alinn resistive ammonia gas sensors

3 Department of Electronic Engineering National University of Tainan, Tainan 700, TAIWAN 4 Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan 7

Accepted Manuscript

Title: AlInN resistive ammonia gas sensors

Authors: W.Y Weng, S.J Chang, T.J Hsueh, C.L Hsu, M.J.

Li, W.C Lai

To appear in: Sensors and Actuators B

Received date: 1-12-2008

Revised date: 14-4-2009

Accepted date: 16-4-2009

Please cite this article as: W.Y Weng, S.J Chang, T.J Hsueh, C.L Hsu, M.J Li, W.C.

Lai, AlInN resistive ammonia gas sensors, Sensors and Actuators B: Chemical (2008),

doi:10.1016/j.snb.2009.04.017

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Accepted Manuscript

AlInN resistive ammonia gas sensors

W Y Weng 1 , S J Chang 1 , T J Hsueh 2,*

C L Hsu 3 , M J Li 3 and W C Lai 4

1

Institute of Microelectronics and Department of Electrical Engineering

Advanced Optoelectronic Technology Center Center for Micro/Nano Science and Technology National Cheng Kung University, Tainan 70101, TAIWAN

2

National Nano Device Laboratories, Tainan 741, Taiwan.

3

Department of Electronic Engineering National University of Tainan,

Tainan 700, TAIWAN

4

Institute of Electro-Optical Science and Engineering, National Cheng

Kung University, Tainan 70101, TAIWAN

Abstract

We report the growth of AlInN epitaxial layer and the fabrication of

AlInN resistive NH3 gas sensor It was found that surface morphology of

the AlInN was rough with quantum dot like nano-islands It was also

found that the conductance of these AlInN nano-islands increased as NH3

gas was introduced into the test chamber At 350oC, it was found that

measured incremental currents were around 105 μA, 127 μA, 147 μA and

157 μA when concentration of the injected NH3 gas was 500, 1000, 2000

and 4000 ppm, respectively

Keywords: AlInN, nano-island, gas sensor, ammonia sensor

Email:tj.Hsueh@gmail.com

TEL: (+886) 6-2757575-62400-1208 ; FAX: (+886) 6-2761854

Accepted Manuscript

Introduction

Ammonia (NH3) is a colorless gas with a special odor It is commonly

used in various industrial sectors [1] Although NH3is extensively used in

our daily life, people may develop a burning sensation in eyes, nose and

throat when exposed to NH3 Inhalation of NH3 vapor could also cause

acute poisoning to people Hence, detecting and measuring NH3 vapor

concentration in the environment is necessary The most commonly used

method to detect gaseous NH3 was either by potentiometric electrodes [2]

or by infrared devices [3] However, these devices are expensive and bulky

It is also possible to detect NH3 vapor concentration by semi-conducting

metal oxide materials [4-9] It has been shown that near surface

conductivity of these materials changes upon exposure to certain gas

molecules Furthermore, it was found that such resistance change is

related to various defects such as oxygen vacancy, metal vacancy or others

[10, 11]

Recently, it was found that III-nitride epitaxial layer can also be used

to detect gaseous butane, propane, ethyl alcohol and carbon monoxide

[12] Although III-nitride-based materials are extensively used as light

emitting diodes [13, 14], ultraviolet photodetectors [15] and high power

electronics [16], only few reports on III-nitride-based sensor for volatile

organic compounds can be found in the literature [12] Compared with

metal oxide sensors, we should be able to integrate III-nitride-based gas

sensors with III-nitride-based photodetectors and electronic devices on the

same chip Other than the binary GaN, ternary AlInN has attracted much

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attention in recent years Compared with AlGaN and InGaN, AlInN is

much less known due to the difficulty in growing high quality crystal [17]

It has been shown that AlInN can be grown lattice matched to GaN with

an indium content of ~17-18% However, it is still difficult to grow high

quality AlInN due to severe phase separation caused by the large disparity

in cation sizes as well as by differences in thermal properties of the binary

constituents [18] It has also been reported that epitaxial AlInN layers are

defective in general with a significant amount of aluminum vacancy,

indium vacancy or nitrogen vacancy Similar to metal oxide sensors, these

defects should be able to enhance the reaction of gas molecular on sample

surface and thus enhance the responsivity of AlInN-based gas sensors In

this study, we report the growth of AlInN Sensing properties of the

fabricated AlInN resistive NH3 gas sensors will also be discussed

Experimental

Samples used in this study were grown on a c-plane (0001) sapphire

(Al2O3) substrate by metalorganic chemical vapor deposition Details of

the growth can be found elsewhere [19] Prior to the growth, we annealed

the sapphire substrates at 1060°C in H2 ambient to remove surface

contamination We then deposited a 30-nm-thick low-temperature GaN

nucleation layer at 530°C, a 2-μm-thick n-type unintentionally doped GaN

(n=3×1016 cm-3) buffer layer at 1020°C, and a 500-nm-thick n-type

unintentionally doped AlInN (n=5×1019 cm-3) active layer at 650°C A

JEOL JSM-7000F field emission scanning electron microscope (FESEM)

operated at 10 keV was then used to characterize structural properties of

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the as-grown AlInN epitaxial layer The cross-sectional image of the

AlInN layer was prepared by an FEI Nova-200 NanoLab Dual-Beam

Focused Ion Beam (DB-FIB) system Crystal qualities of the as-grown

samples were evaluated by a BEDE D1 double-crystal X-ray diffraction

(DCXRD) system The source of X-ray is 1.54056 Å wavelength (Cu Kα)

For the fabrication of NH3 gas sensors, we carefully smeared the

colloidal silver onto the sample surface to serve as contact electrodes The

sample was then annealed at 350°C for 15 min in Ar ambient to form good

ohmic contacts between sliver and the underlying AlInN Figure 1 shows

schematic diagram of the fabricated AlInN resistive gas sensor To

evaluate NH3 gas sensing properties, we placed the fabricated sensor in a

sealed chamber and measured current-voltage (I-V) characteristic of the

sample in air from the two electrodes It should be noted that the sealed

chamber has an inlet port connected to a gas inlet valve and an outlet port

connected to an air pump We first closed the outlet port and injected NH3

gas into the chamber through a gas-injecting syringe At this stage, we

measured I-V characteristic of the sample continuously in the presence of

NH3 gas (i.e., air+ NH3) After the chamber was stabilized, we opened the

outlet port so that the air pump can pump the NH3 gas away At the same

time, we also opened the inlet valve to introduce air into the chamber In

other words, the chamber was kept in atmospheric pressure throughout the

experiment At the end of the experiment, we measured I-V characteristic

of the sample in air again

Result and Discussion

From Hall measurements, it was found that the sheet resistances of our

GaN and AlInN layers were 1.09×106 Ω/sq and 394 Ω/sq, respectively

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These values suggest that parallel conduction which might occur in the

GaN buffer layer should be negligible Figure 2 shows top-view FESEM

image of the AlInN epitaxial layer The inset shows cross-sectional image

of the AlInN layer that prepared by DB-FIB It was found that thickness of

the AlInN epitaxial layer was around 545 nm, which agreed well with our

initial design It was also found that surface morphology of the AlInN was

rough with quantum dot like nano-islands Similar result has also been

reported previously [20] It also was found that the diameter and height of

the nano-islands were around 100 nm and 160 nm, respectively It should

be noted that these nano-islands could provide us a larger surface area,

which in term will result in a large sensor response It should also be noted

that the Pt layer shown in the inset was intentionally deposited to protect

the underneath AlInN and GaN from e-beam etching during DB-FIB

sample preparation No such Pt layer was used during the fabrication of

NH3 sensors Figure 3(a) shows DCXRD spectrum of the sample with two

clear peaks It was found that full-width-half-maximum (FWHM) of the

(0002) GaN peak was around 130 arcsec, suggesting good crystal quality

In contrast, FWHM of the AlInN peak was significantly larger (i.e., 758

arcsec) Based on Vegard’s rule, it was found that indium content in the

AlInN layer was around 62% The fact that no other peaks were observed

suggests that no phase separation occurred in the sample [21] Figure 3(b)

shows energy dispersive spectrum (EDS) measured from the fabricated

devices It can be seen that aluminum, indium and nitrogen peaks could all

be clearly observed in the spectrum It was also found that the atomic

weight percent of indium in the AlInN layer was 62.6%, which agrees well

with that observed from the DCXRD measurement

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Figure 4 shows I-V characteristics of the fabricated sensor measured in

air It can be seen that the measured current increased linearly with the

applied bias Such linear behavior reveals that good ohmic contacts were

formed between the Ag electrodes and the AlInN epitaxial layer For gas

sensing, it is known that oxygen is adsorbed at the N vacancies of

III-nitrides semiconductors [12] Thus, oxygen sorption plays an

important role in electrical transport properties of AlInN epitaxial layer It

is also known that oxygen ionosorption removes conduction electrons and

thus lowers the conductance of AlInN Hence, the sensing mechanism of

AlInN NH3 gas sensor may be described as follows: First, reactive oxygen

species such as O2−, O2− and O− are adsorbed on AlInN surface at elevated

temperatures It should be noted that chemisorbed oxygen species depend

strongly on temperature At low temperatures, O2− is commonly

chemisorbed At high temperatures, however, O− and O2− are commonly

chemisorbed while O2− disappear rapidly [22] The reaction kinematics

can be described as follows [23]:

O2(gas) ↔ O2(absorbed) (1)

O2(absorbed) + e−↔ O2− (2)

O2− +e−↔ 2O− (3)

Thus, the conductance of AlInN nano-islands increased as NH3 gas was

introduced into the test chamber due to the exchange of electrons between

ionosorbed species and AlInN nano-islands The reaction between NH3

gas and the ionic oxygen species can be described by [24]:

2NH3+ 3O− (ads) ↔ 3H2O + N2 + 3e− (4)

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Figure 5 shows response of the fabricated AlInN NH3 gas sensor

measured with 2000 ppm NH3 gas at various temperatures Here, we

define the response as the incremental current, ∆I, before and after the

introduction of NH3 gas With this definition, it was found that measured

∆I were around 15 μA, 52.5 μA, 75.4 μA, 147 μA and 60 μA when the

device was operated at 200oC, 250oC, 300oC, 350oC and 400oC,

respectively Figure 6 shows response variation of the AlInN sensor

exposed to NH3 gas injection and pumping These measurements were

performed by injecting various amounts of NH3 gas into the sealed

chamber following by pumping at 350oC It was found that measured

sensor responses were around 105 μA, 127 μA, 147 μA and 157 μA when

concentration of the injected NH3 gas was 500, 1000, 2000 and 4000 ppm,

respectively In other words, sensor response increased with the increase

of NH3 gas concentration It was also found that measured sensor response

rapidly as we injected NH3 gas into the chamber and pumped them away

Such a result indicates that the response speed of the fabricated sensor is

good

Conclusion

In summary, we report the growth of AlInN epitaxial layer and the

fabrication of AlInN resistive NH3 gas sensor It was found that surface

morphology of the AlInN was rough with quantum dot like nano-islands

It was also found that the conductance of these AlInN nano-islands

increased as NH3 gas was introduced into the test chamber At 350oC, it

Accepted Manuscript

was found that measured incremental currents were around 105 μA, 127

μA, 147 μA and 157 μA when concentration of the injected NH3 gas was

500, 1000, 2000 and 4000 ppm, respectively

Materials and Micro/Nano Science and Technology, National Cheng

Kung University, Taiwan (D97-2700) This work was also in part

supported by the Advanced Optoelectronic Technology Center, National

Cheng Kung University, under projects from the Ministry of Education

Accepted Manuscript

Figure Captions

Figure 1 Schematic diagram of the fabricated AlInN resistive gas sensor

Figure 2 Top-view FESEM image of the AlInN epitaxial layer The inset

shows cross-sectional image of the AlInN layer

Figure 3 (a) DCXRD and (b) EDS spectra of the AlInN layer

Figure 4 I-V characteristics of the fabricated sensor measured in air

Figure 5 Response of the fabricated AlInN NH3 gas sensor measured with

2000 ppm NH3 gas at various temperatures

Figure 6 Response variation of the AlInN sensor exposed to NH3 gas

injection and pumping

Accepted Manuscript

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