CHAPTER 8 SUMMARY AND FUTURE SCOPE

Transcription

1 CHAPTER 8 SUMMARY AND FUTURE SCOPE The potential of room temperature ferromagnetism in many diluted magnetic semiconductors has opened up a new route for realization of spintronic devices. Based on the theoretical calculations, experiments have been extensively carried out on transition metal doping on ZnO. While, number of experimental studies exhibit RTFM, in fact, a clear picture of the origin of magnetism in oxide based diluted magnetic semiconductors has not been obtained yet. Some authors claimed it due to metallic clusters, while others reported the magnetism due to the substitution of doped transition metals into the host matrix. On the theoretical side, it is also not clear which interaction mechanism plays the most important role for the observed intrinsic type of RTFM. Therefore, more work is needed to clarify the origin of the ferromagnetism which requires a more precise control of the transition metal dopant in the host matrix and also a careful structural, magnetic and optical characterizations. Along with magnetic and optical properties, ZnO is an emerging candidate as ferroelectric for future device fabrications. The studies on ferroelectricity of ZnO are also limited. Within the framework of this thesis, ZnO-based DMS systems have been studied in detail. For introducing TM ions into these materials, three different routes (solid atate route, sol-gel route, and thermal decomposition method) were used. Detailed study of the structural, magnetic, optical, dielectric and ferroelectric properties for these TM and non TM ion doped ZnO have been done experimentally by various techniques and theoretically by using different models. The outcomes of these results may have their active participation in various applications. The outcomes of the thesis can be summarized as follow: 8.1 EFFECTS OF TM METAL DOPING ON THE MAGNETIC PROPERTIES OF ZnO Being a ferromagnetic semiconductor with favorable experimental properties, DMSs have potential to suit for the need of spintronic devices. Magnetic properties of II-VI based DMS have attracted much attention after theoretical predictions of RTFM in Mn-doped ZnO. Here, the 169

2 emphasis of chapters 3, 4 and 5 is on the study of magnetic properties of TM (Mn, Co, Ni and Cr) doped ZnO systems prepared by different synthesis routes and tried to search the possible mechanism behind it. First, we studied the effect of synthesis route on magnetic properties of Mn doped ZnO samples. Mn doped ZnO samples were prepared by solid state and by the sol-gel route. The substitution of Zn sites by Mn 2+ ions in wurtzite ZnO was confirmed by XRD for all Zn 1-x Mn x O (x = 0.02, 0.04 and 0.06) samples synthesized by solid state and sol-gel route. FTIR spectroscopic measurements also confirmed the sucessesfull substitution of Mn ions on Zn sites. Blue shift in the band gap has been observed in all samples which is in accordance to the Moss- Burstein band filling effect supported by PL spectra. Weak ferromagnetic behavior is observed in samples prepared by solid state route and ferromagnetism completely disappeared for the sample with 6% Mn concentration. In comparison, the ferromagnetic behavior is improved in samples prepared by sol-gel route. A strong correlation between defect states and magnetic properties was observed. The BMP model was tried to explain the magnetism but the calculated number of BMPs per unit volume calculated be less than the percolation threshold to induce the ferromagnetism in the material some antiferromagnetic ordering was observed in the samples by fitting the the experimental data to Curie Weiss law. Now Co doped ZnO samples were prepared by sol-gel route and studied in detail. XRD measurements reveal that all samples have hexagonal wurtzite structure and no secondary phases were detected. XPS results reveal that Co atoms have successfully incorporated in tetrahedral sites of the wurtzite host matrix without forming any detectable impurity phase. The binding energy separation between Co 2p 3/2 and Co 2p 1/2 levels is determined to be 15.5 ev which is consistent with the divalent state of cobalt homogeneously surrounded by the oxygen tetrahedral. Energy values of Zn 2p 3/2 peaks are below those of the metallic Zn. Thus, the formation of Zn clusters is ruled out. Blue shift in the band gap has been observed which may be ascribed due to the Moss Burstein band filling effect. As, oxygen vacancies are inherently present in our samples, it can be predicted that oxygen vacancy defect constituted BMPs one of the promising candidates for the origin of RTFM in this system. The evolution observed in our case is increase in magnetization with increase in oxygen vacancies, indicating that percolation of BMPs may be responsible for ferromagnetism. The number of BMPs are found to be of the order of /cm 3 which was less than the necessary percolation in ZnO. For the further investigations of the 170

3 magnetic properties, Curie Weiss Law was tried to fit to M-T data and the existence of weak antiferromagnetic (AFM) exchange interaction between Co 2+ ions was observed and the AFM coupling increased with increasing cobalt content. Thus, the Co doped ZnO shows the RTFM behavior with some antiferromagnetic coupling. Ni doped ZnO is another prime candidate from the view point of transparency and magnetism for potential spintronic devices and optical application in short wavelength field. From the research point of view, the magnetic properties of the Ni doped ZnO are not very well understood. Phase pure Zn 1-x Ni x O nanoparticles were successfully prepared by the sol-gel route. The substitution of Zn sites by Ni 2+ ions in wurtzite ZnO was confirmed by XRD and a low intensity peak of NiO is observed for 6% doping concentration. FTIR spectroscopic measurements show a broad band in the range cm -1 composed of three distinct peaks assigned to E 1 (TO), SPM A 1 (TO), and SPM E 1 (TO) modes. A detailed FTIR analysis revealed a decrease in bond length of ZnO with Ni doping which is consistent with XRD results. The blue shift in PL spectra shows strong effect of Ni on ZnO lattice. Magnetic measurements show RTFM, which was explained on the basis of BMP model. The number of BMP s are found to be of the order of per cm 3, which is five order smaller than the required number of BMP s for percolation in ZnO. It may be predicted that oxygen rich stoichiometry with enhanced Zn-O bonding favours the indirect Ni-O-Ni ferromagnetic exchange coupling and reduction of Vo (donors) leading to strong hybridization of Ni in ZnO host matrix responsible for room temperature ferromagnetism. Cr is an attractive material because first-principle calculations indicated that the ferromagnetic (FM) state of Cr-doped ZnO would be more stable than other transition metal doped ZnO systems. It is also very attractive candidate for optoelectronic devices due to large UV-emission. Zn 1-x Cr x O (x=0.02, 0.04, 0.06 and 0.08) samples were prepared by thermal decomposition method. No impurity phase was observed in the sample revealed from XRD and FTIR spectra. Red shift in the band gap of ZnO is observed with the doping of Cr which also affect the magnetic properties of samples. This shift occurs most probably due to band structure deformation by Cr ion doping in the lattice structure of ZnO. In s d and p d exchange interactions, the conduction band edge decreases and the valence band edges increase, resulting reduction of energy bandgap. PL spectra reveals that large defect state are present in the samples along with the sharp peak of UV-emission which also shows the red shift in NBE with increase in doping concentration. All the samples show the RTFM behavior and the magnetization of the 171

4 samples increased with increase in Cr which seems to be defect mediated. The magnetic exchange interaction between O vacancy and Cr ions align all Cr spins around the O vacancy, forming BMPs, thus the magnetization data was nicely fitted in BMP model. The estimated numbers of BMPs are higher than earlier TM doped systems and it has approximately reached to the threshold condition of percolation in DMS. 8.2 OPTICAL PROPERTIES OF Al AND Mg DOPED ZnO NANOPARTICLES ZnO is one of the most popular semiconductors for developing blue and ultra-violet photonic devices. ZnO nanostructures are being used in several industrial applications like, medicine, gas sensors and transparent conducting oxide. Although, a significant understanding on ZnO based micro/nano systems has been created and reported in the literature, the work on pure Al and Mg doped ZnO nanoparticles grown at low temperature is quite insufficient. So, in this thesis work detailed optical and structural properties of Al and Mg doped ZnO nanoparticles are presented in chapter 6. Phase pure Al and Mg doped ZnO nanoparticles were successfully prepared by the DEA assisted thermal decomposition method. The substitution of Zn sites by Al 3+ ions in wurtzite ZnO was confirmed by XRD. The line broadening due to small crystallite size and strain was analyzed by various modified forms of W H equation. It was observed that the strain, stress and energy density varies with Al and Mg doping concentration. FTIR spectroscopic measurements show a broad band in the range cm -1, composed of three distinct peaks assigned to E 1 (TO), SPM A 1 (TO) and SPM E 1 (TO) modes. The calculated effective mass of Zn(Al) O bond decreased with Al substitution because of lower atomic weight of Al than that of Zn. Also, the average force constant decreased with Al substitution which results in an increment in the average Zn (Al) O bond length. The energy band gap of samples shows the variation from 3.03 to 3.21eV for Al doped ZnO and for Mg doped ZnO it varies form ev, increase in the band gap is attributed to the Burstein Moss shift caused by the doping on the Zn site. Room temperature PL measurements illustrate strong NBE emissions, followed by abroad band in the range nm attributed to defects and vacancies. The intensity defect state in Mg doped ZnO nanoparticles is very low i.e. it have the better crystallinity in the samples, when we 172

5 increase the Mg concencentration more than 6% then the intensity of defect states get increases. The tunability of optical properties and defect staes of ZnO nanoparticles by doping of Al and Mg could be useful for potential optoelectronic applications. 8.3 DIELECTRIC AND FERROELECTRIC STUDIES OF Ni And Ba DOPED ZnO SAMPLES Along with optical and magnetic properties, ferroelectric ZnO is an emerging field and has a lot of scope in future device fabrications. There are only few ZnO based FE materials reported in the literature which is highly controversial. So, chapter 7 is devoted to study the properties of Ni and Ba doped ZnO systems for the dilelectric and ferroelectric properties of ZnO. High quality Zn 1 x Ni x O (x=0.02, 0.04 and 0.06) samples were synthesized by the solid state route. The phase purity of samples was analyzed by XRD, which shows the phase purity up to 4% of doping concentration, a low intensity peak of NiO is observed for 6% doping concentration. The FTIR spectroscopic measurements show a broad band in the range cm 1 composed of three distinct peaks assigned to the E 1 (TO), SPM A 1 (TO) and SPM E 1 (TO) modes. A detailed FTIR analysis revealed a decrease in bond length of ZnO with Ni doping which is consistent with XRD results. UV measurement shows the enhancement in the band gap with doping. The blue shift in PL spectra shows strong effect of Ni on ZnO lattice. A transition in the dielectric behavior is observed around 320 C which is higher than earlier reported values but the transition mechanism is still not clear. Also Ba doped ZnO nanoparticles were successfully synthesized by thermal decomposition method. Red shift in band gap and enhanced defect states after Ba doping were observed. High value of dielectric constant and transition temperature (~ 330 C) were observed. The observed high temperature ferroelectricity with high value of remnant polarization (1.01 µccm -2 ) and low coercive field (2.02 kvcm -1 ) may play an important role in the miniaturization of devices and make it potential candidate for nano-optoelectronics, nanostorage and nanoscale memory devices. To fabricate reliable spintronic devices based on DMS materials is the ultimate goal, but it s a long way to go. In future it may be interesting to prepare ZnO nanostructures and thin films for spintronic devices and sensor applications and to optimize the ferroelectric properties of ZnO. 173