ICMCTF2012 Session C4-1: Transparent Conductive Films: Inorganic Oxides, Organic Materials, Metals

Existing applications for zinc oxide thin films include transparent electronic devices, transparent conducting electrodes and ultraviolet photonics. Deposition methods such as molecular beam epitaxy and pulsed laser deposition are normally required to achieve films with sufficient quality for electronic device applications. Unfortunately, these methods typically incur high cost and provide limited scalability. The filtered cathodic vacuum arc (FCVA) deposition technique offering both low cost and high throughput is routinely employed to produce aluminium doped ZnO for degenerate transparent conducting layers. However, the potential for this technique to produce large area, low-defect ZnO films for electronic devices has largely been overlooked.

We report on the structural, optical and electrical characteristics of ZnO films deposited on a-plane sapphire substrates by FCVA. These films exhibit desirable properties including high transparency, moderate intrinsic carrier concentrations (10¹⁷ – 10¹⁸ cm^-3), Hall mobility up to 10 cm²/Vs and low surface roughness (with RMS values typically <5% of the film thickness). These properties can be further improved after annealing the deposited films in oxygen at elevated temperatures. Schottky diodes featuring AgO_x and IrO_x anodes formed on annealed FCVA grown ZnO films exhibit low ideality factors (<1.20) and high rectification (~10⁶). Subsequent production of ZnO MESFET (metal semiconductor field effect transistor) devices with excellent characteristics and yield confirm the potential of these films for electronic device applications.

Since the interface regions within the Schottky diodes and MESFETs are crucial to their performance, cross-sectional transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) have been used to study the microstructure and composition of IrO_x/ZnO Schottky barriers. These barriers were formed by reactively pulsed-laser-depositing IrO_x onto single crystal ZnO wafers. EELS showed that Zn could be found within the IrO_x anode in a region extending up to 10 nm from the interface. This strongly suggests that zinc diffuses across the device interface during the deposition of the anode, leading to the creation of Zn vacancies (acceptors) in the ZnO sub-interface region. Evidence also existed for oxygen passivation near the interface, probably resulting from the presence of active oxygen during the pulsed-laser-deposition process.

Metal oxides are a class of materials playing an increasingly important role in opto-electronic devices, solar cells, low emissivity windows and many other applications. Aluminum doped zinc oxide (AZO) has been identified as an especially promising material due to its great abundance and the low cost of its constituting elements. In previous work we have shown that cathodic arc deposition of AZO resulted in high quality material with resistivities in the mid to low 10^-4 Ωcm and mobilities as high as 60 cm² V^-1 s^-1 [1], comparable to the more expensive indium tin oxide (ITO). To further assist the deposition process by ionization of the gas, we have incorporated a constricted plasma source which is a special kind of glow discharge plasma generator characterized by its simple design and a low kinetic energy of the downstream ions [2]. Preliminary results indicate that increasing the current (or equivalently power) of the constricted plasma source resulted in an increase of the electron mobility of AZO thin films even when grown at relatively low temperatures, a highly desirable feature critical to promote the broad application of AZO.

[1] R.J. Mendelsberg, S.H.N. Lim, Y.K. Zhu, J. Wallig, D.J. Milliron, A. Anders, J. Phys. D-Appl. Phys. 44/23 (2011).

[2] A. Anders, M. Kuhn, Rev. Sci. Instrum. 69/3 (1998) 1340.

The backbones of the current microelectronics industry are components based on Si semiconductors: Modern data processing and telecommunications almost exclusively relies on the use of these single crystalline materials while amorphous or polycrystalline films are used in large scale for TFT devices, for example in flat panel displays. Large sectors of global industry are engaged in their production, further processing and application. However, the perspectives for further developments are limited since the constraints of the material such as non-availability for flexible devices; optical opacity and need for high temperature processing are obvious. The emerging class of oxide semiconductors is able to overcome many of those restrictions, especially because some of them can be prepared as thin (transparent) films under comparatively moderate conditions.

Within this paper, we give an overview on the status of our current research in the framework of the large-scale integrated European research project “Orama” on metal oxides for electronic applications. We utilize a holistic approach starting with first principle materials modelling and ending with certified materials for the main applications indium free oxide TFTs, oxide based “CMOS” devices, advanced chemical sensors and advanced LEDs. A detailed overview on the current state of the research will be presented.

Ga-doped ZnO (GZO) is a promising electrode for use in flat display panels and thin film solar cells. Electrical and optical properties of GZO films are comparable to Sn-doped In₂O₃ (ITO) films. The environmental stability of GZO is an interesting and crucial research topic for practical applications. For a 150-nm-thick or thicker GZO film deposited on glass substrates, it is easy for the variation in electrical resistivity before and after reliability test for 500 h under the condition of 60 °C and 95% relative humidity (RH) to be less than 10%. To date, achieving this stability level has proven to be quite difficult for a 100-nm-thick GZO film. Herein, we report a successful materials design: indium co-doping (GZO:In target: 3 wt% Ga₂O₃ and 0.25 wt% In₂O₃) gives a solution to the crucial issue. The 100-nm-thick GZO:In films were deposited on glass substrates at 200°C by ion plating with direct current arc-discharge under various oxygen gas flow rates (from 0 to 25 sccm). All the samples have an average optical transmittance over 83% in the visible and infra-red regions ranging from 450 to 1200 nm. The reliability test results show that the stability performance is enhanced for GZO:In films. With the oxygen flow rate of 15 sccm, the 100-nm-thick GZO:In film has a resistivity of 4.24×10-4 Ωcm, carrier concentration of 6.37×10²⁰ cm^-3 and Hall mobility of 23.1 cm²V^-1s^-1. The variation in resistivity before and after the reliability test is 7.8%, which is much better than the case without indium co-doping. Room temperature Hall measurement results show that, as oxygen flow rate increased, the carrier concentration in GZO:In film was monotonic decreased, while the Hall mobility had a maximum and resistivity had a minimum in the oxygen flow rate range from 10 to 15 sccm. The role of indium co-doped in GZO film on film stability should be further investigated.

This work was supported by New Energy and Industrial Technology Development Organization (NEDO) under the National Project of Rare Metal Indium In Substitute Materials Development.

Ga-doped ZnO (GZO) is one of promising candidates as transparent electrodes in optoelectronic devices. It is well known that resistivity of GZO strongly depend on their film thickness. However, key parameters, which dominate the thickness dependence, has not been understood yet [1]. Recently, nanosheet seed layers were proposed to control crystal orientation of polycrystalline films on amorphous substrates [2]. By using nanosheets with two dimensional hexagonal lattices, we can control c-axis orientation and lateral crystalline size of GZO films. In this paper, we report temperature dependence of Hall effects in polycrystalline GZO films deposited on glass substrates with and without the nanosheet seed layer.

The GZO films with thickness between 30 and 350 nm were deposited by ion-plating with direct current arc discharge. The Hall effect measurements were performed in temperature range from 11 to 300 K. The resistivity of GZO films at room temperature rapidly decreased with the increase of thickness up to 100 nm. With further increasing the thickness, the resistivity gradually decreased. These observations were common for both of GZO films deposited with and without the nanosheet. In the temperature dependence, the GZO films thicker than 100 nm showed metallic behavior, that is, the resistivity increased with increasing the temperature. While, the GZO films with thickness of 40 nm showed minimum resistivity at around 80 K. The resistivity decreased with increasing the temperature up to 80 K, then turned to increase with further increasing the temperature. This anomaly was caused by increase of carrier concentration with the increase of temperature. The increase in carrier concentration can be attributed to thermal delocalization from some localized states possibly exist in such thin films. For the GZO films with thickness of 100 nm, the carrier concentration was nearly independent on the temperature as expected in degenerate semiconductors. On the other hand, the Hall mobility decreased with increasing the temperature for all the samples. The observation suggests that phonon scattering dominate the temperature dependence of the Hall mobility. The Hall mobility at the lowest temperature should be dominated by static scattering centers such as ionized impurities and some defects. The Hall mobility at the lowest temperature monotonously increased with increasing the film thickness over the range studied here. The variation of the Hall mobility showed strong correlation with the degree of c-axis orientation.

[1] T. Yamada et al., Appl. Phys. Lett. 91, 051915 (2007); J. Appl. Phys. 107, 123534 (2010).

[2] T. Shibata et al., Adv. Mater. 20, 231 (2008).

Zinc oxide belongs to the material class of transparent conductive oxides (TCO) which is both of scientific as well as technical interest, due to the fact that TCO layers are used on a large scale for transparent electrodes in many technical fields: flat panel displays, low emissivity glass coatings [1], organic light emitting diodes (OLEDs) or thin film solar cells [2] . TCOs are degenerately doped (N >> 10¹⁹ cm^-3) n-type compound semiconductors with wide bandgaps (E_g > 3 eV) and low resistivities in the range of 10^-4 to 10^-3 Ωcm [3] . For its application as transparent electrodes they have to be highly conductive and transparent at least in the visible spectral range. The electron density N in ZnO is limited to about 1.5^.10²¹ cm^-3. Higher dopant (electron) concentrations are not possible due to the formation of secondary phases of the dopant and the host atoms. One goal is therefore to achieve a low resistivity ρ = (eNµ)^-1 by maximizing the mobility µ of the electrons. The carrier transport in single-crystalline TCO semiconductors at such high carrier concentrations is limited by ionized impurity scattering [4] . For the application as transparent electrodes, however, polycrystalline TCO films have to be used, which exhibit additional scattering processes: grain boundary scattering and scattering at other crystallographic defects, further reducing the electron mobility. ZnO alloys, like Zn_1-xMg_xO, are of scientific interest, since the addition of other elements changes the band gap. With respect to electrical transport, the so-called alloy scattering has to be taken into account. In this paper the electrical transport in epitaxial and polycrystalline ZnO and ZnMgO films is compared. For the film deposition, magnetron sputtering as a well-known large-area deposition method is used. Since in magnetron sputtering high-energetic negative ions (for instance O^-) occur, which can introduce crystallographic defects or interstitial oxygen atoms, special emphasis is given to the radial variation of the electrical properties and its correlation to the bombardment of the films by negative ions. For this purpose, radially resolved ion distribution functions of negative oxygen ions and other species at a growing film were measured, both for new and eroded targets.

[1] J. Szczyrbowski, G. Bräuer, M. Ruske, H. Schilling, A. Zmelty, Thin Solid Films 351 (1999) 254.

[2] K. Ellmer, A. Klein, B. Rech (Eds.), Transparent Conductive Zinc Oxide: Basics and Applications in Thin Film Solar Cells, Springer, Berlin, 2008.

[3] T. Minami, MRS Bull. 25/8 (2000) 38.

[4] K. Ellmer, R. Mientus, Thin Solid Films 516/30 May (2008) 4620.

P-type conductive TCOs like delafossite type CuAlO₂ films can open the way to transparent electronics when combining them with well-established n-type TCOs. So, it is necessary to investigate into this relatively young field of materials to get an understanding of the basics of the preparation route.

The authors report on experiments on Cu-Al-O-mixtures with compositions slightly deviating from the delafossite stoichiometry CuAlO₂.

Thin films of Cu_xAl_yO_z are prepared by RF-magnetron-cosputtering at room temperature. The stoichiometry is influenced by different sputtering parameters and controlled via lambda-probe setup. After preparation, the films are annealed in inert gas atmosphere at different temperatures. Depending on the film stoichiometry, the annealed films show different TCO properties: They are either transparent or opaque in the visible spectrum. They are either p-type , n-type or non-conducting. And they are crystallized in different phases. So, the preparation way for p-type CuAlO_x is narrow but can be controlled very well.

A matrix will be shown that sums up the experiments and results. Different (narrow) pathways to gain p-type conductive Cu-Al-O-thin films are suggested and implemented into that preparation-route matrix.