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Synthetic Metals 158 (2008) 939–945

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

lectronic properties and junction behaviour of micro- and nano-meter-sizedolyanthranilic acid/metal contacts

run Kumar Singha, Rajiv Prakasha,∗, A.D.D. Dwivedib,1, P. Chakrabartib,1

School of Materials Science and Technology, Institute of Technology, Banaras Hindu University, Varanasi 221005, IndiaCentre for Research in Microelectronics, Department of Electronics Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India

r t i c l e i n f o

rticle history:eceived 17 October 2007eceived in revised form 5 June 2008ccepted 20 June 2008

a b s t r a c t

Self-doped conducting polymer, polyanthranilic acid (PANA) was synthesized and used for the first time forfabrication of contacts with the configurations (Al, Ti, Sn metal)/PANA/indium tin oxide (ITO) coated glassand Al/PANA/Pt-nano-probe using mico- and nano-meter thick PANA layers, respectively. The chemically

vailable online 3 August 2008

eywords:olyanthranilic acidonducting polymerchottky diode

synthesized PANA dissolved in methanol was deposited over the substrate (ITO coated glass plate/Al-flatelectrode) using spin coating technique. The current–voltage (I–V) characteristics and diode performanceparameters of micro- and nano-meter sized devices were compared and contrasted by using micro- andnano-contact arrangements. The I–V characteristics of the above two configurations exhibited a rectifyingcontact with Al and Ti metals and an ohmic contact with Sn. The morphology of polyanthranilic acid filmwas studied using atomic force microscope and UV–vis spectroscopic technique was used to obtain the

ymer

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gmnt

lectronic and junction properties optical bandgap of the pol

. Introduction

The conductivity of conjugated polymers may be tuned fromnsulating regime to metallic regime, by chemical modification,egree of doping and nature of dopants making these materialsttractive for use in the fabrication of electronic devices. These,olymers offer the advantages of lightweight, flexibility, corrosion-esistivity, high chemical inertness, electrical insulation and easef processing. Conjugated polymers such as polyacetylene [1–3],olypyrrole [4–6], polyaniline [7–9] and polythiophene [10–12]ave shown semiconducting behaviour. A variety of semiconduc-or devices such as metal–insulator–semiconductor (MIS) diode,chottky diode, tunnel diode, Schottky gate field effect transistor13–17] have been developed using various semiconducting poly-

ers. As polymer/metal contact is the basic components of all theseevices, the electronic phenomenon at the interface is critically

mportant to the performance and function of such devices. The

undamental device properties are affected by interfacial proper-ies, and it is necessary to understand not only the properties ofonducting polymers itself, but also interfacial electronic phenom-na. The fabrication and characterization of Schottky diode barrier

∗ Corresponding author. Tel.: +91 542 2307047; fax: +91 542 2368707.E-mail addresses: rajivprakash12@yahoo.com (R. Prakash),

tbhu.arun@gmail.com (A.D.D. Dwivedi).1 Tel.: +91 542 2307010; fax: +91 542 2307072.

ivfiTptpSmo

379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2008.06.020

.© 2008 Elsevier B.V. All rights reserved.

sing organic semiconductor and their derivatives have been car-ied out in recent years. Conducting polymers like polyacetylene18], polyaniline and their derivatives [19–23], polypyrrole [24–27]nd polythiophene [28–31] have been investigated for Schottkyiode characteristics. The metal/organic semiconductor junction asn alternative to the metal/inorganic semiconductor junction haseen developed, which has opened the new possibility of replacingonventional inorganic devices by organic ones. The performancef such diode depends on factors such as experimental conditionf preparation of polymer as film, stability of polymer and workunction of metal employed for making contact with conductingolymer.

Among the conducting polymers, polyaniline has receivedreater attention due to advantages over other conducting poly-ers because of its reversible oxidation and reduction (redox

ature) [32–33], excellent environmental stability [34], easy syn-hesis and reverse acid/base doping/dedoping chemistry. However,ts intractable nature due to its insolubility in most of the sol-ents and poor mechanical property such as high brittleness oflms showed its major limitations towards practical applications.he solubility is essential for a polymer in order to facilitateost-synthetic processing. Moreover, the performance of Schot-

ky diode using solution processible polymer [35], or processiblere-polymer or oligomer [36] showed considerable improvement.everal efforts have been made to overcome this problem by poly-erization of aniline derivatives with alkyl, sulphonic acid group,

r carboxyl group [37] substitution. Polyanthranilic acid (PANA)

940 A.K. Singh et al. / Synthetic Metals 158 (2008) 939–945

hn

tasipSio

2

2

gAPuodckpfata

2

stf(tSdvdffFabanbr

2.3. Characterization of semiconducting polymer (PANA)

Fig. 1. Device configuration of (Al, Ti, Sn metal)/PANA/ITO glass assembly.

as overcome this problem due to its solubility in aqueous andon-aqueous solvents.

Recently, we have synthesized PANA using various dopantshrough aqueous and non-aqueous solvents [38]. Polyanthraniliccid synthesized using ammonium peroxodisulphate from diluteulphuric acidic solution showed better electroactivity, solubil-ty and electrical properties. In this paper we report the junctionroperties of polyanthranilic acid in Al/PANA/ITO, Ti/PANA/ITO,n/PANA/ITO and Al/PANA/Pt nano-probe configurations by follow-ng the current–voltage characteristics using micro- and nano-filmsf PANA, respectively.

. Experimental setup

.1. Preparation of sample

Ammonium peroxodisulphate [(NH4)2S2O8], acetonitrile (HPLCrade) and sulphuric acid were obtained from Merck, India.nthranilic acid monomer was obtained from Rolex India Ltd.olyanthranilic acid was synthesized chemically in 0.5N H2SO4sing 0.2 M (NH4)2S2O8. Oxidizing agent dissolved in water wasf same concentration as that of monomer in acid. Solution of oxi-izing agent was added drop-wise in the monomer solution withonstant stirring in dark. After complete addition, the solution wasept for stirring for 3 h and then incubated for 48 h at room tem-erature in dark. The precipitate was washed with 0.5N H2SO4ollowed by water and kept for drying for 2 days in vacuum oven

t 40 ◦C. The water was purified and de-ionised with a Milli-Q sys-em (Millipore Corporation, MA, USA) and the DI-water exhibitedresistivity of 18.6 M� cm at 25 ◦C.

Fig. 2. Device configuration Al/PANA/Pt nano-probe assembly.

cs

Fig. 3. XRD pattern of polyanthranilic acid.

.2. Fabrication of devices

PANA was dissolved in methanol to get two separate sampleolutions of concentrations of 20 mg/ml and 0.5 mg/ml. PANA washen spin-coated as micro-film on ITO glass substrate (with sur-ace resistance of 12 �/cm2) by spin coating (at 600 rpm) techniqueSpin Coater; Model PRS 4000, India) using the first sample solu-ion (20 mg/ml) and dried in vacuum. The three metals Al, Ti andn were deposited with area of 0.5027 mm2 using mask on threeifferent samples of PANA/ITO by vacuum evaporation method byacuum coating system from HIND HIVAC (Model no. 12A4D). Theevices Al/PANA/ITO, Ti/PANA/ITO, Sn/PANA/ITO were not sealed uprom attack of moisture and oxygen, but kept in vacuum desiccatoror further characterizations. The device configuration is shown inig. 1. The nano-film of PANA was subsequently spin coated on flatluminium substrate using the other dilute solution (0.5 mg/ml)y spin coating technique in a similar way. The sample was driednd kept in a desiccator for measurement. Three sets of micro- andano-films of PANA based devices were fabricated in three differentatches in order to test the consistency of the characteristics andepeatability of the production.

The semiconducting polymer PANA was characterized for opti-al and electrical properties. UV–vis study was done by using thepectrophotometer from PerkinElmer, Germany (model no. Lamda

Fig. 4. Cyclic voltammograph of PANA in 0.5N H2SO4 at 20 mV/s scan rate.

A.K. Singh et al. / Synthetic Metals 158 (2008) 939–945 941

ted PA

2Widfi(mo

2

ArHt+at

3

3

ieto3aaiapTp

backbone. Surface morphology of the spin coated polyanthranilicacid was studied using AFM under non-contact mode. Compactpolymer film showing lumps of the polymer is shown in Fig. 5.The lumps resolved in the 500 nm × 500 nm scan size probably

Fig. 5. AFM photograph of spin coa

5). Electrochemical studies were done using Electrochemicalork Station of CH Instrument, USA. X-ray diffraction character-

zation was done with 18-kW rotating anode Rigku powder X-rayiffractometer (XRD). Surface morphologies of micro- and nano-lms were studied by using atomic force microscope (AFM)-STMNT-MDT, Russia made, Model no. PRO 47). The AFM/STM measure-

ent confirmed that the thickness of micro-film to be in the rangef 0.4–0.6 �m and that of nano-film to be less than 100 nm.

.4. Device characterization

Current–voltage (I–V) measurements of devices based onl/PANA/ITO, Ti/PANA/ITO and Sn/PANA/ITO structures were car-ied out with HP Semiconductor Parameter Analyzer (SPA) ofewlett-Packard, USA make, Model no. 4145B at room tempera-

ure (27 ◦C). The applied voltage was scanned between −1.0 and1.0 V. The I–V characteristics of nano-layered device Al/PANA withnano-meter sized Pt tip positioned at nanometric distance from

he PANA film using STM at 27 ◦C as shown in Fig. 2.

. Results and discussions

.1. Structural and electrochemical properties

Polyanthranilic acid synthesized by using (NH4)2S2O8 oxidiz-ng agent and doped with SO4

= was studied for their structural,lectrical and electrochemical properties. Powder X-ray diffrac-ion pattern of PANA shown in Fig. 3 exhibits amorphous naturef the polymer as recorded from 2� = 10◦ to 60◦ with scan rate◦/min. Two characteristic broad peaks of polyanthranilic acidppeared as 10◦ and 25◦ as seen from the figure. The electro-activitynd stability of PANA was supported by cyclic voltammetry. Dop-

ng and dedoping (redox nature) of the polymer was shown byppearance of reversible redox peaks when cyclic voltammetry waserformed for polyanthranilic acid coated ITO plate in 0.5N H2SO4.he voltammogram (Fig. 4) of polyanthranilic acid showed fourairs of redox peaks similar to polyaniline [32] due to polyaniline

Fo

NA film under non-contact mode.

ig. 6. (a) UV–vis spectrum of PANA. (b) (˛h�)2 vs. h� plot for bandgap estimationf PANA.

942 A.K. Singh et al. / Synthetic Metals 158 (2008) 939–945

A/ITO,

sttp

3

scaodrTr[

tsIg[

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wlceius

mtg

3

The current–voltage measurements of the micro-film polymerbased Al/PANA/ITO, Ti/PANA/ITO and Sn/PANA/ITO structures werecarried out by using Semiconductor Parameter Analyser (HP 4145B)at an operating temperature of 27 ◦C, by selecting samples from

Fig. 7. I–V characteristics of (a) Al/PAN

how the bundles of the polymers. The I–V characterization ofhe such bundles for very thin films (nano-sized) for Al/PANAaken with the help of Pt nano-probe are discussed later in thisaper.

.2. UV–vis spectra and estimation of band gap

UV–vis spectra of polyanthranilic acid dissolved in 0.5 M H2SO4howed three peaks as shown in Fig. 6(a). The first peak at 280 nmorresponds to the �–�* transition in benzenoid ring, secondt 351 nm attributed to the excitonic transition due to partialxidation of polymer and can be assigned to represent the interme-iate state between leucoemeraldine form containing benzenoiding and emeraldine form containing the conjugated quinoid ring.hird peak in polyanthranilic acid spera at about 533 nm cor-esponds to exciton-like transition quinoid ring or diimino unit39].

The band gap of polymer is evaluated from the absorbance spec-ra of polymer coated on optically transparent ITO glass [20]. Theubstrate absorbance was corrected by introducing an uncoatedTO glass of the same size as the reference. The optical bandap of polymer was estimated by fundamental relation given by40].

h� = B(h� − Eg)n (1)

here ˛ is the absorption coefficient, h� the energy of absorbedight, n = 1/2 for direct allowed transition and B is proportionality

onstant. Energy gap (Eg) was obtained by plotting (˛h�)2 vs. h� andxtrapolating the linear portion of (˛h�)2 vs. h� to zero, as shownn Fig. 6(b). The bandgap of PANA was estimated to be 3.8 eV bysing this method. It may be pointed out here that the bandgap ofelf-doped PANA is significantly larger than externally doped poly- F

(b) Ti/PANA/ITO and (c) Sn/PANA/ITO.

ers such as polyaniline [38]. This may be accounted for the facthat weaker dopant (•COO−) is used in PANA as against SO4

= or Cl−

enerally used in polyaniline (PANI).

.3. Junction properties

ig. 8. I–V characteristics of Al/PANA nano-layered device using Pt nano-probe.

A.K. Singh et al. / Synthetic Metals 158 (2008) 939–945 943

Table 1Electronic parameters of micro-Schottky contacts

Devices (micro-layered) Work function ˚M (eV) Ideality factor (�) Barrier height �B (eV) Saturation current density J0 (A/cm2)

III

aambacfiiiasfimsbTrfr

eetpcbA

J

wsv

TO/PANA/Al 4.28 1.55TO/PANA/Ti 4.33 1.24TO/PANA/Sn 4.42 Ohmic

ll the three batches. The voltage was scanned between −1.0nd +1.0 V in steps of 0.01 V. The measurements revealed that aajority of samples selected arbitrarily from any one of the three

atches of a single configuration exhibit similar qualitative char-cteristics with minor variation in the quantitative values. The I–Vharacteristics (shown by dotted lines with dots) of the three con-gurations Al/PANA/ITO, Ti/PANA/ITO and Sn/PANA/ITO are shown

n Fig. 7(a–c), respectively. The vertical lines indicate the departuren the measured value of the current for the corresponding volt-ges in terms of the standard deviation in the measured value fromamples taken from different batches. It is clearly seen from thegure that both Al and Ti form rectifying contact with the poly-er, whereas Sn forms an ohmic contact. The values of reverse

aturation current for an applied reverse voltage of −1 V have2

een estimated to be 32.3 and 3.05 �A/cm for Al/PANA/ITO and

i/PANA/ITO, respectively. A lower value of reverse saturation cur-ent for the case of Ti/PANA/ITO would make the device attractiveor low-noise applications. The Sn/PANA/ITO exhibits a contactesistance of the order of 50 �.

fS

J

Fig. 9. ln(J) vs. V for (a) Al/PANA/ITO, (b) Ti/PANA/ITO and

0.68 3.23 × 10−5

0.74 3.05 × 10−6

The metal-semiconductor rectifying contacts (where currentxponentially depends on voltage) can be described by thermionicmission-diffusion theory [41] and/or field emission theory inhe case of heavily doped semiconductor. As the semiconductingolymer behaves as a lightly doped p-type material, the electri-al characteristics of Al/PANA/ITO or Ti/PANA/ITO junction haveeen analysed by assuming the standard emission-diffusion theory.ccording to this theory, the J–V relationship is expressed as

= J0

[exp

(qV

�kT

)− 1

](2)

here J(=I/A) is current per unit area, J0 the saturation current den-ity in absence of external bias, q the electronic charge, V the appliedoltage, T the absolute temperature, � diode quality factor (ideality

actor) and k is the Boltzmann constant. Further J0 is related to thechottky barrier height, �B as

0 = A∗T2 exp(−q�B

kT

)(3)

(c) nano-layered Al/PANA/Pt nano-probe assembly.

944 A.K. Singh et al. / Synthetic Met

Table 2Electronic parameters of nano-Schottky contacts

Devices (Nano-layered) Al/PANA

Work function �M (eV) 4.28IBS

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J

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deality factor (�) 1.16arrier height �B (eV) 0.95aturation current density Jo (A/cm2) 7.58 × 10−10

here A* is the effective Richardson constant. By making use ofree electron value for A* as 120 A cm−2 K−2 [42], �B can be evalu-ted from J0. The value of �B for Al/PANA/ITO, Ti/PANA/ITO basedchottky diode can be evaluated from

B = kT

qln

[A∗T2

J0

](4)

onsidering the forward J–V characteristics for V > kT/q, the Eq. (2)an be approximated as

= J0 exp(

qV

�kT

)(5)

ccording to Eq. (5), ln(J) vs. V should be linear for Al/PANA/ITO,i/PANA/ITO Schottky diode configurations. The variations arehown in Fig. 8a–c. The value of ideality factor for these devicesas determined from the slope of the plot ln(J) vs. V at a particu-

ar temperature (27 ◦C in this case) and reverse saturation currentas determined by the intercept of ln(J) vs. V at V = 0. Using the

alue of J0 the barrier height was determined from the Eq. (4). Thedeality factor, barrier height and reverse saturation current den-ity of micro-layered Al/PANA/ITO and Ti/PANA/ITO Schottky diodesxtracted from the measured J–V characteristics are listed in Table 1.he value of ideality factor (�) for these Schottky diodes is higherhan the ideal value (unity). The deviation of ideality factor fromnity is due to barrier inhomogeneity at the surface of polymerlm.

In the nano-layered Al/PANA configuration, polymer lumps dis-ersed over the metal plates showing high porosity was indicatedy the STM. A single lump was targeted to record the I–V char-cteristic of the nano-layered device using a nano-probe made oflatinum. The measurement was carried out using the arrangementhown in Fig. 2. The I–V characteristics of the nano-layered Al/PANAonfiguration obtained by positioning the nano-sized Pt tip aboveano-lumps/bundle under STM are shown in Fig. 9. The I–V char-cteristics of nano-layered Al/PANA/Pt-nano-tip resembles that oficro-layered Schottky configuration discussed earlier except for

he fact that the reverse saturation current in the former case isuch smaller than the later ones. It is seen that the forward cur-

ent also start increasing sharply at much lower voltage in the casef nano-layered device. The cut-in voltage in the case of nano-ayered Al/PANA has been estimated to be 0.2 V as against 0.5 Vn the case of Al/PANA/ITO and 0.6 V in the case of Ti/PANA/ITO

icro-layered Schottky diode configuration. A low value of cur-ent in the case of nano-layered Al/PANA configuration is attributedo the narrow air-gap existing between the semiconducting poly-

er and the nano-tip Pt electrode which accounts for involvementf tunneling mechanism in the transport of charge carriers. How-ver, in the present analysis we apply the conventional thermionicmission theory of Schottky contact for extracting the parame-ers of the nano-devices. The extracted parameters are listed inable 2.

Further an extremely low value of reverse saturation currentf both micro- and nano-devices in comparison to conventionalemiconductor diodes would make these devices very attractiveor low noise application in the detection of optical signal in theltraviolet region.

[[[

[

als 158 (2008) 939–945

. Conclusions

Polyanthranilic acid a self-doped conducting polymer was syn-hesized and used for the first time for fabrication of Schottky

icro- and nano-devices. The I–V characteristics were measuredith micro- and nano-tips and performance parameters like barriereight, ideality factor and reverse saturation current were calcu-

ated for various metal interfaces for both the devices. The polymerormed rectifying contacts with Al and Ti metal and ohmic con-acts with Sn. The barrier height is estimated to be higher in thease of nano-layered Al/PANA in comparison to the micro-devices.cheaper metal like Al may be used with low cost plastic mate-

ial such as PANA to produce low-cost Schottky diodes. Furtherow value of reverse saturation current and photo-absorbance ofANA in the UV region would make these Schottky diodes attrac-ive for use as low-noise photodetector in the ultra-violet region.he optical characterization of the devices is currently under-ay.

cknowledgement

Authors are thankful to Prof. D. Pandey, SMST, IT, BHU for fruitfuliscussion and suggestions.

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[

[

[

als 158 (2008) 939–945 945

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