Blue-cathodoluminescent layers synthesis by high-dose N, C and B SiO2 implantation

Thermal silicon oxide layers have been implanted at 600 °C with N⁺+C⁺, N⁺+B⁺ and N⁺+C⁺+B⁺ ions. Two different implantation doses have been chosen in order to introduce peak concentrations at the projected range comparable to the SiO₂ density. Some pieces of the samples have been annealed in conventional furnace at 1200 °C for 3 h. After annealing, cathodoluminescence measurements show in all cases a main broad band centered at 460 nm (2.7 eV). High doses of C implantation give rise to an intensity attenuation. Phases formed in the oxides have been investigated by Fourier transform infrared spectroscopy before and after annealing. The spectra suggest that N incorporates as BN and probably as a ternary BCN phase in the triply implanted samples, while C seems to bond mainly with B. Boron is also bonded to O in B-O-Si configuration. Depth structure and quantitative composition of the films were deduced from fittings of the spectroscopic ellipsometry measurements.     

Processing for optically active erbium in silicon by film co-deposition and ion-beam mixing

Techniques of film deposition by co-evaporation, ion-beam assisted mixing, oxygen ion implantation, and thermal annealing were been combined in a novel way to study processing of erbium-in-silicon thin-film materials for optoelectronics applications. Structures with erbium concentrations above atomic solubility in silicon and below that of silicide compounds were prepared by vacuum co-evaporation from two elemental sources to deposit 200-270 nm films on crystalline silicon substrates. Ar⁺ ions were implanted at 300 keV. Oxygen was incorporated by O⁺-ion implantation at 130 keV. Samples were annealed at 600 °C in vacuum. Concentration profiles of the constituent elements were obtained by Rutherford backscattering spectrometry. Results show that diffusion induced by ion-beam mixing and activated by thermal annealing depends on the deposited Si-Er profile and reaction with implanted oxygen. Room temperature photoluminescence spectra show Er³⁺ transitions in a 1480-1550 nm band and integrated intensities that increase with the oxygen-to-erbium ratio.     

Characterization of the laser ablation plasma used for the deposition of amorphous carbon

The plasma produced by laser ablation of a graphite target was studied by means of optical emission spectroscopy and a Langmuir planar probe. Laser ablation was performed using a Nd:YAG laser with emission at the fundamental line with pulse length of 28 ns. In this work, we report the behavior of the mean kinetic energy of plasma ions and the plasma density, as a function of the laser fluence (J/cm²), and the target to probe (substrate) distance. The characterized regimes were employed to deposit amorphous carbon at different values of kinetic energy of the ions and plasma density. The mean kinetic energy of the ions could be changed from 40 to 300 eV, and the plasma density could be varied from 1 × 10¹² to 7 × 10¹³ cm⁻³. The main emitting species were C⁺ (283.66, 290.6, 299.2 and 426.65 nm) and C⁺⁺ (406.89 and 418.66 nm) with the C⁺ (426.65 nm) being the most intense and that which persisted for the longest times. Different combinations of the plasma parameters yield amorphous carbon with different structures. Low levels (about 40 eV) of ion energy produce graphitic materials, while medium levels (about 200 eV) required the highest plasma densities in order to increase the CC sp³ bonding content and therefore the hardness of the films. The structure of the material was studied by means of Raman spectroscopy, and the hardness and elastic modulus by depth sensitive nanoindentation.     

Enhanced photoluminescence of Ar implanted sapphire before and after annealing

In this paper, we studied the changes in the photoluminescence spectra of the Ar⁺ ion implanted mono-crystalline sapphire annealed at different atmospheres and different temperatures. Single crystals of sapphire (Al₂O₃) with the (1 0 1¯ 0) (m-samples) orientation were implanted at 623 K with 110 keV Ar⁺ ions to a fluence of 9.5×10¹⁶ ions/cm². Photoluminescence measurement of the as-implanted sample shows a new emission band at 506 nm, which is attributed to the production of interstitial Al atoms. The intensity of emission band at 506 nm first increased then decreased with increase in annealing temperature. For the same annealing temperature, the intensity of PL peak at 506 nm of the sample annealed in air was higher than the sample annealed in vacuum. The experimental results show that the intensity of the PL peak at 506 nm of Ar-implanted sapphire can be enhanced by subsequent annealing with an enhancement of nearly 20 times. The influence of thermal annealing of the Ar-implanted samples on the new 506 nm emission band was discussed.     

Annealing and amorphous silicon passivation of porous silicon with blue light emission

The photoluminescence (PL) of the annealed and amorphous silicon passivated porous silicon with blue emission has been investigated. The N-type and P-type porous silicon fabricated by electrochemical etching was annealed in the temperature range of 700-900 °C, and was coated with amorphous silicon formed in a plasma-enhanced chemical vapor deposition (PECVD) process. After annealing, the variation of PL intensity of N-type porous silicon was different from that of P-type porous silicon, depending on their structure. It was also found that during annealing at 900 °C, the coated amorphous silicon crystallized into polycrystalline silicon, which passivated the irradiative centers on the surface of porous silicon so as to increase the intensity of the blue emission.     

Chemical reaction of sputtered Cu film with PI modified by low energy reactive atomic beam

Polyimide (PMDA-ODA) surface was irradiated by low energy reactive atomic beam with energy 160-180 eV to enhance the adhesion with metal Cu film. O₂⁺ and N₂⁺ ions were irradiated at the fluence from 5 × 10¹⁵ to 1 × 10¹⁸ cm⁻². Wetting angle 78° of distilled deionized (DI) water for bare PI was greatly reduced down to 2-4° after critical ion flounce, and the surface energy was increased from 37 to 81.2 erg/cm. From the analysis of O 1s core-level XPS spectra, such improvement seemed to result from the increment of hydrophilic carbonyl oxygen content on modified PI surface. To see more carefully correlation of the peel strength with interfacial reaction between Cu and PI, flexible copper clad laminate with Cu (9 μm)/Cu (200 nm) on modified PI substrate (25 μm) was fabricated by successive sputtering and electroplating. Firstly, peel strength was measured by using t-test and it was largely increased from 0.2 to 0.5 kgf/cm for Ar⁺ only irradiated PI to 0.72-0.8 kgf/cm for O₂⁺ or N₂O⁺ irradiated PI. Chemical reaction at the interface was reasoned by analyzing C 1s, O 1s, N 1s, and Cu 2p core-level X-ray photoelectron spectroscopy over the as-cleaved Cu-side and PI side surface through depth profiling. From the C 1s spectra of cleaved Cu-side, by the electron transfer from Cu to carbonyl oxygen, carbonyl carbon atom became less positive and as a result shifted to lower binding energy not reaching the binding energy of C₂ and C₃. The binding energy shift of the peak C₄ as small as 1.7 eV indicates that carbonyl oxygen atoms were not completely broken. From the analysis of the O 1s spectra, it was found that new peak at 530.5 eV (O₃) was occurred and the increased area of the peak O₃ was almost the same with reduced area of the peak carbonyl oxygen peak O₁. Since there was no change in the relative intensity of ether oxygen (O₂) to carbonyl oxygen (O₁), and thus O₃ was believed to result from Cu oxide formation via a local bonding of Cu with carbonyl oxygen atoms. Moreover, from X-ray induced Auger emission spectra Cu LMM which was very sensitive to chemical bonding, Cu oxide or CuOC complex formation instead of CuNO complex was clearly identified by the observation of the peak at 570 eV at higher 2 eV than that of metal Cu. In conclusion, when Cu atoms were sputtered on modified PI by low energy ion beam irradiation, it can be suggested that two Cu atoms locally reacted with carbonyl oxygen in PMDA units and formed Cu⁺O⁻C complex linkage without being broken from carbon atoms and thus the chemically bound Cu was in the form of Cu₂O.     

XPS study of silicon surface after ultra-low-energy ion implantation

Ultra-low-energy ion implantation of silicon with a hydrogen-terminated (0 0 1) surface was carried out using a mass-separated ³¹P⁺ ion beam. The ion energy was 30 eV, the displacement energy of silicon, and the ion doses were 6 × 10¹³ ions/cm². Annealing after the implantation was not carried out. The effects of ion implantation on the surface electrical state of silicon were investigated using X-ray photoelectron spectroscopy (XPS). The Si 2p peak position using XPS depends on the doping conditions because the Fermi level of the hydrogen-terminated silicon surface is unpinned. The Si 2p peak position of the specimen after ion implantation at a vacuum pressure of 3 × 10⁻⁷ Pa was shifted to the higher energy region. It suggested the possibility of phosphorus doping in silicon without annealing. In the case of ion implantation at 5 × 10⁻⁵ Pa, the Si 2p peak position was not shifted, and the peak was broadened because of the damage by the fast neutrals. Ultra-low-energy ion doping can be achieved at ultra-high-vacuum conditions.     

Analysis of the effect of annealing on the photoluminescence spectra of Cu ion implanted ZnS nanoparticles

In the present study, we report the photoluminescence (PL) study of nanoparticles of ZnS implanted with Cu⁺ ions at the doses of 5×10¹⁴, 1×10¹⁵ and 5×10¹⁵ ions/cm² and annealed at 200 and 300 °C. The photoluminescence spectra of the samples implanted at lower doses of 5×10¹⁴ and 1×10¹⁵ ions/cm² and annealed at 200 and 300 °C showed peaks at around 406, 418 and 485 nm. The PL emission peak at 485 nm was attributed to the transition of electrons from conduction band of ZnS to the impurity level formed by the implanted Cu⁺ ions. In the PL spectrum of the sample implanted at the highest dose of 5×10¹⁵ ions/cm², in addition to the emission peaks observed in the PL spectra of the samples implanted at lower doses, a peak at around 525 nm, the intensity of which decreased with increase in the annealing temperature, was observed. The emission peak at 525 nm was attributed to the transitions between sulfur and zinc vacancy levels. The full width at half maximum (FWHM) of the emission peak at 406 nm was observed to decrease with increase in annealing temperature, indicating lattice reconstruction. The observation of copper ion impurity related peak at 485 nm in the PL spectra of samples of the present study indicated that the doping of copper ions into the ZnS lattice is achievable by implanting Cu⁺ ions followed by annealing.     

Low energy oxygen implantation induced improved crystallinity and optical properties of surface modified ZnO single crystals

We report on the low energy oxygen implantation induced improvement in crystallinity and optical properties of surface modified ZnO single crystals. Undoped ZnO (0 0 0 1) single crystal wafers are implanted with 100 keV oxygen ions at a dose of 5 × 10¹³ and 5 × 10¹⁴ cm⁻² and subsequently annealed at 500 and 600 °C in oxygen ambient. The as-implanted and annealed ZnO wafers are studied by Rutherford back scattering spectrometry (RBS), channeling, Raman, photoluminescence (PL), and Fourier transform infrared spectroscopy (FTIR). Channeling studies show a relatively high χ_min (>20%) in the virgin ZnO wafer. After implantation and two-step annealing, RBS studies show improved crystallinity. Raman line width analysis for the mode indicates reduction in strain in the annealed samples as compared to the virgin ZnO wafer. As-implanted samples show drastic quenching of the near band-edge (NBE) PL band due to defects created by the implantation. However, after two-step annealing, the low-dose implanted sample show a five-fold increase in intensity ratio of NBE band (376 nm) to defect related broad band (∼530 nm) at room temperature. Implantation induced changes in the composition and improved crystallinity in the near surface region is accounted for the major improvement in the PL emission.     

Infrared luminescence from spark-processed silicon

The infrared (IR) photoluminescence (PL) emission of spark-processed silicon (sp-Si) was investigated. A broad and strong room temperature PL peak in the 945 nm (1.31 eV) spectral range was observed when sp-Si was excited with an argon laser. This peak is different from the PL commonly reported for anodically etched porous silicon and other silicon-based materials. The PL intensity increases substantially after annealing sp-Si between 350 and 500 °C in air after which it decreases again. The PL wavelength is observed to peak at 1010 nm by annealing sp-Si near 450 °C. It was further found that the most efficient PL occurs for a Si/O ratio of 0.3, for a small spark gap of about 1 mm, and for spark-processing times in the 15-60 min range.A model for the IR PL is proposed which mirrors that for visible PL. Specifically, it is proposed that the electrons which have been pumped by the laser from the ground state into a broad quasi-absorption band (or closely spaced absorption lines between 1.7 and 2.3 eV) revert back to lower IR levels at 1.31 eV by a non-radiative transition from where they revert radiatively to the ground state by emitting the observed 945 nm light.     

Modeling of the transient interstitial diffusion of implanted atoms during low-temperature annealing of silicon substrates

It has been shown that many of the phenomena related to the formation of “tails” in the low-concentration region of ion-implanted impurity distribution are due to the anomalous diffusion of nonequilibrium impurity interstitials. These phenomena include boron implantation in preamorphized silicon, a “hot” implantation of indium ions, annealing of ion-implanted layers et cetera. In particular, to verify this microscopic mechanism, a simulation of boron redistribution during low-temperature annealing of ion-implanted layers has been carried out under different conditions of transient enhanced diffusion suppression. Due to the good agreement with the experimental data, the values of the average migration length of nonequilibrium impurity interstitials have been obtained. It has been shown that for boron implanted into a silicon layer preamorphized by germanium ions the average migration length of impurity interstitials at the annealing temperature of 800 °C can be reduced from 11 nm to approximately 6 nm due to additional implantation of nitrogen. The further shortening of the average migration length is observed if the processing temperature is reduced to 750 °C. It is also found that for implantation of BF₂${\mathrm{BF}}_2$ ions into silicon crystal, the value of the average migration length of boron interstitials is equal to 7.2 nm for thermal treatment at a temperature of 800 °C.     

Influence of nitrogen ion implantation fluences on surface structure and tribological properties of SiC ceramics in water-lubrication

Nitrogen ions were implanted into SiC ceramics by using ion implantation technology (N⁺-SiC). The surface structure and chemical bonds of N⁺-SiC ceramics were determined by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), and their nanohardness was measured by nanoindenter. The friction and wear properties of the N⁺-SiC/SiC tribo-pairs were investigated and compared with those of SiC/SiC tribo-pairs in water using ball-on-disk tribo-meters. The wear tracks on the N⁺-SiC ceramics were observed by non-contact surface profilometer and scanning electron microscope (SEM) and their wear volumes were determined by non-contact surface profilometer. The results show that the N⁺-SiC ceramics were mainly composed of SiC and SiCN phase and SiN, CC, CN and CN bonds were formed in the implantation layer. The highest hardness of 22.3 GPa was obtained as the N⁺-SiC ceramics implanted at 50 keV and 1 × 10¹⁷ ions/cm². With an increase in nitrogen ion fluence, the running-in period of N⁺-SiC/SiC tribo-pairs decreased, and the mean stable friction coefficient decreased from 0.049 to 0.024. The N⁺-SiC ceramics implanted at 50 keV and 5 × 10¹⁷ ions/cm² exhibited the excellent tribological properties in water. In comparison of SiC/SiC ceramic tribo-pairs, the lower friction coefficient and lower wear rate for the N⁺-SiC/SiC tribo-pairs were acquired.     

Influence of the vacuum level upon the growth of carbon nanotubes on silicon carbide surface

We have investigated the influence of the vacuum level upon the growth of carbon nanotubes (CNTs) on 6H-SiC () surface.CNTs of about 160 nm in length were formed densely and uniformly on the 6H-SiC surface during annealing at 1700 °C in a high vacuum (∼10⁻² Pa). CNTs of about 1 μm in length were formed during annealing at 1700 °C in an ultra-high vacuum (∼10⁻⁷ Pa). However, CNTs were not formed and SiO₂ layers were formed on the SiC surface at 1700 °C in air. It is found that longer CNTs can grow up in an ultra-high vacuum, moreover, a little aligned and low-density graphite layers, or carbon nanofibers can also grow up.     

The effect of etching time of porous silicon on solar cell performance

Porous silicon (PS) layers based on crystalline silicon (c-Si) n-type wafers with (1 0 0) orientation were prepared using electrochemical etching process at different etching times. The optimal etching time for fabricating the PS layers is 20 min. Nanopores were produced on the PS layer with an average diameter of 5.7 nm. These increased the porosity to 91%. The reduction in the average crystallite size was confirmed by an increase in the broadening of the FWHM as estimated from XRD measurements. The photoluminescence (PL) peaks intensities increased with increasing porosity and showed a greater blue shift in luminescence. Stronger Raman spectral intensity was observed, which shifted and broadened to a lower wave numbers of 514.5 cm⁻¹ as a function of etching time. The lowest effective reflectance of the PS layers was obtained at 20 min etching time. The PS exhibited excellent light-trapping at wavelengths ranging from 400 to 1000 nm. The fabrication of the solar cells based on the PS anti-reflection coating (ARC) layers achieved its highest efficiency at 15.50% at 20 min etching time. The I-V characteristics were studied under 100 mW/cm² illumination conditions.     

Surface treatments toward obtaining clean GaN(0 0 0 1) from commercial hydride vapor phase epitaxy and metal-organic chemical vapor deposition substrates in ultrahigh vacuum

We studied processes of cleaning GaN(0 0 0 1) surfaces on four different types of wafers: two types were hydride vapor phase epitaxy (HVPE) free-standing substrates and two types were metal-organic chemical vapor deposition (MOCVD) films grown on these HVPE substrates and prepared by annealing and/or Ar ion sputtering in ultra high vacuum. We observed the surfaces through treatments using in situ low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), and Auger electron spectroscopy, and also using ex situ temperature programmed desorption, X-ray photoelectron spectroscopy, X-ray diffraction, and secondary ion mass spectrometry. For HVPE samples, we obtained relatively clean surfaces under optimized three-step annealing conditions (200 °C for 12 h + 400 °C for 1 h + 500 °C for 5 min) without sputtering, after which the surface contamination of oxide and carbide was reduced to ∼20% of that before annealing. Clear GaN(0 0 0 1)1×1 patterns were obtained by LEED and RHEED. STM images showed flat terraces of ∼10 nm size and steps of ∼0.5 nm height. Upon annealing the HVPE-GaN samples at a much higher temperature (C), three-dimensional (3D) islands with facets were formed and the surface stoichiometry was broken down with the desorption of nitrogen in the form of ammonia, since the samples include hydrogen as an impurity. Ar⁺ sputtering was effective for removing surface contamination, however, postannealing could not recover the surface roughness but promoted the formation of 3D islands on the surface. For MOCVD/HVPE homoepitaxial samples, the surfaces are terminated by hydrogen and the as-introduced samples showed a clear 1×1 structure. Upon annealing at 500-600 °C, the surface hydrogen was removed and a 3×3 reconstruction structure partially appeared, although a 1×1 structure was dominant. We summarize the structure differences among the samples under the same treatment and clarify the effect of crystal quality, such as dislocations, the concentration of hydrogen impurities, and the residual reactant molecules in GaN films, on the surface structure.     

Structural and optical characterization of Ce-doped Gd2SiO5 films by sol-gel technique

Cerium-doped Gd₂SiO₅ (GSO:Ce) films have been prepared on (1 1 1) silicon substrates by the sol-gel technique. Annealing was performed in the temperature range from 400 to 1000 °C. X-ray diffraction (XRD), and atomic force microscopy (AFM) were used to investigate the structure and morphology of GSO:Ce films. Results showed that GSO:Ce film starts to crystallize at about 600 °C, GSO:Ce films have a preferential (0 2 1) orientation, as the annealing temperature increase, the (0 2 1) peak intensity increases, the full width of half maximum (FWHM) decreases, and the grain size of GSO:Ce films increases. Emission spectra of GSO:Ce films were measured, results exhibit the characteristic blue emission peak at 427 nm.     

Dependence of photoluminescence from a-Si nanoparticles on the annealing time and exciting wavelength

Thin films of SiO_x having thickness of 0.2 μm and oxygen content x=1.5 or 1.7 are prepared by thermal evaporation of SiO in vacuum. Then some samples are furnace annealed for various times (in the range ) at 770 and 970 K and some others are rapid thermal annealed at 970 K for 30 and 60 s. Photoluminescence (PL) measurements are carried out at room temperature using the 442 nm line of a He-Cd laser and the 488 nm of an Ar laser for excitation. The effect of the annealing conditions and wavelength of the exciting light on the shape of the PL from these films is explored. The deconvolution of the PL spectra measured with the 442 nm line from samples annealed at 770 K for reveals two distinct PL bands peaked at around 2.3 and 2.5 eV, which do not shift appreciably with increasing annealing time. In addition, at longer annealing times, a weak third band is resolved centred in the range 2.0-2.1 eV. It exists in the spectra of all samples annealed at 970 K being more prominent in the samples with x=1.5. The intensity of this band shows different dependences on the annealing time in the films with different initial composition. The results obtained are discussed in terms of radiative recombination via defect states in the SiO_x matrix (the 2.5 eV band) or at the a-Si-SiO_x interface (the 2.3 eV band). The band centred in the 2.0-2.1 eV range is related to recombination in amorphous silicon nanoparticles grown upon annealing.     

Charge compensation on the luminescence properties of ZnWO4:Tb phosphors via hydrothermal synthesis

A phosphor Tb³⁺-doped ZnWO₄ (ZWO:Tb) phosphors were prepared by a hydrothermal method. X-ray powder diffraction (XRD) analysis revealed that the as-obtained sample is pure ZnWO₄ phase. The excitation and emission spectra indicated that the phosphor could be well excited by ultraviolet light (272 nm) and emit blue light at about 491 nm and green light at about 545 nm. Significant energy transfer from WO₄²⁻ groups to Tb³⁺ ions has been observed. Two approaches to charge compensation are investigated: (a) 2Zn²⁺ = Tb³⁺ + M⁺, where M⁺ is a monovalent cation like Li⁺, Na⁺ and K⁺ acting as a charge compensator; (b) 3Zn²⁺ = 2Tb³⁺ + vacancy. Compared with two charge compensation patterns in the ZnWO₄:Tb³⁺, it has been found that ZnWO₄:Tb³⁺ phosphors used Li⁺ as charge compensation show greatly enhanced bluish-green emission under 272 nm excitation.     

Low-loss amorphous silicon-on-insulator technology for photonic integrated circuitry

We report the fabrication of low-loss amorphous silicon photonic wires deposited by plasma enhanced chemical vapor deposition. Single mode photonic wires were fabricated by 193 nm optical lithography and dry etching. Propagation loss measurements show a loss of 3.46 dB/cm for photonic wires and 1.34 dB/cm for ridge waveguides.     

Nonlinear optical absorption and fluorescence of phosphine-substituted bithiophenes in the violet-blue spectral region

The nonlinear optical absorptions of two 5,5′-bis(diphenylphosphino)-2,2′-bithiophene derivatives, Ph₂(X)P(C₄H₂S)₂P(X)Ph₂ (X = O, 1; S, 2), have been investigated by direct transmission measurement with both picosecond and nanosecond laser pulses from 420 nm to 480 nm. Saturated dichloromethane solutions of 1 and 2 exhibit strong nonlinear optical absorptions in this violet-blue spectral region with that of 2 being stronger at all wavelengths. In the picosecond regime, at 420 nm, the transmittance rapidly falls to 50% when the incident fluence is 0.22 J/cm² for 1 and 0.11 J/cm² for 2. Two-photon absorption appears to be the primary mechanism for this nonlinear absorption. The two-photon absorption coefficients β for 1 (2.1 cm/GW) and 2 (4.4 cm/GM) were obtained by fitting the measurement of transmittance as the function of incident beam intensity at 420 nm. These β values are comparable with some of the best results obtained for organic materials in the green, red and infrared spectral region. Both compounds also show fluorescence with an emission peak at 390 nm for 1 and 400 nm for 2. The fluorescence of 1 is considerably stronger than is that of 2. The combination of the wide band gap and strong fluorescence emission of 1 makes it a promising candidate as a host material for blue organic light emitting diodes.