Assessment of surface damage and sidewall implantation in AlGaN-based high electron mobility transistor devices caused during focused-ion-beam milling

The surface amorphization and ion implantation in AlGaN-based high electron mobility transistor (HEMT) model structures caused by ionized gallium during focused-ion-beam milling have been investigated. The extent of Ga+ surface implantation likely to occur during deposition of the surface Pt protective layer was simulated for 30, 5, and 2 keV ion beams. Electron-transparent cross sections of AlGaN/GaN and AlGaN/AlN/GaN HEMT structures were then prepared for electron microscope observation using a dual-beam focused-ion-beam instrument operated at different beam energies. Experimental studies revealed that the upper 9 nm of the AlGaN layer had been amorphized during Pt deposition. Nanoprobe x-ray microanalysis confirmed intermixing with Pt as well as implantation of Ga ions into the upper regions of the foil. Deposition of the first few hundred nanometers of Pt using an electron beam, rather than the usual Ga+ beam, enabled surface damage and ion implantation to be completely avoided. Sidewall damage for specially prepared cross sections was assessed from bright-field and high-angle annular-dark-field images. For final membrane thinning at 30, 5, and 2 keV, the thicknesses of visibly damaged layers were approximately 20, 8, and 4 nm, respectively, roughly twice as large as predicted by simulations.