Effect of growth temperature on the structural and optical properties of few-layer hexagonal boron nitride by molecular beam epitaxy.

We have studied the epitaxy of few-layer hexagonal boron nitride (h-BN) by plasma-assisted molecular beam epitaxy (MBE) using a low growth rate and nitrogen-rich condition. It has been determined that under such conditions, the growth temperature is the factor having the most significant impact on the structural and optical quality of the material. When grown at temperatures <1000 °C, the h-BN film is polycrystalline, and defect-related photoluminescence (PL) emission dominates. Epitaxial domains of exceptional crystalline quality are obtained at elevated substrate temperatures of ~1300 °C, which exhibit strong band-edge PL emission at ~220 nm and negligible defect-related emission at room temperature. Our atomistic calculations reveal that, even though the gap of h-BN is indirect, it luminesces as strongly as direct-gap materials. Experimentally, the luminescence intensity of such a few-layer h-BN sample is measured to be two orders of magnitude stronger than that of a 4-µm thick commercially grown AlN template on sapphire, demonstrating the extraordinary potential of epitaxial h-BN for deep ultraviolet (UV) optoelectronics and quantum photonics.

Electronic and Optical Properties of Two-Dimensional GaN from First-Principles.

Gallium nitride (GaN) is an important commercial semiconductor for solid-state lighting applications. Atomically thin GaN, a recently synthesized two-dimensional material, is of particular interest because the extreme quantum confinement enables additional control of its light-emitting properties. We performed first-principles calculations based on density functional and many-body perturbation theory to investigate the electronic, optical, and excitonic properties of monolayer and bilayer two-dimensional (2D) GaN as a function of strain. Our results demonstrate that light emission from monolayer 2D GaN is blueshifted into the deep ultraviolet range, which is promising for sterilization and water-purification applications. Light emission from bilayer 2D GaN occurs at a similar wavelength to its bulk counterpart due to the cancellation of the effect of quantum confinement on the optical gap by the quantum-confined Stark shift. Polarized light emission at room temperature is possible via uniaxial in-plane strain, which is desirable for energy-efficient display applications. We compare the electronic and optical properties of freestanding two-dimensional GaN to atomically thin GaN wells embedded within AlN barriers in order to understand how the functional properties are influenced by the presence of barriers. Our results provide microscopic understanding of the electronic and optical characteristics of GaN at the few-layer regime.