|
Volume 5 Issue 4 - August 1, 2008  |
|
|
A ZnO nanowire vacuum pressure sensor Shoou-Jinn Chang professor of Institute of Microelectronics, College
of Electrical Engineering and Computer Science, National Cheng Kung
University E-mail: changsj@mail.ncku.edu.tw Nanotechnology 19 (2008) 095505 (4pp) |
||||
  | ||||
 Vacuum pressure sensors are
indispensable devices for applications such as thin film deposition,
biomedical experiment and industrial process control. For example,
thermopile [1], thermocouple [2] and super Pirani sensor [3] have
all been extensively used in various industries. To minimize the
size, it has also been shown that field emission (FE) devices,
micro-opto-electro-mechanical system (MOEMS) devices [4] and
micro-electro-mechanical system (MEMS) devices [5] are also capable
of sensing vacuum pressure. However, the manufacturing processes of
MOEMS and MEMS devices are complex in general. On the other hand, FE
devices are operated at high electric fields. These drawbacks make
it difficult to commercialize MOEMS, MEMS and FE devices as
practical vacuum pressure sensors.In recent years, one-dimensional (1D) ZnO nanowires (NWs) have attracted much attention due to their potential applications in nano-electronics and nano-optoelectronics. In this paper, we report a more detailed study on the properties of these lateral ZnO NWs and the fabrication of vacuum pressure sensors. Properties of the fabricated vacuum pressure sensors will also be discussed. 
Figure 1 Top-view SEM micrograph of the NWs. Figure
1 shows top-view SEM micrograph of the NWs. It was found that ZnO
NWs were grown only on top of the patterned ZnO:Ga film while no NWs
were grown directly on the glass substrate. Similar selective growth
has been reported by Hsu et al. [6, 7]. It was also found that the
ZnO NWs were randomly oriented and some NWs were even grown
laterally. Furthermore, it was found that average length and average
diameter of the laterally grown ZnO NWs were 5 µm and 30 nm,
respectively. Notably, in some cases, a single NW bridged the two
fingers, as shown in figure 1. As a result, the two electrodes were
no longer electrically open. We can thus apply a constant voltage
across the two electrodes and measure the corresponding
current.Figure 2 shows I-V characteristics of the sample measured at low pressures. 
Figure 2 I-V characteristics of the sample measured at
low pressures. It can be seen that measured current increased
linearly with the applied bias for all four cases. Such an
observation indicates that the NW bridged the two electrode remains
pure resistive at low pressures. With the same applied bias, it was
found that measured current increased as the chamber pressure was
decreased. It is known that reactive oxygen species such as O2−, O2−
and O− originated from oxygen gas and/or water vapor are
adsorbed easily on ZnO surface. These adsorbed reactive oxygen
species will become negatively charged by capturing a free electron
from the n-type ZnO NW. Thus, a highly resistive depletion layer
will be formed at the NW surface while the overall conductance of
the NW will become smaller [8]. As we decreased the chamber pressure
through pumping, we significantly reduced the amount of oxygen gas
and water vapor. As a result, the ZnO NW bridged the two electrodes
will become more electrically conductive. Thus, we observed an
increased current at low chamber pressures. From the I-V
relationships shown in figure 2,
Figure 3 Measured resistance of the lateral ZnO NW as a
function of chamber pressure. we can determine the resistance
of the lateral ZnO NW as a function of chamber pressure, as shown in
figure 3. For comparison, resistance of the NW measured at 760 Torr
was also plotted. It was found that measured NW resistance increased
logarithmically as the chamber pressure was increased.In summary, we report the growth and characterization of lateral ZnO NWs on ZnO:Ga/glass templates. Vacuum pressure sensor was then fabricated using one single NW bridged across two electrodes. It was found that measured NW resistance increased logarithmically as the chamber pressure was increased. References
|
||||
|
||||
 | ||||
