Huang, XD

Lai, PT

Shi, RP

X. D. Huang

R. P. Shi

P. T. Lai

Crossref

Applied Physics Letters

Charge-trapping characteristics of fluorinated thin ZrO<sub>2</sub> film for nonvolatile memory applications

HKU Scholars Hub

Title Charge-Trapping Characteristics of Fluorinated Thin ZrO2 Filmfor Nonvolatile Memory ApplicationsAuthor(s) Huang, XD; Shi, RP; Lai, PTCitation Applied Physics Letters, 2014, v. 104, article no. 162905Issued Date 2014URL http://hdl.handle.net/10722/219122Rights Applied Physics Letters. Copyright © American Institute ofPhysics.Charge-trapping characteristics of fluorinated thin ZrO2 film for nonvolatilememory applicationsX. D. Huang,1,a) R. P. Shi,2 and P. T. Lai2,a)1Key Laboratory of MEMS of the Ministry of Education, Southeast University, Nanjing 210096, China2Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road,Hong Kong, China(Received 26 January 2014; accepted 15 April 2014; published online 24 April 2014)The effects of fluorine treatment on the charge-trapping characteristics of thin ZrO2 film areinvestigated by physical and electrical characterization techniques. The formation of silicateinterlayer at the ZrO2/SiO2 interface is effectively suppressed by fluorine passivation. However,excessive fluorine diffusion into the Si substrate deteriorates the quality of the SiO2/Si interface.Compared with the ZrO2-based memory devices with no or excessive fluorine treatment, the onewith suitable fluorine-treatment time shows higher operating speed and better retention due to lessresistance of built-in electric field (formed by trapped electrons) against electron injection from thesubstrate and smaller trap-assisted tunneling leakage, resulting from improved ZrO2/SiO2 andSiO2/Si interfaces.VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4873388]Metal-oxide-nitride-oxide-silicon (MONOS)-type non-volatile memory with discrete traps in the dielectric forcharge storage is considered as a promising candidate toreplace its floating-gate counterpart due to stronger scalingability and higher reliability. Recently, high-k dielectricshave been proposed instead of conventional Si3N4 ascharge-trapping layer (CTL) to achieve lower operating volt-age and higher charge-trapping efficiency.1–8 Among varioushigh-k dielectrics, Hf-based oxides (e.g., HfO2 and HfON)have been extensively investigated as CTL mainly becauseof its large conduction-band offset relative to SiO2(DEc 2.0 eV), high k value (22), and compatibility withCMOS processing.2–4 On the other hand, ZrO2 shows similardielectric properties as HfO2 but has higher k value (37).5Consequently, there is an increasing interest in exploringZrO2 as CTL.5,6 Wu et al. studied the charge-trapping char-acteristics of ZrO2 with and without nitridation and foundthat MONOS device with nitrided ZrO2 showed better per-formance than that without nitridation due to nitrogen passi-vation of the ZrO2 film.5 Besides nitrogen, fluorine is also anexcellent passivant to remove oxide defects and strengthenthe dielectric films due to its very high electronegativity.9Therefore, fluorination is an effective way to improve thecharge-trapping characteristics of dielectrics.7,8 In this work,based on MONOS capacitors, the charge-trapping character-istics of ZrO2 with and without fluorine incorporation arestudied. Detailed analysis of fluorine treatment on the deviceperformance is also carried out.MONOS capacitors with Al/Al2O3/ZrO2/SiO2/Si werefabricated on p-type (100) substrate. After the standard RCAcleaning, 2-nm SiO2 tunneling oxide was grown on thewafers by thermal dry oxidation. Then 3-nm ZrO2 was depos-ited on the SiO2 by sputtering using a Zr target in an Ar/O2(8/1) mixed ambient. Following that, some samples weretreated by a CHF3þO2 (10 SCCM/1 SCCM) plasma at 20Wfor 150 s and 400 s, respectively. The low-concentration O2was used to remove the carbon and hydrogen in the plasma.10Then, 15-nm Al2O3 was deposited by atomic layer depositionusing trimethyl-aluminum (Al(CH3)3) and H2O as precursorsat 300 C. Following that, all the samples went through apost-deposition annealing in N2 at 900C for 30 s. Finally, Alwas evaporated and patterned as gate electrodes followed bya forming-gas annealing at 300 C for 20min. The samplewithout fluorine treatment was denoted as ZrO, while thesamples with 150-s and 400-s fluorine treatment weredenoted as LF-ZrO and HF-ZrO, respectively. In addition,Al/Al2O3/SiO2/Si (denoted as MAOS) and Al/ZrO2/SiO2/Si(denoted as MNOS) capacitors were also fabricated by thesame process to study the charge-trapping characteristics ofthe ZrO2 film. The physical properties of the films with andwithout fluorine treatment were analyzed by transmissionelectron microscopy (TEM), secondary ion mass spectros-copy (SIMS), and X-ray photoelectron spectroscopy (XPS).The electrical characteristics of the devices were measuredby HP4284A LCR meter and HP4156A semiconductor pa-rameter analyzer.Fig. 1 shows the cross-sectional TEM image of theMONOS capacitors with and without the fluorine treatment,where the physical thickness remains the same after the fluo-rine treatment, indicating negligible etching of the ZrO2 filmdue to the low energy of the fluorine plasma treatment.Fig. 2(a) displays the SIMS depth profile of the samples withvarious fluorine-treatment times to evaluate the fluorine dis-tribution. It is clear that fluorine is mainly located in theZrO2/SiO2 stack and decreases rapidly in the Si substrate,indicating strong fluorine passivation in the ZrO2/SiO2 stack.Also, the fluorine atoms diffuse more into the Si substrate forlonger fluorine-treatment time. This is consistent with theXPS data in Fig. 2(b), where the Si 2p spectrum from the sub-strate for the HF-ZrO sample shifts to higher binding energyby 0.05 eV relative to those for the ZrO and LF-ZrO samples,implying the formation of Si-F bonds. Fig. 2(c) shows the Zr3d spectrum combined with curve-fitting lines. For the ZrOsample, the Sr 3d spectrum displays distinct doublet at184.7 eV (Zr 3d3/2) and 182.3 eV (Zr 3d5/2), agreeing withZr–O bonding in the ZrO2 film.11 After the fluorine treatment,a)Electronic addresses: eexdhuang@gmail.com and laip@eee.hku.hk0003-6951/2014/104(16)/162905/5/$30.00 VC 2014 AIP Publishing LLC104, 162905-1APPLIED PHYSICS LETTERS 104, 162905 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  147.8.31.43On: Wed, 23 Sep 2015 01:57:36the spectrum shifts to higher binding energy by 0.5 eV, indi-cating that F is bonded with Zr. Moreover, the Zr spectrumcan be decomposed into two components, corresponding toZrO2 (182.3 eV for Zr 3d5/2) and Zr silicate formed by theZrO2/SiO2 interfacial reaction (182.7 eV for Zr 3d5/2).Compared with the ZrO sample, the much smaller area of thesilicate component for the LF-ZrO and HF-ZrO samples indi-cates negligible formation of the Zr silicate interlayer due tosufficient fluorine passivation. The formation of the interlayerconsumes the SiO2 tunneling oxide, and also it has smallerbandgap (6.6 eV) and more defects than thermally grownSiO2 (8.9 eV).12,13 Therefore, an abrupt interface withoutinterlayer is desirable for good data retention.14Fig. 3(a) displays the C-V loops under 68MV/cmsweeping for the MAOS and MNOS samples. The trapped-charge density (Qox) in the device can be calculated by theexpression belowQox  DVFBCoxq; (1)where DVFB is the memory window from the C-V loops, Coxis the capacitance density of the samples, and q is the elec-tron charge. The Qox for the MAOS and MNOS samples isabout 3.5 1010 cm2 and 1.1 1012 cm2, respectively.The much higher Qox for the MNOS sample than that for theMAOS one at the same operating condition indicates that thecharge-trapping site is mainly in the ZrO2 layer. In addition,compared with the MAOS sample with counterclockwisehysteresis loop, the MNOS one exhibits a clockwise hystere-sis loop, suggesting that charges inject from the Al electrodeinto the ZrO2 layer at the forward sweeping (from8 MV/cm to þ8 MV/cm) mainly due to the smaller barrierheight at the Al/ZrO2 interface (DEC 1.6 eV for Al/ZrO2;3.0 eV for Al/Al2O3).15 Fig. 3(b) depicts the 1-MHz C-Vhysteresis loops of the MONOS devices, where the memorywindow for the ZrO, LF-ZrO, and HF-ZrO samples is 4.8V,3.8V, and 5.1V, respectively. The smaller window for theLF-ZrO sample than the ZrO sample is due to the suppressedformation of interlayer by fluorine passivation,13 while thelargest window for the HF-ZrO sample suggests extra trapsgenerated by excessive fluorine treatment. The C-V curve ofthe HF-ZrO sample shows more severe stretch-out character-istic than the ZrO and LF-ZrO samples, implying its higherinterface-state density (Dit) at the SiO2/Si interface. Usingthe Terman’s method,16 the HF-ZrO sample has an extractedDit of 7.4 1012 cm2, corresponding to 8.8% and 17.5%higher than the ZrO sample (6.8 1012 cm2) and theLF-ZrO sample (6.3 1012 cm2), respectively. Appropriatefluorine incorporation can passivate the SiO2/Si interface,FIG. 1. Cross-sectional TEM image for (a) the HF-ZrO and (b) ZrOMONOS capacitors.FIG. 2. (a) SIMS depth profile ofZrO2/SiO2 on Si substrate with variousfluorine treatments. (b) XPS Si 2pspectrum and (c) XPS Zr 3d spectrumfor the samples with and without fluo-rine treatments.162905-2 Huang, Shi, and Lai Appl. Phys. Lett. 104, 162905 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  147.8.31.43On: Wed, 23 Sep 2015 01:57:36leading to the smallest Dit for the LF-ZrO sample.9However, owing to the high electronegativity of fluorine, ex-cessive fluorine can distort and even cleave the Si-Si bondsat the interface to form dangling Si bonds and Si-F bonds (asdemonstrated in Fig. 2),17,18 resulting in the highest Dit andthus the largest window for the HF-ZrO sample.Fig. 4(a) displays the gate leakage (JG) of the MONOSdevices as a function of electric field across SiO2 (EOX) byapplying positive gate voltage (VG), corresponding to elec-tron injection from the substrate. The LF-ZrO sample has asmaller leakage at low EOX than the ZrO and HF-ZrO sam-ples due to fewer traps at/near its interface and thus reducedtrap-assisted tunneling. Moreover, the JG-EOX curve of theHF-ZrO sample presents an obvious notch (denoted as A),where JG decreases with EOX and then increases again. Thisis associated with its high Dit at the SiO2/Si interface. Due toCoulomb repulsion, electrons trapped by interface states canform a built-in electric field (Ein) opposite to the externalelectric field EOX induced by VG, which is approximatelygiven byEin  14peQitR2; (2)where Qit is the trapped charge; e is the permittivity of thedielectric that Ein passes through; and R represents thedistance between the trapped-charge centroid and the sub-strate. Ein tends to block the electron injection and also elec-trons filled in the shallow traps near the interface would flowback into the substrate due to Ein, both of which can offsetthe electron injection from the substrate. With increasingEOX and establishing a balance between electron trappingand de-trapping, JG increases with EOX again. A similarnotch (denoted as B) is also observed for the ZrO sample dueto high trap density at the ZrO2/SiO2 interlayer and high Ditat the SiO2/Si interface. On the contrary, no notch in theJG-EOX curve for the LF-ZrO sample suggests that its Ein isweak and has little effect on electron injection, resultingfrom the suppressed formation of interlayer at the ZrO2/SiO2interface by the fluorine passivation as well as low Dit at theSiO2/Si interface by avoiding excessive fluorine diffusion tothe Si substrate. For the ZrO and HF-ZrO samples, the trapsgenerated by the Zr-silicate interlayer and high interfacestates lead to larger VFB shift (thus larger Qit) at low electricfield as shown in Fig. 4(b). Moreover, these trapped chargesat the ZrO2/SiO2 and SiO2/Si interfaces make thetrapped-charge centroid closer to the substrate comparedwith those located in the ZrO2 film, thus resulting in strongerEin against electron injection according to Eq. (2). This isconsistent with the phenomenon that the JG of the LF-ZrOFIG. 3. (a) 1-MHz C-V loops of the MNOS and MAOS devices under 68MV/cm sweeping; (b) 1-MHz C-V loops of the MONOS devices under612V sweeping.FIG. 4. (a) JG as a function of EOX for the MONOS capacitors. The simu-lated FN tunneling current through SiO2 tunneling oxide is also shown. (b)VFB shift at low (4 MV/cm) and high (8 MV/cm) EOX. VFB shift isdefined as VFB  VFB0, where VFB is the flat-band voltage under stress, andVFB0 is the flat-band voltage of the fresh device.162905-3 Huang, Shi, and Lai Appl. Phys. Lett. 104, 162905 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  147.8.31.43On: Wed, 23 Sep 2015 01:57:36sample increases more rapidly with EOX than those of theZrO and HF-ZrO samples, which becomes more obvious asthe Fowler-Nordheim (FN) tunneling current dominates JG(FN current is exponentially proportional to electric field ofEOX-Ein).19 It should be noted that the little impact of Ein onelectron injection from the substrate is beneficial to supply-ing sufficient electrons for the memory device, thus contrib-uting to the larger VFB shift (3.1V versus 2.8 V for ZrO,2.5V for HF-ZrO) at high electric field for the LF-ZrO sam-ple in Fig. 4(b). On the other hand, small leakage at low elec-tric field normally indicates suppressed trap-assistedtunneling under retention mode and thus is beneficial fordata retention. Therefore, the steeper JG-EOX characteristicof the LF-ZrO sample is desirable for memory device toachieve high operating speed as well as good data retention.Fig. 5(a) displays the retention characteristics of theMONOS capacitors, and the retention data for the ZrO sam-ple with thicker SiO2 tunneling layer (SiO2 3.5 nm) arealso measured to study charge-loss paths under the retentionmode. The ZrO sample with thicker SiO2 shows excellentdata retention with no charge loss even after 104 s suggeststhat no charge loss happens through the Al2O3 blocking layermainly due to its high quality with negligible traps.Moreover, compared with the ZrO sample with thicker SiO2,the ZrO one with thinner SiO2 (2.0 nm) shows worse dataretention, indicating that the charge loss is mainly deter-mined by the tunneling SiO2 layer. As the operating tempera-ture is raised from 25 C to 125 C, the charge-loss rate after104 s (Qloss) increases from 3.9% to 21.2% for the LF-ZrOsample; For comparison, the corresponding Qloss increasesfrom 19.9% to 32.6% and from 24.9% to 36.8% for the ZrOand HF-ZrO samples, respectively. The lower Qloss of theLF-ZrO sample demonstrates its better data retention. Theactivation energy EA of the charge loss is also extracted fromthe Arrhenius plot of the retention property in Fig. 5(b) togain more insight on the charge-loss mechanism. The muchsmaller EA of the ZrO sample (0.041 eV) and HF-ZrO sam-ple (0.050 eV) indicates that the charge-loss mechanism ismainly based on tunneling process. which hardly depends ontemperature.20 For the ZrO and HF-ZrO samples, the elec-trons trapped at the ZrO2/SiO2 or SiO2/Si interface are closeto the substrate, thus easily escaping back to the substrate,because the tunneling probability increases exponentiallywith decreasing barrier height and tunneling distancebetween the trapped electrons and the substrate. Also, thetraps at the ZrO2/SiO2 and SiO2/Si interfaces can act as amedium to facilitate the escaping of electrons from the CTLto the substrate by trap-assisted tunneling. On the contrary,the trapped electrons in the LF-ZrO sample cannot move eas-ily by tunneling but have to be thermally activated to theconduction band of the CTL before tunneling to the substrate(due to the larger distance of the trapped charges in the ZrO2film from the substrate as well as high-quality ZrO2/SiO2and SiO2/Si interfaces), thus resulting in larger EA(0.17 eV) and better data retention.In summary, the effects of fluorine treatment on thecharge-trapping characteristics of thin ZrO2 film are investi-gated. Compared with the memory devices with no or exces-sive fluorine treatment, the one with suitable fluorinetreatment time shows better characteristics due to suppressedinterlayer growth by fluorine passivation of the ZrO2/SiO2interface and also fewer interface states by avoiding exces-sive fluorine diffusion to the substrate. Therefore, ZrO2 filmwith appropriate fluorine incorporation is a promising candi-date as CTL for high-performance nonvolatile memoryapplications.This work was financially supported by the CRCGSmall Project Funding (No. 201209176095) of theUniversity of Hong Kong.1T. M. Pan, J. S. Jung, and F. H. Chen, Appl. Phys. Lett. 97, 012906(2010).2H. W. You and W. J. Cho, Appl. Phys. Lett. 96, 093506 (2010).3H. J. Yang, C. F. Cheng, W. B. Chen, S. H. Lin, F. S. Yeh, S. P.McAlister, and A. Chin, IEEE Trans. Electron Devices 55, 1417 (2008).4J. Y. Wu, Y. T. Chen, M. H. Lin, and T. B. Wu, IEEE Electron DeviceLett. 31, 993 (2010).5Y. H. Wu, L. L. Chen, Y. S. Lin, M. Y. Li, and H. C. Wu, IEEE ElectronDevice Lett. 30, 1290 (2009).6J. Liu, Q. Wang, S. B. Long, M. H. Zhang, and M. Liu, Semicond. Sci.Technol. 25, 055013 (2010).7X. D. Huang, J. K. O. Sin, and P. T. Lai, IEEE Trans. Electron Devices 58,4235 (2011).8J. C. Wang, C. T. Lin, C. S. Lai, and J. L. Hsu, Appl. Phys. Lett. 97,023513 (2010).9C. R. Hsieh, Y. Y. Chen, and J. C. Lou, Appl. Phys. Lett. 96, 022905(2010).FIG. 5. (a) Data retention for the MONOS capacitors with an initial VFBshift of 3.2V. (b) Arrhenius plot of the retention property.162905-4 Huang, Shi, and Lai Appl. Phys. Lett. 104, 162905 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  147.8.31.43On: Wed, 23 Sep 2015 01:57:3610C. X. Li, C. H. Leung, P. T. Lai, and J. P. Xu, Solid-State Electron. 54,675 (2010).11B. A. V. Hassel and A. J. Burggraaf, Appl. Phys. A 52, 410 (1991).12A. Kawamoto, K. Cho, P. Griffin, and R. Dutton, J. Appl. Phys. 90, 1333(2001).13T. Yamaguchi, H. Satake, and N. Fukushima, Tech. Dig. - Int. ElectronDevices Meet. 2001, 663.14C. H. Lin and Y. Kuo, J. Appl. Phys. 110, 024101 (2011).15G. D.Wilk, R.M.Wallace, and J. M. Anthony, J. Appl. Phys. 89, 5243 (2001).16L. M. Terman, Solid-State Electron. 5, 285 (1962).17M. Morita, T. Kubo, T. Ishihara, and M. Hirose, Appl. Phys. Lett. 45,1312 (1984).18L. Tsetseris, X. J. Zhou, D. M. Fleetwood, R. D. Schrimpf, and S. T.Pantelides, Appl. Phys. Lett. 85, 4950 (2004).19Y. Kim, S. I. Ohmi, K. Tsutsui, and H. Iwai, Jpn. J. Appl. Phys., Part 1 44,4032 (2005).20Y. Liu, S. Tang, and S. K. Banerjee, Appl. Phys. Lett. 88, 213504(2006).162905-5 Huang, Shi, and Lai Appl. Phys. Lett. 104, 162905 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  147.8.31.43On: Wed, 23 Sep 2015 01:57:36

Charge-Trapping Characteristics of Fluorinated Thin ZrO2 Film for Nonvolatile Memory Applications

http://hub.hku.hk/bitstream/10722/219122/1/content.pdf

Charge-Trapping Characteristics of Fluorinated Thin ZrO2 Film for Nonvolatile Memory Applications

Abstract

Similar works

Full text

Available Versions

Crossref

HKU Scholars Hub