Correlation of gate leakage and local strain distribution in GaN/AlGaN HEMT structures
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.
Section snippets
GaN/AlGaN and HEMT structures
An unpatterned 4-inch SiC wafer with GaN/AlGaN epitaxial (EPI) layers was used to optimize parameter settings for the NBED strain mapping. A 3-μm-thick GaN buffer on the SiC wafer had a 25-nm-epitaxial AlGaN barrier on top to form the two-dimensional electron gas (2DEG) channel.
Furthermore, electrically functional, normally-on GaN/AlGaN HEMT devices with Au-Pt-Ni Schottky gates were investigated. The devices were fabricated on 4-inch SiC wafers and passivated with several silicon nitride layers
NBED parameter optimization
The robustness of the NBED strain measurement results against TEM parameters and sample thickness was proved by a 24 factorial design of experiment. The parameters selected were sample thickness, camera length, spot size, and magnification (beam tilt/area of reference). The quantification results were strain and sensitivity. Sensitivity was defined as the standard deviation of measured data in the unstrained areas and was found to be 0.13% ± 0.05%. The parameters were found not to significantly
Discussion
A compressive strain in the AlGaN in [002] direction was present directly under the metal gate. Ni-Pt interdiffusion and Au migration were found which correlated to the emission intensity at the AlGaN interface. Au and Ni, have a repulsive interaction, i.e., Au and Ni do not readily form chemical bonds to form a compound [12]. Similarly, Au and Pt have also mainly repulsive interaction though Au has 15 at.% Pt solubility (at 400 °C) [13]. Since Au does not form compounds with the other elements
Conclusions
It was shown, that a strain distribution underneath Schottky contacts could be measured with NBED. A correlation was achieved between photoelectron emission spots and local Ni-Pt interdiffusion as well as Au migration to the AlGaN interface. The modified Schottky interface should cause local gate leakage currents corresponding to the light emission. The origin of the local interdiffusion of Ni, Pt, and Au could not be clearly identified though. Local strain could be a driving force (stress
Acknowledgements
We thank our colleagues from Integrated Device Technology in Dresden for providing us their equipment for PEM measurements.
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