Structural, electrical, and optical properties of Ti-doped ZnO films fabricated by atomic layer deposition
© Ye et al.; licensee Springer. 2013
Received: 18 November 2012
Accepted: 31 January 2013
Published: 27 February 2013
High-quality Ti-doped ZnO films were grown on Si, thermally grown SiO2, and quartz substrates by atomic layer deposition (ALD) at 200°C with various Ti doping concentrations. Titanium isopropoxide, diethyl zinc, and deionized water were sources for Ti, Zn, and O, respectively. The Ti doping was then achieved by growing ZnO and TiO2 alternately. A hampered growth mode of ZnO on TiO2 layer was confirmed by comparing the thicknesses measured by spectroscopic ellipsometry with the expected. It was also found that the locations of the (100) diffraction peaks shift towards lower diffraction angles as Ti concentration increased. For all samples, optical transmittance over 80% was obtained in the visible region. The sample with ALD cycle ratio of ZnO/TiO2 being 20 had the lowest resistivity of 8.874 × 10−4 Ω cm. In addition, carrier concentration of the prepared films underwent an evident increase and then decreased with the increase of Ti doping concentration.
The application of transparent conductive oxide (TCO) has been found in many areas such as liquid crystal displays, touch panels, and organic light-emitting diodes because of its excellent conductivity and high transmittance for visible light [1–4]. At the moment, indium tin oxide film is the most popular material and has been widely used in many optoelectronic devices. Although it has excellent transparency (greater than 85% transparency in visible spectrum) together with high conductivity (resistivity around 2 to 4 × 10−4 Ω cm) , research on alternative materials is urgent due to the relative scarcity and high cost of indium.
Several alternative TCO materials have been investigated extensively recently. Among them, ZnO seems to be one of the ideal choices due to its low cost. As is well known, ZnO is a II-VI group semiconductor material containing high concentration of native defects, which typically include oxygen vacancies or zinc interstitials. Thus, pure ZnO has excellent conductivity. However, pure ZnO thin films are not very electrically and chemically stable at high temperature . Fortunately, the performance of ZnO thin films can be improved by appropriate impurity doping . For example, it has been reported that Al-doped ZnO film fabricated by atomic layer deposition (ALD) has as high as 80% to 92% transmittance in the visible range and low resistivity around 4 × 10−3 Ω cm . What is more, as is reported by Lin et al., Zr-doped ZnO thin films grown by atomic layer deposition with sapphire substrates have wonderful transparency (>92%) for visible light and high carrier concentration (2.2 × 1020) .
Among the variety of metallic element-doped ZnO films, Ti-doped ZnO films have been investigated recently for their unique electrical, magnetic, and sensing properties. In some previous studies, a number of fabrication techniques such as sputtering, pulsed laser deposition, and chemical vapor deposition (CVD) as well as the structural, morphological, and electrical characteristics of the corresponding films [10–16] have been discussed. However, rare reports focused on Ti-doped ZnO films fabricated by ALD. Furthermore, compared with those of main group metal-doped ZnO films, the conduction mechanisms of ZnO films doped with transition metals such as Ti are still not clearly understood. So it is of greater importance to do research on Ti-doped ZnO (TZO) films grown by ALD. In this work, the effect of Ti doping concentration on the structural, optical, and electrical properties of the deposited TZO films was systematically studied by spectroscopic ellipsometry, X-ray diffraction, atomic force microscopy, transmission spectrometry, and Hall measurement.
TZO thin films were deposited at 200°C in a BENEQ TFS-200 ALD reactor (Vantaa, Finland) using titanium tetraisopropoxide liquid (TTIP), diethyl zinc (DEZ), and deionized (DI) water. TTIP, DEZ, and DI water were used as Ti, Zn, and O sources, respectively. The precursors TTIP and DEZ were separately held in stainless bubblers at 58°C and 18°C, respectively. High-purity quartz, thermally grown SiO2, and silicon served as the substrates. Before loading into the ALD reactor, the quartz glasses were ultrasonically cleaned with acetone and alcohol in sequence for 5 min, and then rinsed with DI water and dried in nitrogen. The silicon substrates were cleaned chemically using a standard Radio Corporation of America solution (New York, NY, USA) and then dipping into the diluted 5% HF solution for 1 min to remove the native oxide layer, followed by rinsing with DI water and drying in N2. The precursors were alternately introduced to the reactor chamber using high-purity N2 (>99.99%) as the carrier gas. A typical ALD growth cycle for ZnO is 0.5-s DEZ pulse/2-s N2 purge/0.5-s H2O pulse/2-s N2 purge, whereas for TiO2, it is 1.0-s TTIP pulse/5-s N2 purge/0.5-s H2O pulse/5-s N2 purge. The TZO films were then achieved in an ALD supercycle mode, which was defined as N ZnO cycles followed by one TiO2 cycle. Supercycles were repeated until the target number of 500 ZnO cycles was reached.
The thicknesses of TZO films were measured by spectroscopic ellipsometry (GES5E, SOPRA, Courbevoie, France) wherein the incident angle was fixed at 75° and the wavelength region from 230 to 900 nm was scanned with 5-nm steps. The crystal structures of films were obtained using an X-ray diffractometer (D8 ADVANCE, Bruker AXS, Madison, WI, USA) using Cu Kα radiation (40 kV, 40 mA, λ = 1.54056 Å). Atomic force microscopy (AFM) using a Veeco Dimension 3100 scanning probe microscope (Plainview, NY, USA) operated in a tapping mode provided surface morphology of the TZO thin films. To obtain the optical transmission spectra, a UV spectrophotometer (UV-3100) in a wavelength range of 200 to 900 nm at room temperature was used in the air. In addition, the electrical properties of TZO films deposited on thermally grown SiO2 are characterized by Hall effect measurements using the van der Pauw method.
Results and discussion
Summary of estimated and measured thicknesses of TZO films with R 2 accuracy greater than 0.995
Number of supercycle
Estimated thickness (nm)
True thickness (nm)
106 ± 2.1
Zn/Ti = 20:1
101 ± 1.7
Zn/Ti = 10:1
95 ± 0.9
Zn/Ti = 5:1
94 ± 1.5
Zn/Ti = 2:1
84 ± 1.4
Zn/Ti = 1:1
80 ± 0.6
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.
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.
Ti-doped ZnO thin films with the thickness of around 100 nm were prepared by ALD at 200°C. The fact that film thicknesses measured by spectroscopic ellipsometry were thinner than expected for samples with ALD cycle ratio of ZnO/TiO2 less than 10 suggested a hampered growth mode of ZnO on TiO2 layer. TZO films synthetized by ALD crystallized preferentially along the  direction. High transparency (>80%) in the visible region was obtained, and the band gap of the TZO films increased with increasing Ti doping concentration due to the Burstein-Moss effect. It was observed that the resistivity of TZO film had a minimum value of 8.874 × 10−4 Ω cm when the ALD cycle ratio between ZnO and TiO2 was 20.
This work is supported by the Important National Science & Technology Specific Projects (2011ZX02702-002), the National Natural Science Foundation of China (no. 51102048), SRFDP (no. 20110071120017), and the Independent Innovation Foundation of Fudan University, Shanghai.
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