Doping effects on physical properties of (1 0 1) oriented tin zinc oxide thin films prepared by nebulizer spray pyrolysis method

Materials Science & Engineering B 248 (2019) 114402

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Doping effects on physical properties of (1 0 1) oriented tin zinc oxide thin films prepared by nebulizer spray pyrolysis method M. Thirumoorthia, J. Thomas Joseph Prakashb, a b

T

⁎

PG and Research Department of Physics, H.H. The Rajah’s College (Affiliated to Bharathidasan University), Pudukkottai 622001, Tamilnadu, India PG and Research Department of Physics, Government Arts College (Affiliated to Bharathidasan University), Trichy 22, Tamilnadu, India

ARTICLE INFO

ABSTRACT

Keywords: ZnO:Sn thin film Nebulizer spray pyrolysis X-ray diffraction AFM Wide band gap

Transparent conducting tin doped zinc oxide thin films were prepared by nebulizer spray pyrolysis technique for different doping concentrations. The structural properties was investigated using X-ray diffraction analysis which ensured that the polycrystalline nature of the zinc oxide with hexagonal wurtzite structure. The surface properties were analyzed by scanning electron microscope and atomic force microscope. The Energy Dispersive X-ray spectroscopy ensures that the presence of Zn, O and Sn elements in the deposited films. Optical transmissions of the films exhibit for undoped film is 75%, which is improved up to 93% for tin doped films and the optical absorption is below 1.7%. The band gap energy of the films is achieved in the range of 3.54–3.67 eV. From the Hall Effect measurements we observed a minimum resistivity (9.2 × 10−4 Ω-cm) and maximum carrier concentration (7.4 × 1021 cm−3).

1. Introduction The rapid developments of opto-electrical semiconductor associated with human requirements are large enough, not only the reduction of the size of the elements in design, and the demand is more convenient, low cost, user-friendly and eco-friendly. The associated researches have been investigated and achieved continuously, among these the transparent conductive oxide (TCO) thin films are widely used in gas sensors [1], piezoelectrical devices [2], surface acoustics wave devices [3] and photovoltaic cells [4]. The minimum requirements of TCO film, which is fulfilled the average transmittance value will be more than 80% and resistivity range of less than 1 × 10−3 Ω-cm. ThIndium-tin-oxide (ITO) is the widely used TCO in displays and solar cell industries. However, a shortage of indium may occur in the future, and its cost is comparatively very high. Zinc Oxide (ZnO) is a promising material for optoelectronic applications such as solar cells, gas sensors, liquid crystal displays and thin film transistors due to its unique properties. ZnO is a II–VI wide band gap semiconductor which has a band gap (≥3.37 eV) at room temperature, high optical transmittance in the visible spectrum and large excitonic binding energy (~60 meV) [5–8]. As required by its applications, its electrical conductivity and optical transmittance can be easily tuned by doping with various dopant materials such as gallium [9], aluminum [10], fluorine [11], indium [12] and tin [13]. Among these elements the tin is suitable dopant material, the electrical conductivity being improved when ZnO is doped with tin. Gallium doped ZnO thin films show an electrical resistivity ⁎

in the range of 1.04 × 10−2 and the energy band gap is 3.27 eV [9]. Aluminum doped ZnO thin films show an electrical resistivity in the range of 1.8 × 10−2 Ω-cm and the transmittance is 85% [10]. The Sn4+ ion has four valence electrons (5s2, 5p2) and the ionic radius of Sn4+ ion (0.69 Å) is slightly smaller than that of Zn2+ ions (0.74 Å), thus we expect that doping of Sn4+ ions in ZnO will lead to improvement in electrical conductivity by increasing carrier concentrations. Furthermore, Zn2+ ions can be easily substituted by Sn4+ ions without large lattice distortion [14]. Tin doped ZnO thin films were fabricated by different techniques such as sol–gel process [15], thermal evaporation [16], remote-plasma reactive sputtering [17], RF sputtering [18], and spray pyrolysis [19,20]. All of the methods mentioned above have many advantages and disadvantages, but the nebulizer spray pyrolysis (NSP) technique has a noticeable advantage; it is a low-cost and non-vacuum technique for large area depositions and can produce high quality film with low precursor volume. The working of NSP method is based on the Bernoulli principle; i.e., when a pressurized flow of air is directed through a constricted orifice, the velocity of the airflow is increased to create a jet stream. The impact of a jet stream with liquid produces aerosol particles (particle size ~2.5 µm) [21]. The mist form of solution is helping to improve the quality of film and obtain a uniform growth due to gradual nucleation with minimum wastage. In previous, Mariappan et al. [22] prepared the tin doped ZnO thin films by nebulizer spray pyrolysis method for different doping concentrations (1%–15%). They reported that the transmittance was increased up to 85%, the energy band gap

Corresponding author. E-mail address: [email protected] (J. Thomas Joseph Prakash).

https://doi.org/10.1016/j.mseb.2019.114402 Received 3 December 2017; Received in revised form 29 April 2019; Accepted 1 August 2019 Available online 22 August 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.

Materials Science & Engineering B 248 (2019) 114402

M. Thirumoorthi and J. Thomas Joseph Prakash

Nomenclature full width at half maximum (rad) crystallite size (nm) wavelength of X-ray (Å) Bragg’s angle (°) lattice spacing (Å) strain miller indices

obtained maximum 3.25 eV and the electrical resistivity obtained in the range of 3.89–5.14 × 106 Ω-cm. But some researchers [19,23] obtained better results than the results obtained by Mariappan et al. [22]. In the present work, the tin doped zinc oxide thin films were prepared by nebulizer spray pyrolysis technique. The structure, surfaces, optical and electrical properties of prepared films were investigated in detail.

absorption coefficient transmittance (%) photon energy (eV) optical band gap (eV) resistivity (Ω cm) carrier concentrations (cm−3) mobility (cm/Vs)

that the films are polycrystalline in nature and has a hexagonal wurtzite structure, which is confirmed by matching the standard diffraction data (JCPDS: 89-0510, Lattice: primitive, Space group: P63mc, 186). It is evident from the XRD analysis that there are no extra peaks corresponding to tin related secondary and impurity phases, which may be attributed to the incorporation of Sn4+ instead of a Zn2+ lattice site. As shown in the XRD patterns, the observed decreases of intensity of diffraction peaks with increasing tin doping concentration, indicates the crystalline quality of films declines gradually. The observed change in crystalline quality is attributed to the strain of the crystal lattice. As shown in the Fig. 2 a shift of the (1 0 1) toward lower 2θ values reveal that the incorporation of Sn4+ ion in ZnO crystal lattice with compositional variations [24]. The crystallite size (D) is calculated using scherrer formula as given below [21],

2. Experiments In this work, the jet nebulizer spray pyrolysis apparatus [21] is used to fabricate ZnO:Sn thin films, which consists of a jet nebulizer, L tube to convey the aerosol, substrate holder with heater and air compressor. We have prepared the tin doped zinc oxide thin films using the following materials. Zinc acetate dihydrates [Zn(CH3COO)2·2H2O], Tin (II) Chloride dihydrates [SnCl2·2H2O] and Acetic acid [CH2COOH] supplied by MERCK company. The mixture of methanol and doubly distilled water (3:1) used as solvent, and Borosilicate glass (1.35 mm thickness) used as substrate respectively. First the zinc acetate dehydrate is dissolved in 100 ml solvent to make 0.5 M starting solutions. Doping of tin was achieved by adding Tin (II) Chloride dehydrates in the starting solution. A few drops of acetic acid were added to obtain a clear and homogeneous solution. The doping level in the solution varied from 0 to 8 wt% in steps of 2 wt%. The mixture was stirred under constant speed for 1 h with a magnetic stirrer under room temperature. Prior to the deposition, glass substrates (1 sq inch) were cleaned with acetone, isopropyl alcohol, and distilled water successively for 15 min in ultrasonicator. The substrate temperature for each deposition was kept at 450 °C in the air atmosphere. The prepared solution was sprayed with a jet nebulizer (HUDSON RCI micro mist, droplet size is ~2.7 μm) on the heated substrate with spray rate 0.5 ml/min using compressed air as a carrier gas. The distance between spray nozzle and substrate surface was maintained at 5 cm. The structural parameters of spray coated ZnO:Sn films were examined by X-ray diffractometer (XRD) using the PANalytical system with Cu Kα1 radiation (λ = 1.54056 Å). Surface morphology and topography of the films were analyzed by the Scanning Electron Microscope (ESEMQUANTA200, FEI-Netherland) and Atomic Force Microscopy in contact mode (Agilent 5500) respectively. Elemental analysis was made by energy dispersive X-ray spectroscopy (attached to SEM). The film thickness was measured using cross-sectional scanning electron microscopy. The optical properties of the films were examined with a double beam spectrophotometer (Oceans optics HR2000-USA) in the UV–Visible regions. The electrical parameters were collected from room temperature Hall Effect measurements (RH2035 PhysTech GmbH) system.

D = (0.9 )/( cos ) The dislocation density (δ) and strain (ε) of the crystal lattice can be calculated by the following formulas [25],

= 1/D 2

= ( cos )/4 where λ is the wavelength (1.54060 Å) of X-ray used to analysis, β is the full width at half maximum (FWHM) and θ is the Bragg’s angle of diffraction. Texture coefficient (TC) measures the relative degree of preferred orientation among lattice planes, which is obtained from following expression [25].

TC (h k l) = [I (h k l)/ Io (h k l)]/[N

1

N (I (h

k l)/Io (h k l))]

where I(h k l)is the measured intensity of the plane (h k l), Io (h k l) is the standard intensity of the respective diffraction plane, according to the JCPDS data card (89-0510) and N is the number of diffraction peaks presented. 1800

8% Sn 6% Sn

1200

4% Sn

1000

600 400

3.1. Structural properties

2% Sn

(002)

200

0% Sn

0

Fig. 1 depict the X-ray diffraction patterns of pure and tin doped ZnO thin films deposited on a borosilicate glass substrate by the nebulizer spray pyrolysis technique at a substrate temperature of 450 °C. There are five diffraction peaks were observed at diffraction angles 31.77°, 34.42°, 36.27°, 47.52°and 62.87°, which are related to (1 0 0), (0 0 2), (1 0 1), (1 0 2) and (1 0 3) diffraction planes of ZnO phase respectively. This result is reveals

25

30

35

40

2

45

(103)

800

(102)

3. Results and discussions

1400

(101)

I n t e n s i t y ( a.u )

1600

(100)

β D λ θ d ε h, k, l

α T hν Eg ρ n µ

50

55

(degree)

60

65

70

Fig. 1. X-ray diffraction patterns of the ZnO:Sn for different doping concentrations. 2

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M. Thirumoorthi and J. Thomas Joseph Prakash

observed in XRD pattern, which is shown in Table 1. It reveals that the preferential orientations of the films are exhibited along the [0 0 2] and [1 0 1] lattice planes. The pure ZnO thin film exhibit a preferential orientation along [0 0 2] plane, which shows that the film is c-axis oriented [27]. The occurring of c-axis orientation may due to the minimization of internal stress and surface energy. The tin doped ZnO thin films exhibit a preferential growth along [1 0 1] plane, which is related to substitution of Sn4+ ions into the Zn sites and declines the crystallinity of the films.

2000 1800

8% Sn

1600

6% Sn

Intensity ( a.u )

1400

4% Sn

1200 1000

2% Sn

800

3.2. Surface and compositional analysis

600 400

(101)

200 0

35.2

35.6

36.0

2

36.4

The surface morphology of thin film may influence the properties such as mechanical, electrical and optical properties. The surface morphology is depends on the deposition technique and its parameters. Scanning electron microscopes (SEM) were employed for analyze the surface morphology in details and measure the thickness of the prepared films. Fig. 3 depicts the surface and cross-sectional (inset) SEM images of pure and tin doped ZnO thin films. It is observed that the surfaces of all the films are covered by the uniform distribution of well defined particles without any cracks. The un-doped ZnO thin film has a hierarchical surface consisting mixture of different shaped particles with small voids and the tin doped ZnO thin films consisting closely packed flower like particles. The thicknesses of the prepared films are decreased gradually from 837.6 nm to 637.5 nm. The voids between the grains and the thickness are decreased with increasing doping concentrations. The variations in thickness may relate to reduction of voids due to strain and also a variation of thickness is common in thin film fabrication process. Moreover, a larger variation in the thickness of the film was associated with a more uniform distribution of residual stress [28]. The reduction of voids will improve the electrical conductivity due to lowering of grain boundary scattering and the optical transmittance due to lowering of incident light scattering. Energy dispersive X-ray spectroscopy (EDX) is an analytical technique used for the elemental analysis of a sample. Fig. 4 shows the EDX spectra of the tin doped ZnO thin films. The spectrum reveals that the

0% Sn 36.8

37.2

37.6

(degree)

Fig. 2. Variations of (1 0 1) peak position as a function of Sn doping concentrations.

The calculated structural parameters of the prepared thin films are presented in Table 1. The crystallite size decreased and the strain increased with increasing doping level. The observed change in average crystallite size is attributed to peak broadening due to strain formed in the crystal lattice. The dislocation density (δ) is to measure the disorder of lattice planes in the crystal structure. The strain arises in the crystal lattice is related to forming of point defect (vacancies and site disorders), dislocations, and extended defects. Here the compositional variations can be a factor in the strain variations [26]. Any deviation of the calculated TC value from unity implies preferred growth, which means the atomic density is higher along that angle than others. The texture coefficient was calculated for all the diffraction peaks Table 1 Structural parameters of prepared films for different doping concentrations. Sample

Crystallite size (nm)

Dislocation density (×1014 lines/m2)

Strain ×10−4

TC

2.8140 2.6032 2.4746 1.9117 1.4769

27.27 79.09 42.5 32.77 36.89 43.70

13.4471 1.5986 5.5363 9.312 7.3482

12.7081 4.3823 8.156 10.577 9.3954

0.1335 1.3387 0.313 0.4809 0.7337

2.8135 2.6027 2.4751 1.9105 1.4768

2.8140 2.6032 2.4779 1.9117 1.4758

33.59 33.82 61.67 34.19 26.93 38.04

8.8629 8.7428 2.6293 8.5546 13.7888

10.3184 10.2478 5.6205 10.138 12.8709

0.6462 1.1086 1.4855 1.2025 0.557

0.265 0.265 0.1689 0.4225

2.8135 2.6027 2.4751 1.9105

2.8183 2.6032 2.4812 1.9117

31.18 31.4 49.5 20.55 33.15

10.286 10.1423 4.0812 23.6797

11.1167 11.0392 7.0018 16.8635

1.0025 1.0948 1.9048 0.9977

[1 0 0] [0 0 2] [1 0 1] [1 0 2]

0.2905 0.246 0.2952 0.475

2.8135 2.6027 2.4751 1.9105

2.8140 2.6032 2.4845 1.9136

28.44 33.82 28.32 18.27 27.21

12.3634 8.7428 12.4684 29.9586

12.1848 10.2478 12.2393 18.9626

1.1449 1.2303 1.5142 1.1104

[1 0 0] [0 0 2] [1 0 1] [1 0 2]

0.2825 0.1652 0.2636 0.511

2.8135 2.6027 2.4751 1.9105

2.8140 2.5995 2.4912 1.9117

29.25 50.37 31.67 16.99 32.07

11.6882 3.9414 9.9648 34.6428

11.8494 6.8809 10.9422 20.3959

1.0072 1.3734 1.8521 0.7671

2θ (°)

Miller indices [h k l]

FWHM (°)

d Space (Å) Standard

Observed

0%

31.773 34.423 36.273 47.523 62.873 Average

[1 0 0] [0 0 2] [1 0 1] [1 0 2] [1 0 3]

0.315 0.1052 0.1968 0.265 0.2525

2.8135 2.6027 2.4751 1.9105 1.4768

2%

31.773 34.423 36.223 47.523 62.923 Average

[1 0 0] [0 0 2] [1 0 1] [1 0 2] [1 0 3]

0.246 0.246 0.1356 0.254 0.346

4%

31.723 34.423 36.173 47.523 Average

[1 0 0] [0 0 2] [1 0 1] [1 0 2]

6%

31.773 34.423 36.123 47.473 Average

8%

31.773 34.473 36.023 47.523 Average

3

Materials Science & Engineering B 248 (2019) 114402

M. Thirumoorthi and J. Thomas Joseph Prakash

Fig. 3. Scanning electron microscopic images of ZnO:Sn as a function of doping concentrations. (Inset) The cross–section images of relevant films.

presence of Zn, O and Sn elements in the prepared films. The weight percentage is almost equal to their nominal stoichiometry within the experimental error as shown in Fig. 4 (inset). Topography is an important physical characteristics of thin film surfaces, which influencing their significant technical properties. Fig. 5 show the 3D–AFM of tin doped ZnO oxide thin films for different

doping concentrations in 3 × 3 μm scale. It can be seen that the three dimensional flower like grain as seen in the SEM images. Fig. 6 show the two dimensional surface profile plots of tin doped ZnO thin films. As shown in Table 2 the surface profile parameters are significantly varied with increasing tin doping concentrations. The surface roughness (Sa) and root mean square (Sq) values are decreased with increasing doping 4

Materials Science & Engineering B 248 (2019) 114402

M. Thirumoorthi and J. Thomas Joseph Prakash

Fig. 4. EDX spectra of prepared ZnO:Sn and the inserted tables of wt% and at%.

concentrations. The average roughness is the mean value of peak and valley as calculated over the measured entire surface area. It is useful for detecting general variations in overall profile height characteristics. Root mean square roughness is the square root of the distribution of surface height and is considered to be more sensitive than the average roughness. The observed reduction in surface roughness of the films will helps to reduce the scattering of incident light and leads to increase of optical transmittance. The surface morphology and topography is depending mainly the deposition method and process parameters.

spectra show lower absorption values in the range of 0–1.7% and have an inverse tendency to transmittance as depicted in Fig. 8. The optical band gap (Eg) of the prepared films are calculated from the extrapolation of linear line portion of the (αhν)2 versus (hν) plot as shown in the Fig. 9(a–e) for the prepared films. The absorption coefficient (α) is calculated using the following relation,

= [ln(1/T)]/d where T is the transmittance and d is thickness of the film. The absorption coefficient (α) and incident photon energy (hν) is related by the Tauc relation

3.3. Optical properties The higher optical transmittance is an important requirement to the transparent conducting oxide thin film. The optical properties of prepared tin doped ZnO thin films were examined from UV–Visible transmittance and absorption spectra. The transmittance spectra of pure and tin doped ZnO thin films were recorded in the wavelength range of 300–1000 nm, which is shown in Fig. 7. It was observed that the optical transmittance of all the films in the visible region between 75 and 93%. This result is revealed that the optical quality of prepared thin films is good. The optical transmittance is increases with increasing tin doping level, which indicates that lowering the scattering of incident light and absorption by surface. As mentioned in the surface properties, the reduction of scattering of incident light may due to decreasing surface roughness. The observed blue shift of absorption edge is suggesting that the band gap was improved due to the tin doping. The absorption

( h ) 2 = A(h

Eg)

where A is the constant and Eg is the optical band gap. The calculated band gap values were plotted as a function of tin doping concentrations as shown in Fig. 9 (f). The calculated band gap values are 3.54, 3.65, 3.67, 3.62 and 3.60 eV for 0, 2, 4, 6 and 8% of tin doping levels respectively, which is higher than reported values [16,23,29]. It reveals that the band gap is increased rapidly up to 4% of tin doping and the decreased for higher doping, a similar trend of band gap variations was observed earlier by Ganesh et al. [29]. The widening of the optical band gap is attributed to increased carrier concentration and which is explained by using the Moss–Burstein effect [30]. The narrowing of band gap for higher tin doping may be due to many-body interaction effects either between free carriers or between free carriers and ionized impurities [31].

5

Materials Science & Engineering B 248 (2019) 114402

M. Thirumoorthi and J. Thomas Joseph Prakash

Fig. 5. AFM – 3D surface images (3 × 3 μm) of ZnO:Sn films.

6

Materials Science & Engineering B 248 (2019) 114402

M. Thirumoorthi and J. Thomas Joseph Prakash

Fig. 6. Surface profile plots of different Sn doped ZnO films.

7

Materials Science & Engineering B 248 (2019) 114402

M. Thirumoorthi and J. Thomas Joseph Prakash

Table 2 Surface profile parameters of ZnO:Sn. Sample of Sn doping (%)

Average Surface Roughness (Sa) (nm)

Root Mean Square (Sq) (nm)

Maximum Peak Height (Sp) (nm)

Coefficient Of Kurtosis (Sku)

Thickness (nm)

0% 2% 4% 6% 8%

36.4 24.1 16 13.8 14.4

46.2 30.4 20.4 17.5 18.2

168 100 78.9 51.7 59.3

3.25 3.05 3.58 3 3.17

837.6 800 712.5 637.5 687.6

the induced electrons in this process helps to improve the electrical conductivity. The induced electrons from the defects cause ZnO to act as a n-type semiconductor. The mobility (µ) and carrier concentration (n) of the films are found from the Hall measurements using the relation,

100 90

70 60

70

50 40 30

0% 2% 4% 6% 8%

20 10 0

µ = 1/n q

80

300

400

500

600

Transm ittance (%)

Transmittance ( % )

80

where q is the charge of an electron and ρ is the electrical resistivity. It can be seen that the resistivity of the prepared thin films is decreased with increasing tin doping concentrations. At the same time the carrier concentration and mobility is increased. It reveals that the electrical property is improved while tin doping in comparison with undoped ZnO thin film. The observed increase of carrier concentration may due to the substitution of Sn4+ in Zn2+ sites of ZnO, which gives two more free electrons to contribute to the electrical conductivity. The mobility of the carriers relates to surface structure and grain boundary of the films. For the higher doping level the resistivity is slightly increased and the mobility is diminished. The reduction of the mobility is attributed to reduction of grain boundary scattering and the increase of the resistivity may relate to the reduction of carrier mobility. We observed a minimum resistivity (9.2 × 10−4 Ω-cm) is observed in the case of 6% of tin doped ZnO and maximum carrier concentration (7.4 × 1021 cm−3) in the case of 8% of tin doped ZnO thin film. The obtained values of electrical resistivity and carrier concentrations are better than the reported values [19,23].

60 50 40 30 20 10 360

380

400

420

440

Wavelength (nm)

700

800

900

1000

Wavelength (nm) Fig. 7. UV–Visible transmittance spectrum of prepared films. Inserted image shows the blue shift of absorption edges. 1.2

0% 2% 4% 6% 8%

Absorption (%)

1.0

0.8

4. Conclusions 0.6

Tin doped ZnO thin films were successfully prepared using a nebulizer spray pyrolysis technique. Structures of the films were analyzed based on XRD measurement which revealed that undoped and tin doped thin films have the hexagonal wurtzite structure. Scanning electron microscopy images depict that the uniform distribution of flower like particles and the surface morphology also varied with tin doping. The EDX spectrum ensures that the presence of Zn, O and Sn elements in the deposited films with their nominal percentage. From the AFM images and the surface profile plots the surface topography of the films is better than undoped films. The surface roughness and RMS roughness values are minimized as increased tin concentrations. Optical transmissions of the films exhibit for undoped film is 75%, which is improved up to 93% of tin doped films and the optical absorption has an opposite trend to transmittance. The energy band gap is improved from 3.54 eV maximum up to 3.67 eV for tin doped films. From the Hall effect measurements we observed a minimum resistivity (9.2 × 10−4 Ω-cm) and maximum carrier concentration (7.4 × 1021 cm−3) for 6% and 8% of tin doped ZnO thin films respectively. From these findings, we conclude that tin doped ZnO thin films are suitable for optoelectronic applications, especially 6% tin doping is optimal value and the jet nebulizer spray pyrolysis technique is suitable for producing uniform thin films with good quality.

0.4

0.2

0.0

300

400

500

600

700

800

900

1000

Wavelength (nm) Fig. 8. UV–Visible absorption spectrum of prepared films.

3.4. Electrical properties The electrical parameters like resistivity, carrier concentration and mobility are examined by Hall Effect measurement. Fig. 10 depicts that the electrical parameters of prepared ZnO thin films as a function of tin doping concentrations. All the films show n-type conductivity as observed from the sign of the Hall voltage. The presence of defects like interstitial zinc atom and oxygen vacancies can be ionized easily and

8

Materials Science & Engineering B 248 (2019) 114402

M. Thirumoorthi and J. Thomas Joseph Prakash

Fig. 9. (a–e) The (αhν)2 versus hν plots of ZnO:Sn thin films for different doping concentrations, and (f) variations of optical band gap as function of Sn concentrations.

9

Materials Science & Engineering B 248 (2019) 114402

M. Thirumoorthi and J. Thomas Joseph Prakash

[11] [12]

[13] [14] [15] [16] [17]

[18]

Fig. 10. Electrical resistivity (ρ), carrier concentration (n) and mobility (μ) of zinc oxide thin films as a function of Sn concentrations.

[19]

Declaration of Competing Interest

[20]

None.

[21]

References

[22]

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