The role of extended and point defects, and key impurities such as C, O, and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation, and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes, and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.

Gan : processing, defects, and devices

The mechanisms of hydrogen-related fracture are briefly reviewed and a few evaluative statements are made about the stress-induced hydride formation, decohesion, and hydrogen-enhanced localized plasticity mechanisms. A more complete discussion of the failure mechanism based on hydrogen-enhanced dislocation mobility is presented, and these observations are related to measurements of the macroscopic flow stress. The effects of hydrogen-induced slip localization on the measured flow stress is discussed. A theory of hydrogen shielding of the interaction of dislocations with elastic stress centres is outlined. It is shown that this shielding effect can account for the observed hydrogen-enhanced dislocation mobility.

Hydrogen-enhanced localized plasticity—a mechanism for hydrogen-related fracture

The major increase in discharge duration and plasma energy in a next step DT fusion reactor will give rise to important plasma-material effects that will critically influence its operation, safety and performance. Erosion will increase to a scale of several centimetres from being barely measurable at a micron scale in today's tokamaks. Tritium co-deposited with carbon will strongly affect the operation of machines with carbon plasma facing components. Controlling plasma-wall interactions is critical to achieving high performance in present day tokamaks, and this is likely to continue to be the case in the approach to practical fusion reactors. Recognition of the important consequences of these phenomena stimulated an internationally co-ordinated effort in the field of plasma-surface interactions supporting the Engineering Design Activities of the International Thermonuclear Experimental Reactor project (ITER), and significant progress has been made in better understanding these issues. The paper reviews the underlying physical processes and the existing experimental database of plasma-material interactions both in tokamaks and laboratory simulation facilities for conditions of direct relevance to next step fusion reactors. Two main topical groups of interaction are considered: (i) erosion/redeposition from plasma sputtering and disruptions, including dust and flake generation and (ii) tritium retention and removal. The use of modelling tools to interpret the experimental results and make projections for conditions expected in future devices is explained. Outstanding technical issues and specific recommendations on potential R&D avenues for their resolution are presented.

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Plasma{material interactions in current tokamaks and their implications for next step fusion reactors

A review is given on the diffusion, solubility and electrical activity of 3d transition metals in silicon. Transition elements (especially, Cr, Mn, Fe, Co, Ni, and Cu) diffuse interstitially and stay in the interstitial site in thermal equilibrium at the diffusion temperature. The parameters of the liquidus curves are identical for the Si:Ti — Si:Ni melts, indicating comparable silicon-metal interaction for all these elements. Only Cr, Mn, and Fe could be identified in undisturbed interstitial sites after quenching, the others precipitated or formed complexes. The 3d elements can be divided into two groups according to the respective enthalpy of formation of the solid solution. The distinction can arise from different charge states of these impurities at the diffusion temperature. For the interstitial 3d atoms remaining after quenching, reliable energy levels are established from the literature and compared with recent calculations.

Transition metals in silicon

Steels for bearings

Hydrogen interactions with imperfections in crystalline metals and semiconductors are reviewed. Emphasis is given to mechanistic experiments and theoretical advances contributing to predictive understanding. Important directions for future research are discussed.

Hydrogen interactions with defects in crystalline solids

We calculate the electronic structures of the hydrogen-boron and hydrogen-aluminum pairs in silicon which are believed to form when the corresponding shallow acceptor activity is neutralized by hydrogen. In particular, computed infrared vibrational frequencies are reported for the first time and compared with recent experimental measurements. Both theory and experiment reveal an interesting reduction in the vibrational frequency for hydrogen in the H-B pair with respect to the isolated monohydride. This we analyze in terms of two- and three-body interactions involving the hydrogen, the acceptor impurity, and the nearby silicon atoms.

Hydrogen-acceptor pairs in silicon: Pairing effect on the hydrogen vibrational frequency.

The single-particle electronic structures and atomic displacements of substitutional nitrogen and oxygen in silicon are treated theoretically in a cluster representation using both the scattered-wave $X\ensuremath{\alpha}$ ($\mathrm{S}\mathrm{W}\ensuremath{-}X\ensuremath{\alpha}$) and modified neglect of diatomic overlap (MNDO) electronic-structure methods. Substitutional carbon and sulfur are treated to a lesser degree for the purpose of comparison. The impurity $X$ and its environment are simulated by the clusters $X{\mathrm{Si}}_{4}{\mathrm{H}}_{12}$ ($\mathrm{S}\mathrm{W}\ensuremath{-}X\ensuremath{\alpha}$ and MNDO) and $X{\mathrm{Si}}_{16}{\mathrm{H}}_{36}$ (MNDO only). In all cases, we find a localized occupied ${a}_{1}$ state in the gap with a localized unoccupied ${t}_{2}$ state above it when the impurity atoms are on center. Using computed MNDO total energies for $X{\mathrm{Si}}_{4}{H}_{12}$ as a function of impurity displacement, we find spontaneous displacement directions correctly predicted for neutral carbon, nitrogen, oxygen, and sulfur, and for negative oxygen. By monitoring the single-particle energies and eigenvectors, we identify the driving force for the spontaneous asymmetric displacements as a pseudo\char22{}Jahn-Teller effect. We further explore the total energy with respect to simultaneous impurity and near-neighbor-silicon displacements, where the effect of the host outside the cluster is simulated by "external springs" connected to the four silicon atoms of the cluster. We find that it is necessary to use effective springs that are quite different from those which follow from a valence-force treatment in order to reproduce the observed spontaneous displacements of the substitutional nitrogen and oxygen impurities. This failure of the simple valence-force treatment is discussed. Our results also suggest a model for the "thermal donor" in silicon, whereby a substitutional oxygen is pushed "on center" by the strain field due to surrounding oxygen interstitials.

Theory of off-center impurities in silicon: Substitutional nitrogen and oxygen

Theory of interstitial transition-metal impurities in silicon

We have applied the self-consistent-field scattered-wave $X\ensuremath{\alpha}$ cluster method to the substitutional group-I impurities ($X$) H, Li, Na, K, ${\mathrm{H}}_{4}$, and ${\mathrm{Li}}_{4}$ (${T}_{d}$ symmetry) in silicon using the cluster $X{\mathrm{Si}}_{4}{\mathrm{H}}_{12}$. We find that for the neutral alkali-metal impurities the electronic structure of the vacancy is essentially preserved except that the ${t}_{2}$ gap state is now occupied by the appropriate number of alkali-metal-atom valence electrons plus the normal neutral-vacancy complement of two electrons. This means that the ${\mathrm{Li}}_{4}$ impurity, for example, is better described as a vacancy ${V}^{4\ensuremath{-}}$ charge compensated by four ${\mathrm{Li}}^{+}$ ions. Hence this suggests that alkali-metal impurities do not passivate vacancy dangling bonds. The electronic structures of the H and ${\mathrm{H}}_{4}$ impurities, on the other hand, are found to be quite different from those of their alkali-metal-impurity counterparts. The single-hydrogen impurity (on-center substitutional hydrogen) is found to have vacancylike ${a}_{1}$ states but crystalline siliconlike ${t}_{2}$ states. Four hydrogen atoms in a vacancy do appear to passivate the vacancy dangling bonds; thus in this case the electronic structure of crystalline silicon is recovered.

Gary G. DeLeo

Papers

Hydrogen interactions with defects in crystalline solids

Hydrogen-acceptor pairs in silicon: Pairing effect on the hydrogen vibrational frequency.

Theory of off-center impurities in silicon: Substitutional nitrogen and oxygen

Theory of interstitial transition-metal impurities in silicon

Electronic structure of hydrogen- and alkali-metal-vacancy complexes in silicon