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Applied Surface Science 257 (2011) 6418–6423

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

ffect of heat treatment on the properties of dc magnetron sputteredaB6/ITO films

an Wang, Lin Zhang, Guanghui Min ∗, Huashun Yu, Yifei Yuaney Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, China

r t i c l e i n f o

rticle history:eceived 31 October 2010eceived in revised form 31 January 2011

a b s t r a c t

Two types of LaB6/ITO films with different deposition times were deposited by direct current mag-netron sputtering. After the films were fabricated, AFM, XRD, FESEM, Step-Height Profiler, UV–VisSpectrophotometer and Hall Measurement Instrument were used to study their performances. After

ccepted 2 February 2011vailable online 24 February 2011

eywords:aB6

TO

400 ◦C’s annealing, morphology of fracture cross-sections of the films exhibit evolutions from stronglycolumnar to the equiaxial (30 min), the films prepared with short time transformed from amorphousto polycrystalline, and Ar pressure generated little effect on the film’s transmissivity. The resistivitydeclined over one order of magnitude, and the films with higher surface roughness value were moreresistant, while that for films unheatreated was just the opposite.

© 2011 Elsevier B.V. All rights reserved.

eat treatmentirect current magnetron sputtering

. Introduction

Lanthanum hexaboride (LaB6) was an excellent thermionic elec-ron emitter material characterized by excellent thermal stability,igh emission density, creep resistance and low work function, and

t had the highest electronic emissivity of the known materials,hich was usually prepared by high-temperature reaction proce-ure. All these brilliant properties made LaB6 resulted in a wideange of applications, such as high-energy optical systems, electronithography, and coatings for resistors [1–4].

Due to their high electrical conductivity and transparency inhe visible region, transparent conductive oxides had been inten-ively studied for their applications in a variety of opto-electronicrofessions. Among these, ZnO and ITO (indium tin oxide) wereost frequently used as the transparent electrodes in various pho-

oelectric devices, for instance organic light-emitting diode (OLED),at panel displays, and liquid crystal displays (LCD), besides own-

ng a wide band gap between 3.5 and 4.3 eV, ITO exhibited the gasensing property of CO and ethanol [5–7]. Magnetron sputteringas one of the most popular techniques for obtaining good quality

lms, it possessed numerous advantages like high sputtering rates,

ow substrate temperature, easier operation of the facility and waseing regularly used for preparation of the alloy, metallic film andultilayer. In this work, firstly we prepared one LaB6 layer, then on

∗ Corresponding author at: School of Materials Science and Engineering, Shan-ong University, Jinan 250061, China. Tel.: +86 13853102305;ax: +86 531 88395639.

E-mail address: ghmin@sdu.edu.cn (G. Min).

169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2011.02.010

Fig. 1. XRD pattern of the LaB6 powder being synthesized in our laboratory.

top of this ITO was deposited, by changing the sputtering parameterAr pressure, before and after annealing LaB6/ITO films’ structural,optical and electrical properties were studied.

2. Experimental

LaB6/ITO films were deposited in a direct current magnetronsputtering system (FJL560C3, Shenyang Scientific InstrumentsResearch Institute, China) on the glasses substrates without inten-

D. Wang et al. / Applied Surface Science 257 (2011) 6418–6423 6419

Fig. 2. AFM images of of the LaB6/ITO films before (a–d) and after annealing (a′–d′) at different Ar pressures: 1.5 Pa, 2.0 Pa, 2.5 Pa, and 3.5 Pa.

6420 D. Wang et al. / Applied Surface Science 257 (2011) 6418–6423

F ter th(

tdwmplIcs

ig. 3. XRD patterns of the LaB6/ITO films at various Ar pressures before (a) and afd) of LaB6/ITO films.

ional heating, the sputter ITO target was sintered an ceramicisc (Ø60 mm, In2O3/SnO2 = 9:1, Weihai Blue Fox Co., Ltd, China)ith purity of 99.99%, and LaB6 target was obtained using self-ade powders in our laboratory, the substrates were placed

aralleling to the targets with 69 mm apart. After the LaB6ayer was completed, the shutter would turn to confrontingTO target, and the whole deposition process was controlled byomputer. The chamber was pumped down to an initial pres-ure of 6 × 10−4 Pa, and the substrates were being ultrasonically

e heat treatment (b). AFM scans of the typical amorphous (c) and crystal structure

cleaned in acetone and absolute EtOH alternately, both the tar-gets were presputterted about 10 min to remove the surface oxides[5,6]. Argon was introduced into the chamber as the workinggas and the flow rates were controlled by the mass flow con-

trollers, the samples were annealed at a thermal annealing furnace(400 ◦C) for 1 h in the air atmosphere, Table 1 depicted the mainexperimental parameters during the deposition, and the crys-talline structure of adopted LaB6 powders could be seen fromFig. 1.

D. Wang et al. / Applied Surface Science 257 (2011) 6418–6423 6421

F (a–d)

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ig. 4. The fracture cross-sections of LaB6/ITO films at different Ar pressures before

Crystallinity of the films was examined by the X-ray diffrac-ion technique (Rigaku, Japan) under beam-accelerating conditionsf 40 kV/200 mA, the selected 2� angle was from 20◦ to 70◦, andhe Ra (surface average roughness) detection was performed withn atomic force microscopy in contact mode under ambient con-

itions (AFM-IIa, Opto-electronics Research Institute of Zhejiangniversity), the optical and electrical properties were obtained by aall Effect Measurement System (HMS-3000, Ecopia) and a UV–Vispectrophotometer (U-4100, Hitachi, Japan) with the wavelengthimit of 200–3300 nm. The applied dc magnetic field was 0.5 T, in

and after (a′–d′) annealing (30 min for each layer): 1.5 Pa, 2.0 Pa, 2.5 Pa, and 3.0 Pa.

order to abandon the influences of substrates to the transmittance,the optical measurement was surveyed with reference to glass,and the fracture surfaces’ morphology was investigated by a highresolution FESEM (3 kV/23 uA, Su-70 Hitachi, Japan) [5–8].

3. Results and discussion

Fig. 2 showed the surface morphology of LaB6/ITO layersdeposited on glass substrates at different Ar pressures, the scannedarea was 3.2 �m × 3.2 �m, and Ra value was presented. Attributed

6422 D. Wang et al. / Applied Surface Science 257 (2011) 6418–6423

Table 1Deposition parameters of the LaB6/ITO films.

Parameters LaB6 ITO

Base pressure 6 × 10−4 Pa 6 × 10−4 PaPower 40 W 48 WAr pressures 1.5, 2.0, 2.5, 3.0, 3.5 Pa 1.5 PaAr flow rate 40 sccm 40 sccmDeposition time 3/3 min (30 min film

was marked

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grain size with more defects leaded to the grain boundary scattering

separately)Distance between target

and substrates69 mm 69 mm

o low substrate temperature, before the heat treatment, all filmsxhibited amorphous structure, and no evident defects such asrinkles and cracks were discovered, the surfaces were smooth

nd uniform. From pictures Fig. 2(a) and (c), amount of submicron-ized rounded grains could be seen, and some researchers indicatedhat small crystallites might nucleate directly on the substrates pre-iously [9]. After the heat processing, all surfaces typically showedhe granular structure, where the agglomerated grains were visi-le, and the Ra value increased from 6 to 16 nm, after the thermalnergy was applied, modifications in the LaB6 layer leaded to thearger grain structure in ITO. The film’s roughness was closely asso-iated with the carrier concentration and surface mobility, betterrystallinity and fewer bonded boundaries could bring in loweresistivity connecting with the results in Fig. 6, on the other hand,hicknesses confirmed by the profiler were 131, 126, 129, 121 nmeparately.

The XRD patterns of LaB6/ITO films at different Ar pressuresefore and after heat treatment were shown in Fig. 3. All thelms revealed weak and wide peaks regardless of the Ar pressuresefore annealing, post-annealing enabled the atoms get rearrangednd form stable polycrystalline structure, which matched with theominating diffraction planes of In2O3 with (1 1 1) and (1 0 0) ashe preferential orientations, and none LaB6 peaks were found inhe plot, basing on our previous work, the heat-treated LaB6 sin-le layer was still amorphous under the same deposition condition10], the (2 1 1) (4 1 1) peaks were nearly invisible. With the increasef Ar pressure, the (4 0 0) and (4 4 0) peak intensities declined, andhe least strength of (2 2 2) peak at 3.5 Pa indicated the bombard-

ent effect of particles tended the films to form disorder structure,nd fine amorphous and crystalline graphs were unfolded as well,he amorphous films were composed of dense network formationncluding some tiny protrusions with no distinct grain boundaries,nd usually their composition was nonstoichiometry, besides theobility of deposited atoms and molecules were low, we could see

he rectangle grains and aggregations after annealing treatment.When the deposition time was extended to 30 min, fractured

urfaces of the films as deposited revealed the compact columnaricrostructure normal to the substrates as shown in Fig. 4, after the

ilayers were heatreated, arrays of the parallel rod-shaped columnsurned into the equiaxial grains. Interrelation between the depo-ition conditions and film growth had been described in diversetructure-zone diagrams (SZDs), and at different fabricating meth-ds of thin films, four basic movement processes for atoms werencluded: shadowing, surface diffusion, bulk diffusion, and desorp-ion, by the diffusive motion the sputtering atoms reached theirquilibrium positions in the lattice [11]. Knuyt et al. [12] proposedhe driving physical process of the texture evolution was intendingo lower surface energy, a column would grow if the surface energyf grain I was smaller than the mean value of that of the surround-

ng grains, and when the Ar pressure was at 2.5 Pa, the structure

as the mostly dense and homogenous.Transmittance spectrum of LaB6/ITO films was closely related

ith the structure and surface morphology, Fig. 5 revealed that

Fig. 5. Transmission spectra of the sputtered LaB6/ITO films for different Ar pres-sures before (a) and after annealing (b).

after annealing the region with higher optical transmittance of thefilms shifted from near infrared to the visible area, and the maxi-mum transmissivity improved from 87.7% at 3.5 Pa to 97% at 2.5 Pa.At high pressures, the working gas’s density increased, the meanfree path reduced causing scattering of the incidenting species, andless particles would arrive at the substrate, and energy of the par-ticles also decreased substantially due to the thermalization. Someresearchers studied that during the deposition process of TiO2 films,if the reactive oxygen concentration was low, the refractive indexand resistivity of the films were discriminating, and in the case ofa stoichiometric oxygen composition, the different films would geta more similar refractive index and density [13], it could be seenafter annealing, all the detective curves displayed a very similarvariation shape, and the Ar pressure generated little effect on theLaB6/ITO films’ transmissivity.

Resistivities and Hall mobilities of the films before and afterannealing relating to Fig. 6 were obtained at room temperature,an order of magnitude difference between the two was found out,and the crystalline film’s resistivity was conspicuously reduced. Asis know to all, the resistivity was closely related with the carrierconcentration and its mobility, and the poor crystallization, smaller

which reduced the carrier mobility. For the amorphous films, therewere only short-range orders of constitutional atoms and ions, andsome electrons might being bounded in the nonuniform networkstructure, and these electrons might get released from the bound

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ig. 6. Hall mobilities (a) and resistivities (b) of the grown LaB6/ITO films at variousr pressures before and after the heat treatment.

ontributing to the fine recombination of atoms after annealing14]. Heat treatment was the most widely used procedure for elim-nating the residual stress, increasing the deposited hardness ordhesion, and reducing the tendency of deformation and crack, inhis work the annealed films with higher surface roughness had areater resistivity, while the films untreated had the best electricalroperty at the pressure of 3.5 Pa.

. Conclusions

LaB6/ITO films were deposited by direct current magnetronputtering method under different Ar pressures, and their optical,

[

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ence 257 (2011) 6418–6423 6423

structural, electrical performances were being studied. After theheatreatment, the amorphous films transformed to the polycrys-talline state, the films’ transmittance had a large improvement,and the resistivity declined over one order of magnitude, thefilms deposited with 30 min showed the fractures’ morphologicevolutions from strongly columnar to the equiaxial, consideringthe photoelectric characteristics of the films, the best capa-bility and uniform structure was obtained at Ar pressure of2.5 Pa.

Acknowledgements

This research was supported by China Postdoctoral ScienceFoundation Funded project (no. 20090461202) and Key Project ofthe Natural Science Foundation of Shandong Province, China (no.Z2007F09).

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