RESUMO
As the electron mobility of two-dimensional (2D) materials is dependent on an insulating substrate, the nonuniform surface charge and morphology of silicon dioxide (SiO2) layers degrade the electron mobility of 2D materials. Here, we demonstrate that an atomically thin single-crystal insulating layer of silicon oxynitride (SiON) can be grown epitaxially on a SiC wafer at a wafer scale and find that the electron mobility of graphene field-effect transistors on the SiON layer is 1.5 times higher than that of graphene field-effect transistors on typical SiO2 films. Microscale and nanoscale void defects caused by heterostructure growth were eliminated for the wafer-scale growth of the single-crystal SiON layer. The single-crystal SiON layer can be grown on a SiC wafer with a single thermal process. This simple fabrication process, compatible with commercial semiconductor fabrication processes, makes the layer an excellent replacement for the SiO2/Si wafer.
RESUMO
We study topological phase transitions in tetragonal NaZnSb[Formula: see text]Bi[Formula: see text], driven by the chemical composition x. Notably, we examine mirror Chern numbers that change without symmetry indicators. First-principles calculations are performed to show that NaZnSb[Formula: see text]Bi[Formula: see text] experiences consecutive topological phase transitions, diagnosed by the strong [Formula: see text] topological index [Formula: see text] and two mirror Chern numbers [Formula: see text] and [Formula: see text]. As the chemical composition x increases, the topological invariants ([Formula: see text]) change from (000), (020), (220), to (111) at x = 0.15, 0.20, and 0.53, respectively. A simplified low-energy effective model is developed to examine the mirror Chern number changes, highlighting the central role of spectator Dirac fermions in avoiding symmetry indicators. Our findings suggest that NaZnSb[Formula: see text]Bi[Formula: see text] can be an exciting testbed for exploring the interplay between the topology and symmetry.