The effect of annealing on structural, optical and photosensitive properties of electrodeposited cadmium selenide thin films

The band gap of as-deposited or unannealed film is higher compared to the annealed CdSe thin films because the deposition at room temperature gives rise to films with smaller crystallite si[r]

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Original Article

The effect of annealing on structural, optical and photosensitive

Somnath Mahatoa,b,*, Asit Kumar Kara

a Department of Applied Physics, Indian Institute of Technology (Indian School of Mines) Dhanbad, 826004 Jharkhand, India

b Saha Institute of Nuclear Physics (Surface Physics and Material Science Division), 1/AF Bidhannagar, Kolkata 700064, India

a r t i c l e i n f o

Article history:

Received 2 December 2016

Received in revised form

7 April 2017

Accepted 9 April 2017

Available online 15 April 2017

Keywords:

CdSe

Thin ﬁlm

Electrodeposition

XRD

Photosensitivity

a b s t r a c t

Cadmium selenide (CdSe) thinfilms have been deposited on indium tin oxide coated glass substrate by simple electrodeposition method X-ray Diffraction (XRD) studies identify that the as-deposited CdSe films are highly oriented to [002] direction and they belong to nanocrystalline hexagonal phase The films are changed to polycrystalline structure after annealing in air for temperatures up to 450C and begin to degrade afterwards with the occurrence of oxidation and porosity CdSe completely ceases to exist at higher annealing temperatures CdSefilms exhibit a maximum absorbance in the violet to blue-green region of an optical spectrum The absorbance increases while the band gap decreases with increasing annealing temperature Surface morphology also shows that the increase of the annealing temperature caused the grain growth In addition, a number of distinct crystals is formed on top of the film surface Electrical characteristics show that the films are photosensitive with a maximum sensitivity

at 350C

© 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

Semiconductors are very important and interesting because of

their technological applications in optoelectronics and

microelec-tronic devices like photodiodes[1], sensors[2], light emitting

di-odes[3], solar cells[4], photoelectrochemical cells[5], photovoltaic

cells[6]and photodetectors for optical communications etc Among

them, Cadmium Selenide (CdSe) is a IIeVI group compound

semi-conducting material of the periodic table This compound is a

highly photosensitive material in the visible region due to their

suitable band gap (1.74 eV)

Different processes such as chemical vapour deposition [7],

physical vapour deposition[8], thermal evaporation technique[9],

spray-pyrolysis[10], chemical bath deposition[11], dip coating[12]

and electrodeposition[13]have been used for depositing cadmium

selenide thinﬁlms However, the electrodeposition process is one

of the simplest and low-cost techniques because it is easy to

manage and it requires very simple arrangement Deposition rate is

easily controlled by changing deposition potential, concentration

and pH value of the electrolyte Many groups are working on cad-mium selenide using the process of electrodeposition[5,7,14e16] The optoelectronic, microelectronic and other applications of cadmium selenide thinfilms depend on their structural and elec-tronic properties affecting device performance These properties are strongly influenced by the deposition parameters such as deposition time, deposition potential, concentration of electrolytic solution, pH of the electrolyte and thermal annealing Thermal treatment is one of the important factors to enhance the efficiency and stability of photosensitive devices Thus, studies of the effect of annealing on structural, optical and electrical properties of thin films are very important in understanding and enhancing device sensitivity[17e19]

The aim of this present work is to prepare cadmium selenide thin films by a simple electrodeposition process on indium tin oxide (ITO) coated glass substrates and to study the effect of annealing temperature (Ta) onfilms' photosensitivity The effect of annealing on crystallinity, morphology and optical absorbance of thefilms are also presented and discussed

* Corresponding author Department of Applied Physics, Indian Institute of

Technology (Indian School of Mines) Dhanbad, 826004 Jharkhand, India.

E-mail address: som.phy.ism@gmail.com (S Mahato).

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.2017.04.001

Journal of Science: Advanced Materials and Devices 2 (2017) 165e171

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2 Experimental

2.1 Film deposition

Cadmium selenide thinﬁlms have been deposited on indium tin

oxide coated glass substrates by using a simple two-electrode

electrodeposition process at room temperature (28 C) Sputter

coated ITO/glass was procured from Macwin India, Delhi The

substrate, having sheet resistance 10U/sq, was used as a working

electrode or cathode, and aﬂat graphite rod was used as an anode

The size of both the electrodes submersed in electrolyte was about

1 1 cm2and hence the area of deposition was about 1 cm2 The

electrodes were separated by a distance of about 1 cm Substrates

were cleaned in acetone within an ultrasonic bath for 15 min and

then cleaned in running distilled water for 5 min and,ﬁnally, they

were dried in air for 15 min before deposition For electrodeposition

of theﬁlms, cadmium chloride (CdCl2) and selenous acid (H2SeO3)

were used as the sources of cadmium and selenium, respectively in

the electrolyte; the molar concentrations of cadmium chloride and

selenous acid were 0.08 M and 0.005 M, respectively The

electro-lyte was continuously stirred for 15 min in a beaker by using a

Teﬂon coated magnetic paddle attached to a stirrer, in order to

perfectly dissolve the ingredients in distilled water All the

chem-icals were procured from Sigma Aldrich and had 99.99% purity The

total volume of the prepared electrolyte was 100 ml pH The value

of the electrolyte was kept at 1.9 by using HNO3 solution The

deposition was conducted for 15 min with aﬁxed deposition

po-tential of 1.80 V for all theﬁlms After deposition, the thin ﬁlm

coated substrates were taken out from the electrolyte, then rinsed

in distilled water and dried in air The as-deposited ﬁlms were

annealed at 250, 350, 450, 550 and 650C in the air for one hour in

a mufﬂe furnace with a ramp up rate of 2 C/min followed by

normal cooling to room temperature

2.2 Reaction mechanism

The reaction mechanism of CdSe thinﬁlm is discussed as

fol-lows The deposition process is based on the slow release of Cd2þ

ions and Se2ions in the solution ion-by-ion basis and settling on

the ITO coated glass substrates The deposition takes place when

the ionic product of Cd2þand Se2is greater than the solubility

product Cadmium selenide is deposited according to the following

over-all net reaction[20]

H2SeO3þ Cd2þþ 6eþ 4Hþ#CdSe þ 3H2O

The rate of the formation of CdSe is determined by the bath

parameters such as pH, concentration and temperature of the

electrolyte[21]

2.3 Film properties

X-ray diffraction (XRD) patterns were recorded using XRD

(BRUKER D8 FOCUS) system with the Cu Karadiation (l¼ 1.5406 Å)

qe2qscan was taken for the range of 10e80with a speed of 0.20/

s and with a step size of 0.030 Optical absorption spectra were

obtained for the region 300 nme900 nm using UVeviseNIR

spectrophotometer The microstructure and composition of the

CdSe thinﬁlms were studied using a scanning electron microscope

(FESEM, Model: JEOL JSM-5800 Scanning Microscope) and energy

dispersive analysis of X-ray (EDAX) module attached with the same

SEM system respectively The electrical resistivity of the samples

was measured by the two-point probe technique Currentevoltage

measurements in dark and illumination were accomplished using a

Keithley 2400 source metre The light source was a 100 W (intensity

3.5 mW/cm2) tungsten bulb controlled by a dc power supply and it was placed 20 cm away from the sample during experiment

3 Results and discussion 3.1 Crystallinity

Cadmium selenide thinﬁlm grown on ITO coated glass substrate

is found to be polycrystalline with hexagonal (wurtzite) crystal structure Fig 1(a) shows the XRD pattern of as-deposited or unannealed CdSe thinﬁlm The peak at 25.99corresponds to the plane (002) which is much stronger than other peaks The intense peak at (002) suggests a dominant orientation of nanocrystalline phase of CdSe thinﬁlm within an otherwise amorphous or nearly amorphous matrix The small hump in the background is due partly

to the amorphous nature of ITO coated glass substrate and also may

be due to some amorphous phase presented in the CdSe thinfilm itself[22].Fig 1(b)e(f) shows the XRD patterns of annealed films Annealing at 250C [Fig 1(b)] makes thefilm more oriented to-wards (002) plane The polycrystalline hexagonal CdSe phase is found after annealing at 350C [Fig 1(c)] Intensity of the most intense peak is continuously found to decrease with the increase of annealing temperature It signifies a gradual change of a highly oriented nanocrystalline phase to a polycrystalline phase Further heat treatment from 450C to 650C shows that the CdSe phase gradually changes to CdO phase [Fig 1(d)e(f)] At the annealing temperature 550C and above, CdSe completely disappears All the XRD patterns from Figs (d)e(f) show the characteristic diffraction peaks of (111) and (200) planes of polycrystalline hexagonal CdO phase Other peaks (211) at 21.88, (222) at 30.91, (400) at 35.68 and (622) at 61.18correspond to ITO This suggests that the after annealing of CdSe thin films in air at a higher temperature [Ta 450C], reaction occurs and chemically a new phase forma-tion takes place; the polycrystalline phase of CdO gradually prevails over the polycrystalline phase of CdSe with increase in tempera-ture XRD plots from (a) to (f) also exhibit gradual reduction in overall peak intensity and hence a rise in background intensity They also demonstrate the appearance of more ITO peaks with enhanced intensity at higher annealing temperature These facts might be related to the gradual loss of CdSe and later CdO [Figs (e) and (f)] from the surface of the thinfilms due to sublimation during annealing and the possibility of diffusion into the substrate may be ruled out

Average crystallite size of CdSe ﬁlms is found to vary from 16.8 nm to 21.9 nm This was calculated from Scherrer's formula using full width at half maximum (FWHM)bof the peaks of XRD proﬁles[23e25]

where D ¼ crystallite size, K ¼ shape factor (0.9), and

l¼ wavelength of Cu Karadiation

The microstrain (ε) values have been calculated by using the following formula:

ε ¼ bhkl

Assuming that, the particle size and strain are independent of each other, equations(1) and (2)may be combined to the following form:

bhklcosq¼Kl

S Mahato, A.K Kar / Journal of Science: Advanced Materials and Devices 2 (2017) 165e171 166

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This is known as WilliamsoneHall formula[26] The graph was

plotted betweenbhklcosqversus 4 sinqas shown inFig 2 From

the linearﬁt to the data, the crystallite size was estimated from the

intercept along ordinate, and strain (ε) was found from the slope of

theﬁt From WilliamsoneHall (WeH) method the average

crys-tallite size is determined to be 31.5 nm for CdSe thinﬁlm annealed

at 450C

The dislocation densitydhas been calculated by using the

for-mula for the highly intense X-ray diffraction peaks

d¼15ε

All the calculated values are shown inTable 1 As expected,

increase in annealing temperature leads to increase in crystallite size, and decrease in strain and dislocation density of theﬁlms 3.2 UVeviseNIR spectroscopy

Fig 3 shows the variations of optical absorbance and trans-mittance (inset) with wavelength of the as-deposited and annealed CdSe thinﬁlms Absorption spectra is strong around the violet to visible region Afterwards, it continuously decreases with increase

in wavelength and becomes almost constant at near infrared (NIR) region for the as-depositedfilm For the annealed films, however, the decrease in absorbance shows a sharp fall at around 700 nm and then it gradually saturates in the NIR region The absorbance Fig 1 XRD patterns of CdSe/ITO thin films at different annealing temperatures: (a) As-deposited, (b) 250 C, (c) 350C, (d) 450C, (e) 550C, and (f) 650C.

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increases and the broad peak shifts from violet to blue-green region

with increasing annealing temperature It may be due to increased

crystallite size in the thinﬁlms The colour of the ﬁlm is found to

change from red-orange to dark black after annealing The values of

the band gap of theﬁlms have been determined from transmission

spectra by using the following relation applicable to near edge

optical absorption of semiconductors:

a¼

K

hn

hn Eg

n

(5)

whereais absorption co-efﬁcient, hnis the photon energy, K is a

constant, Egis the band gap and n is a constant which equals to½

for allowed direct band-gap semiconductor in the present case

[27,28] The band gap energy of CdSe/ITO thin ﬁlm has been

determined by Tauc plot based on the above formula as shown in

Fig 4 The optical band gaps are found to be 2.13 eV, 1.95 eV, 1.91 eV

and 1.88 eV for thinﬁlms of as-deposited and annealed at 250, 350

and 450C temperature respectively The band gap of as-deposited

or unannealedﬁlm is higher compared to the annealed CdSe thin

ﬁlms because the deposition at room temperature gives rise to

ﬁlms with smaller crystallite size So the energy band gap of CdSe

thin ﬁlms tend to decrease as the annealing temperature is

increased due to increased crystallite size of theﬁlms

The value of the extinction coefﬁcient (k) is calculated from the

following relation[29]:

k¼4al

The graphical representation of the variation of extinction

co-efﬁcient with wavelength is shown inFig 5 The graph shows that

even for the photons having energy above band gap, the absorption

coefﬁcient is not constant and strongly depends on wavelength For

photons which have energy very close to that of the band gap, the

absorption is relatively low since only the electrons at the valence

band edge can interact with the photon to cause absorption As the photon energy increases, not just the electrons already having energy close to that of the band gap can interact with the photons, a larger number of other electrons below band edge can also interact with the photons resulting in absorption Thus extinction coef ﬁ-cient has high values near the absorption edge and it has very small values at higher wavelengths

3.3 Surface morphology The surface morphology of as-depositedfilm and annealed films has been studied using FESEM as shown inFig 6(a)e(f) Surface Fig 2 WeH plot for a film annealed at 450 C.

Table 1

Structural parameters for as-deposited and annealed CdSe thin ﬁlms calculated from their corresponding XRD proﬁles.

T a ( C) Crystallite size (nm) Lattice parameters (Å) Strain (ε) Dislocation densityd(10 17 /m 2 )

a Calculated from WeH plot.

Fig 3 UVeviseNIR absorbance and transmittance (inset) spectra of CdSe/ITO thin ﬁlms annealed at different temperatures.

Fig 4 Tauc plots for as-deposited and annealed CdSe/ITO thin ﬁlms.

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topography of as-deposited ﬁlm is shown inFig 6(a) From the

topograph, it is observed that the as-depositedﬁlms are continuous

with homogeneous distribution of densely packed blister-like

particles of nonuniform size varying from several tens of

nano-metre to about 250 nm.Fig 6(b) shows a cross-sectional tilted view

of the ﬁlm annealed at 250 C; spherical nanosized grains of

globule-like structure are observed with several 100 nm in size and

the grains are closely packed with each other to form a crystalline

matrix A wide view of the corresponding area has been presented

inFig 6(e), which covers parts of both cross-sectional and surface

features It appears that after annealing, the particulate features

were more uniform in size, reducing the range of variation observed in as-deposited ﬁlms At the annealing temperature

350C [Fig 6(c)], it is found that thefilms become rougher with the development of some pebble-like crystalline surface features of size varying from about 50 nm to 300 nm Apparently the blister-like features infigure (a) have played the role of growth centres and crystalline features are developed through the process of sur-face and volume diffusion with increase in temperature The SEM micrographs of thefilm annealed at 450C are shown inFigs 6(d) and (f) where the latter represents a wide area view A drastic change in crystalline structural features is observed for 100C in-crease in annealing temperature with respect toFig 6(c) Excellent single crystalline structures of width as big as 1.5mm with various polygon like[30]facets are noticed to evolve on thefilm surface but with very less in number compared to thefilm annealed at 350C. Some pores are also found to develop on the surface of thefilm of irregular shape appearing like crystalline voids Other than the crystals on the surface and the pores, the surface of thefilm appears

to be smooth with clear demarcation of crystalline grains i.e grain boundaries Grain size varies from about 100 nm to 500 nm Top surfaces of the embedded crystalline grains are found to form a nice mosaic pattern

Due to annealing, a number of smaller grains or crystals diffuse and coalesce together to effectively form larger crystalline grains with clear crystallographic faces Above mentioned results demonstrate that the process of annealing induces two parallel grain growth processese one within the volume of the thin film matrixe a primary growth process, and the other over the thin film surfacee a secondary growth process Crystalline nature of CdSe thinfilms is also indicated by XRD measurement

Thickness of thefilms was found to be about 6mm by cross-sectional imaging in SEM Energy dispersive analysis of X-rays (EDAX) confirms the presence of both Cd and Se in the films It also reveals that the thinfilms annealed at different temperatures are nonstoichiometric in nature

Fig 5 Dispersion curves of extinction coefﬁcient (k) for as-deposited and annealed

CdSe/ITO thin ﬁlms.

Fig 6 Scanning electron micrographs of CdSe thin ﬁlms: (a) As-deposited, (b) annealed at 250 C (a tilted cross-sectional view), (c) annealed at 350C, and (d) annealed at 450C,

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3.4 Electrical property

The electrical resistivity of CdSe/ITO thin ﬁlm has been

measured by using dc two probe methods It is determined by

loading a direct current I and measuring a voltage drop V between

two probes which are placed at a distance (s) of 1 mm, using the

following equation[31,32]:

r¼ 2psV

At room temperature the speciﬁc conductance was found to be

of the order of 104(U1cm1).

For photoconductivity measurement of CdSe thinﬁlms, area of

theﬁlm exposed to light was 1 1 cm2 The dark and illumination

IeV characteristics of CdSe thin ﬁlms were recorded as shown in

Fig 7(a) as an example All theﬁlms under dark conditions showed

good rectifying nature They also responded to illumination giving

rise to photocurrent with again rectifying nature or asymmetric

semiconducting nature The characteristic curves demonstrated

that the photo response was sensitive to annealing temperature

The photosensitivity S of the ﬁlms was calculated using the

following formula:

s¼slightsdark

sdark

(8)

wheresphoto is the photoconductivity andsdark is the dark

con-ductivity[9] The as-deposited CdSe thinﬁlms show weak

photo-conductivity and its sensitivity is less (S ~ 3) Annealing at 250C

reveals increased photoconductivity and its sensitivity increases to

~12 The photoconductivity is found to dramatically improve

(S ~ 64) at 350C annealing temperature So it is observed that the

photosensitivity is increased with the increase of annealing

tem-perature as shown inFig 7(b) The reason is associated with the

increased absorbance of the incident light in visible region with

increase in annealing temperature Enhancement in the

photo-conductivity is due to the generation of more electron-hole pairs

excited by the incident light Annealing at 450 C leads the

photoconductivity to fall to zero because of the phase change and

accompanying degradation of CdSe thinﬁlm At this temperature,

microstructural defects like pores and formation of secondary

phase like CdO impair and saturate the conduction of charge

car-riers even after their enhanced generation due to higher

absor-bance The result may be beneﬁcial to the development of large

area, low cost, and good quality CdSe thinﬁlms for photodiode and photovoltaic applications

4 Conclusion The CdSe thin films have been successfully deposited by a simple two electrode electrodeposition method on ITO coated glass substrates The process of annealing in air has been found to change the crystallinity of films from highly oriented nanocrystalline (hexagonal wurtzite) structure to polycrystalline form With annealing globular nanocrystalline grains become bigger and a number of distinct micro-crystals are developed on top of thefilm surface; the crystals grow to a maximum in size at 450C having clear crystallographic faces on their surface For annealing tem-peratures higher than 450C, CdSe is chemically degraded and is converted to CdO The CdSefilms exhibit strong absorbance in the violet to blue-green region With increase in the annealing tem-perature, the band gap decreases from 2.13 eV to 1.88 eV for the as-deposited and 450Cfilms The CdSe films are photosensitive; the sensitivity increases with annealing temperature up to 350C and then decreases

Acknowledgements Authors are grateful to Dr B Pandey, Dr N Das, Dr D Roy and

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