Epitaxial growth of Sc-doped ZnO films on Si by sol–gel route

Ruchika Sharma a, Kiran Sehrawat b, Akihiro Wakahara c, R.M. Mehra a,*a Department of Electronic Science, University of Delhi South Campus, New Delhi 110021, Indiab Department of Physics, Maitreyi College, Chanakyapuri, New Delhi 110021, Indiac Department of Electrical and Electronic Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441-8580, Japan

Applied Surface Science 255 (2009) 5781–5788

A R T I C L E I N F O

Article history:

Received 1 October 2008

Received in revised form 24 December 2008

Accepted 4 January 2009

Available online 10 January 2009

PACS:

�81.20.Fw

61.05.cp

61.05.jh

68.37.Hk

68.37.Ps

68.55.A

Keywords:

Zinc oxide

Scandium

Sol–gel

Annealing

Morphology

Photoluminescence

A B S T R A C T

The epitaxial growth of doped ZnO films is of great technological importance. Present paper reports a

detailed investigation of Sc-doped ZnO films grown on (1 0 0) silicon p-type substrates. The films were

deposited by sol–gel technique using zinc acetate dihydrate as precursor, 2-methoxyethanol as solvent

and monoethanolamine (MEA) as a stabilizer. Scandium was introduced as dopant in the solution by

taking 0.5 wt%1 of scandium nitrate hexahydrate. The effect of annealing on structural and

photoluminescence properties of nano-textured Sc-doped films was investigated in the temperature

range of 300–550 8C. Structural investigations were carried out using X-ray diffraction, scanning electron

microscopy and atomic force microscopy. X-ray diffraction study revealed that highly c-axis oriented

films with full-width half maximum of 0.218 are obtained at an annealing temperature of 400 8C. The

SEM images of ZnO:Sc films have revealed that coalescence of ZnO grains occurs due to annealing.

Ostwald ripening was found to be the dominant mass transport mechanism in the coalescence process. A

surface roughness of 4.7 nm and packing density of 0.93 were observed for the films annealed at 400 8C.

Room temperature photoluminescence (PL) measurements of ZnO:Sc films annealed at 400 8C showed

ultraviolet peak at about (382 nm) with FWHM of 141 meV, which are comparable to those found in

high-quality ZnO films. The films annealed below or above 400 8C exhibited green emission as well. The

presence of green emission has been correlated with the structural changes due to annealing. Reflection

high energy electron diffraction pattern confirmed the nearly epitaxial growth of the films.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

ZnO, an II–VI compound n-type semiconductor with hexagonalwurtzite structure, has a direct band gap of 3.37 eV [1] and largeexciton binding energy of 60 meV at room temperature [2].Epitaxially grown ZnO films have been considered as potentialcandidates for blue and ultraviolet (UV) optical devices, such aslight-emitting diodes (LEDs) and laser diodes (LDs) [3]. Epitaxialgrowth of ZnO has been achieved on a variety of substrates such assilicon, sapphire and SiC using several deposition techniques [4–6].Tiwari et al. [7] attempted the direct epitaxial growth of ZnO on Sisubstrate by pulsed laser deposition that resulted in the formationof an amorphous silica layer, during the nucleation stage of ZnOgrowth process. Introduction of buffer layer of MgO [8] or GaN [9]on Si surface overcomes this problem of oxidation of Si substrate.

* Corresponding author. Tel.: +91 11 24115849; fax: +91 11 24110876.

E-mail address: rammehra2003@yahoo.com (R.M. Mehra).1 Ruchika Sharma, P. K. Shishodia, A. Wakahara and R. M. Mehra, Materials

Science-Poland 27 (2009) Ist issue.

0169-4332/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2009.01.004

Kumar et al. [10] prepared high quality epitaxial ZnO:Al on Sisubstrate by r.f. sputtering and PLD using g-Al2O3 buffer layer, butthe high cost of these techniques limit their use in practicalapplications.

In this work, we attempted to grow epitaxial layer of ZnO on Siwithout buffer layer. We used scandium as dopant in ZnO as ionicradius of Sc is very close to that of ZnO, which might preventdiffusion of oxygen into silicon with improved lattice matching.Minami et al. [11] investigated electrical and optical properties ofSc-doped ZnO films prepared by d.c. magnetron sputteringtechnique. In our experiment, ZnO:Sc films were deposited onp-Si (1 0 0) substrate using low cost and non-vacuum sol–geltechnique [12,13]. In sol–gel derived films, thermal annealingpromotes nucleation resulting in the improvement of structuraland optical performance [14,15] of ZnO films. A study of coarseningprocess in sol–gel deposited ZnO films is important in under-standing the processes involved in improving film quality anddopant activation. Generally, Ostwald ripening is one of thecoalescence phenomena that are always associated with thermalannealing [16]. X-ray diffraction (XRD), scanning electron micro-scopy (SEM) and atomic force microscopy (AFM) were used to

R. Sharma et al. / Applied Surface Science 255 (2009) 5781–57885782

probe the crystalline structure and surface morphology. Therefractive index of ZnO:Sc films has been determined byellipsometry in order to estimate the packing density of ZnO:Scfilms. The PL studies were carried out to ascertain the quality of thefilms. The epitaxial growth has been confirmed by reflection highenergy electron diffraction (RHEED) patterns.

2. Experimental

Sc-doped ZnO films on p-Si (1 0 0) substrates were deposited bysol–gel spin coating technique. Precursor solution of ZnO wasprepared by dissolving zinc acetate dihydrate [(Zn(CH3COO)2�2H2O), purity 99.95%], (GR, Hayashi Pure Chemical Ind. Ltd., Japan),into anhydrous 2-methoxyethanol (AR, Ajax Chemicals, Australia)and monoethanolamine (CP, Bio-Lab, London). The solution contain-ing MEA/Zn (molar ratio of 0.2 M) was stirred for 5 min. A decidedamount (0.5 wt%) of scandium nitrate hexahydrate [(ScNO3�6H2O),purity 99.9%] was introduced as dopant (Sc concentration has beenoptimized in our earlier work [17]) and the mixture wasmagnetically stirred at room temperature for 1 h. The resultantclear, transparent and homogenous solution was kept for gelationfor 48 h. The Si (1 0 0) substrates were cleaned in a boiling mixture ofNH4OH:H2O2:H2O in ratio of 1:1:6 for approximately 10 min. Afterboiling, the substrates were rinsed in overflowing deionized waterfor 10 min and then treated with 2% HF solution to remove the nativeoxide. They were further washed with deionized water for 20 minand dried in nitrogen atmosphere.

Film deposition was carried out in air with spinning speed of2700 rpm for 20 s using the spin coating process. The thermaldecomposition temperature of zinc acetate is 240 8C [18]. Apreheat treatment temperature of 280 8C is sufficient for thecomplete evaporation of organic components and to initiate theprocess of formation and crystallization of the ZnO film. The wetfilms were kept to hydrolyze in air at room temperature for 5 minand then heated for 20 min at 280 8C. In the present work,thickness of the films was in the range of 450–500 nm. The sampleswere annealed in air from 300 to 550 8C for 1 h in steps of 50 8C.Crystallite phase and orientation were evaluated by the X-raydiffraction method (Philips PW 1830 Geiger counter diffract-ometer, PW 1830) using a monochromatized X-ray beam withnickel-filtered CuKa = 1.5418 A). A continuous scan mode wasused to collect 2u data from 308 to 408, with a 0.02 sample pitch and48/min scan rate. Microstructure of the films was investigatedusing the scanning electron microscope (JEOL JSM-6300). Averagesurface roughness of the films was obtained by atomic forcemicroscopy (Burleigh-SPI 3700) with scanned area of5 mm � 5 mm. PL measurements on the films were carried outusing LS55 Luminescence spectrum PerkinElmer using a 325 nmbeam of HeCd laser at room temperature. The nature of epitaxialgrowth and quality of the films were investigated throughreflection high energy electron diffraction.

3. Results and discussion

3.1. Structural properties

The microstructural properties of ZnO:Sc films with Sc 0.5 wt%deposited on p-Si (1 0 0) substrate were investigated by XRD, SEMand AFM measurements.

3.1.1. XRD analysis

Fig. 1a shows XRD patterns for ZnO:Sc films grown on p-Si(1 0 0) substrates, annealed at different temperatures (300–550 8C). Films exhibit (0 0 2) preferential orientation with c-axisand peak intensity increases remarkably with increase in anneal-ing temperature up to 400 8C. This increase in the (0 0 2) peak

intensity may be attributed to greater energy provided by higherannealing temperature to atoms, thus enhancing their mobility.This also removes the defects in the ZnO:Sc films improving theirquality. However, beyond 400 8C, the peak intensity decreased,that might be due to enhanced porosity of the films.

Fig. 1b shows the variation of FWHM as a function of annealingtemperature. The FWHM decreases from 0.368 to 0.218 with theincrease in annealing temperature from 300 to 400 8C, indicatingan improvement in the crystallinity of the films. However, attemperatures (>400 8C), FWHM increases and a decrease inintensity of (0 0 2) peak is observed, showing a degradation ofthe film quality. Moreover, the decrease in FWHM with annealingtemperature implies an increase in crystallite size. Assuming ahomogeneous strain across the films, the crystallite size wasestimated by Sherrer’s equation [19]:

D ¼ 0:9lBcos u

where l, u and B are the X-ray wavelength, Bragg’s diffraction angleand FWHM of the ZnO:Sc (0 0 2) diffraction peak, respectively. Thecrystallite size increases gradually from �21 to 41 nm withincrease in annealing temperature from 300 to 400 8C. However, at400 8C, it appears that small crystallites coalesce together to formlarger crystallites, a process termed as coalescence. Coalescencecauses major grain growth resulting in less surface roughness,which is confirmed directly by our corresponding AFM images(detailed AFM studies in Section 3.2.2).

Fig. 1c shows the variation of 2u and lattice constant ‘c’ as afunction of annealing temperature. For the films annealed at 300and 350 8C, the value of diffraction angle 2u is less than the powdervalue (34.448) indicating that the films are in state of stress withtensile component parallel to c-axis. At 400 8C, the 2u approachespowder value indicating reduction in the tensile stress. The latticeconstant ‘c’ decreases with increase in annealing temperature andapproaches the bulk ZnO value (co = 0.5205 nm) at annealingtemperature of 400 8C. In order to understand the effect of theannealing temperature on stress (s) in ZnO:Sc films, it wasestimated using the formula [22]:

s ¼ �453:6� 109 c � co

co

� �

where co is the strain free lattice constant and c is the latticeconstant [23]. Fig. 1d shows the variation of stress (s) as a functionof annealing temperature. The film annealed at 300 8C has a tensilestress of about �1.72 GPa (the negative sign indicates that thefilms are in a state of tensile stress). At 400 8C, tensile stressreduces to zero. With further increase in annealing temperature,the stress becomes positive, which indicates that the tensile stresshas changed to pressure stress [20,21].

3.2. Surface morphology

3.2.1. Scanning electron micrographs

The SEM images of ZnO:Sc films as grown and annealed at 300–550 8C are presented in Fig. 2a–g. It is seen from Fig. 2a–c that thedensities of grains increase with increase in the annealingtemperature up to 350 8C. Ostwald ripening is a thermal energydependent process and hence, it became significant as thetemperature increased, especially for the film annealed at 300–350 8C (Fig. 2b and c). After annealing at 400 8C (Fig. 2d), grains of220–250 nm size are seen closely bound to each other and roughsintered surfaces are formed. As the annealing temperature isfurther increased to 450 8C (Fig. 2e), the small grains disappear anda dominant sintered surface with clear grain boundaries appearsdue to coalescence. The sintered grains with polyhedral shapes areclosely bound to each other, although a considerable number of

Fig. 1. (a) XRD patterns for ZnO:Sc (Sc:0.5 wt%) films annealed at different temperatures (300–550 8C) grown on p-Si (1 0 0) substrates. (b) Variation of FWHM and crystallite

size as a function of annealing temperature (300–550 8C). (c) Variation of 2u and lattice constant ‘c’ as a function of annealing temperature (300–550 8C). (d) Variation of stress

as a function of annealing temperature (300–550 8C).

R. Sharma et al. / Applied Surface Science 255 (2009) 5781–5788 5783

pores smaller than 100 nm in size are formed in the sintered solid,when the film is annealed at 500 8C (Fig. 2f). As the temperaturewas further increased to 550 8C (Fig. 2g), striking crystal growth isobserved due to cluster migration and the grain size reaches above200 nm.

3.2.2. Atomic force microscopy

The AFM images, given in Fig. 3a–f show the influence ofannealing temperature (300–550 8C) on the morphology ofZnO:Sc (Sc:0.5 wt%) films. The images indicate that the heightand size of grains are building up as the annealing temperatureincreases from 300 to 400 8C. Although the annealing tempera-ture does not have markable influence on the grain size up to350 8C, as is evident from Fig. 3a and b, a process of coalescenceresulting in major grain growth is clearly observed at 400 8C(Fig. 3c). It may be noted that the tips of grains in sample annealedat 400 8C are too sharp to be resolved by AFM. Therefore, the AFMimages do not reveal the exact morphology, as compared to theSEM images.

The average roughness of the surface of ZnO:Sc films as afunction of annealing temperature is depicted in Fig. 4. It isobserved that with increase in annealing temperature, the surfaceroughness decreases dramatically (from 28.8 nm to 4.7 nm) up to400 8C. AFM analysis of ZnO:Sc films annealed at differenttemperatures shows that the particle size in the film treatedunder 300 8C is 90–100 nm, and those in the films treated at 400and 500 8C are 24 and 160 nm, respectively. The particle size inZnO:Sc films analyzed by AFM and SEM are much greater than thatestimated from XRD as summarized in Table 1. This suggests that

AFM gives the grain size (may be few crystallites joined together),while the XRD gives the crystallite size.

3.2.3. Nucleation and growth process

The nucleation and growth process of thin films involve theprocess of nuclei formation, growth, and coalescence. At a lowerannealing temperature, the formation of small grains is kineticallyfavored as illustrated in Fig. 2b and c. The formation of small grainsreduces the super saturation and hence, the possibility of thecoalescence. During annealing at higher temperature, the grainsgain sufficient thermal energy to initiate the coalescence (Fig. 2d).Several mass transport mechanisms have been proposed to explainthe coalescence phenomena, namely, Ostwald ripening, sinteringand cluster migration [24]. In the Ostwald ripening mechanism,the larger particles grow or ‘‘ripen’’ at the expense of the smallerparticles. The size of the smaller particles will shrink or they evendisappear due to the net atomic transport to larger particles. Thelast mechanism ‘cluster migration’ occurs as a result of collisionsbetween separate particles as they move in random motion underexcitation [25,26]. Hence, the coalescence process will bedominated by Ostwald ripening and sintering mechanism. In thetemperature range 300–350 8C, we did not observe the sinteringmechanism as most of the islands appeared independently of oneanother as shown in Fig. 2b and c. However, as the temperaturewas further increased to 400–450 8C, necks connecting the islandsare observed due to sintering process as seen in Fig. 2d and e. Thegrains become quite distinct again at an annealing temperature of500 8C (Fig. 2f). It is believed that at this temperature, theevaporation rate becomes significant due to the fact that (1) small

Fig. 2. SEM images of ZnO:Sc films with different annealing temperature (a) as grown, (b) 300 8C, (c) 350 8C, (d) 400 8C, (e) 450 8C, (f) 500 8C and (g) 550 8C.

R. Sharma et al. / Applied Surface Science 255 (2009) 5781–57885784

Fig. 3. AFM topography of ZnO:Sc films with different annealing temperature (a) 300 8C, (b) 350 8C, (c) 400 8C, (d) 450 8C, (e) 500 8C and (f) 550 8C.

R. Sharma et al. / Applied Surface Science 255 (2009) 5781–5788 5785

sized material has a much lower melting point than its bulkcounterpart [27,28] and (2) the melting temperature of ZnOx isgenerally much lower than that of the ZnO bulk (1975 8C) [29]. As aresult, during thermal annealing, the evaporation process mighthappen simultaneously with the coalescence process.

The packing density (p) of ZnO:Sc films deposited by sol–geltechnique can be expressed as

p ¼ 5

3

n2f � 1

n2f þ 1

" #

Fig. 4. Average roughness of the surface of ZnO:Sc films as a function of annealing

temperature (300–550 8C).

Table 1Estimated grain size of ZnO:Sc films from XRD, SEM and AFM.

Annealed

temperature

(8C)

Grain size (nm) in

crystallographic

plane (0 0 2)

from XRD

Grain size (nm)

estimated from

SEM topographs

Grain size (nm)

estimated from

AFM topographs

300 29.11 30.21 96.18

350 30.38 160.71 162.34

400 41.29 240.84 236.19

450 37.72 200.63 189.36

500 36.61 170.51 154.32

550 38.40 220.12 196.37

Fig. 6. (a) Variation of PL spectra of ZnO:Sc films at different annealing temperature

(300–550 8C). (b) Variation of FWHM and UV emission peak position shifting as a

function of annealing temperature (300–550 8C).

R. Sharma et al. / Applied Surface Science 255 (2009) 5781–57885786

where nf is the refractive index of ZnO:Sc films. Fig. 5 shows thevariation of refractive index (at l = 632.8 nm) and the packingdensities as a function of annealing temperature. The refractiveindex of these films increase from 1.82 to 1.95 as the annealingtemperature is increased from 300 to 400 8C. Above 400 8C, therefractive index starts decreasing. The refractive index may be agood parameter to detect the packing density. From Fig. 5, it is seenthat the packing density increases with increasing annealingtemperature up to 400 8C to 0.97, but then decreases rapidly (from0.972 at 450 8C to 0.906 at 550 8C). Therefore, ZnO:Sc films canobtain high packing densities in the temperature range from 300 to400 8C. This behavior is also confirmed by comparing Fig. 5 withFig. 1 by looking at (0 0 2) peak intensity and therefore, the

Fig. 5. Variation of refractive index (at l = 632.8 nm) and packing densities as a

function of annealing temperature (300–550 8C).

increase in packing density may be attributed to the structuralchanges discussed above.

3.3. Optical properties

3.3.1. Photoluminescence spectra

The PL of the ZnO:Sc thin films carried out at room temperature, asa function of different annealing temperatures (300–550 8C) exhibitstwo emitting bands, namely, the near band edge emission (NBE)centering around 380–386 nm and deep-level emission (DLE)centering around 510–540 nm as shown in Fig. 6a. The NBE inultraviolet range is attributed to the excitonic emission, due to thefact that ZnO has a large excitonic binding energy of 60 meV. Theintensity of NBE increased rapidly with increase in annealingtemperature up to 400 8C and then showed a sharp decrease at450 8C. It has been reported that the DLE is probably related to thevariation of the intrinsic defects in ZnO films [30–32]. There are fivekinds of intrinsic defects in ZnO film, such as zinc vacancy VZn, oxygenvacancy VO, interstitial zinc Zni, interstitial oxygen Oi, and antisiteoxygen OZn. Sun had calculated [33] the energy levels of the intrinsicdefects in ZnO by applying the full-potential linear muffin–tin orbitalmethod. The concentration of the OZn in ZnO:Sc films depends on theannealing temperature. On one hand, metallic Zn cannot be oxidizedcompletely at low annealing temperature (300–350 8C), which

R. Sharma et al. / Applied Surface Science 255 (2009) 5781–5788 5787

results in the increase in interstitial zinc Zni; and on another, too highannealing temperature (500–550 8C) will cause the inter diffusionbetween ZnO and Si substrate; hence, the formation of SiO2, thatresults in the increase in oxygen vacancies VO in the films. The PLemission at different annealing temperature could be explained as:(a) the increase of annealing temperature (300–400 8C) results inimprovement in crystallinity of the film due to the increase of grainsize and crystal orientation, which should reduce the green emissionand increase the UV emission. (b) For the films annealed at lowertemperatures (300–350 8C), the chemical composition was nonstoichiometric, and they usually consisted of excess Zn atoms [34].Therefore, many lattice defects and surface defects exist in lowtemperature annealed ZnO:Sc films. These defects produce variousnon-radiative centers which reduce light emission from the films. Onannealing at highertemperature, thesenon-radiative defectsmaygetre-structured. (c) Annealing at optimized temperature (400 8C)would also reduce the zinc interstitials. Consequently, the visiblelight emission associated with structural defects is suppressed. Suchre-structuring probably made the crystal structure more perfect. (d)Further, annealing films at 550 8C greatly degraded the film (as is alsoconfirmed from XRD), which results in a reduced band edge emissionand increased green emission due to the increase in oxygenvacancies. The degradation of UV luminescence has been attributedto an excessively oxidized layer formation on the surface of ZnO:Scthin film during annealing at high temperature. Therefore, too high(500–550 8C) and low annealing temperatures (300–350 8C) can allincrease the concentration of the OZn in the films, resulting in highintensity of broad DLE as can been seen from Fig. 6a.

Fig. 7. (a) RHEED pattern of ZnO:Sc films on p-Si (1 0 0) substrate annealed at 300 8C. (b) R

pattern of ZnO:Sc films on p-Si (1 0 0) substrate annealed at 500 8C.

Fig. 6b shows that FWHM of NBE is narrowed with the increaseof annealing temperature as a result of improvement in crystal-linity. A 141 meV FWHM was achieved for the film annealed at400 8C, compared to a FWHM of 380 meV for the film annealed at300 8C. This result is in conformation with the results of XRDanalysis (Fig. 1a). Film annealed at (300 and 350 8C) lowertemperature experiences a tensile stress as can be seen fromXRD analysis (Fig. 1c and d), resulting in increase of the band gapand hence, the energy of UV emission shifted from 3.23 to 3.26 eV.Lin et al. [29] observed the green emission centering at 2.38 eVfrom the ZnO film deposited on silicon substrate and suggestedthat green emission should correspond to the electron transitionfrom the bottom of the conduction band to the antisite defect OZn

level. In our films also, the green emission centering at 2.386 and2.318 eV is observed that is similar to Lin et al. results. Vanheusdenet al. [31] believed that the ionized vacancies were responsible forthe green emission. Also, the FWHM is the narrowest at 400 8C(141 meV), that is better than 187 meV reported by PLD [35],which can be correlated well with the structural properties of thefilms. Therefore, both the intensity and FWHM of the UV PL spectraof ZnO:Sc film are found to depend strongly on the microcrystallinestructure. Since films with marked crystallization show stronggreen PL, we assume that the green PL centers are located in theclosely packed crystals of films annealed at 500–550 8C. Such acompact structure is markedly different from the morphology ofthe film prepared using the spray pyrolysis by Studenikin et al.which showed a porous structure throughout the film [36]. Theyhave concluded that the film with maximum porosity has

HEED pattern of ZnO:Sc films on p-Si (1 0 0) substrate annealed at 400 8C. (c) RHEED

R. Sharma et al. / Applied Surface Science 255 (2009) 5781–57885788

maximum green PL, because the reducing gas is able to createoxygen vacancies, i.e., green PL centers in a layer immediatelybelow the crystallite surface. On the other hand, in our case,thermal annealing may cause the dissipation of oxygen atoms,accompanied by the formation of closely packed crystals,producing green PL centers throughout. We, therefore, believethat this method is advantageous for the preparation of a thinphosphor film in low-voltage FEDs, since a compact structure giveslower electrical resistance as compared to a porous structure.

3.4. Reflection high energy electron diffraction

Fig. 7a–c shows the RHEED pattern of ZnO:Sc films on p-Si (1 0 0)substrate annealed at 300, 400 and 500 8C, respectively. Fig. 7aindicates circular ring RHEED pattern with some dots, may be due tothe disordered surface of the films as predicted from SEM images(Fig. 2a). At such a low temperature around 300 8C, the adatommobility is thought to be still insufficient to achieve a two-dimensional growth mode. As a result, we also obtained very smallXRD reflexes for these films (Fig. 1). However, the RHEED pattern of400 8C annealed film shows changing feature relatively closer to astreaky one, which means thermal treatment definitely contributesto the rearrangement of atoms into a smoother surface [37]. Theseresults indicate that ZnO:Sc films with two-dimensional growthmode can be attained only with appropriate annealing temperaturewithout using buffer layers (of MgO or GaN), which were suggestedto decrease unfavorable effects of the large lattice mismatchbetween silicon substrate and ZnO films [8,9]. On the other hand,when the annealing temperature was further increased up to 500 8C,the RHEED patterns turned into a mixed circular ring and spottyshape again as shown in Fig. 7c. It can be argued that the energy ofadatom at 400 8C may be sufficient to initiate surface diffusion forthe two-dimensional growth, but the comparatively high annealingtemperature might enhance the grain growth and as a consequence,the growth of ZnO:Sc thin film was changed into three-dimensionalmode. At a slight higher temperature 500 8C, increase of surfaceroughness due to three-dimensional growths directly reflects on theformation of mixed feature in RHEED pattern (Fig. 4). As wasmentioned already, the annealing temperature of 400 8C providedthe smoothest surface with RMS of 4.7 nm over the scanned size of5 mm� 5 mm for the films.

4. Conclusion

Epitaxial ZnO:Sc films has been grown on p-Si (1 0 0) substratesby inexpensive sol–gel technique without using buffer layer.Oswald ripening and sintering have been found to be the maintransport mechanisms responsible for the changes in surfacemorphology due to annealing.

Acknowledgements

One of the authors Ruchika Sharma gratefully acknowledges thefinancial assistance of AIEJ, Japan, during her visit to ToyohashiUniversity of Technology, Toyohashi, Japan. The authors also wishto acknowledge the financial support of DRDO, Government ofIndia, India.

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