2-DEG Characteristics Improvement by N2-plasma exposure in GaN HEMT heterostructures

The lack of maturity in GaN HEMT processing is strongly limiting transistor expected performances. RF-DC dispersion, known as current collapse, seems to be related with carriers being trapped at the AlGaN surface. The use of a preprocessing N₂-plasma treatment of the surface seems to have no effective improvement, in comparison with that of the pre-passivation processing step. A set of three AlGaN/GaN heterostructures, subject to various surface treatment and cleaning procedures, have been studied. A reduction of DeltaV_th = (1.3 plusmn 0.4) V for the pinch off voltage and of Deltan_s = (-2.4 plusmn 0.3) cm² for the 2DEG charge density were found, for the three samples. The N₂-plasma treated samples showed a modification on the surface states and consequently a worsening of the initial characteristics of the surface. On the other side, an annealing post-processing shows a layer oxidation that modifies the electron distance to the surface DeltaW_B = (-5 plusmn 2) nm, and increases the 2DEG charge density, Deltan_s = (1.4 plusmn 0.3) cm^-2. Finally from XPS results, a contamination of oxygen, carbon and fluorine was found. The stronger cleaning power of the N₂-plasma, aside from the RTA, is used as a great carbon surface cleaner, reducing carbon concentration up to a (70 plusmn 6) %.