Investigating gate metal induced reduction of surface donor density in GaN/AlGaN/GaN heterostructure by electroreflectance spectroscopy

•The surface donor state density of GaN/AlGaN/GaN heterostructures was investigated by electroreflectance spectroscopy•The reduced surface donor density caused by gate metal can be calculated for GaN capped heterostructures•GaN capping layer has a role to play in preventing the reduction of surface donor states of AlGaN layer interface

The internal field of GaN/AlGaN/GaN heterostructure on Si-substrate was investigated by varying the thickness of an undoped-GaN capping layer using electroreflectance spectroscopy. The four samples investigated are AlGaN/GaN heterostructure without a GaN cap layer (reference sample) and three other samples with GaN/AlGaN/GaN heterostructures in which the different thickness of GaN cap layer (2.7 nm, 7.5 nm, and 12.4 nm) has been considered. The sheet carrier density (ns) of a two-dimensional electron gas has decreased significantly from 4.66 × 1012 cm−2 to 2.15 × 1012 cm−2 upon deposition of a GaN capping layer (12.4 nm) over the reference structure. Through the analysis of internal fields in each GaN capping and AlGaN barrier layers, it has been concluded that the undiminished surface donor states (ns) of a reference structure and the reduced ns caused by the Au gate metal are approximately 5.66 × 1012 cm−2 and 1.08 × 1012 cm−2, respectively.