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Optical anisotropy in pulsed laser deposited a-GeSe2 thin films
Journal of Non-Crystalline Solids 246 (1999) 150±154
Letter to the Editor
Optical anisotropy in pulsed laser deposited a-GeSe2 thin ®lms D.I. Florescu *, R.L. Cappelletti Department of Physics and Astronomy, Condensed Matter and Surface Sciences Program, Ohio University, Athens, OH 45701, USA Received 14 December 1998
Abstract Optical anisotropy was observed by dierential polarized optical transmission measurements using polarized 633 nm (sub-bandgap), low incident power (0.9 mW) light on a-GeSe2 thin (<0.4 lm) ®lm samples deposited by laser ablation on silica. Both normal and oblique depositions were studied and no signi®cant dierence was found between them. Neglecting the dierential absorption at this wavelength, optical anisotropy can arise from birefringence. Assuming a simple uniaxial birefringent model, anisotropy axes with a direction change in the 2±15° range were found for each investigated ®lm. Based on this model and considering the multiple re¯ections in both ®lm and substrate, the dierence in refractive index for ordinary and extraordinary rays was evaluated to be in the 0.005±0.015 range. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Transient [1] and metastable photoinduced phenomena (see Ref. [1]) including photodarkening [2], photostructural changes [3] and recently reported a thermal photobleaching [4] have been reported in glassy amorphous semiconductors. One category of photoinduced eects in chalcogenide network glasses are vectoral eects (see Ref. [1]). In order for these vectoral eects to be studied several workers have used an intense laser beam to induce anisotropy, and a weak polarized beam to probe the induced anisotropy [5]. In this study we describe measurements on a set of well-characterized a-GeSe2 samples obtained by pulsed laser deposition (PLD), probing the samples for non-induced or `native' optical anisotropy (OA). Using a low intensity (0.9 mW) probing
*
Corresponding author. Tel.: 1-740 594 4308; e-mail: ¯[email protected]
laser beam and applying the dierential polarized optical transmission technique we found that all our samples exhibit a small but de®nite non-induced OA. 2. Experimental details Bulk GeSe2 glasses used for PLD targets were prepared by the melt-quenching technique as described in previous work [6]. Thin ®lms of GeSe2 were deposited in a dynamic vacuum in the 5± 8 ´ 10ÿ5 Torr range, on chemically cleaned microscope slides (Corning 2948) cut 15 mm wide and 25 mm long. The thickness of these slides was measured to be between 0.976 and 0.983 mm and the index of refraction is n2 1.515 [7]. Several sample ®lms with thicknesses varying from 0.04 to 0.4 lm were produced. Most ®lms were grown at normal incidence, but some were deposited at incident angles 6 45° to promote columnar growth [8]. Columnar growth samples could be expected to show OA.
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 0 8 2 - 4
D.I. Florescu, R.L. Cappelletti / Journal of Non-Crystalline Solids 246 (1999) 150±154
An excimer laser using a krypton-¯uoride mixture was used for PLD. This device, lasing at 248 nm delivers energies upto 200 mJ per pulse, the pulse duration being approximately 10 ns. Deposition was accomplished in 20 min using a repetition rate of 10 Hz. The composition and homogeneity (5%) of the samples were determined by energy dispersive Xray emission using pure element standards for comparison. Film thicknesses were measured by a pro®lometer. For the dierential polarized optical transmission measurements, a linearly (500:1) polarized, 2 mW, He±Ne laser was used. The beam was chopped at a nominal frequency f 50 Hz with an optical chopper. A special wheel was designed with two diametrically opposed small openings into which were ®tted mutually perpendicular polarizing ®lters such that alternating pulses of polarized light were obtained. The sample holder was a rotational-translational stage, which allowed investigation of different spots on the sample without a change in the position of the other optical components. Considering an xyz reference system, the sample was placed in the yz plane and the incident light was parallel to the x direction, normal to the sample, as illustrated in Fig. 1. Using this holder, the sample could be translated along the y direction and also rotated around the x direction varying angle a. The ®lms were exposed in air during illumination at room temperature. The signal was detected with a Si high-speed detector. Two lock-in ampli®ers were used to measure the signal. One was tuned to detect a signal at twice the chopper rotation frequency 2f, and the other one at f. The resulting outputs are proportional respectively to the sum and dierence of the transmitted intensities. The dierence zero signal was set by rotating the laser in the absence of a sample. 3. Results To illustrate the results, three samples with dierent thicknesses are reported here. Two were 0.2 and 0.4 lm respectively, and were deposited at normal incidence. The third one was grown at an
151
Fig. 1. Geometry of the light-sample interaction. 0x is the incident laser radiation direction, where the two mutually orthogonal chopped states of polarization are also represented as a and b. zy is the plane in which the sample lies and a is the angle of rotation of the sample mount in this plane.
angle of incidence of approximately 45°, such that its thickness varied from 0.04 to 0.1 lm from side to side. In Fig. 2, for each sample we show the dierence in the transmitted intensities versus the sample orientation angle a. Two spots on each sample were investigated. The samples were ®rstly rotated in the yz plane varying the angle a until the difference signal became zero at a0 . At this point we can assume that the two transmitted orthogonal polarization intensities are equal. The sample was then rotated to a0 ÿ 45° and the signal was recorded, and then to a0 + 45° and the signal was again recorded. After that, by translation parallel to y, another spot was selected and the procedure repeated. We found the `zero direction' to vary from 2 to 15° for the two investigated spots on each of the samples. Fig. 2 also indicates that the magnitude of the detected dierence signal diers from spot to spot on the same sample. Also there is no signi®cantly dierent behavior of the obliquely deposited sample compared to the normally deposited ones.
152
D.I. Florescu, R.L. Cappelletti / Journal of Non-Crystalline Solids 246 (1999) 150±154
Fig. 2. Dierential polarized optical transmission versus the orientation angle a for two random spots on each sample, after the angle a was set for a null reading at a0 . The sample was then rotated to a0 ÿ 45° and signal was recorded, then to a0 + 45° recording the signal again. Samples are identi®ed by their thicknesses. Incident laser power was 0.9 mW, and the laser spot diameter was 1 mm. The variable thickness sample was obliquely deposited at 45° incidence and shows OA comparable to the normally deposited ®lms. Lines are drawn as guides for the eyes.
4. Discussion The results in Fig. 2 show that our samples exhibit OA. Considering the low value of the absorption coecient (950 cmÿ1 ) [9] of GeSe2 thin ®lms at 633 nm, the dierential absorption is small. We chose to neglect it and analyze the difference signal in terms of optical birefringence. Consider a simple model in which the birefringence is locally uniaxial and the optical axis has a component parallel to the faces of the ®lm, in the yz plane in Fig. 1. Based on this assumption the results in Fig. 2 are easily understood considering three possible orientations of the optical axis with respect to the two orthogonal states of polarization. For a zero signal, the axis is at 45° to both incident orthogonal polarization directions. For maximum positive or negative signal, the axis is parallel to either one or the other of the mutually orthogonal states of polarization. The transmittance for the ordinary and extraordinary rays through a thin, non-absorbing, uniaxially anisotropic ®lm on a transparent, iso-
tropic glass substrate, taking into account multiple re¯ections in both ®lm and substrate is given by Eq. (A.1) in Appendix A. Assuming an optically uniaxial birefringent case, the dierence in refractive index for the ordinary and extraordinary rays is evaluated from dT ; 1 Dn DT dn n2:43 DT is the measured dierence in the two orthogonal transmitted transmittances, Dn is the dierence in refractive index for ordinary and extraordinary rays and (dT/dn)n2:43 is the derivative of the transmittance function with respect to the average refractive index, which for laser-ablated a-GeSe2 thin ®lms is n 2.43 [10]. (dT/dn)n2:43 was evaluated from Eq. (1). Using the data shown in Fig. 2 we evaluated the maximum dierence in refractive index for ordinary and extraordinary rays for each sample. Results are shown below in Table 1. The observed birefringence for normally deposited ®lms is unexpected. We conjecture that residual
D.I. Florescu, R.L. Cappelletti / Journal of Non-Crystalline Solids 246 (1999) 150±154
153
Table 1 Sample thickness (lm)
Dierence in refractive indices, Dn
0.4 0.2 0.04±0.1
0.006 0.001 0.013 0.003 0.007 0.002
stress arising during PLD may be responsible. On the other hand, it was expected that oblique deposition would promote columnar ®lm growth with attendant OA. Evidently either PLD does not promote columnar growth or the associated optical anisotropy is only comparable to the `native' OA. 5. Conclusion In this work we have shown that PLD thin (<0.4 lm) glassy GeSe2 ®lms exhibit non-induced optical anisotropy. Ignoring dierential absorption, optical anisotropy can arise from birefringence. Considering a simple uniaxial model a preferential anisotropy axis with the direction varying from 2 to 15° range was found for each ®lm. No signi®cant OA dierence was found between normal and oblique deposited PLD ®lms.
Fig. 3. Symbolic representation of the air-®lm-substrate-air arrangement used in order to evaluate the transmittance formula in Appendix A. n0 and n2 are the refractive indices for air and glass, respectively, and n is the ®lm average refractive index. h is the angle that the anisotropy axis is making with a direction parallel to the faces of the ®lm; d1 is the ®lm thickness and d2 is the glass substrate thickness.
A transfer matrix approach gives: n Te;0 4 4 cos2 g cos2 g2 :
Acknowledgements
This work was supported by NSF Grant DMR9604921. We thank Professor Pedro Pereyra for useful discussions and suggestions in evaluating the transmittance derivative and Professors Larry Wilen and Martin Kordesch for helpful experimental suggestions. We are especially indebted to Professor J.A. Butcher Jr. for the use of his excimer laser and to Birgit Eey for help in bulk sample preparation.
g1 neff k0 d1 ;
g 2 n2 k 0 d 2 :
2
sin2 g sin2 g2 cos2 g sin2 g2
2 n0 n sin2 g cos2 g2 n n0 n0 n2 n0 n n2 n ÿ4 2 n2 n0 n n0 n n2 sin g cos g sin g2 cos g2
Consider the arrangement shown in Fig. 3. ns and nf are the slow and fast indices for the ®lm. Then we can de®ne the following parameters. 2p ; k0
n0 n2 n2 n0
2
Appendix A
k0
n2 n n n2
g01 ns k0 d1 ;
oÿ1
A:1
with g g1 ; n neff for the extraordinary ray, and g g01 ; n ns for the ordinary ray. Also neff is given by neff
cos2 h sin2 h 2 ns n2f
ÿ1=2 :
154
D.I. Florescu, R.L. Cappelletti / Journal of Non-Crystalline Solids 246 (1999) 150±154
References [1] K. Shimakawa, A. Kolobov, S.R. Elliott, Adv. Phys. 44 (1995) 475. [2] J.P. de Neufville, S.C. Moss, S.R. Ovshinsky, J. NonCryst. Solids 13 (1973) 191. [3] K. Tanaka, Applied Phys. Lett. 26 (1974) 243. [4] D.I. Florescu and R.L. Cappelletti, J. Non-Cryst. Solids 243 (1999) 204.
[5] V.M. Lyubin, V.K. Tikhomirov, J. Non-Cryst. Solids 135 (1991) 37. [6] Birgit Eey and R.L. Cappelletti, Phys. Rev. B, in press. [7] Didarul. Islam, unpublished PhD dissertation (1989), Ohio University. [8] C.A. Spence, S.R. Elliott, Phys. Rev. B 39 (1989) 5452. [9] T. Ojima, S. Adachi, J. Appl. Phys. 82 (1997) 3105. [10] D. Islam, R.L. Cappelletti, Phys. Rev. B 44 (1991) 2516.
Letter to the Editor
Optical anisotropy in pulsed laser deposited a-GeSe2 thin ®lms D.I. Florescu *, R.L. Cappelletti Department of Physics and Astronomy, Condensed Matter and Surface Sciences Program, Ohio University, Athens, OH 45701, USA Received 14 December 1998
Abstract Optical anisotropy was observed by dierential polarized optical transmission measurements using polarized 633 nm (sub-bandgap), low incident power (0.9 mW) light on a-GeSe2 thin (<0.4 lm) ®lm samples deposited by laser ablation on silica. Both normal and oblique depositions were studied and no signi®cant dierence was found between them. Neglecting the dierential absorption at this wavelength, optical anisotropy can arise from birefringence. Assuming a simple uniaxial birefringent model, anisotropy axes with a direction change in the 2±15° range were found for each investigated ®lm. Based on this model and considering the multiple re¯ections in both ®lm and substrate, the dierence in refractive index for ordinary and extraordinary rays was evaluated to be in the 0.005±0.015 range. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Transient [1] and metastable photoinduced phenomena (see Ref. [1]) including photodarkening [2], photostructural changes [3] and recently reported a thermal photobleaching [4] have been reported in glassy amorphous semiconductors. One category of photoinduced eects in chalcogenide network glasses are vectoral eects (see Ref. [1]). In order for these vectoral eects to be studied several workers have used an intense laser beam to induce anisotropy, and a weak polarized beam to probe the induced anisotropy [5]. In this study we describe measurements on a set of well-characterized a-GeSe2 samples obtained by pulsed laser deposition (PLD), probing the samples for non-induced or `native' optical anisotropy (OA). Using a low intensity (0.9 mW) probing
*
Corresponding author. Tel.: 1-740 594 4308; e-mail: ¯[email protected]
laser beam and applying the dierential polarized optical transmission technique we found that all our samples exhibit a small but de®nite non-induced OA. 2. Experimental details Bulk GeSe2 glasses used for PLD targets were prepared by the melt-quenching technique as described in previous work [6]. Thin ®lms of GeSe2 were deposited in a dynamic vacuum in the 5± 8 ´ 10ÿ5 Torr range, on chemically cleaned microscope slides (Corning 2948) cut 15 mm wide and 25 mm long. The thickness of these slides was measured to be between 0.976 and 0.983 mm and the index of refraction is n2 1.515 [7]. Several sample ®lms with thicknesses varying from 0.04 to 0.4 lm were produced. Most ®lms were grown at normal incidence, but some were deposited at incident angles 6 45° to promote columnar growth [8]. Columnar growth samples could be expected to show OA.
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 0 8 2 - 4
D.I. Florescu, R.L. Cappelletti / Journal of Non-Crystalline Solids 246 (1999) 150±154
An excimer laser using a krypton-¯uoride mixture was used for PLD. This device, lasing at 248 nm delivers energies upto 200 mJ per pulse, the pulse duration being approximately 10 ns. Deposition was accomplished in 20 min using a repetition rate of 10 Hz. The composition and homogeneity (5%) of the samples were determined by energy dispersive Xray emission using pure element standards for comparison. Film thicknesses were measured by a pro®lometer. For the dierential polarized optical transmission measurements, a linearly (500:1) polarized, 2 mW, He±Ne laser was used. The beam was chopped at a nominal frequency f 50 Hz with an optical chopper. A special wheel was designed with two diametrically opposed small openings into which were ®tted mutually perpendicular polarizing ®lters such that alternating pulses of polarized light were obtained. The sample holder was a rotational-translational stage, which allowed investigation of different spots on the sample without a change in the position of the other optical components. Considering an xyz reference system, the sample was placed in the yz plane and the incident light was parallel to the x direction, normal to the sample, as illustrated in Fig. 1. Using this holder, the sample could be translated along the y direction and also rotated around the x direction varying angle a. The ®lms were exposed in air during illumination at room temperature. The signal was detected with a Si high-speed detector. Two lock-in ampli®ers were used to measure the signal. One was tuned to detect a signal at twice the chopper rotation frequency 2f, and the other one at f. The resulting outputs are proportional respectively to the sum and dierence of the transmitted intensities. The dierence zero signal was set by rotating the laser in the absence of a sample. 3. Results To illustrate the results, three samples with dierent thicknesses are reported here. Two were 0.2 and 0.4 lm respectively, and were deposited at normal incidence. The third one was grown at an
151
Fig. 1. Geometry of the light-sample interaction. 0x is the incident laser radiation direction, where the two mutually orthogonal chopped states of polarization are also represented as a and b. zy is the plane in which the sample lies and a is the angle of rotation of the sample mount in this plane.
angle of incidence of approximately 45°, such that its thickness varied from 0.04 to 0.1 lm from side to side. In Fig. 2, for each sample we show the dierence in the transmitted intensities versus the sample orientation angle a. Two spots on each sample were investigated. The samples were ®rstly rotated in the yz plane varying the angle a until the difference signal became zero at a0 . At this point we can assume that the two transmitted orthogonal polarization intensities are equal. The sample was then rotated to a0 ÿ 45° and the signal was recorded, and then to a0 + 45° and the signal was again recorded. After that, by translation parallel to y, another spot was selected and the procedure repeated. We found the `zero direction' to vary from 2 to 15° for the two investigated spots on each of the samples. Fig. 2 also indicates that the magnitude of the detected dierence signal diers from spot to spot on the same sample. Also there is no signi®cantly dierent behavior of the obliquely deposited sample compared to the normally deposited ones.
152
D.I. Florescu, R.L. Cappelletti / Journal of Non-Crystalline Solids 246 (1999) 150±154
Fig. 2. Dierential polarized optical transmission versus the orientation angle a for two random spots on each sample, after the angle a was set for a null reading at a0 . The sample was then rotated to a0 ÿ 45° and signal was recorded, then to a0 + 45° recording the signal again. Samples are identi®ed by their thicknesses. Incident laser power was 0.9 mW, and the laser spot diameter was 1 mm. The variable thickness sample was obliquely deposited at 45° incidence and shows OA comparable to the normally deposited ®lms. Lines are drawn as guides for the eyes.
4. Discussion The results in Fig. 2 show that our samples exhibit OA. Considering the low value of the absorption coecient (950 cmÿ1 ) [9] of GeSe2 thin ®lms at 633 nm, the dierential absorption is small. We chose to neglect it and analyze the difference signal in terms of optical birefringence. Consider a simple model in which the birefringence is locally uniaxial and the optical axis has a component parallel to the faces of the ®lm, in the yz plane in Fig. 1. Based on this assumption the results in Fig. 2 are easily understood considering three possible orientations of the optical axis with respect to the two orthogonal states of polarization. For a zero signal, the axis is at 45° to both incident orthogonal polarization directions. For maximum positive or negative signal, the axis is parallel to either one or the other of the mutually orthogonal states of polarization. The transmittance for the ordinary and extraordinary rays through a thin, non-absorbing, uniaxially anisotropic ®lm on a transparent, iso-
tropic glass substrate, taking into account multiple re¯ections in both ®lm and substrate is given by Eq. (A.1) in Appendix A. Assuming an optically uniaxial birefringent case, the dierence in refractive index for the ordinary and extraordinary rays is evaluated from dT ; 1 Dn DT dn n2:43 DT is the measured dierence in the two orthogonal transmitted transmittances, Dn is the dierence in refractive index for ordinary and extraordinary rays and (dT/dn)n2:43 is the derivative of the transmittance function with respect to the average refractive index, which for laser-ablated a-GeSe2 thin ®lms is n 2.43 [10]. (dT/dn)n2:43 was evaluated from Eq. (1). Using the data shown in Fig. 2 we evaluated the maximum dierence in refractive index for ordinary and extraordinary rays for each sample. Results are shown below in Table 1. The observed birefringence for normally deposited ®lms is unexpected. We conjecture that residual
D.I. Florescu, R.L. Cappelletti / Journal of Non-Crystalline Solids 246 (1999) 150±154
153
Table 1 Sample thickness (lm)
Dierence in refractive indices, Dn
0.4 0.2 0.04±0.1
0.006 0.001 0.013 0.003 0.007 0.002
stress arising during PLD may be responsible. On the other hand, it was expected that oblique deposition would promote columnar ®lm growth with attendant OA. Evidently either PLD does not promote columnar growth or the associated optical anisotropy is only comparable to the `native' OA. 5. Conclusion In this work we have shown that PLD thin (<0.4 lm) glassy GeSe2 ®lms exhibit non-induced optical anisotropy. Ignoring dierential absorption, optical anisotropy can arise from birefringence. Considering a simple uniaxial model a preferential anisotropy axis with the direction varying from 2 to 15° range was found for each ®lm. No signi®cant OA dierence was found between normal and oblique deposited PLD ®lms.
Fig. 3. Symbolic representation of the air-®lm-substrate-air arrangement used in order to evaluate the transmittance formula in Appendix A. n0 and n2 are the refractive indices for air and glass, respectively, and n is the ®lm average refractive index. h is the angle that the anisotropy axis is making with a direction parallel to the faces of the ®lm; d1 is the ®lm thickness and d2 is the glass substrate thickness.
A transfer matrix approach gives: n Te;0 4 4 cos2 g cos2 g2 :
Acknowledgements
This work was supported by NSF Grant DMR9604921. We thank Professor Pedro Pereyra for useful discussions and suggestions in evaluating the transmittance derivative and Professors Larry Wilen and Martin Kordesch for helpful experimental suggestions. We are especially indebted to Professor J.A. Butcher Jr. for the use of his excimer laser and to Birgit Eey for help in bulk sample preparation.
g1 neff k0 d1 ;
g 2 n2 k 0 d 2 :
2
sin2 g sin2 g2 cos2 g sin2 g2
2 n0 n sin2 g cos2 g2 n n0 n0 n2 n0 n n2 n ÿ4 2 n2 n0 n n0 n n2 sin g cos g sin g2 cos g2
Consider the arrangement shown in Fig. 3. ns and nf are the slow and fast indices for the ®lm. Then we can de®ne the following parameters. 2p ; k0
n0 n2 n2 n0
2
Appendix A
k0
n2 n n n2
g01 ns k0 d1 ;
oÿ1
A:1
with g g1 ; n neff for the extraordinary ray, and g g01 ; n ns for the ordinary ray. Also neff is given by neff
cos2 h sin2 h 2 ns n2f
ÿ1=2 :
154
D.I. Florescu, R.L. Cappelletti / Journal of Non-Crystalline Solids 246 (1999) 150±154
References [1] K. Shimakawa, A. Kolobov, S.R. Elliott, Adv. Phys. 44 (1995) 475. [2] J.P. de Neufville, S.C. Moss, S.R. Ovshinsky, J. NonCryst. Solids 13 (1973) 191. [3] K. Tanaka, Applied Phys. Lett. 26 (1974) 243. [4] D.I. Florescu and R.L. Cappelletti, J. Non-Cryst. Solids 243 (1999) 204.
[5] V.M. Lyubin, V.K. Tikhomirov, J. Non-Cryst. Solids 135 (1991) 37. [6] Birgit Eey and R.L. Cappelletti, Phys. Rev. B, in press. [7] Didarul. Islam, unpublished PhD dissertation (1989), Ohio University. [8] C.A. Spence, S.R. Elliott, Phys. Rev. B 39 (1989) 5452. [9] T. Ojima, S. Adachi, J. Appl. Phys. 82 (1997) 3105. [10] D. Islam, R.L. Cappelletti, Phys. Rev. B 44 (1991) 2516.