S. Alfieri, N. De Cesare, D. Grassi, E. Perillo, G. Spadaccini, M. Vigilante, Dipartimento di Scienze Fisiche -Universitá ''Federico II'' and INFN, Napoli, ITALY; M. Amann, J.M. Koebel, P. Siffert, CRN/PHASE, Strasbourg, FRANCE; W. Dusi, CNR/TESRE, Bologna, ITALY.
The resistive charge division readout technique, associated with a centroid finding method, already used for HgI linear array detectors, has been extended to two-dimensional arrays. The charge division is obtained by depositing a Ge surface resistive layer between the strips on both sides of the device, with the aim of getting a full-area active region of detection and a spatial resolution of 100 m x 100 m. This device is designed as a position-sensitive detector for imaging in space applications, mainly in the X-ray energy range 15-100 keV, where Bragg concentrators can be used. Interstrip distances and layer resistances were optimized by taking into account the various sources of electronic noise and the input parameters of the specially designed preamplifiers used. Fabrication techniques and preliminary results on energy and spatial resolution, obtained with both X-ray calibrated sources and a pulsed nitrogen laser coupled with a dye, to match the bandgap of the bulk material, will be reported and discussed.
2:00 PM I3.2
POLYCRYSTALLINE MERCURIC IODIDE SEMICONDUCTOR DETECTORS. M. Schieber, A. Zuck, M. Breiman, L. Melekhov, J. Nissenbaum, The Hebrew University of Jerusalem, Jerusalem, ISRAEL; R. Turchetta, W. Dulinski, D. Husson, J. L. Riester, LEPSI (ULP/IN2P3), Strasbourg, FRANCE; T.E. Schlesinger and J. Toney, Dept. of Electr. and Computer Eng., Carnegie Mellon University, Pittsburgh, PA.
Preparation methods of large area polycrystalline mercuric iodide detectors using laser ablation, hot pressing, hot wall vapor deposition and screen printing methods are described. The polycrystallites were characterized using photoluminescence at 4.2K, X-ray diffraction, and response to irradiation with 241Am gamma rays. The screen printed detectors have also been characterized for their radiation hardness to neutrons and to their response to irradiation with beta, soft X-rays such as Sm and harder gamma rays such as Cs, and also to 100 GeV muons. Some imaging results obtained with these detectors will be given. A comparison between the different kinds of mercuric iodide polycrystallites based on these characterizations will be made. Finally a comparison of properties with other polycrystalline wide bandgap semiconductors, such as lead iodide, thallium Bromide, and amorphous Se will also be presented.
2:15 PM I3.3
OPTICAL, ELECTRICAL AND SURFACE CHARACTERIZATION OF MERCURIC IODIDE PLATELETS GROWN IN THE HgI2-HI-H2O SYSTEM. L. Fornaro1, H. Chen2, K. Chattopadhyay2, K.-T. Chen2 and A. Burger2 1Radiochemistry Department, Faculty of Chemistry, Montevideo, URUGUAY; 2Center for Photonic Materials and Devices, Department of Physics, Fisk University, Nashville, TN.
The optical, electrical and surface properties of mercuric iodide platelets grove n from solution in a Hgl2-HI-H2O system were investigated by comparing them with Physical Vapor Transport (PVT) grown crystals.. The absence of bulk imperfections and the uniformity of the as-grown surfaces and the KI solution etched surfaces were confirmed by optical microscopy. The as-grown surface uniformity is higher for solution grown than that of PVT crystals, since the platelets do not have to be cleaved or polished. AFM studies show that the roughness for the cleaved aged and etched surfaces were 0.06 nm, 0.48 nm and 0.3 nm respective. Low temperature photoluminescence properties were measured for the two kind of crystals and will be discussed. However, I-V curves give higher current density and lower apparent resistivity values for the solution grown than for PVT grown crystals. Correlations between optical and surface quality as well as the electrical properties of the crystals grown from both solution and PVT methods are also discussed.
2:30 PM I3.4
ELECTRICAL CHARACTERIZATION OF TRAPS RESPONSIBLE FOR POLARIZATION IN MERCURIC IODIDE X-RAY DETECTORS. Richard J. Anderson, Sandia National Laboratories, Livermore, CA; M. Natarajan, X.J. Bao, TN Technologies, Inc., Round Rock, TX.
Some mercuric iodide spectroscopic x-ray detectors exhibit an erosion of spectral resolution during operation, in the presence of the excitation. This failure is typified by a reversible (but recurring) distortion of the structure of the spectrum, and is thought to be caused by the formation of deep traps, which gives rise to the name trapping polarization attached to this phenomenon. We have performed transient photoconductivity response and thermally stimulated current studies on mercuric iodide x-ray detectors ranging from highly polarizing to non-polarizing. The trap structure of poorly resolving detectors is very different from the high resolution detectors. We will discuss the different trap characteristics of these detectors and their affect on the electronic field internal to the detector crystal, which we infer from transient photoconductivity measurements.
2:45 PM I3.5
ENHANCEMENT OF MERCURIC IODIDE DETECTOR PERFORMANCE THROUGH CRYSTAL GROWTH IN MICROGRAVITY: THE ROLES OF LATTICE ORDER. Bruce Steiner, NIST, Gaithersburg, MD; Lodewijk van den Berg, Constellation Technology Corp., Seminole, FL; Uri Laor, Nuclear Research Centre, Be'er Sheva, ISRAEL.
The hole mobility and carrier lifetime of mercuric iodide detectors have been enhanced through crystal growth in microgravity. The improvement is closely correlated with specific characteristics of the crystal lattice, which can now be approached in terrestrial growth. These structural features have been identified by high resolution synchrotron x-ray diffraction imaging
Gravity affects the uniformity of the crystal lattice in two distinct ways: 1) directly through the deformation that it imposes on the crystal lattice during growth and 2) indirectly through convection, which induces precipitation hardening. These changes modify in different ways the electronic parameters of detectors made from the crystals. As purification procedures are initially extended, the formation of inclusions initially is curtailed, enhancing electronic properties in spite of the softening of the lattice through loss of precipitation hardening. With continued purification, a second phase reappears, this time without severe deterioration in performance.
These studies provide insight into those aspects of mercuric iodide crystal order that are essential to property improvement. As a consequence of the structural changes observed, requirements for starting material have been modified, physical vapor growth procedures have been adjusted, and crystal handling procedures have been changed to achieve superior terrestrial crystals. While the electronic properties of terrestrial crystals have thus already been improved, there is evidence that further augmentation is possible.
3:00 PM I3.6
CRITICAL FACTORS FOR THE PRODUCTIVE GROWTH OF LARGE, HIGH QUALITY MERCURIC IODIDE CRYSTALS. Lodewijk van den Berg, Constellation Technology Corporation, Seminole, FL.
The productive growth of high quality Mercuric Iodide crystals is primarily controlled by two sets of factors: the purity and stoichiometry of the starting material and the temperature gradients in the growth ampoule and in the growing crystal itself.
The purity of the starting material can be optimized by synthesis of the mercuric iodide from prepurified precursor materials and subsequent sublimations. Care should be taken to avoid extreme conditions of temperature and vacuum so that the stoichiometry of the material is maintained. An additional issue in this area is the proper cleaning and bake-out of the growth ampoule so that a minimal amount of inert gas is present during the growth.
Once a good quality seed has been obtained and growth conditions have been established, continued growth can be generated by cooling the crystal. This can be done by either decreasing the temperature of the lower part of the ampoule or by increasing the flow rate of cooling air directed against the bottom of the crystal. The linear and radial temperature gradients which are generated in this way in the crystal need to be carefully balanced to avoid additional spurious nucleation. The critical issue is to maintain at all times a suitable supersaturation at all the growing surfaces.
When the crystal becomes larger and the mass crystallization rates increase it is necessary to increase the vapor transport rate by increasing the temperature of the source and thereby the vapor pressure in the amlpoule. This is especially important to stabilize the surface morphology of the higher index planes which appear at the intersections of the major crystallographic planes.
The identification of the effects of the different growth parameters creates the possibility for future theoretical analysis of the growth process in terms of surface kinetics, optimal supersaturations at the growing surfaces and fluid dynamics calculations of the vapor transport rates.
SESSION I4: CADMIUM ZINC TELLURIDE STRUCTURES I - IMAGERS
Chair: W. Hamilton
Monday Afternoon, December 1, 1997
Salon H/I (M)
3:45 PM *I4.1
CdTe AND CdZnTe ROOM-TEMPERATURE X-RAY AND GAMMA RAY DETECTORS AND IMAGING SYSTEMS. Y. Eisen and A. Shor, Soreq NRC, Yavne, ISRAEL.
Among the semiconductor materials of a wide band gap, CdTe and CdZnTe have attracted most attention as room-temperature X-ray and gamma ray detectors. Suitable CdTe(Cl) materials for nuclear detectors and in particular for spectrometers, have been developed over the past few decades and are mainly grown via the Traveling Heater Method (THM) However, the manufacture of large homogeneous ingots at relatively low cost has not reached yet a proven stage. Cd1-xZnxTe materials mainly grown via the High Pressure Bridgman (HPB) technique, seem to show several advantages over CdTe and appear to better approach the practicality of utilizing large volumes X-ray and gamma ray detectors at moderate cost. Continuing effort is still underway to improve the characteristics of both CdTe and CdZnTe materials in order to achieve homogeneous detectors and monolithic arrays for low and high energy gamma ray spectroscopy and imaging. The following review paper will be divided into three parts: The first part will compare the characteristics of CdTe and CdZnTe nuclear detectors. Characteristics include, Charge collection efficiencies for both electrons and holes indicated by the mobility-trapping time product, energy resolutions, leakage currents, timing properties and robustness in field use. The second part will describe different structural designs of detectors to improve the spectroscopic characteristics. Those include: Hemispherical detectors, coplanar strip-electrode detectors and monolithic two dimensional segmented electrode arrays with pad sizes smaller than their thickness. This part will also describe various electronic methods to compensate for the poor charge collection of holes. The third part will be devoted to field uses of these detectors. Those include: Large volume spectrometers,energy dispersive security imaging system and a new generation nuclear gamma camera for medical diagnostics.
4:15 PM I4.2
PERFORMANCE OF PROTOTYPE SEGMENTED CdZnTe ARRAYS. A. Parsons, D. Palmer1, P. Kurczynski2, L. Barbier, S. Barthelmy1,L. Bartlett3, N. Gehrels, J. Krizmanic3,C.M.Stahle4, J. Tueller, and B. Teegarden, NASA/Goodard Space Flight Center, Greenbelt, MD;1University Space Research Associates, Greenbelt, MD; 2University of Maryland; 3NAS/NRC Resident Research Associate, Greenbelt, MD; 4Orbital Sciences Corp, Greenbelt, MD.
The Burst and All Sky Imaging Survey (BASIS) project is a proposed mission to provide arc second locations of Gamma-Ray Bursts (GRBs). The BASIS coded aperture imaging system requires a segmented detector plane able to detect the position of (10 - 150 keV) photon absorption to less than 100 microns. To develop a detector plane with this fine position resolution we have fabricated many 15 mm x 15 mm x 2 mm 100 micron pitch CdZnTe strip detectors and have assembled them into 2x2 and 6x6 element arrays read out by ASIC electronics. Tests with a 22 keV beam collimated to 30 microns have already demonstrated FWHM position determinations to less than 40 microns and experiments with finely collimated beams continue to higher beam energies. Shadowgrams of fine slit patterns in tungsten will allow tests of position accuracy over a larger areas of the arrays. Measurements of the uniformity and efficiency of the both 2x2 and 6x6 arrays will be presented and applications to the BASIS mission will be discussed.
4:30 PM I4.3
PROTOTYPE IMAGING Cd-Zn-Te ARRAY DETECTORS. P. Bloser, J. Grindlay, Dept of Astronomy, Harvard University, Cambridge, MA; K. Shah, Radiation Monitoring Devices, Inc, Watertown, MA.
We describe initial results of our program to develop and test Cd-Zn-Te (CZT) detectors with a pixellated array readout. Our primary interest is in the development of relatively thick CZT detectors for use in astrophysical coded aperture telescopes with response extending over the energy range 10-600 keV. The coded aperture imaging configuration requires only relatively large area pixels (1-3mm), whereas the desired high energy response requires detector thicknesses of at least 3-5mm. We have developed a prototype detector employing a 10 x 10 x 5mm CZT substrate and 4 x 4 pixel (1.5mm each) readout with gold metal contacts for the pixels and a continuous gold contact for the bias on the opposite detector face. This MSM contact configuration was fabricated by RMD and tested by us for uniformity, efficiency and spatial as well as spectral resolution. We have developed an ASIC readout (IDE-VA-1) and analysis system and report results. A prototype design for a full imaging detector array is discussed.
4:45 PM I4.4
LARGE AREA CZT DETECTOR ARRAY IMAGING SYSTEM: PRELIMINARY RESULTS. Anthony D. Lavletes, G. Joseph Mauger, James H. McQuaid, Lawrence Livermore National Laboratory, Livermore, CA.
This paper describes the preliminary results obtained from the initial design and evaluation of a large area CZT detector imaging system. The prototype detector array is an 8x8 pixel array module with integrated electronics developed by Digirad. The area is approximately 7.25 cm2 with a detector thickness of 3 mm. The development of this array by Digirad includes a new proprietary charge collection technique that substantially reduces low energy tailing characteristics of CZT detectors, thus dramatically increasing the peak-to-valley spectral performance. These new features open the door to new applications including simple operation as large area/volume detectors for various Safeguards applications as well as imaging arrays for computed tomography systems. Both modes of operation are investigated and the performance will be described in detail with comparisons to existing CZT detector technologies.
SESSION I5: CADMIUM ZINC TELLURIDE FABRICATION AND PROCESSING
Chair: Makram Hage-Ali
Tuesday Morning, December 2, 1997
Salon H/I (M)
8:30 AM *I5.1
METAL CONTACT FORMATION ON ZINC CADMIUM TELLURIDE DETECTOR MATERIAL. Arnold Burger, Henry Chen, Kua-Tong Chen, Detang Shi, W. Eugene Collins, Center for Photonic Materials and Devices, Department of Physics, Fisk University, Nashville, TN; and R.B. James, Advanced Electronics Manufacturing Technologies Department, Sandia National Laboratories, Livermore, CA.
The understanding of electric contact formation is critical in exploiting the full potential of CdZnTe material for room temperature detection application. The metal-semiconductor electrical characteristics were shown in this study to be strongly affected by the surface preparation steps prior to metallization (polishing, chemical etching), the choice of the metal and contact deposition technique and by the subsequent surface passivation of CdZnTe. In this paper, we also present the implementation of this study in the detector fabrication resulting in a significant lowering the dark leakage current and improvement in the detector performance.
9:00 AM I5.2
CHARACTERIZATION OF CONTACTS AND INTERFACES ON CdZnTe. Sylvia Mergui, Pierre E. Schmidt, Electrical and Computer Engineering Department, Florida International University, Miami, FL; Makram Hage-Ali, PHASE/CRN, Strasbourg, FRANCE.
The key for a good nuclear detection is a large charge collection efficiency as well as a high carrier mobility. In silicon or germanium technology, this is obtained successfully by using the p-i-n structure. However, due to the specific properties of CdTe and CdZnTe, this structure cannot be used. In order to remedy to the incomplete charge collection resulting from the low mobility of the holes in CdTe, a metal-semiconductor-metal (M1 SM2) structure has been proposed. The first metal contact is obtained by depositing gold or platinum on the semiconductor followed by a surface treatment step which results in a good ohmic contact. The second metal contact is obtained by depositing an indium film followed by a rapid thermal annealing step yielding a hole blocking contact. The latter permits the application of a greater voltage bias resulting in a large uniform electrical field within this type of devices, with a better signal/noise ratio. The defects introduced by the processes have been studied by photoinduced transient spectroscopy PITS (Biorad DL 800). The electrical characterization with temperature of the heterostructure was done to evaluate the contact resistance and the barrier height of the contacts.
9:15 AM I5.3
CHEMICAL ETCHING AND POST-ANNEALING FOR HIGH PERFORMANCE CdZnTe STRIP DETECTORS. Z. Q. Shi, Z. Q. Shi, C. M. Stahle, K. Hu, P. Shu, NASA Goddard Space Flight Center, Greenbelt, MD.
One of the critical issues in CdZnTe (CZT) detector fabrication is the surface treatment. This will not only affect the electrical properties such as leakage current and interelectrode resistance, but also influence the physical properties such as adhesion between the metal and the semiconductor. Good adhesion of the metal film to CZT is essential for high wire bonding yield. Historically, there has been a problem in achieving both low leakage current and excellent wire bonding yield. The leakage current and the adhesion properties were studied as a function of different chemical etchants and post-annealing temperature. Both Pt/Au and electroless Au contacts were studied. We found that a bromine/ethylene glycol chemical etch is suitable for a double sided CZT strip detector process. Keeping a relatively smooth surface after chemical etching is critical for achieving a high yield of good strips. To improve the adhesion of the metal to CZT for wire bonding, the detectors were annealed from 100 C to 175 C for 10 hours in vacuum. It has also been observed that after annealing, the interstrip resistance increased.
9:30 AM I5.4
EVALUATION OF INDIUM DIFFUSED M-i-n CdZnTe DETECTORS. M-A. Jantunen, S. Nenonen, T. Gagliardi, Metorex International Oy, Espoo, FINLAND; L. Aleksejeva, V. Ivanov, Baltic Scientific Instruments, Riga, LATVIA; M. Bavdaz, Space Science Department of the European Space Agency, Noordwiijk, NETHERLANDS.
One of the main electronic noise sources of a room temperature semiconductor detector measuring system is the leakage current of a detector. This can be reduced with a pn-junction type detector structure such as a M-i-n configuration, and with cooling. In this work ten CdZnTe detectors were fabricated with a M-i-n structure by Indium diffusion. The junction characteristics were measured by the current-voltage technique. The detectors' electrical, charge collection and spectroscopic properties were compared to the ones received with the traditional electroless Au contacts, before the junction formation. As a result of the M-i-n detector structure improved leakage current performance was achieved. However, corresponding improvement in the detector energy resolution was not typically observed due to the charge collection properties of CdZnTe material.
SESSION I6: CADMIUM ZINC TELLURIDE CHARACTERIZATION II
Chair: Jack F. Butler
Tuesday Morning, December 2, 1997
Salon H/I (M)
10:15 AM *I6.1
SPATIALLY-RESOLVED MATERIALS AND DETECTOR RESPONSE MEASUREMENTS IN CADMIUM ZINC TELLURIDE. J. M. Van Scyoc, M. S. Goorsky, Univesity of California, Los Angeles, Dept. of Materials Science and Engineering, Los Angles, CA; R. B. James, Sandia National Laboratories, Livermore, CA.
There has been significant advancement in the performance of cadmium zinc telluride (CZT) nuclear radiation detectors in the five years of their development. By carefully selecting material, utilizing specialized device designs, or employing complex signal processing schemes, it is possible to obtain high-performance spectrometers and imagers with a few cubic centimeters of active volume. However, there are still major limitations on the yield of large volume spectrometer grade crystals from the boule. Therefore, there is a continuing research thrust to understand the underlying problems in the material and to develop strategies to improve the material. Because the end goal of these efforts is large volumes of high quality material, measurements that give spatially-resolved information are of most interest. Several characterization tools exist to map large slices from CZT boules, including infrared microscopy, high-resolution x-ray diffraction, high-resolution x-ray topography, infrared polarimetry, photoluminescence, leakage currents, and detector response from excitation by alpha particles, gamma photons, and protons. These measurements provide information ranging from basic properties, such as crystallinity, through electrical properties, such as trap type and density, to device performance, such as localized pulse-height spectral response. This presentation seeks to inventory these measurements, in terms of the information that is obtained from each and the relation of the measurements to one another and the performance of the end product devices. The status of each characterization tool in terms of its potential impact and the actual results obtained thus far will also be discussed.
10:45 AM I6.2
STUDY OF MATERIAL PROPERTIES USING DEPTH SENSING SINGLE-CARRIER CZT DETECTORS. Z. He, G.F. Knoll, D.K. Wehe, Y.F. Du, Department of Nuclear Engineering and Radiological Sciences, The University of Michigan, Ann Arbor, MI.
Poor transport of holes in wide band-gap semiconductors has hindered the application of room-temperature semiconductor gamma-ray detectors for decades. The single carrier charge sensing method has provided a promising opportunity to overcome the problem of hole trapping. An energy resolution of 1.79% FWHM at 662 keV gamma-ray energy has been achieved on a 1 cm3 CdZnTe coplanar grid detector. However, the best energy resolution achieved is still far worse than the statistical prediction based on charge generation. There are other factors which could limit the performance of a single carrier detector, such as the variation of the band-gap energy, variations in the mobility and lifetime of electrons, non-uniformity of electron trapping along different trajectories, possible non-uniformity of the electric field within the detector, and the non-symmetric effect of coplanar electrodes. This paper demonstrates a unique diagnostic technique which combines single carrier charge sensing and depth sensing. This technique has been applied to probe possible causes of poor detector performance. An innovative method for measuring electron mobility and lifetime based on this combining technique is also introduced. The non-uniformity of electron drift velocity has been measured. This measurement provides information on the possible non-uniformity of electron mobility and electric field intensity across the detector area. We have explored correlations between the material properties measured on several large volume CdZnTe crystals to the detector performance. This work should be of interest in designing high performance room temperature semiconductor gamma-ray detectors.
11:00 AM I6.3
USE OF A CAPILLARY X-RAY FOCUSED BEAM TO INVESTIGATE THE CHEMICAL COMPOSITION OF CdZnTe WAFERS WITH HIGH RESOLUTION CdZnTe DETECTORS. Ch. Burggraf, Chr. Burggraf, P. Fougeres*, J.M. Koebel, R. Regal, M. Hage-Ali, P. Siffert, CNRS, Laboratoire PHASE, Strasbourg, FRANCE. *EURORAD, CTT du CNRS, Strasbourg, FRANCE.
The control of the concentration of Zn and its fluctuation in the high pressure Bridgman grown CdZnTe crystals is part of our characterization work on the ternary grown ingots grown in house. In order to reach both high sensitivity and high position resolution, we have developed a new system consisting of a X-ray generator, coupled to a focusing X-ray capillar, delivering intense beams in the micron scale, since the intensity gain is around a factor of 100 compared to conventional methods. The characteristic X-rays are measured through a high resolution CdZnTe detector (220 eV at 5.9 keV FWHM) cooled by a Peltier system. The results of our investigations on different kind of crystals will be discussed.
11:15 AM I6.4
MULTI-PARAMETER HIGH RESOLUTION SPATIAL MAPS OF CdZnTe RADIATION DETECTORS. N.R. Hilton, H.B. Barber, J.D. Eskin, J.M. Woolfenden, University of Arizona, Tucson, AZ; B.A. Brunett, T.E. Schlesinger, Carnegie Mellon University, Pittsburgh, PA; M.S. Goorsky, H. Yoon, UCLA, Los Angeles, CA; R.B. James, J.C. Lund, Sandia National Laboratories, Livermore, CA.
Spatial maps of material properties of CdZnTe detector crystals have yielded useful information for crystal growers. Integrating spatially correlated data from many different experimental modalities allows identification of performance-limiting properties of semiconductor radiation detectors. Our approach is to combine data from X-ray diffraction, photoluminescence, and IR microscopy with data gathered from a 7x7x1.5mm3 CdZnTe detector delineated by photolithography into a 48x48 pixel array. Each pixel is indium bump bonded to a charge sensitive, integrating mutliplexer. This readout method is sensitive to DC currents that are necessary for many of our experiments. The focal plane array is housed in a temperature controlled vacuum dewar. Detector performance characteristics such as energy resolution and quantum efficiency are measured as a function of position in the crystal and correlated with materials properties such as resistivity and electron mobility-lifetime product. Thermally stimulated current and current transient spectroscopy experiments yield data about trapping properties. Because the pixel pitch is only 125 microns, data obtained from the device are fine scale spatial maps. The array is then disassembled to conduct the diffraction, photoluminescence, and microscopy tests. By combining the unique capabilities of our imaging system to make fine spatial maps of various properties as a function of temperature with other well established techniques, we can investigate the relationships of these properties and their consequences for high resolution imaging.
11:30 AM *I6.5
FANO FACTOR DETERMINATION FOR CZT. J.A. Pantazis, A.C. Huber, V.T. Jordanov, R.H. Redus, AMPTEK Inc., Bedford, MA; J.F. Butler, B. Apotovsky, DIGIRAD, San Diego, CA.
The continuous improvement in the manufacturing of CdZnTe (CZT) material has resulted in a practical thermoelectrically cooled detector of very high energy resolution. Such a system was used to determine the CZT Fano Factor at 40C. This system uses a novel reset technique which eliminates the leakage and capacitance of a reset transistor. The resolution for the 5.9 keV55FE peak was measured to be 198 eV FWHM and the Fano factor was determined to be 0.082. The high performance of the CZT material at low energies together with a resolution of 4.2 keV FWHM for the 662 keV peak of 137Cs makes these detectors superior in detecting both X-rays and Gamma rays. Spectra obtained with a battery operated portable MCA and different radioactive sources will be presented.
SESSION I7: DEVICE MODELING AND CALCULATIONAL STUDIES
Chair: Zhong He
Tuesday Afternoon, December 2, 1997
Salon H/I (M)
1:30 PM *I7.1
SIMULATING AND ASSOCIATED EXPERIMENTAL RESULTS OF CdZnTe RADIATION DETECTOR RESPONSE FOR -RAY IMAGING APPLICATIONS. Loïck Verger, Jean-Paul Bonnefoy, Alain Gliere, Patrice Ouvrier-Buffet, Murali Rosaz, LETI-CEA-Technologies Avancdes, Grenoble, FRANCE.
A new model of -ray solid state detector has been developed recently. Induced charge signals and spectral response to gamma rays are simulated using finite differences and finite elements analysis for respectively semiconductor physics and electromagnetism phenomena by conjunction with Monte Carlo calculation for -ray matter interaction. One of the most interesting features of this model is that the charge collection simulation takes into account a non uniform electric field distribution. This paper discusses in details the effects of different physical parameters (mobility, trapping, detrapping, electric field distribution) on CdZnTe detector. In particular, an attempt is made to understand the influence of such parameters on the new fast pulse correction method waveform. This method has been recently developed and is based on the pulse height and rise-time relation of the electron signal. The results are compared to experimental data from thick CdZnTe detectors by using experimental electric field distributions measured by Pockels effect.
2:00 PM I7.2
MODELING OF THE SIGNAL INDUCED BY INCIDENT RADIATIONS IN SEMI-INSULATING GaAs DETECTORS. Adriano Cola, Fabrizio Campanale, Lorenzo Vasanelli, Istituto per lo studio di nuovi Materiali per líElettronica, CNR, Lecce, ITALY; Lino Reggiani, Dipartimento Scienza dei Materiali, Lecce, ITALY.
The incomplete collection of the charge photo-generated by an external radiation is the main drawback in semi-insulating GaAs detectors. The understanding of the physical mechanism at the origin of the above drawback is thus a mandatory issue to improve the efficiency of the detectors. It is known that semi-insulating GaAs is characterized by the high concentration of traps, which affect the electric field profile inside the detector and decrease the charge collection efficiency of the carriers photo-generated by the radiation. Starting from the solution of a drift-diffusion model, which gives the stationary properties of the biased not-irradiated device, the aim of this communication is to present the solution of the dynamic response, represented by the signal induced by the incident radiation. To this purpose we have developed a small signal simulator which enables us to monitor the drift of electrons and holes generated at a given point inside the detector toward the respective electrode. From Ramo-Schokleyí theorem the charge induced in the external circuit has been calculated by accounting for the strong non-homogenous field profile inside the detector. The results allow us to model the collected charge (or current) for an arbitrary time interval, thus investigating the role of trapping/detrapping processes and the dynamics of generated carriers. In particular, different regimes of the drifting carriers are evidenced when they move in the active or in the ohmic region of the detector. In turn, these regimes give rise to fast and slow component in the signal transient, respectively. The relative contribution of these fast and slow components depends on the applied voltage. However, both the corresponding transit times are much lower than standard shaping times, of the order of microseconds , and on this time scale detrapping processes from different traps become effective. A comparison between simulated and experimental results is carried out for a detector where the trap distribution is known.
2:15 PM I7.3
PULSE FILTERING AND CORRECTION FOR CZT DETECTORS USING SIMPLE DIGITAL ALGORITHMS BASED ON THE WAVELET TRANSFORM. J.M. Perez, CIEMAT, Madrid, SPAIN; G. Garcia-Belmonte, Universitat Jaume I, Castellon, SPAIN.
A novel digital version of dual-parameter pulse processing technique for room temperature semiconductor detectors has been previously presented. The double gaussian filtering used in classical works has been implemented by creating a bank of gaussian-like digital filters making use of the wavelet transform. The obtained results for HgI2 experimental detectors have been previously reported, and a preliminary study of the application of this technique to a CZT detector is at press. In this paper the efficiency of the method is consolidated for CZT detectors, reporting detailed information about its spectroscopic capabilities. It is presented the application of the method to a linear array of CZT detectors constructed as a part of a room temperature semiconductor detector matrix developed for a scintific mission mounted on the Spanish MINISAT-01 satellite. Several trapped charge correction levels are compared. Examples of the information obtained from the filtered pulses about detector internal parameters are presented. The simplicity of the method is remarked as one of its most significative advantages. Finally, its hardware implementation in progress is commented.
2:30 PM I7.4
ELEMENTARY THEORY OF LINE SHAPES AND ENERGY RESOLUTION IN SEMICONDUCTOR DETECTORS. James E. Toney, T.E. Schlesinger, Carnegie Mellon University, Pittsburgh, PA; Ralph B. James, Sandia National Labs, Livermore, CA.
We have applied elementary statistical methods to obtain simple, closed-form expressions for line shapes in pulse height spectra obtained with semiconductor detectors. The analysis fully accounts for exponential attenuation and trapping, but excludes noise and statistical fluctuations. The latter can be incorporated in a simple way by convolving the line shape with a Gaussian. In the case of compound semiconductors where the mobility-lifetime product for electrons is much greater than that for holes, we show that the line shape is described by a simple power law, except for a small region very near the peak.The result agrees well with Monte Carlo simulations and can be applied to real pulse height spectra to detect deviations from ideal behavior due to problems such as material non-uniformity. We have also derived an expression for the maximum obtainable energy resolution of a semiconductor detector in the presence of shot noise and intrinsic statistical fluctuations. We have applied this expression to investigate the question of the optimal bandgap for a perfectly compensated material, neglecting surface leakage. Implications of the result regarding the status of currently used materials will be discussed.
2:45 PM I7.5
ON THE ACTIVE VOLUME OF CADMIUM ZINC TELLURIDE DETECTORS. J.C. Lund, N.R. Hilton and B. Brunett, Sandaia National Laboratories, Liveremore, CA.
Cadmium zinc telluride (Cd1-xZnxTe) detectors are being used more widely for gamma-ray sensor applications. lt is widely believed that the active volume of these devices is equal to the physical volume of the detector. However, when these detectors are used as detectors for pulse height spectroscopy, the effective volume of the device is often much less than its physical volume. The difference between the active and physical volume of the device is due to chargee trapping effects. Even for ``electron-only'' devices such as coplanar and Frisch grids the active volume of the device is constrained by electron trapping. We discuss the limitations in effective detector volume for several situations: a planar detector, an electron-only type device and finally a device employing some form of electronic pulse height corrections. These results are then used to predict the maximum active volume attainable by a single Cd1-xZnxTe detector element.
3:00 PM I7.6
MONTE CARLO CODE FOR SEARCHING MOBILITY AND LIFETIME VALUES IN HIGH Z SEMICONDUCTOR DETECTORS. E. Muller, M. Jung, P. Fougeres, M. Hage-Ali, P. Stiffert, CNRS, Laboratoire PHASE, Strasbourg, FRANCE.
The performance of both CdTe and CdZnTe detectors depend strongly on the transport properties of both electrons and holes of the bulk material. Different methods have already been proposed to explore these parameters, like pulse amplitude with alpha particles or risetime measurements. Quite often the observation of the schubweg of holes is rather difficult to observe. In this paper, we intend to valid our Monte Carlo code MCGET, comparing computed pulse height distribution to the experimental spectra recorded when the detectors are bombarded by gamma rays. From the fitting parameters, it becomes possible to get access to the transport properties. Different experimental situations will be described and the limits of this procedure discussed.
3:15 PM I7.7
AN APPROACH TO SUB-PIXEL SPATIAL RESOLUTION IN ROOM TEMPERATURE X-RAY DETECTOR ARRAYS WITH GOOD ENERGY RESOLUTION. W.K. Warburton, X-ray Instrumentation Associates (XIA), Mountain View, CA.
In this paper we examine a recently proposed concept for obtaining sub-pixel spatial resolution in compound semiconductors where hole transport properties are relatively poor.  This approach uses weighted sums and differences of local pixel signals to extract both accurate x-ray energy estimates and interpolate location at the sub-pixel level. A simple analysis, including noise estimates, suggests the possibility of obtaining locations at the 50-100 micron level using 1-2 mm wide stripe electrodes while obtaining 1-2% energy resolution for x-rays up to 100 keV. Following this examination, we will present the most recent experimental results from our program to develop electronics to implement this scheme.
SESSION I8: CADMIUM ZINC TELLURIDE STRUCTURES II
Chair: Carl M. Stahle
Tuesday Afternoon, December 2, 1997
Salon H/I (M)
4:00 PM I8.1
OPTIMIZATION OF FRISCH GRID TYPE CADMIUM ZINC TELLURIDE GAMMA-RAY DETECTORS. J.C. Lund, B.A. Brunett and N.R. Hilton, Sandia National Laboratories, Livermore, CA.
We discuss methods for designing cadmium zinc telluride (Cd1-xZnxTe) detectors containing an internal grid designed to sense the motion of electrons within the device (but not holes). The most direct method for predicting the performance of these devices involves: 1. determine the weighting potential for the collection electrode, 2. determine the trajectories of drifting electrons and holes generated from all volume elements within the device, and 3. compute the induced charge on the contact from each point in the detector by computing the line integral of the weighting potential along with trajectory (multiplied by an exponential factor in time to account for trapping along the trajectory). Using these methods it is possible to numerically predict the charge collection from each point within the detector, and hence the pulse height spectrum that would be produced for any given device design and operating conditions. However this direct method does not provide much insight on how to produce a device optimized for sensitivity and energy resolution. In this paper we discuss optimizing the design of these devices by trading off three quantities: 1. the localization of the weighting function, 2. the flux density at the collecting electrode, and 3. the time required to for electrons to drift from various points in the detector to the collecting electrode.
4:15 PM I8.2
HIGH EFFICIENCY, SPECTROSCOPIC CZT ARRAY. L. Cirignano, M. Klugerman, Y. Dmitriyev and K.S. Shah, RMD, Inc., Watertown, MA.
Compact, efficient, spectroscopic detector arrays which operate at room temperature have applications in many fields including x-ray astronomy, nondestructive testing, and medical imaging. We have fabricated a 16 element CZT array in a geometry that allows for both high efficiency and good energy resolution even at high gamma ray energies. The 4 x 4 array has pixel size of 1.5 mm with 0.2 mm spacing and 5 mm detector thickness. The detector array has been characterized in terms of energy resolution, detection efficiency, and noise properties. The detector has also been operated in current mode. Some potential applications of such a detector array are addressed.
4:30 PM I8.3
AN IMAGING CdZnTe STRIP DETECTOR WITH ORTHOGONAL ANODES.L.A. Hamel, O. Tousignant, J.F. Courville, Groupe de recherche en physique et technologie des couches minces (GCM), University of Montreal, Montreal, CANADA. J.R. Macri, K. Larson, M. Mayer, M.L. McConnell, J.M. Ryan, Space Science Center, University of New Hampshire, Durham, NH.
We present a concept study for a CdZnTe two dimensional imaging spectrometer strip detector where both sets of orthogonal electrodes are anodes. Unlike conventional strip detectors, the cathode signal is not used for imaging or spectroscopy. One set of anodes (collecting strips) is patterned on the CdZnTe substrate. The other set of anodes (non-collecting strips) is patterned on a thin insulating substrate layer adjacent and orthogonal to the collecting strips. The signals from the collecting anode strips are used for the energy measurement and for locating the interaction in one dimension. Signals induced on the non collecting strips from the motion of carriers (electrons) near the collecting strips and are used to locate the interaction in the orthogonal dimension. This technique avoids some of the limitations encountered for strip detectors as a result of very poor hole transport. This technique can be particularly useful in applications requiring thick CdZnTe substrates (>5 mm) operating at energies >300 keV. Collecting and non-collecting anodes signals from a simple prototype detector are shown and compared with signals from a computer model for charge transport and signal generation. Detector fabrication issues and potential applications are discussed.
4:45 PM I8.4
PORTABLE P-I-N CdZnTe DETECTION SYSTEM. P. Bennett, R. Sudharsanan, G. Vakerlis, Spire Corp., Bedford, MA; J. Pantazis, A. Huber, R. Redus, AMPTEK Inc., Bedford, MA.
A portable detection system has been designed and tested for identifying nuclear materials in the field. A PIN CdZnTe detector provides the lowest possible leakage currents leading to improved energy resolution. The detector has a sensitive volume of greater than 100mm3 and has been constructed with novel heterojunction contacts. Thermoelectric cooling is used to reduce leakage currents in both the detector element and the input FET. Risetime discrimination is user selectable to offset the negative effects of incomplete charge collection. The detector probe is connected to a single unit comprising of detector electronics, MCA and portable computer; all operated from battery power, Results from testing with laboratory radioisotopic sources and mixed radioactive materials at a DOE laboratory will be presented.
SESSION I9: POSTER SESSION
Chairs: A. Lavietas and Lodewijk NMN van den Berg
Tuesday Evening, December 2, 1997
Salons E-G (M)
STUDY OF THE HOMOGENEITY OF CdZnTe DETECTORS. H. Hermon, M. Schieber, R.B. James, J. Lund, A.J. Antolak, D.H. Morse, J. Heffelfinger, Sandia National Laboratories, Livermore, CA; N.N.P. Kolesnikov, Yu.N. Ivanov, Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow, RUSSIA; V. Komar, Institute for Single Crystals, National Academy of Science, Kharkov, UKRAINE; M.S. Goorsky, H. Yoon, UCLA, Los Angeles, CA; J. Toney, and T.E. Schlesinger, Carnegie Mellon University (CMU), Pittsburgh, PA.
IR microscopy has been used in the study of the homogeneity of CZT single crystals grown by the vertical high pressure Bridgman (VHPB) method. The presence of black inclusions and lubular hollow pipes has been observed by a rew methods i.e. Photoluminescence (PL), infrared (IR) transmission microscopy and scanning electron microscopy (SEM). We have studied the thermal annealing effects on the electrical performance of the CZT crystals as well as the physical properties using proton induced X-ray emission (PIXE) and X-ray diffraction (XRD). The crystals investigated were grown commercially both in the USA and at the Institute of Solid State Physics, Chernogolska, Russia. We discuss the homogeneity of the various CZT crystals based on the results from these measurement techniques.
INVESTIGATION OF THE X-RAY PLASTIC EFFECT USING A SYNCHROTRON SOURCE. V.F. Petrenko, N.N. Khusnatdinov, and I. Baker, Thayer School of Engineering, Dartmouth College, Hanover, NH.
Recently, it was found that x-rays strongly affect the plastic properties of II-VI crystals, such as ZnTe, CdTe, CdSe, CdS, ZnSe, ZnS, causing significant but almost reversible hardening (the so-called x-ray plastic effect, XPE).1 A standard x-ray generator with a copper target operated at 30 mA and 40 kV produced a relative hardening, / of up to 90% for CdS, and 60% for ZnSe. Illumination of the specimen with white light (photoplastic effect) also caused hardening and reduction in the plastic strain rate. The effects of x-rays and white light were found to be additive. Here we represent a study of the effects of white-beam x-rays of significantly higher intensity (I= 6 1010 photons/(…sec…mA…mrad) with the maximum at 0.8 ) from a synchrotron source (175 mA, E= 2.6 GeV) on the plastic deformation of II-VI compounds. The XPE was found to increase significantly but saturated at 200% for ZnS. It was also found, that contrary to the effects of low intensities of x-rays where the effects of x-rays and light are additive, the simultaneous effects of high intensity x-radiation and light are opposite. In particular, x-radiation extinguishes the photoplastic effect in ZnSe produced by light. Possible mechanisms for this effect are discussed.
FURTHER STUDIES OF CADMIUM ZINC TELLURIDE DETECTOR MATERIAL UTILIZING PICTS TO CORRELATE TRAP INFORMATION TO DETECTOR PERFORMANCE. B.A. Brunett, J.E. Toney, T.E. Schlesinger, Dept of Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA; R.B. James, Sandia National Labs, Livermore, CA; M.C. Driver, E.E. Eissler, eV Products, Saxonburg, PA.
We have applied Photo Induced Current Transient Spectroscopy(PICTS) to pixellated Cadmium Zinc Telluride(CZT) gamma-ray detectors to gain information about variation in trap levels within the material. We have seen some correlation between trap levels and detector performance, but it has been difficult to correlate trap levels with mobility-lifetime products obtained by alpha particle response. In this work we address several questions remaining from previous PICTS and detector operation experiments including: field nonuniformity effects due to excessive carrier generation, dead layer effects in alpha-particle pulse height spectroscopy, effects of excitation wavelength, and the use of more sophisticated analysis techniques to extract closely space trap levels in the presence of alloy broadening.
ROOM-TEMPERATURE CdZnTe GAMMA-RAY DETECTORS. A.A. Melnikov, A.A. Davydov, L.I. Topalova, N.V. Zhavoronkov, and K.A. Vorotilov, Moscow State Institute of Radioengineering, Electronics, and Automation (Technical University), Moscow, RUSSIA.
Uniform CdZnTe monocrystals with high resistivity were grown by vapor phase technique with the use of high-purity polycrystalline CdTe and ZnTe binary compounds. The samples were cut from crystal bars with the diameter of 50 mm and resistivity from 1010 - 8-1010 cm. After that they were polished and etched in a selective etching reagent. Electrical contacts were used for measurements of detectors properties. The estimated value of in the crystals varies from 5-10-6 cmV-1 to 5-10-5 cmV-1 for holes and from 5-10-4 cmV-1 to 8-10-3 cmV-1 for electrons. The energy resolution of 6 with the sensitive volume of 0.125 cm3 was obtained at the line of 59.6 keV 241Am. Detectors with the volume of 1cm3 had the resolution of 5 at the line of 662 keV 137Cs. Detectors with the volume of 4mm 3 showed the 1.5 resolution for -particles (239Pt).Uniform CdZnTe monocrystals with high resistivity were grown by vapor phase technique with the use of high-purity polycrystalline CdTe and ZnTe binary compounds. The samples were cut from crystal bars with the diameter of 50 mm and resistivity from 1010 - 8-1010 cm. After that they were polished and etched in a selective etching reagent. Electrical contacts were used for measurements of detectors properties. The estimated value of in the crystals varies from 5-10-6 cmV-1 to 5-10-5 cmV-1 for holes and from 5-10-4 cmV-1 to 8-10-3 cmV-1 for electrons. The energy resolution of 6 with the sensitive volume of 0.125 cm3 was obtained at the line of 59.6 keV 241Am. Detectors with the volume of 1cm3 had the resolution of 5 at the line of 662 keV 137Cs. Detectors with the volume of 4mm 3 showed the 1.5 resolution for -particles (239Pt).
THE SPECTROMETER CHARACTERISTICS OF CdTe, CdZnTe DETECTORS AND ELECTROPHYSICAL CHARACTERISTICS OF INITIAL CRYSTALS. Peter Dorogov, Victor Ivanov, Alexander Sokolov, Baltic Scientific Instruments Ltd., Riga, LATVIA; Sergey Jakubenja, Byelarussian State University, Minsk, BYELARUS.
Essential progress in perfection of growing methods of CdZnTe (eV PRODUCTS, USA) and CdTe (ACROTEC, Japan) crystals in the last years has allowed to receive detectors with the high spectrometer performance. For further improvement of an initial material for detectors knowledge of parameters, determining quality of detectors, is necessary. There are the various methods for quality surveillance of the widegap semiconductor materials. We used the undestroying contactless method of the local microwaves control, based on measurements of lifetime of nonequilibrum charge carriers or of values of a stationary photoconductivity signal on a local area of a crystal. The given method has not basic restrictions of the control of high-resistance semiconductors. The basic difference of an offered method of the control from existing microwaves methods is by a microwaves resonator of an original design, essentially raising of sensitivity a microwaves method. With the help of the given method were measured time of life of nonequilibrum charge carriers and stationary conductivity of CdTe and CdZnTe of crystals. As the control of uniformity of crystals was carried out measurements of spatial distribution of photoconductivity by the crystals area. From the tested crystals detectors were made and spectrometer characteristics were measured. Moreover values of mobility-lifetime product of charge carriers with using of time of flight method were measured. Dependence of the detectors spectrometer performance from the value of photoconductivity is observed. Measured value of photoconductivity of good detectors in 3-25 times more photoconductivity of bad detectors. Used microwaves method of quality surveillance can be used for selection of crystals intended for detector manufacturing.
SPECTROSCOPIC RESPONSE VERSUS INTERELECTRODIC CHARGE FORMATION POSITION IN CdTe DETECTORS. Natalia Auricchio, Ezio Caroli, Waldes Dusi, Istituto TESRE, Bologna, ITALY; Donato Grassi, Eugenio Perillo, Giulio Spadaccini, Dip. di Scienze Fisiche, Universita' Federico II and INFN, Napoli, ITALY; Makram Hage-Ali, Paul Siffert: Laboratoire PHASE/CNRS, Strasbourg, FRANCE.
The hole-electron pair production energy (4.4 eV) in Cadmium Telluride semiconductor based detectors would allow in principle to achieve spectroscopic performance very close to the Germanium one. In fact the material impurities, crystal lattice defects and low charge mobility can severely degrade the spectroscopy through charge trapping phenomena and really limit the distance between the electrodes. Because of the trapping probability depend on the distance between the charge production position and the collecting electrodes, we have studied the dependence of some CdTe spectroscopic parameters (energy resolution, gain and peak efficiency) from this position. We have performed a set of experimental tests with radioactive sources inside a thick (20 mm) tungsten collimator with a 200 m window. The beam spot was positioned between the two electrodes of a CdTe micro-crystal of 222 mm3. Using a micrometer moving system we performed several scans (with steps 200 m) irradiating at different distance between the electrodes and collecting energy spectra at each step. The results of these measures are presented and discussed in order to emphasise a method to improve CdTe detector spectroscopic capabilities
SCINTILLATOR - PHOTODIODE LINEAR ARRAYS FOR X-RAY INSPECTION SYSTEM. Sergey Ignatov, Victor Potapov, Alexey Fedin, Vladimir Chirkin, Leonid Urutskoev, RECOM Ltd, Kurchatov Institute, Moscow, RUSSIA; Vladimir Gostilo, Vladimir Kondrashov, Alexander Sokolov, Baltic Scientific Instruments Ltd., Riga, LATVIA.
In METORSCAN inspection system high - energy X-radiation (up to 8 MeV) of generator passes through the inspected object (containers, trucks, cars,) and is registered by the arrays of scintillator - photodiode detectors , creating its image .Taking into consideration the dimensions of the objects (up to 4,5 m) the arrays of the detectors in such system can comprise approximately 1500 detectors. Detectors should provide high identity of parameters, low noise level, high sensitivity and linearity of output signal from absorbed dose in wide energy range. The evaluations of the following performances of detectors for design of the radiographic system were carried out by the method of mathematical simulation and experimentally: sensitivity to absorbed energy, noise level, form of instrumental function of detectors array (function of the space response of detectors array to the detection of collimation beam of radiation). The mathematical model included: multiple Compton scattering of gamma - quantums in scintillator, their absorption and formation of electron-positron pairs in interactions; formation of secondary electrons in scintillator and photodiode; an annihilation of positrons and spread of annihilation photons. Calculations of influence of geometry of scintillator and photodiode on detectors noise performance and form of instrument function have shown, that transposition of secondary electrons in sensitive volume of photodiodes can increase noise of the detector in 3-4 times. The results of calculations were used for designing of CdWO4-Si arrays with optimal sizes of scintillator and photodiode for resolution ability. At the given stage the quality of radiometric detectors was evaluated by spectrometer methods. At the bias voltage on photodiode 12 V, the sensitivity of detectors has made up (0,95-1,00) » 10-21 Klb / eV, noise level - (1,50-1,75) »10-16 Klb together with electronic noise. Dynamic range at absorbed energy by separate detector is more than 105, what is sufficient to get required image contrast.
NEW APPLICATION AND ANALYSIS OF AVALANCHE PHOTODIODES AS DETECTORS FOR ELECTRONS RANGING FROM 10 KEV TO 300 KEV. Tetsuji Kodama, Nagoya Univ, Dept of Information Electronics, Nagoya, JAPAN; Nobuyuki Osakabe, Junji Endo, Akira Tonomura, Advanced Research Lab, Hitachi Ltd, Hatoyama, Saitama, JAPAN; Tsuneyuki Urakami, Shinji Ohsuka, Hiroshi Tsuchiya, and Yutaka Tsuchiya, Central Research Lab, Hamamatsu Photonics K.K., Hamakita, JAPAN.
New application of avalanche photodiodes as fast timing detectors for electrons ranging from 10 keV to 300 keV together with an analysis of the silicon avalanche photodiodes to the electrons are reported. Application 1: Quantum mechanical electron intensity correlation, which is the counter part of the photon correlation first observed by Hanbury-Brown and Twiss1 in 1956, has not yet been demonstrated. A theory2 suggests the antibunching of electrons in a coherent beam within the coherence time of the order of 10 fs. The detection requires fast detectors with small dead time. An avalanche photodiode for lightwave communication is thought to be the most suitable detector for the purpose. We3 have confirmed the feasibility of the observation by estimating the sampling time based on the obtainable avalanche photodiode and the digital signal processing (DSP) techniques4. Application 2: Recently a new time-resolved electron microscopy5 have been successfully developed and applied to the study on the vortex transport and the measurement of elastic property of nanoscaled materials. Temporal and spatial correlation function of an electron beam current in the imaging beam modulated by the dynamics of the specimen is measured by means of electron counting technique using avalanche photodiodes and the DSP technique.
HETEROSTRUCTURES METAL OXIDE/SILICON: NEW SOLUTIONS FOR DIFFERENT OPTOELECTRONIC APPLICATIONS. Alexander Malik, Rodrigo Martins, FCT-UNL/CEMOP-UNINOVA, Monte de Caparica, PORTUGAL.
We report for the first time the successful fabrication an ITO/Si and FTO/Si surface barrier photodiodes based on high-resistivity silicon which have been produced by spray pyrolysis deposition technique in air. Three types of photodiodes for low-voltage-bias operating have been developed.
1. X-Ray detectors, including the surface-barrier PIN photodiode with active area 50 mm2, scintillator based on monocrystalline Bi4Ge3O12 and preamplifier (noise of 250 e- RMS). Such detectors show the energy resolution (FWHM/peak) of 16.5 at 661.5 keV (137Cs source) and operating temperature of 176 K.
2. High-speed PIN photodiodes based on thin high resistivity silicon epitaxial structures. Response time at wavelength 0.85 micron is less than 2 ns (10 V bias). With 60 micron diameter light spot, the mean deviation in DC output over the surface of the active area is less than 1.
3. Radiant-tolerant drift epitaxial surface-barrier PIN photodiodes for unbiased operating with exponential impurity distribution in the 10 micron epitaxial layer. The exponential doping profile provides a strong constant build-in electrical field in the epitaxial layer and considerable improvement in the "critical fluence" value (1014 cm-2, for neutron irradiation) was obtained.
APPLICATIONS OF Ge AND Si SPECTROMETERS OPERATED WITHOUT LIQUID NITROGEN. R. Henck, P. Dressler, P. Rohr, Burisys Mesures, Tanneries, FRANCE.
Germanium remains the unsurpassed material for high resolution gamma ray spectroscopy, with the inconvenience to need liquid nitrogen cooling for operation. Results will be presented using two different types of miniaturized cryogenic coolers adapted one (minicryo split Sterling) for small X-ray detectors, the other (Sterling Duet) for medium size coaxial detectors (to about 50% efficiency). Attempts were also made to characterize the operation of Ge detectors at temperatures above LN2, in the range -130 to -100C, cooling being made using Peltier elements. Results obtained with a 5 cm2 planar detector as a function of temperature will be shown. Silicon ion implanted structures were developed to decrease the current room temperature: with 10 mm2, 300 thick samples currents in the range of 50-100 pA are obtained. Associated with electrical reset preamplifiers and a simple Peltier cooling element, resolutions of about 200 eV at 5.9 keV are achieved. Applications in various fields will be discussed. The development started some years ago with ESRF (C. Gauthier, E. Moguiline) in the field of Silicon Drift Detectors (SDD) resulted in the manufacturing of a family of SSD; single channel 10 mm2 SDD, single channel 1 cm2 SDD and 8 channel 2 cm2 SDD. Design and performances will be discussed.
EXTENDING XIA'S DIGITAL SPECTROMETER TECHNOLOGY TO LOW POWER PORTABLE AND REMOTE MONITORING APPLICATIONS. W.K. Warburton, D.A. Darknell, B. Hubbard, and E. Oltman, X-ray Instrumentation Associates (XIA), Mountain View, CA.
The XIA DXP-X12A x-ray spectrometer digitally processes directly digitized preamplifier signals to implement both amplifier and MCA functions in a single, compact module. The constraints under which the DXP-X12A was designed, to accommodate very high count rates at a low cost, coincidentally lead to an instrument which is both very compact and relatively low power (about 6 W/channel), considering its count rate and MCA capabilities. Further, since all functions are digitally controlled, including gain, filter times, pileup inspection, internal calibrations, and MCA binning criteria, the design should be readily adaptable to many applications, including remote access and portable uses with room temperature detectors. In this paper we give the DXP-X12A's functional layout and present approaches to lowering power usage to less than 300 mW/channel for applications where 50,000 cps is an acceptable count rate capability. Several room temperature detector applications will then be examined to explore the possibilities.
COMPOUND SEMICONDUCTOR DETECTORS FOR X-RAY ASTRONOMY: SPECTROSCOPIC MEASUREMENTS AND MATERIAL CHARACTERISTICS. M. Bavdaz, A. Peacock, Space Science Department of the European Space Agency, Noordwijk, NETHERLANDS; S. Nenonen, M.A. Jantunen, T. Gagliardi, Metorex International Oy, Espoo, FINLAND; T. Tuomi, K.T. Hjelt, M. Juvonen, Helsinki University of Technology, Otakaari 1, Espoo, FINLAND; S. Kraft, M. Wedowski, G. Ulm, Physikalisch-Technische Bundesanstalt, Berlin, GERMANY.
The future generation of X-ray Observatories now under preliminary design (XEUS & HTXS) should extend the band in photon energy up to 100 keV. In this region of the X-ray spectrum non thermal emission and absorption mechanisms play an important role - eg. cyclotron emmision lines produced by charged particles radiating in the high magnetic field of a neutron star. This high energy band is also important to constrain emission models responsible for the spectra below 10 keV (limit of current generation missions like XMM and AXAF).
The characteristics of detectors covering the energy band 10 to 100 keV will become an important issue in the overall sensitivity of these future missions.
Clearly issues such as energy resolution and X-ray absorption efficiency will be performance drivers while practical issues such as potential array capability and cooling requirements will also play a role. In this paper we examine the capability of a number of compound semiconductor detectors and provide experimental results on the performance of some of these (CdZnTe,GaAs and TlBr) at these higher photon energies.
By scanning sample detectors with a small, collimated synchrotron radiation beam correlations between the local response and material properties can be determined. Analytical methods employed include photoluminiscence, IR-, Nomarski-and electron-microscopies and synchrotron X-ray topography.An overview of the status of these studies will be given.
MONOLITHICALLY INTEGRATED X-RAY DETECTOR ARRAYS FOR COMPUTED TOMOGRAPHY. R. Sudharsanan,* M. Yoganathan,* P. Bennett,* G.Vakerlis,* B. McCandless,** and R. Birkmire,** *Spire Corporation, Bedford, MA; **Institute for Energy Conversion, University of Delaware, Newark, DE.
The design and performance of a high sensitivity monolithic X-ray detector array, fabricated by depositing CdTe photodiodes on CdWO4 scintillators for computed tomography are presented. Not only do CdTe photodiodes offer the possibility of monolithic integration but they are also better matched to the emission wavelength of CdWO4 scintillators than are Si photodiodes, thus, can yield higher sensitivity. To demonstrate the concept, CdTe photodiodes fabricated on glass were adhesively attached to CdWO4 scintillators The detector exhibited 30% improvement in signal strength over Si photodiodes attached on CdWO4 scintillators. Monolithic X-ray detector arrays were fabricated by depositing a CdTe photodiode structure on CdWO4 by physical vapor deposition . The monolithic devices exhibit dark current densities of 4 to 5 nA/cm2 at -20 mV bias. The effects of deposition and processing conditions on the detector performance will be presented.
CVPE GaAs P-I-N AND VGF GaAs X-RAY DETECTORS. S. Nenonen, M.-A. Jantunen, T. Gagliardi, Metorex International Oy, Espoo, FINLAND; H. Helava, American Xtal Technology, Fremont, CA; M. Bavdaz, Space Science Department, European Space Agency, Noordwijk, THE NETHERLANDS; T. Tuomi, K. Hjelt, M. Juvonen, Helsinki University of Technology, Espoo, FINLAND.
We have fabricated GaAs x-ray detectors from two different types of material: epitaxial material grown by Chemical Vapour Phase Epitaxy (CVPE) and semi-insulating (SI) bulk material grown by Vertical Gradient Freeze method (VGF). The CVPE detectors have a p-i-n contact structure. The structure was made by first growing 30 µm - 40 µm thick intrinsic CVPE layer on a 100 µm thick 2 inch n+ type VGF GaAs wafer and then growing about 5 µm thick p+ CVPE layer on the intrinsic layer. Metallic contacts were deposited on the both sides of the wafer. The p+ side was patterned in such a way that 70 round shape electrical contacts with diameters of 1.5 , 2.5 or 3.5 mm were formed. Then the wafer was cleaved into 70 detectors of size 5 mm x 5 mm. Some of the detectors have guard rings.
8:30 AM *I13.1
DETECTOR GRADE OF CdTe SINGLE CRYSTALS GROWN FROM THE VAPOR PHASE. Michael Fiederle, Material Research Center Freiburg, University of Freiburg, Tobias Feltgen, Matthias Laasch, Thomas Kunz, Wolfgang Joerger, K.W. Benz, Kristallographisches Institut, University of Freiburg; Freiburg, GERMANY.
Different growth methods [1,2] were applied to improve the crystal quality of CdTe towards the application of - and X-ray detectors. The results presented by Laasch for CdTe grown from the vapor phase showed the possibility to produce twin-free, single crystals and to reduce mechanical strain. Several experiments were performed for undoped, gallium and chlorine doped crystals to prove the dopant reproducibility of the growth method. The CdTe:Cl crystals were contacted with a multilayer structure of copper, nickel and gold for ohmic contacts and AuBr3 for rectifying ones. Characterization by Alpha spectroscopy results in a charge collection efficiency of 80 with a low bias voltage of 80 V. The energy resolution was better than 10 % for a detector thickness of 800 m.
9:00 AM I13.2
HIGH ENERGY FLUX DETECTOR USING CdTe P-I-N LAYERS. Madan Niraula, Tomonori Arakawa, Toru Aoki, Yoichiro Nakanishi, Yoshinori Hatanaka, Graduate School of Electronic Science and Technology, Shizuoka University, Hamamatsu, JAPAN; and Hirofumi Kan, Central Research Laboratory, Hamamatsu Photonics K.K., Hamakita, JAPAN.
Cadmium telluride (CdTe) is one of the most promising material for the application in high energy flux detection. For the detector purpose, energy resolution and response speed are the important factors. Good mobility of the carriers in the semiconductor materials, high field to collect the generated carriers and low dark current are the prime factors to give high energy resolution and good response speed of the detector. In this regards, CdTe p-i-n layers appear to be promising. n-and p- type doping of CdTe were studied on the layers (thickness around 300 nm) grown on semi-insulating GaAs substrate by radical assisted metal organic chemical vapor deposition technique working on a low pressure. Dimethylcadmium and diethyltelluride were used for the source materials. For n-type doping, CdTe layers were grown by introducing n-butyliodine into the reaction chamber as a dopant at different flow rates. p-type doping were achieved by evaporating a thin layer (50 nm) of alkali metal such as Na2Te on the surface of the as-deposited CdTe layer and then excimer laser (KrF) annealing was carried out for different laser output energies. The carrier concentration of the both n-and p-type layers thus prepared were increased up to the order of 1016 cm-3 with increasing iodine flow rate and laser output energy respectively, while resistivities were decreased up to few tens of cm in magnitude. This method is very simple and convenient to grow both n-and p-type layers of different resistivities and carrier concentrations. Using this technique, n-and p-type CdTe layers are grown on the CdTe substrate (resistivity > 108 cm so as to form the p-i-n structure. Performance of the detector will be studied using x-ray photon of 25 to 100 keV energy and varying the p-and n- layer resistivity. These results will be discussed in this meeting.
9:15 AM I13.3
A CdTe POSITION SENSITIVE DETECTOR FOR A HARD X- AND GAMMA-RAY WIDE FIELD CAMERA. Ezio Caroli, Giovanni De Cesare, Ariano Donati, Waldes Dusi, John B. Stephen, Gianni Landini, Istituto TESRE/CNR, Bologna, ITALY; Francesco Perotti, IFCTR, Milano, ITALY; Giuseppe Bertuccio, Dip. di Elettronica ed Informatica, Politecnico, Milano, ITALY.
An important region of the electromagnetic spectrum for astrophysics is the hard X- and gamma-rays band between 10 keV and few MeV, where several processes occur in a wide objects variety and with different spatial distribution and time scales. In order to fulfil the observational requirements in this energy range and to take into account the opportunities givens by small/medium size missions (e.g. on the ISS) that several space agency are considering for the next future, we have proposed a compact wide field camera based on a thick (1 cm) position sensitive CdTe detector (PSD). The detector is made of an array of 12896 CdTe microspectrometers with a pixel size of 22 mm2. The basic element of the PSD is the linear module that is an indipendent detection unit with 32 CdTe crystals and monolithic front-electronics (ASIC) supported by a thin (300 m ceramic layer. The expected performance of the PSD over the operative energy range and technical details, in particular of the ASIC functionality, are presented and discussed.
9:30 AM I13.4
NEW CdTe MATRIX AS A 2D QUANTUM SENSOR FOR FAST, WELL SPATIALLY RESOLVED MeV-IMAGING. Jean-Louis Gerstenmayer, Olivier Bonnin, CEA, Bruyeres le Chatel, FRANCE; Vincent Gerbe, Francis Glasser, Olivier Peyret, CEA, Grenoble, FRANCE.
We developed a new, well spatially resolved (millimetric resolution), high sensitivity and high quantum efficiency large size pixelled CdTe(Zn) camera for high energy (>1 MeV) flash radiography applications. Monte Carlo simulations (MCNP4-a) have been used to calculate the energy deposition in the matrix and then to optimize pixels size versus quantum noise and contrast. They give also the quantum efficiency which provides the theoretical limit for the sensitivity when a low noise charge read-out is assumed. We present also the first experimental results which confirm the interest of a such detector for the MeV-flash radiography: the measured sensitivity is better than 0.5 rad (20 MeV - 50 ns pulsed generator with 5 cm lead source filtration).
SESSION I14: VAPOR DEPOSITED SEMICONDUCTORS FOR DETECTORS
Chairs: John A. Pantazis and William K. Warburton
Thursday Morning, December 4, 1997
Salon H/I (M)
10:15 AM *I14.1
MODIFIED VAPOR PHASE GROWTH OF CADMIMUM ZINC TELLURIDE: A PROGRESS REPORT. W.P. Trower, World Physics Technologies, Blacksburg, VA.
We are conducting a series of experiments to realize Cd(1-x)ZnxTe (CZT) by modified Vapor Phase Growth (mVPG) so that the binary CdTe and ZnTe components are kept separate in the ampoule with their vapors passing through channels and openings designed to have them mix stoichiometrically at the growth surface. To date we have obtained polycrystalline CZT boules 50 mm in diameter and 10 mm high, which has been analyzed at several laboratories. The structure of our mVPG CZT is strikingly different from that obtained by standard High Pressure Bridgman methods. Our mVPG properties relevant for detector applications are encouraging. The resistivity is 40 G-cm when the voltage is first applied rising to 66 G-cm after 20 min when the traps are filled. The electron mobility-lifetime is less satisfactory at 2 x 10-3 cm2/V. We will present a more current and complete structural and operational characterization of our mVPG CZT.
10:45 AM I14.2
ZnxCd1-xTe EPITAXIAL GROWTH BY REMOTE PLASMA ENHANCED MOCVD METHOD. Daiji Noda, Toru Aoki, Yoichiro Nakanishi, Yoshinori Hatanaka, Graduate School of Electronic Science and Technology, Shizuoka University, JAPAN.
Metal organic chemical vapor deposition (MOCVD) technique has intensively been investigated in the field of epitaxial films growth. In the compound epitaxial film growth in the ZnTe-CdTe system, composition control of the ratio of Zn/Cd is limited in the range within 0.2. In this study, we have carried out the epitaxial growth of ZnxCd1-xTe in the wide range of the Zn/Cd ratio using the remote plasma enhanced (RPE) MOCVD method. Diethylzinc (DEZn), dimethylcadmium (DMCd) and diethyltellurium (DETe) are used for the source materials. Hydrogen radicals generated by inductively coupled rf remote plasma were introduced into the reaction region. The epitaxial growth of ZnCdTe at the substrate temperature of 200C is observed only when the hydrogen radicals are introduced on the substrate surface region, since the substrate temperature is too low to decompose the metal organic source materials. Hence, the role of hydrogen radicals are considered as to decompose source metal organic materials and to help the surface reaction for the film formation. In this deposition method, we can obtain ZnxCd1-xTe with the composition ratio x in the range of 0 to 1 while varying the ratio of DMCd to DEZn from 0 to 30. The crystallinity of the epitaxial films were 400 to 700 arcsec in FWHM of X ray diffraction measurements. This results suggest that the RPE-MOCVD method can be one of the promising method in preparing ZnCdTe epitaxial films.
11:00 AM I14.3
GROWTH OF BORON PHOSPHIDE FILMS USING MOLECULAR BEAM EPITAXY FOR USE IN SOLID-STATE NEUTRON DETECTORS. D. Buchenauer, D. Dibble, K. McCarty, R. Anderson, M. Clift, D. Medlin, J. Lund, and R. B. James, Sandia National Laboratories, Livermore, CA.
A significant improvement in the detection of thermal neutrons could be made through the use of zinc blende boron phosphide (BP) as the detection medium in a solid-state neutron detector. BP detectors would be more rugged and compact than gas-filled detectors and have superior efficiency compared to 10B coated silicon detectors. BP (band gap 2 eV) is also refractory, hard, and resistant to chemical reactions. The 10B + n + 7Li cross section (3840 barns) is sufficient to stop 95of a thermal neutron flux incident on a 200 m 10BP film. Due to the high melting temperature (3000C) and lower decomposition temperature (1100C) of BP, however, standard crystal growing techniques cannot be used. Epitaxial BP films have been grown using Chemical Vapor Deposition (CVD)1, some with thicknesses up to 300 m2, but no films have yet been characterized as detector quality material for neutron detector applications. Solid Source Molecular Beam Epitaxy (SSMBE) has the potential to grow BP at lower temperatures with improved control over the fluxes, reduced impurity content, and reduced hazards compared with some CVD preparations. We are growing films using electron beam evaporation of boron and thermal cracking of phosphorus vapor using a three-cell EPI cracker. We will report on the stoichiometry and crystallinity of the films as a function of temperature, relative B to P flux, and growth rate. The relationship between the electronic and microstructural properties of the BP films and their performance as solid-state neutron detectors will be examined.
11:15 AM I14.4
EPITAXIAL GROWTH OF HEXAGONAL ZnS AND ITS HETEROSTRUCTURES USING PULSED LASER DEPOSITION. H.C. Ong, J.Y. Dai, and R.P.H. Chang, Department of Materials Science and Engineering, Northwestern University, Evanston, IL.
Epitaxy of cubic ZnS has been prepared by various deposition techniques such as molecular beam epitaxy (MBE) and metalorganic chemical deposition (MOCVD). However, high quality hexagonal ZnS has not yet been reported. Here, we have successfully deposited hexagonal ZnS thin films on (0001) Al2O3 using pulsed laser deposition. The effect of substrate temperature and background pressure on the film growth has been studied in detail. High quality films can be prepared at growth temperature of 450-550C with rocking curve FWHM of (0001) ZnS as narrow as 0.08. High resolution cross-section transmission electron microscopic (TEM) analysis of the films deposited at 500 shows cubic ZnS grains inhabit near interface and therefore the interfacial region is highly defective. However, the surface region, above the critical thickness of 5nm, consists of pure hexagonal structure with a very high level of crystallinity. By using ZnS and MgZnS as active and barrier layers, we have the first time fabricated Mg1-xZnxS/AznS/Mg1-xZnxS quantum well structure. Finally, incorporation of oxygen, nitrogen, and chlorine into ZnS for n-and p- type doping will also be addressed. Our preliminary results show that doping of oxygen can be achieved by introducing molecular oxygen during deposition. Energy dispersive x-ray (EDX) and x-ray diffraction (XRD) show that films with good crystal structure and uniform doping level can be prepared at 0.01 mTorr of oxygen.
11:30 AM I14.5
DIELECTRIC SPECTROSCOPY STUDY OF ZnSe GROWN BY PHYSICAL VAPOR TRANSPORT. Julie R. Kokan and Rosario A. Gerhardt, School of Materials Science and Engineering, The Georgia Institute of Technology, Atlanta, GA; and Ching-Hua Su, MSFC, NASA, Huntsville, AL.
ZnSe, although generally thought of as a wide band gap semiconductor, is insulating in the as-grown state. It is only after heat treatment in a zinc rich atmosphere that semiconductor properties are observed. Therefore, dielectric spectroscopy is an appropriate tool to study the electrical properties of as grown ZnSe. The dielectric properties of large-grained samples of ZnSe grown by physical vapor transport were measured as a function of frequency and temperature. Differences can be seen in the dielectric properties of samples grown under different conditions (such as the effect of a seed and the orientation of the gravity field during growth). The impedance and dielectric spectra of heat treated samples were also acquired and were found to exhibit significant deviations from those of the as grown crystals.
11:45 AM I14.6
NEUTRON DETECTORS MADE FROM CHEMICALLY VAPOUR DEPOSITED SEMICONDUCTORS. F. Foulon, P. Bergonzo, C. Jany, A. Brambilla, R. Marshall, B. Guizard and S. Haan LETI (CEA - Technologies Avancees) DEIN/SPE - CEA/Saclay, Gif-sur-Yvette, FRANCE.
In this paper, we present the results of investigations on the use of semiconductors deposited by chemical vapour deposition (CVD) for the fabrication of neutron detectors. For this purpose, hydrogenated amorphous silicon (a-Si:H) pin diodes and polycrystalline diamond resistive detectors were deposited by radio-frequency and micro-wave plasma enhanced CVD, respectively, a-Si:H and diamond detectors with thicknesses ranging from 10 to 30 m and 10 to 100 m were fabricated. The detectors were coupled to a neutron-charged particle converter: a layer of either gadolinium or boron (isotope 10 enriched) deposited by evaporation. A thermal neutron detection efficiency of about 22 % and 2 % can be achieved by coupling a Si:H diodes and diamond detectors to 4.5 m thick gadolinium (isotope 157) and 3 m thick boron layers, respectively. Neutron flux measurements were carried out using a252Cf source and in a research nuclear reactor at CEA Saclay. We have demonstrated the capability of such neutron detectors to operate at neutron fluxes ranging from 0.1 to 106 neutrons/cm2.s. The advantages of these relatively thin semiconductor detectors include insensitivity to gamma-rays, radiation hardness (4 Mrad for a-Si:H and 100 Mrad for diamond) and for diamond, operation at temperatures up to 500C. These properties enable the use of these devices for neutron detection in harsh environments.
SESSION I15: LEAD IODIDE
Chair: James C. Lund
Thursday Afternoon, December 4, 1997
Salon H/I (M)
1:45 PM *I15.1
CHARACTERIZATION OF X-RAY IMAGING PROPERTIES OF Pbl2 FILMS. K.S. Shah, L. Corignano, Y. Dmitriyev, M. Klugerman, K. Mandal and L.P. Moy, RMD, Inc., Watertown, MA.
This paper describes our recent research in developing vacuum sublimed lead iodide films for X-ray imaging. Lead iodide films are promising for this application due to their low dark current, high stopping efficiency, reasonably good charge transport, low cost, and relatively easy scale-up. Lead iodide films (up to 5 x 5 cm2 area) have been grown and characterized by measuring their X-ray imaging properties such as contrast, spatial resolution, and modulation transfer function. Excellent spatial resolution (> 10 lp/mm with high MTF 50%) has been recorded with Pbl2 films. Relevant detection properties such as signal amplitude for given X-ray energy has also been measured and was found to be about 10 times larger as compared to standard phosphor screens used for X-ray imaging. Charge transport and timing characteristics of these films have been measured and the results indicate that these films should be capable of real-time operation. Application of these films for X-ray imaging such as mammography, fluoroscopy, and X-ray diffraction is addressed.
2:15 PM I15.2
LEAD IODIDE X-RAY AND GAMMA-RAY SPECTROMETERS FOR ROOM AND HIGH TEMPERATURE OPERATION. H. Hermon, R.B. James, M. Schieber, E. Cross, A. Antolak, D.H. Morse, J. Lund, B.A. Brunett, D.L. Medlin, Sandia National Laboratories, Livermore, CA; A. Burger, K.-T. Chen, Fisk University, Nashville, TN; Y.-C. Chang, University of Illinois at Urbana-Chapaign, Urbana, IL; M. Goorsky, Hojun Yoon, J. Van Scyoc, UCLA, Los Angeles, CA; K. Shah, RMD Inc., Watertown, MAS; J. Toney and T.E. Schlesinger, Carnegie Mellon University, Pittsburgh, PA.
In this study we report on the results of the investigation of lead iodide materials properties. The effectiveness of a zone refining purification methods on the material purity is determined by ICP-MS and ICP-OES and correlated to the electrical and physical material properties. We show that this zone refining method is very efficient in removing impurities from lead iodide and we also determine the segregation coefficient for some of these impurities. Triple axis x-ray diffraction (TAD) analysis has been used to determine the crystalline perfection of the lead iodide after applying various cutting, etching and fabrication methods. The soft lead iodide crystal was found to be damaged when cleaved by a razor blade, but by using a diamond wheel saw, followed by etching, the crystallinity of the material was much improved, as observed by TAD. Low temperature photoluminescene also indicates an improvement in the material properties of the purified lead iodide. Electrical properties of lead iodide such as carrier mobility, were calculated based on carrier - phonon scattering. The results for the electrical properties were in good agreement with the experimental data.
2:30 PM I15.3
CALORIMETRIC AND SPECTROSCOPIC CHARACTERIZATION OF ZONE REFINED AND REGROWN LEAD IODIDE. K.-T. Chen, A. Burger, H. Chen, Y.-F. Chen, K. Hansen, L. Suber, S. Wilson, J. Henderson, G.W. Wright and M.L. Cole, Center for Photonic Materials and Devices, Department of Physics, Fisk University, Nashville, TN.
Lead iodide starting material has been purified by zone refining process. The refined ingot has a distinguishable color distribution indicating the accumulation of impurities with distribution coefficients greater or less than 1. The last-to-freeze region show the darkest color following the yellow color for the middle section, and finally the slight dark color for the first-to freeze region The refined ingot was divided into seven sections, and differential scanning calorimetry (DSC), low temperature photoluminescence (PL) and current-voltage techniques (I-V) have been used to investigate the distribution of impurity content and deviation from stoichiometry, as well as their effect on the electrical properties along the refined ingot. The material from the middle region was extracted and used as starting material for crystal growth. Lead iodide single crystals were grown using Bridgman and Physical Vapor Transport (PVT) methods and has been characterized using DSC, I-V and PL measurements. Improvements of the quality of as-grown crystal are presented and discussed. The authors acknowledge the support of NASA Grant NCC8-133 through the Fisk Center for Photonic Materials and Devices, and the support of REU/SRP program from NSF grant DMR 9322437.
2:45 PM I15.4
CONVERTING FILMS FOR X-RAY DETECTORS, APPLIED TO AMORPHOUS SILICON ARRAYS. Steve Ross, George Zentai, Argonne National Laboratory, Argonne, IL.
Hydrogenated amorphous silicon (a-Si:H) is a semiconductor material which can be inexpensively deposited to create a large array detector or readout structure. Combining it with a suitable semiconductor x-ray sensitive converter would produce a large area detector for x-rays with energies 6-20 keV such as used in measurements of diffraction patterns for protein crystallography. In this talk I summarize our results to-date using various semiconductor thin films deposited on either ITO-glass or amorphous silicon. These films include lead iodide, thallium bromide and selenium. Data concerning leakage current, conversion efficiency, response time and temperature effects will be presented. A brief overview of the requirements for a usable detector will be listed.
SESSION I16: OTHER SEMICONDUCTOR DETECTOR MATERIALS
Chair: Michael M. Schieber
Thursday Afternoon, December 4, 1997
Salon H/I (M)
3:30 PM I16.1
SYNCHROTRON X-RAY TOPOGRAPHIC STUDY OF DISLOCATIONS IN GaAs DETECTOR CRYSTALS GROWN BY VERTICAL GRADIENT FREEZE TECHNIQUE. T. Tuomi, M. Juvonen, R. Rantamäki, K. Hjelt, Optoelectronics Laboratory, Helsinki University of Technology, Hut, FINLAND; M. Bavdaz, Space Science Department, European Space Agency, Noordwijk, NETHERLANDS; S. Nenonen, Metorex International Oy, Espoo, FINLAND; P.J. McNally, Microelectronics Research Laboratory, Dublin City University, Dublin, IRELAND; A.N. Danilewsky, Kristallographisches Institut, Universität Freiburg, Freiburg, GERMANY; E. Prieur, M. Taskinen, M. Tuominen, Okmetic Ltd., Espoo, FINLAND.
Large area transmission and section topographs are used for the Burgers vector analysis of dislocations in gallium arsenide wafers grown by the gradient freeze technique. The topographs were made at HASYLAB in Hamburg and at ESRF in Grenoble. Several high-resolution images including stereo pairs of the semi-insulating crystals are obtained on the same film at a time. A typical dislocation line is an arc of a circle which starts from one surface and ends at the same surface. From the disappearance of the dislocation image and using the g b = O criterion it is concluded that the Burgers vector of the most common dislocations is parallel to (110). Rather large volumes of the wafer are dislocation-free. Section topographs of epitaxial wafers show defects and strain fields at the interface between an n-type substrate and the undoped epitaxial layers grown by chemical vapor deposition. The results are compared with those obtained from detector performance measurements.
3:45 PM I16.2
RADIATION DETECTORS BASED ON HIGH PURITY LIQUID PHASE EPITAXIAL GaAs. D.I. Wynne1,2, C.S. Rossington1 and E.E. Haller1,2. 1E.O. Lawrence Berkeley National Laboratory, Berkeley, CA; and 2 Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA.
We report on the growth of high purity n-GaAs using Liquid Phase Epitaxy (LPE) and the fabrication of room temperature p-i-n radiation detectors. Our epilayers are grown from a Ga solvent in a graphite boat in a pure hydrogen atmosphere. Growth is started at a temperature of approximately 800C. Our best epilayers show a net residual donor concentration of 2 1013 cm-3, confirmed by Hall effect measurements. The residual donors have been determined by far infrared spectroscopy to be sulfur and silicon. Epilayers with thicknesses of up to 120 m have been deposited on 650 m thick semi-insulating GaAs substrates and on 500 m thick n+-type GaAs substrates. We report the results obtained with Schottky barrier diodes fabricated from these high purity n-type GaAs epilayers and operated as X-ray detectors. The Schottky barrier contacts consisted of evaporated circular gold contacts on epilayers on n+ substrates. The ohmic contacts were formed by evaporated and alloyed Ni-Ge-Au films on the back of the substrate. Several of our diodes exhibit currents of the order of 1 to 10 nA at reverse biases depleting approximately 50 m of the epilayer. This very encouraging result, demonstrating the possibility for fabricating GaAs p-i-n diodes with depletion layers in high purity GaAs instead of semi-insulating GaAs, is supported by results obtained by several other groups. The consequences of using high purity instead of semi-insulating GaAs will be much reduced charge carrier trapping. Diode electrical characteristics and detector performance results using 55Fe and 241Am radiation will be presented.
4:00 PM I16.3
LOW TEMPERATURE PROCESSED MSM PHOTODETECTORS ON ZnSSe/Si-GaAs(100). Hyesook Hong, Wayne A. Anderson, Eunwha Lee, Huicheng Chang, Myunghee Na and Hong Luo, State University of New York at Buffalo, Center for Advanced Photonic and Electronic Materials, Amherst, NY.
Low temperature (LT) processed ZnSSe MSM photodetectors can be used for detecting Gamma rays or X-ray using scintillation crystals for many space and medical applications. Metal-semiconductor-metal (MSM) photodetectors were fabricated on undoped ZnSSe grown by molecular beam epitaxy (MBE) on semi-insulating (100) GaAs substrates. The MSM photodetectors consist of interdigitated metal fingers with 2 ,m, 3 ,m, and 4 m spacing and width with active area of 100 ,m x 130 ,m on one chip. Probimide and SiO2 thin films were deposited to aid the LT lift-off process before the pattern generation. Interdigitated structure was achieved by photolithography and reactive ion etching. Pd Schottky metal was deposited at a substrate temperature near 77 K using a lift-off technique. The LT metallization provides an improved interface between metal and semiconductor interface. DC signal to noise ratio (SNR) of 2.3x103 was obtained for 2 m interdigitated photodetectors, operated under 130 nW optical power at a wavelength of 400 nm. The detectors showed good DC saturation characteristics indicating a low surface recombination. Saturation current without illumination remained at around 10 pA for a 5 V biasing. Detectors exhibited linearity in light intensity and DC bias voltage suggesting no gain mechanism involved, and showed a high spectral responsivity (0.6 (A/W)) below a wavelength of 450 nm at 5V applied bias.
4:15 PM I16.4
GROWTH, CHARACTERIZATION AND SPECTROSCOPIC INVESTIGATIONS OF InI CRYSTALS FOR OPTICAL AND RADIATION DETECTOR APPLICATIONS. K.C. Mandal, M. Klurerman, L.P. Moy, Y. Dmitriyev, L. Cirignano and K.S. Shah, RMD, Inc., Watertown, MA.
Single crystals of Inl (Eg = 2.01 eV at 300 K) have been grown by vertical Bridgman technique using zone refined (ZR) starting materials. The quality of these crystals has been evaluated by analyzing the impurities within the crystals by inductively coupled plasma spectrometry (ICP), Auger electron spectroscopy (AES), X-ray diffraction (XRD), electron probe microanalysis (EPMA) and photoelectron spectroscopy (XPS). Development of a chemical etching procedure using optimized Br2/methanol solution is discussed. Fabrications and testing of optical and nuclear Inl detectors are also presented.
4:30 PM I16.5
SEMI-INSULATING InP PARTICLE DETECTORS FOR X- AND -RAY DETECTION. F. Dubecky1, R. Fornari2, P.G. Peifer3, J. Darmo1, E. Gombia2, M. Krempasky1, C. Frigeri2, M. Sekaeova1, I. Besse1 and P. Bohaeeck1, 1IEE of Slovak Academy of Sciences, Bratislava, SLOVAKIA; 2MASPEC CNR, Parma, ITALY; 3Department of Physics, University of Florence, ITALY.
Semi-insulating (SI) InP material is important candidate for fabrication of particle detectors for low-energy X- and -rays due to its physical properties. Wide band-gap 1.35 eV allows its application at room temperature (RT), high atomic number of In (49) give short absorption length (0.34 mm for 60 keV photons) and high electron mobility (>3000 cm2/V8 at RT) produce fast signal. Moreover, InP materials are well technologically developed and commercially available. Study of electrical properties and detection performances of SI InP-based particle detectors are presented. Detectors with enhanced Schottky barriers and with Schottky and P+ back contacts give CCE over 70% and energy resolution (FWHM) less than 5% at room temperature (RT). Detection of low energy X- and -rays by the SI InP detectors at RT was not possible due to the large detector noise. Detection performances (CCE, FWHM), detection efficiency) for detection of 60 keV(241 Am) and 122 keV(57Co) photons of detectors cooled by a Peltier down RT are presented. Influence of contact configuration, detector geometry, temperature and material aspects are discussed.
SESSION I17: GROUP IV SEMICONDUCTORS FOR DETECTORS
Chair: Giuseppe Bertuccio
Friday Morning, December 5, 1997
Salon H/I (M)
8:30 AM *I17.1
AMORPHOUS SILICON SENSOR ARRAYS FOR X-RAY AND DOCUMENT IMAGING. R.A. Street, R.B. Apte, D. Jared, P. Mei, S. Ready, Xerox PARC, Palo Alto, CA; T. Granberg, T. Rodericks, R.L. Weisfield, dpiX, Palo Alto, CA.
Research on high performance image sensor arrays based on amorphous silicon TFTs and sensors is presented. Such imagers are important for x-ray medical imaging and document scanning as well as a variety of other applications where large sensor size is important (1). We have implemented various approaches to enhance the performance of the imagers compared to previously reported devices. The size of the imager has been increased to 30x40 cm, which is the largest reported to date, and the number of pixels to >7 million. Processing improvements have nearly double the sensitivity and reduced the data line capacitance which is the principal contribution to the electronic noise of the system. Further improvements in fill factor are accomplished by a change in design to allow a 3-d structure, and some new designs of this type have been made with reduced pixel cross-talk. In addition, a new TFT technology allows both amorphous and polysilicon TFTs made with the identical structure, using laser recrystallization to create the polysilicon TFT. The two types of TFT have threshold voltages within 2V and similar sub-threshold slopes. The reason for the hybrid approach is that a-Si:H devices are preferred for the pixel TFT because of its low leakage current, but the polysilicon devices have the high current needed for peripheral electronics. A new complete imaging system incorporating some of these improvements will be presented.
9:00 AM *I17.2
NEW GENERATIONS OF POSITION SENSITIVE SILICON DETECTORS. Paul Burger, Camberra Semiconductors, Olen, BELGIUM.
The new generation of elementary particle and nuclear physics experiments demand instrumentation with a more precise spatial resolution and a better and faster energy response. Nuclear physics and space experiments need position sensitive pad detectors having very thin entrance windows while high energy physics and medical applications use fast microstrip or drift detectors. Silicon pixel detectors meeting these requirements have been developed and have been further improved by implementing integrated electronics on them. They allow a better X-ray energy resolution and are also used in hybrid photocathode tubes for faster timing and larger dynamic range.
9:30 AM I17.3
APPLICABILITY OF SHALLOW-IMPURITY DOPED SILICON TO PROTON-FLUX SENSORS USING STABLE PARTICLE-INDUCED CONDUCTIVITY. N. Kishimoto, H. Amekura, K. Kono, C.G. Lee, National Research Institute for Metals, Tsukuba, Ibaraki, JAPAN.
High-energy particles incident to a semiconductor sensitively produce electron-hole pairs in the solid. The excited current could be used for a radiation detector, but, in general, the signal rapidly decays with accumulating radiation-induced defects, as recombination centers. Accordingly, semiconductors may not be applicable for detection of high-energy particles to a considerable fluence. In order to improve the situation, we have proposed employment of crystalline Si doped with shallow impurities. Specimens of Cz-Si doped with P or B were irradiated with 17 MeV protons and particle-induced conductivity of doped Si has been in-situ measured, as a function of proton fluence. The proton beam current was 20 - 40 nA/cm2. The deep levels were also examined by DLTS measurement. The increment of particle-induced conductivity showed a fairly stable response to proton flux, against defect accumulation. Although the doping, in general, is not good for the sensitivity unirradiated, the shallow impurities passivate the deep centers of radiation-induced defects and do not significantly deteriorate the carrier lifetime. Radiation tolerance is limited by exhaustion of pre-existed carriers and the more doping may be the better in this context. However, there is a reasonable region of impurity concentration, not to spoil the carrier lifetime. Deep levels of V-O and V-V clusters were responsible for the deterioration. In another aspect, the tolerance limit (fluence) is usable to detect the integrated fluence, by controlling the initial impurity concentration. The mechanism of particle-induced conductivity will be discussed.
SESSION I18: RADIATION DAMAGE
Chair: Anna Cavallini
Friday Morning, December 5, 1997
Salon H/I (M)
10:30 AM I18.1
HIGH COLLECTION EFFICIENCY THIN FILM DIAMOND ALPHA RADIATION DETECTORS. Robert D. McKeag and Richard B. Jackman, University College London, Electronic and Electrical Engineering, London, UNITED KINGDOM; Phillipe Bergonzo, R. Duncan Marshall, F. Foulon, C. Jany and A. Brambilla, LETI (CEA- Centre d'Etudes Nucleaires de Saclay), Gif-Sur-Yvette, FRANCE.
The resilience of diamond within hostile environments makes it an attractive material for the fabrication of radiation detectors. Thin film diamond, grown by CVD methods on non-diamond substrates, is a polycrystalline but commercially accessible for such applications. The grain structure within thin film diamond is inhomogenious; fine grain material exists on the nucleation side whilst very large grains are more typical of the growth side. In most studies to date diamond 'sandwich' structures have been used for alpha detection;relatively poor charge collection effeciencies result (<15%), primarily due to the poor quality of the diamond film over its depth. Etching away the unwanted (poor) region of the film is costly and can also introduce significant damage to the remaining diamond. An alternative approach is to form planar structures where both electrodes are placed on the top side of the diamond film; the first measurements from such device structures are reported in this paper. Interdigitated electrodes (40micron pitch) have been formed on CVD diamond films; operation of these structures as alpha detectors (Am241, 5.5MeV) has been compared to sandwich structures formed on the same CVD material. Collection efficiencies for some incident nalpha particles higher than 70% have been measured on the former, whulst the latter devices were limited to 10%. The devices were not damaged during prolonged exposure to alpha or gamma radiation. The field within the near surface region of the planar device has been simulated; strong modulation of field strength is apparent between the electrodes and this effect is thought to be responsible for the comparatively broad range of collection efficiencies that are apparent. Approaches for improving the designs used will be discussed.
10:45 AM *I18.2
STUDY OF THE RADIATION DAMAGE INDUCED BY HIGH-ENERGY GAMMA-RAYS IN CdTe DETECTOS. W.Dusi, E.Caroli. Istituto TESRE; P.Chirco, M.Zanarini, E.Querzola, M.Giacometti, Universita' di Bologna, Settore di Fisica Biomedica and INFN; A. Cavallini, INFM and Universita' di Bologna, Dipartimento di Fisica; J. Piqueras, Universidad Complutense, Madrid; P.Siffert, M.Hage-Ali, CNRS- Lab. PHASE, Strasbourg.
In recent years the performances of room-temperature semiconductor detectors such as CdTe are improved and they are now suitable candidates for several applications. However, some key parameters that can severely affect such perfomances have not been measured yet. Thus we have studied the damaging of a set of small CdTe detectors irradiated in a Co60 gamma-cell in a wide range of doses and dose-rates. A full characterization of the performances of irradiated detectors has been obtained by means of spectroscopic, electrostatic, photo-induced current transient spectroscopy and photo-deep level transient spectroscopy measurements to quote the energy resolution, the leakage current, the activation energy and capture cross-section of deep level defects, respectively.
11:15 AM I18.3
RADIATION DAMAGES OF Si AVALANCHE PHOTODIODES BY 1-MeV FAST NEUTRONS AND 220-MEV CARBON PARTICLES. H. Ohyama, T. Hakata, E. Simoen1, C. Claeys1, Y. Takami2, K. Hayama, J. Tokuyama, K. Shigaki, H. Sunaga3, K. Miyahara4 and M. Hososhima5, Kumamoto National College of Technology, Kumamoto, JAPAN; 1IMEC, Leuven, BELGIUM; 2Rikkyo University, Kanagawa, JAPAN; 3Takasaki JAERI, Gunma, JAPAN; 4Kumamoto University, Kumamoto, JAPAN; 5Japan Electronic Materials Co., Hyogo, JAPAN.
Results are presented of a study on the performance degradation and induced lattice defects of Si avalanche photodiodes, subjected to 1-MeV fast neutrons and 220-MeV carbon particles. The radiation source dependence of the damage is discussed, taking into account the number of knock-on atoms and the nonionizing energy loss (NIEL). Diodes are fabricated in high resistivity (18 kcm) FZ-Si substrates by a deep B+ p-well implantation, followed by an As n+ ion implantation and subsequent anneal. The diodes are irradiated at room temperature with 1-MeV fast neutrons with fluences ranging from 1011 to 1013 n/cm2 and with 220-MeV carbon particles. The carbon fluence is varied from 1011 to 1013 1/cm2. Current/voltage and capacitance/voltage characteristics are measured together with the induced deep levels m the Si substrate by DLTS. The photocurrent as a function of white light power is also analysed. The influence of the induced lattice defects on the device performance in the avalanche action region is discussed. The recovery behavior of irradiated diodes by isochronal annealing will also be presented in order to clarify the degradation mechanism.
11:30 AM I18.4
DEGRADATION AND RECOVERY OF Si DIODES BY 20-MEV PROTONS AND 220-MEV CARBON PARTICLES. H. Ohyama, T. Hakata, E. Simoen1, C. Claeys1, H. Sunaga2 and M. Hososhima3, Kumamoto National College of Technology, Kumamoto, JAPAN; 1IMEC, Leuven, BELGIUM; 2Takasaki JAERI, Gunma, JAPAN; 3Japan Electronic Materials Co., Hyogo, JAPAN.
Results are presented of a study on the degradation of performance and induced lattice defects of Si square and gated diodes, subjected to 20-MeV protons and 220-MeV carbon particles. The radiation source dependence of the damage is also discussed by comparing to 1-MeV electron and 1-MeV fast neutron irradiation. Diodes fabricated in CZ and FZ-Si substrates are used, m order to investigate possible oxygen-precipitation hardening effects. Some substrates are contaminated by spiking of copper or iron atoms from a NH4OH: H2O2: H2O and HCl solution, respectively. The surface concentrations of the metal contaminants are 2.2 x 1012 and 4.1 x 1012 atoms/cm2, which are realistic values in VLSI processing. The diodes are surrounded by a 600 nm thick LOCOS oxide, making them compatible with a VLSI CMOS process. The gated diodes also have an interdigited structure, with narrow polysilicon fingers in-between the active diode regions. The diodes are irradiated at room temperature with 20-MeV protons with fluences ranging from 1010 to 1014 p/cm2 and with 220-MeV carbon particles. The carbon fluence is varied from 1011 to 1013 1/cm2. Current/voltage and capacitance/voltage characteristics are measured together with the induced deep levels in the Si substrate by DLTS. The influence of the indiffused iron and copper atoms on the device performance is discussed. The recovery behavior of irradiated diodes will also be presented in order to investigate the impacts of carbon and oxygen related lattice defects on the device degradation.
11:45 AM I18.5
RADIATION DAMAGE OF InGaAs PHOTODIODES BY HIGH ENERGY PARTICLES. H. Ohyama, T. Kudou, E. Simoen1, C. Claeys1, Y. Takami2, K. Shigaki, A. Fujii3 and H. Sunaga4, Kumamoto National College of Technology, Kumamoto, JAPAN; 1IMEC, Leuven, BELGIUM; 2Rikkyo University, Kanagawa, JAPAN; 3Kumamoto University, Kumamoto, JAPAN; 4Takasaki JAERI, Gunma, JAPAN.
Results are presented of a study on the performance degradation and the induced lattice defects of In0.53Ga0.47As pin photodiodes, subjected to 20-MeV protons and 220-MeV carbon particles. The radiation damage is also compared to the results for 1-MeV electrons and 1-MeV fast neutrons with respect to the calculation of transfer energy during irradiation and particle range. Pin photodiodes fabricated on In0.53Ga0.47As epitaxial layers grown on (100) InP substrates are used. The nominal thickness of the n-InP window layer and the n--In0.53Ga0.47As epitaxial layers used as light absorbing material are about 1 and 3 m, respectively. TO-18 packaged diodes with the Kovar glass cover removed are irradiated at room temperature with 20-MeV protons with fluences ranging from 1010 to 1014 p/cm2 and with 220-MeV carbon particles. The carbon fluence is varied from 1011 to 1013 1/cm2. The diode performance (current voltage and capacitance/voltage characteristics) and the photo current as a function of a white light power and wavelength are measured together with the induced deep levels in In0.53Ga0.47As epitaxial layers using the DLTS method. The recovery behavior of the induced deep levels and of the diode performance by isochronal annealing will also be presented, in order to investigate the influence of carbon and gallium related lattice defects on the radiation damage.