V. Rotberg

Mechanisms of nitrogen incorporation in GaAsN alloys

We have investigated nitrogen incorporation mechanisms in dilute nitride GaAsN alloys grown by plasma-assisted molecular-beam epitaxy. A comparison of nuclear reaction analysis and Rutherford backscattering spectrometry in channeling and nonchanneling conditions reveals significant composition-dependent incorporation of N into nonsubstitutional sites, presumably as either N–N or N–As split interstitials. Furthermore, we identify the (2×1) reconstruction as the surface structure which leads to the highest substitutional N incorporation, likely due to the high number of group V sites per unit area available for N–As surface exchange.We have investigated nitrogen incorporation mechanisms in dilute nitride GaAsN alloys grown by plasma-assisted molecular-beam epitaxy. A comparison of nuclear reaction analysis and Rutherford backscattering spectrometry in channeling and nonchanneling conditions reveals significant composition-dependent incorporation of N into nonsubstitutional sites, presumably as either N–N or N–As split interstitials. Furthermore, we identify the (2×1) reconstruction as the surface structure which leads to the highest substitutional N incorporation, likely due to the high number of group V sites per unit area available for N–As surface exchange.

Evolution of structural and optical properties of ion-beam synthesized GaAsN nanostructures

We have investigated the evolution of structural and optical properties of GaAsN nanostructures synthesized by N ion implantation into epitaxial GaAs, followed by rapid thermal annealing. Transmission electron microscopy and x-ray diffraction indicate the formation of nanometer-sized crystallites with lattice parameters close to those of pure zincblende GaN. The average crystallite size increases with annealing temperature while the size distribution is self-similar and the volume fraction remains constant, suggesting a coarsening process governed by Ostwald ripening. These GaAsN nanostructures exhibit significant photoluminescence in the near infrared range. The apparent lowering of the fundamental band gap is likely due to the incorporation of a small amount of As in GaN.

A radiation effects facility using a 1.7 MV tandem accelerator

Abstract A facility has been established at the Michigan Ion Beam Laboratory for the study of radiation effects on materials. The capabilities include a broad range of materials (metals, ceramics and polymers), radiation damage rates (10−8 to 10−3 dpa/s) and irradiation temperatures (−196°C to 600°C). The key to the utility of this facility is the control of irradiation dose, dose uniformity, and sample temperature during irradiation. Temperature stability is maintained by simultaneous heating and cooling of the sample stage, and use of a liquid metal interface (for metal samples). The temperature of individual samples in the irradiated area is measured via an infrared pyrometer and thermocouples. Temperature uniformity is confirmed by the pyrometer, while dose uniformity is provided by a split aperture. A total of eight input channels transmit temperature and beam current signals to a 486DX computer to provide feedback to the operator and to record the irradiation history at a frequency of 1 point per second. Continuous irradiations lasting up to 120 hours have been successfully conducted.

Matrix-seeded growth of nitride semiconductor nanostructures using ion beams

We have examined the matrix-seeded growth of narrow-gap nitride nanostructures in nitrogen ion implanted GaAs and InAs. Low-energy implantation followed by rapid thermal annealing (RTA) results in the formation of 2–3 nm sized amorphous precipitates in a crystalline matrix. On the other hand, high-energy implantation results in an amorphous layer, with or without crystalline remnants. When the ion-beam-synthesized amorphous matrix is a continuous amorphous layer, subsequent RTA leads to the formation of 4–5 nm zinc blende (ZB)-GaN-rich crystallites in an amorphous matrix. When this matrix contains crystalline remnants, subsequent RTA leads to the formation of 2–4 nm ZB-GaN-rich crystallites within the amorphous regions. These results suggest that the matrix plays an important role in the nucleation and growth of narrow-gap nitride nanostructures, and that matrix-seeded growth may provide an opportunity to control the structure and properties of the nanostructures.

Formation of buried TiN in glass by ion implantation to reduce solar load

Ti and N were implanted into soda lime glass to doses up to 4.5×1017 cm−2 to reduce solar load and infrared transmission. Analysis of the Ti+N implant distributions by Rutherford backscattering spectrometry and x‐ray photoelectron spectroscopy (XPS) revealed profiles which closely followed each other as designed by the selection of implant energies. XPS, x‐ray diffraction, and selected area electron diffraction in transmission electron microscopy also confirmed the existence of a crystalline B1‐type, cubic TiN layer, 140 nm wide, at doses greater than 9×1016 cm−2. Optical measurements showed that the fraction of infrared radiation reflected was increased by almost a factor of 4 compared to an increase of 1.8 in the visible region. The percentage of the total solar energy rejected reached 80% at the highest dose, indicating that the buried TiN layer is highly effective in reducing solar energy transmission.

Corrosion and mechanical behavior of ion implanted bearing steels for improved fretting behavior

Abstract Ion implantation of AISI 52100 and 1070 steels was conducted in order to improve the corrosion, wear and ultimately the fretting behavior of the steels. Implantations consisted of 1 × 1017 Ta+ cm-2, 3 × 1017 Ti+ cm-2 + 1.5 × 1017 C+ cm-2, and 3.1 × 1017 Ti+ cm-2 + 1.55 × 1017 N+2 cm-2. All implantations were successful in improving the corrosion resistance. On average, the peak anodic current was reduced by over 300 mV, the passivation potential was reduced, and the pitting potential was increased by over 1000 mV in 0.01 M NaCl. Ti + C and Ti + N implantations increased the load-carrying capacity in lubricated scuffing tests by 60% and 40% respectively. Ta produced no improvement in scuffing resistance. Ti + N implantation increased the hardness by 25% over the unimplanted steel and both Ti + C and Ta implantation reduced the surface hardness. Fretting wear was reduced only slightly in the Ta implanted sample and increased in both the Ti + C and Ti + N implanted samples with the latter showing 4–5 times the weight loss as the unimplanted sample. The correlation between fretting and hardness supports a mechanism in which the hard surface layer breaks into fine particles which act as an abrasive under the intense load of the balls.

Analysis of TiN by charged particle beams: Nuclear reaction analysis, nuclear reaction broadening and Rutherford backscattering

Charged particle beams have been used to extract data both on the composition and also on the thickness of two series of TiNx films (about 5 and 2 lira thick respectively), where x varied from 0.3 to 1.0. The technique used were nuclear reaction analysis (NRA), resonance nuclear reaction broadening (NRB) and Rutherford backscattering, together with Auger electron spectroscopy (AES). The data from the NRB depth profiles scale remarkably well with the results from AES. The NRA results have the greatest experimental uncertainty and also indicate a systematic error which is not understood. Both NRA and NRB can return reliable film thickness data. It is also demonstrated that TiC should not be used as a reference standard when analyzing TiN films by AES.

Optical properties of Ti and N implanted soda lime glass

Soda lime glass was implanted sequentially with Ti+ and N+ to doses ranging from 2 to 30×1016 cm−2 in order to study the resulting optical properties. Analysis of the implant distributions was made by using Rutherford backscattering and x‐ray photoelectron spectroscopy and revealed profiles which closely followed each other as designed by the selection of implant energies. Analysis of optical properties showed that the highest dose resulted in an increase in the fraction of infrared reflected by more than a factor of 4 versus 1.7 for the visible regime. The percentage of the total solar radiation rejected exceeded 60% at the highest dose, indicating that the buried layer is highly effective in reducing solar load.

Origins of luminescence from nitrogen-ion-implanted epitaxial GaAs

We have examined the origins of luminescence in N-ion-implanted epitaxial GaAs, using a combination of cross-sectional transmission electron microscopy and low-energy electron-excited nanoscale-luminescence spectroscopy. A comparison of reference, as-implanted, and implanted-plus-annealed samples reveals a variety of emissions. In all samples, we observe the GaAs fundamental band-gap emission, as well as several emissions related to GaAs native defects. In the as-implanted and implanted-plus-annealed samples, an emission related to the implantation-induced defects, is also observed. Interestingly, in the implanted-plus-annealed samples, we identify a near-infrared emission associated with GaAsN nanocrystallites.

Remote Monitoring and Control of Irradiation Experiments

As computer technology plays an increasing important role in particle accelerator facilities, instrumentation systems can be expected to include web connections and other remote capability features. The Michigan Ion Beam Laboratory at the University of Michigan in Ann Arbor has developed remote monitor and control capability by using a combination of commercial software packages and in‐house software development. Irradiation parameters such as ion current on the samples and apertures, sample temperature read from an optical pyrometer, and chamber pressure can all be accessed and monitored remotely through a web site, as can ion source parameters such as power supply currents and voltages or feed gas pressure. With administrator permission, the control parameters of the ion source or the readouts from the irradiation stage can be modified in real time during an experiment. A description will be given of the various ways in which this type of remote monitoring and control has been accomplished at the Michig...