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This paper focuses on the residual stress in a lithium niobate (LN) film layer of a LN-on-insulator (LNOI)/Si hybrid wafer. This stress originates from a large mismatch between the thermal expansion coefficients of the layers. A modified surface-activated bonding method achieved fabrication of a thin-film LNOI/Si hybrid wafer. This low-temperature bonding method at 100 °C showed a strong bond between the LN and SiO2 layers, which is sufficient to withstand the wafer thinning to a LN thickness of approximately 5 μm using conventional mechanical polishing. Using micro-Raman spectroscopy, the residual stress in the bonded LN film in this trilayered (LN/SiO2/Si) structure was investigated. The measured residual tensile stress in the LN film layer was approximately 155 MPa, which was similar to the value calculated by stress analysis. This study will be useful for the development of various hetero-integrated LN micro-devices, including silicon-based, LNOI-integrated photonic devices.

The integer quantum Hall (QH) effects characterized by topologically quantized and nondissipative transport are caused by an electrically insulating incompressible phase that prevents backscattering between chiral metallic channels. We probed the incompressible area susceptible to the breakdown of topological protection using a scanning gate technique incorporating nonequilibrium transport. The obtained pattern revealed the filling-factor (ν)-dependent evolution of the microscopic incompressible structures located along the edge and in the bulk region. We found that these specific structures, respectively attributed to the incompressible edge strip and bulk localization, show good agreement in terms of ν-dependent evolution with a calculation of the equilibrium QH incompressible phases, indicating the robustness of the QH incompressible phases under the nonequilibrium condition. Further, we found that the ν dependency of the incompressible patterns is, in turn, destroyed by a large imposed current during the deep QH effect breakdown. These results demonstrate the ability of our method to image the microscopic transport properties of a topological two-dimensional system. PACS numbers: Valid PACS appear hereA two-dimensional electron system (2DES) subjected to strong magnetic fields forms a quantum Hall (QH) insulating phase with a state lying in a gap between quantized Landau levels (LLs). This gapped phase, the socalled incompressible phase, prevents backscattering between the metallic gapless (compressible) phase counterpropagating along both sides of the 2DES edges [1]. This is the key microscopic aspect of nondissipative chiral transport of the integer QH effect, which is characterized by a longitudinal resistance that vanishes and a universal quantized Hall conductance protected by a topological invariant [2,3]. Topological phases are attracting renewed attention due to the recent discovery of exotic topological materials such as insulators [4][5][6][7], superconductors [8], and Weyl semimetals [9].The formation of incompressible and compressible phases in the QH regime originates from the interplay between Landau quantization and the Coulomb interaction [10], which drives nonlinear screening [11,12]. The spatial configuration depends on the potential landscape. For example, the edge confinement potential, accompanied by strong bending of the LLs, forms spatially alternating unscreening and screening regions due to the Fermi-level pinning at the gap and LLs. These regions respectively result in alternating incompressible and compressible strips near the 2DES edge. The innermost incompressible strip moves and spreads to the bulk as the LL filling factor ν reduces to an integer i from a higher ν. This ν dependency of the edge strips has been microscopically investigated using various imaging techniques such as single-electron transistor imaging [13], Hall-potential imaging [14,15], microwave impedance imaging [16], ca-pacitance imaging [17], and scanning gate imaging [18][19][20][21][22][23][24], and it has bee...