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Thin Solid Films 515 (2

ZnO nanostructured film deposition using the separated pulsed laserdeposition (SPLD) assisted by electric and magnetic drift motion

Kenji Ebihara ⁎, Sang-Moo Park, Koji Fujii, Tomoaki Ikegami

Department of Electrical and Computer Engineering, Kumamoto University, JapanGraduate School of Science and Technology, Kumamoto University, Japan

Available online 16 January 2007

Abstract

We have developed the separated pulsed laser deposition (SPLD) technique to prepare high quality ZnO based films exhibiting uniform anddroplet-free properties. This SPLD consists of an ablation chamber and a deposition chamber which can be independently evacuated underdifferent ambient gases.

The gas species and the pressures in both chambers can be arbitrarily chosen for the specific deposition such as nanostructured films andnanoparticles. The ablation chamber is a stainless steel globe and the deposition chamber is a quartz tube connected to a metallic conic wall withan orifice. We used a KrF excimer laser with λ=248 nm and 25 ns pulse duration. The different gas conditions in two chambers allow us to realizeoptimal control of the plasma plume, the gas phase reaction and the film growth by applying the bias voltage between the conic wall and thesubstrate under the magnetic field. We can expect that at appropriate pressures the electric and magnetic field motion (E×B azimuthal driftvelocity) gives significant influences on film growth.

We have deposited ZnO thin films at various pressures of ablation chamber (Pab) and deposition chamber (Pd). The deposition conditions usedhere were laser fluence of 3 J/cm2, laser shot number of 30,000, Pab of 0.67–2.67 Pa (O2 or Ar), Pd of 0.399–2.67 Pa (O2), and substratetemperature of 400 °C. Particle-free and uniform ZnO films were obtained at Pab of 0.67 Pa (Ar) and Pd of 1.33 Pa (O2). The ZnO film showedhigh preferential orientation of (002) plane, optical band gap of 2.7 eV, grain size of 42 nm and surface roughness of 1.2 nm.© 2006 Elsevier B.V. All rights reserved.

Keywords: Laser ablation; Zinc oxide; Plasma processing and deposition; Nanostructures

1. Introduction

Nanostructured thin films and nanoparticles have attractedmuch attention due to a wide variety of potential applications inelectronic, optical and biomedical fields. Many processingtechniques have been proposed to synthesize nanostructuredmaterials. The pulsed laser deposition (PLD) technique is one ofthe successful tools to create nano-size particles and nanocompo-site thin films. Conventional pulsed laser deposition has been usedto deposit high quality thin films including high Tc super-conducting materials, ferroelectric materials, photonic materialsand organic materials [1]. Although the PLD has been a usefultechnique having many advantages such as stoichiometry,lowering processing temperature and high deposition rate inarbitrary atmosphere, the inherent problems such as the

⁎ Corresponding author.E-mail address: ebihara@cs.kumamoto-u.ac.jp (K. Ebihara).

0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2006.11.168

deposition of droplets, limited deposition area and high cost ofthe system have to be solved for the practical processing system.

Fig. 1. Separated pulsed laser deposition.

Fig. 2. Drift motion of the electric and magnetic fields in the SPLD.

Fig. 4. Optical properties of ZnO films (averaged transmittance for eachsubstrate).

6369K. Ebihara et al. / Thin Solid Films 515 (2007) 6368–6370

We have developed the separated pulsed laser deposition(SPLD) for the preparation of droplet-free and uniform films [2–4]. The SPLD consists of an ablation chamber and a depositionchamber which are independently evacuated at different ambientgasses. The electric field and the magnetic field are applied to thesubstrate in the deposition chamber to control the depositioncondition. This SPLD technique was studied to create ZnOnanostructured materials which are very attractive materials foroptical applications such as ultraviolet laser, blue light-emittingdiodes, and transparent conductive electrodes.

2. Experimental

Fig. 1 shows the geometry of the SPLD system [2,3]. We canindependently control the ambient pressures of the ablationchamber (Pab) and the deposition chamber (Pd). The ablationchamber is a stainless steel globe with 300 mm diameter. Thedeposition chamber is made of a quartz tube of 100 mmdiameter and a metallic conic wall with variable orificediameters was installed on the end of the tube. For the ablationprocess we used a KrF excimer laser with λ=248 nm and 25 ns

Fig. 3. Positions for AFM measurement (•) on the quartz substrates placed onthe holder (400 °C).

pulse duration. Different gas pressures in two chambers havingthe differential evacuation system allow us to provide optimalcontrol of the plasma plume, the gas phase reaction and the filmgrowth by applying the bias voltage (Vb) between the conicwall and the substrate under the magnetic field. The depositionconditions were laser fluence of 3 J/cm2, laser shot number of30,000, Pab of 0.67–2.67 Pa (O2 or Ar), Pd of 0.399–2.67 Pa(O2), and substrate temperature of 400 °C. Fig. 2 is theconfiguration of the components of the SPLD. The magneticinduction (B) is generated by the permanent magnet (Al–Ni–Co:Out. dia=74.5 mm, Inner dia=30 mm, Max. flux=0.1 T) inthe deposition chamber. The positive electric field (E) betweenthe substrate and the conical capsule was directed toward theleft-hand side. Since the radial component of the magneticinduction (Br) crosses vertically with E, the drift motion of ionsand electrons given by

Vd ¼ E � B=B2 m=sð Þ ð1Þ

provides the velocity Vd=E /Br [5].This azimuthal averaged motion causes the plasma to flow

by sweeping over the deposited film. We studied the dynamicsof the plasma plumes of this SPLD assisted by the applied

Fig. 5. XRD patterns of ZnO films when applied bias voltage was (a) AC 500 Vand (b) DC 500 V at a maximum magnetic flux of 0.1 T.

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electric and magnetic fields. The surface morphology of thedeposited films was observed by using atomic force microscopy(AFM) (Seiko:SPI3800N).

3. Results and discussion

In order to evaluate the operation characteristics of thisSPLD system, we deposited the ZnO films by changing thedeposition conditions such as ambient gas pressures in thechambers, electric field (DC, AC) and DC magnetic field.

The AFM images were taken at various locations (r=2 mm,12 mm) on the substrates (a, b, c, d) placed as shown in Fig. 3. Itis shown that near the magnetic axis (r=2 mm) the RMS and thegrain size are in the range of 1.2 nm–3.4 nm and 39.9 nm–45 nm, respectively. At the outside edge of the substrate(r=12 mm) the RMS of 3.0 nm–7.1 nm and the grain size of43.0 nm–72.4 nm were obtained. These results clearly showedthat uniform ZnO films with small grain size can be depositednear the central axis of the magnetic field (a center of the plasmaplume). The total magnetic flux density was about 44 mTon thecenter axis (r∼0) and 20 mT at outside of the substrate edge(r=14 mm). The film quality such as uniformity and crystalstructure were deteriorated apart from the axis of the magneticfield where the E×B drift velocity becomes smaller owing toincreasing the radical component (Br) of the magnetic flux. Themirror type configuration of the magnetic field can enhance theneutral radical deposition on the center axis and most of thecharged particles (ions and electrons) were forced to be movedoff the magnetic field axis.

Fig. 4 shows the typical transmittance averaged over the ZnOfilm deposited on quartz substrate (d). The Tauc plots of foursamples show that the optical band gap is in the range 2.53–2.72 eV. When we applied only the electric field of DC 500 Vwithout magnetic field, higher optical band gap of 3.1 eV wasobtained at the same pressure conditions (Pab=0.67 Pa Ar,Pd=1.33 Pa O2). In our previous work using the conventionalPLD, we also deposited ZnO films indicating optical band gap

of 3.17 eV [4]. Although the existence of the magnetic field caninduce the azimuthal drift motion of the charged particles, theiraxial driving velocities toward the substrate due to the ablation(∼10 mm/μs) are reduced to decrease the deposited energy onthe film for migration and crystal growth. Fig. 5 shows the XRDpatterns of the ZnO films deposited when DC voltage of 500 Vand AC (60 Hz) 500 V were applied under magnetic field of0.1 T. This result shows that the DC bias voltage is effective todeposit the films with high preferential orientation of (002)plane. The alternative bias voltage provided non-crystal(amorphous) structure.

4. Conclusions

The separated pulsed laser deposition (SPLD) was developedto prepare the high quality ZnO films. This SPLD processinghas a characteristic of uniform and droplet-free film depositionwhich is required for composite multilayer device fabrication.The electric and magnetic fields applied to the SPLD showedsignificant effect on the film growth. The SPLD is a promisingtechnique for the preparation of nanostructured films andnanoparticles.

References

[1] D.B. Chrisey, G.K. Hubler, Pulsed Laser Deposition of Thin Films, JohnWiley and Sons, Inc., NY, 1994.

[2] M. Alonso, K. Ebihara, K. Fujii, Y. Narita, S.M. Park, K. Tokumasu, T.Ikegami, Proceedings of The 17th International Symposium on PlasmaChemistry, Canada, Toronto, August 7–12, 2005, p. 543.

[3] K. Ebihara, S.M. Park, K. Fujii, T. Ikegami, Diamond Relat. Mater. 15(2006) 989.

[4] S.M. Park, T. Ikegami, K. Ebihara, Jpn. J. Appl. Phys. 44 (2005) 8027.[5] J.R. Roth, Industrial Plasma Engineering (Vol. 2): Application of

Nonthermal Plasma Processing, Institute of Physics Publishing, Bristol,UK, 2001.