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Single-event burnout (SEB) is experimentally observed in structurally improved vertical Beta-gallium oxide ( $\beta $ -Ga2O3) Schottky barrier diodes (SBDs) with Pt/PtOx Schottky contacts and high-k field-plate (FP) dielectrics. These SBDs are resistant to alpha particle strikes up to the in-air reverse bias voltage of 475 V, but SEB-induced catastrophic failure is observed during Californium-252 (Cf-252) fission fragment exposure at 400 ± 20 V reverse bias. The SEB threshold in Cf-252 testing is improved in PtOx-contact SBDs with 4- $\mu \text{m}$ epitaxial layers and high-k FP dielectrics by ~30%, compared to previously reported results for Pt-contact SBDs with 10- $\mu \text{m}$ epitaxial layers. TCAD simulations show that ion-induced electric fields increase less in devices with FPs than in those without FPs.

Low-energy ion-induced breakdown and single-event burnout (SEB) are experimentally observed in beta-gallium oxide ( $\beta $ -Ga2O3) Schottky diodes with voltages well below those of expected electrical breakdown. Fundamentally different responses were observed among alpha particle, Cf-252, and heavy-ion irradiation. Technology computer-aided design (TCAD) simulations suggest that ion-induced burnout can be triggered at high voltages as a result of a single ion strike, leading to impact ionization, voltage-induced charge separation accentuated by the low intrinsic hole mobility in $\beta $ -Ga2O3, and breakdown. At significantly lower voltages, the cumulative buildup of displacement-damage-induced defects in $\beta $ -Ga2O3 during high-fluence ion irradiation can lead to defect-driven breakdown due to the generation and motion of negatively charged Ga vacancies and O interstitials. First-principles calculations show that defect clusters can be formed, which are much less resistive than the surrounding material. These clusters can be driven deeply into the device by the reverse bias, ultimately forming conduction paths that can facilitate the destruction of the device at reduced voltages.

The introduction of vacancy type point defects by Si doping in InN grown by plasmaassisted molecular beam epitaxy was studied using a monoenergetic positron beam. With the combination of positron lifetime and Doppler broadening measurements, compensating Invacancy VIn acceptors were identified in the material. For increasing Si doping an enhanced formation of VIndefects was observed, up to a concentration of $c_{\rm V} = 7 \times 10^{17} \,{\rm cm}^{ - 3} $in the highest doped sample $n_{\rm e} = 6.6 \times 10^{20} \,{\rm cm}^{ - 3} $. A strong inhomogeneity of the defect profile with a significant increase of the VInconcentration toward the layersubstrate interface could be detected. Additionally, larger vacancy clusters containing several VInare formed in the proximity of the interface.

Wurtzite A and Mplane InN films were grown by molecular beam epitaxy MBE on freestanding GaN substrates. Spectroscopic ellipsometry SE in the photon energy range from 0.56 up to 15 eV was applied in order to determine the ordinary and extraordinary complex dielectric function DF of InN. A distinct optical anisotropy was found over the whole energy range. The extraordinary absorption edge in comparison to the ordinary one is shifted to higher energies confirming previous studies. The investigations in the upper vacuumultraviolet VUV spectral range 9.5–15 eV yielded transition energies for four critical points CPs of the band structure BS which have not been observed so far.

The microstructure of p-type Al0.69Ga0.31N with high Mg-doping was studied by conventional transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). Spike-like domains were observed with an average diameter of ∼10 nm, height of ∼400 nm and density of 2.2 × 1011/cm2. These long and narrow domains nucleated about 200 nm above the Mg-doping boundary and resembled ordinary inversion domains induced by Mg-doping. However, the energy dispersive X-ray spectroscopy line-scan results provided unambiguous evidence that the domains were Ga-rich. The lateral composition segregation between the domains and surrounded matrix was further confirmed by high angular dark field images in STEM. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

N-face AlGaN (cap)/GaN (channel)/AlGaN (barrier)/GaN (buffer) high electron mobility transistors (HEMTs) provide a simple solution for strong confinement of the two-dimensional electron gas (2DEG) from the back since carriers are induced on top of the AlGaN barrier. To reduce the adverse effects of random alloy scattering from the AlGaN barrier, an N-face GaN-spacer HEMT was designed with an epi-structure consisting of an AlGaN (cap)/GaN (channel)/AlN/GaN (spacer)/AlGaN (barrier)/GaN (buffer). The 2DEG is confined at the GaN (channel)/AlN interface, while the GaN-spacer layer separates the 2DEG from the AlGaN barrier. The large polarization-induced electric field of the AlN and its large conduction band discontinuity with GaN provide strong confinement for good pinch-off and low output conductance under high electric fields. Up to 20% improvement in electron mobility was measured in these devices. The devices showed a current gain cut-off frequency (ft) of 24 GHz and power gain cut-off frequency (fmax) of 44 GHz for a nominal gate length of 0.7 μm. An output power density of 4.5 W/mm was measured at 4 GHz, with 34% power added efficiency and a gain of 10.3 dB at a drain bias of 40 V. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The strain in a -plane GaN layers of different thickness grown on r -plane sapphire substrates by hydride vapor phase epitaxy was studied by X-ray diffraction. The layers are found to be under compression in the growth plane and under tension in the growth direction. Therefore, the symmetry of the GaN unit cell is no longer hexagonal but orthorhombic. With increasing layer thickness the strain relaxes and the curvature of the wafer increases. Wafer bending is proposed to be the major strain relaxation mechanism. The anisotropic in-plane strain relaxation is attributed to the elastic and thermal anisotropy of GaN and sapphire. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Intrinsic stresses due to lattice mismatch, high densities of threading dislocations, and extrinsic stresses resulting from the mismatch in the coefficients of thermal expansion, are present in almost all III-Nitride heterostructures. Stress relaxation in the GaN layers occurs in conventional, cantilever (CE) and in pendeo-epitaxial (PE) films via the formation of additional misfit dislocations, domain boundaries, elastic strain and wing tilt. Polychromatic X-ray microdiffraction, high resolution monochromatic X-ray diffraction and SEM analysis have been used to determine the crystallographic properties, misfit dislocations distribution and crystallographic tilts in uncoalesced GaN layers grown by PE and CE. The crystallographic tilt between the GaN(0001) and Si(111) planes was detected in the CE grown samples on Si(111). In contrast there was no tilt between GaN(0001) and SiC(0001) planes in PE grown samples. The wings are tilted upward for both the PE and CE grown uncoalesced GaN layers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN/AlGaN multiple quantum wells (MQWs) of varying well width grown on planar c-plane GaN and a-plane GaN templates fabricated by lateral epitaxial overgrowth (LEO) have been studied by subpicosecond photoluminescence (PL) downconversion and time-correlated single photon counting as a function of pump intensity. Although slower room temperature decays at higher carrier density are observed in both a-plane LEO and c-plane MQWs with 3 nm well width, this behavior is attributed to suppression of electric field-enhanced recombination through the barriers and interfaces at higher carrier density in the c-plane MQW, while in the a-plane LEO MQW the sublinear dependence of the PL signal at t = 0 on pump intensity implies that the radiative lifetime in this high quality sample increases due to a reduction in excitonic oscillator strength associated with phase space filling. The combination of more intense time-integrated PL spectra and shorter PL lifetimes with decreasing well width in the a-plane LEO MQWs for low pump intensity suggests that the radiative lifetime becomes shorter due to the accompanying increase in exciton binding energy and oscillator strength at smaller well width. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

AlGaN based ultraviolet solar blind photodiodes, operating at a peak wavelength of 274 nm with external quantum efficiencies of up to 46% at zero bias, were grown by low pressure MOCVD. The photodiodes operate via backside illumination, allowing the use of Mg doped GaN instead of a high Al mole fraction p-AlGaN layer. The effects of: the individual layer thicknesses, the Al composition difference between the n-type layer and intrinsic layer, the use of an AlGaN composition grade at the i-n interface, and the inclusion of an electron blocking layer at the p-i interface on the device performance were investigated. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN films were grown using metal-organic chemical vapor deposition (MOCVD) on intentionally roughened c-sapphire in order to study the effect of the substrate roughness. Controlled substrate damage was achieved by exposing the substrate surfaces to controlled amounts of trimethylgallium followed by exposure to hydrogen at elevated temperature. The bulk film quality was studied by high-resolution X-ray diffraction and the surface topography was examined by atomic force microscopy. It was observed that mild substrate roughness (surface pit depth up to 30 nm) does not have a significant effect on the bulk film quality nor the GaN surface topography, and that recovery of the surface roughness is observed in very early stages of film growth. Cross sectional transmission electron microscopy confirmed that MOCVD GaN grown on inclined sidewalls etched in c-sapphire maintained the $\{0001\}_{\rm sapphire}\parallel\{0001\}_{\rm GaN}$epitaxial relationship despite the substrate miscut of ca. 15° with respect to the c-axis.

Preliminary results from our work on compact phase shifters using BaSrTiO 3 thin films are presented. A reflection phase shifter utilizing a lumped element resonator achieved 120 phase shift at 10GHz with m 3.3dB insertion loss. A synthetic transmission line true time delay phase shifter at 5GHz was able to achieve a phase shift of 60 with m 1.1dB insertion loss. These results were obtained in a process suitable for integration with conventional passive and hybrid circuit technologies.

In the heteroepitaxial growth of films with large misfit with the underlying substrate, the generation of misfit dislocations and threading dislocations (TDs) is ubiquitous for thicknesses well in excess of the equilibrium critical thickness. Experimental data suggests that, after an entanglement region near the film-substrate interface, there is a fall-off in TD density that is inversely proportional to the film thickness h (applicable to densities in the approximate range 107-109 cm-2), followed by saturation or weak decay of the TD density with further increase in film thickness. In this paper, we build upon a recent framework for understanding TD reduction in terms of their effective lateral motion with increasing film thickness, coupled with the probability that certain pairs of TDs annihilate, reacting to form a single TD, or undergoing a change in line direction. A computer simulation approach is presented that augments a recent theoretical prediction of both the 1/h scaling behaviour and the saturation of TD densities. Specific cases are considered involving TD distributions typical of surface nucleation, island periphery nucleation and combinations thereof. In addition, an analysis is made of the propensity for various TDs to cluster together. Results are presented which suggest that longrange fluctuations in the net Burgers vector content of the local TDs are a cause for saturation behaviour.

Analyses of biopolymer/calcium carbonate composites grown on inorganic abiotic substrates implanted between the shell and the shell-secreting epithelium of live red abalones (Haliotis rufescens) provide detailed spatial and temporal data on the in vivo assembly process that generates the shell. X-ray diffraction and scanning electron microscopy analyses of the growth of these flat pearl composites reveal that biomineralization is initiated by the deposition of an organic sheet on the implanted substrate, followed by the growth of a calcite layer with preferred {10.4} orientation and, finally, by the growth of nacreous aragonite. The calcite layer is structurally similar to the green organic/calcite heterolayer of native shell nacre. It comprises 0.2−2.0-μm-diameter elongated crystallites of typical geological habits in various aggregate arrangements. The shell also contains an external layer of (00.1)-oriented prismatic calcite, which is deposited on one edge of a flat pearl and has a morphology similar to that of the {10.4}-oriented calcite layer. The transition from {10.4}-oriented calcite to aragonite in both the shell and the flat pearl is abrupt. In vitro calcium carbonate growth experiments reveal that a similar calcite-to-aragonite transition is induced by the addition of soluble proteins isolated from the aragonitic nacre. The growth of flat pearls is highly sensitive to physical and chemical properties of the abiotic substrate. Either roughened or hydrophobic substrates result in abnormal arrangements of the basal calcite layer, which are corrected for by a reinitiation of the biomineralization process, beginning with the deposition of an organic sheet. Insertion of flat pearls as substrates, however, results in continued nacre growth without the deposition of an organic sheet and a calcite layer.

The crystal structures of CoCl2-, MnCl2-, NiCl2-, and CuCl2-graphite intercalation compounds (GICs) have been studied with single crystal X-ray diffraction methods. The first three intercalates maintain their pristine in-plane CdCl2 structure after intercalation.The intercalate layers in stage 1 CoCl2-, MnCl2-, NiCl2-GICs stack in an ordered αβTγαβTγ rhombohedral sequence. Additionally, the carbon layers in stage 1 CoCl2-GIC stack in an ordered ABC rhombohedral sequence. For CoCl2-, MnCl2-, and NiCl2,-GICs the intercalate and carbon in-plane lattices exhibit rotational epitaxy. The epitaxy angle is either near 0° or 30°, depending on the intercalate. Current models for rotational epitaxy do not explain the observed rotation angles. The CuCl2 layers have the periodicity of a centred rectangular net and stack, in the stage 1 GIC, in an ordered αβTαβTαβT sequence. The intercalate in stage 1 CuCl2-GIC forms a metrically face centred orthorhombic lattice; however, the symmetry of the intercalate structure is monoclinic. The intercalate interlayer interaction is attenuated by the intervening carbon layers, so that the stacking of the intercalate layers in all of the stage 2 compounds is uncorrelated.

We have examined the role of As precipitates on transport through undoped GaAs grown at low temperature by molecular beam epitaxy and annealed at high temperature. Temperature dependent I–V measurements exhibit two regimes. At temperatures less than ∼200K, transport attributed to point defect-associated hopping conduction is observed even for samples annealed at 750°C. For temperatures greater than ∼200K, the transport is quantitatively consistent with calculations of thermally assisted tunneling emission of electrons from metallic As precipitates acting as buried Schottky barriers.

Ferroelastic domain formation in thin films of rhombohedral La1–xSrxMnO3 (LSMO), coherently grown on (001) or (011) oriented SrTiO3 (STO), is suggested to occur during film deposition at elevated temperatures, unlike other common perovskite films where structural phase transition and domain formation take place only during cooling. Due to a comparatively small lattice parameter misfit, the LSMO/STO system should be an ideal case for the experimental observation of domain formation without additional strain relaxation effects including misfit dislocation formation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Hall mobilities as high as 702 and 1230 cm2/Vs at 300 and 160K along with low dislocation densities of 4.0 × 108cm-2have been achieved in GaN films grown on sapphire by metalorganic chemical vapor deposition. High growth temperatures have been established to be crucial for optimal GaN film quality. Photoluminescence measurements revealed a low intensity of the deep defect band around 550 nm in films grown under optimized conditions.

The structural and morphological evolution of GaN films grown by MOCVD at high temperature (1080°C) on a low temperature grown GaN nucleation layer (NL) on (0001) sapphire were studied using atomic force microscopy (AFM), transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The high temperature (HT) GaN layers were found to grow by initially forming isolated truncated hexagonal islands having {10*BAR*1*BAR*1} facet planes and a top (0001) plane. The non-wetting or partial wetting behavior of the HT GaN on the GaN NL is attributed to both the roughness and predominantly cubic nature of the NL.

The evolution of morphology and associated extended defects in GaN thin films grown on sapphire by metalorganic chemical vapor deposition (MOCVD) are shown to depend strongly on the growth environment. For the commonly used two-step growth process, a change in growth parameter such as reactor pressure influences the initial high temperature (HT) GaN growth mechanism. By means of transmission electron microscopy (TEM), atomic force microscopy (AFM), and high resolution X-ray diffraction (HRXRD) measurements, it is shown that the initial density of HT islands on the nucleation layer (NL) and subsequently the threading dislocation density in the HT GaN film may be directly controlled by tailoring the initial HT GaN growth conditions.