G T Yusuf, MA Raimi, O.E Alaje and AK Kazeem

Abstract
The undoped ZnO, Al doped ZnO and Mg doped ZnO films were deposited by a sol-gel spin coating method onto the glass substrates. 0.3M solution of zinc acetate dehydrates diluted in methanol and deionized water (3:1) was prepared. Equal quantity of Aluminum chloride and tin chloride were added to each solution to serve as dopants. The effect of Aluminum and Magnesium doping on the optical ZnO films was studied. The transparency properties of all thin films are more than 80 % at a visible wavelength of (300-800 nm). The optical band gap of pure ZnO thin film is 3.12ev while the band gap for Al-doped ZnO and Mg-doped films are 3.16eV and 3.26eV respectively. All film parameters changed with dopant types. The variation of optical band gap with doping is well described by Burstein–Moss effect.
Keywords: Band gap; Doping; Films; Transmittance.

Introduction

In this Zinc oxide is an II-VI n-type semiconductor with band gap of approximately 3.3 eV at room temperature and a hexagonal wurtzite structure [1]. Recently, doped zinc oxide thin films have been widely studied for their application as conducting electrode materials in flat-panel displays or solar devices. Unlike the more commonly used indium tin oxide (ITO), zinc oxide is a non-toxic and inexpensive material [1].
Furthermore, pure zinc oxide films are highly transparent in the visible range (light wavelength of 400-700 nm) and have high electrical conductivity. However, non-stoichiometric or impurity (Group III elements or Group IV elements) doped zinc oxide films have electrical conductivities as well as high optical transparent. Non-stoichiometric zinc oxide films have unstable electrical properties at high temperature because the sheet resistance of ZnO thin films increases under either oxygen chemisorptions and desorption [9] or heat treatment in vacuum or in ambient oxygen pressure at 3000C-4000C [27]. Turning to impurity doped ZnO thin films, unlike non-stoichiometric ZnO thin films, impurity doped ZnO thin films possess stable electrical and optical properties. Among the zinc oxide films doped with group II elements such as barium, aluminum, gallium and indium, aluminum-doped zinc oxide (AZO) thin films show the lowest electrical resistivity [11]. Aluminum-doped zinc oxide (AZO) has a low resistivity of 2.4×10-4 Ω cm [11-13], which is quite similar to that of ITO films, which is about 1.2×10-4 Ω cm [14-16] and AZO also shows good optical transmission in the visible and near infrared (IR) regions. Thus, AZO films have been used as transparent conducting electrodes in solar cells [16, 8]. In addition to doping with Group III elements, doping ZnO with Group IV elements such as [9, 10] Ge, Sn, Ti, Si is also a good way to obtain low resistivity transparent materials in order to replace ITO because Ge, Ti, Zr could substitute on the Zn atom site. For example, Sn can serve as a doubly ionized donor with the incorporation of SnO2 as a solute in ZnO and, consequently, provide a high electron carrier concentration. It is, therefore, expected that the Sn doped ZnO (SZO) will have a higher electrical conductivity and better field emission properties compared with undoped ZnO [10].

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A variety of techniques such as DC or RF magnetron sputtering [2], electron beam evaporation [19,20], pulsed laser deposition [21], spray pyrolysis [22,23], chemical vapor deposition [24] and sol–gel processing [25–34,5] have successfully been developed to prepare zinc oxide thin films. Among them, the sol–gel spin coating method is simpler and cost effective. Traditionally, AZO films prepared by this method follow the non-alkoxide route, using metal salts such as acetates, nitrates or chlorides as precursor and dopant, respectively. In addition, organic solvent, such as methanol [20,21], ethanol [16], isopropanol [14], methoxyethanol [11], ethyl glycol and glycerol [10] are widely employed by introducing monoethanolamine (MEA), diethanolamine (DEA) or tetramethyl ammonium hydroxide (TMAH) as stabilizer [10,11,30]. Recently, few studies had reported on the growth of the ZnO thin films with different dopants using sol gel spin coating technique.
Therefore, the aim of this research works however is to study the optical and electrical properties of zinc oxide thin films using different dopants with locally fabricated sol gel spin coating technique.

Experimental

The films have been deposited onto the glass substrates at 400 °C substrate temperature. 0.3M solution of zinc acetate dehydrates diluted in methanol and deionized water (3:1) were prepared and divided into three portions. Aluminum chloride and tin chloride were added to each solution as dopants. A few drops of acetic acid were added to improve the clarity of solution. The concentration of dopants (aluminum chloride AlCl3·6H2O, magnesium nitrate hexahydrate [Mg (NO3)2.6H2O and was 3% and kept constant for all experiments. The starting solutions were mixed thoroughly with magnetic stirrer and filtered by WHATMAN filter paper. The solutions were then spin coated on glass substrates which have been procleaned with detergent and then in methanol and acetone for 10min each using ELA 110277248E/2510E-MT ultrasonic cleaner and then cleaned with de ionized water and heated on hot plate for 600C. The coating solutions were dropped onto the glass substrate which was rotated at 4000rpm 45 each by using Ws- 400 Bz – 6NPP/AS spin coater. After depositing by spin coating, the films were then dried at 3000C for 15minutes in a furnace to evapourate the solvent and remove organic residuals. The optical and electrical properties of the films at each time were investigated. The films were then inserted into a tube furnace and annealed in air at 7500C for 1 hour each. The optical transmission and reflectance of the films were examined by spectrophotometer ranging from 400 to 1000nm. The transmittance T and reflectance R data was used to calculate absorption coefficients of the AZO films at different wavelengths. The relationship between transmittance T, reflectance R, absorption coefficient, α, and thickness d of the film is given by equation (1).
(1)
The absorption coefficient data was used to determine energy band gap, Eg , using equation (2).
(2)
Where is the photon energy, A is a constant thus, a plot of against is a curve line whose intercept on the energy axis gives the energy gap. The band energy gap of the film was then determined by extrapolating the linear regions on the energy axis.
The absorption coefficient,, associated with the strong absorption region of the film was calculated from absorbance A and the film thickness, t, using (3).
(3)
The extinction coefficient, k, was evaluated from (4)
(4)
Where the wavelength of the incident radiation and, t is, is the thickness of the film.
The crystal phase of the films was determined by X-ray diffraction (XRD). The refractive index of the films was determined from the maxima and minima of the reflectance curve.
(5)
Where n is the refractive index, d is the film thickness (nm), is the wavelength (nm) of the incident light, and k is the interference order (an odd integer for maxima and even integer for minima).

Results

The crystal structure of ZnO films was investigated through X-ray diffraction (XRD). The X-ray diffraction spectrum of ZnO, Al-ZnO and Mg-ZnO film annealed at 7500C with prominent reflection planes is shown in figure 1.The peaks in the XRD spectrum correspond to those of the ZnO patterns from the JCPDS data (Powder Diffraction File, Card no: 36-1451) having hexagonal wurtzite structure with lattice constants a=3.24982Å, c=5.20661Å.The presence of prominent peaks shows that the film is polycrystalline in nature. The lattice constants ‘a’ and ‘c’ of the Wurtzite structure of the films were calculated using the relations (6) and (7).
a= √â…“.λ/sin θ(6)
c= λ/sin θ(7)
Figure 2 shows the optical transmittance spectra of ZnO, Al-ZnO and Mg-ZnO thin films in the wavelength range between 300 to 800 nm. The transparency properties of all thin films are more than 80 % at a visible wavelength of (300-800 nm). It is observed that the transmittance varies with dopant types i.e. aluminum and magnesium. The overall spectra shows an emission band with two obvious peaks, where the first peak, the UV peak which also called the emission or near band edge emission contributed to the free exciton recombination [18]. The second broad peak, also known as the green emission corresponds to the recombination of a photon generated hole with an electron in singly ionized [18].

Figure 1: X-ray diffraction patterns for ZnO thin film for aluminum and magnesium dopants
The optical absorbance spectrum measured within the wavelength range of 300–800 nm using a Shimadzu Spectrophotometer is shown in figure 3.

Figure 2: Optical Transmittance of the films for aluminum and magnesium dopants
Approximately, the band gap alteration of the thin film can be deduced from Figure 3. Here, it evidently shows that changes in the absorption edges are in parallel with types of dopant in the thin film. In order to appropriately estimate the optical band gap equation (2) was used. The presence of a single slope in the plot suggests that the films have direct and allowed transition. It is also well known that ZnO is a direct band-gap material [1] and the energy gap (Eg) can thus be estimated by assuming direct transition between conduction band and valance bands. Theory of optical absorption gives the relationship between the absorption coefficients α and the photon energy hν for direct allowed transition as shown in (2) The direct band gap determined using this equation when linear portion of the (αhν)2 against hν plot is extrapolated to intersect the energy axis at α = 0. Plot of (αhν)2 against hν for undoped, Al-doped ZnO and Mg-doped films are shown in figure 3. The optical band of pure ZnO is 3.12ev while the band gap for Al-doped ZnO and Mg-doped films are 3.16eV and 3.26eV respectively. The variation of optical band gap with doping is well described by Burstein–Moss effect [2-5]. For AZO films, compared to pure ZnO films, the contribution from Al3+ ions on substitution sites of Zn2+ ions and Al interstitial atoms determines the widening of the band gap caused by increase in carrier concentration. This is the well-known Burstein–Moss effect and is due to the Fermi level moving into the conduction band. Since doping increases the carrier concentration in the conduction band, the optical band-gap energy increases [2]. Enhancement of band gap thus also ensures that doping was successfully carried out in the ZnO thin films. It is further observed in our present work that an increase in band gap occurs in Mg- doped film as compared with ZnO and Al-ZnO thin films. The absorption properties of the films in UV range are due to the behaviour of ZnO intrinsic optical band gap energy. An absorption coefficient in the UV region significantly varies with types of dopant used. The result suggests improvement in the optical absorption in the UV region with nature of dopant, which provides useful information especially in the optoelectronic devices and device fabrication.
.

Figure 3: Plot of (αhν)2 vs. photon energy (in eV) for aluminum and magnesium as dopants

Conclusions

Transparent conducting thin films (ZnO, Al-ZnO and Mg-ZnO) have been deposited by sol–gel spin coating technique. The optical properties of these films were systematically investigated. X-ray diffraction analysis shows that The peaks in the XRD spectrum correspond to those of the ZnO, Al-ZnO and Mg-ZnO structural patterns is that of hexagonal wurtzite structure with lattice constants a=3.24982Å, c=5.20661Å. The optical transmittance spectra in the wavelength range between 300 to 800 nm shows that all thin films are more than 80 % at a visible wavelength of (300-800 nm). It is observed that the transmittance varies with dopant types i.e. aluminum and magnesium. The optical band of pure ZnO is 3.12ev while the band gap for Al-doped ZnO and Mg-doped films are 3.16eV and 3.26eV respectively. The variation of optical band gap with doping is well described by Burstein–Moss effect.
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