apex-4-055502
TRANSCRIPT
Synthesis of Mica Thin Film by Pulsed Laser Deposition
Yuta Nakasone1, Hirokazu Nakai1, Yumiko Miyake1, Ryosuke Yamauchi1, Nobuo Tsuchimine2,
Susumu Kobayashi2, Yoshifumi Sano3, Nobutaka Takezawa4, Masahiko Mitsuhashi5,
Satoru Kaneko5, Hiroshi Funakubo1, and Mamoru Yoshimoto1;6�
1Depatment of Innovative and Engineered Materials, Tokyo Institute of Technology, Yokohama 226-8503, Japan2TOSHIMA Manufacturing Co., Ltd., Higashimatsuyama, Saitama 355-0036, Japan3YAMAGUCHI MICA Co., Ltd., Toyokawa, Aichi 441-0106, Japan4Tochigi Industrial Technology Center, Utsunomiya 321-3224, Japan5Kanagawa Industrial Technology Center, Ebina, Kanagawa 243-0435, Japan6Patent Attorney, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Received March 8, 2011; accepted April 11, 2011; published online April 28, 2011
Synthesis of crystalline biotite-mica thin films was examined by applying a two-step process: (1) low-temperature growth of the mica film precursor
on an ultrasmooth sapphire (�-Al2O3 single crystal) substrate by pulsed laser deposition using a sintered biotite ceramics target and (2) post-
annealing in vacuum. X-ray diffraction and Raman scattering spectroscopy confirmed that a c-axis-oriented polycrystalline biotite-mica thin film
was obtained by post-annealing the 500 �C-grown film precursor at temperatures above 700 �C in vacuum. An atomic-scale pattern corresponding
to that of the cleaved natural mica surface was observed on the surface of thin film by atomic force spectroscopy.
# 2011 The Japan Society of Applied Physics
In the past few decades, layered compounds such asmetal dichalcogenides,1,2) graphite intercalation com-pounds,3) and high-Tc superconducting cuprates4) have
been extensively investigated, owing to their novel super-conducting and thermoelectric properties, quasi-two-dimensional physics, and material anisotropy.5,6) Amongthe layered minerals, mica is a well-known silicatecompound. Natural or synthetic mica has been usedfor many years in industry as an insulating material, acomposite with glass, a heat-resistant material, a condenser,and among others.
The ideal unit-cell formula of mica minerals is given byI2M4, 5 or 6T8O20(OH)4, where I represents the interlayerelements (e.g., K), M represents the octahedral coordinateelements (e.g., Al, Mg, Fe), and T represents the tetrahedralcoordinate elements (e.g., Si, Al).7) Mica can be easilycleaved in a specific manner because of this distinctivestructure. Typical mica minerals are muscovite [K2Al4-(Al2Si6O20)(OH)4], biotite [K2(Mg,Fe)6(Al2Si6O20)(OH)4],and phlogopite [K2Mg6(Al2Si6O20)(OH)4]. Because naturalbiotite is trioctahedrally coordinated with six Mg2þ and Fe2þ
cations occupying the octahedral sites,7,8) its magnetic andelectrical conduction properties depend strongly on theconcentration of Fe ions.9,10)
For industrial applications, most synthetic mica has beenproduced in bulk or powder form by a melting method.11)
However, there is no reports on fabrication of mica thin filmsvia vapor phase processes. Synthesis of mica thin films isexpected to open new possibilities for electronic devicesbecause of the layer structure and the flat surface withan atomic-scale pattern.12–15) We previously investigatedthe film growth of functional ceramic thin films such asoxides,16–18) borides,19–21) nitrides,22,23) and diamond24) bypulsed laser deposition (PLD). PLD yields high-qualitymulti-component crystalline thin films with compositionscomparable to that of the target.18–26) In this study, we reportthe first successful fabrication of a c-axis oriented biotite-mica thin film by PLD using a sintered target consisting ofnatural biotite powders.
The films were deposited on an ultrasmooth sapphire(�-Al2O3 single crystal) (0001) substrate at 500 �C underan oxygen gas pressure of 1:0� 10�5 Torr by PLD. Thesubstrate had 0.2-nm-high atomic steps and flat terracesand was obtained by annealing commercial substrates at1000 �C in air.25) A focused KrF excimer laser (� ¼ 248 nm,fluence of 3.0 J/cm2, and frequency = 5Hz) was impingedon a biotite target sintered by a hot-press method usingbiotite powders (average particle size: 10 �m in diameter).The excited species in the plasma plume generated duringlaser ablation of the biotite target were examined byin situ optical emission spectroscopy (OES) for determiningthe threshold energy for laser ablation of the biotitetarget. After film growth, the films were subjected topost-annealing at 700–950 �C for 1.5 h in vacuum (�2:0�10�7 Torr).
The composition and crystallinity of the films wereexamined by X-ray photoelectron spectroscopy (XPS), andX-ray diffraction (XRD), respectively. Raman scatteringspectroscopy was also applied for identifying the film’slattice vibration modes. Atomic-scale observation of the filmsurface was conducted in air by atomic force microscopy(AFM).
In the OES measurements, optical emission assigned to allthe elements of biotite components (K, Al, Si, Fe, Mg, O, H)was observed at laser energy densities higher than 1.75J/cm2. At a laser energy density of 3.00 J/cm2, thin filmsof 70 nm thickness were deposited on the ultrasmoothsapphire (0001) substrate at 500 �C. XPS measurements ofthe biotite-mica target indicated a composition ratio ofK : ðMg,FeÞ : ðAl,SiÞ ¼ 1 : 3:18 : 4:34 at.%, while the ratioof the as-deposited thin film was measured from XPS tobe K : ðMg,FeÞ : ðAl,SiÞ ¼ 1 : 3:15 : 6:42 at. %. By compar-ing the compositions of the target and the obtained film,(Al,Si) content was found to be larger than that of the micatarget. This increase of (Al,Si) element for the film mightinduce the defect in the layered structure of the mica. Theobtained thin films grown at 500 �C had no peaks in theXRD pattern, indicating the amorphous state. After theseobservations, the as-deposited film was subjected to post-annealing to obtain solid-state crystallization.�E-mail address: [email protected]
Applied Physics Express 4 (2011) 055502
055502-1 # 2011 The Japan Society of Applied Physics
DOI: 10.1143/APEX.4.055502
Figure 1 shows the XRD patterns of (a) the biotite-micatarget, and (b) the thin film after post-annealing at 950 �Cin vacuum. The inset in Fig. 1(b) shows the result of XRDnarrow scan integrated 40 times. XRD peaks at (001), (005),and (007) as shown in Fig. 1(b) are assigned to biotite-mica.This result indicates that the c-axis oriented biotite thinfilm was crystallized on the sapphire (0001) substrate byannealing at 950 �C in vacuum. Both of XRD peaks at (001)and (005) for the c-axis oriented thin film as shown inFig. 1(b) were found to be shifted slightly to the lower anglethan those of the target in Fig. 1(a). Thus, c-axis length ofthe mica thin film was a little larger than that of the micatarget, probably because of the compositional deviation aswell as the lattice strain for the c-axis oriented thin film.Further experiments are necessary to elucidate the detailedmechanism of c-axis elongation of the thin film. In contrast,a crystalline biotite thin film could not be obtained bypost-annealing in air. Vacuum annealing is thought toenhance the solid-state crystallization of the biotite thin filmprecursor.26,27) After annealing at 700–900 �C, the crystal-lized biotite thin films showed only the biotite XRD peak at(005), suggesting poor crystallinity.
Figure 2 shows the Raman scattering spectra of thebiotite thin film annealed at 950 �C in vacuum. The reportedRaman spectra of mica minerals at 3800–3000, 1150–800,800–600 cm�1, and less than 600 cm�1 can be useful foridentifying the present film.28,29) The Raman peaks at 3800–3000 cm�1 in Fig. 2 are related to the stretching mode ofOH, those at 1150–800 cm�1 are assigned to the stretchingmode of the Si–Onb bond (Onb = non-bridging oxygen) inthe SiO4 tetrahedrals, those at 800–600 cm�1 are assignedto the vibration mode of Si–Ob–Si bonds (Ob = bridgingoxygen) connecting the SiO4 tetrahedral, and those below600 cm�1 are assigned to translational motions of cations inthe octahedral sites.
Figure 3 shows (a) an AFM surface image (1� 1 �m2)and (b) a close-up image (8� 8 nm2) of the biotite thin film
after annealing at 950 �C in vacuum. AFM measurementswere conducted in the constant deflection mode. In Fig. 3(b),the data were filtered to remove high-frequency noise. The
Fig. 1. XRD patterns of (a) the biotite-mica target, and (b) the biotite thin
film deposited on the ultrasmooth sapphire substrate and post-annealed at
950 �C for 1.5 h in vacuum.
Fig. 2. Raman spectra of the biotite thin film deposited on the ultrasmooth
sapphire substrate and post-annealed at 950 �C for 1.5 h in vacuum.
(a)
(b)
Fig. 3. (a) AFM surface image (1� 1 �m2) and (b) AFM surface image
(8� 8 nm2) of the biotite thin film deposited on the ultrasmooth sapphire
substrate and post-annealed at 950 �C for 1.5 h in vacuum.
Y. Nakasone et al.Appl. Phys. Express 4 (2011) 055502
055502-2 # 2011 The Japan Society of Applied Physics
surface morphology of the prepared thin film exhibited aperiodic atomic-scale pattern, as shown in Fig. 3(b). Aspreviously reported,30) an array of hexagonal rings of oxygensurrounding a vacant hexagonal hole can be seen in thesmall-area AFM image of the cleaved basal plane ofmuscovite-mica. The reported center-to-center distancebetween the hexagonal holes was 0.51 nm (�0:02 nm).31)
The periodic distance of the hole array in Fig. 3(b) isestimated to be about 0.5 nm. Thus, the atomic-scale patternof the film surface shown in Fig. 3(b) corresponds well tothat of the cleaved surface of the mica mineral.30,31)
In summary, we fabricated c-axis-oriented polycrystallinebiotite thin films by a two-step process: (1) low-temperaturegrowth of the mica film precursor on an ultrasmoothsapphire substrate by PLD using a sintered biotite ceramicstarget and (2) post-annealing in vacuum. Observations of theXRD pattern, Raman scattering spectra, and AFM surfaceimage confirmed that the thin films were biotite. Theprepared films had an atomic-scale AFM pattern correspond-ing to that of the mica cleavage plane.
Acknowledgment This study was supported in part by the Ministry of
Education, Culture, Sports, Science and Technology of Japan, and the New
Energy and Industrial Technology Development Organization of Japan.
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