Properties of transparent conducting quaternary silver indium tin oxide thin films crystallized with delafossite structure

Materials Chemistry and Physics 199 (2017) 591e596

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Properties of transparent conducting quaternary silver indium tin oxide thin films crystallized with delafossite structure K. Keerthi a, Bindu G. Nair a, M.D. Benoy b, Rakhy Raphael a, Rachel Reena Philip a, * a b

Thin Film Research Lab, U.C. College, Aluva, Kerala, India Department of Physics, Mar Athanasius College, Kothamangalam, Kerala, India

h i g h l i g h t s
The lowest substrate temperature for crystallization of AISO reported till date.
Variations in electrical conductivity is related with the crystallite orientation.
Tuning of band gap is obtained by varying the silver content in the AISO films.
Sn incorporated AIO films are found to have better electrical conductivity.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 March 2017 Received in revised form 16 June 2017 Accepted 13 July 2017 Available online 17 July 2017

A study on crystalline silver indium tin oxide (AISO), a quaternary compound formed by tin incorporation in delafossite AgInO2 is briefed here. X-ray diffraction for structural characterization combined with energy dispersive analysis of X-rays for composition assessment confirms the formation of crystalline thin films with the rhomb-centered rhombohedral delafossite crystalline lattice. The variation in electrical conductivity (101e102 S/cm) and activation energy are correlated with carrier concentration and caxis orientation induced by stoichiometric changes. A band gap tuning from 2.38 eV to 3.23 eV is achieved by the stoichiometry changes. © 2017 Elsevier B.V. All rights reserved.

Keywords: Thinfilms Delafossite Crystallite orientation Bandgap tuning

1. Introduction Delafossite Transparent Conductive Oxides (TCO’s) have a prominent role in the fabrication of transparent semiconductor devices such as built in solar cells on glass window panes, transistors, active matrix liquid crystal displays, touch screen displays and inorganic and organic light emitting diodes in solid-state lighting [1e5]. Wide investigations are being carried out to improve the electrical properties and crystal quality of the TCO films with the aim of fabricating transparent pn junctions that can turn out to be a milestone in oxide electronics. While extensive research has been carried out on ternary n-type and p-type delafossite TCO’s like CuAlO2, CuGaO2, AgGaO2 and AgInO2 [6e9], very few studies are done on quaternary oxides [10e12]. The literature reports suggest low conductivity that fall in the

* Corresponding author. E-mail address: [email protected] (R.R. Philip). http://dx.doi.org/10.1016/j.matchemphys.2017.07.058 0254-0584/© 2017 Elsevier B.V. All rights reserved.

range 105e102 S/cm for crystalline undoped and ~100e101 S/cm for tin doped AIO [9,13,14] and all these studies report that the deposition of crystalline AIO’s or tin doped AIO’s need substrate temperatures above 723 K, which necessitates requirement of substrates like a-Al2O3 (0001) single-crystal substrates that could withstand high temperatures for their deposition. This limits their applications in devices due to the high cost and the shortage in availability of such substrates. Further, a survey through literature shows that studies correlating structure, composition, optical and electrical properties on quaternary AISO are not available till date. A few studies available are on the thermo electric properties of delafossite-type layered oxides AgIn1xSnxO2 that consist of alternating layers of Ag and In1xSnxO2 as well as on DC automotive applications using AISO and tin doped AIO [13e16]. In our previous paper on AIO, deposition of highly conducting AIO films at a substrate temperature of 623 ± 5 K by a two stage process has been reported [17]. The same method is extended in the present study to the successful deposition of crystalline AISO thin

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films. Here, in the first stage, simultaneous evaporation of the pure elements (Ag, In and Sn) in vacuum keeping the substrates at room temperature is done. This is followed by post air annealing treatment at 623 ± 5 K. It is noteworthy that this deposition temperature is ~100 K less than the substrate temperature reported till date for successful crystallization of AISO and hence is carried out on soda lime glass substrates which are cheaper and more available in comparison. 2. Experimental procedure Silver indium tin oxide thin films are prepared by a two stage process, where in the first stage, simultaneous evaporation of 99.999% silver, 99.99% indium and 99.99% tin from electrically heated molybdenum boats is carried out in a multisource vacuum evaporation unit. The evaporation is done at a constant optimized flux onto sodalime glass substrates kept at room temperature, after securing a vacuum of 105 Torr in the unit. The films so prepared at room temperature are then air annealed in a muffle furnace. The annealing of samples are done for a duration of 1hr after raising the substrate temperatures to 623 ± 5 K. For the structural characterizations, a Bruker D8 advance X-ray diffractometer with Cu Ka radiation as source is used. A Carl Zeiss Sigma FE-SEM equipment with an EDAX instrument, is used for morphological and compositional characterization. The room temperature electrical conductivity, photosensitivity and the conductivity variation with temperature are monitored through a Keithley 2611. 2611A source meter coupled with a quartz halogen lamp is used as the source radiation for the photocurrent measurement in white light. Hall measurements are done using Ecopia Hall Effect Measurement System (HMS-3000). Thickness of the films have been determined by Stylus method. A Cary 60 UVeVis spectrophotometer has been used to obtain the optical spectra of films.

3. Results and discussion The X-ray diffraction patterns of ternary silver indium oxide and quaternaries with Ag/In þ Sn ratio varying from silver deficient to silver rich as 0.8, 1.1, 1.3 and 1.5 (within an error limit of ±0.05) respectively named as AIO, AISO 0.8, AISO 1.1, AISO 1.3 and AISO 1.5 are given in Fig. 1. The analysis of the data endows the quaternary films with rhomb-centered rhombohedral structure similar to that of delafossite ternary AIO (JCPDS 21e1077). Although the peak positions of the quaternary are found to be in line with that of the ternary AIO, a variation is observed in the preferential orientation of the grains as the Ag/(In þ Sn) at % is varied from 0.8 to 1.5 in the quaternaries (determined by EDAX measurements). Fig. 2 indicates the EDAX spectrum of AIO and AISO films. Since studies on delafossites like CuInO2 reports a correlation between electrical properties and orientation, this is taken note of here [18]. The AISO 0.8 films show a strong c-axis orientation while AISO 1.5 films exhibit a (009) peak of relatively low intensity. No binaries or any elemental impurity peak could be traced in these samples. The as prepared films of AgInSn before subjecting to the annealing process appear highly reflective and are metallic with electrical conductivity ~104 S/cm. After the annealing of the films at a temperature 623 ± 5 K, the appearance is found to be changed and transparent films are obtained. The formation of AISO is corroborated by the compositional, optical and electrical characterizations that are discussed in the later sections. The determination of lattice parameters (Table 1) from the XRD shows a and c parameters to be almost matching with that of ternary AIO indicating that incorporation of Sn results in substitution of tin atoms at the cation indium site, thus keeping intact the delafossite structure. However, Ag deficient AISO’s show slight lattice relaxation when compared to lattice parameter given in JCPDS file no. 21e1077. The crystallite size (D) is calculated using the Scherrer’s formula

Intensity (arb.units)

(104) (015) (009)

AISO 1.5

(015) (104)

AISO 1.3

(104) (015) AISO 1.1 (009) (104)

20

(116)

(101) (104)

(015)

30

40

AISO 0.8

AIO

50

2 (degrees)

60

70

80

Fig. 1. XRD patterns of the ternary AIO and the quaternary AISO with Ag/In þ Sn ratio 0.8, 1.1, 1.3 and 1.5 named respectively as AISO 0.8, AISO 1.1, AISO 1.3 and AISO 1.5.

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Fig. 2. EDAX spectrum of AIO and AISO film.

D ¼ 0:9l=b cos q

band gap, lies in the range 2.38 eVe3.23 eV which indicates that fine tuning of band gap is possible by stoichiometric changes of the compound. Here, it is to be noted that a decrease in bandgap is observed for AISO with increase in Ag content and the band gap of the quaternary falls between the band gaps reported for ternaries AgInO2 and AgSnO2 as expected. The effect of Ag content to fine tune the band gap has been reported in the case of tungsten oxide thin films by R. Jolly Bose et al. [24] where they observed a decrease in band gap with Ag incorporation. The transmittance of the films varies from 36% to 65% in the visible range for the films, with the maximum transmittance of 65% corresponding to the nearly stoichiometric quaternary (AISO 1.1). It is to be noted that the transmittance of quaternary is higher than that of ternary AIO prepared by this method. Fig. 4d indicates the Urbach energy graph. Urbach energy (EU) arises due to several factors such as the structural defects, compositional fluctuations, inhomogeneous exciton, and thermal effects [25]. Urbach energy is determined from Urbach empirical rule, which is given by the exponential equation:

(1)

where l is the X-ray wavelength, b is the Full Width at Half Maximum (FWHM) of the peak at the diffraction angle q. The grain size calculated by scherrer formula is found to fall within the range 22 nme28 nm for all the samples. This indicates the nano grain formation in these polycrystalline films. The SEM images of AIO and AISO films are shown in Fig. 3. The micrographs reveal close packing of particles that are formed by agglomeration of nanograins in all the AISO films while the agglomerated particles of AIO are found to be larger in size. Fig. 4a depicts the (ahn)2 versus hn graphs and optical transmittance graphs for AISO and AIO films. The band gap variation for films with different Ag/In þ Sn ratio are evaluated from Fig. 4a (inset). Since, as per reports, the direct band gaps of binary and ternary compounds are like Ag2O (~1.2 eV) [19], SnO (~3.7eV) [20], InO2 (2.6e3.2 eV) [21], InSnO (~4.2 eV) [22], AgSnO2 (2.73e3.22 eV) [23] and AgInO2 (2.0e4.4 eV) [9], the combination of these binaries to form quaternaries ought to enable band gap tuning ranging from 1.2 eV to 4.2 eV. Here the direct band gaps of AISO as estimated using the equation

ahn ¼ A hn  Eg


1



a ¼ a0 exp

(2)

2

hn EU

 (3)

where a is the absorption coefficient, (hn) is the incident photon energy and EU is the band tail width (Urbach energy) of the

where A is a constant, hn is the photon energy and Eg is the optical

Table 1 Lattice parameters, room temperature conductivity, optical band gap, carrier concentration and mobility of AIO and AISO thin films.

AIO AISO AISO AISO AISO

0.8 1.1 1.3 1.5

Composition Ag/In þ Sn

a in A0 ±0.001

c in A0 ±0.001

Conductivity S/cm

AgInO2 0.8 1.1 1.3 1.5

3.319 3.314 3.319 3.319 3.319

19.485 19.683 19.485 19.485 19.485

3 6 6 7 2

    

101 102 101 101 102

Eg (eV) ±0.03

Carrier concentration /cm3

Mobility cm2/Vs

3.46 3.23 2.86 2.40 2.38

2.60  1018 1.024  1020 7.954  1019 1.128  1019 7.605  1019

6.97 3.66 4.71 3.87 1.64

    

101 101 100 101 101

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Urbach energy and thereby large disorder and defects in AISO may be due to the presence of an additional impurity atom (replacement of In with Sn) in the crystal lattice. Increase of Ag-content in the thin films of AISO naturally increases the disorder in the structural bonding as more defects are being created. The disorder and defects can introduce localized states at or near the conduction band level, which leads to increase the Urbach energy EU [26,27]. Here, AISO 1.5 has maximum Urbach energy indicating maximum disorder. The room temperature electrical conductivity of the AIO and AISO samples are in the range of 101e102 S/cm and the hot probe and hall effect measurement show all the films to be n-type. Hall measurements are carried out in van der pauw configuration with a constant magnetic field of 0.54 T. Measured value of carrier density and mobility of the films are given in Table 1. The equations to determine carrier concentration (n) and mobility (m) are given below

Carrier concentration; n ¼

Hall Coefficient; RH ¼

1 RH e

Rt B

(4)

(5)

B is the constant magnetic field and t is the thickness. Fig. 3. SEM images of AIO and AISO films.

localized states in the optical energy gap. It is observed that, the values of EU increases as 1.39 (AIO), 1.48 eV (AISO 0.8), 2.19 eV (AISO 1.1), 3.88 eV (AISO 1.3) and 4.29 eV (AISO 1.5) with compositional variations. It is seen that AIO has low Urbach energy and thereby low disorder and defects compared to AISO. The high value of

Mobility; m ¼

s ne

(6)

s is the conductivity of the film. Increase in the conductivity is observed for tin incorporated AIO films with maximum conductivities for AISO 0.8 and AISO 1.5. This could be attributed to the increased amount of carrier concentration of AISO films when compared to that of ternary, a possible

Fig. 4. a. Optical band gap of AISO films, (inset) Ag/(In þ Sn) at % Vs band gap graph, b. Transmittance graph of AISO films, c. Optical band gap of AIO film, (inset) Transmittance graph of AIO thinfilm. d. Urbach energy Graph of AIO and AISO.

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Fig. 5. a. High temperature conductivity of AISO thinfilms, Fig. 5b. High temperature conductivity of AIO thin film (inset) photosensitivity of AIO thin films.

reason being that addition of Sn4þ in the place of In3þ releases more electrons for conduction. The electrical conductivity variation with temperature has been done to check whether defect formation has produced any degeneracy in the films. Ln s Vs 1000/T graph in Fig. 5a for AISO and Fig. 5b for AIO indicate a non-degenrate semiconducting nature for the films. The activation energies are calculated using linear fit to equation Ea

s ¼ s0 e kB T

energy from 0.4 to 0.1 eV. Moreover, it’s observed that the sample with highest c-axis orientation (AISO 0.8) has the least value of activation energy, corroborating the earlier reports [18,28] on Cubased delafossites where they attribute it to the increasing alignment of the BO6 layer along the current direction. It is also noteworthy that the samples with more number of crystallites having caxis orientation show maximum conductivity which is about one order greater than that without any c-axis orientation, which supports the ease of movement of carriers due to the particular alignment along BO6 in such samples, in addition to increase in concentration of carriers. Further, it is evident that the introduction of defects on incorporation of Sn in AIO to produce Ag deficient (in AISO 0.8) as well as Ag excess (in AISO 1.1 to 1.5) AISO has induced a narrowing of band gap in the quaternary. Defect formations can produce band gap narrowing but at this point the exact defect causing this is not predictable although activation energy indicates the donor states to

(7)

where s is the electrical conductivity at temperature T, s0 is the exponential prefactor, kB is the Boltzmann constant and Ea is the thermal activation energy. It is observed that the activation energy observed for the tin doped samples vary from 0.18 eV to 0.49 eV. This is in tune with the observations by Singh et al. [28] in tin doped CuInO2 delafossites where they reported a variation in activation

Photoconductivity (x 101 -1cm-1)

59.6

c

AISO 0.8

59.4

5.68

c

AISO 1.1

5.64 59.2

5.60

59.0

a b 58.8

0

d

4

8

12

16

7.3

20

24

c

5.56

a b

5.52

0

19.8

4

8

12

16

c

AISO 1.5

AISO 1.3

7.2

d

19.6

7.1

19.4

7.0

d

a b 0

10

20

30

40

19.2 19.0 -2

Time (min)

d

b

a 0

2

4

Fig. 6. Photoconductivity of AISO thinfilms under visible light.

6

8

10 12 14

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be moving closer to the conduction band (from 490 meV to 284 meV for AISO 1.3 to AISO 1.5) with increased amount of Ag in Ag rich AISO while the donor state in Ag deficient (182 meV) is much closer. Here optical band gap is found to have an inverse relationship to conductivity as reduction in optical band gap and activation energy is associated with increase in conductivity in the case of Ag rich AISO. Fig. 6 indicates the photoconductivity versus time graph of the AISO films to white halogen light illumination, where ab represents dark current, bc is under illumination and cd represents irradiation cut off region. It is found that both AISO and AIO (Fig. 5b inset) films are photosensitive under visible light which suggests their suitability in optoelectronic and transparent electronic applications. 4. Conclusion Transparent conducting delafossite AISO thin films of good electrical conductivity and transparency are deposited at a temperature 623 ± 5 K, the lowest substrate temperature for crystallization of AISO reported till date. Variations in activation energy and electrical conductivity is correlated with the crystallite orientation of AISO thin films. Sn incorporated AIO films are found to have better electrical conductivity ~101e102 S/cm than the ternary AIO and possess photosensitivity. Tuning of band gap in the range of 2.38 eVe3.23 eV is obtained by varying the silver content in the films. Acknowledgement The authors would like to acknowledge Dr.S.N.Potty and P.Prabeesh C-MET, Thrissur for Hall Measurments and RRP acknowledges Department of Science and Technology (SR/S2/CMP-36/ 2012), India for their funding through a major project. References [1] William C. Sheets, Evan S. Stampler, Mariana I. Bertoni, Makoto Sasaki, Tobin J. Marks, Thomas O. Mason, Kenneth R. Poeppelmeier, Silver deafossite oxides, Inorg. Chem. 47 (2008) 2696e2705. [2] Tadatsugu Minami, New n-type transparent conducting oxides, Mrs. Bull. 25 (2000) 38e44. [3] J. Robertson, R. Gillen, S.J. Clark, Advances in understanding of transparent conducting oxides, Thin Solid Films 520 (2012) 3714e3720. [4] Dean Y. Shahriar, Natasha Erdman, Ulrika T.M. Haug, Matthew C. Zarzyczny, Lawrence D. Marks, Kenneth R. Poeppelmeier, Direct synthesis of AgInO2, J. Phys. Chem. Solids 64 (2003) 1437e1441. [5] Shuxin Ouyang, Naoki Kikugawa, Chen Di, Zhigang Zou, J. Jinhua Ye, A systematical study on photocatalytic properties of AgMO2 (M¼ Al, Ga, In): effects of chemical compositions, crystal structures, and electronic structures, J. Phys. Chem. C 113 (2009) 1560e1566. [6] A.N. Banerjee, K.K. Chattopadhyay, Size-dependent optical properties of sputter-deposited nanocrystalline p-type transparent CuAlO2 thin films, J. Appl. Phys. 97 (1-8) (2005) 084308. [7] Yoshihiko Maruyama, Hiroshi Irie, Kazuhito Hashimoto, Visible light sensitive photocatalyst, delafossite structured a-AgGaO2, J. Phys. Chem. B 110 (2006) 23274e23278.

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