Growth of epitaxial orthorhombic YO1.5-substituted HfO2 thin film

Abstract

YO1.5-substituted HfO2 thin films with various substitution amounts were grown on (100) YSZ substrates by the pulsed laser deposition method directly from the vapor phase. The epitaxial growth of film with different YO1.5 amounts was confirmed by the X-ray diffraction method. Wide-area reciprocal lattice mapping measurements were performed to clarify the crystal symmetry of films. The formed phases changed from low-symmetry monoclinic baddeleyite to high-symmetry tetragonal/cubic fluorite phases through an orthorhombic phase as the YO1.5 amount increased from 0 to 0.15. The additional annular bright-field scanning transmission electron microscopy indicates that the orthorhombic phase has polar structure. This means that the direct growth by vapor is of polar orthorhombic HfO2-based film. Moreover, high-temperature X-ray diffraction measurements showed that the film with a YO1.5 amount of 0.07 with orthorhombic structure at room temperature only exhibited a structural phase transition to tetragonal phase above 450 °C. This temperature is much higher than the reported maximum temperature of 200 °C to obtain ferroelectricity as well as the expected temperature for real device application. The growth of epitaxial orthorhombic HfO2-based film helps clarify the nature of ferroelectricity in HfO2-based films (186 words/200 words).

Ferroelectric HfO2-based materials for next-generation ferroelectric memories

“…Besides the initially reported Si, many other dopants, including Zr, 5,26,27 Y, [28][29][30][31] Al, 32 Gd, 33 Sr, 34 and La 35 have also been reported to induce the ferroelectricity in HfO 2 -based thin films. Doping is able to stabilize the o-phase of HfO 2 with appropriate dopant concentrations, because dopants with ionic sizes different from that of Hf can significantly modify the metal-oxygen bonding and further change the relative stabilities of various HfO 2 phases.…”

“…In this connection, anisotropic stresses are required for the phase transition from tetragonal to orthorhombic, i.e., a compressive stress within the aob plane and a tensile stress along the c-axis exerted on the t-phase. There are many factors reported hitherto to be responsible for causing the anisotropic stresses and thus stabilizing the ferroelectric o-phase during the film growth, such as doping, 4,5,[23][24][25][26][27][28][29][30][31][32][33][34][35] surface energy effect, [36][37][38] island coalescence, 39 thermal expansion mismatch, 17 capping layer effect, 4,38 and formation of oxygen vacancies. 40 The following of this section will discuss each of these factors in order.…”

Ferroelectricity in Simple Binary Crystals

“…The ferroelectricity was confirmed by D-E hysteresis measurements, which revealed P s of 16 µC/cm 2 as shown in Figure 9. The Curie temperature (T c ) was estimated to be about 623 K [39,40]. [18,19,[35][36][37][38].…”

The demonstration of significant ferroelectricity in epitaxial Y-doped HfO2 film

“…Recently, transmission electron microscopy (TEM), scanning TEM (STEM), and convergent electron beam diffraction have shown that the ferroelectric phase in HfO 2 -based materials has a polar structure28. After this study, we grew a single-crystal epitaxial HfO 2 -based films with a single polar orthorhombic phase by pulsed laser deposition (PLD)30. In the previous study, we probed the polar crystal structure by XRD and annular bright-field STEM that visualized the atomic arrangement, including light O atoms30.…”

“…After this study, we grew a single-crystal epitaxial HfO 2 -based films with a single polar orthorhombic phase by pulsed laser deposition (PLD)30. In the previous study, we probed the polar crystal structure by XRD and annular bright-field STEM that visualized the atomic arrangement, including light O atoms30.…”