STUDY OF HfO2 BASED STRUCTURES FOR OPTICAL AND THERMOELECTRIC APPLICATIONS

Abstract

The quest for clean and sustainable energy sources has prompted significant research interest towards the exploration of efficient energy conversion technologies such as thermoelectric (TE) energy harvesting and solid state lighting. The tailored materials required in these technologies are currently at the center stage of material science research. The TE materials can convert waste heat into usable electrical energy and are useful for a wide span of applications ranging from wearable devices to space applications. On the other hand, the good luminescent properties of a material is the first condition for its use in optoelectronic and light emitting applications. The energy conservation based on the light emitting diodes are viable substitute for incandescent bulbs on account of their high brightness, efficiency, fast response rate, and low energy consumption. The distinctive properties of metal oxides make them potential candidates for high temperature TE application and solid state lighting. In this thesis, we have carried out a detailed investigation of structural phase transition, electronic, transport, and optical properties of various phases of HfO2 for enhanced TE performance as well as the optical response using computational and experimental approaches. We realized that carrier concentration optimization is an effective way to enhance the TE performance of HfO2. The feasibility of high p-type carrier concentration (order of ~1022 cm-3 ) was experimentally demonstrated in HfO2. In light of these studies, using the first-principles calculation combined with the semi-classical Boltzmann transport theory, we have reported high TE performance in various polymorphs of HfO2 in a range of carrier concentrations (order of ~ 1018 -1022 cm-3 ) at high temperatures. The highest value of Seebeck coefficient has observed in tetragonal phase at 300 K. The lattice thermal conductivities at room temperature are 5.56, 2.87, 4.32, and 1.75 Wm-1K -1 for cubic (c)-, monoclinic (m)-, orthorhombic (o)- and tetragonal (t)- HfO2, respectively which decrease to 1.58, 0.92, 1.12, 0.53 Wm-1K -1 at 1200 K for respective phases. The low lattice thermal conductivities lead to the high values of the figure of merit, i.e., 0.97, 0.87, 0.83, and 0.77 at 1200 K for the m-, o-, t-, and c- HfO2, respectively, at the optimized carrier concentrations (~1021 cm-3 ). Further to enhance the TE performance of c-HfO2, we have investigated the doping of Ti and S at cationic and anionic sites. The convex hull method has been employed to estimate the stability of various doped structures. Doping leads to create new trap states in the band gap. The band gap with Ti doping decreases more viii sharply as compared to S doping. The magnitude of the Seebeck coefficient is high in Ti doping as compared to S doping. The figure of merit of c-HfO2 with doping enhances ~0.82 at 800 K. We have analyzed the structural phase transition in the HfO2 under the effect of doping and pressure. We discussed the phase transition from a centrosymmetric m-HfO2 to a non centrosymmetric o-HfO2 with Si doping. The reported phase transition pressures i.e., 15GPa, 14 GPa, 8 GPa, and 8 GPa for x = 0, 0.03, 0.06, and 0.09, respectively for Hf1-xSixO2 are in excellent agreement with available experimental results. It has observed that with increasing Si concentrations the transition pressures reduce significantly which is understood in terms of bond length and charge transfer. The thermal stability of the obtained o-HfO2 phase has examined via ab initio molecular dynamics up to its synthesis temperature. The density of states indicates the noticeable appearance of Si states in the lower conduction band and an increase in the extent of hybridization. The effect of Si, along with other dopants such as Ti and S, also examined on the optical properties of HfO2 by calculating the dielectric function and refractive index. The refractive index slightly enhanced with Ti and S doping. The desired and suitable optical properties can be augmented via the suggested doping mechanism. The effect of Si doping on o-HfO2 also broader the UV absorption range but no significant enhancement in refractive index is observed. Our calculations have revealed that the value of refractive index o-HfO2 at 15 GPa can be readily attained at lower pressure i.e., 8 GPa with 9% of Si doing. The optical response of HfO2 under different doping makes it a viable candidate in optoelectronic applications. The structure phase transformation and luminescence properties of undoped and doped HfO2 have been explored experimentally. The samples were synthesized via chemical co precipitation method. The crystal structure and phase analysis of the prepared sample have carried out by XRD and Rietveld refinement studies. The UV-visible spectroscopy has used to analyze the band gap of HfO2. The observed band gap using the TB-mBJ approach is in good agreement with the experimental results. The photoluminescence peaks corresponding to 562 nm, 536 nm, and 450 nm wavelength are attributed to oxygen vacancy. To analyze the vacancy, we have calculated the density of states of O3 and O4 vacancy using density functional theory. The peaks correlated with the total density of states of O4 single vacancy are in close agreement with our experimental observation. A low temperature synthesis of HfO2:xEu3+ (x = 0, 3, 5 and 7 mol%) at 600 ◦C. The XRD results revealed the monoclinic phase in undoped HfO2 and showed mixed phase formation at lower ix concentrations and a dominant cubic phase achieved at 5 mol% doping of Eu in HfO2. The phase transition has also been calculated using density function theory which shows transition point at ~5.11% doping concentration. X-ray absorption spectra has used to identify the oxidation state of Eu ions in the HfO2. Photoluminescence study has demonstrated the emission in the red region with high color purity under different excitation wavelengths from near UV to blue light. The reddish photoluminescence emission with high color purity under different excitation wavelengths from n-UV to blue region may be exploited in solid state lighting-based applications. We hope that our studies regarding structural phase transition in HfO2 help to understand how pressure and doping can be used to optimized various properties of interest. The strategies for enhancing optical and TE properties via doping and carrier concentration optimization may open new avenues for exploring HfO2 for solid state lighting and high temperature TE energy harvesting applications.