D. M. Follstaedt

Thick stress-free amorphous-tetrahedral carbon films with hardness near that of diamond

We have developed a process for making thick, stress-free, amorphous-tetrahedrally bonded carbon (a-tC) films with hardness and stiffness near that of diamond. Using pulsed-laser deposition, thin a-tC films (0.1–0.2 μm) were deposited at room temperature. The intrinsic stress in these films (6–8 GPa) was relieved by a short (2 min) anneal at 600 °C. Raman and electron energy-loss spectra from single-layer annealed specimens show only subtle changes from as-grown films. Subsequent deposition and annealing steps were used to build up thick layers. Films up to 1.2 μm thick have been grown that are adherent to the substrate and have low residual compressive stress (<0.2 GPa). The values of hardness and modulus determined directly from an Oliver–Pharr analysis of nanoindentation experimental data were 80.2 and 552 GPa, respectively. We used finite-element modeling of the experimental nanoindentation curves to separate the “intrinsic” film response from the measured substrate/film response. We found a hardness ...

Finite-element modeling of nanoindentation

Procedures have been developed based on finite-element modeling of nanoindentation data to obtain the mechanical properties of thin films and ion-beam-modified layers independently of the properties of the underlying substrates. These procedures accurately deduce the yield strength, Young’s elastic modulus, and layer hardness from indentations as deep as 50% of the layer thickness or more. We have used these procedures to evaluate materials ranging from ion implanted metals to deposited, diamond-like carbon layers. The technique increases the applicability of indentation testing to very thin layers, composite layers, and modulated compositions. This article presents an overview of the procedures involved and illustrates them with selected examples.

Effect of threading dislocations on the Bragg peakwidths of GaN, AlGaN, and AlN heterolayers

We develop a reciprocal-space model that describes the (hkl) dependence of the broadened Bragg peakwidths produced by x-ray diffraction from a dislocated epilayer. We compare the model to experiments and find that it accurately describes the peakwidths of 16 different Bragg reflections in the [010] zone of both GaN and AlN heterolayers. Using lattice-distortion parameters determined by fitting the model to selected reflections, we estimate threading-dislocation densities for seven different GaN and AlGaN samples and find improved agreement with transmission electron microscopy measurements.

Low-dislocation-density GaN from a single growth on a textured substrate

The density of threading dislocations (TD) in GaN grown directly on flat sapphire substrates is typically greater than 10{sup 9}/cm{sup 2}. Such high dislocation densities degrade both the electronic and photonic properties of the material. The density of dislocations can be decreased by orders of magnitude using cantilever epitaxy (CE), which employs prepatterned sapphire substrates to provide reduced-dimension mesa regions for nucleation and etched trenches between them for suspended lateral growth of GaN or AlGaN. The substrate is prepatterned with narrow lines and etched to a depth that permits coalescence of laterally growing III-N nucleated on the mesa surfaces before vertical growth fills the etched trench. Low dislocation densities typical of epitaxial lateral overgrowth (ELO) are obtained in the cantilever regions and the TD density is also reduced up to 1 micrometer from the edge of the support regions.

The effect of H2 on morphology evolution during GaN metalorganic chemical vapor deposition

In situ optical reflectance transients reveal that the morphology evolution of the initial low-temperature buffer layer strongly influences the structural and electrical quality of the high-temperature GaN films. Moreover, the morphology evolution of that buffer layer, specifically evolution of the spatial and orientational distributions of the nuclei, is strongly affected by H2. The growth conditions for which surface smoothness is maintained throughout the two-step growth do not necessarily produce the best quality final GaN films; instead, there may be an optimal roughness and incubation period en route to the best quality final films.

Interaction of copper with cavities in silicon

Copper in Si was shown to be strongly bound at cavities formed by He ion implantation and annealing. Evolution of this system during heating was observed by Rutherford backscattering spectrometry and transmission electron microscopy. Results were mathematically modeled to characterize quantitatively the binding of Cu in the cavities and, for comparison, in precipitates of the equilibrium silicide, η‐Cu3Si. Binding of Cu to cavities occurred by chemisorption on the walls, and the binding energy was determined to be 2.2±0.2 eV relative to solution in Si. The heat of solution from the silicide was found to be 1.7 eV, consistent with the published phase diagram. These findings suggest the use of cavities for metal‐impurity gettering in Si devices. Hydrogen in solution in equilibrium with external H2 gas displaced Cu atoms from cavity walls, a mechanistically illuminating effect that is also of practical concern for gettering applications.

Cavity formation and impurity gettering in He-implanted Si

Cavity microstructures formed in Si after ion implantation of He (30 or 130 keV) and annealing at 700°C or above are examined with cross-section transmission electron microscopy. A threshold concentration of 1.6 at.% He is identified as required to form cavities that survive such anneals. The cavities coarsen with a constant volume corresponding to ∼0.75 lattice sites per implanted He atom and have surface areas 3-7 times that of the wafer area for fluences of 1 × 1017 He/ cm2. Transition metal atoms (Cu, Ni, Co, Fe, Au) are shown to be strongly trapped (1.5–2.2 eV) on the cavity walls by chemisorption. Whereas Cu, Au, and Ni are bound more strongly to the cavity sites than to their respective precipitated phases, Co and Fe are more strongly bound to their silicides; nonetheless, appreciable trapping of Co and Fe does occur in equilibrium with the silicides. Cavity trapping appears to be an effective gettering mechanism at low impurity levels, as needed to meet future microelectronics device requirements.

Brittle-ductile relaxation kinetics of strained AlGaN/GaN heterostructures

The authors have directly measured the stress evolution during metal organic chemical vapor deposition of AlGaN/GaN heterostructures on sapphire. In situ stress measurements were correlated with ex situ microstructural analysis to directly determine a critical thickness for cracking and the subsequent relaxation kinetics of tensile-strained Al{sub x}Ga{sub 1{minus}x}N on GaN. Cracks appear to initiate the formation of misfit dislocations at the AlGaN/GaN interface, which account for the majority of the strain relaxation.

Misfit dislocation formation in the AlGaN∕GaN heterointerface

Heteroepitaxial growth of AlxGa1−xN alloy films on GaN results in large tensile strain due to the lattice mismatch. During growth, this strain is partially relieved both by crack formation and by the coupled introduction of dense misfit dislocation arrays. Extensive transmission electron microscopy measurements show that the misfit dislocations enter the film by pyramidal glide of half loops on the 1∕3⟨1123⟩∕{1122} slip system, which is a well-known secondary slip system in hcp metals. Unlike the hcp case, however, where shuffle-type dislocations must be invoked for this slip plane, we show that glide-type dislocations are also possible. Comparisons of measured and theoretical critical thicknesses show that fully strained films can be grown into the metastable regime, which we attribute to limitations on defect nucleation. At advanced stages of relaxation, interfacial multiplication of dislocations dominates the strain relaxation process. This work demonstrates that misfit dislocations are important mec...

Trapping and surface permeation of deuterium in He‐implanted Fe

Iron was ion implanted with He and deuterium (D) and then heated at a constant rate of 2 K/min. The evolving depth distribution of the D was monitored using the nuclear reaction 2D(3He,p)4He, and resulting data were analyzed by applying the diffusion equation with appropriate trapping terms. Two defect traps for D were identified, the respective binding enthalpies being 0.53±0.07 and 0.71±0.07 eV when referenced to a solution site. The weaker of these traps is believed to be a monovacancy, while the stronger may be a vacancy cluster. A third type of trap, with strength 0.78±0.08 eV, was found to be associated with He. It is proposed that the responsible entities are ∼1 nm He bubbles which were observed by transmission electron microscopy, and that the D is bound to the walls of these bubbles by a mechanism similar to chemisorption. The analysis also yielded an estimate of the D recombination coefficient at the electropolished and air‐exposed Fe surface, ∼5×10−19 cm4/s at a temperature of 500 K.