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 N
₂
-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 ΔV
_th
=(1.3±0.4)V for the pinch off voltage and of Δn
_s
= (-2.4±0.3) cm
^-2
for the 2DEG charge density were found, for the three samples. The N
₂
-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 ΔW
_B
= (-5±2) nm, and increases the 2DEG charge density, Δn
_s
= (1.4±0.3) cm
^-2
. Finally from XPS results, a contamination of oxygen, carbon and fluorine was found. The stronger cleaning power of the N
₂
-plasma, aside from the RTA, is used as a great carbon surface cleaner, reducing carbon concentration up to a (70±6)%.