Vacuum/volume
47lnumber IO/pages 1211 to 121311996
Copyright Q 1996 Elsevier Science Ltd
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Pergamon
PII: 80042-207X196)00144-3
Holographic investigations of photoinduced
in PECVD Ge-Se thin films*
changes
V Boev,” E Slee~kx,~ M Mitkova,” P Markovsky,” P Nagelsb and K Zlatanova,” “The Bulgarian Academy of
Sciences, Central Laboratory of Electrochemical Power Sources, Acad G Bontchev Str, Bl. 70, Sofia 1113,
Bulgaria; bf?UCA University ofAntwerp 2020, Antwerp, Belgium; “The Bulgarian Academy of Sciences, Central
Laboratory of Optical Storage and Processing of Information, Acad G Bontchev Str, P 0 Box 95, Sofia 17 13,
Bulgaria
received 20 November
1995
The photoinduced changes in thin films of the Ge-Se system, where the ratio of Ge:Se = 1:2, prepared by
PECVD technology, are investigated by holographic methods.
The temporal dependence of diffraction efficiency at constant intensity of a laser beam is investigated for
scalar and polarization recording. A typical initial maximum is observed, attributed to a change in
transmission. The recording has amplitude-phase
character.
The maxima/ value of the diffraction efficiency in both cases is approximately IO-*%. The calculations show
that the photoinduced change in the refractive index In,,- n,,l z 2.1 0m3. In order to induce photoanisotropy,
longer exposure is needed and (n,,- n,( z 2.10p3. The electron microscopicanalysis
of the irradiated (exposure
E w 250 J/cm*) and non-irradiated sample does not give information of a substantial change in the film
topology.
The results obtained are compared to those of films prepared by vacuum-thermal evaporation. They show
that there is no substantial difference in the value of photoinduced changes. That indicates that the medium
order of the films that is associated with the method of their synthesis is not of considerable importance for
the promotion ofphotoinduced
changes. The results are interpreted in view of the theory of soft configurations
and defects in disordered systems. Copyright 0 ‘1996 Elsevier Science Ltd
Introduction
The photoinduced
changes in chalcogenide
glasses possess features that attract the researchers’ interest. They are associated
with the lone-pair electrons and low coordination
number of
the chalcogenide atoms that are the reason for strong electronphonon coupling and cause great mobility of a number of local
atomic configurations.
These phenomena have been investigated
in many aspects and, among the different chalcogenide
glasses,
the greatest attention has been given to the germanium chalcogenides.‘,2 The main object of these investigations
have been
thin films of the foregoing, prepared usually by vacuum evaporation.3.4 One of us has previously reported results on the preparation of thin films of the systems As-S and Ge-Se by plasma
enhanced chemical vapour depositions,6 and the great advantage
of producing films with exact stoichiometry
in a wide range of
combinations
of the two elements.
The present work reports on holographic investigations
of the
* Based on paper presented at the 9th International
School on Vacuum
Electron and Ion Technologies, 14-17 September 1995, Sozopol, Bulgaria.
photoinduced
phenomena in thin films prepared by PECVD. The
interest of the holographic technique as a tool for elucidating the
mechanism of photoinduced
changes, has increased due to its
high sensitivity and unique ability to follow the course of the
photoinduced
changes by measuring in real time the diffracted
light from induced gratings. Owing to its high sensitivity, the
holographic
method is especially suitable for analysis of low
sensitivity materials and thin films.
Experimental
The investigated
films were deposited on glass substrates in a
plasma discharge stainless steel reactor. The precursor gases were
high purity hydrides, H,Se and GeH,, pure or diluted in hydrogen
(15 mol % of the hydrides). A low pressure plasma, varying
between 0.1 to 1 mbar, was excited by an RF discharge (13.56
MHz) between two parallel plate electrodes, 8 cm in diameter as
described in Ref. 6.
The composition
of the films were determined by an electron
microprobe analyzer was close to the ratio Ge:Sex 1: 2 and film
thickness was about 1 pm.
1211
V Boev et a/: Holographic
investigations
of photoinduced
changes
A standard
interferometric
configuration
with a spatial
frequency of 200 mm-’ was used and the diffraction efficiency
measured in real time with a 1.7 mW He-Ne laser at A:= 632.8
nm. The arrangement
permitted both scalar and polarization
holographic recording as described in Ref. 7. The thin films were
investigated before and after illumination by scanning electron
microscopy.
Results
Figure 1 shows the change in transmission
with time at 1 = 488
nm and intensity of an Ar+ laser Z = 2.5 W/cm’. Initially, a strong
increase in transmission is observed after which it nearly restores
its original value in about 25 s. Further illumination of the sample
brings about an insignificant increase in transmission.
Similar
behaviour was observed by Song et ~1.’ The oscillations at the
beginning of the illumination are not fully understood.
We suppose that a non-radiative
recombination
occurs in the chalcogenide glasses that is released via self-trapped
excitons that
return to ground state after excitation (maybe thermal) caused
by the illumination,
i.e. these effects are due mainly to photostimulated change in covalent bonds and, hence, to the creation
of new defects. This hypothesis is supported
by the electron
microscope investigations
that proved no substantial change in
the film topology from illuminated (exposure E = 250 J/cm’) and
non-illuminated
samples (Figure 2(a) and (b)).
Figure 3 shows the temporal
behavior of the diffraction
efficiency in scalar recording. A typical maximum is observed,
brought about by the change in transmission,
the recording having amplitude-phase
nature. Further illumination
results in an
increase in diffraction efficiency and is explained, above all, by
the photoinduced
change in the refractive index. In this case the
maximum value of the diffraction efficiency is approximately
IO-*%. The calculations show that the photoinduced
change in
the refractive index is In, -n,l x 2.10m3. A possible explanation
of the mechanism of these phenomena
is offered by Fritzsche’s
model for the reversible and irreversible photostructural
changes
in chalcogenide glasses9 In chalcogenide thin films these changes
are promoted by the rapid localization of photo-excited
carriers,
the low energy of valence alternation defect pairs and the steric
freedom of low coordination
atoms of changing their positions
and bond configurations.
In these processes, the lone-pair elecwith each
trons play an essential role, ‘O changing interaction
other and the surrounding bonds due to the photoexcitation.
Figure 2. SEM photographs of (a) illuminated (exposure E = 250 J/cm2)
and (b) non-illuminated sample with composition Ge,, $e,, +
o.oqn
0
--rmTmTiTi?-“T
100
200
300
400
t, set
Figure 3. Temporal behaviour of the diffraction efficiency in scalar recording for the sample with composition
Ge,, $e,, 5.
0.00 ? I
0
it
1,
I-n,rrrTr-~7Tm,
40
‘_
80
120
160
t, set
Figure 1. Change in transmission with time at I = 488 nm and intensity
I = 2.5 W/cm* for the sample with composition Ge,, $e,* 5.
1212
Figure 4 shows the diffraction efficiency dependence on time
in polarization
recording. It is evident from the curves that in
order to induce photoanisotropy
(birefringence), longer exposure
is needed and (n,,-n,l x2.10-‘.
According to Fritzsche,”
the
light-induced
optical anisotropy
in chalcogenide
thin films is
produced by non-radiative
geminate recombination
events that
occur in the absorbing microvolumes
and cause local changes in
atomic bond configurations.
It may be suggested that polarized
light not only creates new defects, but orients the existing intrinsic
defects mainly in one direction along a particular optical axis.
V Boev et al: Holographic
investigations
of photoinduced
changes
- 6.0
0
‘;;
i
F
4.0
m
lg t, set
Figure 4. Diffraction efficiency dependence
on time in polarization
cording for the sample with composition
Ge,, See8 s.
re-
Adding the complexes to the chalcogenide glasses may enhance
these effects as we have found for thin films of Se,,Ag,Jis’*
in
which the maximum value of the photoinduced
birefringence is
An = 0.016, i.e. eight times greater than that of the samples that
we measured in the present investigation. That is probably related
to the appearance of Weigert-effect in these films.
The diffraction
efficiency behaviour
after cessation of the
illumination is shown in Figure 5. Relaxation processes, typical
of glassy materials are observed.
It may be shown that the dark relaxation of the photoinduced
changes is described by the fraction-exponential
law of Kohlrausch exp( - t/r)’ where 0 < y < 1 and r is relaxation time,
depending on the value of the photoinduced
changesI
Such a
relaxation dynamic is attributed to the structural elements, i.e. in
the beginning the smallest structural elements relax. It may be
assumed that these are selenium chains with limited length,
6.01
4.0-l -nrn
0
<
2
r(rrmrrrrTmm
10
20
t, set
.
r-rrpn-rrrrq
30
40
Figure 5. Diffraction efficiency behaviour after cessation of the illumination at polarization
(curve 1) and scalar (curve 2) holographic recording
for the sample with composition
Ge,, See8 5.
enabling the relaxation of increasingly bigger elements, which
are tetrahedral
units of selenium with germanium atoms. The
hierarchical limitation of the photoinduced
anisotropy dynamic
explains its slower decrease with time, as shown in Figure 5.
With these investigations
we practically observed all kinds of
photoinduced
phenomena, both in films of pure Se,14 and in GeSe films of various compositions,”
including those identical to
films investigated
and produced by vacuum evaporation.
The
results for the scalar recording show that the values of the photoinduced changes or films prepared by PECVD, have the same
order of magnitude as films produced by vacuum evaporation.
Bearing in mind that the method of film preparation
affects
mainly the medium order, I6 it may be concluded that it is not
of considerable
importance
for the photoinduced
processes in
chalcogenide
glasses. This is further proof for the applicability
of the suggested mechanisms, treating the phenomena within the
chemical bond, by which the open structure of chalcogenides
permits low coordination
atoms and coordination
defects to
swing and flip into adjacent void regions to form new band
configurations.”
Similar results are obtained on comparison
of the vacuum
evaporated As2S3 and ASS, as well as spin-coated As2S3 and ASS
thin films.18
Acknowledgements
This work was sponsored
by the Ministry of Education
and
Science, Contract NSF X-221, and Copernicus program, Contract ERBCIPACT940107.
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