Author name used in this publication: P. F. LeeAuthor name used in this publication: J. Y. Dai2005-2006 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Dai, J

Gong, XG

Hou, ZF

Koo, KM

Lau, WM

Lee, PF

Li, Q

Applied Physics Letters

PolyU Institutional Repository

APPLIED PHYSICS LETTERS 88, 182903 2006DownlEffects of Al addition on the native defects in hafniaQuan Li,a K. M. Koo, and W. M. LauDepartment of Physics, The Chinese University of Hong Kong, Shatin, New Territory, Hong Kong, ChinaP. F. Lee and J. Y. DaiDepartment of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University,Hung Hom, Kowloon, Hong Kong, ChinaZ. F. Hou and X. G. GongbSurface Physics Laboratory & Department of Physics, Fudan University, 200433-Shanghai, ChinaReceived 21 October 2005; accepted 10 March 2006; published online 3 May 2006Two occupied native defect bands are experimentally detected in pure HfO2. The density of statesof band one in the middle of the band gap reduces drastically with the Al addition, while that of bandtwo slightly above the valence-band maximum remains rather unaffected. We attribute the twobands to the charged oxygen vacancy, and the oxygen-interstitial-related defect states of the HfO2,respectively. We demonstrate that the added Al passivates the VO+ induced midgap states but haslittle effect on other aspects of the electronic structure of the material. © 2006 American Institute ofPhysics. DOI: 10.1063/1.2196470The scaling requirements in the complementary metal-oxide-semiconductor CMOS integrated circuit technologypredict the inadequacy of SiO2 as gate dielectric material fora system smaller than 0.1 m, which leads to an extensivesearch of the high-dielectric constant high-K materials aspotential replacements for SiO2 Ref. 1. Among these high-K dielectrics, pseudobinary Hf-based materials appear to beone of the most promising material systems and they haveindeed demonstrated various desirable electrical behaviors.In such a pseudobinary system, the addition of a third ele-ment usually Si or Al to HfO2 is expected to increase,relative to the properties of pure HfO2, the Si/dielectric in-terfacial stability,2 crystallization temperature,3 band gap,and both the valance- and conduction-band offsets to SiRefs. 4 and 5.On the other hand, there are still practical deficienciesin the HfO2 system. In particular, various native defectsin HfO2, such as oxygen vacancy and interstitial, have beenboth theoretically predicted6,7 and experimentallyobserved.8,9 Once formed, these defects not only introducedefect levels in the band gap, but also serve as charge traps.They can thus severely affect the electrical behavior of thematerials, such as leakage current and charge scattering.6,10These effects become even more complicated in the pseudo-binary system because the aforementioned addition of thethird element in HfO2 may interact with the native defects ofHfO2, modify the defect states, and thereby either improve ordeteriorate the electrical performance of the gate dielectrics.To address these outstanding issues, we have carried outa systematic investigation on the electronic structure changesof a representative pseudobinary Hf-based material-hafniumaluminate as a function of aluminum concentration. In thisletter, we report our experimental detection of the presenceof at least two broad occupied defect bands in the band gapof the nominally pure HfO2 films referred to as pure HfO2therein, their individual changes in density of states as afunction of aluminum addition, and a theoretical explanationaElectronic mail: liquan@phy.cuhk.edu.hkbElectronic mail: xggong@fudan.edu.cn0003-6951/2006/8818/182903/3/$23.00 88, 18290oaded 07 Apr 2011 to 158.132.161.52. Redistribution subject to AIP licof these observations with ab initio studies.The hafnium aluminate thin films with thickness ofabout 20 nm were deposited by pulsed laser deposition onp-type 100 Si substrates using a high-purity hafnium alu-minate target with different Hf/Al ratios Table I. The sili-con substrates were treated by an HF etch just before thin-film deposition, an etch which is known to leave the siliconsurface terminated by hydrogen. The X-ray photoelectronspectroscopy XPS measurements were carried out using aPHI Quantum 2000 system, equipped with a monochroma-tized Al K source, a surface charge neutralizer, and a low-energy argon ion gun. All high-resolution scans were taken ata photoelectron takeoff angle of 90° and calibrated to Au4f7/2 at 84.0 eV from a sputter-cleaned gold foil. Measure-ments of the Fermi-level position in the band gap were alsocalibrated by the threshold of photoemission of the sputter-cleaned gold reference under the same spectral resolutionconditions. The microstructures of the thin films were inves-tigated by transmission electron microscopy TEM, Tecnai20ST. The valence electron energy-loss measurements ofthe films were performed using the Gatan GIF system at-tached to the TEM with an energy resolution of 0.7 eV. Thespectra were acquired in the diffraction mode at small mo-mentum transfer with an angular resolution about 0.2 mrad.The Hf/Al ratios of the films are determined from theXPS survey scan using the corresponding peak areas, as il-lustrated in Table I. Scanning transmission electron micros-TABLE I. Hafnium/aluminum ratio in the hafnium aluminate target and theas-deposited thin films.SampleHf/Al ratios inthe targetsHf/Al ratios in theas-deposited thin films1 1/0 1/02 2/1 4/13 1/1 1.5/14 1/2 1/25 1/4 1/66 0/1 0/1© 2006 American Institute of Physics3-1ense or copyright; see http://apl.aip.org/about/rights_and_permissions182903-2 Li et al. Appl. Phys. Lett. 88, 182903 2006Downlcopy and transmission electron diffraction suggest similarthickness 20 nm and morphology of the all as-depositedfilms the films are amorphous and no interfacial layer isdetected.The loss functions Im−1/ of the films Fig. 1a areobtained by removing the plural scattering from the corre-sponding energy-loss spectra using a direct deconvolutionmethod.11 The band gap of pure HfO2 is estimated to be6.0 eV. Little change of the band gap is identified in theas-deposited films as the Al concentration increases, untilpure alumina is obtained, for which the band gap is estimatedas 6.5 eV, also consistent with the literature value of amor-phous alumina.3 An obvious midgap band with excitationenergy of 3–6 eV appears in the pure HfO2 films. Such afeature significantly decreases when aluminum is introducedin the films, and completely diminishes in the pure aluminafilm.Detailed information of the occupied midgap statesabove the valence-band maximum VBM are obtainedusing high-resolution XPS scan in the low-energy range0–10 eV. The most predominant features in the spectra arethe two midgap bands centered at 2.5 eV band I and0.5 eV band II above the VBM observed in pure HfO2as marked by arrows in Fig. 1b. It is interesting to notethat the two bands behave differently with the Al addition.The intensity of band I decreases drastically as the Al con-centration increases in the films, and becomes undetectablewhen the Hf/Al ratio reaches 1/6. In comparison, littlechange is observed for the intensity of band II as the filmcomposition varies, and such band only disappears in purealumina.From these observations, we conduct ab initio calcula-tions on defects in monoclinic HfO2 within the generalizedgradient approximation GGA Ref. 12, using a total-energy plane-wave pseudopotential VASP code13 and a super-cell of 96 atoms of monoclinic HfO2. Atoms are removedfrom or added to the supercell in order to model the vacancyor interstitial defects, respectively. To simulate the Al addi-tion, the Al subsitutional defect AlHf is studied in the casethat one of the Hf atoms in the supercell is substituted by anAl atom. The defect pairs VO–AlHf, Oi–AlHf, or O2–AlHfare formed by a substitutional Al atom and an adjacent oxy-gen vacancy or interstitial. The defect structures are relaxedin their various charge states until forces on all atoms were14less than 0.01 eV/Å, using the ultrasoft pseudopotentialsoaded 07 Apr 2011 to 158.132.161.52. Redistribution subject to AIP licand a plane-wave cutoff of 500 eV. Due to the large sizeof the supercell, a single  point was employed fork-point sampling, which resulted in energy convergce of9 meV/HfO2. We identify the defect levels in HfO2 by cal-culating the vertical ionization energies VIP, Ip, and re-laxed electron affinities REA, e, with respect to the bot-tom of the conduction band of HfO2 using the equations andprocedure described in Ref. 6.The calculated ionization energies and relaxed electronaffinities of these defects are summarized in Table II. Thosevalues of the pure HfO2 are in good agreement with theliterature.6 The formation of a neutral oxygen vacancy intro-duces a fully occupied defect level with two electrons VO0 in the middle of the band gap. The highest occupied state islocated at 2.02 eV above the VBM as indicated by our cal-culated ionization potential Table II.The addition of neutral oxygen interstitials Oi0 and O20has two effects in general. First, the defects contribute tooccupied and unoccupied defect states in the band gap. Sec-ond, they may also interact with VO and thereby alter thephysical properties of oxygen vacancy. Table II shows thatthe VIP of Oi0 and O20 are 0.04 and 0.08 eV above the VBM.Their respective REA are at 3.71 and 4.64 eV below theconduction-band minimum CBM. Since their unoccupiedstates are below the highest occupied states of both VO andFIG. 1. Color online a Loss functions and b XPSlow loss spectra of the Hf–Al–O thin films with differ-ent Al concentrations.TABLE II. Ionization energies and electron affinities of defects in differentcharged states, which are corrected by the difference between the calculatedband gap Eg=3.86 eV and experimental data Egexp=5.68 eV Ref. 15.DefectsIpDq eDqThis work Ref. 6 This work Ref. 6VO0 3.66 3.41 ¯ ¯VO+ 3.48 3.75 3.01 2.76Oi0 5.64 5.55 3.71 3.95Oi− 5.37 5.38 5.02 4.75O20 5.60 5.53 4.64 4.67O2− 5.42 5.46 5.13 5.06VO–AlHf0 2.97 ¯ 2.61 ¯VO–AlHf+ 5.75 ¯ 2.33 ¯Oi–AlHf0 5.62 ¯ 5.43 ¯Oi–AlHf− 5.61 ¯ 1.76 ¯O0–AlHf0 5.47 ¯ 5.07 ¯O0–AlHf− 5.203 ¯ 4.50 ¯ense or copyright; see http://apl.aip.org/about/rights_and_permissions182903-3 Li et al. Appl. Phys. Lett. 88, 182903 2006DownlVO+, they can draw electrons from the oxygen vacancies andform Oi− and O2−, i.e., formation of Oi O2 will compensateVO, removing electrons from its defect level. The Fermi levelof the system will drop accordingly. The actual behavior andelectrical properties will obviously depend on the actualamounts of these defects in a real sample.By comparing the energy levels obtained in theoreticalcalculation, we propose that the two defect bands observed2.5 and 0.5 eV above the VBM in the XPS spectrum ofthe pure HfO2 films can be assigned to the oxygen vacancyand interstitial related defects, respectively. These results areconsistent with the relevant theoretical and experimental datain the literature.6,8 Due to our HfO2 films being amorphous,the observed defect band at 2.5 eV above the VBM appearsas a broad band of occupied states in our XPS spectrum,which suggests that the Fermi level of our pure HfO2 shouldbe at the “head” of this band near CBM, i.e., about 4 eVabove the VBM. This implies that the amount of VO must begreater than that of oxygen interstitials Oi0 and O20. Theaforementioned defect interactions among the vacancies andinterstitials would therefore leave VO+ as a half occupied de-fect states with the smallest VIP in the band gap of pureHfO2, and thus a corresponding Fermi level in between theenergy level of VO+ and the CBM if we also note that nointerface states are observed in our HfO2 films.Regarding the effects of Al addition, substitution of Al atthe Hf site, which is energetically favorable especially underoxygen-rich conditions, will create a hole state near theVBM because Al has three valence electrons and Hf has four.This acceptor level will compensate the VO donor state, andremove electrons from the donor level. When Al concentra-tion is more than twice as large as the oxygen vacancy con-centration, all the electrons in the oxygen vacancy donorlevels will be removed, i.e., VO is 2+ charged. Indeed thischange is demonstrated by the observed diminishing band Iwith Al addition in the XPS data in Fig. 1b, although wecould not exclude the possibility that the oxygen vacancy canbe suppressed due to the growth condition of films in oursamples from 2 to 5 Table I. On other hand, although the Aladdition also changes other occupied defect states near theVBM, the changes on their VIP are relatively small. Further-more, AlHf itself also creates occupied defect levels nearVBM contributing to the XPS signal. This explains the per-sistent appearance of band II in the XPS spectra of the HfO2films with Al addition Fig. 1b. In short, our experimentaland theoretical results suggest that ionized oxygen vacanciesand interstitials are the major defects observed in our experi-ments. This is also supported by the C-V measurement of thecorresponding films after annealing at 850 °C, which sug-oaded 07 Apr 2011 to 158.132.161.52. Redistribution subject to AIP licgests the existence of large amounts of positively chargeddefects.16In conclusion, we found that two defect bands centeredat 2.5 and 0.5 eV above the VBM in pure hafnia can beidentified as originating from the oxygen vacancies and oxy-gen interstitials, respectively. Introducing Al to the pureHfO2 leads to interaction between Al and these defects,which passivates the occupied VO gap states, but has littleeffect on other aspect of the electronic structure of the ma-terial. The current study, therefore, suggests an alternativeapproach to eliminate these defects, i.e., through defect en-gineering, by introducing an appropriate dopant to achievethe goal of gap state reduction and even elimination.The authors are thankful for the critical comments ofS.-H. Wei and D. S. Wang. X.G.G. is partially supported bythe NSF of China, the national program for the basic re-search and research project of Shanghai. Q.L. acknowledgesthe RGC grant under Project No. CUHK 402105. Z.F.H. ac-knowledges support from Shanghai Postdoctoral ScienceFoundation under Grant No. 05R214106. The computationwas performed at Shanghai Supercomputer Center and Su-percomputer Center of Fudan.1G. D. Wilk, R. M. Wallace, and J. M. Anthony, J. Appl. Phys. 89, 52432001.2M.-H. Cho, H. S. Chang, Y. J. Cho, D. W. Moon, K.-H. Min, R. Sinclair,S. K. Kang, D.-H. Ko, J. H. Lee, J. H. Gu, and N. I. Lee, Appl. Phys. Lett.84, 571 2004.3W. J. Zhu, T. Tamagawa, M. Gibson, T. Furukawa, and T. P. Ma, IEEEElectron Device Lett. 23, 649 2002.4H. Y. Yu, M. F. Li, B. J. Cho, C. C. Yeo, M. S. Joo, D.-L. Kwong, J. S.Pan, C. H. Ang, J. Z. Zheng, and S. Ramanathan, Appl. Phys. Lett. 81,376 2002.5V. V. Afanas’ev, A. Stesmans, and W. Tsai, Appl. Phys. Lett. 82, 2452003.6A. S. Foster, F. Lopez, A. L. Shluger, and R. M. Nieminen, Phys. Rev. B65, 174117 2002.7K. Xiong and J. Robertson, Microelectron. Eng. 80, 408 2005.8H. Takeuchi, D. Ha, and T. King, J. Vac. Sci. Technol. A 22, 1337 2004.9A. Y. Kang, P. M. Lenahan, and J. F. Conley, Appl. Phys. Lett. 83, 34072003.10V. V. Afanas’ev and A. Stesmans, J. Appl. Phys. 95, 2518 2004.11J. Daniels, C. von Festenberg, H. Reather, and K. Zeppenfeld, OpticalConstants of Solids by Electron Spectroscopy, Springer Tracts in ModernPhysics Vol. 54 Springer, New York, 1970, p. 78.12P. Perdew, J. A. Chevary, S. H. Vosko, K. A. Jackson, M. R. Pederson, D.J. Singh, and C. Fiolhais, Phys. Rev. B 46, 6671 1992.13G. Kresse and J. Furthmüler, Comput. Mater. Sci. 6, 16 1996; G. Kresseand J. Furthmüler, Phys. Rev. B 54, 11169 1996.14D. Vanderbilt, Phys. Rev. B 41, 7892 1990.15M. Balog, M. Schieber, M. Michiman, and S. Patai, Thin Solid Films 41,247 1977.16G. D. Wilk and D. A. Muller, Appl. Phys. Lett. 83, 3984 2003.ense or copyright; see http://apl.aip.org/about/rights_and_permissions

Effects of Al addition on the native defects in hafnia

Effects of Al addition on the native defects in hafnia

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