Adjustment of threshold voltage in AlN/AlGaN/GaN high-electron mobility transistors by plasma oxidation and Al2O3 atomic layer deposition overgrowth

Abstract

We discuss possibilities of adjustment of a threshold voltage VT in normally off GaN high-electron mobility transistors (HEMTs) without compromising a maximal drain current IDSmax. Techniques of a low power plasma or thermal oxidation of 2-nm thick AlN cap over 3-nm thick AlGaN barrier are developed and calibrated for a thorough oxidation of the cap with a minimal density of surface donors at the inherent oxide-semiconductor interface. It has been shown that while a thermal oxidation technique leads to the channel and/or interface degradation, low density of surface donors and scalability of VT with additionally overgrown Al2O3 may be obtained for plasma oxidized HEMTs. With 10-nm thick Al2O3 deposited at 100 °C by atomic-layer deposition, we obtained VT of 1.6 V and IDSmax of 0.48 A/mm at a gate voltage of VGS = 8 V. Density of surface donors was estimated to be about 1.2 × 1013 cm−2, leaving most of the negative polarization charge at the semiconductor surface uncompensated. Further reduction of surface donors may be needed for even higher VT.

Impact of GaN cap on charges in Al2O3/(GaN/)AlGaN/GaN metal-oxide-semiconductor heterostructures analyzed by means of capacitance measurements and simulations

“…Further, so far, a partial suppression of the compensating charge (manifested by readable positive shift in MOS-HEMT V th in respect to corresponding SB-HEMT structure) has been achieved by careful electrochemical oxidation or O 2 plasma treatment of the III-N barrier layer. 6,7,10 This indicates that oxidation of barrier surface does not necessary lead to the compensating charge formation, instead, the oxide/III-N interface quality seems to be a key factor. Defects such as dangling bonds at oxide/III-N interface may play an important role for positive compensating charge formation.…”

“…6,7,10 In contrast, MOCVD of Al 2 O 3 at 600 C followed by annealing at 700 C used here or thermal oxidation at 900 C used in TABLE I. Summary of the effective Al 2 O 3 /barrier charge Q ef extracted from the V th -t ox dependence shown in Fig.…”

“…[3][4][5] While these devices initially targeted RF power applications, 4 more and more effort is being devoted towards development of high-voltage power switching MOS-HEMT devices. [5][6][7][8] This is because MOS gate structure inherently provides suppressed gate leakage at positive gate voltage override and potentially higher positive threshold voltage (V th ) of an enhancement mode (or normally off) transistor, when compared to Schottky barrier (SB) or junction gated structure. 9 However, further improvements of this technology are hampered by difficulties in technological control over the MOS-HEMT's V th .…”

“…Despite several successful approaches to process normally off GaN MOS-HEMTs, [5][6][7][8][9][10] there is still a lack of understanding on the charge distribution in MOS-HEMT structures. As recently pointed out, 11 charges across the MOS-HEMT structure can be separated into those residing in the oxide and/or its interface with semiconductor and charges belonging to III-N heterostructure.…”

Controlling the interface charge density in GaN-based metal-oxide-semiconductor heterostructures by plasma oxidation of metal layers

“…5,6,8 For the control of N it (and bulk oxide charge N ox which can work in the same way), various methods have been presented, such as gate-recess, 9 plasma oxidation of AlN, 10 or fluorination of a gate oxide. 11 Our group has also shown first results indicating that thermal annealing of a plasma-oxidised Al thin film as gate dielectric leads to V th shifts to more positive values due to the presence of oxide-related charge.…”