Imaging and identifying defects in nitride semiconductor thin films using a scanning electron microscope

In this paper we describe the use of electron backscatter diffraction (EBSD) mapping and electron channeling contrast imaging—in the scanning electron microscope—to study tilt, atomic steps and dislocations in epitaxial GaN thin films. We show results from epitaxial GaN thin films and from a just coalesced epitaxial laterally overgrown GaN thin film. From our results we deduce that EBSD may be used to measure orientation changes of the order of 0.02°, in GaN thin films. As EBSD has a spatial resolution of ≈ 20 nm, this means we have a powerful technique with which to quantitatively map surface tilt. We also demonstrate that channeling contrast in electron channeling contrast images may be used to image tilt, atomic steps and threading dislocations in GaN thin films.

In 1967, Coates discovered the electron channelling contrast of backscattered electrons (BSEs) in scanning electron microscopy, and by this the possibility to investigate arrangements of lattice defects in deformed microstructures of materials. Since that time, a straightforward development of the scanning electron microscopes as well as of the electron channelling contrast technique took place. Nowadays, the performance of scanning electron microscopes is high enough that the resolution of electron channelling contrast imaging (ECCI) micrographs is comparable with conventional bright field transmission electron microscopy (TEM) micrographs. In the first part of the present paper, a historical review on the development of the ECCI technique starting from its discovery more than 45 years ago up to the combination with other advanced methods of scanning electron microscopy like electron backscatter diffraction or high-resolution selected area channelling patterning in the last few years is given. Major important investigations using this technique for the visualization of individual lattice defects like stacking faults (SFs) and dislocations or dislocation arrangements are chronologically summarized. The second part demonstrates that nowadays, ECCI micrographs taken in high-resolution scanning electron microscopes can be called high-resolution ECCI (HR-ECCI). It is shown that the resolution of individual SFs and dislocations in the HR-ECCI micrographs is comparable to that of conventional TEM (about 15 nm defect image width). Furthermore, the paper is demonstrating that HR-ECCI micrographs can be obtained for various types of materials after different mechanical loadings and different grain sizes ranging from large grain size of 500 μm (cast steel) down to less than 2 μm (γ-TiAl).

Grain boundary migration is an important mechanism of microstructural modification both in rocks and in metals. Combining detailed cathodoluminescence (CL) and electron backscatter diffraction (EBSD) analysis offers the opportunity to relate directly changes in crystallographic orientation to migrating boundaries. We observe the following features in naturally heated quartz grains from the thermal aureole of the Ballachulish Igneous Complex (Scotland, U.K.): (a) propagation of substructures and twin boundaries in swept areas both parallel and at an angle to the growth direction, (b) development of slightly different crystallographic orientations and new twin boundaries at both the growth interfaces and within the swept area and (c) a gradual change in crystallographic orientation in the direction of growth. All these features are compatible with a growth mechanism in which single atoms are attached and detached both at random and at preferential sites, i.e. crystallographically controlled sites or kinks in boundary ledges. Additionally, strain fields caused by defects and/or trace element incorporation may facilitate nucleation sites for new crystallographic orientations at distinct growth interfaces but also at continuously migrating boundaries. This study illustrates the usefulness of combined CL and EBSD in microprocess analysis. Further work in this direction may provide detailed insight into both the mechanism of static grain growth and the energies and mobilities of boundaries in terms of misorientation and grain boundary plane orientation.

Influence of strain amplitude on the development of dislocation structure during cyclic plastic deformation of 304 LN austenitic stainless steel

A potential of low energy (< 1keV) electron channeling contrast imaging (LE‐ECCI) by scanning electron microscopy (SEM) is demonstrated to characterize the crystallographic stacking sequence of hexagonal 4H‐SiC single crystal within the first unit cell thickness from the topmost surface (< 1nm). It is also revealed that the LE‐SEM signal intensity associated with the crystallographic orientation has enough sensitivity to even the change in Si‐atom and C‐atom (i.e. the change in surface polarity) at a primary electron energy of 0.4keV. The obtained data can be explained by an electron multiple scattering cluster (EMSC) theory where the interference of both incident and diffracted electron waves is considered within a finite size (a few nanometers) of an atomic cluster. So far, for characterizing local crystallographic orientation of bulk crystals using SEM, ECCI with a higher energy (> 10keV) of primary electron beam has been utilized in order to obtain larger interaction volume due to its deeper penetration depth [1]. The electron channeling contrast (ECC) from only a shallower surface region with lower energy has not been applied yet because of the consideration attributed to the lack of enough interaction volume in the lower energy regime. In fact, to our knowledge, the critical smallest volume (specimen minimum thickness) has not been investigated yet associated with ECC. In our previous study [2], a crystallographic orientation contrast from two bilayers thick surface layers of 4H‐SiC single crystal was observed by changing an incident LE (< 1keV) EB direction with respect to the orientation of an atomically flattered 4H‐SiC surface (Figs. 1). This contrast is considered to be the ECC in an extremely low electron energy regime. This result brings about the assumption that even the two bilayers thick interaction volume can contribute to the formation of ECC. In order to verify this assumption, in this study, crystallographic orientation dependence of SEM signal intensity was quantitatively measured on an atomically flat 4H‐SiC surface. Furthermore, in order to exploit the Z ‐information, different polar faces, Si‐terminated (0001) Si‐face, and C‐terminated (000‐1) C‐face were compared, where atomic sites of Si‐ and C‐atoms are switched in the crystal. The obtained results were compared with the EMSC theory in which the concept of short‐range periodicity is included [3]. This theoretical model is more applicable to reproduce the effect of shallower information depth of the LE‐EB, compared with a Bloch‐wave model utilized in the conventional ECCI in the higher electron energy regime. Fig.2 shows the representative SEM signal intensity from 4H‐SiC Si‐face and C‐face as a function of the sample tilting angle obtained at a primary electron energy of 0.4 keV. Although the profile shapes seem to be different at a glance, peak positions are almost the same. Considering the fact that the atomic arrangement below the surface is identical both in Si‐face and C‐face SiC, except Si‐ /C‐atomic sites in the crystal, the difference in the profile shape is ascribed to the contrast of backscattered electron yield [4]. The similar trend was also reproduced by the EMSC calculation.

Ultrawide bandgap (UWBG) gallium oxide (Ga2O3) represents an emerging semiconductor material with excellent chemical and thermal stability up to 1400 oC. It has a band gap of 4.5-4.9 eV, much higher than that of the GaN (3.4 eV) and 4H-SiC (3.2 eV). It exhibits high transparency in the deep ultraviolet (DUV) and visible wavelength region due to its very large bandgap. The monoclinic b-phase Ga2O3 represents the thermodynamically stable crystal among the known five phases (α, β, γ, δ, ε). The breakdown field of β-Ga2O3 is estimated to be 8 MV/cm, which is about three times larger than that of 4H-SiC and GaN. These unique properties make β-Ga2O3 a promising candidate for high power electronic device and solar blind photodetector applications. More advantageously, single crystal β-Ga2O3 substrates can be synthesized by scalable and low cost melting based growth techniques such as edge-defined film-fed growth (EFG), floating zone (FZ) and czochralski methods. For β-Ga2O3 thin film synthesis, both molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) have been demonstrated to produce high quality and controllable doping films but with slow growth rates (2-10 nm/min). Halide vapor phase epitaxy (HVPE) using chloride precursors was demonstrated to grow β-Ga2O3 films with fast growth rates (>5 μm/hr). Recently, we have developed a low pressure chemical vapor deposition (LPCVD) method to grow high quality β-Ga2O3 thin films on both native Ga2O3and c-sapphire substrates with controllable doping and fast growth rates up to 10 μm/hr. In this talk, we present a study on the defects in β-Ga2O3 thin films grown via LPCVD. The β-Ga2O3 thin films were grown on native β-Ga2O3 substrates and sapphire substrates using high purity gallium and oxygen as the precursors, and argon (Ar) as the carrier gas. The growth temperature ranged between 850 ˚C and 950 ˚C. The β-Ga2O3 thin films were characterized by using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). FESEM images were taken with Helios 650. High resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction (SAED) were taken using a FEI Tecnai F30 at 300 kV. Bright field, dark field, combined with two-beam condition TEM were used to characterize the defects in the as-grown films. From our studies, β-Ga2O3 thin films grown on (010), (001) and (-201) β-Ga2O3 substrates have shown different growth rates as well as different properties of interfacial defects. Improved LPCVD growths to suppress defects in β-Ga2O3thin films grown on different substrates will be discussed. Room temperature Hall measurements will be performed to understand the dependence of carrier transport on defects. In summary, the fundamental defect studies are performed on β-Ga2O3 thin films grown via LPCVD on Ga2O3 and sapphire substrates. The results from this study are important knowledge for power device applications.

Through recent advances in ultrahigh resolution FE-SEM with novel electron optics and multidetection systems [1], where secondary electrons, high-angle backscattered electrons, and channeling backscattered electrons (BSE) are detected separately, the world of electron microscopy and practical surface analysis, which had been fairly static in the 1980s and 1990s, may be changed drastically. Thus, by reducing the accelerating voltage to about 1.0–2.0 kV and through selective detection of secondary electrons (SE), topographic details of both conductive and nonconductive surfaces are revealed at a lateral resolution of about 1.5 nm under uncoated conditions. Simultaneously and by selective detection of high-angle BSE, having compositional and channeling information, the lateral resolution of BSE images has also been improved dramatically, with an excellent atomic number contrast (Z-contrast) sensitivity; a lateral resolution comparable to, or even better than, that of in-lens SE imaging, i.e., ~1.5 nm, is often realized at an accelerating voltage of about 1.0–2.0 kV range. Further, electron channeling contrast imaging, through selective detection of channeling BSEs, i.e., the primary electrons that underwent Mott scattering, allows subgrains, fine precipitates of sizes down to several nanometers, or even dislocations to be imaged clearly and at high resolution in the bulk sample in SEM; the information of these kinds has been obtained previously by transmission electron microscopy of thin foil specimens prepared through slow and tedious sample preparation procedures.

Electron channelling contrast imaging (ECCI) was used in the cold-field emission scanning electron microscope (CFE-SEM) to image the microstructure on deformed bulk specimen. Imaging was conducted with a pole-piece mounted silicon photodiode detector at 5 keV to collect backscattered electrons generated from a low-tilted (0 – 3 degrees) specimen. Broad ion beam milling surface preparation technique was used to remove surface layers and reveal near-surface deformation features. The uniaxial hot-compression tests were conducted on Mg-0.3 wt% Al-0.2 wt% Ca alloy. ECCI observations on deformed bulk specimen showed irregular and complex channelling contrast variations inside parent grains and low angle grain boundaries originated from parent grain boundaries. ECCI on an ion milled prepared surface provides non-destructive and rapid visualisation and characterisation of strain fields along with near-surface deformation substructures in CFE-SEM.

Electron channeling contrast imaging (ECCI) was used to characterize coherently strained La0.7Sr0.3MnO3 (LSM) films grown on (110)cubic-SrTiO3 (STO) and (100)orthorhombic-NdGaO3 (NGO). We focus on the characterization of a relatively low density (1–3 μm/μm2) of meandering loops (MLs) found in the LSM film on STO and absent in the film on NGO. The MLs exhibit a uniform contrast variation from the background and a strong contrast dependence on the diffraction vector g. The MLs are quantitatively consistent with LSM anti-phase boundaries (APBs) having a displacement vector R=12[001]LSM. These APBs are consistent with a “double positioning” degeneracy of tilted octahedra along [001]LSM on untilted octahedra along [001]STO. The results highlight the non-destructive capacity of ECCI to characterize extended defects in oxide films.

Crystallographic orientation and microstructural changes of ZnO thin films deposited by rf-magnetron sputtering were investigated with changing deposition parameters. Positional dependence of texture orientation and microstructural features, which might be attributed primarily to bombardment of high energetic oxygen species on the film in the region of the target erosion area, were observed. The films deposited at low temperature and/or using pure argon had (002) preferred orientation and a smooth columnar structure. At increasing substrate temperature and/or oxygen partial pressure, c-axis orientation of the film was severely degraded and abnormal elongated crystallites were also developed. As the films became thicker, c-axis orientation changed from normal to the substrate to parallel to the substrate. In this case, the microstructure was altered from a smooth fine columnar structure to abnormally grown elongated crystallites that were developed to a faceted texture structure.

Thermo-mechanical fatigue (TMF) of the duplex stainless steel SAF2205 (X2CrNiMoN22-5-3) was studied in the temperature range of 100–350°C. The tests were carried out on the duplex steel and on single-phase ferritic (X6Cr17, AISI 430) and austenitic steels (X2CrNiMo18-14-3, AISI 316L) similar to the two phases of the duplex steel for comparison. The mechanical behaviour of the three steels is analysed and discussed together with microstructural investigations by scanning electron microscopy, including electron backscatter diffraction and electron channelling contrast imaging.

Two-dimensional grain boundary sliding and mantle dislocation accommodation in ODS ferritic steel

Hydrogen-assisted failure in Ni-based superalloy 718 studied under in situ hydrogen charging: The role of localized deformation in crack propagation

Microscopic and nanoscopic study on subsurface damage and fatigue crack initiation during very high cycle fatigue

Effect of ball size on steady state of aluminum powder and efficiency of impacts during milling