Surface Review and Letters, Vol. 15, No. 5 (2008) 681–688c© World Scientific Publishing Company

LOWER TEMPERATURE FORMATION OF ALUMINA

THIN FILMS THROUGH SOL–GEL ROUTE

S. RIAZ∗,†, S. SHAMAILA†, B. KHAN‡ and S. NASEEM∗,§

∗Centre for Solid State Physics,

Punjab University, Lahore 54590, Pakistan†State Key Laboratory of Magnetism,

Chinese Academy of Sciences, Beijing 100080, China‡Department of Chemistry, LCW University,

Lahore, Pakistan§shahzad naseem@yahoo.com

Received 29 February 2008

Bayerite sol is spun onto single crystal Si substrate, after synthesis and optimization, to obtainfilms of thickness ∼ 0.2 µm. The deposited films are room temperature dried and then heatedup to a temperature of 350◦C in order to obtain Al2O3. Surface and structural changes, duringheating, are observed with optical microscopy. Scanning electron microscopy (SEM) and X-raydiffraction (XRD) are used for post-treatment analyses/characterization. The as-deposited andheated samples’ surfaces are smooth as seen with optical and scanning electron microscopein case of optimized conditions. XRD patterns show the change from amorphous to crystallinebehavior of these films when heated under various conditions. The most stable form of aluminumoxide, i.e. α-Al2O3, is obtained when samples are heated up to a temperature of as low as350◦C. The thin films are also deposited onto sodalime glass substrates in order to confirmAl2O3 formation through band gap probing. Photoconduction is used to find the energy bandgap, which comes out to be 4.7 eV; lower value is correlated to the defect induced states in theband gap.

Keywords: Alumina; sol–gel; corrundum; low-temperature.

1. Introduction

Aluminum oxide or alumina is one of the groups of

inorganic chemicals currently produced in very large

volumes.1 Alumina (Al2O3) exhibits several attrac-

tive properties such as good corrosion resistance,

high dielectric strength, resistance to the diffusion

of impurity, and high chemical and thermal stabil-

ity. These characteristics motivate its use for var-

ious applications, such as electrical,2 optical,3 wear

resistant coating for cutting tools,4 and diffusion bar-

rier coating for nonvolatile memories5 where dense

and closed structures are required, and also as cata-

lyst support where, in contrast, highly porous films

are required.6

Several routes such as chemical vapor deposition,7

electrochemical anodic oxidation,8 electroless plat-

ing,9 and spray pyrolysis of metal-organics10 have

been developed for preparation of nano-materials.

Among these, the sol–gel method is an emerg-

ing route with high promise for very homogeneous

films, which can be carried out at relatively low

temperatures.11,12

§Corresponding author.

681

682 S. Riaz et al.

Al2O3 exists in more than 15 distinct crystallo-

graphic phases, and it can undergo a variety of tran-

sitions until the most-stable corundum structure, i.e.

α-Al2O3, forms at high temperature, i.e. 1200◦C.13

However, this temperature is reduced by the intro-

duction of additives such as CuO and Fe2O314–16 or

α-Al2O317–20 before gelation. Hence, the transforma-

tion is observed at 1050◦C.11 Another alternative is

the modification of the sol formulation,21–25 which

allows conversion to α-Al2O3 without additives or

seeding. In this process, the presence of water in the

prepared sol retards the conversion to α-Al2O3,23

favoring transition via θ-Al2O3, which is remarkably

stable.19 Tsay et al.24 demonstrated that sols pre-

pared with a reduced amount of water experienced a

direct transformation to α-Al2O3, whereas the water-

containing sol passed through several transition

structures, such as γ-Al2O3 and θ-Al2O3. Indeed, the

measured activation energy of θ-Al2O3 → α-Al2O3

transition is 650kJ/mol, which is in contrast to that

of the induced γ-Al2O3 → α-Al2O3 transformation

(360–431kJ/mol).19 Therefore, the key to perform-

ing the α-Al2O3 transformation at lower tempera-

tures is to avoid the transition via θ-Al2O3.

As sol–gel precursors, aluminum alkoxides dif-

fer significantly from other metal-alkoxides in terms

of their chemical reactivity and complex-forming

ability. These differences dictate the adoption of dif-

ferent strategies for the creation of alumina based

sol–gel products. The sol–gel reactions in usual

metal-alkoxides/silica-based system are rather slow

and often require the use of catalysts to acceler-

ate the process. This is explained by the fact that

aluminum alkoxides (e.g., aluminum sec-butoxide)

are very reactive toward nucleophilic reagents like

water.26 They readily undergo hydrolysis, which

results in a very fast sol–gel process, as shown in

Fig. 1. Even if the solution of aluminum sec-butoxide

is stirred vigorously, the rates of these reactions

are so high that large agglomerated alumina parti-

cles precipitate out instantaneously when the sol–

gel ingredients are mixed together. Such fast pre-

cipitation makes it difficult to reproducibly prepare

alumina-based sol materials.

The hydrolysis activity of aluminum sec-butoxide

can be controlled by chelating it with various β-

ketoesters (or R-acetoacetates) through a steric

effect, e.g., ethyl acetoacetate,28 acetylacetone,29

which are capable of undergoing keto–enol

Al

OCH(CH3)2

(H3C)2HCO OCH(CH3)2

+ 3H2O Al

OH

HO OH

+ 3(CH3)2CHOH

Aluminum Isopropoxide Aluminum Hydroxide

(a)

Al

OH

HO OH

+Al

OH

HO

n

OH

Al

O

HO OH

( O Al )n

O Al

OO

(b)

Al

OCH(CH3)CH2CH3

H3CH

2C(H

3C)HCO OCH(CH

3)CH2CH3

+ 3H2O Al

OH

HO OH

+ 3CH3CH2(CH3)CHOH

Aluminum sec-butoxide Aluminum Hydroxide

(a)

Al

OH

HO OH

+Al

OH

HO

n

OH

Al

O

HO OH

( O Al )n

O Al

OO

(b)

Fig. 1. Chemical reactions of alumina sols show (a)hydrolysis and (b) poly-condensation for both Al-isopropoxide and Al-sec-butoxide (adapted from Ref. 27).

tautomerism. The formation of the stabilized

chelated complex reduces the hydrolysis and conden-

sation rates of aluminum sec-butoxide by decreasing

the number of available alkoxy groups.

Thus, as compared to other metals, alumina is

capable of both ion and ligand exchange.27 The lig-

and exchange ability of alumina originates from the

presence of Lewis acid sites on the surface, i.e. coor-

dinatively unsaturated Al3+, and water molecules or

other easily displaced ligands coordinatively bonded

to the sites, as shown in Fig. 2.27 Lewis basic ana-

lytes containing polar functional groups such as car-

boxylic, phenolic-OH or amino groups can substitute

for the surface hydroxyl group or coordinated water

molecules and form complexes with the metal ions

of the oxide surface.30 Therefore, precursors of alu-

mina, such as aluminum sec-butoxide and aluminum

iso-propoxide, are very attractive and are chosen, for

Lower Temperature Formation of Alumina Thin Films Through Sol–Gel Route 683

H2O2(aq)H2O(l) + 1/2 O2(g)

+ HO

H

Al

X

O

H2O2(aq)H2O(l)

OH

Al

O

OH

Al

O

Strong Bronsted acid site Weak Bronsted Acid Site

Fig. 2. A representation of Lewis acid sites for alumina.

the current work, to prepare alumina films by the

sol–gel method that may lead to novel applications.

In this paper, we report on the detailed charac-

terization of sol–gel based alumina thin films from

locally synthesized precursors. According to the best

of our knowledge this is the first alumina report

in which the most stable phase, i.e. corundum, is

achieved at a reaction temperature of 350◦C.

2. Experimental Details

2.1. Synthesis of alumina sol

Aluminum foil was first activated chemically so that

it may become reactive for the next steps. This

pre-activated aluminum foil went through different

chemical reactions at various stages to produce the

desired product. Finally, aluminum hydroxide solu-

tion was obtained from two different sources sepa-

rately, i.e. aluminum iso-propoxide and aluminum

sec-butoxide by following the hydrolysis and poly-

condensation. The chemical reactions/chains can be

followed through flowcharts of Figs. 3(a)–3(c).

2.2. Deposition of alumina

thin films

Silicon (100), 5×5mm, substrates were used for thin

film deposition. An ultrasonic bath for 30min with

acetone, and 20min with isopropyl alcohol was given

to the substrates in order to make them free of any

Aluminum Foil HgCl2

distilled water

Heatedand aged at room temperature

Activated aluminum foil

washed with distilled water

(a)

StirringRefluxing

Activated Al foil Isopropyl Alcohol

Alumnum Iso-propoxide

Adding CCl4

Vacuum distilation

Stirring

Refluxing

Activated Al foil Absolute sec-butanol

Aluminum sec-butoxide

Adding CCl4

Vacuum distilation

(b)

Aluminum Iso-propoxide HNO3 based solution

Stirring

Refluxed

Sol A(AlOOH)

HNO3 basedsolutionAluminum sec-butoxide

Stirring

Refluxed

Sol B (AlOOH)

(c)

Fig. 3. Flowcharts of sol–gel processes.

contamination and gas residues on the surface after

giving an HF etch. After cleaning, the substrate was

held by vacuum on a homemade spinner (Fig. 4).

Sol (A&B separately) is dropped on surface of the

substrate and spun for half a minute at a speed of

4000 rpm. Thickness of 0.2µm was achieved in this

way. These samples were then dried at room temper-

ature for 20min. After drying, the samples were sub-

jected to heat treatment at different temperatures for

varying times. The whole process can be understood

in a better way diagrammatically as shown in Fig. 5.

2.3. Characterization of alumina

thin films

Leica DM4000 optical microscope, equipped with a

heating stage, was used to observe the general surface

character and to check the ongoing surface changes

with variation in temperature. The current to the

heating stage was provided by the current controller

system that utilizes Testo 925 digital temperature

684 S. Riaz et al.

Fig. 4. Photographs of the homemade spinner; (a) com-plete spinner set-up along with a rotary pump and(b) magnified image of spinner stage.

As-deposited AlOOH

thin film

Characterizations

Wet gel

Heat treatment at different

temperaturesDried

Aged at room temperature

Spin Coating of Sols (A & B)

Fig. 5. Block diagram of film deposition.

display. Hitachi S-3400N scanning electron micro-

scope equipped with EDX was used to check the sur-

face and composition of these sol–gel prepared thin

films. Alumina thin films were characterized struc-

turally with the help of Rigaku D/MAX-IIA X-ray

diffractometer. For measuring the photoconductivity,

thin films were prepared on glass substrate. Ohmic

contacts were made on the thin film of Al2O3 by

evaporating semi-transparent and thick aluminum

films and then pasting copper wires with conduct-

ing silver epoxy.

Samples were mounted on a test shield model

5104 by Keithley Instruments and Bausch & Lomb

monochromator was used for photoconduction mea-

surements. A 610-C Digital Electrometer was

attached with this assembly in order to measure the

current. The voltage was applied using power supply

model IZS 5165. The photocurrents were normalized

to unit radiant flux incident on the sample at any

wavelength.

3. Results and Discussion

Viscosity of both the sols was checked periodically

and it was found that both sols were very stable for

a long time period as shown by Fig. 6.

The freshly spun transparent sol of aluminum

hydroxide is left to cure at room temperature for

at least 20min. This room temperature cured sam-

ple is then transferred to metallurgical optical micro-

scope equipped with a heating stage. The sample is

then heated, mostly up to 350◦C, while the surface

being micrographed every 15min. The heat treat-

ment, at the optimum temperature, lasts usually

60min, which varies from sample to sample. A typ-

ical optical micrograph of the as-deposited sample

(sol A) is shown in Fig. 7. The surface is quite

smooth and featureless but as the reaction tempera-

ture is increased, chained structure appears as shown

in Fig. 8. Appearance of chained structure is indica-

tive of chemical reaction occurring at early stages

of the film growth. Scanning electron micrograph

Fig. 6. Viscosity vs. time plotted for alumina sols.

Lower Temperature Formation of Alumina Thin Films Through Sol–Gel Route 685

Fig. 7. Optical micrograph of as-deposited thin film ofsol A (×50).

Fig. 8. Optical image of the chained structure ofdeposited film during heat treatment (sol A).

(Fig. 9) shows featureless surfaces at higher magnifi-

cations for the film prepared from sol A. However, in

case of sol B film, there are some voids observed in

the electron micrograph (Fig. 10). These voids show

the poly-condensation of alumina molecules and thus

support the formation of chained network. These

voids have been found to be on the surface alone

rather than through the film.

Fig. 9. SEM image of optimized Al2O3 thin film syn-thesized from sol A.

Fig. 10. SEM image of optimized Al2O3 thin film syn-thesized from sol B.

The aluminum based sols A and B, spin coated

onto silicon (100) substrates, were checked for their

crystal structure, phase transformation, and confir-

mation of the finally synthesized product at various

stages. Figure 11 shows XRD patterns of the as-

deposited (room temperature aged) and heated sam-

ples prepared from sol A. It is evident, from the pat-

tern [Fig. 11(a)], that initially the film exhibits amor-

phous behavior. However, the few emerging peaks

belong to Al(OH)3, which are indicative of the for-

mation of Bayerite sol.31 The aluminum hydroxide

phase is transformed to α-alumina32 after heating

at 350◦C for 60min as shown in Fig. 11(b). Tsay

et al.24 have demonstrated that sols prepared with a

686 S. Riaz et al.

Fig. 11. XRD patterns of “Sol A” for (a) as-depositedAl-hydroxide thin film and (b) heated at 300◦C for60 min.

reduced amount of water experienced a direct trans-

formation to α-Al2O3, whereas the water-containing

sol passed through several transition structures, such

as γ-Al2O3 and θ-Al2O3. This means that sol A pre-

pared in our case was water-free since a direct tran-

sition to α-Al2O3 is observed. This alpha phase of

alumina is more favored in the dielectric applications.

Figure 12 shows the XRD patterns of the

deposited (room temperature aged) and heated sam-

ples of alumina film prepared from sol B. This pat-

tern (Fig. 12(a)) also shows the amorphous behavior

of the deposited film along with the presence of alu-

minum hydroxide peaks of varying intensities. It can

be seen from the comparison of Figs. 11(a) and 12(a)

that the peaks of Al(OH)3 are smaller in case of sol

B. This may be due to the reason that the sol synthe-

sized from Al-sec butoxide based precursor does not

readily react with other constituents of the product

under these conditions. Figure 12(b) shows the XRD

pattern taken after heating of aluminum hydroxide

Fig. 12. XRD patterns of “Sol B” for (a) as-depositedAl-hydroxide thin film and (b) heated at 300◦C for60 min.

(sol B) at 350◦C for 60min. Various peaks of alu-

mina are observed in this figure due to hydrolysis

followed by condensation at this temperature. The

major peak though belongs to α-Al2O3,32 few peaks

of θ-Al2O333 are also present. This means that sol B

contained some water content as discussed above.

The grain size of these alumina films was

calculated from the Williamson–Hall plot of the

relation34,35:

2ωf cos θ

Kλ=

1

D+

4e

Kλsin θ,

where 2ωf is in radians, K is the shape factor of the

crystalline particles, λ is the wavelength, e (= ∆d/d)

is the microstrain, D is the particle size, and θ is the

Bragg angle.

The extrapolation method for the elimination of

the instrumental broadening of diffraction lines has

been used since it does not require a standard sam-

ple. This method is valid for the conventional powder

X-ray diffractometer with the Bragg–Brentano focus-

ing geometry36,37 that was used in the present case.

The resultant grain size, for both the sols, comes out

to be of the order of 12–15nm.

Lower Temperature Formation of Alumina Thin Films Through Sol–Gel Route 687

Fig. 13. Photoconduction measurements of Al2O3 thinfilm.

The band gap of these Al2O3 films was deter-

mined by photoconduction measurements. Photo-

conductivity of the sample is plotted in Fig. 13,

which shows the relative photocurrent vs. energy of

the incident photons. The optical band gap (Eg) of

Al2O3 thin films was determined from the straight

line intercept at the photon energy axis. For an

applied potential of 0.5V, the energy band gap is

4.7 eV. At room temperature, the energy band gap

of bulk Al2O3 is 6.2 eV.38 The discrepancy of about

1.5 eV in the energy band gap of Al2O3 is due to the

fact that we have studied Al2O3 in thin film form.

In addition, lowering of the band gap value may also

be associated with defect induced states in the band

gap.39 This idea is supported by the peaks at 3.8 eV

and 4.3 eV in Fig. 13.

4. Conclusions

Alumina sol was spin coated onto silicon sub-

strates after optimization of synthesis of the sol. Al-

isopropoxide based sol (A) and Al-sec-butoxide sol

(B) were optimized for deposition of alumina thin

films through sol–gel route. Bayerite [Al(OH)3] phase

of both the sols was observed in the room temper-

ature aged samples as confirmed by the XRD pat-

terns. Direct transition of sol A into α-Al2O3 was

observed at a temperature of 350◦C whereas sol

B exhibited indirect transition to α-Al2O3 through

θ-Al2O3. Optical and scanning electron micrographs

showed continuous film surfaces resulting from opti-

mized conditions. The energy band gap value of the

alumina film was 4.7 eV, which has been attributed

to defect induced states in the band gap.

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