Strain measurements at AlGaN/GaN HEMT structures on Silicon substrates

High Electron Mobility Transistors (HEMTs) based on AlGaN/GaN are of great interest due to their high electrical performance and the related applications. The high carrier density, electron mobility, breakdown voltage, and the good thermal stability of AlGaN/GaN are great benefits for high power and high frequency technologies. The high electron mobility is a consequence of a two dimensional electron gas (2DEG) which is formed at the interface between GaN and AlGaN. The source, drain, and gate of the transistors are realized by metal contacts on top of the semiconductor. Strain in the transistor structures may arise due to thermal processing steps or applied passivation layers on top of the HEMT structure. Especially the thermal processes may cause strain due to the mismatch in the coefficients of thermal expansion between the metal contacts and the GaN/AlGaN. Additional stress may be induced by the substrate material. In order to reduce material costs, such as those associated with power electronic applications, the usage of silicon substrates to replace the expensive silicon carbide and sapphire are under development. The disadvantage of GaN on silicon is the lower quality of the deposited AlGaN/GaN layers caused by the mismatch of the lattice parameters which differ by 17%. This can cause higher defect densities and residual strain in the AlGaN/GaN epi-layers.