ARTICLE IN PRESS

Journal of Crystal Growth 312 (2010) 483–486

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jcrysgro

Fabrication of high-electron-mobility ZnO epilayers by chemical vapordeposition using catalytically produced excited water

Hiroshi Nishiyama a, Hitoshi Miura b, Kanji Yasui b, Yasunobu Inoue a,n

a Department of Materials Science and Technology, Nagaoka University of Technology, Nagaoka 940-2137, Japanb Department of Electrical Engineering, Nagaoka University of Technology, Nagaoka 940-2137, Japan

a r t i c l e i n f o

Article history:

Received 27 August 2009

Received in revised form

22 October 2009

Accepted 20 November 2009

Communicated by M. TischlerZnO epilayers exhibited average transparency higher than 90% in the visible and infrared regions

2 �1 �1

Available online 3 December 2009

Keywords:

A3. Chemical vapor deposition processes

A3. Catalytic processes

A3. Hot water

A3. Thin film

B1. Oxides

B2. Semiconducting materials

48/$ - see front matter & 2009 Elsevier B.V. A

016/j.jcrysgro.2009.11.041

esponding author. Tel.: +81 258 47 9832; fax

ail address: inoue@analysis.nagaokaut.ac.jp (Y

a b s t r a c t

Hot H2O jet evolved by the exothermic reaction of H2 with O2 on a Pt-dispersed ZrO2 catalyst was

employed for gas-phase hydrolysis of dimethyl zinc to fabricate thin ZnO films. The X-ray diffraction

pattern and photoluminescence spectra showed that the ZnO epilayers directly grown on a-sapphire

substrate at 873 K had a defect free crystal structure close to a single crystal. The as-grown

(400–2000 nm). The epilayers had high electron mobilities of 140–170 cm V s and low residual

electron concentrations of 1.7–6�1017 cm�3 that are significantly better than those for ZnO films so far

reported by conventional chemical vapor deposition and any other energy-consuming physical method

such as pulse laser deposition and molecular beam epitaxy. The present method uses the chemical

energy from only H2 and O2, which is energy-saving and ecologically friendly, while it is superior in

high-quality ZnO fabrication. It is also pointed out that the method raises unlimited possibilities for a

wide range of the fabrication of high-quality metal oxide epilayers, because of the availability for

various volatile alkyl metals.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Zinc oxide is a highly useful material for producing thin filmsfor optoelectronic applications, such as acoustic and acousto-opticdevices [1], light waveguides [2], photoconductive devices [3],and transparent electrodes [4]. In addition, ZnO is composed ofsafe and abundant elements, making low-cost applications ofZnO possible. As an example of its multifunctional properties,applications in novel devices that exploit excitonic effects haverecently received considerable attention. The exciton bindingenergy of ZnO is 60 meV, which is significantly greater than thosefor GaN (25 meV) and ZnSe (22 meV), while its band gap in directexcitation is 3.37 eV at room temperature [5]. Thus, ZnO is a verypromising material for novel exciton devices operating at roomtemperature and for short-wavelength optoelectronics in thevisible and ultraviolet regions. For realizing ZnO light-emittingdiodes and laser diodes operating in the visible and ultravioletregions, fabrication of p-type ZnO is critical [6]. However, thin filmZnO tends to exhibit intrinsic n-type conductivity, having lowelectron mobility and high residual electron density, because ofdeviations from stoichiometry due to intrinsic defects, such as O

ll rights reserved.

: +81 258 47 9830.

. Inoue).

vacancies and Zn interstitials. Thus, the first essential step isestablishing technology for fabricating intrinsic ZnO epitaxial filmswith high electron mobility and low residual electron concentrations.

A wide variety of growth techniques, including molecularbeam epitaxy (MBE), pulsed laser deposition (PLD), chemicalvapor deposition (CVD), and radiofrequency magnetron sputter-ing have been employed for preparing thin ZnO films [7–10].Although MBE and PLD have both been used to grow high-qualityZnO films, these methods have been limited to the laboratorybecause of their high costs and the difficulty in scaling them up.CVD has many technical advantages for industrial applications,including being able to achieve high rate growths on large surfaceareas, a wide selection of metal-organic source materials, easypurification by evaporation, and low-cost production. The CVDtechniques developed so far use either dimethyl zinc (Zn(CH3)2)or diethyl zinc (Zn(C2H5)2) as the source for Zn, and O2[11,12],H2O [13,14], CO2[15], N2O [14] or (CH3)2CHOH [16] as oxidationagents. Unfortunately, ZnO films produced using conventionalCVD methods yield low-quality films compared to those preparedby MBE and PLD. Thus, the critical need in the mass production ofthin films is a revolutionary advancement in the CVD techniquethat will enable the fabrication of high-quality ZnO epilayers withhigh electron mobility and low residual electron concentrations.Our consideration is that low-quality ZnO film production by theconventional CVD is due to the incomplete reaction of alkyl zinc in

ARTICLE IN PRESS

Skimmer plate

Reactionzone

High qualityZnO epilayer

H2O*

Nozzle

Pt/ZrO2

(CH3)2Zn

H2

O2

Fig. 1. A schematic representation of CEW-CVD technique. The jet flow of hot H2O

is focused through several slits and is allowed to react with (CH3)2Zn ejected from

another nozzle in the reaction zone.

H. Nishiyama et al. / Journal of Crystal Growth 312 (2010) 483–486484

the gas phase: ZnO layer growth mainly occurs via thermalsurface reactions on substrates, leading to high concentrations ofdefects and grain boundaries. However, if hot water excited in ahigh energy is used to hydrolyze alkyl zinc, ZnO molecules, or atleast reactive ZnO precursors, will be produced in the gas phase,making the growth of ZnO epilayers feasible, in a similar fashionto PLD and MBE. Fig. 1 shows the concept behind this technique.The unique aspect of the present approach is the use of thePt-catalyzed exothermic H2–O2 reaction to obtain high-energyH2O that induces a very rapid gas-phase reaction with dimethylzinc. The whole reaction scheme is given as follows:

H2þO2 -Pt catalyst

H2O�

H2O�þZnðCH3Þ2-ZnOþ2CH4

Here, H2On represents hot water. Because of exothermicH2�O2 recombination reaction, the temperature of the catalystincreases instantaneously from room temperature due to thereaction heat evolved and produces H2On in a high energy. Herewe report that this approach has successfully developed thetechnique of providing high-quality ZnO epilayers. Since thistechnique uses hot H2O catalytically activated by the exothermicreaction of H2 and O2, we have termed it catalytically producedexcited water chemical vapor deposition (CEW-CVD).

2. Experimental section

In the CEW-CVD method, high purity H2 and O2 were introducedto a Pt-dispersed ZrO2 catalyst in a high vacuum chamber. CeramicZrO2 was used as a support on which Pt metal was dispersed by aconventional impregnation method. The catalyst temperatureincreased very rapidly from room temperature to 1700 K due tothe heat generated by the exothermic reaction. The hot H2O wasevolved in the gas phase from the high temperature Pt catalystsurface, and was ejected as a hot H2O jet from a fine nozzle into thereaction zone through skimmer plates. Then, the H2O jet wasallowed to collide with flowing dimethyl zinc, Zn(CH3)2, ejectedfrom another fine nozzle. Distance between the substrate andreaction zone was usually 10 mm. The direction of hot water flowwas perpendicular to the substrate surface. The ZnO epilayer wasgrown directly on an a-plane sapphire substrate at 873 K. Thea-plane sapphire substrate was degreased with acetone, etchedwith a H2SO4+H3PO4 solution, and then rinsed with deionizedwater. The Hall mobilities and residual carrier concentrations of the

ZnO epilayers were measured by the van der Pauw method at amagnetic field of 0.447 T and at room temperature.

3. Results and discussion

The growth rates of ZnO films by CEW-CVD method were in therange 0.02�0.08 mm min�1, which was significantly larger thanthe rates for other deposition methods, such as PLD and MBE. TheX-ray diffraction pattern of an as-grown ZnO film exhibited intense(0 0 0 2) and (0 0 0 4) peaks from the ZnO(0 0 0 1) index plane. Thelattice constant c was 0.5204 nm, which was close to that of singlecrystal ZnO, indicative of a nearly stress-free crystal structure. Thefull-width at half-maximum (FWHM) value of the o-rocking curveof ZnO(0 0 0 2) was less than 0.11. This indicated that the ZnO filmwas composed of grains with a small fluctuation of tilt angles.Fig. 2(A) shows the transmittance spectrum of an as-grown 1 mm-thick ZnO film in the wavelength range 300–2000 nm. The averagetransparency was higher than 90% in the visible and infraredregions (400–2000 nm) and peaks at 98% at around 1000 nm.Fig. 2(B) shows a photograph of an as-grown 1 mm-thick ZnO film.No coloring was observable and the as-grown film was sotransparent that the words underneath the film were clearlyvisible. Fig. 2(C) shows an atomic force microscopic image of theas-grown ZnO film. Large grains with a clear hexagonal structurecan be observed. The grain sizes were in the range 0.2–0.6 mm. Thefacets of the large crystal grains and the film surface were fairlyflat. The average roughness of the film surface was less than 9 nm.

Fig. 3 shows photoluminescence (PL) spectra of the as-grownZnO film. The PL spectrum at room temperature exhibited a sharppeak at 3.29 eV with a FWHM of 109 meV. This peak was assignedto band edge emission. Green luminescence from deeper levels[17] was not observed in significant levels; its intensity was as lowas 0.2% the band edge emission intensity. This clearly indicates thatthe defect density is negligible. The PL spectra at 16 K yielded astrong emission peak at 3.357 eV, attributed to the neutral donor-bound exciton Do

x. The FWHM of the peak was as low as 3.6 meV,which was much smaller than 10 meV [17] for ZnO obtained byCVD (the reaction of Zn(C2H5)2 with N2O). Interestingly, the FWHMvalue was also smaller than those for ZnO fabricated by MBE(5.5 meV) [18], by PLD on a sapphire (0 0 0 1) plane (5.1 meV) [19],and by laser MBE (LMBE) on a ScAlMgO4 substrate (4.8 meV) [20].The free exciton emission appeared at 3.373 eV. The two-electronsatellite recombination lines of the Do

x emission appeared at 3.330and 3.318 eV. The longitudinal optical phonon replicas of the Do

x

emission were clearly observed from the first to the fourthemissions. The PL spectra indicated that the crystal structure ofthe ZnO epilayer was almost defect free.

The Hall mobility and residual carrier concentration of ZnOepilayers by CEW-CVD were in the ranges 140–170 cm2 V�1 s�1

and 1.7–6�1017 cm�3, respectively. Fig. 4 shows plots of electronmobility as a function of electron concentration for the ZnOepilayers. A comparison was made with ZnO films previouslyprepared on sapphire substrates by the other methods. Theexperimental values for ZnO films grown directly by PLD were40–90 cm2 V�1 s�1 for electron mobility and 7�1016–3�1017

for electron concentration [20]. The electron mobilities andconcentrations of ZnO films directly grown by MBE [21] andLMBE [22] were scattered over wide ranges of 8–90 cm2 V�1 s�1

and of 1.6�1016–2.4�1018 cm�3, respectively. To minimizelattice mismatch with the substrate and to improve the growthof ZnO layer, buffer layers (precoated ZnO and MgO/ZnO layers)and the step growth technique were frequently employed. Forexample, the technique increased the electron mobility from 30 to141 cm2 V�1 s�1 for ZnO directly grown by PLD [23] and from 60to 125 cm2 V�1 s�1 by MBE [24]. In the case of CVD, application of

ARTICLE IN PRESS

1

2

3

1

2

3µm µm

Wavelength / nm

Tra

nsm

ittan

ce /

% (a) (b)

Fig. 2. (A) A transmittance spectrum of an as-grown ZnO film. The inset shows an expanded view (y-axis direction) of the overall transmittance spectrum. (B) A photograph

of an as-grown ZnO film. The sample on the left (a) is a-plane sapphire, whereas the sample on the right (b) is a sapphire with an as-grown 1 mm ZnO film. The words

(CEW-CVD and Technology) under the film can be clearly seen. (C) An atomic force microscopic image of an as-grown ZnO film.

Inte

nsity

/arb

. uni

ts

Photon energy /eV

FXTES

D0X-1LO

D0X-4LO

D0X-3LO

D0X-2LO

10

16K

D0X

R.T.

Fig. 3. Photoluminescence spectra of an as-grown ZnO film measured at room

temperature (inset) and at 16 K. The four peaks of D0x-1LO–D0

x-4LO as a phonon

replica of D0x appear at 3.2856, 3.2149, 3.1440, and 3.0731 eV, respectively.

Electron concentration /cm-3

Ele

ctro

n m

obili

ty /c

m2 V

-1s-1 NA/ND = 0

0.1

0.50.60.70.80.9

MBE, (24) MBE, (26)

PLD, (23)

MOCVD, (13) MBE, (28)

MOCVD, (14)

LMBE, (22)

PLD, (ref.20)

CEW-CVD

Fig. 4. Plots of electron concentration vs. electron mobility for ZnO epilayers

produced by CEW-CVD, and comparisons with data previously reported for ZnO

grown on sapphires by various physical and chemical methods. Open circles are

for samples produced by CEW-CVD in the present study. Filled triangles (red: Ref.

[24]; blue: Ref. [26]; black: Ref. [28]), squares (black: Ref. [23]) and diamonds

(blue: Ref. [13]; green: Ref. [14]) represent data for ZnO films grown on buffer

layers, accompanied by step scan growth by MBE, PLD and CVD, respectively. Open

triangles (Ref. [22]) and squares (Ref. [20]) indicate direct growth by LMBE and

PLD, respectively.

H. Nishiyama et al. / Journal of Crystal Growth 312 (2010) 483–486 485

the same techniques produced ZnO layers [13,25] with mobilitiesof 20–71 cm2 V�1 s�1 and large electron concentrations of1018–1019 cm�3, although a considerably higher mobility of100 cm2 V�1 s�1 was obtained for the reaction of (C2H5)2Zn+H2O+NH3+H2[20]. Thus, it should be noted that even thoughCEW-CVD was used without buffer layers and step growthtechniques, it is able to provide a significantly higher electronmobility, compared with PLD, MBE, and CVD combined with thosetechniques.

Fig. 4 also shows theoretical lines for the electron mobilityas a function of electron concentration for different compen-

sation ratios of NA/ND, where NA and ND are the densities of theacceptors and donors, respectively. The correlations were derivedby considering five scattering mechanisms such as ionizedimpurity scattering, piezoelectric scattering, deformation poten-tial scattering, polar optical phonon scattering, and dislocationscattering [26,27]. The lines in Fig. 4 are obtained by assuming

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H. Nishiyama et al. / Journal of Crystal Growth 312 (2010) 483–486486

that dislocation scattering is small. A small NA/ND ratio isdesirable for intrinsic ZnO epilayer formation. Our data are inclose agreement with the theoretical correlation curves forNA/ND=0.5–0.7. Although some of the data points for the ZnOfilms by PLD, MBE, and LMBE are located on the curve forNA/ND=0.8–0.9, most of them lie far from the curve. These resultsclearly demonstrate that the CEW-CVD method is very effectivefor suppressing generation of unintentional acceptor impuritiesderived from intrinsic defects, and is superior to PLD and MBE. Itis evident that employing hot H2O is a key for achieving high-quality ZnO growth. Although a detailed analysis of the epilayergrowth mechanism still needs to be performed, the presentresults provide plenty of encouragement for doing further workon producing high-quality thin ZnO films.

4. Conclusion

In conclusion, CEW-CVD has been demonstrated to be a verypromising technique for fabricating high-quality ZnO epilayers.Because it uses the chemical energy from only H2 and O2, it isenergy-saving and ecologically friendly, while the electronicproperties of ZnO are superior to those produced by energy-consuming physical methods, such as PLD and MBE. Thus, thismethod is extremely advantageous for commercial applications.Furthermore, CEW-CVD utilizing a hot water jet generated bycatalysts also raises unlimited possibilities for a wide range offabrication processes for high-quality thin metal oxide films,because of the availability of hydrolysis for volatile alkyl metals.

Acknowledgement

This work was supported by Tokyo Electron Ltd.

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