Abstract:
A method for synthesizing I-III-VI 2  compounds, including: melting a Group III element; adding a Group I element to the melted Group III element at a rate that allows the Group I and Group III elements to react thereby providing a single phase I-III compound; and adding a Group VI element to the single phase I-III compound under heat, with mixing, and/or via vapor transport. The Group III element is melted at a temperature of between about 200 degrees C. and about 700 degrees C. Preferably, the Group I element consists of a neutron absorber and the group III element consists of In or Ga. The Group VI element and the single phase I-III compound are heated to a temperature of between about 700 degrees C. and about 1000 degrees C. Preferably, the Group VI element consists of S, Se, or Te. Optionally, the method also includes doping with a Group IV element activator.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    The U.S. Government has rights to the present disclosure pursuant to Contract No. AC05-000R22800 between the U.S. Department of Energy and Babcock and Wilcox Technical Services Y-12, LLC. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates generally to methods for synthesizing semiconductor quality chalcopyrite crystals for nonlinear optical and radiation detection applications and the like. More specifically, the present disclosure relates to methods for synthesizing a semiconductor detector of ionizing electromagnetic radiation, neutrons, and energetic charged particles. 
       BACKGROUND OF THE DISCLOSURE 
       [0003]    The present state-of-the-art in semiconductor radiation detection is silicon diodes, high purity germanium (cooled by liquid nitrogen), and compound semiconductors, such as cadmium zinc telluride (CZT) and mercuric iodide. Each of these materials has one or more significant drawbacks related to its use. Silicon has a low atomic number and is therefore primarily useful for the detection of energetic charged particles and atomic x-rays emitted by low atomic number elements. Germanium has a higher atomic number but, because of its low band gap energy, must be cooled by liquid nitrogen in a bulky, expensive, and potentially dangerous cryogenic system in order to reduce thermally generated noise. Compound semiconductors, such as CZT and mercuric iodide, have sufficiently high band gap energy to be useful at or near room temperature. However, CZT has been plagued by production problems, resulting in polycrystalline ingots with twins, inclusions, and grain boundary defects. These defects can never be completely removed and are a consequence of CZT being a solid solution, rather than a true compound. The result is that spectroscopy grade crystals must be mined from bulk material. Mercuric iodide suffers from low carrier mobility, short carrier lifetime, space charge polarization, and surface degradation. In addition, mercuric iodide is an extremely soft material that is easily damaged by the slight pressure of an electrical connection and by temperatures over sixty degrees Celsius. In general, these compound semiconductors do not interact with neutrons such that they must be coupled with a thin layer of a neutron absorbing material, such as  6 LiF or  10 B. A reaction between  6 Li or  10 B occurs in the thin absorber layer, which creates alpha particles that are detected by a semiconducting substrate. The absorber layer must be thin in order for the semiconducting substrate to detect the resultant alpha particles.  3 He gas filled tube detectors are the state-of-the-art for thermal neutron detection. 
         [0004]    As a result, U.S. Pat. No. No. 7,687,780 (Bell et al.) provides a semiconductor detector of ionizing electromagnetic radiation, neutrons, and energetic charged particles. The detecting element includes a compound having the composition I-III-VI 2  or II-IV-V 2 , where the “I” component is from column 1A or 1B of the periodic table, the “II” component is from column 2B of the periodic table, the “III” component is from column  3 A of the periodic table, the “IV” component is from column 4A of the periodic table, the “V” component is from column 5A of the periodic table, and the “VI” component is from column 6A of the periodic table. The detecting element detects ionizing electromagnetic radiation by generating a signal proportional to the energy deposited in the element, and detects neutrons by virtue of the ionizing electromagnetic radiation emitted by one or more of the constituent materials subsequent to capture. The detector may contain more than one neutron sensitive component. 
         [0005]    Related to the I-III-VI 2  compounds, however, improved methods for combining the elemental constituents in a multistep synthetic process are still required, providing improved purity and homogeneity and more precisely controlling the reaction rate and yielding a I-III-VI 2  charge with a single phase stoichiometry. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The present disclosure is illustrated and described herein with reference to the various drawings, in which like reference numbers are used to denote like method steps, as appropriate, and in which: 
           [0007]      FIG. 1  is a flowchart illustrating one exemplary embodiment of the method for synthesizing I-III-VI 2  compounds of the present disclosure, specifically utilizing a heating process; 
           [0008]      FIG. 2  is a flowchart illustrating another exemplary embodiment of the method for synthesizing I-III-VI 2  compounds of the present disclosure, specifically utilizing a mixing process; and 
           [0009]      FIG. 3  is a flowchart illustrating a further exemplary embodiment of the method for synthesizing I-III-VI 2  compounds of the present disclosure, specifically utilizing a vapor transport process. 
         BRIEF SUMMARY OF THE DISCLOSURE 
         [0010]    In various exemplary embodiments, the present disclosure provides a multistep synthetic process for synthesizing an inorganic compound with unique electrical and optical properties. This compound is semiconducting and can be developed for nonlinear optical applications, as well as radiation detection. The I-III-VI 2  stoichiometry crystallizes into a chalcopyrite type structure, and when the Group I element is lithium, the material is potentially ideal for the room temperature detection of neutrons, for example. The best known synthesis method, described in U.S. Pat. No. 7,687,780 (Bell et al.), involves heating stoichiometric quantities of the three elemental powders simultaneously to form the I-III-VI 2  compound. For the lithium containing compounds, lithium reactivity is difficult to control, leading to poor phase homogeneity. Thus, the present disclosure provides improved methods for combining the elemental constituents in a multistep synthetic process. These methods provide improved purity and homogeneity and more precisely control the reaction rate, thereby yielding a I-III-VI 2  charge with a single phase stoichiometry. 
           [0011]    In one exemplary embodiment, the present disclosure provides a method for synthesizing I-III-VI 2  compounds, including: melting a Group III element; adding a Group I element to the melted Group III element at a rate that allows the Group I and Group III elements to react thereby providing a single phase I-III compound; and adding a Group VI element to the single phase I-III compound and heating. The Group III element is melted at a temperature of between about 200 degrees C. and about 700 degrees C. One (1) mole of the Group I element is added to 1 mole of the Group III element. Preferably, the Group I element consists of a neutron absorber, such as  6 Li, and the Group III element consists of In or Ga. The Group VI element and the single phase I-III compound are heated to a temperature of between about 700 degrees C. and about 1000 degrees C. Two (2) moles of the Group VI element are added to the single phase I-III compound. Preferably, the Group VI element consists of S, Se, or Te. Optionally, the method also includes doping with a Group IV element activator. 
           [0012]    In another exemplary embodiment, the present disclosure provides a method for synthesizing I-III-VI 2  compounds, including: melting a Group III element; adding a Group I element to the melted Group III element at a rate that allows the Group I and Group III elements to react thereby providing a single phase I-III compound; and adding a Group VI element to the single phase I-III compound under heat while rotating all constituents at an angle. The Group III element is melted at a temperature of between about 200 degrees C. and about 700 degrees C. One (1) mole of the Group I element is added to 1 mole of the Group III element. Preferably, the Group I element consists of a neutron absorber, such as  6 Li, and the Group III element consists of In or Ga. The Group VI element and the single phase I-III compound are heated to a temperature of between about 700 degrees C. and about 1000 degrees C. Two (2) moles of the Group VI element are added to the single phase I-III compound. Preferably, the Group VI element consists of S, Se, or Te. Optionally, the method also includes doping with a Group IV element activator. 
           [0013]    In a further exemplary embodiment, the present disclosure provides a method for synthesizing I-III-VI 2  compounds, including: melting a Group III element; adding a Group I element to the melted Group III element at a rate that allows the Group I and Group III elements to react thereby providing a single phase I-III compound; and adding a Group VI element to the single phase I-III compound under heat via vapor transport. The Group III element is melted at a temperature of between about 200 degrees C. and about 700 degrees C. One (1) mole of the Group I element is added to 1 mole of the Group III element. Preferably, the Group I element consists of a neutron absorber, such as  6 Li, and the Group III element consists of In or Ga. The Group VI element and the single phase I-III compound are heated to a temperature of between about 700 degrees C. and about 1000 degrees C. Two (2) moles of the Group VI element are added to the single phase I-III compound. Preferably, the Group VI element consists of S, Se, or Te. Optionally, the method also includes doping with a Group IV element activator. In this exemplary embodiment, the single phase I-III compound and the Group VI element are physically separated in the reaction vessel. The reaction takes place through vapor transport of the Group VI element into the single phase I-III compound melt by maintaining the Group VI element region at a higher temperature than the single phase I-III compound melt region. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Again, related to U.S. Pat. No.  7 , 687 , 780  (Bell et al.), the detecting element includes a compound having the composition I-III-VI 2  or II-IV-V 2 , where the “I” component is from column 1A or 1B of the periodic table, the “II” component is from column 2B of the periodic table, the “III” component is from column 3A of the periodic table, the “IV” component is from column  4 A of the periodic table, the “V” component is from column 5A of the periodic table, and the “VI” component is from column 6A of the periodic table. A more concise manner of description is that the crystal is formed from elements in the group of 1A-3A-6A, 1B-3A-6A, or 2B-4A-5A of the periodic table. An example from group 1A-3A-6A is lithium-gallium-selenium. An example from group 1B-3A-6A is copper-gallium-selenium. An example from group 2B-4A-5A is cadmium-germanium-antimony. Crystals formed from groups 1B-3A-6A and 2B-4A-5A are chalcopyrites. The detecting element detects ionizing electromagnetic radiation by generating a signal proportional to the energy deposited in the element, and detects neutrons by virtue of the ionizing electromagnetic radiation emitted by one or more constituent materials subsequent to capture. The detector may contain more than one neutron sensitive element. 
         [0015]    The detecting system generally includes a semiconductor crystal onto which conducting electrodes are deposited on opposing surfaces of the crystal. The semiconductor material may be intrinsic material or doped to produce intrinsic material. Intrinsic material, when referring to semiconductors, refers to a semiconductor material in which the majority and minority charge carriers in the material are balanced and the material does not display either negative (n-) or positive (p-) type conductivity. Doping is the process of introducing small amounts of impurities (typically in the amount of parts per million (ppm)) for the purpose of altering the electrical properties of the material to force a desired density of positive and negative charge carriers. The electrical contacts may be ohmic, or may be Schottky. An ohmic contact is a metal semiconductor contact with very low resistance independent of the polarity of the applied voltage. A Schottky contact is a metal semiconductor contact used to form a potential barrier. The resulting detecting element forms a p-n, or p-i-n diode, or simply a bulk semiconducting material. A p-i-n diode is a semiconductor structure comprised of p- and n-type materials with an intrinsic material in between. The addition of the intrinsic layer changes the properties of the p-n junction. 
         [0016]    In a preferred embodiment, a voltage is applied between the electrodes by a suitable means, such as a battery. A resistor is provided in line between one of the electrodes and the battery. Any signal generated in response to radiation is extracted through a junction between the crystal and the resistor. If the detecting element is simply bulk semiconducting material, then the polarity of the voltage with respect to the resistor is of no consequence. Although the resistor connected between the negative terminal of the battery and the crystal is contemplated, the resistor may also be connected between the positive terminal of the battery and the crystal with the signal taken from the junction between the resistor and the crystal. If the crystal is realized as a diode (p-n, p-i-n, or Schottky), then the connection must be such that the diode is reverse biased by the battery. 
         [0017]    When the crystal is exposed to ionizing electromagnetic radiation, electron-hole pairs are created in the bulk of the material. These charges are separated by the applied voltage and the resulting charge pulse is sensed as a current pulse or a voltage pulse. The amplitude of the charge pulse is proportional to the energy deposited in the crystal by the radiation. In this mode, the crystal realizes a detector of alpha, beta, gamma, and x-ray radiation, in addition to cosmic rays. 
         [0018]    In one arrangement, the crystal may be fabricated with one element that reacts with neutrons and subsequently emits ionizing electromagnetic radiation. The crystal then also serves to detect neutrons. For example, if the “I” element is silver (Ag), an element from column 1B of the periodic table, then exposure to neutrons results in transmutation of the silver nuclei to radioisotopes with short half-lives. These isotopes decay by the emission of beta particles (electrons), which create ionization in the detector just as would radiation originating outside the detecting element. Analysis of the spectrum of pulse amplitudes and the temporal behavior of the count rate yields a signature of the presence of neutrons. 
         [0019]    In another arrangement, if the “I” element is lithium, an element from column 1A of the periodic table, then exposure to neutrons results in the exothermic reaction  6 Li(n,α) 3 H. The energetic triton and alpha particles liberate charge as they decelerate, ultimately depositing their entire energy in the crystal. Analysis of the resulting spectrum of pulse amplitudes yields a signature of the presence of neutrons. 
         [0020]    In a further arrangement, if the “III” (3A) element of the crystal is indium (In), behavior similar to that described for silver is observed. In a still further arrangement, more than one element may be neutron sensitive. For example, if both silver and indium are used, then multiple half-lives are observed in the count rate, and spectra of beta particles characteristic of both elements are observed in the pulse height spectrum. Analysis of such data can give information on the spectral characteristics of the incident neutron flux. 
         [0021]    In operation, the semiconductor radiation detecting apparatus works in the following manner. Means are provided to convert current or charge pulses to a digital value. The electrical charge signal generated in response to radiation passes from the junction through a capacitor to a charge integrating pre-amplifier whose output signal, in turn, is directed to a shaping amplifier. The shaping amplifier produces an approximately Gaussian shaped pulse. The pulse is directed to an analog-to-digital converter (ADC), which translates the analog voltage developed by the shaping amplifier into a digital value. The digital values from the converter are directed to a processor and display. The processor records the number of times each value occurs during a measurement. This accumulates a histogram of the magnitudes of the pulses produced by the incident radiation. The processor compares these values to known values and provides an indication of the incident radiation based on the comparison. The use of the amplifiers, converter, and processor to condition signals and create an indication of the incident radiation is well known to those of ordinary skill in the art and does not require detailed explanation. 
         [0022]    In an alternate embodiment, the pre-amplifier simply provides gain without integration and the shaping amplifier is replaced by a voltage comparator and gated integrator. The voltage comparator triggers the gated integrator to integrate the voltage pulse from the pre-amplifier. The gated integrator signals the ADC to perform a conversion when the integration period is complete. The processor and display perform the same functions as described above. 
         [0023]    In another alternate embodiment, a battery supplies bias to the crystal. The charge generated by incident radiation in the crystal is separated by the potential developed by the virtual ground at the inverting terminal of the operational amplifier, and the resulting current pulse is forced through a feedback resistor. In this manner, the current pulse is converted to a voltage pulse and is then directed to the pre-amplifier. 
         [0024]    These crystals are useful as radiation detectors, and as semiconductors, for the following reasons. Carrier mobility in the range of 500-10,000 cm 2 /V-s has been reported and band gaps between 1.2 and 2.7 eV have been produced. These values are comparable to or better than those of germanium and indicate that performance at room temperature should exceed that of CZT. In addition, some of the constituent materials have high neutron absorption cross sections, conferring simultaneous sensitivity to ionizing electromagnetic radiation and neutrons. 
         [0025]    There are a number of advantages to using chalcopyrites. There is a ready availability of high purity, oriented, crack free, single chalcopyrite crystals produced for use in optical applications. These crystals are used in infrared nonlinear optical equipment to effect second harmonic generation or optical parametric oscillation. The finished materials have improved properties that should continue to improve as a result of research and development spurred by the interest of the military in using chalcopyrites in high powered lasers. Chalcopyrites have physical properties that permit their use as semiconductor radiation detectors at room temperature. As such, they operate according to the same physics as do silicon, CZT, and mercuric iodide. However, they differ from these materials in that the average atomic number is much larger than silicon, making them useful at higher energies than silicon. They can be grown in large, crack free single crystal boules (unlike CZT and mercuric iodide). They also are an improvement over mercuric iodide in that the use of mercuric iodide is limited to applications in which the temperature does not exceed 80 degrees Celsius, while chalcopyrites can withstand temperatures up to several hundred degrees Celsius. 
         [0026]    Without limitation, it may be advantageous to deposit more than two electrodes onto the crystal to control the shape of the internal electric field. In another example, sandwiching the crystal between spring loaded contacts enables the application of a voltage without the deposition of electrodes. In yet another example, the pre-amplifier, shaping amplifier, and ADC may be replaced by a charge-to-digital converter. 
         [0027]    In view of the above, the present disclosure involves separating I-III-VI 2  synthesis from the constituent elements into two distinct steps, and three associated embodiments are contemplated. The highest quality elemental starting materials are required to achieve successful synthesis of I-III-VI 2  compounds. Group I elements typically have the lowest purities as starting materials. Referring to  FIGS. 1-3 , in all exemplary embodiments, the most reactive element (Group I) is reacted with the Group III element, forming a binary alloy with equal stoichiometry  5 . The Group III element is melted in a crucible under inert atmosphere. The Group I element is then slowly added to the melt, allowing the small amount of Group I element to react before another addition. This process minimizes overheating of the reaction, which causes additional alloy stoichiometries to form in the melt. The result is a highly crystalline, single phase I-III material. Two moles of a Group VI element are then added to the I-III compound at elevated temperature to form the chalcopyrite I-III-VI 2 . The method by which the Group VI element is added is different in each of the three exemplary embodiments. 
         [0028]    Referring specifically to  FIG. 1 , in the first exemplary embodiment  10 , the Group VI elemental powder is added directly to the I-III alloy and heated to 700-900 degrees C. (depending on the group VI element) in a crucible  12 . The reaction is allowed to proceed to completion and then cooled. Because the I-III compound is formed in an initial reaction step, the stoichiometry is defined as one mole for each element. Addition of the Group VI element for the final I-III-VI reaction has lower overall stoichiometric variability throughout the charge. 
         [0029]    Referring specifically to  FIG. 2 , in the second exemplary embodiment  20 , the addition of the Group VI element into a single crucible is as with the first exemplary embodiment; however, the constituents are mixed at elevated temperature with constant crucible rotation at an angle  22 , for example about 20 degrees. Rotation further promotes mixing during the synthesis to reduce phase variability. 
         [0030]    Referring specifically to  FIG. 3 , the third exemplary embodiment  30  involves vapor transport of the Group VI element  32 . The previously prepared I-III compound is placed in one well of a crucible, while the Group VI element is placed in an adjacent well. As the mixture is heated to 700-900 degrees C., the Group VI element slowly vaporizes and is transported through thermal currents to the melted group I-III well. The reaction then occurs to form I-III-VI 2 . Vapor transport further slows the reaction to minimize overheating and promote a single phase synthetic charge. 
         [0031]    It should be noted that the methods of the present invention are not limited to the synthesis of semiconductor materials. Doping with an activator (e.g., Group IV element) may be performed to create a scintillator material, for example. 
         [0032]    Although the present disclosure has been illustrated and described herein with reference to preferred embodiments and specific examples thereof, it will be readily apparent to those of ordinary skill in the art that other embodiments and examples may perform similar functions and/or achieve like results. All such equivalent embodiments and examples are within the spirit and scope of the present disclosure, are contemplated thereby, and are intended to be covered by the following claims.