- Email: [email protected]
ZnSe optical materials by implantation of 1 MeV hydrogen ions
1232
Nuclear
Instruments
and Methods
in Physics
Research
B59/60
(1991) 1232-1235 North-Holland
Surface hardening of &Se, ZnS, and ZnS/ZnSe implantation of 1 MeV hydrogen ions Richard
L.C. Wu. A.W. McCormick
Universal Energr Systems.
Joseph Keeley MorronInternational/CL’D
optical
materials
by
and P.P. Pronko
Dayton. OH. USA
Inc.. Wohurn. MA. USA
Zinc selenide and zinc sulfide are excellent optical materials for applications in the visible and infrared regions of the spectrum. Since these materials possess low mechanical strength and toughness, they are vulnerable to erosion and impact damage in severe environments. In order to improve their physical hardness without degrading their spectral transmission, high-energy (1 MeV) hydrogen ions were implanted into the surfaces of ZnSe. water-clear ZnS (CleartranTM), and ZnS/ZnSe composite (TuftranTM). A systematic study of the effect of ion fluence (0.1 to 3XlO”/cm’) on surface hardness and optical transmission was performed. Substantial improvement in microhardness has been found which increases with dosage. The hardness of TuftranTM leveled off at a dose of 3 x lO”/cm’. while that of ZnSe and CleartranTM continued to improve with ion fluence. The hardness of ZnSe was increased by a factor of 78% at a dose of 3 x lO”/cm”. without degrading important optical properties.
1. Introduction Zinc selenide (ZnSe) and zinc sulfide (ZnS) are excellent optical materials for applications throughout the infrared and visible regions of the spectrum. Since these materials possess low mechanical strength (the flexural strength of ZnSe and ZnS is 55 and 203 MPa, respectively) and toughness (the hardness of ZnSe and ZnS is 120 and 240 kg/mm’, respectively), they are vulnerable to erosion and impact damage in severe environments. The improvement of the mechanical properties without degrading their spectral transmission is an important step for the future application of these optical window materials. A number of approaches have been investigated to improve the toughness of ZnS and ZnSe, such as composite whiskers [l]. coatings [2.3], transformation toughening [l], and ion implantation [1.4.5]. Recently, ion implantation has been widely recognized as a method for modification of surfaces in general. The implantation of light ions ( < Li) at energies between 100 and 150 keV has been demonstrated to improve the hardness of ZnS [4,5]. The Vicker’s hardness of ZnS was increased slightly from 210 to 229 kg/mm’ by implanting He+ at 100 keV and a dose of 2 X 10” ions/cm* [5]. At present, no study has shown the effect of implanted dose on the hardness and spectral transmission of water-clear ZnS. ZnSe. and composite ZnS/ZnSe at high-energy (MeV) by hydrogen (H ’ ) ion implantation. where electronic stopping is a dominant effect. The 0168-583X/91/$03.50
C’ 1991 - Elsevier Science Publishers
purpose of this study was to determine the extent to which the hardness of CVD ZnSe, CleartranTM (ZnS). and TuftranTM (ZnS/ZnSe) could be improved by implantation of H + ions at 1 MeV and up to a dose of 3 X 10” ions/cm* without degrading their spectral transmission.
2. Experimental CVD ZnSe. CleartranTM, and TuftranTM materials used in these experiments were manufactured by Morton International/Advanced Materials/CVD. Inc. CleartranTM IS a water-clear form of ZnS. It is made by subjecting CVD ZnS to high temperature and pressure. The post-deposition treatment of CVD ZnS improves the optical transmittance. However. it does lead to a mechanically weak and soft material compared to ZnS. TuftranTM is a ZnS/ZnSe composite material which combines the optical properties of ZnSe with the hardness and abrasion resistance of ZnS. Ion implantation was performed with 1 MeV H+ ions from a 1.7 MV General Ionex 4117A Tandetron available at Universal Energy Systems. Inc. (UES). Fluences ranged from 1 x lOI to 1 x 10” ions/cm’ and the ions were implanted normal to the surfaces of C1eartranTM, ZnSe, and TuftranrM. The penetration ranges for 1 MeV hydrogen ions in ZnSe. CleartranTM, and TuftranTM calculated using computer code TRIM-88
B.V. (North-Holland)
1233
R. L. C. Wu et al. / Surface modification of ZnSe and ZnS optical materials
Table 1 The Knoop hardness of ZnSe, CleartranTM and TuftranTM, measured before and after implantation of 1 MeV H+ ions
[6] are 10.7 + 0.6. 12.4 i 0.5, and 12.5 f 0.6 pm, respec-
tively. The Knoop hardness was measured by a Clark Model MHT-1 Hardness Tester. All hardness measurements were made using a 25 g load. A total of ten indentations were measured for each sample before and after the ion implantation. The infrared transmission was measured from 2.5 to 50 pm using a Perkin-Elmer Infrared Spectrometer (Model 1330R). This instrument is accurate to within + 5%. The IR transmission characteristics of each sample were measured before and after the implantation of Ht at each dose.
Material
Ion fluence [l x lOI ions/cm*]
Knoop hardness [KH,, kg/mm’]
Hardness improvement [%I
ZnSe
0 1 3 6 10 30
104+ 125f 135k 148& 159* 185f
7 7 6 9 7 7
0 20 30 42 53 78
CleartranTM
0 1 3 6 10
162+ 178f 200* 208+ 216+
7 5 4 7 7
0 10 23 28 33
TuftranTM
0 1 3 6 10
209fll 224+ 8 256 f 14 242+ 8 255 f 12
0 7 22 16 22
3. Results 3.1. Effects of fruence on the hardness of ZnSe, clear ZnS, and composite ZnS/ ZnSe The Knoop hardness of each substrate was found to increase as the fluence of 1 MeV H+ increased from 1 x lOI6 up to 1 x 10” ions/cm2. The hardness results are shown in table 1. The errors are the standard deviation derived from ten hardness measurements for each data point. The percentage of hardness improvement at each dose for each material is shown in fig. 1. The results demonstrate that the hardness of ZnSe
dosage up to 3 x 10” 7 kg/mm2 to 185 f 7 The hardness of waterup to 1 x 10” ions/cm’.
continuously increased with ions/cm2 going from 104 + kg/mm2, a 78% improvement. clear ZnS increased with dosage
,’
CLEARTRANT’
I
.
1
2
3
4 H+
5 ION
6
-
-
TUFTRANTM
/
I
78910
FLUENCE
I
2
(I
~10’~
3
‘oNS/cm2
4
5
II,
678:
)
Fig. 1. The Knoop hardness improvement of &Se. CleartranTM and TuftranTM as a function of 1 MeV H+ ion implantation. IX. INSULATORS/CERAMICS/POLYMERS
R. L. C. Wu et al. / Surface modification of ZnSe and ZnS optical materials
1234
Table 2 The transmission of ZnSe. CleartranTM implantation with 1 MeV H+ at different Sample
Transmission
ZnSe CleartranTM TuftranTM
and TuftranTM samples cumulative doses
1 x 10” ions/cm2
71.5 74.2 62.5
71.5 74.5 62.8
71.5 73.5 62.6
71.5 74.2 63.5
71.5 74.2 62.0
of ZnSe,
80 70
40
5 Y
30
g
20
+-
IO 0 400
r 800
and
6 x 10’6 ions/cm’
90
vl
implantation
3x10’6 ions/cm2
100
60
ion
1 x 10’6 ions/cm2
There was no visual damage on the surface of each material after the 1 MeV H+ ion implantation with various dosages up to 1 X 10” ions/cm2. The optical transmission of ZnSe, water-clear ZnS, and composite ZnS/ZnSe was measured from 2.5 to 50 pm. Within the experimental uncertainty. there was no change for each material before and after the high-energy H+ ion implantation. Table 2 shows the transmission at 10 pm for the samples at each dosage level. As the H+ ion fluence was increased to 3 x 10” ions/cm2 implanted on ZnSe, the color of the ZnSe surface appeared to darken. The optical transmission of ZnSe before and after the implantation is shown in fig. 2. There is a slight decrease in the transmission in the visible region, while there is
50
before
1200 WAVELENGTH
Fig. 2. The optical transmission MeV H+ ion implantation
after
ion
[%I
3.2. Effects of fluence on optical transmmion clear ZnS. and composite ZnS/ ZnSe
9
at 10 pm
Before implantation
from 162 i_ 7 kg/mm2 to 216 + 7 kg/mm2, an improvement of 33%. For ZnSe and water-clear ZnS, there was no sign of the hardness leveling off. The hardness of composite ZnS/ZnSe, which was implanted with H+ ions on the ZnS side, was observed to level off above 3 X lOI ions/cm*. The hardness of ZnS/ZnSe increased from 209 k 11 kg/mm2 to 255 + 12 kg/mm2, an improvement of 22%.
P 0
measured
1600
2Om
2400
(nm ) of ZnSe before and of 3 X 10” ions/cm2.
after
1
no change in the IR region spectrometer.
within
the accuracy
of the
4. Discussion In this study, the hardness of ZnSe, water-clear ZnS, and composite ZnS/ZnSe has been demonstrated to be increased as the 1 MeV H+ ion fluence increased. The use of the light ion H+ is very attractive because of the large penetration depths (> 10 km) that can be obtained at MeV energies being comparable to the IR wavelength region of interest. These energies provide modification of a much thicker surface layer than is otherwise possible with heavy ion implantation. Also. light ions produce virtually no sputtering, thus maintaining the surface polish. The apparent surface hardening in this study may be due to ionization radiation effects, since most of the energy dissipation for hydrogen ions at high energies will be in electronic excitation. However, a smaller, but significant number of cascade displacement of lattice atoms through knock-on and secondary collision will also occur. In addition, atomic displacements through electronic excitation are known to occur in ionic solids that have structures similar to ZnS and ZnSe. The mechanisms for hardening of these materials by high-energy proton bombardment (H+) is not completely clear at this time. There is no likely precipitate hardening effect in this instance, and therefore, it can be ruled out. However, it is probable that at high doses, sufficient point defects such as vacancies and interstitials form that collapse into line defects such as dislocation loops, networks, and clusters or small voids. These defects can interact with each other impeding their motion through the crystal lattice, thus producing a hardening effect that is similar to work hardening. Whatever the effect, it is clear that hardening by MeV light ion implantation has the advantage of producing thick hardened layers on the surface, in contrast to a thinner layer by conventional heavy ion implantation at keV energies. The optical transmission of ZnSe. CleartranTM. and TuftranTM do not change after H+ ion implantation of 1 X lOI ions/cm’ over the entire spectral region of 2.5
R.L. C. Wu et al. / Surface modification of ZnSe and ZnS optical materials to 50 pm. It is demonstrated that there is no optical damage in the crystal structure during high-energy proton ion implantation below the dose of 1 X 10” ions/cm*. However, as the dose increased to 3 X 10” ions/cm*, the color of ZnSe darkened, and the optical transmission was decreased in the visible region as shown in fig. 2. It was demonstrated that in this high dose implantation regime, damage to the ZnSe structure began, but the level of damage did not affect its optical transmission in the infrared region.
5. Conclusion It is demonstrated that high-energy (> 1 MeV) proton ion implantation substantially improved the hardness of ZnSe and CleartranTM surfaces without degrading their optical transmission in the infrared region. It is an attractive method for hardening the surfaces of these IR window materials for use in the erosion and impact damage environments.
1235
References
VI Fifth Annual
Program Review and Workshop Materials for Long Wave Infrared Windows and Domes, Naval Weapons Center. China Lake, CA, USA (1988). PI J.T. Keeley and R.L.C. Wu, Proc. 1st Int. Symp. on Diamond and Diamond-like Films, vol. 89-12 (Electrochemical Society, 1989) p. 250. [31 A.H. Lettington, J.C. Lewis, C.J.H. Wort, B.C. Monachan and A.J.N. Hope. Proc. Europ. MRS Meeting, vol. 17 (1987) p. 469. [41 Erosion Resistant FLIR Windows: Colorless ZnS, CVD Final Report, AFWAL Contract F33615-81-C-5076 (April 1984). [51 M.J. Mirtich, D. Nir, D. Swec and B. Banks, J. Vat. Sci. Technol. A4 (1986) 2680. Kl J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids (Pergamon, 1980).
IX. INSULATORS/CERAMICS/POLYMERS
Nuclear
Instruments
and Methods
in Physics
Research
B59/60
(1991) 1232-1235 North-Holland
Surface hardening of &Se, ZnS, and ZnS/ZnSe implantation of 1 MeV hydrogen ions Richard
L.C. Wu. A.W. McCormick
Universal Energr Systems.
Joseph Keeley MorronInternational/CL’D
optical
materials
by
and P.P. Pronko
Dayton. OH. USA
Inc.. Wohurn. MA. USA
Zinc selenide and zinc sulfide are excellent optical materials for applications in the visible and infrared regions of the spectrum. Since these materials possess low mechanical strength and toughness, they are vulnerable to erosion and impact damage in severe environments. In order to improve their physical hardness without degrading their spectral transmission, high-energy (1 MeV) hydrogen ions were implanted into the surfaces of ZnSe. water-clear ZnS (CleartranTM), and ZnS/ZnSe composite (TuftranTM). A systematic study of the effect of ion fluence (0.1 to 3XlO”/cm’) on surface hardness and optical transmission was performed. Substantial improvement in microhardness has been found which increases with dosage. The hardness of TuftranTM leveled off at a dose of 3 x lO”/cm’. while that of ZnSe and CleartranTM continued to improve with ion fluence. The hardness of ZnSe was increased by a factor of 78% at a dose of 3 x lO”/cm”. without degrading important optical properties.
1. Introduction Zinc selenide (ZnSe) and zinc sulfide (ZnS) are excellent optical materials for applications throughout the infrared and visible regions of the spectrum. Since these materials possess low mechanical strength (the flexural strength of ZnSe and ZnS is 55 and 203 MPa, respectively) and toughness (the hardness of ZnSe and ZnS is 120 and 240 kg/mm’, respectively), they are vulnerable to erosion and impact damage in severe environments. The improvement of the mechanical properties without degrading their spectral transmission is an important step for the future application of these optical window materials. A number of approaches have been investigated to improve the toughness of ZnS and ZnSe, such as composite whiskers [l]. coatings [2.3], transformation toughening [l], and ion implantation [1.4.5]. Recently, ion implantation has been widely recognized as a method for modification of surfaces in general. The implantation of light ions ( < Li) at energies between 100 and 150 keV has been demonstrated to improve the hardness of ZnS [4,5]. The Vicker’s hardness of ZnS was increased slightly from 210 to 229 kg/mm’ by implanting He+ at 100 keV and a dose of 2 X 10” ions/cm* [5]. At present, no study has shown the effect of implanted dose on the hardness and spectral transmission of water-clear ZnS. ZnSe. and composite ZnS/ZnSe at high-energy (MeV) by hydrogen (H ’ ) ion implantation. where electronic stopping is a dominant effect. The 0168-583X/91/$03.50
C’ 1991 - Elsevier Science Publishers
purpose of this study was to determine the extent to which the hardness of CVD ZnSe, CleartranTM (ZnS). and TuftranTM (ZnS/ZnSe) could be improved by implantation of H + ions at 1 MeV and up to a dose of 3 X 10” ions/cm* without degrading their spectral transmission.
2. Experimental CVD ZnSe. CleartranTM, and TuftranTM materials used in these experiments were manufactured by Morton International/Advanced Materials/CVD. Inc. CleartranTM IS a water-clear form of ZnS. It is made by subjecting CVD ZnS to high temperature and pressure. The post-deposition treatment of CVD ZnS improves the optical transmittance. However. it does lead to a mechanically weak and soft material compared to ZnS. TuftranTM is a ZnS/ZnSe composite material which combines the optical properties of ZnSe with the hardness and abrasion resistance of ZnS. Ion implantation was performed with 1 MeV H+ ions from a 1.7 MV General Ionex 4117A Tandetron available at Universal Energy Systems. Inc. (UES). Fluences ranged from 1 x lOI to 1 x 10” ions/cm’ and the ions were implanted normal to the surfaces of C1eartranTM, ZnSe, and TuftranrM. The penetration ranges for 1 MeV hydrogen ions in ZnSe. CleartranTM, and TuftranTM calculated using computer code TRIM-88
B.V. (North-Holland)
1233
R. L. C. Wu et al. / Surface modification of ZnSe and ZnS optical materials
Table 1 The Knoop hardness of ZnSe, CleartranTM and TuftranTM, measured before and after implantation of 1 MeV H+ ions
[6] are 10.7 + 0.6. 12.4 i 0.5, and 12.5 f 0.6 pm, respec-
tively. The Knoop hardness was measured by a Clark Model MHT-1 Hardness Tester. All hardness measurements were made using a 25 g load. A total of ten indentations were measured for each sample before and after the ion implantation. The infrared transmission was measured from 2.5 to 50 pm using a Perkin-Elmer Infrared Spectrometer (Model 1330R). This instrument is accurate to within + 5%. The IR transmission characteristics of each sample were measured before and after the implantation of Ht at each dose.
Material
Ion fluence [l x lOI ions/cm*]
Knoop hardness [KH,, kg/mm’]
Hardness improvement [%I
ZnSe
0 1 3 6 10 30
104+ 125f 135k 148& 159* 185f
7 7 6 9 7 7
0 20 30 42 53 78
CleartranTM
0 1 3 6 10
162+ 178f 200* 208+ 216+
7 5 4 7 7
0 10 23 28 33
TuftranTM
0 1 3 6 10
209fll 224+ 8 256 f 14 242+ 8 255 f 12
0 7 22 16 22
3. Results 3.1. Effects of fruence on the hardness of ZnSe, clear ZnS, and composite ZnS/ ZnSe The Knoop hardness of each substrate was found to increase as the fluence of 1 MeV H+ increased from 1 x lOI6 up to 1 x 10” ions/cm2. The hardness results are shown in table 1. The errors are the standard deviation derived from ten hardness measurements for each data point. The percentage of hardness improvement at each dose for each material is shown in fig. 1. The results demonstrate that the hardness of ZnSe
dosage up to 3 x 10” 7 kg/mm2 to 185 f 7 The hardness of waterup to 1 x 10” ions/cm’.
continuously increased with ions/cm2 going from 104 + kg/mm2, a 78% improvement. clear ZnS increased with dosage
,’
CLEARTRANT’
I
.
1
2
3
4 H+
5 ION
6
-
-
TUFTRANTM
/
I
78910
FLUENCE
I
2
(I
~10’~
3
‘oNS/cm2
4
5
II,
678:
)
Fig. 1. The Knoop hardness improvement of &Se. CleartranTM and TuftranTM as a function of 1 MeV H+ ion implantation. IX. INSULATORS/CERAMICS/POLYMERS
R. L. C. Wu et al. / Surface modification of ZnSe and ZnS optical materials
1234
Table 2 The transmission of ZnSe. CleartranTM implantation with 1 MeV H+ at different Sample
Transmission
ZnSe CleartranTM TuftranTM
and TuftranTM samples cumulative doses
1 x 10” ions/cm2
71.5 74.2 62.5
71.5 74.5 62.8
71.5 73.5 62.6
71.5 74.2 63.5
71.5 74.2 62.0
of ZnSe,
80 70
40
5 Y
30
g
20
+-
IO 0 400
r 800
and
6 x 10’6 ions/cm’
90
vl
implantation
3x10’6 ions/cm2
100
60
ion
1 x 10’6 ions/cm2
There was no visual damage on the surface of each material after the 1 MeV H+ ion implantation with various dosages up to 1 X 10” ions/cm2. The optical transmission of ZnSe, water-clear ZnS, and composite ZnS/ZnSe was measured from 2.5 to 50 pm. Within the experimental uncertainty. there was no change for each material before and after the high-energy H+ ion implantation. Table 2 shows the transmission at 10 pm for the samples at each dosage level. As the H+ ion fluence was increased to 3 x 10” ions/cm2 implanted on ZnSe, the color of the ZnSe surface appeared to darken. The optical transmission of ZnSe before and after the implantation is shown in fig. 2. There is a slight decrease in the transmission in the visible region, while there is
50
before
1200 WAVELENGTH
Fig. 2. The optical transmission MeV H+ ion implantation
after
ion
[%I
3.2. Effects of fluence on optical transmmion clear ZnS. and composite ZnS/ ZnSe
9
at 10 pm
Before implantation
from 162 i_ 7 kg/mm2 to 216 + 7 kg/mm2, an improvement of 33%. For ZnSe and water-clear ZnS, there was no sign of the hardness leveling off. The hardness of composite ZnS/ZnSe, which was implanted with H+ ions on the ZnS side, was observed to level off above 3 X lOI ions/cm*. The hardness of ZnS/ZnSe increased from 209 k 11 kg/mm2 to 255 + 12 kg/mm2, an improvement of 22%.
P 0
measured
1600
2Om
2400
(nm ) of ZnSe before and of 3 X 10” ions/cm2.
after
1
no change in the IR region spectrometer.
within
the accuracy
of the
4. Discussion In this study, the hardness of ZnSe, water-clear ZnS, and composite ZnS/ZnSe has been demonstrated to be increased as the 1 MeV H+ ion fluence increased. The use of the light ion H+ is very attractive because of the large penetration depths (> 10 km) that can be obtained at MeV energies being comparable to the IR wavelength region of interest. These energies provide modification of a much thicker surface layer than is otherwise possible with heavy ion implantation. Also. light ions produce virtually no sputtering, thus maintaining the surface polish. The apparent surface hardening in this study may be due to ionization radiation effects, since most of the energy dissipation for hydrogen ions at high energies will be in electronic excitation. However, a smaller, but significant number of cascade displacement of lattice atoms through knock-on and secondary collision will also occur. In addition, atomic displacements through electronic excitation are known to occur in ionic solids that have structures similar to ZnS and ZnSe. The mechanisms for hardening of these materials by high-energy proton bombardment (H+) is not completely clear at this time. There is no likely precipitate hardening effect in this instance, and therefore, it can be ruled out. However, it is probable that at high doses, sufficient point defects such as vacancies and interstitials form that collapse into line defects such as dislocation loops, networks, and clusters or small voids. These defects can interact with each other impeding their motion through the crystal lattice, thus producing a hardening effect that is similar to work hardening. Whatever the effect, it is clear that hardening by MeV light ion implantation has the advantage of producing thick hardened layers on the surface, in contrast to a thinner layer by conventional heavy ion implantation at keV energies. The optical transmission of ZnSe. CleartranTM. and TuftranTM do not change after H+ ion implantation of 1 X lOI ions/cm’ over the entire spectral region of 2.5
R.L. C. Wu et al. / Surface modification of ZnSe and ZnS optical materials to 50 pm. It is demonstrated that there is no optical damage in the crystal structure during high-energy proton ion implantation below the dose of 1 X 10” ions/cm*. However, as the dose increased to 3 X 10” ions/cm*, the color of ZnSe darkened, and the optical transmission was decreased in the visible region as shown in fig. 2. It was demonstrated that in this high dose implantation regime, damage to the ZnSe structure began, but the level of damage did not affect its optical transmission in the infrared region.
5. Conclusion It is demonstrated that high-energy (> 1 MeV) proton ion implantation substantially improved the hardness of ZnSe and CleartranTM surfaces without degrading their optical transmission in the infrared region. It is an attractive method for hardening the surfaces of these IR window materials for use in the erosion and impact damage environments.
1235
References
VI Fifth Annual
Program Review and Workshop Materials for Long Wave Infrared Windows and Domes, Naval Weapons Center. China Lake, CA, USA (1988). PI J.T. Keeley and R.L.C. Wu, Proc. 1st Int. Symp. on Diamond and Diamond-like Films, vol. 89-12 (Electrochemical Society, 1989) p. 250. [31 A.H. Lettington, J.C. Lewis, C.J.H. Wort, B.C. Monachan and A.J.N. Hope. Proc. Europ. MRS Meeting, vol. 17 (1987) p. 469. [41 Erosion Resistant FLIR Windows: Colorless ZnS, CVD Final Report, AFWAL Contract F33615-81-C-5076 (April 1984). [51 M.J. Mirtich, D. Nir, D. Swec and B. Banks, J. Vat. Sci. Technol. A4 (1986) 2680. Kl J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids (Pergamon, 1980).
IX. INSULATORS/CERAMICS/POLYMERS