Chinese Materials Research Society

Progress in Natural Science: Materials International

Progress in Natural Science: Materials International 2013;23(1):64–69

1002-0071 & 2013 Ch

http://dx.doi.org/10.1

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ORIGINAL RESEARCH

Structural and optical properties of Cr doped ZnO

crystalline thin films deposited by reactive electron

beam evaporation technique

Amjid Iqbala,n, Arshad Mahmood

a, Taj Muhammad Khan

a, Ejaz Ahmed

b

aNational Institute of Laser and Optronics (NILOP), P.O. Nilore, Islamabad 45650, PakistanbPhysics Division PINSTECH, P.O. Nilore, Islamabad 45650, Pakistan

Received 28 July 2012; accepted 27 November 2012

Available online 23 February 2013

KEYWORDS

Cr doping;

Thin film;

ZnO thin films;

Roughness;

Ellipsometry;

Optical constants

inese Materials R

016/j.pnsc.2013.01

thor. Tel.: þ92 5

.

amjid821@gmail.co

r responsibility of

Abstract ZnO and Cr-doped ZnO thin films are grown on to glass substrates using reactive

electron beam (e-beam) evaporation technique. Variation of structural, morphological, and optical

properties with Cr doping is investigated. X-ray diffraction (XRD) studies show that the films are

polycrystalline in nature with single phase. Energy dispersive spectroscopy (EDS) results

demonstrate that Cr ions are substitutionally incorporated into ZnO. Atomic force microscopy

(AFM) reveals that the films present a compact surface and root mean squared (RMS) roughness

increased with Cr contents. The optical band gap energy Eg of the films has been determined using

Transmission data by spectrophotometer and ellipsometry. The band gap energy found to be

decreased with increasing Cr doping concentration. The optical constants (refractive index,

extinction coefficient) are calculated using ellipsometry and found to increase with Cr doping

concentration.

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1. Introduction

Zinc oxide which is group II–IV semiconductor material with

a wide direct band gap of 3.37 eV and large exciton binding

energy of 60 meV has numerous applications in the fields

of ultraviolet light emitting devices, ultraviolet laser diodes,

transparent conducting films and solar cell [1,2]. ZnO nano

crystalline structures are attractive due to its fantastic electro-

nic, optical and magnetic properties [3]. It is highly desired to

prepare ZnO nano materials to finely tune its electrical, optical

sting by Elsevier B.V. All rights reserved.

Structural and optical properties of Cr doped ZnO crystalline thin films 65

and magnetic properties for its potential applications in spin-

optoelectronics devices. Doping with proper elements is an

effective method to tune surface state, energy levels and

transport performance of carriers in semiconductors, enhan-

cing their electrical, optical and magnetic properties [4–7]. The

doping of transition metal elements in ZnO offers a feasible

means of fine tuning the band gap to make use as UV detector

and light emitters [8]. Cr is a typical transition metal element

with special abundant electron shell structure and close ionic

radius of Cr3þ (0.063 nm) to that of Zn2þ (0.074 nm), which

means that Cr3þ can easily substitute into ZnO lattice [9].

However, Cr doped ZnO thin films has received little experi-

mental attention, in spite of, favorable for doping, chemical

stability against etching and existence of ferromagnetism at

room temperature [9–11]. Another important feature of this

material is that it has magnetic, semiconducting and optical

properties. The samples can be synthesized in the bulk and

thin film forms and a wide range of magnetic properties

including room temperature ferromagnetism have been

reported and can be applied to short-wave magneto-optical

devices. Cr-doped ZnO is a promising material for spintronics

applications and can be made a room temperature transparent

ferromagnetic semiconductor. The control tuning of the

optical band-gap with Cr doping in ZnO can be applied in

the fabrication of devices such as detector and light emitters.

Much effort has been placed on the fabrication of ZnO based

dilute magnetic semiconductor materials, their structural and

magnetic characterization. There are no so many reports about

the influences of Cr doped ZnO thin films on the lattice structure

and optical properties. Nevertheless, the optical properties like

optical constants (n, k) and transmittance in UV regions for this

material is less reported, despite of its essential importance for

applications in spin-optoelectronic devices. Cr doped ZnO thin

films have been prepared by a variety of techniques such as, RF

magnetron sputtering, co sputtering [12,13], and pulse laser

deposition [14]. To our knowledge, none of the report has

mentioned the growth of Cr doped ZnO by electron beam

(e-beam) technique.

In this work we have reported the influence of Cr doping on

structural, optical and morphological properties of Cr doped

ZnO thin films using reactive (e-beam) evaporation technique.

In addition to this, we achieved tunebility in physical proper-

ties with Cr doping using (e-beam) technique.

Fig. 1 XRD pattern of (a) pure ZnO, (b) 3.3 at% Cr and (c)

7.7 at% Cr doped ZnO thin films.

2. Experimental detail

The targets of ZnO and Cr doped ZnO were prepared by the

conventional solid state reaction route method. The desired

amount of highly pure ZnO and Cr2O3 (0, 3, 6 mol%) powder

was taken and mixed for 3 h by pestle and mortar and made

its pellets using pellet presser. Then, the pellets were sintered

at the temperature of 950 1C for 6 h. The sintered pellets were

used as targets for the deposition of the films. The rotary and

diffusion pumps were used to evacuate the vacuum chamber of

e-beam system up to 2� 10�5 mbar. The substrate to target

distance was 35 cm and to gain homogeneous films, the

substrates were rotating continuously with speed 15 cycles/

min. Deposition process was carried out for 6 min at 300 1C

substrate temperature in oxygen environment. Structural and

phase identification of the thin films were carried out by

grazing angle X-ray diffraction technique using Bruker D-8

Discover X-ray diffractometer with a CuKa-radiation(l¼1.54186 A) as an X-ray source and 2y range from 251 to

701. Concentrations of Cr, Zn and O were determined by EDS

installed with leo440i SEM. Surface morphology of thin films

was investigated by atomic force microscope (AFM) (Quesant

Universal SPM, Ambios Technology, USA) (QScopeTM 350)

in non contact mode. The optical transmittance measurements

were carried out using UV-NIR Spectrophotometer (Hitachi

U-4001) in the spectral range from 300 to 900 nm. Film

thickness and optical constants (e.g. refractive index, extinc-

tion coefficient) were carried out using Jawoolam 200VI

Ellipsometer. C and D spectra were measured at the incidence

angle 701 in the spectral wavelength range from 370 to 900 nm.

3. Results and discussion

3.1. XRD study

The deposited polycrystalline thin films were subjected to

XRD analysis for phase and structural identification as shown

in Fig. 1. The characteristic peaks with high intensities

corresponding to the planes (1 0 0), (0 0 2), (1 0 1) and lower

intensities at (1 0 2), (1 1 0), (1 0 3) and (1 1 2) indicate the

films is of high quality of hexogonal ZnO wurtzite structure.

It is evident from the XRD data that there are no extra peaks

due to chromium, other oxides or any zinc chromium phase,

indicating that as synthesized samples are single phase. The

peaks of diffraction patterns of doped samples are shifted to

right as compared to the undoped ZnO. This shows that small

variation in lattice parameters occur as Cr concentration in

sample increase. The lattice constant C was calculated using

the following equation for thin films [15]:

1

d2¼

4

3

h2 þ hkþ k2

a2

� �þ

l2

c2ð1Þ

C values are presented in Table 1. As can be seen, with

increase of Cr doping lattice constant C decrease from

0.526 nm to 0.521 nm. A decrease in the lattice parameters

can be expected when Zn2þ ions are replaced by Cr3þ ions

because of the smaller radius of Cr3þ ions (0.063 nm) than

Zn2þ ions (0.074 nm) [9], so the substitutional incorporation

Table 1 Calculated parameters for ZnO and Cr doped ZnO thin films using XRD analysis.

Sl. No. Cr composition (at%) Grain size (nm) Lattice constant C (nm)

1 0 28.47 0.526

2 3.3 40.13 0.523

3 7.7 42.26 0.521

Fig. 2 EDX spectrum of (a) 3.3 at% Cr and (b) 7.7 at% Cr doped ZnO films.

Fig. 3 AFM micrographs of (a) ZnO, (b) 3.3 at% Cr and (c) 7.7 at% Cr doped ZnO thin films.

A. Iqbal et al.66

Fig. 5 Schematic diagram of Cauchy and EMA layer model.

Fig. 6 Experimental and modeled values of C and D (a) ZnO, (b) 3.3 at% Cr doped ZnO and (c) 7.7 at% Cr doped ZnO.

Fig. 4 (a) Transmission spectra of pure and Cr doped ZnO films and (b) band gap of pure and Cr doped ZnO thin films.

Structural and optical properties of Cr doped ZnO crystalline thin films 67

of Cr dopant results in modification of the lattice constant C.

The contraction in lattice constant C and slight shift in XRD

peaks of different concentrations could be attributed to Cr

incorporation and is indicative of Cr doping in to the ZnO matrix.

The average crystallite size (D) of the first three major peaks

was calculated using the Scherer’s formula [16]. The calculated

grain size is listed in Table 1. The table reveals that the moderate

amount of Cr doping in ZnO thin films reinforce the grain growth.

Fig. 7 (a) Variation of optical constants (a) refractive index and (b) extinction coefficient in Cr-doped ZnO thin films.

Table 2 Comparison of optical band gaps calculated by ellipsometry and spectrophotometry.

Sl. No. at% of Cr Band gap (eV) ellipsometry Band gap (eV) spectrophotometry

1 0 3.32 3.35

2 3.3 3.28 3.31

3 7.7 3.19 3.26

A. Iqbal et al.68

3.2. EDS analysis

The amount of Cr-doping was examined by EDS. Fig. 2(a) and

(b) shows the EDS spectrum of 3.3 at% Cr and 7.7 at% Cr

doped ZnO thin films respectively. From the figures it is well

evident that the deposited thin films consist of Cr, Zn and O

compositions. The presence of Si peak is due to glass substrate.

Therefore, the EDS results further indicate that Cr is incorpo-

rated in to ZnO, which is also, consistent with XRD results..

3.3. AFM study

Fig. 3(a–c) shows the AFM micrographs with scanning area of

1 mm� 1 mm for pure ZnO and Cr doped ZnO thin films,

respectively. All the samples have smooth morphologies with

root mean squared (RMS) roughness ranging from 5.06 nm to

24.04 nm. Similar results are observed by Ren-Chuan Chang

et al. in Mg doped ZnO thin films [17]. It is observed that the

RMS roughness increases with Cr doping which means the

surface becomes rough and grain size increases. The observed

increase in RMS with Cr doping may be due to no proper

agglomeration of crystallites..

4. Optical study

4.1. Spectrophotometry

Optical transmission spectra of the films were taken in the

wavelength of 300–900 nm using UV-NIR Spectrophotometer.

Fig. 4(a) displays the transmittance of pure and Cr doped ZnO

thin films. Transmission spectra of the samples divulge that

the transmittance decreases with Cr concentration which may

be due to the thickness of the films and doping effect. The

optical band gap energy (Eg) was determined from transmis-

sion measurements by analyzing the optical data with the

expression for the optical absorption coefficient, and the

photon energy hn using the following equation for direct band

gap materials [18]:

ðahnÞ2 ¼Aðhn�EgÞ ð2Þ

where a is the absorption coefficient, h is the Planck’s

constant, A is a constant and Eg is the optical band gap

energy. The optical band gap was obtained by extrapolating

the linear portion of the plots of (ahv)2 versus hn to a¼0. Forpure ZnO optical band gap was found to be 3.35 eV. We

observed the tuneability of the optical band gap from 3.35 eV

to 3.26 eV with Cr contents in ZnO material. Similar results

have also been reported for cobalt and Ni doped ZnO thin

films [19,20]. The decrease in the optical band gap in transition

metal doped II–IV semiconductor compounds has been

observed and can be best interpreted in terms of sp–d spin

exchange interaction between band electrons and the localized

d electrons of the transition metal ions substituting the cations

[19,21].

5. Spectroscopic ellipsometry

Film thickness and optical constants (e.g. refractive index,

extinction coefficient) were measured using spectroscopic

ellipsometry. C and D spectra were measured at the incidence

angle 701 in the spectral wavelength range of 370–900 nm.

Cauchy layer model was used for ZnO thin film to calculate

thickness and optical constants. Cauchy dispersion relation for

this model is given by the equation [22]:

n lð Þ ¼An þBn

l2þ

Cn

l4ð3Þ

The A, B, C parameters are variable fitted parameters that

determine the index dispersion. For Cr doped ZnO films effective

medium approximation (EMA) layer model was used to calculate

film thickness and optical constants. Fig. 5 shows the layer models

used for fitting experimental and theoretical data. Fig. 6 shows the

curve fitting for theoretical and experimental data. The film

Structural and optical properties of Cr doped ZnO crystalline thin films 69

thickness was found to 62 nm for ZnO, 105 nm for 3.3 at% Cr

and 125 nm for 7.7 at% Cr doped ZnO.

Fig. 7(a) and (b) shows the refractive index and extinction

coefficient of pure and Cr doped ZnO, which manifests an

increasing trend in optical constants with respect to increasing

Cr contents. This may be attributed to the increased surface

roughness which is clearly seen from AFM micrographs in Fig. 3.

Because of the surface roughness, the optical scattering will

increase and results in increase of optical constants. In this study,

besides the Cr incorporation, the film thickness size plays an

important role in the high refractive index value. That is, it can be

clearly seen that the refractive index increase with increasing film

thickness. A comparison of the optical band gaps was made

calculated by two different techniques, ellipsometry and spectro-

photometry. The comparison is shown in Table 2.

From Table 2 we can predict that the optical band gaps

calculated from the transmission data (3.35 eV) is much close

to the actual value than calculated from ellipsometry data.

This discrepancy in the calculated values and experimental

results from ellipsometry and spectrophotometry respectively

is due to the fact that ellipsometry data models based on

assumptions for data fitting between experimental and theo-

retical data. Moreover the values calculated by ellipsometry

are close to others experimental reported data [23,24].

6. Conclusion

ZnO and Cr doped ZnO crystalline thin films were prepared

on glass substrate using e-beam evaporation technique at

300 1C of substrate temperature. Moreover effect of Cr

concentration on band gaps modulation, film roughness,

strain and optical constants (n, k) was discussed. We observed

that Cr doping have great impact on the structural and optical

properties of ZnO thin film. XRD confirmed single phase,

polycrystalline thin films. Grain size and surface roughness

increases with Cr concentration as observed from XRD and

AFM. Transmittance spectra showed that thin films are

transparent which decreases with increase in Cr contents.

The band gaps modulation from 3.35 eV to 3.26 eV was

observed and reflects the sp–d exchange in ZnO with Cr. A

comparison of band gaps was made by spectrophotometry and

ellipsometry. The band gaps energy calculated from transmis-

sion data and ellipsometry are closely matched. Refractive

index and extinction coefficient increases with Cr contents.

AFM study shows the formation of thin film and also

confirmed the increase in surface roughness with increase in

Cr contents.

Acknowledgments

The authors gratefully acknowledge the assistance from

National Institute of Laser and Optronics (NILOP), Pakistan

and Optics Laboratory Pakistan.

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