Extrinsic nd intrinsic chrge trpping t the grphene/ferroelectric interfce Supporting Informtion M. Humed Yusuf, Bent Nielsen, M. Dwber nd X. Du* Deprtment of Physics nd Astronomy, Stony Brook University, Stony Brook, New York 11794, USA Ferroelectric superlttices were grown using n off-xis RF mgnetron sputtering technique. The oxide mterils were grown from 1.3 trgets, contining compressed powder of the mteril in the correct stoichiometric rtios. The following growth conditions were used for the mterils: Tble 1. Growth conditions for n p PTO / n s STO superlttices SrRuO 3 PbTiO 3 SrTiO 3 Temperture ( C) 630 560 560 Pressure (Torr) 0.1 0.18 0.18 Flow O 2 (SCCM) 6 7 7 Flow Ar (SCCM) 32 16 16 Gun Power (W) 30 35 35 Growth Rte (mins / unit cell) 1.5 4.04 2.0 1
Implementing the bove growth conditions ensures high structurl perfection of the superlttices, s seen from the HRTEM (Fig. S1) nd X-ry diffrctogrm (Fig. S1b). To minimize contmintion from mbient dsorbtes before grphene deposition, the n p PTO/ n s STO superlttices should be immeditely put on the hot plte lredy set t 250 C upon growth. To ccount for the high substrte temperture used in the deposition of grphene, tpefree method is used. A thin foil of highly oriented pyrolytic grphite (HOPG, ZYA grde from Momentive Performnce Mterils) is then peeled from the bulk mteril, nd trnsferred onto the superlttice. The foil is then pressed onto the substrte with pressurized (30 psi) nitrogen flow through needle tht is 0.033 in dimeter (Needles with Luer Lock hub from McMster- Crr) for ~ 5 s. The nitrogen gs is extrcted from the boil-off of liquid nitrogen, in dewr connected to nitrogen line, ensuring high-purity nitrogen for grphene deposition. The flkes re then blown wy nd grphene flkes cn be found left on the substrte. This process is repeted until few grphene flkes re identified. To identify grphene flkes for subsequent nno-fbriction, LbView bsed opticl microscope imge cquisition/processing system is used. This system is cpble of performing in situ imge cquisition, color plne filtering, nd imge verging nd contrst enhncement. Grphene, from single to multiple lyers nd on rbitrry substrtes, cn be esily visible with the system by proper tuning of the prmeters. After identifiction, the grphene is immeditely coted with PMMA. AFM inspection cn be done to verify the grphene lyer thickness before spinning PMMA. After inspection, lignment mrks were written on the surfce of these superlttices in the vicinity of the grphene using electron bem lithogrphy. E-bem lithogrphy ws done twice more to ccount for the lekge-current insulting Al 2 O 3 pds nd to write the two or fourterminl configurtions. The dosge of the electron bem on the photoresist (either MMA or 2
PMMA or both) ws properly ssigned by doing dose test. Subsequently, the smples went through the stndrd processes of development (using 3:1 IPA/MIBK solution), metlliztion (35 nm of plldium) nd lift-off (using wrm cetone nd IPA). The metlized smples were finlly coted with PMMA. b 100000 log 10 (intensity) [counts] 10000 1000 100 10 1 STO (002) SL SRO SL-1 SL-2 42 43 44 45 46 47 48 2thet(degrees) Figure S1. () High Resolution Trnsmission Electron Microscopy (HRTEM) showing individul bilyers. (b) θ 2θ X ry diffrctogrm of 14 bilyer 15 PTO/ 3 STO superlttice. b SL Grphene 3
Figure S2. () SL indictes single-lyer grphene, s seen on the surfce of n p PTO/ n s STO superlttice. The 70um X 70um re is enclosed by four lignment mrks written on the surfce of the bre superlttice. (b) SEM (scnning electron microscope) imge of four-terminl device with Al 2 O 3 insultion (shded drker) underneth the lrge gold pds. Inset: A mgnified SEM imge of the 150 nm thick Al 2 O 3 insultion. Figure S3. Pek Position vs. gte rmping rnge (± V BG ) for 2 smples, S1 nd S2, where grphene deposition ws crried out t 250 C nd t room temperture respectively. Arrows indicte the rmping direction for the respective pths. A common feture for ll the devices, with grphene deposition t 250 C, is tht the forwrd rmping pek hs little rmping rnge dependence, giving more credibility to symmetricl electron trpping. On the other hnd, for smples in which grphene ws deposited t mbient temperture, both the peks re seen to devite for the vrious ± V BG rnges. 4
-1.5 V 1.5 V Cross-over 580 Hz V G 250 Hz 6 V r (-1.5V) to 6V 6V to V r (-1.5V) 10 Hz T = 300K -2-1 0 1 2 V BG (V) Vr = -1.5V R(kohms) 3 2.25V to 6V (Initil) -3-2 -1 0 1 2 3 4 5 6 7 VBG(V) Figure S4. () Unlike the chrge-trpping on PZT gte oxides observed previously, chrge trpping in the PTO/STO superlttice system is reltively fst phenomenon nd it tkes into effect within the order of few milliseconds. (b) Dirc pek positions s function of bckwrd rmped voltges (Vr) for 15/3 PTO/STO nti-hysteretic smple. After n initil forwrd sweep to 6V, bckwrd sweep is crried out from 6V to Vr. After reching Vr ech time, the loop is completed by yet nother forwrd sweep to 6V. The bckwrd Dirc pek position nd the forwrd Dirc pek position (for the second forwrd sweep) re subsequently noted down for ech run. This gives informtion bout the de-trpping of electrons tht were trpped during the initil forwrd sweep. For this prticulr smple, this effect ws observed by drmtic chnge in the forwrd pek positions between 0V nd -2V. Inset: This shows the gting curve of grphene for one prticulr dt point when Vr = -1.5V. 5