A self-referenced interferometer for in situ cryogenic wafer curvature measurements.

A self-referenced interferometer to measure time-varying curvature in mechanically unstable environments is needed in many applications. One application that demands this measurement technique with fast data acquisition, 2D sensitivity, and insensitivity to vibration is the measurement of thermal strain in thin films in operational environments. The diverging beam interferometer described here demonstrates an angular sensitivity to the local curvature using interferograms captured by a CMOS camera. Two-dimensional Fourier analysis is used to extract curvature changes. The interferometer demonstrates an experimental sensitivity to curvature changes on the order of 10-4 m-1 and is used to measure thermal stresses in a cryogenic environment of a polycrystalline titanium nitride thin film on a silicon wafer that exhibits anisotropic curvature.

Band Alignment of ScxAl1-xN/GaN Heterojunctions.

ScAlN is an emergent ultrawide-band-gap material with both a high piezoresponse and demonstrated ferroelectric polarization switching. Recent demonstration of epitaxial growth of ScAlN on GaN has unlocked prospects for new high-power transistors and nonvolatile memory technologies fabricated from these materials. An understanding of the band alignments between ScAlN and GaN is crucial in order to control the electronic and optical properties of engineered devices. To date, there have been no experimental studies of the band offsets between ScAlN and GaN. This work presents optical characterization of the band gap of molecular beam epitaxy grown ScxAl1-xN using spectroscopic ellipsometry and measurements of the band offsets of ScxAl1-xN with GaN using X-ray photoemission spectroscopy, along with a comparison to first-principles calculations. The band gap is shown to continuously decrease as a function of increasing ScN alloy fraction with a negative bowing parameter. Furthermore, a crossover from straddling (type-I) to staggered (type-II) band offsets is demonstrated as Sc composition increases beyond approximately x = 0.11. These results show that the ScAlN/GaN valence band alignment can be tuned by changing the Sc alloy fraction, which can help guide the design of heterostructures in future ScAlN/GaN-based devices.