The analysis of wide band gap semiconductors using Raman spectroscopy

One of the main issues that must be addressed in the development of AlGaN/GaN HEMTs is the concern over device reliability. Due to the heteroepitaxial growth and other processing methods, mechanical stresses are inherent in these devices that can negatively impact device performance. Stressinduced effects on device performance due to Al content, (Zhang et al, J Appl Phys, 87:7981-7987, 2000), passivation, and substrate-induced residual stress in the GaN layer have been reported. In addition to impacting the 2-DEG, excessive strain in the AlGaN layer can induce crystallographic defects which lead to degraded device performance and shortened lifetimes. Under high-voltage conditions, the large electric field in the vicinity of the drain side gate edge is responsible for generation of large in-plane tensile strains in the AlGaN layer. These strains can result in device degradation via the formation of electrically active defects or trap states near the gate in the AlGaN. The direct measurement of the residual strain in the AlGaN layer is not simple due to its thickness (on the order of 25 nm). Instead, observation of the through-thickness variation of residual strain in the GaN layer allows a firstorder estimation of the imposed strains in the AlGaN layer. In this chapter, we detail the use of Raman spectroscopy for characterizing AlGaN/GaN HEMTs.