Structural , Optical , Electrical , and Photoresponse Properties of Postannealed Sn-Doped ZnO Nanorods

Tin (Sn) doped ZnO nanorods were synthesized on glass substrate using a sol-gel method. The synthesized nanorods were postannealed at 150, 350, and 500C.The surfacemorphologies of Sn-doped ZnO nanorods at different postannealing temperatures were studied using scanning electron microscope (SEM). XRD results show that as the postannealing temperature increased from 150C to 500C, the c-axis orientation becomes stronger. Refractive indices and dielectric constants were calculated on the basis of different relationships by utilizing bandgap values. These bandgap values were obtained in terms of optical absorption by using a UV-Visible spectrophotometer. The enhancing effects of annealing temperatures on electrical properties were observed in terms of current-to-voltage measurements. Resistivity decreases as postannealing temperature increases from 150C to 500C. Annealed samples were evaluated for UV-sensing application. The samples exhibit a responsivity of 1.7 A/W.


Introduction
Zinc oxide is an n-type semiconductor with bandgap energy of 3.37 eV and large exciton binding energy of ∼60 meV.ZnO and its alloys are having vast device applications such as light emitting diodes (LEDs), solar cells, and optical waveguides [1][2][3].ZnO is nontoxic, highly transparent along with high mechanical and thermal stability.Several researchers explore that the dopants can improve the electrical and optical properties of ZnO such as Al, In, and Sn [4][5][6].When Sn was added into ZnO for doping, Sn +4 substitutes Zn +2 site in the ZnO crystal structure resulting in two more free electrons to contribute to the electric conduction [7].The ionic radius of Sn +4 (0.069 nm) is smaller than Zn +2 (0.074 nm); therefore, Sn +4 ions can replace Zn +2 ions in substitution sites [8].
ZnO can be prepared by varieties of different techniques such as pulsed laser deposition (PLD) [9], RF sputtering [10], chemical vapor deposition (CVD) [11], sol-gel [12], metal organic chemical vapor deposition (MOCVD) [13], and molecular beam epitaxy (MBE) [14].Among them, sol-gel is the mostly used technique for ZnO preparation; it is low cost, needs low temperature, and can be implemented for largescale preparation as compared to other techniques [15].
In the present study, Sn-doped ZnO nanorods were successfully synthesized using sol-gel method.We also investigated the effects of postannealing temperatures on structural, optical, electrical, and photoresponse properties.The results of this investigation contribute to the application of synthesized nanostructures in optical devices and heterojunction solar cells.

Experimental
ZnO seed solution was prepared by using low cost sol-gel spin coating method as reported in our previous research [16].Briefly, 3.84 g zinc acetate dihydrate was dissolved in 50 mL 2-methoxyethanol and 1.069 g monoethanolamine.Monoethanolamine acts as a stabilizer and was added dropwise under stirring and constant temperature.Thus, a ZnO solution of concentration 0.35 M was prepared.The solution was stirred for 2 hours at 60 ∘ C until a transparent and homogeneous solution was obtained.The prepared solution was stored overnight at room temperature.The aged solution was spin-coated on glass substrate using spin coater which was rotated at 3000 rpm for 30 s.The film was heated at 250 ∘ C for 20 min after each coating.The coating-to-drying process was repeated for five times, and thus produced films were annealed at 500 ∘ C for 1 hour.
The ZnO nanorods were grown on seeded glass substrate using hydrothermal growth method.The growth solution was prepared by mixing zinc nitrate hexahydrate (0.1 M), tin chloride dihydrate (0.001 M) and hexamethyltetramine (0.1 M) in 150 mL deionized water (DI).The solution was sonicated for 30 min at 50 ∘ C in an ultrasonicator (Delta DC200H).The seeded substrates were placed vertically so that the seeded side was facing down in the growth solution.The growth was done in a preheated oven at 93 ∘ C for 6 hours.After the growth, samples were washed with DI water to remove the residual salts and dried using N 2 gun.The synthesized samples were postannealed at temperatures of 150, 350, and 500 ∘ C for 1 hour under ambient environment.Silver electrodes were deposited on the top of Sn-doped ZnO nanorods using hard mask.The metal was evaporated via Edwards Auto 306 thermal evaporation in a vacuum environment.
The surface morphology of Sn-doped ZnO nanorods was examined using scanning electron microscopy (SEM JEOL6460LA).The crystal structure and orientation of Sndoped ZnO nanorods were investigated using X-ray diffractometer (XRD, Bruker AXS D8 Advance).The UV-Visible (UV-Vis) spectra of Sn-doped ZnO nanorods were obtained using a Lambda35 UV-Vis spectrometer (Perkin Elmer).A Keithley 2400 source meter was used to determine currentto-voltage (I-V) and current-to-time (I-t) characterization.Optical properties were determined by UV-Vis spectroscopy (HP 8453, Agilent).A digital lock-in amplifier (SR 830) and a 500 W Xenon lamp (SVX 1450) were used to characterize the spectral response.All of the measurements were performed in an ambient environment.
No other peaks of impurities such as zinc nitrate hexahydrate and hexamethyltetramine were observed in the spectra.It was observed that all the postannealed Sn-doped ZnO nanorods have preferential growth at (002) orientation.The intensity of peak appeared at (002) plane found increased with an increase in postannealing temperature.The intensification of peak that appeared at (002) plane indicates that crystal quality of the synthesized Sn-doped ZnO nanorods gets improved with the increase in postannealing temperature.The average crystalline size was calculated using Scherrer's formula [22][23][24]: where  is the crystalline size,  is the wavelength of X-ray,  is the angle of diffraction, and  is the FWHM of the observed peak.The calculated crystallite size values were 30, 32, and 36.9 nm with respect to postannealing temperatures of 150, 350, and 500 ∘ C, respectively, which revealed that crystallite size increased as the postannealing temperature increased.
The increment in crystalline size may be attributed to particles merging activity at high postannealing temperature [25].Figure 2 illustrates the SEM images of Sn-doped ZnO nanorods postannealed at temperatures of 150, 350, and 500 ∘ C, respectively.It was notified that all the substrates were densely covered with vertically aligned nanorods.As the postannealing temperature increased from 150 to 500 ∘ C, the Sn-doped ZnO nanorods showed noticeable variations on the surface morphology as shown in Figures 2(b) and 2(c).Based on dynamics growth and thermodynamics process, particles likely move more disorderly at maximum postannealing temperature than at other temperatures; as a result, the anisotropic growth of crystals is perturbed because of the surrounding driving force; hence, Sn-doped ZnO nanorods show a granular shape (Figures 2(b) and 2(c)) [26].Furthermore, Sn-doped ZnO nanorods at a higher postannealing temperature are more agglomerated and spherical than those at a lower postannealing temperature; this result indicates that the samples are highly crystalline in nature [27].
The inset of Figure 3 shows the absorbance spectra, which displayed that as the postannealing temperature increases, the absorbance edge shifts towards the lower wavelength, indicating expansion in the optical bandgap.The optical bandgap of direct-transition semiconductor Sn-doped ZnO nanorods was calculated using Tauc model [28] and compared with the reported values [19,20]: where  is the absorbance coefficient, ℎV is the photon energy,  is an energy independent constant, and   is the energy gap.The optical bandgap of Sn-doped ZnO nanorods which were postannealed at temperatures of 150, 350, and 500 ∘ C was determined by extrapolating the straight section of the plot of (ℎV) 2 versus photon energy as shown in Figure 3.
The calculated values of the optical bandgap energy were 3.18 eV, 3.21 eV, and 3.33 eV for the samples postannealed at temperatures of 150, 350, and 500 ∘ C, respectively.The change in bandgap energy may be attributed to the electron-electron and electron impurity scattering [29,30].
Our knowledge of refractive index  is important in the advancement of different materials in optoelectronic research.Therefore,  as a function of wavelength is an important parameter and should be evaluated for the design of various devices, such as integrated optical switches, filters, and modulators.Efforts have been devoted to relate  to energy gap (  ) based on simple relationships [31][32][33][34][35].In the present study, various relationships between  and   were reviewed.Ravindra et al. [35] suggested that different relationships are present between bandgap and high-frequency , in which a linear form of  as a function of   is presented as follows: where  = 4.048 and  = −0.62 eV −1 .Light refraction and dispersion are also considered.Hervé and Vandamme [36] proposed an empirical relationship as follows: where  = 13.6 eV and  = 3.4 eV.Ghosh et al. [37] showed an empirical relationship for groups II to VI semiconductors on the basis of the band structure and quantum dielectric considerations of Penn [38] and Van Vechten: where  = 25  + 212,  = 0.21  + 4.25, and (  + ) refer to an appropriate average optical energy gap of the material.The calculated  of the end-point compounds and   (Table 1) are consistent with experimental [39,40] and theoretical [41][42][43][44] values.These results are verified by calculating the optical dielectric constant  ∞ , which is dependent on ; hence,  ∞ =  2 [45].The  obtained using the Hervé and Vandamme model [36] is important to enhance the detecting and sensing of ZnO nanostructure.Therefore, high absorption may be attributed to an increase in sensor efficiency.Current-to-voltage characteristic was carried out to study the postannealing effect on the electrical properties of Sndoped ZnO nanorods.The effect of postannealing temperature can be visually inspected in current-voltage characteristic curves of Figure 4.
The linear relationship between current-to-voltage curves demonstrates the ohmic nature of Sn-doped ZnO nanorods.
The resistivity  at postannealing temperatures of 150, 350, and 500 ∘ C can be calculated from the I-V curves by using the following relation: where  is the resistance,  is the area of electrodes, and  is the length of nanorods.Resistivity values of Sn-doped ZnO nanorods annealed at temperatures of 150, 350, and 500 ∘ C were (316.75 × 10 6 ), (55.89 × 10 6 ), and (20.95 × 10 6 ) Ω-cm, respectively.It can be revealed that the decrement in electrical conductivity was a causative factor due to reduced grainboundary scattering and an increase in electron mobility with grain size growth [46].Our X-ray diffraction result is consistent with those in previous studies [47][48][49][50][51], in which crystallinity is improved with respect to annealing temperature.Photoresponse ( on /  off ) increases as annealing temperature increases [52][53][54][55][56][57][58].In ZnO-type materials, photoresponse is generally caused by oxygen desorption and adsorption process originating from either a bulk-or surface-related process [52,59,60].
Figures 5(a) and 5(b) show the responsivity and photoresponse curves of the Sn-doped ZnO nanorods postannealed at 150, 350, and 500 ∘ C. In this study, the sharp cutoff in the responsivity curves decreased in magnitude at approximately 370 nm.The maximum responsivity of our sensor at an applied bias of 5 V was 1.7 A/W, which is higher than previously reported values [61,62].The photoresponse spectra also showed that UV light response occurs faster at higher postannealing temperature than at other temperatures.This phenomenon may occur possibly because crystallinity is improved, defect ratio decreases, and barrier height increases at maximum postannealing temperatures [55].This improved crystallinity also creates a well-interconnected network [53] of Sn-doped ZnO nanorods, thereby enhancing the electronic flow along the nanorod/nanowire  axis [63,64].However, electrons experience resistance along the nanorod axis at low annealing temperatures; as a result, photoresponse is decreased.

Conclusion
Sn-doped ZnO nanorods were successfully synthesized on glass substrate by low temperature sol-gel method.The effect of postannealing temperature on the structural, optical, electrical, and photoresponse properties of Sn-doped ZnO nanorods is investigated using X-ray diffraction, UV-Vis spectroscopy, I-V, and I-t measurements, respectively.The crystallinity and c-axis orientation of the Sn-doped ZnO nanorods were increased with annealing temperatures.The optical bandgap energy of Sn-doped ZnO nanorods decreased as the annealing temperature increases.Electrical characteristics reveal the effect of annealing temperature on the resistivity and photoresponse of Sn-doped ZnO nanorods.Hence, the proposed Hervé and Vandamme model and the improved photoresponse of postannealed samples are applicable in optoelectronic devices.also like to thank the technical staff of Institute of Nano ElectronicEngineering and School of Microelectronic Engineering, Universiti Malaysia Perlis for their kind support to smoothly perform the research.

Figure 5 :
Figure 5: (a) The spectral responsivity and (b) current-to-time photoresponse curves of Sn-doped ZnO nanorods at different postannealing temperatures.