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SnSe thin films synthesized by solid state reactions
Thin Solid Films, 149 (1987) 197-203 PREPARATION AND CHARACTERIZATION
197
SnSe T H I N FILMS S Y N T H E S I Z E D BY SOLID STATE REACTIONS DANG TRANQUAN Groupe de Physique Cristalline, Unitk associb au CNRS 040804, Universit~ de Rennes I, Campus de Beaulieu, 35042 Rennes Ckdex (France)
(ReceivedJune 2, 1986;revisedSeptember16, 1986;acceptedNovember20, 1986)
Thin SnSe films were synthesized by solid state reactions on glass substrates. X-ray diffraction data, electrical transport properties and optical absorption coefficient data were measured. The grain length, conductivity and Hall mobility were higher than those of evaporated SnSe thin films by factors of 20, 5 and 7 respectively. An exponential-inverse-temperature law is observed for the variations in resistivity, Hall mobility and carrier density. The results are explained in terms of a grain boundary potential barrier mechanism. The optical absorption results indicate the absorption edge of the synthesized SnSe thin films is due to an allowed direct transition of energy about 1.195 eV.
1. INTRODUCTION SnSe is a semiconductor with a band gap of about 1 eV which can be an efficient solar material1-4. In earlier papers 5-7 we have examined the transport and optical properties of evaporated SnSe thin films. We have shown that for photovoltaic thin film structures, the grain size and the electrical transport properties (Hall mobility) of these films need improvement. In the work reported here SnSe thin films were prepared by solid state reaction between the constituents in thin films form. The grain size and the transport characteristics of these films were greatly improved. We present the structural and electrical properties and the optical absorption of these films synthesized on glass substrates. 2. EXPERIMENTALDETAILS 2.1. S y n t h e s i s o f S n S e f i l m s
The SnSe films were synthesized by solid state reaction from thin films. Layers of elemental tin and selenium (99.999~o pure) were sequentially deposited at room temperature onto Pyrex substrates in a dual-source evaporation system. The typical pressure during an evaporation was about 1 × 10 -4 Pa. The evaporation rate from 0040-6090/87/$3.50
© ElsevierSequoia/Printedin The Netherlands
198
DANG TRAN QUAN
each source was independently monitored and controlled to achieve the desired layer thickness. The layer thickness varied from 10 to 50 nm and the number of layers varied from 3 to 13. The overall composition was determined by varying the relative thickness of the layers. Selenium has a high vapour pressure. At the synthesis temperature of the SnSe film (400 °C), the vapour pressure of selenium is about 13 Pa while that of tin is negligible s. Thus, to prevent selenium sublimation loss during thermal treatments, the tin layers were deposited first and last. Sandwiches (Sn/Se/Sn) and multilayer (Sn/Se/Sn/Se/Sn... Sn) structures were prepared. They were treated in situ at 400 _+ 5 °C for 60 min. The rate of heating of the film substrate (from 20 to 400 °C) is about 10 °C min-1. The vacuum was kept below 10 -4 Pa during the entire experiment. Three samples were synthesized, one from a sandwich structure and the other two from multilayer structures. The thickness of the tin and selenium layers were as follows: in the sandwich structure (Sn/Se/Sn), esn = 37.5 nm and ese = 84.8 nm; in the first multilayer structure (five tin layers and four selenium layers), esn = 23 nm and ese = 32.5 nm; in the second multilayer structure (seven tin layers and six selenium layers), es, = 22 nm and es, = 24.9 nm. The synthesized films have a metallic appearance. Their thickness was in the range 150-330 nm. Their composition was checked by electron microprobe analysis. 2.2. Transport and optical absorption measurements These measurements were taken under vacuum (10-1 Pa) in a separate cell. The standard Van der Pauw techniques and the four-probe resistivity technique were employed. Ohmic contacts were made by evaporating high purity aluminium through masks onto the film. The electrode surfaces were parallel to that of the substrate. The ohmic nature of the contacts was confirmed by the linear current-voltage characteristic throughout the temperature range studied (150-330 K). The electric current and the voltage were measured using Keithley 616 electrometers. Hall voltages were determined only at temperatures above 210 K. Taking into account the errors involved in the measurements of the sample current, film thickness, Hall voltage etc., the overall error in the Hall mobility is estimated to be about 5~o. The optical measurements were carried out at room temperature using a dual-beam Beckman D K 2A spectrophotometer. The transmittance was measured at wavelengths from 0.5 to 2.5 lam. 3. FILM CHARACTERIZATION Over the film thickness range investigated (150-330 nm), very few differences in the experimental results were observed from sample to sample. We report only the results for an SnSe film 245 nm thick, which was obtained from a multilayer structure (five tin layers and four selenium layers). 3.1. Structural characterization In the X-ray diffraction diagram of the synthesized SnSe thin films (Fig. 1, curve b), only the (040) peak and its equivalents (020), (060) and (080) were observed and these were indexed in terms of an orthorhombic SnSe lattice.
SnSe FILMS MADE BY SOLID STATEREACTIONS
199
(D O
o9
t"4
,,¢~
ttn
,4.
CO
o
¢q O
-~
,D
//
#
^
O
//-----
Fig. 1. X-ray diffraction spectra of SnSe: curve a, polycrystallineSnSe; curve b, an SnSe thin film with preferential orientation (040) (the (020), (040), (060) and (080) orientations are equivalents),formed by solid state reaction from a multilayer structure of fivetin layers and four seleniumlayers with es, = 23 nm and ese= 32.5 nm. These SnSe films have a strongly preferred orientation. Their crystallites are perpendicular to the (040) plane. It should be n o t e d that this preferential o r i e n t a t i o n is different from the (111) o r i e n t a t i o n of evaporated SnSe thin films 7.
3.2. Scanning electron microscopy Figure 2 shows a s c a n n i n g electron m i c r o g r a p h of the synthesized SnSe film. It can be seen that the film is composed of long grains. The width of these grains as d e t e r m i n e d from the m i c r o g r a p h is a b o u t 140 n m while their length can exceed 1 lain. Their thickness is equal to that of the film (245 nm).
200
DANG TRAN QUAN
Fig. 2. Scanning electron micrograph of an SnSe film synthesized from a multilayer structure of five tin layers and four selenium layers with esn = 23 nm and ese = 32.5 nm.
In the case of evaporated SnSe thin films, the crystallites are small; their size is less than 50 nm. Thus we can note that the grain size is greatly increased. In particular, the grain length has been increased by a factor of 20.
3.3. Optical absorption The absorption coefficient ~ can be calculated from measurements of the transmission T, the film thickness d and the reflectivity R. Neglecting interferences effects, Tis given by 9 T=
(1 -- R)2(1 + k2/n 2) exp(~d) - R E e x p ( - ~d)
where n and k are the refractive index and the extinction coefficient respectively. In experiments on semiconductors k 2 ~ n 2 SO that the average transmission and reflectivity are related by T = (1 - R) 2 expi-c~d) To eliminate R, the absorption coefficient ~ at a wavelength 2 was determined from the measured transmittivity for various thicknesses d. The spectral variation in at 300 K was determined over the energy range 0.95-1.30 eV. The intrinsic absorption edge of the synthesized SnSe film at 3 0 0 K was examined in terms of a direct transition using the equation of Bardeen et al. 1° which states that the absorption coefficient g is given by
~hv = B(hv
-- Eg) x
(1)
where x = 1/2 for a direct allowed transition and x = 3/2 for a direct forbidden transition. It is seen from Fig. 3 that, over the energy range 0.95-1.3 eV, the spectral variation in ~ can be described by eqn. (1) with x = 1/2. W h e n the linear portion is extrapolated to ~hv=O, we find that the direct energy gap Eg is 1.195 _+ 0.005 eV.
SnSe
FILMS MADE BY SOLID STATE REACTIONS
201
/
u
.t:
016
018
do
° o°
1
1.2
I'.4-
h'v
(eV)'
Fig. 3. The absorption edge at 300 K of a synthesized SnSe film plotted a s (~hv) 2 against the photon energy (film prepared from a multilayer structure of five tin layers and four selenium layers with es. = 23 n m and ese = 32.5 nm).
This estimated value is of the same order as the energy gap (1.210 eV) of evaporated SnSe thin films 7.
3.4. Electrical properties A typical plot of the resistivity p on a logarithmic scale against the reciprocal temperature is shown in Fig. 4. It is seen that there are two distinct regions, each obeying an equation of the form P = Po exp where Ep is the resistivity activation energy. Figure 4 also illustrates typical results for the carrier density and the Hall mobility against 1/T. The mobilities of synthesized SnSe thin films are a factor of 7 higher than those of evaporated SnSe films while the resistivities are a factor of 5 lower (i.e. the conductivities increased by a same factor) 7. The mobility and carrier density increase with increasing temperature, which indicates the predominance of the grain boundary potential barrier mechanism in these films. The grain boundary potential barrier model is based on the assumption that the grain boundaries have an inherent space charge region (due to the lattice discontinuity). Band bending occurs and potential barriers to the charge transport result. The exponential temperature dependence of the mobility and the carrier concentration can be represented by1 t-t 5 /'
.. = .o exp,,-
q~b"~
202
DANG TRAN QUAN
P
? (~1.cm)
cc~
16 10
103
1J
I¢
/
.
~H " ~ b
cm~,; =o,ollev
~
"
'o' I~/TIK] Fig. 4. Resistivity p, Hall mobility #H and carrier density p vs. reciprocal temperature for an SnSe film synthesized from a multilayer structure of five tin layers and four selenium layers with %, = 23 n m and %° = 32.5 nm.
and
( Ep)
P = P0 exp --
where ~b is the grain boundary barrier potential and #o is the grain-boundary-limited mobility for the case of no potential barrier. Ep is the carrier activation energy for p-type films. The values of q~b and Ep can be estimated from plots of log/t n and log p against 1/T. The values of q~b and Ep thus calculated are q~b = 0.011 eV and Ep = 0.225 eV. The relationships between the conductivity, mobility and carrier concentration activation energies predicted by the grain boundary potential barrier model ls-av are observed: E~ (0.240 eV) ~ Ep (0.225 eV) + q ~ b ( 0 . 0 1 1 eV). 4. CONCLUSION
SnSe thin films were synthesized by solid state reaction. The characteristics of
SnSe FILMS MADE BY SOLID STATE REACTIONS
203
the synthesized SnSe thin films were greatly superior to those of evaporated SnSe thin films. The grain length, the conductivity and the mobility were increased by factors of 20, 5 and 7 respectively. At room temperature, the resistivity, the Hall mobility and the carrier concentration were found to be 8 f2 cm, 59 cm 2 V - 1 s - 1 and 9.5 × 1016 c m - 3 respectively. The crystallite length exceeds 1 ~tm. These SnSe films were found to possess a direct band gap with an energy of 1.195 + 0.005 eV. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
J.J. Loferski, J. Appl. Phys., 27 (1956) 777. J.J. Loferski, Proc. IEEE, 51 (1963) 667. M. Rodot, Acta Electron., 18 (1975) 345. M. Rodot, Rev. Phys. Appl., 12 (1977) 411. A. Bennouna, P. Y. Tessier, M. Priol, Dang Tran Quan and S. Robin, Phys. Status Solidi B, 117 (1983) 51. A. Bennouna, A. Seignac, M. Priol, Dang Tran Quan and S. Robin, Proc. 7th Int. Conf. on Vacuum Ultraviolet Radiation Physics, Jerusalem, August 1983, in Ann. Israel Phys. Soc., 6 (1983) 258. Dang Tran Quan, Phys. Status Solidi A, 86 (1984) 421. L.I. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGraw-Hill, New York, 1970. T.S. Moss, OpticalProperties of Semiconductors, Butterworths, London, 1959. J. Bardeen, F. J. Blatt and L. H. Hall, in R. Breckenridge, B. Russel and T. Hahn (eds.), Photoconductivity Conf., Wiley, New York, 1956. R.L. Petriz, Phys. Rev., 104 (1956) 1508. R.H. Bube, Annu. Rev. Mater. Sci., 5 (1975) 201. L.L. Kazmerski, W. B. Berry and C. W. Allen, J. Appl. Phys., 43 (1972) 3515. H. Berger, Phys. Status Solidi, 1 (1961) 739. R.G. Mankarious, Solid-State Electron., 7 (1964) 702. L.L. Kazmerski and Y. J. Juang, J. Vac. Sci. Technol., 14 (1977) 769. L.L. Kazmerski, F. R. White, M. S. Ayyagary, Y. J. Juang and R. P. Patterson, J. Vac. Sci. Technol., 14 (1977) 65.
197
SnSe T H I N FILMS S Y N T H E S I Z E D BY SOLID STATE REACTIONS DANG TRANQUAN Groupe de Physique Cristalline, Unitk associb au CNRS 040804, Universit~ de Rennes I, Campus de Beaulieu, 35042 Rennes Ckdex (France)
(ReceivedJune 2, 1986;revisedSeptember16, 1986;acceptedNovember20, 1986)
Thin SnSe films were synthesized by solid state reactions on glass substrates. X-ray diffraction data, electrical transport properties and optical absorption coefficient data were measured. The grain length, conductivity and Hall mobility were higher than those of evaporated SnSe thin films by factors of 20, 5 and 7 respectively. An exponential-inverse-temperature law is observed for the variations in resistivity, Hall mobility and carrier density. The results are explained in terms of a grain boundary potential barrier mechanism. The optical absorption results indicate the absorption edge of the synthesized SnSe thin films is due to an allowed direct transition of energy about 1.195 eV.
1. INTRODUCTION SnSe is a semiconductor with a band gap of about 1 eV which can be an efficient solar material1-4. In earlier papers 5-7 we have examined the transport and optical properties of evaporated SnSe thin films. We have shown that for photovoltaic thin film structures, the grain size and the electrical transport properties (Hall mobility) of these films need improvement. In the work reported here SnSe thin films were prepared by solid state reaction between the constituents in thin films form. The grain size and the transport characteristics of these films were greatly improved. We present the structural and electrical properties and the optical absorption of these films synthesized on glass substrates. 2. EXPERIMENTALDETAILS 2.1. S y n t h e s i s o f S n S e f i l m s
The SnSe films were synthesized by solid state reaction from thin films. Layers of elemental tin and selenium (99.999~o pure) were sequentially deposited at room temperature onto Pyrex substrates in a dual-source evaporation system. The typical pressure during an evaporation was about 1 × 10 -4 Pa. The evaporation rate from 0040-6090/87/$3.50
© ElsevierSequoia/Printedin The Netherlands
198
DANG TRAN QUAN
each source was independently monitored and controlled to achieve the desired layer thickness. The layer thickness varied from 10 to 50 nm and the number of layers varied from 3 to 13. The overall composition was determined by varying the relative thickness of the layers. Selenium has a high vapour pressure. At the synthesis temperature of the SnSe film (400 °C), the vapour pressure of selenium is about 13 Pa while that of tin is negligible s. Thus, to prevent selenium sublimation loss during thermal treatments, the tin layers were deposited first and last. Sandwiches (Sn/Se/Sn) and multilayer (Sn/Se/Sn/Se/Sn... Sn) structures were prepared. They were treated in situ at 400 _+ 5 °C for 60 min. The rate of heating of the film substrate (from 20 to 400 °C) is about 10 °C min-1. The vacuum was kept below 10 -4 Pa during the entire experiment. Three samples were synthesized, one from a sandwich structure and the other two from multilayer structures. The thickness of the tin and selenium layers were as follows: in the sandwich structure (Sn/Se/Sn), esn = 37.5 nm and ese = 84.8 nm; in the first multilayer structure (five tin layers and four selenium layers), esn = 23 nm and ese = 32.5 nm; in the second multilayer structure (seven tin layers and six selenium layers), es, = 22 nm and es, = 24.9 nm. The synthesized films have a metallic appearance. Their thickness was in the range 150-330 nm. Their composition was checked by electron microprobe analysis. 2.2. Transport and optical absorption measurements These measurements were taken under vacuum (10-1 Pa) in a separate cell. The standard Van der Pauw techniques and the four-probe resistivity technique were employed. Ohmic contacts were made by evaporating high purity aluminium through masks onto the film. The electrode surfaces were parallel to that of the substrate. The ohmic nature of the contacts was confirmed by the linear current-voltage characteristic throughout the temperature range studied (150-330 K). The electric current and the voltage were measured using Keithley 616 electrometers. Hall voltages were determined only at temperatures above 210 K. Taking into account the errors involved in the measurements of the sample current, film thickness, Hall voltage etc., the overall error in the Hall mobility is estimated to be about 5~o. The optical measurements were carried out at room temperature using a dual-beam Beckman D K 2A spectrophotometer. The transmittance was measured at wavelengths from 0.5 to 2.5 lam. 3. FILM CHARACTERIZATION Over the film thickness range investigated (150-330 nm), very few differences in the experimental results were observed from sample to sample. We report only the results for an SnSe film 245 nm thick, which was obtained from a multilayer structure (five tin layers and four selenium layers). 3.1. Structural characterization In the X-ray diffraction diagram of the synthesized SnSe thin films (Fig. 1, curve b), only the (040) peak and its equivalents (020), (060) and (080) were observed and these were indexed in terms of an orthorhombic SnSe lattice.
SnSe FILMS MADE BY SOLID STATEREACTIONS
199
(D O
o9
t"4
,,¢~
ttn
,4.
CO
o
¢q O
-~
,D
//
#
^
O
//-----
Fig. 1. X-ray diffraction spectra of SnSe: curve a, polycrystallineSnSe; curve b, an SnSe thin film with preferential orientation (040) (the (020), (040), (060) and (080) orientations are equivalents),formed by solid state reaction from a multilayer structure of fivetin layers and four seleniumlayers with es, = 23 nm and ese= 32.5 nm. These SnSe films have a strongly preferred orientation. Their crystallites are perpendicular to the (040) plane. It should be n o t e d that this preferential o r i e n t a t i o n is different from the (111) o r i e n t a t i o n of evaporated SnSe thin films 7.
3.2. Scanning electron microscopy Figure 2 shows a s c a n n i n g electron m i c r o g r a p h of the synthesized SnSe film. It can be seen that the film is composed of long grains. The width of these grains as d e t e r m i n e d from the m i c r o g r a p h is a b o u t 140 n m while their length can exceed 1 lain. Their thickness is equal to that of the film (245 nm).
200
DANG TRAN QUAN
Fig. 2. Scanning electron micrograph of an SnSe film synthesized from a multilayer structure of five tin layers and four selenium layers with esn = 23 nm and ese = 32.5 nm.
In the case of evaporated SnSe thin films, the crystallites are small; their size is less than 50 nm. Thus we can note that the grain size is greatly increased. In particular, the grain length has been increased by a factor of 20.
3.3. Optical absorption The absorption coefficient ~ can be calculated from measurements of the transmission T, the film thickness d and the reflectivity R. Neglecting interferences effects, Tis given by 9 T=
(1 -- R)2(1 + k2/n 2) exp(~d) - R E e x p ( - ~d)
where n and k are the refractive index and the extinction coefficient respectively. In experiments on semiconductors k 2 ~ n 2 SO that the average transmission and reflectivity are related by T = (1 - R) 2 expi-c~d) To eliminate R, the absorption coefficient ~ at a wavelength 2 was determined from the measured transmittivity for various thicknesses d. The spectral variation in at 300 K was determined over the energy range 0.95-1.30 eV. The intrinsic absorption edge of the synthesized SnSe film at 3 0 0 K was examined in terms of a direct transition using the equation of Bardeen et al. 1° which states that the absorption coefficient g is given by
~hv = B(hv
-- Eg) x
(1)
where x = 1/2 for a direct allowed transition and x = 3/2 for a direct forbidden transition. It is seen from Fig. 3 that, over the energy range 0.95-1.3 eV, the spectral variation in ~ can be described by eqn. (1) with x = 1/2. W h e n the linear portion is extrapolated to ~hv=O, we find that the direct energy gap Eg is 1.195 _+ 0.005 eV.
SnSe
FILMS MADE BY SOLID STATE REACTIONS
201
/
u
.t:
016
018
do
° o°
1
1.2
I'.4-
h'v
(eV)'
Fig. 3. The absorption edge at 300 K of a synthesized SnSe film plotted a s (~hv) 2 against the photon energy (film prepared from a multilayer structure of five tin layers and four selenium layers with es. = 23 n m and ese = 32.5 nm).
This estimated value is of the same order as the energy gap (1.210 eV) of evaporated SnSe thin films 7.
3.4. Electrical properties A typical plot of the resistivity p on a logarithmic scale against the reciprocal temperature is shown in Fig. 4. It is seen that there are two distinct regions, each obeying an equation of the form P = Po exp where Ep is the resistivity activation energy. Figure 4 also illustrates typical results for the carrier density and the Hall mobility against 1/T. The mobilities of synthesized SnSe thin films are a factor of 7 higher than those of evaporated SnSe films while the resistivities are a factor of 5 lower (i.e. the conductivities increased by a same factor) 7. The mobility and carrier density increase with increasing temperature, which indicates the predominance of the grain boundary potential barrier mechanism in these films. The grain boundary potential barrier model is based on the assumption that the grain boundaries have an inherent space charge region (due to the lattice discontinuity). Band bending occurs and potential barriers to the charge transport result. The exponential temperature dependence of the mobility and the carrier concentration can be represented by1 t-t 5 /'
.. = .o exp,,-
q~b"~
202
DANG TRAN QUAN
P
? (~1.cm)
cc~
16 10
103
1J
I¢
/
.
~H " ~ b
cm~,; =o,ollev
~
"
'o' I~/TIK] Fig. 4. Resistivity p, Hall mobility #H and carrier density p vs. reciprocal temperature for an SnSe film synthesized from a multilayer structure of five tin layers and four selenium layers with %, = 23 n m and %° = 32.5 nm.
and
( Ep)
P = P0 exp --
where ~b is the grain boundary barrier potential and #o is the grain-boundary-limited mobility for the case of no potential barrier. Ep is the carrier activation energy for p-type films. The values of q~b and Ep can be estimated from plots of log/t n and log p against 1/T. The values of q~b and Ep thus calculated are q~b = 0.011 eV and Ep = 0.225 eV. The relationships between the conductivity, mobility and carrier concentration activation energies predicted by the grain boundary potential barrier model ls-av are observed: E~ (0.240 eV) ~ Ep (0.225 eV) + q ~ b ( 0 . 0 1 1 eV). 4. CONCLUSION
SnSe thin films were synthesized by solid state reaction. The characteristics of
SnSe FILMS MADE BY SOLID STATE REACTIONS
203
the synthesized SnSe thin films were greatly superior to those of evaporated SnSe thin films. The grain length, the conductivity and the mobility were increased by factors of 20, 5 and 7 respectively. At room temperature, the resistivity, the Hall mobility and the carrier concentration were found to be 8 f2 cm, 59 cm 2 V - 1 s - 1 and 9.5 × 1016 c m - 3 respectively. The crystallite length exceeds 1 ~tm. These SnSe films were found to possess a direct band gap with an energy of 1.195 + 0.005 eV. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
J.J. Loferski, J. Appl. Phys., 27 (1956) 777. J.J. Loferski, Proc. IEEE, 51 (1963) 667. M. Rodot, Acta Electron., 18 (1975) 345. M. Rodot, Rev. Phys. Appl., 12 (1977) 411. A. Bennouna, P. Y. Tessier, M. Priol, Dang Tran Quan and S. Robin, Phys. Status Solidi B, 117 (1983) 51. A. Bennouna, A. Seignac, M. Priol, Dang Tran Quan and S. Robin, Proc. 7th Int. Conf. on Vacuum Ultraviolet Radiation Physics, Jerusalem, August 1983, in Ann. Israel Phys. Soc., 6 (1983) 258. Dang Tran Quan, Phys. Status Solidi A, 86 (1984) 421. L.I. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGraw-Hill, New York, 1970. T.S. Moss, OpticalProperties of Semiconductors, Butterworths, London, 1959. J. Bardeen, F. J. Blatt and L. H. Hall, in R. Breckenridge, B. Russel and T. Hahn (eds.), Photoconductivity Conf., Wiley, New York, 1956. R.L. Petriz, Phys. Rev., 104 (1956) 1508. R.H. Bube, Annu. Rev. Mater. Sci., 5 (1975) 201. L.L. Kazmerski, W. B. Berry and C. W. Allen, J. Appl. Phys., 43 (1972) 3515. H. Berger, Phys. Status Solidi, 1 (1961) 739. R.G. Mankarious, Solid-State Electron., 7 (1964) 702. L.L. Kazmerski and Y. J. Juang, J. Vac. Sci. Technol., 14 (1977) 769. L.L. Kazmerski, F. R. White, M. S. Ayyagary, Y. J. Juang and R. P. Patterson, J. Vac. Sci. Technol., 14 (1977) 65.