www.elsevier.com/locate/apsusc

Applied Surface Science 253 (2007) 8661–8668

Mg2+ substitutions in ZnO–Al2O3 thin films and its effect

on the optical absorption spectra of the nanocomposite

Soumen Das, Subhadra Chaudhuri �,*

Nanofilm Lab, Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700032, India

Received 27 February 2007; received in revised form 26 March 2007; accepted 18 April 2007

Available online 3 May 2007

Abstract

ZnO–Al2O3 nanocomposite thin films were prepared by sol–gel technique. The room temperature synthesis was mainly based on the successful

peptization of boehmite (AlO(OH)) and Al(OH)3 compounds, so as to use it as matrix to confine ZnO nanoparticles. The relative molar

concentrations of xZnO to (1 � x) Al2O3 were varied as x = 0.1, 0.2 and 0.5. The optical absorption spectra of the thin films showed intense UV

absorption peaks with long tails of variable absorption in the visible region of the spectra. The ZnO–Al2O3 nanocomposites thin films were doped

with MgO by varying its molar concentrations as y = 0.05, 0.75, 0.1, 0.125, 0.15 and 0.2 with respect to the ZnO present in the composite. The MgO

doped thin films showed suppression of the intense absorption peaks that was previously attained for undoped samples. The disappearance of the

absorption peaks was analyzed in terms of the crystalline features and lattice defects in the nanocomposite system. The bulk absorption edge, which

is reportedly found at 3.37 eV, was shifted to 5.44 eV (for y = 0.05), 5.63 eV (for y = 0.075) and maximum to 5.77 eV (for y = 0.1). In contrast,

beyond the concentration, y = 0.1 the absorption edges were moved to 5.67 eV (for y = 0.125), 5.61 eV (for y = 0.15) and to 5.49 eV (for y = 0.2).

This trend was explained in terms of the Burstein–Moss shift of the absorption edges.

# 2007 Elsevier B.V. All rights reserved.

PACS : 81.20.Fw; 78.66.Hf; 61.72.Vv; 78.66.�w

Keywords: Sol–gel; Nanocomposite; Thin films; Optical absorption

1. Introduction

The wide band gap of ZnO (3.37 eV for the bulk ZnO [1])

can be altered if the particle sizes of the system lie in the

nanometric region. This large band gap of ZnO coupled with its

large excitonic binding energy (0.060 eV) have made it a

potential candidate in flat panel devices, light emitting diodes,

LASERs in the ultraviolet region [2,3] and as transparent

conducting oxide. Above this, the band gap of ZnO also

depends on the size of the crystallites. This phenomenon is

called quantum confinement (QC) effect and is analyzed and

explained by several authors [4–6]. It is observed that the higher

surface to volume ratio of nanocrystals introduces various

surface related defects and disorders in the system. So, they are

* Corresponding author. Tel.: +91 33 2473 4971; fax: +91 33 2473 2805.

E-mail addresses: soumen0208@gmail.com (S. Das), mssc2@iacs.res.in

(S. Chaudhuri).� Deceased.

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.04.072

embedded in insulating matrix to minimize these defects. High

band gap materials like MgO, SiO2 or Al2O3 are also used to

restrict the growth rate of the nanocrystals and in the process

enhance the optical band gap of the system. The quantum

confinement effect, and the introduction of Al3+ or Mg2+ ions

into the lattice of ZnO are used fruitfully to tune the band gap of

ZnO nanocrystals [7–9]. This latter mode of tuning is called the

Burstein–Moss Shift, and it is dependent on the diffusibility of

the dopant into the crystal lattice [10].

ZnO with MgO (Eg = 7.3–7.7 eV [11]) can produce

MgxZn1�xO alloy having higher band gap and improved

optical and photoluminescence properties [12,13]. The suit-

ability of MgxZn1�xO as a multilayer quantum well structure

has also been studied in various earlier reports [14,15].

According to the phase diagram of the ZnO–MgO binary

system the solubility of MgO in the ZnO lattice is less than 4%

in the bulk form [16], but for thin film the solid solubility is as

high as 33% [17]. The degree of solubility of Mg in ZnO is

found to depend largely on the deposition techniques and on the

processing conditions [18–20]. The ionic radius of Mg2+

Fig. 1. The TEM images of the (a) alumina sol network, (b) the cylindrical

fibrillar structure of alumina hydroxide annealed at 300 8C.

S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–86688662

(0.57 A) is very close to that of Zn2+ (0.72 A) [21], so at relatively

higher annealing temperatures the Mg ions diffuse into the ZnO

lattice, and replace each other. For higher concentration of MgO

it has been shown that the MgxZn1�xO alloys form a metastable

alloy [17]. These metastable phase and the degree of

metastability are the influencing factors for the potential

applications of MgxZn1�xO based devices. On the other hand,

the accumulation of MgO on the grain boundaries of ZnO has an

influence on the surface states of the nanoparticles [22].

In this article, we have undertaken a comparative study of

the UV–vis optical absorption spectra of the single layered

undoped and MgO doped ZnO–Al2O3 nanocomposite thin

films. We have also concentrated on the shift of the optical

absorption edge from its bulk band gap value and the

broadening of the optical spectra at the absorption edge. We

found out that these phenomena largely depend on the role of

the MgO as dopant, its accumulation on the surface of ZnO and

its solubility in the nanocomposite.

2. Experimental details

ZnO–Al2O3 nanocomposites with relative molar concentra-

tions of ZnO to Al2O3 as 10:90, 20:80 and 50:50 were prepared

by sol–gel technique. For Al2O3 part, the aqueous solution of

aluminum nitrate (Al(NO3)3�6H2O) was refluxed for 1 h and

then treated with NH3 solution (25%). The white precipitate (a

mixture of aluminum hydroxide and boehmite) was dissolved

in (12:1) volume ratio of ethanol and water. Three cubic

centimeter of acetic acid was added to it drop wise under

constant stirring. The pH of the sol was measured as 2.75. At

the end of 3 h of stirring a completely transparent and viscous

sol was obtained. The detail of the preparation of the composite

sol and the subsequent deposition of the thin films were

reported by the authors elsewhere [23]. At this point we would

like to mention that the sol–gel preparation of the alumina was

mainly based on the Yoldas method [24], as described in

numbers of well-cited papers [25,26]. The main difference in

our method from the prescribed one was that we accomplished

the entire preparation at room temperature, without resorting to

heat treatment during peptization. In fact, heat treatment at

80 8C was a crucial parameter in Yoldas method.

In the latter section of this article, the nanocomposites will

be denoted by sample SA (10:90), SB (20:80) and SC (50:50).

The thin films were annealed at 300, 500 and 700 8C in air.

Later, sample SA was doped with MgO at molar concentrations

y = 0.05, 0.075, 0.10, 0.125, 0.15 and 0.20 with respect to the

molar concentration of ZnO present in the composite. For

doping, the required amount of Mg-acetate was mixed directly

with the ZnO–Al2O3 sol under stirring. The thin films were dip-

coated (5 cm/min) and annealed in air at 300 8C for 25 min. In

another case, sample SC was also doped with y = 0.05 MgO and

was annealed at 300 and 700 8C.

The surface morphology of the thin films was determined by

atomic force microscopy (AFM) (Nanoscope IV scanning

probe microscope controller). The morphological attributes of

the nanocomposite samples were determined by transmission

electron microscope (TEM) (JEOL 2010 electron microscope)

along with the selected area electron diffraction (SAED)

pattern. The composition analysis and the presence of the Mg2+

in the nanocomposite thin films were confirmed by the energy

dispersive X-ray analysis (EDAX) attachment to a JEOL JSM

J2010 Field emission scanning electron microscope (FESEM).

The optical absorption spectra of the products were recorded by

an Schimadzu 2401spectrophotometer.

3. Results and discussion

3.1. The TEM study

Fig. 1 shows the TEM images of the alumina sol taken on a

Cu grid and annealed at 300 8C. The image shows a network

S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668 8663

like build up of the aluminum based hydroxide in the sol.

Earlier, Yoldas [24] showed that depending on certain

parameters, like the type of alkoxides used, the ratio of

alkoxide to water, the rate of hydrolysis, pH standard and the

mode of drying there are may be several structural variations of

the final alumina gel. The microstructural images in Fig. 1a

shows interconnected branch like extension of the hydroxide

consisting of closely knit grains forming thick impenetrable

structures. These dangling branches contribute to the density,

the pore size and the specific area of a dry gel. Colloidal gels

obtained from aluminum precursors comprise of two types of

oxo-bridges, Al–O–Al and Al–O(OH)–Al(OH). The dry gel in

this case forms a random network of neighbouring particles

where the proportion of linear linkage between particles varies.

The pore presents in the ‘‘cylindrical fibrillar structure’’ also

Fig. 2. The TEM images of the ZnO–Al2O3 nanocomposite (a,b) the network like app

annealed at 300 8C, inset figure shows the ZnO grains consisting of agglomerated Z

rings from ZnO nanocrystals.

produces rooms for the guest nanoparticles to get confined and

to grow during sintering, as we will observe in Fig. 2.

In Fig. 2a and b the ZnO–Al2O3 nanocomposite (sample SA)

showed similar branch like structures. That the ZnO nano-

particles are embedded and are randomly dispersed in the

Al2O3 matrix can be observed from Fig. 2c. The black dots are

agglomerated grains of ZnO nanoparticles and can be well

resolved at the inset figure, which shows the high-resolution

image of a single ZnO grain consisting of a number of

agglomerated nanoparticles. The average size of the ZnO

nanoparticles was determined as around 9.0 nm. In Fig. 2d the

SAED image shows spots in the diffused diffraction rings

indicating the polycrystalline nature of the nanoparticles. In our

previous report [23] we have seen that the average particle size

was 10.0 nm for sample SB and around 18.0 nm for sample SC

earance, (c) the ZnO nanoparticles embedded on the amorphous alumina matrix

nO nanoparticles. (d) The SAED images of the composite showing diffraction

Fig. 4. The XRD spectra of the alumina gel dried at different temperatures or

1 h in air medium. The image shows the transition alumina at different

temperatures.

S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–86688664

processed under similar conditions. Thus the nanoparticles

grow in size when the proportion of the alumina is lesser in the

composite. This is because; the presence of larger quantity of

ZnO inside the pores of the alumina matrix facilitates the

growth to larger grains when treated at different annealing

temperatures.

3.2. X-ray diffraction study

The crystalline phase of the nanocomposite thin films were

studied with the XRD measurements. The XRD spectra of

sample SA, SB (annealed at 300–700 8C, not shown in this

report) did not show significant difference in appearance from

each other, and characteristic peaks either of alumina, or ZnO

were absent. As in the subsequent TEM images revealed the

polycrystalline nature of the samples, we inferred that the

absence of the characteristic peaks was due to the low

resolution of the XRD measuring instrument. The characteristic

peaks of ZnO are observed for sample SC annealed at 300 and

700 8C as is shown in Fig. 3. The peaks are identified as (1 0 0),

(1 0 1) and (2 0 0) [JCPDS-File No. 35-0664]. It is seen that the

peak intensity increases at higher annealing temperature and

are more prominent in the lower spectra (for films annealed at

700 8C). The average radius of the confined nanoparticle is

measured from the highest peak in the spectra and is calculated

as 8.2 nm, which is close to the size we obtained for sample SC

from TEM (reported in [23]). We would also like to point out

that in the XRD spectra of the nanocomposite thin films no peak

of Al2O3 is observed. The reasons for this absence may be two.

The first is the amorphous nature of the sample. To examine this

possibility, the powder form of the alumina gel was annealed at

various temperatures and due to this a vast phase transformation

is observed as a function of temperature. The observation is

shown in Fig. 4. The dry gel at room temperature mainly

consisted of boehmite (AlO(OH)) and aluminum hydroxide

(Al(OH)3). This is because, Al, with an oxidation number of III,

can easily polymerize in a variety of different polycations

and constitute various solid phases. In the polycondensation

reaction, if the experimental conditions are such that the

synthesis temperature is below 80 8C, the structure that is

Fig. 3. The XRD spectra of the ZnO–Al2O3 nanocomposite thin films annealed

at 300 8C and 700 8C.

formed is Al(OH)3. On the other hand if the synthesis

temperature is above 80 8C, the structure is similar to that of

boehmite, or AlO(OH) [27]. This phase is retained up to a

drying temperature of 200 8C for 1 h. At even higher

temperatures, constant dehydration or, the removal of chemical

water from this gel, left the structure disarrayed and the

transition to alumina is stalled up to 800 8C. The XRD spectra

in the intermediate steps are more like those for amorphous

samples. In the first stage these gel produces a transition

alumina at a temperature of 800 8C, where the phase is

identified as g-Al2O3. Complete conversion of this g-Al2O3 to a

more stable form of a-Al2O3 was attained at 1100 8C. As

intermediate phases, the u-Al2O3 and d-Al2O3 phases appear at

1000 8C. To further check the transition from amorphous phase

Fig. 5. The XRD spectra of the alumina gel dried at 600 8C and 700 8C for

different duration of time. The drying did not result in transition alumina.

S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668 8665

to g-Al2O3 we further annealed the gel at 600 8C and at 700 8Cfor different time. The result is shown in Fig. 5. The treatment

did not result in transition phase of alumina and the phase

remained amorphous. Thus we conclude that the phase

transition of the alumina dry gel is mainly dependent on the

annealing temperature. So, the nanocomposite thin film

annealed at 700 8C may not have resulted in the crystalline

phase of alumina and no sign of characteristic alumina peak

is observed. Another observation is that alumina thin film

drawn on quartz substrate crystallizes only after rapid thermal

processing (RTP) at high temperature [28]. This is because

during regular annealing the slow diffusion of Si4+ ion in the

alumina lattice site breaks the periodicity of the crystal and

XRD spectra for this thin film is similar to that coming from

amorphous samples.

3.3. The AFM study

The surface topography of the undoped nanocomposite and

the MgO doped thin film samples are examined by the atomic

force microscopy (AFM). The horizontal sequence of the

images shows the surfaces of an area equal to 1.0 mm �1.0 mm, 500 nm � 500 nm and 200 nm � 200 nm for the

undoped samples SA (Fig. 6a–c), and also y = 0.05 MgO

Fig. 6. The surface morphology of the MgO doped ZnO–Al2O3 nanocomposite thi

content.

doped sample (Fig. 6d–f) and y = 0.15 MgO doped sample

(Fig. 6g–i) annealed at 300 8C in air. The grain sizes of all the

samples are in the nanometric region, and the grains are smooth

and regular looking as seen in Fig. 6g–i. The dark and light

patches on the surface, which is more in Fig. 6a–c, decrease in

the subsequent Fig. 6d–i indicating an increase in the

smoothness of the top surfaces. The smoothness of the top

surface of the thin film is measured in terms of the roughness,

which is a basic parameter indicating the deviation of a surface

with respect to a perfect plane. The root mean square roughness

R (rms) is defined as

Rrms ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXN

i¼1

ðZi � ZavÞ2

N

vuut (1)

where Zi is the Z value of each point, Zav the average of the Z

values and N is the number of points. Sequentially the rms

values of the surfaces are calculated as (i) 2.11, 1.97 and

1.86 nm (undoped SA), (ii) 4.05, 3.16 and 2.01 nm (y = 0.05

MgO doped SA) and (iii) 1.70, 1.45 and 0.99 nm (0.15 MgO

doped SA). As the annealing temperature of the thin films was

same, thus the smoothness of the films increases with the

doping which may be the result of the accumulation of the

n films. The figures show that the surface gets smoother with increasing MgO

Fig. 7. The representative EDAX spectra show the presence of Mg2+ in the

ZnO–Al2O3 nanocomposite thin film.

Fig. 8. The normalized optical absorbance spectra of the ZnO–Al2O3 nano-

composite thin films annealed at 300 8C, 500 8C and 700 8C. (a) sample SA, (b)

sample SB and (c) sample SC.

Fig. 9. A comparative study of the normalized absorbance spectra for the

composite thin films annealed at 300 8C.

S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–86688666

MgO particles on the surface of ZnO nanograins. A represen-

tative EDAX spectrum for the MgO doped ZnO–Al2O3 nano-

composite thin film is shown in Fig. 7.

3.4. UV–vis optical absorption

Fig. 8 shows the normalized absorbance spectra of sample

SA, SB and SC annealed in the temperature range of 300, 500

and 700 8C. The thickness (t) of these single layered ZnO–

Al2O3 nanocomposite thin films is determined as 119 nm. The

spectra are normalized in order to compare the intensity of the

absorbance peaks obtained for different samples at different

synthesis condition. The spectra indicate (a) a long tail with

varying absorbance at lower energies, (b) sharpening of the

absorption peak with increasing annealing temperature, and (c)

broadening of the absorption peak with higher concentration of

ZnO in the nanocomposite. In an earlier report by Das et al. [23]

the absorbance spectra of ZnO–Al2O3 was discussed in detail. It

is evident from the figure that the temperature dependence on

the absorption intensity is more prominent for sample SB and

SC, whereas for sample SA, the intensities of absorbance for all

the three annealing temperature are almost same. In Fig. 9a

comparison of the intensity of the absorption peaks is shown for

the nanocomposite thin films annealed at 300 8C. It is observed

from the figure that the absorbance intensity is largest for

sample SA and least for sample SC, whereas for sample SB the

intensity is intermediate of the other two. In nanometric

dimension the surface to volume ratio is much higher compared

to that for larger particles. This large surface area introduces

various surface related disorders and defects in the materials. In

the host–guest system, for a well-confined nanoparticle these

defects are comparatively lesser. The defects and disorders

creep into the nanocomposite more rapidly when the

confinement effect is weak, this happens when the concentra-

tion of the matrix is comparatively lesser. Thus from Fig. 9, we

Fig. 10. The normalized absorbance spectra of the MgO doped sample SA. The

blue shift and the red shift of the absorbance edge are shown in the figure. (For

interpretation of the references to colour in this figure legend, the reader is

referred to the web version of the article.)

Fig. 11. (a) The (ahn)2 vs. hn plots for ZnO–Al2O3 nanocomposite thin films

with different molar content of MgO. The extrapolation of straight lines on the

energy aixs determines the ban gap of the system. (b) The variation DEabs with

n2/3 is shown; the accuracy of the observed band gap and corresponding ionic

concentration are determined. The linear fit shows the Burstein–Moss shift of

the absorption edges.

S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668 8667

may infer that the confinement for sample SA is superior to that

for sample SB, and it is less effective for sample SC. We may

also conclude that the randomness and disorder are less for

sample SA, compared to those for sample SB or, SC [29].

In Fig. 10 the absorption spectra of the MgO doped sample

SA are shown. The molar concentrations of MgO were kept at

y = 0.05, 0.075, 0.10, 0.125, 0.15 and 0.20 of the total molar

concentration of ZnO present in the ZnO–Al2O3 nanocompo-

site. The doped thin films were annealed at 300 8C. The

absorbance spectra show that the high intensity peaks, which

are characteristic for undoped sample SA, are reduced in

the doped samples. A relative ratios of the intensities of the

absorption peak for the MgO doped films and that for sample

SA stand at 0.53, 0.45, 0.33, 0.35, 0.47 and 0.49. So, the

absorption peaks first decrease and then increase with the MgO

concentration in the nanocomposites. It is also observed that the

absorption edge of the thin films shifted with the MgO

concentration in the nanocomposite. To calculate these shifts,

in Fig. 11a we have plotted the (ahn)2 versus hn plots of the

MgO doped ZnO–Al2O3 nanocomposite thin films. The band

gap was calculated by extrapolating the straight part of the plot

to (ahn)2 = 0. The intersection of the straight lines on the

energy axis gives the value of the respective band gaps of the

composites. The observed absorption edges for y = 0.05, 0.075

and 0.1 were calculated as 5.44, 5.63 and 5.77 eV, respectively.

For other concentrations, that is for y = 0.125, 0.15 and 0.20,

the band gaps are derived as 5.67, 5.61 and 5.49 eV,

respectively. Thus the apparent shifts of the band gap from

the bulk band gap value of ZnO (�3.37 eV) are put as 2.07,

2.26, 2.4, 2.3, 2.24 and 2.12 eV. Thus we observed that the shift

in the obtained band gap with respect to its bulk value first

increase to reach a maximum and then decrease with further

increase in the MgO concentrations in the composite.

This shifting of the ZnO absorption edge may be due to two

reasons, (a) an increase in the free carrier concentration due to

doping which causes the Burstein–Moss shift [10], or (b) due to

the decrease in the nanoparticle sizes. Since we did not observe

significant size reduction after the composite was doped with

MgO, we observe that the widening of the band gap for those

relatively lower concentrations of MgO in the nanocomposite

can be explained by the Burstein–Moss shift. According to the

parabolic band theory [30], the absorption edge shift DEabs is

given by

DEabs ¼�

�h2

2m�

�½3p2n�2=3 � Ebgr (2)

where Ebgr is due to the band-gap-reduction effect. In Fig. 11b

we have plotted the variation of DEabs with n2/3 by neglecting

the last term and using 1/m = 1/me + 1/mh, where me and mh are

the electron and hole effective mass, respectively. The straight

line that appears confirms the assumed Burstein–Moss shift

with molar concentrations of MgO. The accuracy of the experi-

mentally obtained band gaps calculated in the above method

and that of corresponding ionic concentrations are also illu-

strated in Fig. 11b. The error in DEabs is determined by repeated

experimental observations and by considering the overall

deviation from the mean of the obtained values. To calculate

the error in the ionic concentration (n) of MgO, we have used

Eq. (2), along with the error in DEabs.

The density of electrons is decreased once the Mg2+ ions are

substituted into the Zn2+ ion site in the composites. As a result

Fig. 12. The transmittance spectra of ZnO–Al2O3 nanocomposite thin films of

sample SC annealed at 500 8C and 700 8C. The spectra for MgO doped

(y = 0.05) films are shown in dotted lines.

S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–86688668

of this, an increase in the Fermi-level in the conduction band of

degenerate semiconductors leads to widening of the energy

bands. On the contrary, the observed decrease in the band gap

with the molar concentrations of MgO as y = 0.125, 0.15 and

0.20 may be due to excess Mg atoms in the nanocomposites

those are segregated onto the grain boundary. These segregated

Mg atoms do not act as dopant and consequently do not assist in

the process of band gap widening. It was indicated that the high

concentration of extrinsic elements, e.g., Al and Mg, introduces

non-equilibrium defects into ZnO films and those defects are

the reason for the crystalline degradation and thermal

instability of the films [31]. This also causes the observed

decrease in the optical absorption coefficient for the MgO

doped ZnO–Al2O3 nanocomposites.

The effect of MgO doping in sample SC is shown in Fig. 12.

The solid lines indicated the transmittance spectra of the

undoped thin films annealed at 300 and 700 8C, whereas the

dotted lines are those for y = 0.05 MgO doped samples.

The spectra reveal that the absorption edges for these thin films

moved towards lower values of the band gap with the annealing

temperatures for both doped and undoped films, and thus

Burstein–Moss shift is not observed in this case. So, in sample

SC having higher ZnO concentration, MgO does not act as

dopant and is agglomerated on the surface of ZnO nanoparticles

as is explained in the previous case.

4. Conclusion

The polycrystalline ZnO–Al2O3 nanocomposite was synthe-

sized by sol–gel technique. The sol was doped subsequently

with MgO and the surface morphology of the thin films was

seen to improve with the concentration of dopant. The observed

UV absorption peaks were very intense for undoped nano-

composite though the broadening effect dominating for

composite with equal molar concentrations of ZnO and

Al2O3. The observed shifting of the absorption edge with

MgO doping was ascribed to the Burstein–Moss shift. The

effect of the solubility limit of MgO in the composite was put as

10% of the molar concentration of ZnO for smaller grains of

ZnO whereas for larger grains it is below 5% of the molar

concentration of ZnO.

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