ICMCTF2012 Session C2-2/F4-2: Thin Films for Photovoltaics and Active Devices: Synthesis and Characterization

Transparent conductive oxide (TCO) films are a widely used group of functional materials that can be found in a large variety of applications such as consumer electronics (e.g. flat panel displays and touch-screens), solar cells and smart glass/windows. Indium Tin Oxide (ITO) – the most widely applied TCO, exhibits excellent electrical and optical properties and good corrosion resistance. The drawback of ITO is that it is too expensive for the majority of low cost applications and, due to the rarity of In, has a potential to become even more expensive in the near future. Aluminum-doped Zinc Oxide (AZO) is considered to be one of the most viable alternatives to ITO in many application areas.

Deposition of AZO by sputtering can be done non-reactively from ceramic targets or reactively from metal Al doped Zn targets. Reactive deposition mode, as compared to sputtering ceramic targets, offers significant cost savings on the target material and an increase in production rates. Operation in reactive mode however requires a stable and well controlled process. This concerns both, the reactive gas delivery (i.e. partial pressure) and plasma substrate interaction.

Impurity-doped ZnO thin films that would be suitable for transparent electrode applications in Si-based thin-film solar cells must necessarily attain not only a decrease of plasma resonance frequency by lowering the carrier concentration while retaining a low resistivity, but also a significant scattering of the incident visible and near-infrared light by surface texturing the film. This paper describes the influence of the kind and content of dopant on the electrical properties and their stability as well as on the light management obtainable by surface texturing in impurity-doped ZnO thin films. Al-, Ga- and B-doped ZnO (AZO, GZO and BZO) thin films were prepared on OA-10 glass substrates at a temperature of room temperature (RT) or 200^oC using a pulsed laser deposition (PLD) and magnetron sputtering deposition (MSD). AZO, GZO and BZO thin films were prepared by PLD using an ArF excimer laser. AZO and GZO thin films were also prepared by direct current MSD with superimposed radio frequency power. The electrical properties were evaluated for impurity-doped ZnO thin films prepared with a thickness in the range from 0.5 to 2 μm, because the obtainable electrical properties were considerably dependent on the film thickness. In addition, for impurity-doped ZnO thin films prepared on substrates at 200^oC, the following characteristics were found to be considerably dependent on the content of impurity doped into the thin films, irrespective of the deposition methods used: the obtainable carrier concentration, Hall mobility and their stability in long-term moisture-resistance tests as well as the surface texture structure formed by wet-chemically etching and its resulting haze properties. It should be noted that differences in the obtainable Hall mobility among AZO, GZO and BZO thin films prepared with a low resistivity on the order of 10^-4 Ωcm by the different deposition methods were attributed to the content rather than the kind of impurity doped into the thin films. In addition, the influence of rapid thermal annealing (RTA) on surface texture formation as well as on light management was investigated in the impurity-doped ZnO thin films. The RTA treatment significantly improved the obtainable light management in thin-films prepared on substrates at 200^oC over that found in thin-films prepared at RT. Comparing AZO, GZO and BZO, the AZO thin films prepared with an appropriate Al content on substrates at 200^oC were found to be the most suitable for transparent electrode applications in Si-based thin-film solar cells.

Ga-doped ZnO (GZO) is one of the promising materials for transparent electrodes in flat panel displays and solar cells. Highly transparent GZO polycrystalline films with low resistivity can be deposited at low temperatures below 473 K. Very little work is currently available in the published literature on temperature dependence of electrical properties; resistivity, r, carrier concentration, n, and Hall mobility, m, of GZO films. In this paper, we have investigated the subject to discuss carrier transport through grain boundaries in the films.

GZO films with a thickness of 200 nm were deposited by ion-plating with DC-arc discharge. Sintered ZnO tablets with different Ga₂O₃ contents ranging from 0.003 to 4 wt.% were used as resources. During the growth process, an oxygen gas was introduced into the deposition chamber to compensate oxygen deficiencies. Hall measurements were carried out using the van der Pauw method at 83-343 K under the magnetic field of 0.47 T.

Analysis of data obtained by the Hall measurements shows that the characteristics of μ-T curve gradients, ⊿μ/⊿T, plotted as a function of n can be divided into three regions: (1) Region I; n values below n=1×10²⁰ cm^-3, the ⊿μ/⊿T > 0 (grain-barrier-limited transport) and the values decreased with increasing n, (2) Region II; n values ranging from 1×10²⁰ to 5×10²⁰ cm^-3, ⊿μ/⊿T=0: μ values were independent of T and (3) Region III; n>1×10²¹cm^-3, the ⊿μ/⊿T < 0 (metal-like behavior). This tendency is probably due to the change in dominant scattering mechanism from grain boundary scattering to phonon scattering with increasing n.

In the case of Region I described above, based on the model that carrier mobility is mainly determined by the potential barrier, V_B, at the grain boundary, we applied a charge trapping model to understand the nature of grain boundary scattering on carrier transport for polycrystalline GZO films. As a result, we find the values of the V_B of less than 0.02 eV for all samples in Region I. Considering that even GZO films in the Region I have the V_B values smaller than kT at room temperature, tunneling current can makes a major contribution to the carrier transport trough grain boundaries for all GZO films under investigation.

Cadmium oxide has a high intrinsic electron mobility compared to all other conventional transparent conductive oxides (TCOs) such as ZnO:Al and indium-tin-oxide. Unintentionally doped CdO films can have Hall mobilities greater than 150 cm²/Vs and carrier concentrations in the high 10¹⁹ cm^-3 resulting in resistivities in the 10^-4 Ω cm range. However, the small band gap (2.2 eV) severely restricts the visible transmission of undoped CdO, thus limiting its application as a TCO. However, the optical absorption edge can be considerably widened by increasing the electron concentration due to the Burstein-Moss shift which occurs when the Fermi level shifts to higher energy and more conduction band states are filled with electrons from the donor dopants. In this work we report on high quality films of CdO, undoped and doped with indium, deposited by filtered cathodic arc deposition (FCAD) and by pulsed laser deposition (PLD). The properties of films synthesized by both methods will be compared. Using these highly non-equilibrium synthesis methods, we have achieved extremely low resistivities (less than 5 x 10^-5 Ω-cm) for In-doped CdO films with an electron concentration greater than 1x10²¹ cm^-3 and mobilities as high as 200 cm²/Vs. Moreover, due to the high electron concentration, these films have a band gap >3.2 eV. The high optical transmittance (>85%) of this material in the spectral range extending from the visible to 1300 nm in the infrared makes it an ideal TCO for thin film photovoltaics, especially in multijunction solar cells where high infrared transmittance is essential.

Overview of the research activities in the CIGS PV technology will be presented. In addition, the current status and future prospects of the CIGS technology will be discussed based upon our research results.

High-efficiency CIGS solar cells and submodules have been developed in our research group. In order to fill the efficiency gap between small-area cells and commercial modules, the multi-stage evaporation technique has been applied to fabricate the monolithically integrated CIGS submodules. The conversion efficiencies of integrated submodules on 10x10cm² sodalime glass and flexible ceramics substrates have been improved up to h=16.6% and h=15.9%, respectively. These results indicate that the CIGS technologies are competitive with the current Si and CdTe technologies in terms of both cost as well as performance.

At the moment, CIGS (CuInGaSe₂) solar cells show the highest efficiency among industrial scale produced thin film solar cells. Given the present strong increase in production, however, the availability and price of indium will become an issue because of its low abundance in Earth's crust. Therefore, there is strong interest in alternative indium free absorber materials. Kesterites CZTS (Cu₂ZnSn(S_xSe_1-x)₄) attracted most attention owing to the fact that relatively high efficiencies have already been demonstrated and also due to the similarity to CIGS.

In this contribution we report on reactive sputtering for deposition of CZTS precursors. In order to avoid Sn loss at elevated temperatures a two stage process, synthesis of precursor films at relatively low substrate temperature followed by annealing, is used. Depositions are performed by pulsed DC magnetron sputtering from two targets, a CuSn alloy and Zn, in a mixture of Ar and H₂S. The film structure is evaluated by X-ray diffraction and Raman spectroscopy while the composition is analysed by RBS, XRF, EDS and EPMA. Internal stress is measured by deflection of thin substrates. Characteristics of the deposition process are discussed with respect to the discharge power, total pressure, substrate temperature and H₂S flow on the film structure and composition.

Sulphur incorporation can be readily controlled by H₂S flow with the structure changing from amorphous to columnar with increasing S content. The main issue encountered in the depositions is related to the film composition as the ratio between Cu and Sn does not correspond to the target composition. This effect is discussed in detail with respect to the sputtering and transport through the gas phase. Finally, material properties after annealing are briefly summarized.

This paper studies the reset behaviors of resistive random access memory devices with the Pt/SiO_x:Gd/TiN structure. The ReRAM device has treated a same stopping voltage with ac and dc modes respectively for reset process. Different reset method can influence the oxidation rate of the filament and the oxygen anions escaping rate from the oxygen reservoir, causing different high resistive states. When increasing stopping voltage, more triggered oxygen anions can produce to oxidize with the filament. Furthermore, the device also exist the unipolar switching phenomenon with an atypical reset behavior, and this reset behavior is dissimilar to the typical unipolar reset phenomenon undergoing the dc sweeping method. Utilizing the fast IV measurement simulating the AC operation mode analyzes the reset behavior of bipolar and unipolar. Undergoing the increase ramp speed condition respectively to reset the device on bipolar switching mode, a raising-time dependent relation affects the reset behavior such as reset voltage and stopping current. A mechanism is proposed to explain the bipolar unipolar reset characteristics.