ICMCTF2015 Session C3: Transparent Conducting Oxides and Related Inorganic and Organic Materials

In this paper, we describe the present status and prospects for further development of high-efficiency Al-doped ZnO (AZO)/n-type oxide semiconductor/p-type Cu₂O heterojunction solar cells that feature a structure that is fabricated by inserting an n-metal oxide thin film between an AZO transparent electrode and a Cu₂O sheet. The Cu₂O sheets and the AZO and n-oxide thin films were prepared by oxidizing Cu sheets using a heat treatment process and by using a pulsed laser deposition (PLD) method, respectively. To achieve a higher efficiency in AZO/n-oxide semiconductor/p-Cu₂O solar cells, we have reported that it was necessary to improve the interface at the n-oxide/p-Cu₂O heterojunction as well as reduce the series resistance and increase the parallel resistance of the solar cells. In addition, we have investigated the effect of the inserted n-metal oxide semiconductor thin film on the obtainable photovoltaic properties in the Cu₂O-based solar cells by inserting various kinds of n-oxide thin films such as binary compounds and multi-component oxides, prepared under various deposition conditions. As a result, we have recently reported that Cu₂O-based p-n heterojunction solar cells with an energy conversion efficiency over 5% could be fabricated by using the PLD method to deposit AZO and n-Ga₂O₃ thin films at a low temperature on Cu₂O sheets [1]. We have more recently found that an improvement of obtainable photovoltaic properties in n-oxide semiconductor/p-Cu₂O heterojunction solar cells could also be achieved by both decreasing the resistivity of Cu₂O sheets without changing the Hall mobility of 100-110cm²/Vs as well as optimizing the chemical composition of multi-component oxides such as Ga₂O₃-Al₂O₃ and Ga₂O₃-ZnO systems, used as the n-semiconductor layer. The for mer was achieved by incorporating Na into the Cu₂O sheets by heat treating, in an Ar gas atmosphere, Cu₂O sheets covered with an appropriate Na compound, resulting in an increase of hole concentration from 10¹³ to 10¹⁸ cm^-3, as recently reported by us. As a result, we have achieved a conversion efficiency over 6% in a MgF₂/AZO/(Ga₂O₃)_0.975-( Al₂O₃)_0.025/Cu₂O heterojunction solar cell fabricated by depositing a (Ga_0.975Al_0.025)₂O₃ thin film on a Cu₂O sheet with a resistivity on the order of 10Ωcm.

[1] T. Minami, Y. Nishi, and T. Miyata, Appl. Phys. Express 6 (2013) 044101.

Gallium (Ga)-doped zinc oxide (GZO) is one of the most conceivable candidates for the alternative material for tin-doped indium oxide (ITO). Many different types of growth methods have been reported for obtaining polycrystalline GZO films. We have paid attention to ion-plating using a DC arc discharge because the method enables the deposition of GZO films on large area substrates at high deposition rates with low plasma damage. In our previous papers, we reported carrier concentration dependences of carrier-scattering mechanism [1] and of photoluminescence (PL) properties [2]. In this paper, to clarify the relationship between the structural and electrical properiies, we study the structural properties of the GZO films systematically in terms of the oxygen (O₂) gas flow rate during the deposition process and the Ga contents.

200-nm-thick GZO polycrystalline films were deposited on alkali-free glass substrates at 200 ºC by the ion-plating. The source GZO pellets were prepared by sintering of the ZnO powder containing the several amounts of Ga₂O₃ powder (0.003-4 wt%). The O₂ gas was introduced into the deposition chamber. The O₂ gas flow rate was varied in the range from 0 to 30 SCCM.

The out-of-plane θ-2θ scan X-ray diffraction (XRD) patterns of all the GZO films were dominant by the (002) diffraction peak, indicating highly c-axis orientation perpendicular to the glass substrates. With increasing O₂-gas flow rates and/or the Ga contents, the (002) diffraction peak shifted higher angles. This showed a decrease in the lattice constant of the c-axis. On the other hand, the in-plane 2θ_χ-ϕ scan XRD measurements revealed an increase in the lattice constant of the a-axis with increasing O₂-gas flow rates and/or the Ga contents. The results showed the biaxial stress of GZO films: the compressive stress acting along the c-axis direction together with the tensile stress acting along the a-axis direction. We found strengthened stress with an increase in O₂-gas flow rate and/or Ga contents. Considering that all of the as-deposited samples were deposited on the same type of substrates with a fixed substrate temperature, the behavior of the residual stress should be discussed in terms of the changes of intrinsic stress. We found that the electrical properties strongly depend on both O₂-gas flow rates and Ga contents. We will demonstrate the relationship between the structural and electrical properties.

Reference: [1] T. Terasako, H. Song, H. Makino, S. Shirakata, T. Yamamoto, Thin Solid Films 528 (2013) 19. [2] T. Terasako, Y. Ogura, S. Fujimoto, H. Song, H. Makino, M. Yagi, S. Shirakata, T. Yamamoto, Thin Solid Films 549 (2013) 12.

WO₃ is known for its electrochromic function by which the material can switch between color (dark blue) and bleach states depending on the imposed electrical voltage. Such switch of multiple states can be achieved by electrochemical insertion of cations and electrons into the tungsten oxide. In this study, a stack of flexible electrochromic device made of NiO –Ta₂O₅ - WO₃ is deposited by sputtering with various supplies of argon and/or oxygen. The stack is sandwiched by two electrodes made of In₂O₃ and on top of polyethylene terephthalate (PET) substrate. Each film was individually fabricated at first to determine the optimal parameters of deposition. Structures, compositions and properties of deposited films were examined and analyzed by the following tools: Surface profiler and FESEM to inspect the thickness and surface morphology; XRD and Raman spectrometers for the study of microstructures; EDX and XPS to examine the chemical compositions; four-point probe and Hall effect sensor to measure the electrical resistivity; UV-visible-NIR spectrometer for the assessment of optical properties. After the optimal parameters were determined, a stack of films was fabricated and used the electrochemical insertion of Li cations and electrons into the WO₃ film to make it electrochromisic. Our goal is to utilize the current physical deposition process to fabricate a flexible device with optimal performance. The fabricated electrochromic WO₃ stack shall be flexible enough, which can fit the requirement of incoming generation of mobile devices.

Diluted magnetic semiconductors (DMSs) based on ZnO have attracted a great deal of attention due to their potential application in magnetic semiconductor devices. In particular, they are predicted to display ferromagnetic properties at Curie temperature above room temperature and have large magnetization. (Ga, Co)-ZnO films for use as DMSs have been fabricated using various techniques, including molecular beam epitaxy (MBE) [1], pulsed laser deposition (PLD) [2] and inductively coupled plasma enhanced physical vapor deposition (ICP-PVD) [3], etc. However, there are very few reports about (Ga, Co)-ZnO thin films deposited using radio frequency (rf) magnetron sputtering .

In this study, (Ga, Co)-ZnO [Co_xGa_yZn_(1-x-y)O] films with different Ga contents are co-sputtered on glass substrates by rf magnetron sputtering. The x content [Co/(Ga+Co+Zn)] in the films is ~ 0.05. The content of y [Ga/(Ga+Co+Zn)] varies from 0 to 0.032. Using Hall effect analysis, the resistivity (ρ) of the film is 42.9 Ω-cm when y content is 0. When the content of y increases to 0.032, the ρ value drops greatly to 4.93 × 10^-3 Ω-cm. In photoluminescence analysis, all the oxygen-rich (Ga, Co)-ZnO films contain acceptor-like defects. In magnetic properties analysis, when low Ga contents are doped and with low carrier concentration range (< 2.48 × 10¹⁷/ cm³), saturation magnetization (Ms) is established by Bound Magnetic Polaron (BMP) due to defects and magnetic atoms in the films. However, Ms is created by the free electrons and Co atoms that produce the exchange coupling effect when high Ga concentration is present in the films and in high carrier concentration range (> 7.34 × 10¹⁸/ cm³). In the transition zone, since the two mechanisms mentioned above are overlapped and exchanged, Ms value drops when the carrier concentration in the films increases from 2.48 × 10¹⁷/ cm³ to 7.34 × 10¹⁸/ cm³.

Keywords：rf magnetron co-sputtering, diluted magnetic semiconductor s, (Ga, Co)-ZnO films, optical properties, electrical properties, magnetic properties.

Reference

[1] Zhonglin Lu, Hua-Shu Hsu, Yonhua Tzeng, Fengming Zhang, Youwei Du and Jung-Chun-Andrew Huang , Appl. Phys. Lett. 95 (2009) 062509.

[2] Liping Zhu, Zhigao Ye, Xuetao Wang, Zhizhen Ye and Binghui Zhao, Thin solid films 518 (2010) 1879-1882.

[3] Xue-Chao Liu, Zhi-Zhan Chen, Bo-Yuan Chen, Er-Wei Shi and Da-Qian Liao, J. Cryst. Growth 312 (2010) 2871-2875

InTaO₄ thin films are transparent with photocatalytic function if appropriate compositions are obtained. The photocatalytic function is due to the formation of intermediate bands between conduction and valance bands in the original In₂O₃ and Ta₂O₅ structures. Such intermediate bands provide the opportunity for electron-hole separation under which free electrons can move to free surfaces or grain boundaries to combine with radials there. This is the basic mechanism for photocatalytic function. Although single oxide is possible to be photocatalytic, the combination of two transition metal oxides in principle should be much easier to achieve such function with higher efficiency. Current understanding about the mixture of transition metal oxides is that the structure should be homologous, i.e. a mix of crystallites of In₂O₃ and Ta₂O₅ to give the apparent compositions InTaO₄. Such structure can guarantee the formation of intermediate bands and be easily achieved by sputtering. To realize this idea and characterize the properties of combined films, we prepared the InTaO₄ thin films by sputtering through a specifically designed process. A multilayer In₂O₃ and Ta₂O₅ thin films were alternately deposited on heated glass substrates. Each layer is about 5-7 nm and 8 – 10 layers of each oxide were deposited to have total thickness around 100 nm. After deposition, the whole multilayer film was rapidly annealed at 700°C to have a compound mixture. To examine this annealed compound films, we shall use surface profiler and FESEM to inspect the thickness and surface morphology; XRD and TEM to study the microstructures; EDX or XPS to examine the chemical compositions; UV-visible-NIR spectrometer to assess the optical properties. The photocatalysis will be conducted by the electrochemical test to evaluate the decomposition of methylene blue/methylene orange under visible light radiation. It is expected this newly fabricated compound films shall have applications in many biomedical devices.