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Determination of Existing Stress in Silicon Films on Sapphire Substrate Using Raman Spectroscopy

Raman spectroscopy is used to determine built-in stresses in silicon on sapphire (SOS) devices. The method is direct, nondestructive and can be applied at various temperatures. For epitaxial silicon films on sapphire substrates a built-in stress of 7.0±0.3 kbar at room temperature and 8.7±0.3 kbar at liquid nitrogen temperature was measured.

We studied thermal annealing effects of low-temperature-grown GaN (LT-GaN) on Si substrate by use of the two step growth technique. Difficulties were encountered when LT-GaN films were annealed up to the temperature of 1050°C on Si substrate. However, the LT-GaN on sapphire was stable after the same annealing condition. The decomposition of LT-GaN and the wavy surface were observed. Thus, thermal stability of LT-GaN on Si was lower than that on sapphire. To improve this low thermal stability, LT-GaN on Si was coated by middle-temperature-grown GaN (MT-GaN). However, swellings were observed on the surface after annealing to the temperature of 1050°C and there were hollows, which indicated that the corrosion of Si occurred below the temperature of the melting point of Si, under swellings. Energy dispersive X-ray (EDX) analysis indicated that a large amount of Ga was detected in swellings. Ga is responsible for the corrosion of Si substrate and the possible reason for the formation of hollows was the meltback etching of Si by Ga.


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