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Study of microstructure of amorphous Ge-Se films using spectroscopic ellipsometry
Thin Solid Films, 164 (1988) 51-56
51 STUDY OF M I C R O S T R U C T U R E OF A M O R P H O U S Ge-Se FILMS USING SPECTROSCOPIC ELLIPSOMETRY* SATYENDRA KUMARt, D. K. PANDYA AND K. L. CHOPRA~ Thin Film Laboratory, Physics Department, Indian Institute of Technology, Hauz Khas, New Delhi 110016 (India)
The dielectric function spectra ~ =/31 --ig 2 of evaporated a m o r p h o u s G e - S e films in the energy range 2-5.2 eV have been studied using spectroscopic ellipsometry (SE). The ~ spectra of GexSel_~ (0.2~
I. INTRODUCTION
Amorphous chalcogenides, in contrast to a m o r p h o u s silicon and germanium, with their lower coordination and highly polarizable lone-pair (LP) orbitals exhibit electronic and structural flexibility which is responsible for m a n y observed phenomena such as the photostructural changes peculiar to this class of materials. Our laboratory has reported I a number of interesting phenomena when these films are exposed to band gap photons, energetic electrons and ions. In particular, photoexposure of a m o r p h o u s chalcogenide films leads to changes in the optical constants and a shift in the optical absorption edge 2. The studies 1 of optical constants in the vicinity of the absorption edge have yielded significant information on the role of various atoms in the chalcogenide network. A direct determination of the dielectric function in the interband transition region is expected to provide information on the electronic band structure and microstructural aspects of virgin and irradiated chalcogenide films. Ellipsometry being sensitive to the surfaces can detect small changes in composition and microstructure at the film surface. Spectroscopic ellipsometry (SE) has earlier 3 been employed to study the optical spectra of crystalline and glassy GeSe2 films. In this paper, we report the results of a detailed SE study on the optical properties and microstructure of a m o r p h o u s GexSel_ x films as a function of composition, angle of deposition and photoexposure. * Paper presented at the 7th International Conferenceon Thin Films, New Delhi, India, December 7 11, 1987. t Present address: Laboratoire de Physique des Interfaces et des Couches Minces, Ecole Polytechnique, 91128 Palaiseau C6dex, France. :~Present address: Indian Institute of Technology, Kharagpur 721302, India. 0040-6090/88/$3.50
© ElsevierSequoia/Printed in The Netherlands
52
S. KUMAR, D. K. PANDYA, K. L. CHOPRA
2. EXPERIMENTAL DETAILS
The bulk alloy glasses ofGexSel _x (at x = 0.20, 0.25 and 0.33) were prepared by melt quenching. Thin films of various compositions were deposited on one side of polished quartz substrates by evaporation of bulk glasses from a molybdenum boat in a background pressure of about 10-6Torr. Films of uniform thickness were deposited at various angles of deposition. Typical deposition rates were 5-10 ~ s 1 and typical thicknesses were in the range 1-2 I~m. Ellipsometric data in the spectral range 2-5.2 eV were recorded using a rotating polarizer spectroscopic ellipsometer (SOPRA model ES2G). The angle of incidence ~bof the light beam was kept constant at 70.0 °. Details of the SE data acquisition and analysis techniques are reported elsewhere 4. To study the photoinduced effects, the films were exposed to UV light using a 250 W mercury lamp at an intensity of about 30 m W c m - 2. In order to prevent heating of the films during exposure, the samples were kept on a water-cooled copper platform. 3.
RESULTS AND DISCUSSION
3.1. Effect o f composition The real and imaginary parts of dielectric function spectra of an as-deposited amorphous GeSe2 film are shown in Fig. 1. The extension of the data in the low energy range (broken lines) was estimated from the interference oscillations in the data calculated in a two-phase model 4. The ~2 spectrum is characterized by two distinct broad peaks at about 3.6 eV and about 5 eV. This do,able-peak nature is in contrast to the single-peak behaviour of amorphous films of tetrahedrally coordinated group IV and III-V semiconductors. The two-peak nature is a peculiarity of amorphous L P semiconductors such as selenium and tellurium 5. The first increase in the ez spectrum has been attributed 3'6 to electronic transitions from the LP states of the selenium atoms to the s-like antibonding lower conduction band and the second peak to the transitions between p bonding valence states and the s antibonding conduction band states. Our spectra are in good agreement with the data reported by Aspnes et al. 3 on evaporated glassy GeSe2 films. The slightly lower magnitude of ez in our case could be due to the difference in microstructure of the films. Figure 2 shows the composition dependence of the e2 spectra of amorphous GexSel -x films. With increasing selenium concentration the absorption edge shifts towards higher energy and the strength of the e2 spectrum in the interband transition region increases. With increasing selenium (decreasing x) the e z spectrum moves gradually towards that of selenium (also shown in Fig. 2) where the first peak should occur between 2 and 6 eV (LP band-antibonding) with the second peak starting at about 6 eV up to about 10 eV (bonding-antibonding). The threshold energy of the second peak can be seen to shift towards higher energy from about 4 eV for GeSe2 to about 4.5 eV for GeSe 4. A similar trend has been predicted from the band structure calculations 6'7. Further, the absorption edge becomes sharper with increasing selenium because of enhanced structural flexibility and associated removal of dangling bonds. The shift in absorption edge is also consistent with earlier observations 8 using standard reflection and transmission measurements.
STUDY OF
Ge-Se FILMS BY SPECTROSCOPIC ELLIPSOMETRY
8.0
o-Ge ' ~
DEPOST IED
Se2AS
E:I"
6.0
- -
GeSe
.... 6.0 ........
53
2
GeSe 3 Oe Se4
--
....
..._.~.;." ;" F,-"
4.0 E:2 2.0
~.0
2.0
l"
2 I.
I
I
2 O
i
I
3.0
l
I
S ,O
2.0
I
4.0 E(eV)
I
I
4 ,
3.0
6 I
4.0
8 i
10 I
s.o
E (eV)
Fig. 1. Measured dielectric function ~ = el
--i•2
spectra of an as-deposited amorphous GeSe 2 film.
Fig. 2. Imaginary part of the measured dielectric function spectra of as-deposited amorphous GexSel _x (x = 0.20, 0.25 and 0.33) films. The e2 spectrum of amorphous selenium is also shown 5.
3.2. Effect of obliqueness Figure 3 shows the ez spectra for a set of as-deposited amorphous GeSe3 (a-GeSe3) (Ge2sSev5) films prepared at various angles of deposition in the same deposition run. There is a large decrease in the magnitude of e2 for obliquely deposited films. Unlike a-Ge 4, films deposited below 60 ° do not show much change and are not shown in the figure. The shape of the dielectric function spectra is retained even in films deposited at 80 °, indicating that the basic bonding configuration remains unchanged on oblique deposition. Oblique deposition gives rise to an enhanced columnar structure with associated voids and dangling bonds compared with films deposited at 0 ° (normal incidence). An increase in defects causes tailing of the localized states at the band edges which is responsible for the observed decrease in slope of the absorption edge for obliquely deposited films. Similar data were obtained on films deposited with other compositions. 6.C A S DEPOSITED 0 - Oe Se 3
•
J
^
.
:;5:2(
_----_::-:
g }7" 0
i 2.0
I 3.0
I
E(eV)
I 4.0
I 5.0
Fig. 3. Imaginary part ofthe measured dielectric function spectra of a-GeS% (Ge2sSeTs) films deposited at different angles of the incident vapour beam.
The Bruggeman effective medium approximation (EMA) together with linear regression analysis (LRA) were employed to calculate the density deficit in obliquely deposited films. The dielectric function spectra of the film deposited at 0 ° were taken as the standard reference for the EMA-LRA calculations. The analysis of films
54
s. KUMAR, D. K. PANDYA, K. L. CHOPRA
deposited at 80 ° yielded a void percentage in bulk (density deficit) of 21.7%_ 0.7% ( + 0.7% being the 90% confidence limits). The value of the unbiased estimator tr was 0.012 and this indicates a good fit. The density deficit calculated by SE data analyses is in agreement with earlier s gravimetric measurements. It should be noted that a pronounced columnar microstructure accompanied by a large density deficit (about 40%) is obtained 4 in a-Ge films deposited at 80 °. In contrast, because of the network flexibility on introduction of twofold-coordinated selenium atoms, the void structure in the germanium-based chalcogenide films is considerably reduced. We should mention that the SE data were also recorded by rotating the sample on its axis as has been earlier reported 4 for a-Ge films. A biaxial optical anisotropy was observed in obliquely deposited films. However, the extent of anisotropy was smaller than that of obliquely deposited pure Ge films.
3.3. Photoinduced effects The dielectric function spectra of an a-GeSe3 (Ge25Se75) film deposited at 0 ° and exposed to UV light for l0 h together with those of an as-deposited film are compared in Fig. 4. The two broad peaks in the e2 spectrum, typical of as-deposited films, collapse into a single broad peak onphotoexposure. The magnitude of e 2 increases considerably in the lower energy range and decreases in the higher energy region compared with the spectrum of the as-deposited film. Corresponding changes in the optical constants are shown in Fig. 5. An increase in the value ofn on exposure has been reported by various workers 1'2"9. However, a decrease in the high energy region has not been observed in earlier photometric measurements. It should be noted that ellipsometry provides information about the sample volume depending on the penetration depth of photons. In our case, the data obtained for energy above 3.5 eV are from a sample thickness of less than 500 ~. E.1 ~ , 8.0 -
,, ',,
\
- ....
0 ° a - G e Se3 AS D E P O S I T E D UV E X P O S E D
/ / " - ~
--
AS
DEPOSITED
~
n
6,0
~
z,.O
2.0
........
k
g
o
/ 2.0
~
3. O E(eV)
I
I
4,0
5.O E(eV)
Fig. 4. Measured dielectric function spectra of a normally deposited a-GeSe3 film before ( - - ) ( - - ) long UV light exposure. Fig. 5. Optical constants (n and k) spectra of an a-GeSe3 film deposited at 0 ° before ( long UV light exposure.
and after
) and after ( - - - )
If we recall that the dielectric function is defined in terms of polarizability per unit volume, it is clear that the increase in magnitude of e2 on long exposure to UV light (Figs. 4 and 5) indicates either a more efficient packing of the basic tetrahedrai units Ge(Sel/2)4 or an increase in their polarizability at lower energies. A densification of about 10% would be required to account for the observed increase in
STUDY OF
Ge-Se FILMS BY SPECTROSCOPIC ELLIPSOMETRY
55
~2 at 3 eV which seems unlikely in the case of films deposited at 0 °. Further, densification alone does not explain the single-peak behaviour and the decrease in e 2 at higher energy. A change in polarizability would require a rearrangement in the bonding configuration. Aspnes et al. 3 have observed a single broad peak in the e2 spectrum of silver-photoactivated a-GeSe2 with an overall increase in the magnitude. They argued that the photoactivation by silver must be reducing the medium-range order by primarily interacting with selenium atoms. Further, the polycrystalline-toamorphous transition in silicon is also accompanied by a loss of the two-peak structure in the e 2 spectrum to a single broad peak with a concomitant shift in the absorption edge towards lower energy x°. This is also a clear evidence of the loss of long-range order. A single broad peak behaviour will also be expected if photoexcitation can induce a shift in the LP band so that the L P - b o n d i n g band separation is negligible. In As2Se 3 the L P and bonding bands are known ~1'~2 to overlap and no separation is observed in the optical spectra. Another possibility is to shift the second threshold peak towards higher energy, i.e. further separation of L P - b o n d i n g bands, as is the case of pure selenium 5 (Fig. 2). We deposited an approximately 100 ,~ thick pure selenium film on top of an asdeposited a-GeSe3 film. Strikingly, the apparent e 2 spectrum of the stack (a-Se/a-GeSe3) calculated in a two-phase model reproduces qualitatively the data for UV exposure. This experiment suggests that long UV exposure of as-deposited films produces a selenium-rich surface layer. However, SE data in the limited energy range are not sufficient to distinguish clearly the bulk photostructural changes. Further, the extrinsic effects such as oxidation need more careful analyses. Oblique deposition of films is known ~ to enhance the photoinduced effects but it is difficult to obtain reliable ellipsometric data on photoexposed a-Ge-Se films deposited at 80 ° owing to a considerable loss of reflectivity in the energy range of interest. 4. CONCLUSIONS The dielectric function spectra of amorphous GexSel-x films have been determined using SE. The experimental data are consistent with the earlier band structure calculations. SE has yielded significant information on the microstructure of a-Ge-Se films. Obliquely deposited a-Ge-Se films show a smaller variation in microstructure than a-Ge films. The dielectric function spectra of the films in the interband transition region are found to be sensitive to photoexposure. The SE data suggest a change in bonding configuration at the film surfaces on photoexposure. REFERENCES 1 K.L. Chopra and L. K. Malhotra, in D. Adler, H. Frirzsche and S. R. Ovshinsky (eds.), Physics of Disordered Materials, Plenum, New York, 1985, p. 215, and references therein.
2 S.Rajag•pa•an•B.Singh•P.K.Bhat•D.K.PandyaandK.L.Ch•pra•J.App•.Phys.•5•(•979)489. 3 D.E. Aspnes, J. C. Phillips, K. L. Tai and P. M. Bridenbaugh, Phys. Rev. B, 23 (1981) 816. 4
K. L. Chopra and S. Kumar, in M. A. Kastner, G. A. Thomas and S. R. Ovshinsky (eds.), Disordered Semiconductors, Plenum, New York, 1987, p. 327. 5 J. Stuke, J. Non-Cryst. Solids, 4 (1970) 1. 6 M. Lannoo and M. Bensoussan, Phys. Rev. B, 16 (1977) 3546. 7 T. Shimizu and R. Negeishi, J. Non-Cryst. Solids, 31 (1979) 287.
56 8 9 I0 11 12
s. KUMAR, D. K. PANDYA, K. L. CHOPRA S. Rajagopalan, Ph.D. Thesis, Indian Institute of Technology, Delhi, 1981. L.K. Malhotra, A. Kumar and K. L. Chopra, Thin Solid Films, 124 (1985) 309. S. Kumar, B. Drevillon and C. Godet, J. Appl. Phys., 60 (1986) 1542. H.L. Althaus, G. Weiser and S. Nagel, Phys. Status Solidi B, 87 (1978) 117. R. Zallen, R. E. Drews, R. L. Emerald and M. L. Slade, Phys. Rev. Lett., 26 (1971) 1564.
51 STUDY OF M I C R O S T R U C T U R E OF A M O R P H O U S Ge-Se FILMS USING SPECTROSCOPIC ELLIPSOMETRY* SATYENDRA KUMARt, D. K. PANDYA AND K. L. CHOPRA~ Thin Film Laboratory, Physics Department, Indian Institute of Technology, Hauz Khas, New Delhi 110016 (India)
The dielectric function spectra ~ =/31 --ig 2 of evaporated a m o r p h o u s G e - S e films in the energy range 2-5.2 eV have been studied using spectroscopic ellipsometry (SE). The ~ spectra of GexSel_~ (0.2~
I. INTRODUCTION
Amorphous chalcogenides, in contrast to a m o r p h o u s silicon and germanium, with their lower coordination and highly polarizable lone-pair (LP) orbitals exhibit electronic and structural flexibility which is responsible for m a n y observed phenomena such as the photostructural changes peculiar to this class of materials. Our laboratory has reported I a number of interesting phenomena when these films are exposed to band gap photons, energetic electrons and ions. In particular, photoexposure of a m o r p h o u s chalcogenide films leads to changes in the optical constants and a shift in the optical absorption edge 2. The studies 1 of optical constants in the vicinity of the absorption edge have yielded significant information on the role of various atoms in the chalcogenide network. A direct determination of the dielectric function in the interband transition region is expected to provide information on the electronic band structure and microstructural aspects of virgin and irradiated chalcogenide films. Ellipsometry being sensitive to the surfaces can detect small changes in composition and microstructure at the film surface. Spectroscopic ellipsometry (SE) has earlier 3 been employed to study the optical spectra of crystalline and glassy GeSe2 films. In this paper, we report the results of a detailed SE study on the optical properties and microstructure of a m o r p h o u s GexSel_ x films as a function of composition, angle of deposition and photoexposure. * Paper presented at the 7th International Conferenceon Thin Films, New Delhi, India, December 7 11, 1987. t Present address: Laboratoire de Physique des Interfaces et des Couches Minces, Ecole Polytechnique, 91128 Palaiseau C6dex, France. :~Present address: Indian Institute of Technology, Kharagpur 721302, India. 0040-6090/88/$3.50
© ElsevierSequoia/Printed in The Netherlands
52
S. KUMAR, D. K. PANDYA, K. L. CHOPRA
2. EXPERIMENTAL DETAILS
The bulk alloy glasses ofGexSel _x (at x = 0.20, 0.25 and 0.33) were prepared by melt quenching. Thin films of various compositions were deposited on one side of polished quartz substrates by evaporation of bulk glasses from a molybdenum boat in a background pressure of about 10-6Torr. Films of uniform thickness were deposited at various angles of deposition. Typical deposition rates were 5-10 ~ s 1 and typical thicknesses were in the range 1-2 I~m. Ellipsometric data in the spectral range 2-5.2 eV were recorded using a rotating polarizer spectroscopic ellipsometer (SOPRA model ES2G). The angle of incidence ~bof the light beam was kept constant at 70.0 °. Details of the SE data acquisition and analysis techniques are reported elsewhere 4. To study the photoinduced effects, the films were exposed to UV light using a 250 W mercury lamp at an intensity of about 30 m W c m - 2. In order to prevent heating of the films during exposure, the samples were kept on a water-cooled copper platform. 3.
RESULTS AND DISCUSSION
3.1. Effect o f composition The real and imaginary parts of dielectric function spectra of an as-deposited amorphous GeSe2 film are shown in Fig. 1. The extension of the data in the low energy range (broken lines) was estimated from the interference oscillations in the data calculated in a two-phase model 4. The ~2 spectrum is characterized by two distinct broad peaks at about 3.6 eV and about 5 eV. This do,able-peak nature is in contrast to the single-peak behaviour of amorphous films of tetrahedrally coordinated group IV and III-V semiconductors. The two-peak nature is a peculiarity of amorphous L P semiconductors such as selenium and tellurium 5. The first increase in the ez spectrum has been attributed 3'6 to electronic transitions from the LP states of the selenium atoms to the s-like antibonding lower conduction band and the second peak to the transitions between p bonding valence states and the s antibonding conduction band states. Our spectra are in good agreement with the data reported by Aspnes et al. 3 on evaporated glassy GeSe2 films. The slightly lower magnitude of ez in our case could be due to the difference in microstructure of the films. Figure 2 shows the composition dependence of the e2 spectra of amorphous GexSel -x films. With increasing selenium concentration the absorption edge shifts towards higher energy and the strength of the e2 spectrum in the interband transition region increases. With increasing selenium (decreasing x) the e z spectrum moves gradually towards that of selenium (also shown in Fig. 2) where the first peak should occur between 2 and 6 eV (LP band-antibonding) with the second peak starting at about 6 eV up to about 10 eV (bonding-antibonding). The threshold energy of the second peak can be seen to shift towards higher energy from about 4 eV for GeSe2 to about 4.5 eV for GeSe 4. A similar trend has been predicted from the band structure calculations 6'7. Further, the absorption edge becomes sharper with increasing selenium because of enhanced structural flexibility and associated removal of dangling bonds. The shift in absorption edge is also consistent with earlier observations 8 using standard reflection and transmission measurements.
STUDY OF
Ge-Se FILMS BY SPECTROSCOPIC ELLIPSOMETRY
8.0
o-Ge ' ~
DEPOST IED
Se2AS
E:I"
6.0
- -
GeSe
.... 6.0 ........
53
2
GeSe 3 Oe Se4
--
....
..._.~.;." ;" F,-"
4.0 E:2 2.0
~.0
2.0
l"
2 I.
I
I
2 O
i
I
3.0
l
I
S ,O
2.0
I
4.0 E(eV)
I
I
4 ,
3.0
6 I
4.0
8 i
10 I
s.o
E (eV)
Fig. 1. Measured dielectric function ~ = el
--i•2
spectra of an as-deposited amorphous GeSe 2 film.
Fig. 2. Imaginary part of the measured dielectric function spectra of as-deposited amorphous GexSel _x (x = 0.20, 0.25 and 0.33) films. The e2 spectrum of amorphous selenium is also shown 5.
3.2. Effect of obliqueness Figure 3 shows the ez spectra for a set of as-deposited amorphous GeSe3 (a-GeSe3) (Ge2sSev5) films prepared at various angles of deposition in the same deposition run. There is a large decrease in the magnitude of e2 for obliquely deposited films. Unlike a-Ge 4, films deposited below 60 ° do not show much change and are not shown in the figure. The shape of the dielectric function spectra is retained even in films deposited at 80 °, indicating that the basic bonding configuration remains unchanged on oblique deposition. Oblique deposition gives rise to an enhanced columnar structure with associated voids and dangling bonds compared with films deposited at 0 ° (normal incidence). An increase in defects causes tailing of the localized states at the band edges which is responsible for the observed decrease in slope of the absorption edge for obliquely deposited films. Similar data were obtained on films deposited with other compositions. 6.C A S DEPOSITED 0 - Oe Se 3
•
J
^
.
:;5:2(
_----_::-:
g }7" 0
i 2.0
I 3.0
I
E(eV)
I 4.0
I 5.0
Fig. 3. Imaginary part ofthe measured dielectric function spectra of a-GeS% (Ge2sSeTs) films deposited at different angles of the incident vapour beam.
The Bruggeman effective medium approximation (EMA) together with linear regression analysis (LRA) were employed to calculate the density deficit in obliquely deposited films. The dielectric function spectra of the film deposited at 0 ° were taken as the standard reference for the EMA-LRA calculations. The analysis of films
54
s. KUMAR, D. K. PANDYA, K. L. CHOPRA
deposited at 80 ° yielded a void percentage in bulk (density deficit) of 21.7%_ 0.7% ( + 0.7% being the 90% confidence limits). The value of the unbiased estimator tr was 0.012 and this indicates a good fit. The density deficit calculated by SE data analyses is in agreement with earlier s gravimetric measurements. It should be noted that a pronounced columnar microstructure accompanied by a large density deficit (about 40%) is obtained 4 in a-Ge films deposited at 80 °. In contrast, because of the network flexibility on introduction of twofold-coordinated selenium atoms, the void structure in the germanium-based chalcogenide films is considerably reduced. We should mention that the SE data were also recorded by rotating the sample on its axis as has been earlier reported 4 for a-Ge films. A biaxial optical anisotropy was observed in obliquely deposited films. However, the extent of anisotropy was smaller than that of obliquely deposited pure Ge films.
3.3. Photoinduced effects The dielectric function spectra of an a-GeSe3 (Ge25Se75) film deposited at 0 ° and exposed to UV light for l0 h together with those of an as-deposited film are compared in Fig. 4. The two broad peaks in the e2 spectrum, typical of as-deposited films, collapse into a single broad peak onphotoexposure. The magnitude of e 2 increases considerably in the lower energy range and decreases in the higher energy region compared with the spectrum of the as-deposited film. Corresponding changes in the optical constants are shown in Fig. 5. An increase in the value ofn on exposure has been reported by various workers 1'2"9. However, a decrease in the high energy region has not been observed in earlier photometric measurements. It should be noted that ellipsometry provides information about the sample volume depending on the penetration depth of photons. In our case, the data obtained for energy above 3.5 eV are from a sample thickness of less than 500 ~. E.1 ~ , 8.0 -
,, ',,
\
- ....
0 ° a - G e Se3 AS D E P O S I T E D UV E X P O S E D
/ / " - ~
--
AS
DEPOSITED
~
n
6,0
~
z,.O
2.0
........
k
g
o
/ 2.0
~
3. O E(eV)
I
I
4,0
5.O E(eV)
Fig. 4. Measured dielectric function spectra of a normally deposited a-GeSe3 film before ( - - ) ( - - ) long UV light exposure. Fig. 5. Optical constants (n and k) spectra of an a-GeSe3 film deposited at 0 ° before ( long UV light exposure.
and after
) and after ( - - - )
If we recall that the dielectric function is defined in terms of polarizability per unit volume, it is clear that the increase in magnitude of e2 on long exposure to UV light (Figs. 4 and 5) indicates either a more efficient packing of the basic tetrahedrai units Ge(Sel/2)4 or an increase in their polarizability at lower energies. A densification of about 10% would be required to account for the observed increase in
STUDY OF
Ge-Se FILMS BY SPECTROSCOPIC ELLIPSOMETRY
55
~2 at 3 eV which seems unlikely in the case of films deposited at 0 °. Further, densification alone does not explain the single-peak behaviour and the decrease in e 2 at higher energy. A change in polarizability would require a rearrangement in the bonding configuration. Aspnes et al. 3 have observed a single broad peak in the e2 spectrum of silver-photoactivated a-GeSe2 with an overall increase in the magnitude. They argued that the photoactivation by silver must be reducing the medium-range order by primarily interacting with selenium atoms. Further, the polycrystalline-toamorphous transition in silicon is also accompanied by a loss of the two-peak structure in the e 2 spectrum to a single broad peak with a concomitant shift in the absorption edge towards lower energy x°. This is also a clear evidence of the loss of long-range order. A single broad peak behaviour will also be expected if photoexcitation can induce a shift in the LP band so that the L P - b o n d i n g band separation is negligible. In As2Se 3 the L P and bonding bands are known ~1'~2 to overlap and no separation is observed in the optical spectra. Another possibility is to shift the second threshold peak towards higher energy, i.e. further separation of L P - b o n d i n g bands, as is the case of pure selenium 5 (Fig. 2). We deposited an approximately 100 ,~ thick pure selenium film on top of an asdeposited a-GeSe3 film. Strikingly, the apparent e 2 spectrum of the stack (a-Se/a-GeSe3) calculated in a two-phase model reproduces qualitatively the data for UV exposure. This experiment suggests that long UV exposure of as-deposited films produces a selenium-rich surface layer. However, SE data in the limited energy range are not sufficient to distinguish clearly the bulk photostructural changes. Further, the extrinsic effects such as oxidation need more careful analyses. Oblique deposition of films is known ~ to enhance the photoinduced effects but it is difficult to obtain reliable ellipsometric data on photoexposed a-Ge-Se films deposited at 80 ° owing to a considerable loss of reflectivity in the energy range of interest. 4. CONCLUSIONS The dielectric function spectra of amorphous GexSel-x films have been determined using SE. The experimental data are consistent with the earlier band structure calculations. SE has yielded significant information on the microstructure of a-Ge-Se films. Obliquely deposited a-Ge-Se films show a smaller variation in microstructure than a-Ge films. The dielectric function spectra of the films in the interband transition region are found to be sensitive to photoexposure. The SE data suggest a change in bonding configuration at the film surfaces on photoexposure. REFERENCES 1 K.L. Chopra and L. K. Malhotra, in D. Adler, H. Frirzsche and S. R. Ovshinsky (eds.), Physics of Disordered Materials, Plenum, New York, 1985, p. 215, and references therein.
2 S.Rajag•pa•an•B.Singh•P.K.Bhat•D.K.PandyaandK.L.Ch•pra•J.App•.Phys.•5•(•979)489. 3 D.E. Aspnes, J. C. Phillips, K. L. Tai and P. M. Bridenbaugh, Phys. Rev. B, 23 (1981) 816. 4
K. L. Chopra and S. Kumar, in M. A. Kastner, G. A. Thomas and S. R. Ovshinsky (eds.), Disordered Semiconductors, Plenum, New York, 1987, p. 327. 5 J. Stuke, J. Non-Cryst. Solids, 4 (1970) 1. 6 M. Lannoo and M. Bensoussan, Phys. Rev. B, 16 (1977) 3546. 7 T. Shimizu and R. Negeishi, J. Non-Cryst. Solids, 31 (1979) 287.
56 8 9 I0 11 12
s. KUMAR, D. K. PANDYA, K. L. CHOPRA S. Rajagopalan, Ph.D. Thesis, Indian Institute of Technology, Delhi, 1981. L.K. Malhotra, A. Kumar and K. L. Chopra, Thin Solid Films, 124 (1985) 309. S. Kumar, B. Drevillon and C. Godet, J. Appl. Phys., 60 (1986) 1542. H.L. Althaus, G. Weiser and S. Nagel, Phys. Status Solidi B, 87 (1978) 117. R. Zallen, R. E. Drews, R. L. Emerald and M. L. Slade, Phys. Rev. Lett., 26 (1971) 1564.