A Study of Ozone as an Oxygen Source for the Growth of High-Κ Dielectric Films for Gate Dielectric on GaN/AlGaN/GaN

Abstract

GaN is a promising alternative to silicon technology for the next-generation high-power and high-frequency electronics. The choice stems from the intrinsic properties of GaN of a wide bandgap and consequently high breakdown voltage, high saturation electron velocity and good thermal conductivity. Spontaneous and piezoelectric polarization effects cause accumulation of a high density of carriers at III-Nitride heterointerfaces enabling engineering of high mobility channels.

The primary factor inhibiting the further growth of GaN HEMTs is the high leakage current leading to device unreliability. The MIS-structure used in Si-CMOS processing has been adapted and shown to reduce leakage current in GaN technology. However, the introduction of an insulator adds another interface which suffers from poor quality due to innumerable traps with varying time constants. This leads to device threshold voltage instability and drain current collapse, while decreasing the device transconductance due to the increased gate-to-channel spatial separation. High-K dielectrics have been shown to reduce leakage current with smaller decrease in transconductance in Si-CMOS technology and therefore, applied to GaN technology. ALD is recognized as a novel method for high-κ gate dielectric deposition, where H2O is primarily used as the oxygen source for growth; excellent properties have been reported. However, ozone-grown films show further suppressed leakage current and offer better interfacial quality on silicon.

In this study, MOSCaps have been developed on GaN/AlGaN/GaN heterostructures with PECVD Si3N4 and ALD HfO2 as the passivation layer and gate dielectric, respectively. HfO2 was grown using either H2O or ozone as the oxygen source. XPS analysis, capacitance-voltage, conductance-voltage and leakage current-voltage characteristics have been used as probes to study the quality of the film and its interface with the III-N semiconductor.

It is observed that due to the sufficient supply of oxygen, ozone helps in the formation of a better bulk dielectric by more complete oxidation. However, the interface is degraded by uncontrolled surface oxidation of the barrier layer or/and penetration of oxygen impurities, creating shallow donor traps aiding in leakage. The overall leakage current with the ozone-grown dielectric is reduced by almost half-an-order of magnitude due to the better bulk dielectric achieved.