Introduction

Zinc oxide (ZnO) thin film has attracted much attention in the research community due to the coexistence of conductivity and transparency in the visible region of the electromagnetic radiation spectrum. A major driving force of research in zinc oxide thin film is its prospective use as thin film solar cells (Martinez et al. 1997), thin film transistors (Fortunato et al. 2005), gas sensors (Sahay and Nath 2008) and optoelectronic devices in the UV region (Moon et al. 2005).

Many methods have been employed to obtain good quality of ZnO thin films. R.F sputtering (Jeong et al. 2003), pulsed laser deposition (Craciun et al. 1994), spray pyrolysis technique (Prasad and Kumar 2009), sol–gel method (Chien et al. 2010), etc. are some of the examples. Despite the interest in ZnO thin films, not much attention has been paid toward its preparation by thermal evaporation technique. Thermal evaporation technique is a relatively simple method and is a low-cost procedure. It does not require any catalyst or high temperature growth. The temperature for further oxidation of ZnO film in this method is also moderate enough to be easily applied to thin film technology. Much of previous studies on ZnO thin films deposited by thermal evaporation technique concentrated on the effect of annealing and oxidation mechanism of films (Aly et al. 2001; Aida et al. 2006).

A great deal of activity has been focused recently on the development of doped ZnO thin films. Since zinc oxide is an n-type semiconductor, n-type doping is much easier compared to p-type doping. Doping with aluminum (Al), gallium (Ga) and indium (In) has been attempted by many groups using different deposition techniques and with different sources of dopants resulting in high-quality ZnO thin films (Sahay and Nath 2008; Prasada and Kumar 2010; Hafdallah et al. 2011; Ma et al. 2007; Tokumoto et al. 2002). The room temperature conductivity of indium-doped ZnO thin film has been already reported (Hafdallah et al. 2011; Lee et al. 1996; Lucio et al. 2006). However, there are very few reports on the study of doping of ZnO thin films using different sources of dopants.

However to the best of our knowledge, this is the first attempt to improve the electrical conductivity of ZnO thin films obtained by thermally evaporating pure ZnO powder through doping with indium (In) and indium oxide (In2O3) separately. We report the growth and characterization of ZnO thin films obtained by thermally evaporating pure ZnO powder under vacuum. Further, the conductivity of the films is improved by doping it with 5 % In and 5 % In2O3. Metallic indium and indium oxide are separately used as the source of n-type dopants. In both the cases, indium being a third group element is expected to occupy the zinc lattice site. This work is an attempt to study the nature of indium as a dopant in its pure metallic state as well as in the oxide state.

Experimental work

Undoped ZnO thin films were obtained by thermally evaporating pure ZnO powder (99.99 %, Alfa Aesar, Ward Hill, USA). These ZnO thin films were coated on clean glass slides maintained at room temperature under vacuum of the order of 10–5 torr, using tungsten as the evaporating source.

For the preparation of doped films, a mixture of 95 % of ZnO powder and 5 % of dopant is evaporated maintaining other deposition parameters the same as those of undoped condition. In the present study, metallic indium powder and indium oxide powder (99.9 %, Alfa Aesar, Ward Hill, USA) were used as the two dopants. The thickness of films obtained is measured using gravimetric method and the thickness of all film samples was found to be 200 nm.

During the process of evaporation, ZnO powder decomposes into zinc and atomic oxygen according to the kinetics of evaporation of ZnO, and oxygen will be released as the primary by-product of the process (Boris et al. 2004). Thus, films obtained are expected to be oxygen deficient in the as-deposited condition and they are found to be dark brown in color and opaque in nature. Hence, all undoped and doped films were then annealed at 300 °C for 2 h for further oxidation (Aida et al. 2006) and all films turned transparent. This is due to the re-oxidation of the film and hence the improvement in the stoichiometry of the film. Some reports have proved the improvement in stoichiometry of thermally evaporated ZnO thin films after annealing (Aly et al. 2001; Aida et al. 2006; Bouhssira et al. 2006). The annealed films were then subjected to different characterizations as follows.

Structural characterization of the obtained ZnO thin films was studied by X-ray diffractometer using JEOL diffractometer with a scanning rate of 1°/min. The scanning electron microscopy (SEM) to study the surface morphology of the films was performed with JEOL JSM 6380 system. Transmittance and absorbance measurements were carried out relative to the uncovered substrate at normal incidence in a spectral range of 250–850 nm using Ocean Optics Inc SD 2000 UV–VIS spectrometer. The electrical conductivity of the ZnO thin films was found out on coplanar structures consisting of two silver strips evaporated on the film surface and measurements were carried out with computer-assisted Keithley source meter (2400) setup.

Results and discussion

X-ray diffractogram showed that all doped and undoped films obtained were amorphous in nature in the as-deposited condition as well as after annealing. This amorphous nature leads to flat and smooth surface of the film and low internal stress; such films are well suited for the application of flat panel displays (Shigesato 2010).

The scanning electron micrographs of the surface of the undoped and doped ZnO films are shown (Figs. 1, 2, 3). It is clear from these micrographs that there is no modification in the surface topography of the films after doping. It is observed that all undoped and doped films have a smooth, continuous and pinhole-free microstructure.

Fig. 1
figure 1

SEM image showing the surface topography of undoped ZnO thin film

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Fig. 2
figure 2

SEM image showing the surface topography of In-doped ZnO thin film

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Fig. 3
figure 3

SEM image showing the surface topography of In2O3-doped ZnO thin film

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The transmittance spectra of the films, taken after annealing at 300 °C for 2 h, shows an excellent transmittance of an average of 90 % in the visible region of the electromagnetic spectrum. From the spectra, it was shown that doping did not decrease the transmittance of the films and all undoped, In-doped and In2O3-doped films had almost equal transmittance in the visible region (Fig. 4).

Fig. 4
figure 4

Transmittance spectra of a undoped, b In2O3-doped and c In-doped ZnO thin films

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Since ZnO is a direct band gap semiconductor allowed direct transitions can be assumed, and the spectral dependence of the absorption coefficient α can be described using Eq. (1) (Khairnar et al. 2003).

$$ \alpha h\nu = B\left( {h\nu - E_g} \right)^{1/2} . $$
(1)

The variation of (αhν)2 with hν is plotted and the linear portion of the plot is extrapolated to zero to find the band gap of the film (Fig. 5). It is observed that the band gap of the undoped film is 3.26 eV.

Fig. 5
figure 5

Plot (αhν)2 as a function of (hν) for a undoped, b In2O3-doped and c In-doped ZnO thin films

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When the film is doped with 5 % of In metal, the band gap is reduced to 3.2 eV. This decrease in the band gap is due to the formation of donor levels below the conduction band, which will absorb the photons having energy lower than the band gap value.

The band gap is widened to 3.3 eV when the film is doped with 5 % of In2O3. This widening of optical band gap is generally attributed to the Burstein–Moss shift (Kykyneshi et al. 2010), which results from the filling of electronic states near the bottom of the conduction band, due to the increase of carrier concentration in the ZnO film. In this process, the apparent band gap of a semiconductor is increased as the absorption edge is pushed to higher energies, because of all states close to the conduction band being populated. Tokumoto et al. (2002) also made the same observation when a ZnO thin film grown by spray pyrolysis method was doped with indium.

The I–V characteristics of undoped and doped ZnO films show the stability of current over a range of voltage from –5 to 5 V and the reduction in the resistance of the samples due to doping (Fig. 6).

Fig. 6
figure 6

I–V Characteristics of a undoped, b In-doped and c In2O3-doped ZnO thin films

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The conductivity of the present films was measured in a coplanar structure obtained with evaporation of two silver stripes on the film surface.

The method is mainly based on the formula.

$$ \sigma = \frac{l}{RA} $$

where σ is the conductivity of the film, A is the area between two contacts, R is the resistance of the film and l is the length of the contacts.

The area between the silver contacts can be written as the product of thickness of the film (t) and gap between the two contacts (d).

Thus, the conductivity of the film will be equal to

$$ \sigma = \frac{l}{Rtd}\,\Upomega^{ - 1} \,{\text{cm}}^{ - 1} . $$

The room temperature conductivity of the undoped film is found to be 90 Ω–1 cm–1. This value of conductivity is found to be considerably high as compared to previously reported results on the conductivity of undoped ZnO thin films (Craciun et al. 1994; Banerjee et al. 2006; Rusu et al. 2007).

Oxygen vacancies present in the film even after the annealing process may be the major reason for this high conductivity of the film. It is well known that the conductivity of ZnO thin film can be improved by the creation of intrinsic donors by the lattice defect. Oxygen vacancies present in the film will act as intrinsic defects and are responsible for the conduction electron carrier generation in ZnO. The electron generation can be explained using Kroger–Vink notation according to Eq. (2) (Kykyneshi et al. 2010).

$$ {\text{O}}_{\text{o}}^{X} \to {\text{V}}_{\text{o}}^{\prime\prime} + 2{\text{e}}^{\prime } + {\raise0.5ex\hbox{$\scriptstyle 1$} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{$\scriptstyle 2$}}{\text{O}}_{2} \left( {\text{g}} \right). $$
(2)

From Eq. (2), it is clear that when an oxygen anion escapes from the crystal structure (½O2) from the occupied oxygen site, a doubly ionized vacancy site (Vo) is created with the liberation of two electrons. Thus, multiple carriers are created from a single defect and good n-type conductivity can be obtained. Hence, low resistivity at room temperature in ZnO thin film can be achieved by the creation of intrinsic donors by lattice defects such as oxygen vacancies.

However, these intrinsically doped films will not be stable at ambient conditions. At high temperature, these films will undergo oxidation and there will be a reduction in the oxygen vacancies, leading to the significant increase in the film resistivity. Moreover, the resistivity of the intrinsically doped film can be reduced only up to 10–2–10–3 Ω cm. Thus, these intrinsically doped films are not well suited for device application (Elmer 2010).

After doping the film with 5 % of metallic indium, the conductivity of the film is increased to 580 Ω–1 cm–1. When the film is doped with 5 % of In2O3, a very good improvement in the conductivity is observed and the room temperature conductivity of the film is found to be 3.5 × 103 Ω–1 cm–1.

The high conductivity of indium oxide-doped ZnO can be explained by considering Eq. (3).

According to Eq. (3), when In2O3, which is a group III oxide, is added to ZnO, it is assumed that, being a group III dopant atom, indium will occupy the zinc lattice site spending the additional electrons which are not required for the bonding to the conduction band (Elmer 2010):

$$ {\text{In}}_{2} {\text{O}}_{3} \to 2{\text{In}}_{\text{Zn}} + 2{\text{e}}^{\text{I}} + \left( \raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} \right){\text{O}}_{2} + 2{\text{O}}_{\text{o}} . $$
(3)

These additional electrons will contribute to improving the n-type conductivity of the film. Hence, the conductivity of the films doped with In2O3 is found to be very high as compared to that of films doped with metallic indium. The conductivity and percentage transmittance of indium-doped ZnO thin films obtained by different methods and using different dopant sources reported by other research groups are shown along with the present results in Table 1.

Table 1 Conductivity and percentage transmittance of indium-doped ZnO thin films obtained by different methods and using different dopant sources along with the present results
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Conclusions

We have used thermal evaporation method as a simple and low-cost technique for the preparation of ZnO thin films. Due to the oxygen vacancy in the ZnO thin film which acts as intrinsic defect, a high conductivity of up to 90 Ω–1 cm–1 was achieved in the undoped state itself. On doping the film with metallic indium, decrease in the optical band gap was observed, whereas with indium oxide band gap widening was noticed. When the film is doped with metallic indium, room temperature conductivity of the order of 580 Ω–1 cm–1 is obtained, and when the film is doped with indium oxide, an excellent improvement in the conductivity of up to 3.5 × 103 Ω–1 cm–1 is achieved due to the liberation of extra electrons during the process of doping. In all the cases, visible region transparency of above 90 % is attained without significant change due to doping. Structural characterization studies have shown the amorphous nature and smooth surface topography of all undoped and doped films.