Ferroelectric materials have unique properties such as ferroelectric hysteresis, high permittivity, high piezoelectric effect and pyroelectric coefﬁcient, strong electro-optic effect and anomalous temperature coefﬁcient of resistivity . A well-known member of oxide ferroelectric materials is lead zirconate titanate (Pb[ZrxTi1-x]O3) which possesses excellent ferroelectric, piezoelectric, and electromechanical properties. Due to these superior properties, ferroelectric materials have a high potential for nonvolatile memories, capacitors, sensors, actuators and resonant wave devices, infra-red detectors .
Recently, the integration of PZT with III-V semiconductors such as gallium nitride (GaN) has attracted great deal of attention in scientific community. Gallium Nitride (GaN) semiconductor is an important material with many potential applications such as field effect transistor in high power and high frequency devices due to its direct, wide band gap of 3.45 eV at room temperature and it possesses high chemical and mechanical stability, fast carrier transport, high thermal conductivity and breakdown voltage . Furthermore, GaN-on-silicon offers the advantages, such as good electrical and thermal conductivity, large size availability, mass production, and significantly low cost for GaN optoelectronic and microelectronic devices . Fig. 1 (a) shows the AFM image of GaN/Si substrate which consist of steps and terraces structures. The step height of GaN surface is ~0.26 nm. The GaN surface has two dimensional RHEED patterns, which means that GaN surface roughness is quite low, at room temperature and 675 oC, as shown in Fig. 2 (a) and (b). RHEED sensitive to surface roughness and surface reconstructions, thus it gives about surface information. As a result of these surface characteristics shown in Fig. 1 and Fig. 2, GaN/Si substrates allow the deposition of highly crystalline thin films. The integration of PZT with GaN could lead to potential novel or new generation device applications. Some devices such as FeFET, PZT gate on AlGaN/GaN (two dimensional electron gas (2DEG) system), and microwawe devices have already been studied by using PZT/GaN heterostructures . The integration of these two materials has been limited because of the different crystal structures and large lattice mismatch of perovskite PZT and wurtzite GaN . If PZT films are grown directly on GaN, the non-ferroelectric pyrochlore phase is formed. The growth of epitaxial perovskite PZT films on GaN/Si substrates can be achieved using buffer layers such as MgO, TiO2, STO, PbO, PbTiO3 [7, 8]. Both the diffusion of Pb and Ti atoms into GaN and leakage current can be prevented as well as the crystallization of the perovskite-phase PZT films on GaN/Si can be promoted using buffer layers.
This project focuses on the integration of oxide ferroelectric materials (Pb[ZrxTi1-x]O3) with III/V. In order to grow epitaxial PZT thin films on GaN/Si substrates, pulsed laser deposition (PLD) has been used. We focus mainly on rocksalt oxide materials (MgCaO, MgO) as a candidate buffer layers on GaN/Si to achieve epitaxial growth of PZT films and to reduce the lattice mismatch between PZT and GaN. In order to understand the growth mechanism and interface structure of epitaxial PZT films grown on buffered GaN/Si, we use different characterization techniques such as XRD (X-Ray Diffraction), XPS (X-ray photoelectron spectroscopy (XPS), AFM (Atomic force microscopy), HRSEM (High Resolution Scanning Electron Microscopy), HRTEM (High-resolution transmission electron microscopy), and RHEED (Reflection high-energy electron diffraction). In phase of this project, we are going to realize device fabrication and characterization for PZT/GaN/Si systems.
Fig. 1 (a) AFM image and (b) step height of GaN/Si substrate.
Fig. 2 RHEED patterns of the surface of GaN/Si substrate at (a) room temperature and (b) 675 oC.
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