high energy ion implantation induced electrical effects in bulk amorphous as2se3

High energy ion implantation induced electrical e�ects in bulkamorphous As2Se3

1

Mahender Singh b, K.L. Bhatia a,*, Nawal Kishore a, R.S. Kundu a, D. Kanjilal c

a Non-Crystalline Semiconductor Laboratory, Physics Department, Maharshi Dayanand University, Rohtak-124 001, Indiab Physics Department, Government College Panchkula, Haryana, India

c Nuclear Science Centre, New Delhi-110 067, India

Received 5 November 1997

Abstract

Bulk amorphous chalcogenide semiconductor, As2Se3, has been irradiated at room temperature with 75 MeV energy

Ni, Ge and Ag ions at ¯uences in the range 1� 1013±1� 1014 ions=cm2. The ion-induced e�ects on the electronic prop-

erties have been monitored by measuring the dc conductivity and frequency dependent ac conductivity (500 Hz ± 10

kHz) as a function of temperature (180±450 K). It is found that the electrical e�ects in the samples bombarded with

Ni ions are quite di�erent from the ones bombarded with Ge/Ag ions. Ion-irradiation induced defect states near the

Fermi level play a dominant role in the variable range hopping conduction. Bipolaron hopping conduction appears

to be a�ected less by ion-irradiation. It is interesting to see that as small an ion-dose as 5� 1013 ions=cm2

is quite ef-

fective to modify the electrical transport behaviour of the glass. Ó 1998 Elsevier Science B.V.

1. Introduction

Amorphous arsenic triselenide �As2Se3� is oneof the stable stoichiometric and technically impor-tant chalcogenide glasses. The e�ect of impuritieson the electronic properties of chalcogen basedamorphous semiconductors has been a controver-sial issue ever since their discovery [1]. Earlier stud-ies showed that they are insensitive to variousadded impurities because of the presence of pinned

Fermi level in them [2]. However, more recent ex-periments reveal that some added impurity atomsin large concentrations may be situated in siteswith unusual con®gurations which do not allowthem to satisfy their valencies and they, therefore,can behave in an electrically active manner leadingto a donor-like or an acceptor-like behaviour [3±6].

Two types of methods have been used to incor-porate the electrically active impurity atoms intothese semiconductors:

(a) Thin ®lms prepared by cold methods, suchas photodi�usion [7], thermal co-evaporation andrf-sputtering [8]. During the process of introduc-tion of the doping elements by these methods,

Nuclear Instruments and Methods in Physics Research B 140 (1998) 349±360

* Corresponding author. Fax: 06221516540.1 Part of the results was presented at the workshop on Swift

Heavy Ions in Materials Science, Indian Institute of Science,

Bangalore, India, March 10±11, 1997.

0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved.

PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 1 1 5 - 3

the temperature remains not higher than the melt-ing temperature of the amorphous substance and itcannot reach a structural or a con®gurationalequilibrium.

(b) Bulk glassy semiconductors prepared bymelt-quenching. In this method, certain metallicatoms (Pb, Bi, Ni) are added in large concentra-tions during the synthesis of the glass [4±6]. Sincethe concentration of the added impurity atoms isquite large, the resulting semiconductor is moreappropriately termed as ``chemically modi®ed''semiconductor.

Recently, it has been shown that keV energy Niions implanted in thin ®lms of a±As2Se3 can inducedoping e�ects [9]. The electrical properties of theamorphous semiconducting thin ®lms can thus bemodi®ed.

Incorporation of impurities into the semicon-ductors by high energy heavy ions is a non-equilib-rium process which can result in intriguing nearsurface and deep into the surface property changesdepending upon the energy of the ion [10,11]. Theion-solid interaction processes can lead to themodi®cation of the composition, structure, elec-tronic properties and the topography of the semi-conductor.

In this context, a programme of investigation ofion-irradiated amorphous semiconductors by hea-vy ions of MeV energy, has been initiated by us[12]. In the present work, we have irradiated the bi-nary chalcogenide glass, As2Se3, with di�erenttypes of ions (Ag, Ni, Ge) of 75 MeV energy andstudied the ion-induced electrical e�ects in thebombarded semiconductors. Our ®rst results arereported in this communication.

2. Experimental procedure

The amorphous composition As2Se3 was pre-pared from high purity (99.999%) elements, Asand Se, by the conventional melt-quenching tech-nique, following the procedure laid down in ourearlier communication [13], and its non-crystallinenature was ascertained from X-ray di�raction pat-terns. Samples for conductivity studies were pre-pared in the form of discs/chips of thicknesslying in the range 0.4±0.6 mm and the surface area

of about 10 mm2. For irradiation of bulk samples,75 MeV energy ion-beams from NEC 16 MV Vande Graa� type Electrostatic Accelerator (Pelletron)[14] of the Nuclear Science Centre, New Delhi,were used. Di�erent ion-species used for irradia-tion were Ni, Ag and Ge. The beam current waskept at 10±14 nA and the ¯uence in the range1� 1013±1� 1014 ions=cm

2. All the irradiation

work was done at room temperature. E�ectivebeam dimensions were kept at 4 mm� 5 mm bydefocussing the ion-beam so that the entire surfaceof the semiconductor received a uniform ion-dose.For electrical measurements, aquadag paint wasapplied on the two opposite sides of the bombard-ed specimen to make good ohmic contacts. To as-certain the ohmic nature of the contacts, I±Vcharacteristics were measured. The dc conductivitywas measured in sandwich geometry using aKeithley electrometer (Model 610C) and a stabi-lized power supply. The ac conductivity was mea-sured using a three-electrode geometry with aGeneral Radio Capacitance Bridge Assembly(Model 1620A), in the frequency range of 500Hz±10 kHz and in the temperature span of 180±450 K. A specially designed metal cryostat ®ttedwith liquid nitrogen trap (for measurements atlow temperatures) and the heater (for higher tem-perature measurements) was used to measure thetemperature dependence of conductivity. Twomeasurements were made for each sample to checkthe reproducibility of the results. The dc and acconductivities were measured with the same sam-ple and in the same geometry. The annealing ofthe samples was carried out in the cryostat undervacuum (of the order of 10ÿ3 Torr) up to a maxi-mum temperature of 150�C, which is less thanthe glass-transition temperature, Tg, of the sampleTg � 185�C).

3. Results

In Fig. 1, the experimental results of the tem-perature dependence of dc conductivity of Ni ionbombarded sample (at a dose of5� 1013 ions=cm

2) are presented. The curve for

unbombarded sample is also included for compar-ison. The virgin (unbombarded) sample follows

350 M. Singh et al. / Nucl. Instr. and Meth. in Phys. Res. B 140 (1998) 349±360

thermally activated transport behaviour, r �r0 exp�ÿDE=kT ), where DE is the thermal activa-tion energy and r0 is pre-exponential factor. Irra-diation of As2Se3 induces tail in the conductivityplot at low temperatures. E�ect of sequential an-nealing (at di�erent temperatures) is also shownin this ®gure by plotting the corresponding data.Annealing makes low temperature tail less shal-low; the value of conductivity is reduced and thehigh temperature part of the curve becomes dis-tinct with its slope �DE� always being less thanthe virgin As2Se3 sample. E�ect of ion-dose varia-tion is shown in Fig. 2 by plotting the dc conduc-tivity data of Ni-ion bombarded (annealed at150�C) samples at various doses (1� 1013; 5�1013 and 1� 1014 ions=cm

2). The ion-induced

changes are more prominent at a dose of5� 1013 ions=cm

2. The results of the temperature

dependence of rdc in Ag and Ge ion-irradiatedsamples are presented in Figs. 3 and 4, respective-ly. The e�ect of annealing on the conductivity ofthese samples is also shown in the ®gures by plot-ting the corresponding data. Species-dependenceof irradiation on the electrical transport is exhibit-ed in Figs. 5 and 6 where results of temperaturedependence of dc conductivity of the samplesbombarded with 75 MeV Ge, Ag and Ni ions, res-

pectively, are presented. The asbombarded (i.e.bombarded and unannealed) samples show muchless e�ect at low temperatures when Ge or Ag ionswere used as projectiles. Irradiation of sample withNi ion exhibits larger changes in the low tempera-ture conduction. This is an interesting feature ofthe present study. E�ect of annealing on the con-ductivity of the samples irradiated with Ge and

Fig. 2. Dose dependence of ion-induced changes in the dc con-

ductivity �rdc� for bulk amorphous As2Se3 samples bombarded

(annealed at 150�C) with 75 MeV energy Ni ions.

Fig. 1. E�ect of sequential annealing on the temperature dependence of dc conductivity �rdc� for bulk amorphous As2Se3 sample bom-

barded with 75 MeV energy Ni ions at a dose of 5� 1013 ions=cm2. The data for unbombarded sample are also plotted.

M. Singh et al. / Nucl. Instr. and Meth. in Phys. Res. B 140 (1998) 349±360 351

Ag ions is also not so signi®cant. The developmentof low temperature tail in the electrical conductiv-ity plots is found in all the three cases; but the ef-fect is more striking in the case of Ni-bombardedsample.

The typical results of measurement of frequen-cy and temperature dependences of ac conductivi-

ty in the unbombarded and the Ni-ion bombardedand annealed (at 150�C) samples are compared inFigs. 7 and 8, respectively. The plotted ac conduc-tivity is rac�x� � r�x� ÿ rdc, where r�x� is the to-tal measured conductivity under ac ®eld and rdc isthe dc part of conductivity. The corresponding val-ue of rdc is also plotted in these ®gures for compar-ison. The changes produced in the frequency andtemperature dependences of rac�x� are not verydrastic. Irradiation increases rac�x� by about oneorder of magnitude. The dispersion with frequencyis reduced on bombardment.

Fig. 4. E�ect of sequential annealing on the temperature depen-

dence of dc conductivity �rdc� for bulk amorphous As2Se3 sam-

ple bombarded with 75 MeV energy Ge ions at a dose of

5� 1013 ions=cm2. The data for unbombarded sample are also

plotted.

Fig. 5. Temperature dependence of dc conductivity �rdc� for

bulk amorphous As2Se3 samples bombarded with di�erent ions

(Ni, Ge, Ag) of 75 MeV energy.

Fig. 6. Temperature dependence of dc conductivity �rdc� for

bulk amorphous As2Se3 samples bombarded (annealed at

150�C) with di�erent ions (Ni, Ge, Ag) of 75 MeV energy.

Fig. 3. E�ect of annealing on the temperature dependence of dc

conductivity �rdc� for bulk amorphous As2Se3 sample bom-

barded with 75 MeV energy Ag ions at a dose of

1� 1014 ions=cm2. The data for unbombarded sample are also

plotted.


4. Discussion

4.1. Ion±solid interaction

An MeV energy heavy ion, while passingthrough a solid state target, loses its energy mainlyin two ways: (a) nuclear displacements and (b)electronic excitations. At MeV energy, the elec-tronic energy loss becomes important and is re-sponsible for reversible/irreversible excitations inthe electronic sub-system of the target near the sur-face region. The nuclear energy loss introduces dis-order deep into the surface. In fact, a deep buriedregion where material modi®cation takes place, isexpected to be produced. According to the tradi-

tional ballistic mechanism of defect production[15], MeV energy ion implantation produces negli-gible concentration of defects at a depth consider-ably smaller than the projected range, Rp, of theenergetic ion. However, defects produced at depthsof the order of Rp by the preceding ion can migrateto the surface under the relaxations brought onthem by electronic excitations introduced by sub-sequent ions which may increase the mobility ofdefect forming atoms near the surface [16]. An in-creased mobility of defect forming atoms near thesurface where the electronic energy loss of fast ionsis maximum, may also lead to the fast recombina-tion or annealing. Which of the two mechanisms

Table 1

The values of electronic energy loss (dE/dx)e, nuclear energy loss (dE/dx)n, the projected range Rp, longitudinal straggling DRp and

lateral straggling DRL for 75 MeV Ni, Ge, Ag ions in As2Se3 as determined from TRIM calculations. The values of percentage elec-

tronic and nuclear energy losses are also given

Ion (dE/dx)e (eV/�A) (dE/dx)n (eV/�A) Rp (lm) Straggling (lm) % (dE/dx)e % (dE/dx)n

DRp DRL

Ni 1.06 ´ 103 2.74 11.04 0.62 0.78 99.74 0.26

Ge 1.13 ´ 103 4.36 11.33 0.68 0.89 99.62 0.38

Ag 1.29 ´ 103 12.07 10.12 0.72 0.92 99.07 0.93

Fig. 8. Temperature dependence of ac conductivity of bulk

amorphous As2Se3 samples bombarded (and annealed at

150�C) with 75 MeV energy Ni ions at a dose of

5� 1013 ions=cm2

at di�erent frequencies: 500 Hz (h), 1 kHz

(n), 2 kHz (D), 5 kHz (s) and 10 kHz (d). Continuous solid

lines represent theoretical curves for bipolaron hopping conduc-

tion. The dc conductivity �rdc� is also plotted for comparison.

Fig. 7. Temperature dependence of ac conductivity of bulk

amorphous As2Se3 (unbombarded) sample at di�erent frequen-

cies: 500 Hz (h), 1 kHz (n), 2 kHz (D), 5 kHz (s) and 10 kHz

(d). Continuous solid lines represent theoretical curves for

bipolaron hopping conduction. The dc conductivity �rdc� is also

plotted for comparison.


will dominate and at what dose, depends largelyon the depth from which defects migrate to thesurface, i.e., essentially on the value of Rp. In thepresent case, concepts of theoretical TRIM(Transport of Ions in Materials) calculations [17]were used to estimate the projected range, Rp,straggling, DRp, and nuclear and electronic energylosses, etc., of the 75 MeV energy Ni, Ag and Geions in As2Se3 composition. A total of 1000 inci-dent ions were taken for calculations in each case.Values of the parameters so obtained are presentedin Table 1. The value of Rp comes out to be about11 microns deep inside the surface. It is to be notedthat the study of ion-implanted oxide glasses [18]has shown that defects are also produced at adepth considerably greater than the projectedrange. The region of extensive damage is charac-terized by very high displaced atom concentrationswhere some physical properties like stress, hard-ness, etc., reach maxima. The transition in depthfrom such a region into the damaged underlyingregion must be accompanied by large gradientsof stress. Further, there may also be a contributionfrom momentum transfers to the displaced atomsat the normal end of the ion-trajectory. In suchan altered structure, it is expected that there willbe much increased di�usion coe�cient for the mo-bile species (ion-induced defects/complexes) [18].When such a system existing in some sort of meta-stable equilibrium state is subjected to sequentialthermal annealing cycles, the ion-related defects

are very likely to spread much deeper into the un-damaged portion of the semiconductor lying be-low the highly damaged portion. The net resultof such physical changes should be strongly re¯ect-ed in the electrical transport behavior of the bom-barded sample. Though the theoreticallycalculated damaged region is buried about 11 lmdeep inside the sample surface, and is backed byabout 400±600 lm thickness of the undamagedsample, the observed electrical conduction behav-iour represents the overall e�ect of the in¯icteddamage, migrated/di�used ion-induced defects/complexes into the underlying undamaged port-

Fig. 9. Depth dependence of electronic energy loss �dE=dx�eand nuclear energy loss �dE=dx�n for 75 MeV energy Ni ions

in amorphous As2Se3 using TRIM calculations.

Fig. 10. TRIM calculated depth pro®le and vacancy distribution of 75 MeV energy Ni ions in amorphous As2Se3.


ion, under the in¯uence of stress induced gradientand thermal annealing.

A number of experiments have shown [19] thatan MeV energy ion posseses damaging capacity atpenetration depth considerably smaller than thatat Rp. Energy for the production of defects canbe obtained only from the relaxation, on the nucle-ar subsystem, of electronic excitation, which storesthe greater part (more than 90%) of the energy ofions being stopped. Using TRIM calculations, re-sults of electronic and nuclear energy losses andthe damage-depth pro®le for As2Se3 sample irradi-ated with 75 MeV Ni, Ge and Ag ions have beendetermined and typical plots for Ni-irradiatedsample are shown in Figs. 9 and 10, respectively.

4.2. Electrical transport

Amorphous As2Se3 is a widely studied chalcog-enide semiconductor. E�ect of addition of varioustypes of impurities on its electrical transport andoptical properties have been investigated [20].The changes induced in the density of localizedstates by the addition of three groups of impuri-ties: (a) Mn, Fe, Ni, Cu transition metal; (b) Zn,Ga, In and (c) Na, K, T1, have been found to bequite di�erent. The added group (a) of transitionmetal impurities (Mn, Fe, Ni, Cu) produces a widedistribution of localized states near the Fermi lev-el. As is well known, a chalcogenide glass hascharged defect centres: D� and Dÿ [21]. A sche-matic diagram [20] illustrating the features isshown in Fig. 11. In the diagram, the density ofDÿ charged defects is sketched. The crosshatchedpart in the diagram represents the additional local-ized states introduced by transition metal impuri-ties near Fermi level. Apparently, this shouldfavour the presence of hopping conduction (vari-able range) among these new states near the Fe-rmi-level. This property of transition metalimpurity in As2Se3 is re¯ected in the present elec-trical data of Ni implanted As2Se3 glass also as dis-cussed below.

4.2.1. DC conductivityIn Fig. 12, the temperature dependence of dc

conductivity in Ni ion-irradiated As2Se3 bulk sam-ple (annealed at 150�C) is compared with the sim-

ilar conductivity data of Ni co-sputtered As2Se3

thin ®lm [8]. E�ect of 25 keV energy (Ni) implan-tation in As2Se3 thin ®lm [9] is also shown in the®gure.

The virgin (unbombarded) bulk sample behavesin a similar way as the virgin (unimplanted/un-doped) thin ®lm. However, MeV energy Ni-ion- ir-radiation produces quite di�erent changes ascompared to Ni doping or keV energy Ni-ion im-

Fig. 11. Schematic energy level diagram for As2Se3 doped with

transition metal impurity. The shaded portion corresponds to

the density of localized states and the crosshatched portion rep-

resents the density of localized states introduced by the dopant.

The density charged defects �Dÿ� which correspond to trapping

centres, is also indicated.


plantation in thin ®lms. Perusal of the ®gure showsthat the value of dc conductivity is increased andthe activation energy is decreased due to doping/keV energy Ni ion-implantation. Unfortunately,the available conductivity data of keV energy Niion- implanted or of Ni doped thin ®lm samplesare upto about room temperature only; no lowtemperature data are reported in the literature.Drastic e�ect of MeV energy Ni ion-irradiationon the conductivity behaviour is imminent. Exten-sive tailing observed in the conductivity plot atlower temperature is quite striking.

We propose that the electrical transport in theion-irradiated specimen takes place via two modes:

rdc � re � rh; �1�

where re and rh represent extended state and vari-able range hopping conduction, respectively,which are given by

re � r0 exp�ÿDE=kT �; �2�

rh � rho exp ÿ �T0=T �1=4h i

; �3�where T0 � 16r3=kN�EF� is a hopping parameter[22], rho is the pre-exponential constant for hop-ping conduction, k is the Boltzmann's constant,N�EF� is the density of states at the Fermi leveland rÿ1 is the decay length of a localized wavefunction at Fermi level which is taken as 10ÿ9 mfor electrons in Ni atoms [22] and the same valueis used for other ions (Ge, Ag) as well. Here DEis thermal activation energy for extended state

Fig. 12. Temperature dependence of dc conductivity �rdc� for (a) bulk amorphous As2Se3 sample (unbombarded) (h±h), (b) As2Se3

bombarded (annealed at 150�C) with 75 MeV Ni ions at a dose of 5� 1013 ions=cm2

(n±n), (c) undoped As2Se3 thin ®lm [8] (s±s), (d)

As2Se3 doped (co-sputtered) with 5.3 at% Ni [8] (D±D), (e) unimplanted As2Se3 thin ®lm [9] (d±d) and (f) As2Se3 thin ®lm implanted

with 25 keV energy Ni ions at a dose of 1� 1015 ions=cm2

[9] (.±.).


band conduction and r0 is the pre-exponentialconstant. The former �re� predominates in the re-gion (referred to as region II) at higher tempera-tures while the latter �rh� in the region (referredto as region I) at lower temperatures.

Plot of low temperature part �rh� of rdc as afunction of Tÿ1=4 (region I) gives a good ®t tothe experimental data. Typical plots oflog10 rh ÿ Tÿ1=4 for two doses (5� 1013 and1� 1014 ions=cm

2) are presented in Fig. 13. From

the ®tting, the density of defect states N�EF� takingpart in the variable range hopping process hasbeen estimated �1021±1022=cm3� and the valuesare presented in Table 2.

The data of region II (band conduction region)was ®tted with Eq. (2) and the value of the

estimated ®tting parameters DE and r0 are alsosummarised in Table 2 for Ni ion-irradiated sam-ples. The annealing behaviour of ion-irradiatedsamples has some typical features. On annealing,conductivity is reduced and the low temperaturehopping region I (tail in the conductivity plot) be-comes less shallow. This feature suggests that asubset of Ni ion-irradiation induced defect statescontributing to the transport process are gettingeliminated/annealed out. This trend is opposite tothat in Ni ion- irradiated ternary glass,Pb20Ge19Se61, as reported by us earlier [12]. The se-quential annealing e�ect on the electrical transportin Ni ion-irradiated Pb20Ge19Se61 is reproduced inFig. 14, for comparison.

The values of electrical parameters DE (thethermal activation energy), r0 (pre-exponentialfactor) and N�EF� (density of defect states near Fe-rmi level) summarized in Table 2 provide some ad-ditional features of Ni ion-irradiation induceddefects. The irradiation with increasing dose andsubsequent annealing decreases DE from 0.71 (invirgin As2Se3) to 0.3 (in sample irradiated with1� 1014 ions=cm

2). The associated value of pre-

exponential factor r0 is reduced from0:6 cmÿ1Xÿ1 (in virgin As2Se3) to about2:1� 10ÿ6 cmÿ1Xÿ1 (in the irradiated sample).The low temperature value of conductivity is in-creased by several orders of magnitude on ion- ir-radiation. This is due to increased hoppingconduction. Following the argument of Gomi etal. [21], high value of r0 (� 102±104 cmÿ1 Xÿ1) in-dicates mainly extended state contribution to theelectrical conduction. Low values of r0

(10ÿ2±10ÿ4 of lower) suggests the presence of widedistribution of localized gap and tail states. Obvi-

Fig. 13. Plots of log10rh ÿ Tÿ1=4 for 75 MeV energy Ni ion

bombarded (annealed at 150�C) samples at various doses:

1� 1013 ions=cm2

(s), 5� 1013 ions=cm2

(h) and

1� 1014 ions=cm2

(d). The continuous lines represent the least

square ®tting to the corresponding experimental data.

Table 2

Values of parameters for dc conductivity and ac conductivity of bulk amorphous As2Se3 sample bombarded (annealed at 150°C) with

75 MeV energy Ni ions at various doses. The data for unbombarded sample (i.e. at a dose of 0 ions/cm2) are also included

Dose DE (eV) r0 (Xÿ1cmÿ1) N(EF)

(eVÿ1cmÿ3)

e1 Wm (eV) N (defects/cm3)

0 ions/cm2 0.71 6.00 ´ 10ÿ1 ) 25.92 1.8 4.9 ´ 1020

1 ´ 1013 ions/cm2 annelaed at 150°C 0.42 6.85 ´ 10ÿ6 1.5 ´ 1022 ) ) )5 ´ 1013 ions/cm2 annelaed at 150°C 0.30 3.79 ´ 10ÿ6 9.4 ´ 1021 28.93 2.0 2.8 ´ 1021

1 ´ 1014 ions/cm2 annelaed at 150°C 0.30 2.14 ´ 10ÿ6 1.3 ´ 1022 24.57 1.9 8.9 ´ 1020


ously, in the present case the ion-irradiation pro-cess induces wide distribution of localized statesin the band gap. Accordingly, the decrease in DE

is associated with the increased tailing of bandedges and production of localized defect statesnear Fermi-level, as depicted in Fig. 11. The ion-dose dependence of rdc, at various temperatures,

Fig. 15. Dose dependence of rdc for 75 MeV energy Ni ions ir-

radiated As2Se3 glass (annealed at 150�C), at di�erent tempera-

tures; 225 K (h), 276 K (n), 300 K (D), 360 K (s) and 400K

(d).

Fig. 16. Dose dependence of DE and ro for 75 MeV energy Ni

ion irradiated As2Se3 glass (annealed at 150�C).

Fig. 14. Temperature dependence of rdc for bulk amorphous Pb20Se19Se61 sample [12]: (a) unbombarded (h), (b) bombarded with 75

MeV energy Ni ions at dose of 5� 1013 ions=cm2

(s). The remaining curves show the e�ect of sequential annealing when the bombard-

ed sample is annealed at various temperatures: (c) 100�C (´), (d) 150�C (D), (e) 200�C (d) and (f) 220�C (D).


in Ni ion bombarded and annealed (at 150�C)sample is presented in Fig. 15. The conductivityexhibits maximum value at a dose of5� 1013 ion=cm

2. A minimum in rdc is developed

at a dose of 1� 1013 ions=cm2

at higher tempera-ture. The dose dependence of DE and r0 is shownin Fig. 16. Both these parameters decrease withdose and saturate at higher doses. It is to be notedthat electrical e�ects of keV energy and MeV ener-gy ion-irradiation in As2Se3 are quite di�erent, seeFig. 12. This is due to the fact that at MeV energy,electronic energy loss process is the prominentmode for energy loss of energetic ions passingthrough the solid target. The prominent exhibitionof Ni ion-irradiation induced e�ects in the electri-cal transport is associated with the di�erent elec-tronic structure of the transition metal (Ni) ion.It seems that most of the Ni projectiles are elec-tronically active in the As2Se3 target.

4.2.2. AC conductivityThe temperature and frequency dependences of

ac conductivity presented in Figs. 7 and 8 are dis-cussed now.

The correlated-barrier-hopping (CBH) model[23] has been extensively applied to the chalcog-enide glassy semiconductors. According to Elliott[23], the conduction occurs via bipolaron hoppingprocess wherein two electrons simultaneously hopover the potential barrier between two charged de-fect states (D� and Dÿ) and the barrier height iscorrelated with the intersite separation via a Cou-lombic interaction. In the CBH model [23], theelectrons in charged defect states hop over theCoulombic barrier whose height W is given (inSI units) as W � Wm ÿ ne2=�pe1e0r�, where Wm isthe maximum barrier height, e1 the bulk dielectricconstant, r the distance between hopping sites andn is the number of electrons involved in a hop(n � 2 for bipolaron hopping process). The relax-ation time s for the electrons to hop over a barrierheight of W is given by s � s0 exp�W =kT �, wheres0 is a characteristic relaxation time which is ofthe order of the atomic vibrational period and kis the Boltzmann constant. The ac conductivityrac�x� originating from intimate D�±D pairshaving a non-random distribution can be written[23] as

rac�x� � np3e1e0NNpxR6x

6exp

e2

4pe0e1kTgRx

� �:

�4�where

Rx � ne2

pe1e0Wm

1� kTWm

ln�s0x�� ÿ1

; �5�

N is the density of localized states at which carriersexist, Np the density of localized states to whichcarriers hop and Tg the glass transition tempera-ture.

Using the above concepts of CBH model [23]for bipolaron hopping conduction in the localizedstates, and following the procedure laid down inour earlier paper [11], theoretical ®tting of the dataof rac�x� was carried out. In Figs. 7 and 8, thecontinuous curve represents theoretical ®t andthe various symbols correspond to experimentaldata points at di�erent frequencies. The ®t is rea-sonable within about 10±15% at all frequencies.This shows the prevalence of bipolaron hoppingconduction in the unbombarded and bombarded(annealed) samples. The ®tting parameters: maxi-mum barrier height Wm and the participating de-fect density NNp �N � NP�, are indicated in theTable 2. Irradiation and subsequent thermal an-nealing increases defect density taking part inbipolaron hopping conduction by about one orderto magnitude. The defect density estimated fromdc conductivity data of variable range hopping is1±2 orders of magnitude higher than that deter-mined by rac�x� data. This may be due to the factthat not all the irradiation induced defects takepart in bipolaron hopping process.

5. Conclusions

From the experimental data presented here, it isevident that the electrical e�ects of Ni ion-irradia-tion are quite di�erent from that of Ge or Ag ion-irradiation. The Ni ion-irradiation induced defectsstates near Fermi level play a dominant role invariable range hopping conduction. There seemsto be less e�ect on the bipolaron hopping conduc-tion as estimated by the measurements offrequency dependent rac�x�. Small dose (5�


1013 ions=cm2) is quite e�ective to modify the elec-

trical transport property of As2Se3 glass.

Acknowledgements

This work was carried out under UGC-NSCproject No. S-30 for use of the Pelletron accelera-tor at Nuclear Science Centre (NSC), New Delhi,India. The authors thank Prof. G.K. Mehta, Di-rector, NSC, New Delhi and Dr. A.K. Sinha, Con-vener, Accelerator User's Committee, NSC, NewDelhi, for providing the Pelletron accelerator facil-ity. Technical assistance of N.K. Abbi, MadanSingh and Jug Lal of M.D. University, Rohtak,is thankfully acknowledged.

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high energy ion implantation induced electrical effects in bulk amorphous as2se3

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