In this work, Al/Al 2O 3 nanocomposite thin film is deposited on Si substrate by radio frequency sputtering to form a metal-insulator-semiconductor structure. It is found that the current conduction at low fields is greatly enhanced with temperature. The current increase can be attributed to the decrease in the tunneling resistance and/or the formation of some tunneling paths due to the release of some measurement-induced charges trapped in the thin film as a result of increase in the temperature. The current conduction evolves with a trend toward a three-dimensional transport as the temperature increases. © 2011 American Institute of Physics.published_or_final_versio

Chen, TP

Ding, L

Fung, S

Liu, Y

Liu, Z

Wong, JI

Yang, M

Journal of Applied Physics

HKU Scholars Hub

Title Temperature dependence of current transport in Al/Al 2O 3nanocomposite thin filmsAuthor(s) Liu, Y; Chen, TP; Ding, L; Yang, M; Liu, Z; Wong, JI; Fung, SCitation Journal Of Applied Physics, 2011, v. 110 n. 9Issued Date 2011URL http://hdl.handle.net/10722/143386Rights Creative Commons: Attribution 3.0 Hong Kong LicenseTemperature dependence of current transport in Al/Al2O3 nanocompositethin filmsY. Liu,1,a) T. P. Chen,2,b) L. Ding,2 M. Yang,2 Z. Liu,2 J. I. Wong,2 and S. Fung31State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science andTechnology of China, Chengdu, Sichuan, 610054, People’s Republic of China2School of Electrical and Electronic Engineering Nanyang Technological University, Singapore 6397983Department of Physics, The University of Hong Kong, Pokfulam, Hong Kong(Received 7 July 2011; accepted 26 October 2011; published online 15 November 2011)In this work, Al/Al2O3 nanocomposite thin film is deposited on Si substrate by radio frequencysputtering to form a metal-insulator-semiconductor structure. It is found that the current conductionat low fields is greatly enhanced with temperature. The current increase can be attributed to thedecrease in the tunneling resistance and/or the formation of some tunneling paths due to the releaseof some measurement-induced charges trapped in the thin film as a result of increase in thetemperature. The current conduction evolves with a trend toward a three-dimensional transport asthe temperature increases.VC 2011 American Institute of Physics. [doi:10.1063/1.3663313]INTRODUCTIONIntensive research have been carried out on Al2O3 dueto its excellent electrical properties such as low leakage cur-rent, high dielectric constant (9), and high breakdown volt-age (9 MV/cm). The material is considered as a promisingcandidate for the gate dielectric of field-effect transistors.1–3Various techniques such as radio-frequency magnetron sput-tering,4 atomic-layer-deposition (ALD),5 and plasma-enhanced atomic-layer deposition (PEALD)6 have been usedto prepare the Al2O3 films. Recently, it was reported that Al-rich Al2O3 thin film can be used to realize resistive memorydevice7 and floating gate non-volatile memory device.8 Asthose memory devices may work at elevated temperatures,the temperature dependence of current conduction in the Al-rich Al2O3 layer of the metal-isolator-metal structure forresistive memory device or in the gate dielectric for the float-ing gate memory device is important. On the other hand,some studies on the current conduction mechanism of semi-conductor nanocrystals embedded in oxide film have beenreported.9–11 Mechanisms including Fowler–Nordheim (FN)tunneling, nanocrystal-assisted tunneling, Frenkel–Pooleemission and nanocrystal-assisted FN tunneling have beenproposed.9 However, the mechanism of conduction in metal/dielectric nanocomposite films has seldom been discussed.In this work, Al2O3 film is deposited on Si substrate by radiofrequency (RF) sputtering of Al target in O2 ambient to forma metal-insulator-semiconductor (MIS) structure. Transmis-sion Electron Microscopy (TEM) image shows that Al/Al2O3 nanocomposite thin films are formed. The influence oftemperature on current transport at low fields in Al/Al2O3nanocomposite films has been studied. It is found that thecurrent conduction is greatly enhanced with temperature.The current-voltage (I-V) characteristics at low fields followa power law. As the temperature increases, the systemevolves with a trend toward a three-dimensional (3D) trans-port. The phenomena can be explained by a model of chargetransport in the Al nanocrystal arrays.One hundred nm thick Al-rich Al2O3 films were depos-ited on p-type, (100)-oriented Si wafers via radio frequencymagnetron sputtering. The deposition was carried out with RFsputtering of Al target (purity> 99.999%) in O2/Ar ambient ata power of 310 W. The Ar to O2 ratio is set to 60:1. The filmswere annealed with rapid thermal process at 500 C for 2 min.A 200 nm aluminum layer was then deposited on the top ofthe thin films to form gate electrodes. The backside electrodewas formed with a 500 nm aluminum layer after removing ini-tial oxide. The high resolution transmission electron micros-copy (HRTEM) image shows the existence of Al nanocrystals(nc-Al) (or nanoparticles) in the Al oxide layer, as illustratedin Fig. 1. The I-V measurements were carried out with aKeithley 4200 semiconductor characterization system, andcapacitance-voltage (C-V) measurements were carried outwith a HP4284 A LCR meter at the frequency of 1 MHz.Figure 2 shows the I-V characteristics measured at lowfields at 25 C, 75 C, and 100 C, respectively. As can beseen in the figure, the current conduction is greatly enhancedby increasing the temperature. The conduction enhancementcan be explained as following. Electron tunneling can takeplace between adjacent uncharged Al nanocrystals, andmany such nanocrystals form tunneling paths connecting theSi substrate to the metal gate.9,12,13 As reported in our previ-ous works13,14 electrons can be trapped in the nanocrystalswhen they transport along the tunneling paths during a cur-rent measurement, causing a reduction in the current conduc-tion due to the Coulomb interactions, whereas the release ofthe trapped charges leads to an increase in the current con-duction. In the Al/Al2O3 nanocomposite thin film, even inthe initial state, there may be some electrons trapped in nano-crystals during the electrical measurement. The trappedcharges can be released when the temperature is increased,making more nanocrystals neutral (uncharged). With the ex-istence of more tunneling paths formed by the unchargeda)Author to whom correspondence should be addressed. Electronic mail:yliu2008@e.ntu.edu.sg.b)Electronic mail: echentp@ntu.edu.sg.0021-8979/2011/110(9)/096108/3/$30.00 VC 2011 American Institute of Physics110, 096108-1JOURNAL OF APPLIED PHYSICS 110, 096108 (2011)Downloaded 28 Feb 2012 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsnanocrystals, the current conduction of the thin film systemis enhanced. The charge release is confirmed by the C-Vmeasurements, as shown in the inset of Fig. 2. With tempera-ture increasing, the C-V characteristic is shifted to thenegative-voltage side, which indicates electron release fromthe nanocomposite thin film at a higher temperature. Theflatband voltage shift is proportional to the amount of thecharges released from the Al/Al2O3 nanocomposite film. Thecharge release leads to an enhancement in the current con-duction as well as a higher dimensional transportation in theoxide thin film, which will be discussed later.Figure 2 shows that the I-V characteristics at low fields fol-low a power law. The power-law behavior could be explainedby a model similar to the one of collective charge transport inarrays of normal-metal quantum dots (QDs).15 The currentthrough an array of metallic quantum dots separated by tunnelbarriers is shown to follow the power-law relationship15,16I ¼ I0ðV  VthÞf; (1)where f is the scaling exponent, and Vth is the threshold volt-age. The scaling exponent f¼ 1 and 5/3 correspond to theone- and two-dimensional arrays of QDs, respectively.15,16Equation (1) can be used to fit our experimental result of thesystem of Al/Al2O3 nanocomposite film, and the fitting canyield the values of the factors including f, Vth and I0 ofEq. (1). The threshold voltage is found to be approximatelyzero. Figure 3 shows Arrhenius plot of ln(I0) and f obtainedfrom the fittings. f is found to be larger than 5/3. This sug-gests that the current conduction in the system is a quasi-3Dtransport. With temperature increasing, some of trappedcharges are released from the nanocrystals, leading to forma-tion of some new tunneling paths. With the formation of newtunneling paths, the current conduction of the system willevolve toward a higher dimensional transport, i.e., quasi-3Dtransport. On the other hand, I0 is observed to increase withtemperature. For the case shown in Fig. 3, I0 increasesgreatly from the initial I0¼ 4.12 109AV-f at 25 C toI0¼ 5.60 106 A V-f at 200 C.As indicated in Fig. 3, the temperature increase leads toan increase in both I0 and f. The increase in I0 is attributed tothe decrease of the tunneling resistance and/or the increasein the number of tunneling paths as a result of the chargerelease at a higher temperature. The activation energyobtained from the Arrhenius plot of I0 is 0.52 eV, indicatingthat the barrier for charge releasing is around 0.5 eV. On theother hand, f increases from 1.79 at 25 C to 2.10 at 200 C,showing an evolution with a trend toward the 3D transportwhen more charges are released at a higher temperature.As mentioned above, the increases in both f and I0 aredue to the release of the charges trapped in the nanocrystals ata higher temperature. To confirm the charge release, C-V mea-surement is carried out to determine the flatband voltage shiftwhich is proportional to the amount of the charges released.As shown in the inset of Fig. 2, the C-V curve shifts towardthe negative-voltage side when the temperature increases,showing a negative flatband voltage shift. The negative flat-band voltage shift indicates a reduction in the amount of elec-trons trapped in the nanocomposite film. The electron releaseis due to the electron escape from nanocrystals at a highertemperature. Note that the C-V measurement is affected bythe charge trapping in the oxide layer and is most sensitive tothe charges in the oxide region near the Al2O3/Si interface.FIG. 2. I-V characteristics at low fields measured at 25 C, 75 C, and100 C, respectively. The inset shows the C-V characteristics at the corre-sponding temperatures. The solid lines show the fittings based on Eq. (1).I0¼ 4.11 109 AV-f and f¼ 1.79 at 25 C; I0¼ 6.90 108 AVf andf¼ 1.98 at 75 C; and I0¼ 1.64 107 AV-f and f¼ 2.00 at 100 C. FIG. 3. Arrhenius plots of I0 and f.FIG. 1. TEM image of the Al/Al2O3 nanocomposite thin film.096108-2 Liu et al. J. Appl. Phys. 110, 096108 (2011)Downloaded 28 Feb 2012 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsTherefore, the electron release from the oxide layer isdescribed with the equivalent areal charge density (Qri) calcu-lated from the flatband voltage shift (~VFB), i.e.,Qri¼CoxDVFB where Cox is the oxide capacitance per area.Note that the initial flat-band voltage at room temperature(25 C) is taken as the reference for calculating DVFB and thusQri is zero at room temperature. Assuming the charge uni-formly distributes in the oxide film, the total amount ofcharges released from the film is proportional to Qri. Thus, thetemperature dependence of ~VFB is translated into the temper-ature dependence of Qri. To study the effect of charge releaseon the current transport, electron releases with differentamounts from the nanocrystals are achieved by raising thetemperature to different levels. For a given temperature, Qri isobtained from the flatband voltage shift based on the C-V mea-surement as discussed above, and I0 and f are obtained fromthe fittings to the I-V characteristics. In this way, we are ableto examine the influence of electron release from nanocrystalson I0 and f. Figure 4 shows I0 and f as a function of Qri.Electron tunneling can take place between adjacentuncharged nanocrystals, and many such nanocrystals formconduction paths connecting the Si substrate to the metalgate.12,13 As nanocrystals distribute in the oxide matrix ran-domly in three dimensions, with the formation of moreuncharged nanocrystals due to more electron release at ahigher temperature, the electron tunneling via the unchargednanocrystals will go beyond the two-dimension regime, mak-ing f larger than 5/3. Therefore, the current conduction ofthe system will evolve toward the three-dimension transportwith temperature. For example, f increases from 1.79 forQri¼ 0 at 25 C to 2.10 for Qri¼ 3.69 1011 e/cm2 at thetemperature of 200 C, but it is always beyond the upperlimit f¼ 5/3 for two dimensional transport. This indicatesthat the current conduction of the system is a qusi-3D trans-port and evolves toward the 3D transport with more chargerelease. As can be observed from Fig. 4, the charge releaseleads to an increase in I0 also, which is due to the decrease ofthe tunneling resistance and/or the increase in the number oftunneling paths as a result of the charge release at a highertemperature.12 Fittings to the data of I0 and f shown in Fig. 4yield the following phenomenological mathematical expres-sions: I0¼ 4.12 109exp(1.95 1011Qri) andf¼ 7.7 1013 Qriþ 1.82. As given by the expressions, forthe initial state at room temperature, i.e., Qri¼ 0, I0 is4.12 109 AV-f, which is actually the I0 at room tempera-ture; and f¼ 1.82, which is larger than 5/3, suggesting thatthe current transport at room temperature is beyond the two-dimension regime and is a quasi-3D transport.Note that charging and discharging can occur duringelectrical measurement and they are random processes. Assuch, I-V measurement at a given temperature is not neces-sarily repeatable. For example, the I-V characteristic afterback from the measurement at 100 C to room temperature(25 C) can be different from the initial I-V characteristic atroom temperature. The current level of the former can be ei-ther higher or lower than that of the latter.In conclusion, Al/Al2O3 nanocomposite filmes are pre-pared with RF magnetron sputtering of Al target in O2 ambi-ent. MIS structures are formed by coating an Al layer on thetop of the films. The I-V characteristic of the system at lowfields follows a power law, i.e., I¼ I0 (V-Vth)f where thethreshold voltage Vth  0 and the factors I0 and f increasewith the charge release from the nanocrystals when the tem-perature increases. The charge release from nanocrystalscauses an increase of I0 because of the decrease in the resist-ance of the tunneling paths and/or the formation of more tun-neling paths due to more neutral nanocrystals available. Onthe other hand, f also increases, showing an evolution of thecurrent conduction with a trend toward the 3D transport.ACKNOWLEDGMENTSThis work has been financially supported by NSFCunder project No. 60806040, the Fundamental ResearchFunds for the Central Universities under project No.ZYGX2009X006, the Young Scholar Fund of Sichuan underproject No. 2011JQ0002 and the National Research Founda-tion of Singapore under project NRF-G-CRP 2007-01.1Z. Jin, H. S. Kwok, andM.Wong, IEEE Electron Device Lett. 19, 502 (1998).2O. Auciello, W. Fan, B. Kabius, S. Saha, J. A. Carlisle, R. P. H. Chang, C.Lopez, E. A. Irene, and R. A. Baragiola, Appl. Phys. Lett. 86, 042904 (2005).3J.-C. Chiang and J.-G. Hwua, Appl. Phys. Lett. 90, 102902 (2007).4S. Choi, S. S. Kim, H. Yang, M. Chang, S. Jeon, C. Kim, D. Y. Kim, andH. Hwang, Microelectron. Eng. 80, 264 (2005).5S. K. Kim, Y. Xuan, P. D. Ye, S. Mohammadia, J. H. Back, and M. Shim,Appl. Phys. Lett. 90, 163108 (2007).6J. B. Koo, C. H. Ku, S. C. Lim, S. H. Kim, and J. H. Lee, Appl. Phys. Lett.90, 133503 (2007).7W. Zhu, T. P. Chen, Z. Liu, M. Yang, Y. Liu, and S. Fung, J. Appl. Phys.106, 093706 (2009).8S. Nakata, K. Saito, and M. Shimada, Appl. Phys. Lett. 87, 223110 (2005).9J. I. Wong, T. P. Chen, M. Yang, Y. Liu, C. Y. Ng, and L. Ding, J. Appl.Phys. 106, 013718 (2009).10M. Yang, T. P. Chen, Z. Liu, J. I. Wong, W. L. Zhang, S. Zhang, and Y.Liu, J. Appl. Phys. 106, 103701 (2009).11R. Krishnan, Q. Xie, J. Kulik, X. D. Wang, S. Lu, M. Molinari, Y. Gao, T.D. Krauss, and P. M. Fauchet, J. Appl. Phys. 96, 654 (2004).12C. Y. Ng, Y. Liu, T. P. Chen, and M. S. Tse, Smart Mater. Struct. 15, S43(2006).13Y. Liu, T. P. Chen, C. Y. Ng, M. S. Tse, S. Fung, Y. C. Liu, S. Li, andP. Zhao, Electrochem. Solid-State Lett. 7, G134 (2004).14Y. Liu, T. P. Chen, P. Zhao, S. Zhang, S. Fung, and Y. Q. Fu, Appl. Phys.Lett. 87, 033112 (2005).15A. A. Middleton and N. S. Wingreen, Phys. Rev. Lett. 71, 3198 (1993).16H. E. Romero and M. Drndic, Phys. Rev. Lett. 95, 156801 (2005).FIG. 4. I0 and f as a function of Qri. Note that Qri¼ 0 at room temperature,which is taken as the reference. The change of Qri is realized by changingthe temperature. The solid lines show the fittings.096108-3 Liu et al. J. Appl. Phys. 110, 096108 (2011)Downloaded 28 Feb 2012 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

Temperature dependence of current transport in Al/Al 2O 3 nanocomposite thin films

Y. Liu

T. P. Chen

L. Ding

M. Yang

Z. Liu

J. I. Wong

S. Fung

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Temperature dependence of current transport in Al/Al<sub>2</sub>O<sub>3</sub> nanocomposite thin films

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Temperature dependence of current transport in Al/Al 2O 3 nanocomposite thin films

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