Study of spin coated high antimony content Sn–Sb oxide films on silica glass

MA TE RI A L S CH A R A CT ER IZ A TI O N 59 ( 20 0 8 ) 5 7 8–5 8 6

Study of spin coated high antimony content Sn–Sb oxide films on silica glass L.K. Dua, A. De 1 , S. Chakraborty, P.K. Biswas⁎ Sol–Gel Division, Central Glass and Ceramic Research Institute, 196, Raja S. C. Mullick Road, Kolkata-700 032, India

AR TIC LE D ATA

ABSTR ACT

Article history:

Antimony doped tin oxide (ATO) films of 326–1714 ± 3Å physical thickness were deposited

Received 13 October 2006

onto silica glass substrate by the sol–gel spinning technique. The chosen atomic ratios of Sn:

Received in revised form

Sb was varied as 90:10, 70:30, 50:50 and 30:70. Nanostructured surface feature was observed

23 April 2007

in the SEM micrographs of ATO films of 10 to 30 at.% Sb. The nanocluster size was dependent

Accepted 23 April 2007

on antimony content. Cassiterite phase of SnO2 was observed at low content of Sb. The films were absorbing in the UV region. Two direct band gaps and one indirect band gap for each

Keywords:

system were evaluated from their absorption spectra. The two direct band gap values were

Antimony doped tin oxide

in the range, 3.73–5.20 eV while the indirect band gap values were in the range, 2.54–3.46 eV.

Sol–gel

In the case of single layer system, Moss–Burstein shift in both direct and indirect band gaps

Band gap

was observed with increase in at.% of Sb content. Electrical resistivity of the films was in the

Thermal emissivity

range, 1.19 × 10− 3 to 155.59 × 10− 3 Ω cm. Minimum resistivity was obtained for 30 at.% Sb. Transmissivity of the films in the visible region was in the range, 80–97%. Total thermal emissivity (λ range, 5.0–20.0 μm) values were in the range 0.78–0.86. © 2007 Elsevier Inc. All rights reserved.

1.

Introduction

Tin dioxide (SnO2) film has been widely used as a transparent conducting oxide (TCO) material for various applications such as solar cells, liquid crystal displays, optoelectronic devices, heat mirrors, gas sensors etc. [1–4]. Electrical conductivity of the film usually increases significantly if a small amount of suitable dopant (Sb, Cd etc.) be incorporated and this does not interfere optical transparency of the material. On the contrary, addition of large amount of dopant diminish the conductivity, optical transparency and crystallinity of the films [5] for a number of factors such as formation of mixed phases of host and dopant oxides, creation of different types of defect centers, generation of amorphous and crystalline states of the oxides, creation of disorder in the matrix [6] which governs the optical band gap of the materials. The conventional techniques such as CVD,

PVD have been followed widely [2,5,7] to study the effect of high dopant concentration on structural, optical and electrical properties of the doped material. But little work [8–10] has been done on the sol–gel based antimony content Sn–Sb oxide (ATO) film. The sol–gel processing exhibits good uniformity and better-controlled composition [11] for thin film deposition by the spinning method. Hence, this work is focused on sol–gel spin coated ATO film with wide variation of at.% of Sb because most of the works restrict dopant level up to 10 at.% or mol% of Sb with respect to SnO2 [11–14]. Wettable precursor solutions for Sn–Sb oxide films have been prepared starting with SnCl4, 5H2O and SbCl3. The atomic ratio of Sn:Sb has been varied up to a maximum of 70 at.% Sb. The developed films were characterized to investigate the effect of Sb content on surface morphology, optical band gap, electrical, and thermal emissivity properties of the films.

⁎ Corresponding author. Tel.: +91 33 2483 8082x3303; fax: +91 33 2473 0957. E-mail addresses: [email protected], [email protected] (P.K. Biswas). 1 Presently in Department of Chemistry, Krishnath College, Berhampore-742101, Murshidabad, West Bengal as Lecturer. 1044-5803/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2007.04.017

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2.

Experimental

3.

2.1.

Preparation of Precursors

Physical thickness of the ATO films was measured ellipsometrically (Gaertner L116 B, USA) at 632.8 nm. Thermogravimetric analysis (TGA) was done by Shimadzu make (Japan) T-50 thermal analyzer. Infrared (IR) study of the isolated precursor powders at different temperatures was done by using Thermo electron corporation make Nicolet 5700 FTIR. Phase identification was made with a Philips (Expert pro) X-ray diffractometer with CuKα radiation. Transmission and absorption spectra were recorded by UV–VIS–NIR spectrophotometer (Shimadzu, UV3101 PC, Japan). Microstructure was studied utilizing a scanning electron microscope (LEO S 430 I) and the elemental presence in the film was identified by EDX analysis using the same equipment calibrating with cobalt standard. The directional hemispherical transmittance (Th) and reflectance (Rh) of the coated glass in the wavelength range 5–20 μm was determined at ambient temperature by the integrating sphere (IS) method utilizing Thermo Nicolet, 5700 FTIR coupled with Labsphere make I.S. The I.S. has two apertures (Fig. 1). For measuring reflectance, the sample is placed at one aperture (position A) while the incoming radiation passed through the aperture B and reflected from the sample. In the case of measuring transmittance the incoming radiation is passed through the same aperture A where the sample has been placed and the aperture B is kept closed by reflection standard. The directional hemispherical spectral emittance (ελ) was calculated from the Eq. (1).

Sols corresponding to 6 wt.% equivalent SnO2–Sb2O3 maintaining Sn:Sb atomic ratios as 90:10, 70:30, 50:50 and 30:70 (corresponding sol designations L1, L2, L3 and L4) were prepared in alcoholic medium. Required amount of recrystallized hydrated stannic chloride (Loba Chemie) was dissolved in a solvent mixture (1:1, by volume) of ethyl alcohol (dehydrated, Bengal Chemicals and Pharmaceuticals Ltd., India) and 1-propanol (E-merck India Ltd., for synthesis) and stirred for 15 min. Similarly, required amount of Antimony (III) chloride (E-merck India Ltd., purity ∼ 98%) was dissolved in the same solvent mixture and stirred separately for 15 min. Next, the antimony chloride solution was added to the tin chloride solution dropwise. Required amount of 10.5 (M) HCl was then added to the precursors for dissolution of SbCl3 maintaining the mole ratio of water:SbCl3 = 1:1. The solution was stirred for 1 h and aged for 48 h in ambient air before coating.

2.2.

Preparation of Films

The ATO films (Table 1) were deposited on Heraus, Germany make pure silica glass substrate (suprasil grade, dimensions, 25 mm × 25 mm) using the precursors of different Sn:Sb ratios by using the spinning (rpm, 1500) technique (Convac, 1001). Sample No. L11, L12, L13 correspond to the films obtained after one, two, three layer depositions respectively from L1 sol. Similarly, L21, L22, L23 are derived from L2 sol, L31, L32, L33 are derived from L3 sol and L41, L42, L43 are derived from L4 sol. After each deposition the films were cured in air at 480 °C for 0.5 h and the whole process was repeated several times to increase the physical thickness of the films.

Characterization of Films

ek ðk; h ¼ 0-; T ¼ Tamb Þ ¼ 1  Rh ðk; h ¼ 0-; T ¼ Tamb Þ

ð1Þ

 Th ðk; h ¼ 0-; T ¼ Tamb Þ

where θ = angle of incidence, T = temperature in Kelvin, Rh and Th are directional hemispherical reflectance and transmittance respectively.

Table 1 – Sample designation, number of layer/operation, physical thickness, direct and indirect band gaps, %T at 0.55 μm, % R at 10 μm and thermal emissivity of ATO films of different Sn:Sb ratios Sample no.

L11S L12S L13S L21S L22S L23S L31S L32S L33S L41S L42S L43S

Number Thickness Sn:Sb in Sn:Sb of (Å) (± 3) precursor (from operation/ EDX) layer

1 2 3 1 2 3 1 2 3 1 2 3

953 1362 1714 421 852 1256 326 614 1005 345 574 965

90:10

97:03

70:30

88:12

50:50

76:24

30:70

47:53

Band gap (Eg) (eV) Direct hν range (2.5– 5.0 eV) (Ebulk)

hν range (4.0– 7.0 eV) (Econf)

4.22 4.35 4.24 4.16 4.22 4.22 4.13 4.17 4.17 3.73 3.80 3.93

5.05 5.11 5.20 5.08 5.09 5.16 5.04 5.04 5.18 4.22 4.41 4.48

Indirect (Edef)

3.19 3.39 3.41 2.89 3.46 3.42 2.77 3.44 3.33 2.54 2.70 3.16

%T at %R at 0.55 μm 10 μm

92.6 90.5 96.6 94.3 90.4 92.5 93.2 84.4 85.3 91.7 86.5 81.5

22.1 25.3 30.5 22.3 26.5 27.9 22.1 24.2 25.1 20.6 20.6 20.6

Total thermal emissivity (ε) (wavelength range; 5– 20 μm) 0.85 0.84 0.78 0.85 0.84 0.82 0.86 0.85 0.84 0.85 0.85 0.85

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Fig. 1 – Measurement set up with IS (integrating sphere). The inner surface of the IS is coated with high reflecting gold sheath, which reflects the radiation diffusely; (A) measurement of the directional hemispherical reflectance and (B) measurement of the directional–hemispherical transmittance.

For nontransparent samples Th = 0 and then the directional hemispherical emittance, ελ can be calculated from Eq. (2).

It is not unlikely to generate similar species by the reaction of SbCl3 with water molecule and solvents.

ek ðk; h ¼ 0-; T ¼ Tamb Þ ¼ 1  Rh ðk; h ¼ 0-; T ¼ Tamb Þ:

SbCl3 þ H2 O ¼ SbðOHÞCl2 þ HCl

ð8Þ

SbCl3 þ C2 H5 OH ¼ SbðOC2 H5 ÞCl2 þ HCl

ð9Þ

SbCl3 þ C3 H7 OH ¼ SbðOC3 H7 ÞCl2 þ HCl

ð10Þ

ð2Þ

As silica glass is not transparent in the infrared region (05– 20 μm), Eq. (2) will be valid for determination of thermal emissivity of glass. Total thermal emissivity, ε (T) in the wavelength range (a, b) was evaluated by using Planck's equation (Eqs. (3) and (4)). Rb eðTÞ ¼

a

ek ðTÞIk ðTÞdk Rb a Ik ðTÞdk

ð3Þ

Ik ðTÞ ¼

2khc2 =k5 ½expðhc=kTkÞ  1

ð4Þ

where, Iλ is the spectral emittance of energy emitted from an object (say, film) with temperature above 0 K; h, the Planck's constant; c, the speed of light; k, Boltzman constant; T, temperature in Kelvin and λ, wavelength in meter. Hall mobility (μ), free electron carrier concentration (N) and resistivity (ρ) in a magnetic filed of 0.51 T (Tesla) of the samples were measured at room temperature by HEM 2000 (EGK Corporation, Korea) using four probe van der Pauw method.

4.

Results and Discussion

The precursors of different Sn–Sb contents were prepared from their metal salts. There may be a number of constituents (Eqs. (5)–(7)) [15,16] of Sn(IV) in the precursors as the system has water molecule as the water of crystallization, ethyl alcohol (C2H5OH) and n-propyl alcohol (C3H7OH) as solvents. SnCl4 þ H2 O ¼ SnðOHÞCl3 þ HCl

ð5Þ

SnCl4 þ C2 H5 OH ¼ SnðOC2 H5 ÞCl3 þ HCl

ð6Þ

SnCl4 þ C3 H7 OH ¼ SnðOC3 H7 ÞCl3 þ HCl

ð7Þ

When these precursors were used for deposition of layers, it is expected that the Sn(IV) compounds transformed to SnO2 (Eqs. (11)–(13)) on thermal curing while major part of the metalloorganic Sb(III) compounds and even the generated antimony oxides [17,18] sublimed (Eqs. (14)–(16)) as we obtained only about 3 at.% Sb in the case of films derived from Sn:Sb = 90:10 precursor (Table 1). IR study of the powder samples (not shown here) obtained by calcination at different temperatures as per thermogravimetric analysis revealed disappearance of Sb–O asymmetric stretching frequency [19] at 120 °C which suggests about the sublimation of metalloorganic Sb(III) compounds. The TGA also shows a number of reaction steps from gel to oxide transformation. SnðOHÞCl3 þ H2 O ¼ SnO2 þ 3HCl

ð11Þ

SnðOC2 H5 ÞCl3 þ H2 O ¼ SnO2 þ C2 H5 Cl þ 2HCl

ð12Þ

SnðOC3 H7 ÞCl3 þ H2 O ¼ SnO2 þ C3 H7 Cl þ 2HCl

ð13Þ

SbðOC2 H5 ÞCl2 þ H2 O→SbðOC2 H5 ÞCl2 ↑

ð14Þ

SbðOC3 H7 ÞCl2 þ H2 O→SbðOC3 H7 ÞCl2 ↑

ð15Þ

SbðOHÞCl2 þ 1=2H2 O ¼ 1=2Sb2 O3 þ 2HCl

ð16Þ

An attempt has been made to understand the compositional effect of the films by the quantitative EDX analysis. Fig. 2 depicts the EDX spectra of the films, L13, L23, L33 and L43, obtained after third layer deposition. Appearance of the

M A TE RI A L S CH A RACT ER IZ A TI O N 59 ( 20 0 8 ) 5 7 8 –5 8 6

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Fig. 2 – EDX spectra of ATO films deposited on silica glass substrate from precursors of different Sb contents. The characteristics L lines of Sn and Sb:1 (Sn L1); 2 (Sb Ln + Sn Lα1); 3 (Sb Lα1 + Sn Lβ1); 4 (Sb Lβ1 + Sn Lβ2); 5 (Sb Lβ2 + Sn Lγ1).

peaks at 3.045 keV (Sn L1), 3.444 keV (Sn Lα1), 3.905 keV (Sn Lβ2), 4.101 keV (Sb Lβ2) and 4.348 keV (Sb Lγ1) confirmed the presence of both Sb and Sn elements in the film. Area under Sb L lines has been apparently increased with increase in Sb content in the films. Table 1 shows how much Sn:Sb is present in the films. This would corroborate the reaction steps as discussed earlier (detailed study is in progress). Film thickness of any particular Sb concentration increased with number of layers (Table 1). As for example, in case of Sn: Sb = 90:10 composition, thickness of the films increased from

953 Å (L11) to 1362 Å (L12) and finally to 1714 Å (L13) for successive deposition of layers. X-ray diffraction analysis of the films of relatively high thickness (L13, L23, L33 and L43) containing different at.% Sb suggested the presence of tetragonal cassiterite SnO2 phase (Fig. 3). Reflections from the (110), (101), (200) and (211) planes of tetragonal SnO2 [20,21] were mainly observed which diminished with increasing Sb content (≥30 at.%). Adhesion of successive layers is supported from the change of (110) shoulder in L22 to sharp peak in L23 sample. Characteristic X-ray feature for single layer deposition

Fig. 3 – X-ray diffraction patterns of ATO films deposited on silica glass substrate utilizing the sols of different Sn:Sb ratios.

Fig. 4 – Absorption spectra of ATO films deposited on silica glass substrate utilizing the sols of different Sn:Sb ratios; (a) 90:10, (b) 70:30, (c) 50:50, (d) 30:70.

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from L2 sol was not shown as the diffractogrammes appeared as X-ray amorphous. For this particular Sb concentration a distinct 2θ line appeared at 45°. This was possibly from the (211) plane of tetragonal Sn metal [21] because if the film contains a composite oxides of Sn and Sb along with the Sn and Sb metals then no other entity than Sn metal would be responsible for the reflection at 2θ ≈ 45°. As we have observed only (211) plane of tetragonal Sn it may be inferred that Sn metal has an orientation effect along (211) plane. But the intensity of the reflection line is not considerably high in support of orientation effect. This was possibly due to the formation of relatively low content of Sn metal. If the content of Sb be N30 at.%, then metallic tin could not be detected by XRD. The amorphous nature of the films at this stage could occur due to formation of solid solution of Sn–Sb oxides. The ATO films of four different compositions do not show any significant absorption in the visible region (Fig. 4), but sharp absorption was observed below 350 nm. Absorption coefficient (α) of the films were evaluated from the optical

density data and the direct and the indirect band gaps of the films were determined using α in Eq. (5) [22] 1

ðahmÞn ¼ Aðhm  Eg Þ

ð17Þ

where hν is the incident photon energy, A is a constant and Eg is the band gap of the material and the exponent n depends on the type of transition; n = 1/2, 2, 3/2, and 3 corresponding to allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions respectively. In the present case, two types of plots, (i) (αhν)2 versus hν when n = 1/2 and (ii) (αhν)1/2 versus hν when n = 2 were done for direct and indirect band gap evaluation respectively. Linear extrapolation of the parabolic curve to hν axis resulted in two direct band gap values (Fig. 5, Table 1). Out of the two different band gap values, one is in the relatively lower photon energy (2.5–5.0 eV) region and the other one is in the higher photon energy (4.0–7.0 eV) region. The band gap values (3.73–4.35 eV) in the low photon energy (2.5–5.0 eV) (Fig. 6) region of single

Fig. 5 – Plot of (αhν)2 versus hν to evaluate direct band gaps in the photon energy range; 1.5–7.0 eV for Sn–Sb oxide films of different doping levels.

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Fig. 6 – Moss–Burstein shift of direct allowed transitions of the Sn–Sb films of different doping levels in the photon energy range (1.5 to 5.0 eV).

layer system showed blue shift with decrease in Sb content. This can be identified as a typical Moss–Burstein [23] shift if we consider the increasing carrier concentration values with decrease in Sb content (Table 2) of the films. As this band gap closely resembles to the band gap of pure and doped SnO2/ Sb2O3 [9,24–28], this may be termed as Ebulk, the band gap of bulk system which may be either in bulk or in thin film form. The relatively high direct band gap values of the ATO films up to 50 at.% Sb were around 5.0 eV (Table 1, Fig. 5) which was considerably high with respect to the band gaps of bulk system. This behaviour was possibly for the development of nanostructured Sn–Sb oxide films and their confinement behaviour [29,30]; hence this band gap may be termed as Econf. Econf: ¼ EgðbulkÞ þ h2 ð1=m⁎e þ 1=m⁎h Þ=8R2  1:8e2 =eR þ : : :: : : ;

ð18Þ

where Econf. is effective band gap of nanocluster (quantum dot), Eg(bulk) is the direct band gap energy of bulk system, R is the nanocluster radius, ε is the dielectric constant of the system, me⁎ and mh⁎ are the effective masses of electron and hole of exciton [31,32] of the system when it is irradiated by electromagnetic radiation. The nanostructured feature is clearly evident from the SEM micrographs (Fig. 7). The SEM image of the ATO film of Sn:Sb = 90:10 exhibits formation of Sb doped SnO2 nano clusters of average dimension of 35 nm. The average dimension was determined from the histograms taking into account of about 300 clusters (Fig. 8) of different

dimensions. It is interesting to note that the cluster size increased homogeneously to about two times (Fig. 7) on further increasing Sb content (Sn:Sb = 70:30). The average cluster size was found to be 80 nm, which was determined in similar way as described above. On the other hand, further increase of Sb content in the film (Sn:Sb = 50:50), the cluster size increased to 120 nm (average). In this case scanning electron microscopic image displays the clusters suspended in an amorphous medium, which implies that solid solution of the oxides has possibly been started to develop at this stage. It is true that well defined X-ray diffraction peaks should appear with increasing grain size, but this was not observed. The reason for this anomalous behaviour was not understood. The SEM image of maximum Sb content (Sn:Sb = 30:70) confirms the existence of amorphous medium of solid solution of Sn–Sb oxides as evident from the X-ray amorphous nature (Fig. 2) of the film. The evaluated indirect band gap values for the films were in the range, 2.54 to 3.46 eV. The relatively small values may be caused by the high degree of disorder in the film [33,34] creating sublevels in between conduction and valence bands which may be termed as Edefect. In the case of single layer system the indirect band gap also shifted to the blue region with decrease in Sb content which correspond to increase in carrier concentration supporting similar M–B shift to occur as predicted for Ebulk system. The films deposited from the precursors of different Sb content showed about 80 to 97% transmission at 0.55 μm (Table 1), the wavelength of maximum luminous efficiency of human eye [23]. The (%) transmission of the films of almost similar thickness (∼ 928 ± 80 Å) decreased (Fig. 9) with increasing Sb content. This occurred due to red shift of indirect band gap (Table 1) with increasing Sb content from 30 at.% with an exception to the system of Sn:Sb = 90:10. The tailing effect of the indirect absorption decreases the %T at 0.55 μm. The anomalous behaviour of %T with thickness was observed in few cases, for example, L13S and L11S (Table 1). The thicker film (L13S) exhibits relatively high transmission at 550 nm. This is due to the interference effect. The measured R.I. of L13S and L11S are 1.820 (± 0.005) and 1.714 (±0.005) respectively. As the L13S layer of 1.820 R.I. and 1714 Å physical thickness is deposited on to a glass of R.I. 1.512, there will be reflections at 1248 nm, 416 nm, 250 nm due to interference assuming there is no dispersion. Hence in the transmission curve, the minima will be at 1248 nm, 416 nm. This implies that there will be a trend of increasing transmission at 550 nm. On the other hand, in the case of L11S layer of 953 Å physical thickness and R.I., 1.743 deposited on to glass of R.I. 1.512, the reflections due

Table 2 – Sample designation, carrier concentration, resistivity and Hall mobility of ATO films of different Sn:Sb ratios Sample no.

Sn:Sb

Number of operation

Carrier concentration (N) ×1019 (cc– 1)

Resistivity (ρ) × 10− 3 (Ω cm)

Hall mobility (μ) (cm2/V s)

L11S L12S L13S L21S L22S L23S L31S L32S L33S

90:10

1 2 3 1 2 3 1 2 3

2.79 15.60 106.23 2.15 13.62 42.23 1.85 10.92 32.28

155.59 17.08 8.70 97.12 14.64 1.19 55.24 26.03 2.32

1.44 2.34 7.27 2.99 3.13 12.36 6.10 2.19 8.35

70:30

50:50

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Fig. 7 – SEM micrographs of the Sn–Sb films obtained from the precursors of different antimony contents; (a) 10 at.% (b) 30 at.% (c) 50 at.% (d) 70 at.%.

to interference would appear at 664 nm, 221 nm. This would imply appearance of minima at 664 nm in the transmission spectrum which would result in increasing transmission at around 550 nm. But if we consider the wavelength difference of corresponding minima of L11S and L13S near to 550 nm then we find relatively large difference for L13S. Hence, % transmission will be relatively high in case of L13S. On the other hand, as the peak of blackbody radiation curve at ambient temperature lies at ∼10 μm [23] thrust was mainly given on the observation of hemispherical reflectance at this particular wavelength. Directional hemispherical reflectance (Rh) at 10 μm increased with increasing physical thickness of the films (Table 1) in each system. The only exception in this regard was the films derived from Sn:Sb = 30:70 precursor where (Rh) was independent of thickness. The above reflection (Rh) depends [35] on resistivity of the films. The spectral emittance of the films in the range 5.0–20.0 μm was obtained from the spectral reflectance (Eq. (2)). After 10 μm the spectral emittance was found to increase with Sb content (Fig. 10), but this sequence was not observed in the 5.0–10.0 μm wavelength range. Total thermal emissivity was evaluated from spectral emittance by utilizing the Eqs. (3) and (4). The film derived from Sn:Sb = 90:10 precursor exhibited maximum Rh (30.5%) at 10 μm and minimum total thermal emissivity (ε) at ambient temperature (0.78, λrange 5.0–20.0 μm). On the contrary, the films derived from Sn:Sb = 70:30 exhibited relatively less reflection (28%) at 10 μm although minimum resistivity

(sample no. L23) was obtained in this case. However, we also observed that Rh increased with increase in physical thickness for the decreasing trend in resistivity (Fig. 11). It is interesting to note that there is a trend to decrease the gap between Ebulk and Edefect with increasing physical thickness which may also be the reason to increase the electron density at the conduction level resulting in decrease in resistivity. The high electron scattering (γ) (Eq. (19)) in the films of relatively low thickness may be expected for low mobility (μ) (Table 2). g¼

e m⁎ A

ð19Þ

where e and m⁎ are charge and effective mass of electron respectively. The low scattering may be observed for the films of relatively high thickness (Table 2) for obtaining relatively high mobility [7]. In the case of single layer, the Hall mobility does not change significantly with Sb content but with increasing number of layers, Hall mobility differs significantly with Sb content. It is also observed that the carrier concentration for a particular composition increased with increasing thickness of the film.

5.

Conclusion

ATO films with wide variation of Sb (10 to 70 at.%) were deposited on silica glass by sol–gel spinning technique. The Sb

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585

Fig. 10 – Spectral directional hemispherical emissivity (εd) of Sn–Sb oxide films of different Sn:Sb atomic ratios.

Fig. 8 – Distribution of particle size of the Sn–Sb films of different Sn:Sb ratios, (a) 90:10 and (b) 70:30.

content present in the film was found to be always less than that in the precursor due to sublimation effect of Sb compounds occurring during thermal curing. Only cassiterite phase of SnO2 was observed in the films of maximum Sb

Fig. 9 – Variation of (%) Transmission of the Sn–Sb films (92.8 ± 8 nm thickness) at 0.55 μ with different Sb doping concentrations.

content, 30 at.%, while films with Sb N 30 at.% were X-ray amorphous. Films of each composition exhibited two direct and one indirect band gap values which were termed as the band gaps for bulk (Ebulk), confinement (Econf) and defect (Edefect) systems. The films are of nanostructured form as evident from SEM images. Moss–Burstein shift towards direct band gap was observed if Sn concentration be increased. Carrier concentration was in the range, 1019–1021/cc and the electrical resistivity was in the range, 10− 3–10− 1 Ω cm. The films of relatively low resistivity of Sn:Sb = 90:10 system exhibited minimum thermal emissivity, 0.78. Attempt has been made to relate the optical, electrical properties with the optical band gap of the films.

Acknowledgments Authors are thankful to Dr. H. S. Maiti, Director, CGCRI, Kolkata for his constant encouragement to carry out this work. One of the authors (LKD) thanks CSIR, India for offering him a research internship. This work was done under the CTSM programme (CMM 0019).

Fig. 11 – Variation of electrical resistivity of Sn–Sb oxide films with film thickness.

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