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Accepted Manuscript

Influence of Al doping on structural and optical properties of Mg-Al co-doped

ZnO thin films prepared by solgel method

Dongyu Fang, Kui Lin, Tao Xue, Can Cui, Xiaoping Chen, Pei Yao, Huijun Li

PII: S0925-8388(13)02791-6

DOI: http://dx.doi.org/10.1016/j.jallcom.2013.11.061

Reference: JALCOM 29890

To appear in:

Received Date: 12 October 2013

Revised Date: 9 November 2013Accepted Date: 12 November 2013

Please cite this article as: D. Fang, K. Lin, T. Xue, C. Cui, X. Chen, P. Yao, H. Li, Influence of Al doping on structural

and optical properties of Mg-Al co-doped ZnO thin films prepared by solgel method, (2013), doi:http://dx.doi.org/

10.1016/j.jallcom.2013.11.061

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Structural, and optical properties of Mg-Al co-doped ZnO thin films

1



Dongyu Fanga, Kui Lin

a, Tao Xue

b, Can Cui

a, Xiaoping Chen

b, Pei Yao

a,b,*,

Huijun Lic

aSchool of Materials Science and Engineering, Tianjin University, Tianjin

300072,

P. R. China

b Center for Analysis and Tests, Tianjin University, Tianjin 300072, P. R. China

cFaculty of Engineering, University of Wollongong, Northfields Avenue,

Wollongong, NSW 2522, Australia

Correspondence information: Corresponding author: Pei Yao

E-mail address: [email protected]


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Dongyu Fanga, Kui Lina, Tao Xueb, Can Cuia, Xiaoping Chenb, Pei Yaoa,b,*, Huijun

Lic

aSchool of Materials Science and Engineering, Tianjin University, Tianjin 300072,P. R. China

bCenter for Analysis and Tests, Tianjin University, Tianjin 300072, P. R. China

cFaculty of Engineering, University of Wollongong, Northfields Avenue,

Wollongong, NSW 2522, Australia

Abstract

Mg-Al co-doped ZnO (AMZO) thin films were successfully deposited onto quartz

glass substrates by solgel spin coating method. The structure, surface morphology,

composition, optical transmittance, and photoluminescence properties of AMZO thin

films were characterized through X-ray diffraction, scanning electron microscopy

with energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy,

UVVISNIR spectrophotometry, and fluorescence spectrophotometry. The results

indicated that AMZO thin films exhibited preferred orientation growth along the


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c-axis, and the full width at half maximum of the (002) diffraction peak decreased

first and subsequently increased, reaching a minimum of approximately 0.275at 3%

Al content. The calculated crystallite size increased from 30.21 nm to 40.73 nm. Al

doping content increased from 1% to 3% and subsequently reached 19.33 nm for Al

doping content at 5%. The change in lattice parameters was demonstrated by the c/a

ratio, residual stress, bond length, and volume per unit cell. EDS analysis confirmed

the presence of Mg and Al elements in ZnO thin films. The atomic percentage of Mg

and Al elements was nearly equal to their nominal stoichiometry within the

experimental error. In addition, the optical transmittance of AMZO thin films was

over 85% in the visible region, and the optical band gap increased with increasing Al

doping concentration. Room temperature photoluminescence showed ultraviolet

emission peak and defect emission peak. The defect emission peak of AMZO thin

films blue-shifted from 612 nm to 586 nm, with Al doping content increasing from

1% to 5%.

PACS (optional, as per journal):75.40.-s; 71.20.LP

Keywords:Mg-Al co-doped ZnO thin films; Al doping; solgel method; optical band

gap; photoluminescence


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1. Introduction

Wide-band gap materials have been receiving considerable attention because of

their applications in optoelectronic devices. Zinc oxide (ZnO) thin films are one of the

most commonly used wide band gap materials. Considering their band gap of 3.37 eV

and exciton binding energy of 60 meV at room temperature, ZnO thin films have

elicited much interest because of their potential applications in laser-emitting diodes,

laser diodes, surface acoustic devices, diluted magnetic semiconductors, solar cells,

and organic electroluminescence devices, among others [1-3]. ZnO thin films have

been prepared using various growth techniques, such as pulsed laser deposition [4],

magnetron sputtering [5], molecular beam epitaxy [6], spray pyrolysis [7] and solgel

method [8]. Among them, the solgel method has drawn much attention because of its

advantages, such as homogeneity at the molecular level, accurate compositional

control, low cost, lower crystallization temperature and easy reproducibility [9, 10].

Normally, pure ZnO thin films are n-type semiconductors, and their optical and

electrical properties are lower and more unstable [11]. Therefore, doping is usually

preferred to improve the optical and electrical properties of ZnO thin films. To date,

many groups have investigated the optical and electrical properties of ZnO thin films

by doping them with Mg, Cd, Al, Ga, Sn, and In elements [12-15]. In single-doped

ZnO thin films, the optical and electrical properties could not be improved

simultaneously. Thus, co-doping is an effective method to simultaneously enhance the

optical and electrical properties of ZnO thin films. The band gap and electrical

property of ZnO thin films can be improved by Mg and Al co-doping. Most studies


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have investigated the optical and electrical properties of Al-doped Zn1-xMgxO thin

films with different Mg doping concentrations. For example, Matsubara et al. [16]

reported that the maximum band gap of Al-doped Zn1-xMgxO thin films with

resistivity 1 10-3cm was 3.97 eV, and the average transmittance of all the

films was over 90% in the wavelength region. Yang et al. [17] demonstrated that the

wide band gap and resistivity of optimized ZnMgAlO thin films were 4.5 eV and

1.610-3cm, respectively. Prathap et al. [7] reported that the energy band gap of

Zn0.76Mg0.24O:Al thin films varied from 3.79 eV to 3.68 eV when Al doping

concentration varied from 0% to 6%, which could be attributed to the formation of

localized states in the band gap. Duan et al. [18] reported that the optical

transmittance of Al-doped Zn1-xMgxO thin films with Al doping content at 1 at.%

annealed in nitrogen increased to 70% to 80% compared with that of films annealed in

vacuum (50% to 60%). The optical band gap of Al-doped Zn1-xMgxO thin films

annealed in nitrogen increased with increased Mg doping concentration from 0% to

8%. However, the effect of Al doping on the structural and optical properties of

Mg-Al co-doped ZnO thin films has not been reported.

The thermodynamic solid solubility of MgO in ZnO is less than 4 mol%,

according to the phase diagram of ZnOMgO binary system. Therefore, in the present

study, Mg doping content of Mg-Al co-doped ZnO (AMZO) thin films was 3 at%.

AMZO thin films with different Al doping contents were deposited onto quartz glass

substrates using solgel spin coating method. The main goal of this work was to

investigate the influence of different Al doping contents on structural, surface


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morphology, optical band gap, and photoluminescence properties of AMZO thin

films.

2. Experimental details

Mg-Al co-doped ZnO thin films doped with different Al contents were deposited

onto quartz glass substrates by solgel spin coating method. Zinc acetate dihydrate

[Zn(CH3COO)22H2O] was used as starting material. 2-Methoxyethanol (C3H8O2),

monoethanolamine (MEA), aluminum nitrate nonahydrate [Al(NO3)39H2O], and

magnesium acetate [Mg(CH3COO)24H2O] were used as solvent, stabilizer, and

dopant source, respectively. The concentration of the solutions was 0.75 mol/L, and

the molar ratio of MEA to metal ions was maintained at 1:1. Al-doped MgZnO thin

films with Al/Zn nominal molar ratio of 1%, 3%, and 5% were prepared under a

constant Mg/Zn nominal molar ratio of 3%. First, zinc acetate dihydrate, aluminum

nitrate nonahydrate, and magnesium acetate were added to the 2-methoxyethanol

solution, after which monoethanolamine was added to the solution during stirring.

The resultant solution was stirred at 60 C for 2 h to yield a clear and homogeneous

solution. The solution was aged for 24 h for subsequent use in spin coating. Second,

the quartz glass substrates were ultrasonically cleaned in acetone, ethanol, and

deionized water, respectively. The surfaces of the cleaned quartz glass substrates were

modified with 1 mol/L potassium hydroxide (KOH) solution to obtain good wetting

characteristics. Third, the films were spin coated at 2000 rpm for 20 s. After each

layer was deposited, the substrate was dried in air at 180 C for 20 min. The coating


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procedure was repeated 10 times to obtain the desired thickness. Finally, the films

were annealed at 500 C for 1 h in ambient atmosphere.

The crystal structure was analyzed by X-ray diffraction (XRD) measurements

(Rigaku D/Max 2500PC diffractometer) with Cu-K1 (=1.54056 ) radiation. The

surface morphology, cross-section film thickness, and composition of the films were

observed by field-emission scanning electron microscopy (FE-SEM: FEI NOVA

NanoSEM 430) with energy-dispersive X-ray spectroscopy (EDS). Furthermore, the

films were studied by transmission electron microscopy (TEM: FEI Tecnai G2 F20)

operating at 200 kV. The optical transmittance was determined by UVVISNIR

spectrophotometer (Shimadzu UV-3101) in the wavelength range of 300 nm to

800 nm. The room temperature photoluminescence spectra were measured using a

fluorescence spectrophotometer (Jobin Yvon FL3-21) with Xe lamp (450 W) as light

source excited at 325 nm.

3. Results and discussion

3.1 Structural characterization of AMZO thin films

Figure 1 shows the XRD patterns of pure ZnO and AMZO thin films doped with

varying Al contents. All the films have three diffractive peaks corresponding to the

(100), (002), and (101) diffraction peaks of ZnO. All diffractive peaks can be indexed

into the ZnO hexagonal wurtzite structure, indicating that Mg and Al doping did not

change the wurtzite ZnO structure. The intensity of the (002) diffraction peak


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increased with increasing Al doping content up to 3%, and then decreased with further

increasing Al doping content. This finding suggests that AMZO thin films with Al

doping content of 3% have the best crystal quality among all the AMZO thin films.

No secondary phases related to Mg, Al, Al2O3, MgO, or other impurity phases were

found in the AMZO thin films within the XRD detection limits, indicating that Mg2+

and Al3+would substitute into the Zn2+sites or incorporate into interstitial sites in the

ZnO lattice. However, no secondary phases were detected by XRD analysis, and the

existence of secondary phases cannot be completely excluded because of the

limitations of this characterization technique [19].

Figure 2 shows the variation of diffraction peak intensity along the (002) plane

as a function of 2value from 33.6to 35.6. The diffraction peak position of AMZO

thin films slightly shifted toward higher value of diffraction angle compared with pure

ZnO thin films. Similar results have been reported [9, 20, 21], which may be caused

by the fact that doping of Mg and Al ions induced the change of lattice strain [21]

because of the smaller ionic radii of Mg2+(0.066 nm) and Al3+(0.053 nm) compared

with Zn2+ (0.074 nm). Moreover, the full width at half maximum (FWHM) of the

(002) diffraction peak for AMZO thin films decreased first and subsequently

increased, reaching a minimum of approximately 0.275 at 3% Al content. These

results indicate that suitable Al doping content could improve the crystal quality of

AMZO thin films.

The texture coefficient of the pure ZnO and AMZO thin films was estimated

from the following equation [22]:


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( ) 010

( ) / ( )

( ) / ( )n

I hkl I hklTC hkl

N I hkl I hkl

=

(1)

where I(hkl) represents the measured relative intensity of plane (hkl) and I0(hkl)

represents the standard intensity of plane (hkl) according to the JCPDS card (36-1451).

N represents the reflection number and nrepresents the number of diffraction peaks.

TC(hkl)=1 suggests that a sample possesses randomly preferred oriented crystallite.

The calculated texture coefficients TCare presented in Fig. 3. The films have larger

TC values for the (002) plane, suggesting that all the films have c-axis preferred

orientation.

The average crystallite size of the films was calculated from the diffraction peaks

of (002) plane using DebyeScherrers formula [23]:

0.9

cosD

= (2)

2 2 2measured instrumental = (3)

whereDrepresents the crystallite size, represents the X-ray wavelength (1.54056 ),

represents the instrumental corrected FWHM of the diffraction peak, represents

Braggs diffraction angle, measured represents the FWHM of the diffraction peak

measured from the samples, and instrumentalrepresents the instrumental FWHM of the

diffraction peak from a standard material such as silicon.

The lattice constants of the aand caxes of pure ZnO and AMZO thin films were

determined by using the following equations [24]:

2 2 2

2 2 2

1 4

3hkl

h hk k l

d a c

+ += +

(4)

2 sinhkld n = (5)


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where (hkl) represents the miller index, dhkl represents the interplanar spacing for

indices (hkl), and aand crepresent the lattice constants of the films.

The lattice strain () of the films can be calculated by using the following

formula [25]:

4tan

= (6)

The residual stress () in the plane of the films for hexagonal crystals can be

determined using the following biaxial strain model [21]:

213 33 11 12 0

13 0

2 ( )

2

c c c c c c

c c

+ = (7)

where cij represents the elastic stiffness constants (c11=208.8 GPa, c12=119.7 GPa,

c13=104.2 GPa, and c33=213.8 GPa) [26] and c0represents the lattice constant of bulk

ZnO (5.2071 ) [3]. The positive sign of indicates that the stress is tensile in nature.

The average crystallite size, lattice constants a and c, c/a ratio, lattice strain,

residual stress, bond length, and volume per unit cell obtained from the XRD data for

pure ZnO and AMZO thin films are presented in Table 1. The crystallite size

increased with increasing Al doping content up to 3%, and subsequently decreased

with further increase in Al doping content. The lattice constant cof AMZO thin films

decreased first and subsequently increased with increasing Al doping content. The

results were attributed to the lattice strain induced by Mg2+and Al3+substitution into

Zn2+

sites. Furthermore, doping of Mg and Al elements into ZnO leads to increased

residual stress in AMZO thin films compared with pure ZnO thin films. The smaller

residual stress of AMZO thin films was found to be 0.2730 GPa when the Al doping


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content was 3%. The volume of unit cell for hexagonal crystal structure is calculated

from the following equation [24]:

2( ) 0.866Volume V a c= (8)

The ZnO bond length (l) can be calculated by using the following formula [27]:

2221

3 2

al u c

= +

(9)

where2

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3

au

c= + represents the potential parameter of the hexagonal crystal

structure. Table 1 shows that the bond length decreased with increasing Al doping

content up to 3%, which is attributed to the fact that Mg or Al ions replaced Zn ions

sites, and Mg-O bonds or Al-O bonds were formed in Zn-O lattice whose bond length

was smaller than that of the Zn-O bonds. The volume of the unit cell of AMZO thin

films increased first with increased Al doping content up to 3% and subsequently

increased with further increase in Al doping concentration. These results may be

attributed to the residual stress produced by substitution of Al ions into Zn ion lattice

sites or redundant incorporation of Al ions into the crystal lattice interstitial sites.

3.2 Morphological characterization of AMZO thin films

The surface morphology of pure ZnO and AMZO thin films are shown in Fig. 4.

Al doping concentration exerts a noticeable influence on the surface morphology of

AMZO thin films. The surface of pure ZnO thin films showed compact, uniform grain

size distribution, and the presence of some cracks. As the Al doping content increased,

the surface of AMZO thin films exhibited smoother surfaces and the cracks gradually

disappeared. However, some grain sizes became larger with increased Al doping


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concentration, indicating that appropriate Al doping content could improve the

surface morphology of AMZO thin films. These findings were different from Huangs

results [28], which found that a film undergone a transition from the columnar to

granular structure when Al doping content increased from 0 to 1.5 at.%, the granular

transition to nanorods with increasing the Al content from 1.5 at.% to 4 at.%. These

results may be related to the Mg doping content. The inset in Fig. 4(c) shows that the

thickness of AMZO thin films with Al doping content at 3% was approximately

335 nm.

Figure 5 shows the plane-view TEM patterns of pure ZnO thin films. The image

reveals that the crystallites have similar spherical shape. The particle size of ZnO thin

films varied between 28 and 45 nm, which is approximately in agreement with the

XRD results. Furthermore, the selected area electron diffraction (SAED) pattern

shows discontinuous diffraction rings from the inset of Fig. 5, indicating that pure

ZnO thin films were polycrystalline.

3.3 EDS studies of AMZO thin films

The compositions of pure ZnO and AMZO thin films were determined through

EDS, which showed the presence of Al, Mg, Zn, and O as elementary components.

The typical EDS spectra of pure ZnO and AMZO thin films are shown in Fig. 6,

confirming the presence of Mg and Al elements in ZnO thin films and showing that

the intensity of Al element increased with increasing Al doping content. The atomic

percentages of the compositional elements, such as Zn, Mg, Al and O, are presented


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in Table 2. The values of the atomic percentages of Mg/metal ion ratio and Al/metal

ion ratio were very close to their nominal stoichiometry and were within the

experimental error.

3.4 Optical properties of AMZO thin films

The transmittance spectra of pure ZnO and AMZO thin films are illustrated in

Fig. 7. The spectra show that all the films have sharp ultraviolet absorption edges in

the UV region. A blue shift of ultraviolet absorption edges with increasing Al doping

content is demonstrated in the inset of Fig. 7. This phenomenon is mainly attributed to

the BursteinMoss effect [7, 16]. In addition, all the films have average optical

transparency of over 85% in the visible range. AMZO thin films with Al doping

content at 1% and 2% show slightly higher transparency than the other films. The low

transmittance may be attributed to the optical scattering caused by surface

morphology [20, 29].

The optical band gap energy (Eg) of pure ZnO and AMZO thin films is given by

the following equation [9]:

( ) ( )1

2g

h A h E = (10)

where h is the photon energy of the incident photons, A is a constant, and is the

absorption coefficient. The absorption coefficient () can be calculated from the

transmittance of the films with the formula =(1/d)ln(1/T), where dis the thickness of

the films and Tis the transmittance. Therefore, the optical bandgap is evaluated by the

intercept of the linear region in the plot of (h) versus photon energy (h), as shown


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in Fig. 8. The bandgap (Eg) as a function of Al doping content is shown in the inset of

Fig. 8. The bandgap of AMZO thin films was bigger than that of pure ZnO thin films.

Furthermore, the bandgap of AMZO thin films shifted toward higher value of band

gap energy compared with that of pure ZnO thin films. These results were differ from

Sharmas results [30], which the decreased in the band gap of AZO thin films was

attributed to an increase in the stress present in the film. The broadening of the

bandgap for AMZO thin films may be attributed to be the fact that the Fermi level

moves into the conduction band caused by the increase of carrier concentration in

AMZO thin films because of the substitution of the Al and Mg ions into Zn ion sites.

According to the Pauli principle which states that electrons prevent states from being

doubly occupied and optical transitions are vertical [31], when electrons located at the

valence band require an additional energy to be excited to higher energy states in the

conduction band, the optical band gap is broadened. Similar results have also been

reported [31-33].

3.5 Photoluminescence studies

Figure 9 shows the room temperature PL spectra of pure ZnO and AMZO thin

films excited by 325 nm. Typically, two emission bands appeared in the spectra, the

UV emission centered at approximately 376 nm originates from the radiative

recombination of free excitions corresponding to the near-band edge emission of ZnO,

and the broad visible emission ranging from 587 nm to 612 nm relates to the

impurities and structural defects of crystals. Figure 10 exhibits the UV peak position,


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visible peak position, and the intensity ratio of UV peak to visible peak as a function

of Al doping concentration. Figure 9 demonstrates that the intensity of UV emission

of AMZO thin films increased with increasing Al doping concentration up to 3%, and

subsequently decreased with further increase in Al doping concentration. The UV

peak of AMZO thin films slightly shifted toward shorter wavelengths with the

increase in Al doping concentration compared with that of pure ZnO thin films. These

results may be attributed to the fact that more free electrons produced by substitution

of Mg and Al ions Zn ions would occupy the energy levels located at the bottom of

the conduction band. When these electrons are excited, the excitons would occupy the

higher energy levels at the bottom of the conduction band and the radiative

recombination of excitons would lead to a shift toward shorter wavelength of UV

emission peak [20, 34]. Moreover, the intensity ratio of UV peak to visible peak

(IUV/Ivis) is mainly influenced by the crystal quality of the films, and the energy level

of defects can be determined by the peak position in PL spectra [35]. Figure 10

demonstrates that the intensity ratio of UV peak to visible peak increased initially

with increasing Al doping concentration and subsequently decreased with further

increase in Al doping content, reaching a maximum value with Al doping content at

3%. AMZO thin films with Al doping content at 3% were found to have the best

crystal quality. These results agree with the XRD results in Fig. 1.

Generally, the deep-level emission peak of ZnO thin films has been ascribed to

various types of impurities and intrinsic defects, such as zinc vacancy (VZn),

interstitial zinc (Zni), oxygen vacancy (VO), oxygen interstitial (Oi), oxygen antisite


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(OZn), zinc antisite (ZnO) and donor-acceptor pair (DAP) [36, 37]. Zni, VO, and ZnO

are donors, whereas VZn, Oi, and OZn are acceptors [38]. The origins of the

defect-related emission peak in PL spectra have been studied for a long time. Xu et al.

[37] calculated various defect state positions of ZnO thin films by the full-potential

linear Muffin-tin orbital method, and found that the green emission was attributed to

the complex VOZni, Oi, and OZn defects. Djurisic et al. [39] found that the red

emission of ZnO nanostructures that is related to defect complexes includes zinc

vacancy complexes. Wu et al. [40] found that the green and yellow emissions of ZnO

films grown on sapphire were attributed to the radiative recombination of a

delocalized electron close to the conduction band with a deeply trapped hole inOV

+

andi

O , respectively. Wang et al. [41] found that yellow emission of Al-doped ZnO

thin films prepared by solgel method originated from the oxygen interstitial. Chen et

al. [42] reported that the broad orange emission (approximately 600 nm) of Al-doped

ZnO nanostructures was attributed to the interstitial oxygen ions. Shi et al. [43]

reported that the red emission of Mg-doped ZnO phosphors prepared using solgel

method was attributed to zinc vacancies and oxygen vacancies.As shown in the above

results, the mechanism of defect emission peak of ZnO and ZnO doped with different

elements is still unclear.

Figure 9 shows that the visible emission peak of pure ZnO thin films was a red

emission centered at 612 nm. The defect emission peaks of AMZO thin films were

red emission centered at 612 nm, orange emission located at 604 nm, and yellow

emission corresponding to 586 nm, respectively. The schematic diagram of the


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energy-level position of defects in ZnO thin films from Xus calculation are shown in

Fig. 11. The red emission of pure ZnO and AMZO thin films with Al-doped content

at 1% was attributed to the electron transition from Zn ito Oi, or VOZnicomplexes to

Oi. The yellow emission of AMZO thin films with Al-doped content at 5% could be

attributed to the electron transition from VOZni complexes to VZn. However, the

orange emission of AMZO thin films with Al-doped content at 3% can be ascribed to

the deep-level intrinsic defects of oxygen interstitials [42]. In addition, the defect peak

of AMZO thin films shifted toward the shorter wavelengths (Fig. 10), which came

from the defect site differences related to the intrinsic defects or extrinsic defects

produced by Mg and Al ion substitution into Zn ion sites. Simultaneously, the

existence of tensile stress related to the defects in AMZO thin films have important

functions in the blue shift of the defect peak.

4. Conclusions

Mg-Al co-doped ZnO thin films with different Al doping concentrations were

deposited on quartz glass substrates using solgel spin coating method. Experimental

results showed that AMZO thin films possessed a preferential orientation along the

(002) plane, and the intensity of (002) diffraction peak increased initially with

increase in Al doping concentration up to 3% and subsequently decreased with further

increase in Al doping concentration. Al doping concentration was observed to have

great influence on the surface morphology. Results from EDS spectra demonstrated

the presence of Mg and Al elements in ZnO thin films. Moreover, the optical

transmittance of AMZO thin films was over 85% in the wavelength of 400 nm to


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800 nm. The optical band gap of AMZO thin films increased with increasing Al

doping concentration. The room temperature PL spectra showed that the visible

emission peak of AMZO thin films blue shifted from 612 nm to 586 nm, with increase

in Al doping concentration from 1% to 5%. These results indicated that the defects in

the films could be controlled by varying the Al doping concentrations. High-quality

AMZO thin films with good optical properties can be used for optoelectronic devices

in the future.

Acknowledgements

The authors are grateful for the financial support from the Natural Science

Foundation of Tianjin, P. R. China (Grant Nos. 07JCZDJC00600 and

07JCYBJC06000).

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Figure Captions

Fig. 1. XRD patterns of pure ZnO and AMZO thin films with various Al doping

concentrations.

Fig. 2. XRD patterns of (002) diffraction peak for pure ZnO and AMZO thin films.

Fig. 3. TC values of pure ZnO and AMZO thin films with various Al doping contents.

Fig. 4. SEM images of pure ZnO and AMZO thin films with the following Al doping

contents: (a) ZnO, (b) 1% Al, (c) 3% Al, and (d) 5% Al; the inset of (c) shows

the thickness of AMZO thin films with Al doping at 3%.

Fig. 5. TEM images of pure ZnO thin films; the inset shows the SAED pattern.

Fig. 6. Typical EDS spectra of pure ZnO and AMZO thin films with the following Al

doping contents: (a) ZnO, (b) 1% Al, (c) 3% Al, and (d) 5% Al.

Fig. 7. Transmittance spectra of pure ZnO and AMZO thin films with various Al

doping contents.

Fig. 8. Plot of (h) versus hcurves of pure ZnO and AMZO thin films. Inset shows

the optical band gap as a function of Al doping concentration.

Fig. 9. Room temperature PL spectra of pure ZnO and AMZO thin films with

different Al doping concentrations.

Fig. 10. Variation of UV peak position, visible peak position, and intensity ratio of

UV peak to visible peak with Al doping concentration.

Fig. 11. Schematic diagram of energy-level position of defects in ZnO thin films.


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Table Captions

Table 1. Average crystallite size (D), lattice constants a and c, c/a ratio, strain (),

residual stress (), , volume of the unit cell, and bond length (l) of pure ZnO

and AMZO thin films with different Al doping contents.

Table 2. Compositional ratio of AMZO thin films with different Al doping

concentrations.


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Fig. 1


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Fig. 2


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Fig. 3


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Fig. 4


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Fig. 5


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Fig. 6


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Fig. 7


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Fig. 8


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Fig. 9


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Fig. 10


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Fig. 11


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Table 1

Lattice

parameter ()

Samples

Averagecrystallite

size (D)

(nm) a c

c/a

ratio

Strain

()

Residual

stress

(GPa)

Bond

length

(l)()

Volumeper unit

cell

()3

ZnO 31.03 3.2506 5.2066 1.6017 0.8522 0.0224 0.3799 1.9781 47.6430

Zn0.96Mg0.03Al0.01O 30.21 3.2506 5.2008 1.6000 0.8745 0.2819 0.3802 1.9774 47.5899

Zn0.94Mg0.03Al0.03O 40.73 3.2467 5.2010 1.6020 0.6486 0.2730 0.3799 1.9758 47.4768

Zn0.92Mg0.03Al0.05O 19.33 3.2546 5.1980 1.5971 1.3660 0.4072 0.3807 1.9788 47.6826


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Table 2

Atomic (%)

SamplesZn Al Mg O

Al/(Zn+Al+Mg)

(%)

Mg/(Zn+Al+Mg)

(%)

Zn0.96Mg0.03Al0.01O 44.67 0.57 1.19 53.57 1.2 2.6

Zn0.94Mg0.03Al0.03O 44.9 1.39 1.27 52.44 2.9 2.7

Zn0.92Mg0.03Al0.05O 40.07 2.24 1.11 56.59 5.2 2.6


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Highlights:(1)Mg-Al co-doped ZnO (AMZO) thin films with various Al doping concentrations

were prepared by sol-gel spin coating method(2)The effects of different Al doping concentrations on the structural, surface

morphology, and optical properties were investigated.

(3)

The EDS spectra confirmed presence of Mg and Al elements in AMZO thin films.(4)The optical band gap of AMZO thin films increased with Al doping concentrationincreased.

(5)The origin of the photoluminescence emissions was discussed.

sd paper reprint

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