We discuss trends in residual stress as a function of film thickness, growth temperature, and substrate orientation for GaN/AlN/6H-SiC heterostructures. Films are mostly compressive for samples less than about 0.7 m thick, are tensile up to about 2 m, then abruptly become less tensile with stress values near 1 kbar thereafter. We interpret this as a successive relief of lattice mismatch and thermal stresses culminating in a catastrophic relief by unknown mechanisms at moderate thicknesses. These data indicate that relaxation processes in these heterostructures are not as well understood as previously supposed.

Pendeo-epitaxy of individual GaN and Al x Ga 1-x N films and single-and multi-layer heterostructures of these materials have been achieved on a columnar GaN seed layer using metallorganic vapor phase epitaxy. These structures have been characterized using scanning electron microscopy and atomic force microscopy. The RMS roughness value of the grown side wall plane 1120 ( ) of these structures was 0.099 nm.

Superconducting ultrathin films grown epitaxially onto crystalline substrates exhibit strained epitaxial growth due to lattice mismatch, which can have a significant effect on their superconducting properties. We present a complete correlation of the surface morphology, crystal growth, strain, microstructure, and superconducting properties in single-crystal Nb(110) thin films sputter deposited on a-plane sapphire substrates. Notably, we observe that the lattice mismatch between Nb and sapphire induces the formation of a hexagonal surface structure during the first three atomic layers. This is followed by a strained bcc Nb(110) phase whose in-plane lattice parameter progressively relaxes to bulk value. Similar lattice relaxation was also observed in the direction perpendicular to the interface using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Significant perpendicular strain in films up to 30 nm thick was found to ultimately affect the superconducting properties of the Nb thin films as demonstrated with AC susceptibility measurements, where dissipative effects in the lattice associated with the presence of strain and associated defects were identified.

Superconducting Radio Frequency (SRF) cavities provide enhanced efficiency and reduced energy consumption in present-day particle accelerators. Niobium is the material of choice for SRF cavities due to its high critical temperature and critical magnetic field. In order to understand why certain treatments, especially a low temperature bake, improve performance, it is important to study Nb surface characteristics and identify elemental contaminants which may affect the performance of the cavity. [1] Initial studies using SIMS and Focused Ion Beam (FIB) prepared specimens for Transmission Electron Microscopy (TEM) have helped to characterize the Nb surface and measure the surface oxide layer thickness.[2] C, N and O are of particular interest as interstitial contaminants and earlier studies suggested very high H concentration. In the present study, ion implants of C, N, O and deuterium (D) in Nb and Si were analyzed using SIMS. D was implanted to characterize H while avoiding interference from the high H background. The D implant was easily detectable in Si, but showed a constant value and no implant shape in Nb. This result implies either that D (and by implication, hydrogen) has a high mobility in Nb, or that there is movement of D due to the primary ion beam. Nevertheless, C, N, and O could be quantified using the ion implants. Depth profiles of polycrystalline and single crystal Nb samples were also obtained. Keywords: SIMS; depth profiling; SRF Nb; deuterium; residual gas species Introduction SRF cavities are frequently used to achieve the desired acceleration in particle accelerators. Since high field gradients requiring high power are required, efficiency must be a primary consideration in cavity performance. Superconductors have thus replaced conventional copper cavities due to their ability to provide dramatically low resistance loss even when taking into account the cost of refrigerating at cryogenic temperature. Cavities use a RF standing wave with the frequency chosen to give particles an accelerating push as they pass through. Nb is currently the material of choice for these cavities because of its high critical temperature (9.2• K), high critical thermal-dynamic magnetic field (∼200 mT) and easy formability. Since the penetration range of the electromagnetic fields into Nb is only approximately 60 nm, understanding surface and near-surface region properties of Nb is of great importance. [1] In our initial studies of the Nb surface [2] , transmission electron microscopy (TEM) was used to characterize the oxide and suboxide regions on the Nb surface. Since Nb is a soft material, sample preparation by traditional TEM polishing methods is problematic since polishing can smear the surface and polishing media can become embedded in the material. For our study, [2] TEM liftout sample preparation using a gallium-focused ion beam (FIB) system was used to obtain high-quality TEM samples. [3,4] The cross-sectional TEM images obtained were used to measure oxide thickness on single crystal samples, which showed a ...

Single crystal niobium specimens of (100), ( 110) and (111) crystal orientations have been analyzed using TEM and SIMS. The TEM specimens were prepared using Focused Ion Beam (FIB) and show niobium oxide thicknesses ranging from 4.9 to 8.3 nm for the three specimens after buffer chemical polishing. The oxide layers appear uniform and no significant sub-oxide region was noted. SIMS analysis was made for all three orientations on hydrogen, carbon, and oxygen before and after heat treatments at 90, 600, and 1250ºC. Hydrogen is at a high level between the oxide layer and niobium, but at a relatively low level in the oxide. No high oxygen concentration region was noted in the niobium below the oxide. C contamination on the surface is detected mainly at the surface. Analysis after heat treatments showed some decrease in hydrogen after the 600ºC heat treatment, and significant oxidation of the niobium after the 1250ºC heat treatment.