Rigorous Investigation of AlGaN/GaN Heterostructure Surface Treatments with Si Thin Films

Wide bandgap AlGaN/GaN High Electron Mobility Transistors (HEMTs) are currently the subject of intensive research and have already demonstrated an unprecedented performance in high power, high frequency and high temperature areas of applications. In order to improve ohmic contacts and to address the performance degradation due to surface states on this material, several research groups have used Si-containing films on the semiconductor surface.;A remnant SiNx film with the approximate thickness between 1.0 and 2 0 nm was identified by SIMS and XPS on the Si-treated AlGaN surfaces after the Si film was removed with an aggressive HF:HNO3 wet etch. The water contact angle measurement has revealed the similarity between Si-coated and Si-treated samples, and the results were in sharp contrast to the measurements on the control samples.;The Si treatments and coatings were found to pre-passivate the AlGaN surface, significantly improving the Two-Dimensional Electron Gas (2DEG) sheet resistances and current densities before silicon nitride passivation. The best ohmic contact resistance values were not found to be affected significantly by the treatments or coatings and remained in the range of 0.5 - 0.6 O-mm (rhoc ≈ 6.3 x 10-6 - 9.0 x 10-6 O-cm2). However, Si treatments and coatings were found to significantly improve the ohmic contact resistance values in adverse or poorly optimized processing conditions. Both the treatments and the coatings have resulted in higher leakage currents and more negative threshold voltages in fabricated devices.;It was concluded that the solid state diffusion was not responsible for the electrical enhancements in fabricated devices. The electrical changes observed in this study were attributed to the remnant SiNx film on Si-treated and Si film on Si-coated AlGaN surfaces.;In this study a rigorous investigation of the "treated" and "coated" AlGaN/GaN surfaces with two different Si-containing thin films was carried out. "Treatment" is defined as the deposition of Si-containing thin film, encapsulation with SiO2, Rapid Thermal Annealing (RTA) and the films removal with a wet etch. "Coating" is defined as the deposition of Si film on the AlGaN/GaN surface left in place for all further processing. Treated, coated and control samples were examined with Secondary Ion Mass Spectroscopy (SIMS), X-ray Photo-electron Spectroscopy (XPS), Hall and contact angle measurements in order to establish the physical mechanism responsible for the electrical changes on the Si-treated surfaces. Previous workers have attributed the improvement in the electrical properties to deep diffusion of Si species into the GaN or AlGaN epilayers, as indicated by SIMS profiling. In this work, it is demonstrated that this detection of Si was due to a SIMS profiling artifact.