Ion beam induced amorphization and recrystallization of Si/SiC/Si layer systems

Epitaxial, buried silicon carbide (SiC) layers have been fabricated in (100) and (111) silicon by ion beam synthesis (IBS). In order to study the ion beam induced epitaxial crystallization (IBIEC) of buried SiC layers, the resulting Si/SiC/Si layer systems were amorphized using 2 MeV Si²⁺ ion irradiation at 300 K. An unexpected high critical dose for the amorphization of the buried layers is observed. Buried, amorphous SiC layers were irradiated with 800 keV Si⁺ ions at 320 and 600°C, respectively, in order to achieve ion beam induced epitaxial crystallisation. It is demonstrated that IBIEC works well on buried layers and results in epitaxial recrystallization at considerably lower target temperatures than necessary for thermal annealing. The IBIEC process starts from both SiC/Si interfaces and may be accompanied by heterogenous nucleation of poly-SiC as well as interfacial layer-by-layer amorphization, depending on irradiation conditions. The structure of the recrystallized regions in dependence of dose, dose rate, temperature and crystal orientation is presented by means of TEM investigations.     

Ion beam synthesis and doping of photonic nanostructures

Optically-active silica nanowires are produced by metal-induced growth on silicon substrates using ion-implantation, with two different strategies employed for their fabrication. The first is based on Er implantation of nanowires produced by a thin-film Pd catalyst layer, and the second employing implanted Er as both the catalyst and dopant. The luminescence properties of the resulting Er-doped silica nanowires are reported and compared with similarly implanted fused silica samples. Comparison shows that the luminescence lifetime of Er is increased by incorporation within the nanowires due to a reduction in the density of available optical states in these structures. Additional details of the synthesis, structure and properties of these functionalised nanowires are also presented.     

Ion beam erosion of amorphous materials: evolution of surface morphology

In this work we present our results concerning the formation of self-organized nanoscale structures during the bombardment with a low-energy defocused Ar ion beam. We studied glass surfaces because of their physical properties, technological interest and cheapness. The evolution of sample surface was studied ex situ by atomic force microscopy. We found, in agreement with Bradley and Harper, a morphology characterized by a regular ripple structure with the wave vector perpendicular or parallel to the ion beam direction. This structure periodicity was found to vary in the range 90–350 nm with a linear time evolution. In order to gain further information about the sputtering process and for comparison with the existing continuum theories of surface erosion, we studied the scaling behaviour of surface roughness.     

Ion beam induced nucleation in amorphous GaAs layers during MeV implantation

To investigate the nonlinear dose dependence of the thickness of the recrystallized layer during ion beam induced epitaxial recrystallization at amorphous/crystalline interfaces GaAs samples were irradiated with 1.0 MeV Ar⁺, 1.6 MeV Ar⁺ or 2.5 MeV Kr⁺ ions using a dose rate of 1.4 × 10¹² cm⁻² s⁻¹ at temperatures between 50°C and 180°C. It has been found that the thickness of the recrystallized layer reaches a maximum value at T_max = 90°C and 135°C for the Ar⁺ and Kr⁺ implantations, respectively. This means that the crystallization rate deviates from an Arrhenius dependence due to ion beam induced nucleation and growth within the remaining amorphous layer. The size of the crystallites depends on the implantation dose. This nucleation and growth of the crystallites disturbes and at least blocks the interface movement because the remaining surface layer becomes polycrystalline. Choosing temperatures sufficiently below T_max the thickness of the recrystallized layer increases linearly with the implantation dose indicating that the irradiation temperature is too low for ion induced nucleation.     

Ion beam synthesis of SiC by C implantation into SIMOX(1 1 1)

We report the conversion of a 65 nm Si(1 1 1) overlayer of a SIMOX(1 1 1) into 30–45 nm SiC by 40 keV carbon implantation into it. High temperature implantation (600 °C) through a SiO₂ cap, 1250 °C post-implantation annealing under Ar ambient (with 1% of O₂), and etching are the base for the present synthesis. Sequential C implantations (fluence steps of about 5 × 10¹⁶ cm^?2), followed by 1250 °C annealing, has allowed to estimate the minimum C fluence to reach the stoichiometric composition as ～2.3 ×  10¹⁷ cm^?2. Rutherford Backscattering Spectrometry was employed to measure layer composition evolution. A two-sublayers structure is observed in the synthesized SiC, being the superficial one richer in Si. Transmission electron microscopy has shown that a single-step implantation up to the same minimum fluence results in better structural quality. For a much higher C fluence (4 × 10¹⁷ cm^?2), a whole stoichiometric layer is obtained, with reduction of structural quality.     

C—N多晶的离子束合成及分析

介绍了用离子束合成法研制新型材料β－Ｃ３Ｎ４的条件及过程，用ＲＢＳ、ＸＲＤ等手段分析研究该方法生成物的配比、结构和物相，观察到ＣＮ化合物多晶。     

Thin amorphous gallium nitride films formed by ion beam synthesis

Ion implantation and plasma enhanced chemical vapour deposition (PECVD) have been used to synthesise an amorphous gallium nitride compound (a-GaN) within an amorphous silicon nitride (a-SiN_x:H_y) matrix by implanting Ga⁺ into a-SiN_x substrates. This route may enable the synthesis of large area a-GaN substrates for the use as possible seed layers for the growth of crystalline GaN as well as an amorphous semiconductor in its own right. A study of an entire range of a-SiN_x with different compositions ‘x’ has enabled the choice of the most suitable type of target substrate. It has been shown that nitrogen-rich a-SiN_x has a high stress as well as a steady incorporation of N. X-ray Photoelectron Spectroscopy (XPS) and Rutherford Backscattering Spectroscopy (RBS) studies yield information on the chemistry and elemental depth profiles of the material synthesised. Low temperature annealing, compatible with large area glass substrates is then used to increase the thickness of the a-GaN layer and transform more of the nitrogen rich a-SiN_x.     

Controlling the density distribution of SiC nanocrystals for the ion beam synthesis of buried SiC layers in silicon

The depth distribution of SiC nanocrystals formed during high-dose implantation of carbon ions into silicon at conditions suitable for the ion beam synthesis of buried SiC layers in silicon is studied in this paper. For implantation temperatures of 400–600°C and dose rates of 10¹² − 10¹³ C⁺/cm²s, SiC precipitates in crystalline silicon are observed to be of approximately equal size, independent of the depth position beneath the surface. Ballistic destruction of small precipitates and difficulties in precipitate growth are thought to be responsible for the observed narrow size distribution. The destruction of precipitates may lead to the simultaneous release of a superthreshold concentration of carbon atoms resulting in a carbon-induced amorphization of the silicon host lattice. The local reduction of the number density of SiC nanocrystals involved with this amorphization can be used to tailor discontinuous depth distributions of oriented SiC precipitates providing ideal starting conditions for the synthesis of well-defined single-crystalline SiC layers in silicon.     

Ion beam propagation and focusing

Inertial confinement fusion with ion beams requires the efficient delivery of high energy (1 MJ), high power (100 TW) ion beams to a small fusion target. The propagation and focusing of such beams is the subject of this paper. Fundamental constraints on ion beam propagation and focusing are discussed, and ion beam propagation modes are categorized. For light ion fusion (LIF), large currents (2–33 MA) of moderate energy (3–50 MeV) ions of low atomic number (1A12) must be directed to a target of radius 1 cm. The development of pulsed power ion diodes for LIF is discussed, and the necessity for virtually complete charge neutralization during transport and focusing is emphasized. Fornear-term LIF experiments, the goal is to produce pellet ignition without the standoff needed for the ultimate reactor application. Ion diodes for use on Sandia National Laboratories Particle Beam Fusion Accelerators PBFA-I (2–4 MV, 1 MJ, 30 TW, operational) and PBFA-II (2–16 MV, 3.5 MJ, 100 TW, scheduled for operation in 1985) are discussed. Ion beam transport from these diodes to the pellet is examined in reference to the power brightness . While values of =2–5 TW/cm²/sr have been achieved to date, a value of 100 TW/cm²/sr is needed for breakeven. Research is now directed toward increasing , and means already exist (e.g., scaling to higher voltages, enhanced ion diode current densities, and bunching), which indicate that the required goal should be attainable. Forfar-term LIF applications, the goal is to produce net energy gain with standoff suitable for a reactor. This may be achieved by ion beam transport in preformed, current-carrying plasma channels. Channel transport research is discussed, including experiments with wire-initiated, wall-initiated, and laser-initiated discharge channels, all of which have demonstrated transport with high efficiency (50–100%). Alternate approaches to LIF are also discussed, including comoving electron beam schemes and a neutralized beam scheme. For heavy ion fusion (HIF), moderate currents (10 kA) of high energy (10 GeV) ions of high atomic number (A200) must be directed to a target of radius 0.3 cm. Conventional accelerator drivers for HIF are noted. For a baseline HIF reactor system, the optimum transport mode for low charge state beams is ballistic transport in near vacuum (10^–4–10^–3 Torr lithium), although a host of other possibilities exists. Development of transport modes suitable for higher charge state HIF beams may ultimately result in more economical HIF accelerator schemes. Alternate approaches to HIF are also discussed which involve collective effects accelerators. The status of the various ion beam transport and focusing modes for LIF and HIF are summarized, and the directions of future research are indicated.     

Ion beam enhanced etching of LiNbO3

Single crystals of z- and x-cut LiNbO₃ were irradiated at room temperature and 15 K using He⁺- and Ar⁺-ions with energies of 40 and 350 keV and ion fluences between 5 × 10¹² and 5 × 10¹⁶ cm⁻². The damage formation investigated with Rutherford backscattering spectrometry (RBS) channeling analysis depends on the irradiation temperature as well as the ion species. For instance, He⁺-irradiation of z-cut material at 300 K provokes complete amorphization at 2.0 dpa (displacements per target atom). In contrast, 0.4 dpa is sufficient to amorphize the LiNbO₃ in the case of Ar⁺-irradiation. Irradiation at 15 K reduces the number of displacements per atom necessary for amorphization. To study the etching behavior, 400 nm thick amorphous layers were generated via multiple irradiation with He⁺- and Ar⁺-ions of different energies and fluences. Etching was performed in a 3.6% hydrofluoric (HF) solution at 40 °C. Although the etching rate of the perfect crystal is negligible, that of the amorphized regions amounts to 80 nm min⁻¹. The influence of the ion species, the fluence, the irradiation temperature and subsequent thermal treatment on damage and etching of LiNbO₃ are discussed.     

Ion beam modification of metals

Energetic ions beams may be used in various ways to modify and so improve the tribological properties of metals. These methods include: — ion implantation of selected additive species; — ion beam mixing of thin deposited coatings; — ion-beam-assisted deposition of thicker overlay coatings.

The first of these techniques has been widely used to modify the electronic properties of semiconductors, but has since been extended for the treatment of all classes of material. Tool steels can be strengthened by the ion implantation of nitrogen or titanium, to produce fine dispersions of hard second-phase precipitates. Solid solution strengthening, by combinations of substitutional and interstitial species, such as yttrium and nitrogen, has also been successful. Both ion beam mixing (IBM) and ion-beam-assisted deposition (IBAD) use a combination of coating and ion bombardment. In the first case, the objective is to intermix the coating and substrate by the aid of radiation-enhanced diffusion. In the latter case, the coating is densified and modified during deposition and the process can be continued in order to build up overlay coatings several μm in thickness. The surface can then be tailored, for instance to provide a hard and adherent ceramic such as silicon nitride, boron nitride or titanium nitride. It is an advantage that all the above processes can be applied at relatively low temperatures, below about 200° C, thereby avoiding distortion of precision components. Ion implantation is also being successfully applied for the reduction of corrosion, especially at high temperatures or in the atmosphere and to explore the mechanisms of oxidation. Ion-assisted coatings, being compact and adherent, provide a more substantial protection against corrosion: silicon nitride and boron nitride are potentially useful in this respect. Examples will be given of the successful application of these methods for the surface modification of metals and alloys, and developments in the equipment now available for industrial application of ion beams will also be reviewed.     

Monitoring interstitial fluxes by self-assembled nanovoids in ion-implanted Si/SiGe/Si strained structures

We report on the effect of the rapid thermal annealing (RTA) ambiance on evolution of self-assembled voids of nanometer size. The spherically shaped voids are produced in molecular beam epitaxially grown Si/SiGe/Si strained structures with in-situ implantation of 1 keV Ge ions followed by RTA at 800 or 900 °C. The voids are of nanometer size and are exclusively assembled in the narrow SiGe layer. During the RTA, the voids grow in size in a nitrogen ambiance and shrink in an oxygen ambiance. The evolution of the voids correlates well with oxidation-induced injection of excess interstitials. Prospects for point defect monitoring are discussed.     

Electron-beam radial distribution analysis of irradiation-induced amorphous SiC

Advanced electron microscopy techniques have been employed to examine atomistic structures of ion-beam-induced amorphous silicon carbide (SiC). Single crystals of 4H-SiC were irradiated at a cryogenic temperature (120 K) with 300 keV Xe ions to a fluence of 10¹⁵ cm⁻². A continuous amorphous layer formed on the topmost layer of the SiC substrate was characterized by energy-filtering transmission electron microscopy in combination with imaging plate techniques. Atomic pair-distribution functions obtained by a quantitative analysis of energy-filtered electron diffraction patterns revealed that amorphous SiC networks contain heteronuclear Si–C bonds, as well as homonuclear Si–Si and C–C bonds, within the first coordination shell. The effects of inelastically-scattered electrons on atomic pair-distribution functions were discussed.     

Ion beam synthesis of gold nanoclusters in SiO2: Computer simulations versus experiments

A kinetic 3D lattice Monte-Carlo (KLMC) method is applied to describe nucleation and growth of nanoclusters from a space- and time-dependent supersaturated solid solution of impurity atoms formed by high-dose ion implantation. Within the framework of homogeneous nucleation, the dependence of the nanocluster size distribution on implantation temperature T_imp and ion current j is shown. The simulation results are consistent with size and depth distributions of gold nanoclusters in SiO₂ observed after implantation at different T_imp. The influence of changes of T_imp or j during implantation on the size distribution is simulated and the results are discussed with respect to corresponding implantations.     

Ion beam studies of InAs/GaAs quantum dots after annealing

The microstructure changes of self-assembled InAs/GaAs quantum dots during RTA treatment was investigated using ion channeling and photoluminescence (PL). A small blueshift of the PL emission is observed for annealing temperatures of 650-800 °C and an obvious blueshift at 850 °C. The yield of channeled spectra decreased as annealing temperature was increased, but the yield increased while temperature above 800 °C in RTA. These results imply the strain of QD varied during RTA treatments. In addition, the As/Ga atomic ratio near the surface was determined from the surface peaks of the channeled spectrum.     

Ion beam studies of InAs/GaAs self assembled quantum dots

Self assembled InAs/GaAs quantum dots (QD’s) emit in the wavelength range (1.3-1.55 μm) revealing an enormous potential to become the active elements of low threshold lasers and light emitting diodes for communication systems. However, the luminescence is dramatically quenched at room temperature (and even below) due to the defects in the GaAs matrix which open non-radiative recombination paths.In this study we combine Rutherford backscattering/channelling (RBS-C) and high resolution X-ray diffraction (HRXRD) to study the structural properties of the InAs/GaAs structures. The InAs/GaAs QD heterostructures were grown by atmospheric pressure metal organic vapour phase epitaxy. Channelling measurements reveal a good crystalline quality along the main axial directions with minimum yields in the range of 4-6% through the entire capping layer. An increase on the dechannelling rate was observed in the region where the InAs quantum dots were buried. The channelling results also give evidence for the presence of defects preferentially oriented. Detailed angular scans in a structure with a 28 nm cap allowed the study of the In orientation with respect to the GaAs matrix and a perfect alignment was found along the growth direction. The strain in the dots shifts the angular curves along the tilt directions.     

Ion beam induced luminescence of materials

Ion beam interactions with many insulating materials produces strong luminescence and this may be collected and analysed to gain information on the defect structures present. The use of light ions such as H⁺ and He⁺ with MeV energies allows penetration to a depth of several microns in most materials. The variation of ion species and energy allows precise control of ion-induced damage and ionisation within the material. This control of depth, ionisation, and damage can be superior to that of the traditional cathodo-luminescence technique and comparison with spectra from the two methods often yields differences in sensitivity to different emission bands and features.     

Ion beam characterization of Fe-implanted GaN

Fe ion implantation in GaN has been investigated by means of ion beam analysis techniques. Implantations at an energy of 150 keV and fluences ranging from 2 × 10¹⁵ to 1 × 10¹⁶ cm^?2 were done, both at room temperature and at 623 K. Secondary Ions Mass Spectrometry was used to determine the Fe implantation profiles, whereas Rutherford Backscattering in channeling conditions with a 2.2 MeV ⁴He⁺ beam allowed us to follow the damage evolution. Particle Induced X-ray Emission in channeling conditions with a 2 MeV H⁺ beam was employed to study the lattice location of Fe atoms after implantation. The results show that a high fraction of Fe-implanted atoms are located in high symmetry sites in low fluence implanted samples, where the damage level is lower, whereas the fraction of randomly located Fe atoms increases by increasing the fluence and the resulting damage. Moreover, dynamical annealing present in high temperature implantation has been shown to favor the incorporation of Fe atoms in high symmetry sites.