Feasibility Study of UV Laser Assisted 3D-Atom Probe Analysis of AlGaN/GaN HEMTs.

摘要

Field evaporation of Air Force Research Laboratory AlGaN/GaN high electron mobility transistors was attempted using a 3-D atom probe system with a 343nm UV wavelength laser. Previous attempts to analyze these samples using a Local Electrode Atom Probe system with a 532nm wavelength laser were unsuccessful. This work was completed to study (1) the factors (temperature, laser power, ambient, and sample orientation) affecting field evaporation of AlGaN/GaN high electron mobility transistors and (2) using laser assisted field evaporation to measure the chemical composition of gate metal/AlGaN interface region on the nano-scale in order to correlate changes in composition with electrical and physical degradation using site specific regions identified with infra-red spectroscopy, photoemission electron microscopy, and electron beam induced current analysis techniques. Additionally, previous studies using this 343nm UV wavelength system on ceramic oxide materials, Li ion battery powders, and (Ga,Mn)As thin films on GaAs substrates which exhibit poor electrical and thermal properties like the wide bandgap AlGaN/GaN epilayers have been successfully. However, in this study every atom probe tip from the devices fractured before a successful 'complete' field evaporation consisting of the gate metal, interfacial layer, and AlGaN/GaN epilayers occurred. This tip fracture issue is possibly due to the presence of a thick interfacial layer between the gate metal and AlGaN epilayer. This interfacial layer could lower the adhesion strength between the gate and AlGaN leading to the fracture and delamination observed during the laser assisted field evaporation due the magnitude of the electric field on the tip apex. However, it is noted that increasing temperature, lowering laser power, and introducing of a slight pressure of Ne gas helped improve parts of the field evaporation of the samples.