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

Abstract

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.

Practical Issues for Atom Probe Tomography Analysis of III-Nitride Semiconductor Materials

“…In order to systematically evaluate FIB implantation caused by sample preparation, the following procedures were adopted: An InAlN/GaN sample was first coated with a ~50 nm thick Pt layer using a sputter coater before loading into the FIB chamber. A strip of Pt with dimensions of about 20 µ m (length)×2 µ m (width)×1 µ m (thickness) was further deposited on the sample surface using electron beam (e-beam) assisted Pt deposition at voltages of 5–10 kV with beam currents of 1.4–2.7 nA. This approach was used to avoid possible Ga implantation into the sample surface, which could occur during the FIB-induced Pt deposition (Thompson et al, 2007; Cullen & Smith, 2008). Following standard protocols (Thompson et al, 2006, 2007), hereafter, trench-cutting, extraction and mounting of the sample wedge were performed at 30 kV (FIB) with variable beam currents of 9.3 nA to 24 pA. From this sample wedge a number of sections were mounted onto the Si posts of a standard flat-top microtip array coupon. The samples were then sharpened using variable beam currents (0.44–0.13 nA) at a 15 kV FIB voltage, making sure to retain at least ~80 nm of the Pt layer. The final FIB “clean-up” procedure was carried out at different voltages (5, 3.5 and 1 kV) on each individual sample.…”

“…A strip of Pt with dimensions of about 20 µ m (length)×2 µ m (width)×1 µ m (thickness) was further deposited on the sample surface using electron beam (e-beam) assisted Pt deposition at voltages of 5–10 kV with beam currents of 1.4–2.7 nA. This approach was used to avoid possible Ga implantation into the sample surface, which could occur during the FIB-induced Pt deposition (Thompson et al, 2007; Cullen & Smith, 2008).…”

FIB-SEM cathodoluminescence tomography: practical and theoretical considerations

“…From the simulations it follows that the electron beam interaction volume wherein CL emission is generated in transparent materials significantly exceeds the volume affected by the ion beam. Especially when taking into account that milling a cross section involves milling at an edge, which can reduce the affected volume (Giannuzzi et al , 2005; Cullen & Smith, 2008). This means that, at least on a theoretical basis, FIB‐SEM tomography with CL imaging should be possible, provided that the altered surface layer does not negatively influence the CL emission from the greater volume sampled by the electron beam.…”

FIB Method of Sectioning of III-V Core-Multi-Shell Nanowires for Analysis of Core/Sell Interfaces by High Resolution TEM

“…This quantification is possible due to the final 2 kV final cleaning of lamella to minimize the incorporation of gallium to side wells. The literature data concerning simulations of Ga atoms implantation shows that during FIB cleaning at 5 kV, the penetration depth into silicon is about 4 nm, with maximum atomic concentration of 2 at.% [11,12]. Thus, in the case of performing 2 kV final cleaning, implantation of gallium can be neglected for EDS quantification when 30-50 nm lamella is used.…”