Normally-Off AlGaN/GaN Mos-Hemts Using ZrO2 Gate Dielectric with Tuneable Charge
Authors: Anderson Travis J , Wheeler Virginia D. , Shahin David I , Tadjer Marko J. , Luna Lunet E , Koehler Andrew D. , Hobart Karl D , Kub Francis J , Eddy Charles R. 
Published: February 2020

GaN-based high electron mobility transistors (HEMTs) are of significant interest for next-generation RF power amplifiers and monolithic microwave integrated circuits (MMICs). The integration of a gate dielectric in a MOS-HEMT device has been shown to offer significantly reduced gate leakage, resulting in improved reliability and reduced off-state power consumption. ZrO2 has attracted increasing attention as a candidate gate insulator for GaN-based HEMTs, due to a high dielectric constant (25), large bandgap (5.9 eV), and high breakdown voltage (15-20 MV/cm). We have previously reported high positive threshold voltage in unpassivated HEMT structures with recessed barrier layers and ZrO2 gate dielectrics deposited using zirconium (IV) tert-butoxide (ZTB) as the Zr precursor [1]. In this work, we compare these devices to structures incorporating the more common tetrakis(dimethylamino)zirconium (TDMA-Zr) precursor, which is expected to exhibit less fixed oxide charge. Combined with the optional barrier recess step, we demonstrate threshold voltage control over a range of 7V for a given HEMT layer structure. We also report the integration of SiNX passivation layers and an evaluation of the dynamic switching performance of the devices.

AlGaN/GaN HEMT devices structures were grown on Si substrates by MOCVD, and devices were fabricated following standard GaN device process steps described elsewhere [1]. ZrO2 layers were deposited by atomic layer deposition using both ZTB and TDMA-Zr precursors and deionized water. Ellipsometry and X-ray photoelectron spectroscopy (XPS) were used to verify thickness and identify film stoichiometry. Capacitance-voltage measurements were initially used to characterize the oxide. A dielectric constant of 25 was extracted, and interface trap density was on the order of ~1x1012, dependent upon the nature of the oxide and the surface upon which it was deposited (HEMT structure vs GaN MOS capacitor). Gated Hall measurements were used to characterize the 2DEG mobility and sheet carrier density, both in the HEMT structure and in the recessed region under the gate.

The non-recessed devices exhibit comparable current density, transconductance, and ON-resistance. The threshold voltage is shifted positive for device structures incorporating the ZTB-derived ZrO2 film, approaching enhancement mode even without the barrier recess and demonstrating an exceptionally high +4V with a barrier recess. The mechanism for this is proposed to be the presence of negative charge in the oxide film, potentially due to excess oxygen in the film. The presence of such charge was verified on GaN capacitor structures. In contrast, the TDMA-Zr-derived ZrO2 films exhibit a negative VT shift due to the thicker effective barrier, just reaching enhancement mode operation even with a full barrier recess. The breakdown voltage was comparable among all device structures both with and without recess. As expected, the gate leakage was suppressed over 5 orders of magnitude compared to the reference Schottky gated HEMT. Pulsed I-V measurements indicate a degraded current collapse behavior in the ZTB-based devices, which is expected as the negative charge from the oxide would be expected to enhance the charge trapping effect. In contrast, the TDMA-Zr-based device structures exhibit comparable current collapse to the reference device, and possibly even represent an improvement due to reduced trapping under the gate in the MOS structure. Similar behavior in the recessed gate structure indicates minimal permanent damage from the plasma recess etch.

In conclusion, we have demonstrated enhancement mode AlGaN/GaN MOS-HEMTs with a thin ALD-ZrO2 gate oxide and barrier recess. The integration of this particular high-k dielectric in the device structure results in a positive threshold voltage shift due to negative charge in the oxide film and at the interface, which when integrated with a barrier recess enables a VT ~+4V while suppressing gate leakage by 4 orders of magnitude compared to a reference Schottky-gated HEMT.

1. T.J. Anderson, et. al. Appl. Phys. Express 9, 071003 (2016)