Transport Studies of AlGaN/GaN Heterostructures of Different Al Mole Fractions With Variable $\hbox{SiN}_{x}$ Passivation Stress

The influence of passivation by silicon nitride (SiN
x
) of different stress states (compressive, neutral, and tensile) is presented as a function of varying Al mole fraction (
x
). All types of SiN
x
passivant induced, as expected, an increase in 2-D electron gas (2DEG) concentration. In addition, however, the 2DEG mobility increased after passivation for the low-
x
(0.15) sample, and the more tensile the film stress is, the greater the relative increase. This led to a very highly measured 2DEG mobility of 2360 cm
²
V
^-1
s
^-1
at 300 K. In higher
x
samples, however, mobility was decreased after passivation, increasingly so as
x
increased, and to a varying extent with different stresses. It is apparent from the results that there is a complex relationship between the stress in the SiN
x
layer, the mole fraction, and the transport properties of the 2DEG. Thus, tailoring of the passivant deposition conditions, and not simply passivant dielectric choice, to optimize transport properties, is critical for a given passivant, deposition tool, and Al mole fraction, to maximize device performance.