The van der Waals heterostructures of two-dimensional (2D) atomic crystals constitute a new paradigm in nanoscience. Hybrid devices of graphene with insulating 2D hexagonal boron nitride (h-BN) have emerged as promising nanoelectronic architectures through demonstrations of ultrahigh electron mobilities and charge-based tunnel transistors. Here, we expand the functional horizon of such 2D materials demonstrating the quantum tunneling of spin polarized electrons through atomic planes of CVD grown h-BN. We report excellent tunneling behavior of h-BN layers together with tunnel spin injection and transport in graphene using ferromagnet/h-BN contacts. Employing h-BN tunnel contacts, we observe enhancements in both spin signal amplitude and lifetime by an order of magnitude. We demonstrate spin transport and precession over micrometer-scale distances with spin lifetime up to 0.46 nanosecond. Our results and complementary magnetoresistance calculations illustrate that CVD h-BN tunnel barrier provides a reliable, reproducible and alternative approach to address the conductivity mismatch problem for spin injection into graphene.

We study the properties of Si-doped AlN films grown on 6H-SiC(0001) by plasma-assisted molecular beam epitaxy. Whereas nominally undoped AlN films are invariably insulating in nature, Si-doped films are found to be semiconducting with an electron concentration up to 7.4×1017cm−3, and a resistivity approaching 1 Ω cm at room temperature. Even heavy Si-doping (1×1020cm−3) does not degrade the structural properties of the AlN films. The morphology of these films is characterized by Si-induced step-bunching, but remains smooth with a rms roughness of about 1 nm.

The impact of macroscopic sample inhomogeneity and contact placement on the measured carrier type and concentration in the widely used van der Pauw–Hall method is investigated using finite-element analysis of the electrostatics. A variety of likely macroscopic inhomogeneities in mobility or carrier concentration across a square-shaped sample are considered. Inhomogeneities in mobilities do not affect the measured carrier type and concentration, as long as the carrier concentration remains homogeneous. Inhomogeneities in carrier concentrations can result in an incorrect assignment of the carrier type. However, when contacts are placed at the sample corners and not inside the sample area, the correct carrier type is recovered. Our calculations provide an explanation for recent measurements on ZnO [Ohgaki et al., J. Mater. Res. 23, 2293 (2008)]. Guidelines for avoiding incorrect interpretation of van der Pauw–Hall measurements are provided.

Hexagonal boron nitride (h-BN) is a large bandgap insulating isomorph of graphene, ideal for atomically thin tunnel barrier applications. In this letter, we demonstrate large area chemical vapor deposited (CVD) h-BN as a promising spin tunnel barrier in graphene spin transport devices. In such structures, the ferromagnetic tunnel contacts with h-BN barrier are found to show robust tunneling characteristics over a large scale with resistances in the favorable range for efficient spin injection into graphene. The non-local spin transport and precession experiments reveal spin lifetime ≈500 ps and spin diffusion length ≈1.6 μm in graphene with tunnel spin polarization ≈11% at 100 K. The electrical and spin transport measurements at different injection bias current and gate voltages confirm tunnel spin injection through h-BN barrier. These results open up possibilities for implementation of large area CVD h-BN in spintronic technologies.