Thin Solid Films 517 (2008) 661–669

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Substrate temperature dependence of growth mode, microstructure and opticalproperties of highly oriented zinc oxide films deposited by reactive sputtering

Sukhvinder Singh a, Tapas Ganguli b, Ravi Kumar b, R.S. Srinivasa c, S.S. Major a,⁎a Department of Physics, Indian Institute of Technology Bombay, Mumbai-400076, Indiab Semiconductor Laser Section, RRCAT, Indore-452013, Indiac Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, Mumbai-400076, India

⁎ Corresponding author. Tel.: +91 22 25767567; fax: +E-mail address: syed@phy.iitb.ac.in (S.S. Major).

0040-6090/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.tsf.2008.07.040

a b s t r a c t

a r t i c l e i n f o

Article history:

Polycrystalline ZnO films w Received 26 December 2007Received in revised form 21 July 2008Accepted 24 July 2008Available online 3 August 2008

Keywords:ZnOReactive sputteringMicrostructureOptical propertiesX-ray diffractionTransmission electron microscopy

ere deposited on quartz substrates by reactive sputtering of zinc target. X-raypowder diffraction, pole figure analysis and high resolution measurements along with transmission electronmicroscopy, Raman and photoluminescence studies were carried out to study the microstructure,crystallinity and optically active defects in the films. All the films deposited in the substrate temperaturerange from room temperature to 600 °C exhibited strong c-axis preferred orientation. The changes inpreferred orientation of crystallites with substrate temperature were attributed to its being determined bypreferential nucleation at lower temperatures and surface diffusion at higher temperatures. A detailedmicrostructural analysis showed that with increase in substrate temperature from 300 °C to 600 °C, asignificant reduction in micro-strain to ∼10−3 takes place, along with a marginal increase in crystallite size.Raman and photoluminescence studies have shown that the films deposited below 300 °C possessed poorcrystalline quality. The film deposited at 600 °C yielded the most intense and narrow (∼102 meV) band edgeluminescence at room temperature, though it did not exhibit the strongest c-axis orientation of crystallites.This is attributed to its superior crystalline quality and absence of oxygen-deficiency related defects.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

ZnO is a wide band gap semiconductor with large exciton bindingenergy (60 meV), making it one of the most potential materials torealize the next generation optoelectronic devices, operating in shortwavelength region [1–3]. Several growth techniques, such as,sputtering, pulsed laser deposition (PLD), metal-organic chemicalvapour deposition (MOCVD), Sol–gel process and spray pyrolysis havebeen used for the deposition of polycrystalline ZnO films. In recentyears, high quality epitaxial ZnO films have also been grown oncrystalline substrates. Most of this work has been reviewed in Refs.[1,2]. As a deposition technique, sputtering offers several advantagesdue to its simplicity, versatility and scalability, and has beenextensively used for the deposition of ZnO films [4]. Epitaxial ZnOfilms [5–9] as well as high quality polycrystalline ZnO films on non-crystalline substrates [10–12] have been deposited by various forms ofsputtering.

In a recent work [13] on reactively sputtered ZnO films on quartzsubstrates, it has been shown that all the films deposited in thesubstrate temperature range of room temperature to 600 °C exhibited

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a single low order XRD peak due to (0002) reflection, except for thefilm deposited ∼200 °C, which exhibited multiple peaks with adominant (0002) reflection. Thus all the ZnO films were c-axispreferentially oriented. However, the intensity variation of (0002)peak of the films deposited at different substrate temperaturesshowed an interesting variation. The film deposited at 300 °C showedthe strongest (0002) peak. In contrast, all the films deposited at lowertemperatures showed much less intense (0002) peaks (by nearly twoorders), while those deposited at higher temperatures showed a smalland steady decrease in the intensity of (0002) peak with increase ofsubstrate temperature. The films deposited below 300 °C possesseduniform strain (N10−3), which became negligible in the filmsdeposited above 300 °C. Most interestingly, it was found that theZnO film deposited at 600 °C exhibited the sharpest absorption edgeand a strong and narrow (∼100 meV) room temperature band edgephotoluminescence (PL), though it exhibited a lesser degree of c-axispreferred orientation than the film deposited at 300 °C.

In order to understand the above mentioned features, an extensivemicrostructural investigation of ZnO films deposited on quartzsubstrates at different temperatures has been undertaken, using X-ray pole figure analysis, high resolution X-ray diffraction (HRXRD) andtransmission electron microscopy (TEM). Raman spectroscopy andphotoluminescence measurements were used to obtain additionalinformation on microstructure, crystallinity and defects in the films. A

Fig. 1. XRD patterns of ZnO films deposited at different substrate temperatures (asindicated in the figure). All the films were deposited at rf power of 400 W with 20%oxygen in the sputtering atmosphere.

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comprehensive attempt has thus been made to establish the role ofmicrostructural parameters in determining the optical quality ofpolycrystalline ZnO films deposited by reactive sputtering of a zinctarget.

2. Experimental details

ZnO films were deposited on quartz substrates by reactive rfmagnetron sputtering using argon–oxygen mixture. A 99.9% pure Zntarget of 3-in. diameter was used. The target to substrate distance was55 mm. The base pressure was 1×10−3 Pa. The flow rates of argon (24SCCM) and oxygen (6 SCCM)were controlled bymass flow controllers.Deposition was carried out at a working pressure of ∼1 Pa, after pre-sputtering with argon for 10 min. The sputtering power wasmaintained at 400 W during deposition. The depositions were carriedout in the temperature range of room temperature to 600 °C.

X-ray diffraction (XRD) studies were performed with a PANalyticalX'Pert PRO powder diffractometer using Cu Kα radiation. X-ray polefigures were measured using a PANalytical MRD four-circle diffract-ometer using CuKα radiation. HRXRDmeasurements were carried outin ω and ω–2θ scan geometries using PANalytical X'Pert MRD system.The incident beam optics had a 4-bounce hybrid monochromator,which ensured CuKα1 (1.54056A°) output collimated to about 20 arcsec in the plane of scattering. A 1/2° slit was placed at the outputbefore the detector. Transmission electron microscopy (TEM) studieswere carried out using a PHILIPS Model CM200 SupertwinMicroscopeoperated at 200 kV. Raman spectra of the films were recorded at roomtemperature in back scattering geometry, using JOBIN YVON HORIBAHR-800 confocal micro-Raman spectrometer equipped with a 20 mWAr+ laser (514.5 nm). Photoluminescence (PL) measurements werecarried out at room temperature using a 325 nm He–Cd laser and aJOBIN YVON HR-460 monochromator. Ambios XP-2 surface profil-ometer was used for thickness measurements.

3. Results and discussion

3.1. Microstructural studies

All the studies were carried out on films of approximately thesame thickness (650±50 nm). The deposition rate was nearly thesame (∼1 μm/h) for all the films. Typical XRD patterns of the filmsdeposited at 100 °C, 150 °C, 200 °C, 250 °C, 300 °C and 600 °C areshown in Fig. 1. As reported earlier [13], the films deposited upto100 °C showed a single loworder peak due to (0002) reflection, henceonly a typical case is shown in Fig. 1. No XRD peaks corresponding toreflections other than (0002) and (0004) planes of hexagonal ZnOwere observed in these cases. However, all the films deposited in therange of 150–250 °C, clearly showed the presence of low intensitypeaks due to (101̄0) and (101̄1) reflections, with a dominant (0002)reflection. The film deposited at 200 °C showed relatively higherintensity of (101̄0) and (101̄1) peaks. In contrast, the films depositedat higher substrate temperatures again showed only (0002) peakwith much higher intensity, though the intensity decreased mono-tonically with increase of substrate temperature from 300 °C to600 °C. Typical cases of the films deposited at 300 °C and 600 °C areshown in Fig. 1. Thus all the ZnO films exhibit a strong c-axispreferential orientation of crystallites, though the extent of preferredorientation appears to be the highest in the filmdeposited at 300 °C. Itis also seen from Fig. 1 that the position of (0002) peak shiftsmonotonically towards the bulk value with increase of substratetemperature and the uniform strain in the films deposited above300 °C is practically negligible, as reported earlier [13].

Fig. 2 shows the pole figures of (0002) reflection for ZnO filmsdeposited on quartz substrates at different temperatures. All the filmsshow a very pronounced (0002) texture, with rotational symmetry.The films deposited at room temperature and 100 °C show broad pole

figures, which become sharper with initial increase in substratetemperature. The films deposited at 300 °C and 400 °C thus show verysharp and high intensity pole figures, confirming the presence of astrong and nearly complete c-axis orientation of crystallites. Interest-ingly, at higher substrate temperatures of 500 °C and 600 °C, the polefigures again become broader, indicating enhanced mis-orientation of(0002) planes of crystallites with respect to the film surface. Thisexplains the steady decrease in the intensity of (0002) XRD peak withincrease in substrate temperatures above 300 °C, as mentioned aboveand reported earlier [13].

TEM studies were carried out on typical ZnO films, which werelifted-off from quartz substrates. The film on quartz substrate wasimmersed in dilute HF for few secs, which resulted in etching at thefilm-substrate interface and thinning of the film. Subsequently, thefilm on substrate was carefully transferred to a bath of de-ionizedwater, which resulted in peeling-off of the film and floating on watersurface. The film was transferred onto a 200 mesh carbon coated

Fig. 2. (0002) peak pole figures of ZnO films deposited at different substrate temperatures (as indicated in the figure).

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copper grid for TEM studies. Both bright and dark field images wereexamined in all the cases. Fig. 3 presents typical TEM results in theform of bright and dark field images, along with the correspondingdiffraction patterns and crystallite size distributions for the filmsdeposited at room temperature, 300 °C and 600 °C. Electrondiffraction data obtained for all the films confirmed the presence ofpolycrystalline hexagonal ZnO. The lattice constants (‘a’ and ‘c’)obtained from the diffraction patterns are listed in Table 1 for typicalcases. The value of ‘c’, decreases with increasing substrate tempera-ture and becomes close to the bulk value for the films deposited at300 °C or higher temperatures. These observations are in agreementwith the above described XRD results. The lattice constant ‘c’ however,remains close to the bulk value, irrespective of the substratetemperature. The average lateral crystallite size of the films wasobtained from dark field TEM images. The values are listed in Table 1.The results are in good agreement with the corresponding bright fieldimages. The crystallite size of the film deposited at room temperatureis (21±14) nm, which increases slightly to (25±20) nm for the film

deposited at 300 °C. However, for the film deposited at 600 °C, thecrystallite size increases substantially to (43±30) nm.

The above described microstructural features of ZnO films,especially the pattern of change in preferred orientation of crystalliteswith substrate temperature is unusual and interesting. The unusualreduction in the extent of c-axis preferred orientation in sputteredZnO films at intermediate temperatures and its subsequent enhance-ment at higher temperatures has been reported earlier [14], thoughwithout much explanation. There are also some reports on ZnO filmsdeposited by sputtering [10,15] and MOCVD [16], showing a strong c-axis preferred orientation near room temperature, followed by itsreduction with increase in substrate temperature to the range of 150–300 °C. In a recent review, Kajikawa [17] has extensively analyzed theissue of preferred orientation in sputtered ZnO films by compiling therelevant experimental observations and explanations put forth for thepreferred orientation behaviour. A comprehensive understanding ofthe complex dependence of c-axis preferred orientation of sputteredZnO films on processes such as, preferential nucleation, preferential

Fig. 3. Bright field and dark field TEM images of ZnO films deposited at differentsubstrate temperatures (as indicated in the figure). The diffraction pattern and theanalysis of the crystallite sizes are shown as insets.

Table 1Characteristic parameters of ZnO films deposited at room temperature, 300 °C and600 °C on quartz substrates

Substratetemperature

From TEM From HRXRD

Lateralcrystallitesize (nm)

‘c’ (Å)(±0.01)

‘a’ (Å)(±0.01)

Tilt(degree)

Verticalcoherencelength (nm)

Micro-strain

Room temp. 21±14 5.25 3.26 – – –

300 °C 25±20 5.21 3.25 2.6 140±15 2.0×10−3

600 °C 43±31 5.20 3.26 11.1 65±15 1.1×10−3

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crystallization, sticking, surface diffusion and grain growth appears tobe lacking. However, it is generally believed (though, this has alsobeen debated [18,19]) to originate from preferential (0002) nucleation

[17,20,21] driven by minimization of surface energy. The preferredorientation can also be significantly affected at the growth stage bytwo processes, namely, sticking and surface diffusion of ad atoms.Fujimira et al. [21] have proposed that the differences in the stickingprobabilities of growing species on different planes determinepreferred orientation. For example, in the case of sputtered ZnOfilms [22,23], the existence of Zn and O in the sputtering gas causes a-axis orientation, while the higher sticking probability of Zn–O speciescauses c-axis preferred orientation. In the present work, it has beennoticed that the preferred orientation effects show a strongdependence on substrate temperature, but are practically indepen-dent of rf power (in the range of 300–400 W) and oxygen content (inthe range of 10–30%) of the sputtering atmosphere. Hence, effects dueto sticking coefficients of growing species have been ignored, whileexplaining the preferred orientation behaviour.

The preferred orientation behaviour of sputtered ZnO films seen inthe present work can be explained in the light of the above discussion.As shown in Fig. 1, the films deposited up to 100 °C, exhibit a singleXRD peak indicating (0002) preferred orientation, though the peakintensity in these cases are nearly two orders of magnitude smaller,compared to the strongly (0002) oriented film deposited at 300 °C.Further, Fig. 2 shows that the corresponding pole figures for the filmsdeposited up to 100 °C are very broad, indicating significant mis-orientation of the crystallites. TEM studies of these films have alsoindicated that the lateral crystallite size is relatively small (∼20 nm).On the other hand, in these films, the crystallite size along the growthdirection (c-axis) is relatively larger (∼100 nm), as obtained from the(0002) peak width. The broad and low intensity pole figures and smalllateral crystallite size indicate that although these films exhibitpreferred c-axis orientation, the degree of crystallinity and overallstructural order in these films is not very high. The presence ofdisordered component in the films can also not be ruled out. This canbe explained by assuming negligible surface diffusion at lowersubstrate temperatures. Thus, it is primarily due to preferential(0002) nucleation that the c-axis oriented crystallites grow along withsome poorer crystallinity material.

As the substrate temperature is increased to the range of 150–250 °C, the XRD patterns (Fig. 1) of the corresponding films showmultiple peaks. Though in all these cases, the (0002) peak remainsdominant, but it shows a significant increase in its half width. For thefilm deposited at 200 °C, the (0002) peak shows the largest widthcorresponding to a value of ∼50 nm for the crystallite size alonggrowth direction (c-axis). This behaviour can be explained byconsidering that at these temperatures, the preferred orientation ofcrystallites is no longer determined by preferred nucleation. Instead, itis significantly affected by the growth process, controlled essentiallyby enhanced surface diffusion due to increase in substrate tempera-ture. However, at relatively low substrate temperatures, surfacediffusion may be limited to being within the grains, i.e., among planesonly. According to Kajikawa et al. [17,20], when surface diffusion takesplace among planes within a grain, higher surface energy planes maybecome preferred orientation planes, because surface diffusionessentially occurs to conceal the plane with higher surface energy.

Fig. 4. (a) ω–2θ scans for ZnO films deposited at substrate temperatures of 300 °C and600 °C for (0002), (0004) and (0006) reflections. (b) Broadening in the reciprocal spaceas obtained from the symmetric scans along ω–2θ-axis (qz) for (0002), (0004) and(0006) reflections.

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Such a preferred orientation behaviour has been recently reported[24] and discussed in detail for sputtered hexagonal GaN films. Theappearance of (101̄0) and (101̄1) reflections along with (0002)reflections in the XRD patterns of these films, indicating the presenceof differently oriented crystallites with respect to the film surface, isthus attributed to the surface diffusion among planes within thegrains. It is noteworthy that the AFM studies of these films reported inRef. [13], showed that the surface morphological features of two vastlydifferent sizes were present in the film deposited at 200 °C, unlike thefilms deposited at other substrate temperatures. It is also noticed fromFig. 2 that the pole figure for the film deposited at 200 °C is relativelysharp, which indicates that there is an increased tendency of the(0002) oriented crystallites to align with the growth direction. Thesefeatures, together with the decrease of crystallite size along thegrowth direction to ∼50 nm indicate that a constrained growth of c-axis oriented crystallites along with crystallites with other orienta-tions takes place in this temperature range.

At higher substrate temperatures (300 °C and above), thesignificantly increased surface diffusion coefficient is expected totake the growth process to the regime where surface diffusion amonggrains becomes dominant. In this temperature regime, the grains withhigher energy surface shrink and those with lower energy surfacegrow laterally to determine the nature of preferred orientation.Consequently, the lowest surface energy plane of ZnO, i.e., (0002)acquires a preferential orientation, as has been observed in thepresent case at substrate temperatures of 300 °C or higher. Thedominance of surface diffusion and tendency of lateral growth in thesefilms is also evident from the increase in lateral crystallite size (Fig. 3)and the large surface features seen in their AFM images, reportedearlier [13]. However, an interesting feature of the films deposited atand above 300 °C is the monotonic decrease in the intensity of (0002)peak (Fig. 1) and the broadening of corresponding pole figures (Fig. 2)with increase in substrate temperature. The films deposited at 300 °Cand 600 °C were analyzed by high resolution X-ray diffraction(HRXRD) studies, which have provided additional insight into thisbehaviour. The films deposited below 300 °C could not be studied,owing to poor signal intensity. The HRXRD results are discussedbelow.

A strongly oriented polycrystalline film on amorphous substratescan be considered to consist of crystallites, with certain mean verticaland lateral dimensions. For materials having hexagonal structure,usually the crystallites are oriented with their b0002N axis along thegrowth direction and are rotated randomly about this direction.Information about lateral and vertical coherence lengths (crystallitesizes), crystallite tilt andmicro-strain can be estimated from full widthat half maximum (FWHM) of the omega (ω) andω–2θ scans of (000 l)reflections, using Williamson–Hall plots [25]. The terms vertical andlateral refer respectively, to the growth direction and a directionperpendicular to it, in the plane of the film. The finite crystallite size,tilt and micro-strain cause the broadening of reciprocal space points[26]. The broadening due to finite size is invariant in reciprocal spacewith higher orders of reflection (increasing scattering vector, q), butthe broadening due to both tilt and micro-strain increase withincreasing magnitude of q [26]. The finite vertical coherence lengthand micro-strain cause the broadening, Δqz of (000l) reflections in ω–

2θ scans, which can be respectively, estimated from the intercept andslope of the linear Williamson–Hall plots of Δqz (FWHM along qzdirection) vs. q. Here, q is the magnitude of the position of (000 l)point in the reciprocal space. Similarly, the finite lateral coherencelength and tilt cause broadening, Δqx in the qx–qy plane, which can bemeasured by the spread of (000l) reflections in ω-scan. The lateralcoherence length and tilt can be estimated respectively, from theintercept and slope of the linear Williamson–Hall plots of Δqx (FWHMalong qx direction) vs. q.

ω–2θ scans of symmetric (0002), (0004) and (0006) reflections fromZnO films deposited at substrate temperatures of 300 °C and 600 °C are

shown respectively, in Fig. 4(a). For all the cases, the values of FWHM inreciprocal space (Δqz) are also indicated. As discussed above, the finitevertical crystallite size and micro-strain were calculated from William-son–Hall plots (Δqz vs. q), shown in Fig. 4(b). The values of theseparameters are also listed in Table 1. It may be noted that the verticalcrystallite sizeof thefilmdepositedat300 °C is∼140nm, as compared tothe value of ∼65 nm for the film deposited at 600 °C. The micro-strain(non-uniform strain) is found to be 1.1×10−3 in the film deposited at600 °C, which is nearly half the value of 2.0×10−3, measured in the filmdeposited at 300 °C.

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Fig. 5(a) shows the ω scans of symmetric (0002), (0004) and(0006) reflections from ZnO films deposited at 300 °C and 600 °C. Forall the cases, the values of FWHM in reciprocal space (Δqx) are alsoindicated. Theω scans of the film deposited at 600 °Cwere found to bemuch broader compared to the film deposited at 300 °C. The cor-responding Williamson–Hall plots of Δqx vs. q are shown in Fig. 5(b),for both the films. The linear fits for the two films show significantly

Fig. 5. (a)ω-scan for ZnO films deposited at substrate temperatures of 300 °C and 600 °Cfor (0002), (0004) and (0006) reflections. (b) Broadening in the reciprocal space asobtained from the symmetric scans along ω-axis (qx) for (0002), (0004) and (0006)reflections.

different slopes, fromwhich the values of tilt have been estimated andlisted in Table 1. The average tilt value of∼2.6° for the film deposited at300 °C is significantly smaller than the value of ∼11.1° for the filmdeposited at 600 °C. These results are in tune with the qualitativeinformation obtained from the corresponding pole figures (Fig. 2). Theintercepts of Δqx vs. q plots however, possess large uncertaintiesowing to the broadω curves, especially in case of the film deposited at600 °C. This implies the presence of a large error in the estimation oflateral coherence length, hence the lateral crystallite size in thesefilms has been taken as measured by TEM, and listed in Table 1.

The microstructural features of the films deposited at 300 °C and600 °C can now be compared. The average vertical (along growthdirection) and lateral crystallite sizes of the film deposited at 300°Care ∼140 nm and ∼25 nm, respectively. This film also exhibits a verysharp (0002) pole figure (Fig. 2) and correspondingly a small value(∼2.6°) of tilt. It clearly possesses the highest quality in terms of c-axiscrystallite orientation, in comparison to all the other films. As thesubstrate temperature is increased, the pole figures become broader,indicating that the mis-orientation of (0002) planes of crystalliteswith respect to the film surface increases. Consequently, the filmdeposited at 600 °C shows a much larger value of tilt (∼11.1°). Further,the average vertical and lateral crystallite sizes for this film are∼65 nm and ∼43 nm, respectively, indicating that with increase insubstrate temperature, the vertical crystallite size decreases, but thelateral crystallite size increases. It can thus be inferred that the filmdeposited at 600 °C consists of relatively equiaxed crystallites withslightly larger overall crystallite volume compared to the filmdeposited at 300 °C. These observations can be explained inaccordance with the structure-zone model, proposed originally byThornton [27]. According to this model [28], surface diffusioncontrolled columnar growth usually takes place at intermediatetemperatures. At higher substrate temperatures, lattice and grainboundary diffusion processes begin to dominate, leading to formationof equiaxed re-crystallized grains. The results presented above showthat a transition from columnar crystallites to equiaxed crystallitestakes place with increase of substrate temperature. The increase in tiltseen in the films deposited above 300 °C is thus indicative of mis-orientation amongst equiaxed crystallites, which is attributed to theabsence of columnar growth in sputtered films deposited onamorphous substrates, at relatively higher temperatures.

The HRXRD studies also show that the micro-strain present in thefilm deposited at 600 °C is nearly half of that in the film deposited at300 °C. The higher micro-strain in the film deposited at 300 °C isindicative of a larger presence of point defects in this film. It may berecalled from earlier work [13], that the uniform strain in the filmsdeposited below 300 °C was considered intrinsic to the growthprocess. The intrinsic strainwas attributed to the O− ions formed at thetarget, which may have sufficient energies to bombard the growingfilm [29], causing implantation, displacement or removal of surfaceatoms. The relatively large micro-strain present in the film depositedat 300 °C is also attributed to bombardment due to O− ions. Thesignificant decrease in micro-strain seen in the film deposited at600 °C is attributed to annealing effects, leading to reduction in pointdefects.

3.2. Raman spectroscopy studies

Fig. 6 shows the Raman spectra of ZnO films deposited at dif-ferent substrate temperatures. The spectrum of the quartz substrate isalso shown for comparison. All the ZnO films deposited in thesubstrate temperature range of room temperature to 250 °C show twovery broad peaks centered ∼400 cm−1 and ∼570 cm−1. The broad peak∼400 cm−1 is attributed to a combination of A1(TO) and E1(TO)modes (TO denotes transverse optical) of ZnO, which are reported at380 cm−1 and 407 cm−1, respectively [30,31]. This broad peak has ashoulder on the higher frequency side at ∼440 cm−1, which is assigned

Fig. 6. Raman spectra of ZnO films deposited at different substrate temperatures (asindicated in the figure), along with the spectrum of the quartz substrate.

Fig. 7. PL spectra of ZnO films deposited at different substrate temperatures (asindicated in the figure).

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to E2-high mode [30,31]. These features are rather unexpected, sincein a c-axis oriented ZnO film in backscattering geometry, only a strongE2-high mode and a weak A1(LO) mode (LO denotes longitudinaloptical) are expected to be present [1,30]. The broad peak ∼570 cm−1

is assigned to a combination of A1(LO) and E1(LO) modes, which arereported at 574 cm−1 and 583 cm−1, respectively [30,31]. Thesefeatures of Raman spectra of ZnO films are indicative of poorcrystallinity, arising from lattice strain, structural defects and disorder[32–34]. These observations, especially the presence of weak E2-highmode as a shoulder, indicate that though most of the films depositedbelow 300 °C showed [13] a single and intense (0002) peak, indicatingc-axis preferred orientation of crystallites, these films actually possesspoor crystalline quality. The features seen in the Raman spectra arethus consistent with the significantly low intensity of (0002) XRDpeaks (Fig. 1), broad pole figures (Fig. 2), small lateral crystallite size(Fig. 3) and the high uniform strain (N10−3) exhibited by these films(reported in Ref. [13]).

The film deposited at 300 °C however, shows drastically dif-ferent spectral features. An enhancement of E2-high intensity isclearly seen, though a shoulder due to TO modes and a broad banddue to LOmodes continue to exist. The increase in the prominence ofE2-high peak corroborates well with the significant improvement inc-axis orientation of crystallites, as also seen from the correspondingXRD patterns and pole figures. The films deposited at highersubstrate temperatures show a monotonic increase in the intensityand sharpness of the E2-high peak and decrease in the intensity of TOand LOmode peaks. Interestingly, the film deposited at 600 °C showsonly an intense E2-high peak along with a very weak and broad A1

(LO) mode at ∼575 cm−1. As mentioned above, such features arecharacteristic of strong c-axis orientated ZnO films [1,31]. The smallshift of LO modes towards higher frequencies and the reduction intheir intensity are attributed to decrease in point defects [31]. Thedecrease in point defects correlates well with the significantdecrease in micro-strain and the small increase in crystallite size,for the film deposited at 600 °C. It is interesting to note that suchfeatures are seen in the ZnO film deposited at 600 °C, which, as

compared to the film deposited at 300 °C, shows a much larger tilt(mis-orientation of crystallites). This result clearly shows that themis-orientation of crystallites does not significantly affect theintrinsic quality of crystallites in terms of point defects and strain.Raman results have thus shown that the mere appearance of a single(0002) XRD peak in ‘a film’, which is usually taken as indicative of astrong c-axis orientation of crystallites and hence its ‘high’ crystal-line quality, can be misleading. It is clear that additional data isrequired to make such assertions.

3.3. Photoluminescence studies

Absorption studies on ZnO films have shown [13] that all the filmsexhibited band gaps ∼3.3 eV. It was however, observed that withincrease in substrate temperature, the absorption edge becamesharper and sub-band gap absorption was significantly reduced. Itwas also shown that the ZnO film deposited at 600 °C exhibited astrong and narrow band edge photoluminescence peak. In thissection, a detailed study of the near band edge and defect levelluminescence at room temperature is presented for the ZnO filmsdeposited at different substrate temperatures.

668 S. Singh et al. / Thin Solid Films 517 (2008) 661–669

Fig. 7 shows the room temperature PL spectra of ZnO filmsdeposited at different substrate temperatures. All the films showstrong emission at ∼376 nm, attributed to near band edge lumines-cence of ZnO [35]. Weak and broad bands are also seen in visibleregion, which are known to arise from oxygen vacancies and zincinterstitials [36,37]. The intensity of band edge luminescence is foundto increase with substrate temperature, accompanied by narrowing ofthe peak, as reported earlier for sputtered ZnO films on Si [38]. Thefilms deposited at room temperature, 100 °C and 200 °C showed veryweak band edge emission and negligible luminescence in the visibleregion. This is attributed to poor crystallinity and large disorder in thefilms, as seen earlier from UV–visible absorption studies [13], whichshowed large sub-band gap absorption and is in tune with the Ramanresults. It has also been reported that ZnO films with smallercrystallites show poorer luminescence, which has been attributed tonon-radiative relaxation through surface states [38,39]. With increasein substrate temperature, the intensity of band edge emission beginsto increase together with the defect related luminescence in thevisible region. The film deposited at substrate temperature of 400 °Cthus showed comparable intensities of band edge and defectluminescence. The increase in both band edge as well as defectluminescence together, can be attributed to decrease in non-radiativerelaxation processes. With further increase in substrate temperatureto 500 °C, the intensity of band edge emission begins to increase fasterthan the defect related luminescence. The band edge luminescenceintensity however, shows a drastic increase for the film deposited at600 °C, while the defect related luminescence bands remainpractically unchanged and weak. The half width of the band edgeluminescence peak is also significantly reduced to ∼102 meV. Thesharp and intense band edge PL from the film deposited at 600 °C is inline with the microstructural and Raman studies, which indicatedreduction of point defects in this film. This is further supported by thepresence of weak defect related luminescence in the visible region. Ithas been reported [36] that the visible emission in ZnO originatesfrom oxygen vacancy or zinc interstitial related defects, and getssignificantly reduced by the improvement in stoichiometry of ZnOfilms.

It may be pointed out that the band edge PL peakwidth of 102meVat room temperature is quite comparable to the reported values forZnO films deposited by MBE [40], MOVPE [35,41] and PLD [37], onsingle crystalline substrates. It is also interesting to note that the mostintense and narrow PL does not come from the ZnO film deposited at300 °C, which exhibited the strongest and nearly complete c-axispreferred orientation of crystallites. This result is important andinteresting, as it indicates that themain cause of the enhancement andnarrowing of band edge luminescence in polycrystalline ZnO films isthe decrease of micro-strain, which results from the reduction in pointdefects, such as oxygen vacancies and zinc interstitials. It may also bementioned that the film deposited at 600 °C possessed equiaxedcrystallites with a slightly larger overall size, as compared to the filmdeposited at 300 °C. This may also partly contribute to its superioroptical quality.

4. Conclusions

Polycrystalline ZnO films were deposited on quartz substrates byreactive sputtering of zinc target. The films deposited below 300 °Cshowed preferred c-axis orientation of crystallites but Raman and PLstudies indicated their poor crystallinity and optical quality. The filmsdeposited up to substrate temperatures of 100 °C, show c-axispreferred orientation due to preferential nucleation. The appearanceof multiple orientations in the substrate temperature range of 150–250 °C is attributed to surface diffusion between planes within thegrains. The film deposited at 300 °C and higher temperatures showed astrong c-axis orientation of crystallites due to the dominance of surfacediffusion between grains. However, with increase in substrate

temperature from 300 °C to 600 °C, the c-axis orientation was againfound to steadily decrease, owing to transition from columnarstructure to equiaxed structure of crystallites. Themicro-strain presentin the film deposited at 600 °C was nearly half of that in the filmdeposited at 300 °C, which is attributed to lesser point defects. Ramanand PL studies also show that the film deposited at 600 °C possesseslesser oxygen-deficiency related point defects, which correlates wellwith the observed decrease in its micro-strain to ∼10−3. As aconsequence of reduction in optically active point defects, the ZnOfilmdeposited at 600 °C showed a high intensity andnarrowband edgeluminescence with FWHM of ∼102 meV at room temperature.Polycrystalline ZnO films deposited by reactive sputtering of a metalliczinc target have not been usually reported to possess such structuraland optical quality. The reduction in defects is attributed to thepresence of energetic zinc ad atoms having high surface mobility aswell as annealing effects. It is also inferred that the micro-strainpresent within the crystallites plays the most crucial role compared toothermicrostructural parameters, in determining the optical quality ofpolycrystalline ZnO films.

This work has shown that the mere appearance of a single andintense (0002) peak in ‘a film’ is not a sufficient indicator of its ‘high’crystalline quality. This is particularly relevant to the case ofpolycrystalline ZnO films, which under most conditions of depositionexhibit a single or intense (0002) XRD peak. It is also clear that ingeneral, there is a need to examine other aspects of microstructure,such as, crystallite size, crystallite tilt and uniform as well as non-uniform strain (micro-strain) and also independently assess thepresence of optically active defects, before drawing meaningfulconclusions about the ‘quality’ of polycrystalline semiconductor films.

Acknowledgements

The financial support from MHRD (Govt. of India) for this work isgratefully acknowledged. Sukhvinder Singh is thankful to CSIR, NewDelhi (India) for Senior Research Fellowship. Tapas Ganguli and RaviKumar are thankful to Dr. S. M. Oak for support and encouragement.The National OIM Texture Facility of IIT Bombay is gratefullyacknowledged for X-ray pole figure measurements. SAIF and CRNTS,IIT Bombay are respectively acknowledged, for providing TEM andRaman facilities.

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