- Nano Express
- Open Access
The heterojunction effects of TiO2 nanotubes fabricated by atomic layer deposition on photocarrier transportation direction
© Chang et al; licensee Springer. 2012
- Received: 3 November 2011
- Accepted: 23 April 2012
- Published: 23 April 2012
The heterojunction effects of TiO2 nanotubes on photoconductive characteristics were investigated. For ITO/TiO2/Si diodes, the photocurrent is controlled either by the TiO2/Si heterojunction (p-n junction) or the ITO-TiO2 heterojunction (Schottky contact). In the short circuit (approximately 0 V) condition, the TiO2-Si heterojunction dominates the photocarrier transportation direction due to its larger space-charge region and potential gradient. The detailed transition process of the photocarrier direction was investigated with a time-dependent photoresponse study. The results showed that the diode transitioned from TiO2-Si heterojunction-controlled to ITO-TiO2 heterojunction-controlled as we applied biases from approximately 0 to -1 V on the ITO electrode.
- Anodic Aluminum Oxide
- Atomic Layer Deposition
- Reverse Bias
- Forward Bias
In recent years, nanostructure materials have attracted much interest due to their remarkable physical and chemical properties. Among these nanostructure materials, TiO2 nanostructures have emerged as one of the most promising materials for optoelectronic devices because of the variety of growth methods and their high melting point (1,855°C), chemical inertness, physical stability, indirect band gap (3.2 eV), high photoconversion efficiency, and photostability. Based on its excellent optical properties, TiO2 has been utilized for many applications, such as photoelectrochemical water splitting , photoelectrochemical generation of hydrogen , dye-sensitized solar cells , and photocatalysis .
Unfortunately, the inherent high band gap of 3.2 eV limits the optical application of TiO2. Therefore, most of the research efforts have been focused on modifying the material properties with the hope of enhancing the absorbability of TiO2 to extend from the ultraviolet (UV) region to the visible region through a doping process [5, 6]. However, in addition to modifying material properties, it is essential to understand and pay special attention to heterojunctions in the study of traditional semiconductors because the heterojunction effect determines a device's ultimate performance. Furthermore, when nanoscale materials are utilized, the heterojunction effects are magnified and become even more critical. For heterojunction semiconductor devices, the type of contact determines the carrier transportation direction, and the space-charge region and the potential gradient of a junction determine the magnitude of the photocurrent as the devices are illuminated by a light source [7, 8]. Therefore, apart from the modification of the intrinsic material, heterojunction studies of devices or sensors under UV light illumination should not be neglected. Wang et al. reported that N-TiO2/C heterojunctions could increase absorption in the visible light region and exhibit a higher photocatalytic activity than pure TiO2 . Zhang et al. reported that the heterojunction of Bi2MoO6/TiO2 shows effective separation of photo-generated carriers driven by the photo-induced potential difference generated at the Bi2MoO6/TiO2 heterojunction interfaces . Lee et al. reported that a TiO2/water solid-liquid heterojunction exhibits a high photosensitivity, excellent spectral selectivity, linear variations in photocurrent, and fast responses with ultraviolet . As mentioned above, the study of TiO2 heterojunctions has focused on single heterojunctions. However, there are two heterojunctions in many devices, such as solar cells . Therefore, more studies in this field are necessary, especially in nanostructure systems.
In this research, we employed an anodic aluminum oxide (AAO) template and atomic layer deposition (ALD) nanotechnology to prepare TiO2 nanotube arrays. A thin ITO electrode was deposited on top of the TiO2 to form a Schottky contact , and the heterojunction effects on the photoconductive characteristics of TiO2 nanotubes under forward and reverse biases were investigated. To explain the change of the carrier transportation direction, we discussed the mechanism using an energy band diagram.
To fabricate TiO2 nanostructures, first, AAO was prepared on p-type (100) silicon substrates with two-stage anodization. The detailed fabrication process for the AAO was presented elsewhere . The diameters of the AAO pores are 70 nm. In particular, the AAO barrier has been removed after the pore widening treatment so that the deposited TiO2 nanostructure can adhere to the substrate after the removal of the AAO template.
To deposit the TiO2 nanostructure by ALD, Si substrates with the AAO templates were first placed into a quartz tube reactor with the operating environment maintained at 1.6 × 10-1 Torr and 400°C. The precursors of TiCl4 and H2O, kept separately in a canister at 30 ± 1°C and 25 ± 1°C respectively, were used as Ti and O sources, respectively. Pure Ar gas (99.999%) was used as a carrier gas and purge gas. To prepare TiO2 nanotube arrays, a 300-cycle deposition parameter was introduced. Each deposition cycle consisted of eight steps, which included TiCl4 reactant, pump-down; Ar purge, pump-down; H2O reactant, pump-down; and Ar purge and pump-down. Typical pulse times for the TiCl4 and H2O precursors were 1 s, and the purge time was 3 s. To remove the residual reactants and by-products efficiently, the pump-down process was added after each step. Then, with mechanical polishing, the TiO2 film on the top surface of AAO was removed. Finally, the AAO template was selectively removed by a 0.1 wt% sodium hydroxide (aqueous) solution, and TiO2 nanotube arrays were fabricated on the Si substrate.
Highly ordered self-aligned TiO2 nanotubes can be fabricated using the template of (AAO) on p-type (100) Si substrates and the ALD technique . To prepare TiO2 nanotube arrays, a 300-cycle deposition parameter was adopted. An ITO film (450 nm) and Al film were chosen as the electrode and the back electrode for ohmic contact and were deposited using an e-gun evaporation system and a thermal evaporation coater, respectively. The current characteristics of the specimens were recorded by a Keithley 2400 sourcemeter (Keithley Instruments Inc., Cleveland, Ohio). Due to the measuring requirement, a small voltage of approximately 10-6 V is applied automatically by the Keithley 2400 sourcemeter when the equipment is set up to 0 V bias. Nevertheless, it is regarded as a short circuit (approximately 0 V) condition because 10-6 V can be considered negligible. To perform the study, the photoresponse was measured under UV illumination of approximately 21 mW/cm2 (λ = 365 nm) in air at room temperature. A field-emission scanning electron microscope (FESEM JSM-6500 F, JEOL Ltd., Tokyo, Japan) and transmission electron microscopy (TEM) were utilized to examine the morphology of the TiO2 nanotube arrays.
Although the fast photoresponse can be observed at the UV on/off moment, the gradual increase and the slow recovery of the photocurrent under UV on/off illumination were recorded when the bias was -0.4 V. However, when a -0.6 V bias was applied, no net photocurrent magnitude was observed under UV illumination, except for the gradual increase and slow recovery of the photocurrent, as shown in Figure 4c. This phenomenon means that we have reached the transition point because the photocurrent generated from the ITO-TiO2 heterojunction was enhanced at an increased reverse bias, whereas the current generated from the TiO2-Si heterojunction was reduced at an increased forward bias. The two currents reached equal magnitudes at the transition point, thus canceling each other out. After the bias was enlarged to -1 V, the current was increased from 0.476 to 0.485 mA under UV illumination, as shown in Figure 4d. The photocurrent originating from the ITO-TiO2 heterojunction was large enough to counteract the effects of the photocurrent resulting from the TiO2-Si heterojunction, finally governing the photocarrier transportation direction. However, the gradual increase and slow recovery in the photocurrent were still observed. Nevertheless, the important finding of our study is that we have observed that the transition point of the photocurrent transportation direction changes from TiO2-Si heterojunction-controlled to ITO-TiO2 heterojunction-controlled and from short circuit to -1 V.
From the above results, we speculate that the photocarrier transportation may be dominated by the space-charge region and the potential gradient of the heterojunctions. However, the area ratio of the heterojunction of TiO2-Si to ITO-TiO2 should be considered. We suppose that more heterojunction area would contribute more photocarriers when UV illumination is used. The photocarriers move in opposite direction for the TiO2-Si and ITO-TiO2 heterojunctions. From Figure 1b, c, the heterojunction area ratio can be calculated by the following equation:
where rTiO2 is the dimension of the TiO2 nanotubes and tTiO2 is the wall thickness of the TiO2 nanotubes. The voltage of the transition point is -0.6 V when the heterojunction area ratio is 4.1. The voltage at the transition point may decrease as the junction area ratio decreases. Nevertheless, further studies are required to clarify this point.
In summary, we studied the heterojunction effects on the UV photoresponse of TiO2 nanotubes fabricated by ALD on a Si substrate using ITO as the electrode. In the short circuit (approximately 0 V) condition, photocurrents of 0.13 and -0.13 mA were measured when the positive and the negative electrodes were connected to the ITO, respectively. When a negative bias was applied, the net photocurrent changed from off to on and UV illumination decreased from 0.13 mA to 3 μA as the negative bias was decreased from short circuit (approximately 0) to -0.4 V. The photocurrent reached a transition point when -0.6 V was applied, which is where current generated by the ITO-TiO2 heterojunction equals the current generated by the TiO2-Si heterojunction. These intriguing results may be attributed to the depletion regions in the ITO-TiO2 and Si-TiO2 heterojunctions. More studies are needed to clarify which heterojunction dominates the photocurrent.
The authors would like to thank the National Science Council of the Republic of China, Taiwan, for the financial support of this research under Contract No. NSC-96-2628-E-009-010-MY3.
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