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Synthetic Metals 157 (2007) 231–234

Influence of temperature on charge transport and deviceparameters in an electrospun hybrid organic/inorganic

semiconductor Schottky diode

Raul Perez a, Nicholas J. Pinto a,∗, Alan T. Johnson Jr. b

a Department of Physics and Electronics, University of Puerto Rico, Humacao, PR 00791, USAb Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA

Received 22 January 2007; received in revised form 12 February 2007; accepted 13 February 2007Available online 6 April 2007

bstract

The temperature dependence of a Schottky diode fabricated from an electrospun doped polyaniline nanofiber on an inorganic n-doped siliconubstrate has been studied in the temperature range 180 K < T < 300 K. The standard thermionic emission model of a Schottky junction with andithout a series resistance was utilized to analyze the data. No significant difference in the values of the device parameters were observed via these of either method. Charge transport in the ON state of the diode was compared to that in an isolated electrospun doped polyaniline nanofiber

nd the temperature dependence of the diode resistance was seen to deviate from the quasi 1-D variable range hopping that characterizes chargeransport in doped polyaniline. The constrained diode architecture permits the simultaneous existence of multiple charge transport mechanismsnder normal operating conditions making this an interesting system for further study.

2007 Elsevier B.V. All rights reserved.

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eywords: Fiber; Diode; Electrospinning

. Introduction

The synthesis of �-conjugated conducting polymers that aretable in air has stimulated their use as active components in elec-ronic devices. These devices are considered by many in the fieldo shape the next generation of cheap and disposable electronicnventions. Most polymer based devices typically are fabricatedaving the active organic component in the form of thin filmsith 2-D geometry [1,2]. The ability to prepare nanofibers of

uch polymers via electrospinning has opened a new route tohe fabrication of devices and sensors with quasi 1-D structures3–6]. The simplest and easiest polymer based device to fabricates a hybrid organic/inorganic Schottky diode in which a junctions formed between a p-doped polymer and an n-doped inorganic

emiconductor. This construction has been achieved via electro-hemical polymerization [7–10] or spin coating [11] the polymernto the n-doped semiconducting substrate. We reported earlier

∗ Corresponding author. Tel.: +1 787 850 9381; fax: +1 787 850 9308.E-mail address: nj pinto@uprh.edu (N.J. Pinto).

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379-6779/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2007.02.002

n the fabrication of such a Schottky diode using a n-dopedi/SiO2 substrate and an electrospun fully doped polyanilineanofiber at room temperature [12–14]. Being a majority carrierevice there is no diffusion capacitance associated with a for-ard biased Schottky diode [15]. In addition, the relative easef their fabrication using our technique and the compact size ofhese diodes makes them ideal candidates for use in low powerigh frequency applications.

Under thermal equilibrium conditions with no appliedxternal bias the Fermi levels of the n-doped Si and that of the-doped polyaniline in the device must be coincident leadingo band bending at the junction via the flow of charge fromhe semiconductor into the polymer. This process creates aarrier to charge flow at the interface of the nanofiber andhe semiconductor whose height is determined by both theolymer metal work function and the surface states on theemiconductor [16]. In this paper, the influence of temperature

n a Schottky diode has been studied and the temperatureependence of the device parameters reported. The standardhermionic emission model of a Schottky junction with andithout a series resistance was utilized to extract the device

2 Metals 157 (2007) 231–234

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Fig. 1. (a) Schematic of the prepatterned and then cleaved n-doped Si/SiO2

substrate and the electrospun polymer nanofiber making contacts to the goldelectrode above and to the doped Si below the oxide layer. The real fiber isfleo

nbpdhutas temperature is decreased. The diode turn-on voltage obtainedby extrapolating a linear fit to the data in the forward biasedregime (VB > 1 V) to the voltage axis at zero current is seen to

Fig. 2. Current–voltage characteristics at selected temperatures of a Schottky

32 R. Perez et al. / Synthetic

arameters. No significant difference in the inferred devicearameters is observed via the use of either method. Theemperature dependence of the dynamic resistance of the dioden the ON state is compared to that of an isolated polyanilineanofiber and has been analyzed using the quasi 1-D variableange hopping model that has been shown to be appropriate foronducting polymers [17]. Charge transport in the diode is seeno show deviations from 1-D variable range hopping implyinghat a combination of other charge transport mechanisms maye operative in the device under normal operating conditions.

. Experimental

A commercially available n-doped Si wafer (<1,1,1>,.1–1.0 �-cm) with a 200 nm thermally grown oxide layer wassed as the substrate and formed part of the diode construction.fter prepatterning gold electrodes over the oxide via standard

ithography and lift-off techniques the substrate is cleaved inir through the electrodes. The exposed cleaved surface has thedge of the gold electrode separated from the doped Si by thensulating oxide layer. A doped polyaniline nanofiber (diam-ter ∼ 100 nm) was then deposited over the wafer edge vialectrospinning and completed the Schottky diode construction.he polyaniline nanofiber was prepared as follows: 100 mg ofmeraldine base polyaniline was doped with 129 mg of cam-horsulfonic acid (HCSA) and dissolved in 10 ml CHCl3 for aeriod of 4 h. The resulting deep green solution was filtered and0 mg of polyethylene oxide (PEO) having molecular weight00,000 was added to the solution and stirred for an additionalh. PEO was added to assist in fiber formation and the solutionas then filtered using a 0.45 �m PTFE syringe filter. Usingvery simple electrospinning technique reported earlier [3,18]

ndividual, charged, dry and flexible polyaniline nanofibers wereeposited over the wafer edge making contacts to the gold andhe doped Si and that are stable with no apparent degradationr oxidation. The resulting Schottky diode is formed along theertical edge of the substrate at the nanofiber-doped Si interface.uch a vertical orientation may offer higher levels of integration

n circuitry than that provided by in-plane horizontal structures.xternal electrical contacts to the device were made via the usef gold wire (diameter ∼ 25 �m) and silver paint. The device washen mounted inside a closed cycle helium refrigerator whoseemperature was controlled via a Cryo Con Model 32B tempera-ure controller. The device current–voltage (I–V) characteristicsere measured at fixed temperatures using a Keithley Model517A electrometer in a vacuum of ∼10−4 Torr. Fig. 1(a) showsschematic of the device together with the external electrical cir-uit and Fig. 1(b) shows a scanning electron microscope (SEM)mage of a polyaniline nanofiber at the wafer edge in a typicalevice. The fiber appears continuous with no visible signs ofracture.

. Results and discussion

Fig. 2 shows the I–V characteristic curves at a few selectedemperatures when the positive terminal of VB was connected tohe gold electrode and the negative terminal was connected to the

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exible and does not fracture as it bends over the substrate edge. The externallectrical connections are also shown. (b) Scanning electron microscope imagef the electrospun polyaniline nanofiber at the wafer edge in a typical device.

-doped Si. Reversing the polarity of VB results in the forwardias diode response lying in the third quadrant [12]. As tem-erature is lowered the forward bias current is seen to decreaseue to an increase in the dynamic series resistance of the diodeowever the current under reverse bias appears to be relativelynaffected by temperature. As a result of this current loweringhe device rectification ratio calculated at ±2 V also decreases

iode when the positive terminal of VB was connected to the gold electrode. Inset:lots of the diode turn-on voltage-VON (�) and the rectification ratio-ION/IOFF

©) calculated at ±2 V as a function of temperature. The diode turn-on voltages obtained by extrapolating a linear fit to the data in the forward biased regimeVB > 1 V) to the voltage axis at zero current.

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ave a weak temperature dependence and increases slightly asemperature is lowered. These results are shown in the inset toig. 2.

In order to quantitatively analyze the diode characteristics wessume the standard thermionic emission model of a Schottkyunction as follows [19]

= Js

[exp

(qVB

nkT

)− 1

](1)

s = A∗T 2 exp

(−qφB

kT

)(2)

here J is the current density, Js is the saturation current den-ity, q is the electron charge, k is the Boltzmann constant, Ts the absolute temperature, φB is the barrier height and n ishe ideality factor which takes into account corrections to theriginal simple model, e.g. image–force barrier lowering. Theichardson’s constant (A∗ = 4πqm∗ k2/h3) is calculated to be20 A2/K2-cm2 assuming m* is the bare electron mass. The inseteft plot in Fig. 3 shows a representative semilogarithmic plotf the diode current versus applied voltage under forward biasonditions at 300 K. At low biases a linear variation of the cur-ent is observed consistent with Eq. (1) while the deviation frominearity at higher bias voltages generally is related to Ohmicosses due to the diode series resistance. Extrapolating the lin-ar portion of the semi-log plot to zero bias yields a saturationurrent density of 5.95 × 10−2 A/cm2 and the diode ideality fac-or calculated from the slope of the linear portion of the plot asollows

= q

kT

(∂VB

∂ ln J

)(3)

s n∼5. Using these equations we calculate the barrier heightf 0.49 eV. The variation in n and φB as a function of temper-ture is shown in Fig. 3. The ideality parameter appears to be

ig. 3. Temperature dependence of the diode ideality parameter (n) and thearrier height (φB). Symbols (�) and (©) represent the ideality parameter andymbols (�) and (�) represent the barrier height calculated using the simplehermionic emission model of a Schottky diode with (filled symbols) and withoutempty symbols) the series resistance respectively. No significant difference isbserved in the results via the use of the two methods. Inset: (left) representativeemilog plot at 300 K of the forward biased current vs. VB. (Right) representativelot at 300 K of the derivative of the bias voltage with respect to J vs. J for

B > 1 V and is used to extract parameters according to Ref. [22].

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ls 157 (2007) 231–234 233

elatively constant in the temperature range 250 K < T < 300 Kelow which it increases slightly. The high values (n > 1) of thedeality parameter have been attributed to several factors thatnclude the recombination of holes and electrons in the deple-ion layer [20], the presence of an interfacial layer and interfacetates at the polymer–semiconductor interface [21] or even aunneling process [11]. Since in the diode fabrication processhe silicon substrate is not vacuum cleaved there exists a strongossibility of the presence of dangling Si bonds at the cleavedurface that could interact with the fiber leading to the interfacialayer mentioned above.

In an alternate approach that includes the dynamic seriesesistance of the diode (Rs) Eq. (1) can be rewritten as [22]

= Js

[exp

q(VB − JRsAeff)

nkT− 1

](4)

here Aeff is the effective area of the Schottky junction∼7.85 × 10−11 cm2). This equation takes into account the facthat some of the applied bias voltage (VB) is dropped across theeries resistor so that the voltage across the Schottky diode isess than VB. In order to extract the device parameters from aingle IV measurement we have utilized the method below asuggested by Cheung [22] and compared these values to thosextracted using Eqs. (1)–(3). By defining β = q/kT , and usingq. (2), we can rewrite Eq. (4) as follows for VB > 3kT/q

B = RsAeffJ + nφB +(

n

β

)ln

(J

A∗T 2

)(5)

Taking the derivative of Eq. (5) with respect to ln(J) we obtain

d(VB)

d(ln J)= RsAeffJ + n

β(6)

subsequent plot of d(VB)/d(lnJ) versus J will yield RsAeff and/β from the slope and the intercept respectively from which Rsnd n can be determined. The inset right plot in Fig. 3 showsuch a representative plot at 300 K. The barrier height can alsoe obtained by defining H(J):

(J) ≡ VB −(

n

β

)ln

(J

A∗T 2

)(7)

nd rewriting Eq. (5) as follows

(J) ≡ RsAeffJ + nφB (8)

rom which the plot of H(J) versus J will yield RsAeff and nφBrom the slope and intercept, respectively. Using the value of nbtained above one can then calculate φB and further, the valuef Rs can be double checked for consistency with that obtainedarlier. Part of the results of this analysis to obtain n and φB arehown in the inset right plot of Fig. 3. There is good agreementith the n and φB values obtained using both approaches as

een in Fig. 3. It is interesting to note that the series resistanceid not affect the values of the ideality parameter or the barrier

otential significantly. The Rs values obtained using the aboveethod were found to be consistent to within 10%.In order to understand the charge transport behavior the

ynamic resistance of the diode in the ON state (VB > 1 V) has

234 R. Perez et al. / Synthetic Meta

Fig. 4. Temperature dependence of the normalized resistance of the diode calcu-lated for VB > 1 V using the method suggested in Ref. [22] (�). The normalizedresistance of a single isolated polyaniline nanofiber (�) is also shown forcpr

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omparison. Inset: I–V curves at selected temperatures for a single isolatedolyaniline nanofiber that makes contact to two Au leads at either end. A linearesponse indicates Ohmic contacts with Au.

een calculated via Eq. (5) and its temperature dependence isompared to the temperature dependence of the resistance of aingle electrospun polyaniline nanofiber contacted at either endy gold leads. The I–V curves for the single nanofiber are shownn the inset to Fig. 4 at a few selected temperatures and are seen toe linear indicating Ohmic contacts with the gold leads. In fullyoped polyaniline it is generally accepted that charge transportccurs via quasi-1D variable range hopping (VRH) where theemperature dependence of the resistance is given by [23]

(T ) = Ro exp

(To

T

)1/2

(9)

here To is associated with the characteristics of electronic statesnvolved in charge transport and is also a measure of disorder.ig. 4 shows the logarithm of the resistance normalized to theesistance at 300 K as a function of T−1/2 for the diode and thatf the single nanofiber. The values of To are calculated to be2,000 and 16,500 K for the diode and nanofiber, respectively.he diode resistance is seen to deviate from the quasi 1-D VRHodel at lower temperatures. A higher value of To for the diode

ndicates more charge localization due to increased disorder inhe current path in the device and the deviation of the data fromhe quasi 1-D model of charge transport indicate the presence of

ultiple charge transport mechanisms, viz. thermionic emission,RH, tunneling, etc.

. Conclusions

A simple technique of fabricating organic/inorganic Schottkyiodes in air and within seconds is presented. The tempera-

[[[

ls 157 (2007) 231–234

ure dependence of the electrospun Schottky diode has beennalyzed using the simple thermionic emission model withnd without the device series resistance. The device parame-ers were seen to be similar using both approaches mentionedbove. Since the diode architecture consists of the Schot-ky junction in series with a finite segment of a polyanilineanofiber, the charge transport in the device deviates from theuasi 1-D model of charge transport under normal operatingonditions and suggests that multiple mechanisms of chargeransport are present making it an interesting system for furthertudy.

cknowledgements

This work was supported in part by NSF under grants353730, 0402766 and by DoD under grant W911NF-06-1-519. A.T.J. acknowledges support from NSF under grant520020.

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