Electrical properties of polymer/Si heterojunctions

ELSEVIER Thin Solid Films 313-3-M (1999) 469475 Electrical properties of polymer/Si heterojunctions Abstract Metal/conjugated polymer film/silicon structures have been fabricated. Current-voltage measurements have been performed with both AI and Au metal electrodes on n- and p-type Si substrates. The conjugated polymer was regioregular poly @octylthiopheneI (P30T). Rectificalion ratios as high as - 10” have been observed, with turn on voltages of -i.5 V. Evidence of Schottky barrier formation at the Al/polymer interface, and Fowler-Nordheim tunneiling at the PjOT/p-Si interface is shown. Capacitance-voltage characteristics were obtained in the frequency range 10 H2 to I MHz, Strongfrequencydispersion is observed which is indicative of the presence of traps. A model is proposed to explain this dispersion which shows good agreement with the experimental results. Preliminaq optical results indicate that thesediodesmaq’ have applications as optical sensors. Q 1999 Elsevier Science S.A. AU rights reserved. k’r~~:~&: Schottky barrier; Polgi3-o~rylthiophene); Semiconductor interfke 1. Introduction Conjugated polymers [ 1,2] have found applications ranging from batteries [3]: light emitting diodes [4], field effect transistors [5], and lasers[G] and most recently field emitting devices [7]. They are particularly attractive because of their ease and cheapness of fabrication. An area that has not been fully explored is the incorporation of these materials in Si technology. In this paper we have investigated diodes,fabricated on Si using regioregular poly 13-octylthiophene) (P30T) films. This conjugated polymer is a new generationpolythiophene derivative, which with its high ordering [S], and excellent environmenta stability is a good candidate for the active part of electronic devices. 2. ExperimentaI Regioregular P30T is describedby McCuIIough et al. [9]. NMR studieshave shown P30T with a head--tail concentration of >9@?&,indicating a more ordered polymer than the conventional P30T oiH-T < 50% [IO]. The absorption coefficient for thesefilms was obtained using the method of transmission and reflection by spectrophotometry, from which the Taut band gap was found to be -1.79 eV. The technique has been describedelsewhere[ 1I]. Thin films of thickness200-300 nm, as determinedusing a Tencor Itistru* Corresponding author. Tel.: t 41-151-794-2000; fax: t 3-1-151-794 4.540. E-mail address: [email protected] (I. Muss) ment model 200 alpha step 200 thickness profiler, where spin coated on a clean wafer of Si wafer, in a cIean nonvacuum environment. n-Si and p-Si, 100) wafers were used of 5 CL-’ cm-’ conductivity. After spinning the wafers were immediately dried for 24 h in a vacuum of IO-” Torr, before electrical measurementswere performed. Circular metallic electrodesof aluminium or gold were evaporated, through a mask. in a vacuum of -IO-’ Torr. The electrode area was measured to be -3.14 X IO-’ cm”. The DC and AC measurementsR’ereperformed asdescribedelsewhere[14]. 3. Results and discussion Fig. la shows a typical I-k’ piot of an Al/P3OT/p-Si diode. A rectification ratio of almost six orders is typical with a turn on voltage of - 1.5 V. Fomzrd current is defined with positive biasapplied to the p-Si substraterelative to the Ai electrode. Some evidence that the diode.sbecome bulk limited by the P30T polymer film is observed at high voltages since the conductivity estimated from this plot is -10-6 a-1 cm-‘: a typical value calculated by gap-ceil measurementson the polymer film. Fig. lb shows a Fowler-Nordheim (F-N) [I21 plot, obtained by plotting I/ F’ versus l/F, of the A.W30T/p-Si diodes at forward current (as defined above), where I is the current and F is the applied electric field. It is apparentthat when the voltage applied is greater than 1.5 V a reasonablefit to the F-Nplot is observed.Wnen the data for voItages < 1.5 V are plotted (insert of Fig. Ib) there is an exponential dependenceof current on voltage. There is also the possibility of thermio- 0040~6090/99/$ - see front matter Q 1999 ElseGer Science S.A. All righrs resened. HI: soo40-6090195)O 166 l-7 170 IO’“8 1 -3.5 I, I, -3.0 1, -2.5 1, -2.0 *, -1.5 ), -1.0 1, -0.5 Applied ‘I 0.0 1, 0.5 ’ 1.0 Voltage I’ 1.5 I 2.0 ’ I 2.5 ’ I 3.0 ’ I 3.5 [V] 10"' lo'- 1 0.0 1.0~10‘~ 2.0~10'" 3.0~10'" (Electric 4.0~10"' 5.0x10" 6.0~10" Field)-‘(h) 04 Fig. 1. iai I-I:cllarxt~ri~tic~ I .5 V. to the diode equarion. for .AlP?OT~p-Si diode and tb) the T-Nplot: a good fit is seen abole d voltage of 1.5 V. The msc’t hw~ rhe tit, for w.hgr~ < 171 0.8 -4 -3 -2 -1 0 Applied Voltage 1 2 3 i 4 M Fig. 2. (a) Reverse i-i’characterktic for Ai/P3OT/-psi diode, where a good fit to (I’C,,P - #I,; - kT!q‘l”’ is observed. The inset is the temperature dependence of the reverse current: (b) the effect of vying the metal electrode and substrate type. For metal/poIymer/p-Si diodes the voItage polarity reflects on Si and for the Al/polymer/n-Si the voIiage poiariry reflects on the aluminium electrode. nit emissionpreceding the F-N type mechanism[ 131. We have recently shown that Al forms a rectifying contact to P30T polymer films [ 141.in addition we have also demonstrated that image force lowering occurs [141.This requires lhat the reverse current of the Al/P30T Schottky barrier vary as p,‘j [is] as in Fig. 2a. &-ET in the polymer is found to be approximately 0.25 eV. This is reasonable since the activation energy obtained for the bulk P30T polymer film is 0.23 eV [IGj. Therefore, it appearsthat initially, for voltages Cl.5 V, most of the applied bias falls across the AUP30T barrier which dominates the current. The temperature dependenceof the reverse current (inset of Fig. 2aj indicates that thermionic emissionis less important tian diffusion from P30T to Al, and for which the linear dependenceof I on i/T is expected [ 17J. The latter piot then enablesthe barrier height to be obtained [ 171.This btier height was estimated to be -0.47 eV. The barrier height (#bj will be given by & - &, t $,- ahere #P is ET E,: &i is the built in potential at the AL’polpmer interface and 4POiis the polymer work function. given by: xFo; -t EC-E,- = 1.75425 eV. ba, is the work function of Al (4.3 eV) [IS] and xLal is the electron affinity of the polymer film (a) lo2 10’ Frequency (w [Hz] 1 -0-4OKHz -c3-6OKHz " 80KHz -v--1OOKHz 200KHz -+-400KHz 60OKHz "0 E 0cp .m...EOOKHz ---1MHz 5.0x1 o"O 0.0 I Applied Fig. 3. rai LOU frequency diode. dependence of conductance and tbl rhe C-l’char~creria~ics (-3 eV) [ 191. If we consider. to a first approximation. that the work function differences are valid it follows that d+,0.15 eV agreeing closely with the value obtained experimentally. For voltages > 1.5 V this barrier acts like a p-n junction at forward current and so the dominant mechanismwill possi- Voltage for frquensie~ [V ] rangin from 40 kHz to I mHz for the AUP30T/n-Si bly be at the P3OT/p-Si. For the polymer, EC-E, - 0.25 eV, and assumingthat the p-5 work function is -4.9 eV [20] the band bending at the P3OT/p-Si valence band will be 0.3 eV. This suggeststhat only a small barrier to holes will exist at the p-Si/P30T. If this barrier is assumedto be approximately triangular then the F-N tunnelling mechanismmay 6.0x10-'5.OX1O's A 4.0x10~"3.0x10-'2.0x10a31.0x1 o‘Q 7 Frequency [Hz] (a) I 0.’ IO" 1 0“ IO" 1 oeB -* A A IO” IO“ . Dark 1o’o A Light 1O"O IO"' Applied Voltage [V] tb) Fig. 4. (a) Frequency dependence pF, C&p - 16 pF, Cr -7nFandT- of the capacitance for ALP?OT/p-Si diode. The brokenline is the theorcrical plot ofEq. ilj, using parameters 1 ms and ibJ ihe I-Vcharacteristics of the iil/p3OT/p-Si in the dark and under illumination. CPniy - 3700 then occur. as shown in Fig. lb. The barrier height deduced, assuming a perfect mirror surface from this plot is 0.02 eV. However. we have evidence from atomic force microscopy and scanning electron microscopy that the films are not smooth and so the assumption that the field enhancement factor c/3) = 1 is not applicable and will therefore much greater; hence the barrier height will be much greater than 0.01 eV. These low values for the bal-r-ier height are common in other amorphous semiconductors and reflect the difficulty in knowing all the microscopic parameters that determine the final barrier height. Further evidence for this contact behaviour is provided by considering the effect of replacing the Al by Au. We have demonstrated that Au/P30T contacts are ohmic (I. ,Musa. W. Eccleston. unpublished data). From Fig. 2b an increase of the reverse current bq; almost five orders of magnitude is observed, whilst the forward current (for voltages > 1.5 V) remains the same. as expected from the above argument. To eliminate the role of the P30Tpolymer/silicon interface the aluminium electrode was replaced by gold and p-Si replaced by n-Si. The resultant total current is shown in Fig. 2b. When the Al contact is changed to Au an increase of reverse current by almost three orders is apparent. In addition. there is also an increase in current for voltages < 1.5 V. This may be due to the change in the shape of the conduction band edge next to the Schottky barrier. and would be consistent with there being a relatively small amount of Fermi level pinning. Above I.5V the forward current does not change signifcantly. When the p-Si substrate is replaced by n-Si, there is an increase in reverse current by almost five orders of magnitude. The AC conduction process was also investigated. Fig. 3a shows a typical plot of conductance versus frequency in the range 10 Hz to 6 kHz. Two regions are apparent: at low frequencies up to 100 Hz. the conductance is constant and is similar to the DC value. The second region is seen to vary as 0”“‘. Amorphous semiconductors also give conductance proportional R” with k a constant [11,3?]. It has been suggested that the second region is characteristic of trapped carriers hopping between filled and empty states at the Fermi level. k is a measure of the number of carriers that are empty. which suggests in our case that 86% of traps are empty [23]. Fig. 3b shows typical C-I/characteristics of the Al/P30T/n-Si diodes. Under forward current. the capacitance is constant and the diodes look like metal/insulator/ semiconductor (MIS) structures. Under reverse bias. the capacitance decreases as the Si becomes depleted. Fig. la shows the capacitance as a function of frequency. This dispersion is typical of materials with traps [24]. From Fig. -!a the capacitance saturates at high frequency. which decreases as the traps start to respond to the signal. At high frequencies the traps can no longer respond and the capacitance approaches the geometrical value (-100 pF’). as in a MIS structure. This frequency dependence of the capacitance can be qualitatively explained with the equivalent circuit in the inset of Fig. la. This circuit was proposed by Nicollian and Goetzberger [Z]. tance C(II’) is 1 C Ci< ,i + c, x The equivalent capaci- (C:,,& + cc,.,, + c,)LJYc‘,c,,(c,: (q ,,,,i\ + C‘k,, + CJ+‘D%yc,*d, ,1,, + C&J + C!/J I (1) where C,,,, is the capacitanceof the polymer film. C,,, is the depletion capacitance.C, is the capacitance associatid with the traps, R is the signal frequency and T is the carrier lifetime and -RTCr where RT is an affective resistanceassociated with generation-recombinationprocess.The effect of white light on the Al/P30T/p-Si diodesis shown in Fig. -lb. The reverse current increasesby almost three orders when white light is incident on the device. The structure could be attractive as a low impedanceoptical detector when integrated with a polymer TFT. Acknowledgements 1-M is grateful to M.N. Sedghi-Monofared for useful discussions.Kevin Molly for his assistancein the clean room and the UK Engineeringand Physical ResearchCouncil (EPSRC) for financial support. References [I] I. hlua. S.J. Higgins. M;. Ecclr~ron. J. Appl. Phys. 81 (1997) 2288. [2] I. Muss. IV. Ecclrs[on. S.J. Hlgzms, J. Appl. Phys. S3 (199s) 5558. [3) B. Scrosari, Applicxion\ of Elrcrnxztiue Polymers, Chapman and Hall. London. 199-l. [41 V.L. Calvin. M.C. Schlamp. A.P. Alivisutob. Nature 370 (1994) 354. [S] D.M. de Lecuw. M.M.J. Simrnon. A.R. Broun. R.EF. Einerhand. Synth. %le[. 87 (1997) 53. [6] N. Tcssler. G.J. Dcnton. R.H. Friend. Synth. hlrt. 84 (1997) 175. [7] 1. Musn. D.A.I. Munindralu~~. G.A.J. Amararunga. W. Eccleston. Nature 39.5 II9981 362. [Si T.-A. Chen. W. 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