LASER INDUCED OSCILLATORY PHENOMENA IN AMORPHOUS GeSe2 FILMS

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LASER INDUCED OSCILLATORY PHENOMENA IN AMORPHOUS GeSe2 FILMS

J. Hajtó

To cite this version:

J. Hajtó. LASER INDUCED OSCILLATORY PHENOMENA IN AMORPHOUS GeSe2 FILMS.

Journal de Physique Colloques, 1980, 41 (C4), pp.C4-63-C4-69. �10.1051/jphyscol:1980410�. �jpa- 00219925�

JOURNAL DE PHYSIQUE Colloque C4, suppMment au n ^O 5 , Tome 41, mai 1980, page C4-63

LASER INDUCED OSCILLATORY PHENOMENA IN AMORPHOUS GeSe2 FILMS

J. HajtB

Central Research Institute for Physics H-1525 Budapest, P. 0. B. 49, Hungary.

Abstract.- Studying the optical properties of 5 p to 10p thick GeSe2 glassy films under the influence of focussed, continuous He-Ne laser beams a region of medium intensity 11.4 to 2.7 kb7/crn2) is found where a low frequency (3 to 5 Hz) periodic pulsation in the absorption coefficient sets in. A tentative explanation is sugges- ted in terms of laser induced self-trapped exciton states of the material.

1. Introduction.- The oscillatory behaviour of the transmittance of amorphous GeSez films illuminated by a continuous He-Ne la- ser beam was reported in a recentletter/l/.

Oscillation can be observed when the inci- dent light intensity is within a certain range (1.4-2.7 kW/cm2 for a 6.4 thick film for example). The frequency (3-50 Hz) and amplitude of oscillation depend on the.in- cident light intensity, an increase in the laser power is followed by an increase in the amplitude of the transmittance change and a decrease in oscillation frequency.

Two models were presented to explain the observed oscillation which are: a) the ef- fect is due to reversible amorphous-crys- talline-amorphous phase changes /I/; b) os- cillation is connected with the appearance of total internal reflection due to diffe- rent metastable and excited states in the material /2/.

Although the physical grounds of the two suggested mechanisms are different, both ones require a simultaneous increase of reflexion or light scattering duringthe decreasing transmittance period of oscil- lation. It has been shown in a recent pa- per /3/ that there are no phase difference in the reflected, transmitted and scattered light signals during the oscillation i.e.

the decrease in the transmittance is accom- panied by a simultaneous.decrease of re- flectance and scattered light. Therefore the origin of oscillation has not cleared yet.

This paper is concerned with the in- vestigation of optical properties (trans- mittance, reflectance and scattering) and

temperature of vacuum evaporated GeSez under the influence of laser irradiation.

2, Experimental.- GeSe2 films were vacuum evaporated onto water cooled silica subs- trates. Deposition rates usually ranged

0

from 20 to 40 A/s, the thicknesses were measured by a quartz crystal monitor during evaporation.

The temperature of the films was mea- sured using vacuum evaporated Au-Ni ther- mocouples /4/

The continuous beam of a 30 mW output

0

power He-Ne laser (1 = 6328 A) was focussed on the samples to a spot diameter of 15-20 microns. The incident light intensity could be varied by a polarizer and the transmit- ted, reflected and scattered light signals were displayed on a storage oscilloscope.

In this way in situ measurements on the optical data could be performed during the oscillation.

3. Results.- The changes induced by laser irradiation can be of three kinds (see Table I) depending on the input power den- sity /2/. The crystallization is not dis- cussed in this paper.

Table I, Processes caused by laser light of diffe- rent intensities in 6p thick a-GeSe2 films

~ntensity (kw/cm2 Response 0.001-1.4 Photobleaching 1.4-2.7 Oscillation over 2.7 Crystallization

3.1. gho_tgbleachinq.- Starting with modes intensities the first effect of interest is the "photobleaching" of the films even without focusing of the laser beam. This

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980410

JOURNAL DE PHYSIQUE

means, for example an increase in transmit-

0

tance from 30 % to 70 % (at X = 6328 A) in the case of layer of 6.4 p thickness. In polarized light the photobleached spots are darker (Fig. lb) or brighter (Fig. lc) by rotating the analyzer than photographed in non-polarized light (Fig. la). This fact indicates that illumination of the film leads to its becoming optically ani- sotropic which is to our knowledge the first observation in evaporated chalcoge- nide films.

Figure 1. Microphotos of photobleached spots in GeSen film.

From a = 10' cm-I to a = 10' cm-' the optical absorption coefficient varies exp- nentially with the photon energy as

a = aoexp[rfiw] (Urbach edge) (Fig. 2)

.

Illumination causes a permanent shift of the Urbach edge to higher photon energies and an increase of the slope of the edge.

The shift of the absorption edge of the as deposited films is irreversible and perma- nent, heat treatment of the layers causes a similar shift. de Neufville as presented a "localized heating" model for the rever- sible photostructural. change observed in chalcogenide films /5/ assuming that the reversible photo induced structural change is characterized by an increase in random- ness in atornic configurations and a ther- mal recovery corresponds to the restora- tion of the initial short range order. This mechanism is not applicable for the irre- versible changes observed in our case, because even at very low incident light intensities (I<1 K/cm2) photobleaching occurs. At intensities below 1 \v/cm2 the

direct heating effect of the beam is negli- gible as measured by evaporated Au-Ni thermocouples.

- as depostted

--- ~ l l u m ~ n o t e d

- - - h e a t treated (200°C 2hr)

crystalhne GeSe2

Figure 2. Absorption coefficient versus photon energy of GeSe2 films.

The other optical parameter of inte- rest was the refractive index (n) as a function of photon energy between 1 and 2 eV. In this low absorption portion of the optical spectrum n2 (w) = E 1 (w)

.

was calculated from the measured reflexion and transmission data using the method of Brattain and Briggs / 6 / where multiple reflexions were taken into account. n(w) can be fitted by the T~Ternple-Di Domenico /7/ dispersion relation-ship.

€ 1 (w) = n2 (w) = 1 + ^{E E} d 0 / [E;'

-

fitting constants which measure the oscil- lator energy and strength respectively.

Eo for chalcogenide glasses corresponds to the mean energy of transitions from valence band (lone-pair) states to conduc- tion band states /8/. Ed measures the

strength of interband optical transitions and is related to the charge distribution within each unit cell. E primary depends

d

on the coordination number and valency.

By plotting (n2-1) versus (ha) (Fig. 3)

Figure 3. (n2-1 )

-'

(-l/Ed Eo). It is clear that the decrease in n accompanying exposure and annealing of GeSe2 films is associated with an in- crease in Eo rather than be a change in Ed (Table 11).

Table 11. Index of refraction (at 6 w = 1.96 eV) and oscillator parameters of GeSes films Sample E~ ( e ~ ) E~ ( e ~ ) n An=nl-nz

1 as deposited 4.91 22.96 2.472

2 exposed 5.1 23.0 2.451 -0.021 3 annealed 5.12 23.0

The measured changes on the absorption coefficient and the refractive index indi- cate a reduction of the disorderness of atomic bounding between the neighrest and second neighrest rleighbours due to the evaporation process and therefore a decrea- se of the extent of the localized tail states adjacent to the band edge (increase in the slope of the absorption edge and in the oscillator energy Eo).

3.2. Self induced oscillation of light absor~tjgg.- To produce pulsations in the transmitted and reflected light the inci- dent power density should be increased until a dark centre develops in the middle of the image. This can be distinguished from crystallization since on reducing the intensity it disappears completely, without delay, leaving no sign of permanent phase changes. On the other hand, the intensity (averaged over the 20 p spot) should not exceed an upper limit (about 2 - 7 kh;/cm2 for 6 thick films) otherwise the central region quickly becomes crystalline and thereafter doesn't show further optical changes.

The transmittance, reflectance and light scattering pulsations measured simul- taneously have the same character (Figs.

4-g). ^A decreasing of transmittance is accompanied by a simultaneous decreasing of the reflectance and a same decrease of the scattered light independently on the angle of scattering.

Fig. 4. Transmittance (upper signal) and reflec- tance (lower signal) pulsations under focussed continuous He-Ne laser beam (I = 1 .9 kw/cm2)

.

Ordinate: intensity of transmittance and reflec- tance; abscissa: time 200 ms/div.

Using the expressions of Bruttain and- Briggs /6/ one can calculate the changes of the absorption coefficient and refrac- tion index of the films from the measured transmittance and reflectance changes during the oscillation periods (Pig. 6).

The absorption coefficient of the GeSe2 films has two characteristic values during the oscillation period, Both level B

( a = 2.4 x lo2 cm-l) and level C ( a = 5.1 x 10' cm-')(see Fig. 6) are metastable

C4-66 JOURNAL DE PHYSIQUE

states of the material and exist only It is possible to measure the tempe- under the influence of intensive laser rature of the GeSez films during oscilla- irradiation. The value of the absorption tion using vacuum evaporated thin film coefficient decrease to the stable photo- Au-Ni thermocouples because the thermocou- bleached state (level A: a = 1.9 x 10~cm-') ple transparent enough to detect the

transmitted laser beam (Fig. 7).

Au d=100 8.

silica substrate

Fig. 5. Transmittance (A), small angle (B) and large angle (C) scattered light pulsations under focussed continuous He-Ne laser beam (I = 2.5 kw/cm2)

independently of the momentary state of the oscillation when the exciting laser light is switched off. The refractive in- dex changes in opposit direction as the absorption coefficient and has a minimum value as a reaches the maximum indicating that a real absorption change does occur during the oscillation period.

light c l i ht ligM

6 ^{of? on} t off

c 4 'E

I

" ^t

Fig. 6. Oscillations of absorption coefficient and refractive index (at fiu = 1.96 eV) under the influence of continuous laser irradiation

(I = 2.7 kw/cm2)

2.4

2.3

..*

2.1

Fig. 7. Experimental arrangement for simultaneous observation of transmittance and temperature oscillation.

-

-

1

-wpm r

-

I

The thermovoltage and the transmittance were recorded simultaneously on a storage oscilloscope (Fig. 8). As the transmittan- ce increases the temperature of the GeSe2 film decreases exponentially and the decrease of the transmittance (increasing absorption) is accompanied by a simulta- neous exponential increasing of the tempe- rature.

Fig. 8. Transmittance (A) and temperature of GeSen film under the influence of laser irradia- tion (I = 2.7 kw/cm2.)

Oscillation can't be induced usingthe

0 0

green (A = 5145 A) and blue (A = 4880 A) wavelengths of a continuous Ar-ion laser, The irradiated spots become darker with increasing intensities without any sign of oscillation and then changed over to crystalline state.

We tried to induce similar pulsations in 6 p thick self-supporting films (remo- ving the substrate) at the same experimen- tal conditions. Permanent photobleaching was obtained but no transmittance or reflectance pulsations occurred. Instead of any oscillatory response the transmit- tance of the films decrease gradually under the influence of continuous laser irradiation (Fig. 9). The duration of :steps is depending on the illumination condi- tions, The steps are shorter (second si- gnal in Fig. 9) when the sample is illumi- nated again 0.8 s after the first gradual decreasing of the transmittance (first signal in Fig, 9) and are longer when the sample was kept in darkness for 8 s (third signal in Fig. 9).

Fig. 9. Step by step decreasing of the transmit- tance of the GeSe:! film under the influence of continuous laser irradiation (I = 0.5 kw/cm2j

4. Discussion.- 4.1. Photob1eachinq.- The energy of laser light ( f i w = 1.96 eV at

0

A = 6328 A) used to induce the photoblea- ching and oscillation is slightly less than the value of optical energy gap of GeSez films

(e

.

'3

assumption that the electrons are mostly excited from the localized tail states of the valence band edge is plausible.

In the freshly evaporated GeSez films a large number of defect states are expec- ted (Ge-Ge, Se-Se bonds, dangling bonds /9/ which contributes substantionally to the observed absorption edge, Upon illumi- nation or annealing a local rearrangement of these defect states (weaker bonds) occurs forming stronger Ge-Se bonds. The extent of the localized tail states decrea- ses as a result of the reduction of the defect states. Therefore one can observe a shift of the absorption edge to the direction of bulk well annealed glass sta- te (see Fig. 2). The slope of the Urbach edge increases upon illumination or annea- ling indicating the decrease of internal electric fields created by charged impuri- ties in the material /lo/.

The increase of the mean energy (E

0

see Table 11) of transitions from valence band (lone-pair) states to conduction band states similarly should be attributed to the reduction of the localized tail states.

This conclusion is supported by the fact that the oscillator strength constant (Ed) haven't changed upon illumination indica- ting that there is no sufficient change in the average coordination or in the char- ge distribution i.e. only the defect

states are involved in the photobleaching.

4.2. Absorption oscillation.- It is clear from the measured optical data that a real absorption oscillation occurs at the low absorption region of the Urbach tail in the GeSez films.

There are two important features of the oscillation: a) oscillation can't be induced using higher energy light

( 6 w = 2.2-2.4 eV) ; b) there are at least two optically metastable states existing only under the influence of intensive la- ser irradiation therefore the effect of temperature has to be considered.

According to the Onsager model /11/ after photoexcitation the electron and hole will diffuse apart to a distance R deter- mined by the diffusion constant and the thermalization time which is proportional to the excess kinetic energy. The bonding energy between the electron and hole is

C4-68 JOURNAL DE PHYSIQUE

e2/4 If R is larger than the critical distance Ro where e 2 / 4 ~ ~ ~ o = kT then the electron and hole will diffuse apart.

Because of the energy of excitation used in our experiments (?iw = 1.96 eV) repre- sents a low energy excitation for the GeSez glasses i.e. R < Ro exciton forma- tion occurs. The mobilities in GeSe2 are less than 1 cm2/volts /12/ thus the charge carriers (excitons) move slowly and spend a comparatively long time at a given site between hops. Due to the strong electron- phonon coupling a metastable self trapped excition is formed. The configurational coordinate diagram for the excition has been described by /13/ and used by /14/

for the explanation of photostructural phenonena in chalcogenides (Fig. 10)

.

Fig. 10. Configurational coordinate diagram for chalcogenide glasses.

In this model two distinct mechanisms of optical excitations are expected. One is a transition to the uncoupled exciton state and the second is to the self trap- ped excition state. We assume that the

increased absorption during the oscilla- tion could be accounted to the formation of self trapped excitofi states gene- rating by high intensity low energy exci- tation of continuous laser light. Because of the higher absorption, the temperature of the film raises too. At higher tempera- ture the thermal recombination of self trapped charge carriers becomes important.

As a result of thermal release of self trapped excited states the absorption de- creases again because the energy of ground state lies below the energy level of the self-trapped excition states (see Fig. 10).

As the absorption decreases the temperatu- re decreases too (see Fig, 8 ) , the material cools down and the process should start again resulting in a continuous oscilla- tion of light absorption.

The quantitative treatment of the oscillation has not been completed so far, we present this model merely as an interes- ting example for consideration. This qua- litative model is in good accordance with a number of observations an amorphous GeSe2 and it explains why this particular material is efficient in producing a pe- riodic pulsing effect. Amorphous GeSe2 has T x 400 OC high enough to resist crystal-

9

lization under fairly intense light irradiation. Its optical absorption edge varies just around the wavelength of the exciting He-Ne laser. Due to its large Se content the fraction of dangling bonds can rise considerably and in the glassy state the narest neighbour distance has a greater degreee of freedom to change accor- ding to the dominant charged defect states, than it would have in the crystalline phase. Indeed the corresponding density fluctuations in the irradiated volume element have been observed by scanning electron microscopy in a-GeSez /15/. In principle, other amorphous chalcogenide compounds, having an absorption edge in the visible range should behave similarly.

we believe that the clue of the oscilla- tion is laying on some unique properties of the amorphous state, More work is needed in this field to find other subs- tances or modifiers to GeSez raising its sensitivity.

Acknowledgements.- The author wish to thapk I. Kdsa Somogyi and J. Gazs6 for their interest and help in connection with the work and G. Zentai who helped in re- solving the experimental diffdculties involved in the work.

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by B.. 0. Seraphin (North Holland Publ.

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