CuAlS2 thin films Dip coating deposition and characterization 2017 Journal of Science Advanced Materials and Devices

CuAlS2 thin films Dip coating deposition and characterization 2017 Journal of Science Advanced Materials and Devices tài...

Original Article

P G Department of Physics, Sardar Patel University, Vallabh Vidyanagar, Gujarat 388 120, India

a r t i c l e i n f o

Article history:

Received 6 October 2016

Received in revised form

12 April 2017

Accepted 14 April 2017

Available online 24 April 2017

Keywords:

CuAlS 2

Thin film

Dip coating

XRD

Microscopy

Electrical transport properties

a b s t r a c t

CuAlS2thinfilms were deposited by a dip coating technique at room temperature The X-ray energy dispersive (EDAX) and X-ray diffraction (XRD) analysis showed that the deposited CuAlS2thinfilm is nearly stoichiometric and possesses a tetragonal unit cell structure The crystallite sizes determined from the XRD data employing Scherrer's formula and modified forms of HalleWilliamson relation like the uniform deformation model (UDM), uniform stress deformation model (USDM), uniform deformation energy density model (UDEDM), and the sizeestrain plot method (SSP) were in good agreement with each other The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) studies

of the thinfilm revealed that the deposited film is uniform without any cracks and the film covers the whole of the substrates The atomic force microscopy (AFM) of the as-synthesized thinfilm surfaces showed spherical grains having coalescences between them The optical absorbance spectrum analysis showed that the thinfilms possess both direct and indirect band gaps The semiconducting and p-type nature of the thinfilms was confirmed from dc e electrical resistivity versus temperature, room tem-perature Hall effect, and Seebeck coefficient versus temtem-perature measurements The effect of the different illuminations on the CuAlS2thinfilm showed that it can be used as a material for absorption of ultra-violet radiation All the obtained characterization results are deliberated in detail

© 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

The ternary chalcopyrites belonging to MI-MIII-C2VI(MIe Cu, Ag;

MIIIe Al, Ga, In; CVIe S, Se, Te) compound semiconductor family

have received wide interest because of convenient band structures

suitable for optically active devices[1] They have been synthesized

in single crystal form[2,3], but more recently experimental

in-vestigators have focused on thinfilms due to high potential for

large area photovoltaic modules The CuAlS2is one of the members

of the ternary chalcopyrite family having the direct optical band

gap of 3.5 eV[1,3] The optical band gap of this compound is the

highest among those of all the chalcopyrite compound

semi-conductors making it an interesting material for applications Due

to the wide optical band gap, the CuAlS2has found potential

ap-plications in solar cells[4], in photovoltaic [5], as light emitting

devices in the blue region of the spectrum[6], as window layers of

solar cells[7]and in laser diodes operating in a short wavelength

region[8] The CuAlS2 thinfilms have been used as oxygen gas

sensor operating at room temperature showing an enhanced

sensitivity with the aging of the film[9] Nanocrystals of CuAlS2 have been employed in targeted“in-vitro” imaging of cancer cells after nano-engineering their surface [10] The CuAlS2micro- and nano-particles have been used as the catalyst in cellulose pyrolysis

[11] An additional major advantage of CuAlS2is that its constituent elements are copious in nature and are non-toxic Inspired by the importance and potential applications of CuAlS2[12]a study on this material in the thin film form has been undertaken in this investigation

Till now, a number of methods have been employed to deposit CuAlS2 thin films These methods include iodine transport [13], metal organic decomposition (MOD) [14], single source thermal evaporation[9], sulfurization of precursors in H2Sflow[15], sul-furization of sputtered metallic precursors by sulphur vapours in hermetically sealed ampoules [16], thermal evaporation of elemental mixture [17], spray pyrolysis [18,19], pulsed plasma deposition[20], horizontal Bridgman method[21], chemical bath deposition (CBD)[22,23], two stage thermal evaporation[24]and electron beam evaporation[25] The literature shows no report of deposition or study of CuAlS2thinfilms by dip coating technique The advantage of dip coating deposition is that it is a low cost so-lution deposition technique mainly used for uniform coating of large areas[26]and to synthesize thinfilms of high quality[27,28]

* Corresponding author.

E-mail address: sunilchaki@yahoo.co.in (S.H Chaki).

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.2017.04.002

2468-2179/© 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

Journal of Science: Advanced Materials and Devices 2 (2017) 215e224

The high quality thinfilms by dip coating technique are achieved

due to the layer-by-layer growth during each dip of the substrate in

the aqueous solution In each dip of the substrate, the individual

layer forms through ion-by-ion adsorption Thus the dip coating

thinfilm formation is by ion-by-ion adsorption leading to

layer-by-layer deposition in every dip, consequently to the possibility of

minimization or even elimination of defects and imperfections in

the synthesized thinfilms The other advantages of dip coating are

the control over deposition rate andfilm thickness by means of

control on dipping time during each dip and by regulating the

number of dips, respectively Other deposition parameters like dip

speed, withdrawal speed and dry duration; the time the substrate is

out of solution between consecutive dips can be a handle to control

the film deposition In this study, CuAlS2 thin films have been

deposited on glass substrates by dip coating technique The

as-deposited dip coating thinfilms were comprehensively

character-ized for elemental composition, crystal structure, surface

morphology, optical and electrical properties

2 Experimental

Cupric chloride (CuCl2$2H2O) [S D Fine Chem Ltd., Mumbai,

India], triethanolamine (TEA) (C6H15NO3) [Sisco Chem Pvt Ltd.,

Mumbai, India], aluminium chloride (AlCl3$6H2O) [Oxford

Labora-tory, Mumbai, India], ammonia liquid (NH3) [Chiti-Chem

Corpora-tion, Vadodara, India] and thiourea (NH2CSNH2) [Chiti-Chem

Corporation, Vadodara, India] were used for the synthesis of

CuAlS2thinfilms by dip coating technique The chemicals were all

of AR grade and were used without any further purification or

processing

In the synthesis of CuAlS2thinfilms by using dip coating

tech-nique,firstly 10 ml of 0.5 M cupric (II) chloride (CuCl2$2H2O)

so-lution was mixed with 5 ml of 3.7 M TEA soso-lution in a 100 ml clean

dry glass beaker under continuous stirring for 5 min The

CuCl2$2H2O acts as precursor for Cu and TEA acts as complexing

agent to slow down the release of the metal ions resulting in slow

precipitation of the compound by ioneion reaction and to prevent

the agglomeration of the desired metal ions Then, in the above

solution, 16 ml of 10 M NH3 solution was added and stirred for

5 min Here NH3(liquid ammonia) is used as reagent to adjust the

pH of the solution The pH of the solution was kept at 9.5, the

reason for this being if pH< 7 the solution becomes acidic due to

which it can corrode the deposited thinfilms Under continuous

stirring, 10 ml of 0.7 M aluminium chloride (AlCl3$6H2O) solution

was added and stirred for 5 min Finally, 10 ml of 1.0 M thiourea

solution was mixed and stirred for 5 min At last, thefinal solution

was made to reach 100 ml by adding appropriate amount of

deionized water The final solution of 100 ml volume was kept

under programmed dip coating unit apparatus [Dip Coating Unit,

Model No: HO-TH-02; Holmarc Opto-Mechatronics Pvt Ltd., Kochi,

Kerela, India] for thinfilms deposition The dip coating parameters

were maintained for the CuAlS2thinfilms depositions as: dipping

speede 9 mm/s; withdrawal speed e 9 mm/s; dip duration e 10 s;

dry duratione 5 s and total number of dips e 600 In case of certain

characterizations the numbers of dips were increased to increase

thefilm thickness

During the deposition of CuAlS2thinfilms the following reaction

was expected to have occurred:

CuCl2$2H2Oþ 2NH4OHþ TEA / [Cu (TEA)]2 þþ 2OH1 þ 2NH4Clþ

2H2O

AlCl3$6H2Oþ 3NH4OH/ Al3 þþ 6H2Oþ 3OH1 þ 3NH4Cl

2(NH)CSþ 2OH1/ 2CH N þ 2HOþ 2HS1

2HSþ 2OH1/ 2S2þ 2H2O [Cu (TEA)]2þþ Al3þþ 2S2/ CuAlS2Y þ TEA The average thicknesses of the dip coating deposited thinfilms were determined by the gravimetric weight difference method

[27,28] In the average thin film thickness calculation, the film density was taken as 3.48 g/cm3determined from the XRD data analysis and will be discussed later in this paper

3 Results and discussion 3.1 X-ray energy dispersive analysis The chemical compositions of the as-deposited CuAlS2thinfilms were determined by the energy dispersive analysis of X-ray (EDAX) technique The EDAX analysis was done atfive different spots of the thinfilms.Fig 1(a) shows the EDAX spectrum The average weight %

of the elements fromfive different spots of the as-deposited thin films with standard values are tabulated as inset of theFig 1(a) The observed extra peaks of other elements like Si, Na, Mg, O, Ca etc in the EDAX spectrum are due to the glass substrate The values

of Cu, Al and S are tabulated after deleting the glass substrate ele-ments The obtained data clearly states that the deposited CuAlS2 thinfilm under this analysis is nearly stoichiometric but slightly rich in aluminium and deficient in sulphur

3.2 Structural analysis

Fig 1(b) shows the XRD patterns of CuAlS2thinfilms taken by the Philips X-pert-MPD X-ray diffractometerðl¼ 1:54056 ÅÞ Here CuKa(1.5405Å) radiation without any filter was used as the X-ray source The step size (2q) employed was 0.050with the default slit setting and receiving slit height of 0.15 mm The scan speed employed was 0.2/sec

All peaks observed on the XRD patterns could be indexed as those of CuAlS2with tetragonal unit cell structure The lattice pa-rameters determined using the Powder e X software from the recorded XRD patterns are: a¼ b ¼ 5.33 Å and c ¼ 10.40 Å They are

in good agreement with the reported values of a¼ b ¼ 5.325 Å and

c¼ 10.390 Å; according to JCPDS Card No 25-0014 Other param-eters like the Miller indices, 2qangle, interplanar spacing (d) and %

d errors for prominent XRD peaks are tabulated inTable 1 The error of 1.57% for %d may be due to the presence of defects arising owing to grain size The X-ray density‘r’ of the as-deposited CuAlS2thinfilm was calculated to be 3.48 g/cm3 This calculated value is in good agreement with the reported value of 3.43 g/cm3 for bulk CuAlS2[29]

The peak broadening in the XRD pattern occurs due to the decrease of crystallite size arising as a result of the dislocation generated lattice strains [25] The crystallite size in the as-deposited CuAlS2 thinfilms was determined from the XRD peak broadening employing Scherrer's formula[30], given by:

where D is the crystallite size, K is shape depending parameter and

is taken here as 1 considering the particles to be spherical in shape,

lis the X-ray wavelength (1.5405Å),bis the angular line width at half maximum intensity, andqis the Bragg angle in degree The value of the crystallite size D was evaluated from the slope

of the Scherrer's plot ofcosq

l versus 1

bhklfor as-deposited CuAlS2thin films and results are shown in Fig 2(a) The graphically and analytically determined crystallite sizes are tabulated inTable 2

The source of strains in the thinfilms is due to the crystalline

imperfections, distortions and dimensional constraints The

dependence of the full width at half maxima (FWHM) on the strain

and grain size is related by the HalleWilliamson relation [31],

which represents the Uniform Deformation Model (UDM) The

materials strain properties are independent of the crystallographic

direction because the strain was assumed to be uniform in all

crystallographic directions

The graph of

4sinq versus

bhkl cosq l

 was plotted for the promi-nent XRD peaks of CuAlS2thinfilms, and is shown inFig 2(b) The slope and the ordinate intercept of thefitted line give the strain and the crystallite size, respectively The positive slope value reveals the presence of tensile strain produced due to the tensile stress This external tensile force tends to increase the inter-atomic distance as observed from the values of the lattice parameters derived from XRD data The origin of the extrinsic stress in a thinfilm comes mainly from the adhesion to the substrate, while the intrinsic stress comes from the defects, such as dislocations in thefilm The results

of the UDM analysis for the CuAlS2 thin films are tabulated in

Table 2 The Hooke's law gives the linear proportionality relation be-tween the stressðsÞ and the strain ðεÞ as

Fig 1 (a) e EDAX spectrum along with inset table of chemical composition; (b) e XRD pattern of a CuAlS 2 thin film.

Table 1

Miller indices, 2qangle, inter-planar spacing (d), and %d error.

Fig 2 (a) e Scherrer's plot, (b) e Plot of the modified form of HalleWilliamson analysis representing UDM, (c) e Plot of the modified form of HalleWilliamson analysis using USDM; (d): Plot of the modified form of HalleWilliamson analysis using UDEDM, and (e) e The SSP plot of CuAlS films.

S.H Chaki et al / Journal of Science: Advanced Materials and Devices 2 (2017) 215e224 217

where Yhklis the modulus of elasticity or the Young's modulus.

Eq.(3)is valid for a significantly small strain Assuming a small

strain to be present in the deposited CuAlS2thinfilms, Hooke's law

can be employed Applying the Hooke's law approximation to

HalleWilliamson relation, the equation is:

bhklcosq



K

D



þs4sinq

lYhkl



(4)

Eq (4) is known as the Uniform Stress Deformation Model

(USDM) For a tetragonal unit cell structure, Young's modulus[32]is

evaluated by the following Eq.(5),

where L¼ al/c, a and c are the lattice parameters; h, k and l are

Miller indices taken from XRD analysis The elastic compliance

constants Sij(m2/N) of CuAlS2were taken from the reported values

[32]and are presented inTable 3

The determined value of the Young's modulus, Yhkl, for the

CuAlS2 thin films having tetragonal unit cell turned out to be

102.52 GPa, which is nearly equal to the reported value 102.13 GPa

[32]and 106.91 GPa[33] An USDM plot of



bhkl cosq l

 versus

 4sinq

lY hkl



for the CuAlS2thinfilms is shown inFig 2(c) The parameters like,

stress calculated from the slope of thefitted line, the strain

calcu-lated using Eq.(3)and crystallite size determined from the

inter-cept are tabulated inTable 2 They are in good agreement with the

values obtained from UDM

Another model known as Uniform Deformation Energy Density

Model (UDEDM) was used to determine the crystallite size, strain

and stress The energy density can also be determined by this

model For an elastic system that follows Hooke's law, the energy

density (u) can be given as[32],

u¼ε2Yhkl

The equation of HalleWilliamson relation[25], can be rewritten

using Eq.(6)as:

bhklcosq

 K D



þpffiffiffiu 4sinq

l

ffiffiffiffiffiffiffiffiffi 2

Yhkl

The plot of bhkl cosq

l versus

4sinq ffiffiffiffiffiffi 2

hkl

q 

of Eq (7) is shown in

Fig 2(d) The square of the slope of thefitted line gives the energy density u and the reciprocal of the y-intercept indicates the crys-tallite size D Then stress and strain were calculated using Eqs.(3) and (6), respectively All the obtained values are tabulated in

Table 2 The value of the crystallite size determined using UDEDM is

in good agreement with the values determined using other models

The grain size and the strain can also be evaluated using the SizeeStrain Plot (SSP) method In this estimation, it was assumed that the crystallite size profile is described by a Lorentzian function and the strain profile by a Gaussian function[32] Hence,

ðdhklbhklcosqÞ2¼K

D

d2hklbhklcosq

2

2

where K is a constant that depends on the shape of the particles; for spherical particles it is taken, e.g as 1 InFig 2(e), the graph of

ðdhklbhklcosqÞ2versusðd2

hklbhklcosqÞ is plotted by using Eq.(8)for the prominent XRD peaks taken on the CuAlS2thinfilms In this case, the crystallite size is derived from the slope of the line and the square root of the y-intercept will give the value of the strain The obtained values are tabulated inTable 2

The grain size (D) and the dislocation density (d) of thefilms were calculated for the preferential orientations to have informa-tion about their crystallinity levels The dislocainforma-tion density (d),

defined as the length of dislocation lines per unit volume of the film, was evaluated by Eq.(9) [34],

The crystallization levels of the as-deposited thinfilms are good because of their smalldvalues derived from the Scherrer's formula,

Table 2

Crystallite size, strain, stress, energy and dislocation density in CuAlS 2 thin films.

Crystallite size D (nm) Strain ε  10 3 Stresss(MPa) Energy density u (kJ/m 3 ) Dislocation densityd 10 4 (nm)2

Table 3

Elastic constants of CuAlS 2 thin films.

Yhkl¼



h2þ k2þ L22

s11

h4þ k4

þ ð2s12þ s66Þh2k2þ ð2s13þ s44Þh2þ k2

the HeW plot, and the SSP plot, and collected inTable 2, which

represents also the amount of defects in thefilms

The TEM and SAED images of the as-deposited CuAlS2thinfilms

are shown inFig 3(a) and (b), respectively The TEM image and

SAED pattern of as-deposited CuAlS2thinfilms were recorded using

the Philips, TECNAI 20 Transmission Electron Microscope The TEM

and SAED samples were prepared by scratch removing the thin

film The scratch removed films were allowed to float on the

distilled water in a petri dish Thefloating thin films were then

swiftly taken on a copper grid The wet copper grid with thefilm samples was dried by keeping it on a piece offiltering paper The copper grid along with sample was then inserted into the electron microscope for TEM and SAED analysis

The TEM image shows that the deposited thinfilm is uniform without any cracks The selected area electron diffraction (SAED) pattern for CuAlS2 thin film (Fig 3(b)), shows a concentric ring pattern along with spots, revealing that the deposited thinfilms are polycrystalline with large grain size in nature The rings were

Fig 3 (a) e TEM image, (b) e SAED pattern, (c): SEM image of large area, (d) and (e) e SEM images of small selected areas, (f) e 2D AFM image; (g) e height profile, (h) e 3D (x-y-z)

e 3D (y-x-z) AFM image of the as-deposited CuAlS films.

S.H Chaki et al / Journal of Science: Advanced Materials and Devices 2 (2017) 215e224 219

indexed as (112), (200), (220), (312), (116) and (400) indices, which

are associated with the tetragonal structure All indexed planes

except the (400) one are in agreement with the XRD data

TheFig 3(c, d and e) present the SEM images of CuAlS2 thin

films deposited on glass substrates at room temperature.Fig 3(c)

clearly shows that the film covers the whole surface of the

sub-strates having a pocketed morphology variation Fig 3(d and e)

show a magnified image of pocketed morphology of the thin films

Fig 3(d) clearly shows the presence of a rod like structure whereas

Fig 3(e) shows beautiful bunches of rods originating from thefilm

surface

The AFM images of the as-deposited CuAlS2 thin films were

recorded by the Nano Surf Easyscan-2 in the tapping mode.Fig 3(f)

shows the two dimensional (2D) andFig 3(h and i) show the three

dimensional (3D) AFM images of the as-deposited CuAlS2 thin

films, respectively The height profile variation is shown inFig 3(g)

TheFig 3(f) shows the 2D image of afilm area of 1.98mm 1.98mm

This 2D image shows clearly the presence of spherical grains having

coalescences between them The Fig 3(h and i) present the 3D

image of afilm area of 256mm 256mm This 3D image shows

obvious structures like hills and mountains having valleys between

them The height profile parameters, illustrated inFig 3(g), taken

along the horizontal line of the AFM images of CuAlS2thinfilms are

tabulated inTable 4 The parameters such as peak p, valley z or v

(Rp-v), root mean square (rms), roughness (Rq) and the average

roughness (Ra) values indicate the roughness in the vertical

di-rection.Fig 3(g) shows the rise in heights at the two ends of the

viewed horizontal scale These heights increase may be due to

presence of bunch of nanorod features at the sites as observed in

the SEM image

3.3 Optical analysis

The optical absorption spectrum, shown inFig 4(a), of CuAlS2

thin films deposited by dip coating has been recorded in the

wavelength range 200 nme3200 nm The spectrum shows high

absorption in the ultra violet range with the absorption edge lying

at 290 nm corresponding to an energy of 4.28 eV

The energy band gap Egwas determined from the optical ab-sorption data using the near-band edge abab-sorption relation, given

by the Eq.(10) [31]below,

ða$h$vÞn¼ Ah$v  Eg



(10)

where, n characterizes the transition For allowed and forbidden direct transitions, n¼ 2 and 2/3 respectively, and n ¼ 1/2 and 1/3 for allowed and forbidden indirect transitions, respectively The ab-sorption coefficient ‘a’ was calculated employing the BeereLambert

Eq.(11) [35e37]

where A is the absorbance of light passing through the sample, t is the path length of light which travels through the CuAlS2thinfilm sample (average thickness of the thinfilm in the measurement was

260 nm)

The analysis of Eq.(10)shows that n¼ 2 and ½ fits well for the as-deposited CuAlS2thinfilms stating that the as-deposited CuAlS2 thinfilms possess direct and indirect allowed optical band gaps The plots of (a$h$n)2versus h$nand (a$h$n)1/2versus h$nare shown

inFig 4(b) The value of the direct allowed optical band gap was determined by extrapolating the straight line portions of (a$h$n)2 versus h$n The obtained value of the direct optical band gap is 3.82 eV for the CuAlS2thinfilms in the present investigation which

is greater than the reported value of 3.49 eV for bulk material[1] This shows that the blue shift occurred due tofilm thickness The value of indirect allowed optical bandgap of 3.11 eV was evaluated

by extrapolating the straight line portions of (a$h$n)1/2versus h$n

for the as-deposited CuAlS2thinfilms

The transmittance (T%) and the reflectance (R%) spectra of the as-deposited CuAlS2thinfilms are shown inFig 5(a) The drop in the transmittance for wavelengths higher than 700 nm may pre-sumably be due to the absorption by free carriers After 1200 nm wavelength, the transmittance is stable and so this material can be utilized as an infrared window The data from the spectra have been used to determine the optical constants of thefilm The refractive index is an important parameter for materials to be used for optical applications In the region of the inter-band transition that has strong absorption, the refractive index of thefilm can be deter-mined by the Eq.(12) [38]below, only when the illuminations of electromagnetic waves are perpendicular to the surface of thefilm,

h¼1þ

ffiffiffi R p

The plots of the refractive index (h) and the extinction coef fi-cient (k¼a$l/4p) versus wavelength (l) are shown inFig 5(b)

Table 4

Surface and line roughness analysis of the AFM profiles.

e Absorbance spectrum, (b) e Plot of direct and indirect band gap of CuAlS films.

The plots show that the refractive index (h) and the extinction

coefficient (k) vary with the wavelength in the range 290e3200 nm

for the as-deposited thin films The variation shows that the

refractive index decreases in the wavelength range of 290 nm to

nearly 700 nm The static refractive indexh(0) determined using

the optical dispersion relationship has been found to beh(0)¼ 1.84

This obtained value is less than the reported value of 2.12[38] This

variation may be due to surface dissimilarity of the as-deposited

thinfilms and the reported investigated thin films The dielectric

constant dependence on frequency is defined by the Eq.(13)below,

whereεrandεiare the real and the imaginary parts of the dielectric

constants, respectively, and these values were calculated using the

formulas of Eq.(14) [39]below,

εrðuÞ ¼ n2ðuÞ  k2ðuÞ and εi¼ 2nðuÞkðuÞ: (14)

The variations of theεrandεivalues of the as-deposited CuAlS2

thinfilms with wavelength are shown inFig 5(c) Theεrvalues are

higher than that ofεivalues

3.4 Electrical analysis

The dce electrical resistivity variation with temperature in the

temperature range from ambient to 423 K was studied on CuAlS2

thinfilms using a four-probe set-up of the Model DFP-02 (Scientific

Equipment& Services, Roorkee, India) Using the measured voltage

while keeping the current constant, the resistivity (r) at each

temperature value was evaluated by taking into consideration the

correction factor The average thickness of the thin films was

327 nm The plots of logrversus 1000/T for as-deposited CuAlS2

thin films are shown inFig 6(a) The resistivity decreases with

increasing temperature, implying the thin film material to be

semiconducting in nature The activation energy determined for

the linear portion of the plot arrived at a value of 0.81 eV, which is

in reasonable agreement with the reported value of 0.70 eV[40]

Hall Effect analysis at room temperature was carried out on the

as-deposited CuAlS2thinfilms by using the Hall Effect setup, model

DHE-22 (Scientific Equipment, Roorkee, India) Graphite conductive

adhesive alcohol-based (Alfa Aesar) paste was used for making the

Ohmic contacts in the van der Pauw geometry The Ohmic nature of

the electrical contacts made on the sample were confirmed by

measuring IeV characteristics between R12,12, R23,23, R34,34 and

R41,41contacts of the thinfilms (seeFig 6(b)) for both polarities in

the current range from5mA toþ5mA

The sample under investigation was kept in an applied magnetic

field which modifies the path of the majority carriers which

pro-duce Hall voltage.Fig 6(c) shows the graph of Hall voltage (V )

versus magneticfield (B) The Hall coefficient (RH), the mobility of charge carriers (mH) and the charge carrier concentration (h) were evaluated employing the standard formulae using the value of the slope of the plot inFig 6(c), the thickness of the samples and the constant measuring current The average thickness of the thinfilms used for the Hall measurement was 318 nm The values obtained are tabulated inTable 5 The positive value of the Hall coefficient implies that the deposited thinfilms are of p-type in nature which was also confirmed by the hot probe method The evaluated carrier concentration of thin films turned out to be in the order of

1016 cm3also revealing the samples to be semiconductors The value for the hole mobility determined from the Hall Effect mea-surement was 4.39 cm2/Vs for the as-deposited CuAlS2thinfilms This value is in good agreement with the reported one (<5 cm2/

V1s1)[40] The variation of the thermoelectric power ‘S’ as a function of temperature was measured on the as-deposited CuAlS2thinfilms using the experimental set up, TPSS-200, (Scientific Solution, Mumbai, India) The average thickness of the thinfilms employed for the thermoelectric power measurement was 298 nm The variation of the potential difference between the two probes at a constant temperature difference (DT) of 7 K was measured in the temperature range from 300 K to 423 K

The determined Seebeck coefficient (S) as a function of the in-verse of temperature (1000/T) is shown in Fig 6(d) for the as-deposited CuAlS2thinfilms The plot shows that the values of the Seebeck coefficient increase with temperature revealing the sem-iconducting nature of the samples[41] The absolute values of the Seebeck coefficient at all evaluated temperatures is positive implying the sample to be p-type in nature, which further uphold the results of the Hall Effect and hot probe methods The p-type nature of the dip coating as-deposited CuAlS2 thin films as confirmed by Hall Effect, the Seebeck coefficient and the hot probe measurements are due to intrinsic acceptor defects arising owing

to metal rich condition[42] The metal rich CuAlS2having more Al compared to Cu as confirmed by EDAX data, leads to AleCu sub-stitutional defect dominance than that of copper vacancy and Cu-Al substitutional defects[42] This substitutional imperfection gives rise to acceptor defects leading to p-type behaviour of the as-deposited CuAlS2 thin films The carrier concentration of

~1016 cm3as obtained in the present study and presented in

Table 5, matches the reported data[42]for Al rich thinfilms and is less by a factor of ~1000 for the Cu-rich CuAlS2 material, thus substantiates Al-Cu defects dominance leading to the p-type na-ture Moreover, Tell et al.[43]stated that more atomic percentage concentration of Al compared to Cu atoms in CuAlS2leads to the p-type behaviour The present metal rich and sulphur deficient as-deposited dip coating CuAlS2 thin films as confirmed by the EDAX data also corroborate to its p-type behaviour The values of Fermi energy (E) and constant (A) were evaluated from the slope

Fig 5 (a) e Transmittance (T) and reflectance (R) spectra, (b) e Plots of the refractive index (h) and the extinction coefficient (k) versus wavelength, (c) e Variation of real and imaginary part of the dielectric constant with wavelength of the as-deposited CuAlS 2 thin films.

S.H Chaki et al / Journal of Science: Advanced Materials and Devices 2 (2017) 215e224 221

and the intercepts of theFig 6(d), respectively Using the value of

‘A’ and the carrier concentration (h) obtained from the Hall Effect

measurements, the scattering parameter (s), the effective density

of states (NA) and the effective mass of holes (mh*) were evaluated

by employing standard equations The calculated values are

tabu-lated inTable 6

The IeV characteristics study on CuAlS2thinfilms was carried

out in dark as well as under white and UV electromagnetic

illu-minations The sample setup for IeV measurement was prepared by

taking as-deposited dip coating CuAlS2thinfilms being deposited

on one side of a rectangular glass substrate of the dimension

35 mm 26 mm The average thickness of the films was 314 nm

The four contacts on the four vertices of the rectangular sample

were made usingflexible thin copper wires bonded with graphite

conductive adhesive alcohol-based (Alfa Aesar) paste The copper

wires used for the contacts were very thin and the silver paste

contacts on the thinfilms were kept minimal with contacts at the periphery of the set-up to avoid blocking of illumination The IeV measurements under white illumination (Philips) was made using

a 4 W lamp providing an illuminating intensity on the sample surface of 6614 Lux, whereas the UV illuminated (Model: UVSL-14P, Ultra-Violet Products Ltd Cambridge CB4 1TG, UK) IeV measure-ment was carried out with a 4 W lamp providing illuminating in-tensity of 31 Lux on the sample surface The inin-tensity was measured with a light Luxmeter (Model: MECO-930, MECO Meters Pvt Ltd., Navi Mumbai, India) The recorded IeV characteristics in dark, under white and UV illuminations are shown inFig 7 In case of dark and white illumination, the sample just behaves as a simple resistor unaffected by the external illumination, thus both IeV plots nearly overlap The results of the UV illumination of the thinfilms show IeV characteristics deviations from those of the dark and white illumination The UV illuminated curve shows that a small

Table 5

The room temperature Hall parameters of as-deposited CuAlS 2 thin films.

Hall coefficient R H (cm 3 /C) Carrier concentrationh(cm3) Hall mobilitym(cm 2 /Vs) Semiconductor type

Table 6

Values of Fermi energy (E F ), constant (A), scattering parameter (s), room temperature Seebeck coefficient (S), effective density of states (N A ) and effective mass (m*) of as-deposited CuAlS 2 thin films.

Fermi energy E F (eV) A Scattering constant (s) S (mV/K) N A (cm3) m h * (kg)

Fig 6 (a) e Logrversus 1000/T plot, (b) e The IeV characteristics between different pairs of contacts, (c) e Hall voltage induced as a function of applied magnetic field, (d) e Plot of Seebeck coefficient (S) versus 1000/T of the CuAlS 2 thin films.

increase in voltage induces a large increase in the current This

indicates that the UV-illumination produces more charge carriers

This relates to the optical energy direct band gap of the deposited

CuAlS2thinfilm being 3.82 eV which matches to the wavelength of

the exciting UV radiation (~325 nm) The observed effect of the

illumination on the CuAlS2thinfilm suggests that this film can be

used as a photovoltaic material for absorption of UV radiation

4 Conclusion

Thin CuAlS2films on glass substrates have been fabricated by

the room-temperature dip coating technique Various

character-ization and analysis measurements, among others including also

optical absorbance, Hall effect, hot probe, dce resistivity, Seebeck

coefficient and optical illumination responses, etc have been

car-ried out to study and determine the structure, morphology and

other intrinsic physical properties of the as-deposited thinfilms

Results obtained have confirmed that the as-deposited thin films

are p-type semiconductors of tetragonal structure with activation

energy of 0.81 eV, a carrier concentration of about 1016cm3and

the band gaps of 3.82 eV and 3.11 eV for the allowed direct and

indirect transitions, respectively All measurements and analyses

have consistently revealed the p-type semiconducting nature of the

as-deposited thin films and suggested that these materials are

potentially applicable for absorption of the ultra-violet radiations

Acknowledgements

Two of the authors (SHC and MPD) are thankful to the Gujarat

Council on Science and Technology (GUJCOST), Gandhinagar for

providingfinancial assistance through Research Project; vide letter

Nos GUJCOST/MRP/2016-17/433 dated 27/06/2016 & GUJCOST/

MRP/16-17/300 dated 20/06/2016 for carrying out this research

work One of the authors, TJM, is thankful to University Grants

Commission (UGC), New Delhi for the award of Maulana Azad National Fellowship (MANF) to carry out this research work References

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