HRBS/channeling studies of ultra-thin ITO films on Si

High-resolution Rutherford backscattering spectroscopy (HRBS)/channeling techniques have been utilized for a detailed characterization of ultra-thin indium tin oxide (ITO) films and to probe the nature of the interface between the ITO film and the Si(0 0 1) substrate. Channeling studies provide a direct measure of the lattice strain distribution in the crystalline Si substrate in the case of amorphous over layers. The measurements on DC magnetron sputtered ITO films have been carried out using the recently installed HRBS facility at the Centre for Ion Beam Applications (CIBA). The thickness of the ultra-thin (∼9.8 nm) ITO films was calculated from the HRBS spectra having an energy resolution of about 1.4 keV at the superimposed leading (In + Sn) edge of the ITO film. The films were near stoichiometric and the interface between ITO film and Si was found to include a thin SiO_x transition layer. The backscattering yields from (In + Sn) of ITO were equal in random and channeling directions, thereby revealing the non-crystalline nature of the film. Angular scans of HRBS spectra around the off-normal [1 1 1] axis clearly showed a shift in the channeling minimum indicative of compressive strain of the Si lattice at the SiO_x/Si interface. The observed strain was about 0.8% near the interface and decreased to values below our detection limits at a depth of ∼3 nm from the SiO_x/Si interface.     

Interface strain study of thin Lu2O3/Si using HRBS

The interface of thin Lu₂O₃ on silicon has been studied using high-resolution RBS (HRBS) for samples annealed at different temperatures. Thin rare earth metal oxides are of interest as candidates for next generation transistor gate dielectrics, due to their high-k values allowing for equivalent oxide thickness (EOT) of less than 1 nm. Among them, Lu₂O₃ has been found to have the highest lattice energy and largest band gap, making it a good candidate for an alternative high-k gate dielectric. HRBS depth profiling results have shown the existence of a thin (∼2 nm) transitional silicate layer beneath the Lu₂O₃ films. The thicknesses of the Lu₂O₃ films were found to be ∼8 nm and the films were determined to be non-crystalline. Angular scans were performed across the [1 1 0] and [1 1 1] axis along planar channels, and clear shifts in the channeling minimum indicate the presence of Si lattice strain at the silicate/Si interface.     

Ion beam induced modifications in electron beam evaporated aluminum oxide thin films

Al₂O₃ thin films find wide applications in optoelectronics, sensors, tribology etc. In the present work, Al₂O₃ films prepared by electron beam evaporation technique are irradiated with 100 MeV swift Si⁷⁺ ions for the fluence in the range 1 × 10¹² to 1 × 10¹³ ions cm⁻² and the structural properties are studied by glancing angle X-ray diffraction. It shows a single diffraction peak at 38.2° which indicates the γ-phase of Al₂O₃. Further, it is observed that as the fluence increases up to 1 × 10¹³ ions cm⁻² the diffraction peak intensity decreases indicating amorphization. Surface morphology studies by atomic force microscopy show mean surface roughness of 34.73 nm and it decreases with increase in ion fluence. A strong photoluminescence (PL) emission with peak at 442 nm along with shoulder at 420 nm is observed when the samples are excited with 326 nm light. The PL emission is found to increase with increase in ion fluence and the results are discussed in detail.     

Differential cross-section measurements for the C(d, p0)C reaction and applications to surface analysis of materials

This work involves surface analysis by nuclear techniques, which are non-destructive, and computer simulation. The “energy analysis” method for nuclear reaction analysis is used. Energy spectra are computer simulated and compared to experimental data, giving target composition and concentration profile information. Measured values are presented for the differential cross-section of the ¹²C(d, p₀)¹³C reaction in the deuteron energy range 0.81-2.07 MeV for laboratory detection angles of 165° and 135°, using self-supported two-layered targets consisting of high purity thin films of typically 13 μg/cm² natural carbon and 65 μg/cm² gold. The error in the absolute differential cross-section values is generally ∼6%. The method, using these values, is successfully applied to determination of uniform concentration profiles of ¹²C, along considerable depths, for a thick flat target of high purity pyrolitic graphite. It is characterised a thin surface film of carbon on a thick flat quartz target. Uniform concentration profiles of ¹⁶O are also obtained from (d, p) and (d, α) reactions.     

Electronic and atomic structure modifications of copper nitride films by ion impact and phase separation

We have studied electronic and atomic structure modifications of Cu₃N films under 100 keV Ne and 100 MeV Xe ion impact. Cu₃N films were prepared on R(11-2 surface)-cut-Al₂O₃ substrates at 250 °C by using a RF-magnetron sputter deposition method. X-ray diffraction (XRD) shows that unirradiated films are polycrystalline with (1 0 0) orientation of cubic structure. We find that the electrical resistivity (∼10 Ω cm before ion impact) decreases by more than two orders of magnitude after the Ne impact at a fluence of ∼10¹³ cm⁻², where no Cu phase separation is observed. For further ion impact (larger than ∼10¹⁵ cm⁻²), XRD shows Cu diffraction peak (Cu phase separation), and the resistivity decreases further (three orders of magnitude). Decomposition and phase separation are discussed based on these results, as well as temperature dependence of the resistivity and optical absorption. The results of 100 MeV Xe ion impact are compared with those of Ne ion impact.     

Photon emission mechanisms of different phosphors

A comparison is made between the different mechanisms involved in the electron and photon excitation processes of four different phosphors. The green luminescence peak of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor is normally asymmetrical and tails towards the longer wavelengths. The observed peak in the longer wavelength region is associated with preferential alignment of one of the Eu²⁺ d-orbitals. Four Gaussian peaks fitted to the cathodoluminescent (CL) and photoluminescent (PL) spectra of broad band emission spectra of X₁-Y₂SiO₅:Ce phosphor may be attributed to the two different sites (A1 and A2) of the Ce³⁺ ion in the host matrix and the difference in orientation of the neighbour ions in the complex crystal structure. Co-activation of Ce³⁺ with Eu³⁺ quenches the red emission from Eu³⁺ and considerably increases the blue emission from Ce³⁺ in a SiO₂ matrix. In nanoparticulate PbS the emission data show a blue shift from the normal emission at ∼3200 nm in PbS bulk to ∼560-700 nm. The blue shifting of the emission wavelengths from 3200 to ∼560-700 nm is attributed to quantum confinement of charge carriers in the restricted volume of nanoparticles.     

Desorption of gold nanoclusters (2-30 nm) under low excitation of their electronic subsystem by Ar ions (14 keV/nm)

Gold nanodispersed targets with islands-grains sized 2-30 nm were irradiated by Ar⁷⁺ ions with the energy of 45.5 MeV and (dE/dx)_e = 14.2 keV/nm in gold. The desorbed gold nanoclusters were studied by TEM method. For all the targets desorption of intact gold nanoclusters is observed. However, for inelastic stopping of monatomic Ar ions in gold of 14.2 keV/nm desorption of nanoclusters is observed only up to ∼25 nm. The yield of the desorbed nanoclusters considerably decreases from 3 to 0.02 cluster/ion with the increase of the mean size of the desorbed nanoclusters from 3 to 14.2 nm. The results are discussed.     

Middle-frequency magnetron sputtering for GaN growth

A middle-frequency magnetron sputtering system was designed and constructed for GaN growth, in which a pair of back cooled pool-shaped twin magnetrons were used for Ga metal targets. GaN films were prepared using this system under various gas pressure (0.5-3.0 Pa) in a mixture of N₂ and Ar with N₂/Ar ratio of 6:1. X-ray diffraction showed that the GaN films had a strong (0 0 0 2) orientation, and the film deposited at 1.5 Pa had two more weak peaks attributed to and . The full width at half maximum (FWHM) of the (0 0 0 2) peak for the GaN film deposited at 1.5 Pa and 0.5 Pa is ∼721 and ∼986 arcsec, respectively. The deposition rate was in the range of 43.5-87.8 nm/min and was mainly influenced by the deposition pressure. The films deposited at higher pressures are columnar in structure. A root-mean-square roughness of 4.4 nm was obtained from the atomic force microscopy (AFM) surface morphology of the film deposited at 0.5 Pa.     

Electroluminescence from metal-oxide-silicon tunneling diode with ion-beam-synthesized β-FeSi2 precipitates embedded in the active region

A metal-oxide-silicon (MOS) tunneling light-emitting diode is fabricated with ion-beam-synthesized β-FeSi₂ precipitates embedded in the active region. Fe ions were implanted into p⁻-100 silicon substrate at cryogenic temperature (∼−120 °C), followed by rapid thermal oxidation (RTO). Under constant voltage biased in accumulation and at temperatures down to 80 K, electroluminescence (EL) with wavelength peaking at ∼1.5 μm is observed at a current density of about 2.0 A/cm². Light output increases linearly with current density. Temperature dependence of the EL shows that the luminescence is due to interband recombination in the crystalline precipitates. The strain in these isolated precipitates may contribute to the luminescence properties of β-FeSi₂ in silicon.     

Large-area and high-density silicon nanocone arrays by Ar sputtering at room temperature

The large-area, high-density of ∼1-2 × 10⁹/cm² silicon nanocone arrays by ion-irradiation with incident angle of 75° have been achieved by using carbon-cone-mask. The scanning electron microscopy (SEM) images show that the width of silicon nanocones is ∼150 nm and the height is ∼400 nm. The investigation of SEM shows that the formation of the silicon nanocones proceeds through three periods, carbon nanocones-nanocones with carbon on the top and silicon at the bottom-silicon nanocones.     

Irradiation effect on dielectric properties and electrical conductivity of Au/SiO2/n-Si (MOS) structures

The Au/SiO₂/n-Si (MOS) structures were exposed to beta-ray irradiation to a total dose of 30 kGy at room temperature. Irradiation effect on dielectric properties of MOS structures were investigated using capacitance−voltage (C−V) and conductance−voltage (G/ω−V) characteristics. The C−V and G/ω−V measurements carried out in the frequency range from 1 kHz to 10 MHz and at various radiation doses, while the dc voltage was swept from positive bias to negative bias for MOS structures. The dielectric constant (ε′), dielectric loss (ε″), loss factor (tan δ) and ac electrical conductivity (σ_ac) were calculated from the C−V and G/ω−V measurements and plotted as a function of frequency at various radiation doses. A decrease in the ε′ and ε″ were observed when the irradiation dose increased. The decrease in the ε′ and ε″ of irradiated MOS structures in magnitude is explained on the basis of Maxwell−Wagner interfacial polarization. Also, the σ_ac is found to decrease with increasing radiation dose. In addition, the values of the tan δ decrease with increasing radiation dose and give a peak. From the experimental results, it is confirmed that the peak of loss tangent is due to the interaction between majority carriers and interface states which induced by radiation.     

Structural studies of silicon oxynitride layers formed by low energy ion implantation

Silicon oxynitride (Si_xO_yN_z) layers were synthesized by implanting ¹⁶O₂⁺ and ¹⁴N₂⁺ 30 keV ions in 1:1 ratio with fluences ranging from 5 × 10¹⁶ to 1 × 10¹⁸ ions cm⁻² into single crystal silicon at room temperature. Rapid thermal annealing (RTA) of the samples was carried out at different temperatures in nitrogen ambient for 5 min. The FTIR studies show that the structures of ion-beam synthesized oxynitride layers are strongly dependent on total ion-fluence and annealing temperature. It is found that the structures formed at lower ion fluences (∼1 × 10¹⁷ ions cm⁻²) are homogenous oxygen-rich silicon oxynitride. However, at higher fluence levels (∼1 × 10¹⁸ ions cm⁻²) formation of homogenous nitrogen rich silicon oxynitride is observed due to ion-beam induced surface sputtering effects. The Micro-Raman studies on 1173 K annealed samples show formation of partially amorphous oxygen and nitrogen rich silicon oxynitride structures with crystalline silicon beneath it for lower and higher ion fluences, respectively. The Ellipsometry studies on 1173 K annealed samples show an increase in the thickness of silicon oxynitride layer with increasing ion fluence. The refractive index of the ion-beam synthesized layers is found to be in the range 1.54-1.96.     

Effect of crystallinity on the memory effect of Ge nanocrystals synthesized by atom beam sputtering

The effect of crystallinity of Ge nanocrystals on the charge storage properties of the metal oxide semiconductor (MOS) structure has been investigated. MOS structure with Ge nanocrystals embedded in the oxide has been fabricated by using atom beam sputtering technique. After annealing at 600 °C in Ar + H₂ atmosphere, capacitance-voltage (C-V) measurements show flat band voltage shift of ∼0.9 V. It which is a clear indication of the memory effect of Ge nanocrystals, while unannealed structure doesnot show any hysteresis in the C-V curve. Micro Raman spectroscopy and X-ray diffraction (XRD) analyses show that crystalline content of Ge nanoparticles in the MOS structure has increased after annealing.     

Tailoring optical and electrical properties of MgO thin films by 1.5 MeV H implantation to fluences

Thin films of magnesia (MgO) with (1 0 0) dominant orientations were implanted with 1.5 MeV H⁺ ions at room temperature to various fluences of 10¹³, 10¹⁴ and 10¹⁵ ions/cm². X-ray analysis unambiguously showed crystallinity even after a peak damage fluence of 10¹⁵ ions/cm². Rutherford backscattering spectrometry combined with ion channeling (RBS/C) was used to analyze radiation damages and defect distributions. Optical absorption band observed at 5.7 eV in implanted films was assigned to the anion vacancies and the defect was completely disappeared on annealing at 450 °C. Number of F-type defects estimated was 9.42 × 10¹⁵ cm⁻² for the film implanted with 10¹⁵ ions/cm². DC electrical conductivity of 4.02 × 10⁻⁴ S cm⁻¹ was observed in the implanted region which was three orders higher than the as-deposited films. In unison, film surface was modified as a result of the formation of aggregates caused by the atomic mixing of native matrix atoms (Mg and O) and precipitated hydrogen.     

Ion-induced electron emission monitoring of the structure and morphology evolution in HOPG

The temperature dependences of the ion-induced electron emission yield γ of highly-oriented pyrolytic graphite (HOPG) under high-fluence (10¹⁸-10¹⁹ ions/cm²) 30 keV Ar⁺ ion irradiation at ion incidence angles from θ = 0^o (normal incidence) to 80^o have been measured to trace both the structure and morphology changes in the basal oriented samples. The target temperature has been varied during continuous irradiation from T = −180 to 400 ^oC. The surface analysis has been performed by the RHEED and SEM techniques. The surface microgeometry was studied using laser goniophotometry (LGF). The dependences of γ(T) were found to be strongly non-monotonic and essentially different from the ones for Ar⁺ and N₂⁺ ion irradiation of the polygranular graphites. A sharp peak at irradiation temperature T_p ≈ 150 ^oC was found. A strong influence of electron transport anisotropy has been observed, and ion-induced microgeometry is discussed.     

Swift heavy ion irradiated InGaAs/InP multi quantum wells: Band-structure, interface and surface modifications

The band-structure, interface and surface modification by swift heavy ion irradiation of In_0.55Ga_0.45As/InP multi quantum wells have been studied using photoluminescence, high resolution X-ray diffraction and atomic force microscopy. Three distinct photoluminescence peaks were observed for as-grown samples at low temperature and at room temperature the peaks merge together. Detailed analysis has been carried out to understand the origin of additional satellite peaks. A peak shift of about 23 nm was observed for irradiated samples after annealing. Highly-ordered satellite peaks were observed in X-ray scans of as-grown and Ag ion irradiated samples. In comparison, Au ion irradiated sample showed stronger interfacial degradation as seen by the diminished satellite peaks. The peak position of the irradiated samples shifted to the compressive side and was broadened in comparison with as-grown samples. The as-grown and annealed samples show smooth surfaces whereas irradiation results in nano-sized dot/island types of structures at the surface. The results are discussed in the light of complementary information provided by these techniques.     

Characteristics of the electron-emission defects introduced in Si-SiO2 structures by MeV electron irradiation

Defects induced by high-energy electrons in Si-SiO₂ structure have been studied by the optically stimulated electron emission (OSEE) method. Si-SiO₂ structures with oxide thickness of 100 nm are irradiated with 23 MeV electrons for different durations. It is shown that most of the defects created by electron irradiation at the interface and in the oxide bulk are vacancies like E′-centers. Most of the photoemission activity changes are observed during low doses electron irradiation. Some uncharged defects like diamagnetic oxygen-deficient centers are also observed, together with E′-centers.     

Alkali ion scattering from Ag(0 0 1) and Ag thin films at low and hyperthermal energies

We have investigated the scattering of K⁺ and Cs⁺ ions from a single crystal Ag(0 0 1) surface and from a Ag-Si(1 0 0) Schottky diode structure. For the K⁺ ions, incident energies of 25 eV to 1 keV were used to obtain energy-resolved spectra of scattered ions at θ_i = θ_f = 45°. These results are compared to the classical trajectory simulation safari and show features indicative of light atom-surface scattering where sequential binary collisions can describe the observed energy loss spectra. Energy-resolved spectra obtained for Cs⁺ ions at incident energies of 75 eV and 200 eV also show features consistent with binary collisions. However, for this heavy atom-surface scattering system, the dominant trajectory type involves at least two surface atoms, as large angular deflections are not classically allowed for any single scattering event. In addition, a significant deviation from the classical double-collision prediction is observed for incident energies around 100 eV, and molecular dynamics studies are proposed to investigate the role of collective lattice effects. Data are also presented for the scattering of K⁺ ions from a Schottky diode structure, which is a prototype device for the development of active targets to probe energy loss at a surface.     

Effect of swift heavy ion irradiation on nanocrystalline CaS:Bi phosphors: Structural, optical and luminescence studies

Luminescence studies of CaS:Bi nanocrystalline phosphors synthesized by wet chemical co-precipitation method and irradiated with swift heavy ions (i.e. O⁷⁺-ion with 100 MeV and Ag¹⁵⁺-ion with 200 MeV) have been carried out. The samples have been irradiated at different ion fluences in the range 1 × 10¹²-1 × 10¹³ ions/cm². The average grain size of the samples before irradiation was estimated as 35 nm using line broadening of XRD (X-ray diffraction) peaks and TEM (transmission electron microscope) studies. Our results suggest a good structural stability of CaS:Bi against swift heavy ion irradiation. The blue emission band of CaS:Bi³⁺ nanophosphor at 401 nm is from the transition ³P₁ → ¹S₀ of the Bi³⁺. We have observed a decrease in lattice constant (a) and increase of optical energy band gap after ion irradiation. We presume this change due to grain fragmentation by dense electronic excitation induced by swift heavy ion. We have studied the optical and luminescent behavior of the samples by changing the ion energy and also by changing dopant concentration from 0.01 mol% to 0.10 mol%. It has been examined that ion irradiation enhanced the luminescence of the samples.     

Incidence-angle dependent Cs incorporation in Si during low-energy bombardment: A dynamic computer simulation study

The implantation of Cs atoms in silicon was investigated by dynamic computer simulations using the Monte-Carlo code T-DYN that takes into account the gradual change of the target composition due to the Cs irradiation. The incorporation of Cs atoms was studied for incidence angles ranging from 0° to 85° and for four impact energies (0.2, 0.5, 1 and 3 keV). The total implantation fluences were (1-2) × 10¹⁷ Cs/cm², well above the values required to reach a stationary state. The steady-state Cs surface concentrations exhibit a pronounced dependence on impact angle and energy. At normal incidence, they vary between ∼0.57 (at 0.2 keV) and ∼0.18 (3 keV), but decrease with increasing incidence angle. Under equilibrium, the partial sputtering yield of Si exhibits the typical dependence on incidence angle, first increasing up to a maximum value (at ∼70°-75°) and declining sharply for larger angles. For all irradiation conditions a strongly preferential sputtering of Cs as compared to Si atoms is found, increasing with decreasing irradiation energy (from 4.6 at 3 keV to 7.2 at 0.2 keV) and for nearer-normal incidence.