A study of ZnO and ZnO:Al thin films deposited by RF plasma jet at atmospheric pressure

Chichina Mariya1,2, Churpita Oleksandr2, Hubicka Zdenek2, Kment Stepan2, Milan Tichy1.
1 Charles University in Prague, Faculty of Mathematics and Physics, Department of Electronics and Vacuum Physics, V Holešovičkách 2, 180 00 Prague, Czech Republic
2 Institute of Physics, Division of Optics, Academy of Sciences of the Czech Republic, Na slovance 2, 182 21 Prague 8, Czech Republic
Abstract. We report on the application of barrier-torch plasma jet system – the novel plasma deposition system capable of working at atmospheric pressure in open air – for deposition of transparent and conducting ZnO thin layers.
Keywords: barrier-torch discharge, ZnO thin film, atmospheric pressure, doping additions.
PACS: 52.75.Hn, 52.77.-j, 52.80.Pi.
The atmospheric plasma deposition of various thin films is subject of great interest. A large number of systems for atmospheric PECVD (Plasma Enhanced Chemical Vapour Deposition) have been developed recently. Low temperature dielectric-barrier discharges [1-6] are very often applied for PECVD of polymer and other kinds of thin films [7-10].
The atmospheric RF barrier-torch plasma jet apparatus has been described in [11]. The system (Figure 1) works at the atmospheric pressure; it is just encased in acrylic glass box, which is provided by a pipe leading the used working gases out of the building.
FIGURE 1. RF plasma jet system at atmospheric pressure used for deposition of ZnO thin films.
We applied single-jet system for deposition of ZnO and ZnO:Al conductive oxide thin films at atmospheric pressure on quartz glass substrates. The quartz glass tube with internal diameter 1.5 mm is surrounded by the stainless steel RF powered electrode. The RF electrode was connected with the 13.56 MHz RF power generator via the matching unit.
Helium and Nitrogen gases were fed into the quartz nozzle. The distance between nozzles outlet and the substrate was 4-13 mm.
Due to the high RF field at the edge of the powered electrode the RF barrier-torch discharge is generated in the mixture of He and N2 gases. Vapours of precursors were fed directly into the nozzle powered by RF electrode. As the growth precursors, vapours of Zn-acetylacetonate
(Zn[C5H7O2]2) were used for the deposition of ZnO thin films. Al-acetylacetonate (Al [C5H7O2] 3) vapours were used as the accessory precursors for Al doping. These chemicals were placed into the separate containers, kept at electronically stabilized temperature up to 110°C. The additions of doping to growth precursor Zn-acetylacetonate were expected to give rise to the thin films conductance via the N type conductivity.
The stable precursor temperature resulted in stable precursor flow rate (stable He-N2-precursor mixture ratio) even if we could not directly measure the precursor flow rate. Quartz glass with thickness 2.2 mm were used as substrates. Coating of the larger area was provided by motor-driven x-y movement of the grounded Al substrate holder with the water-cooling system. The photograph of the typical ignited atmospheric single plasma jet stream can be seen in Figure 2.
Plasma system excited by RF source worked in pulse modulated mode. This modulation allowed exciting of the high density plasma in the active part of the duty cycle and simultaneously keeping the neutral gas in the plasma jet near the substrate reasonably cold thus protecting polymer substrate from thermal damages.
ZnO films in the role of transparent conductive oxide films gained recently wide interest due to their stability. They have applications in solar cell technology, gas detection, and many others TCO (Transparent Conducting Oxide) applications. For understanding of the thin film structures we investigated the ZnO films with help of X-Ray Diffraction (XRD) method. These analyses have shown that the ZnO thin films on quartz glass substrates have a pronounced hexagonal crystal structure with preferable crystallites orientation with ‘c’ axis perpendicular to the substrate surface, see Figure 3. The chemical composition of the films was very close to that of stoichiometric ZnO. FIGURE 2. Photography of single plasma jet system.
For successful deposition of ZnO and ZnO: Al films we had to determine the optimal parameters of the deposition process. Figures 4-7 demonstrate partial results of this effort.
Dependence of the deposition rate on the temperature of the precursor vapours in case of ZnO films is shown in Figure 4. As we noted, the temperature of the precursor determined the quantity of the chemical elements at first in the plasma zone and also in the film. For creating the right stoichiometric and conductive ZnO structure it is important to keep relation between the deposition area on the substrate and the quantity of precursor vapours. The deposition area is the place of the contact between the plasma jet and the substrate.
Dependence of the deposition rate on the N2 gas flow rate can be seen in Figure 5. The flow rate of the gas determines the time spent by the precursor particles in the plasma zone. During that time the excitation and other processes can occur. Hence, the particles were inactivated before landing on the substrate in the case of a very low flow rate (the lifetime of excited particles was limited) and the particles did not have enough time to become excited in case of very high flow rate.
Using the Van der Pauw four-point method [12] we performed measurements of the conductivity of the deposited ZnO thin films in dependence on the deposition conditions (precursor temperature, distance from the plasma jet to the substrate, gas flow rate etc.) aiming at achieving of the maximum conductivity of the deposited ZnO:Al layer. The temperature of the container with the accessory precursor (Al-acetylacetonate) changed the quantity of the precursors in the plasma zone and affected the conductivity of the ZnO films as shown in Figure 6. It can be seen the abnormal conductivity in Figure 6 at the temperature 91°C for the accessory precursor. We obtained much the same.

The conductivity of the ZnO:Al thin film depends on the distance between nozzle outlet and the substrate in almost linear manner, see Figure 7. The point corresponding to s = 6mm and QN2/He= 450/450 sccm corresponding to conductivity around 0.5 S/cm was omitted from the graph in Figure 7 for better visibility of the other data points (it would reduce substantially the scale on ordinate axis).
Atomic force microscopy (AFM) technique allowed us to visualize structure of ZnO thin films. Height image data obtained by the AFM is three-dimensional. The usual method for displaying the data uses colour mapping for height, for example darker colour for low features and brighter colour for high features. A popular choice of colour scheme is shown on the left hand side of the paragraph. The investigation was carried out in contact mode. In this mode the tip makes “physical contact” with the sample, and is essentially dragged across the sample surface to make a topographic image. Figure 8 shows AFM morphology for ZnO thin films. The typical average surface roughness of these films is between 80-150 nm and size of patterns between 500-700 nm.
This work is a part of the project MSM 0021620834 that is financed by the Ministry of Education, Youth and Sports of the Czech Republic. Thanks are also due to Czech Science Foundation, grants No. 202/03/H162, 202/05/2242 and 202/06/0776.
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