I declare that the matter included in this dissertation is the result of research carried out by me at the Department of Physics, Indian Institute of Technology Guwahati, Guwahati, India under the supervision of Prof. This confirms that the work in the thesis entitled "Studies on third-order optical nonlinearity, photoluminescence and random laser radiation in Zn1-xAlxO (0≤x≤0.10) and Zn1-xTixO (0≤x≤0, 05), applied with a pulsed laser" Mr. Gyan Prakash Bharti (Student No. Department of Physics, Indian Institute of Technology Guwahati for the award of the degree of Doctor of Philosophy has been carried out under my supervision and the same has not been submitted elsewhere for the award of any other degree.
Acknowledgement
Commdt., BSF and Vinod Kumar, Asst.. Commdt., CRPF, Anu bhabhi and Priyanka bhabhi) for their love and blessings which always motivated and inspired me. Finally, I would like to thank my parents, brothers and sisters for their endless love and blessings and patience throughout the journey.
Abstract
Introduction
- Crystal structure of ZnO
- Band gap engineering
- Burstein Moss (BM) effect
- Quantum confinement effect
- Temperature effect
- Photoluminescence in ZnO
- Non Linear optical properties of ZnO thin film
- Random lasing in ZnO
- Fabrication of pure and doped ZnO thin films
- Organization of present thesis
In the case of transition metals (Al. Mn, Mg, etc.), the reduction of the DLE emission band in ZnO is linked to the reduction of the defect states. Ben Ayadi et al., “The Properties of Aluminum Doped Zinc Oxide Thin Films Prepared by RF Magnetron Sputtering from Nanopowder Targets,” Materials Science and Engineering: C.
Experimental details
- Sintering of pellets
- Preparation of substrate
- Pulsed laser deposition (PLD) set-up
- Characterization of PLD thin films
- Energy dispersive X-ray spectroscope (EDX)
- Field emission scanning electron microscope (FESEM)
- X-Ray diffraction
- Raman spectroscopy
- Stylus profilometer
- UV-VIS-NIR spectrophotometer
- Fluorescence spectrometer
- Modified Z-Scan set-up
- Multiphoton absorption induced photoluminescence (MPA-PL) experimental set-up set-up
- Low temperature photoluminescence experimental set-up
- Random lasing experimental set-up
The ablation from the target material depends on the material properties such as absorption coefficient (), reflectivity (R), specific heat (Cv) and thermal conductivity (K) etc. The schematic diagram of the pulsed laser deposition (PLD) setup used for deposition of Zn1-xAlxO ( 0≤x≤0.10) and Zn1-xTixO (0≤x≤0.05) thin films in the present work are shown in fig. The turbo pump was connected to the bottom of the PLD chamber through a 100 CF port.
In the present case, the real part of the linear refractive index, n0′= n, is the linear extinction coefficient, n0′′= (αλ/4π), the real part of the nonlinear refractive index n2′= n2. A temperature controller (Cryo-con 32B) was attached to the sample holder to control the temperature of the sample.
Fabrication and characterization of
- Experimental details
- Energy dispersive X-rays spectra of Zn 1-x Al x O (0≤x≤0.10) thin films
- XRD spectra of Zn 1-x Al x O (0≤x≤0.10) thin films
- Raman spectra of Zn 1-x Al x O (0≤x≤0.10) thin films
- UV-VIS-NIR spectra of Zn 1-x Al x O (0≤x≤0.10) thin films
- Photoluminescence spectra of Zn 1-x Al x O (0≤x≤0.10) thin films
- Conclusion
The wt% of Al content in the film is almost the same as that of the corresponding target, Fig. 1 is also observed in pure and Al-doped ZnO thin films, which is due to the silent B1 Raman mode [13]. The blue shift in the band gap of AZO thin films can be explained via the Burstein Moss (BM) effect [17].
Therefore, the band gap is blue-shifted with the increase of x in the AZO film. The optical transmittance in the ZnO film decreases with the increase of Al content.
Fabrication and characterization of
The estimated amount of Ti (wt%) from EDX measurements in Zn1-xTixO thin films (0≤x≤0.050) is plotted against the amount of Ti (wt%) in the corresponding TZO (bulk) powder form and is shown in Fig. All data points are the average of three different locations of individual thin film samples. A reasonable match of the Ti content indicates a near-stoichiometric transfer from the target to the PLD films of TZO.
The distortion in the (002) peak may be due to the excess free electrons occupying the vacant space between the lattice positions and void formation due to the addition of a relatively large amount of Ti. The average crystallite size (D) in the (002) plane in TZO thin films shows a large variation in the range of 7.8–26 nm. The decrease in grain size in Ti-doped ZnO thin film is mainly due to the smaller discrepancy in the ionic radii of the Ti4+ and Zn2+ ions.
This will favor the adsorption of the clashing atoms and lead to the larger crystallite size. A small deviation of the lattice parameters in the Zn1-xTixO film from the pure ZnO may be due to the difference in ionic radii of the participating species (r(Zn2+)=60 pm, r(Ti4+)=60.5 pm) [ 14].
It is observed that x=0.020 is a transition point where the substitution of Ti4+ ion by Zn2+ ion is most suitable for the crystalline quality of the film. The ratio of lattice parameters (c/a) is estimated to be ∼1.60, which is closed to the hexagonal crystal structure. The estimated lattice constants in the present case are smaller than the lattice constants of strain-free bulk ZnO crystal (a0=3.253Å and c0=5.209Å) [JCPDS data files], indicating the compressive stress in the films.
The suppression of A1(LO) peak in TZO films suggests the reduction of the defect density in the system. The clear appearance of E2(low) and E2(high) peaks in the presence of Ti ions indicates the wurtzite crystal structure.
A marginal increase in band gap energy is observed in TZO thin films with x up to x=0.020, while it decreases slightly at higher Ti content (x>0.020). The increase in the optical gap energy for the TZO film is due to the quantum confinement effect. It is very obvious that the permeability of TZO thin films decreases with increasing Ti concentration.
At higher Ti concentration (x>0.020), the complete absence of interference fringes in the spectrum indicates the deterioration of the film quality which is also indicated by the XRD and Raman measurements, Fig 4.3 & 4.4. Therefore, the linear refractive indices (n) in the TZO thin films could be estimated using Swanepoel envelope method, eq.
The linear refractive indices in Zn1-xTixO thin films increase for 0≤x≤0.02, as shown in the figure. Upon incorporation of Ti into the film, the morphology of the ZnO, the shapes of the particles are almost spherical and eventually become marigold flower similar structure for x=0.020 film as shown in the inset of Figure 4.9(d). The grain size distribution in Zn1-xTixO thin films (0≤x≤0.050) is shown in Fig.
The average particle size, estimated by fitting the particle size distribution in the Zn1-xTixO (0≤x≤0.050) films, is shown in Table 4.3. It is observed that the distribution of the nanostructured particles is Gaussian in nature for pure ZnO (x=0.000) and Zn1-xTixO (x>0.020) films, while that of 0.005≤x≤0.020 fits well with the Lorentzian function.
In the case of a higher concentration of Ti (x>0.010), there is a rapid increase in NBE emission. At higher values of x (>0.020) there is not much variation in the intensity of the NBE band and it is observed that it is comparable to the intensity at x=0.020. The nominal reduction of the DLE peak occurs due to the improvement of the crystal quality, while the damping of the DLE peak may be due to the quantum confinement effect.
Therefore, the densities of optically active defects (VO, VZn, Oi, Zni and OZn) are changed with Ti concentration in ZnO thin films. In the present case, no clear signature of the TiO2 phase is observed in the XRD spectra, Fig.
Conclusion
3] Zhixin Wan et al., “Electrical and optical properties of Ti-doped ZnO films grown on glass substrate by atomic layer deposition”, Materials Research Bulletin. 7] Haixia Chen et al., "Optical properties of Ti-doped ZnO films synthesized via magnetron sputtering", Journal of Alloys and Compounds. 13] Kristin Bergum et al., "Thickness-dependent structural, optical and electrical properties of Ti-doped ZnO films prepared by atomic layer deposition", Applied Surface Sci.
Sridhar et al., "Spectroscopic study and optical and electrical properties of Ti-doped ZnO thin films by spray pyrolysis", Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy. Liu et al., "Effects of Ti-doped concentration on the microstructures and optical properties of ZnO thin films", Superlattices and Microstructures.
Low temperature
- Low temperature photoluminescence spectra of Zn 1-x Al x O (x=0.00 and x=0.05) thin films
- DLE band in ZnO and AZO films
- NBE band in TZO thin films
- DLE band in TZO thin films
- Conclusion
Therefore, the transition involving the Zni and VZn defect increases, resulting in the enhancement of the blue band in the AZO thin film compared to the pure ZnO thin film. We observe that the peak intensities gradually increase with decreasing sample temperature. This may be due to a decrease in radiative recombination with decreasing temperature.
The extended spectra of NBE bands in TZO thin films in the spectral range from 350 to 400 nm are shown in Fig. The second DLE band in TZO thin films in the spectral range from 450 nm to 720 nm is shown in Fig.
Nonlinear absorption and refractive
- Conclusions
This is due to the thermal origin of the nonlinearity in the films exposed via a continuous wave laser (cw). It is observed that the position of the UV emission in multi-photon absorption is slightly blue-shifted w. The relative narrowing of FWHM upon increasing Al concentration may be due to the increase in free carrier concentration in the Zn1-xAlxO films, as shown in Fig.
As the Al concentration is increased, the free carrier concentration is increased, leading to the. Therefore, it is observed that the TPA conversion efficiency is improved with the increase in the Al concentration.
Random lasing action in pulsed
- Random lasing in ZnO pellet
- Effect of Al concentration (x) on Random lasing signal in Zn 1-x Al x O (0≤x≤0.10) thin films thin films
In this chapter, the random laser emission of the pure ZnO grain and pulsed laser-deposited thin films of Zn1-xAlxO (0≤x≤0.10) pumped by 3rd harmonic of a Q-switched Nd:YAG laser are presented . The thin films were subjected to field emission scanning electron microscope measurement to reveal the surface morphology of the thin films to get an idea about the possible cavity formation. However, the estimated average particle size is found to be significantly larger compared to that of the average crystallite size, Table 3.1, Ch.
At very high Al concentration (x=0.10) the surface structure of the film indicates the diffuse structures, fused nanorods and nanowires. However, the average diameter of the nanowire in 10 wt% Al-doped ZnO film is quite large compared to 5 wt% Al-doped film, and so is the facet size of nanorods, 348 nm and 448 nm for x=0, respectively, 05 and 0.10.
