Investigation of the passivation-induced V_TH shift in p-GaN HEMTs with Au-free gate-first process

Abstract

In this work, we observe the distinct V_TH characteristics in the Au-free gate-first processing p-GaN/AlGaN/GaN HEMTs with two commonly used passivation layers, i.e., SiN and SiO₂. The lower incorporated H was found in the p-GaN/AlGaN/GaN heterostructure with higher activation anneal temperature (i.e., 700 °C). Furthermore, Photoluminescence (PL) spectrum demonstrates a higher blue luminescence (BL) intensity after higher annealing treatment. The X-ray photoelectron spectroscopy (XPS) spectrum near valence band edge depicts a similar valence band maximum (VBM) characteristic, by means no impact on p-GaN surface bending by using distinct thermal treatment. The device with SiN shows a depletion-mode (D-mode) characteristic (V_TH ~ -5 V) whereas the device with SiO₂ passivation exhibits an enhancement-mode (E-mode) characteristic (V_TH ~ +0.7 V). Moreover, Transmission Line Model (TLM) devices are fabricated to investigate the effects of the passivation on two-dimensional electron gas (2DEG) in p-GaN/AlGaN/GaN stack. The results indicate that a low R_sh is obtained while passivating device surface with SiN layer, suggesting that 2DEG is present, which is most probably due to an unfunctional p-GaN layer. The secondary ion mass spectrometry (SIMS) results indicate a high hydrogen intensity in the p-GaN/AlGaN/GaN stack with a SiN passivation layer. Thus, the p-GaN deactivation process that correlates to the formation of complex MgH after SiN passivation is proposed to explain the D-mode characteristic in the device with a SiN passivation layer.

Introduction

Nowadays, the GaN-based High Electron Mobility Transistors (HEMTs) have been considered as a promising technology for high-efficient power switching applications. Over more than 20 years' development, the GaN power devices have demonstrate for the applications such as electrical vehicle (EV) inverter, laptop adaptor, wireless power transfer, UPS, or even space applications [[1], [2], [3], [4]]. However, the reliability still remains the challenges in GaN power devices. In addition to the evaluation of typical device parameters and Safe Operation Area (SOA), various reliability tests need to be considered, i.e., hard switching [5], semi-on stress [6,7], high temperature gate bias (HTGB) [8], time dependent dielectric breakdown (TDDB) [9,10], Mean-time-to-failure (MTTF) [11], and JEDEC standard qualifications [12].

Generally, the GaN-based HEMTs have the superior characteristics, featuring with a low R_DS,on due to an inherent two-dimension electron gas (2DEG) between AlGaN and GaN layer, wide bandgap (E_G) of 3.4 eV, low gate charge (Q_G), nearly zero reverse recovery charge (Q_rr), and high critical electric field (E_C ~ 3.3MV/cm) [13].

Considering the common usage preference, an E-mode characteristic is adopted for commercial applications due to an easier integration and fail-safe operation. Various approaches have been proposed to achieve a normally-off operation in GaN-based HEMTs, e.g., fluorine-based treatment [14], gate recess [[15], [16], [17]], hybrid-MIS structure [18], p-GaN/AlGaN gate [19,20]. Recently, AlGaN/GaN HEMTs with a p-GaN Schottky gate attract lots of attentions toward an enhancement-mode characteristic. High performance AlGaN/GaN HEMTs with a p-GaN Schottky gate with robust stability have been demonstrated so far [21,[23], [24], [25]]. Moreover, the surface passivation is used to modulate the strain in access region, further reducing the access resistance due to the additional polarizations [26]. The surface passivation is also effective to improve the breakdown performance [27] and stability of dynamic on-resistance [28,29]. Nevertheless, the impact of the passivation on the characteristics of p-GaN gate HEMTs still needs more investigation.

In this work, the secondary ion mass spectrometry (SIMS), photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS) analyses are used to analyze the p-GaN/AlGaN/GaN stack and the p-GaN gate HEMTs are fabricated with two different passivation layers, i.e., SiN and SiO₂, to evaluate the impacts of the passivation on the V_TH characteristics. To further understand the effects of passivation layer, we fabricated the Transmission Line Model (TLM) devices in p-GaN/AlGaN/GaN stack. Finally, a physical model is proposed to explain the p-GaN deactivation mechanisms after the device is passivated by SiN layer.