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Optical Property Study of Charge Compensated (Si, Na) Co-doped ZnO

ZnO is co-doped with Na+ and Si4+ in the ratio 2:1. The ratio was intentionally chosen so that net valence state of dopant theoretically matches that of host. This is to avoid dependence in the amount of oxygen vacancies/interstitials arising out of cationic valence state of the dopant. With such a combination, modifications in structural and optical properties do not depend on excess or deficit of the dopant charge state. For lower doping, Na+ ions behave as interstitial sites which enhance strain, lattice disorder and thereby creating defects. Formation of interstitial defects leads to reduction in bandgap energy and produce orange-red luminescence. For higher doping, Na+ starts substituting at Zn2+ site which helps in reducing strain and lattice disorder and thereby increases bandgap. Inspite of presence of Si4+ with higher charge, there is a gradual increase in oxygen vacancies due to lattice disorder.

visible region of solar spectrum, which is primarily important in solar cell fabrications. Doping offers a method to tailor electrical, optical and magnetic properties of ZnO 9 . As an extrinsic semiconductor, n-type conductivity is common in ZnO 10 . For achieving n-type conductivity in ZnO, group III elements (B, Al, Ga and In), group IV elements (Si, Ge, Sn) on Zn-site and group VII elements (F,Cl) on O-site has been reported [11][12][13][14][15] . But, p-type conductivity in ZnO is difficult to achieve 10 . However, doping group I elements (Li, Na, K, Cu, Ag) on Zn-site can result in p-type nature 16,17 . Similarly Zn vacancies and group V elements (N, Sb and As) at Osite may also produce p-type conductivity in ZnO [18][19][20] .
Three factors, i.e., dopant formation energy, dopant ionization energy and dopant solubility determine properties of modified ZnO 21  In a previous work it was reported that Si 4+ doped ZnO leads to enhancement in bandgap and reduction in oxygen vacancies due to higher charge of Si 4+ in comparison to Zn 2+ 30 . In this manuscript, Na + is co-doped in Si 4+ doped ZnO to see effect of compensation of extra charge due to Si. For every Si 4+ two Na 1+ are needed. Hence, a ratio of Na:Si= 2:1, was chosen intentionally to achieve this goal. Therefore, Zn1-xNa2x/3Six/3O was synthesized using sol-gel process. Defect structure and optical properties of these material has been studied in detail.

Experimental:
Zn1-xNa2/3Si1/3O with x=0 (Z0), 0.013 (ZSN1), 0.020 (ZSN2) and 0.027 (ZSN2) respectively have been synthesized by standard Pechini sol-gel process followed by solid state sintering. ZnO powder is dissolved in HNO3 (Alfa Aesar, purity 99.9%) and an appropriate amount of orthosilicate ((C2H5O)4Si) and Sodium nitrate (NaNO3) is added. A homogeneous solution was prepared by prolonged stirring. A polymeric solution was formed by heating 1 g of citric acid (Alfa Aesar, 98%) and 1 mL of 20% concentrated glycerol (Alfa Aesar, 99.9%) in de-ionized water at 70 °C for 4 h. These Zn/Si/Na solutions were then added to polymeric solutions. Resultant solutions were stirred and heated on hot plates at ~60 °C. To polymeric chains, Zn/Si/Na ions homogenously got attached. Solutions gradually dehydrated to form gels in ~4hr, which were burnt on hotplates in ambient conditions to form dark powders. Resultant powders were decarbonized and denitrified by heating in air at 450 °C for 6 h and then annealing at 600°C for 2 h.
X-ray diffraction studies were carried out using a Bruker D2-Phaser diffractometer.
Agilent UV-vis spectrometer (model Carry 60) was used to analyse electronic bandgap. Room temperature photoluminescence measurement was investigated using Dongwoo Optron DM 500i.

Result and Discussion:
A dominant hexagonal wurtzite ZnO structure was revealed from XRD studies. Some minor reflections coming from zinc blend was also observed (Figure 1 (a)). No secondary phases were found related to simple or complex oxides of Zn, Si and Na. However, for higher substitution (x ≥0.027) secondary phases appear. Average crystallite size was calculated using Scherer formula (inset of figure 1

(a)). Field Emission Scanning Electron Microscopy (FESEM)
images show agglomerated particles for all samples. Particle size (average) was estimated using Image J software. Average particle sizes first decrease from 0.7 to 0.148μm but thereafter increase to 0.3 μm (Figure 1(b)). This trend is consistent with crystallite size obtained from XRD data. However, XRD values are much smaller than values obtained from SEM studies which reveal larger agglomerated particles composed of smaller crystallites. Rietveld refinement was done using GSAS software to estimate changes in lattice parameter, strain and crystallite size with silicon substitution. Pseudo-Voigt profile was chosen for peak shape. Background was modelled using a 6-coefficient polynomial function. For lower doping (ZNS1), strain increases but lattice parameters decrease ( figure 2(a, b)). However, for higher doping, strain reduces but lattice parameters increase. Increased strain inhibits particle growth and reduces particle as well as crystallite size. Reverse phenomenon happens for higher doping. Reports 31,32 on Na doped ZnO suggested that Na prefers to occupy interstitial sites whereas higher doping leads to increase in substitutional site. Similar trend is observed in b)

a)
Si 4+ /Na + doped ZnO samples. With minimal Si 4+ /Na + doping (ZSN1), Na + prefers to occupy interstitial sites which pressurizes lattice, reduces lattice parameters and thereby enhances lattice strain of system. For higher doping, substitution increase due to proper rearrangement and coupling of Na and Si in lattice, reducing strain. Due to higher ionic radii of Na, volume as well as lattice parameter increases. Optical bandgap properties were studied using UV-Vis spectroscopy ( figure 3(a)).
Absorption coefficient, α, is related to bandgap of a material. As ZnO is a direct bandgap material, extrapolation of linear part of (αhν) 2 vs hν graphs were plotted until it meets x-axis gives value of bandgap. Band gaps were found to be decreasing from 3.12 eV in Z0 to 3.11 eV in ZSN1. Thereafter, bandgap increases for higher doping ( figure 3 (b)).
To understand effect of structural disorder on bandgap, Urbach energy (EU) has been calculated. A plot of ln(α) vs hν was plotted and a linear fitting was done on linear part of curve ( figure 4(a)). Urbach energy (EU) increases from 83 meV in Z0 to 125 meV in ZSN1 and then decreases to 60 meV for ZSN3. These changes in Urbach energy (Eu) (structural disorder) effectively modify bandgaps. respectively. First peak is due to donor acceptor pair while second peak is due to free excitons. To understand contribution of each colour emission to total spectrum, area of each peak was added and grouped into AUV, AB, AG, AY, and AO-R for UV, blue, green, yellow, and orange-red. Colour fraction PN= Pcolour/PUV of each colour was calculated using already published procedure 30 .

Figure 6: Variation of (a) INBE/IDLE (b) PNBE/PDLE with x (c) PN Vs x
Green emission steadily increases from undoped samples to highly doped samples.

Conclusion:
Co-doping of Na + /Si 4+ in ZnO at ratio 2:1 is done to achieve charge difference compensation between dopant and host ion. At lower doping (ZSN1), Na enters into interstitial sites which increase strain and Urbach energy thereby reducing bandgap of material. Increase in interstitial defects is also confirmed by enhancement in orange-red emission in photoluminescence spectra. For higher doping (ZSN2 and ZSN3), Na 1+ and Si 4+ substitute Zn 2+ which reduces strain and Urbach energy thereby increasing bandgap of material. Further reduction in orange-red emission indicates interstitial defects reduction. Inspite of charge difference compensation and presence of higher charge dopant (Si 4+ ), there is enhancement in oxygen vacancies as confirmed by increase in green emission.