Structural transition and recovery of Ge implanted β-Ga2O3, EA Anber, D Foley, AC Lang, J Nathaniel, JL Hart, MJ Tadjer, KD Hobart, …, Applied Physics Letters 117 (15), 152101
Abstract
Ion implantation-induced effects were studied in Ge implanted β-Ga2O3 with the fluence and energy of 3 × 1013 cm−2/60 keV, 5 × 1013 cm−2/100 keV, and 7 × 1013 cm−2/200 keV using analytical electron microscopy via scanning/transmission electron microscopy, electron energy loss spectroscopy, and precession electron diffraction via TopSpin. Imaging shows an isolated band of damage after Ge implantation, which extends ∼130 nm from the sample surface and corresponds to the projected range of the ions. Electron diffraction demonstrates that the entirety of the damage band is the κ phase, indicating an implantation-induced phase transition from β to κ-Ga2O3. Post-implantation annealing at 1150 °C for 60 s under the O2 atmosphere led to a back transformation of κ to β; however, an ∼17 nm damage zone remained at the sample surface. Despite the back transformation from κ to β with annealing, O K-edge spectra show changes in the fine structure between the pristine, implanted, and implanted-annealed samples, and topspin strain analysis shows a change in strain between the two samples. These data indicate differences in the electronic/chemical structure, where the change of the oxygen environment extended beyond the implantation zone (∼130 nm) due to the diffusion of Ge into the bulk material, which, in turn, causes a tensile strain of 0.5%. This work provides a foundation for understanding of the effects of ion implantation on defect/phase evolution in β-Ga2O3 and the related recovery mechanism, opening a window toward building a reliable device for targeted applications.
β-Ga2O3 has drawn substantial attention due to its wide band gap, high electric breakdown field, and high thermal stability.1–13 These properties make β-Ga2O3 a promising material for applications in harsh environments and, in particular, space missions where single event upsets may occur.6 However, the structural complexity of β-Ga2O3, including the two different crystallographic positions of Ga, and numerous crystallographic polymorphs lead to a large number of complex defects which can form due to radiation exposure,7 including space-based radiation effects. These lattice defects, such as dislocations6,8 and voids,6–8 act as scattering sites for carriers, which, in turn, influence the electronic properties of Ga2O3.14
Doping via ion implantation has been shown to improve the performance of Ga2O3 devices by lowering contact resistance.15 Both Si- and Sn-ion implantation doping have been studied.14 Wong et al.16–19 demonstrated the use of implantation of deep level impurities (N, Mg) to produce current blocking regions in vertical MOSFETs. The annealing temperatures employed have ranged from 900 to 1150 °C, but there is little evidence that all the structural damage from implantation has been recovered under these conditions. Therefore, understanding implantation-induced damage in β-Ga2O3 is crucial. Wendler et al.20 reported the formation of point defects and defect clusters in β-Ga2O3 after P, Ar, and Sn implantation. They also showed that the damage clusters are not amorphous, highlighting the possibility of the formation of another Ga2O3 phase post implantation. Another study of Ga2O3 implanted using Eu,21 however, claimed the formation of the amorphized zone within the implanted layers where the amorphization started at the surface at a fluence of ∼1 × 1015 at/cm2 and became deeper by increasing the fluence to ∼ 4 × 1015. Annealing of implanted Ga2O3 at temperatures above 1000 °C shows almost complete recovery of ion-induced defects;22,23 however, the distribution of these dopants may happen randomly, increasing the chance of segregation near the surface.21,22 Besides, Tadjer et al.24 observed that the lattice parameters of Sn-implanted Ga2O3 did not completely recover after annealing at 1150 °C for 60 s, suggesting the need for a higher annealing temperature.
While these results marked a step forward in understanding the radiation tolerance of β-Ga2O3, a comprehensive understanding of the nature of the damage zone requires more advanced characterization. In this study, we examine β-Ga2O3 in three conditions: pristine, as-implanted with Ge with a concentration of ∼1020 cm−3 (supplementary material Fig. S3),23 and Ge implanted-annealed at 1150 °C for 60 s. We analyze these samples using analytical electron microscopy via scanning/transmission electron microscopy (S/TEM) and electron energy loss spectroscopy (EELS). Additionally, precession electron diffraction (PED), via Topspin, was utilized to measure strain induced from complex defects formed upon radiation. The technique provides a reliable strain measurement with a spatial resolution down to ∼2 nm.25 Our TEM results show an isolated band of damage after Ge implantation, which extends ∼130 nm from the sample surface and corresponds to the projected range of implantation. After annealing, TEM imaging shows that implantation-induced damage is partially recovered with the remaining defect zone minimized to 17 nm. However, even in the “recovered” areas, we observe differences in strain and O K-edge fine structure compared to the pristine material.
The starting material was a 650 μm thick Sn-doped (n = 3.6 × 1018 cm−3) β-Ga2O3 single crystal [(001) surface orientation, Tamura Corporation, Japan] grown by the edge-defined film-fed technique,12 and 20 μm Si doped β-Ga2O3 epitaxial layer grown on top of this substrate. The x-ray diffraction full width at half maximum was <150 arc sec for both the substrate and the epi layer. After growth, the epi-surface was planarized by Chemical Mechanical Polishing. The n-drift region was grown by halide vapor phase epitaxy with a carrier concentration of 1.62 ×× 1016 cm−3, obtained from capacitance–voltage measurements. The samples were implanted with Ge at room temperature with a 7° tilt with respect to the beam normal, near uniform concentrations of 1019 cm−3, with energy and fluence conditions of 3 × 1013 cm−2/60 keV + 5 × 1013 cm−2/100 keV + 7 × 1013 cm−2/200 keV. Ge was chosen as the element for implantation as it represents a donor with the mass intermediate between Si and Sn. Focused ion beam (FIB) was performed in a FEI Strata DB 235, which was used to fabricate the TEM lamella for all three conditions (as-grown, as-implanted, and implanted-annealed β-Ga2O3). The samples were prepared via a typical FIB in situ liftout procedure using 30 kV Ga-ions and were thinned to electron transparency at 2 kV. Microstructural analysis was carried out using Scanning-TEM (STEM) mode on a JEOL 2100 field emission TEM, equipped with a high-resolution pole piece, direct detection electron energy loss spectroscopy (DD-EELS) and precession electron diffraction using TopSpin.
Figure 1 presents cross-sectional TEM images of β-Ga2O3 in the pre-implantation condition where stalking faults [Figs. 1(a) and 1(b)], with an average density of 1.5 × 1010 cm−2, and an individual dislocation [Fig. 1(d)] are observed. The same specimen region, shown in Fig. 1(d), was imaged again under two beam conditions where dislocation is found to be visible if g [1⎯⎯1¯10] is applied and invisible in the case of [001] diffraction vector. From g*b measurements (listed in Table I) and based on dislocation visibility, the Burgers vector must be parallel to either [010] or [100]. The dislocation line is oriented along [001] and, therefore, is an edge dislocation.
FIG. 1. Cross-sectional high resolution TEM (HRTEM) image of the uppermost region of the pristine β-Ga2O3 epi layer, which shows the presence of stacking faults (a) and Bragg-filtered image filtered using the (200) and (400) planes (b) and related [110] selected area diffraction pattern (c). The TEM image shows the presence of an individual dislocation (d) and corresponding weak beam dark field (WBDF) TEM image obtained from different two beam conditions using g(1⎯⎯1¯00) (e) and g(001) (f). SF density was determined by counting the number of imaged SFs in a cross-sectional TEM sample.
