El-Kady, Y L

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Indian J. Phys. 7 0 A  ( 4 ) ,  5 0 7 - 5 1 6  ( 1 9 9 6 )I J P  A— an international journalT h e th r e sh o ld  sw itch in g  in  sem ico n d u ctin g  g lass G e2 iS e i7 T e6 2Y  L  E l - K a d yDepartment of Physics, Faculty of Science, Menoufia University, EgyptReceived 11 July 1995, accepted 29 January 1996Abstract : The preliminary measurements show the material Ge2iSenTe62 is CCNR with memory type. The I-V characteristics have been carried out for virgin bulk samples as a function of the temperature and the thickness of the sample in the range 27-60°C and 0.41- 0.86 mm respectively. Empirical equations for the dependence of the threshold resistance, the current and the electric field across the sample terminals on the thickness have been proposed. The conduction at threshold may be due to the transport of carriers excited into localized states at the edges of the valence or the conduction band. The specific heat and fhe thermal conductivity at threshold are temperature independent through out the measured temperature range. The dependence of dc conductivity on temperature shows the amorphous- crystalline transformation at 435 K.Keywords : Semiconducting glasses, threshold switching, electrical properties PACS Nos. : 72 80.Ng, 72.20.-i, 81.40.-Rs1. IntroductionT h e  c h a l c o g e n i d e  g l a s s e s  i n  G e - S e - T e  s y s t e m  h a v e  b e e n  o b t a i n e d  e a r l i e r  [ 1 ]  b y  q u e n c h i n g  t h e  m e l t  f r o m  h i g h  t e m p e r a t u r e  i n t o  i c e  w a t e r  [ 2 ] .  T h e  g l a s s  f o r m i n g  r e g i o n  o f  t h e  s y s t e m  h a s  b e e n  s t u d i e d  [ 3 - 5 ] .  T h e  D T A  m e a s u r e m e n t s  f o r  t h e  t e l l u r i u m - r i c h  g l a s s  o f  t h e  s y s t e m  [ 6 ]  a n d  t h e  o p t i c a l  p r o p e r t i e s  o f  G e —S e —T e  g l a s s  f i b e r s  [ 7 , 8 ]  h a v e  b e e n  c a r r i e d  o u t .T h e  e l e c t r i c a l  c o n t r o l l e d  s w i t c h i n g  a n d  t h e  m e m o r y  e f f e c t s  f o r  t h e  c h a l c o g e n i d e  g la s s e s  h a v e  b e e n  o b t a i n e d  e a r l i e r  [ 6 ] .  T h e  t h r e s h o l d  s w i t c h i n g  o f  t h e  I - V  c h a r a c t e r i s t i c  o f  i h e  c h a l c o g e n i d e  g l a s s e s  h a s  b e e n  a f f e c t e d  b y  m a n y  f a c t o r s  ( e.g. t h e  a m b i e n t  t e m p e r a t u r e ,  t h e  t h i c k n e s s  o f  t h e  s a m p l e ,  t h e  s t a t i c  p r e s s u r e  o n  t h e  s a m p l e ,  t h e  k i n d  o f  t h e  m a t e r i a l  a n d  t h e  a r e a  o f  t h e  e l e c t r o d e  etc.) [ 9 ] .T h e  s t u d y  o f  t h r e s h o l d  s w i t c h i n g  p h e n o m e n a  o f  G e 2 i S e j 7T e 62 u n d e r  t h e  e f f e c t  o f  t h e  t e m p e r a t u r e  a n d  t h e  t h i c k n e s s  g i v e s  m o r e  i n f o r m a t i o n  o n  i t s  p h y s i c a l  n a t u r e ,  w h i c h  i s  o f©  1 9 9 6  I A C S70A(4)-7508 Y L  E l K a d ymuch scientific and technological interest. Our main interest is to study the nature of the conduction mechamism and the dependence of the electrical parameters at threshold, on the ambient temperature (below Tg) and the thickness of the sample.2. Experimental detailsAppropriate constituents of spectroscopic pure Ge, Se and Te have been sealed in silica tube under a vacuum of 10“3 tonr. The tube was heated at 1000°C for 6 hours and shaken from time to time, then quenched in ice water. The amorphous state has been assured by X-ray diffraction technique. Using differential thermal analysis (DTA) the glass transition and crystallization temperatures are about 152 and 255°C respectively. The current and the voltage across the sample have been measured at different constant temperatures for virgin samples with thicknesses 0.41, 0.55, 0.72 and 0.86 mm with an accuracy of ± 0.005 mm. The temperature range is 27°C-60°C with an accuracy of ± 1.0°C. The current and the voltage have been measured using digital electrometer (Keithley Y-616) and vacuum tube voltameter (Simpson-USA) respectively.The specific heat at threshold (Sft) has been calculated using the equation^th Ah* * =  4.186. a x d S ^  ( T^  — 7b), (1)where V* and /ft are the voltage and the current at threshold, t is the time necessary for the voltage turnover (threshold) and 10~5 sec [9], a is the cross sectional area of the electrode s  the cross sectional area of the filament current passing at threshold, the density of the material d  = 5.51 gm/cc [10], x is the thickness of the sample, Ta is the ambient temperature which is equal to the temperature of the sample and 7th is the temperature of the material when the threshold current passes through it [10].Also the thermal conductivity at threshold Kth has been determined according to the ralationKih = eS,h. (2)where e= 8.4 x 10~3 cm/gm sec.This equation can be applied in the temperature range where the phonon propagation may be absent.3. Results and discussionThe general I-V characteristic curve of the studied chalcogenide glass in shown in Figure 1 in which two distinct states exist, a high resistance and a low resistance state. The curve shows that the material is CCNR with memory according to the classes of the switching effects [9]. The I-V traces were reversible unless we exceed the negative resistance state (NR) and reach the holding voltage. The material transits to the ON state (memory) above the holding voltage.The I-V curves for a sample with thickness x = 0.41mm at different constant ambient temperatures are shown in Figure 2. Simillar behavior of I-V curves have been(Amp.IT h e  th r e s h o ld  sw itc h in g  in  s e m ic o n d u c tin g  g la s s  G e 2 ,S e ,7T e(l> 509Figure 1. The general behaviour of I V charactet istics toi CiC2 |Sej 7Tef,9Figure 2. The 1 V eliaracteristics curves at different constant tempetatuie 7(, for the thickness a = 0.41 mm.o b t a i n e d  f o r  t h e  o t h e r  t h i c k n e s s e s .  A t  t h r e s h o l d ,  V (h d e c r e a s e s  w h i l e  / , h i n c r e a s e s  b y  i n c r e a s i n g  Ta. T h e s e  a r e  r e a l i z e d  f o r  a l l  m e a s u r e d  s a m p l e s .  T h e  d e c r e a s i n g  o f  V th  o r  t h e  i n c r e a s i n g  o f  7 lh  w i t h  t h e  t e m p e r a t u r e  i s  e x p o n e n t i a l  f o r  e a c h  t h i c k n e s s .  T h e5 1 0 Y L E l - K a d yFigure 3(a). Ln 7,^  versus I U a for different constant thickness xFigure 3(b). Ln versus x at different constant temperature Tabetween In / th  and l/Ta give parallel straight lines as shown in Figure 3a, the constant slope of these relations comfirms the experimental data where the slope is a thickness independent.It was found that the threshold current increases exponentially by increasing the thickness at constant ambient temperature. The relations between ln /th and x at different constant temperature are shown in Figure (3b). The obtained straight lines have a constant slope («). These mean that the variation of In /,h with the thickness is independent ofa m b i e n t  t e m p e r a t u r e .  A  p r o p o s e d  e q u a t i o n  o f  t h e  t h r e s h o l d  c u r r e n t  d e p e n d e n c e  o n  t h e  s a m p le  t h i c k n e s s  i n  t h e  m e a s u r e d  r a n g e  c a n  b e  w r i t t e n  a sAh =  B enx> ( 3 )w h e r e  B a n d  n a r e  c o n s t a n t s ,  t h e s e  c o n s t a n t s  a r e  t e m p e r a t u r e  d e p e n d e n t  a n d  i n d e p e n d e n t  r e s p e c t i v e l y .T h e  d e p e n d e n c e  o f  t h e  t h r e s h o l d  r e s i s t a n c e  Rlh o n  t h e  t e m p e r a t u r e  i n d i c a t e s  a  s e m i c o n d u c t i n g  b e h a v i o u r  a n d  i t  o b e y s  a n  A r r h e n i u s  e q u a t i o n ,  Rxh =  Ra e x p  ( W/KTa ) ,  w h e r e  R0 i s  c o n s t a n t  a n d  W t h e  a c t i v a t i o n  e n e r g y .T h e  r e l a t i o n  o f  I n  Rxh versus \/Ta f o r  d i f f e r e n t  t h i c k n e s s e s  g i v e  s t r a i g h t  l i n e s  w i t h  c o n s t a n t  s l o p e  a s  s h o w n  i n  F i g u r e  ( 4 a ) .  I n  o t h e r  w o r d s ,  t h e  t h r e s h o l d  c o n d u c t i v i t y  i n c r e a s e s  b y  i n c r e a s i n g  t h e  t e m p e r a t u r e  ( Ta ) .  T h e  v a l u e  o f  a c t i v a t i o n  e n e r g y  a t  t h r e s h o l d  i s  ~  0 . 1 9  e v  w h i c h  i s  s m a l l  c o m p a r e d  t o  t h e  v a l u e  o b t a i n e d  f r o m  d c  c o n d u c t i v i t y  m e a s u r m e n t s  ( 0 . 4 9  e v ) .  B y  c o n s i d e r i n g  t h e  e f f e c t  o f  t e m p e r a t u r e  o p  t h e  m o d e  o f  c o n d u c t i o n  i n  a m o r p h o u s  s e m i c o n d u c t o r ,  p a r t i c u l a r l y  t h e  c h a l c o g e n i d e  g l a s s e s  [ 1 1 ] ,  t h e  c o n d u c t i o n  m e c h a n i s m  a t  t h r e s h o l d  m a y  b e  d u e  t o  t h e  t r a n s p o r t  o f  c a r r i e r s  e x c i t e d  i n t o  l o c a l i z e d  s t a t e s  a t  t h e  e d g e s  o f  t h e  v a l e n c e  o r  c o n d u c t i o n  b a n d .T h e  d e p e n d e n c e  o f  Rxh o n  t h i c k n e s s  i s  a l s o  e x p o n e n t i a l ,  w h e r e  t h e  r e l a t i o n s  b e t w e e n  In  Rxh a n d  x a t  d i f f e r e n t  c o n s t a n t  t e m p e r a t u r e  a r c  l i n e a r  a s  s h o w n  i n  F i g u r e  ( 4 b ) .  T h e  s l o p e  o f  t h e  l i n e a r  d e p e n d e n c e  o f  I n  / ? th  o n  x i s  t e m p e r a t u r e  i n d e p e n d e n t .  A  p r o p o s e d  e m p i r i c a l  e q u a t i o n  o f  t h e  t h r e s h o l d  r e s i s t a n c e  o n  t h e  t h i c k n e s s  c a n  b e  w r i t t e n  a sRih =  C e x p [ ( w  -  n) X]. ( 4 )w h e r e  C i s  a  c o n s t a n t  w h i c h  i s  t e m p e r a t u r e  d e p e n d e n t  a n d  m  a n d  n a r e  c o n s t a n t s  i n  c a s e  o f  V[h a n d  / th r e s p e c t i v e l y  w h i c h  h a v e  p o s i t i v e  v a l u e s .  F i g u r e  ( 4 b )  s h o w s  a  d e c r e a s e  o f  Rxh w i t hT h e th r e s h o ld  s w itc h in g  in s e m ic o n d u c tin g  g la s s  G e 2jS e I7Te62 511Figure 4(a). Ln versus MTa for different constant thickness.512 Y  L  E l-K a d yX mm-Figure 4(b). Ln versus x  at different constant temperatureincreasing sample thickness, hence the threshold conductivity will increase with increasing thickness.The experimental data indicate an exponential dependence of the threshold electric field across the sample on the ambient temperature and the thickness of the sample. The temperature dependence shows a decrease of F* with increasing the temperature at constant thickness as shown in Figure (5a) where the values of F,h are large for the smaller thickness. The slopes of the linear relations in Figure (5a) are constant. Also the thickness- dependence shows a decrease of Fth with increasing thickness at an ambient temperature. Figure (5b) shows the linear relations between In F,h and x where the values of F(h arc larger for the lower temperature. These relations have a constant slope (m).The decrease of F,h with increasing thickness at constant temperature means an increase of the threshold conductivity 0th will1 thickness. As a result, the current path increases and the electric field across the sample will decrease as shown from the experimental results. At threshold, the thermal initiation as a result of an increase of the ambient temperature and the Joule heating due to the electric current passing through its path introduce a small thermal energy. This energy is not sufficient to make a sensible change in the amorphousity of the material identified by X-ray diffraction. Accordingly, it can be said that at threshold, the non-equilibrium carriers generated by the field increases by increasing the thickness.u FtkT h e  th r e s h o ld  s w itc h in g  in s e m ic o n d u c tin g  g la s s  G e 2iS e ,7T e62 513Figure 5(a). Ln versus V T a for different constant thickness.The threshold power Pa, shows an exponetial dependence on the sample thickness and is independent of the ambient temperature as shown in Figure 6. The obtained specific heat at threshold S* for each sample at different constant ambient temperature shows514 Y  L  E l - k a d yFigure 6. The threshold power /*,(, dependence of thickness at different constant temperature.Table 1. The calculated specific heat at thresholdT(°C) X(mm) Sx 105 (CaVgm8C)T(°C) X(mm) Sx 10s (Cal/gm°C)33 0.41 6.4 46 0.41 6.0•0.55 6.2 0.72 6.00.72 6.4 0.86 6.20.86 6.239 0.41 6.4 52 0.41 6.20.55 6.0 0.55 6.00.72 6.0 • 0.72 6.00.86 6.2temperature independence (see Table 1) where its value is nearly constant through the measured temperature range. Consequently, the calculated thermal conductivity at threshold according to equation (2) will be temperature independent.The dependence of dc conductivity on the temperature, indicates a semiconducting behaviour as shown in Figure 7. This dependence obeys an Arrhenius equation of the form O —G0 erWKT where W is the activation energy. Figure 7 shows the amorphous-crystalline transformation at transition temperature (7'|Cil)|C) = 435°K which is higher than Tg.The obtained activation energies of amorphous and crystalline states are 0.49 and 1.21 eV respectively. The conduction in the former state may be a transport of c a r r ie r sT h e  th r e s h o ld  s w itc h in g  in  se m ic o n d u c tin g  g la s s  G e 2iS e l7T e62 515Figure 7. The dc conductivity dependence of temperature.excited beyond the mobility edges into non-localized (extended) states at Ec or E „  where W = (£c -  E F) or (Ep -  E v) where E c and E v are the energy of conduction and the valence band mobility edges respectively and E F is the Fermi energy. In the later state, the transport of carriers may be in the conduction band, where W ~(EC-  E,?) +  (EF -  Ev). It can be said that the Fermi energy lies near to the mid-gap in the amorphous state.4. C onclusionThe resu lts  o f  I -V  ch arac te ristics ind icate  the sem iconducting glass Ge2 iS en T e62 is a current contro lled  negative resistance (CCNR) device w ith memory. The threshold current, voltage, resistance, electric field  across the sam ple are exponential dependent on the am bient tdrnperature (in the range o f 27-60°C ) and the thickness (in the range of 0.41-0.86* m m ), w h ile  th e  th resho ld  pow er is tem perature-independent. The proposed em p irica l eq u a tio n s  fo r the th ickness-dependence have been  deduced experimentally.The conduction mechanism at threshold may be attributed to the transport of carriers excited into localized states at the edges of the valence or the conductionA^A/4\ o516 Y L E l-K a d yconduction in amorphous state may be a transport of carriers excited beyond the mobility edges into non-localized states.References[1] J A Muir and R J Cashman J. Opt. Soc. Am. 157 (1967)[2] A A Obraztov IZV, Akad. Nauk. SSSR. Neorg. M ater. 8 337 (1972)[3] M Noda and S Marnno Yogyo-Kyokai-Shi 82-4 56 (1974)[4] D J Sarrach, J P de Neufville and W  L  Haworth J. Non Cryst. Solids 22 245 (1976)|5] J Wieder and S Aronson J. Non Cryst. Solids 33 405 (1979)[6] D Pearson J. Non cryst. Solids 2 1  (1970)[7] J Nishij, T  Yamashita and T  Yamagishi Appl. Opt. 28 no. 23 5122 (1989)[8] T  Katsuyama and H Matsumura J. Appl. Phys. I S  2743 (1994)[9] D Adler Amorphous Semiconductors (Cleveland, O h io: CRC Press) Chap. V ll (1971)[10] MM El-Hawary PhD Thesis (Faculty of Science, Menoufia University, Egypt) (1992)[11] N F Mott and E A Davis Electronic Processes in Non-Crystalline Materials 2nd edn. (Oxford : Garcndon) Chap. 6 and 9 (1979)

The Threshold Switching in Semiconducting Glass Ge21Se17Te62

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The Threshold Switching in Semiconducting Glass Ge21Se17Te62

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