Strain Metrology of GaN/AlN Heterostructures and Devices
Access to this document is restricted. Some items have been embargoed at the request of the author, but will be made publicly available after the "No Access Until" date.
During the embargo period, you may request access to the item by clicking the link to the restricted file(s) and completing the request form. If we have contact information for a Cornell author, we will contact the author and request permission to provide access. If we do not have contact information for a Cornell author, or the author denies or does not respond to our inquiry, we will not be able to provide access. For more information, review our policies for restricted content.
The perturbative nature of strain has far-reaching impacts on a semiconducting crystal’s structural, electronic, and photonic properties, which have historically been studied to advance semiconductor technology. Impacts of strain are routinely observed in III-nitride lattice-mismatched heterostructures, ranging from defect generation to enhancement of properties like piezoelectricity and energy-band deformation. Rapid miniaturization of III-nitride devices has resulted in device dimensions in the micron and sub-micron regimes, where strain effects can also lead to new and unexpected behavior. Strained GaN technology has recently received attention in the form of epitaxially strained GaN grown on single-crystal AlN for applications in devices like the AlN/GaN/AlN high electron mobility transistor (HEMT) and the GaN/AlN p-channel Field Effect Transistor (pFET). Straining GaN has also been predicted to enhance hole mobilities in GaN by an order of magnitude, thus generating further interest in strain engineered GaN devices. Consequently, reliable and non-destructive strain metrology becomes a crucial necessity for the further development of III-nitride technology. This work studies the relationship between compressive strain and optical phonon frequencies in GaN/AlN heterostructures grown using plasma-assisted molecular beam epitaxy (PA-MBE). Deviation from the conventionally used phonon deformation potential theory is observed at high strain regimes, which is accounted for with an empirical model. Using this model in combination with confocal Raman microscopy, strain relaxation over a sub-micron length scale at the mesa-edge of the channel in an AlN/GaN/AlN HEMT device is quantified. Solid mechanics-based models and simulations are used to provide a compact prescription for the measured strain relaxation. The mesa-edge strain relaxation effect is also qualitatively verified using 4D scanning transmission electron microscopy (4D-STEM) measurements. In conclusion, this work develops a framework for non-destructive strain metrology to probe sub micron length-scale strain variations in complex III-nitride heterostructure-based devices and concludes with insights on the exploitation of the mesa-edge strain relaxation effect to engineer strain in GaN/AlN heterostructure-based devices.