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 (SiNx) of different stress states (compressive, neutral, and tensile) is presented as a function of varying Al mole fraction (x). All types of SiNx 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 cm²V^-1s^-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 SiNx 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.