Nowadays, numerous applications in the electronics and/or optoelectronics field need transparent thin films presenting a good electrical conductivity. The transparent conductive oxides (TCO) which reveal a large band gap and a good electrical conductivity fulfil these requirements. Recently, due to the significant increase of the demand, the prize of the most employed TCO, namely indium tin oxide (ITO) has strongly increased. Therefore, an alternative to this material becomes necessary. Among all the candidates, ZnO:F and Cd₂SnO₄ present the best performance in term of transparency and electrical conductivity. For obvious environmental reasons the latter cannot be considered. Therefore, ZnO:F is indentified as the best candidate to replace ITO in TCO applications.

The most employed techniques for the synthesis of ZnO:F are Chemical Vapor Deposition and Spray Pyrolisis which both require organometallic precursor and high temperature processing. Another drawback of these technologies is the low chemical purity of the synthesized films because of the presence of the precursor decomposition products. At the contrary, reactive magnetron sputtering is an environmentally friendly technology allowing the synthesis of thin films with very fine control of the chemistry. Therefore, the aim of this works is to study the reactive magnetron sputtering of ZnO:F.

Thin films were prepared by DC reactive sputtering using a zinc target in an Ar/O₂/F₂ mixture. In a first attempts, ZnO films have been synthesized in order to optimize the matrix properties in terms of cristallinity and transparency. The studied parameters were the DC power (P_DC), the total pressure (P_Tot) and the O₂ flow (f_O2). Our data reveal that the ZnO films presenting the best features are prepared for P_DC = 70 W, P_Tot = 30 mTorr and f_O2 = 3 sccm.

The second step was to introduce fluorine in this matrix. Therefore, we have studied the crystallographic, chemical, electrical and optical properties of the deposited films as a function of the fluorine content. In our deposition window, all films present a high transmission in the visible (> 80%). Our XRD data reveal decrease of the crystallite size with the increase of the fluorine content. Above a fluorine concentration of 2-3%, the ZnO:F cristallinity decreases. Our XPS and XRD data suggest that F atoms substitute O atoms in the ZnO structure. Finally, the electrical properties have been investigated by Hall effect measurements. For the optimal synthesis conditions (~ 2% of fluorine in the film), a charge carrier density of ~ 10²⁰ cm^-3, an electrical resistivity of 10^-2 Ω.cm and charge mobility of about 4 cm²/V.sec have been measured.

Tin doped indium oxide (In₂O₃) aka ITO is one of the most important transparent conducting oxides, yet it is only recently that many fundamental aspects the bulk and surface physics of indium oxide itself and of ITO have been addressed [1-3]. We have an ongoing programme concerned with growth of In₂O₃ on Y-stabilised ZrO₂ by oxygen plasma assisted molecular beam epitaxy and will review our most recent work in this area. Issues that will be addressed include the following:

• The influence of surface energies and strain on the growth of In₂O₃ on low index zirconia surfaces. Mechanisms for relief of strain, including crystallographic tilting and development of nanostructures during high temperature MBE growth.
• The influence of strain on the optical properties of ultrathin In₂O₃ films.
• Surface structure and surface physics of In₂O₃ and ITO surfaces, including development of electron accumulation layers for material with low bulk doping levels.

References

1 P D C King et al., Physical Review Letters 2008 101 116808

2 A Walsh et al., Physical Review Letters 2008 100 167402/1-4

3 K H L Zhang et al., Chemistry of Materials 2009 21 4353-4355

The ITO films deposited by magnetron sputtering and facing target sputtering at low substrate temperature have quite different distributions of film properties. However, their origin was still not clear. In this paper, we clarify the origin of the different distributions between the sputtering methods, and will propose a sputtering method to deposit the ITO films with good uniformity.

In the deposition of ITO films by a conventional planar magnetron sputtering, the films deposited at the center of the substrate have higher oxygen content than the films deposited at the end of the substrate. It should be noted that the film deposited by a facing target sputtering has much lower oxygen content than the films deposited by conventional planar magnetron sputtering. As a result, the ITO films with poor transparency were always obtained by the sputtering in pure Ar gas, when a Facing Target Sputtering system was used.

These phenomena can be explained by the following model; When sputter-deposition of ITO films was performed at a low temperature, only oxygen atoms produced by the sputter-emission from the target surface promote the oxidization of indium atoms on the film surface. In other words, oxygen molecules cannot oxidize the indium atoms at a low temperature.

In addition, oxygen atoms sputter-emitted from the target have different angular distributions than indium atoms have. That is, the emission ratio of oxygen atoms to indium atoms sputter-emitted from the target surface changes depending on the emission angle and gradually decreases with an increase of the emission angle. This phenomenon mainly causes the distribution of the properties of ITO films on the substrate, although bombardment of high energy negative oxygen ions also affects the distribution of film properties in planar magnetron sputtering.

In order to deposit uniform film on the substrate, compensation of the angular distribution in the emission ratio of oxygen atoms to indium atoms is necessary. Use of two sputtering sources arranged like a facing target sputtering system is one of the solutions to compensate the distribution and obtain the films with more excellent uniformity.

Among various materials for thin film transparent conductor applications, Al-doped ZnO (AZO) is a particularly attractive material because of its excellent properties, such as higher thermal stability, good resistance against damage by hydrogen plasma and potentially, low cost compared to indium tin oxide (ITO). Of the various available deposition techniques, Atomic layer deposition (ALD) provides superb control at the nanoscale for thickness, uniformity, conformality and Al doping of AZO films. This is particularly attractive for use in nanostructures, as well as in more conventional applications such as liquid crystal displays.

We report here results for structural, optical and electrical properties of ultrathin ALD AZO films as a function of at% Al. AZO films of ~ 100nm thickness were deposited on quartz substrates at 150C using a commercial BENEQ TFS 500 reactor using diethyl zinc (DEZ) and H2O as precursors for ZnO, and trimethyl aluminum (TMA) and H2O as precursors for Al2O3. Al-doping was incorporated in a film by introducing a single cycle of TMA-H2O after fixed cycles of DEZ-H2O pulses. This ‘super’ cycle was repeated until the desired thickness was achieved. Al-doping was varied from 0.0at% to 24.5at%, on various samples, as determined by EDX. In addition, XRD, AFM, UV-Vis spectroscopy and temperature-dependent (80K-340K) Hall measurements were carried out to understand the structural, optical and electrical properties in these films.

Strong texture effects were observed in the AZO films on the quartz substrates as the films preferentially crystallized along the [100] direction. This texturing effect is different from the [002] normally reported for AZO films deposited using established methods other than ALD. Crystallinity and electrical conductivity peaked at 3at% Al, consistent with previous published work. AFM results show a dramatic drop in surface roughness with Al doping. Optical transmittances of over 80% were obtained for all films in the visible region.

Calculation of lattice parameter constants from XRD data and analysis within the framework of the Burstein-Moss effect, reveal that AZO films act as substitutionally doped films for Al doping less than ~7.3at%. Beyond this value of doping, phase segregation and possible formation of a low conductivity phase cause a reduction in the concentration and mobility of free carriers and hence a degradation of the electrical properties.