Measurements of strain in AlGaN/GaN HEMT structures grown by plasma assisted molecular beam epitaxy

• AlGaN layers were grown by PAMBE.
• Structure and strain were determined by X-ray and HRTEM.
• Strain is distributed anisotropically: uniform and shallow in GaN.
• Strain is distributed anisotropically: nonuniform and deep in AlGaN.

The AlGaN/GaN high electron mobility transistor (HEMT) structures were grown on the (0001) HVPE bulk GaN substrates using plasma-assisted molecular-beam epitaxy (PAMBE). The AlGaN layers of 12% or 21% Al were grown to nominal 20 nm thickness after which the 3 nm thick GaN cap was added. High-resolution X-ray diffraction (HRXRD) and TEM measurements were used to determine crystallographic quality, composition and strain distribution in these samples. It was shown that 12% Al samples are uniform while 21% samples exhibit large compositional nonuniformity. The mismatch strain is localized close to interface on GaN side while on AlGaN side penetrates deeply, in more nonuniform way, especially for higher Al content sample.