Electrical properties of nickel-doped arsenic trisulphide

Transcription

1 PRAMANA jornal of physics Printed in India Vol. 40, No. 5, May 1993 pp Electrical properties of nickel-doped arsenic trislphide NAVDEEP GOYAL, RAJNI SHUKLA and MANOHAR LAL Department of Physics, Panjab University, Chandigarh ~60014, India MS received 1 Febrary 1993 Abstract. Reslts of temperatre and freqency dependent a.c. condctivity of pre and nickel-doped a-as2s 3 are reported. The a.c. condctivity of pre As2S s obeys a well-known relationship: aac oc t~ s. Freqency exponent s is fond to decrease with increasing temperatre. Correlated barrier hopping (CBH) model sccessflly explains the entire behavior of a.c. condctivity with respect to temperatre and freqency for pre As2S 3. Bt a different behavior of a.c. condctivity has been observed for the nickel doped As2S3. At higher temperatres, distinct peaks have been observed in the plots of temperatre dependence of a.c. condctivity. The freqency dependent behavior of a.c. condctivity (ao oc co s) for nickeldoped As2 $3 is similar to pre As2 $3 at lower temperatres. Bt at higher temperatres, In ~rac vs In f crves have been fond to deviate from linearity. Sch a behavior has been explained by assming that nickel doping gives rise to some netral defect states (D O' ) in the band gap. Single polaron hopping is expected to occr between these D O' and D + states. Frthermore, all D, D O' pairs are assmed to be eqivalent, having a fixed relaxation time at a given temperatre. The contribtion of this relaxation to a.c. condctivity is fond to be responsible for the observed peak in the plots of temperatre dependence of a.c. condctivity for nickel-doped As2 $3. The entire behavior of a.c. condctivity with respect to temperatre and freqency has been explained by sing CBH and "simple pair" models. Theoretical reslts obtained by sing these models, have been fond to be in agreement with the experimental reslts. Keywords. Chalcogenide glasses; amorphos semicondctors; a.c. condctivity; electrical properties. PACS Nos 72-20; Introdction The stdy of doped amorphos semicondctors is of great interest both to the scientists as well as engineers. Most of the imprities have a little or no effect on the electrical properties of the chalcogenide glasses I-1, 2]. Mott [1] explained this fact by his 8N rle. According to this rle, imprities in chalcogenide glasses are coordinated in sch a way that the bonding reqirements are satisfied and therefore they do not form donors or acceptors. However the incorporation of transition and certain other metals (Ag, Bi) cases prononced changes in electrical properties of chalcogenide glasses [3]-[7]. It has been sggested [1, 8] that in these cases the addition of imprities lead to creation of charged dangling bonds giving rise to certain peaks in the density of states. These charged states can npinn the Fermi level and the activation energy of the condctivity cold drop to two-third of its original vale. One conseqence of the incorporation of(metallic) imprities cold be the dielectric loss peaks, sperimposed pon the backgrond loss. Sch loss peaks have been 377

2 Navdeep Goyal, Rajni Shkla and Manohar Lal observed in Ag-doped As2S a [9], Ag-doped As-Ge-Te [] and Mn-doped As2Se 3 [3]. A possible explanation for this Debye-like loss peak can be that the metal imprities form electrically active close pairs with a single fixed relaxation time for the carrier transport. More experimental work is reqired to nderstand the role of this type of imprities in chalcogenide glasses. It is therefore desirable to stdy the effect of sch type of imprities in chalcogenide glasses. The work reported in this paper is a step in that direction. We report here some measrements of the temperatre and freqency dependence of a.c. and d.c. condctivity of Ni-As2S 3 system with a view to obtain reliable information on the effects of doping on gap states and condction mechanism. An attempt has been made to find ot the energy levels and density of states for Ni-doped As2 $3 by fitting the experimental data in CBH model [11-13] calclations. 2. Experimental Details The glasses of As2 S3-Ni system were prepared in the athors laboratory by meltqenching method. For this prpose As2S 3 and 99"99% pre nickel were weighed in the desired weight percentages. The materials sealed in qartz ampoles nder vaccm were kept in a rocking frnace. The temperatre of the frnace was gradally increased to 650 C and kept constant for abot 48 h and later raised to 750 C. The ampoles were qenched in water. The amorphos natre of the samples was confirmed sing X-ray diffraction. Homogeneity of the materials was confirmed by the elemental analysis of the ingots at different points sing EDAX techniqe. Blk samples in thin pellet form, with sharp edges (diameter 0"61 cm, thickness 0"06 cm) were obtained by grinding the glass ingots and then compressing the powder in a die nder hydralic pressre of the order of 6Kg/m 2. The pellets were annealed at 400 K for abot 24 h. Faces of the annealed pellets were coated with aqadag (a condcting emlsion). Measrements of d.c. and a.c. condctivity were made by holding the samples in between steel electrodes in a three terminal cell assembly (figre 1). The temperatre of the cell cold be maintained constant within 0.1 C. The a.c. condctivity measrements were carried ot sing a general radio bridge (model 1620) over a freqency range of 500 Hz to 0kHz and temperatre range of 173 K to 423 K. Megohm-meter (Model RM 160/3, British Physical Laborato~'ies) was sed for d.c. condctivity measrements. To stdy the effect of doping on band gap of As 2 S 3, UV-VIS-NIR spectra of these samples were'taken on Hitachi spectrophotometer (Model 330). 3. Reslts and discssion The optical absorption spectra obtained for compositions (As 2 S 3)0- x Nix( x = 0, 0.4, 1"0, 4"0 and 7.0) in the visible range are shown in figre 2. The optical band gap was determined with the help of the plots shown in figre 2, and the reslts are listed in table 1. These vales have been sed to estimate the maximm barrier height WM for these materials. Figre 3 shows the temperatre dependence of d.c. condctivity for As2S3-Ni glasses for different percentages of Ni. It is clear from the figre that as the doping percentage of Ni increases, the slope of the crve (a,.c. vs 00/T), and hence the d.c. activation energy decreases. The freqency dependent behavior of a.c. condctivity at different temperatres 378 Pramana - J. Phys., Vol. 40, No. 5, May 1993

3 Electrical properties of Ni-doped As 2 S a " A ~0 /J A- BNC Connector B- Nozte for Vacm C- Wire in "TefLon Bsh 0- Teflon Bsh E- Upper Electrode F- Sompte 6- Lower Etectrode H- Nicrometer I- Thermocopte J- Oter Jacket Figre 1. Sketch of the sample holder sed for the condctivity measrements. for pre As2S a glasses is shown in figre 4. It is clear from figre 4 that pre As2S a obeys a well-known empirical relation tra~ = A~o ~ (1) at all temperatres. Figre 5(a-d) show the freqency-dependent bchavior of a.c. condctivity (aac) at two widely different temperatres for As2Sa-Ni system. It is evident from these figres that at lower operating temperatre, the well-known power law (ac oc co s) is obeyed, bt at higher temperatre In a vs In fcrve deviates from linearity. This deviation was consistently observed in all the concentrations stdied. Frther, the temperatre dependence of a.c. condctivity in As2Sa-Ni glasses at khz, depicted in figre 6, shows that at lower temperatres, the behavior of Nidoped AsES 3 is similar to that of ndoped material. Bt, at higher temperatres, a peak appears in the temperatre dependent behavior of Ni-doped As 2 S a. We fond that the peak is persistent at all the freqencies, also the temperatre at which the Pramana - J. Phys., Vol. 40, No. 5, May

4 Navdeep Goyal, Rajni Shkla and Manohar Lal 0 80 [ ~60 Z C) I-- a. o 40 ID 20 Y AszS 3 Pre o 0A% Ni x 1.0 % Ni 4%Ni ~7%Ni 0 5 Figre 2. I I I h ~ (ev) Optical absorption spectra for varios glasses in AszSa-Ni system. Table 1. Parameters sed for model fitting. B WM W, W 2 U~t, N r N~r20 T e Glass (ev) (ev) (ev) (ev) (ev) (cm -s) (cm-l) 3 ( C) As2S 3 2"21 2"2 0"5 1"0 0"65 5'0 X (As253)99.6Nio. 4 2"27 2"1 0"6 0"8 0"7 8"0 x 17 2"3 213 (As2Sa)99.oNil. o 2"17 2"1 0"6 0" "85 x l017 4"2 213 (As2Sa)96.oNi4.o 2"14 2"1 0"6 0"8 0"7 1"2 x l0 Is 3"5 205 (As2S3)ga.oNiT.o 2"14 2"1 0"6 0"8 0"7 1"5 x 18 3"5 209 Vales of B (band gap) were obtained experimentally, all other parameters have been estimated to obtain a good fit for the experimental vales. peak appears is independent of doping concentration. In the following discssion, an attempt has been made to explain these new observations. The a.c. condctivity in semicondctors has been interpreted in terms of pair approximation 13]. In this approximation, it is assmed that a pair consists of two localized states between which carriers hop back and forth. Each pair has its own relaxation time z which is related to the transition probability of the carrier. The contribtion of one sch pair to a.c. condctivity is given by C~o2~/(1 + o~2z2), where C is independent of to and z. If the relaxation time is same for all the pairs then the above relation will hold good in the simple condctivity I-3]. Bt in amorphos semicondctors, localized states are generally considered to be randomly distribted in 380 Pramana - J. Phys., Vol. 40, No. 5, May 1993

5 Electrical properties of Ni-doped As2S 3 'E U T -11 Ni Content (at %) Z~ ;.0 a 7.0 b ~ I I I ('K -1 ) Figre 3. D.C. condctivity for glasses in As2S3-Ni system. lo--9 l _ J i E o Figre 4. I! I lo 3 lo ~, lo 5 Freqency (Hz) Freqency dependence of ~ for pre As2Sa glass. Pramana- J. Phys., Vol. 40, No. 5, May

6 -9 - Navdeep Goyal, Rajni Shkla and Manohar Lal i! (o) 0.~ % 203 K -/ o 25 (b) 1.0 % 153 K /~ o 233K ~ ~0-11 / L-" I E 7 -lz -9 (c) 7.0 % 153 K 0 I /" I I (d) 4.0% 128 K o 2t~3K I I I I I 3 G 5 z 3 ~ 5 Freqency (Hz) Freqency (Hz) Figre 5. Freqency dependence of aa for As 2 S3-Ni glasses (a) (As 2 S 3 )99'6 (b) (AsS)99.0Nit.o(c) (As2S3)93.0NiT.0(d) (As2S3)96.oNia.o. Nio-4 space and, therefore, pairs with different relaxation times exist. Conseqently the a.c. condctivity is the sm of contribtions from all sch pairs and is given by the relation [12, 14, [ wm- ktlnll/,o,o)q tr, = ~n NNpe,oR6 exp 8kTg _1 (2) which can be redced to a more familiar form of (1). 382 Pramana -J. Phys., Vol. 40, No. 5, May 1993

7 Electrical properties of Ni-doped As 2 S 3 Ni Content (wt %) a 7.0 % 1# 9 ~.0 % o 1.0 % & 0.0% 7 E 1~ ~ 7 C b o ~o-ll.12 I ~ I I I /1. ~,K -~ ) Figre 6. Temperatre dependence of a.c. condctivity for different glasses of As2S3-Ni system at khz. The hopping length R,~ in (2) is temperatre dependent and is written as [12] 4he 2 R~, = (3) e - { WM -kt ln(l/~oz0)} In (2) and (3), n is the nmber of polarons involved in the hopping process, e is the dielectric constant. NNv is proportional to the sqare of concentration of states and is also a temperatre dependent term given by [12, 13] NN e = N2/2 (for bipolaron hopping) (4) NNv = N2/4exp( - U ff/2kt) (for single polaron hopping) (5) For bipolaron hopping, between D and D-, the barrier height is maximm and is designated as W~ which is replaced by W 1 for single polaron hopping between D O and D- and W 2 for single polaron hopping between D O and D as shown in figre 7. Freqency exponent s is evalated from (2) and (3) as [12] d(ln a) 6k T T ~- = (6) d In co W - ktln(1/ogzo) 8Tg Eqation (6) explains a decrease in the vale of s with increasing temperatre for pre As 2Sa, as illstrated in figre 4. The total a.c. condctivity is the combined contribtions of bipolaron and single polaron hopping and in general [13] oac = ab + O'se + ash (7) where a b is the contribtion of bipolaron hopping and ash and ase are the contribtions Pramana - J. Phys., Vol. 40, No. 5, May

8 Navdeep Goyal, Rajni Shkla and Manohar Lal Process I... Process II Process III Figre 7. The three hopping processes involved in hopping condction in As2 $3. -9 P I E v 1o-IO O o n C~ o o v v 50KHz b o e,,v o o v 7:, o- KHz ~o- o n n_ ZKHz I 0o/T (K -1) Figre 8. Temperatre dependence of a.c. condctivity for As2S3, (points represents experimental data and solid line represents CBH fitting). I from single polaron hopping between D O and D- and D O and D + defect states respectively. Figre 8 shows the experimental data and calclated crves (from the CBH model fitting) for pre As2S3. Parameters sed in these calclations are shown in table 1. W M was taken to be slightly less than the band gap obtained from the optical stdies. CBH model shows that the single polaron hopping is dominant at higher temperatres [13], so the vale of s at high temperatres (> 4 K) was sed to calclate the vale of W 1 by sing (6). The figre clearly indicates that the experimental points 384 Pramana - J. Phys., Vol. 40, No. 5, May 1993

9 Electrical properties of Ni-doped As 2 Sa and the calclated crve are in good agreement with each other for pre As2S 3. Bt the loss peak observed in the temperatre dependent a.c. condctivity behavior of Ni-doped As2S 3 (figre 6) cannot be explained on the basis of CBH model for randomly distribted centres. To explain this we assme that Ni-doping gives rise to some netral defect states (D ') in the band gap. The assmption seems valid becase Ni + ion, acting as an ionized donor, traps an electron from a neighboring D- centre, thereby forming a (Ni + -D-) (or D ') centre. Single polaron hopping is expected to occr between these D O` and D + states. Unlike the original D O states of the material, the newly formed netral defects are not thermally activated, becase the concentration of these states depends only pon the doping concentration. All these D + and D o paired states are taken to be eqivalent, having a fixed relaxation time at a particlar temperatre. The contribtion to the a.c, condctivity de to all the defect pairs (as.p) is given by [3] e2 Nsr 2 afl z ' s'p = 12kT 1 + (D2r 2 (8) where Ns is the concentration of pairs and r0 is the hopping distance. Takanao et al [31 reported a similar peak in a.c. condctivity for Mn-doped As2 Sea. They observed that if the condctivity of pre As2Se 3 is sbtracted from that of Mn-doped As2 Sea, then the freqency dependence of this increment to the a.c. condctivity is well explained by (8) for a fixed vale of z. In Mn-doped system, the magnitde of the a.c. condctivity remained almost the same even after doping at lower temperatres. Bt, in Ni-doped As2 $3, we observed a large increase in the a.c. condctivity. Therefore we sed a slightly different approach (described below), to explain the increment in a.c. condctivity on the basis of simple pairs. At lower temperatres, the contribtion to a.c. condctivity is predominantly de to bipolaron hopping and it is almost temperatre independent [12, 15]. Therefore, if a.c. condctivity, at lower temperatres, is sbtracted from the vales obtained arond the peak region, then the freqency dependence of this increment is expected to be explained by (8) for varios doping percentages. Theoretical calclations were made by sing (8) for the freqency dependence of increment in a.c. condctivity de to simple pairs. It is clear from figre 9 that freqency dependence of the increment in a.c. condctivity, so obtained, is in agreement with the experimental reslts obtained by s as discssed above. It is therefore expected that Ni doping in As2S3 prodced simple pairs which possess fixed relaxation time. When a v in (8) is considered to be a fnction of z with constant co then a.p has a peak at cox = 1. Therefore the prononced peak observed in the temperatre dependence of a.c. condctivity, as shown in figre 6, is jstified by (8). Figre shows a plot of the freqency against temperatre at which peaks are observed. Vales z0(= 5 x -14s) and W(= 0.48 ev) are estimated from the intercept at lit = 0 and the slope of the line of the crve (figre ). It has been fond that the relaxation z(= 1/o~) follows the relation: = ~oexp(w/kt) (9) Since the CBH is known to be a major condction mechanism in ndoped samples, the a.c. condctivity at lower temperatres for Ni-doped samples is similar to that of ndoped sample. Whereas, at intermediate temperatres, it has simple pair contribtions. Conseqently, the a.c. condctivity for Ni doped As 2 S 3 is expected to be Pramana - J. Phys., Vol. 40, No. 5, May

10 Navdeep Goyal, Rajni Shkla and Manohar Lal Q Theoretical Crve Experimental Points 15 lo O -11._= Q "lz -13 z i I i 3 /~ 5 Freqency (Hz) Figre 9. Freqency dependence of increment in a.c. condctivity de to simple pairs for (As2S3)99.oNil, o. 5 ~' ~ Z 0 I,,,= ~o Figre. I /1. (K -1 ) Freqency vs peak temperatre plot for (As2S3)99.oNil.o glass. 386 Pramana - J. Phys., Vol. 40, No. 5, May 1993

11 given by Electrical properties of Ni-doped As 2 S a O'ac = O's. P "[- (7 b.3 L O'se + O'sh 0o) where a.p is the simple pair contribtion, ab is the bipolaron hopping contribtion and ash and ase are the single polaron hopping contribtions. Figres show the experimental and theoretical [ 161 reslts of the temperatre -9 I(~ I0 I E r= 1~" o b o ~ ZO KHz e, o c KHz o O--o- 5KHz 0 o ZKHz o o4 1KHz 1(~ "lz 1( 13 2 I I I I I IO00/T (K "~ ) Figre 11. Temperatre dependence of a.c. condctivity for (As2Sa)99.6Nio. 4 (O experimental points, -... CBH model fitting, --- simple pair contribtion, - - total calclated condctivity) 'E '~ 16" b o -12 et 0 0 ~ 0 0 / st, 0 v r) ZO KHz o- KHz o 0 o 5 KHz Z KHz 1 KHz 16' I I I ~ I tooo/t (K -1) Figre 12. Temperatre dependence of a.c. condctivity for (As2S3)99.0Ni1. 0 ( experimental points, -... CBH model fitting, --- simple pair contribtion, - - total calclated condctivity). Pramana - J. Phys., Vol. 40, No. 5, May

12 Navdeep Goyal, Rajni Shkla and Manohar Lal -9,~,~o T s / " ZO K Hz Q ~ KHz ~,,~ ~.' ~ 5 KHz Z KHz -~ ~'~" 1KHz \ -1z I 2 3 I I I /1 (K "1) Figre 13. Temperatre dependence of a.c. condctivity for (As2S3)96.oNi4.o ((3 experimental points,-... CBH model fitting,--- simple pair contribtion, - - total calclated condctivity).,o-sl I E J O o ~'".- ~ ".' ~, Q Q g 2.0 KHz o, n o o o-- KHz O S KHz Z KHz o-xr"7--,, "-''v-" --' "--'~-- ~ O-- 1 KHz i "12,. 7000/T (K-l) Figre 14. Temperatre dependence of a.c. condctivity for (AszS3)93.0NiT.0 ((3 experimental points, -... CBH model fitting, --- simple pair contribtion, - - total calclated condctivity). I dependence of a.c. condctivity for Ni-doped As 2 S 3 system at different freqencies. The vales of varios parameters sed in calclation are listed in table 1. The figre clearly indicates that the theoretical crves (solid lines) are in good agreement with the experimental vales (data points sperimposed on solid crves) at all the freqencies. Therefore the behavior of Ni-doped As2 $3 seems to be well explained by introdcing the concept of simple paired states having a fixed relaxation time. The contribtion of simple paired states, formed as a reslt of doping as shown figres 11-14, increases p to I% concentration of Ni. Thereafter it starts decreasing and bipolaron hopping, once again starts dominating. This also explains the deviation of a.c. condctivity (figre 5) from (1). 388 Pramana - J. Phys., Voi. 40, No. 5, May 1993

13 4. Conclsions Electrical properties of Ni-doped As2S 3 The a.c. condctivity behavior of As2S 3 can be explained well by CBH model involving randomly distribted paired states. Bt doping of As2 $3 with nickel, leads to a peak in the temperatre dependence of the a.c. condctivity of the material. This is possibly becase nickel prodces netral states which form closely eqivalent simple pairs with D states with almost same intersite separation. This leads to a fixed relaxation time for these simple paired states at a particlar temperatre. The combined contribtions of the bipolaron hopping states, single polaron hopping states and simple pairs explain the a.c. condctivity behavior of As2 S3-Ni glasses. Acknowledgements The athors are thankfl to Prof. K K Srivastava for sefl discssion and sggestions. One of s (RS) is thankfl to CSIR, New Delhi, for providing the financial assistance in the form of a fellowship dring corse of this work. References [1] N F Mott, Philos. Mag. 19, 835 (1969) [2] N F Mott and E A Davis, Electronic processes in non-crystalline materials, 2nd edn, (Clarendon Press, Oxford, 1979) [3] Y Takanao, M Kitao and S Yamada, Philos. Mag. B55, 515 (1987) [4] K Hirata, M Kitao and S Yamada, J. Phys. Soc. Jpn. 52, 1317 (1983) [5] Y Takanao, M Kitao and S Yamada, Jpn. J. Appl. Phys. 24, L190 (1985) [6] J T Edmond, J. Non-Cryst. Solids 1, 39 (1968) [7] E A Owen, Glass Ind. 48, 637 (1967) [8] N F Mott, Philos. Mag. 36, 33 (1977) [9] P L Sherrell and J C Thompson, J. Non-Cryst. Solids 24, 69 (1977) [] O S Panwar, M Radhakrishna, K K Srivastava and K N Lakshminarayan, J. Non-Cryst. Solids 33, 411 (1979) [11] K Shimakawa, J. Phys. (Paris) 42, 167 (1981) [12] S R Elliott, Philos. Mag. B36, 1291 (1977) [13] K Shimakawa, Philos. Mag.!146, 123 (1982) [14] S R Elliott, Philos. Ma#. B38, 325 (1978) [15] S R Elliott, Adv. Phys. 36, 135 (1987) [16] Theoretical reslts have been obtained by sing eq. (). Pramana - J. Phys., Vol. 40, No. 5, May