Bhuiyan, A H

Chowdhury, F-U-Z

English

IACS Institutional Repository

'Ill J phys. 76A (3). 239-244 (2002)U Pan international journalThe dc electrical conduction mechanism of heat-treated plasma-polymerizeddiphenyl (PPDP) thin filmsF-U-Z Chowtlhury** and A H Bhuiyan*"“ D epartm en t o f Physics, Bangladesh Institute o f Technology. C hittagong,C h itta g o n g -4 3 4y . BangladeshD epartm en t o f  Physics, Bangladesh U nivers ity  o f Lng inceriiig  A  Technology,D h a k a -loot). BangladeshE -m a il laruquc_.LCg>bitctg netReceived 20 Novenihet 200J. uctepled 12 Fehnuny 2002Mistraci ‘ Th in , un ifo rm  and pm holc-Irce p lasm a-po lym cn/ed  diphenyl (P P D P ) films were deposited using a capacitively coupled glow discharge rL‘,iLii)r The infrared ( IR )  analysi.s reveals that the heat-treated P P D P  h im  structure is d ille re n t from  that ol the m onom er d iphenyl. ,Study on dc Jail lull coiuluetion m echanism  has been perform ed on heat-treated P PD P films of d ille ren t thicknesses It is seen that Poole-Prenkcl mechanism is Iniiml m he operative in the heat-treated  P P D P  film sK m iords  E le c tr ic a l conductio n , in frared  spectroscopy, plasm a po lym erization  r \ ( ’S Nos. 73.,SO Fq, 7K 30.Jw , Kl l5 .G h1. Litrodiiction1!il pindiiclion ol polymer thin films of organic vapours by jfow discharge techniques has been known for many years [ 1- d The plasma polymers and plasma-modified surfaces [4, 5] ki\c gamed considerable interest among the researchers. Plasma p o l y m c i s  have few additional advantages compared to the u iivcniionally synthesized polymers, like excellent adhesion I" ilic substrate, ease of preparation in completely dry iimnsplicrc and possibility of preparing extremely thin, uniform md pmholc-lrcc films. Owing to the wide range of possible luhiiological applications of these films in various electronic devices, an elucidation of the underlying carrier generation and I'iiiispnri processes in them is important. Because of their high 'esishviiy, high dielectric breakdown strength and cost (^ llceiivcness, these materials have found important applications ihm film insulators and capacitors in integrated niicmelccironics, passivation layers for integrated circuits, dry I'diography for VLSI, electron beam or X-ray lithography, proicctivc coatings and photoconductive devices [6-8].^'Any scientists have investigated the electrical properties various organic compounds produced by the glow discharge^oritsponcJing A u th o rtechnique. They have observed that three different conduction processes are involved. The first one is the injection of earners from the electrode by means of thermal and field assisted emission, and is referred to as Schottky mechanism. 1Tie second process, in which the carriers arc produced by the dissociation of donor-acceptor centres in the bulk of the material, is called Poolc-Frcnkcl (P-F) conduction. If the generation proee.sscs are slower than the transport of carriers through a dielectric, the conduction is controlled by either the Schottky or the P-F mechanism. On the other hand, when the transport is slower than the generation of carriers, it constitutes the rate-determining step and the conduction can then be described as a space charge limited (SCL) conduction mechanism. A procedure commonly used to distinguish among these mechanisms is to examine the dependence of the current (I) or current density (J) on the appl ied voltage (V) and the sample thickness (d) and to compare these relationships with those predicted by the respective theories I9-14].Particular interest has been established in the phenyl oligomers since the discovery that poly-p-phenylene could be made conducting by doping with acceptor or donor f 15|. It may be noted that the reported intrinsic dc conductivity value of poly p-phcnylene is of the order of I0~’^  m"* that could be©20021ACS240 F-U’Z Cho\v(llnir\ and A H Hhuiyanincreased lo over 10** 2^"* m"‘ by eloping. Allhough, intcrcsl has been shown on diphenyl (16, I7| ilierc are scanty ol rcporls on synthesis and characlcri/alion. Recently, the optical [I8J, chcTTiical composition [ 19) and dielectric propcilics 120] of PPDP thin films have been investigated. In this paper, an attempt is made to elucidate the probable dc conduction mechanism of PPDP thin films heai-lrcalcd at 423 K.2. ExpcrimcnLil details2. /. Sample preparation :Monomer diphenyl ) of BDH Chemicals Ltd., Hnglandwas purchased from local market. The PPDP thin films were prepared using a belPjar type capacitivcly coupled glow discharge system. Chemically cleaned microscope glass slides were used as substrates. The film deposition conditions utilized m this investigation arc (i) substrates were placed on the top of the bottom electrode, (ii) maximum deposition time was 1 hour,(iii) electrode separation was 0 04 m, (iv) deposition power was about 40 W al 50 Hz, and (v) pressure duiing deposition was about 30 Pa. The more details of plasma polymerization system and the control of deposition paiamcters used m this study has been reported elsewhere |2 1).Transparent light yellow colour PIU^P thin films up to a few hundred nm thick were deposited onto the glass subsiiates. Samples were prepared under the same deposition condition. The thickness was determined by producing a well-tlefmed film outline using mask and then measuring the step height by interfcromelry technique.2.2 IR Spectroscopy :The PPDP deposited on to the substrates was collected by scrapping off from the substrate before heat treatment. The KBi pellets of monomer diphenyl and PPDP films heat-treated at 473 K (for two hours) prepared and IR spectra were recorded al room temperature on an IR spectrometer (IR-470, Shimadzu Corporation, Japan) All the spectra were recorded in transmittance (%) mode m the wave number region 4(X)0 to 4(K) cm' *.2.3 dc measurements :Aluminium (Al of purity 4N (British Chemical Standard) was coated by vacuum evaporation al about 10^ Pa to act as lower electrode onto chemically cleaned glass substrates by Edward’s coating unit (model E-306). The PPDP thin films were deposited over the lower electrode and finally Al was evaporated on PPDP thin lilm as bclore lo act as upper electrode. A number of PPDP lilms were hcai-ircalcd al 423 K for a period of two hours in a furnace.Fordc measurement, samples of sandwiched (A l/PPDP/Al) structure with vacuum evaporated Al metal thin film electrodes on both sides were used. The electrical measurements were carried out in a stainless steel chamber under inert (argon)atmosphere to avoid oxidation. The circuit diagram for curicm voltage measurements is shown in Figure I. The current uck, the hcat-ircaicd PPDP thin films was measured at diff. /  temperatures by a digital Kcithlcy electrometer 614, and tii! voltage was supplied using a stabilized dc power supply (|h 5()(K)N of Ogawa Seiki Co. Ltd., Japan).Fif>urc 1. A  schematic c ircu ii d iagram  o f dc m casuiemcntT h e  samples were healed by healing the specimen chdinhc. externally by a heating coil during measurement. A vaiuhi,. resistance was used lo control the current through the coil )1k> icmperalure was recorded by a Cr-A I thermocouple placed close lo the sample which was connected lo a Keithlc\ i‘i7 \ digital mullinieter.3. Results and discussion3 ./ IR spec troscopy .\I'hc IR spectra of monomer (diphenyl) and PPDP heal iicaic 1 n 473 K lor two hours arc presented in Figure 2. The IR spcLii.ioi diphenyl and heat-treated PPDP film are rcprcsenicd hy speuu A and B, respectively. These spectra reveal the hcai-iuMiui PPDP structure is different from that of monomer.W avenum ber (cm) ’F ig u re  2. The IR  spectra nl m onom er ( (C T D d -  ^ " ‘1 bl"*^ "treated ai 4 73  KThe spectrum A is well matched with the standard IR spectrum of diphenyl [22]. The band assignments ol nionoiiK’i diphenyl are depicted in Table I. In the spectrum B of hcai treated PPDP, a broad band around 3450 cm“' is due U) du presence of 0 -H  stretching vibration for absorbed water m PPDP film. The vibration al 2915 cm”' is owing lo the C H stretching of aromatic ring structure. The wide band at 1450 cm '' can be assigned lo aromatic ring stretching vibraimn for C = C in the structure and lo the stretching vibration ofC = ^The dc e lectrica l conduction mechanism etc Table 1. assignment o f  m onom er ((C ^ H ),) ,.241Peak position ( c n r ‘ ) Band assignment: ‘M 51 9 0 0 -1 6 6 7  1 6 S 0 - I4 5 0  1 1 7 0 -1 0 4 0  740 -6 8 0, ( C - H )  o f arom aiic  ring  O v e r tones o f  arom aiie  ring  ,(C  =  C ) o f aromatic ring  g(C =  H ) in-planc  j (=  C - H )  o u l-o f-p lan c, -sirciching, 6 -b e n d in gjioup which IS typical for plasma polymer. The weak bands at 1140 950 cm ' correspond to the C = H m-planc bending and hands SOO, 760 and 700 cm“' correspond to the = C-H out-of- plane phenyl ring bending. Thus, the IR spectrum of heat-treated PPDP sht^ws the main absorption band features of the monomer Jiphcnyl except for bands corresponding to C = O and O-H 118|. The above observations reveal that the PPDP films contain an aioiualic ring structure with CH side groups. Thus, the hcat- Dcaicd PF’DPthm film composed not only of the corresponding monomer bands but also of new bands which were absent in the monomer spectrum, namely C = O and O-H stretching vibration. I hc band assignments of heat-treated PPDP film are summarized 111 lable 2.|.ibU‘ 2. IR  band assignm cni o f hca l-lrcatcd  PPD P  filml \  .ik position (cm  ')U  SOsISSO I4 S 01 I lO-OSOKOO7no/()()Band assignment( O - H )  |« e u l( C - H )  o f a iom ntic  ring  ^(C =  C ) o f aiom alic  ring and (C  =  0 ) [new]^ (C = H ) in -p lane  ^(= C - H )  o u l-o f-p la n c  ^(= C " H )  o u t-u l-p la n c  C - H )  ou t-o f-p lancetc. The criterion for establishing the predominance of a particular mechanism is to analyze the dependence of the current density on these parameters.The detailed information about the transport mechanism through the heat-treated PPDP may be obtained from the curicnl density vs. applied voltage (J~-V) characteristics analysis. The J -V  characteristics of heat-treated PPDP thin films of different thicknesses and at different lemperatuies were investigated in the dc voltage range 0.2-15 0 V. The voltage dependence of current density or cuiTcnt can be expressed asJ o c V \ (I)where n is a power factor. When /i = I, the conduction is ohmic and n > 2 .  the conduction is non-ohmic.Typical J -V  characteristics of these samples are presented in h'iguie 3. In these plots, two diffeient voltage regions are observed with slopes n -  1 and ~ 2 in ihc lower (ohmic) and higher voltage regions (non-ohmic), respectively. In the first region, current density is observed to be proportional to the applied voltage (ohmic region). In the second region, a power dependence of current density on the applied voltage i,e. / *x V" occur with 2.02 < n < 2.33 This is a ccmimon feature of most organic layers with low mobility and high resistivity. It is observed that the value of the exptmenl n decreases with increasing temperature.' MiiMchiiig, 8 -b c n d in gThe lormation of carbonyl group is usually attributed to the ‘^ xitlaiion of the hydrocarbon part of the PPDP after exposure to iiir owing to oxygen reactions with a radical species (dangling honds) trapped in the structure of plasma polymer. Cross-linking niay also (^ccur between different carbons of the chains due to iIk‘ loss of hydrogen, particularly in the plasma-polymerized hims because of the impact of energetic particles within the plasma during deposition.- Electrical properties :the modes of carrier transport in insulating polymer films, 'anous mechanisms has been suggested based on the current density dependence on voltage, temperature, film thickness.F ig u re  3. Plots o f  currcni density against applied voltage ai d iffe ie n i m easurem ent lempcraiurc.s for hcai-ircated PPD P film  {d = 2 00  nm )The slopes arc found to decrease with increasing thickness in both regions. This type of change in conduction process is not unusual because of the carrier motion may be thickness dependent, ie ., the conduction current in the heat-treated PPDP films may be inversely proportional to the power of the sample thickness; this will be discussed below. A slope n > 2  suggests242 F U-Z Chowdhury and A H Bhuiyanthe possibility of space charge limited (SCL) conduction [23) and the current density can be presented as :9 (2)where is mobility of charge carriers, e is permittivity of the dielectric material, £ q is permittivity of free space, Vis the applied voltage and d  is the thickness.TTic voltage dependence of current suggests that the current may be SCL in the heat-treated PPDP films above 5.0 V. The thickness dependence of the cunent in the SCL region follows the relationJ ^ d ' (3)where 1 is a parameter depending upon the trap distribution; in the presence of traps, J > 3.In Figure 4, the J  at room temperature is plotted against d. The slopes are found to be 1.25 and 1.63 in the first and secondJ = JqexpP F ''^ - < pkT (4)whereto is the low field currem density; p  isthecoerricicm p is the static electric field and is equal to V/d; V is the appi^j voltage, d  is the film thickness; (p is the ionization energy d localized centres in Poolc-Frenkcl conduction and Coulombie barrier height of the electrode-polymer interface in Schmi  ^conduction; T  is the absolute temperature, and k is Boltzmann constant. The coefficient f j  for the Schottky-typ  ^mechanism is known as Schottky coefficient, f}^, and is defined as1/2Ps =e47t€Qe (5)For the P-F mechanism, it is called Poole-Frenkel coc(Tiiieni P Pf. , and is defined asP p F  -{  1 ^ e1/2where e is the electronic charge, £ is the high frequenu dielectric constant (4.71 for heat-treated PPDP sample luivin^  ^thickness, d -  2(X) nm) and £ q is the perm ittiv ity  of free sjkilcFor a constant applied voltage at a particular tcnipcrjiun the current density expression (cq. (4)) can be written aslo g 7 = < r ''^  or ( / 'Thus, for the P - F  or the Schottky mechanism, the ploi ot the log 7 against the square root of the film thickness, d' . should be a straight line. Figure 5 shows a plot of 7 versus 7' for heat-treated PPDP thin films. It is observed iluii the experimental points fit a straight line, in accordance \m i1i cq.(7).Thickness, d (nm)F ig u re  4. Plois ot ciirrcnl density apiimsi thickness for the heat-treated  P PD P film s 111 the ohm ic and non-ohm ic regionsvoltage regions, respectively. In the .second region, the slope is much less than that corresponding to SCL conduction. These observations suggest that the conduction in heat-treated PPDP may be different from SCL conduction. As the PPDP were grown in a plasma discharge, they arc likely to contain a high concentration of trapping sites. Since SCL conduction does not occur in this material, an alternative assumption is that the conduction mechanism may either be Poole-Frenkel (P-F) or Schottky type. The general expression for the 7 that is equally valid for both the P-F and Schottky mechanism is of the formFigure 5. The dependence of current density J on square rooi olsample thickness for the heat-treated PPDP films in the region.The dc electrical conduction mechanism etc 243Purihermore, in order to ascertain the most probable induction mechanism in heat-treated PPDP films, plots of J v e r s u s  at different temperatures arc shown in Figure 6. ItVoltage (V ’'»)hRiirc 6. V aria tio n  o f  c u rre n t d e n s ity  J  w ith  square root o f  ap p lied  volUi'L- V ' -  fo r the h ea t-trea ted  P P D P  film s  at d iffe re n t tem peraturesu/ -  200 nm)matching between the values of /3 (experimental) is observed with the Ppfr (theoretical). This again indicates the dominance of P-F type conduction in the heat-treated PPDP films.4. ConclusionsThe IR analysis and electrical conduction mechanism of heat- treated PPDP have been discussed. The IR analysis indicates that the structure of heat-treated PPDP thin film deposited by glow discharge technique is different from that of monomer because of possible cyclizalion/aggrcgation and cross-linking in the film structure. On heat treatment, cycli/ation/aggregaiion may occur by the removal of radicals and/or dangling bonds. The study of the current-voltage characteristics reveals that the dc conduction of heat-treated PPDP thin film is governed by Poole-Frcnkel mechanism.AcknowledgmentsThe authors arc thankful to the authority of Bangladesh University of Hnginccring & Technology, Dliaka for providing the financial support. One of us (FUZC) is grateful to Bangladesh Institute of Technology, Chittagong for study leave.T a b le  3 . Values o f d, f ' ,  T, P,P^ and for hcal-Ircaied P PD P ihin film sF ilmthickness  d (n m )D ie le c tr icc o n stan t€'M easurem ent tem p e ra tu re  T  (K )E x p e r im e n ta lly  observed P (eV -m '^ 'VValues o f p^ from  eq ^(cV -m '^ -V  "-)Values ol p  ^lio m  eq 4 pf.f. ( e V -m ''V4 5 0 2 H3 X 10-'4 0 0 3 16 X 10-'3 0 0 2 ,62  X 1 0 '2 0 0 4 .7 1 2 9 8 2 6 0  X 10 ' 1.74 X 10-' 3.48 X 10-'2 0 0 4 7H 3 2 3 2 72 X 10-' 1 73 X 10 ' 3 46 X 10 '2 0 0 4 85 3 7 3 2 80 X 10^' 1 72 X 10-' 3 44 X 1 0 '2 0 0 4 9 3 4 7 3 2 .98  X 10 ' 1 71 X 1 0 ' 3 42 X 10-'2 0 0 5 10 5 2 3 3 29  X 10-' 1 68 X 10 ' 3 36 X 10 'can be seen that these curves are linear, again indicating the prchcncc of P~F or Schottky type mechanism. To identify the cltJmmant mechanism, the values of P  (experimental) were ‘’^ l^aincd from the slopes of the plots in Figure 6. The values of Ps theoretical) and P pp (theoretical) arc calculated using eq. (5) and (6), respectively. The experimental p  values of different *'•"1 ihicknesses at different temperatures are compared with the ilicorciical p ^  and Ppp  values in Table 3. From the different 'alucs of p  (experimental) and P (calculated from eq (4)) as depicted in Table 3 that the p  (experimental) is in between the P s  and Ppp value. But p  (experimental) values increases both thickness and measurement temperature. BetterReferences[1 ]  J Goodm an J . Ptdxin S a  44  S 5I ( I9 6 0 )[2 ]  P W h ile  Insulation 13 52 (1 9 6 7 )[3 ]  L  V  G regor Thin Solid Film 2 235 (1 9 6 8 )14] F  F  Shi Surf Coat. Technol. 82  I (1 9 9 6 )15 ] N In a g a k i in Plusmu Surface M odification and Plasma Polymerization (U S A  : Tcchnom ic Publishing Co Inc ) p 144 ( 1 9 9 6 )[6 ]  R C hristy  J . Appl Phys 31 1680 (1 9 6 0 )[7 ]  T  H ira i and O  Nnkada Jpn. J . Appl Phys. 7 112 (1 9 6 8 )[8 ]  D  Brosset, Bui A i and Y. Segui Appl. Phys. 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A B M O  Islam and A H  Bhmyan Vai 43 (2000)[20] F -U -Z  Chowdhury and A H Bhuiyan Thin Solid Films 3?o t( 2000 ) '[21] F - U - Z  Chowdhury PhD  Thesis  (Bangladesh Urm oisn^  Engineering & Technology) (2000)[22] Robert T Conley in Infrared Spei troscop\\ 2nd edn (Llnsm,. MA , Allyn and Bacon) p IK) ( 1972)(23| M A L.umparl J AppI Phys. 35 2738 ( 1964)

The DC Electrical Conduction Mechanism of Heat-treated Plasma-polymerized Diphenyl (PPDP) Thin Films

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The DC Electrical Conduction Mechanism of Heat-treated Plasma-polymerized Diphenyl (PPDP) Thin Films

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