E L S E V I E R Surface and Coatings Technology 82 (1996) 291-293

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Deposition and microstructural characterization of MgO thin films by a spray pyrolysis method

Xie Yi a, Wang Wenzhong a, Qian Yitai a, Yang Li b, Chen Zhiwen ~' a Department of Applied Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China b Structure Research Laboratory, University of Science and Technology of China, HeJei, Anhui 230026, People's Republic of China

Received 31 July 1995; accepted in final form 6 October 1995

Abstract

Thin films of MgO (150 nm) were deposited by a spray pyrolysis method. Ethanol water solutions of magnesium acetylacetonate were ultrasonically nebulized and thermally decomposed on to Si(ll 1) and NaCI(100) in the temperature range 400-450 '=C. The microstructure of as-deposited MgO thin films was studied by X-ray diffraction, scanning electron microscopy and transmission electron microscopy.

Keywords: Magnesium acetylacetonate; MgO thin films; Spray pyrolysis

1. Introduction

Magnesium oxide is an attractive candidate for thin insulating layers in electronic devices owing to its high dielectric constant [ 1 ]. MgO has been widely used as a substrate material for thin film growth of high T~ super- conductors because of its lattice matching to YBazCu3Ov.~ and its low chemical reactivity [2]. Various methods have been used to deposit MgO buffer layers on silica and fused silica [3 -5] .

Spray pyrolysis processing has recently emerged as a variable deposition process capable of producing high quality thin films of a variety of oxides as well as sulphides [6,7]. Several oxide thin films have been deposited by ultrasonically nebulizing and thermally decomposing solutions of the corresponding metal deriv- atives of acetylacetone. Appropriate choice of the precur- sor as well as the operating conditions of the reactor can lead to film deposition from the vapour phase resulting in high quality thin films [6] .

In this study, thin films of magnesium oxide were deposited on S i ( l l l ) and freshly cleaved NaCI(100) substrates by spray pyrolysis. The microstructure of the films was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

2. Experimental details

Magnesium acetylacetonate, Mg(CH3COCHCO- CH3h (Mg(acac)2), was chosen as precursor for the

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preparation of MgO films. The process of preparation of Mg(acac)2 was similar to that of Al(acac)3 [8] . An appropriate amount of Mg(NO3)z.6H20 was dissolved in 15 cm 3 of distilled water and 10 cm 3 of methanol. This solution was immersed in an ice bath with constant stirring and 3 cm 3 of acetylacetone and 4 cm 3 of propyl- ene oxide were added. Concentrated ammonium hydrox- ide was added to the solution dropwise until the pH was approximately 7 and a fluffy white precipitate of Mg(acac)2 was formed. The precipitate and solution were refrigerated overnight, filtered and dried.

An ethanol-water solution of Mg(acac)2 was formed by dissolving 0.0025 mol Mg(acac)2 in 100 cm 3 ethanol in a 250 cm 3 volumetric flask which was then filled to capacity with distilled water. The Si(111) substrate was ultrasonically treated in solutions of H20:NH4OH:H202 and H20:HCl:H202 in the volume ratio 6 : 1 : 1, followed by ultrasonic cleaning in distilled water just prior to deposition [9]. The NaCI(100) substrate, of thickness 0.1 cm, was cleaved to 0.7 x 0.7cm 2 just prior to deposition.

The solution of Mg(acac)2 was ultrasonically nebu- lized, sprayed on the substrates and thermally decom- posed in the temperature range 400 450 °C in the reactor [10]. The solution was nebulized by a commercial ultrasonic humidifier and the resulting mist was swept into the reactor by compressed air as carrier gas at a flow rate of 3.5-4.01 min -1. Experiments showed that thin films of high quality could be deposited when the

292 Xie ~7 et al./Suffiu'e and Coatings Technolo~" 82 (1996) 291 293

distance from the nozzle to the substrate was between 8.0 and 9.0 cm.

The phase of the films was studied by X-ray powder diffraction. The X-ray diffraction (XRD) pattern was determined with a Rigaku D/MAX-TA diffractometer using monochromatic high intensity Cu K:~ radiation (2=1.5418 ?,). Micrographs of the surface and cross- section of the films were taken with a Hitachi X-650 scanning electron microscope operating at 25 kV.

In order to study the fine structure of the films, the surface morphology of the films on NaCI(100) was also examined by TEM. The films were mounted on 200 mesh copper grids after floating them in distilled water [11]. TEM images were taken with a Hitachi H-800 transmission electron microscope using an accelerating voltage of 200 kV.

The resistivities of the thin films were measured by the standard four-probe method with soldered indium contacts at room temperature.

3. Re su l t s and d i scuss ion

Smooth and homogeneous magnesium oxide films have been formed on both S i ( l l l ) and NaCI(100) substrates. The films had good adherence to both sub- strates. They appeared uniform and shiny with a bright blue colour.

Typical XRD patterns of as-deposited films are shown in Fig. 1. Apart from the strong peaks of the Si(111) and NaCI(100) planes, only two broad peaks at 20= 43 ° and 62.Y ~ appear. They correspond to the 200 and 220 reflections of MgO with the rock salt structure respectively. The size of the crystallites comprising the films is about 15 nm as calculated from the halfwidths of the diffraction peaks using Scherrer's formula.

The SEM image of the cross-section of an MgO film on S i ( l l l ) substrate (Fig. 2) shows an extremely homo-

o

10 20 30 40 50 60 70 2 0

Fig. 1. XRD patterns of as-prepared MgO thin fihns on (a) S i ( l l l ) and (b) NaCI(100) substrates.

f+Im + + + + ++++++++++++++++++++++++++ + :+ +++++++++++++++ + ++++

+++++++++++++s++m ++++ ..... , , , + + +++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++

+ +++++++++++++++ +++ ++++

Fig. 2. SEM image of cross-section of MgO iihn on Si( 111 ) substrate.

geneous layer of thickness 150 nm. The thickness of films on NaCI(100)is also 150 nm.

Fig. 3 shows SEM images of the surface of MgO thin films deposited on Si(111) and NaCI(100). SEM obser- vation shows that both films are dense, smooth and homogeneous without visible pores and defects. There are no visible particles in the micrograph (Fig. 3a) of the film on Si(111), showing that the particles are less than 20 nm in size, which is consistent with the broadening of the XRD pattern. The particles in the film on NaCI(100) are uniform spherical crystallites 100 nm in size, which is much larger than indicated by the XRD pattern. The difference in morphology shows that the kind of substrate influences the growth of particles in thin films.

TEM was performed to investigate the difference from XRD and SEM. Fig. 4 shows TEM images of an MgO film deposited on NaCI(100). The TEM image with lower resolution (Fig. 4a) shows that the film consisted of spherical particles of size 100 nm, which tallies with the result of SEM. However, the higher resolution micro- graph (Fig. 4b) indicates that these particles in fact comprise densely packed nanocrystallites of size less than 10 nm.

The resistance measurements show that as-deposited MgO thin films are insulating.

4. C o n c l u s i o n s

Magnesium oxide thin films of thickness 150 nm were deposited on both Si(111) and NaCI(100) by a spray pyrolysis method using an ethanol water solution of magnesium acetylacetonate as source material. The resulting films were identified as MgO films by X-ray diffraction. Uniformity of the films was confirmed by SEM and the bright uniform colour of the films. SEM and TEM investigations showed that the MgO films on S i ( l l l ) consisted of particles of size less than 20nm,

Xie Yi et al./SurJaee and (batings Teehnology 82 (1996) 291-293 293

Fig. 3. SEM images of surface of MgO films on (a} Si(111) and (b) NaCl(100) substrates.

Fig. 4. TEM images of surface of MgO film on NaCl(100) substrate.

while the films on NaCI(100) consisted of spherical particles of size 100 nm and these particles in fact comprised densely packed nanocrystallites of size less than 10 nm.

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