Aluminum Thin Films

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COVER SHEET

Tesfamichael, T. and Motta, N. and Bostrom, T. and Bell, J. M. (2006) Development of Porous MetalOxide Thin Films by Co-evaporation. Applied Surface Science 253(11):pp. 4853-4859.

Accessed from http://eprints.qut.edu.au

Copyright 2 006 Elsevier


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Development of Porous Metal Oxide Thin Films by Co-evaporation

a*T. Tesfamichael, aNunzio Motta, bThor Bostrom, and cJ. M. Bell

a Faculty of Built Environment and Engineering, School of Engineering Systems,

b Faculty of Science, Analytical Electron Microscopy Facility

cCentre for Built Environment and Engineering Research

Queensland University of Technology, 2 George Street, Brisbane, QLD 4000,

Australia

Fax: +61 7 38641516

*Corresponding author : Dr Tuquabo Tesfamichael, Phone: +61 7 38641988,

[email protected]

Abstract

This paper focuses on the development of mixed metal oxide thin films and physical

characterization of the films. The films were produced by co-evaporation of titanium

oxide and tungsten oxide powders. This allowed the development of titanium oxide-

tungsten oxide films as analyzed using XPS. Examination in the SEM and AFM

showed that the films were nano-porous with the pore size and pore orientation

varying as a function of the deposition angle. UV-Vis spectra of the films show an

increase of transmittance with increasing deposition angle which is attributed to the

structure and porosity of the films. Raman analysis indicated that the as-deposited

films have broad and weak Raman characteristics, attributed to the nanocrystal

nature of the films and the presence of defects, and the peak broadening deceases

after annealing the film, as expected.

PACS: 81.05.Rm; 61.43.Gt; 78.66.Qn; 68.55.Jk; 07.07.Df; 61.72.y


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Keywords: metal oxide gas sensors; oblique deposition; co-evaporated TiO 2-WO 3

thin film; surface characterization; porosity

1. Introduction

To protect people and property from gas-related accidents, gas sensors with higher

detecting capacity and gas selectivity are needed. A common sensing mechanism

involves measuring an electrical conductance change caused by gas adsorption.

This requires a sensitive layer that converts a change in the chemical state of some

species into an electrical signal. Metal oxide materials such as TiO 2, SnO 2, ZnO and

WO 3 have been used for gas-sensing devices because of their simplicity and low

cost [1-9]. However, they have poor selectivity and are affected by humidity and thus

commonly operate at temperatures between 200-400 oC. The gas sensing properties

of these oxides are determined by their intrinsic properties, but can be modified by

addition of impurities and/or modifying the microstructure including particle size,

distribution and orientation, surface morphology and porosity [6; 8; 10-13]. Thematerials have band gap energies between 2.6 to 3.3eV at room temperature. They

behave like n-type semiconducting oxides due to non-stoichiometry and they show

good chemo-sensitivity toward reducing gas pollutants. Among these oxides SnO 2 is

one of the most widely used sensors but recent research shows the other metal

oxides are able to detect pollutants and toxic gas emissions such as ammonia,

carbon monoxide, hydrogen sulfide, nitrogen dioxide. The response behavior of

various metal oxide films to a specific gas suggests that oxygen vacancy and charge

transport mechanisms are dependent on film microstructure [14]. Thin films are

usually compact and the active layer is limited to the surface whereas thick films are

commonly porous and hence the whole layer can interact with the gas species. If a

controlled porosity can be achieved in thin films, then the gas sensing properties can


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be enhanced. Gas detecting sensitivity also depends on the reactivity of film surface

as sensors are strongly influenced by the presence of oxidizing or reducing gases on

the surface. The reactivity can be enhanced by impurities, defects and active species

on the surface of the films, increasing the adsorption of gas species. It has been

shown that inclusion of different doping metals in the oxide films increased the

sensitivity to specific gases [3; 10-12; 15-17].

Gas sensitivity and selectivity can also be enhanced by appropriate choice of

material and proper control of the film microstructure. Each material has its own

response and co-deposition of various materials can add selectivity to specific gas

species and also often improves sensor quality and performance [8; 18]. Various

techniques have been used to develop films for gas sensor applications. This

includes sol-gel, chemical vapor deposition (CVD) and physical vapor deposition

(PVD) [19-24]. Each of the film deposition techniques has its own advantages and

limitations. TiO 2-WO 3 thin film sensors have been produced by sputtering and sol-gel

techniques [18; 25]. In this work, we expand the use of metal oxide sensors by co-

evaporation of titanium oxide and tungsten oxide producing porous metal oxide thin

films. The experiment investigated the development of titanium oxide-tungsten oxide

films by adjusting deposition angle to optimize the microstructure of the thin film to

achieve high performance gas sensors. Deposition onto a substrate held at an angle

to the incoming atom or ion flux is known to create columnar structures which often

exhibit higher porosity and rougher surfaces than films deposited with the ion or atom

flux normal to the substrate [26]. The combination of co-deposition to provide doping

and off-normal deposition to change the porosity and surface roughness should

provide real opportunities to improve sensitivity and selectivity.


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Physical characterization of various films to determine film structure, composition,

crystallinity and optical properties have been performed. Atomic Force Microscopy

(AFM) and Scanning Electron Microscope (SEM) were used to study the surface

structure and morphology of the films. The chemical composition was investigated

using X-Ray Photoelectron Spectroscopy (XPS). The optical properties and

crystalline nature of the films have been characterized using UV-Vis spectroscopy

and Raman spectroscopy, respectively.

2. Experimental

2.1. Thin Film Preparation

The present work focuses on a development of nanoporous metal oxide thin film by

evaporation method. A conducting SnO 2:F glass substrate (Libbey-Owens Ford TEC

8 8 /square nominal resistance) was obtained from Libbey-Owens Ford (USA).

The substrate was cut in to 5.5x5.5 cm squares and cleaned using methylated spirit.

The substrate was attached to a variable-angle substrate holder and mounted 10 cm

above a heat resistive boat (see Figure 1). High purity commercial titanium oxide

(99.9%) and tungsten oxide (99.9%) pellets were obtained from Pure Tech Inc.

(USA). The pellets of equal proportion (by volume) were crushed to a fine powder,

mixed and put onto the boat in a chamber (Figure 1). The chamber was evacuated

by mechanical and diffusion pumps to a base pressure of about 1.5 x10 -4 Torr. Co-

evaporation of the mixture was carried out at a partial pressure of about 10 -2 Torr by

applying an electric current through the boat. Film thickness was controlled using

quartz thickness monitor and films of about 100 to 200 nm thick were obtained by

controlling the deposition rate and time. The samples analyzed in this paper have film

thickness of about 200 nm. Various samples were evaporated at different angles of

incidence between normal angle and 80 degree. One sample was annealed in air in


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a temperature-substrate-specimen controlled oven at 450 oC for 24 hours to

investigate the change in crystalline property of the film. The effect of annealing on

crystallinity and gas selectivity of the films will be studied systematically at a later

stage.

2.2. Film Characterization

The surface microstructure of the films deposited at various angles of incidence was

examined using an FEI Quanta 200 scanning electron microscope operated at 25kV.

The samples for SEM were coated with a very thin layer of gold in a sputter coater to

provide electrical conduction and thereby reduce any charging effects. AFM images

of the film surface were obtained using an NT-MDT Solver P47 scanning probe

microscope (NT-MDT Co., Moscow, Russia) with "Golden" Si cantilevers operated in

semi-contact (vibrating tip) mode. Optical properties of the film were measured using

Varian Cary 50 UV-Visible conventional spectrophotometer in the wavelength range

300 to 1100 nm. As a reference, zero and 100% baseline signals were displayed

before each measurement of transmittance. The composition of the film was

determined using an Kratos AXIS ULTRA multi-technique X-ray Photoelectron

Spectrometer (XPS) incorporating a 165 mm hemispherical electron energy analyser.

XPS measurements were made at the Brisbane Surface Analysis Facility at the

University of Queensland. Survey (wide) scans were taken at an analyser pass

energy of 160 eV. Survey scans were carried out over 1200-0 eV binding energy

range with 1.0 eV steps and a dwell time of 100 ms and atomic concentration of each

element in the film were determined from peak areas of the profile. Narrow multiplex

high resolution scans were run with 0.1 eV steps and 250 ms dwell time. Raman

measurements were carried out using Renishaw System-1000 Raman spectrometer

to determine the chemical structure and physical state of the film. A HeNe laser


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excitation source of wavelength 633 nm with a 10 mW power at the sample was

used. A Raman shift between the wavenumbers 200 cm -1 to 1000 cm -1 has been

measured.

3. Results And Discussions

3.1 Scanning Electron Microscopy

Figure 2 shows SEM images of titanium oxide-tungsten oxide films deposited at

various angles of incidence. Figure 2a shows a film deposited at near normal angle

of incidence. A dense film with a range of submicron particle sizes from

approximately 100 nm to 500 nm can be observed.

Figures 2b and c show images of titanium oxide-tungsten oxide films deposited at

70 and 80 angle of incidence. From the images, it can be observed that at higher

angles of incidence nanoporous films have been obtained. The microstructures of

these films are considerably different from the film deposited at near normal angle of

incidence (Figure 2a). The particle size is similar, with some smaller crystallites

evident in the films deposited at 80 angle of incidence, but there is considerable

porosity evident at the surface of the film, with pores of about 20 nm to 100 nm in

size distributed across the film. From the results it can be concluded that the porosity

of the co-evaporated films was found to increase with increasing angle of deposition

and those films which possess nanopores are especially desirable for gas-sensor

design [10; 27; 28].

3.2 Atomic Force Microscopy

AFM images of four samples deposited between 30 to 80 degrees of incidence are

shown in Figure 3a-d. The porosity of the films was found to increase with increasing

deposition angle which is consistent with the SEM results discussed above. During

deposition of the films, the particles cluster together forming bigger grains with grain


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boundaries clearly shown between particles and also between grains. From the

images, it is apparent that angular deposition at a higher angle can significantly

change the porosity and morphology of the film. The effect of surface morphology on

gas sensitivity has been reported elsewhere [13].

Using the Nova and Image Analysis software from NT-MDT, the average grain size

was calculated from the AFM scans and is shown in Table 1. Generally the grain size

remained similar as a function deposition angle with pronounced clustering of the

individual particles at lower angles. At the highest angle of deposition (80 degree) the

clustering of the particles was lower and as a result the average grain size was

reduced (Table 1). In order to improve the gas-sensing characteristic of a film,

optimizations of the grain size and porosity are important factors. Overall the

analyses have shown that a change in deposition angle leads to the change in film

microstructure.

3.3 X-Ray Photo-electron Spectroscopy

Figure 4a shows XPS spectra obtained from survey scans on the surface of the co-

evaporated film between binding energies 0 to 1200 eV. As expected elements of C,

O, Ti and W were detected. From the spectra, impurities of K and P were also

detected. From high resolution spectra (Figure 4b), the core level of Ti 2p 1/2 and Ti

2p 3/2 are found at binding energy of 465.6 eV and 459 eV, respectively with a peak

separation of 6.6 eV. Figure 4c shows the core level of W 4f 5/2 (38.2 eV) and W 4f 7/2

(36 eV) obtained from high resolution measurements and the two peaks are

separated by 2.2 eV. The Ti 2p 3/2 peak obtained from the co-evaporated film (459

eV) is in close agreement with the literature value (459.2 eV) which corresponds to

the fully oxidized Ti +4 in titanium dioxide [29]. This shows that the Ti in the metal

oxide film can be assigned to the TiO 2. The binding energy of W 4f 7/2 obtained in this


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work (36 eV) was found to be in reasonable agreement with the corresponding

characteristic binding energy of W 4f (35.8 eV) in tungsten trioxide film reported

elsewhere [5; 14; 24]. The peak position of O 1s was at 531 eV (see table 2) but this

value is shifted, for example by 1.1 eV when compared with the core level of O 1s

(529.9 eV) of the anatase TiO 2 film reported elsewhere [30]. This can be due to the

nanocrystal nature of the mixed oxide thin film which may cause a shift of the core

levels.

From the peak area profile and the FWHM, the atomic concentrations of the

elements before and after etching the film using argon gas have been determined. As

shown in Table 2 and Figure 5, the amount of C decreased significantly after 5

minutes of etching which indicated that the C was not introduced during the

deposition process. However, the concentrations of K and P remained significant

after etching for up to 40 minutes. The source of these impurities was not known. The

slight shift of the core level peaks of Ti 2p and W 4f observed above may be

influenced by the K and P impurities in the nano-porous metal oxide film. The

concentrations of Ti and W across the depth of the film remained fairly constant with

about 8 at% and 19 at%, respectively. This is equivalent to a volume fraction of about

0.3 of TiO 2 compared to WO 3 in the film.

3.4 Raman spectroscopy

Raman spectroscopy was used to characterize the chemical and crystalline nature of

the film. As shown in Figure 6, the as-deposited film has broad and weak Raman

characteristic peaks around 645 and 696 cm -1. Raman peak broadening is

associated with various structural and microstructural features, including nano-size

crystals, presence of grain boundary and defects [31]. The 645 cm-1

peak position


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corresponds to anatase titanium oxide Raman active mode E g and the 696 cm -1 peak

corresponding to the vibrational frequency Raman mode of monoclinic tungsten

trioxide. From literature the expected vibrational frequencies of the most intense

Raman mode of anatase titanium oxide and monoclinic tungsten oxide are 639 cm -1

and 700 cm -1 respectively. The results for these films show a small shift from the

anticipated results [18; 32; 33]. This can be due to the nano-structure of the film that

can have a wide distribution of Ti-O and W-O bond angles which may cause a shift of

Raman peaks. In Figure 6, Raman spectrum of the film after annealing at 450C for

24 hours is also shown. The intensity of the absorption bands at 645 and 696 cm -1

increased and the peak width deceases. This indicates a more crystalline film after

heat treatment. In addition a weak Raman peak appeared at about 320 cm 1 which is

most likely from the tungsten trioxide hexagonal phase, which is normally seen at

310 cm -1 [18].

3.5 Optical Spectrophotometer

Transmittance of various films deposited at different angles of incidence measured in

the wavelength range 300 to 1100 nm is shown in Figure 7. From the figure, the

transmittance increases significantly with increasing deposition angle over the

measured wavelength range. The most likely cause for the increase is the reduced

overall electron density as the porosity increases. Luminous transmittance of the

films were determined by weighting the transmittance of the films (Figure 7) in the

wavelength range 380 to 760 nm to the intensity of the corresponding visible

spectrum for air mass 1.5 as shown in Table 3. The films deposited at higher angle

are fairly transparent having a luminous transmittance of about 60% which is closer

to the transmittance value of the conducting glass substrate (65%). The anatase

phase of titanium oxide has a band gap energy of 3.2 eV whereas tungsten trioxide


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has a band gap energy of 2.6 eV. Their corresponding optical transition wavelengths

for these oxides are 385 nm and 475 nm, respectively. The co-evaporated

nanocrystalline metal oxide films containing defects thus have not shown a sharp

absorption edge (see Figure 7).

4. Conclusions

Co-evaporated titanium oxide-tungsten oxide films with submicron grain sizes have

been deposited at various angles of incidence. With angular deposition a nanoporous

film was produced with an increase of pore size and orientation at higher deposition

angles. Optimum porosity is one of the parameters in metal oxide films that best

control gas response and sensing properties. The transmittance of the films

increased with increasing angle of deposition most likely due to the reduced overall

electron density as the porosity increases. The XPS and Raman analyses show a

mixed oxide film containing anatase TiO 2 and monoclinic WO 3 phases. Due to the

mixed nature of the TiO 2-WO 3, it is anticipated that the material would have a broad

gas selectivity. The gas sensing properties and selectivity of the films will be further

investigated.

Acknowledgments

This work was financially supported by Queensland University of Technology (QUT).

The authors are indebted to Dr Barry Wood for assistance with the XPS analyses.

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Figures and Figure Captions

Figure 1. Schematic diagram of the evaporative deposition as a function of angles of

incidence, .


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Figure 2SEM images of titanium oxide-tungsten oxide film co-evaporated at (a) near normal

(b) 70 degree and (c) 80 degree angles of incidence.


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b

a


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Figure 3 AFM images of co-evaporated thin films deposited at (a) 30 degree, (b) 50degree, (c) 70 degree and (d) 80 degree angles of incidence.

d

c


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Figure 4 XPS spectra of co-evaporated titanium oxide-tungsten oxide film: (a)

survey (wide) scan between binding energy 0 to 1200 eV on the surface of the film

before etching, and (b), (c) narrow high resolution peaks for Ti 2p and W 4f,

respectively after etching the film.


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0

10

20

30

40

50

60

70

0 10 20 30 40

Etching Time (min)

A t o m

i c C o n c e n

t r a

t i o n

( % )

W

O

P

Ti K

C

Figure 5 Atomic concentrations of C 1s, O 1s, Ti 2p, W 4f, K 2s and P 2p as a functionof etching time for titanium oxide-tungsten oxide film analyzed using high resolutionXPS.

Figure 6 Raman shift of co-evaporated film before and after annealing at 450 oC for 24hours measured using 633 nm HeNe laser source and a power of 10 mW at thesample.


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0

10

20

30

40

50

60

70

80

300 500 700 900 1100

Wavelength (nm)

T r a n s m

i t t a n c e

( % )

Substrate

30o

50o

80o

Figure 7 Transmittance of titanium oxide-tungsten oxide films deposited at different

angles of incidence as a function of wavelength between 300 nm to 1100 nm. For

comparison the transmittance of the substrate (conducting glass) is shown.


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Tables and Table captionsTable Captions

Angle of deposition Average grain size

30 300 nm

50 346 nm

70 387 nm

80 250 nm

Table 1. Grain sizes of titanium oxide-tungsten oxide film deposited between 30and

80 angles of incidence, determined from AFM scans.

Element Peak

Position

Be (eV)

at % (0

min)

at % (5

min)

at % (10

min)

at % (20

min)

at % (30

min)

at % (40

min)

C 1s 285 40.7 11.4 9.2 6.8 5.1 9.4

O 1s 531 43.2 55.9 61.7 62.6 56.6 54.8Ti 2p 459 3.3 7.3 7.7 7.7 9.1 8.0

W 4f 36 8.7 15.7 18.7 21.4 20.5 19.9

K 2s 378 1.2 6.9 1.1 0.2 4.8 5.6

P 2p 134 3.0 2.8 1.7 1.3 3.9 2.3


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Table 2. Atomic concentration of co-evaporated titanium oxide-tungsten oxide film for

various etching times obtained using XPS analysis.

Deposition

Angle

Luminous

Transmittance

30 o 53%

50 o 55%

70 o

59%

80 o 60%

substrate 65%

Table 3. Luminous transmittance (%) of co-evaporated titanium oxide-tungsten oxide

films for various deposition angles.

Aluminum Thin Films

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