Thesis title: Piezoelectric properties control in nanostructured ceramic materials and device fabrication
This PhD thesis explores the control of the piezoelectric properties of two different ceramic materials such as Zinc Oxide Nano-Rods (ZnO-NR) for energy harvesting application and Gallium Nitride (GaN) for MMIC application. As far as ZnO-NRs are concerned, the goal is to develop a nanostructured device with marked piezoelectric properties while as regards to the GaN substrate, the goal is the control of the piezoelectric properties to investigate the influence on the performance of the MMIC device.
The work of this thesis is divided by the following:
1. Enhancing piezoelectric properties of the nano-engineered thin film of a vertically oriented array of Zinc Oxide Nano-Rods (ZnO-NRs) for energy harvesting application;
2. Piezoelectric properties control of the GaN/AlGaN substrate for MMIC application.
The first research activity reports the development and the piezoelectric characterization of a nano-engineered thin film of a vertically oriented array of Zinc Oxide Nano-Rods (ZnO-NRs) and Polyvinylidene Fluoride (PVDF). The fabrication of the device has been possible thanks to the optimization of the production techniques of individual constituent of the device, to obtain the best final piezoelectric response. The work was divided into three main parts:
1. The fabrication and characterization of an array of vertically oriented zinc oxide nanorods on a flexible substrate (PET-ITO). ZnO-NRs were produced by Chemical Bath Deposition (CBD) which is a user friendly and green technique. The piezoelectric response of the produced samples was investigated by evaluating the piezoelectric coefficient (d33), through Piezo-response Force Microscopy (PFM). We found that the piezoelectric coefficient of vertically oriented ZnO-NRs is (6.89±0.63) pm/V.
2. The fabrication of PDVF thin film with enhanced piezo-response were produced through spin-coating and phase inversion by quenching. We studied how the piezoelectric performance of PVDF films is affected by (i) the polymer/solvent mass ratio; (ii) the rotational spin speed; and (iii) the
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quenching temperature. The thin films obtained was characterized through the following techniques: FE-SEM (Field Emission-Scanning Electron Microscopy) and EDS (Energy Dispersion X-ray spectrometry) for morphological and chemical characterization, FT-IR (Fourier Transform Infrared Spectroscopy) for structural analysis, the rheometric measurement for rheological behaviour, and PFM (Piezo-response Force Microscopy) for the piezoelectric response. The best value of d33 (⁓30 pm/V) was obtained through higher polymer concentration (i.e., 30 wt.%), quenching in liquid nitrogen.
3. Fabrication of nano-engineering thin film with enhanced piezoelectric properties consisting of an array of vertically oriented ZnO-NRs grown on the flexible PET-ITO substrate embedded in a PVDF thin film matrix. In this case, the PVDF, besides acting as a protective matrix for the nanostructures, has a synergic function in the piezo-response leading a benefit in terms of the piezoelectric coefficient (d33). The thin film obtained was characterized by FE-SEM (Field Emission-Scanning Electron Microscopy) and Piezo-response Force Microscopy (PFM). We found that the piezoelectric coefficient of the nano-engineered thin film is (13.01±1.6) pm/V.
As regards the second research activity, the influence of the piezoelectric properties of the GaN/AlGaN substrate on the performance of GaN (Gallium Nitride)-based High Electron Mobility Transistors (HEMTs) on SiC was evaluated. The present research activity reports the optimization of low-strain Si3N4 (Silicon Nitrite) thin film obtained by PECVD (Plasma Enhanced-Chemical Vapor Deposition) for GaN-HEMT devices, where the influence of surface residual stress on the performance of GaN/AlGaN-HEMT have been evaluated. When the substrate is subjected to mechanical strain, the piezoelectric nature of the AlGaN/GaN substrate will produce a variation of 2-DEG (Two-Density Electron Gas) which it will influence the performances of the device itself. The mechanical stress shall be further amplified by high-temperature treatments like Rapid Thermal Annealing (RTA), requested during the manufacturing of the HEMT for Ohmic Contact formation, caused by molecular recombination of Si3N4. For these
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reasons, this work is also looking for a minimum and stable Si3N4 thin film delta stress before/after high-temperature thermal treatment. Design Of Experiment (DOE) approach has been employed to better set-up the experiments and to reach a robust and stable flat area of interest. The thin films physical characteristics are characterized through the following measurements:
1. Thickness and refractive index measurement by ellipsometry spectroscopy, before and after thermal treatment;
2. Stress measurement by a profilometer, by differential measurements of stress before and after the thermal treatment;
3. FTIR (Fourier-transform infrared spectroscopy) analysis.
In particular, the performances and the reliability of the HEMT device was evaluated by
• I-V measurement to evaluate the breakdown voltage and gate leakage;
• C-V measurement to evaluate the free carrier concentration (2-DEG);
We compared four Si3N4 thin films characterized by different tensile residual stress after the thermic treatment (RTA). We have shown the piezoelectric properties of the substrate influence the 2-DEG density and thus the performance of the device.