Elemental, structural and optical properties of nanocrystalline Zn1-xCuxSe films deposited by close spaced sublimation technique

The decrease in the dielectric constant with annealing temperature may be due to the lower compactness of annealed ZnSe ﬁlms than as-deposited ZnSe ﬁlms.. Similar results have been repor[r]

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Original Article

Elemental, structural and optical properties of nanocrystalline

a School of Chemical and Materials Engineering, National University of Sciences and Technology, H-12, Islamabad, Pakistan

b National Institute of Lasers and Optronics, P.O Nilore, 45650, Islamabad, Pakistan

c Experimental Physics Labs, National Centre for Physics, Quaid-e-Azam University, Islamabad 45320, Pakistan

a r t i c l e i n f o

Article history:

Received 17 October 2016

Received in revised form

17 January 2017

Accepted 18 January 2017

Available online 25 January 2017

Keywords:

X-ray diffraction

Morphology

Dielectric constant

Spectroscopic ellipsometer

Energy band gap

a b s t r a c t

The elemental composition, film thickness and concentration depth profiles of as-deposited and annealed Zn1xCuxSefilms were studied by the Rutherford backscattering spectrometer (RBS) technique Thefilms were deposited on glass substrates by close spaced sublimation (CSS) technique As-deposited films of about 250e300 nm thickness were then annealed in air at temperatures of 200C and 400C for

1 h Structural characterization including crystal structure, crystal orientation, stacking fault energy (ҮSFE) and surface morphology were carried out by using X-ray diffraction (XRD) and atomic force mi-croscopy (AFM) XRD studies revealed that the fabricatedﬁlms are polycrystalline with a zinc-blende structure and a strong (111) texture plane Surface roughness was observed to be enhanced with annealing temperature with a decrease in stacking fault energy (ҮSFE) Spectroscopic ellipsometry has been utilized for the estimation of band gap energy (Eg) and dielectric constant (ε1) Band gap energy of theﬁlm increased with increasing annealing temperature while the dielectric constant decreased

© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Pursuant to the reported literature, ZnSe is the most prominent

material in optoelectronics and optical coating applications,

particularly in the UV region[1e8] It has a direct band gap of 2.7 eV

and its thin ﬁlms are transparent over a wide range of visible

spectrum therefore, it is used as a window layer for the fabrication

of thinﬁlm solar cells[9] ZnSe based solar cells has an efﬁciency

greater than 11% by transmitting higher energy photons to the

absorber layer of the solar cell[10,11] A number of approaches have

been applied to tailor the physical properties of ZnSe thinﬁlms and

extract their peculiar properties[12e15] The properties of a thin

ﬁlm are directly determined by composition, structure and

micro-structure which can be varied with growth conditions such as

growth temperature, layer thickness and composition as well[16]

In this work, physical properties of Cu enriched ZnSeﬁlms with

different concentrations of Cu and annealed at 200C and 400C

temperature were studied in detail In order to obtain precise

knowledge of the structural and optical properties, one has to keep the exact composition and stoichiometry of the fabricated ﬁlm layers especially for designing modern optoelectronic and optical devices Unfortunately, so far just the elemental composition and the thickness had been investigated for the Cu/ZnSeﬁlms[17,18] without any insight description of the stoichiometry and struc-ture of interfaces for the deposited layers

Multiple approaches have been employed for the deposition of ZnSe:Cu films like lyothermal method[19]two-sourced thermal evaporation [20,21], spray pyrolysis deposition technique [22], layer-by-layer assembly with anionic and cationic alternating polymer layers [23], chemical synthesis[24], and chemical bath deposition[25] Irrespective of the deposition technique, investi-gation of the microstructure and morphology evolution in poly-crystalline Zn1 xCuxSe films is of key importance to develop a deeper understanding of the performance of devices employing these layers To characterize accurately such parameters, very diverse and in some cases very complicated diagnostic methods are needed Rutherford backscattering spectrometer (RBS) is a well established surface analyzing technique which can be used for element analysis and for depth profiles of major and minor con-stituents of thinfilms in the near-surface region[26]

* Corresponding author.

E-mail address: shani_788@yahoo.com (M Arslan).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

http://dx.doi.org/10.1016/j.jsamd.2017.01.004

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Previously, we have reported the structural and optical analysis

of annealed Zn1xCuxSe thinﬁlms[27] In the present study, we

demonstrate several new results of the RBS analysis of the

depos-ited and annealedﬁlms using high-energy 2 MeV Heþion beams

from a pelletron tandem-type ion accelerator Qualitative and

quantitative information about stoichiometry and structure of

in-terfaces doped semiconductor as a function of depth, Cu

concen-tration and annealing effect has been discussed with further details

Results of this investigation have been correlated with the ﬁlms

structural and optical properties Among the structural properties,

surface roughness and morphology, and crystalline quality of the

ﬁlms have been obtained by using the XRD and AFM methods

Additionally, optical properties such as band gap tune-ability and

dielectric constant have been determined by spectroscopic

ellipsometer

2 Experimental

Zn1 -xCuxSe thinﬁlms were deposited on glass substrates by

close spaced sublimation technique at room temperature

Com-plete experimental detail for the deposition of the ﬁlms and

annealing procedure is discussed somewhere else[27] In this study

a total of 15 samples for as-deposited and annealedﬁlms are

pre-sented according to Cu concentration as 0.00 x 0.20 as given in

Table 1

Film composition and concentration depth proﬁles were

determined through RBS Data for all the samples were recorded by

a 5 MeV pelletron tandem accelerator (5UDH-2, NEC) using a

2.023 MeV Heþcollimated beam (2 mm diameter) The sample was

mounted on aﬁve-axis adjustable goniometer with an accuracy of

0.01in a vacuum chamber The backscattering ions were recorded

by a surface barrier detector (energy resolution is 25.8 keV)ﬁxed at

a backscattering angle of 170 A beam integral connected with the

sample holder was used to receive charge on sample from beam

and make sure the experiments are comparable and repeatable

(dose 15 mC) The structural properties were examined by

PAN-alytical 3040/60 X, Pert PRO X-ray diffraction unit with CuKa

(0.154 nm) radiation Surface morphology of the samples was

studied by AFM (Quesant Universal SPM, Ambios Technology, USA)

(QScope™ 350) in non contact mode An AFM tip of silicon nitride

was used having an approximate radius of curvature 10 nm Both

topography and phase images were recorded simultaneously in the

scanning areas of 2e5mm2 All the images were collected in air at

scan rate of 1.0 Hz with 600 600 pixels resolution AFM images were analyzed by using Nova Px software (NT-MDT Co.) thus generating root mean square (RMS) surface roughness Spectro-scopic ellipsometer (J A Woolam M-200VI) was employed to determine the band gap energy (Eg) and dielectric constant (ε1) The ellipsometer consists of QTH lamp as the light source and a dual grating scanning monochromator (370e1670 nm) Incident light was focused on the sample at room temperature in air to determine the optical constants All the spectra were taken at an angle of incidence of 70

3 Results and discussion 3.1 Rutherford backscattering spectrometry (RBS) The Rutherford backscattering spectrometry (RBS) experiments were carried out for as-grown and annealed Zn1 xCuxSefilms of various compositions and thicknesses The collected RBS spectra were thenfitted by the code RUMP[28]tofind the relative con-centrations of various elements in thefilm.Fig 1(aec) shows the RBS spectra of as-grown and annealed films of Zn1xCuxSe (x¼ 0.10, 0.15 and 0.20) which shows that the simulated spectra of the depositedfilms are in good agreement with the measured data The energy spectrum of the emitted ion yields information about the concentration depth profiles The composition has been altered and copper was seemingly introduced into the ZnSe matrix

as a substitutional metallic participant as seen through RBS spectra and the data collected after RUMP code simulation The composi-tion of the samples was calculated and compared with the initial percentage which shows that composition is nearly stoichiometric and the accurate incorporation of added copper is observed The spectra shows that the depositedﬁlms mainly have a Zn(Cu)xSe chemical composition, with x varying from 0.10 to 0.20 at the surface region and then decreasing with depth The maximum concentration of the added Cu is present at the surface (~100e200 nm) layers It consists of three superimposed elemental yields of three atomic species at different channels Moreover, it can

be seen that there is no impurities or contamination in the as-depositedﬁlms though small percentage of oxygen and silicon is seen in the spectra which are coming from the substrate signal The peaks of the heavy elements (Zn, Se and Cu) can be clearly sepa-rated by the Heþbeam and can thus be used to determine the relative thickness of theﬁlms (by using the nominal density of the

Table 1

Film thickness and compositional analysis investigated by spectroscopic ellipsometer (SE) and Rutherford backscattering spectroscopy (RBS) of Zn1xCu x Se thin ﬁlms for various Cu concentrations (x).

M Arslan et al / Journal of Science: Advanced Materials and Devices 2 (2017) 79e85 80

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bulk materials) as well as their stoichiometry The highest channel

number corresponds to the highest backscattered energy of Heþion

from the heaviest element Se present in the compound The peak

occurring at a channel below 950 is due to the silicon contained in

the glass substrate The energy channel correspondence to zinc is

1285; selenium is 1344 while for copper is 1152

After annealing the intensity of the RBS spectra increases and

become slightly broader This shows that annealing plays an

important part in altering theﬁlm thickness and prominent change

in elemental ratio with respect to stoichiometry within the layers

has been observed (shown inTable 1) As the progress in semi-conductor thinfilm technology is advancing, the thickness of de-vice circuits is becoming very thinner and thinner Therefore, an accurate measurement forfilm thickness is required For thin tar-gets, the scattering is proportional to the target thickness Angular frequency changes as the mass of the constituent's changes within the sample from which we can estimate the thickness accurately by using RBS The broadness observed in the RBS peaks with increasing annealing temperature indicates the increase in thick-ness of thefilms[29] It was noticed that a protuberant peak occurs

at ~1152 elemental yield for copper content which is only promi-nent for 0.20 Cu as shown inFig 1(c) This peak arises due to the excessive enrichment of Cu in the ZnSe matrix and corresponds to the unbound Cu content which subsists on the surface By increasing annealing temperature this peak shows declination and fades at 400C annealed temperature From this it is certain that Cu diffuses into ZnSe matrix with increasing annealing temperature According to the RUMP simulation, thefilms annealed at 400C exhibits the most homogeneous Cu concentration as a function of depth compared with the other studied samples The thickness of thefilms determined by RBS technique, quartz crystal and spec-troscopic ellipsometry were in good agreement Moreover, the depositedfilms were uniform and well adherent with the substrate The analyzed compositions and thicknesses of the compositefilms are shown inTable 1

3.2 Structural studies 3.2.1 XRD results

To investigate the crystal structure, composition and phase, as-deposited and annealed Zn1xCuxSe thinﬁlms were characterized

by using X-ray diffraction (XRD) The X-ray diffraction pattern of pristine and heat treatedﬁlms are shown inFig 2(a,b) which shows that the crystal planes are preferentially orientated along the (111) plane with zinc-blende structure.Fig 2(a) shows the XRD pattern

of the Zn1 xCuxSe ﬁlms annealed at 400 C with two different copper concentrations (0.00 and 0.15) whileFig 2(b) shows XRD pattern of as-deposited and annealed (200 C and 400 C)

Zn1 xCuxSe thinfilms for 0.05 Cu concentration XRD graphs indi-cate that the increase of annealing temperature leads to improve-ment of the thinfilms growth in the (111) plane orientation This is due to the decrease in film stresses and grain coarsening as described in detail in our published paper[27]

It is observed that the annealing temperature of 200 C and

400 C did not affect the predominant (111) crystallographic texture No secondary phase is observed after annealing, however, the intensity of (111) peaks increases with the annealing temper-ature This increase of the peak intensity is due to the improvement

of clusters, relocation of atoms and elimination of defects formed during thefilm deposition After annealing, the intensity of (220) and (311) reflection decreases while the preferred orientation in (111) direction increases radically at 400C Slight modification of the crystal structure is observed by annealing at temperature of

200 C however the crystal structure changes signiﬁcantly after annealing at 400C A small decrease in diffracting angle 2qfor 0.15 annealed at 400 C is also observed which conﬁrms that after annealing the grains are recrystallized and coalescence is assumed

to happen

Fig 3shows the effect of annealing on the FWHM andҮSFEfor (111) orientation FWHM can be increased or decreased depending

on coalescence and recrystallization of grains Recrystallization may help (111) orientation and peak growth, making the FHWM smaller It is observed that FWHM and ҮSFE decrease with increasing annealing temperature, which is mainly due to grain growth and improvement in crystallinity.Ү for the as-deposited Fig 1 Rutherford backscattering spectra of as-deposited and annealed (200 C and

400C) Zn1xCu x Se (a) 0.10, (b) 0.15, (c) 0.20 ﬁlms.

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and annealed ﬁlms is calculated for (111) plane by using the

relation[30]

"

2p2

45ð3 tanqÞ1=2

#

where‘q’ is the Bragg's angle and ‘b’ is the full width half maxima

(FWHM) The stacking fault energies (Ү ) were calculated from

Fig 2 XRD pattern of (a) 400C annealed Zn1xCu x Se (0.00, 0.15) and (b) as-deposited

and annealed (200C and 400C) Zn1xCu x Se (0.05) thin ﬁlms.

Fig 3 Variations of full width half maxima (FWHM) and Stacking fault energy ( Ү SFE ) of

as-deposited and annealed (200 C and 400C) Zn1xCu x Se thin ﬁlms with Cu

concentration.

Fig 4 AFM 2-D and 3-D images of (aeb) as-deposited, (ced) 200 C annealed and (eef) 400 C annealed Zn1xCu x Se (0.00 and 0.10) thin ﬁlms.

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the shift of the peaks of the X-ray lines of theﬁlms with reference to

the 2003 JCPDS database No: 89-7130, using Eq.(1)

ҮSFEalso decreases gradually with increasing copper

concen-tration up to 0.10 and crystal growth becomes sharp while an

opposite trend is observed beyond this Cu concentration The

minimum values ofҮSFEare obtained at 0.10 and 400C annealed

temperature while the maximum at 0.20 Cu concentration The

smaller value of theҮSFE(0.0377 J/m2) obtained at 0.10 Cu exhibits

excellent crystalline quality of CuxZn1 xSeﬁlms There is no report

on theҮSFEof Zn1 xCuxSe thinﬁlms deposited by closed space

sublimation technique All the structural parameters are

summa-rized inTable 1

3.2.2 AFM results

The morphology of as-deposited and annealed samples have

been analyzed with the help of AFM diagnostic tool The

topog-raphy of the surface in 2D and 3D image is shown inFig 4for all

samples with a root mean square (RMS) roughness are listed in

Table 2.Fig 4(a,b) indicates the surface morphology of as-deposited

Zn1xCuxSe (x¼ 0.00, 0.10) ﬁlms.Fig 4(a) presents a low roughness

surface with an RMS value of 1.12 nm for 0.00 Cu concentration

ﬁlm, over a scan size of 2mm2, which suggests the formation of very

smooth surface The roughness increases to 1.39 nm for 0.10 Cu

concentration The increase in RMS roughness with Cu is due to the

grain growth and improved crystallinity as corroborated from our

XRD results These formations are in agreement with those

re-ported by Mazon-Montijo et al for CdSﬁlms[31]

Fig 4(cef) shows the surface morphology of the annealed

Zn1xCuxSe (x¼ 0.00, 0.10) thin ﬁlms at 200 and 400C over a scan

size of 5mm2 A careful comparison between both annealed samples

reveals that the micro features on the 400C annealedﬁlm surface

are almost similar in shape to 200C annealedﬁlms except size of the

particles are reduced after 400C annealing The islands formed on

the surface of theﬁlms annealed at 400C shows more improvement

in particle size along withﬁner micro-asperities The addition of Cu

contents and annealing enhance the grain growth and roughness

This is useful for solar cell applications as rough surface trap more

light Light trapping is widely used to enhance the absorption in the

absorber layer of thinﬁlm solar cells and therefore to increase the

current density The most prevalent light-trapping technology is

introducing nano-textured interfaces into the solar cells[32]

3.3 Spectroscopic ellipsometry

Spectroscopic ellipsometer has been employed to determine the

band gap (E) and dielectric constant (ε) of our target thinﬁlms A

beam of polarized light is illuminated onto the sample and polar-ization change is measured from reﬂection spectra The polariza-tion change in the reﬂection signal is measured and then characterized by two quantities, psi (J) and delta (D) parameters for amplitude and phase changes respectively

tanðjÞ$eiD¼rrp

where tan (J) is the magnitude of the reﬂectivity ratio, rpis the

reﬂectivity for p-polarized light and rs is the reﬂectivity for s-polarized light The experimental psi (j) and delta (D) spectra were recorded as a function of wavelength over the range (400e800) nm

at an incidence angle of 70 These parameters are correlated with thinﬁlm optical properties by the above expression and then make

a comparison between the experimental and simulated data by utilizingﬁtting functions[33]

The extinction coefﬁcient (k) and refractive index (n) measured ellipsometry data already reported for the same samples in our published paper[27] However, in this article we determined the band gap energy by using extinction coefﬁcient (k) spectra obtained from Ellipsometer The optical band gap energy (Eg) was calculated using the following relations

a¼ 4pk=l

ahn2¼ AEg hn whereais the absorption coefficient, h is Planck's constant,yis frequency and A is proportionality constant To estimate the band gap of thesefilms, (ahn)2was plotted against hyusing the above equation for as-deposited and annealedfilms of different compo-sitions Extrapolation of the linear portion to the (ahn)2¼ 0 axis gives the value of band gap energy as shown inFig 5(aec) For all compositions, the band gap energy increases with annealing tem-perature as shown in Fig 5(aec), while decreases with the in-creases of Cu concentration in the ZnSe matrix as shown inFig 5c (inset) Our calculated values of optical band gap by spectroscopic ellipsometer are slightly greater than reported by J Kvietkova et al and Dahmani et al by using spectroscopic ellipsometer[34,35] The band gap energies estimated by SE technique is significantly different than transmission data The reason for this is that the

reﬂectivity obtained by the SE is very different from that of optical probing via a spectrophotometer in terms of the spatial fre-quencies Moreover, measurement of the optical parameters by the

SE requires large-scale approximation for theﬁtting of the model

Table 2

Peak position (2q), full width half maxima (FWHM), stacking fault energy (SFE), Band gap (E g ) and mean square roughness (RMS) of as-deposited and annealed (200 C and

400 C) Zn1xCu x Se thin ﬁlms for various Cu concentrations (x).

Cu concentration Nature of ﬁlms 2qdeg [27] FWHM (2q) Stacking fault energy (J/m 2 ) Band gap (eV) Roughness (nm)

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while transmission is real time measurement Comparing the SE

and RBSﬁlm thickness, the thickness is approximately 5% greater

than those calculated by the RBS as listed inTable 1 This variation

in theﬁlm thickness by the two different techniques is due to the

diversity in theﬁtting software's and the mode of simulating the data

Furthermore, dielectric quantity (ε1) of Zn1 xCuxSe ﬁlms ob-tained from the ellipsometryﬁt is shownFig 6(aec) ε1is the real part of the complex dielectric function,ε ¼ ε1þ ίε2which represents how much a material is polarized due to creation of electric dipoles

Fig 5 Band gap energy of as-deposited and annealed (200C and 400C) Zn 1x Cu x Se

(a) 0.00, (b) 0.05, (c) 0.15 ﬁlms, determined from k-spectra Inset in Fig 6 (c) shows the

variations of band gap energy of as-deposited and annealed (200 C and 400 C)

Zn1xCu x Se ﬁlms with copper concentration.

Fig 6 Dielectric constant ( 3 1 ) of as-deposited and annealed Zn1xCu x Se (a) 0.00 (b) 0.05, (c) 0.20 ﬁlms, determined by spectroscopic ellipsometer Inset in Fig 6 (c) shows the variations of dielectric constant of as-deposited and annealed (200C and 400C)

Zn Cu Se ﬁlms with copper concentration.

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in the material by applying electricﬁeld Change in the polarization

of any material directly affects the dielectric properties of the

ma-terial This change in the dielectric constant is measured by the

ﬁtting of ellipsometry parameters psi (j) and delta (D) SE measures

the dielectric constant (ε1) by the following equation[36]

Dielectric studies show that the dielectric constant (ε1) values

decrease with increasing annealing temperature as shown in

Fig 6(aec), while increases with Cu contents addition as depicted

in Fig 6c (inset) Post-annealing treatment of ﬁlms plays an

important role and considerably affects the dielectric properties of

the prepared Zn1 xCuxSe ﬁlms The decrease in the dielectric

constant with annealing temperature may be due to the lower

compactness of annealed ZnSeﬁlms than as-deposited ZnSe ﬁlms

Similar results have been reported for annealed ZnSe ﬁlms by

Venkatachalam et al.[37]

4 Conclusion

The effect of post deposition thermal annealing on the

compo-sitional stoichiometry and depth concentration of deposited layers

of Zn1 xCuxSe thin ﬁlms was investigated by RBS technique By

increasing the annealing temperature of ﬁlms from 200 C to

400C the physical properties have improved signiﬁcantly

Varia-tions in stoichiometry found with RBS technique is complemented

by micro analysis characterizations performed by XRD and AFM,

while the optical results obtained by using SE XRD data predicts

the improvement of crystallinity with an increase in FWHM value

while stacking fault energy decreases with increasing annealing

temperature AFM suggests the formation of very smooth surface;

larger grains are replaced by smaller and ﬁne grains

(micro-as-perities) at 400C accompanied by an increase in surface

rough-ness Spectroscopic ellipsometry analysis reveals that the bang gap

increases while dielectric constant decreases with the increase in

annealing temperature

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