John H. Barrett

Role of correlations of lattice vibrations in channeling

Abstract Computer simulations of channeling have been done using correlated thermal displacements of the lattice atoms. For the channeling minimum yield and half angles, results are given as a function of temperature. For the surface yield, results are given as a function of angle from the axial direction to supplement earlier results as a function of temperature. In all quantities correlations cause effects qualitatively similar to a reduction in vibration amplitude, although the reduction varies from quantity to quantity. These variations are consistent with the idea that correlations will be most important for a trajectory aligned with the rows and of decreasing importance as the direction of a trajectory approaches a random direction. The largest effect occurs for the surface yield wherein there is a reduction of about 15% for the cases studied so far.

Methods of channeling simulation

Abstract Many computer simulation programs have been used to interpret experiments almost since the first channeling measurements were made. Certain aspects of these programs are important in how accurately they simulate ions in crystals; among these are the manner in which the structure of the crystal is incorporated, how any quantity of interest is computed, what ion-atom potential is used, how deflections are computed from the potential, incorporation of thermal vibrations of the lattice atoms, correlations of thermal vibrations and form of stopping power. Other aspects of the programs are included to improve the speed; among these are table lookup, importance sampling and the multiparameter method. It is desirable for programs to facilitate incorporation of special features of interest in special situations; examples are relaxations and enhanced vibrations of surface atoms, easy substitution of an alternate potential for comparison, change of row directions from layer to layer in strained-layer lattices and different vibration amplitudes for substitutional solute or impurity atoms. Ways of implementing all of these aspects and features and the consequences of them are discussed.

Radiation from channeled leptons

Abstract Radiation from planar- and axial-channeled positrons (56 MeV) and electrons (28 and 56 MeV) in Si has been observed. In axial channeling a large, almost featureless, low-energy enhancement is observed, but in planar channeling relatively sharp features occur. For planar-channeled positrons the energies of the spectral peaks (ranging from 35 to 50 keV) agree well with a theory based on transitions between eigenstates in an almost-harmonic potential. A Monte-Carlo simulation of the positron motion coupled with a classical calculation of the emitted radiation gives agreement with the results and also gives the classical equivalent of coherent bremsstrahlung (CB). It further predicts strong coupling between planar oscillation frequencies and CB frequencies with the formation of strong sidebands. For planar-channeled electrons sets of spectral peaks are observed in the range from 30 to 130 keV. An analysis based on a V 0 exp[− k | x |] potential yields energies in close agreement with experiment.

Studies of defects and surfaces by channeling

Uses of channeling to study defects and surfaces are reviewed briefly with emphasis on ways in which they are currently evolving. Consideration is first given to normalization of yields for channeling directions to the yield for a random direction, this normalization being important to the accuracy of both defect and surface studies. The discussion of atom location concentrates on the use that can be made of quantitative calculations of flux peaking. The treatment of surfaces includes accurate determination of the area of the surface peak as well as basic ways of extracting from experiments information on the positions of surface atoms.

Mechanism of ion dechanneling in compound semiconductor superlattices

Computer simulations of channeling have been used to evaluate a model proposed by Saris et al. to explain some of their ion dechanneling measurements in InAs‐GaSb superlattices. Their model, which involves slight offsets of atomic rows at each interface, produces much less dechanneling than observed experimentally. However, the simulations show that the observed dechanneling can be produced by much larger offsets which would probably be spread over several atomic layers near each interface.

SURFACE EFFECTS AND LOW FREQUENCY LOSSES IN HARD SUPERCONDUCTORS.

The power dissipation in a hard superconductor based on a model consisting of a bulk flux pinning region and a surface sheath is calculated. The results indicate that the losses are somewhat greater than previously calculated.

Diffusion and strain relaxation in silicon/silicon-germanium/silicon structures studied with Rutherford backscattering spectrometry

Abstract The thermal stability of strained Si/Si1−xGex/Si structures grown by molecular beam epitaxy was investigated by resistive heating and in situ Rutherford backscattering spectrometry. Ge profiles obtained from a 50 nm Si1−xGex layer on a Si(100) substrate capped with 50 nm Si were evaluated for different Ge concentrations after sequential heating periods at a particular temperature between 850 and 1010° C. The diffusion coefficients, calculated from the increase in signal in the tail of the Ge profile, proved to be comparable to the value for Ge in bulk Si. A more pronounced decrease of the signal at the center of the Ge profile indicated a faster diffusion within the SiGe layer which was confirmed by analysis of the FWHM of the Ge profile. Ion channeling measurements were used to characterize tetragonal strain in the buried SiGe layers. Angular scans through the 〈111〉 direction were interpreted with Monte Carlo channeling calculations and used to study strain relaxation in dislocation-free and partially relaxed layers.

A method for the determination of atomic displacements in compound crystals by means of RBS-PIXE-channeling experiments

Abstract A method which combines proton-induced X-ray emission (PIXE), Rutherford backscattering (RBS) and ion channeling, which enables the determination of static or dynamic root-mean-square displacements of constituent atoms in compound crystals is described. Full PIXE and RBS channeling angular scan curves are measured simultaneously by scanning through a major axis of the crystal under study. The experimental data are interpreted by using two different methods: A) a simple semi-analytical model which takes advantage of the depth information contained in RBS spectra to estimate depth correction factors needed to analyze the PIXE angular scan data, and B) a Monte Carlo simulation program of channeling, which takes into account the special structural features of the materials studied and allows for a direct comparison of simulated results with experimental PIXE data. The use of the technique is demonstrated by deducing rms displacements of constituent atoms of some II–VI (binary and ternary) semiconductors analyzed by both methods. The present results agree well with vibrational amplitude data obtained from X-ray diffraction measurements available from the literature.

Monte Carlo calculations of channeling radiation and comparison with experiment

Abstract Results of classical Monte Carlo calculations are presented for the radiation produced by ultra-relativistic positrons incident in a direction parallel to the (110) plane of Si in the energy range 30–100 MeV. The results all show the characteristic CR (channeling radiation) peak in the energy range 20–100 keV. Plots of the centroid energies, widths, and relative total yields of the CR peaks as a function of energy show the power law dependences of γ 1.45 , γ 1.7 , and γ 2.5 with uncertainties in the exponents of ±0.05, ±0.3 and±0.5 , respectively. Agreement with experimental data is good for the centroid energies but only rough for the widths. Adequate experimental data for verifying the yield dependence on γ does not yet exist. The slight departure from a pure γ 1.5 dependence, in the case of the centroid energies, can be accounted for by the effects of anharmonicity.

Channeling simulation in Ni3Al

Measurements using ion channeling and Rutherford backscattering have indicated that Hf and Ta dopants in ordered Ni3Al alloys are located predominately on the Ni sites. Because other experiments have indicated the dopants to be on the Al sites, computer simulations of the ion channeling and scattering processes have been performed to see whether some unusual factor could lead to erroneous conclusions when the standard methods to interpret channeling results are used. Consideration has been given to the effects of a higher average charge on the Al sites if high Z dopants are located there, to the effects of displacement amplitudes for the dopants that differ from those of the host atoms, and to the effects of mosaic spread in the crystal. It is concluded that the most straightforward way to interpret the ion scattering results is that the Hf and Ta are mostly on the Ni sites in the alloys. However, it appears that the impurities could be mostly on the Al sites if the impurity atoms have a substantially smaller displacement amplitude than do the host atoms in the alloys.