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Thin Solid Films 516 (2

Hydrothermal growth and characterization of ZnO thin film on sapphire(0001) substrate with p-GaN buffer layer

Trilochan Sahoo a, Ju-Won Jeon a, V. Kannan a, Cheul-Ro Lee a, Yeon-Tae Yu a,Yong-Won Song b, In-Hwan Lee a,⁎

a School of Advanced Materials Engineering and Research Center of Industrial Technology and Semiconductor Physics Research Center,Chonbuk National University, Chonju 561-756, Republic of Korea

b Department of Nano-Optic Engineering, Korea Polytechnic University, Gyeonggi 429-793, Republic of Korea

Received 19 August 2007; received in revised form 12 January 2008; accepted 2 March 2008Available online 7 March 2008

Abstract

Monocrystalline ZnO thin films on p-GaN/sapphire (0001) substrate were grown by single step hydrothermal technique at 90 °C. ZnO thin filmswere grown in aqueous solution of zinc nitrate and ammonium hydroxide. The X-ray diffraction, scanning electron microscopy and roomtemperature photoluminescence analysis were carried out to characterize structural, morphological and optical properties of the films. The thin filmsrevealed single crystalline nature and wurtzite symmetry. The films were c-axis oriented and have honeycomb like pitted surface morphology. Theepitaxial relationship between ZnO film and p-GaN buffer layer was observed to be (0001)[11 2̄ 0]ZnO||(0001)[11 2̄ 0]GaN. Sharp luminescent peakcentered at 376 nm due excitonic emission and broad deep level emission peak were obtained from room temperature photoluminescencemeasurement of the ZnO thin film.© 2008 Elsevier B.V. All rights reserved.

Keywords: Epitaxy; Hydrothermal deposition; Thin films; p-type gallium nitride; Photoluminescence; X-ray diffraction; Zinc oxide

1. Introduction

Among the wide band gap semiconductors, the ZnO hasattracted a lot of attention for its versatile properties. ZnO is adirect, wide band gap semiconductor with band gap of 3.37 eVand has very high exciton binding energy of 60 meV [1–3]. Thehigh exciton binding energy is very promising for realization oflight emitting devices such as ultra-violet (UV) light emittingdiode and UV laser beyond room temperature [4,5]. ZnO haslong been in use for wave guide transducers [6]. It is also apotential material for gas sensors, transparent electrodes andspintronics application [7–9]. Good quality ZnO thin film isessential for fabrication of ZnO related devices. Epitaxial ZnOthin film is generally grown by vapor phase processes like pulsedlaser deposition, chemical vapor deposition, molecular beamepitaxy, metal organic chemical vapor deposition (MOCVD)

⁎ Corresponding author. Tel.: +82 632703614; fax: +82 632702305.E-mail address: ihlee@chonbuk.ac.kr (I.-H. Lee).

0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2008.03.001

and sputtering [10–14]. In comparison to vapor phase depositiontechniques, solution phase growth processes are less expensive,do not require much stringent conditions and as such are carriedout at relatively lower temperatures. Chemical bath deposition,hydrothermal and electrochemical deposition are being adoptedas solution phase techniques for growth of ZnO thin film [15–17].Among these solution phase techniques, the hydrothermalprocess is the most attractive as a comparatively simple andindustrially economical method. Hydrothermal route has longbeen followed for growth of ZnO powder, single crystals [18,19].Nanostructures of ZnO are also being synthesized by hydro-thermal technique [20]. However epitaxial growth of singlecrystalline ZnO thin film by hydrothermal technique is yet toestablish and there are only a few reports on this kind of work.Andeen et al. have reported the epitaxial growth of ZnO thin filmon spinel (111) substrate by hydrothermal technique [16]. Asurface modifier assisted two step hydrothermal method forgrowth of epitaxial ZnO thin film on undoped GaN bufferedsapphire (0001) substrate has recently been reported [21].

Fig. 1. XRD θ–2θ scan of the ZnO thin film on p-GaN/sapphire (0001). (a) pH10.8, Zn2+: 0.05 M, 90 °C and 12 h and (b) pH 10.9, Zn2+: 0.0375 M, 90 °C and24 h.

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In this work we have reported the epitaxial growth and cha-racterization of single crystalline ZnO thin film on p-GaN/sapphire (0001) substrate by single step hydrothermal technique.The anomalous growth of ZnO thin film on p-GaN/sapphire(0001) compared to undoped GaN is discussed. This could fur-ther improve the understanding of epitaxial growth of ZnO thinfilm by hydrothermal technique and help in fabrication of ZnOrelated heterostructures.

2. Experimental section

The reagents used for the growth of ZnO thin film in thishydrothermal process were zinc nitrate hexahydrate [Zn(NO3)2·6H2O] and ammonium hydroxide [NH4OH (28%)].Aqueous solution of zinc nitrate hexahydrate of differentconcentrations was prepared with deionized water. The pH ofthe solution was adjusted by addition of ammonium hydroxide.20 ml of the precursor solution was then transferred into a 35 mlTeflon container. The p-GaN/sapphire (0001) substrate wasplaced in the precursor solution at approximately 10 mm abovethe bottom, with help of Teflon insert such that the p-GaN layerwould face downward. The Mg doped p-GaN thin film onsapphire (0001) substrate, was previously grown by MOCVDtechnique. The Hall effect measurement of the p-GaN thin filmexhibited hole concentration of 4.5×1017 cm−3, the holemobility of 8.5 cm2/Vs and the resistivity of 1.7 Ω cm. Beforeplacing the substrate in the precursor solution, it was thoroughlycleaned by acetone, ethyl alcohol and deionized water. TheTeflon container was sealed in a stainless steel autoclave, whichwas then placed in an oven at 90 °C for specific period of time.On completion of the reaction, the container was cooled down toroom temperature and the sample was collected. The sample wasrinsed with deionized water and dried. Two ZnO thin filmsamples were grown under different conditions and depicted assamples A and B. Sample A was grown at pH 10.8, initial Zn2+

concentration of 0.05M in 12 h, while sample B was grown at pH10.9, initial Zn2+ concentration of 0.0375 M in 24 h. Bothsamples were grown at same temperature of 90 °C.

The phase and in plane orientation of the as grown ZnO thinfilm on p-GaN/sapphire substrate were characterized by θ–2θand ϕ-scan of X-ray diffraction (XRD) technique with Cu_Kα1

radiation. The surface morphology and cross section of the thinfilms was analyzed by scanning electron microscopy (SEM)operated at 20 kV. The room temperature photoluminescence(PL) spectra were obtained using He–Cd laser (325 nm) forexcitation with power densities up to 10 mW/cm2. The lumi-nescence was dispersed by a 0.75 m monochromator and detec-ted by a photon multiplier interfaced with a lock-in amplifier.

3. Results and discussion

The XRD (θ–2θ scan) patterns of the ZnO thin films on p-GaN/sapphire (0001) substrate are shown in Fig. 1. The XRDpattern 1a corresponds to the ZnO thin film sample A and 1b tosample B. The XRD pattern consisted of peaks corresponding to(0002) and (0004) reflections of ZnO thin film and p-GaN bufferlayer. The (0002) and (0004) reflection peaks of ZnO thin film

are merged with the respective reflection peaks of p-GaN bufferlayer due to close lattice matching. Thus the XRD result revealedthat the as grown ZnO thin films had wurtzite structure and wereoriented along the c-axis. The single crystalline nature of the ZnOthin filmswere also confirmed from theXRDanalysis. Fig. 2a andb shows the XRD (ϕ-scan) patterns of the ZnO thin film and p-GaN buffer layer. Fig. 2a refers to sample A, while Fig. 2b tosample B. From the ϕ-scan patterns of the thin films, the six foldsymmetry of the wurtzite structure of ZnO and p-GaN is clearlyindicated. The similar angular positions of the respective peaks ofZnO and p-GaN reveals that the ZnO thin film and the p-GaNbuffer layer possessed epitaxial relationship of (0001)[112̄ 0]ZnO||(0001)[112̄ 0]GaN. Similar epitaxial relationship between ZnOand GaN is reported for ZnO thin film grown on GaN by hydro-thermal and other method [21,22].

Figs. 3 and 4 show the plane and cross sectional SEM micro-graphs of the ZnO thin films grown on p-GaN/sapphire substrate(0001). Fig. 3a and b shows the SEM images of the surfacemorphology and cross section of sample A. The surface mor-phology and cross section of sample B are displayed in Fig. 4aand b respectively. The figures show that the ZnO thin films werecontinuous and the film surface had honeycomb like pittedmorphology. The pits were more pronounced on the thin filmsurface grown at lower initial Zn2+concentration. The presenceof partially coalesced pits indicates that the thin film growthmight have initiated with nucleation of densely distributed ZnOislands which finally have merged with each other to form con-tinuous film. Further the aligning of the lines of pits with eachother is an indication of uniform orientation of nuclei producingthe ZnO thin film [16]. It is interesting to note the hexagonalshape of the pits on the thin films surfaces in both of the cases,which is a hint of six fold symmetry nature of the ZnO wurtzitecrystal structure as revealed by the XRD ϕ-scan data. The crosssectional SEM images displayed in Figs. 3b and 4b show that thethickness of the thin films were in the range of 3–4 µm. Thethickness of the film was found to increase with initial precursorconcentration. The fact that the thickness of the thin filmincreases with increase in initial Zn2+ concentration supports the

Fig. 3. (a) Surface morphology and (b) cross sectional SEM micrograph of ZnOthin film (pH 10.8, Zn2+: 0.05 M, 90 °C and 12 h).

Fig. 2. XRD ϕ-scan of the ZnO thin film on p-GaN/sapphire (0001). (a) pH10.8, Zn2+: 0.05 M, 90 °C and 12 h and (b) pH 10.9, Zn2+: 0.0375 M, 90 °C and24 h.

Fig. 4. (a) Surface morphology and (b) cross sectional SEM micrograph of ZnOthin film (pH 10.9, Zn2+: 0.0375 M, 90 °C and 24 h).

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earlier observation by Andeen et al., that film thickness is afunction of initial zinc concentration [16]. Formation of suchcontinuous thin film at this higher pH value is not expectedaccording to earlier works [21,23]. It is reported that at similarpH values as in the present case, only elongated ZnO rods weregrown on undoped GaN buffer layer.

Though the mechanism of the epitaxial growth of ZnO thinfilm by vapor phase techniques is extensively studied, the solu-tion phase growth process is not well understood owing to verycomplex growth environment. The growth habit of ZnO crystalin solution phase is determined by a number of parameters suchas pH, temperature, molecular strength of soluble species, su-persaturation, ligands and presence surface modifiers [24,25].The soluble zinc species in the aqueous solution are ZnOOH−,Zn(OH)4

2−, ZnO2− etc., which under hydrothermal conditions at

supersaturation dehydrates to nucleate ZnO. Elongated growthof ZnO crystallites are normally observed under hydrothermalgrowth environment [18]. However ZnO crystallite with variousshapes and sizes can be obtained by using different kinds ofsurface modifiers [26]. In addition to crystal growth habit, theepitaxial growth of thin films in solution phase is influenced bythe nature of substrate. The density and size of the ZnO crys-tallites grown on pre-grown ZnO, gold coated glass, glass and

Fig. 5. RT PL of ZnO thin film grown on p-GaN/ sapphire (0001) substrate (a)pH 10.8, Zn2+: 0.05 M, 90 °C and 12 h and (b) pH 10.9, Zn2+: 0.0375 M, 90 °Cand 24 h.

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single crystalline sapphire were observed to vary from one sub-strate to the other under similar growth conditions [23,25]. It isreported that the electrostatic interaction between soluble zincspecies and the substrate play important role for epitaxial growthof thin film [21]. It is proposed that the undoped GaN surface iscomposed of negatively charged surface sites at higher pHvalues (pHN10), which repels the negatively charged solublezinc species leading to scattered growth of ZnO rods. However,the proposal is based on the speculated iso-electric point of un-doped GaN, which is not known yet. As at similar pH values, thegrowth of continuous thin film instead of scattered crystallites onp-GaN buffer layer was observed in this case. Therefore, it maybe possible that at such higher pH values the p-GaN surface,unlike the undoped GaN surface composed positively chargedsites, which would be attracting the negatively charged solublezinc complexes leading to dense nucleation for growth of con-tinuous ZnO thin films.

The room temperature (RT) PL spectra of the ZnO thin filmon p-GaN/sapphire are shown in Fig. 5. Spectrum (a) corres-ponds to sample A and (b) to sample B. The room temperaturePL spectra of the ZnO thin film consisted of two emission peaks.The sharp PL peak at 376 nm (~3.297 eV) in UV band due toband edge emission is attributed to excitonic recombination [27].The small full width at half maximum (15 nm–130 meV) of theexcitonic peak is comparable to gas phase grown ZnO thin films[11]. Another broad peak related to deep level transition was alsoobserved at 575 nm (~2.156 eV). The defects related to the deeplevel may have resulted from oxygen vacancy or zinc interstitialsin the ZnO thin film [28].

4. Conclusions

Single crystalline zinc oxide thin films were grown on p-GaN/sapphire (0001) substrate by simple hydrothermal techni-que at 90 °C. The ZnO thin films exhibited wurtzite symmetryand were oriented along the c-axis. The in plane orientationbetween the ZnO and p-GaN buffer layer was observed to be[112̄ 0]ZnO||[112̄ 0]GaN. The thin films were continuous and hadpitted surface morphology. The nature of the pits indicates thatthe continuous thin filmmight have resulted from coalescence ofZnO islands. The anomalous growth of thin film suggests thatthe film growth in aqueous solution depends on the nature of the

substrate. The room temperature PL measurement revealedemission peaks at 376 nm and 575 nm corresponding to exci-tonic recombination and deep level transition respectively.

Acknowledgements

This work was supported by the Korea Research FoundationGrant funded by the Korean Government (MOEHRD)(KRF-2007-211-D00075) and was supported by the grant of Post-Doc.Program, Chonbuk National University (the second half term of2006).

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