In view of increased use of fuel cells as a new clean and viable energy source
to replace petroleum, hydrogen sensors are strongly demanded to avoid hazardous
explosion. Since, so-called sensor networks are making a rapid progress, the
sensor material is preferably a semiconductor which can realize an integrated
on chip with ultra low power processing electronics and wireless RF circuit
such as used for radio frequency identification (RFID) chip (Usami
and Ohki, 2003). Until now, there have been many reports on chemicals sensors
using metal-oxide compound semiconductors, such as SnO2 and ZnO (Yamazoe
and Miura, 1992; Morrison, 1982). However, the sensing
mechanism of these compound semiconductors is related to various defects such
as oxygen vacancy and metal vacancy.
High temperature operation and long term stability are important requirements
for gas sensor. Silicon-based gas sensors cannot be operated above 250°C,
preventing them for applications at high temperature (Luther
et al., 1999). GaN and SiC based materials are known as wide-bandgap
semiconductors that show great promise for electronic devices operating at high
temperatures. Simple Schottky diode or field-effect-transistor structure fabricated
on GaN and SiC (Casady et al., 1998) are sensitive
to a number of gases, including hydrogen and hydrocarbons (Kim
et al., 2003a). To make such gas sensors, a catalytic metal such
as Pd or Pt can be used since they show stable sensing response and also suitable
for high temperature operation.
AlGaN/GaN heterojunction has been shown to form a potential well and a two-dimensional
electron gas (2DEG) at the lower heterointerface. These structures are well-known
for possessing high electron mobility in the 2DEG channel, highest sheet carrier
concentration among III-V material system, high saturation velocity, high breakdown
voltage and good thermal stability. The expected advantages of using undoped-AlGaN/GaN
compared to doped structures are; (1) lower gate leakage current, (2) lower
pinch-off voltage and (3) less noise due to less impurities in AlGaN barrier
layer. These are the reasons why many groups prefer non-modulation doped nitride
HEMT structures (Rizzi and Luth, 2002). In addition,
higher mobility can be obtained by undoped-material compared to doped material.
Therefore, it is expected to provide faster time-transient sensing response.
The mobility of the studied material was confirmed to be two times higher than
n-doped AlGaN/GaN structure reported in ref. (Matsuo et
al., 2005). The 2DEG concentration and mobility of the samples at room
temperature are 6.61x1012 cm-2 and 1860 cm2/Vsec,
In this study, the sensing characteristics of hydrogen gas sensor utilizing Pt/AlGaN/GaN HEMT structure studied at various temperatures are reported.
An undoped-AlGaN/GaN HEMT structure used for hydrogen gas sensor is shown in Fig. 1. The AlGaN/GaN epitaxial layers are grown by metal organic chemical vapor deposition (MOCVD) on 430 μm-thick c-plane sapphire substrate. The epitaxial structures consist of a 25 nm-thick undoped AlGaN, a 2 μm-thick undoped GaN and a buffer layer.
Figure 2a and b shows the device structure
fabrication after certain processes, the device fabrication process starts with
100 nm-thick SiO2 deposition using plasma-enhanced chemical vapor
deposition (PECVD) at 280°C with a SiH4/NH3/He gas
system to act as mesa patterning mask. Then, a mesa is formed by inductive-coupled
plasma (ICP)-assisted reactive ion beam etching with a Cl-based gas system consisting
of BCl3, Cl2 and Ar gases. The etching pressure is 5 mTorr
and the etching rate is around 0.1 μm min-1. Next, the removal
of SiO2 mask is carried out before ohmic contact formation.
The ohmic contacts are formed by deposition of Ti/Al/Ti/Au (20 nm/50 nm/20
nm/150 nm) multilayers followed by rapid thermal annealing at 850°C for
30 sec in N2 ambient. After that, the device surface is covered with
300 nm-thick SiO2 film using PECVD in order to realize a planar connection
from circular Pt Schottky contact to external contact pad for electrical measurement
||Material structure used for hydrogen gas sensor
||Schematic structure after each step of fabrication process.
(a) top view and (b) cross-sectional view
||(a) Fabricated device (top view) and (b) schematic of cross
sectional device structure
Consequently, this SiO2 dielectric layer at Schottky contact area
is etched out using BHF etchant so that a Schottky contact can be formed on
AlGaN surface. Then, Pt circular Schottky contact with a thickness of 5 nm and
a diameter of 600 μm is formed by electron-beam evaporation. Finally, Ti/Au
is deposited to form an electrical interconnection from Schottky contact to
external circuit. The completed fabricated device is shown in Fig.
THE SENSING PERFORMANCE
The current-voltage (I-V) measurement is carried out to evaluate the response to hydrogen gas. Typical I-V characteristics measured in vacuum and high purity hydrogen ambient at room temperature are shown in Fig. 4a and b. It can be understood here that the rectifying characteristics of fabricated diode degrade towards ohmic-like characteristics where large reverse currents are generated.
As shown in Fig. 4a and b, both the forward and reverse currents
show a slight change of current upon exposure to hydrogen. The slight change
of current may due to the diffusion rate for hydrogen atom through the catalytic
metal is very slow at room temperature (Kim et al.,
2003b). Thus, it can be said that the sensitivity of gas sensor is quite
low at room temperature.
The responses of the fabricated diodes are also investigated at various temperatures
ranging from room temperature to 200°C. As shown in Fig. 5,
a large current change by the same amount of H2 concentration is
observed as the temperatures increase up to 200°C.
||I-V characteristics of fabricated gas sensor measured at room
temperature in vacuum and high purity hydrogen ambient (a) linear and (b)
||I-V characteristics of fabricated gas sensor measured in high
purity hydrogen ambient at room temperature, 50, 100, 150 and 200°C
||The cyclic time response of current at T = 200°C and V
= 1 V for the Pt/AlGaN/GaN Schottky diode
This change is considered to be due to more effective catalytic dissociation
of H2 on the Pt surface can be realized at higher temperature (Song
et al., 2005). However, as shown in Fig. 4a and b,
the device shows ohmic-like characteristics where large reverse currents are
generated. We suggest that it should be a reason for discrepancy where the curve
of reverse current measured at room temperature show higher values than reverse
current measured at 50 and 100°C.
Figure 6 shows the time-transient response measured temperature
of 200°C and forward bias, Va of 1 V. It can be seen that there
is sufficient cracking of H2 for the diode to be a sensitive gas
||Increment and decrement speed of current during absorption
and desorption of hydrogen
The slopes are very steep which show that the response of the sensor is relatively
fast at each cycle. Figure 7 shows the increment and decrement
speeds of current at each cycle. The data are extracted from the slopes at each
cycle shown in Fig. 6. Constant speed is obtained at each
cycle where the average of increment and decrement speed of current is estimated
to be 27.6 and 17.6 nA sec-1, respectively. These increment and decrement
speeds of current correspond to the hydrogen adsorption and desorption rates,
respectively. The increment speed is much faster than the decrement speed for
each cycle meaning that the absorption of H2 is faster than desorption.
This is because a desorption process requires thermal energy supply, leading
to a longer decrement time.
Pt-circular Schottky diode was successfully fabricated for gas sensor application. Both the forward and reverse currents of the device increase upon exposure to hydrogen and both currents also increase with the temperature. This indicates that the diffusion rate for hydrogen atom through the Pt metal is enhanced with the increase of temperature. The time transient response shows a constant current increment and decrement speed for each cycle where the average of increment and decrement speed is estimated to be 27.6 and 17.6 nA sec-1, respectively. The increment speed is much faster than the decrement speed for each cycle, meaning that the absorption process of H2 is faster than desorption process.
The authors wish to extend their thanks for the support provided by the Ibnu Sina Institute, Universiti Teknologi Malaysia and Nano-Optoelectronics Research Laboratory, Universiti Sains Malaysia. This work was supported by the Ministry of Science, Technology and Innovation under Science Fund Grant 03-01-06-SF0281. We wish to thank our colleagues for useful discussions, particularly, Assoc. Prof Dr. Zulkafli Othman at Universiti Teknologi Malaysia.