A Comparison Study on Hydrogen Sensing

Performance of MoO3 Nanoplatelets Coated with a

Thin Layer of Ta2O5 or La2O3

J. Yu*, Y. Liu, F. X. Cai, M. Shafiei, G. Chen, N. Motta, W. Wlodarski,

K. Kalantar-zadeh and P. T. Lai

1Department of Electrical and Electronic Engineering, The University of Hong Kong,

Hong Kong, SAR2School of Chemistry, Physics and Mechanical Engineering, Institute of Future Environments,

Queensland University of Technology, Australia3School of Electrical and Computer Engineering, RMIT University, Australia

Abstract

There has been significant interest in developing metal oxide films with high surface

area-to-volume ratio nanostructures particularly in substantially increasing the performance of

Pt/oxide/semiconductor Schottky-diode gas sensors. While retaining the surface morphology of these

devices, they can be further improved by modifying their nanostructured surface with a thin metal

oxide layer. In this work, we analyse and compare the electrical and hydrogen-sensing properties of

MoO3 nanoplatelets coated with a 4 nm layer of tantalum oxide (Ta2O5) or lanthanum oxide (La2O3).

We explain in our study, that the presence of numerous defect traps at the surface (and the bulk) of the

thin high- layer causes a substantial trapping of charge during hydrogen adsorption. As a result, the

interface between the Pt electrode and the thin oxide layer becomes highly polarised. Measurement

results also show that the nanoplatelets coated with Ta2O5 can enable the device to be more sensitive (a

larger voltage shift under hydrogen exposure) than those coated with La2O3.

Key Words: Hydrogen, Gas Sensor, Metal Oxide, Heterostructure

1. Introduction

The rise of nanotechnology and the ability to or-

ganise and restructure matter at the nanoscale, have

brought forth the development of cheap, low-power,

miniaturized sensors with enhanced performance. As

such, hydrogen sensors are important and have become a

core component in clean and renewable energy related

instruments. Devices which convert hydrogen gas to en-

ergy, such as internal-combustion engines, turbines and

fuel cells, all require high-performance and responsive

sensors, due to the highly volatile nature of hydrogen

gas, especially when applied as a fuel [2].

Sensors based on Pt/metal oxide/semiconductor

Schottky diode have arisen as one of the most favourable

choices for industrial applications due to their simplicity,

lightweight and portability. They have demonstrated

many innovative advantages such as sizable down-scal-

ing and also adaptability and compatibility with existing

electronic based technologies [3]. The performance of

sensors that have implemented nanostructured metal ox-

ide materials have been shown to excel far beyond those

with bulk-based materials in terms of sensitivity and re-

sponse time [4,5].

In terms of sensing performance, as we continue to

scale the dimensions of a sensor down to incredibly

smaller size, it would inevitably assign a decrease in the

surface area available for interaction with a gas. Thus,

Journal of Applied Science and Engineering, Vol. 17, No. 1, pp. 3138 (2014) DOI: 10.6180/jase.2014.17.1.05

*Corresponding author. E-mail: jcwyu@hku.hk, yu.jerrycw@gmail.com

the main advantage in implementing a nanostructured

thin film for sensing applications comes from addition of

a metal oxide layer which the first necessary step that can

be used to increase the surface area-to-volume ratio of

the morphological surface. Whilst preserving the surface

morphology, we can apply a second step by the modifi-

cation of the surface quality by adding an ultrathin metal

oxide layer to maximise the effectiveness of the mate-

rials surface.

In this work, the authors designate two high- metal

oxides (Ta2O5 and La2O3) to be deposited as an ultra-thin

layer on MoO3 nanoplatelets. By adding this thin layer,

the resulting structure of the sensor resembles that of

thin-film devices based on quantum heterostructures [6].

A high- layer is commonly implemented in many high-

performance electronic devices to suppress the problem-

atic leakage current. However, in this work, we aim to

make use of the defects in the high- metal oxide for ef-

fectively trapping the hydrogen adsorbate charge to in-

duce a stronger polarisation at the interface between the

Pt electrode and the metal oxide layer. In addition, this

can provide a new method for low-temperature (or RT)

sensor and thus save power.

2. Experimental

2.1 Preparation of Substrates

The nanoplatelet MoO3 films were grown by thermal

evaporation on 6H-SiC n-type substrates (Tankeblue Co.),

the motivations behind the choice of our deposition pa-

rameters are explained in our previous work [7,8]. The na-

tive SiO2 layer was removed by etching the wafers in 10%

HF + H2O and an electrode of Ti and Pt, with thicknesses

of 40 and 100 nm, respectively were deposited on the

backside of the wafers using electron beam evaporation.

The wafers were diced into 5 5 mm2 substrates and an-

nealed at 500 C to form the ohmic contact. A second

cleaning process was performed using the same HF pro-

cess to remove the SiO2 layer from the polished side of the

substrate, which is formed during the annealing process.

Subsequently, the prepared substrates were cleansed in

acetone, isopropanol, DI water and blown dry with N2 gas.

2.2 Growth of Nanoplatelets Films

The source was prepared by weighing 10 mg of MoO3

powder (China Rare Metal Material) on an alumina boat,

placing it at the centre of a (18 mm inner diameter and 680

mm length) quartz tube furnace. The SiC substrates were

placed on another alumina boat at a distance of 16 cm from

the source. A flow of gas with 10% O2 in Ar at 800 sccm

was initiated and the thermal deposition was performed

by heating to 770 C for 30 min at a rate of 2 C/min. The

cooling process was controlled at the same rate.

2.3 Deposition of Thin Sensing Layer

The fabrication of Ta2O5 and La2O3 thin film layers

were performed by RF sputtering deposition in a Denton

vacuum discovery sputtering system as per our works

referenced in [9,10]. The thickness of the Ta2O5 and

La2O3 high- layers were calibrated by depositing at a

power of 15W RF separately for 350 sec approximately

on dummy wafers. The thickness achieved was approxi-

mated as 3.7 nm (with a mean square error of 0.623) as

measured by a VB400 Spectroscopic Ellipsometer soft-

ware (Cauchy model). Deposition of the high- layers

were then repeated for the nanoplatelet films.

2.4 Material Characterisation

The surface morphology of the deposited MoO3

nanostructures was characterised by scanning electron

microscopy (SEM) via a FEI Nova NanoSEM. X-ray dif-

fraction (XRD) analysis was conducted using a Brucker D8

Advance to determine the difference in crystallographic

structure for the uncoated MoO3 films, and those coated

with Ta2O5 and La2O3. Transmission electron micro-

scopy (TEM) was performed via a JEOL 1010 determine

the thickness of the RF sputtered high- layer.

2.5 Formation of Schottky Contact and Testing

Methodology

Using a shadow mask, a circular pad of Pt with dia-

meter of 1 mm was deposited by the PECS sputter, via

a lift-off process. The deposited thickness was 30 nm as

determined by a calibrated built-in quartz crystal thick-

ness monitor.

In this work, the configuration of the developed sen-

sors is of the form Pt/high- thin film/MoO3 nano-

platelets on SiC. Electrical and hydrogen-sensing tests

were performed in a multi-channel gas testing system as

explained in our previous work [710].

32 J. Yu et al.

3. Results and Discussion

3.1 Surface Morphology and Crystallographic

Structure

Analysis of the surface morphology of the as-de-

posited MoO3 nanoplatelets conducted by SEM is

shown in Figure 2. The insets of the figure illustrate the

TEM analysis of the selected area of the MoO3 nano-

platelets subsequent to the RF sputtering of Ta2O5 or

La2O3 (Figure 2).

The arrows in the micrograph indicate the thickness

of the deposited layer to be approximately 4 nm, verify-

ing the modelling estimate as measured by ellipsometry.

The crystallographic structure of the MoO3 nanoplate-

lets, with Ta2O5 and La2O3 as measured by XRD is

shown in Figure 3. The results from the XRD also show

that the original MoO3 crystal structure is preserved [1,8]

with the exception of additional peaks of Ta2O5 for the

sputtered Ta2O5 layer. Similar observations can be made

for the MoO3 nanoplatelets with sputtered La2O3 layer.

These characterisation results indicate that the sputtered

Ta2O5 and La2O3 layers can be deposited onto the MoO3

nanoplatelets as a thin layer while preserving the surface

morphology.

3.2Electrical andStaticHydrogenSensingProperties

The measured current-voltage (I-V) characteristics

of the developed hydrogen sensors as exposed to syn-

thetic air and dilute hydrogen ambience at various tem-

peratures are shown in Figure 3.

The hydrogen sensing mechanism is based on the

catalytic dissociation of hydrogen and the hydrogen ad-

sorption mechanism as per described in literature [11].

Hydrogen atoms that adsorb into the Pt catalyst metal

polarise and induce a dipolar charge layer at the interface

between the Pt electrode and the surface of the metal

oxide. We will subsequently characterise the build-up of

this dipolar charge in terms of barrier height lowering at

the interface and also change in the dielectric constant of

the metal oxide in the next section.

To simply characterise the static performance of the

sensors developed in this work, we calculate the sen-

sitivity parameter (SF) under forward-bias mode of op-

A Comparison Study on Hydrogen Sensing Performance of MoO3 Nanoplatelets Coated with a Thin Layer of Ta2O5 or La2O3 33

Figure 1. SEM of the MoO3 nanoplatelets deposited onto SiCby thermal evaporation (insets shows TEM of the 4nm RF sputtered Ta2O5 and La2O3 thin film).

Figure 2. XRD diffractograms of the MoO3 nanoplatelets [1]deposited onto SiC by thermal evaporation with thepresence of Ta2O5 (as indicated by hollow squares)and La2O3 (as indicated by the filled circles).

Figure 3. Current-voltage characteristics of the pure MoO3nanoplatelets, La2O3 and Ta2O5 coated nanoplate-lets based hydrogen sensor as exposed to an air anddiluted hydrogen ambience at (a) 25 C; (b) 140 C;(c) 220 C; (d) 300 C.

eration. SF is defined as [12]:

(1)

where IH2 and Iair are the forward current measured in a

hydrogen ambience and in air ambience, respectively.

The value for SF as determined by Equation (1), and

voltage shift (V) under a 100 A constant current

mode of operation, are shown in Table 1. The results in-

dicate that the Ta2O5 coating can give the device signifi-

cantly higher hydrogen sensitivity than the La2O3 coat-

ing. We will further analyse the electrical properties in

terms of two different conduction mechanisms.

3.3Change inBarrierHeight andDielectricConstant

In further analysis on the electrical properties of

these sensors, we assume that the current density (J) can

be modeled by the Schottky diode theory. Therefore, based

on the thermionic emission (TE) conduction mechanism,

charge carriers flow by climbing over an energy barrier

(the barrier height) at the interface according to the for-

ward J-V equation as given by [13]:

(2)

where A** is the effective Richardson constant; T is the

absolute temperature; q is the electron charge; B is the

effective barrier height; VF is the forward applied volt-

age; is the ideality factor; and k is Boltzmanns con-

stant.

Using Equation (2) from the TE model, the barrier

height and the change in barrier height () with respect

to the exposure to hydrogen gas (1% concentration in

air) for different temperatures were calculated from the

J-V data in Figure 3. The low-field region between 0.1 V

and 0.3 V was selected for these calculations at 100 A,

in order to increase the accuracy of the results. The cal-

culated values for the above parameters and are plotted

in Figure 4.

From the results in Figure 4, we anticipated that the

change in barrier height would increase with respect to

increasing temperature. However, it appears that in Fig-

ure 5, the results (not shown) increasingly diverage and

exceed the values that have been considered reasonable

in the literature [10,14]. Therefore, to explain this dis-

crepancy, we assume that there is an additional conduc-

tion mechanism other than TE. By calculating the ideal-

ity factor (as presented in Table 2) from the J-V data (Fig-

ure 3) using Equation 2, we observe that the nanoplate-

lets with either Ta2O5 or La2O3 coating exhibit a far st-

ronger non-ideal behaviour than the pure MoO3 nano-

platelets. This indicates that there could likely be surface

34 J. Yu et al.

Table 1. The sensitivity (SF) and voltage change (V) at

100 A, exhibited from the MoO3, La2O3/MoO3and Ta2O5/MoO3 nanoplatelets based MIS-type

sensor diodes exposed to air and 1% diluted

hydrogen ambience at different temperatures

MoO3 [1] La2O3/MoO3 Ta2O5/MoO3

T (C) SF V (V) SF V (V) SF V (V)

25 0.74 0.03 0.95 0.09 0.27 0.15

140 0.22 1.22 0.41 2.85 0.74

220 0.53 3.71 0.81 9.32 1.20

300 0.07 7.52 1.13 9.43 1.11

Figure 4. Effective barrier height B (as denoted by the hol-low circles) and dielectric constant (as denoted bythe filled squares) for the MoO3 nanoplatelets, withand without La2O3 and Ta2O5 coating at differenttemperatures. The thermionic emission (TE) modelis found to be dominant allowing for an accuratecalculation of the effective barrier height, from 25C to 350 C for the uncoated MoO3 and from 25 Cto 140 C for the MoO3 with the coatings. Thus,from 180 C to 300 C, the Poole Frenkel (PF)mechanism is dominant, allowing for the calcula-tion of the dielectric constant for the nanoplateletswith La2O3 and Ta2O5.

and bulk traps in the thin coating layer which signifi-

cantly influences the flow of current. Therefore, for sim-

plicity we will consider these traps as defect traps and

examine the electrical properties in terms of the trap-as-

sisted conduction mechanism (also known as Poole

Frenkel emission).

In classical mechanics, we presume that if numerous

defects are present in the thin high- layer, charge car-

riers that flow in their proximity to these defects can

undergo trapping and de-trapping processes at the inter-

face. In quantum mechanics, electrons have a probability

where they can exist inside the potential well (defect

trap) before tunneling through to the other side. With suf-

ficient thermal energy, these traps can be thermally acti-

vated and provide carriers and alternative pathway to

flow through the energy barrier. Poole-Frenkel (PF)

emission models this behaviour quite effectively from a

classical viewpoint. In this model, the dielectric constant

() can be deduced from the slope of the forward bias

semi-log J-V1/2 characteristics using Equation (3) as

given by [15]:

(3)

where C is a proportionality constant; WT is the activa-

tion energy of the traps; E is electric field across the thin

film; and 0 are the dielectric constant of the thin film

and the permittivity of vacuum, respectively.

In this work, we have selected a high-field region

between 1.5 V and 2.0 V to obtain a reasonable -value,

as the PF model is suited for this region of data as plotted

in Figure 4.

Secondly, the TE mechanism that is present in these

coated nanoplatelets can overlap with the Poole Frenkel

(PF) emission due the traps in the RF sputtered high-

layer. The forward bias condition was selected instead of

reverse bias as our previous investigations [7] and [10]

assumed pure TE to be present, however in this case, by

applying a high reverse bias voltage there can also be

contribution by Fowler-Nordheim tunnelling which can

often dominate over PF emission and therefore affect the

accuracy of our interpretation using the PF emission

model [16].

The significance of the values for the barrier height

and -value of the sensors as calculated in Table 2 and

Table 3 tells us that at temperatures below 180 C, the

dominant conduction mechanism should be TE. Although

A Comparison Study on Hydrogen Sensing Performance of MoO3 Nanoplatelets Coated with a Thin Layer of Ta2O5 or La2O3 35

Table 2. The calculated values for the ideality factor of

the MoO3, La2O3/MoO3 and Ta2O5/MoO3nanoplatelets based sensor diodes

T (C) MoO3 [ref] La2O3/MoO3 Ta2O5/MoO3

25 1.77 5.22 5.92

140 1.20 4.42 4.40

180 1.12 3.97 4.24

220 4.03 4.19

260 1.15 3.93 4.26

300 1.06 3.43 4.26

Figure 5. Plot of the effective change barrier height B (asdenoted by the hollow circles) and effective changein dielectric constant (as denoted by the filledsquares) for MoO3 nanoplatelets, and Ta2O5 andLa2O3 coated MoO3 nanoplatelets.

Table 3. Dielectric constant () and effective change in

dielectric constant () of La2O3/MoO3 and

Ta2O5/MoO3 nanoplatelets based sensors as they

are exposed to air and 1% diluted hydrogen

ambience at different temperatures. Values that

are highlighted with a * indicate a possible

overlap in conduction mechanism with

Thermionic Emission (TE)

T (C) La2O3/MoO3 Ta2O5/MoO3

25 053.6* .-4.4* 064.4* 00.6*

140 035.7* .-0.6* 035.3* 00.8*

180 28.8 1.4 33.0 0.9

220 29.6 1.0 32.1 1.0

260 28.2 0.3 33.2 1.1

300 20.7 1.6 33.1 1.2

the calculated change in effective barrier height gives a

reference to how much in Figure 5 continues to in-

crease with respect to temperature, it does not provide

enough information to conclusively speculate whether

these results are reasonable. However, from the PF re-

sults we can argue that there is evidence to suggest that at

temperatures roughly above 140 C, there is a clear tran-

sition of results making PF emission the dominant con-

duction mechanism. This is due to the activation of the

defect traps (assuming they are from the Ta5+ or La3+ at-

oms) at the surface (and bulk) of the high- metal oxides,

and the results show consistency in the estimated -value

as approximately 28 for La2O3 and 33 for Ta2O5 -coated

MoO3 nanoplatelets. We have displayed the results

which should be invalid below 180 C in the PF plot to

show the validity of this argument.

3.4 Dynamic Hydrogen Sensing Performance

In comparing the operating speed for hydrogen sens-

ing applications, the dynamic response of the Ta2O5 and

La2O3 -coated MoO3 nanoplatelets is shown in Figure 6.

The sensors are operated below 300 C as the morpho-

logy of the nanoplatelets begin to deform above these

temperatures [8]. Thus, we chose to study and compared

the sensors at 220 C in terms of a potential difference

(voltage shift). This voltage shift is induced by the di-

polar charge caused by the adsorption of hydrogen and

as compared to our previous work, the additional coating

layer increases the trapping of charge as per proposed in

this investigation [10,14].

From the dynamic response (as shown in Figure 6),

we can see that the voltage shift of the Ta2O5-based sen-

sor is larger than that exhibited by the La2O3-based sen-

sor. This implies that the Ta2O5 layer has a better capabil-

ity to hold the adsorbate charge as compared to La2O3.

The implications of our results and principal advan-

tage in modifying the metal oxide nanostructured surface

is to further increase the sensitivity (change in voltage),

which effectively also allows the resolution of the sensor

to be improved. Yet, in our previous work we observed

that there is clearly a trade-off between the above advan-

tages and dynamic performance, in particular the

response and recovery time.

The aforementioned advantages are also dependent

on the operating temperature. As we have shown in this

work there is a transition from TE to PF emission and in

some instances, PF has undesirable effects, as it can

cause the consumption of applied potential difference in

order to maintain a constant current to flow across the

traps [17]. However in this case, as the PF emission is

clearly dominant at temperatures greater than 140 C and

thus, we have selected a temperature 220 C for the anal-

ysis of the sensing performance as at low temperatures

the sensitivity is very low. Our results also implies that

these traps will need to be thermally activated and thus

give the sensor an optimal operating temperature range.

We have calculated our voltage shift, response and

recovery time from Figure 6 as presented in Table 4.

We can deduce from the response and recovery time

results, that the La2O3 based sensor reaches saturation

quicker than the Ta2O5 based sensor across all hydrogen

concentrations. As the La2O3 sensor responds quicker,

the results suggest that the sensor based on this material

can accumulate charge at across the Pt/La2O3 interface

and the La2O3 layer at a greater rate than at the Pt/Ta2O5

interface. We also consider that when adsorbate charge is

accumulated at the Pt/oxide interface, it can also induce

an image force charge at the other side of the thin layer

[13] which increases the intensity of the overall change

in polarisation (or change in potential difference) over

the thin layer.

However, the results from the recovery time also

show that when the hydrogen source is removed, charge

is depleted from the Pt/La2O3 interface at a slower rate as

compared to that from Pt/Ta2O5 interface. These results

36 J. Yu et al.

Figure 6. Dynamic hydrogen sensing performance of theTa2O5 and La2O3 coated MoO3 nanoplatelets as ex-posed to different concentrations of H2 gas for 5min at 220 C, under 100 A bias current.

could signify that the traps in La2O3 are located deeper

than the traps in Ta2O5 and further work is required to in-

vestigate and understand the origin of this behaviour. In

terms of the response magnitude, the results indicate that

the Ta2O5 coating provides a stronger response which

signifies a better sensitivity and resolution for the sensor.

3.5 Future Work

In this work, we have examined the effects of adding

a thin layer of Ta2O5 or La2O3 on MoO3 nanostructured

metal oxide. As we can see from the results, we can ob-

serve that there is a significant improvement in terms of

sensitivity, however dynamic response is still rather slow

(up to three minutes) as compared to other previous

works [79]. Therefore, there is a need to further im-

prove sensors that employ a high- metal oxide as its sur-

face layer, by examining the effects of passivation me-

thods on the defect traps or rather to dope the layer with

other materials to increase its overall conductivity. The

implications of such work can serve as a basis for future

studies in the fields of electronics and gas sensing.

4. Conclusions

In this work, we have presented how the addition of

Ta2O5 and La2O3 thin films substantially improves the

hydrogen sensing performance of MoO3 nanoplatelets.

By comparing their results, we establish that there is a

transition in the dominant conduction mechanism from

thermionic emission to Poole-Frenkel emission between

140 C and 180 C. By examining the changes in barrier

height and dielectric constant, we can conclude that the

Ta2O5 layer provides better trapping of the adsorbate

charge than the La2O3 layer. As metal oxide based sensor

technologies will likely continue to employ high- di-

electrics in the future, it will be necessary to further our

control and understanding of defect traps in order to

further improve sensor functionality and performance.

Acknowledgements

The authors of this work would like to acknowledge

the CRCG Small Project Funding (201109176240) and

the University Development Fund (Nanotechnology Re-

search Institute, 00600009) of The University of Hong

Kong.

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Manuscript Received: May 24, 2013

Accepted: Feb. 22, 2014

38 J. Yu et al.