UC Santa Barbara

(Al,Ga,In)N semiconductor materials are widely used in high-frequency, high-power electronics due to their wide bandgaps. Both metal- and N-polar AlGaN/GaN high-electron-mobility transistors (HEMTs) demonstrated excellent performances as high-frequency signal amplifiers. While the majority of today’s III-N transistors are based on metal-polar heterostructures, N-polar materials have gained attention following the breakthrough in the deposition of high quality films. Compared to their metal-polar counterparts, N-polar HEMT structures improve the scalability of devices, increase the electron confinement and reduce contact resistance, exhibiting great potentials in high-frequency device fabrications.

In order to suppress alloy scattering in the HEMT structures, a thin AlN interlayer is usually introduced between the AlGaN barrier and the GaN channel. However, a significant amount of unintentional Ga incorporation was observed in AlN films grown by metal-organic chemical vapor deposition (MOCVD), one of the major techniques to produce the HEMT epi structures. In the first part of my thesis, the impact of impure AlN interlayers on HEMTs was examined, explaining the significant improvement in electron mobility despite of the high Ga concentration of ~ 50%. Moreover, both metal-polar and N-polar AlN films grown by MOCVD under various conditions were investigated, the results of which indicated that the major source of unintentional Ga was the former Ga deposition on the susceptor in the same run. It was also observed that N-polar AlN films contained less Ga compared to metal-polar ones when they were grown under same conditions. Methods to suppress the Ga were also discussed. In addition, the morphological and electrical properties of the GaN/AlN/GaN heterostructures with AlN films grown under different conditions were analyzed by atomic force microscopy (AFM) and room temperature Van der Pauw hall measurement.

Following the study of AlN interlayers in the HEMT structures, the development of N-polar HEMT epitaxial structures with highly-scaled channel thicknesses was discussed in detail. Small channel thickness is critical to prevent short channel effects when scaling down the lateral size of N-polar HEMT devices. By modifying the Si doping level in the back-barrier and the Al composition of the AlGaN cap, the channel thickness of the conventional N-polar HEMT structure with pure GaN channel was successfully scaled down to 8 nm. To further reduce the channel thickness, a thin InGaN layer was introduced between the channel and the AlGaN cap, leading to a decrease of the electric field in the channel and an increase of the distance between the centroid of the 2DEG and the AlN/GaN interface, which suppressed the scattering at the interface and significantly improved the electron mobility. The sheet charge density also increased due to the net positive polarization charge at the GaN/InGaN interface. The design was demonstrated by MOCVD. An increase of 73% in electron mobility from 606 to 1141 cm2/(V·s) was observed when the 6 nm thick pure GaN channel was replaced by a 4 nm GaN / 2 nm In0.1Ga0.9N composite channel. The smallest applicable channel thickness was decreased to 4 nm with the composite channel design.