Influence of the oxygen flow rate on the plasma parameters in reactive magnetron sputtering plasma using Zn target
Aluminum-doped zinc oxide (AZO) and zinc oxide (ZnO) are promising materials which have been investigated for photovoltaic and photosensor applications. The applications of ZnO based thin films are due to several unique material properties where it is a II-IV semiconductor with a wide direct bandg...
| Main Authors: | , , , |
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| Format: | Conference or Workshop Item |
| Published: |
2012
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| Subjects: | |
| Online Access: | http://eprints.uthm.edu.my/6140/ http://eprints.uthm.edu.my/6140/1/Influence_of_the_oxygen_flow_rate_on_the_plasma.pdf |
| Summary: | Aluminum-doped zinc oxide (AZO) and zinc oxide (ZnO) are promising materials which have
been investigated for photovoltaic and photosensor applications. The applications of ZnO based thin films
are due to several unique material properties where it is a II-IV semiconductor with a wide direct bandgap
of 3.2 eV at room temperature, which is almost similar to titanium dioxide (TiO2) material. In many
researches, ZnO based thin films have been grown by various deposition technique, such as sputtering,
chemical bath deposition, pulsed laser deposition, and thermal chemical vapor deposition. In such
options, the reactive sputtering technique using Zn metal target represents one of the simplest and most
effective techniques which have been proposed by many researchers[1,2].
Researchers have studied and showed the influence of deposition recipe to properties of deposited
film. However, very few researchers investigated the details of the reactive Ar+O2 magnetron sputtering
plasma properties employing Zn target. Thus, in the present work, we investigate the properties of
magnetron sputtering plasma using Zn target as a function of oxygen gas ratio.
The magnetron sputtering plasmas were produced using radio frequency (rf) power supply in
Ar+O2 as ambient gas. Figure 1 shows the experimental setup. A Langmuir probe was used to collect the
ion and electron currents from the plasma. The position of the probe is at the downstream region of
magnetron sputtering plasma in which the magnetic field is very small. Figure 2 shows the example of
current-voltage property collected from Langmuir probe measurement. The measurements was repeated 5
times for each condition and averaged. From the current intensity, the electron density and electron
temperature were calculated. The properties of reactive magnetron Zn sputtering plasma as a function of
oxygen gas flows were studied.
Figure 3 shows the electron temperature as a function of oxygen gas flow at various total gas
pressure. The rf discharge power and Ar gas flow was fixed at 200 W and 40 sccm, respectively. As
shown in Fig.3, the electron temperature increased from approximately 1 eV to 2 eV when the oxygen gas
flow increased. The increment of electron temperature was due to the balance of electron energy which is
used to ionize the gases in vacuum chamber. The ionization energy of argon and oxygen were 15.7 eV
and 13.5 eV, respectively. Therefore, at higher oxygen flow rate (oxygen ratio) lower energy is used to
ionize the discharge gas.
Figure 4 shows the electron density as a function of oxygen gas flow at various total gas pressure. The
rf discharge power and Ar gas flow was fixed at 200 W and 40 sccm, respectively. As shown in Fig. 4,
the electron density increased when the oxygen flow rate increased from 0 to 5 sccm. Then, the electron
density decreased after the oxygen flow rate exceed 5 sccm. During the early injection of oxygen, the
sputtering discharge is in the transition mode from metallic mode to dielectric mode. This results in the
inefficient loss of electron in the discharge plume. Therefore, the electron density increased at the early
injection of oxygen. On the other hand, when the oxygen ratio exceed 5 sccm, the electron attachment on
oxygen to form negative oxygen ion become dominant. As a result, electron density decreased drastically
after the oxygen flow rate exceed 5 sccm. Further investigation on this speculation is needed.
This work was supported by Ministry of Higher Education of Malaysia under Fundamental
Research Grant Scheme (FRGS 2009-2011) and Universti Tun Hussein Onn Malaysia under Graduate
Incentive Scheme (GIS 2009-2011). |
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