ZRO2 as high-K gate dielectric for GaN-based transistors
Date of Issue2016
School of Electrical and Electronic Engineering
GaN-based high electron mobility transistors (HEMTs) have shown their excellent performance in high power, high frequency and low noise applications. One of the critical issues that further limits the performance and reliability of GaN HEMTs is the high gate leakage current. The high gate leakage current will reduce the power added efficiency and breakdown voltage and increase the minimum noise figure (NFmin) for GaN-based HEMTs. To mitigate above issues, GaN Metal-Insulator-Semiconductor HEMTs (MISHEMTs) using high-k materials as gate insulator and passivation layer are investigated. Owing to its large band gap (~5.6 eV) and high dielectric constant (~25), ZrO2 is a promising candidate to be utilized as the gate dielectric for GaN MISHEMTs. In order to effectively suppress gate leakage and passivate surface traps, understanding of band alignments and interfacial properties between ZrO2 and GaN-based semiconductor substrates is important. In this thesis, the studies on the band alignment between ZrO2 and GaN were carried out. The effect of device processing on interfacial properties between ZrO2 on (Al)GaN was investigated. The application of ALD-ZrO2 as high-k gate dielectric for GaN-based HEMTs was also explored. The major contributions of this thesis are summarized below: (1) The band alignment between atomic layer deposited (ALD) ZrO2 and GaN was experimentally evaluated by utilizing angle-resolved X-ray photoelectron spectroscopy (XPS) combined with numerical calculations by taking into account of GaN surface band bending and gradient potential in ZrO2 layer. Valence band discontinuity ΔEV of 1 eV and conduction band discontinuity ΔEC of 1.2 eV at ZrO2/GaN interface were determined. (2) The interface states related to the interfacial sub-oxide layer between ALD-ZrO2 and (Al)GaN were affected by various fabrication processing treatments. The effect of surface pre-treatment before ALD deposition and post-deposition annealing on interfacial chemical bonding states for ALD-ZrO2 on (Al)GaN were analyzed by X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HR-TEM). (3) AlGaN/GaN MISHEMTs on high-resistivity Si substrate using 10 nm thick ALD-ZrO2 as gate dielectric were demonstrated. Due to the application of ALD-ZrO2, the reverse gate leakage current was effectively suppressed and forward gate input bias was extended to a high value. The interface trap state density was also evaluated by AC conductance and “Hi-Lo frequency” methods. Reduction of interface trap state density Dit value from 4×1011 cm-2∙eV-1 and 3×1011 cm-2∙eV-1at energy of -0.29 eV to 7×1010 cm-2∙eV-1 and 1×1011 cm-2∙eV-1at energy of -0.38 eV was observed by AC conductance method and “Hi-Lo” frequency method, respectively.