Nathaniel Findling

The formation of an ohmic p-contact represents a critical step in the processing of GaN-based devices such as light-emitting diodes (LEDs). Thermal annealing in an O2-containing atmosphere at high temperature of a GaN/Ni–Au contact is mostly used today, while nonannealed contacts have been reported to provide much lower electrical performances. We experimentally evidence that the quality of the contact before annealing strongly depends on the nature of the Ni/GaN interface. Electrical results are correlated to microstructural and physicochemical analysis using high-resolution transmission scanning electron microscopy coupled with energy dispersive x-ray spectroscopy for this purpose. Using an annealed contact formed in standard conditions as a reference, we evidence from in situ treatment of the interface before metal evaporation that interfacial Ga-Oxide plays a detrimental role. We show that the metal evaporation conditions may strongly impact the physicochemical composition of the interface before annealing. We obtain nonannealed contact with electrical characteristics close to that of an annealed, standard one, using a reduced evaporation rate, in our evaporation conditions. This result is of interest for the processing of p-GaN contacts at a low thermal budget.

Superlattices (SLs) with spatially modulated elastic properties-particularly those based on III–V semiconductors-have emerged as key platforms for reaching the ultrahigh-frequency (GHz–THz) regime in nanoacoustic devices. The exploration of alternative materials with multifunctional properties remains a rich and promising area of research. In this work, we investigate the structural, magnetic, and acoustic properties of nanometric Co/Pt SLs. X-ray reflectometry reveals a well-defined periodicity, while scanning transmission electron microscopy combined with local compositional analysis shows that the SLs exhibit a compositional modulation rather than sharp interfaces. The polycrystalline nature of the structures is confirmed by both x-ray diffraction and transmission electron microscopy. Magnetization measurements indicate the presence of perpendicular magnetic anisotropy at higher Co concentrations. Picosecond acoustic experiments demonstrate that the SLs support short-lived acoustic modes up to 900 GHz, along with up to seven acoustic echoes with frequencies ranging from 40 to 250 GHz. These findings open a path toward investigating phonon-driven magnetization dynamics in such structures, establishing a foundational step toward the development of magnetoacoustic devices operating at ultrahigh frequencies.