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High contrast laser writing in insitu textured germanium films via a novel chemical oxide formation
Volume 65, number 4
OPTICS COMMUNICATIONS
15 February 1988
HIGH CONTRAST LASER WRITING IN INSITU TEXTURED GERMANIUM FILMS VIA A N O V E L C H E M I C A L O X I D E F O R M A T I O N L. K A M E S W A R A R A O Indian Institute of Science, Instrumentation and Services Unit, Bangalore 560012, India
Received 21 August 1987
A high contrast laser writing technique based on laser induced efficient chemical oxidation in insitu textured Ge films is demonstrated. Free running Nd-YAG laser pulses are used for irradiating the films. The irradiation effects have been characterised using optical microscopy, electron spectroscopy and microdensitometry. The mechanism for the observed contrast has been identified as due to formation of GeO2 phase upon laser irradiation using X-ray initiated Auger spectroscopy (XAES) and X-ray photoelectron spectroscopy (XPS). The contrast in the present films is found to be nearly five times more than that known due to GeO phase formation in similar films.
1. Introduction Recently, films with textured surfaces, have attracted many researchers as potential candidates for optical image storage, particularly for write once and read immediate applications [ 1 ]. The early experiments demonstrated the optical storage potential o f such films, by laser modification o f surface texture, giving rise to high reflectance contrast with high sensitivity [ 2 ]. The films used in such studies, however, were relatively thick, required multiprocesses for preparation and critical control over various process parameters for obtaining the required texture. In order to reduce the film preparation time and complexity of the control, the technique o f oblique deposition of films has been suggested for preparation of textured films [ 3 ]. These films possess insitu texture due to their characteristic columnar structure and are relatively thin (less than 5000 ,~). Laser irradiation o f such films gives rise to appreciable optical contrast o f writing via certain novel effects [ 3 - 5 ] . The process responsible for the optical contrast in such films is found to be dependent upon the irradiation conditions as well as on the material o f the film. Two such processes that are already reported are selective oxidation in germanium films [3,4] and photodarkening due to morphological reordering in PbTe films [ 5].
Oxidation is an important process step for the fabrication of microelectronic devices. G o o d quality dielectric thin films are essential for achieving best performance from these devices. Conventional high temperature techniques to produce such films, involve long duration o f thermal treatment and high temperature furnaces, which result in substrate warpage, dopant redistribution and defect generation and propagation, thereby limiting the performance of the device [ 6 ]. Laser oxidation has been studied by many researchers as a fast and cold technique to overcome these problems [7]. The laser formed oxide films have been mostly assessed for the quality o f their electrical properties. First attempt to utilize laser oxidation as a process for fabrication o f optical memories and optical imaging is reported in germanium films [ 3 ]. In obliquely deposited films of germanium, the laser irradiation leads to simultaneous structural and chemical transformations, in the irradiated region, giving rise to appreciable optical contrast. It is found experimentally that the contrast in these films is predominantly due to laser induced chemical oxidation, in the irradiated region. The chemical oxide phase is identified as GeO. The interesting feature of this oxide phase is that it is normally thermodynamically unstable phase and hence is difficult to grow over appreciable thickness by conventional methods [ 8 ].
0 030-4018/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
239
Volume 65, number 4
OPTICS COMMUNICATIONS
15 Februa~, 1988
However it has been grown over appreciable thickness with ease only in obliquely deposited films of Ge, via laser irradiation. Characteristically the irradiated region enriched with GeO phase shows reduced transmittance relative to the virgin film. The present communication, reports interesting transformations in the transmittance characteristics and the associated optical contrast of the irradiated region as the laser power density has been increased, in obliquely deposited films of Ge.
2. Experimental results and discussion Fig. 1. Optical micrograph of the laser irradiated region ( × 80). The advantages and characteristics of the obliquely deposited films have already been discussed in an earlier report [ 3 ]. Thin films of 3000 A thickness are deposited on microscopic glass plates in a vacuum of 10 6 torr at an angle of 80 °. A pulsed free running Nd:YAG laser with a typical pulse width of 300 gs has been used for irradiating the films. The absorption coefficient of the films lies between 103 to 104 cm ~ for the wavelength of irradiation used. The corresponding optical penetration depth of the films is nearly 105 to 104 A. As this depth of penetration exceeds the thickness of the films, the transformation in the film due to laser irradiation are expected to be homogeneous throughout the thickness of the film. The observed contrast due to irradiation has been analysed by (i) optical microscopy, (ii) microdensitormeter and (iii) X-ray initiated Auger and photo electron spectroscopy. The films have been irradiated at power density level of 11 kW/cm 2. At this level the films were found to become nearly totally transparent in the irradiated region as compared to the virgin film. Fig. 1 shows the typical optical micrograph of the irradiated region when such a transformation has occurred. The optical density difference between the irradiated region and the unirradiated region is measured by an optical microdensitometer. The measured values are 1.58 and 0.09 in the irradiated and unirradiated regions respectively. This corresponds to a transmission contrast of nearly 75%, with the irradiated region becoming more transparent relative to virgin film. This effect of increased transmission in the irradiated region in these films stands totally different to what was reported earlier in sim240
ilar films, wherein the transmission in the irradiated region has been found to be reduced [3]. As it has already been found [ 3 ] that obliquely deposited germanium films undergo significant chemical transformations upon laser irradiation and the optical contrast is predominantly due to these chemical changes, it is suggested that the observed effect in the present experiment could also be due to the formation of a new chemical phase other than the GeO phase. In order to identify the new chemical phase, the films are subjected to Auger (XAES) and XPS studies. The binding energy of the electrons at various core levels are sharply defined for all the elements and is highly sensitive to the chemical state as well as the chemical bond. Chemical changes upon laser irradiation should result either in formation of new chemical bonds or/as well as changes in existing bonds. Such changes will result in the shifting of the binding energy, by a small value, and this shift is a characteristic of the chemical phase formed. By measuring these shifts one can identify the new chemical phase. The chemical phases in the present film, both before irradiation and after irradiation are identified through comparison with reported values of binding energies for various phases. Fig. 2 shows the XAES spectra of the Ge films, both before irradiation and after irradiation. The ~G peak at the kinetic energy of 1146 eV in trace (a) corresponds to elemental Ge. The trace (b) of the fig. 2 shows the transformation in the XAES spectrum upon irradiation with the appearance of a pronounced peak at 1137.2 eV at the expense of the peak at 1146 eV. This
Volume 65, n u m b e r 4
OPTICS C O M M U N I C A T I O N S
15 February 1988
% +'-" {'4
I , . . , 4Y
,rro0io, 0
,%
°~
(B)
I
i
t
t
11
1215
I
I
t
t
t
1220 BE (eV)
I
I
1225
Fig. 3. XPS of Ge film near Ge(2p) level.
1150
1145
1140 KE (eV)
1135
Fig. 2. A] Kc~ X-ray initiated Auger spectra near Ge(L3VV) region.
suggests that upon irradiation, the XAES spectral peak of the elemental Ge undergoes a chemical shift of nearly 8.8 eV towards lower kinetic energy. This shift of 8.8 eV in XAES spectrum in the Ge(L3VV) region is a finger print of formation of GeO2 [9]. This is further confirmed by comparison of XPS spectra of the film, with the reported data. Fig. 3 shows the XPS spectra of the Ge film for Ge(2p3/2) level, both before and after irradiation. An examination of the spectra clearly shows that the binding energy of the Ge(2p3/2) level shifts from 1217.5 eV to 1220.8 eV. Fig. 4 shows the XPS spectra near O(ls) region. Upon irradiation the binding energy near O(ls) level shifts from 531 eV to 532.5 eV. These shifts of 3.3 eV in Ge(2p) level and 1.5 eV in O (I s) respectively also confirm the identity of GeO2 phase formation in the irradiated region [3,9]. This clearly suggests that the observed increase in the transmittance of the irradiated region is essentially due to formation of a new chemical phase i.e. GeO2 due to laser irradiation. The measured transmittance contrast due to for-
mation of GeO2 phase is nearly 75%, which is nearly five times more than that reported earlier via chemical oxidation to GeO phase in similar films. The photograph in fig. 1 has been selected to show simultaneously, the relative contrast effects due to for-
i rrodiated 1o
c/s,,c/ \ unirrodiated I
I
J
I
~
t
f
530
J
i
535 BE (eV)
Fig. 4. XPS of the Ge film near O ( l s ) level.
241
Volume 65, number 4
OPTICS COMMUNICATIONS
15 February 1988
efficiently at these unsaturated sites resulting in the f o r m a t i o n o f the GeO2 phase, giving rise to very high contrast o f writing. The intrinsic resolution limits o f these films could be very high, since the starting asdeposited films are a m o r p h o u s a n d hence do not suffer from grain size limitations. Although a Nd:laser operating at 1.06 ~tm has been used to d e m o n s t r a t e the effect, a shorter wavelength laser could be used for an i m p r o v e d resolution in practical applications.
3. Conclusions Fig. 5. Optical micrograph of irradiated region (×80) with abundant GeO phase. m a t i o n o f G e O phase ( p r e s e n t as dark sports at the b o u n d a r y o f the i r r a d i a t e d sport), GeO2 phase (characterised by the t r a n s m i t t i n g p o r t i o n with large n u m b e r o f cracks) a n d due to f o r m a t i o n o f hole via laser e v a p o r a t i o n (shown in the center o f the photograph). F o r comparison, the optical m i c r o g r a p h o f the laser writing via p r e d o m i n a n t l y G e O phase form a t i o n ( r e d region) is shown in the fig. 5, wherein the central bright p o r t i o n corresponds to f o r m a t i o n o f GeO2. The exposure conditions for the photographs in both the figs. 1 and 5 are identical. The near total t r a n s m i t t a n c e characteristic o f the i r r a d i a t e d region suggests that the Ge to GeO2 phase transform a t i o n is almost complete a n d homogeneous. It is interesting to note that in the n o r m a l l y deposited films o f g e r m a n i u m , a t t e m p t s to form G e O a n d GeO2 phase have been unsuccessful using similar e x p e r i m e n t a l set up a n d procedure. This indicates obliquely d e p o s i t e d films with their characteristic c o l u m n a r structure and the associated porosity are ideally suited for efficient o x i d a t i o n reaction. The starting films for this e x p e r i m e n t are found to be a m o r p h o u s in phase, through an exa m i n a t i o n o f electron diffraction patterns. P h o t o n energy o f Nd:laser pulses exceeds the b a n d g a p energy o f the Ge films. I r r a d i a t i o n o f these films by Nd:laser pulses results in breaking up o f G e - G e b o n d s leading to f o r m a t i o n o f u n s a t u r a t e d dangling b o n d s at a large n u m b e r o f sites. Laser pulses simultaneously raise the t e m p e r a t u r e o f the samples transiently, a n d if this t e m p e r a t u r e crosses a critical value, e n h a n c e d photo o x i d a t i o n seems to take place 242
In s u m m a r y , it has been d e m o n s t r a t e d that controlled i r r a d i a t i o n o f textured Ge films, near I I k W / c m 2 leads to efficient o x i d a t i o n in the i r r a d i a t e d region, giving rise to an appreciable optical contrast. This contrast is nearly five times more than the earlier r e p o r t e d value due to a similar oxidation effect in similar films. Such a contrast is interesting for applications in image storage, optical m e m o r i e s and microfabrication.
Acknowledgements The author wishes to acknowledge the help o f Dr. M.S. Hegde, Dr. K.S. H a r s h a v a r d h a n a n d Prof. E.S. Raja G o p a l during the course o f this work.
References [ 1] S.Y. Sub and G. Craighead, Appl. Optics 24 (1985) 208. [2] H.G. Craighead and R.E. Howard, Appl. Phys. Lett. 39 (1981) 352. [3] L. Kameswara Rao, K. Soloman Harshavardhan, A. Selvarajah and M.S. Hedge. Appl. Phys. Lett. 49 (1986) 826. [4] T.S. Sampathkumar, L. Kameswara Rao and M.S. Hedge, Appl. Surf. Sci. 27 (1986) 255. [ 5 ] L. Kameswara Rao and A. Selvarajan, in: South West Optics 87, Topical meeting on lasers in material diagnostics, Optical Society of America, 1987, Albuquerque, New Mexico, USA. [6] S. Su, Solid State Technology 24 (1981) 72. [7] T.E. Orlowski and H. Richter, Appl. Phys. Lett. 45 (1984) 241. [8] E.E. Crisman, Y.M. Eriel, J.J. Loferski and P.J. Stiles, Jl. Electrochem. Soc. 129 (1982) 1845. [9] J.E. Castle and D. Epler, Proc. R. Soc. London Ser. A.339 (1974) 49.
OPTICS COMMUNICATIONS
15 February 1988
HIGH CONTRAST LASER WRITING IN INSITU TEXTURED GERMANIUM FILMS VIA A N O V E L C H E M I C A L O X I D E F O R M A T I O N L. K A M E S W A R A R A O Indian Institute of Science, Instrumentation and Services Unit, Bangalore 560012, India
Received 21 August 1987
A high contrast laser writing technique based on laser induced efficient chemical oxidation in insitu textured Ge films is demonstrated. Free running Nd-YAG laser pulses are used for irradiating the films. The irradiation effects have been characterised using optical microscopy, electron spectroscopy and microdensitometry. The mechanism for the observed contrast has been identified as due to formation of GeO2 phase upon laser irradiation using X-ray initiated Auger spectroscopy (XAES) and X-ray photoelectron spectroscopy (XPS). The contrast in the present films is found to be nearly five times more than that known due to GeO phase formation in similar films.
1. Introduction Recently, films with textured surfaces, have attracted many researchers as potential candidates for optical image storage, particularly for write once and read immediate applications [ 1 ]. The early experiments demonstrated the optical storage potential o f such films, by laser modification o f surface texture, giving rise to high reflectance contrast with high sensitivity [ 2 ]. The films used in such studies, however, were relatively thick, required multiprocesses for preparation and critical control over various process parameters for obtaining the required texture. In order to reduce the film preparation time and complexity of the control, the technique o f oblique deposition of films has been suggested for preparation of textured films [ 3 ]. These films possess insitu texture due to their characteristic columnar structure and are relatively thin (less than 5000 ,~). Laser irradiation o f such films gives rise to appreciable optical contrast o f writing via certain novel effects [ 3 - 5 ] . The process responsible for the optical contrast in such films is found to be dependent upon the irradiation conditions as well as on the material o f the film. Two such processes that are already reported are selective oxidation in germanium films [3,4] and photodarkening due to morphological reordering in PbTe films [ 5].
Oxidation is an important process step for the fabrication of microelectronic devices. G o o d quality dielectric thin films are essential for achieving best performance from these devices. Conventional high temperature techniques to produce such films, involve long duration o f thermal treatment and high temperature furnaces, which result in substrate warpage, dopant redistribution and defect generation and propagation, thereby limiting the performance of the device [ 6 ]. Laser oxidation has been studied by many researchers as a fast and cold technique to overcome these problems [7]. The laser formed oxide films have been mostly assessed for the quality o f their electrical properties. First attempt to utilize laser oxidation as a process for fabrication o f optical memories and optical imaging is reported in germanium films [ 3 ]. In obliquely deposited films of germanium, the laser irradiation leads to simultaneous structural and chemical transformations, in the irradiated region, giving rise to appreciable optical contrast. It is found experimentally that the contrast in these films is predominantly due to laser induced chemical oxidation, in the irradiated region. The chemical oxide phase is identified as GeO. The interesting feature of this oxide phase is that it is normally thermodynamically unstable phase and hence is difficult to grow over appreciable thickness by conventional methods [ 8 ].
0 030-4018/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
239
Volume 65, number 4
OPTICS COMMUNICATIONS
15 Februa~, 1988
However it has been grown over appreciable thickness with ease only in obliquely deposited films of Ge, via laser irradiation. Characteristically the irradiated region enriched with GeO phase shows reduced transmittance relative to the virgin film. The present communication, reports interesting transformations in the transmittance characteristics and the associated optical contrast of the irradiated region as the laser power density has been increased, in obliquely deposited films of Ge.
2. Experimental results and discussion Fig. 1. Optical micrograph of the laser irradiated region ( × 80). The advantages and characteristics of the obliquely deposited films have already been discussed in an earlier report [ 3 ]. Thin films of 3000 A thickness are deposited on microscopic glass plates in a vacuum of 10 6 torr at an angle of 80 °. A pulsed free running Nd:YAG laser with a typical pulse width of 300 gs has been used for irradiating the films. The absorption coefficient of the films lies between 103 to 104 cm ~ for the wavelength of irradiation used. The corresponding optical penetration depth of the films is nearly 105 to 104 A. As this depth of penetration exceeds the thickness of the films, the transformation in the film due to laser irradiation are expected to be homogeneous throughout the thickness of the film. The observed contrast due to irradiation has been analysed by (i) optical microscopy, (ii) microdensitormeter and (iii) X-ray initiated Auger and photo electron spectroscopy. The films have been irradiated at power density level of 11 kW/cm 2. At this level the films were found to become nearly totally transparent in the irradiated region as compared to the virgin film. Fig. 1 shows the typical optical micrograph of the irradiated region when such a transformation has occurred. The optical density difference between the irradiated region and the unirradiated region is measured by an optical microdensitometer. The measured values are 1.58 and 0.09 in the irradiated and unirradiated regions respectively. This corresponds to a transmission contrast of nearly 75%, with the irradiated region becoming more transparent relative to virgin film. This effect of increased transmission in the irradiated region in these films stands totally different to what was reported earlier in sim240
ilar films, wherein the transmission in the irradiated region has been found to be reduced [3]. As it has already been found [ 3 ] that obliquely deposited germanium films undergo significant chemical transformations upon laser irradiation and the optical contrast is predominantly due to these chemical changes, it is suggested that the observed effect in the present experiment could also be due to the formation of a new chemical phase other than the GeO phase. In order to identify the new chemical phase, the films are subjected to Auger (XAES) and XPS studies. The binding energy of the electrons at various core levels are sharply defined for all the elements and is highly sensitive to the chemical state as well as the chemical bond. Chemical changes upon laser irradiation should result either in formation of new chemical bonds or/as well as changes in existing bonds. Such changes will result in the shifting of the binding energy, by a small value, and this shift is a characteristic of the chemical phase formed. By measuring these shifts one can identify the new chemical phase. The chemical phases in the present film, both before irradiation and after irradiation are identified through comparison with reported values of binding energies for various phases. Fig. 2 shows the XAES spectra of the Ge films, both before irradiation and after irradiation. The ~G peak at the kinetic energy of 1146 eV in trace (a) corresponds to elemental Ge. The trace (b) of the fig. 2 shows the transformation in the XAES spectrum upon irradiation with the appearance of a pronounced peak at 1137.2 eV at the expense of the peak at 1146 eV. This
Volume 65, n u m b e r 4
OPTICS C O M M U N I C A T I O N S
15 February 1988
% +'-" {'4
I , . . , 4Y
,rro0io, 0
,%
°~
(B)
I
i
t
t
11
1215
I
I
t
t
t
1220 BE (eV)
I
I
1225
Fig. 3. XPS of Ge film near Ge(2p) level.
1150
1145
1140 KE (eV)
1135
Fig. 2. A] Kc~ X-ray initiated Auger spectra near Ge(L3VV) region.
suggests that upon irradiation, the XAES spectral peak of the elemental Ge undergoes a chemical shift of nearly 8.8 eV towards lower kinetic energy. This shift of 8.8 eV in XAES spectrum in the Ge(L3VV) region is a finger print of formation of GeO2 [9]. This is further confirmed by comparison of XPS spectra of the film, with the reported data. Fig. 3 shows the XPS spectra of the Ge film for Ge(2p3/2) level, both before and after irradiation. An examination of the spectra clearly shows that the binding energy of the Ge(2p3/2) level shifts from 1217.5 eV to 1220.8 eV. Fig. 4 shows the XPS spectra near O(ls) region. Upon irradiation the binding energy near O(ls) level shifts from 531 eV to 532.5 eV. These shifts of 3.3 eV in Ge(2p) level and 1.5 eV in O (I s) respectively also confirm the identity of GeO2 phase formation in the irradiated region [3,9]. This clearly suggests that the observed increase in the transmittance of the irradiated region is essentially due to formation of a new chemical phase i.e. GeO2 due to laser irradiation. The measured transmittance contrast due to for-
mation of GeO2 phase is nearly 75%, which is nearly five times more than that reported earlier via chemical oxidation to GeO phase in similar films. The photograph in fig. 1 has been selected to show simultaneously, the relative contrast effects due to for-
i rrodiated 1o
c/s,,c/ \ unirrodiated I
I
J
I
~
t
f
530
J
i
535 BE (eV)
Fig. 4. XPS of the Ge film near O ( l s ) level.
241
Volume 65, number 4
OPTICS COMMUNICATIONS
15 February 1988
efficiently at these unsaturated sites resulting in the f o r m a t i o n o f the GeO2 phase, giving rise to very high contrast o f writing. The intrinsic resolution limits o f these films could be very high, since the starting asdeposited films are a m o r p h o u s a n d hence do not suffer from grain size limitations. Although a Nd:laser operating at 1.06 ~tm has been used to d e m o n s t r a t e the effect, a shorter wavelength laser could be used for an i m p r o v e d resolution in practical applications.
3. Conclusions Fig. 5. Optical micrograph of irradiated region (×80) with abundant GeO phase. m a t i o n o f G e O phase ( p r e s e n t as dark sports at the b o u n d a r y o f the i r r a d i a t e d sport), GeO2 phase (characterised by the t r a n s m i t t i n g p o r t i o n with large n u m b e r o f cracks) a n d due to f o r m a t i o n o f hole via laser e v a p o r a t i o n (shown in the center o f the photograph). F o r comparison, the optical m i c r o g r a p h o f the laser writing via p r e d o m i n a n t l y G e O phase form a t i o n ( r e d region) is shown in the fig. 5, wherein the central bright p o r t i o n corresponds to f o r m a t i o n o f GeO2. The exposure conditions for the photographs in both the figs. 1 and 5 are identical. The near total t r a n s m i t t a n c e characteristic o f the i r r a d i a t e d region suggests that the Ge to GeO2 phase transform a t i o n is almost complete a n d homogeneous. It is interesting to note that in the n o r m a l l y deposited films o f g e r m a n i u m , a t t e m p t s to form G e O a n d GeO2 phase have been unsuccessful using similar e x p e r i m e n t a l set up a n d procedure. This indicates obliquely d e p o s i t e d films with their characteristic c o l u m n a r structure and the associated porosity are ideally suited for efficient o x i d a t i o n reaction. The starting films for this e x p e r i m e n t are found to be a m o r p h o u s in phase, through an exa m i n a t i o n o f electron diffraction patterns. P h o t o n energy o f Nd:laser pulses exceeds the b a n d g a p energy o f the Ge films. I r r a d i a t i o n o f these films by Nd:laser pulses results in breaking up o f G e - G e b o n d s leading to f o r m a t i o n o f u n s a t u r a t e d dangling b o n d s at a large n u m b e r o f sites. Laser pulses simultaneously raise the t e m p e r a t u r e o f the samples transiently, a n d if this t e m p e r a t u r e crosses a critical value, e n h a n c e d photo o x i d a t i o n seems to take place 242
In s u m m a r y , it has been d e m o n s t r a t e d that controlled i r r a d i a t i o n o f textured Ge films, near I I k W / c m 2 leads to efficient o x i d a t i o n in the i r r a d i a t e d region, giving rise to an appreciable optical contrast. This contrast is nearly five times more than the earlier r e p o r t e d value due to a similar oxidation effect in similar films. Such a contrast is interesting for applications in image storage, optical m e m o r i e s and microfabrication.
Acknowledgements The author wishes to acknowledge the help o f Dr. M.S. Hegde, Dr. K.S. H a r s h a v a r d h a n a n d Prof. E.S. Raja G o p a l during the course o f this work.
References [ 1] S.Y. Sub and G. Craighead, Appl. Optics 24 (1985) 208. [2] H.G. Craighead and R.E. Howard, Appl. Phys. Lett. 39 (1981) 352. [3] L. Kameswara Rao, K. Soloman Harshavardhan, A. Selvarajah and M.S. Hedge. Appl. Phys. Lett. 49 (1986) 826. [4] T.S. Sampathkumar, L. Kameswara Rao and M.S. Hedge, Appl. Surf. Sci. 27 (1986) 255. [ 5 ] L. Kameswara Rao and A. Selvarajan, in: South West Optics 87, Topical meeting on lasers in material diagnostics, Optical Society of America, 1987, Albuquerque, New Mexico, USA. [6] S. Su, Solid State Technology 24 (1981) 72. [7] T.E. Orlowski and H. Richter, Appl. Phys. Lett. 45 (1984) 241. [8] E.E. Crisman, Y.M. Eriel, J.J. Loferski and P.J. Stiles, Jl. Electrochem. Soc. 129 (1982) 1845. [9] J.E. Castle and D. Epler, Proc. R. Soc. London Ser. A.339 (1974) 49.