GOX 2022 Session AC-TuM: Advanced Characterization & Microscopy

Due to the low crystal symmetry, gallium oxide can display formation of unique defects ranging from point defects to phase transition that are important to understand, as such defects directly correlate to the properties of the material and performance of gallium oxide-based devices. This presentation will overview the recent progress in the atomic scale characterization of various defects in gallium oxide and aluminum gallium oxide using scanning transmission electron microscopy. We make a direct connection between the atomic structure of these defects and important properties of gallium oxide materials and devices, including growth characteristics of the films as well as their electric and thermal properties. The topics will include: (i) formation of point defects and complexes, (ii) alloy incorporation and phase stability in aluminum gallium oxide, (iii) formation of 2D defects, such as stacking faults and twins, (iv) phase transformation induced by incorporation (or diffusion) of impurity atoms, (v) defects at interfaces with metal contacts and their influence on thermal interface resistance, and (vi) defects created by ion implantation of gallium oxides. The new information that we summarize in this presentation is expected to help achieve atomic scale control of defects in gallium oxide materials and devices for the next generation power electronics applications.

(Al_xGa_1–x)₂O₃ is an emerging ultra-wide bandgap semiconductor with a bandgap tunability of 4.8 - 8.7 eV and highly promising for high power electronics [1]. The Al inclusion limit in (Al_xGa_1–x)₂O₃ varies with growth orientation. While (010)–(Al_xGa_1–x)₂O₃ is single β–phase stable till 27% Al, (-201) and (100)–(Al_xGa_1–x)₂O₃ exhibit β–phase for >50% Al [2]. The Al incorporation in (Al_xGa_1–x)₂O₃ for different orientations are limited by phase segregations, chemical heterogeneity and domain rotations due to difference in surface free energy. The higher the surface free energy, the lower the Al incorporation at the growth surface. Also, as the surface free energy varies for different growth orientation, the binding energies would be different which play a significant role in Al inclusion range in (100), (-201) and (010)–(Al_xGa_1–x)₂O₃ films. Therefore, a comprehensive understanding of the film’s structural-chemical morphology and properties (surface energy, binding energy and bond lengths) of (100), (-201) and (010)–(Al_xGa_1–x)₂O₃ is needed to achieve films with high Al% for high power transistors.

Here, we employed atom probe tomography (APT), a nanoanalytical tool combining microscopy to provide chemical imaging and spectroscopy to reveal qualitative binding energy/bond length information of material. The nanoscale structure-chemistry of (100), (-201) and (010)–(Al_xGa_1–x)₂O₃ varying Al composition was probed. From the in-plane lateral Al/O distribution, (Al_xGa_1–x)₂O₃ layers with 20% Al are found to be homogeneous in (100), (-201) and (010) orientation while (Al_xGa_1–x)₂O₃ layers with 50% Al are relatively less homogeneous in each case. This is attributed to the higher surface migration length of Al atoms compared to that of Ga atoms. The APT spectroscopy was used to determine the relative bond length information of Ga–O and Al–O for (100), (-201) and (010)–(Al_xGa_1–x)₂O₃ films varying Al content. The observed APT spectroscopy result reveals that the bond length of Ga–O and Al–O changes as the (Al_xGa_1–x)₂O₃ growth orientation varies.

This work will provide critical understanding and insights on the structural chemistry and bond lengths of (Al_xGa_1–x)₂O₃ films with different growth orientations and will aid in optimizing the growth towards developing (Al_xGa_1–x)₂O₃ films with high Al%.

Acknowledgment: NSF (Grant No. 2114595; 1810041 and 2019753) and AFOSR (FA9550-18-1-0479)

Reference: 1. Bhuiyan et al. APL Materials, 8, 031104 (2020); 2. Bhuiyan et al. Appl. Phys. Lett. 117, 142107 (2020)

Ga₂O₃ is an ultra-wide bandgap semiconductor that has larger values of bandgap, Baliga’s figure of merit, and breakdown electric field than SiC and GaN. There are four commonly accepted polymorphs of Ga₂O₃, namely trigonal α (corundum structure), monoclinic β, orthorhombic κ, and cubic γ (cation deficient spinel structure) phases. Of those four polymorphs, β-Ga₂O₃ has been the most investigated, as this phase is the thermodynamically stable phase from room temperature to the melting point at atmospheric pressure^1-2. However, the lower symmetry of β-Ga₂O₃ results in anisotropic optical and electronic properties. Compared with β-Ga₂O₃, κ-Ga₂O₃ has higher symmetry and some unique properties. κ-Ga₂O₃ shows spontaneous polarization (Psp) parallel to the c-axis and thus a high-density two-dimensional electron gas can be formed at the interface without doping. The reported values of Psp are 0.23 C/m^{2 3} and 0.242 C/m^{2 4}, respectively, which are about an order of magnitude higher than those of GaN and AlN. In this study, nominally phase pure κ-Ga₂O₃ films were successfully grown on vicinal c-plane sapphire (0.15° offcut toward m-plane) by low-pressure metal-organic chemical vapor deposition⁵. Phase and microstructural characterizations were conducted using a complementary suite of tools. High-angle annular dark-field scanning transmission electron microscopy of a κ-Ga₂O₃ film grown under optimum conditions revealed the pseudomorphic growth of 3-4 monolayers of α-Ga₂O₃ at the interface, followed by a 20-60 nm transition layer containing a mixture of β- and γ-Ga₂O₃ which was covered by an ~700 nm-thick layer of phase-pure κ-Ga₂O₃. The occurrence of these phases and their sequence of formation will be presented. X-ray diffraction (XRD) and scanning electron microscopy investigations showed that the top layer varied between ∼100% κ-Ga₂O₃ and ∼100% β-Ga₂O₃, depending on the growth temperature and the growth rate. XRD φ scans showed in-plane epitaxial relationships and the presence of the three rotational domains in the κ-Ga₂O₃. Atomic force microscopy investigations revealed a smooth surface morphology with a root-mean-square roughness of ~3.5nm for optimum growth conditions. In summary, growth conditions have been established that yield 700 nm-thick films, above a thin transition layer, comprising phase-pure κ-Ga₂O₃; whereas the β-phase is favored at higher growth temperatures and lower growth rates.

Recently, beta-gallium oxide (β-Ga₂O₃) has attracted attention for high power devices due to its high bandgap of 4.9eV and possibility of manufacturing large diameter wafers using quasi-equilibrium melt-based techniques, which can have low defects. Extended defects such as dislocations, stacking faults, inclusions, dislocation slip bands etc have proven to have detrimental effects on power and RF device performance and reliability. Defect identification and their mitigation is necessary to fabricate devices that can reach the predicted breakdown and on-state resistance performance for Ga₂O₃ devices. Investigation of extended defects over large diameter Ga₂O₃ wafers, including defect delineation and micro-structural properties can be obtained using high resolution x-ray topography (XRT). In this study, various extended defects were investigated in 100 mm diameter Ga₂O₃ wafers with 10μm thick epitaxial layers using multiple reflection XRT characterization to identify defect types and distinguish defects in both substrates and epitaxial layer.

For this study, a 100 mm diameter, edge-defined, film-fed (EFG) growth Ga₂O₃ wafer with 10 mm epitaxial layer grown via halide vapor phase epitaxy (HVPE) was obtained from Novel Crystal Technology. XRT imaging was performed on a Rigaku XRTMicron system equipped with a 1.2kW Cu/Mo dual rotating anode, high precision X, Y, f goniometer and 5.4mm/2.2mm pixel dual X-ray cameras. Imaging was performed using Mo ka₁ in transmission geometry with g=(020) and in reflection geometry with g=(-809), (607), and (-44,10). Imaging using Cu ka1 was also performed in reflection geometry with g=(224) and (514). Using these various imaging conditions the penetration depth of the X-rays was controlled in the sample. Hence, identification and delineation of a variety of extended defects from both the epitaxial layers as well as the substrates was performed.

A distribution of basal plane dislocations (BPD) was observed across the wafer with a density ~3x10³ cm^-2. These BPDs are primarily within the substrate. Few of the BPDs were observed to propagate into the epitaxial layers. Slip bands were observed emanating from the edge of the wafer in several regions and are within the epitaxial layers only. These are likely due to residual damage in the wafer edge processing. Additionally pits were identified within the epitaxial layer, which could be due to pitting occurring by Ga droplets during the HVPE process. Other defects such as inclusions, surface dislocations, and scratches were also observed. Detailed micro-structure and dislocation analysis will be presented on the extended defects observed in the multiple XRT images.