The growth per cycle (GPC) of pure ZnO and TiO2 films are tested to be 0.2 and 0.025 nm/cycle, respectively. Measured thicknesses of TZO films are then listed in Table 1 together with the expected thicknesses, which are given by
In Equation 1, it is assumed that the GPC for a given material has no business with the material deposited in the previous cycle. Since the GPC of ZnO is much greater than that of TiO2, the estimation of the film thickness is accurate provided that ZnO encounters no barrier to grow on TiO2. As an example, for the TZO film with N = 20, the measured thickness is 101 nm, which is very close to the expected one. However, with further increase of Ti doping concentration, the measured film thicknesses are found to be off-target. Especially, in the case of the sample with N = 1, the measured thickness was found to be around 80 nm, which was much smaller than the ideal one (115.0 nm). Thus, it is inappropriate for us to assume the GPC of ZnO on TiO2 to be 0.2 nm/cycle. The black squares in Figure 1 show the true thickness as a function of N.
To model the true growth process of ALD-ZnO film on TiO2 layer, a method similar to that reported by Banerjee et al.  was employed. The decrease of the GPC of ZnO may result from the reduced adsorption of DEZ on TiO2. Thus, it is appropriate to assume that the GPC of ZnO follows an exponential behavior given by
represents the GPC of ZnO in TZO film, A is the GPC of pure ZnO film, the independent variable i is the i th cycle number after TiO2 deposition, and the parameter n refers to the number of cycles it needs for GPC′
to reach 63.2% of the ideal growth rate of ZnO. According to Equation 2, the GPC′
would be close to that observed in pure ZnO films after enough number of ZnO cycles. It is also appropriate to assume that GPC′TiO 2 remains unchanged throughout the whole process since TiO2 is always deposited on ZnO. Considering all the assumptions above, the total thickness of the film can be given by
where T denotes the total thickness and the constant t is the GPC of TiO2. Using this function model to fit the measured data, the parameter n can be calculated to be approximately 1 while t is approximately 0.024 nm/cycle. Thus, it can be concluded that TiO2 encounters little barrier to grow on ZnO.
Figure 2 shows the XRD patterns of as-deposited TZO films on quartz. As is displayed in Figure 2a, the crystallinity of the films depends on the N. No phases related to TiO2 or Zn2TiO4 are detected in the scanning range. Usually, the  direction, i.e., the c-axis, is the preferential orientation commonly occurring in pure ZnO films and doped ZnO films prepared by other fabrication techniques such as sol–gel, CVD, and sputtering . However, in the current samples, the (100) peak gradually becomes dominant and the (002) peak turns to be weaker as Ti doping concentration increases. The (100) peak reaches a maximum for the sample with N = 5. However, no peak can be observed in the samples with N = 2 and 1, indicating that the TZO films become amorphous with too much Ti doping. It is well known that the (002) plane of ZnO consists of alternate planes of Zn2+ and O2− and thus is charged positively or negatively, depending on surface termination. On the other hand, the (100) plane is a charge neutral surface consisting of alternate rows of Zn2+ and O2− ions on the surface. Thus, it is conceivable that the layer-by-layer growth during ALD may cause the Ti4+ ions to disturb the charge neutrality of the (100) plane, thereby affecting its surface energy and causing its preferential growth .
In addition, the locations of the (100) diffraction peaks shift towards lower diffraction angles as Ti concentration increases, as shown in Figure 2b. To understand this phenomenon, it is worthwhile to notice that the valence of Ti tends to be +4 in the TZO films made by atomic layer deposition. Along the  direction, the film layer is composed of the line of Zn2+ ions or the line of O2−. If Ti4+ ions take the place of Zn2+ sites, the repulsive force in this direction would increase because of extra positive charge. This effect can enlarge the interplanar spacing along the  direction, thus leading to the observed decrease of the diffraction angle.
The AFM images of the films deposited on silicon substrate were measured to further characterize the effect of Ti doping concentration on the surface morphology of TZO films. Figure 3 shows the AFM images of these films and their root mean square (rms) surface roughness in a scan size of 2 × 2 μm2. It was found that the rms roughness value of the films except for the sample with N = 1 is in the range of 1.65 to 1.80 nm. The surfaces of these films are evidently smoother than those deposited by RF reactive magnetron sputtering . It highlights the potential use of TZO films made by ALD as TCO, where precise control over film uniformity and smoothness is required. The film with N = 1 shows the lowest surface roughness with its rms roughness value to be 0.59 nm. In addition, no etching effect on the film is found in the experiment .
Figure 4 displays the transmission spectra of TZO films grown on quartz. It is obvious that an average optical transmittance more than 80% in the visible range is obtained for the samples with N from 20 to 2, which is valuable for TCO applications such as liquid crystal displays. The relatively low transmission for the sample grown with N = 1 resulted from the high concentration of Ti in the TZO films. Moreover, all the films show a sharp absorption edge in the ultraviolet range, which shifts to the lower wavelength side with Ti concentration increase. The optical band gap of TZO thin films can be calculated according to the transmission spectra. As a direct-band gap material , it is reasonable to assume that the absorption coefficient (α) is proportional to − ln(T), where T is the optical transmission. According to the Tauc relationship, the relation between the optical band gap (Eg) and absorption coefficient is given by 
where h is Planck's constant and v is the frequency of the incident photon. The Eg of the TZO films can be obtained by drawing the plot of (α × hv)2 versus the photon energy (hv) and extrapolating a straight line portion of this plot to the axis of photon energy, as is indicated in the inset of Figure 4. It can be found that the band gap energy increases from 3.26 eV for pure ZnO film to 3.99 eV for the film with N = 1. The widening of band gaps with the increase of titanium concentration is generally attributed to the Burstein-Moss band-filling effect. Excessive carriers induced by the doped Ti would fill the conduction band edge, so the optical band gap is widened [20, 21].
To investigate the electrical properties of the TZO thin films, Hall measurements are carried out at room temperature. The thermally grown SiO2 was chosen as the substrate since the substrate needs to be insulative. The dependence of carrier density, resistivity, and mobility on Ti contents in the TZO films is shown in Figure 5. It should be noted that the resistivity of the sample with N = 1 is so large that its mobility and carrier concentration cannot be measured accurately. As is displayed, the resistivity, mobility, and carrier concentration for pure ZnO films prepared by ALD are 2.14 × 10−3 Ω cm, 1.4 × 1020 cm−3, and 22.5 cm2/V · s, respectively. The resistivity of the TZO film with N = 20 at first drops to a minimum value of 8.874 × 10−4 Ω cm and then goes up with the increase of the Ti contents. It suggests that the conductivity of ZnO film can be improved significantly with appropriate Ti doping concentration. On the other hand, the maximum carrier concentration of 6.2 × 1020 cm−3 is achieved for the sample with N = 10, which is higher than that reported by Park and Kim . However, carrier concentration of the TZO film undergoes an abrupt drop when more Ti impurities are introduced into the TZO film. The decrease in the carrier concentration can be interpreted as follows: As the Ti doping concentration continues to increase, some titanium atoms tend to aggregate near grain boundaries to form TiO2 instead of taking the place of Zn2+ to generate more free carriers . The widening of band gap is also generally considered as a dominant mechanism contributing to the decrease of carrier concentration [20, 21]. In addition, the mobility of TZO films decreases from 21.7 cm2/s for pure ZnO to 2.3 cm2/s for the sample with N = 2. The decrease in mobility is apparently due to the increase of carrier scattering, the deterioration in the crystalline quality, and formation of TiO2 at the grain boundaries.