Band Alignments of Ternary Wurtzite and Zincblende III-Nitrides Investigated by Hybrid Density Functional Theory.

Band gaps and electron affinities of binary and ternary, wurtzite (wz-) and zincblende (zb-) III-nitrides are investigated using a unified hybrid density functional theory, and band offsets between wz- and zb- alloys are calculated using Anderson's electron affinity model. A conduction (and valence) band offset of 1.85 (0.89) eV has been calculated for zb-GaN/InN heterojunctions, which is 0.25 eV larger (and 0.26 eV smaller) than that of the wz- counterpart. Such polarization-free zb-GaN/InGaN/GaN quantum well structures with large conduction band offsets have the potential to suppress electron leakage current and quantum-confined Stark effects (QCSEs). Contrarily, the conduction (and valence) band offset of zb-AlN/GaN heterojunctions is calculated to be 1.32 (0.43) eV, which is 1.15 eV smaller (and 0.13 eV larger) than that of the wz- case. The significant reduction in zb-AlN/GaN band offsets is ascribed to the smaller and indirect band gap of zb-AlN-the direct-to-indirect crossover point in zb-Al X Ga1-X N is when X ∼ 65%. The small band gap of the zb-AlN barrier and the small conduction band offsets imply that electrons can be injected into zb-AlN/GaN/AlN quantum well heterostructures with small bias and less energy loss when captured by the quantum wells, respectively, i.e., loss as heat is reduced. The band gap of ternary III-nitrides does not linearly depend on alloy compositions, implying a nonlinear dependence of band offsets on compositions. As a result, the large bowing of the conduction band offset is identified and ascribed to the cation-like behavior of the conduction band minimum, while the linear dependence of the valence band offset on compositions is attributed to the anion-like character of the valence band maximum.

Confined and interface optical phonon emission in GaN/InGaN double barrier quantum well heterostructures.

In GaN-based high electron mobility transistors (HEMTs), the fast emission of longitudinal optical (LO) phonons can result in the formation of hot spots near the gate region where high electric fields produce hot electrons. In this work, we investigate the probability of phonon emission as a function of electron energy for confined and interface (IF) phonon modes for wurtzite GaN/InGaN/GaN heterostructures. Hot electrons radiate optical phonons which decay, anharmonically, into acoustic phonons that are essentially heat carriers. Herein, phonon engineering concepts are introduced which facilitate thermal management through the production of polar optical phonons. Some of the electrons near a semiconductor gate which manifests a strong electric field, are accelerated and the resulting hot electrons will produce confined and interface modes when the electrons are incident on a suitably-placed quantum well. This paper focuses on the production of confined and interface phonons. It is shown that interface modes may be preferentially produced which lead to elongated, lower-temperature hot spots.

Electron Scattering via Interface Optical Phonons with High Group Velocity in Wurtzite GaN-based Quantum Well Heterostructure.

Here we present a detailed theoretical analysis of the interaction between electrons and optical phonons of interface and confined modes in a wurtzite AlN/GaN/AlN quantum well heterostructure based on the uniaxial dielectric continuum model. The formalism describing the interface and confined mode optical phonon dispersion relation, electron-phonon scattering rates, and average group velocity of emitted optical phonons are developed and numerically calculated. The dispersion relation of the interface phonons shows a convergence to the resonant phonon frequencies 577.8 and 832.3 cm-1 with a steep slope around the zone center indicating a large group velocity. At the onset of interface phonon emission, the average group velocity is small due to the large contribution of interface and confined mode phonons with close-to-zero group velocity, but eventually increases up to larger values than the bulk GaN acoustic phonon velocity along the wurtzite crystal c-axis (8 nm/ps). By adjusting the GaN thickness in the double heterostructure, the average group velocity can be engineered to become larger than the velocity of acoustic phonons at a specific electron energy. This suggests that the high group velocity interface mode optical phonons can be exploited to remove heat more effectively and reduce junction temperatures in GaN-based heterostructures.

High Internal Quantum Efficiency Ultraviolet Emission from Phase-Transition Cubic GaN Integrated on Nanopatterned Si(100).

Ultraviolet emission characteristics of cubic (c-) GaN enabled through hexagonal-to-cubic phase transition are reported. Substrate patterning and material growth are shown to affect phase purity and emission characteristics of c-GaN as studied by electron backscatter diffraction, and photo- and cathodoluminescence, respectively. Raman study shows a tensile strain in the c-GaN. Time-resolved photoluminescence reveals c-GaN band edge emission decay time of 11 ps. The ultraviolet emissions from both phases of GaN are linearly polarized in the same direction, which is along the ⟨112̅0⟩ and ⟨110⟩ directions of hexagonal GaN and c-GaN, respectively. Temperature-dependent (5.7 to 280 K) cathodoluminescence studies reveal an internal quantum efficiency of ~29% at room temperature along with intrinsic and extrinsic defect energy levels of ~124 and ~344 meV, respectively, of the phase-transition c-GaN. Using the IQE value and carrier decay lifetime, a radiative lifetime of 38 ps is extracted. Overall, photonic properties of phase-transition c-GaN and their dependence on substrate patterning and material growth are reported.

Principle of direct van der Waals epitaxy of single-crystalline films on epitaxial graphene.

There are numerous studies on the growth of planar films on sp(2)-bonded two-dimensional (2D) layered materials. However, it has been challenging to grow single-crystalline films on 2D materials due to the extremely low surface energy. Recently, buffer-assisted growth of crystalline films on 2D layered materials has been introduced, but the crystalline quality is not comparable with the films grown on sp(3)-bonded three-dimensional materials. Here we demonstrate direct van der Waals epitaxy of high-quality single-crystalline GaN films on epitaxial graphene with low defectivity and surface roughness comparable with that grown on conventional SiC or sapphire substrates. The GaN film is released and transferred onto arbitrary substrates. The post-released graphene/SiC substrate is reused for multiple growth and transfer cycles of GaN films. We demonstrate fully functional blue light-emitting diodes (LEDs) by growing LED stacks on reused graphene/SiC substrates followed by transfer onto plastic tapes.