Correlation of gate leakage and local strain distribution in GaN/AlGaN HEMT structures

Highlights

• Leaky HEMT gate areas are analyzed with local strain and elemental mapping.

• Interdiffusion of Schottky contact metallization is found at the leaky locations.

• Strain maps display greater variation in the leaky locations compared to the non-leaky locations.

Abstract

GaN/AlGaN HEMT structures are observed to undergo a reversible, drastic change in the leakage current when covered with an additional polymer passivation layer. The polymer layer induces a stress on the HEMT structures, which initiates material migration processes and the formation of structural defects, influencing the electrical performance. Local strain measurements were performed in the semiconductor, at the critical HEMT gate electrode, to evaluate the impact of the stress on the Schottky gates. The strain distributions in the structures were measured with nanobeam electron diffraction from electron-transparent samples at cross sections and longitudinal sections at the positions of high leakage currents. A variation of the strain distribution underneath the gate electrode was detected in a cross-sectional sample. On the contrary, only minor differences in the strain values were measured in the longitudinal sections at different photoemission sites. Finally, localized metal interdiffusion was detected at the sites with the highest photoemission intensities.

Introduction

III-V compound semiconductors have raised considerable attention for several years in the areas of telecommunication and high-power devices. Especially GaN/AlGaN high-electron-mobility transistors (HEMTs) are attractive owing to their higher bandgap and better break down performance, hence having the ability to operate under high power and elevated temperature. Furthermore, the high carrier mobility enables operation at high frequencies required in high-speed telecommunication applications [1].

Unfortunately, due to the lattice mismatch in hetero epitaxy it is assumed that these devices suffer from defects such as dislocations and strain at the interfaces of the dissimilar materials. Therefore, any additional strain introduced in the transistor structures may compromise the electrical performance and reliability of HEMTs. Therefore, it is of utmost importance to thoroughly understand both the nature of the strain-introducing or strain-generated defects and the magnitude of the strain.

Recently, Zanoni et al. [2] published a review of various failure mechanisms observed in GaN/AlGaN HEMTs. A number of different mechanisms were identified, which may act simultaneously or independently, and differentiating the dominating root-cause may be challenging. Nevertheless, the various identified physical mechanisms can be summarized to be stress-related defect generation [3], charge trapping [4], electrochemical degradation [5], and interdiffusion of metal-metal and metal-semiconductor interfaces [6]. The stress/strain-assisted phenomena and the interdiffusion are further pursued in this discussion.

Strain-induced effects are investigated by analyzing GaN/AlGaN HEMT devices with different leakage currents. The leakage currents were mapped from a full wafer. Thereafter, the leakage paths are visualized along the gate structures by photoemission microscopy (PEM). At conspicuous areas, transmission electron microscopy (TEM) investigations are conducted to characterize structural defects at the gate and in the channel region, and to map local strain fields with nanobeam electron diffraction (NBED). Furthermore, scanning TEM (STEM) energy dispersive X-ray spectroscopy (EDX) is applied to investigate possible interdiffusion phenomena. The objective of these investigations is to correlate the leaky areas in the channel region to any defects and especially to local anomalies in strain.