Strong Coupling of Epsilon-Near-Zero Phonon Polaritons in Polar Dielectric Heterostructures.

We report the first observation of epsilon-near-zero (ENZ) phonon polaritons in an ultrathin AlN film fully hybridized with surface phonon polaritons (SPhP) supported by the adjacent SiC substrate. Employing a strong coupling model for the analysis of the dispersion and electric field distribution in these hybridized modes, we show that they share the most prominent features of the two precursor modes. The novel ENZ-SPhP coupled polaritons with a highly propagative character and deeply subwavelength light confinement can be utilized as building blocks for future infrared and terahertz nanophotonic integration and communication devices.

Near-UV electroluminescence in unipolar-doped, bipolar-tunneling GaN/AlN heterostructures.

Cross-gap light emission is reported in n-type unipolar GaN/AlN double-barrier heterostructure diodes at room temperature. Three different designs were grown on semi-insulating bulk GaN substrates using molecular beam epitaxy (MBE). All samples displayed a single electroluminescent spectral peak at 360 nm with full-width at half-maximum (FWHM) values no greater than 16 nm and an external quantum efficiency (EQE) of ≈0.0074% at 18.8 mA. In contrast to traditional GaN light emitters, p-type doping and p-contacts are completely avoided, and instead, holes are created in the GaN on the emitter side of the tunneling structure by direct interband (that is, Zener) tunneling from the valence band to the conduction band on the collector side. The Zener tunneling is enhanced by the high electric fields (~5 × 106 V cm-1) created by the notably large polarization-induced sheet charge at the interfaces between the AlN and GaN.

Plasma-assisted Molecular Beam Epitaxy of N-polar InAlN-barrier High-electron-mobility Transistors.

Plasma-assisted molecular beam epitaxy is well suited for the epitaxial growth of III-nitride thin films and heterostructures with smooth, abrupt interfaces required for high-quality high-electron-mobility transistors (HEMTs). A procedure is presented for the growth of N-polar InAlN HEMTs, including wafer preparation and growth of buffer layers, the InAlN barrier layer, AlN and GaN interlayers and the GaN channel. Critical issues at each step of the process are identified, such as avoiding Ga accumulation in the GaN buffer, the role of temperature on InAlN compositional homogeneity, and the use of Ga flux during the AlN interlayer and the interrupt prior to GaN channel growth. Compositionally homogeneous N-polar InAlN thin films are demonstrated with surface root-mean-squared roughness as low as 0.19 nm and InAlN-based HEMT structures are reported having mobility as high as 1,750 cm2/V∙sec for devices with a sheet charge density of 1.7 x 1013 cm-2.