Source: Dissertation Abstracts International, Volume: 76-11(E), Section: B.

Adviser: T. P. Ma.

Gallium Nitride (GaN) has been attracting much attention both in academia and industry as an excellent material for optical and electronic devices, due to its attractive properties such as wide band gap (∼3.4eV), high electron saturation velocity, and high breakdown strength. So far, its application in optical devices, led by light-emitting-diode (LED), has been successfully demonstrated: the blue and green light GaN LEDs have been commercialized. Comparatively, GaN-based electronic devices are still largely in research stage. As the performance of Si-based power devices is reaching its plateau, GaN-based power devices become particularly attractive due to their material properties as mentioned previously.

This work focuses on the process/device issues on two gate stacks associated with promising GaN-based transistors: Al2O3 and AlGaN. It is worth mentioning here that high-k materials, such as Al2O 3, have been proposed for GaN-based Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), and AlGaN on GaN has been widely accepted as the gate stack for High Electron Mobility Transistors (HEMTs).

To realize the benefit of high-k dielectrics, such as Al2O 3, on GaN, a low interface-trap density is required. To this end, two relevant fabrication process steps are examined: GaN surface treatment and post deposition anneal (PDA). It's observed that the NH4OH surface treatment, prior to Atomic Layer Deposition (ALD) of the Al2O 3, reduces the interface-trap density, by reducing the density of Ga-O bonds and Carbon, as revealed by the XPS study. In the PDA study, we have found an inverse correlation between the hydrogen content, obtained from the Secondary Ion Mass Spectrometry (SIMS), and the interface-trap density. To characterize the interface-trap density over a large energy range (>1eV) into the bandgap of GaN, various measurement techniques, such as photo-assisted Capacitance Voltage measurement, high temperature AC conductance methods, Deep Level Transient Spectroscopy (DLTS), etc. are used. Through optimized fabrication process conditions, an average interface-trap density of less than 2 x 1012 eV-1cm-2 is realized.

We study AlGaN on GaN primarily by utilizing the Inelastic Electron Tunneling Spectroscopy (JETS) technique. Two types of traps are identified: Type I is associated with trap assisted tunneling, and Type II is related to carrier trapping. We have found that electrically active Type I traps exist in a trap band 0.5eV below the conduction band minimum of AlGaN, and they give rise to trap assisted tunneling features in the spectra. By measuring the currents through the gate at various temperatures, we have found that the electron conduction through the AlGaN barrier layer follows the Frankel-Poole (F-P) mechanism. Carrier-trapping related Type II traps are further studied by the use of X-ray irradiation, where IETS spectra for the samples undergoing post irradiation anneals at various temperatures are investigated.

In addition, mobility curves of AlGaN/GaN HEMTs and MOS-HEMTs at various temperatures are extracted and studied. At relatively low temperatures (80K to 300K), the phonon limited mobility component is a weak function of temperature, due to the two dimensional nature of the dominant phonons from the gate stack. As temperature increases up to 520K, the mobility curves show a stronger temperature dependence, due to the phonons in the GaN substrate (similar to the SiO 2/Si MOSFETs).