The report was given the Raytheon internal number S 6. The report was submitted by the authors 31 March 5. Cadmium Telluriue State -of-the -Art 4 1. Fabrication, 4 2. Optical properties 8 C.
Chemical Vapor Deposition Process 12 1. Advantages 13 2. Static and dynamic CVD systems 13 3. Conventional and transport chemical vapor deposition 15 D.
CVD system for CdTe 19 3. Evolution of mixing chamber and mandrel design 23 4. CVD of CdTe using metal-organics 34 5. Physical Data 48 III. Introduction 54 B. Elimination of Internal Voids 54 C. Extrapo- lated values of surface loss range from a minimum of 0. For the latter applications single crystals are needed, while for laser windows, both single crystal and polycrystalline material are usable. In the fabrication processes used prior to this work difficulties were experienced in controls the structural perfection and purity of the material, and as a result.
The chemical vapor deposi ion process offers the promise of overcoming some of the difficulties experienced to date.
Thus, the primary objective of the program was to establish the technology base that would allow one to determine the usefulness of the proces. The program was of 18 months duration and was carried out in several phases. The specific processes investigated an evaluated were; 1 dimethyl cadmium and dimethyl tellurium vapors; 2 dimethyl cadmium and tellurium metal vapors; 3 dimethyl cadmium vapor and HjTe gas; 4 cadmium metal and dimethyl tellurium vapors; an 5 Cd and Te vapors; and 6 H 2 Te gas and Cd vapor.
The second phase involved the task of process optimization. The most promising methods yielding satisfactory material under the first phase were studied more thoroughly. The process parameters which were de er mined to be of Importance were; deposition temperature, furnace pressure, and molar ratio of the reactants.
The third phase of this program ran concurrently with the first two phases. It involved the evaluation and measurement of the various process parameters such as deposition rate and profile, as well as quantative and qualitative determinations concerning the structure and properties of CdTe. The material was characterized optically using infrared transmission. Laser 1 calorimetry was used to determine the absorption coefficient of CdTe at Properties such as microhardness were studied to determine the effect of various deposition conditions on the mechanical properties of the material.
The results of these studies indicate that the chemical vapor deposition process can yield theoretically dense cadmium telluride. Further more, the results indicate that it will be feasible to fabricate an improved state-of-the-art material using this process. Summary of Inherent Problems in Producing CdTe Detailed studies on the growth and properties of synthetic CdTe have been in progress since , although interest in the growth of large, single crystals with a high p t product for gamma -ray detectors did not occur until The most recent emphasis on production of large, low- loss, low-scatter optical components is part of the development of high power In , CdTe was introduced as an infrared window material in the form of a hot pressed poly crystalline aggregate.
This form, which is adequate for use in passive infrared systems, is far fiom satisfying the more stringent laser requirements. In principle, the material properties required in an infrared window should be attained more easily than those called for in high -resolution gamma- ray detectors. In the latter case the utility of the material is affected by charge transport properties and is sensitive to both intrinsic and extrinsic lattice and crystal defects on an atomic scale.
For windows, however, the predicted free carrier absorption intensity at 1C. Thus, a reduction of carrier concentration by a factor of only 10 2 would result in a bulk loss coefficient of 0. Furthermore, the extrapolated multiphonon loss curve predicts an intrinsic bulk absorption coefficient of 10 cm at Impurity and defect suppression in crystals grown from the melt, or by condensation at high temperatures, is difficult to achieve. Chemical vapor deposition CVD offers the one technique wherein theoretically dense material can be produced at temperatures well below the melting point, and where surface migration is relied upon to give pore -free material.
Cadmium Telluride State -of-the -Art 1. In addition to studying the temperature -composition and pres sure -temperature projections of the binary phase diagram, he made resistivity and Hall -coefficient measurements on doped and nominally-undoped single crystals which had been quenched after being annealed over a range of temperatures and cadmium partial pressures.
On the basis of this work and much subsequent work it has been shown that CdTe can undergo stoichiometric deviations when grown at high temperatures, and that such changes in composition are caused mainly by lattice defects which determine the level of conductivity and carrier type.
A recent analysis of the state-of-the-art in CdTe single crystals has been 4 published by Strauss. The two main processes that have been used to fabricate high quality CdTe single crystals and in some cases polycrystals are growth from a liquid-phase and growth from the vapor phase.
Of the two processes liquid phase growth has been most extensively used. In conven- tional vapor deposition, CdTe or elements of the compound after purification and sintering are sublimed or evaporated and transported to a cooler portion of the furnace where they are condensed on an appropriate substrate. The high temperatures involved in both processes cause substantial concentrations of native vacancies to form because the composition deviates from its stoichiometric value.
In cadmium telluride cation vacancies pre- dominate over anion vacancies. The equilibrium concentration of vacancies is a function of temperature, composition of the equilibrium gas, and the concentration of foreign donors and acceptors.
As the crystal is cooled both processes the allowed range of deviation from the stoichiometric composition decreases as given by the solid-solubility field of the phase diagram Figure 2.
Precipitation of the major vacancies occurs because the vacancies of the bulk crystal cannot be cooled rapidly enough to quench them; therefore, the majority of vacancies cluster at nucleation points within the host lattice. Because the minority host atoms have no place to escape they remain within the same volume of the condensed vacancies. In CdTe, cadmium vacancies condense with an equal number of tellurium atoms, forming voids that are partially filled with elemental tellur- ium.
The formation of vacancies can be suppressed by imposing a sufficiently high pressure of the element corresponding to the majority vacancy at the growth temperature. In the case of vapor deposition this results in the suppression of sublimation so that reasonable growth rates cannot be attained.
Slow cooling results in the formation of a relatively small number of large pre- cipitates while fast cooling results in a larger number of small precipitates; the total volume of the precipitates is approximately equal in both cases. Alternatively, a proper dopant can be added to the melt or vapor to compensate the precipitant. Optical properties Cadmium telluride in high -resistivity form is essentially transparent between the indirect electronic edge at 1. The refractive index at The situation is summarized in Figure 3.
We estimate the enveloping exponential -8 "1 line to pass through the CdTe, Such a ma- -8 -1 terial should give a free-carrier absorption at The state-of-the-art CdTe loss coefficient measured calorimetrically at Another possible source of loss is scattering. Calorimetry on ZnSe Figure 4 material shows that loss for conventionally polished surfaces is typically in the range 0. In ZnSe, this figure is more likely to be derived from a scatter and reabsorption process than from a direct absorption, although water and organic -film contamination is also possible.
In any case, the CdTe loss figure for a 1 cm sample is approaching the anticipated surface loss. The practical limit for these losses must be considered. The material must also be free of internal metallic particles or voids that can act as scatter centers. For particles much larger than the operative wavelength, the scatter cross-section of imbedded particles is equal to their geometric cross-section.
Malmberg and A. Maryott An equal ratio arm, capacitance-conductance bridge, operated at frequencies below kilocycles per second, was used to measure the dielectric constant of water with an accuracy of better than 0. The experimental method and sources of error are considered in some detail. Introduction Numerous determinations of the dielectric con- stant of water have been reported in the literature during the past half century.
Several of the more recent investigators [1,2,3,4,5], 2 employing varied experimental techniques in which the accuracy was stated or has been inferred to be of the order of 0. Discrepancies amounting to a percent or more exist at higher temperatures. A low-frequency bridge method with a Wagner earth to permit the use of three-terminal dielectric cells was employed. The various methods of measure- ment, generally some form of bridge or resonance method, are all more or less subject to errors assoc- iated with the residual impedances of the network that are enhanced when the medium possesses appreciable conductivity.
The present assembly has the advantage of facilitating the minimization, ready control, and accurate evaluation of these residual errors, and of simplifying certain problems regarding the design of cells suitable for accurate absolute measurements. The values of dielectric constant reported in this investigation are significantly lower than previously assigned values , about 0.
Apparatus The a-c bridge is of the equal-ratio arm, capaci- tance-conductance type, with a Wagner earth. The Wagner earth permits use of either two- or three-terminal cells, and in conjunction with proper 1 For jan extensive compilation and comparison of data prior to , see N. A schematic diagram of the circuit is shown in figure 1.
The basic network and its components, with the exception of the unknown arm and standard capaci- tor, are substantially the same as those described previously .
The unknown arm contains a shielded 3,ohm resistor, i? The capaci- tance of the test cell is measured in terms of C m by connecting the cell in parallel first with C and then with Z 5 of the Wagner earth by means of the switch, K.
Jn the latter case the eel] is effectively removed from the main bridge circuit because its earth- potential electrode is connected to ground, and admittances from any corner of the bridge to ground do not affect the conditions of balance.
Figuee 1. Schematic diagram of the bridge circuit. Intercomparisons between the three cells, using media of various dielectric con- stants, indicated that these capacitances varied linearly with the dielectric constant of the medium. Because accurate measurements become increas- ingly difficult as the conductivity of the medium increases, the materials of construction of the two cells, A and B, used with water, were selected to minimize electrolytic contamination.
Cell C was used only for intercomparisons with liquids of lower dielectric constant and conductivity. Cell A is a fixed, three-terminal, cylindrical capaci- tor.
A schematic drawing is shown in figure 2. The base material is copper, with the electrode surfaces coated with pure tin. The central, or high- potential, electrode, H, is supported by the grounded guard electrode, G. Electrical insulation and a liquid-tight seal are provided by 0.
Teflon gaskets on each side of the silvered coppei spacer, F, and by a Bakelite bushing on the upper side. The high -potential terminal is electrostatically iso- lated from the case by the removable shield, S.
The guard electrode is supported on the outer case, which, in part, serves as the earth-potential electrode D, by means of a Pyrex-glass ring and 0. To pre- vent the trapping of gas in the annular space between the guard and the earth potential electrodes when the cell is being filled with liquid, four 1-mm holes were drilled through G at level E.
Sectional view of cell A. Coupling between the exposed, earth-potential electrode and other com- ponents of the network is prevented by proper shielding. As lead, shield, and guard capacitances are all effective directly to ground, only the capaci- tance between the test electrodes, D and H, is measured.
Cell B, shown at the right in figure 3, is of the differential type. It was constructed from a Gen- eral Radio type D variable air capacitor modified in several respects. The steatite insulators were replaced w r ith Pyrex glass and par- tially shielded to minimize any effect of the insulation on the differential capacitance.
The unit was mounted in a gold-plated brass case. The rotor control handle is variable between fixed stops mounted on the cover. The high-potential stator plates bracket the earth-potential rotor plates when interleaved to avoid varying the stray fields through the insulating supports.
Both are isolated from the case except when the cell is used as a two- terminal capacitor. Then the rotor plates and the case are connected together.
A comparison table 1 of the values of dielectric constant obtained for a given medium when the cell is used as both a two- and a three-terminal capacitor provides a check of the linearity of its replaceable interval of capacitance with respect to dielectric constant. Love Resurrection. Alison Moyet. Sophie B. True Colors. Cyndi Lauper. En Vogue. This Moment Is Mine. Mary J. Gettin in the Way. Jill Scott. Track Listing - Disc 2. Daylight in Your Eyes. No Angels. Gotta Tell You.
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