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.     

A comparative EPR study of ion implantation induced damage in Si, Si1−xGex (x ≠ 0) and SiC

Electron Paramagnetic Resonance (EPR) measurements have been made to investigate the build up of damage in silicon in relaxed crystalline Si_1−xGe_x (x = 0.04, 0.13, 0.24, 0.36) and in 6H-SiC as a result of increasing the ion dose from low levels (10¹² cm⁻²) up to values (10¹⁵ cm⁻²) sufficient to produce an amorphous layer. Si, Si_1−xGe_x (x ≠ 0) and SiC were implanted at room temperature with 1.5 MeV Si, 2 MeV Si and 0.2 MeV Ge ions respectively. A comparison is made between the ways in which the type and population of paramagnetic defects depend on ion dose for each material.     

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.     

Structural changes in anatase TiO2 thin films irradiated with high-energy heavy ions

In order to investigate possible structural changes due to high-density electronic excitation, anatase TiO₂ thin film specimens were irradiated with 230 MeV ¹³⁶Xe¹⁵⁺ ions and 200 MeV ¹⁹⁷Au¹³⁺ ions. X-ray diffraction (XRD) patterns were measured before and after irradiation. The intensity of the XRD peak assigned to the (0 0 4) planes of anatase TiO₂ decreases in an exponential manner as a function of ion-fluence. This result can be explained by the formation of the cylindrical damaged regions (i.e. ion tracks) with diameters of 9.6 and 16.3 nm for 230 MeV Xe and for 200 MeV Au ion irradiations, respectively. The difference in the track diameter between Xe ion irradiation and Au ion irradiation can be attributed to the difference in the electronic stopping power (and to the ion-velocity effect, if any). For 200 MeV Au ion irradiation, splitting of the (0 0 4) peak is observed. The original (0 0 4) TiO₂ peak remains in the same position, but the new peak shifts to higher angles as fluence increases.     

Argon irradiation damage at a Cu/Al2O3 interface for different alumina structures

The evolution of damages at a Cu/Al₂O₃ device interface after Ar⁺ irradiation, depending on alumina structure, and the effect of surface roughness on sputtering have been studied. A polycrystalline Cu/Al₂O₃ bilayer and polycrystalline Cu on amorphous alumina were irradiated with 400 keV Ar⁺ ion beam at doses ranging from 5 × 10¹⁶ to 10¹⁷ Ar⁺/cm² at room temperature. The copper layer thicknesses were between 100 and 200 nm. RBS analysis was used to characterize the interface modification and to deduce the sputtering yield of copper. The SEM technique was used to control the surface topography. A RBS computer simulation program was used to reproduce experimental spectra and to follow the concentration profile evolutions of different elements before and after ion irradiation. A modified TRIM calculation program which takes into account the sputtering yield evolution as well as the concentration variation versus dose gives a satisfactory reproduction of the experimental argon distribution. The surface roughness effect on sputtering and the alumina structure influence at the interface on mixing mechanisms are discussed.     

Radiation damage effects in CMR manganite materials

Polycrystalline La_0.5Pb_0.5Mn_1−xCr_xO₃ (x = 0.075 and 0.15) samples have been irradiated with 50 MeV Li³⁺ ions with different fluences and the effects on the transport properties have been studied by means of the temperature and magnetic field dependent resistivity measurements. Due to Li³⁺ ion irradiation, the resistivity increases and the metal–insulator transition temperature (T_mi) decreases. At low temperatures (below T_mi), a dominant contribution of the electron–magnon scattering process is observed for all the irradiated and unirradiated samples. The low temperature resistivity behavior as well as the magnetoresistance is modified due to irradiation. The changes in the magnetotransport properties due to irradiation have been compared with the changes caused due to Mn site substitution.     

Effect of N_2 and O_2 on OH radical production in an atmospheric helium microwave plasma jet

UV-pulsed laser cavity ringdown spectroscopy of the hydroxyl radical OH(A–X)(0–0)band in the wavelength range of 306–310 nm was employed to determine absolute number densities of OH in the atmospheric helium plasma jets generated by a 2.45 GHz microwave plasma source.The effect of the addition of molecular gases N_2 and O_2 to He plasma jets on OH generation was studied.Optical emission spectroscopy was simultaneously employed to monitor reactive plasma species.Stark broadening of the hydrogen Balmer emission line(H_β)was used to estimate the electron density nein the jets.For both He/N_2 and He/O_2 jets, newas estimated to be on the order of 10~(15)cm~(-3).The effects of plasma power and gas flow rate were also studied.With increase in N_2 and O_2 flow rates, netended to decrease.Gas temperature in the He/O_2 plasma jets was elevated compared to the temperatures in the pure He and He/N_2 plasma jets.The highest OH densities in the He/N_2 and He/O_2 plasma jets were determined to be 1.0?×10~(16)molecules/cm~3 at x?=?4 mm(from the jet orifice)and 1.8?×?10~(16)molecules/cm~3 at x=3 mm, respectively.Electron impact dissociation of water and water ion dissociative recombination were the dominant reaction pathways, respectively, for OH formation within the jet column and in the downstream and far downstream regions.The presence of strong emissions of the N_2~+ bands in both He/N_2 and He/O_2 plasma jets, as against the absence of the N_2~+ emissions in the Ar plasma jets, suggests that the Penning ionization process is a key reaction channel leading to the formation of N_2~+ in these He plasma jets.     

Synthesis of carbon-nitride films using a fast-switched dual-source low energy ion beam deposition system

Thin C_xN_y films were deposited in UHV using alternating low energy ion beams of C⁺ and N⁺ or N₂⁺ in the energy range of 5 to 100 eV. The ion beam deposition system is equipped with two Freeman ion sources, mass analysis and fast automated beam switching, allowing perpendicular bombardment of the target with a single ion beam at a time. The composition and density of the films were studied by ARS (in situ), XPS and RBS. The dependence of the film properties and growth mechanisms on ion energy, beam switching rate, and C-to-N arrival ratio have been investigated. The influence of the deposition parameters on the film stoichiometry is discussed. Exposure of the film to atmosphere leads to oxygen incorporation, resulting in a lowered surface concentration of nitrogen. The XPS N 1s and C Is binding energies vary in a relatively broad range indicating that several bond states may be present. The influence of the substrate material on film growth has also been studied. On Si{100}, film growth commences with the formation of an interfacial silicon nitride. No film growth was observed on gold, however deposition was possible on tantalum and molybdenum.     

Influence of SHI irradiation on the formation of buried SiC

Silicon carbide (SiC) precipitates buried in Si(1 0 0) substrates were synthesized by ion implantation of 50 keV and 150 keV C⁺ ions at different fluences. Two sets of samples were subsequently annealed at 850 °C and 1000 °C for 30 min. Fourier transform infrared (FTIR) spectroscopy studies and X-ray diffraction (XRD) analysis confirmed formation of β-SiC precipitates in the samples. Ion irradiation with 100 MeV Ag⁷⁺ ions at room temperature does not induce significant change in the precipitates. It could be interpreted from the FTIR observations that ion irradiation may induce nucleation in Si + C solution created by ion implantation of C in Si. Modifications induced by swift heavy ion irradiation are found to be dependent on implantation energy of C⁺ ions.     

Radiation damage induced by 5 keV Si ion implantation in strained-Si/Si0.8Ge0.2

The damage distributions induced by ultra low energy ion implantation (5 keV Si⁺) in both strained-Si/Si_0.8Ge_0.2 and normal Si are measured using high-resolution RBS/channeling with a depth resolution better than 1 nm. Ion implantation was performed at room temperature over the fluence range from 2 × 10¹³ to 1 × 10¹⁵ ions/cm². Our HRBS results show that the radiation damage induced in the strained Si is slightly larger than that in the normal Si at fluences from 1 × 10¹⁴ to 4 × 10¹⁴ ions/cm² while the amorphous width is almost the same in both strained and normal Si.     

Surface swelling of sapphire induced by MeV and GeV heavy ions

-Al₂O₃ single crystals were bombarded with MeV xenon ions from 10¹⁵ to 10¹⁷ ions cm⁻² and GeV uranium ions from 10¹¹ to 10¹³ ions cm⁻² to study the surface swelling of sapphire at 77 and 300 K due to atomic collision processes (Xe) and electronic energy loss processes in the 20–45 keV/nm regime (U). The induced damage was studied by channeling Rutherford backscattering. Surface swelling was measured with a profilometer. The step height induced by nuclear cascades of MeV xenon increases with the ion fluence and saturates. With GeV uranium, an electronic stopping power threshold for surface swelling was observed and the step height increased with the damage for dE/dx higher than this threshold.     

Defect evolution during ion bombardment of amorphous Si probed by in situ conductivity measurements

In this work we use in-situ conductivity measurements during ion irradiation as a sensitive probe of the defect structure of amorphous Si. Electronic transport in amorphous Si occurs by hopping at the high density ( 10²⁰ cm⁻³ eV⁻¹) of deep lying localized states introduced by the defects in the band gap. In-situ conductivity measurements allow to follow directly the defect generation and annihilation kinetics during and after ion bombardment of the material. Amorphous Si layers, patterned to perform conductivity measurements, were annealed at 500°C in order to reduce the defect density by about a factor of 5. Defects were subsequently reintroduced by high energy ion irradiation at different temperatures (77–300 K). During irradiation the conductivity of the layer increases by several orders of magnitude and eventually saturates. Turning off the beam results in a decrease of the conductivity by a factor of 2 in times as long as a few hours even at 77 K. The effects of different ions (He, C, Si, Cu, and Au) and different ion fluxes (10⁹–10¹² ions/cm² s) on these phenomena have been explored. These data give a hint on the mechanisms of defect production and annihilation and demonstrate a strong correlation between electrical and structural defects in amorphous silicon.     

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.     

Study of 160 MeV Ni ion irradiation effects on electrodeposited polypyrrole films

Conducting polymer polypyrrole thin films doped with LiCF₃SO₃, [CH₃(CH₂)₃]₄NBF₄ and [CH₃(CH₂)₃]₄NPF₆ have been electrodeposited potentiodynamically on ITO coated glass substrate. The polymer films are irradiated with 160 MeV Ni¹²⁺ ions at three different fluences of 5 × 10¹⁰, 5 × 10¹¹ and 3 × 10¹² ions cm⁻². An increase in dc conductivity of polypyrrole films from 100 S/cm to 170 S/cm after irradiation with highest fluence is observed in four-probe measurement. X-ray diffractogram shows increase in the crystallinity of the polypyrrole films upon SHI irradiation, which goes on increasing with the increase in fluence. Absorption intensity increase in the higher wavelength region is observed in the UV–Vis spectra. The SEM studies show that the cauliflower like flaky microstructure of the surface of polypyrrole films turns globular upon SHI irradiation at fluence 5 × 10¹¹ ions cm⁻² and becomes smooth and dense at the highest fluence used. The cyclic voltammetry studies exhibit that the redox properties of the polypyrrole films do not change much on SHI irradiation.     

Investigation of the radiation hardness on semiconductor devices using the ion micro-beam

Variation of the ion beam induced charge (IBIC) pulse heights due to ion irradiation was investigated on a Si pn diode and a 6H-SiC Schottky diode using a 2 Mev He⁺ micro-beam. Each diode was irradiated with a focused 2 MeV He⁺ micro-beam to a fluence in the range of 1×10⁹–1×10¹³ ions/cm². Charge pulse heights were analyzed as a function of the irradiation fluence. After a 2 MeV ion irradiation to the Si pn junction diode, the IBIC pulse height decreased by 15% at 9.2×10¹² ions/cm². For the SiC Schottky diode, with a fluence of 6.5×10¹² ions/cm², the IBIC pulse height decreased by 49%. Our results show that the IBIC method is applicable to evaluate irradiation damage of Si and SiC devices and has revealed differences in the radiation hardness of devices dependent on both structural and material.     

Electron microscopy and Rutherford backscattering spectrometry characterisation of 6H SiC samples implanted with He

6H SiC single crystals were implanted at room temperature with 1 MeV He⁺ up to a fluence of 2 × 10¹⁷ at./cm². RBS-channeling analysis with a 2 MeV He⁺ beam indicated the formation of extended defects or the generation of point defects at a constant concentration over a depth of about 1 μm. Electron microscopy characterisation revealed the presence of two amorphous buried layers at depths of about 1.75 and 4.8 μm. They are due to the implantation and to the analysing RBS beam, respectively. No extended planar or linear faults were found in the region between the surface and the first amorphous layer. However, at the surface, a 50 nm thick amorphous layer was observed in which crystalline inclusions were embedded. Electron diffraction and HREM data of the inclusions were typical for diamond. These inclusions were even found in the crystalline SiC material below this layer, however at a reduced density.     

钙钛矿太阳能光伏电池及其抗质子辐照性能的仿真研究

由于具有低成本、高性能等优点,目前钙钛矿太阳能电池被广泛关注。此外,SnO₂具有禁带宽度适中、载流子迁移率高、减反射性能优异等优点,因此被广泛用作电子传输层。本工作主要分析电子传输层厚度、空穴传输层厚度、钙钛矿吸收层厚度、环境温度、串联电阻以及并联电阻对器件性能的影响。研究结果表明,最优的SnO₂电子传输层厚度为40 nm,最优的Spiro-OMeTAD空穴传输层厚度为70 nm。随着钙钛矿吸收层厚度增加,太阳能电池的光电转化效率不断提高,当钙钛矿吸收层厚度达到650 nm时光电转化效率达到饱和。此外,环境温度的上升导致填充因子以及短路电流的下降,从而使得器件性能退化。太阳能电池的寄生串联电阻和并联电阻分别会导致短路电流和开路电压下降。最后,我们的仿真结果表明,FTO/SnO₂/CH₃NH₃PbI₃/Spiro-OMeTAD/Ag结构的钙钛矿太阳能电池可以耐受高达10¹³ cm^-2的质子束辐照。本研究为发展高性能钙钛矿太阳能电池及其空间应用提供了重要的理论指导。     

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.     

Atomic mixing at metal–oxide interfaces by high energy heavy ions

We study the atomic mixing at metal (Bi or Au)/oxide (SiO₂ or Al₂O₃) interfaces under 150–200 MeV heavy ion irradiation. Irradiation-induced interface mixing state is examined by means of Rutherford backscattering spectrometry (RBS). For Bi/Al₂O₃ interfaces, the heavy ion irradiations induce a strong atomic mixing and the amount of the mixing increases with increasing the electronic stopping power for heavy ions. By comparing the results with that for 3 MeV Si ion irradiation, we conclude that the strong atomic mixing observed at Bi/Al₂O₃ interfaces is attributed to the high-density electronic excitation. On the other hand, for other interfaces (Bi/SiO₂, Au/Al₂O₃ and Au/SiO₂), atomic mixing is rarely observed after the irradiation. The dependence of atomic mixing on combinations of irradiating ions and interface-forming materials is discussed.