The formation of compound layers in silicon by ion beam synthesis

The technique of ion beam synthesis (IBS) using high doses of energetic ions has been successfully implemented to produce a variety of compounds, the physical properties of which are dependent on the implanted species and range from insulators, e.g. SiO₂, through semiconductors, e.g. SiC, to conductors, e.g. CoSi₂. In this paper we study the evolution of these compounds and compare and contrast their methods of formation. To demonstrate the versatility of the technique we look at three examples of IBS layers: (1) To date most of the interest in IBS has concentrated on the production of buried oxide layers for silicon-on-insulator (SOI) device applications. Recently it has been shown that by using a series of sequential implants and high-temperature anneals the defect density in the silicon overlayer can be dramatically reduced. To study how this process occurs, we followed the redistribution of the implanted species during implantation and annealing using both ¹⁶O⁺ and ¹⁸O⁺. (2) Buried CoSi₂ layers can be fabricated in (100) single-crystal silicon by implanting high doses of energetic cobalt ions at elevated temperatures. For the higher doses (≥ 4 × 10¹⁷ O⁺/cm² at 350 keV), a continuous coherent layer of CoSi₂ grows epitaxially during implantation. For lower doses, precipitates of both A- and B-type CoSi₂ are observed. After annealing at 1000° C for 30 min, single-crystal aligned layers are produced for the higher doses, while for lower doses discrete octahedral A-type precipitates are formed. (3) The microstructures of synthesized SiC layers are more complex than analogous synthesized oxide or silicide layers. Unlike buried oxide layers, the carbon concentration at the peak of the implanted distribution does not saturate at a value equivalent to that in the stoichiometric compound, but continues to rise, reflecting the lower diffusivity of the C in the synthesized compound layer. To achieve chemical segregation of the implanted carbon, very-high-temperature (≥ 1300°C), long-time (typically 20 h) anneals are required. At the interface with the silicon substrate the synthesized layer grows with a degree of epitaxy. This is also found to occur during implantation if the temperature is ≥ 650° C.     

Surface exfoliation and defect structures in Si induced by 160 keV He and 110 keV H ion implantation

Cz n-type Si(100) wafers were implanted at room temperature with 160 keV He ions at a fluence of 5 × 10¹⁶/cm² and 110 keV H ions at a fluence of 1 × 10¹⁶/cm², singly or in combination. Surface phenomena and defect microstructures have been studied by various techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM). Surface exfoliation and flaking phenomena were only observed on silicon by successive implantation of He and H ions after subsequent annealing at temperatures above 400 °C. The surface phenomena show strong dependence on the thermal budget. At annealing temperatures ranging from 500 to 700 °C, craters with size of about 10 μm were produced throughout the silicon surface. As increasing temperature to 800 °C, most of the implanted layer was sheared, leaving structures like islands on the surface. AFM observations have demonstrated that the implanted layer is mainly transfered at the depth around 960 nm, which is quite consistent with the range of the ions. XTEM observations have revealed that the additional low fluence H ion implantation could significantly influence thermal growth of He-cavities, which gives rise to a monolayer of cavities surrounded by a large amount of dislocations and strain. The surface exfoliation effects have been tentatively interpreted in combination of AFM and XTEM results.     

Investigations of lattice location and mobility of carbon in GaAs using the channeling technique

In order to get information about the lattice location and the mobility of ¹²C in GaAs wafers, the channeling technique in combination with nuclear reaction analysis (NRA) is a powerful method. The targets were implanted with ¹²C ions (2.6 × 10¹³−2.6 × 10¹⁵cm⁻²) at energies between 60 and 3000 keV. This corresponds to implantation depths of about 0.1 to 3 μm. Using the nuclear reaction ¹²C(d, p)¹³C a depth distribution of the implanted carbon is obtained. The relationship between the concentration of ¹⁴C in random and along the 100 and 110 axial directions gives information about the substitutional lattice location of carbon within the GaAs crystal. In addition, we measured lattice defect depth distributions with a 1.5 MeV ⁴He⁺ beam before and after thermal annealing at temperatures up to 600°C.     

Ion beam synthesis and recrystallization of amorphous SiGe/SiC structures

Si_1−xGe_x amorphous layers implanted with different doses of carbon (between 5 × 10¹⁵ and 2 × 10¹⁷ cm⁻² and annealed at 700°C and 900°C have been analyzed by Raman and Infrared spectroscopies, electron microscopy and Auger electron spectroscopy. The obtained data show the synthesis of amorphous SiC by implanting at the highest doses. In these cases, recrystallization only occurs at the highest annealing temperature (900°C). The structure of the synthesized SiC strongly depends on the implantation dose, in addition to the anneal temperature. For the highest dose (2 × 10¹⁷ cm⁻²), crystalline β-SiC is formed. Finally, a strong migration of Ge towards the Si substrate is observed from the region where SiC precipitation occurs.     

Observation of micro-cavities in nickel during He and D irradiation and post-irradiation annealing

Changes in sizes and morphology of small cavities in nickel irradiated by 25 keV helium ions and 20 keV deuterons were investigated during irradiation and on annealing after irradiation by means of transmission electron microscopy. In the early stage of He⁺ irradiations at 600 and 700° C, roundish cubes appeared, gradually changed to octahedra. and, then, by the truncation of apexes, finally reached cubo-octahedra. Nucleation and growth behavior of cubic cavities in D⁺ irradiated nickel was different from the case of He⁺ irradiation. On annealing of the He⁺ irradiated specimen, only octahedral cavities showed marked growth, finally changing to roundish cubes at 750° C. Cavities of roundish cubes and cubo-octahedra did not grow nor change their shapes remarkably by the annealing. The cubic cavities formed by D⁺ irradiation at 360° C showed gradual shrinkage on annealing at 600° C and disappeared at 625° C. The changes of cavities during irradiation and on annealing were interpreted by the effect of the internal gas pressure.     

Low temperature epitaxial regrowth of mercury implanted sapphire

Hg ions were implanted into sapphire at room temperature and 80 keV energy to a fluence of 1 × 10¹⁵ Hg⁺ / cm². This fluence was enough to produce an amorphous surface layer. The annealing behaviour was studied combining RBS/channeling and hyperfine interaction techniques. Surprisingly, the RBS/channeling results show there is an epitaxial regrowth of the damaged layer after annealing at 800°C for 20 min. Although some of the implanted Hg segregates to the surface during the epitaxial regrowth, a significant fraction is incorporated into regular sites along the c-axis. The hyperfine interactions results, obtained after implantation of a dose of 5 × 10¹² Hg⁺ / cm², show that a small fraction of Hg is probably bound to oxygen. This result is in agreement with the RBS/channeling measurements which also show that the system formed after annealing is stable even at high temperatures.     

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.     

Effect of annealing on the optical absorption of Ni nanoparticles in MgO single crystals

Ni⁺ ion implantation with an energy of 64 keV in MgO single crystals was conducted at room temperature up to a fluence of 1 × 10¹⁷ ion/cm². The as-implanted crystals were annealed isochronally at temperatures up to 900 °C. Optical absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) have been utilized to characterize the changes of optical properties and the microstructure of the annealed samples. XPS results showed that the charge state of implanted Ni was still mainly in metallic Ni⁰ after annealing at 900 °C. TEM analysis revealed metallic Ni nanoparticles with depth-dependant dimensions of 1–10 nm in the annealed sample. Optical absorption spectroscopy indicated that the Ni nanoparticles exhibited a broad surface plasmon resonance absorption band in annealed samples and the band shifted to a longer wavelength with the increasing annealing temperature.     

Carbon clustering in Si1−xCx formed by ion implantation

Silicon-carbon alloys were formed by multiple energy implantation of C⁺ ions in silicon and in Silicon on Sapphire (SOS). The ion fluence ranged between 5 × 10¹⁶ − 3 × 10¹⁷ ions/cm² and the energy between 10–30 keV in order to obtain constant carbon concentration into a depth of 100 nm. The carbon atomic fraction (x) was in the range 0.22–0.59 as tested by Rutherford backscattering spectrometry (RBS). Thermal annealing of the implanted films induced a transition from amorphous to a polycrystalline structure at temperatures above 850°C as detected by Infrared spectrometry (IR) in the wavenumber range 600–900 cm⁻¹. The optical energy gap and the intensity of the infrared signal after annealing at 1000°C depended on the film composition: they both increased linearly with carbon concentration reaching a maximum at the stoichiometric composition (x = 0.5). At higher carbon concentration the IR intensity saturated and the optical energy gap decreased from the maximum value of 2.2 to 1.8 eV. The behaviour at the high carbon content has been related to the formation of graphitic clusters as detected by Raman spectroscopy.     

Electrical and optical properties of Co ion implanted a-Si1 − xCx:H alloys

Low resistivity a-Si_1 − xC_x:H alloy films have been formed by high dose Co⁺ ion implantation. The influence of the carbon content of the films on the resistivity has been studied and the lowest values, of the order of 10 Ω/Sq, have been observed for the carbon free films. Even lower resistivities, a further reduction of up to 50%, have resulted from annealing at temperatures up to 500°C. Changes in the optical and structural properties of the implanted a-Si_1 − xC_x:H films have been studied by means of IR and Raman spectroscopy. Results show that the implantation produces considerable structural and chemical modifications. The formation of, and the transition to, a possible CoSi₂ phase has been observed by examining the IR and Raman spectra as a function of implant dose.     

RBS studies of the lattice damage caused by 1 Me V Si implantation into Al0.3Ga0.7As/GaAs superlattices at elevated temperature

The lattice damage accumulation in GaAs and Al_0.3Ga_0.7As/GaAs superlattices by 1 MeV Si⁺irradiation at room temperature and 350°C has been studied. For irradiations at 350°C, at lower doses the samples were almost defect-free after irradiation, while a large density of accumulated defects was induced at a higher dose. The critical dose above which the damage accumulation is more efficient is estimated to be 2 × 10¹⁵ + Si/cm² for GaAs, and is 5 × 10¹⁵ Si/cm² for Al_0.8Ga_0.7As/GaAs superlattice for implantation with 1.0 MeV Si ions at 350°C. The damage accumulation rate for 1 MeV Si ion implantation in Al_0.3Ga_0.7As/GaAs superlattice is less than that in GaAs.     

XPS and optical studies of Xe-implanted and annealed YSZ single crystals

Xe⁺ ion implantation with 200 keV was completed at room temperature up to a fluence of 1 × 10¹⁷ ion/cm² in yttria-stabilized zirconia (YSZ) single crystals. Optical absorption and X-ray photoelectron spectroscopy (XPS) were used to characterize the changes of optical properties and charge state in the as-implanted and annealed crystals. A broad absorption band centered at 522 or 497 nm was observed in the optical absorption spectra of samples implanted with fluences of 1 × 10¹⁶ ion/cm² and 1 × 10¹⁷ ion/cm², respectively. These two absorption bands both disappeared due to recombination of color centers after annealing at 250 °C. XPS measurements showed two Gaussian components of O_1s spectrum assigned to Zr–O and Y–O, respectively, in YSZ single crystals. After ion implantation, these two peaks merged into a single peak with the increasing etching depth. However, this single peak split into two Gaussian components again after annealing at 250 °C. The concentration of Xe decreased drastically after annealing at 900 °C. And the XPS measurement barely detected the Xe. There was no change in the photoluminescence of YSZ single crystals with a fluence of 1 × 10¹⁷ ion/cm² after annealing up to 900 °C.     

Effect of forming gas anneals on the photoluminescence from nanocrystalline silicon formed by Si implantation into SiO2 matrix

The blue region of the room temperature photoluminescence spectrum from Si nanocrystallites formed in SiO₂ by Si⁺ ion implantation has been observed for the first time after annealing in a forming gas (10% H₂ + 90% N₂) ambient. Thermally grown SiO₂ on Si substrates were implanted with a dose of 2 × 10¹⁷ Si⁺ cm⁻² at energies of 200 keV and 400 keV. For reference purposes, quartz silica was implanted also with the same dose of 200 keV Si⁺ ions. The implanted samples were annealed in nitrogen and forming gas at 900°C for 3 to 180 min. Both the SiO₂ and quartz samples exhibited luminescence at about 380 nm which was weak, but detectable, before annealing. During extended anneals in forming gas, the intensity increased by a factor of about 2 above that recorded after a nitrogen anneal but the peak position was unchanged. The intensity was greater in samples annealed in forming gas which is due to the additional hydrogen. It would seem that this blue luminescence originates from new luminescent centres in the matrix caused by the Si⁺ ion implantation.     

Helium bubbles in nickel annealed at T>0.7Tm

Helium-induced cavities in fusion materials are considered to be detrimental. Valuable information on He bubbles and on the basic mechanisms underlying their evolution is obtained by post-implantation annealing, subsequent to He implantation at about 300 K. For Ni and Cu, it has been shown that up to annealing temperatures T_a≤0.7T_m (T_m is the melting point), highly overpressurized bubbles form in the volume and coarsen very slowly by migration and coalescence, whereas near vacancy sources the overpressure relaxes and the coarsening occurs rapidly by Ostwald ripening (OR) which leads to the appearance of small and large He cavities. Annealing of He loaded Ni at T_a from 0.72T_m to 0.92T_m leads to the formation of only one population of nearly equilibrium bubbles which is related to the recovery of the ability of dislocations to emit vacancies into their surroundings. The effective activation energy of the increase of the mean bubble radius was found to be 0.60±0.02 eV, which is lower than that for the OR at lower temperatures (1.1 eV). Analysis shows that the mechanism covering the coarsening of He bubbles at very high temperatures is still OR, but limited by the rate of ledge nucleation on the bubble walls.     

氦离子辐照烧结碳化硅损伤效应研究

利用中国科学院近代物理研究所320 kV高压平台提供的氦离子辐照烧结碳化硅,辐照温度从室温到1 000 ℃,辐照注量为1015~1017 cm-2。辐照完成后,进行退火处理,然后开展透射电子显微镜、拉曼光谱、纳米硬度和热导率测试。研究发现,烧结碳化硅中氦泡形核阈值注量低于单晶碳化硅。同时,氦泡形貌和尺寸与辐照温度、退火温度有关。另外,对辐照产生的晶格缺陷、元素偏析进行了研究。结果表明,辐照产生了大量的缺陷团簇,同时氦泡生长也会发射间隙子,在氦泡周围形成间隙型位错环。在晶界处,容易发生碳原子聚集。辐照导致材料先发生硬化而后发生软化,且热导率降低。     

Characterization of extended defects in SiGe alloys formed by high dose Ge implantation into Si

The synthesis of SiGe/Si heterostructures by Ge⁺ ion implantation is reported. 400 keV Ge⁺ ions were implanted at doses ranging from 3 × 10¹⁶ to 10 × 10¹⁶ ions/cm² into (001) Si wafers, followed by Si⁺ amorphisation and low temperature Solid Phase Epitaxial Regrowth (SPER). TEM investigations show that strained alloys can be fabricated if the elastic strain energy (E_el) of the SiGe layer does not exceed a critical value (E′_el) of about 300 mJ/m², which is independent of the implantation energy. Our analysis also suggests that “hairpin” dislocations are formed as strain relieving defects in relaxed structures. A “strain relaxation” model is proposed to explain their formation.     

Optical behaviour of Er doped rutile by ion implantation

Rutile single crystals were implanted at room temperature with fluences of 5 × 10¹⁵ Er⁺/cm² ions with 150 keV energy. Rutherford backscattering/channeling along the 0 0 1 axis reveals complete amorphization of the implanted region. Photoluminescence reveals the presence of an optical centre close to the intra-ionic emission of Er³⁺ in the as-implanted samples. After annealing at 800 °C in air no changes were observed in the aligned RBS spectrum. On the contrary, annealing in reducing atmosphere (vacuum) induces the epitaxy of the damage layer. These results are unexpected, since for implantations of other ions under the same conditions, epitaxial recrystallization of the damage region occurs at this temperature. On the other hand, photoluminescence studies show the presence of new Er-related optical centres with high thermal stability in the samples annealed under oxidizing conditions. Annealing at 1000 °C in vacuum leads to the complete recrystallization of the damaged region. At this temperature a large fraction of Er segregates to the surface.     

Temperature and dose dependences of nitrogen implantation into aluminium

It has been established that nitrogen implantation into metals can alter their surface properties such as friction, wear, corrosion, etc. Recent studies have shown that nitrogen implantation into aluminium leads to the formation of aluminium nitride which has interesting tribological, electronic and optical properties. For a given implantation energy, the characteristics of the nitrogen profile, e.g. thickness, shape and concentration, depend strongly on the experimental conditions during the implantation. In order to study the influence of the implantation parameters, aluminium samples have been bombarded with ¹⁵N⁺₂ of 100 keV to different doses ((1–20) × 10¹⁷ N⁺/cm²), at several temperatures (25–300° C). Distributions of the implanted species were investigated by nuclear reaction analysis (NRA) and by Rutherford backscattering spectroscopy (RBS). The chemical bonds of aluminium in the matrix were studied by using low-energy electron-induced X-ray spectroscopy (LEEIXS). It is shown that aluminium nitride is formed and that the nitrogen distribution presents a surface peak when the implantation temperature is higher than 200° C.     

Carbon ion-implanted optical waveguides in Nd:YLiF4 crystal: Refractive index profiles and thermal stability

We report on the optical planar waveguides in Nd:YLiF₄ laser crystals fabricated by 6.0 MeV C³⁺ ion implantation at doses of 1 × 10¹⁵ or 2.5 × 10¹⁵ ions/cm², respectively. The refractive index profiles, which are reconstructed according to the measured dark mode spectroscopy, show that the ordinary index had a positive change in the surface region, forming non-leaky waveguide structures. The extraordinary index is with a typical barrier-shaped distribution, which may be mainly due to the nuclear energy deposition of the incident ions into the substrate. In order to investigate the thermal stability of the waveguides, the samples are annealed at temperature of 200–300 °C in air. The results show that waveguide produced by higher-dose carbon implantation remains relatively stable with post-irradiation annealing treatment at 200 °C in air.     

Surface regrowth of Sb ion implanted Si(100)

Thermal regrowth of a Si(100) surface, damaged by 80 keV Sb implantation, was monitored by angular resolved photoemission (ARUPS), Rutherford backscattering (RBS) and channelling. It was found that regrowth in UHV at 650°C does not result in a well ordered surface. Annealing at higher temperatures (700–1100°C) results in densities of surface defects of (2.5 ± 0.4) × 10¹⁵ at./cm². A well ordered Si(100)2 × 1 reconstructed surface can be formed only after removal of a 10 nm thick layer by Ne ion bombardment, and heat treatment at 600°C. These observations can be explained by the formation of a surface layer with misoriented domains simultaneously with the solid phase epitaxy.