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Among the rapid growth in application for III-nitride materials, there is a partial interest in GaN/AlN multiple quantum wells for intersubband transitions. Because of the large conduction band offset (~ 2 eV) and the very short relaxation time (140-370 fs), GaN/AlN multiple quantum wells are promising candidates for developing ultrafast all-optical switches operating at Tbit/s. However, due to the large lattice constant mismatch (~ 2.5 % along the a-axis) and the thermal expansion coefficient difference (~ 30 % at room temperature) between GaN and AlN, large strain is generated inside the GaN/AlN multiple quantum well. This causes cracks in the sample surface which may be harmful for devices. In this work we have studied the crack density with the GaN buffer layer thickness, the GaN cap layer thickness, the number of quantum wells and the rate of temperature ramp down after growth as parameters. The GaN/AlN multiple quantum well structures were grown in a Varian Gen II MBE machine and the substrate was 2.5 um GaN (on c-plane sapphire) template grown by metal-organic vapor-phase epitaxy. The well and barrier widths were about 2.7 and 3.2 nm respectively and the number of quantum wells varied from 3 to 10. After growth the sample surface was studied by atomic force, scanning electron, and optical microscopy. We found that line cracks with hexagonal pattern appeared after 4 period quantum wells and the crack density increased exponentially with the number of periods. The different rates, 10 and 60 °C/min, of temperature ramp down had no effect on 5 period quantum wells. We also found the 100 nm GaN buffer layer reduced the crack density and it depended on the cap layer thickness, which had a minimum density for a cap layer thickness of about 280 nm. |