Thermodynamics of Ga2O3 Heteroepitaxy and Material Growth Via Metal Organic Chemical Vapor Deposition.

Heteroepitaxy of gallium oxide (Ga2O3) is gaining popularity to address the absence of p-type doping, limited thermal conductivity of Ga2O3 epilayers, and toward realizing high-quality p-n heterojunction. During the growth of ß-Ga2O3 on 4H-SiC (0001) substrates using metal-organic chemical vapor deposition, we observed formation of incomplete, misoriented particles when the layer was grown at a temperature between 650 °C and 750 °C. We propose a thermodynamic model for Ga2O3 heteroepitaxy on foreign substrates which shows that the energy cost of growing ß-Ga2O3 on 4H-SiC is slightly lower as compared to sapphire substrates, suggesting similar high-temperature growth as sapphire, typically in the range of 850 °C-950 °C, that can be used for the growth of ß-Ga2O3 on SiC. A two-step modified growth method was developed where the nucleation layer was grown at 750 °C followed by a buffer layer grown at various temperatures from 920 °C to 950 °C. 2θ-ω scan of X-ray diffraction (XRD) and transmission electron microscope images confirm the ß-polymorph of Ga2O3 with dominant peaks in the (-201) direction. The buffer layer grown at 950 °C using a "ramp-growth" technique exhibits root-mean-square surface roughness of 3 nm and full width of half maxima of XRD rocking curve as low as 0.79°, comparable to the most mature ß-Ga2O3 heteroepitaxy on sapphire, as predicted by the thermodynamic model. Finally, the interface energy of an average Ga2O3 island grown on 4H-SiC is calculated to be 0.2 J/m2 from the cross-section scanning transmission electron microscope image, following the Wulff-Kaishew theorem of the equilibrium island shape.

Epitaxial Growth of (-201) ß-Ga2O3 on (001) Diamond Substrates.

Heteroepitaxial growth of ß-Ga2O3 on (001) diamond by metal-organic chemical vapor deposition (MOCVD) is reported. A detailed study was performed with Transmission Electron Microscopy (TEM) elucidating the epitaxial relation of (-201) ß-Ga2O3||(001) diamond and [010]/[-13-2] ß-Ga2O3 ||[110]/[1-10] diamond, with the presence of different crystallographically related epitaxial variants apparent from selected area diffraction patterns. A model explaining the arrangement of atoms along ⟨110⟩ diamond is demonstrated with a lattice mismatch of 1.03-3.66% in the perpendicular direction. Dark field imaging showed evidence of arrays of discrete defects at the boundaries between different grains. Strategies to reduce the density of defects are discussed.

Managing dose-, damage- and data-rates in multi-frame spectrum-imaging.

As an instrument, the scanning transmission electron microscope is unique in being able to simultaneously explore both local structural and chemical variations in materials at the atomic scale. This is made possible as both types of data are acquired serially, originating simultaneously from sample interactions with a sharply focused electron probe. Unfortunately, such scanned data can be distorted by environmental factors, though recently fast-scanned multi-frame imaging approaches have been shown to mitigate these effects. Here, we demonstrate the same approach but optimized for spectroscopic data; we offer some perspectives on the new potential of multi-frame spectrum-imaging (MFSI) and show how dose-sharing approaches can reduce sample damage, improve crystallographic fidelity, increase data signal-to-noise, or maximize usable field of view. Further, we discuss the potential issue of excessive data-rates in MFSI, and demonstrate a file-compression approach to significantly reduce data storage and transmission burdens.

Tungsten oxide nanorod growth by pulsed laser deposition: influence of substrate and process conditions.

Tungsten oxide nanorods (NRs) have been grown on W, Ta and Cu substrates following 193 nm pulsed laser ablation of a WO3 target in a low background pressure of oxygen. The deposited materials were analysed by scanning and (high resolution) transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), X-ray diffraction, Raman and X-ray photoemission spectroscopy, and tested for field emission. In each case, HRTEM analysis shows NR growth along the [100] direction, and clear stacking faults running along this direction (which are also revealed by streaking in the SAED pattern perpendicular to the growth axis). The NR composition in each case is thus determined as sub-stoichiometric WO(3-δ), but the NR morphologies are very different. NRs grown on W or Ta are short (hundreds of nm in length) and have a uniform cross-section, whereas those grown on a Cu substrate are typically an order of magnitude larger, tapered, and display a branched, dendritic microstructure. Only these latter NRs give significant field emission.

Electro-induced orientational ordering of anisotropic pigment nanoparticles.

The response of anisotropic pigment particle suspensions to externally applied electric fields has been explored for possible application in reflective display technologies. Three different types of pigment particle were suspended in dodecane, using a polymeric stabilizer, and showed Schlieren textures between crossed polarizers at high concentrations (greater than 25-30 wt%), indicating the formation of colloidal nematic phases. Orientational order parameters were determined by X-ray scattering, and the influence of polydispersity on the values is discussed. X-ray scattering measurements also demonstrated a change in the structure factor consistent with the onset of a colloidal nematic phase. In addition, the pigment particles were dispersed into various liquid crystal hosts at low concentrations (less than 5 wt%) with and without the presence of mesogenic mimic stabilizers. However, the influence of these stabilizers on orientational ordering could not be confirmed. The electro-induced ordering determined via scattering was related to the electro-optical response of each suspension using a simple model. The particles in nematic hosts not only showed a high degree of orientational ordering at lower electric field strengths, but also showed a reduction in stability. Although these systems have shown strong orientational ordering, the optical response has been limited by the intrinsic shape of the pigment particles and the distribution of the transition dipoles moments within them. Nevertheless, the feasibility of developing materials for display applications has been demonstrated.

Investigating the role of microbes in mineral weathering: nanometre-scale characterisation of the cell-mineral interface using FIB and TEM.

Focused ion beam (FIB) sample preparation in combination with subsequent transmission electron microscopy (TEM) analysis are powerful tools for nanometre-scale examination of the cell-mineral interface in bio-geological samples. In this study, we used FIB-TEM to investigate the interaction between a cyanobacterium (Hassallia byssoidea) and a common sheet silicate mineral (biotite) following a laboratory-based bioweathering, incubation experiment. We discuss the FIB preparation of cross-sections of the cell mineral interface for TEM investigation. We also establish an electron fluence threshold (at 200keV) in biotite for the transition from scanning (S)TEM electron beam induced contamination build up on the surface of biotite thin sections to mass loss, or hole-drilling within the sections. Working below this threshold fluence nanometre-scale structural and elemental information has been obtained from biotite directly underneath cyanobacterial cells incubated on the biotite for 3 months. No physical alteration of the biotite was detected by TEM imaging and diffraction with little or no elemental alteration detected by STEM-energy dispersive X-ray (EDX) elemental line-scanning or by energy filtered TEM (EF-TEM) jump ratio elemental mapping. As such we present evidence that the cyanobacterial strain of H. byssoidea did not cause any measurable alteration of biotite, within the resolution limits of the analysis techniques used, after 3 months of incubation on its surface.

Raman scattering in Me-doped ZnO nanorods (Me = Mn, Co, Cu and Ni) prepared by thermal diffusion.

We have investigated normal and resonant Raman scattering in Me-doped ZnO nanorods (Me = Mn, Co, Cu and Ni) prepared by thermal diffusion. Experimental results show that the normal Raman spectra consist of the conventional modes associated with wurtzite ZnO and impurity-related additional modes. Under resonant conditions, only longitudinal optical (LO) phonon scattering and its overtones are observed. The number of LO phonon lines and their relative intensity depend on the doping element and level. For the nanorods doped with Cu and Ni, we have observed LO phonon overtones up to eleventh order. This situation does not happen for the Mn-doped nanorods, which show only five LO phonon modes. By co-doping Mn and Co into the ZnO host lattice, however, the LO phonon overtones up to eleventh order are observed again. The nature of this phenomenon is explained by means of the study of XRD, TEM and photoluminescence.

Investigation of the charge on threading edge dislocations in GaN by electron holography.

We present direct evidence for the charging around end-on threading edge dislocations in n-type GaN doped with silicon by off-axis electron holography in a transmission electron microscope. It is shown that the inner potential is reduced by up to 2.5 V within 10 nm of the dislocation, consistent with a negatively charged core. The results, which can be fitted with an unscreened potential, are consistent with a line charge of about 2 electrons/c, where c = 0.52 nm is the unit cell parameter of GaN. The origin of this line charge is discussed. The application of the method to other types of dislocation is also considered.