Wafer-scale hBN crystalline films: Although wafer-scale polycrystalline films of insulating hexagonal boron nitride (hBN) can be grown, the grain boundaries can cause both scattering or pinning of charge carriers in adjacent conducting layers that impair device performance. Lee et al. grew wafer-scale single-crystal films of hBN by feeding the precursors into molten gold films on tungsten substrates. The low solubility of boron and nitrogen in gold caused micrometer-scale grains of hBN to form that coalesced into single crystals. These films in turn supported the growth of epitaxial wafer-scale films of graphene and tungsten disulfide. Science , this issue p. 817

The large-scale crossbar array is a promising architecture for hardware-amenable energy efficient three-dimensional memory and neuromorphic computing systems. While accessing a memory cell with negligible sneak currents remains a fundamental issue in the crossbar array architecture, up-to-date memory cells for large-scale crossbar arrays suffer from process and device integration (one selector one resistor) or destructive read operation (complementary resistive switching). Here, we introduce a self-selective memory cell based on hexagonal boron nitride and graphene in a vertical heterostructure. Combining non-volatile and volatile memory operations in the two hexagonal boron nitride layers, we demonstrate a self-selectivity of 10¹⁰ with an on/off resistance ratio larger than 10³. The graphene layer efficiently blocks the diffusion of volatile silver filaments to integrate the volatile and non-volatile kinetics in a novel way. Our self-selective memory minimizes sneak currents on large-scale memory operation, thereby achieving a practical readout margin for terabit-scale and energy-efficient memory integration.