Forescattered electron channeling contrast imaging (ECCI) offers the potential for imaging and analyzing extended defects in a scanning electron microscope (SEM). Indeed, it is shown that ECCI is able to determine the Burgers vector of threading dislocations with the aid of carefully determined experimental parameters and accompanying image simulations. Simulations are compared with ECC images from samples with features that are relatively easily studied and modeled: those based on specially engineered 4H-SiC mesa substrates. These mesas serve as substrates for both homoepitaxial 4H-SiC layers and heteroepitaxial GaN layers in which images of threading dislocations (TDs) have been recorded using ECCI and found to strongly resemble diffraction contrast simulations of TD intensity profiles.

An approach to fabricate a set of simultaneously operating dual-UV-wavelength detectors is described. The fabrication flow relies on the confined-epitaxy growth method. The confined epitaxial AlxGa1-xN-layer stacking approach is used to establish simultaneous multiple UV-wavelength detection. The chosen stoichiometries of specific epitaxial layers set the wavelength sensitivity at approximately 355 nm for pixel A and 320 nm for pixel B. Spectral responsivity plots of the detectors clearly show the dual-UV-color sensitivity of the pair. The detectors have signal-to-noise ratios of 15 and 17 and spectral responsivity values of 0.12 AAV and 0.05 AAV for pixel A and pixel B, respectively.

During the growth of silicon carbide (SiC) by hot-wall chemical vapor deposition, there is a competition between etching and growth processes due to hydrogen etching of the SiC surface, which is a function of temperature and the etching atmosphere. The interplay of the etch rates and growth rates and the influence of the doping level of the growing layer on these rates must be taken into account when growing device structures with stringent thickness constraints. In this work, a comparison of secondary ion mass spectrometry data with etch rates for a variety of individual epilayers is used to differentiate the contribution of etching processes from that of site competition effects between precursor and incorporated dopant species on the net growth rate. It is found that the presence of nitrogen at the growing surface decreases the net growth rate by competing with carbon species for incorporation into surface lattice sites at a level far beyond what would be expected for most dopant atoms. This is a result of the high nitrogen flux required at the surface due to the inefficiency of nitrogen incorporation at growth temperatures. Furthermore, etch rates of SiC epilayers were found to increase with increasing carrier concentrations for both p- and n-type 4H-SiC. Finally, small additions of propane to the hydrogen atmosphere suppresses the etch rate of SiC; however, too much propane leads to degradation of the surface morphology. The activation energy for the hydrogen–propane etching mechanism is determined to be 120.5kcalmol−1, in good agreement with values reported in the literature.

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

We report evidence for non-destructive determination of dislocation type and Burgers vector direction in GaN using electron channeling contrast imaging (ECCI) inside a scanning electron microscope. Forescattered electron intensity fluctuations generated by threading dislocations exhibit characteristic spatial profiles indicative of dislocation type (screw, edge) and Burgers vector direction. Simulated channeling contrast features by two-beam dynamical diffraction calculations show qualitative agreement with recorded images. Forescatter ECCI sensitivity to atomic steps and low-angle grain boundaries in GaN allow for additional confirmation. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Plasmonic‐enhanced emission was achieved from a III‐nitride nano‐wire optoelectronic device. The III‐nitride system has been widely applied to light emitters in the blue, violet and ultra‐violet portions of the electromagnetic spectrum. One limit to its application has been the efficiency reduction associated with the inherently defective nature (particularly threading dislocations) of the material that is typically grown by hetero‐epitaxy. The VLS fabrication technique inherently yields defect‐free material that serves as an excellent emission and gain media. A two‐step approach was developed in a metalorganic chemical vapor deposition system to grow nano‐wires initially vertically by the VLS mechanism and, when necessary, laterally by altering the growth conditions. Furthermore, various silver coatings were employed to enhance the plasmonic‐based transfer of electromagnetic energy generated in the nano‐wire to the external near‐field. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A two-step deposition process was developed in a metal organic chemical vapor deposition system to create a defect-free p-type structure consisting of a GaN nanowire (NW) core surrounded by a distinct AlGaN radial shell. The m-plane growth direction yields a nanowire with an isosceles triangular cross section with two identical {-110-1} facets and a third (0001) facet. The AlGaN shell introduces a polarization-induced sheet charge at the AlGaN/GaN interface and may shield the NW from surface-state induced depletion. Transmission electron microscopy confirmed that single-component NWs and sufficiently thin NW heterostructures are free of extended defects. High hole currents and current modulation were demonstrated with back-gated NW transistor structures.

A novel erbium chloride seeded growth process was developed for the growth and fabrication of erbium doped GaN nanocrystals. This erbium chloride seeded technique simplifies the delivery of erbium into a metal organic chemical vapor deposition system. Additionally, this selective growth of the GaN nanocrystal only occurred in the presence of the ErCl3 seed. Strong green emission, distinctive of the Er3+ ion, was observed in the GaN nanocrystals.

Future integrated circuit technology will feature a fusion of optical and optoelectronic components with traditional electronic devices. Information can be rapidly transmitted as light in dielectric waveguides, photonic crystal guides and metallic nanoarrays. This paper presents a description of electromagnetic propagation in semiconductor and metallic nanostructures. Diffraction effects will dominate the propagation of light when the dimension of the cavity or device approaches its wavelength. The plasmonic effect circumvents this problem by propagating the tight wave through highly localized conduction electrons in a noble metal [1].

During GaN growth on an on-axis SiC substrate, a large density of dislocations (similar to 10(9) cm(-2)) is unavoidably generated by the GaN/SiC misfit, uncontrolled misorientation in the substrate and defects present at the surface of the substrate. Recently a unique SiC substrate was developed with step-free and stepped mesas as well as continuous surface areas similar to a standard wafer all in close proximity. It is now possible to isolate the influence of defects and steps on the deposition of GaN on SiC. This paper provides a link between the dislocation environment and the strain state in the GaN film as well as the change of this strain with increasing thickness.

Different protective caps (AlN, MgO, graphite) are investigated for their feasibility for protecting GaN surfaces during ultra-high-temperature (>1300 °C) microwave annealing. Compared to other capping materials, pulsed-laser-deposited AlN is found to protect the GaN surface more effectively, during ultra-fast microwave annealing at temperatures as high as 1500 °C. The RMS surface roughness (0.6 nm) of the GaN sample annealed at 1500 °C with an AlN cap in place is similar to the value (0.3 nm) measured on the as-grown sample. The photoluminescence and electrical measurements have indicated a decrease in the compensating deep donor concentration for the increasing microwave annealing temperature, as long as the surface integrity of the GaN epilayer is preserved. These results indicate the attractiveness of the AlN cap for microwave annealing of ion-implanted GaN.

An approach to reduce vertical threading dislocations in the active regions of III-nitride devices is described. The approach involves confined homo- or heteroepitaxy of GaN materials using sputtered oxide masks to delineate growth regions and conventional metal-organic chemical vapor deposition. The resulting confined epitaxial material is terminated with equilibrium crystal facets, which form hexagonal mesas, and contains a reduced dislocation density and reduced strain compared to the underlying template layer for homoepitaxial growth. Characterization of pn junction diodes grown with this approach reveals significantly reduced leakage currents in as-grown, unpassivated structures (as low as 1×10−7Acm−2).

A novel rare-earth chloride seed was employed as a catalyst for growth of GaN nano- and micro-crystals on c-, a- and r-plane sapphire. The ErCl3 seed on the substrate surface enhanced the growth rate and density of the GaN crystals. Distinctive green photoluminescence was measured, confirming that Er3+ ions were active in the GaN matrix. This technique can be adapted to selectively grow GaN crystals with emission tailored to the particular optical transitions of the rare-earth seed. (C) 2007 Elsevier B.V. All rights reserved.

Threading dislocations in metal-organic chemical-vapor grown GaN films were imaged nondestructively by the electron channeling contrast imaging (ECCI) technique. Comparisons between ECCI and cross-sectional transmission electron microscopy indicated that pure edge dislocations can be imaged in GaN by ECCI. Total threading dislocation densities were measured by ECCI for various GaN films on engineered 4H-SiC surfaces and ranged from 107to109cm−2. A comparison between the ultraviolet electroluminescent output measured at 380nm and the total dislocation density as measured by ECCI revealed an inverse logarithmic dependence.

The plasmonic response from a nanotextured silver coating was utilized to enhance the transfer of ultraviolet light generated in a group-III nitride nanowire emitter. A two-step approach was developed in a metal-organic chemical vapour deposition system to grow nanowires initially vertically by the vapour-liquid-solid mechanism and, subsequently, laterally by increasing the growth temperature and the group-V/III reactant ratio. This controllably produced a 20 nm GaN:Si core with a 200 nm outer-diameter AlGaN:Mg sheath structure. Solvothermal chemistry based on an ethylene glycol solvent was employed to deposit a silver coating that approximated a dense packing of metallic nanospheres. Nanoscale emission and plasmonically enhanced transfer of this energy were simulated to aid the development and understanding of this system.

A set of three 4H-SiC wafers with manufacturer specified micropipe density of 0-5 cm-2 were characterized by x-ray diffraction (XRD) maps before and after final chemical-mechanical polish. After final polish, the wafers were also investigated with atomic force microscopy, radius of curvature measurements and cross-polarization (x-pol) mapping. It was found that there was largely a lack of correlation between the XRD and x-pol maps, which strongly suggests that x-pol is insensitive to crystalline imperfections to which XRD is sensitive.

The impetus for dislocation motion in thin films is generally understood in terms of Peach-Koehler forces. For the case of III-nitride films grown on step-free 4H-SiC mesas, however, it is the gradient of the strain energy from the mesa edge that is capable of driving misfit dislocations. Using the strain profile as a function of the distance from the mesa edge and the line tension of the c-plane threading arms, we have calculated the excess stress driving the half loop from the mesa edge into the mesa interior. We have also compared the half-loop excess stress with the excess stress driving the tilt of threading edge dislocations, which has been proposed as one of the principal strain relief mechanisms in III-nitride films. The excess stress driving c-plane half loops ranges from a few 1000MPa at the mesa edge to few 100MPa towards the mesa interior, while the excess stress driving the tilt of threading edge dislocations is in excess of 20000MPa. The greater excess stress driving dislocation tilt, however, does not dominate strain relief for III-nitride films on step-free SiC mesas due to the difficulty in nucleating threading dislocations in the absence of interfacial steps.

Using transmission electron microscopy, we have analyzed dislocations in AlN nucleation layers and GaN films deposited by metalorganic vapor phase epitaxy on the (0001) surface of epitaxially grown 4H-SiC mesas with and without steps. For 4H-SiC substrates free of SiC surface steps, half-loop nucleation and glide parallel to the AlN/SiC interfacial plane play the dominant role in strain relief, with no mechanism for generating threading dislocations. In contrast, 4H-SiC mesa surfaces with steps give rise to regions of high stress at the heteroepitaxial interface, thereby providing an environment conducive to the nucleation and growth of threading dislocations, which act to accommodate misfit strain by the tilting of threading edge dislocations. We compare the excess stress associated with strain relief for each mechanism and find that the driving force for plastic flow is much greater for threading dislocation tilt than for half-loop propagation.