Silicon CCDs have limited sensitivity to particles and photons with short penetration depth, due to the surface depletion caused by the inherent positive charge in the native oxide. Because of surface depletion, internally-generated electrons are trapped near the irradiated surface and therefore cannot be transported to the detection circuitry. This deleterious surface potential can be eliminated by low-temperature molecular beam epitaxial (MBE) growth of a delta-doped layer on the Si surface. This effect has been demonstrated through achievement of 100% internal quantum efficiency for UV photons detected with delta-doped CCDs. In this paper, we will discuss the modification of the band bending near the CCD surface by low-temperature MBE and report the application of delta-doped CCDs to low-energy electron detection. We show that modification of the surface can greatly improve sensitivity to low-energy electrons. Measurements comparing the response of delta-doped CCDs with untreated CCDs were made in the 50 eV-1.5 keV energy range. For electrons with energies below 300 eV, the signal from untreated CCDs was below the detection limit for our apparatus, and data are presented only for the response of delta-doped CCDs at these energies. The effects of multiple electron hole pair (EHP) production and backscattering on the observed signals are discussed.

Multiple interaction computer simulations have been used to determine the properties of collision cascades in liquid In targets induced by normally incident 5 keV Ar+ ions. Below the first atomic layer the cascade becomes Thompson-like relatively quickly. However, within the first atomic layer the angular distribution of moving atoms became forward peaked by 150 fs and remained so until,∼300 fs. Energy and angle resolved (EARN) spectra were calculated for the ejected atoms. The peak of the energy distribution shifted to lower energies at larger ejection angles, and the angular distributions became broader for lower energy particles. Both results agree with recent experimental data, and with a simple model proposed bg Garrison. Our results suggest that the detailed structure of the surface layer is very important in the sputtering process.

ABSTRACTMeV ion irradiation effects on semiconductor crystals, GaAs(100) and Si (111) and on an insulating crystal CaF2(111) have been studied by the x-ray rocking curve technique using a double crystal x-ray diffractometer. The results on GaAs are particularly interesting. The strain developed by ion irradiation in the surface layers of GaAs (100) saturates to a certain level after a high dose irradiation (typically 1015/cm2), resulting in a uniform lattice spacing about 0.4% larger than the original spacing of the lattice planes parallel to the surface. The layer of uniform strain corresponds in depth to the region where electronic energy loss is dominant over nuclear collision energy loss. The saturated strain level is the same for both p-type and n-type GaAs. In the early stages of irradiation, the strain induced in the surface is shown to be proportional to the nuclear stopping power at the surface and is independent of electronic stopping power. The strain saturation phenomenon in GaAs is discussed in terms of point defect saturation in the surface layer.An isochronal (15 min.) annealing was done on the Cr-doped GaAs at temperatures between 200° C and 700° C. The intensity in the diffraction peak from the surface strained layer jumps at 200° C < T ≤ 300° C. The strain decreases gradually with temperature, approaching zero at T ≤ 500° C.The strain saturation phenomenon does not occur in the irradiated Si. The strain induced in Si is generally very low (less than 0.06%) and is interpreted to be mostly in the layers adjacent to the maximum nuclear stopping region, with zero strain in the surface layer. The data on CaF2have been analysed with a kinematical x-ray diffraction theory to get quantitative strain and damage depth profiles for several different doses.

ABSTRACTMeV oxygen ion implantation in GaAs/AlGaAs has been shown to provide a simple and very promising technique for quantum well laser fabrication. A l0μm stripe single quantum well (SQW) graded-index separation confinement heterostructure (GRINSCH) laser made in this way has achieved high performance with high quantum differential efficiency, low threshold current and good electrical isolation characteristics. MeV oxygen ion implantation with optimum thermal annealing produces a deep buried electrical isolation layer in n-type GaAs and reduces optical absorption in GaAs/AlGaAs quantum well structures. Ion implantation stimulated compositional disordering as well as implanted oxygen-related deep level traps may be considered as important effects for electrical and optical modification of interfaces in GaAs and AIGaAs.

We report on the use of 20 MeV C14+ ion beams for pre-irradiation of Si, SiO2 and Al2O3 surfaces with resulting alterations in the adhesion of subsequently deposited Au and Ag thin films. The effect does not follow the same pattern for Au and Ag films, nor is there a simple correlation to the previously observed adhesion effect induced by ion irradiation after film deposition. Possible mechanisms, both for pre-irradiation and post-irradiation effects, are discussed, as is the importance of chemical structures with respect to interfacial bond strength.

A velocity spectrum of neutral sputtered particles as well as a low resolution mass spectrum of sputtered molecular ions i" has been measured for 4.74 MeV 19F+2 incident on UF4 . The r velocity spectrum is dramatically different from spectra taken with low energy (keV) bombarding ions, and is shown to be consistent with a hot plasma of atoms in thermal equilibrium inside the target. We propose a "thermalized ion explosion" model for high energy sputtering which is also expected to describe track formation in dielectric materials. The model is shown to be consistent with the observed total sputtering yield and the dependence of the yield on the primary ionization rate of the incident ion. t Supported in part by the National Aeronautics and Space Administration [NGR 05-002-3331, the National Science Foundation [PHY79-23638) and the Department of Energy [EX-76G-03-1305]. 1 ONE OF THE BAND AIb PREPRINT SERIFS IN ATOMIC & APPLIED PHYSICS April 1980

Simulations were performed describing the motion and breakup of energetic C-60 ions interacting with crystalline targets. A hybrid algorithm was used that employs a binary collision model for the scattering of the carbon ions by the atoms of the solid, and molecular dynamics for the Coulomb interactions of the 60 carbon ions with one another. For the case of yttrium iron garnet ( YIG), directions such as [110], [100], [010] and [001] demonstrate channelling for a large fraction of the C ions. For directions such as [111], [211] and [753] the trajectories show no more channelling than for random directions. The effects of tilt, shielding and wake-field interactions were investigated for YIG and alpha-quartz.

We present here detailed simulations of the interaction of energetic C10 and C5 clusters at the energies of 1, 2, and 4 MeV per carbon atom with an amorphous carbon target. The spatial evolution of the cluster components is simulated accounting for both scattering and Coulomb explosion. The former is calculated by means of the Monte Carlo method while the latter is computed by means of molecular dynamics. The charge state of the individual cluster components is calculated as a function of penetration depth, and is determined by the competition between electron ionization and recombination. The results of calculations of the effect of the neighbouring cluster components on the suppression of the values of the charge state are presented and compared to the experimental values of Brunelle et al. Charge state suppression calculations for the 2 MeV/C clusters for both C10 and C5 agree well with the experimental results for penetration depths of less than about 500 and 250 Å respectively, assuming the intracluster Coulomb potential is screened by four target valence electrons. At 4 MeV/C the results are similar although less screening is required; a possible explanation is the inability of the plasma to completely screen the higher velocity projectiles. The 1 MeV/C calculated results however differ in their behaviour from the 2 and 4 MeV/C cases.

Two types of excitation and emission spectra with ultraviolet excitation are shown at 11 K for Lu/sub 2/(SiO/sub 4/)O:Ce (LSO). Since there are two crystallographically independent lutetium sites in LSO, the existence of two Ce activation centers (two-activation-center model) is proposed. The existence of two different decay time constants of the order of tens of nanoseconds, which are characteristic for fast Ce emission, are shown at 77 K. Although the temperature dependence of the decay time and the luminescence efficiency could not be measured for the Ce2, the fact that the gamma-ray-excited emission can be reconstructed from a weighted combination of the Ce1 and Ce2 emission spectra supports the two-activation-center model for LSO.<>

We present an efficient algorithm able to predict the trajectories of individual cluster constituents as they penetrate relatively thick amorphous targets. Our algorithm properly treats both the intracluster Coulomb repulsion and the collisions between cluster constituents and target atoms. We have compared our simulation predictions to experimental measurements of the distribution of lateral exit velocities, and demonstrated that the in-target Coulomb explosion of 2MeV/atom carbon clusters in carbon foils must be shielded with a screening length of less than 2.5 Angstrom. We also present a simple phenomenological model for the suppression of the exit-side charge of ions in clusters which depends on the enhanced ionization potential that on electron near an ion feels due to the ion's charged comoving neighbors. By using our simulation algorithm we have predicted the exit correlations of the cluster constituents and verified that the charge suppression model fits the observed charge suppression of ions in clusters to within the experimental uncertainties.

We present the results of simulations of cluster formation during Ar+ --> In-Ga (liquid) sputtering events, We find that the indium concentration in small k-atom clusters (k less than or equal to 4) reflects the concentration in the target at the depth from which the clusters were sputtered. We find a strong correlation between the production of clusters and the size of the events responsible for their production. A simple model for the recombination of uncorrelated emissions into small clusters accurately predicts the production of small k-clusters during events of size N to be Y-k(N) similar to N-k. However, this uncorrelated recombination model does not predict the proper energy spectra for clusters nor does it predict the oft experimentally observed power law decay of the yield of clusters, We discuss how a model for the recombination of correlated emissions may more readily explain these features.

Current-driven vortex dynamics of type-II superconductors in the weak-pinning limit is investigated by quantitatively studying the current-dependent vortex dissipation of an untwinned YBa2Cu3O7 single crystal. For applied current densities (J) substantially larger than the critical current density (Jc), non-linear resistive peaks appear below the thermodynamic first-order vortex-lattice melting transition temperature (Tm), in contrast to the resistive hysteresis in the low-current limit (J < Jc). These resistive peaks are quantitatively analysed in terms of the current-driven coherent and plastic motion of vortex bundles in the vortex-solid phase, and the non-linear current - voltage characteristics are found to be consistent with the collective flux-creep model. The effects of high-density random point defects on the vortex dynamics are also investigated via proton irradiation of the same single crystal. Neither resistive hysteresis at low currents nor peak effects at high currents are found after the irradiation. Furthermore, the current-voltage characteristics within the instrumental resolution become completely ohmic over a wide range of currents and temperatures, despite theoretical predictions of much larger Jc-values for the given experimental variables. This finding suggests that the vortex-glass phase, a theoretically proposed low-temperature vortex state which is stabilized by point disorder and has a vanishing resistivity, may become unstable under applied currents significantly smaller than the theoretically predicted Jc. More investigation appears necessary in order to resolve this puzzling issue.

The production of MeV/amu heavy-ion and MeV cluster-ion beams has allowed continuous damage tracks to be made in a wide variety of materials. Using simple phenomenological models of the track-formation process one can estimate in advance the morphology of the tracks that will result from a particular set of irradiation parameters, i.e., target material, ion type and energy. In this talk I shall discuss the use of these models and how they are applied in a specific example: the pinning of quantized magnetic-flux vortices in a high-temperature superconductor. For this application one must also employ models for the interaction of the vortex and the column of damage. On the basis of such simulations it is found that although damage tracks are extremely useful for increasing the flux pinning, and hence the critical current, it would be even better if one could control the track positions and radii over a wider range of values. A new development in nanotechnology will be discussed that may, indeed, allow this to be accomplished easily and inexpensively.

Racetrack Playa in Death Valley, California is home to a group of stones which have, on occasion, slid hundreds of meters across the dry lake bed. New data on wind shear from a similar playa, Owens Dry Lake, about 80 km SSE of Racetrack Playa, and data on the velocities of wind gusts across these playas are consistent with threshold coefficients of static friction that allow for the motion of 30 of the 31 rocks for which data are available.