Strain measurements at AlGaN/GaN HEMT structures on Silicon substrates

Abstract

High Electron Mobility Transistors (HEMTs) based on AlGaN/GaN are of great interest due to their high electrical performance and the related applications. The high carrier density, electron mobility, breakdown voltage, and the good thermal stability of AlGaN/GaN are great benefits for high power and high frequency technologies. The high electron mobility is a consequence of a two dimensional electron gas (2DEG) which is formed at the interface between GaN and AlGaN. The source, drain, and gate of the transistors are realized by metal contacts on top of the semiconductor. Strain in the transistor structures may arise due to thermal processing steps or applied passivation layers on top of the HEMT structure. Especially the thermal processes may cause strain due to the mismatch in the coefficients of thermal expansion between the metal contacts and the GaN/AlGaN. Additional stress may be induced by the substrate material. In order to reduce material costs, such as those associated with power electronic applications, the usage of silicon substrates to replace the expensive silicon carbide and sapphire are under development. The disadvantage of GaN on silicon is the lower quality of the deposited AlGaN/GaN layers caused by the mismatch of the lattice parameters which differ by 17%. This can cause higher defect densities and residual strain in the AlGaN/GaN epi‐layers. In this work the local residual strain distribution in the AlGaN/GaN layers of HEMT structures is characterized. Investigations were conducted utilizing Nano Beam Electron Diffraction (NBED) which is a well‐established and sensitive method for strain analysis in semiconductors. The experiments were performed with an image Cs‐corrected TEM (Titan 3 G2 60‐300, FEI) equipped with a 3‐condenserlens system and a small condenser aperture (10 µm) which is crucial for NBED experiments. The NBED data were further analyzed using the FEI Epsilon Nanobeam Diffraction Strain Analysis Package (v1.1.0.39). Fig. 1a shows a dark field STEM overview of a normally‐on AlGaN/GaN HEMT representing the typical arrangement of the source, gate, and drain metal contacts and the field plate. In Fig. 1b a detailed image of the gate contact can be seen where the AlGaN‐layer on top of the GaN substrate is visible. Fig. 1c presents the elemental mapping of the gate Schottky contact in order to depict the different contact metals and partially the field plate. The contact is out of gold with a thin layer of nickel metallization underneath to form the Schottky contact to the active layer. No abnormalities at this gate structure could be found by TEM and EDX analysis. Nevertheless, electrical measurements of the investigated HEMT show a significant gate‐drain leakage current. Consequently, despite no irregularities were discovered with high resolution TEM of the interface region of the gate contact, NBED results showed local strain at the area (Fig. 2b). The compressive strain in [002] direction starts at the Schottky interface of the gate structure and runs through the AlGaN layer to the GaN bulk material. This may implicate a low resistance electron path from the gate into the 2DEG and must be further investigated.