Nanophase precipitates of CdS formed in amorphous SiO2 by ion implantation and thermal processing have recently been found to exhibit a “hollow-particle” or “shell-like” microstructure. The present investigations show that this hollow-particle microstructure can be reproduced for a variety of materials other than CdS, and these results provide new insight into the mechanisms responsible for the formation of hollow precipitates embedded in solid hosts. Various elemental metal nanocrystals were formed in (100)-oriented crystalline Si hosts by ion implantation coupled with thermal treatments in which the annealing parameters were varied to investigate the “hollow-particle” formation conditions. The results indicate that depending on the melting points and vapor pressure of the precipitates or on the initial state of the host material, several processes acting either independently or in concert can lead to hollow precipitate formation. First, the implantation of materials having a high vapor pressure, either at the implant temperature or when heated during annealing, can lead to the formation of cavities in the crystalline host. Hollow precipitates can then form by a partial filling and coating of the cavity walls by the implanted species in a diffusion-based gettering/ripening process. Internal void formation can also occur or be enhanced by volume contraction during cooling if the particle solidifies from a liquid phase.

Ion implantation coupled with annealing is a versatile and flexible approach to creating ferromagnetic near-surface nanocomposites that represent a wide range of particle/host combinations. We have used ion implantation and thermal processing to create a layer of Co nanoparticles in a sapphire host that was subsequently irradiated with Xe, Pt, or Pb in order to systematically modify the magnetic properties of the composite. Transmission electron microscopy (reported in an accompanying paper in this volume) was used to carry out a detailed characterization of the microstructure of the resulting near-surface composites whose magnetic properties were determined using SQUID magnetometry or magnetic circular dichroism. These composites exhibit magnetic hysteresis with coercivities ranging from near zero (i.e., superparamagnetism) up to 1.2 kG - depending on the composition and microstructure. We also present the results of preliminary experiments in which we attempt to control the spatial distribution of magnetic elements within ion-implanted ferromagnetic nanocomposites. The results demonstrate methods for tailoring the magnetic properties of nanocomposites produced by ion implantation for specific applications.

The bombardment of C with 100 keV and 1 MeV W at normal incidence is studied as a function of the incident W fluence experimentally and by computer simulation with the program TRIDYN. Calculated oscillations in the amount of retained W and in the target weight change are confirmed experimentally for 100 keV at room temperature. XPS investigations show W2C formation during ion implantation already at room temperature. RBS depth profiles for 1 MeV bombardment show W mobility and surface segregation even at liquid nitrogen (LN2) temperatures. At elevated temperatures W clusters to form nano-particles at the surface and the oscillations in the retained amount of W disappear.

Silica samples were implanted at multiple energies with Se and Ge ions producing implanted layers ~ 2.4 µm in depth starting at ~ 0.5 µm below the surface. In each case, the concentration of implanted species was ~ 0.05 atomic %. The optical absorption of the samples was measured from 2.7 to 6.5 eV. In all spectra, local maxima at 5.0 eV with shoulders at 5.9 eV were observed. The spectra have been assumed to be describable as a superposition of Gaussian absorption bands with mean energies taken from the literature of 4.8, 5.01, 5.17, 5.88, and 7.15 eV. The relative strengths of each of these bands have been obtained by linear regression. These fits show that additional bands at 3.7 and 6.4 eV are required to fit the data for the Se samples, while bands at 5.54 and 6.4 eV are needed to fit the data for the Ge samples.

Ion implantation was used to form compound semiconductor nanocrystal precipitates of ZnS, CdS, and PbS in both glass and crystalline matrices. The precipitate microstructures and size distributions were investigated by cross-sectional transmission electron microscopy techniques. Several unusual features were observed, including strongly depth-dependent size variations of the ZnS precipitates and central void features in the CdS nanocrystals. The morphology and crystal structure of the nanocrystal precipitates could be controlled by selection of the host material. The size distribution and microstructural complexity were significantly reduced by implanting a low concentration of ions into a noncrystalline host, and by using multi-energy implants to give a flat concentration profile of the implanted elements.

The formation of vacancy-related defects in n-type silicon has been studied immediately after implantation of He, Si, or Ge ions at 85 K using in-situ DLTS. A-center concentrations in He-implanted samples reach a maximum immediately after implantation, whereas, with Si or Ge ion implanted samples they continuously increase during subsequent anneals. It is proposed that defect clusters, which emit vacancies during anneals, are generated in the collision cascades of Si or Ge ions. An illumination-induced suppression of A-center formation is seen immediately after implantation of He ions at 85 K. This effect is also observed with Si or Ge ions, but only after annealing. The suppression of vacancy complex formation via photoexcitation is believed to occur due to an enhanced recombination of defects during ion implantation, and results in reduced number of vacancies remaining in the defect clusters. In p-type silicon, a reduction in K-center formation and an enhanced migration of defects are concurrently observed in the illuminated sample implanted with Si ions. These observations are consistent with a model where the injection of excess carriers modifies the defect charge state and impacts their diffusion.

The evolution of defects in Czochralski and epitaxial p- and n-type silicon wafers following irradiation with He. Si or Ge ions at 80 K has been investigated by in situ deep level transient spectroscopy (DLTS). Defect annealing and formation reactions have been observed over the temperature range 80–350 K. In p-type silicon, new species-dependent levels are observed immediately after implantation, but these levels anneal out at or below room temperature. The wellknown divacancy and interstitial defects, usually reported after room temperature implantation, are revealed in the DLTS spectra only upon annealing at 160–200 K. In n-type silicon, vacancy-oxygen pairs are observed immediately after implantation. However, vacancy-related defects continue to form over a broad temperature range in samples implanted with Si or Ge. These observations are consistent with a model whereby vacancies and interstitials are released from defect clusters at temperatures >200 K to form divacancies and other defect pairs which are stable at room temperature.

Amorphous, polycrystalline, and single crystal nanometer dimension particles can be formed in a variety of substrates by ion implantation and subsequent annealing. Such composite colloidal materials exhibit unique optical properties that could be useful in optical devices, switches, and waveguides. However colloids formed by blanket implantation are not uniform in size due to the nonuniform density of the implant, resulting in diminution of the size dependent optical properties. The object of the present work is to form more uniform size particles arranged in a 2-dimensional lattice by using a finely focused ion beam to implant identical ion doses only into nanometer size regions located at each point of a rectangular lattice. Initial work is being done with a 30 keV Ga beam implanted into Si. Results of particle formation as a function of implant conditions as analyzed by Rutherford backscattering, x-ray analysis, atomic force microscopy, and both scanning and transmission electron microscopy will be presented and discussed.

Ion implantation is a versatile technique by which compound semiconductor nanocrystals may be synthesized in a wide variety of host materials. The component elements that form the compound of interest are implanted sequentially into the host, and nanocrystalline precipitates then form during thermal annealing. Using this technique, we have synthesized compound semiconductor nanocrystal precipitates of ZnS, CdS, PbS, and CdSe in a fused silica matrix. The resulting microstructures and size distributions were investigated by cross-sectional transmission electron microscopy. Several unusual microstructures were observed, including a band of relatively large nanocrystals at the end of the implant profile for ZnS and CdSe, polycrystalline agglomerates of a new phase such as γ-Zn 2SiO4, and the formation of central voids inside CdS nanocrystals. While each of these microstructures is of fundamental interest, such structures are generally not desirable for potential device applications for which a uniform, monodispersed array of nanocrystals is required. Methods were investigated by which these unusual microstructures could be eliminated.

Silica composites containing nanometer dimension colloids have been fabricated by implantation of Ag ions followed by Sb ions, and by implantation of Sb ions followed by Ag ions. Doses for the sequential element implantations were in ratios of 9 : 3 Ag : Sb and 3 : 9 Sb : Ag with the total dose held constant at 12 × 1016 ions/cm2. Energies of implantation were 305 keV for the Ag ions and 320 keV for the Sb ions. Single element colloids were also fabricated by implantation of Ag or Sb using the same nominal dose and implantation energy of the sequential implantations. Approximately spherical particles were formed in all implanted samples. Microstructures of the nanoclusters in the various samples were markedly different. Selected area diffraction techniques revealed that alloyed phases of Ag–Sb were formed in some of the sequential implantations. The microstructure and the optical response of the nanocluster glass composites were found to be strongly dependent upon the order of the ion species implanted. The optical spectra of the 3 : 9 Sb : Ag sample displays two resonance peaks indicative of a Ag resonance peak and a resonance peak of an alloyed phase of Ag–Sb. Optical spectra for the 9 : 3 Ag : Sb sample displays two broad absorption peaks indicative of coated particles.

Sequential ion implantation followed by thermal annealing has been used to form encapsulated CdS, ZnS, and PbS nanocrystals in SiO2 and Al2O3 matrices. In SiO2, nanoparticles are nearly spherical and randomly oriented, and ZnS and PbS nanocrystals exhibit bimodal size distributions. In Al2O3, nanoparticles are facetted and oriented with respect to the matrix. Initial photoluminescence (PL) results are presented.

We have fabricated Se nanoparticles in silica substrates by ion implantation followed by thermal annealing up to 1000°C, and studied the Se nanoparticle formation by optical absorption spectroscopy, Rutherford backscattering spectrometry, X-ray diffraction, and transmission electron microscopy. The sample with the highest dose (1 × 1017 ions/cm2 ) showed the naonparticle formation during the ion implantation, while the lower dose samples (1 and 3 × 1016 ions/cm2) required thermal treatment to obtain nano-sized particles. The Se nanoparticles in silica were found to be amorphous. After thermal annealing, the particle sizes became larger than the exciton Bohr radius for bulk Se. Thus, the absorption edges for different doses approached the value of bulk after thermal annealing. The temperature dependent absorption spectra were also measured for this system in a temperature range from 15 to 300 K.

Complex nonlinear optical materials comprising elemental, compound or alloy quantum dots embedded in appropriate dielectric or semiconducting hosts may be suitable for deployment in photonic devices. Ion implantation, ion exchange followed by ion implantation, and pulsed laser deposition have all been used to synthesize these materials. However, the correlation between the parameters of energetic-beam synthesis and the nonlinear optical properties is still very rudimentary when one starts to ask what is happening at nanoscale dimensions. Systems integration of corplex nonlinear optical materials requires that the mesoscale materials science be well understood within the context of device structures. We discuss the effects of beam energy and energy density on quantum-dot size and spatial distribution, thermal conductivity, quantum-dot composition, crystallinity and defects — and, in turn, on the third-order optical susceptibility of the composite material. Examples from recent work in our laboratories are used to illustrate these effects.

The formation and optical properties of nanometer dimension metal colloid composites formed by the sequential implantation of Cd then Ag and by single element implantations of Cd and Ag in silica were characterized by transmission electron microscopy (TEM) and optical spectroscopy. A nominal dose of 6×1016 ions/cm2 as determined by current integration was used for both ion species. The doses used for the sequential implantations were a 1 to 1 ratio of Cd to Ag. Sequential implantations of Cd and Ag led to the formation of both multi-component metal nanoclusters and elemental nanoclusters. The electron diffraction measurements indicated that the polycrystalline particles of Ag5Cd8 and elemental Ag were formed. The optical response was consistent with results expected from effective medium theory.

Carbon is implanted into fused silica with doses of 1, 3, 6, 10×1016 ions/cm2. Infrared spectroscopy has identified the formation of CO and CO2 molecules in the implanted glasses. The relationships among concentrations of carbon dioxide, carbon monoxide and carbon doses are established by the infrared measurements. Annealing under different atmospheres have dramatic effects on CO and CO2 concentrations.

High-dose ion implantation, followed by annealing, has been shown to provide a versatile technique for creating semiconductor nanocrystals encapsulated in the surface region of a substrate material. We have successfully formed nanocrystalline precipitates from groups IV (Si, Ge, SiGe), III-V (GaAs, InAs, GaP, InP, GaN), and II-VI (CdS, CdSe, CdSxSe1x, CdTe, ZnS, ZnSe) in fused silica, Al2O3 and Si substrates. Representative examples will be presented in order to illustrate the synthesis, microstructure, and optical properties of the nanostructured composite systems. The optical spectra reveal blue-shifts in good agreement with theoretical estimates of size-dependent quantum-confinement energies of electrons and holes. When formed in crystalline substrates, the nanocrystal lattice structure and orientation can be reproducibly controlled by adjusting the implantation conditions.

The linear and nonlinear optical properties of a series of Sb nanometer dimension metal colloids in silica were measured. The colloids were fabricated by the implantation of Sb in silica with nominal doses of 3, 6 and 9x1016 ions/cm2 at 320 keV. The linear optical response was measured from 200 to 900 nm. The nonlinear optical properties were measured using the z-scan technique at wavelengths of 596 and 617 nm. The results are compared to linear and nonlinear properties of a sample implanted with 6x1016 Ag ions/cm 2. Both absorption and the nonlinear index of refraction increase with increasing volume fraction for the Sb implanted samples. The lower linear absorption and comparable nonlinear index of refraction for the Sb colloid-glass composites may be better for devices than composites using metal colloids that have a surface plasmon resonance.

Zinc ion implanted silica with controlled thermal annealing has been investigated. Low temperature optical measurements indicate the presence of Zn clusters in the as-implanted silica. Optical spectra of the annealed sample under a reducing environment suggest Zn cluster and Zn metal colloid formation. The absorption peak at ∼5.3 eV may be due to the surface plasma absorption of Zn metal colloids in silica. The oxidized samples (10 and 6 x 1016 ions/cm2) show an absorption peak at ∼4.3 and ∼4.8 eV, respectively and imply ZnO quantum dot formation. The blueshift in exciton absorption can be attributed to the quantum confinement effects.

In this work, we report the formation of a new type of active or “smart” surface that is produced by ion implantation and thermal processing. By co-implanting vanadium and oxygen into a single-crystal sapphire substrate and annealing the system under appropriate conditions, it was possible to form buried precipitates of vanadium dioxide that were crystallographically oriented with respect to the host AI2O3 lattice. The implanted VO2 precipitate system undergoes a structural phase transition that is accompanied by large variations in the optical transmission which are comparable to those observed for thin films of VO2 deposited on sapphire. Co-implantation with oxygen was found to be necessary to ensure good optical switching behavior.

The optical, structural, and thermodynamic properties of materials can be changed by reducing their dimensions. We sequentially implanted In and As into fused silica windows in order to investigate the formation and properties of InAs nano-particles. The UV/VIS/NIR, FTIR in mid-IR, and far-IR spectroscopy were mainly used to study the change in electronic transitions and in vibrational modes (phonons) of the nanoparticle InAs. The phonons can be confined to the surface of nanoparticles and have frequencies falling between the transverse and longitudinal optical modes of the bulk material. Thermal annealing developed the formation of InAs quantum dots from as-implanted In+As system. At a certain annealing temperature, we observed a change in UV/VIS transmission spectra and IR reflectance spectra indicating the formation of InAs quantum dots. This is particularly evident from the absorption in the IR, and surface phonon bands are observed confirming the presence of quantum confined InAs.