Fabrication And Investigation Of Gan Nanostructures And Their Applications In Ammonia Gas Sensing
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Date
2015-07
Authors
Beh, Khi Poay
Journal Title
Journal ISSN
Volume Title
Publisher
Universiti Sains Malaysia
Abstract
In this work, gallium nitride (GaN) nanowires, porous GaN (PGaN), and ammonia
(NH3) gas sensors have been fabricated and studied. The GaN nanowires samples in
this work were grown using chemical vapour deposition (CVD) method, additionally
employing vapour-liquid-solid (VLS) growth mode. For the studies of GaN
nanowires, VLS growth mechanism, particularly the effects of metal catalyst was
focused upon. Prior to that, several growth parameters that suits the CVD system
have to be determined first. This comprised of several works, which were nitridation
effects towards gallium (III) oxide (Ga2O3), growth substrates [silicon (Si) and cplane
sapphire], and optimum NH3 flow rate. From the aforementioned works,
Ga2O3 and c-plane sapphire have been chosen as the precursor and substrate
respectively, while NH3 flow rate was set to 250 standard cubic centimeter per
minute (sccm) in the subsequent works. From the studies of catalyst effects towards
GaN nanowires growth, the saturation state of metal catalyst could be deduced using
x-ray diffraction (XRD) results along with available phase diagrams. It was revealed
that iron (Fe) saturated around Fe6Ga5, under solid-liquid mixture state. However,
the liquid content of the alloy decreases with temperature, subsequently become solid
at 900˚C. The state transitions were believed to promote the growth of (h00) facets
thus resulted in curled and bended nanowires. For nickel (Ni), the alloy remained
solid entirely, while Ni5Ga3 was believed to be the saturation state. Interestingly, Ni-
Ga could be oversaturated, resulted in the formation of nanoribbons as seen at
1000˚C. The saturation state of gold (Au) was difficult to determine, since very little
amount of gallium (Ga) (< 1 at%) sufficient to saturate it. Coupled with the growth
conditions, the resulting nanowires have a unique morphology that strongly
suggested c-oriented growth. On the physical characteristics, the nanowires diameter
for Ni catalyst ranging from 60 to 80 nm; while that of Fe catalyst 100 to 160 nm.
Au catalyst produced nanowires of greatest size (diameter), ranging from 140 to 200
nm. Photoluminescence (PL) results suggested the presence of interfacial stress and
surface disorder, while the first order Raman bands under Selection rule revealed the
GaN nanowires to be hexagonal wurtzite structure, along with additional modes due
to nanosize effects from the nanowires. PGaN was produced by photo-enhanced
anodization technique with duration as variable. Porous morphology had been
obtained and prolonged anodization resulted in breakdown. A growth mechanism
has been proposed for that. It was found that the average individual pore area of
sample anodized for 5, 10, and 20 minutes was about 1566, 2575, and 2885 nm2
respectively. The XRD and PL results showed relaxation of biaxial compressive
stress in porous samples. Meanwhile, two prototype NH3 gas sensors were made and
have a rectifying behaviour. On that of GaN nanowires exhibited increased
sensitivity with working temperature, while comparison between as-grown and
PGaN showed the latter being superior in sensing. The sensing mechanism was
similar in both samples, where based on the changes between NH3 and oxygen (O2)
concentrations. For GaN nanowires sample, the sensitivity (SF) (at 3V, 350°C) was
109% with average response and recovery time (tresponse, trecovery) about 10 and 2s
respectively. Meanwhile, SF (at 5V, 350°C) of as-grown (PGaN) was 48.2% (26.1%),
while that of tresponse and trecovery about 17s (35.3s) and 19.2s (8.2s) respectively.
Description
Keywords
Gas ammonia