B. Hourahine

Coincident electron channeling and cathodoluminescence studies of threading dislocations in GaN

We combine two scanning electron microscopy techniques to investigate the influence of dislocations on the light emission from nitride semiconductors. Combining electron channeling contrast imaging and cathodoluminescence imaging enables both the structural and luminescence properties of a sample to be investigated without structural damage to the sample. The electron channeling contrast image is very sensitive to distortions of the crystal lattice, resulting in individual threading dislocations appearing as spots with black-white contrast. Dislocations giving rise to nonradiative recombination are observed as black spots in the cathodoluminescence image. Comparison of the images from exactly the same micron-scale region of a sample demonstrates a one-to-one correlation between the presence of single threading dislocations and resolved dark spots in the cathodoluminescence image. In addition, we have also obtained an atomic force microscopy image from the same region of the sample, which confirms that both pure edge dislocations and those with a screw component (i.e., screw and mixed dislocations) act as nonradiative recombination centers for the Si-doped c-plane GaN thin film investigated.

Characterization of the blue emission of Tm/Er co-implanted GaN

Comparative studies have been carried out on the cathodoluminescence (CL) and photoluminescence (PL) properties of GaN implanted with Tin and GaN co-implanted with Tin and a low concentration of Er. Room temperature CL spectra were acquired in an electron probe microanalyser to investigate the rare earth emission. The room temperature CL intensity exhibits a strong dependence on the annealing temperature of the implanted samples. The results of CL temperature dependence are reported for blue emission (similar to 477 nm) which is due to intra 4f-shell electron transitions ((1)G(4)-> H-3(6)) associated with Tm3+ ions. The 477 nm blue CL emission is enhanced strongly as the annealing temperature increases up to 1200 degrees C. Blue PL emission has also been observed from the sample annealed at 1200 degrees C. To our knowledge, this is the first observation of blue PL emission from Tin implanted GaN samples. Intra-4f transitions from the D-1(2) level (similar to 465 nm emission lines) of Tm3+ ions in GaN have been observed in GaN:Tm films at temperatures between 20-200 K. We will discuss the temperature dependent Tm3+ emission in both GaN:Tm,Er and GaN:Tm samples.

SITE MULTIPLICITY OF RARE EARTH IONS IN III-NITRIDES

This presentation reviews recent lattice location studies of RE ions in GaN by electron emission channelling (EC) and X-ray absorption fine structure (XAFS) techniques. These studies agree that RE ions at low concentrations (whether they are incorporated during growth or introduced later by ion implantation) predominantly occupy Ga substitutional sites, as expected from considerations of charge equivalence. We combine this result with some examples of the welldocumented richness of optical spectra of GaN:RE3+ to suggest that the luminescence of these materials may be ascribed to a family of rather similar sites, all of which feature the REGa defect.

Energy-weighted dynamical scattering simulations of electron diffraction modalites in the scanning electron microscope

Transmission Kikuchi diffraction (TKD) has been gaining momentum as a high resolution alternative to electron back-scattered diffraction (EBSD), adding to the existing electron diffraction modalities in the scanning electron microscope (SEM). The image simulation of any of these measurement techniques requires an energy dependent diffraction model for which, in turn, knowledge of electron energies and diffraction distances distributions is required. We identify the sample-detector geometry and the effect of inelastic events on the diffracting electron beam as the important factors to be considered when predicting these distributions. However, tractable models taking into account inelastic scattering explicitly are lacking. In this study, we expand the Monte Carlo (MC) energy-weighting dynamical simulations models used for EBSDxa0[1] and ECPxa0[2] to the TKD case. We show that the foil thickness in TKD can be used as a means of energy filtering and compare band sharpness in the different modalities. The current model is shown to correctly predict TKD patterns and, through the dictionary indexing approach, to produce higher quality indexed TKD maps than conventional Hough transform approach, especially close to grain boundaries.

Dynamical simulations of transmission Kikuchi diffraction (TKD) patterns

Truly nanostructured materials pose a significant spatial resolution challenge to the conventional Electron Backscatter Diffraction (EBSD) characterization technique. Nevertheless, the interaction volume can be reduced by the use of electron transparent samples and the acquisition of electron backscatterlike patterns (EBSP) in transmission mode instead. These transmission Kikuchi diffraction (TKD) patterns are typically acquired by mounting a thin foil, similar to transmission electron microscopy (TEM), and tilting it at a slight angle (20◦ -30◦ ) from horizontal towards a standard EBSD camera.

Non-destructive Imaging of Extend Defects in III-nitride Thin film Structures Using Electron Channelling Contrast Imaging

Irrespective of the substrates, the growth plane, or the growth conditions employed, extended defects such as threading dislocations, stacking faults, misfit dislocations and grain boundaries are generally observed in the as–grown III-nitride thin film structures. Often extended defects are electrically active and are problematic for minority carrier devices, such as AlGaN based ultraviolet light emitting diodes and high electron mobility transistors. This is why structural characterisation techniques which are simultaneously rapid to use, and structurally definitive on the nanoscale become a prerequisite. Electron channelling contrast imaging (ECCI) performed in a scanning electron microscope (SEM) is a quick and non-destructive structural characterisation technique for imaging, identifying and quantifying extended defects in nitride thin films [1, 2].

Electron channeling contrast imaging of defects in III-nitride semiconductors

C.Trager-Cowan 1 , G. Naresh-Kumar 1 , N. Allehiani 1 , S. Kraeusel 1 , B. Hourahine 1 , S. Vespucci 1 , D.Thomson 1 , J. Bruckbauer 1 , G. Kusch 1 , P. R. Edwards 1 , R. W. Martin 1 , C. Mauder 2 , A. P. Day 3 , A. Winkelmann 4 , A.Vilalta-Clemente 5 , A. J. Wilkinson 5 , P. J. Parbrook 6 , M. J. Kappers 7 , M. A. Moram 7,8 , R. A. Oliver 7 , C. J. Humphreys 7 , P. Shields 9 , E. D. Le Boulbar 9 , D. Maneuski 10 , V. OShea 10 , K. P. Mingard 11 .