Fabrication of polycrystalline aluminum

Fabrication of polycrystalline aluminum oxide thin films via hydrolysis and

hydrothermal reactions in solutions

XiaoFei Duan a

, Irving Liaw a

, Nguyen H. Tran b

, Robert N. Lamb a,⁎

a

School of Chemistry, The University of Melbourne, Victoria, 3010, Australia

b

School of Natural Sciences, The University of Western Sydney, Parramatta, NSW, Australia

article info abstract

Article history:

Received 20 September 2010

Received in revised form 9 March 2011

Accepted 4 June 2011

Available online 14 June 2011

Keywords:

Aluminum oxide

Polycrystalline

Thin films

Sol–gel

Hydrothermal

Polycrystalline Al2O3 thin films were fabricated through a method combining urea hydrolysis and

hydrothermal reactions. The overall growth temperature of these films was achieved as low as 150 °C.

Although cracks occurred across the gel film after hydrolysis, a subsequent nucleation under elevated

pressure and temperature resulted in a closely packed morphology. Moreover, the hydrothermal treatment

led to high oxide content and an increase in crystallinity within the films.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Al2O3 thin films have been seen in a variety of applications from

electronics, [1,2] biological implantation, [3,4] to mechanical coatings.

[5,6] Conventional deposition methods require high thermal energy, i.e.

high deposition temperature. A reduction in deposition temperatures

opens up opportunities for coupling of electrical and mechanical

properties of polycrystalline Al2O3 films with soft, thermally unstable

substrates such as polymers. Even though atomic layer deposition can

be used for a low temperature deposition (≤300 °C), [7,8] controls over

multiple sources and instrumentation often result in experimental

complications. Sol–gel deposition method, on the other hand, has been

extensively employed for fabrication of Al2O3 thin films, [9] due to low

deposition temperature and inexpensive equipment. The method

involves the hydrolysis of precursors such as aluminum alkoxide to

produce the hydrated Al(OH)3 films at temperatures below 100 °C.

[10,11] However, high temperature annealing treatment was still

required to form oxide films (N350 °C [12,13]), metastable crystalline

phases (N800 °C [10]) and thermodynamically stable phase (N1000 °C

[11,14,15]). Annealing films at high temperatures often causes micro￾cracks across the film on the substrate due to differences in thermal

expansion coefficients. An alternative transformation method is a low

temperature hydrothermal treatment. At an elevated pressure, this

method demonstrated effective formation of a polycrystalline ZrO2 thin

film at a low temperature of ~200 °C [16].

Prior to the structural transformation, urea hydrolysis reaction is

employed to produce Al(OH)3. This method can effectively deliver

OH− and form Al hydroxide with Al3+, which has been used for the

synthesis of Al(OH)3 particles and powders. [17–19] But the formation

of thin films has to date not been reported. In this work, we demonstrate

the formation of polycrystalline Al2O3 thin films at a temperature as low

as 150 °C using a hydrolysis reaction followed by a carefully controlled

hydrothermal treatment. With this combined process, a transformation

of thin films containing micro-cracks to closely packed, crack-free films

were achieved.

2. Experimental

A sequential three-step reaction was used to produce polycrystalline

Al2O3 thin films. Firstly, Al(NO3)3 and urea (5.2 mmol : 5.8 mmol) were

dissolved in demineralized water (20 ml). The clear solution was heated

and maintained at 80 °C. A Si wafer (10×10 mm) was cleaned with

demineralized water and immersed in the solution for 2 h. Secondly,

after removing the Si wafer from the solution, the wafer was tilted

against filter paper to remove excess solution. It was then heated and

dried at 100 °C for N1 h in a furnace. A thermally dehydrated film (TDF)

with observable diffraction bands was visible on the surface of the Si

substrate. Finally, this sample was transferred into a Parr reactor, to

which demineralized water (500 μl) was added. The reactions were run

at 150 (4.7 atm), 200 (15.3 atm) and 250 °C (39.2 atm), respectively, for

24 h, and then gradually cooled at room temperature. The hydrother￾mally treated film (HTF) was then dried at 100 °C for 1 h. TDF and HTF

were characterized using X-ray photoelectron spectroscopy (XPS),

Thin Solid Films 520 (2011) 25–29

⁎ Corresponding author. Surface Science and Technology Group, School of Chemistry,

The University of Melbourne, VIC, 3010, Australia.

E-mail addresses:[email protected](X. Duan), [email protected](R.N. Lamb).

0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2011.06.003

Contents lists available at ScienceDirect

Thin Solid Films

j o u r n a l h om e p a g e : www. e l s ev i e r. c om / l o c a t e / t s f