Wahab, L A

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Indian J. Phys. 68A (6), 521 -  528 (1994)U P  A— an inicmaiionaJ journalCharacterization of chalcohalide (A siS63)9515 semiconductor glassL A WiihabN a t i o n a l  C e n t e r  t o r  R a d i a t i o n  R e s e a r c h  a n d  T e c h n o l o g y ,  N a s r C n y ,  C a i r o ,  E g y p t  Rei eiveci 15 Mutch 19^4, accepted 3 September 1994A b s t r a c t  : C h a l c o h a l i d e  g l a s s e s  a r e  r e l a t i v e l y  u n c o m m o n  i n o r g a n i c  g l a s s e s  c o m p a r e d  to  c h a l c o g e n i d c  a n d  h a l i d e  g l a s s e s  In  t h e  p r e s e n t  w o i k ,  A s o S e ^  a n d  ( A s 2 S c t ) i ) s  I s  g l a s s e s  h a v e  b e e n  p i c p a r e d  a n d  a n a l y z e d  b y  d i t t e r e n t  t e c h n i q u e s  ( X ^ r a y  p o w d e r  d i f f r a c t i o n  d e n s i t y  d e l e i m i n a t i o n ,  v i c k e r  h a r d n e s s  a n d  t h e i m a l  a n a l y s i s )  A l s o  t h e  t e m p e r a l u i e  d e p c i u l c n c c  o f  t h e  d c  c o n d u c t i v i t y  a s  w e l l  a s  t h e  o p t i c a l  a b s o r p t i o n  a t r o o m  t c m |r c Y a tu r c  h a v e  lK*en m e a s u r e dK e y w o r d s  : A m o r p h o u s  s c m i c o n d u c i o r s ,  c h a l c o g e n i d c  a n d  c h a l c o h a l i d e  g l a s s e s ,  p r e p a r a t i o n  a n d  a n a l y s i sP A C S  N o s .  : 6 1  1 0  L x ,  7 2  2 0 - 1, 7K  6 6  J g1. IntroductionChalcogenidc and halide glasses have aUraclcd a great deal of attention during the past 30 years because of their special IR properties.Chalcogenidc (sulfide, selenide and telluride) glasses arc well known for their chemical durability and mid IR transmittance up to 15 |im. However, these glasses possess high refractive indices and intrinsic optical loss, as well as contamination from parasitic absorption by groups such as OH, SH, etc'. On the other hand, the relatively poor chemical durability of halide glasses [IJ together with their low-glass transition temperatures, pose serious problems for practical application. The structural aspect is interesting from a fundamental view point since chalcogenidc glasses are predominantly covalent and halide glasses arc predominantly ionic in character.At first sight, it might seem possible to improve the chemical durability ol halide glasses by incorporation into a chalcogenidc glass. Secondly, a possible decrease in the refractive index of the chalcogenide glass would reduce the intrinsic scattering losses in the IR[2], ,© 1994 lACS522 LA WahabSince halide and chalcogenide glasses are both transparent in the IR region, it is probable that the mixed chalcohalide glasses are IR transmitting also. Pioneering work on chalcohalidc glasses were carried out in 1960 by Flaschcr e ta l  [3], Pearson [4], Hopkins etal [5| and Dembovskii e ta l  [6], mainly concerned on ternary glassy systems, such as As-S-X, Gc-S-X, Sb“S-X (X is Cl, Br or I). Also, the glass formation and optical properties ot multi-component chalcohalide glasses based on AsSe-AsTe-PbCl-Tl were studied in 1960 [7]. A general review of the properties of chalcohalide has been provided by diffcren! authors 11,H,91 in the literature.In chalcohalide systems, the anions have a great influence on chemical bond characteristics and also on glass formation. It seems that closer the electronegativity value between group VI and group VII elements, the more easy is the formation of chalcohalide glasses.2. Experimental techniques2. / Material prefxiration :2 1.1. Bulk materialFirstly, As2.Sc  ^ was prepared from highly pure arsenic and selenium (99.999%) by placing the appropriate proportions into fused silica tubes scaled under 10“^ ton. The evacuated tube was then placed in a furnace and the temperature was raised in steps to 800°C. The synthesis time was about 4 hrs, during which the molten material was vigorously shaked. At the end of the synthesis process, the molten material was quenched in ice-water mixture. The ternary (As2Sc 3)9sI^  was prepared from the binary As2Se3 by adding the correct amount of iodine and maintaining at 6(X)''C for 6 hrs in an evacuated (10“^ torr) silica tube. The ampoule was occasionally shaken during the synthesising period to ensure homogeneity of the material Quenching was then carried out in air for few seconds followed by immersion in icc-waler mixture (to ensure less porosity within the obtained samples).2.1.2. Thin filmThe electron beam evaporation was chosen for preparing the thin film sample. An Edward 306A coating unit with a carbon boat was used for evaporation. The thickness of the sample was 5000 A.2.2. Measu rement techniques :The amorphous character of the investigated samples was confirmed by Philips X-ray diffractometer (PW 1140190) with X-ray goniometer of the type PW 1373. Thermal experiments were carried out on a Shimadzu instrument (type DT-50). The differential scanning calorimetry (DSC) measurements w^ ere carried out in a nitrogen atmosphere (50 ml/min), using a-Al203 as the reference material. Thermogravcmetric (TGA) measurements were also perfonned to evaluate the activation energy of decomposition using the method proposed by Ozawa [9]. In this method, TGA curves were obtained at different heating rates ( 0  s) and the following equation was used to obtain E jCharacterization of chalcohalide semiconductor lass 523log<I>, + 0.4568£,//?7'j = \og0^  -H 0 A 561E JR T ^  =where T(K) is the temperature at which the sample is decomposed at a healing rate 0 .  In the above equation, when the common logarithms of the heating rates arc plotted against the reciprocal of absolute temperatures, a straight line will be obtained in the range where the decomposed ratios arc equal. Accordingly, the activation energy of decomposition E j  can be evaluated from the slope of the line.Measurements of dc conductivity were made in a dry nitrogen atmosphere using specially designed cells provided with a temperature-controlling system in the temperature range from 160 K to below of the sample. Kcilhly 616 digital electrometer was used for measurements of correct down U> lO"'"* amp.The vickers microhardness of the glasses (V^ )^ was measured at room temperature, using a vickers microhardness tester (Shiinadzu lype-M) attached to a data processor (Shimadzu Dutaletty 150)The optical absorption was measured at room temj>eralurc using Uvikon 860 Koniron.3. Results and discussion3 1. Physical properties : 'The XRD patterns of the prepared powder glasses As280  ^ and (AS2SC itre shown in I'lgLire 1. The figure shows that the samples arc amorphous.Figure 2 shows DSC thermograms for the two glasses measured at a constant healing rate of 20 deg/min. The figure shows the three phenomena of interest : the glass transition (at524 LA Wahaba temperature T^ ,), the crystallization exotherm (with the maximum crystallization at the temperature 7f) and the melting endotherm (with a melting point 7^). The glass transitionFigure 2. DTA thermogrnins of AS2SC3 and (As2863) ^ Is glassesoccurred at 177^ 0^ for AS2SC3 and at I54''C for (As2Se3>)5ls. After 7’^,, the exothermic peak, due to crystallization starts at temperature 333°C for AS2SC3 and at 297°C for (As2Sc3)9sls. The endothermic peak due to melting occurred at 375°C for AS2SC3 and 371.5'^C for (AS2SC 3)9sls-The density (d )  of the rn-prepared glassy As2wSc3 and (As2Sc3)(^ l<^  samples was determined by the hydrostatic method and the results are 4.49 and 4.47 gm/cm ' respectively, The measurements for each composition were carried out on three different parts of the same ingot. The^ 7 values obtained were reproducible within 0.05%, indicating high homogeneity of the quenched ingots,The value of vickers hardness (V//) decreases from 137.7 (A.S2Se3) to 136 Kgm mm‘“ ((As2Se 3)9515) with standard deviation 2.5. Such a change in corresponds to the change in nature and concentration of the chemical bonds favoured for each composition.For the same quenching rate, the glass forming tendency differs for different materials. Hruby and Stourac [10] suggested an appropriate measure for the glass forming ability, that can be determined from DTA (DSC) of a glassy sample. This is defined aswhere is the glass transition temperatures, and are the start of the crystallization and melting temperatures respectively, in a DTA (DSC) thermogram. The values of K^i$ ofAs2Sc3 and (As2Se3 )9 5 1 5  which are obtained from their DSC scans at 20 dcg/min, arc 3.7 and 1.92 eV respectively.The results indicate that I tor the two glasses and irdecreases by adding 1,which means that the tendency to torm crystalline solids increases with the addition of iodine.The activation energy of decomposition is useful for evaluating the thermal stability of materials. By plotting lo g 0  versus 1/7 and applying the least square fitting, Ej can be calculated (Figure 3). The Ej values obtained arc 2 cV for As2Sc3 and 1.08 cV for (As2Se3V;5 l5 , which show that £,/decreases by adding I.Characterization of chalcohalide (AsjSe semiconductor glass 525Fipiire 3. Plot o( 1/7’The decomposition energy is correlated with the type and strength of the possible chemical bonds. The chalcogenidc glasses arc predominantly covalent while the halide glasses arc predominantly ionic in character. By adding iodine, the covalent nature of the bonds may get weakened which in turn, means that with the addition of iodine the glass needs less energy to decomposeThe average heat of atomi/alion (in Kilocalorie per gram-atom) is defined for a compound a direct measure of the cohesive energy and thus the average bondstrength ( 1 1 1 can be given as= [ a H ^  + j i n ” + y w f)/(«  + y3+y).In the present work, for As2 Sc3 and (As2Se3>9 5 l 5 are found to be equal to 57.54 and 57.016 K Cal/gm atom, respectively.3.2. Electrical conduction :The glas.ses As2 Sc 3 and (As2Se3 ) 9 5 1 5  have high resistivity at room temperature which makes measurements of conductivity very difficult at lower temperatures.Figure 4 shows the plot of \ogcJversus MT lor deposited film of As2 S c 3 , which indicates a deviation from linearity at low temperatures = 220 K, The value of the activation energy of conduction has been computed from the slope of least-square fits tor the function68A(6)-3526 LA Wahah\ o g o -  j{ \/T ) . The value of AE and (Jq estimated from the linear part in the temperature range 22Q-330 K, are 0.866 eV and 5.37 x I0^(Q cm)“*, respectively.Figure 4. Temperature dependence of eicctrica! conductivity of AsiSe^ and (As 2Se3)(j^  I5; glasse.s.For the deposited film of (AsjSe,),,!,. the plot of the relation logcr = 5^ 1/7') in the temperature range 160-333 K, exhibits two distinct slopes, showing two mechanisms ol conduction. It was found that AE = 0.75 eV and Oq = 2.6 x 10^  (12 cm)^ ' for the high temperature range (256-330 K) and AE  = 0.24 eV and (Tj = 2.016 x 10“^ (Q cm)”* for the low temperature range (220-256 K).The value of AE decreases in the order AS2SC3 and (As2Se3)9^ 15. The gap of (As2Se3)9515 seems to be closed by localized slates band tails resulting from the compositional disorder created by the addition of I. This gives rise to the hopping conduction at lower temperatures as a second mechanism.3,3. Optical absorption :The absorption of light by amorphous solids depends on the energy (hv)  ot the incident photon and the optical gap of the material. For most amorphous semiconductors, this means photons in the visible region ol the spectrum since their band gaps often lie in the range 1-3 eV. This behaviour can be explained by using the general equation adopted by Hasegawa e ta l  [12], which relates the absorption coefficient (a) to the optical gap of amorphous solids.(n v a Twhere a  is the optical absorption coefficient.Characterizjcition of chalcohalide (As2Se semiconductor glass 527Figure 5 shows the photon energy dependence of the absorption coefficient for the deposited film of (As2Sc3)9515 which indicates an exponential behaviour. It is clear that the optical absorption for amorphous semiconductors near absorption edge usually consists of(Nthree regions [ 13). Our results lie in the second region. This region corresponds to transitions between extended states in both valence and conduclion bands and the absorption coefficient a  is expressed asa  =  A C n v - i T t v ,  where A is a constant.Figure 6. Absorption coefncient a  of fresh and ^nealed (As2Sc3)95l5Thus, (ofHv)'  ^ v^  Tiv plot gives as the extrapolation value of liv' intersecting a  = 0 which is equal to 1.66 eV. for As2Sc3 is found to be 1.74 eV [11’. The addition of I to528 LA WahabAs2& 1 creais more coinposilional disorder and consequently more localized state in the gap. The present of high density of localized states increases the transition probabilities and hence lowers. the optical energy gap. So addition of I to AS2SC3 decrease the width of the energy gap. This result is consistent with the results obtained from the conductivity experiments.By annealing the sample at 160°C for different limes (up to 65 min.), it is found that remains the same but the absorption coefficient decrease with time (Figure 6).4. ConclusionThe chalcohalidc glass (AS2SC3XBI5 is found lo have a high glass forming ability and interesting optical and electrical properties. Addition of small amounts of iodine to (AsiSe^ lysl.-! is found to improve its optical and electrical properties and makes it more suitable for different applications.Rclrreiu'cs \LI I G Fuxi J Non^Cnst Snhds 140 IK4 (19^ )2)|21 J S Sanghcni, J Heo and J D Macken/i / Non-Cryst. Solids 10 155 (1988)PI S S Hasdicn, A D Pearson and W R Norihovcr J Appl Phys 31 219 (I960)[41 A D Pearson Modern Aspects of the vitreous slate Vol 3 ed J Mackenzie (London Butierworths) p 40(1964)[5] T \l Hopkins, R A Pnicrnak, F S Goull and J R Herndon J Phys Chetn 66 733 (1962)[6] S A t)ciiibovskii, V V Kinlenkoand Y A Buslaev f:v Akad Nank SSSR, Neorf* Matei 7 328 (1971). 637(1970) ^[7] G Fuxi, X Mao and Z Sun J ('in Ceram Soi 3 167 (1964)(81 H ZhaniJ, G Fonlcnlcau and J Lucas J Non-Cryst Solids 104 38 (1988)|9J T Ozawa Polymers 13 150 ( 1971) f lOJ A Hruby and L Siourac Maier Res Bull 6 465 (1971)[111 M Yamaguchi Phil Maf> 51 651 (1985)(I2J S Hasepawa. S Ya/aki and T Shimizu Sol St. Comniun 26 4070 (1978)(13) J Tauc Amorphous and Liquid Semicondiit tors cd J Tauc (London and New Yoik Plenum) ( 1974)

Characterization of Chalcohalide (As2Se3)95 I5 Semiconductor Glass

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Characterization of Chalcohalide (As2Se3)95 I5 Semiconductor Glass

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