ALBERT

Notes: Epitaxial PbTiO3 films were prepared by metalorganic chemical-vapor deposition on MgO(001)-, SrTiO3(001)-, and LaAlO3(001)-oriented substrates. Four-circle x-ray diffraction, transmission electron microscopy, Rutherford backscattering (RBS) channeling, and optical waveguiding were performed to characterize the deposited films. Epitaxial, single-crystal films were obtained on all three substrate materials under the same growth conditions. However, the defect structure of the films, including grain tilting, threading dislocation density, and 90° domain formation, was strongly dependent on the choice of substrate material. Films grown on MgO(001) and LaAlO3(001) (pseudocubic indices) substrates are nominally c-axis oriented; however, the PbTiO3 grains in the film form a fourfold domain structure, with the grains tilted ∼0.6° and ∼0.7°, respectively, toward the [100] directions (cubic or pseudo-cubic) of the substrates. In addition, these films contain a significant volume fraction of 90°-domain (a-axis) structures with a critical thickness hc for domain formation below the detection level of our experiments (hc≥100 A(ring)). Together, these structural defects result in a low RBS channeling yield reduction. In contrast, films grown on SrTiO3(001) substrates showed no tilting of the c-axis grains and a minimum RBS channeling yield of as low as ∼3%. In addition, we observed that below a critical film thickness of hc∼1500 A(ring), 90° domain formation was completely suppressed resulting in a nearly perfect single-crystal structure.The refractive indices and optical birefringence of the films were measured as a function of wavelength using the film-prism coupling method. Both the ordinary and extraordinary refractive indices for films grown on MgO(001) and LaAlO3(001) were higher than that of single-crystal PbTiO3; however, the optical birefringence of films grown on MgO(001) was reduced from that of the bulk. For films grown on SrTiO3(001), the ordinary refractive index was very close to that of single-crystal PbTiO3. We correlate the refractive index values and the reduced birefringence to the degree of residual strain and the volume fraction of 90° domains in the samples, respectively. © 1995 American Institute of Physics.

Notes: We have grown aluminum nitride thin films by ultrahigh vacuum reactive sputter deposition on Si(111) and Si(001) substrates. We show results of film characterization by Raman scattering, ion beam channeling, and transmission electron microscopy, which establish the occurrence of epitaxial growth of wurtzitic aluminum nitride thin films on Si(111) at temperatures above 600 °C. In contrast, microstructural characterization by transmission electron microscopy shows the formation of highly oriented polycrystalline wurtzitic aluminum nitride thin films on Si(001). Real-time substrate curvature measurements reveal the existence of large intrinsic stresses in aluminum nitride thin films grown on both Si(111) and Si(001) substrates.

Notes: The damage produced at the interfaces in a sample of GaAs/Al0.6Ga0.4As/GaAs that has been subjected to ion implantation at 77 and 293 K with 1 MeV Kr+ ions has been investigated using a combination of ion channeling and transmission electron microscopy (TEM) techniques. Low temperature ion-channeling spectra obtained from samples implanted at 77 K, to an ion dose of 1014 ions cm−2, were similar to the random spectrum, indicating that the GaAs and Al0.6Ga0.4As layers had sustained a considerable degree of damage. An asymmetric signal developed in the He+ ion-channeling spectrum as the sample warmed to room temperature. The backscattering yield corresponding to the bottom interface (i.e., Al0.6Ga0.4As grown on GaAs) resembled that of the random yield, whereas that from the top interface (GaAs grown on Al0.6Ga0.4As) decreased, shifting toward the unirradiated channeled spectrum. This observation suggests that the damage produced near the top of the Al0.6Ga0.4As layer is thermally unstable. Cross-sectional TEM images reveal a greater amount of damage in the form of extended defects and amorphous regions at the bottom interface than at the top one. This difference is sufficient to account for the observed asymmetry in the channeling spectra. Increasing the ion dose to 1015 ions cm−2 produced a damage state throughout the Al0.6Ga0.4As layer that was stable at both 77 and 293 K. TEM examination revealed that at this ion dose the GaAs and Al0.6Ga0.4As layers were both amorphous. Room-temperature implantation to a dose of 1×1016 ions cm−2 was also performed. Planar defects were observed at both interfaces, although their density appeared to be greater near the bottom interface.In addition, the bottom interface was rougher than the top. The difference in the damage states at the bottom and top interfaces can be attributed to a variation in the number of displacement cascade events as a function of depth through the Al0.6Ga0.4As layer. This variation in the number of cascades results in different amounts of ion mixing at the top and bottom interfaces. © 1995 American Institute of Physics.

Notes: Experiments were carried out to investigate strong traps (binding enthalpies〉0.50 eV) for deuterium that were found earlier when pure nickel was implanted with argon and krypton at temperatures ≤500 °C. In one experiment, three different areas of a nickel specimen were implanted with a fluence of 2×1016 cm−2 krypton at 600, 700, and 800 °C; a second experiment involved implanting krypton at three different fluences (2, 6, and 20×1015 cm−2) in nickel at 500 °C. Retained deuterium was monitored during subsequent isochronal anneals using nuclear reaction analysis. In a third experiment, depth profiles for both implanted krypton and trapped deuterium were determined simultaneously using secondary ion mass spectrometry. Traps for deuterium in krypton-implanted nickel were found with binding enthalpies ranging from 0.5 to 1.1 eV. At implantation temperatures 〈500 °C, the traps are in the same location as the krypton atoms. At implantation temperatures 〉700 °C, the traps are found where the krypton was originally implanted in spite of the fact that a significant fraction of the krypton apparently escapes through the surface during the implantation process. The observed fluence and temperature dependence indicate that the traps with binding enthalpies between 0.5 and 1.1 eV are associated with the surfaces of cavities formed during the implantation.

Notes: Krypton ions with energy 600 keV were implanted in nickel to fluences of 2 × 1016 cm−2 under three different conditions. Deuterium was subsequently introduced into the implanted regions by electrolysis at room temperature. After the diffusible deuterium was permitted to escape, the 2H(3He,1H)4He nuclear reaction was used to analyze for the trapped deuterium during an isochronal annealing program. The region implanted at 100 °C with no higher temperature anneal had the largest number of traps; the region implanted at 100 °C and annealed for 100 min at 500 °C had considerably less; the region implanted at 500 °C had the least. Electron diffraction patterns confirmed the existence of solid crystalline krypton in all three regions. Transmission electron microscope studies revealed precipitates with an average diameter of 8 nm in the region implanted at 500 °C. The two regions implanted at 100 °C contained smaller precipitates. Trap binding enthalpies were obtained by math modeling. In addition to the traps with binding enthalpy of 0.55 eV reported earlier by other investigators for helium implanted in nickel, a smaller number of traps with binding enthalpies up to 0.83 eV were also found. The trapping of deuterium by various types of imperfections, including the solid krypton precipitates, is discussed.

Notes: Argon ions with energy 250 keV were implanted at fluences of 2×1016 cm−2 at temperatures of 500, 250, and 21 °C, in a specimen of relatively pure polycrystalline nickel. Deuterium was introduced into the surface and implanted regions by making the specimen the negative electrode of an electrolytic cell containing 1-N pure deuterated sulfuric acid. Deuterium trapped in the vacancy complexes of the implanted regions was analyzed as a function of temperature using the 2H(3He,1H)4He nuclear reaction during an isochronal annealing process. The results indicate that the types of traps and trap densities found in the regions implanted at 21 and 250 °C were essentially identical while the trap density found in the region implanted at 500 °C was approximately 40% of that found in the other regions. Math model comparison with the experimental results suggests the existence of at least two types of traps in each region. Trap binding enthalpies used in the math model to fit the experimental data were slightly higher for the region implanted with argon at 500 °C than for the regions implanted at the lower temperatures. TEM studies revealed the presence of small voids in the region implanted at 500 °C as well as dislocation loops similar to those found in the regions implanted at the lower temperatures.

Notes: A marked softening in the shear elastic properties of GaAs following 1.5-MeV Kr ion irradiation is observed. Ion channeling and Raman scattering are used to monitor the accumulation of damage, which for 1.5-MeV incident energy extends to a depth of ∼8000 A(ring). Both techniques show that the sample is amorphized by a dose of 2×1014 ions/cm2. Brillouin scattering reveals that a shear elastic modulus of this near-surface irradiated region decreases by 43% as the material transforms from crystalline to amorphous. The observed results are discussed and compared with similar measurements on Si and intermetallic compounds.

Notes: A grain-size-dependent reduction in the room-temperature thermal conductivity of nanocrystalline yttria-stabilized zirconia is reported for the first time. Films were grown by metal-organic chemical vapor deposition with controlled grain sizes from 10 to 100 nm. For grain sizes smaller than approximately 30 nm, a substantial reduction in thermal conductivity was observed, reaching a value of less than one-third the bulk value at the smallest grain sizes measured. The observed behavior is consistent with expectations based on an estimation of the phonon mean-free path in zirconia.© 2000 American Institute of Physics.

Notes: To determine the influence of interface type on the accumulation of damage and ion mixing in GaAs/AlxGa1−xAs heterostructures, the damage produced by ion implantation at 77 K in single-layer (GaAs/AlxGa1−xAs/GaAs) and double-layer (GaAs/AlxGa1−xAs/GaAs/ AlxGa1−xAs/GaAs) heterostructures has been investigated by using a combination of Rutherford backscattering spectrometry and transmission electron microscopy. In the single-layer geometry, the degree of disorder increases with depth and the mixing is greater at the AlxGa1−xAs on GaAs interface than at the GaAs on AlxGa1−xAs one. The damage distribution in the sample with the double-layer geometry was different in the two layers, but overall it was similar to that in the single-layer geometry. These trends were observed in samples with x=0.6 and 0.2. These results indicate that migration of charged defects due to the presence of an implantation-induced electric field is not responsible for the asymmetry in the damage accumulation across the layer, the interface disorder and ion mixing, and the initiation of amorphization at interfaces. Instead, these effects can be better understood in terms of the depth dependence of the density of cascade events. © 1996 American Institute of Physics.

Notes: Diffusion-induced grain boundary migration (DIGM) was observed in Au/Cu bilayers irradiated with 1.5 MeV Kr at T≥400 K. Rutherford backscattering spectrometry (RBS) showed nearly uniform distributions of Cu present throughout polycrystalline Au films after irradiation and after annealing treatments. Irradiation increased the amount of Cu relative to annealed-only areas. Cross-section transmission electron microscopy combined with x-ray energy dispersive spectroscopy (XEDS) identified alloyed zones (14–20 at. % Cu), confirming DIGM in the Au film of an ion bombarded bilayer. A description of DIGM is presented relating RBS measurements of the film-averaged Cu composition with treatment time, average grain size, and film thickness. Application of this model to the experimental results in combination with XEDS work indicates that irradiation enhances DIGM by increasing the grain boundary velocity.