Current Applied Physics 10 (2010) 164–170

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Current Applied Physics

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Epitaxial growth of highly transparent and conducting Sc-doped ZnO filmson c-plane sapphire by sol–gel process without buffer

Ruchika Sharma a, Kiran Sehrawat b, R.M. Mehra a,*

a Department of Electronic Science, University of Delhi South Campus, New Delhi 110 021, Indiab Department of Physics, Maitreyi College, Chanakyapuri, New Delhi 110 021, India

a r t i c l e i n f o

Article history:Received 30 January 2009Received in revised form 15 May 2009Accepted 18 May 2009Available online 21 May 2009

PACS:81.05.Dz61.05.cp61.05.jh68.37.Hk68.37.Ps81.15.Kk

Keywords:Zinc oxideScandium dopingSol–gelThin filmsAnnealing temperature

1567-1739/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.cap.2009.05.013

* Corresponding author. Tel.: +91 11 24115849; faxE-mail address: rammehra2003@yahoo.com (R.M.

a b s t r a c t

Highly transparent and conductive scandium doped zinc oxide (ZnO:Sc) films were deposited on c-planesapphire substrates by sol–gel technique using zinc acetate dihydrate [Zn(CH3COO)2�2H2O] as precursor,2-methoxyethanol as solvent and monoethanolamine as a stabilizer. The doping with scandium isachieved by adding 0.5 wt% of scandium nitrate hexahydrate [(ScNO3�6H2O)] in the solution. The influ-ence of annealing temperature (300–550 �C) on the structural, optical and electrical properties was inves-tigated. X-ray Diffraction study revealed that highly c-axis oriented films with full-width half maximumof 0.16� are obtained at an annealing temperature of 400 �C. The surface morphology of the films wasjudged by SEM and AFM images which indicated formation of grains. The average transmittance wasfound to be above 92% in the visible region. ZnO:Sc film, annealed at 400 �C exhibited minimum resistiv-ity of 1.91 � 10�4 X cm. Room-temperature photoluminescence measurements of the ZnO:Sc filmsannealed at 400 �C showed ultraviolet peak at �3.31eV with a FWHM of 11.2 meV, which are comparableto those found in high-quality ZnO films. Reflection high-energy electron diffraction pattern confirmedthe epitaxial nature of the films even without introducing any buffer layer.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction tron sputtering technique. Chichibu et al. [10] reported that ZnO

Recently developed large area crystalline ZnO thin films havebeen considered as potential candidates for blue and ultravioletoptical devices, such as light-emitting diodes (LEDs) and laserdiodes (LDs) [1]. Crystalline ZnO thin films have been attemptedon a variety of substrates such as c-plane sapphire by plasma en-hanced metal organic chemical vapor deposition (PEMOCVD) [2],MgAl2O4 by metal organic vapor phase epitaxy [3] and SiC by pulselaser deposition techniques [4]. Direct growth of crystalline ZnOthin films on sapphire suffers from poor structure due to large lat-tice mismatch (16.8%) between ZnO and sapphire [5,6]. EpitaxialZnO film has been grown using an epi-GaN buffer layer on c-planesapphire substrate by sol–gel technique because lattice mismatchbetween ZnO and GaN is just 2.2% [7]. Kumar et al. [8] attemptedthe direct epitaxial growth of ZnO:Al thin film on r-plane sapphireby pulse laser deposition technique that resulted in resistivity of2.14 � 10�3 X cm. Minami et al. [9] investigated electrical andoptical properties of Sc doped ZnO films prepared by d.c. magne-

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: +91 11 24110876.Mehra).

epilayers were successfully grown on air-annealed sapphire sub-strates by helicon-wave-excited-plasma sputtering.

In this work, we attempted to grow epitaxial layer of ZnO:Scfilms on c-plane sapphire without buffer layer by low cost andnon-vacuum sol–gel technique [11,12]. In sol–gel derived films,thermal annealing promotes nucleation resulting in the improve-ment of structural and optical performance [13–15]. The epitaxialgrowth of our films has been confirmed by the measurement ofreflection high-energy electron diffraction (RHEED) pattern. X-raydiffraction (XRD), scanning electron microscopy (SEM) and atomicforce microscopy (AFM) were used to probe the crystalline struc-ture and surface morphology of ZnO:Sc films at different annealingtemperatures. The influence of annealing temperature on electrical(resistivity, carrier concentration and Hall mobility) and optical(transmittance, band gap) properties has been reported. The PLstudies were carried out to ascertain the quality of the films.

2. Experimental details

A precursor solution of ZnO, about 0.2 M in concentration, wasprepared from zinc acetate dihydrate [Zn(CH3COO)2�2H2O (purity

R. Sharma et al. / Current Applied Physics 10 (2010) 164–170 165

99.95%)] and dissolved in anhydrous 2-methoxyethanol (AR, AjaxChemicals, Australia). Scandium nitrate hexahydrate[(ScNO3�6H2O), purity 99.9%] (0.5 wt%) was used for scandiumdoping [16] and MEA (CP, Bio-Lab, London) was as a stabilizer.The resultant solution was stirred at room temperature for 4 h toyield a clear and homogeneous solution. The solutions were leftto age for 48 h. The sapphire substrate was degreased by boilingin acetone and methanol for 5 min and dried by blowing nitrogengas. The substrate was then heated up to 700 �C for thermaldesorption of surface contaminants. At this temperature, the sub-strate was exposed to activated nitrogen for 10 min for nitridation.Film deposition was carried out in air with spinning speed of2700 rpm for 20 s using the spin coating process. The films weredried on a hot plate at 300 �C for 10 min. This process was repeatedseveral times to deposit films of desired thickness (450–500 nm).The films were annealed for 1 h at different temperatures (300–550 �C) at the rate of 4 �C/min and cooled in air. The crystal orien-tation of the films was evaluated by XRD using Cu Ka(k = 1.5405 A�) radiation ((D/MAX 2100H, Rigaku, Japan). The cur-rent and voltage of XRD was maintained at 20 A and 40 V, respec-tively during the measurement. The crystal orientation of the filmswas evaluated by RHEED using HITACHI-H 300 Electron Micro-scope. The surface morphology of the film was observed by AFMusing SPI 3700/SPI 300; Seiko instruments Co. Tokyo, Japan in dy-namic force (tapping) mode. The SEM micrographs of thin films areinvestigated by a scanning probe microscope (SPA-400, SeikoInstrument, Japan). The optical properties of the deposited filmswere investigated using dual beam UV–vis spectrophotometer inthe wavelength range 190–1500 nm [Shimatzu UV 336]. Photolu-minescence (PL) measurements were done at room temperatureusing LS55 Perkin Elmer Luminescence Spectrum Analyzer. Thefilms were excited by a 325 nm beam of He–Cd laser with an out-put power of 30 mW. The emitting light from the sample is focusedinto the entrance slit of a monochromator, and it is picked up byPMT. A cutoff filter is used to suppress the scattered laser radiation.The cutoff wavelength of the filter at the ultraviolet side was340 nm. The electrical resistivity (q) and the Hall coefficient (RH)were measured by the van der Pauw technique. The sign of the Hallcoefficient confirmed the n-type conduction of the films.

Fig. 1. (a) XRD patterns for ZnO:Sc films grown on c-plane sapphire substrates,annealed at different temperatures (300–550 �C). (b) Variation of FWHM andcrystallite size as a function of annealing temperature (300–550 �C). (c) Shows thevariation of 2h and lattice constant ‘c’ as a function of annealing temperature (300–550 �C).

3. Results and discussion

3.1. Structural properties

The microstructural properties of ZnO:Sc films on c-plane sap-phire substrate were investigated by XRD, RHEED, SEM and AFM.

3.1.1. XRD analysisFig. 1a shows XRD patterns for ZnO:Sc films grown on c-plane

sapphire substrate, annealed at different temperatures (300–550 �C). Films exhibit (0 0 2) preferential orientation with c-axisand peak intensity increases remarkably with increase in annealingtemperature up to 400 �C. Annealing temperature has the effect ofnarrowing the diffraction peak, and shifting the (0 0 2) peaks tohigher 2h angles, a result of the partial relief of intrinsic stresswithin the annealed films [17]. Therefore, at some appropriateannealing temperature (400 �C), compressive stress due to thermalmismatch and tensile-intrinsic stress of ZnO:Sc film would canceleach other. But when the annealing temperature is high(>400 �C), the adatoms are decomposed and re-evaporated fromthe surface. The ZnO:Sc films becomes thermodynamically unsta-ble resulting in decrease in intensity of the films and crystallinequality of the films degraded.

Fig. 1b shows the variation of full width at half maximum(FWHM) and crystallite size as a function of annealing tempera-

ture. The FWHM decreases from 0.41� to 0.16� with the increasein annealing temperature from 300 to 400 �C, indicating animprovement in the crystallinity of the films. However, at temper-atures (>400 �C), FWHM increases and peak intensity decreases,showing a degradation of the film quality. Moreover, the decreasein FWHM with annealing temperature implies an increase in crys-tallite size. Assuming a homogeneous strain across the films, thecrystallite size was estimated by Sherrer’s equation [18]:

D ¼ 0:9kB cos h

ð1Þ

where k, h and B are the X-ray wavelength, Bragg’s diffraction angleand FWHM of the ZnO:Sc (0 0 2) diffraction peak, respectively. The

Fig. 2. (a) RHEED pattern, (b) SEM and (c) AFM of ZnO:Sc film annealed at 300 �C.

Fig. 3. (a) RHEED pattern, (b) SEM and (c) AFM of ZnO:Sc film annealed at 350 �C.

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crystallite size increases gradually from �29 to 47 nm with increasein annealing temperature from 300 to 400 �C. The increase ofannealing temperatures is in favour to the diffusion of atoms ab-

sorbed on the substrate and accelerates the migration of atoms tothe energy favorable positions, resulting in the enhancement ofthe crystallinity and c-axis orientation of ZnO:Sc films, which is

R. Sharma et al. / Current Applied Physics 10 (2010) 164–170 167

indicated by the increase of (0 0 2) peak strength and decrease ofFWHM value for ZnO:Sc films annealed upto 400 �C.

Fig. 1c shows the variation of 2h and lattice constant ‘c’ as afunction of annealing temperature. For the films annealed at 300and 350 �C, the value of diffraction angle 2h is less than the powdervalue (34.44�) indicating that the films are in state of stress with

Fig. 4. (a) RHEED pattern, (b) SEM and (c) A

Fig. 5. (a) RHEED pattern, (b) SEM and (c) A

tensile component parallel to c-axis. At 400 �C, the 2h approachesthe powder value indicating reduction in the tensile stress. The lat-tice constant ‘c’ decreases with increase in annealing temperatureup to 500 �C. It approaches the bulk ZnO value (co = 0.5205 nm) atthe annealing temperature of 400 �C. However, at 550 �C latticeconstant decreases slightly due to the large co-efficient of linear

FM of ZnO:Sc film annealed at 400 �C.

FM of ZnO:Sc film annealed at 450 �C.

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expansion of c-plane sapphire substrate (5.00 � 10�6/�C) in com-parison with ZnO films (3.02 � 10�6/�C).

3.1.2. Surface morphology (RHEED, SEM and AFM)Fig. 2a–c represent RHEED pattern, SEM image and AFM image

of ZnO:Sc film annealed at 300 �C, respectively. A weak streakyRHEED pattern was observed. A beginning of larger and twistedgrain growth, with poor connectivity between them, was seenthrough the SEM micrograph. Surface features of uniform lateralsize of 50–70 nm were clearly observed in the AFM image.

As the annealing temperature was increased to 350 �C, theRHEED pattern showed a mixture of thicker line and spotty pat-tern, indicating the formation of an island (Fig. 3a). It is clear fromSEM image that the film posses a uniform defined structure ofgrains with size of 60–100 nm, and the ‘hallos’ disappeared(Fig. 3b). The AFM image clearly indicated an increase in the graingrowth with a network of densely packed small particles (Fig. 3c).

A diffused spotty RHEED pattern is seen in the film annealed at400 �C (Fig. 4a). SEM micrograph shows distinctly different surfacemorphology, as large features of �250 nm along with smaller fea-tures of 60–100 nm (Fig. 4b). A large number of 3D island with reg-ular hexagonal shapes and rough step edges randomly distributedon the film surface were obtained from AFM image (Fig. 4c).

As the film was further annealed at 450 �C, the streaky line inthe RHEED pattern was superimposed on the spotty pattern, sug-gesting the growth of the island (Fig. 5a). SEM image revealed in-crease in the lateral size of morphological features withannealing temperature up to 450 �C (Fig. 5b). The surface rough-ness decreases with increase in annealing temperature (Fig. 5c).

When the film was annealed at 500 �C, the intensity of the spottypattern gradually increased, whereas that of the streaky line dimin-ished and almost disappeared (Fig. 6a). SEM image revealed a mix-ture of small (40–80 nm) and large (150–200 nm) surface features(Fig. 6b). The AFM image of the same film indicates that the grainsgrow to form as a hexagonal prism with a pyramidal point (Fig. 6c).

A sharp and streaky pattern was well observed for the film an-nealed at 550 �C (Fig. 7a) and this indicates two-dimensional epi-taxial growth. SEM indicates an agglomeration of particles along

Fig. 6. (a) RHEED pattern, (b) SEM and (c) A

with pinholes (Fig. 7b). Least rms value of surface roughness wasfound for the film annealed at 550 �C (Fig. 7c). Thus, it is seen thatthe mixed RHEED patterns comprising of streaks and spots at low-er annealing temperatures, showed changing feature relatively clo-ser to a spoty one after annealing at higher temperature along withthe rearrangement of atoms into a smoother surface [19,20].

3.2. Optical properties

3.2.1. TransmittanceThe optical transmission spectra (300–1500 nm) for ZnO:Sc

films deposited on c-plane sapphire substrate as a function ofannealing temperatures (300–550 �C) are shown in Fig. 8a. Theinterference fringes were clearly seen in all the samples suggestingthat the films were homogeneous and uniform. The average trans-mission is around 84% in the film annealed at 300 �C and showed asystematic increase �92% with increase in annealing temperatureup to 400 �C. A small decrease in the transmission is observed forthe films annealed at temperatures (>400 �C). The higher transmis-sion may be attributed to the presence of low surface roughness inthe ZnO:Sc film and thereby resulting in less scattering of light. It isimportant to point that a well defined and sharp absorption edge ataround 390 nm is observed in all the samples annealed in the range300–550 �C. The presence of interference pattern for the films an-nealed at different temperatures indicates the stability of interfacebetween ZnO:Sc and c-plane sapphire.

3.2.2. BandgapThe optical absorption coefficient (a) of a direct band gap semi-

conductor near the band edge, for photon energy hv greater thanthe band gap energy Eg of the semiconductor, is given by the rela-tion [21]

ahm ¼ Affiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðhm� EgÞ

qð2Þ

where h is Planck’s constant and v is the frequency of the incidentphoton. The value of a is determined from transmittance spectra.The plot of (ahv) 2 versus photon energy (hv) (Tauc’s plot) for

FM of ZnO:Sc film annealed at 500 �C.

Fig. 7. (a) RHEED pattern, (b) SEM and (c) AFM of ZnO:Sc film annealed at 550 �C.

Fig. 8. (a) Variation of transmittance (T%) as a function of annealing temperature(300–550 �C). (b) Tauc’s plots of ZnO:Sc films as a function of annealing temper-ature (300–550 �C).

Fig. 9. PL spectra of ZnO:Sc films at different annealing temperature (300–550 �C).

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ZnO:Sc films annealed at different temperature is shown in Fig. 8b.Eg was obtained by extrapolating the linear part of the Tauc’s plot to

intercept the energy axis at (ahv)2 = 0. The Eg increases from 3.20 to3.26 eV with increase in annealing temperature from 300 to 550 �C.

3.2.3. Photoluminescence (PL)Fig. 9 shows PL spectra for ZnO:Sc films annealed at tempera-

tures from 300 to 550 �C. The dominant peak at round 3.26–3.35 eV corresponding to near band emission was observed in allthe annealed samples and was due to radiative recombination offree exciton in ZnO. However, a broad band related to deep levelemission was also observed in the ZnO:Sc film annealed at 300and 350 �C. This broad emission tail is generally associated withthe presence of native defects in ZnO films in the form of oxygenvacancies or point defects [22]. It is important to point out thatthe broad band due to deep level emission in ZnO:Sc film com-pletely disappeared after annealing at 400 �C (Fig. 9). The PL spec-tra of ZnO:Sc film annealed at 400 �C shows maximum intensity ofthe peak corresponding to 3.31 eV with relatively low FWHM�11.2 meV. This confirms the growth of good quality and defectfree epitaxial ZnO:Sc film without buffer layer on c-plane sapphiresubstrate.

Fig. 10. Variation of r, n and l of ZnO:Sc films with annealing temperature (300–550 �C).

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3.3. Electrical properties

The influence of the annealing temperature from 300 to 550 �Con resistivity (q), carrier concentration (n) and Hall mobility (lH)of ZnO:Sc films is shown in Fig. 10. The resistivity of the film de-creases from 8.32 � 10�3 to 1.91 � 10�4 X cm with increase inannealing temperature from 300 to 400 �C, whereas a slight in-crease in the resistivity of the films was observed beyond 400 �C.The carrier concentration of the film increases from 1.9 � 1019 to8.9 � 1020 cm�3 as the annealing temperature increases from 300to 400 �C. As the annealing temperature was increased above400 �C, slight decrease in carrier concentration was observed. Thevariation of Eg with annealing can be analyzed in terms of bandgap widening due to Burstein–Moss (BM) effect [23]. Accordingto this effect, there is widening of the Eg due to increase in the car-rier concentration. However, in the present case, Eg is found to in-crease continuously with the increase in annealing temperaturewhere as the carrier concentration decreased at higher annealingtemperature. It may be mentioned that for the ZnO:Sc films grownon corning glass, the variation of band gap with annealing couldnot be accounted by BM effect [16]. The Hall mobility also showedsimilar behavior as the carrier concentration, i.e., it is found to in-crease with the annealing temperature up to 400 �C. The decreasein the resistivity up to 400 �C is related to the increase in carrierconcentration and Hall mobility. The increase in mobility and car-rier concentration with the annealing temperature can be attrib-uted to the reduction in grain boundary scattering [24] due tothe improvement of crystallinity and an increase in the crystallitesize of ZnO:Sc films with the increase in the annealing temperature(as observed by XRD measurements). The observed slight increasein the resistivity above 400 �C could be due to the structural degra-dation (decrease in peak intensity of (0 0 2) peak and increase inthe FWHM as shown in Figs. 1a and b). The decrease in resistivitywith annealing temperature has also been reported for Al dopedZnO films [25,26].

4. Conclusion

The deposition parameters to grow highly conducting and epi-taxial ZnO:Sc films using sol–gel process on c-plane sapphire hasbeen investigated. The best film was obtained at annealing temper-ature of 400 �C. The absence of deep level emission in PL andRHEED pattern confirms the high quality of epitaxial nature ofZnO:Sc film. The successful growth of conducting and epitaxialZnO:Sc films on sapphire without using buffer layer demonstratesthe feasibility of utilizing these films for optoelectronic devicesapplications at low cost and on large area substrate.

Acknowledgements

One of the authors, Ruchika Sharma, gratefully acknowledgesthe financial assistance of AIEJ, Japan, during her visit to ToyohashiUniversity of Technology, Toyohashi, Japan. The authors also wishto acknowledge the financial support of DRDO, Government of In-dia, India, under the Project No. ERIP/ER/0103325/M/01.

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