A Comparative Study of Gallium-, Xenon-, and Helium-Focused Ion Beams for the Milling of GaN

Abstract

The milling profiles of single-crystal gallium nitride (GaN) when subjected to focused ion beams (FIBs) using gallium (Ga), xenon (Xe), and helium (He) ion sources were investigated. An experimental analysis via annular dark-field scanning transmission electron microscopy (ADF-STEM) and high-resolution transmission electron microscopy (HRTEM) revealed that Ga-FIB milling yields trenches with higher aspect ratios compared to Xe-FIB milling for the selected ion beam parameters (30 kV, 42 pA), while He-FIB induces local lattice disorder. Molecular dynamics (MD) simulations were employed to investigate the milling process, confirming that probe size critically influences trench aspect ratios. Interestingly, the MD simulations also showed that Xe-FIB generates higher aspect ratios than Ga-FIB with the same probe size, indicating that Xe-FIB could also be an effective option for nanoscale patterning. Atomic defects such as vacancies and interstitials in GaN from He-FIB milling were suggested by the MD simulations, supporting the lattice disorder observed via HRTEM. This combined experimental and simulation approach has enhanced our understanding of FIB milling dynamics and will benefit the fabrication of nanostructures via the FIB technique.

Keywords: focused ion beam; gallium nitride; milling profiles; molecular dynamics; transmission electron microscopy.

Figure 1

The perspective view of the MD simulation model, the FIB milling process, and preparation of TEM-FIB lamellar. (a) The model is composed of a substrate layer, a thermal layer, and a fixed layer. The GaN (0001) facet is normal to the direction of the bombardment of the ion particles. (b) Trenches milled via the FIB line-scan mode (30 kV, 42 pA) at different milling times (10 s, 20 s, 30 s, 40 s, 50 s). (c) Pt protection layer with electron beam deposition. (d) FIB lamellar attached to the lift-out Cu grid. (e) Thinned FIB lamellar showing the cross-sectional profiles of trenches created by FIB line scan.

Figure 2

ADF-STEM images showing the trench profiles created by (a1–a5) Xe-FIB (30 kV, 42 pA), (b1–b5) Ga-FIB (30 kV, 42 pA), and (c1–c5) He-FIB (30 kV, 22 pA) line-scans at various milling times. AR represents the aspect ratio of the trench and is defined as the ratio between the depth and the width at the half depth of the trench.

Figure 3

TEM images (a,b) of milling profile by Ga-FIB (30 kV, 42 pA, 10 μm line scan for 10 s) and the corresponding FFTs of different regions in (b) showing three regions: Crystalline, Crystalline-Amorphous Transition, Amorphous.

Figure 4

TEM image (a) of milling profile by He-FIB (30 kV, 22 pA, 10 μm line scan for 50 s) and HRTEM images (insets are the corresponding FFTs) showing different regions: (b) Crystalline, (c) Defected, (d) Fourier-filtered (0002) diffraction high-magnification image of a cropped region in (c).

Figure 5

STEM images showing the milling profiles milled for 10 s by (a) Xe-FIB with 3 nm, (b) Ga-FIB with 1 nm, (c) He-FIB with 0.5 nm probe sizes, and the corresponding cross-sectional IDS defect distributions under 0, 50, 100, 200, 300 impacts obtained by MD simulations.

Figure 6

Cross-sectional views of 1 nm thick central slice and top views of 1 nm thick slice at 3 nm below the top surface (for Ga and Xe ions) and 1 nm thick slice from the top surface (for He ion) of GaN structures after 500 impacts of (a) Ga with 1 nm, (b) Xe with 1 nm, (c) Xe with 3 nm, (d) He with 1 nm, and (e) He with 0.5 nm probe sizes. The arrows with the color gradient represent the magnitude of the displacement vectors of atoms from their static positions, ranging from 0 Å to 30 Å.

Figure 7

Different views of point defects distribution within the GaN structure after 300 impacts of He with 1 nm probe size obtained by MD simulations.