Applied Surface Science 154–155 Ž2000. 135–139
www.elsevier.nlrlocaterapsusc
Polarization-dependent laser crystallization of Se-containing
amorphous chalcogenide films
V. Lyubin
b
a,)
, M. Klebanov a , M. Mitkova
b
a
Department of Physics, Ben-Gurion UniÕersity of the NegeÕ, Beersheba 84105, Israel
Central Laboratory of Electrochemical Power Sources, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
Received 1 June 1999; accepted 29 July 1999
Abstract
We report about the observation of influence of the light polarization on the photocrystallization process in the
Se 70 Ag 15 I 15 , elementary Se and Se 80Te 20 films. Irradiation with linearly polarized He–Ne and Arq laser light results in the
formation of polycrystalline films with strong optical anisotropy Ždichroism., the sign of which is determined by the
direction of the electrical vector of light. In the laser-crystallized Se 80Te 20 films we observed also the anisotropy of the
photoconductivity. The results obtained indicate that the laser-induced crystallization has a nonthermal character. q 2000
Elsevier Science B.V. All rights reserved.
PACS: 78.66.Jg; 42.70.Gi
Keywords: Amorphous chalcogenide films; Photoinduced phenomena; Photocrystallization; Polarization-dependent effects
1. Introduction
Amorphous chalcogenide films are known to exhibit several photoinduced phenomena, both scalar
Žphotodarkening, photorefraction, photodoping. and
vectorial, Žphotoinduced anisotropy, photoinduced
gyrotropy, photoinduced light-scattering. which are
connected with photoinduced structural transformations, defects creation and diffusion of metallic atoms
into the film w1–5x. These phenomena are already
applied in various fields of modern microelectronics
and electrooptics w1,3x. Another interesting photoinduced phenomenon is the photocrystallization first
)
Corresponding author. Tel.: q972-7-6461249; fax: q972-76472903.
E-mail address: vlyubin@bgumail.bgu.ac.il ŽV. Lyubin..
observed by Dresner and Stringfellow in amorphous
Se films w6x and then studied by different authors
w7–10x on various chalcogenide films.
Recently, studying the photoinduced processes in
the amorphous Se–Ag–I films, we observed a new
effect, i.e., the influence of light polarization on the
photocrystallization process and on the resulting
properties of the crystallized films w11x. This phenomenon was then observed also in amorphous selenium Ža-Se. films w11–14x.
In the present paper we report the results of a
detailed study of laser-induced crystallization of
amorphous Se 70 Ag 15 I 15 , elementary Se and Se 80Te 20
films. Laser-induced crystallization of the latter films
was shown to be accompanied not only by optical
anisotropy but also by the appearance of electrical
anisotropy.
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 4 3 7 - 7
136
V. Lyubin et al.r Applied Surface Science 154–155 (2000) 135–139
2. Experimental
Amorphous Se 70 Ag 15 I 15 films were prepared by
vacuum evaporation from a vaporiser with a small
opening described in Ref. w15x. The a-Se and Se 80 Te 20
films were fabricated by standard thermal vacuum
evaporation at a pressure of 3 = 10y6 Torr. The
thickness of all chalcogenide films was in the range
300–2000 nm. Study of photodarkening and photoinduced anisotropy was carried out using an experimental set-up with two linearly polarized laser beams
Žinducing and probing beams. illuminating the same
area of the film. A low-power He–Ne laser beam
Ž; 0.4 mWrcm2 at l s 633 nm. was applied as a
probing beam, while as a photoinducing beam we
used either an He–Ne laser beam Ž; 2.75 Wrcm2 ,
l s 633 nm. or an Arq ion laser beam Ž; 0.3
Wrcm2 , l s 488 nm.. The photodarkening was
recorded by monitoring the decrease of the transmitted light intensity. The photoinduced anisotropy Ždichroism D . was determined from the equation D s
2 Ž I I yI H .rŽ I I qI H ., where I I and I H are correspondingly the intensities of the probing beam transmitted through the film for the parallel and perpendicular polarized components of the electric vector
of the inducing beam.
For investigation of the anisotropy of the photoconductivity we used the samples in which three Au
electrodes were deposited on the oxide substrate
before forming the chalcogenide film ŽFig. 1.. The
electrical circuit shown in Fig. 1 was adjusted by
shifting the position of the illuminating light beam in
such a manner that the nano-amperemeter G showed
Fig. 1. Scheme of electrical circuit and electrodes used for the
study of anisotropic photoconductivity. Ž1–3. Electrodes; Ž4. position of the laser beam; V1 and V2, electrical batteries; G, nanoamperemeter.
a zero current by illumination of the sample with
nonpolarized light. By irradiation with strong Žinducing. linearly polarized light, the device G showed a
current if the photoconductivity became anisotropic,
as the electrical fields between electrodes 1–2 and
2–3 were directed either in parallel or perpendicular
to the electrical vector of probing light.
In the structural investigations we used the standard methods of optical microscopy, scanning electron microscopy ŽSEM. and X-ray diffraction ŽXRD..
All experiments were accomplished at room temperature.
3. Results
3.1. Se7 0 Ag 15 I15 films
The photodarkening under the action of both He–
Ne- and Arq- ion laser light was not observed in the
as-prepared Se 70 Ag 15 I 15 films, as seen from Fig. 2.
Excitation of such films by the linearly polarized
light led to the generation of linear photoinduced
dichroism with properties analogous to that observed
in the AsSe films studied previously w16x. At the
same time, storage of the films at temperature of
30–358C during 2–3 weeks or the artificial aging
Žannealing at 708C for 2 h. resulted in a noticeable
change of photoinduced processes in them. The prolonged photodarkening appeared under irradiation of
the films by both He–Ne- and Arq-laser beams ŽFig.
2.. The dichroism excited by the light with a constant direction of the polarization vector changed its
sign after 10–20 min of irradiation and grew slowly,
achieving very large values of D s 15–18% as shown
in Fig. 3. Relaxation of dichroism after interruption
of the exciting light Žat various D values. was not
observed ŽFig. 3.. Annealing of irradiated Žphotodarkened. films at a glass transition temperature of
558C and even at 80–908C for several hours did not
lead to a thermobleaching and to the destruction of
dichroism, opposite to the results in the AsSe films
where annealing was accompanied by the increase of
transparency Žbleaching of the film. and by the
disappearance of dichroism w4,5,16x. These results
allowed us to assume that we do not deal here with
the usual photodarkening and photoinduced anisotro-
V. Lyubin et al.r Applied Surface Science 154–155 (2000) 135–139
Fig. 2. Kinetics characteristics of transmission changes in as-prepared and thermally treated Se 70 Ag 15 I 15 film induced by linearly
polarized He–Ne and Arq laser light.
py but with another photoinduced effect, i.e., photocrystallization w6–10x. This assumption was confirmed also by structural investigations. X-ray analysis of the irradiated films showed for instance the
presence of Se, Ag 2 Se and AgI microcrystallites.
To check if the observed phenomenon is characteristic not only for amorphous Se–Ag–I films but
also for other amorphous chalcogenide films, we
investigated the films of ‘‘classical’’ amorphous materials displaying the photocrystallization phenomenon such as amorphous selenium Ža-Se. and
Se–Te films w6–10x.
3.2. a-Se and Se 8 0 Te2 0 films
The whole set of characteristics shown above for
the Se 70 Ag 15 I 15 films was also observed in the a-Se
Fig. 3. Kinetics characteristics of dichroism generation in thermally treated Se 70 Ag 15 I 15 film stimulated by linearly polarized
He–Ne laser light — horizontal Ž1. and vertical Ž2. directions of
the electrical vector. Dichroism relaxation after interruption of the
exciting light at various dichroism values Ž3–5. are also shown.
137
and Se 80 Te 20 films. As obtained by the deposition
process, these films did not display photodarkening
at room temperature, but only after a thermal treatment, i.e., annealing at 50–708C for 1 h, they could
be photodarkened by a He–Ne laser beam with
power 2.75 Wrcm2 . The annealed a-Se and Se 80 Te 20
films displayed large photoinduced dichroism, and
the sign of the dichroism changed with changing the
polarization direction of the inducing light to the
orthogonal one. Such reorientation of dichroism in
the studied films was very different from the same
reorientation in the AsSe films w4,16x. At the beginning of the experiment, the vertical polarization vector stimulated an increase of positive dichroism while
the horizontal polarization vector diminished the positive dichroism and led finally to a negative dichroism. In the next stages the reverse processes were
observed as shown for the Se 80 Te 20 films in Fig. 4.
Photodarkening and photoinduced dichroism were
stable, i.e., they did not relax in the darkness and
could not be erased in the process of annealing at the
glass transition Ž418C and 608C, respectively, for
a-Se and Se 80 Te 20 films. and at higher temperatures.
The appearance of photoinduced crystallization in
a-Se and Se 80 Te 20 films was confirmed by SEM and
XRD analyses.
Especially important is that in the Se 80 Te 20 films
Žwhich had a much smaller electrical resistance than
the Se 70 Ag 15 I 15 and a-Se films. we observed a
photoinduced anisotropy of photoconductivity. The
results of the study of the Se 80 Te 20 samples, using
Fig. 4. Kinetics characteristics of dichroism generation and reorientation in a thermally treated Se 80 Te 20 film stimulated by linearly polarized He–Ne laser beam with horizontal Ž x . and vertical
Ž y . directions of the electrical vector.
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V. Lyubin et al.r Applied Surface Science 154–155 (2000) 135–139
Fig. 5. Kinetics characteristics of photocurrent D I in a thermally
treated Se 80 Te 20 film induced by linearly polarized He–Ne laser
beam with horizontal Ž x . and vertical Ž y . directions of the
electrical vector.
the special electrical circuit, shown schematically in
Fig. 1, are demonstrated in Fig. 5. It can be seen that
the irradiation of the sample by the inducing linearly
polarized light beam leads to the appearance and
growth of a photocurrent D I in the circuit. Changing
the polarization direction of the inducing light was
always accompanied by corresponding reversible
jumps of the current. Kinetics of photocurrent is
quite analogous to the kinetics of the optical dichroism changes shown in Fig. 4.
4. Discussion
The polarization-dependent laser-induced crystallization was observed in the Se 70 Ag 15 I 15, a-Se and
Se 80 Te 20 films, showing that this effect is typical for
quite a number of selenium-based chalcogenide materials. Polycrystalline films prepared by linearly polarized light irradiation displayed a large linear
dichroism with a sign which depended on the direction of the electrical vector of light and was opposite
in sign to the photoinduced dichroism in the as-prepared films. In the laser-crystallized Se 80 Te 20 films
we observed also an anisotropy in photoconductivity.
To the best of our knowledge, this is the first reported case where the photoinduced optical anisotropy in chalcogenide films is accompanied also by
electrical anisotropy. Discussing the peculiarities of
the dichroism kinetics, we can assume that the early
stages of the process can be related to the kinetics of
polarization-dependent laser-induced crystallization
and the later stages are related to the anisotropy of
photoconductivity in the photocrystallized Se 80 Te 20
films.
The results obtained show that the laser-induced
crystallization has a nonthermal character and is
indeed a photonic process as the thermal process
cannot be polarization-dependent. The results also
can be related to the model of photocrystallization,
proposed in Ref. w7x which assumes a rupture of
interatomic covalent bonds parallel to the electrical
vector of photoinducing light, resulting in the directional growth of crystallites. This mechanism is discussed in our previous paper w11x. Large values of
photoinduced dichroism accompanying the laser
crystallization can be used in modern optoelectronics
and photonics w11x.
5. Conclusion
We have observed the dependence of laser-induced crystallization of amorphous Se 70 Ag 15 I 15 , elementary Se and Se 80 Te 20 films on the polarization
state of the inducing light beam. Polycrystalline films
formed as a result of irradiation by linearly polarized
light beam displayed large linear dichroism with a
sign which depends on the direction of the electrical
vector of light. Optical anisotropy was shown to be
accompanied by the appearance of photoconductivity
anisotropy. The results obtained support the model of
photoinduced crystallization based on the photoinduced rupture of interatomic covalent bonds.
Acknowledgements
The authors would like to thank Dr. T. Petkova
for preparation of the Se 70 Ag 15 I 15 samples. This
work was supported by a grant from the Israeli
Ministry of Science as well as by grant X-558 of the
Bulgarian Ministry of Education, Science and Technology.
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