S. M. Myers

Mechanisms of transition-metal gettering in silicon

The atomic process, kinetics, and equilibrium thermodynamics underlying the gettering of transition-metal impurities in Si are reviewed. Methods for mathematical modeling of gettering are discussed and illustrated. Needs for further research are considered.

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.

Ion-beam studies of hydrogen-metal interactions

Abstract Experiments utilizing ion implantation and ion-beam analysis have provided a large body of new quantitative information on hydrogen interactions within metal matrices and at metal surfaces. Investigated matrix interactions include trapping by vacancies, vacancy-solute complexes, He bubbles, and oxide precipitates, together with such phase-change reactions as hydride formation, hydrogen-bubble precipitation, and hydrogen reduction of precipitated oxides. Extracted information encompasses mechanisms, binding enthalpies, microstructures, and local atomic configurations. In the area of surfaces, ion-channeling analysis has yielded the lattice positions of chemisorbed hydrogen on a variety of metals. Additionally, ion-implantation experiments have quantified surface-limited uptake and release for bare, chemisorbed, and oxidized metal surfaces. Theoretical studies coupled with the ion-beam experiments have produced advances in four areas of hydrogen behavior. First, effective-medium theory has been shown to yield a quantitative description of hydrogen energies and positions at matrix defect traps and surface chemisorption sites. Second, a new, analytical treatment of trapping kinetics gives quantitative trapping rates for arbitrary trap volume fraction and arbitrary spatial correlation between traps. Third, new theoretical studies of surface barriers have clarified the dominant processes of release for bare, chemisorbed, and oxidized metal surfaces. Finally, general transport formalisms have been developed to predict overall hydrogen behavior in terms of individual matrix and surface processes.

Chemical kinetics of hydrogen and (111) Si‐SiO2 interface defects

Electron paramagnetic resonance (EPR) measurements and theoretical considerations have yielded a unified model for the hydrogen chemistry of silicon dangling bond Pb defects at the (111)  Si‐SiO2 interface. Previous EPR measurements indicated that passivation of Pb centers with H2 proceeds by the reaction H2+Pb→HPb+H with an activation energy of 1.66±0.06 eV. New EPR studies reported here show that HPb centers dissociate by the reaction HPb→Pb+H with an activation energy of 2.56±0.06 eV. When combined, these two reactions yield H2→H+H, which in vacuum requires an energy input of 4.52 eV. Comparison of these energies indicates that the reverse reactions H+HPb→Pb+H2 and H+Pb→HPb occur with essentially no energy barrier and are controlled by the local availability of atomic hydrogen.

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.

Quantitative comparisons of dissolved hydrogen density and the electrical and optical properties of ZnO

Recent density functional theory calculations indicate that hydrogen is soluble in ZnO, effectively forming a shallow donor state. It has been suggested that these donors are responsible for the large increases in electron concentration seen in ZnO samples annealed at elevated temperatures in H2 gas. In order to make a quantitative connection between the amount of dissolved hydrogen and any observed changes in electrical properties, we have annealed single crystal ZnO samples from several sources in H2 and D2 gas at 750 °C and compared the observed changes in carrier concentration with nuclear reaction analysis and secondary ion mass spectrometry profiles of deuterium. We find that the amount of deuterium remaining in our gas-charged samples is ∼3.6–5.5×1017 cm−3, substantially larger than the increase seen in conduction band electron densities at 350 K. Our modeling indicates that these gas treatments produce a hydrogen-related donor state at 0.036±0.004 eV below the conduction band minimum and also caus...

Control and Elimination of Cracking of AlGaN Using Low-Temperature AlGaN Interlayers

We demonstrate that the insertion of low-temperature AlGaN interlayers is effective in reducing mismatch-induced tensile stress and suppressing the formation of cracks during growth of high-temperature AlGaN directly upon GaN epilayers. Stress evolution and relaxation is monitored using an in situ optical stress sensor. The combination of in situ and ex situ characterization techniques enables us to determine the degree of pseudomorphism in the interlayers. It is observed that the elastic tensile mismatch between AlGaN and GaN is mediated by the relaxation of interlayers; the use of interlayers offers tunability in the in-plane lattice parameters.

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.

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.

Multiple hydrogen occupancy of vacancies in Fe

The binding of multiple deuterium (D) atoms to vacancies in iron (Fe) is investigated both experimentally, by use of ion‐beam techniques, and theoretically, by use of the effective‐medium scheme. The experimental D‐release stages are accounted for by trapping at vacancies with a binding enthalpy that depends on occupancy. It is found that for 1–2 D in a vacancy, the trap strength is 0.63 eV, whereas the binding enthalpy for 3–6 D in a vacancy is 0.43 eV. These results are in good agreement with predictions from the effective‐medium theory.