有机场效应晶体管中累积电荷分布检测方案(其它光谱仪)

智能文字提取功能测试中

e-Journal of Surface Science and Nanotechnologye-J. Surf. Sci. Nanotech. Vol. 16 (2018) 364-3699 August 2018Conference - ISSS-8 - e-Journal of Surface Science and NanotechnologyVOLUME 16 (2018) Mapping Accumulated Charges at the Semiconductor/Insulator Interface ofOrganic Field-Effect Transistors by Sum-Frequency Generation Spectroscopy* Haiya Yang and Takayuki Miyamae Nanomaterials Research Institute， National Institute of Aduanced Industrial Science and Technology (AIST)， 1-1-1 Higashi，.Tsukuba， Ibaraki 305-8565， Japan Masato Miyashita Research Association of High-Throughput Design and Development for Aduanced Functional Materials (ADMAT)， 1-1-1 Higashi，Tsukuba， Ibaraki 305-8565， Japan (Received 8 January 2018； Accepted 22 July 2018； Published 9 August 2018) Charges accumulated at the semiconductor/insulator interface of a top-contact bottom-gate organic field-effecttransistors (OFET) with a channel length/width of 1000 um/1000 um while applying a negative gate voltage arevisualized in a probe region of 4 mm× 4 mm by electric-field induced sum-frequency generation spectroscopy. Itis found the charges are accumulated not only inside but also outside the channel of the OFET. The accumulatedcharges are also found to be uniformly distributed on the semiconductor/insulator interface. The resolution ofthis mapping technique is explored to be 0.34 mm in the horizontal direction and 0.32 mm in the vertical. DOI：10.1380/ejssnt.2018.364 Keywords： Organic field-effect transistors； Charge accumulations； Sum-frequency generation spectroscopy； Electric field；Semiconductor/insulator interface I. INTRODUCTION Due to their widespread applications in industry suchas displays and imagers， organic field-effect transistors(OFET) have been interesting topics since 30 years ago[1]. Since the semiconductor/insulator interface is closeto the conducting channel where accumulated charges aredriven from source to drain electrodes during operation，charge transport is highlydependent on the quality of thesemiconductor/insulator interface of OFET 2]. Second-harmonic generation (SHG) [3] and sum-frequency gener-ation (SFG) [4] are intrinsically and specifically surface-sensitive techniques to probe molecular interactions onsurfaces and buried interfaces where the inversion sym-metry is necessarily broken. Electric field is required tobe designedly induced into SHG and SFG while study-ing interfaces of OFET during operation. Electric-fieldinduced SHG 5， 6 and SFG 7-9 are utilized to studyinterfaces of OFET and correlate their electrical proper-ties and SHG or SFG measurements. Visualizing chargecarriers in OFET during operation is an interesting andmeaningful work among them， because this could helpus monitor performances and clarify mechanisms relatedto charge transport of OFET. It is first successfully ob-served that charge carriers accumulated at the semicon-ductor/insulator interface of OFET fabricated with pen-tacene films and polyvinyl phenol dielectric layers whileapplying a negative gate voltage with an SFG microscope[8]. However， the probe region of the observed chargecarriers in the SFG microscope is limited to 0.2 mm ×0.2 mm， which is too tiny to observe OFET samples forobservation and investigation of the homogeneity of theinterfaces of spin coating films. In this study， electric-field induced SFG spectroscopy ( * This paper was p resented at t he 8th International Symposiumon S urface S cience， T sukuba International Congress Center，Tsukuba， Japan， October 22-26， 2017. ) ( T Corresponding author： t-miy a mae@ a is t .go . jp ) is applied into observing charge accumulations and distri-butions at the semiconductor/insulator interface of OFETunder operation， by mapping the position distributions ofSFG intensity. It is found that during applying a negativegate voltage， the non-resonant background of the SFG in-tensity at the semiconductor/insulator interface of OFETincreases substantially because of the increased electricfield caused by positive charges accumulated at the semi-conductor/insulator interface of OFET， while the SFGspectral shapes do not show any distinct changes at thesame time. This increasing is found not only inside theconducting channel between source and drain electrodes，but also outside of them. The non-resonant SFG intensityshows an almost uniform increasing in the entire area ofOFET， due to the charge accumulation. This work couldhelp us clarify OFET mechanisms of charge accumulationduring operation， and provide a guide about how to de-sign OFET devices with much better performances. Thiswork also extends the applications of SFG in the indus-try of organic electronics and facilitates the process of thedevelopment of this surface-detectable technique. II. EXPERIMENTAL A. OFET An OFET sample is a typical top-contact bottom-gate structure fabricated with two gold electrodes of~30 nm thick at the top and a semiconductor layerof 2，7-dioctyl[1]benzothieno[3，2-b]benzothiophene((C8-BTBT) with a thickness of ~50 nm below preparedthrough a spin-coating method (1000 rpm， 30 s) witha C8-BTBT solution in chloroform (Wako， pure grade)of a concentration of 0.8 wt%. AA silicon substratewasused as a gate electrode with a SiO2 layer of~200 nm thick coated on its surface as an insulator.There are two types of channels in the OFET； one islength/width of 1000 um/1000 um， while the other is thatof 500 um/500 um. A precision source/measure (Agilent， B2901A) was used to apply available gate voltages from+100 V to -100 V into OFET. The gate electrode is con-nected to high voltage end， whereas those two gold elec-trodes connected with each other are connected to ground. B. Sum-frequency generation spectroscopy An SFG spectrometer with a picosecond laser unit (Ek-spla， PL2231-50) of 50 Hz repetition rate， a harmonic unit(Ekspla， SFGH500-2H)， and an optical parametric gen-eration unit (Ekspla， PG501-DFG1P) was used in thisstudy to generate a visible beam at 532 nm and a tunableinfrared (IR) beam ranging from 2.5 um to 10 um， re-spectively. The polarization combination was set at PPPduring measurements in which the IR， visible， and SFGbeams were polarized in the plane of incidence. The in-cident angles for visible and IR beams were 65° and 55°from the surface normal， respectively. The output ener-gies of lasers were reduced to the minimum possible levelto avoid any damage on the sample. The step for the scan-ning ofthe XY-maps was 0.2 mm for Fig. 3 and 0.01 mmfor Fig. 4 for both X and Y directions using mechanicalstepping motors. The SFG signal intensity was averagedby 100 times of acquisitions per point. All measurementswere performed under ambient conditions. The tempera-ture was kept at 20°C during all SFG experiments. III. RESULTS AND DISCUSSION A. Vg dependence on SFG spectra of OFET As shown in Fig. 1， we obtained SFG spectra of thechannel region of OFET in PPP polarization combinationduring applying a series of gate voltages of +40 V， 0 V，and -40 V. In this experiment， because Vas =0， there is FIG. 1. The SFG spectra in PPP polarization combinationtaken at the center of the channel region of the OFET duringapplying a series of gate voltages of +40 V， 0 V， and -40 V.Each SFG spectrum is fitted based on Eq. (2)； the inset showsa linear relationship of XNR versus Vg； the arrow pointed at3000 cm-is discussed in the text of Sec. III C. no bias voltage between source and drain electrodes. Sev-eral SFG peaks observed in Fig. 1 should be derived fromasymmetric and symmetric vibrations of alkyl chains ofC8-BTBT in OFET. The SFG spectra under gate volt-ages of 0 Vand +40 V are almost mutually overlapped inwhich the strongest SFG peak at 2967 cm-1 should be as-signed to the asymmetric stretch vibration of CH3. How-ever， with applying a gate voltage of -40 V， the overallbackground of the SFG spectrum starts to increase obvi-ously in contrast to those of 0 V and +40 V， although thestrengths of SFG peaks themselves do not show such anapparent change. That the overall background of the SFG spectra ofOFET increases while applying a negative gate voltage， aswell as its explanation have been comprehensively demon-strated elsewhere [8， 10]. Here we only make a brief andsupplemental explanation in this study by citing a relatedequation given by [10-12] where PsFG is nonlinear polarization of the SFG beam，x(2) and x(3) are the second- and third-order nonlinearsusceptibilities， Evis and EIR are electric fields of visi-bleand IR beams， and Eo is an applied DC electric field，respectively. Since C8-BTBT in OFET is a p-type semi-conductor 13， hole carriers are injected and accumulateat the semiconductor/insulator interface under applyinga negative gate voltage， which produces an electric-fieldeffect there. So， the electric field of Eo at this interface in-creases， resulting in an increasing ofx(3)EvIs EIR Eo andthus leading to an increasing of PsFG. But such kindof carriers injection and accumulation do not happen asabove if a reversely positive gate voltage is applied for ap-type semiconductor of C8-BTBT. This is the dominantreason for the unchanged SFG spectrum under the gatevoltage of +40 V shown in Fig. 1 as compared with thatof0 V. The results of SFG spectra under different gatevoltages of -40 V，0 V， and +40 V in Fig. 1 are exactly inaccord with transfer characteristics of the OFET samples.In other words， when V≥0 V， there is no drain currentIds generated in p-typed OFET； but when V<0 V， draincurrent Ids would be generated there if absolute value ofVg exceeds that of threshold voltage Vth， which is around-20 V as we measured. It is required to make a furthermore deeper discussionabout Vg dependence on the measured SFG spectra of theOFET. The intensity of SFG output is given by [14] where XNR is non-resonant contribution to the suscepti-bility， wIR is the frequency of IR beam， Aq， wq， and Iqare the amplitude， the frequency， and the damping con-stant of the qth vibrational mode， respectively. Basedon Eqs. (1) and (2)， the nonlinear polarization of theSFG beam of PsFG consists of non-resonant contribu-tions of XNR and vibrationally resonant contributions ofAgt2q wIR-Whqat are+il both supposed to be theoreticallyaffected by the electric-field effect of Eo. The linear cor-dotrelation of XNR| versus Vg that can be directly inferred FIG.2.A schematic illustration of three active interfaces ofOFET probed by SFG spectroscopy and corresponding chargedistribution. from Eqs. (1) and (2) is experimentally evidenced by Yeet al.[10] and us as shown in the inset of Fig. 1. However，for vibrationally resonant contributions， different from thereported correlation between ratios of CH2/CH and Ve[7， 10]， no outstanding changes in strengths of SFG peaksor ratios of CH2/CH3 are observed in our experimentsshown in Fig. 1. This observation clearly indicates thatthere are no distinct orientation changes of alkyl chainsof C8-BTBT in OFET under applying a negative gatevoltage. We should note that the strengths of the vibra-tionally resonant SFG peaks do not show any changes byapplying the voltage. This observation is different fromthe phenomenon observed in the electric field inducedSFG taken from the organic light emitting diodes (OLED)under operation [15， 16]. In the case of the OLED， theintensities of the vibrational peaks were also increased byapplying the voltage， while such behavior was not foundin C8-BTBT of OFET. Since the SFG measurement underthe doubly-resonant condition was used for the measure-ment of the electric field induced SFG of the OLED， theintensities of the vibrational resonant SFG peaks were in-creased with the increase of the applied electric field. Onthe other hand， since the electronic transition of the alkylchromophore is far from the visible wavelength region，and the current SFG measurements is not performed un-der doubly resonance condition， the electric field inducedeffect is observed just as an increase in the non-resonantbackground intensity. Here we would like to discuss the contributions ofmeasured SFG spectra in Fig. 1 from three differentactive interfaces in OFET. As shown in Fig. 2， sincethe OFET in this work has a multilayer structure withthree interfaces from top to bottom， namely， the topmostair/semiconductor interface of 0， the buried semiconduc-tor/insulator interface of 1， and the buried insulator/gateinterface of 2， SFG signals could be generated from all ofthese three interfaces， including both resonant vibrationsand non-resonant backgrounds from not only the inter-face 0 but also the interface 1， and only non-resonantbackgrounds from the interface 2 because there are noresonant vibrations coming from SiO2 at measured IR re-gion [17-20]. The interface 1 is commonly regarded as themost important interface of OFET because it is the veryplace for charge accumulate and charge transport [21]. Inreported work related to SFG measurements for OFET，the measured SFG signals are attributed to dominantly come from the interface 1 instead of the others， although the total SFG signals may include contributions from theinterfaces 0 and 2 in terms of multiple interference effects[8， 10]. In our experiment， it is consistently believed themeasured SFG spectra and their Vg dependence shouldcome from mainly the interface 1， with our explanationsshown as follows..While applying a negative gate volt-age， injected holes ought to accumulate at the interface 1and generate an increased electric field there. This makesthe total background of SFG intensities to increase as ex-plained early based on Eq.(1). But such movement ofcharge accumulation does not happen at the other twointerfaces of 0 and 2. Injected holes do not go to the interface 0 and stay there under a negative gate voltage.Although electrons generated in the gate electrode at thesame time because of injected holes accumulate at the in-terface 1， and some of them certainly go to and stay atthe interface 2， they do not aggregate there as the sameway as the injected holes at the interface 1.In otherwords， the roles these electrons play on the electric fieldat the interface 2 are too weak to make a comparablylarge enhancing of non-resonant background there whileapplying the gate voltage， different from those of injectedholes at the interface 1. Consequently， while applying agate voltage of -40 V， the increased total background ofSFG intensity in Fig. 1 are mainly caused by the chargeaccumulation that could only happen at the interface 1. B. Charge accumulations in OFET mapped bysum-frequency generation spectroscopy In order to explore the position dependence of theSFG intensity on the semiconductor/insulator interface ofOFET without and with applying a negative gate voltage，we show the in-plane SFG signal intensity distributionaround the source and drain electrodes in Fig. 3. In thismeasurement， the wavelength of 3000 cm-is selected asa probe wavelength (see the arrow in Fig. 1). We scannedits SFG intensity around two gold electrodes and theirsurroundings including the channel with gate voltages of0 V and -80 V relative to the source and drain contacts(Vas =0 V). In this experiment， the applied gate volt-age is enhanced from -40 V to -80 V for observing moredrastically increased SFG intensities in order to obtain anXY-map with a sharper contrast， although -40 V is se-lected to be the gate voltage to observe the electric-fieldeffect in the SFG data taken in Fig. 1. IBased on theSFG intensity at 3000 cm-1， an XY-map of SFG inten-sity with and without applying a gate voltage of -80 Vis obtained. The wavelength of 3000 cm-1 is preferred tothose of SFG peaks such as 2965 cm-because the SFGsignals at 3000 cm-is dominantly contributed from non-resonant background， which has an exactly proved lin-ear relationship with V 10]. For the wavelength of SFGpeaks like 2965 cm，although resonant SFG signals areassumed to be also affected by the electric field， the in-tensities of resonant SFG signals change less radically asVg varies [7， 10]. As shown in Fig. 3(a， b)， charge accumulations at thesemiconductor/insulator interface and the electric fieldbetween source and drain electrodes are obviously visual-ized by mapping the position distributions of SFG inten- FIG. 3.The XY-maps of the OFET surface based on the SFG intensities at the wavelength of 3000 cmin PPP polarizationcombination without (a) and with (b) applying a gate voltage of -80 V(c) the positon dependence from source to drainelectrodes of SFG intensities at the wavelength of 3000 cm- without and with applying a gate voltage of -80V. sity at 3000 cm-1. This is a dot-by-dot scanning of SFGintensities at 3000 cm-in PPP polarization combinationon the sample surface. The SFG intensities generated atthe gold electrodes and C8-BTBT are largely differentunder the same experimental conditions， since the goldsurfaces generate much stronger frequency-independentSFG signals than organic thin film interfaces 22]. Basedon this significant difference， we can effortlessly locatethe semiconductor layer， the gold electrodes， the channelregion， and the region outside the channel， respectively.Because SFG intensity is plotted against X and Y posi-tions， the colors in this XY-map will change drasticallyonce there is any change on the SFG intensities in thisarea. Without applying a negative gate voltage into theOFET， the white region is considered to be gold elec-trodes， whereas the blue regions outside the white regionis the semiconductor layer. The SFG signal intensitiesseems to be uniformly distributed. The absence of in-tensity variations in the plane of the semiconductor areaindicates that the spin-coating film is almost uniform atleast within the measured area.While applying a gatevoltage of -80 V， the blue region changes into cyan notonly inside the channel but also outside the channel， whichmeans the SFG intensity there increased under applyingthe gate voltage in contrast to that without applying thenegative gate voltage. The reason for the increased SFGintensity has been explained in Sec. III A that positivecharges accumulated at the semiconductor/insulator in-terface. Charge accumulations；ddistributed1homogenouslyaround source and drain electrodes in a region of 4 mm ×4 mm are first mapped out by electric-field induced SFGspectroscopy as shown in Fig. 3(a， b). It provides us anadditional way of electric-field induced SFG spectroscopyto visualize accumulated charge carriers and electric-fielddistributions at the semiconductor/insulator interfacein OFET in a much larger probe region， as well as inOFET with much longer and wider channels than typicalones with a channel length/width of 95 um/145 um 8].OFET mechanisms related to charge carriers could alsobe clarified by such kind of visualizations， which wouldprovide a guidance for designing new ones to improveAFperformances of OFET. One big difference in this XY-map from that of Mat- sumoto’s group [8] is there are some color changes evenoutside the conducting channel. This probably dependson the construction of OFET instead of SFG techniques.In their OFET configuration， the gold electrodes might beembedded into the semiconductor and blocked the chan-nel from its outside region.But in our case， the goldelectrodes are actually paved on the semiconductor， andthe entire semiconductor layer beneath the gold electrodescould be the channel. So， while applying a negative gatevoltage， the color of the outside region of the channelalso changes into cyan， which means charges are alsoaccumulated here. Surely， mapping the OFET surfaceonly by SFG intensity generates some minor errors at thesame time. For example， before and after applying Vg，the shape of gold electrodes seems to have some slightchanges， as shown in Fig. 3(a， b)， but this could not hap-pen in fact. In any case， this XY-map by SFG intensityscanned with SFG spectroscopy could help us accuratelylocate the source and drain electrodes， the semiconductorlayer， and the conducting channel conveniently. Another obvious difference from the result of SFG mi-croscopy is that accumulated charges are almost uni-formly distributed from drain to source electrodes. IInorder to illustrate this situation more clearly， Fig. 3(c)is plotted based on the result of the channel region inFig. 3(a，b). As shown in Fig. 3(c)， the changes in thecurve of Vg=-80 V from that of Vg=0 V are within er-rors of them， which illustrates that accumulated chargesare almost uniformly distributed from drain to source elec-trodes， and even the entire surface of the semiconductorof OFET. Here we make an explanation about this ho-mogenous distribution of charges at the interface. Beforeapplying the gate voltage Vg， the OFET is in the depletestate； after adding Vg， it is in the accumulated state. Atthis time， Vas =0 V， so there is no bias voltage betweensource and drain electrodes， which means the electric fieldbetween these two electrodes should be homogenous dis-tributed. However， it could be expected that when ap-plying a strong bias voltage Vas between source and drainelectrodes of OFET during operation， there would be anexpected bias position dependence of SFG intensity be-tween source and drain electrodes caused by the diagonaldistribution of the quantity of charge carriers between twoelectrodes. This will be our future work. cu FIG. 4. The X-position and Y-position dependences from C8-BTBT to the gold electrode of SFG intensities at the wavelengthof 3000 cm in PPP polarization combination. Corresponded photos of the scanning processes on gold electrodes of OFETare also shown above. C. The resolution in the XY-map of sum-frequencygeneration spectroscopy The SFG intensities obtained by SFG spectroscopy areutilized to map the charge accumulations at the semicon-ductor/insulator interface of OFET； Therefore， we haveto illustrate the resolution of this mapping technique ofSFG. The same method as that in the SFG microscopyimaging is selected to calculate the horizontal and verticalresolution for the SFG spectroscopy during its mappingprocess shown in Fig. 4， in which the SFG beam is allowedto scan a region consisted of two flats with extremely dif-ferent SFG intensities horizontally and vertically. It isconsidered that the distance between 10% and 90% in-tensity is the resolution for both horizontal and vertical23]. As shown in Fig. 4， the resolution in our technique isevaluated to be 340 um for the horizontal and 320 um forthe vertical edge of the gold electrode. Although this res-olution is still too large to see the local defects or chargetraps inside the OFET channel， especially far from thesmallest one of 0.48 um for a sum-frequency generationconfocal microscope reported [24， it is enough for us toprobe a mapped region of 4 mm × 4 mm of the semicon-ductor/insulator interface of OFET， which indicates thatour conclusion is reasonable and convincing. It is feasibleto furthermore improve the resolution of the mapping ofSFG spectroscopy evaluated in this kind of experiment inour future work. [1]I. Kymissis， Organic Field Effect Transistors： The-ory， Fabrication and Characterization (Springer， Boston，2009). In this work， SFG spectroscopy is utilized to ob-served charge accumulations occurring at the semiconduc-tor/insulator interface of OFET during the application ofa negative gate voltage. It is found that charge accumula-tions are mainly distributed around the source and drainelectrodes not only inside the channel between source anddrain electrodes but also outside the channel， and thisSFG signals almost homogenous distribute everywhere es-pecially between gold electrodes， which reflects the elec-tric field is homogenously distributed around the chan-nels. The resolution of this mapping technique is also ex-plored and calculated to be 340 um for the horizontal and320 um for the vertical edge. The accumulated chargesand the electric field around the electrodes in OFET arevisualized by spectroscopic SFG technique and its map-ping. This work adds a new electric-field induced SFGspectroscopic way to observe uniformity/nonuniformitydistribution of the accumulated charges in OFET， andhelp us clarify OFET mechanisms related to charges andthe deciding factors of the performances of OFET. Thiswork extends the applications of SFG in the industry oforganic electronics， and will facilitate the process of thedevelopment of this surface-detectable technique. ACKNOWLEDGMENTS This study is supported by New Energy and IndustrialTechnology Development Organization (NEDO) througha commissioned project (P16010). ( 2 G. H o rowitz， i n ： Organic E l ectronics. A duances in Poly-mer Science， Vol. 223， edited by T. Grasser， G. Meller，and L . L i ( Springer， Berlin， Heidelberg，2009) p . 1 13. ) ( 3] Y .R. Shen， Annu. Rev. P hys. Chem. 40，327 (1989). ) ( 4 Y . R . 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Appl.Phys. 5 1， 1 22402 (2012). ) SSN The Japan Society of Vacuum and Surface Science -Stage： https：//www.jstage.jst.go.jp/browse/ejssnt/       Charges accumulated at the semiconductor/insulator interface of a top-contact bottom-gate organic eld-effect transistors (OFET) with a channel length/width of 1000 um/1000 um while applying a negative gate voltage are visualized in a probe region of 4 mm  4 mm by electric-eld induced sum-frequency generation spectroscopy. It is found the charges are accumulated not only inside but also outside the channel of the OFET. The accumulated charges are also found to be uniformly distributed on the semiconductor/insulator interface. The resolution ofthis mapping technique is explored to be 0.34 mm in the horizontal direction and 0.32 mm in the vertical.