Optical Properties HfO2

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  • Materials Chemistry and Physics 126 (2011) 515523

    Contents lists available at ScienceDirect

    Materials Chemistry and Physics

    journa l homepage: www.e lsev ier .com/ lo

    Inuen rtie

    M.F. Al-K eer,Physics Depart

    a r t i c l

    Article history:Received 13 AReceived in reAccepted 10 Ja


    Keywords:Hafnium oxidee-Beam evaporationHydrogen annealingStructural propertiesChemical propertiesOptical propertiesElectrical prop

    eposiion, Xtry, aal, an. Annremee banreecf ann

    1. Introdu

    Hafniumexcellent csesses a higthat is transpectral rative indexThus, HfO2interferenceelectronic aconstant, ality with silcandidate fooxide-semiapplicationsensors [8]

    Thermalvoids in betion of the

    CorresponE-mail add

    0254-0584/$ doi:10.1016/j.erties


    oxide (HfO2) is a material that is characterized byhemical, thermal, and mechanical stability, and pos-hmelting point [1]. It is awide band gap semiconductorsparent from the deep ultraviolet to the mid infrarednges [2]. Moreover, it has a relatively high refrac-and shows the highest laser damage threshold [3].has been used extensively in optical coatings, such aslters [4] and anti-reection coatings [5]. For micro-

    pplications, HfO2 is characterized by a high dielectricong with good thermodynamic and mechanical stabil-icon [6]. Therefore, HfO2 has emerged as the leadingr the replacement of silicon oxide as the gate in metal-

    conductor eld effect transistors (MOSFETs) [7]. Others have been reported for hafnium oxide, including gasand protective coatings [4].ly evaporated lms consist of cylindrical columns withtween [9]. Such a microstructure leads to deteriora-properties of the lms, such as optical homogeneity,

    ding author. Tel.: +966 3 860 3747; fax: +966 3 860 2293.ress: [email protected] (M.F. Al-Kuhaili).

    environmental stability and adhesion [9]. Solutions to this prob-lem include using deposition methods with high particle energy,bombardment with energetic ions during deposition, and sub-strate heating [911]. Another solution is post-deposition thermalannealing,which leads to densication of the lms, and thus higherpacking density [911]. Another advantage of thermal annealing isthe crystallization of the lms, and consequently a reduction in thedensity of structural defects.

    Annealing in hydrogen has been widely used in the microelec-tronics industry to remove native oxides and passivate danglingbonds [12]. Recently, numerous advantages of annealing in hydro-gen have emerged. For example, it has been established thatannealing zinc oxide thin lms in hydrogen led to a reduction ofthe resistivity of the lms by several orders of magnitude with-out reduction in transmittance [13].Moreover, hydrogen annealingwas suggested as a practicalmethod for controlledn-typedoping ofzinc oxide [14]. Annealing silicon in hydrogenwas used to fabricatemicrodisks with quality factors in excess of 3105 [12]. Annealingof wide band gap semiconductors, such as gallium nitride and sili-con carbide, in hydrogen led to signicant improvement in surfaceand interface quality [15]. Finally, hydrogen annealing was used toremove oxides from elemental nanowires [16].

    Previous studies have mainly focused on annealing HfO2 thinlms in nitrogen [6,1727] or oxygen [24,2830]. Several studies

    see front matter 2011 Elsevier B.V. All rights reserved.matchemphys.2011.01.036ce of hydrogen annealing on the prope

    uhaili , S.M.A. Durrani, I.A. Bakhtiari, M.A. Dastagment, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

    e i n f o

    ugust 2010vised form 18 October 2010nuary 2011

    a b s t r a c t

    Thin lms of hafnium oxide were dannealed in hydrogen. X-ray diffractphotoluminescence, spectrophotometigate the structural, chemical, opticamorphous and nearly stoichiometricchiometry. Photoluminescencemeasuthe lmswere transparentwith awidover, the lms were suitable as anti-was signicantly reduced as a result ocate /matchemphys

    s of hafnium oxide thin lms

    M.B. Mekki

    ted by electron beam evaporation, and were subsequently-ray photoelectron spectroscopy, atomic force microscopy,

    nd currentvoltage measurements were performed to inves-d electrical properties of the lms. As-deposited lms wereealing led to crystallization of the lms, and reduction of stoi-nts revealed thepresenceofoxygen-relateddefects.Optically,d gap, and thiswas not affected by hydrogen annealing.More-tion coatings on silicon. The electrical resistivity of the lmsealing.

    2011 Elsevier B.V. All rights reserved.

  • 516 M.F. Al-Kuhaili et al. / Materials Chemistry and Physics 126 (2011) 515523

    were reported on the annealing of ultrathin (thickness 1700 C) [20]. Under some special conditions, such aseposition, polymorphs can exist at much lower tem-20]. This was veried in several studies [6,29,32]. Theand existence of polymorphs depend on the substrateand temperature, deposition technique, and anneal-

    here and temperature. Moreover, it was shown thatiticalminimum thickness for the onset of crystallization

    ows XRD patterns of hafniumoxide thin lms depositednd annealed at different temperatures in hydrogen. Then of the as-deposited lms showed a large and broadwith no diffraction peaks. Such a pattern is character-rphous lms. At an annealing temperature of 400 C,

    ad an XRD pattern with small peaks superimposed on

  • M.F. Al-Kuhaili et al. / Materials Chemistry and Physics 126 (2011) 515523 517

    hin l

    that the lmshows thelms.

    Fig. 2 shooxide thinshows a unthe annealewith the comean-squacolumns (Hroughness,There are tFirst, the ias revealedthe lms belms becamroughness,The reducthad a dom800 C, theannealed possibly du(Table 1).



    he coitiouing tthic


    Table 2Summary of th

    Ta (C)


    a 4f is the eb 4d is the ec is the end r is the ratFig. 2. Two- and three-dimensional AFM images of as-deposited hafnium oxide t

    s annealed at 400 C were still amorphous. This clearlyinuence of the substrate on the crystallinity of the

    ws AFM images of as-deposited and annealed hafniumlms. The morphology of the as-deposited lmsiform columnar microstructure. The morphology ofd lms shows a distorted columnar microstructure,lumn consisting of spherical grains. The average root-re roughness (Rrms) and the maximum height of the) are given in Table 1. The as-deposited lms had low

    3.2. Ch

    Thesurveywere tadventindicatativelyhigh-r1s corewhich decreased as the lms were annealed at 400 C.hree effects taking place upon annealing at 400 C.ntensity of columnar microstructure was decreased,by the reduction of the maximum height. Second,came more compact, with lower thickness. Third, thee polycrystalline. The rst two effects tend to reducewhereas the third effect tends to increase roughness.ion of roughness suggests that the rst two effectsinant role. When the lms were annealed at 600 orroughness increased. Since the thicknesses of the

    lms were comparable, the increase in roughness wase the increase in crystallite size, as revealed by XRD

    results areThe Hf 4

    are due to sHf 4f spectGaussian/Locorrection,4f7/2 peak wthe value othe spin-oragreementSeveral XPSthe Hf 4f7/2are two rea

    e chemical properties of the lms.

    Binding energies (eV)

    Hf 4f7/2 Hf 4f5/2 Hf 4d5/2 Hf 4f3/2 O 1s (A) O 1s (B

    16.6 18.1 212.9 223.5 530.2 531.816.4 18.0 213.0 223.7 530.0 532.116.4 18.0 212.9 223.6 530.1 531.516.2 17.8 212.3 223.7 530.2 532.0

    nergy separation between the Hf 4f7/2 and 4f5/2 levels.nergy separation between the Hf 4d5/2 and Hf 4d3/2 levels.ergy separation between the Hf 4f7/2 and O 1s levels.ion of the B component to the total O 1s peak.ms (a), and lms annealed at 800 C in hydrogen (b).

    al properties

    ical state of the lmswas investigated usingXPS.Wides of the lms revealed that the only elements presentnstituent elements (hafniumand oxygen) in addition tos carbon. No silicon (from the substrates) was detected,hat the substrateswere completely obscured by the rel-k lms. In addition to the wide survey scans, detailedtion spectra were obtained in the Hf 4f, Hf 4d, and Ol regions. The spectra are shown in Fig. 3, and the XPS

    summarized in Table 2.f spectrum consists of two peaks (4f7/2 and 4f5/2) thatpin-orbit splitting. In order to resolve these peaks, therum was de-convoluted into two components using arenzianmixed function employing Shirley backgroundas shown in Fig. 3. The binding energy (BE) of the Hfas 16.40.2 eV. This value is in close agreement withf 16.20.4 eV, reported for bulk HfO2 [1]. Moreover,bit splitting (4f) was 1.51.6 eV, and thus is in closewith the reported values of 1.4 eV [5] and 1.7 eV [33].studies on ultrathin HfO2 lms reported that the BE ofpeak was almost 1.0 eV higher than our values. Theresons for this difference. The rst reason is the value of

    Energy separations (eV) Ratios

    ) 4fa 4db c rd [O/Hf]

    1.5 10.6 513.6 0.236 2.061.6 10.6 513.6 0.234 1.941.6 10.6 513.7 0.221 1.911.6 10.5 514. 0 0.140 1.91

  • 518 M.F. Al-Kuhaili et al. / Materials Chemistry and Physics 126 (2011) 515523

    Fig. 3. XPS sp(b), and O 1s r

    the BE of thSecond, thethe formati

    The oxyponents: aa high-BE ccomponentues are 529separation4f7/2 peak iis 513.70physi-sorbebe related tnent to thepeaks, is givprogressive

    The atomthe normalaccount the4d5/2 peak wthe Hf 4d3/the [O/Hf] rratio, as depresence ofthe high-BEthat probes[O/Hf] ratio

    results suggest the existence of oxygen vacancies. Nevertheless, the[O/Hf] ratio (Table 2) showed the expected trend, i.e. it decreasedupon annealing in hydrogen. This behavior is consistent with thevariation in the values of the separation between theHf 4f7/2 and