Developing high-transmittance heterojunction diodes based on NiO/TZO bilayer thin films
© Huang et al.; licensee Springer. 2013
Received: 8 March 2013
Accepted: 5 April 2013
Published: 1 May 2013
In this study, radio frequency magnetron sputtering was used to deposit nickel oxide thin films (NiO, deposition power of 100 W) and titanium-doped zinc oxide thin films (TZO, varying deposition powers) on glass substrates to form p(NiO)-n(TZO) heterojunction diodes with high transmittance. The structural, optical, and electrical properties of the TZO and NiO thin films and NiO/TZO heterojunction devices were investigated with scanning electron microscopy, X-ray diffraction (XRD) patterns, UV-visible spectroscopy, Hall effect analysis, and current-voltage (I-V) analysis. XRD analysis showed that only the (111) diffraction peak of NiO and the (002) and (004) diffraction peaks of TZO were observable in the NiO/TZO heterojunction devices, indicating that the TZO thin films showed a good c-axis orientation perpendicular to the glass substrates. When the sputtering deposition power for the TZO thin films was 100, 125, and 150 W, the I-V characteristics confirmed that a p-n junction characteristic was successfully formed in the NiO/TZO heterojunction devices. We show that the NiO/TZO heterojunction diode was dominated by the space-charge limited current theory.
Transparent electronics is an advanced technology concerning the creation of invisible electronic devices. To realize transparent electronic and optoelectronic devices, transparent conducting oxides (TCOs) have been widely utilized [1–3]. Zinc oxide (ZnO) is an n-type semiconductor with a large binding energy of 60 meV and a wide bandgap of 3.3 eV. Doped ZnO thin films are promising alternatives to replace indium-tin oxide (ITO) thin films as TCOs due to the former's stable electrical and optical properties. The low resistivity of ZnO-based thin films arises from the presence of oxygen vacancies and zinc interstitials . Aluminum (Al) , gallium (Ga) , and indium (In) [7, 8] have been widely studied as dopants to enhance the n-type conductivity of ZnO-based thin films. ZnO-based TCO materials have numerous potential applications in electronic and optoelectronic devices, such as solar cells , light-emitting diodes , blue laser diodes , and flat-panel displays . Trivalent cation-doped ZnO thin films present good electrical conductivity and transparency over the visible spectrum. In the past, Chung et al. investigated the properties of Ti-doped ZnO thin films with different TiO2 concentrations and reported that the lowest resistivity of TZO thin films was achieved when the Ti concentration was 1.34 mol% . Lin et al. studied the effect of substrate temperature on the properties of TZO thin films by simultaneous radio frequency (RF) and DC magnetron sputtering . Wang et al. examined the effects of substrate temperature and hydrogen plasma treatment on the characteristics of TZO thin films .
Nickel oxide (NiO) is a p-type semiconductor TCO material with a wide range of applications: it has been used in transparent conductive films  and electrochromic devices  and as a functional layer material in chemical sensors . NiO has a wide bandgap of 3.6 to 4.0 eV at room temperature; hence, a NiO thin film is also transparent in the range of visible light . According to the literature, TZO and NiO thin films can be prepared by sputtering [16, 20], chemical vapor deposition [21, 22], and the sol-gel process [23, 24]. Among these methods, sputtering is the most widely used. In this paper, the fabrication and characterization of an optically transparent p-n heterojunction diode by deposition of NiO thin films on TZO thin films are presented, with an emphasis on device performance, including transparent and current-voltage characteristics. In addition, the structural, optical, and electrical properties of the NiO/TZO heterojunction diodes were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) patterns, UV-visible spectroscopy, and Hall effect measurement.
Results and discussion
Figure 4 shows the XRD patterns of the NiO thin films deposited as a function of deposition power. No matter what deposition power was used, the only (200) diffraction peak was observed in the NiO thin films, and the 100 W-deposited NiO thin films had the optimal crystallization. XRD patterns of the NiO/TZO heterojunction diodes for TZO thin films deposited at different deposition powers are shown in Figure 5. All patterns exhibited the (002) and (004) diffraction peaks of the ZnO (TZO) crystallization preferential orientation along the c-axis at diffraction angles (2θ) near 34.28° and 72.58°, with a hexagonal structure; no peak characteristic of TiO2 was found. The diffraction peak revealed that a 2θ value of 36.74° corresponded to the (111) plane of the NiO thin film with a cubic structure, which was different from the result in Figure 4. The result in Figure5 is an important proof that as the NiO thin films is deposited on the TZO thin films with the (002) and (004) diffraction peaks, the crystalline structure of the NiO thin films will be controlled by TZO thin films. For that, the main diffraction peak is changed from the (200) plane to the (111) plane, and then the TZO thin films will dominate the crystalline structure (Figure 1a). Figure 5 also shows that both the diffraction intensity ratio of 2θTZO(002)/2θNiO(111) and the diffraction intensity of the TZO thin films increased with increasing deposition power. In addition, as the deposition power increased from 75 to 150 W, the full width at half maximum (FWHM) values decreased from 0.417 to 0.314, as shown in the inset of Figure 5. Those results reveal that the crystallization of TZO thin films is enhanced at higher deposition powers. This finding proves that the resistivity of TZO thin films closely depends on variations in deposition power (see Figure 3) because the crystallization of TZO thin films increases as the FWHM value decreases .
Equation 3 shows that the Burstein-Moss shift of the absorption edge to the shorter wavelength region is due to the increase in carrier concentration (n e ), as demonstrated in Figure 3.
From the above results, we know that the variations in forward voltage are similar to the turn-on voltages of the NiO/TZO heterojunction diodes. In the high forward voltage region (III), the voltages are large 4.7, 1.3, and 2.1 V for TZO thin film deposition powers of 100, 125, and 150 W, respectively, and those results are dominated by the SCLC mechanism. The transition region (II), between regions (I) and (III), often appears in SCLC-dominated I-V characteristics when traps are used. The presence of trap bands with different energies is responsible for different slopes in the different regions of the I-V characteristics. The results obtained in this study indicate that the charge transport mechanism of the investigated diodes can be influenced by the SCLC.
In this study, the resistivity of TZO thin films linearly decreased from 1.3 × 10−2 to 2.2 × 10−3 Ω cm, and the average transparency of TZO thin films was about 90% in the wavelength range from 400 to 1,200 nm as the deposition power increased from 75 to 150 W. Transparent p-n heterojunction diodes were successfully fabricated using NiO and TZO thin films. These NiO/TZO heterojunction diodes had an average transparency of over 82% in the visible region. For TZO thin films deposited at 75 W, the symmetrical I-V curve of the NiO/TZO heterojunction diodes was not a typical characteristic of a p-n junction diode. The forward currents of the NiO/TZO heterojunction diodes abruptly increased when the turn-on voltages were over 2.57 V (deposition power 100 W), 1.83 V (125 W), and 2.05 V (150 W), demonstrating that these I-V curves are a characteristic of a typical p-n junction diode. The log scale of these I-V curves indicated that the SCLC dominates the current transport.
The authors acknowledge the financial support from NSC 101-2221-E-005-065, 101-2221-E-244-006, and 101-3113-S-244-001.
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