Keywords

24.1 Introduction

Transparent electronics is an advanced technology concerning the creation of visible electronic devices. To realize transparent electronic and optoelectronic devices, transparent conducting oxides (TCOs) have been widely utilized [1, 2]. Good TCOs should have wide optical band gap (>3.5 eV), low electrical resistivity (<10−3 Ω-cm), high optical transmittance (>80 % in the visible region), and good etching property. Impurity doped indium oxides, tin oxides, and zinc oxides are known to satisfy these conditions well [3, 4]. Especially, impurity-doped indium oxide systems, such as tin-doped indium oxide (ITO), have been most widely used for numerous optoelectronic applications. However, in order to have high electrical conductivity and high transmittance, ITO films must be deposited and then annealed at high temperature (>300 °C). This high temperature makes the ITO films rough due to the crystallization, which leads to significant deterioration of the device reliability [5, 6].

Zinc oxide (ZnO) is an n-type semiconductor with a large binding energy and a wide bandgap. Doped ZnO films are promising alternatives to replace ITO films as TCOs due to the former’s stable electrical and optical properties. The low resistivity of ZnO-based films arises from the presence of oxygen vacancies and zinc interstitials [7]. Al [8], Ga [9], and Ti [10] have been widely studied as dopants to enhance the n-type conductivity of ZnO-based films. Also, the ZnO-based films can be deposited using variety of techniques, such as sputtering [11], pulsed laser deposition (PLD) [12], metal organic chemical vapor deposition (MOCVD) [13], and spray pyrolysis (SP) [14]. Indium-doped zinc oxide (IZO) films can be deposited by RF magnetron sputtering technique which has been widely used due to its advantageous features including simple apparatus, high deposition rates, and low deposition temperature.

In this study, IZO films of a fixed composition (ZnO = 98 mol % and In2O2 = 2 mol %) were deposited by radio frequency magnetron sputtering from an oxide target mixed with ZnO and In2O3, near which composition the lowest resistivity was observed when processed at room temperature. The structural, optical, and electrical properties of IZO films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) pattern, UV–visible spectroscopy, and Hall measurement.

24.2 Experimental Procedures

Raw materials (ZnO and In2O2, 99.99 % in purity) were weighed according to the composition formula ZnO = 98 mol % and In2O2 = 2 mol % (IZO), and ball-milled with deionized water for 1 h. After being dried and ground, the powder was calcined at 800 °C for 2 h and ground again. The calcined powder was uniaxially pressed into a 2-inch plate in a steel die, and sintering was carried out at 1400 °C in air for 2 h. 20 mm × 20 mm × 1 mm Corning 1737 glass were used as substrates, then the substrates were cleaned with acetone, isopropyl alcohol, deionized (D.I.) water, and dried under blown nitrogen gas, then IZO films were deposited on glass substrates. The working distance between the substrate and target was fixed at 10 cm. The base pressure was 5 × 10−6 torr and the working pressure was maintained at 5 × 10−3 torr. The deposition temperature of IZO films was kept at room temperature, the RF power was 100 W, and the deposition times varied from 30 to 90 min. The crystalline structures of IZO films were determined with an X-ray diffractometer using CuKα radiation (K = 1.5418 Å). The surface morphology and thickness of IZO films were measured by using the field emission scanning electron microscope (FE-SEM). The mobility, carrier concentration, and resistivity were obtained from Hall-effect measurements using the Van der Pauw method. Optical transmittance was measured by a UV spectrophotometer in the wavelength range of 200–1100 nm.

24.3 Results and Discussion

Figure 24.1 shows the surface SEM images of IZO films as a function of deposition time. The surface micrographs of the 30, 50, and 70 min-deposited IZO films had the similar morphologies. The surfaces of IZO films are smooth and compact, the average grain sizes of the 30, 50, and 70 min-deposited IZO films were about 12.3, 22.6, and 45.5 nm, respectively. However, as deposition time was increased to 90 min, the grain size of IZO films increased and the abnormal particles were also formed, and the roughness would critically increase. These results suggest that the deposition time of IZO films shorter than 90 min is better. The cross-section FE-SEM images of IZO films with different deposition times are shown in inset Fig. 24.1. The thickness of IZO films increased with deposition time is considerable. The thickness increased from 237 to 389 nm as the deposition times increased from 30 to 90 min. In addition, the crystallization IZO film reveals that preferential orientation growth with the columnar structure, as shown in Fig. 24.2a–d.

Fig. 24.1
figure 1

Surface SEM images and cross-section SEM images (inset) of the IZO films as a function of deposition times. a 30 min, b 50 min, c 70 min, and d 90 min

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Fig. 24.2
figure 2

Carrier concentration, resistivity, hall mobility, and thickness of the IZO films as a function of deposition times

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Figure 24.2 shows the mobility, carrier concentration, and resistivity of IZO films as a function of deposition time. As the deposition time increased from 30 to 90 min, the mobility increased from 19.26 to 43.69 cm2/V-s. This result cause by the better crystal quality and larger particle sizes of IZO films. Therefore, a reduced grain boundary barrier is obtained, leading to an increase in carrier mobility. The carrier concentration of IZO films slightly decreased from 9.95 × 1018 to 1.21 × 1018 cm−3 as the deposition time increased from 30 to 90 min. The resistivity of TCO films is proportional to the reciprocal of the product of carrier concentration (Ne) and mobility (μ):

$$ \rho = 1/{\text{N}}_{\text{e}}\upmu $$
(24.1)

which is a combined result of both the mobility and the carrier concentration. The resistivity of IZO films decreased from 9.08 × 10−3 to 2.85 × 10−3 Ω-cm when film thickness increased from 237 nm (deposition time was 30 min) to 389 nm (90 min), as shown Fig. 24.2.

XRD patterns of IZO films with different deposition times are shown in Fig. 24.3. All patterns exhibited the (002) peaks of IZO crystallization preferential orientation along the c axis at diffraction angles (2θ) near 34.1°, with a hexagonal structure; no characteristic peak of In2O3 phase was found. As deposition time increased from 30 to 90 min, the full width at half maximum (FWHM) values decreased from 0.459 to 0.413. The decrease in FWHM value suggests the crystallization of IZO films increases with increasing deposition time. Those results reveal that the crystallization of IZO films is enhanced as longer deposition time is used, for that the resistivity of IZO films will closely depend on the deposition time. Compared the diffraction spectra show that the 2θ value of the (002) peak shifted from 34.29° to 34.05° as the deposition time of IZO films increased from 30 to 90 min, it means that the lattice constant c increased with increasing deposition time. This result is caused by that the ionic radius of In3+ (80 pm) is larger than that of Zn2+ (74 pm), and longer deposition time the more In3+ ionic will influence the characteristics of IZO films, then the 2θ value of the (002) peak is expected to shift downwards.

Fig. 24.3
figure 3

XRD patterns of IZO films as a function of deposition times: a 30 min, b 50 min, c 70 min, and d 90 min

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The optical transmittance spectra of IZO films in the wavelength range of 200–1100 nm are shown in Fig. 24.4. The average transmittance rate of IZO film is about 85 % in the 400–700 nm range as deposition time was 30 min, and the average transparency of IZO films slightly decreased as deposition time increased. In the ultraviolet range, all the IZO films showed a sharp absorption edge and exhibited a stronger redshift phenomenon with increasing deposition time. This redshift can be explained by the Burstein-Moss shift, a shift of the Fermi level into the conduction band, the energy of which enhances the optical bandgap [15, 16]:

$$ \varDelta E_{g}^{BM} = \frac{{\hbar^{2} k_{F}^{2} }}{2}\left( {\frac{1}{{m_{e} }} + \frac{1}{{m_{h} }}} \right) = \frac{{\hbar^{2} k_{F}^{2} }}{{2m_{vc}^{*} }} $$
(24.2)

where kF stands for the Fermi wave vector and is given by kF = (3π2ne)1/3, me is the effective mass of electrons in the conduction band, and mh is the effective mass of holes in the valence band, which can be simplified as the reduced effective mass m *vc . ΔE BMg can be rewritten by inducing kF for the carrier concentration (ne):

$$ \varDelta E_{g}^{BM} = \frac{{\hbar^{2} k_{F}^{2} }}{{2m_{vc}^{*} }}(3\pi^{2} n_{e} )^{2/3} $$
(24.3)

Equation (24.3) shows that the Burstein-Moss shift of the absorption edge to the longer wavelength region is due to the decrease in carrier concentration, as demonstrated in Fig. 24.2.

Fig. 24.4
figure 4

Transmittance spectra of the IZO films as a function of deposition times

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The corresponding optical bandgap (Eg) of IZO films were determined by applying the Tauc model and the Davis and Mott model [17] using Eq. (24.4):

$$ \left( {\alpha {\text{h}}\upnu} \right)^{ 2} = {\text{ c}}\left( {{\text{h}}\upnu - {\text{E}}_{\text{g}} } \right) $$
(24.4)

where α is the optical absorption coefficient, c is the constant for direct transition, h is Planck’s constant, and ν is the frequency of the incident photon. Figure 24.5 shows (αhν)2 vs. hν plots of IZO films. Their Eg values decreased when the deposition time of IZO films increased from 30 to 90 min. The variations in Eg values roughly agree with the variations of the carrier concentrations shown in Fig. 24.2 and Eq. (24.3).

Fig. 24.5
figure 5

The energy gap of the IZO films as a function of deposition times

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24.4 Conclusions

In this study, crystalline indium zinc oxide (IZO) films were deposited on glass substrates using RF magnetron sputtering technique. The (200) preferred orientation of IZO films increased as the deposition time increased, which would cause the better conductive properties in IZO films. The resistivity of IZO films linearly decreased from 9.08 × 10−3 to 2.85 × 10−3 Ω-cm with increasing in the thickness of IZO films, which could be explained by an increase of carrier mobility with decreasing grain boundary scattering. The average transmittance rate of IZO films was about 75 % in the wavelength range from 400 to 800 nm as the deposition time increased from 30 to 90 min.