1 Introduction

Zinc oxide (ZnO) is an II–VI compound semiconductor material with a wurtzite structure and wide direct band gap (≥ 3.1 eV). Due to its important properties including high transparency, low electrical resistivity, good thermal stability in hydrogen plasma environment, and non-toxicity, the zinc oxide is a promising material for a wide range of applications in a variety of optoelectronic devices like solar cells [1,2,3], light-emitting diodes (LEDs) [4,5,6], transparent electrodes for flat panel displays [7, 8], and gas sensors [9,10,11]. For solar cells and flat panel displays applications, a transparent conducting oxide (TCO) is a necessary component. The most commonly used TCO’s are tin oxides and indium tin oxide (ITO) due to their outstanding characteristics of high optical transmittance and high electrical conductivity. However, the toxicity of indium element as well as the high cost and scarce has greatly limited its practical applications in the above fields. In view of the depletion of ITO, zinc oxide (ZnO) is the most promising alternative candidate, since the zinc and oxygen elements are practically costless and available; moreover, its transparency and resistivity are comparable to ITO’s.

Many deposition techniques such as sputtering [12], plasma-enhanced chemical vapor deposition (PECVD) [13], pulsed laser deposition [14], sol–gel [15], and spray pyrolysis [16, 17] have been utilized to prepare ZnO thin films with adequate performance for TCO’s applications. Sputtering technique has been intensively used for ZnO thin film synthesis because it offers much more advantages in comparison with the other methods. This method provides uniform and adherent film at near room temperature. In the present work, the optimal discharge conditions obtained with Si substrates were used [18, 19].

The electrical resistivity and the optical transparency usually decrease with the thickness of TCO layer [20]. The trade-off between these two requirements is an important issue for specific applications. In the present research, our aim is to establish a relationship between the film thickness and its properties.

2 Experimental procedure

ZnO films were prepared by RF magnetron sputtering system using a ZnO target at room temperature on glass and silicon substrates, the deposition conditions are summarized in Table 1.

Table 1 Deposition conditions for ZnO thin films
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The film thickness was measured by profilometry and estimated from a film cross-section observation performed by scanning electron microscopy (SEM). The crystalline structure of the films was characterized by X-ray diffraction (XRD ) in θ−2θ scanning mode and by rocking curve (RC) mode on a D5000 Siemens, using a CuKα radiation (λ=1.5406 Å).

The structural study of the ZnO films was carried out by high-resolution transmission electron microscopy (HRTEM) (HF2000-FEG) with an acceleration voltage of 200 kV and 0.23 nm resolution. Energy dispersive spectroscopy (EDS) associated with Jeol 5400 scanning electron microscope was used to determine the chemical composition of the films.

The dc electrical resistivity measurements in dark and at room temperature were performed with a four-point probe and appropriate correction factors. Film optical transmittance was recorded by using CARRY UV–Vis–NIR scanning spectrophotometer.

The calculation details of the optical band gap (Eg), the band tail width or energy of disorder commonly called Urbach energy (Eu), and the internal stress were described elsewhere [18,19,20] and the grain size of the films is calculated by using Scherer formula [D = 0.9λ/(FWHM*cosθ)].

3 Results and discussions

Figure 1 shows the growth rate of intrinsic ZnO films, deduced from the evolution of the thickness of the layers with the deposition time. We notice a first part that increases rapidly at the beginning of the deposition and thereafter it becomes slower until saturation, explainable probably by the nucleation process which reaches a steady state.

Fig. 1
figure 1

Variation of deposition rate versus film thickness

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The chemical composition of the films was characterized by X-ray energy dispersion spectroscopy (EDX). Figure 2 shows the EDX spectrum for different thicknesses. We note the presence of the characteristic peaks of zinc and oxygen for all the thin layers of zinc oxide. It appears that the concentrations obtained by this technique are not influenced by the variation in thickness, considering the same concentrations practically: 51.2 at.% Of Zn and 48.8 at.% O for all layers (the Zn/O ratio = 1.05).

Fig. 2
figure 2

EDS spectrum for ZnO films

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Also, it should be noted that the electron scattering pear involves a much greater depth than the width of the analyzed area. In this case, the analyzed zone includes a part of the substrate on which the samples were deposited. That’s why we distinguish the characteristic peak of silicon with a higher intensity for the thinnest layers (40 nm).

The X-ray diffraction study shows that the studied films are well crystallized in the preferred orientation (002), which indicates the hexagonal wurtzite crystal structure (JCPDS card No: 036-1451). This preferential orientation is due to the low energy required for (002) plane formation [21]. The shift of the (002) peak to the large angles when the thickness increases explains the decrease in the inter-reticular distance (Fig. 3) and implies that the films are under strain. The stress is compressive and varies from 3 (GPa) to nearly free of stress when the thickness increases from 40 to 1400 nm (Fig. 4). This is attributed to the improvement in crystallinity. Similar value of stress has been reported in the literature [12, 22]. In the same Fig. 4, there is an increase in the full width at half maximum (FWHM) of the peak up to about 260 nm in thickness. Beyond 260 nm, there is a decrease in the FWHM. This can be explained by the evolution of the growth rate, due to the presence of a preliminary phase during which the ZnO molecules do not uniformly cover the surface of the substrate where the film consists of small grains.

Fig. 3
figure 3

X-ray diffraction patterns of ZnO films with different thickness

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The ZnO film electron diffraction picture presented in Fig. 5 shows spots characteristic of the crystalline plans present in the structure: we identify the plans (100), (110), and (002). These plans are characteristic of zinc oxide structure. The growth of the layer seems mainly to take place in the direction [002] (which is perpendicular to the substrate plan) since the spot (002) is the most intense. This result is in good agreement with X-ray diffraction results. The same plans were observed for the 132-nm-thick ZnO films prepared by DC magnetron sputtering in the study by HRXTEM made by Kiriakidis et al. [23].

Fig. 4
figure 4

Variation of FWHM, grain size, and intrinsic stress with thickness

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The columnar structure is evident in dark field diffraction for the spots (100) and (002) as shown in Fig. 5b and c. Under the diffraction conditions of the spot (100), it is clear that it is somewhat intense compared to the dark field diffraction of (002), so it is negligible compared with the latter meaning that the interface is crystallized mainly according to (002).

Fig. 5
figure 5

Localized SAED patterns (a) and HRTEM micrograph performed in dark field (b, c) of ZnO film grown on silicon substrate

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To confirm this hypothesis of growth, the texture of ZnO films was studied using the rocking curves technique. Figure 6 shows the rocking curves spectra of the diffraction peak (002) for different thicknesses; it is clear that as the thickness increases the width at half maxima (FWHM) decreases. The small angle (< 1.4°) for the layers thicker than 260 nm indicates the good texturing of the films along the ‘c’ axis. On the other hand, for the thin layers, the rocking curves indicate the contribution of the other peaks identified in the electron diffraction pictures in the selected area. It is for this reason that the stresses decrease by increasing the thickness.

Fig. 6
figure 6

Rocking curves diagram as function of thickness

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The obtained values of FWHM using rocking curves in this work (1.3°−2.3°) are less than those reported in the literature [24] where the work of Yasui et al. (the order of 3.14°) deals with ZnO:Al thin films and Sayago et al. [25] (the order of 4.67°) for ZnO thin films prepared by RF magnetron sputtering.

Figure 7 shows the optical transmittance spectra obtained from the ZnO films in the wavelength ranging 250–1000 nm and its variation with the thickness. We notice an average transmittance of the order of 90% for all films in the visible range. For thin films (d < 40 nm), we do not observe the interference fringes caused by multiple reflections (at interfaces substrate/film and film/air). The transmission drops by about 5% for the thickness of 1400 nm.

The fall in T% for wavelengths less than 380 nm corresponds to the absorption in ZnO due to the transition of electrons between the valence and the conduction bands. Not only the gap varies, but also the shape of the curve around the gap. Indeed, the curve for thin films (d < 90 nm) does not increase abruptly at the height of the gap but rather progressively. This gradual transition of the transmission at the gap level could come from the contribution of the glass substrate, which is not probably uniformly covered with ZnO at the start of its growth.

Fig. 7
figure 7

Optical transmittance of ZnO films with different thickness

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The increase in the optical band gap of the films with the thickness caused by the decrease in the Urbach energy (Fig. 8) is remarkable until reaching a thickness of 260 nm. The values of EU (0.08–0.22 eV) obtained in the present study are similar to those reported for ZnO prepared by sol–gel [26] and CVD [27] techniques.

Fig. 8
figure 8

Variation of band gap and band tail width versus film thickness

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The absorption edge blue shift with increasing thickness is explained by the “Burstein-Moss” effect [28, 29]. The charge carriers additionally induced in the layers fill the energy bands situated just above the conduction band, causing a widening of the optical gap described as the energy difference between the conduction and the valence bands. So, this broadening in the gap with thickness is initially due to the increase in the concentration of charge carriers. Then the decrease in band gap from 3.30 eV for 260 nm, to only 3.20 eV for 1400 nm, can be correlated with the grain size [30] which increases with the thickness.

The variation of the resistivity as a function of film thickness is presented in Fig. 9. The resistivities of the films are between 1 and 4*10− 2 (Ω cm), which are consistent with the reported values of resistivity obtained for ZnO prepared by reactive sputtering [22]. During the first deposition phase, the resistivity (ρ) is very high at a film thickness about 40 nm. This is due to the fact that the film is not being sufficiently thick for conductibility. Then it decreases rapidly to very small values and remains relatively stable at values of the order of 4*10− 2 (Ω cm). We attribute this behavior to an oxygen deficit (oxygen vacancy) and/or an excess of zinc atoms in the interstitial positions, which generates more free electrons. As we can correlate this variation in resistivity with the degree of crystallization, in our analysis by XRD, this is interpreted by the evolution of the grain size (decrease of FWHM). There are, therefore, fewer grain boundaries which are a limiting factor in mobility (act as traps for free carriers and as barriers against transport).

Fig. 9
figure 9

Dependence of electrical resistivity in ZnO films on thickness

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4 Conclusion

In summary, highly transparent conductive ZnO thin films with different thicknesses were deposited at room temperature on silicon and glass substrates using RF magnetron sputtering technique. All the ZnO samples are polycrystalline with preferred orientation along (002) direction. It is clear that as the thickness increases the FWHM in rocking curves decreases. The small angle (< 1.4°) for the layers thicker than 260 nm indicates the good texture of the films along the 'c'’ axis. On the other hand and for the thin layers, the rocking curves indicate the contribution of the other peaks identified in the electron diffraction pictures in the selected area. It is for this reason that the stresses decrease by increasing the thickness. From the optical and electrical characterizations, it was found that the obtained ZnO films are greater than 90% transparent in the visible region of light and have low electrical resistivity (~ 10− 2 Ω cm).

The combination of high visible transmittance, low resistivity, and good structural and morphological properties makes the ZnO thin films, for thickness > 90 nm, very suitable candidates for usage in many optoelectronic devices and solar cells applications.