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Cadmium telluride infrared transmitting material
Infrared Physics, 1966, Vol. 6, pp. 145-151. Pergamon Press Ltd., Printed in Great Britain.
CADMIUM
TELLURIDE INFRARED TRANSMITTING MATERIAL LE ROY S. LADD Eastman Kodak Company,
Rochester,
(Received 22 February
New York
1966)
Abstract-Cadmium telluride was developed as an infrared transmitting material covering the broad spectral region from 1 to 30 p. High transmitting windows up to 3f in. dia. have been formed by the hot-pressing method. Property measurements of cadmium telluride are presented. A comparison is made of cadmium telluride and other well known and available materials that transmit from 1 to 30 p. Cadmium telluride appears to be the superior material, particularly for applications under severe environmental conditions. INTRODUCTION
years ago we conducted a preliminary investigation of cadmium telluride (CdTe) as an infrared transmitting material. Recently this work was extended to the development of CdTe for a special long wavelength application requiring a number of 2) in. dia. and + in. thick windows. Property requirements included transmittance from 1 to 30 p and a combination of strength, thermal and environmental properties not found in currently available materials that transmit over this broad spectral region. Our earlier investigation indicated that CdTe would meet the requirements of this application. The development work was greatly facilitated by our experience in developing the family of infrared transmitting materials that are marketed under the tradename Kodak Irtran Optical Materials. These materials are formed into dense, polycrystalline windows of high infrared transmittance by hot-pressing at temperatures well below the melting point of the specific material. This method was chosen for fabricating CdTe. Very recently, CdTe has been added to this group under the designation of Irtran 6. Until the introduction of this new material, Irtran 4, which is useful to about 20 p, transmitted to the longest wavelength. CdTe supplements the Irtran family by extending the transmittance range to 30 CL. SEVERAL
RAW
MATERIAL
AND
FABRICATION
of CdTe raw material obtained from eleven suppliers were tested and evaluated. Most of the samples were prepared by fusion reaction of high purity Cd and Te elements. One supplier provided raw material that was zone refined from fusion reaction material. Two samples were furnished that had been prepared by wet chemical methods. For test and evaluation, a portion of the sample was ground to a fine powder, then hotpressed to small O-8 in. dia. windows for infrared transmittance determinations. The specimens were also examined with an infrared viewer. Opaque inclusions, internal cracks, and general optical homogeneity could be observed with this instrument. Fusion reaction CdTe received from three suppliers was found satisfactory for infrared Samples
145
146
LEROY
S.LADD
optical use. The zone-refined material consistently produced specimens of high transmittance but the cost was prohibitive. CdTe prepared by wet chemical methods was not satisfactory. Concurrently with the raw material investigation, a study of optimum hot-pressing procedures and parameters was conducted on the small O-8in. dia. test specimens. Hot-pressing work was then extended to pressings which were 24 in. in dia. and up to 0.400 in. thick. With good starting material, 2t in. dia. pressings were soon produced with consistently high transmittance. Pressing size has recently been extended successfully to 32 in. dia. and work to scale up to 6 in. dia. is under investigation. CdTe can be worked in the optical shop with conventional techniques used on optical glass. Because the material is rather soft, some care must be used during final polish to remove scratches. TRANSMITTANCE
CHARACTERISTICS
Figure 1 shows a typical transmittance curve for a 2$ in. dia. and 8 in. thick CdTe window. In observing the transmittance level, it should be noted that the two-surface reflection loss for CdTe is approximately 35 % as calculated from refractive index data. Thus, the theoretical external transmittance maximum is approximately 65 ‘A. The Fig. 1 specimen is essenti-
100 90 $
80 70
0
2
4
6
8
IOWavele&, 12
16
18
20
22
24
26
28
30
microns FIG. 1. State-of-the-art
transmittance
of hot-pressed
cadmium telluride. Specimen thickness = 0.125 in.
ally at the theoretical transmittance limit from 2 to 28 p and is quite free from absorption bands in this region. The drop in transmittance from 2 to 1 p is attributed mainly to the influence of the short wavelength transmittance cut-off at 0.85 p due to electronic absorption. Small amounts of porosity and other inclusions may contribute in a very small degree to reduced transmittance by scattering in this region. The drop in transmittance from 28 to 30 TVis an absorption edge due to lattice absorption associated with multiphonon processes. PROPERTIES
OF HOT-PRESSED
CADMIUM
TELLURIDE
A compilation of property measurements made during the project are presented in Table 1. Some characteristics which are marked “preliminary data” will be remeasured or extended to a greater temperature range. Modulus of rupture and modulus of elasticity were determined by four-point bending. Nine specimens were measured at each of the three test temperatures. Measurements were also attempted at 200°C but plastic flow was encountered at this temperature.
Cadmium telluride infrared transmitting
141
material
TABLE 1. PROPERTYMEASUREMENTS OF HOT-PRESSED CADMIUMTELLURIDE A. Modulus of rupture and modulus of elasticity (preliminary data)
Temperature
Modulus of rupture (psi)
Modulus of elasticity (psi X 106)
4,590 4,540 5,880
5.2 5.3 45
Liquid nitrogen (- 196°C) Room temperature (25°C) 100°C -
B. Thermal conductivity (preliminary data)
Temperature -
(“C)
Thermal conductivity (Cal. sec-1cn-20C-1cm)
40 20 0 20 40 60 80 100 120 140
0.013 0012 0.010 OGO98 0+)095 O+TO92 0.0090 OGO88 0.0086 0,0085
C. Thermal expansion (preliminary data)
Range of temperature
(“C)
Coefficient of linear thermal expansion (“c-r x 10-G)
25-100 25-200 25-250
5.5 59 6.2
D. Thermal shock properties sample size: (2.020 in. dia. x 0.145 in. thick) Test conditions 1. Quenched from room temperature (25°C) to liquid nitrogen temperature (- 196%) 2. Quenched from 170°C to water at room temperature (25°C) 3. Quenched from 180°C to water at room temperature (25°C) E. Environmental Test conditions
-___-
1. No cracks or other effects 2. No cracks or other effects 3. Axial cracks
properties Results
Exposure to 96% relative humidity at 90°F temperature Exposure to air at elevated temperature Exposure to vacuum of approximately elevated temperatures.
Results
100~ at
1. No change in transmittance after 11 days exposure 2. No change in transmittanceafter 16 hr at3OO”C. After 16 hr at 350°C, a light gray oxidation coating developed on the specimen surfaces. 3. No change in transmittance after 7 hr at 300%. At 4OO”C, thermal etching was observed after approximately 5 min.
148 TABLE 1 -
LEROY S. LADD
continued
F. Refractive index Wavelength (p)
Refractive index
1.0 1.5 2.0 2.5 3.0 3.5 4.0 5.0 6.0 7.0 8.0 9.0 10.0
2.839 2.142 2.713 2.702 2.695 2.691 2,688 2.684 2.681 2.679 2,677 2,674 2,672 G. Density
.~____.__~
Density = 5.8511 g/cm3 _ H. Hardness Hardness--45
Knoop
The thermal conductivity measurements were made early in the project on a specimen that had somewhat lower transmittance than was attained in later specimens. There may have been some porosity and impurities in the measured specimen that contributed to lower thermal conductivity than may be expected for later, high transmitting material. Thermal expansion was measured with a Leitz dilatometer. The thermal expansion of CdTe matches the thermal expansion of commonly used mounting or retaining assembly materials much more closely than other infrared materials that transmit from 1 to 30 p. Forming hermetic seals to the mount or retainer is facilitated by closely matching thermal expansions. The thermal shock and environmental property measurements demonstrate that CdTe may be used under severe operating conditions. Refractive index was measured by well known refractive techniques. A prism of hotpressed CdTe was prepared and the angle between prism faces was accurately measured. Radiation of desired wavelength from a Perkin Elmer monochromator was passed through the prism which was mounted on a Gaertner spectrometer table. The refracted beam was focused on a thermocouple detector until a maximum signal was obtained. The angle of minimum deviation was thus determined. Knowing the angle of minimum deviation and the angle of the prism, refractive index could be calculated. It was not possible to measure beyond 10 p with available equipment. Density was determined by Archimedes method on specimens of hot-pressed CdTe with
480
2,390
-
KRr* KI* CsBr*
cs1+
0.8
3.9 4.6 2.3
2.3
-5.3
Modulus of elasticity (psi x 106)
50
43 43 48
58
-5.5
Thermal expansion (“C-l x 10ee)
621
730 723 636
415
_1,090
Melting point (“C)
44
53 128 124
0.05
Insoluble
Water solubility (g/100 gs HaO)
-
I 20
40
_45
(Knoop)
Hardness
4.5
28 3.1 4.4
14
8 ii 12
28
35
5-85 7.4
(% at 1014
WCC)
Density
Reflection loss
Water soluble, soft
High reflection loss, relatively soft Flows under pressure, soft Water soluble, soft Water soluble, soft Water soluble, soft
Shortcomings
I~trumentat~n January 1959.
* Data from the University of Michigan, Willow Run Laboratories, Report No. 2389-11-S State of-the-Art-ReportOptical Materials for Infrared
18,100
4,540
KRS-5*
CdTe
Material
Modulus of rupture (Psi)
TABLE 2. COMPARISONOF PROPERTIESOF INFRAREDTRA~SMI~IN~ MATERIALSFOR THE l-30p REGION
i? S z!. c
P’
1
ii: c s. 8 a ti % Ei
p
150
LEROY S. LADD
good transmittance and weighing from 47 to 70 g. The average density for four specimens was 58511 g/c3 with a range from 5.8509 to 58514 g/cc. In Table 2, a comparison is made of the properties of CdTe and other well known and available infrared materials that transmit usefully from 1 to 30 p. The more favorable characteristics of each material are underlined in the table. For use under severe operating conditions, CdTe appears to be the superior material. The four alkali halides are limited by water solubility to applications in carefully controlled atmospheres. KRS-5 is limited in its applications by tendency to flow under pressure. It would not be suitable for instrumentation where one side of the window is subjected to high vacuum. All of the materials listed in Table 2 are soft and easily scratched but the hardness data indicate that CdTe is slightly harder than the others. The most serious shortcoming of CdTe is its high reflection loss. However, low reflection coatings can be applied to reduce surface reflection losses to a few percent. Work is continuing in this area, and durable and efficient coatings should result. Transmittance of hot-pressed CdTe as a function of elevated temperature was measured. The specimen was heated in a cylindrical, resistance wound furnace mounted in a Perkin Elmer Model 13 spectrophotometer. This instrument chops the source energy before impinging on the specimen. Emitted radiation from the specimen does not influence the measurements since the detection system is sensitive only to chopped energy. The specimen temperature was measured with a thermocouple contacted against the edge of the sample. Measurements were made from 1 to 15 p using a NaCl prism and from 15 to 30 p with a CsBr prism. Elevated temperature transmittance at 100” and 200°C from 1 to 15 TVand at 100°C 200°C and 300°C from 15 to 30 p is shown in Fig. 2. The 2 % drop in transmittance from
- 60 P
22
FIG. 2. Transmittance
of hot-pressed
CdTe at elevated temperatures.
24
26
28
Specimen thickness
30
= 0.125 in
1 to 15 p at 100°C and 200°C may be measurement error. The only significant change in transmittance occurred in the 23-30 p region where there was a progressive drop in transmittance with increasing temperature. The loss in transmittance is attributed to shift of the lattice absorption edge to shorter wavelengths with increasing temperature. The transmittance loss was transient; transmittance was at the original level when measured at room temperature after completion of the elevated temperature measurements. Transmittance at liquid nitrogen temperature was also measured. The specimen was cooled to liquid nitrogen temperature in a dewar equipped with CsBr windows. Transmittance from 1 to 15 p was measured on a Perkin Elmer Model 21 spectrophotometer with a NaCl prism and from 15 to 30 p on a Perkin Elmer Model 221 with a CsBr prism.
Cadmium telluride infrared transmitting
151
material
Transmittance at liquid nitrogen temperature is shown in Fig. 3. There was no measurable change in transmittance from 1 to 25 p. As expected, from 25 to 30 p there was an increase in transmittance at liquid nitrogen temperature due to shifting of the absorption edge to longer wavelengths with decreasing temperature.
100
s
90 80
70 $- 60 5 c
50 40
E 30 s g20
"""""""""""'1""'
IO 0
2
4
6
8
IO
14 12 Wavelength,
16 I8 microns
20
22
24
26
28
30
FIG. 3. Transmittance of hot-pressed CdTe at liquid nitrogen temperature (- 196°C) Specimen thickness = 0.125 in. CONCLUSIONS
CdTe was successfully developed as a long wavelength infrared transmitting material. With increasing interest at wavelengths beyond 14-15 p, it is expected that this material will find use in a variety of military and commercial applications. The transmittance of CdTe over the broad region from 1 to 30 P and its desirable strength, thermal and environmental properties indicate that it should be suitable for use under a wide range of environmental conditions. Hot-pressing was entirely feasible for forming windows up to 34 in. dia. Work to scale up to 6 in. dia. is currently being carried on. Hot-pressed
Acknowledgement-Most of the work was supported by Space Systems Division, Air Force Systems Command, Los Angeles, California 90045 under Contract AFO4(695)-546.
CADMIUM
TELLURIDE INFRARED TRANSMITTING MATERIAL LE ROY S. LADD Eastman Kodak Company,
Rochester,
(Received 22 February
New York
1966)
Abstract-Cadmium telluride was developed as an infrared transmitting material covering the broad spectral region from 1 to 30 p. High transmitting windows up to 3f in. dia. have been formed by the hot-pressing method. Property measurements of cadmium telluride are presented. A comparison is made of cadmium telluride and other well known and available materials that transmit from 1 to 30 p. Cadmium telluride appears to be the superior material, particularly for applications under severe environmental conditions. INTRODUCTION
years ago we conducted a preliminary investigation of cadmium telluride (CdTe) as an infrared transmitting material. Recently this work was extended to the development of CdTe for a special long wavelength application requiring a number of 2) in. dia. and + in. thick windows. Property requirements included transmittance from 1 to 30 p and a combination of strength, thermal and environmental properties not found in currently available materials that transmit over this broad spectral region. Our earlier investigation indicated that CdTe would meet the requirements of this application. The development work was greatly facilitated by our experience in developing the family of infrared transmitting materials that are marketed under the tradename Kodak Irtran Optical Materials. These materials are formed into dense, polycrystalline windows of high infrared transmittance by hot-pressing at temperatures well below the melting point of the specific material. This method was chosen for fabricating CdTe. Very recently, CdTe has been added to this group under the designation of Irtran 6. Until the introduction of this new material, Irtran 4, which is useful to about 20 p, transmitted to the longest wavelength. CdTe supplements the Irtran family by extending the transmittance range to 30 CL. SEVERAL
RAW
MATERIAL
AND
FABRICATION
of CdTe raw material obtained from eleven suppliers were tested and evaluated. Most of the samples were prepared by fusion reaction of high purity Cd and Te elements. One supplier provided raw material that was zone refined from fusion reaction material. Two samples were furnished that had been prepared by wet chemical methods. For test and evaluation, a portion of the sample was ground to a fine powder, then hotpressed to small O-8 in. dia. windows for infrared transmittance determinations. The specimens were also examined with an infrared viewer. Opaque inclusions, internal cracks, and general optical homogeneity could be observed with this instrument. Fusion reaction CdTe received from three suppliers was found satisfactory for infrared Samples
145
146
LEROY
S.LADD
optical use. The zone-refined material consistently produced specimens of high transmittance but the cost was prohibitive. CdTe prepared by wet chemical methods was not satisfactory. Concurrently with the raw material investigation, a study of optimum hot-pressing procedures and parameters was conducted on the small O-8in. dia. test specimens. Hot-pressing work was then extended to pressings which were 24 in. in dia. and up to 0.400 in. thick. With good starting material, 2t in. dia. pressings were soon produced with consistently high transmittance. Pressing size has recently been extended successfully to 32 in. dia. and work to scale up to 6 in. dia. is under investigation. CdTe can be worked in the optical shop with conventional techniques used on optical glass. Because the material is rather soft, some care must be used during final polish to remove scratches. TRANSMITTANCE
CHARACTERISTICS
Figure 1 shows a typical transmittance curve for a 2$ in. dia. and 8 in. thick CdTe window. In observing the transmittance level, it should be noted that the two-surface reflection loss for CdTe is approximately 35 % as calculated from refractive index data. Thus, the theoretical external transmittance maximum is approximately 65 ‘A. The Fig. 1 specimen is essenti-
100 90 $
80 70
0
2
4
6
8
IOWavele&, 12
16
18
20
22
24
26
28
30
microns FIG. 1. State-of-the-art
transmittance
of hot-pressed
cadmium telluride. Specimen thickness = 0.125 in.
ally at the theoretical transmittance limit from 2 to 28 p and is quite free from absorption bands in this region. The drop in transmittance from 2 to 1 p is attributed mainly to the influence of the short wavelength transmittance cut-off at 0.85 p due to electronic absorption. Small amounts of porosity and other inclusions may contribute in a very small degree to reduced transmittance by scattering in this region. The drop in transmittance from 28 to 30 TVis an absorption edge due to lattice absorption associated with multiphonon processes. PROPERTIES
OF HOT-PRESSED
CADMIUM
TELLURIDE
A compilation of property measurements made during the project are presented in Table 1. Some characteristics which are marked “preliminary data” will be remeasured or extended to a greater temperature range. Modulus of rupture and modulus of elasticity were determined by four-point bending. Nine specimens were measured at each of the three test temperatures. Measurements were also attempted at 200°C but plastic flow was encountered at this temperature.
Cadmium telluride infrared transmitting
141
material
TABLE 1. PROPERTYMEASUREMENTS OF HOT-PRESSED CADMIUMTELLURIDE A. Modulus of rupture and modulus of elasticity (preliminary data)
Temperature
Modulus of rupture (psi)
Modulus of elasticity (psi X 106)
4,590 4,540 5,880
5.2 5.3 45
Liquid nitrogen (- 196°C) Room temperature (25°C) 100°C -
B. Thermal conductivity (preliminary data)
Temperature -
(“C)
Thermal conductivity (Cal. sec-1cn-20C-1cm)
40 20 0 20 40 60 80 100 120 140
0.013 0012 0.010 OGO98 0+)095 O+TO92 0.0090 OGO88 0.0086 0,0085
C. Thermal expansion (preliminary data)
Range of temperature
(“C)
Coefficient of linear thermal expansion (“c-r x 10-G)
25-100 25-200 25-250
5.5 59 6.2
D. Thermal shock properties sample size: (2.020 in. dia. x 0.145 in. thick) Test conditions 1. Quenched from room temperature (25°C) to liquid nitrogen temperature (- 196%) 2. Quenched from 170°C to water at room temperature (25°C) 3. Quenched from 180°C to water at room temperature (25°C) E. Environmental Test conditions
-___-
1. No cracks or other effects 2. No cracks or other effects 3. Axial cracks
properties Results
Exposure to 96% relative humidity at 90°F temperature Exposure to air at elevated temperature Exposure to vacuum of approximately elevated temperatures.
Results
100~ at
1. No change in transmittance after 11 days exposure 2. No change in transmittanceafter 16 hr at3OO”C. After 16 hr at 350°C, a light gray oxidation coating developed on the specimen surfaces. 3. No change in transmittance after 7 hr at 300%. At 4OO”C, thermal etching was observed after approximately 5 min.
148 TABLE 1 -
LEROY S. LADD
continued
F. Refractive index Wavelength (p)
Refractive index
1.0 1.5 2.0 2.5 3.0 3.5 4.0 5.0 6.0 7.0 8.0 9.0 10.0
2.839 2.142 2.713 2.702 2.695 2.691 2,688 2.684 2.681 2.679 2,677 2,674 2,672 G. Density
.~____.__~
Density = 5.8511 g/cm3 _ H. Hardness Hardness--45
Knoop
The thermal conductivity measurements were made early in the project on a specimen that had somewhat lower transmittance than was attained in later specimens. There may have been some porosity and impurities in the measured specimen that contributed to lower thermal conductivity than may be expected for later, high transmitting material. Thermal expansion was measured with a Leitz dilatometer. The thermal expansion of CdTe matches the thermal expansion of commonly used mounting or retaining assembly materials much more closely than other infrared materials that transmit from 1 to 30 p. Forming hermetic seals to the mount or retainer is facilitated by closely matching thermal expansions. The thermal shock and environmental property measurements demonstrate that CdTe may be used under severe operating conditions. Refractive index was measured by well known refractive techniques. A prism of hotpressed CdTe was prepared and the angle between prism faces was accurately measured. Radiation of desired wavelength from a Perkin Elmer monochromator was passed through the prism which was mounted on a Gaertner spectrometer table. The refracted beam was focused on a thermocouple detector until a maximum signal was obtained. The angle of minimum deviation was thus determined. Knowing the angle of minimum deviation and the angle of the prism, refractive index could be calculated. It was not possible to measure beyond 10 p with available equipment. Density was determined by Archimedes method on specimens of hot-pressed CdTe with
480
2,390
-
KRr* KI* CsBr*
cs1+
0.8
3.9 4.6 2.3
2.3
-5.3
Modulus of elasticity (psi x 106)
50
43 43 48
58
-5.5
Thermal expansion (“C-l x 10ee)
621
730 723 636
415
_1,090
Melting point (“C)
44
53 128 124
0.05
Insoluble
Water solubility (g/100 gs HaO)
-
I 20
40
_45
(Knoop)
Hardness
4.5
28 3.1 4.4
14
8 ii 12
28
35
5-85 7.4
(% at 1014
WCC)
Density
Reflection loss
Water soluble, soft
High reflection loss, relatively soft Flows under pressure, soft Water soluble, soft Water soluble, soft Water soluble, soft
Shortcomings
I~trumentat~n January 1959.
* Data from the University of Michigan, Willow Run Laboratories, Report No. 2389-11-S State of-the-Art-ReportOptical Materials for Infrared
18,100
4,540
KRS-5*
CdTe
Material
Modulus of rupture (Psi)
TABLE 2. COMPARISONOF PROPERTIESOF INFRAREDTRA~SMI~IN~ MATERIALSFOR THE l-30p REGION
i? S z!. c
P’
1
ii: c s. 8 a ti % Ei
p
150
LEROY S. LADD
good transmittance and weighing from 47 to 70 g. The average density for four specimens was 58511 g/c3 with a range from 5.8509 to 58514 g/cc. In Table 2, a comparison is made of the properties of CdTe and other well known and available infrared materials that transmit usefully from 1 to 30 p. The more favorable characteristics of each material are underlined in the table. For use under severe operating conditions, CdTe appears to be the superior material. The four alkali halides are limited by water solubility to applications in carefully controlled atmospheres. KRS-5 is limited in its applications by tendency to flow under pressure. It would not be suitable for instrumentation where one side of the window is subjected to high vacuum. All of the materials listed in Table 2 are soft and easily scratched but the hardness data indicate that CdTe is slightly harder than the others. The most serious shortcoming of CdTe is its high reflection loss. However, low reflection coatings can be applied to reduce surface reflection losses to a few percent. Work is continuing in this area, and durable and efficient coatings should result. Transmittance of hot-pressed CdTe as a function of elevated temperature was measured. The specimen was heated in a cylindrical, resistance wound furnace mounted in a Perkin Elmer Model 13 spectrophotometer. This instrument chops the source energy before impinging on the specimen. Emitted radiation from the specimen does not influence the measurements since the detection system is sensitive only to chopped energy. The specimen temperature was measured with a thermocouple contacted against the edge of the sample. Measurements were made from 1 to 15 p using a NaCl prism and from 15 to 30 p with a CsBr prism. Elevated temperature transmittance at 100” and 200°C from 1 to 15 TVand at 100°C 200°C and 300°C from 15 to 30 p is shown in Fig. 2. The 2 % drop in transmittance from
- 60 P
22
FIG. 2. Transmittance
of hot-pressed
CdTe at elevated temperatures.
24
26
28
Specimen thickness
30
= 0.125 in
1 to 15 p at 100°C and 200°C may be measurement error. The only significant change in transmittance occurred in the 23-30 p region where there was a progressive drop in transmittance with increasing temperature. The loss in transmittance is attributed to shift of the lattice absorption edge to shorter wavelengths with increasing temperature. The transmittance loss was transient; transmittance was at the original level when measured at room temperature after completion of the elevated temperature measurements. Transmittance at liquid nitrogen temperature was also measured. The specimen was cooled to liquid nitrogen temperature in a dewar equipped with CsBr windows. Transmittance from 1 to 15 p was measured on a Perkin Elmer Model 21 spectrophotometer with a NaCl prism and from 15 to 30 p on a Perkin Elmer Model 221 with a CsBr prism.
Cadmium telluride infrared transmitting
151
material
Transmittance at liquid nitrogen temperature is shown in Fig. 3. There was no measurable change in transmittance from 1 to 25 p. As expected, from 25 to 30 p there was an increase in transmittance at liquid nitrogen temperature due to shifting of the absorption edge to longer wavelengths with decreasing temperature.
100
s
90 80
70 $- 60 5 c
50 40
E 30 s g20
"""""""""""'1""'
IO 0
2
4
6
8
IO
14 12 Wavelength,
16 I8 microns
20
22
24
26
28
30
FIG. 3. Transmittance of hot-pressed CdTe at liquid nitrogen temperature (- 196°C) Specimen thickness = 0.125 in. CONCLUSIONS
CdTe was successfully developed as a long wavelength infrared transmitting material. With increasing interest at wavelengths beyond 14-15 p, it is expected that this material will find use in a variety of military and commercial applications. The transmittance of CdTe over the broad region from 1 to 30 P and its desirable strength, thermal and environmental properties indicate that it should be suitable for use under a wide range of environmental conditions. Hot-pressing was entirely feasible for forming windows up to 34 in. dia. Work to scale up to 6 in. dia. is currently being carried on. Hot-pressed
Acknowledgement-Most of the work was supported by Space Systems Division, Air Force Systems Command, Los Angeles, California 90045 under Contract AFO4(695)-546.