Amplitude of photostructural changes in chalcogenide vitreous semiconductors

Solid State Communications, Vol 51. No 8. pp 6 4 7 - 6 5 0 , 1984 Pnnted In Great Bntalla

01/38-1098/84 $3 00 + O0 Pergamon Press Ltd

AMPLITUDE OF PHOTOSTRUCTURAL CHANGES IN CHALCOGENIDE VITREOUS SEMICONDUCTORS V K Mahnovsky. A P Sokolov and V G Zhdanov Institute of AUtolnatton and Electrometry, Siberian Branch of the U S S R Ac Scl 630090, Novoslblrsk, Unlversttetsky p r , I. U S S R (Received I 0 March 1984 by G S Zhdanoi') it is shown that a position of an optical absorption edge (OAE) of amorphous AsSe and As2S3 films irradiated b5 hght up to saturation is independent on Tex p the temperature at wtuch the sample is exposed, and the amplitude of a reversible photomduced shift o! OAE ,..XEis determined by its thermal shift ,..kEas It IS heated from Texp up to the glass-transition temperature Tg So, in order to obtain maxmaum photomduced changes we need to use materials wlth maximum thermal variations of the forbidden zone width, and to tend to a greater difference between Tg and Tex p The obtained results are well explained althln the scope of the local heating model REVERSIBLE PHOTOINDUCED changes of the optical properties have been discovered ahnost in all of chalcogenlde vitreous semiconductors ChVS [ 1 , 2 ] The amplitude of the photomduced changes depends on the temperature of a sample, intensity and spectral composition of excitation hght [ 1 - 4 ] It has been shown before [3,4] that optical properties of a ChVS film illuminated up to saturation does not depend on the prehlstory, ~ e on the original states, and a shaft of the optical absorption edge (OAE) o f the sample exposed at some telnperature Tex p increases with decreasing Texp [1,3] If we measure the absorption coefficient ot o f the sample illuminated up to saturation at the salne temperature T = Tex p at winch it was exposed, then some new regularities of photodarkenmg are discovered position of the optical absorption edge of the Ilhanunated sample is practically Independent on the exposure temperature T~xp (Fig 1). The amplitude of the photolnduced shaft of the OAE over the temperature range T~xp = I 0 0 - 3 0 0 K was studied m this paper Vacuum evaporated and wellannealed (at T = 180°C dunng two hours) AsSe and As2Ss samples were used. The absorption spectra were measured with a "'Specord UV-VIS'" spectrometer As the AsSe sample is cooled the optical absorpuon edge shafts towards l'ugh energies la ~ 2a ~ 3a ~ 4a (Fig 1 ) Subsequent exposure at the gwen temperatures (293, 2 0 0 , 1 0 0 K) leads to photodarkenlng. In this case. as can be seen from Fig 1, for the fllununated sample the absorption edge posmon, measured at the temperature of exposure, does not change (2b, 3b, 4b). only the inchnatlons of the Urbach taft are different Note that the OAE of the annealed film occupies the same positron at T = 430 K

The Ineasurements taken x~tth As:S3 films revealed a similar dependence of photodarketung on the exposure temperature But for AszS3 the position of the absorpnon edge of an illuminated sample corresponds to the position of the OAE of annealed film at T = 400 K (Fig 1 ) Our results are in agreement with those obtained before m paper [5]. where it ~as shown that over the temperature range Texp = 20 - 8 0 ° C the absorption coefficient of the illuminates As2S3 film does not depend on exposure T From the above results x,,e can conclude that position of the optical absorption edge of the exposed sample corresponds to position of OAE of the annealed film at some characteristic temperature T~ (Tk ~ 430 K for AsSe and Th "~ 400 K for As2Sa) and Is nearly independent on Texp We tr3, to understand the meaning of this experllnental fact in the k,netlc theory of v~tnficat~on the concept of "'fictlve" temperature T r IS Introduced to characterize an individual property P of vureous state T r is defined as the temperature at whach equilibrium hquld has the same value of the property Pas the given glass (Fig 2) [6, 7] As the external parameters vary sharply the system relaxation to a new equilibrium state occurs for the fimte ume r The relaxation time is "r ~ exp (V/kT) [6] where I," is some energy barrier, and it mfinltel~ grows as the temperature decreases At some temperature ~"becomes more than the characteristic time of cooling,and the structure "freeze'" in the nonequlhbrlum state Naturally, the more the cooling rate q, the hagher the "'fiCtlVe" temperature T t It is qualitatively reflected on the P - T diagram (Fig 2), where OD are the properties of eqtttllbrlum liquid, OA are the properties of wellannealed and slow-cooled glass, OB and OC are the properties of cooled samples at various rates of cooling

647

648

AMPLITUDE OF PHOTOSTRUCTURAL CHANGES P

]

s Se / 4~10 ~

"5

As z

_l

.'.,.,7,o/

,o;17,, s# #

2b N,,,/l= [ h ~ l

/

4~'o"i~/j l

"

Pi P 18

S~I/if i

S

",,,,r,,',"1,o/p,y.I I/ 2

2xlO ~

T

¢/)

Vol. 51, No 8

I

2o

I E(eVi

Fig 1 Absorption coefficient for at temperatures 430 K - I a, 300 100 K - 4a and illuminated AsSe 300 K - 2b, 200 K - 3b, 100 K As2S3, except for I a - 400 K

:>2

C

n

!

1

Eoot {

.N

__

~'/,

2,4

the annealed AsSe film K - 2a, 200 K - 3a, films at temperatures 4b. Similarly, for

The kinetic v,mficatton theory takes on as a relaxation parameter, such properties o f glasses which variations, if only over a short temperature range, are proportional to the temperature variations [6]. Density, length of a sample, refractive Index, etc are usually used m the experiment. In our case for the property P we can choose Eop t - optical width o f the forbidden zone It vanes proportionally to T over a rather large temperature range Under slow cooling from the melt the variations o f E o p t correspond to the curve OA (Fig 2) While under sharp cooling, depending on the rate of cooling, OB, OC, etc can be realized which is really observed m the experiment as a shift of the optical absorption edge towards the long wavelength region (darkening) [ 8 , 9 ] . In so doing, the Increase in the cooling rate is accompanied with increase In the amphtude of the long wavelength shift AE [8, 9] We suppose that similar processes occur in chalcogenide vitreous semiconductors m the case of photostructural translormattons The results presented In Fig I can be easily explained within the scope of the local heating model [3] It is based on the assumption that absence o f a long-range order is amorphous semiconductors can lead to localization o f the energy o f nonradiative recombination o f photoexcited carriers The released energy hco ~ 2 0 - 2 5 eV locahzes in the region wtuch size is of the order o f the structural correlation range R c ~ 5 - 1 0 A [10, 11 ] This assumption is supported by previously obtained estimations of value of the mtcroreglon whach transforms to a new state as a hght quantum is absorbed (v) ~ 5 x 10-22-5 × 10 -23 c m 3 [3] So, the nonradlatwe recomblnauon In amorphous semiconductors results in high level o f excitation of the irucrovolume (i> 5 x 10-2eV per atom), ~,hlch corresponds to a high amphtude o f v,brahons o f atoms

T,L T,M Tm

Tq

Z

Fig 2 Dependence o f the property P on temperature at various coohng rates q (qA < qB < qc). Ta is the value o f "fictlve" temperature o f the property P m the t state This excitation can be considered as heating o f this microregJon (v) (local heating) up to T > T a In so doing the optical properties of the mlcroregion (v) are close to those o f the heated sample. The next step Is sharp coolmg o f the rmcrovolume (v) due to energy dismpatlon to surrounding glass network (a curve similar to OC is reahzed, Fig 2), which results in freezing of structure with properties eqmvalent to the e q m h b n u m structure properties at the temperature close to the glass-transition temperature. The coohng rates reahzed In t i m case are, probably, maximum possible and at temperatures of the sample T ~ Tg they weakly depend on T Together with decreasing Eop t the photostructural transformations are accompanied with reversible changes m the refractive index M and sample thickness d [ l , 4] The analysis of the results obtained in [4] shows that the value (nd) o f exposed .66253 samples remains nearly constant for vanous exposure temperatures and corresponds to Tk ~ 400 K, which is in a good agreement with our results Freezing of structure near the glass-transmon temperature, arising under local heating should lead to increase In an average i n t e r a t o r m c &stance that corresponds to its changes as the e q m h b n u m structure is heated up to T ~ Ta, which provides a macroscopic increase in the sample thickness Ad/d

= f3AT = {3(Tg - rexp),

(l)

where 13is a thermal expansion coefficient, and T e x p is the temperature at which the illumination occurs Substituting for As2Sa the value/3 = 2.46 x 10 -s (l/degree) [12] and Tg = 450 K, we obtain A d / d 039%atT=298KandAd/d=071%atT=163K, which ts m a good agreement with the results o f [1 ] (0 3 8 - 0 51% and 0 71%, respectively). The local heating mechanism connects the properties of the samples exposed up to saturation to the

Vol. 51, No 8

649

AMPLITUDE OF PHOTOSTRUCTURAL CHANGES

Table I

A E ( 1 0 O K ) e V x 10 -3

AE ( 2 0 0 K ) eV x 10 -3 A E ( 2 9 3 K ) e V x 10 -3 Tg CC)

ASloSgo

ASlsS82

As2.TS73

AS3sS65

As~S6o

GeS2

86---5 70 +--5 15 -+ 5 30

93"+5 71 + 5 22 -+ 5 80

100"+ l0 80 -+ 5 38"+ 5 130

110"+5 85 -+ 5 42"+ 5 170

130"+ 10 90 +- 10 45 -+ 5 180

-

45O

40(3

~

35c

_

/ / /

3OO

250

I

I

I

I

iO

20

30

40

As(%)

Fig 3. Value of the parameter Tk for AsxSi-x composition exposed at temperatres (o) - 293 K, (~) 200 K, (t~) _ 100 K There are also values of the temperatures I - glass-transition Tg [15] and 2 - optical recording erasure Ten [4] for the above compositions values of the same properties of the annealed structure at "fictwe" temperature Tk ~ Tg Thus, the variation of the glass-transition temperature should lead to corresponchng varmtlon m Tk Correlation of the dynamical range of photodarkenmg and Tg for GexSel-x composmons has been described earlier in [I 3] We have measured the photomduced shift of the optical absorption edge for ASxS l_x (0 I <~x ~< 0 4) compositions at T = 2 9 3 , 2 0 0 and 100 K. The obtained results are gwen in Table 1, where AE was measured at the level ofct = 104cm -~. At T = 293 K the results agree with those given in [4, 14] the magnitude of the variations is proportional to the arsenic content Th~s resulted m the erroneous conclusion of the authors of [14] on the mfluence of the A s - A s bonds on photostructural transformations At lower temperature. however, the amphtude AE depends, but weakly, on the composition and varies 1.2-1.4 times as x increases 4 times, wtuch indicates to the absence of correlation between the magmtude o f photodarkenmg and As content. On the other hand, the obtained values o f T~ correlate with dependence o f Tg on the composition [15] (Fig. 3). The best agreement Is observed between the parameter 7"# and the temperature of optical

75 470

recording erasure Ten [4], which In the gven case characterizes the energy barrier V or relaxation of optical properties The results obtamed for vanous composluons revealed that the position of the optical absorption edge (characterized in tlus case by the parameter Tk) appears to be dependent on the temperature of the sample under dlummatlon (Fig 3) The dispersion of Tk(T) for AsxSi-x Increases as x decreases Tius regularity can be seen from the diagram in Fig 2 as the As content decreases the glass-transition temperature approaches T = 293 K, which corresponds to approaching the point N on the curve OC to Tg The dispersion of the "'fictive" temperature Tf(T) in this case increases sharply (Fig 2) The given results illustrate that the value AE of photomduced shift of the optical absorption edge is determmed by thermal variation in Eopt as the sample is heated from the exposure temperature to the glasstransition temperature This provides increase in the dynamical range of photomduced changes with decreasing temperature Thus, to obtain maxamum photoreduced changes in optical properties of ChVS one should use materials with the maximum coefficient of thermal vanatton of forbidden zone width dEg/dT (such as AsSe) and tend to have a greater difference between the exposure temperatre and the glass-transition temperature From the compositions presented m Table 1 it can be seen that GeS2 has the maximum magnitude of photomduced changes (the data are taken from [I I), which in our opinion, is associated to a high value of

T~ [161 In conclusion it should be noted that photostructural transformations are inherent in many amorphous semiconductors, they have been discovered almost in all ChVS This phenomenon is based on the nature of amorphous state - the absence of a long-range order, which can lead to energy localization under nonradiative recombination of photolnduced carriers The local heating model [3], that takes into account a specific character of the amorphous structure, allows to explain the main features of optical recording in chalcogemde vatreous semiconductors, particularly, the position of the absorption edge of exposed samples and the dependence of the magnitude of the photomduced changes on temperature and composition of ChVS

650

AMPLITUDE OF PHOTOSTRUCTURAL CHANGES

Acknowledgements - The authors thank L V Samsonova for preparing the ChVS samples

7 8

REFERENCES 1. 2. 3 4 5 6.

K Tanaka,J Non-Cryst Sohds 35 & 36, 1023 (1980) V.L. Averyanov, A.V Kolobov, B.T. Kolorruets & V M Lybm,J Non-Cryst Sohds45,343 (1981) V G Zhdanov, V.K Mahnovsky & A P Sokolov, A vtometrtya NS, 3 (I 981 ) V.G Tsukerman, m Novle regtsmruyushtes sredt dlyagolografit, p.45 Leningrad (1983) K Kamura, H. Nakata, K Murayama & T. Nmomtya. Sohd State Commun. 40, 551 ( 1981 ) 0 V. Mazunn,Steklovame t stabthzatso'a neorgamschesktkh stekol, p 62 Nauka, Leningrad (1978)

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