Abstract
The present invention provides a radiation detection system for detecting X-ray and gamma rays featuring Cd.sub.1-xMg.sub.xTe in solid solution as a crystal semiconductor and electrical connection means. The crystal has a composition in the range of Cd.sub.0.99Mg.sub.0.01Te to Cd.sub.0.71Mg.sub.0.29Te and may be doped with indium or another Group III element, which may be suitable for use at room temperature as well as controlled temperatures. The present invention further provides a method for detecting X- or gamma ray radiation by (a) providing a solid solution Cd.sub.1-xMg.sub.xTe crystal in the composition range of Cd.sub.0.99Mg.sub.0.01Te to Cd.sub.0.71Mg.sub.0.29Te; (b) providing an electrical contact means for connecting the Cd.sub.1-xMg.sub.xTe crystal to an amplification, measurement, identification or imaging means; and (c) detecting the presence of the X- or gamma ray radiation.
Claims
1. A detection system for X-ray or gamma ray radiation comprising a solid solution Cd.sub.1-xMg.sub.xTe crystal in a composition range Cd.sub.0.99Mg.sub.0.01Te to Cd.sub.0.71Mg.sub.0.29Te, an amplification, measurement, identification or imaging means, and an electrical contact means for connecting the Cd.sub.1-xMg.sub.xTe crystal to the amplification, measurement, identification or imaging means.
2. The detection system of claim 1 wherein the crystal has been annealed.
3. The detection system of claim 1 further comprising an insulating layer between the electrical contact means and the crystal.
4. The detection system of claim 1 wherein the crystal is doped with a Group III element.
5. The detection system of claim 4 wherein the Group III element is Indium.
6. The detection system of claim 1 wherein the amplification, measurement, identification or imaging means is a spectrum.
7. The detection system of claim 4 wherein the Group III element is present in an amount of 110.sup.17 cm.sup.3 to 310.sup.17 cm.sup.3.
8. A method for detecting X- or gamma ray radiation comprising (a) providing a solid solution Cd.sub.1-xMg.sub.xTe crystal in the composition range of Cd.sub.0.99Mg.sub.0.01Te to Cd.sub.0.71Mg.sub.0.29Te; (b) providing an electrical contact means for connecting the Cd.sub.1-xMg.sub.xTe crystal to an amplification, measurement, identification or imaging means; and (c) detecting the presence of the X- or gamma ray radiation.
9. The method according to claim 8 further comprising providing a Group III element dopant to the crystal.
10. The method according to claim 9 wherein the dopant is Indium.
11. The method according to claim 9 wherein the dopant is present in an amount of 110.sup.17 cm.sup.3 to 310.sup.17 cm.sup.3.
12. The method according to claim 8 wherein the amplification, measurement, identification or imaging means is a spectrum.
13. The method according to claim 8 performed at approximately room temperature.
14. The method according to claim 8 wherein the detecting is performed using a system comprising a bias voltage source to provide positive and negative voltage.
15. The method according to claim 8 wherein the detecting is performed using a system comprising one or more preamplifiers.
16. The method according to claim 8 wherein the detecting is performed using a system that is a coaxial, a cross-strip plate, a Bolotinikov, a co-planar, a pixilated, or a pad configuration, or a Frisch ring.
Description
BRIEF DESCRIPTION OF THE FIGURES
[0024] FIG. 1 is a photograph showing the defects in a typical Cd.sub.1-xZn.sub.xTe crystal boule. Grains and twins, which are crystal defects, are labeled. This photograph illustrates a typical Cd.sub.1-xZn.sub.xTe crystal boule. The crystal defects mean that only small sections of the boule (ingot) can be used for detectors and these must be laboriously selected and cut from the boule. This poor crystal structure creates a large amount of waste and increases detector production costs.
[0025] FIG. 2 is a photograph of a low defect 20 mm diameter crystal boule (ingot) of solid solution Cd.sub.1-xMg.sub.xTe. The surface of the boule is free of voids and that there is no external indication of defects such as grains or twins.
[0026] FIG. 3 is a photograph of a cross section slice cut from the Cd.sub.1-xMg.sub.xTe boule shown in FIG. 2. The crystal is free of the grains and twins that typify a Cd.sub.1-xZn.sub.xTe crystal boule as shown in FIG. 1.
[0027] FIG. 4 is a graph showing the lattice constant and the dependence thereon of the energy bandgap of various alloys including CdTe and MgTe. The lattice constants of CdTe and MgTe are very similar whereas the lattice constants of CdTe and MnTe and CdTe and ZnTe are more dissimilar. The almost identical lattice constants of CdTe and MgTe assures that Cd.sub.1-xMg.sub.xTe crystals will be of excellent quality and will have fewer grains and boundaries than Cd.sub.x-1Mn.sub.x Te and Cd.sub.x-1Zn.sub.xTe and will also have a larger band gap.
[0028] FIG. 5 is a graph showing the segregation coefficient of Mg in CdTe as being close to 1.0 compared to 1.35 for Zn in CdTe, which is an important advantage over CZT for assuring a more homogeneous distribution of Mg in CdTe throughout the ingot as compared to a less homogeneous distribution of Zn throughout a CZT ingot.
[0029] FIG. 6 A-D shows a comparison (Top) of the IR transmission images obtained from a Nikon multi-function microscope of an as grown unannealed Cd.sub.1-xMg.sub.xTe crystal (top Left) with an annealed Cd.sub.1-xMg.sub.xTe crystal. The black spots are inclusions of Te, and both samples are sufficient for detector applications and show fewer inclusions than are found in CZT crystals. FIG. 6A represents IR transmission image obtained from Nikon multi-function microscope of an as-grown Cd.sub.0.92Mg.sub.0.08Te crystal. FIG. 6B represents IR transmission image obtained from Nikon multi-function microscope of an annealed Cd.sub.0.92Mg.sub.0.08Te crystal. FIG. 6C shows reconstructed 3D distributions of Te inclusions along the thickness of as-grown CdMgTe crystal at five different points over the 18 mm diameter wafer. FIG. 6D shows reconstructed 3D distributions of Te inclusions along the thickness of annealed CdMgTe crystal at five different points over the 18 mm diameter wafer. The dimension of each 3D image is 1.11.53 mm.sup.3. It shows that there are very few inclusions above 20 microns before annealing and none after annealing. The annealed crystal has fewer inclusions and therefore fewer locations where electrons and holes can recombine prematurely. Annealing may be done in the presence of an excess of Cd and/or Mg.
[0030] FIG. 7 shows the spectral response from 241 Am source of a 2 s pulse of a 873 mm.sup.3 planar in doped Cd.sub.1-xMg.sub.xTe detector. The spectra show the counts at channel numbers in the presence of three voltages 200 volts (blue), 250 volts (red) and 300 volts (green) as detected and plotted by a multichannel analyzer.
[0031] FIG. 8 shows a current versus bias voltage curve from which the resistivity of a planar In doped Cd.sub.1-xMg.sub.xTe detector was calculated at 210.sup.10 ohm-cm. Although 210.sup.10 ohm-cm is sufficient for a detector, resistivity can be increased as a function of crystal perfection and also improves with optimal dopant concentration.
[0032] FIG. 9 shows the mobility lifetime of a planar In doped Cd.sub.1-xMg.sub.xTe detector at 710.sup.4 cm.sup.2/V. This parameter can also be optimized by increasing crystal purity and we have to date achieved 810.sup.4 cm.sup.2/V as shown in Example 2.
[0033] FIG. 10 A-D shows the development of crystal quality over a mere eight (8) growth cycles as growth parameters were adjusted and purity was increased. The second (FIG. 10B) and third (FIG. 10C) ingots show progressive improvement compared to the first (FIG. 10A) while the last (FIG. 10D) ingot, which was ingot numbr 8, shows that the growth process has evolved to the point where the ingot is almost entirely single crystal.
[0034] FIG. 11 shows a Cd.sub.1-xMg.sub.xTe planar detector and a detection system comprising a preamplifier, a shaping amplifier and a multi-channel analyzer. When incident protons from a radiation source charge the carrier the electrons and holes move in opposite directions. The signal is then conducted to a preamplifier that produces a voltage pulse with an amplitude (height) proportional to the energy of the incoming photon, then to a shaping amplifier that amplifies the signal and converts the signal to a Gaussian pulse, followed by a multi channel analyzer that generates a spectrum of the incoming proton.
[0035] FIG. 12 shows a micro-scale response map of an indium doped Cd.sub.0.92Mg.sub.0.08Te detector exposed to a low energy x-ray source of 25 keV at a scan resolution of 100 microns, and demonstrates that Cd.sub.1-xMg.sub.xTe can be used as an x-ray detector with good resolution.
[0036] FIG. 13 shows a Cd.sub.1-xMg.sub.xTe planar detector comprising Au electrical contacts, a voltage source and voltage regulation means.
[0037] FIG. 14 A-C shows three common configurations of detectors used with CZT as the semiconductor material. From top to bottom are shown a planar detector (FIG. 14A), a co-planar grid detector (FIG. 14B) and a pixelated or pad detector (FIG. 14C).
[0038] FIG. 15 A-D shows three configurations of coaxial detectors in which the semiconductor crystal is rod shaped and is surrounded by the electrical contact surfaces. Three configurations are shown from left to right, being a true coaxial detector (FIG. 15A), a closed-ended coaxial detector (FIG. 15B) and closed-ended bulletized coaxial connector (FIG. 15C). Beneath the three configurations is a cross section of this type of detector (FIG. 15D) appearing as concentric circles in which the semiconductor crystal is the central circle.
DETAILED DESCRIPTION OF THE INVENTION
[0039] The present invention provides a radiation detector having Cd.sub.1-xMg.sub.xTe in a solid solution and electrodes. This detector is a significant improvement over the detectors now available such as those based upon Germanium, Silicon, Mercury Iodide, and Cadmium Zinc Telluride, because it functions at room temperature due to its large bandgap and also enables elimination of crystal defects which not only increases yields but also increases electron and hole lifetime.
[0040] A low defect solid solution Cd.sub.1-xMg.sub.xTe semiconductor in the compositional range Cd.sub.0.99Mg.sub.0.01Te to Cd.sub.0.71Mg.sub.0.29Te enables production of large volume crystals which are processed and configured with electrical contact means as room temperature detectors for X- and gamma rays. The material may be doped with an element to increase resistivity chosen from Group III (Al, Ga, In) which compensate Cd acceptors. Group III dopants are used to compensate Cd vacancies for the purpose of increasing the resistivity of the solid solution because Group III elements have additional donor electrons. Doping may be achieved by adding an element to the melt during growth or synthesis or by diffusion into the surface of a crystal after growth. Resistivity is reduced in the ternary compound Cd.sub.1-xMg.sub.xTe when the compostion moves from stoichiometry to an excess of Te and creates vacancies of Cd. It is of course preferable to use no or minimal doping and to instead develop crystalline perfection and purity, because although doping increases resistivity it also increases the recombination rate.
[0041] Cadmium Magnesium Telluride (either Cd.sub.1-xMg.sub.xTe or CMgT) and doped Cd.sub.1-xMg.sub.xTe is a material that possesses all required properties for a radiation detector viz. elements with high atomic numbers: [0042] Cd48 [0043] Mg12 [0044] Te52
[0045] CMgT also has high resistivity (210.sup.10 -cm as measured and theoretically much higher depending on crystal quality) as shown in FIG. 8, and it has high electron transport properties that have already been measured to at (710.sup.4 cm.sup.2/V) in preliminary tests as shown in FIG. 9. Moreover, this material offers several distinct advantages over Cd.sub.1-xZn.sub.xTe. Among those advantages are: [0046] (1) the optimal energy bandgap of 1.7-2.1 eV is attainable using less Mg in CdTe, to produce a useful solid solution Cd.sub.1-xMg.sub.xTe crystal compared to the amount of Zn needed in CdTe to produce a useful solid solution Cd.sub.1-xZn.sub.xTe crystal, because MgTe has Eg=3.5 eV compared to Eg=2.2 eV for ZnTe. The energy bandgap of Cd.sub.1-xMg.sub.xTe increases about 17 meV per atomic percent Mg compared with 6.7 meV per atomic percent of Zn in respect of Cd.sub.1-xZn.sub.xTe. Therefore 12 at. % Mg in CdTe, produces 1.7 eV compared to the 30 at. % Zn needed in CdTe and 29 at. % Mg in CdTe, produces 2.0 eV. The lower Mg content required to produce the desired bandgap in Cd.sub.1-xMg.sub.xTe compared to the higher requirement in Cd.sub.1-xZn.sub.xTe reduces the composition defects of Cd.sub.1-xMg.sub.xTe crystals. [0047] (2) The segregation coefficient of Mg in CdTe is mostly reported as 1.0 compared to 1.35 for Zn in CdTe, which is an important advantage over CZT for assuring a homogeneous distribution of Mg in CdTe throughout the ingot. (Woodbury et al., J. Cryst. Growth, 1971; 10:6; Lorenz et al., J. Electrochem. Soc., 1966; 113:559; Yang et al., Physical Review, 2009; B79: 245202). [0048] (3) The almost identical lattice constants of CdTe (6.48 ) and MgTe (6.42 ) yields good crystallinity and the value of the lattice parameters ratio of MgTe is indicative of zinc blende crystal structure.
[0049] The characteristics described in (2) and (3) above, viz. segregation coefficient of 1.0, nearly identical lattice constants and tendency to zinc blend crystal structure, include a combination of parameters that yield low defect single crystals that can be grown in large volume, thereby reducing the production costs of devices such as detectors and, in particular, and also enabling production of low cost large-area devices.
[0050] The combination of good uniformity, good crystallinity, and tendency to form zinc-blended structures provide low defect large-volume CMgT single crystals, and ultimately reduce the production costs of large-area devices.
[0051] In some instances, the radiation detector includes a planar detector having a Cd.sub.1-xMg.sub.xTe crystal in the composition range Cd.sub.0.99Mg.sub.0.01Te to Cd.sub.0.71Mg.sub.0.29Te with gold contacts deposited on the planar crystal faces as shown in FIG. 13. There may be an insulating layer between the crystal and all or part of the electrical contact as in a Frisch-ring detector. There may be an insulating layer on some or all of the longitudinal surface of the crystal such as is described in U.S. Pat. No. 8,063,378, the disclosure of which is herein incorporated by reference, for instance as in FIG. 3A, 3B, 4 and 7 which insulating layer may also extend to the exterior of an electrode or cathode and further be shielded by a conducting shield such as a contacting ring or Frisch ring around and electrically insulated from the detector crystal body (a Bolotinikov detector). The detector may be part of a complex system such as a system of the type shown in FIG. 11 wherein there is a bias voltage source to provide positive and negative voltage as shown in FIG. 14 where there are one or more preamplifiers, shaping amplifiers and single or multi-channel analyzers or in system with arrays of detectors such as a medical imaging system. The configuration of the detector may be in any form including, for example, coaxial, co-planar and pixelated or pad configurations, Frisch ring, Bolotnikov, arrays of detectors and cross-strip plates as shown, for example, in FIGS. 1, 2, 3 and 4 of U.S. Pat. No. 8,063,380, the disclosure of which is herein incorporated by reference (cross-strip plate detectors). The Cd.sub.1-xMg.sub.xTe crystal may be doped with Group III elements or undoped and that doping means may include diffusion.
[0052] Crystals may be synthesized in a one zone vertical tube furnace with various compositions in the CdTe-MgTe phase diagram. Cd, Mg, Te, and dopants (Al, Ga, In) are mixed with respective compositions in a crucible with a cone bottom, which is inserted into a quartz ampoule and vacuum sealed. Care must be taken during heating to avoid explosions due to a high pressure of elements in the ampoule, which may be achieved by slowly melting the elements.
[0053] Single crystals may be grown from synthesized element doped Cd.sub.1-xMg.sub.xTe ingots using the zone melting with solvent method. Tellurium acts as a solvent in accordance with the Gibb's phase rule in CdMgTe system. Single crystals may be grown by moving the melt zone from the cone bottom to the top of the crucible, but other means such as the traveling heater method may be used by those skilled in the art.
[0054] A detector is normally a part of a detection system having a semiconductor crystal with electrodes deposited on its surface (the detector) and situated within an electrical field. For example, the crystal may have electronically biased (cathode and anode) electrodes. The signal may be conducted to a preamplifier producing a voltage pulse with an amplitude (height) proportional to the energy of the incoming photon, then to a shaping amplifier that amplifies the signal and converts the signal to a Gaussian pulse, followed by a multi channel analyzer that generates a spectrum of the incoming proton.
[0055] Detectors may be constructed in many different configurations, such as planar (FIGS. 10, 11, 3, 14), co-axial (FIG. 15), pixelated (FIG. 14), Frisch-ring, Bolotinikov, and cross-strip plate detectors. The common feature of all of these detectors is a semiconductor and electrical contact means, although in Frisch ring and Bolotinikov detectors there is a non-contacting portion of the electrode as there exists a thin layer of insulation between it and the crystal. For example, a planar detector as shown in FIG. 13 is constructed from a Cd.sub.1-xMg.sub.xTe crystal with gold contacts applied to its surface. Typical detector contacts are gold, platinum, copper and aluminum. The contacts may be deposited by various means, but a common technique for applying gold and platinum contacts is electroless metal deposition using solutions of AuCl.sub.3 or PtCl.sub.4. The solution creates a chemical reaction with the surface of the crystal which deposits the film on the crystal. Detectors may also be used in arrays, such as is an x-ray imaging system. There are also surface preparation requirements to construct the detector, the most prominent of which is etching of the surface of the semiconductor to eliminate surface stress and improve surface perfection. Stress would the source of recombination of electrons and also a source of reduced resistivity at the perimeter which could create current noise. Etching may be done with a Bromine-methanol solution. Etching also improves the surface quality and it is known that surface defects can act as trapping centers and can result in surface current leakage.
[0056] As described herein, it is possible to provide a defects-free large-volume semiconductor detector for X- and gamma rays suitable for operation at room temperature, thereby reducing detector manufacturing and production costs due to increased yields.
EXAMPLE 1
[0057] Crystals were grown by the zone melting with solvent method. An In-doped ingot Cd.sub.0.92Mg.sub.0.08Te 18 mm diameter 34 mm length was grown with excess Te. The growth was carried out as described above. The ingot was mostly single crystal with very good crystalline perfection. From the single crystal thus grown samples were manufactured which had high resistivity 210.sup.10 ohm.cm (FIG. 8). Samples were fabricated into 873 mm planar detector configured and as shown in FIG. 7 a good spectral response was achieved at 3 voltages (200 v, 250 v, and 300 v) from 241 Am, a commonly used gamma ray source for energy calibration (Glemen F. Knoll, Radiation Detection and Measurement, John Wiley & Sons 2000, p. 486-488) and the electron mobility-lifetime value was determined to be 710.sup.4 V/cm.sup.2 (FIG. 9).
EXAMPLE 2
[0058] Crystals were grown by the zone melting with solvent method. An In-doped ingot Cd.sub.0.95Mg.sub.0.05Te 20 mm diameter 42 mm length was grown with excess Te. The growth was carried out as described above. The ingot was mostly single crystal with very good crystalline perfection. From the single crystal thus grown samples were manufactured which had high resistivity 310.sup.10 ohm.cm. Samples were fabricated in 873 mm planar configured in the same manner as Example 1 above. It was confirmed that the detector had a good spectral response from a 241 Am source, and the electron mobility-lifetime value was determined to be 810.sup.4 V/cm.sup.2.
EXAMPLE 3
[0059] Crystals were grown by the zone melting with solvent method. An In-doped ingot Cd.sub.0.71Mg.sub.0.29Te 18 mm diameter 34 mm length was grown with excess Te. The growth was carried out as described above. The ingot was mostly single crystal with very good crystalline perfection. The calculated band gap at this composition is 2.0 eV.
[0060] Crystals were grown by the zone melting with solvent method. An In-doped ingot Cd.sub.0.70Mg.sub.0.30Te 18 mm diameter 34 mm length was grown with excess Te. The growth was carried out as described above. The ingot had a poor crystalline perfection. Similarly bad crystalline perfection was demonstrated with the composition Cd.sub.0.69Mg.sub.0.31Te. Cd.sub.1-xMg.sub.xTe solid solutions in with concentrations of Mg in excess of Cd.sub.0.70Mg.sub.0.30Te exhibit some evidence of wurtzite structure coexisting with zinc blend. This structure creates flaws which our as grown examples demonstrate preclude single crystal growth of a perfect zinc blend structure at concentration 30 at. % and higher.
[0061] These 2 failed crystallinity experiments coupled with the knowledge that 29 at. % produces an energy bandgap of 2.0 eV which is well within in the optimal bandgap range (1.7 eV to 2.2 eV) for room temperature detection, together establish the outer boundaries of the compositional range of Cd.sub.1-xMg.sub.xTe solid solution crystals used in a detector at 30 at. %.
