Amorphous-like Raman spectra of semiconductor microcrystals

The size dependence of Raman scattering from gas-evaporated Si, Ge and GaP microcrystals, for which a free boundary condition can be assumed, has been investigated. As the microcrystals become sufficiently small (smaller than about 100A), spectra very similar to those of amorphous materials are observed, even though electron microscopy proves that they are crystalline. The amorphous-like Raman signals are believed to come from the surface layers of microcrystals.     

Mechanism of necking as revealed by ultrasonic cavitation of polyethylene single crystals

Ultrasonic cavitation of polyethylene single crystals and single-crystal aggregates in the form of cakes results in lamella fragmentation and necking involving the transformation of lamellar crystals into fibrillar crystals between 20 to 400 Å in diameter. The smaller fibrils (～20–30 Å) have a very smooth appearance, whereas the larger ones (～100–400 Å) contain a beady structure about 100 Å periodically spaced along the fibrils. The smoother microfibrils are suggested to contain extended chains that are formed by unfolding of molecules directly from the chain-folded lamellae as well as from the folded-chain crystals contained within the beady fibrils. The presence of the chain-folded crystals within the larger beady fibrils is shown in numerous instances to be due to incorporation of mosaic crystalline blocks originally present, but weakly connected to one another, in the lamellar single crystals. The necking process is deduced by observation to involve primarily a mechanical shearing of mosaic crystalline blocks along the c-axis plus a rotation into the fibril direction. Observation of extreme resistance of lamellae in the overgrowth regions to cavitation damage suggests the presence of tie molecules and/or interpenetrating cilia between these lamellae. The suggestion finds strong support from additional studies carried out on lamellae that have been tied together at the folds by cross-linking with γ rays.     

Structural and vibrational properties of PVT grown Bi2Te3 microcrystals

High-quality Bi₂Te₃ microcrystals have been grown by physical vapor transport (PVT) method without using a foreign transport agent. The microcrystals grown under optimal temperature gradient are well facetted and they have dimensions up to ～100 μm. The phase composition of grown crystals has been identified by X-ray single crystal structure analysis in space group R3?m, a=4.3896(2) Å, b=30.5019(10) Å, Z=3 (R=0.0271). Raman microspectrometry has been used to describe the vibration parameters of Bi₂Te₃ microcrystals. The FWHM parameters obtained for representative Raman lines at 61 cm^?1 and 101 cm^?1 are as low as 3.5 cm^?1 and 4.5 cm^?1, respectively.     

Raman scattering from GaP nanowires

A complete Raman study of GaP nanowires is presented. By comparison with the Raman spectra of GaP bulk material, microcrystals and nanoparticles, we give evidence that the Raman spectrum is affected by the one-dimensional shape of the nanowires. The Raman spectrum is sensitive to the polarization of the laser light. A specific shape of the overtones located between 600 and 800 cm^-1 is actually a signature of the nanowires. Some phonon confinement and thermal behavior is also observed for nanowires.     

A Rutherford scattering study of the chemical composition of native oxides on GaP

The chemical compositions of two types of protective oxide films on single crystal GaP have been studied by Rutherford backscattering (2 MeV He⁺) combined with ion channeling. Films of 350–1400 Å thickness were grown by immersing GaP slices in hydrogen peroxide, with or without externally applied anodic bias. Films grown by a galvanic coupling process have compositions of about 1:1·1:4·5 (Ga:P:O) and are believed to be vitreous mixtures of Ga₂O₃+P₂O₅. Films grown with anodic bias exhibited a deficiency of Ga in a ～200 Å region at the GaP-oxide interface.     

Surface disorder in c-Si induced by swift heavy ions

The disorders induced in crystalline silicon (c-Si) through the process of electronic energy loss in the swift heavy ion irradiation were investigated. A number of silicon <1 0 0> samples were irradiated with 65 MeV oxygen ions at different fluences, 1×10¹³ to 1.5×10¹⁴ ions/cm², and characterized by the Raman spectroscopy, the optical reflectivity, the X-ray reflectivity, the atomic force microscopy (AFM) and the X-ray diffraction (XRD) techniques. The intensity, redshift, phonon coherence length and asymmetric broadening associated with the Raman peaks reveal that stressed and disordered lattice zones are produced in the surface region of the irradiated silicon. The average crystallite size, obtained by analyzing Raman spectrum with the phonon confinement model, was very large in the virgin silicon but decreased to<100 nm dimension in the ion irradiated silicon. The results of the X-ray reflectivity, AFM and optical reflectivity of 200–700 nm radiation indicate that the roughness of the silicon surface has enhanced substantially after ion irradiation. The diffusion of oxygen in silicon surface during ion irradiation is evident from the oscillation in the X-ray reflectivity spectrum and the sharp decrease in the reflectivity of 200–400 nm radiation. The rise in temperature, estimated from the heat spike model, was high enough to melt the local silicon surface. The results of XRD indicate that lattice defects have been induced and a new plane <2 1 1> has been formed in the silicon <1 0 0>after ion irradiation. The results of the present study show that the energy deposited in crystalline silicon through the process of electronic energy loss ～0.944 keV/nm per ion is sufficient to induce disorders of appreciable magnitude in the silicon surface even at a fluence of ～10¹³ ions/cm².     

The role of phase-matching and nanocrystal-size effects in three-wave mixing and CARS processes in porous gallium phosphide

Nonlinear-optical interactions, such as second-harmonic and sum-frequency generation and coherent anti-Stokes Raman spectroscopy (CARS), are investigated in porous GaP for the first time by means of a novel laser source based on mode-locked picosecond Nd³⁺:YVO₄ laser and subsequent continuum generation in an optical fiber. The efficiency of the former two nonlinear optical processes is shown to be strongly dependent on the wavelengths of the interacting waves and tends to increase with the decrease of the excitation wavelength. The power of the generated second-harmonic and sum-frequency increases by a factor of 2 and 30, respectively, compared to the crystalline GaP. In contrast, the CARS signal in porous GaP is found to be less efficient than one in crystalline GaP. The observed results are explained in terms of competition of the phase-matching effects in GaP nanocrystals and the enhanced photon lifetime in scattering porous GaP layers. PACS 42.25.Dd; 42.65.Ky; 42.70.Nq; 81.07.Bc     

Absolute photoabsorption cross sections for H2O and D2O from λ 180–790 Å

Absolute photoabsorption cross sections for H₂O and D₂O have been measured photoelectrically from λλ 180 to 790 Å using synchrotron radiation. The cross sections increase smoothly with wavelength to ～λ610 Å, with both H₂O and D₂O displaying a broad absorption band extending above a nearly linear background from λλ 400 to 490 Å. The continuum has a maximum of ～ 22.5 Mb at λ 640 Å. Above λ 615 Å, superimposed on the continuum, a diffuse structure appears which is similar to the vibrational structure of the ²B₂ states of H₂O⁺ and D₂O⁺ as observed in photoelectron spectra. The structure is believed to arise from excitation of a 1b₂ electron to the vibrational levels of a Rydberg orbital with n¹ ≈ 2.64.     

Fabrication and characterization of GaP/polymer nanocomposites for advanced light emissive device structures

GaP nanoparticles have been prepared using white P and a mild aqueous synthesis at decreased temperature followed by ultrasonication and stored as the suspension in water–ethanol mixture. They were characterized by standard methods of X-ray diffraction, transmission electron microscopy, Raman light scattering, and photoluminescence. Properties of GaP nanoparticles were compared with industrial and specially grown perfect GaP single crystals. It was shown that the GaP nanoparticles in suspension are the most suitable for high quality GaP/polymers nanocomposites because only they are uniform with dimensions of about 10 nm which is optimal for appearance of the pronounced quantum confinement effect. Polyglycidyl methacrylate (PGMA), polyglycidyl methacrylate-co-polyoligoethyleneglycol methacrylate (PGMA-co-POEGMA), and biphenyl vinyl ether (BPVE) polymers were used to prepare GaP polymer nanocomposites. The thickness of the polymer nanocomposite film was about 250–300 nm defined from AFM scratch experiment. The resulting nanocomposites yielded a bright luminescence at room temperature in a broad band with the maximum ranging from 2.5 to 3.2 eV and showed pronounced quantum confinement effects and other interesting and important for application phenomena leading to dramatic 1 eV expansion of GaP luminescence to the UV spectral region.     

Coherent anti-Stokes Raman scattering and normal anti-Stokes Raman scattering due to LO phonons in GaP using cw diode lasers

Coherent anti-Stokes Raman scattering (CARS) and normal anti-Stokes Raman scattering (NARS) have been measured in (001) GaP at room temperature due to the 403 cm⁻¹ LO phonons using a continuous wave (CW) 785.0 nm fixed-wavelength pump laser and a CW Stokes laser tunable in the 800-830 nm wavelength range. CARS measurements are normally made using pulsed lasers. The use of CW diode lasers allows a more accurate comparison between the measured and calculated values of the CARS signal. The pump and Stokes laser beams were linearly polarized perpendicular to each other, same as the pump and normal Stokes/anti-Stokes scattered light for the GaP sample used in this work. The pump and Stokes laser powers incident upon the GaP sample, located in the focal plane of a 20 mm effective focal length lens, were <20 and 50 mW, respectively. The diameter of the laser beams in the focal plane of the focusing lens was determined to 40±5 μm. The pump and Stokes laser beam intensities incident upon the 0.3 mm thick GaP sample were <2 and 5 kW cm², respectively. The powers of the CARS and NARS signals were measured using a Raman spectrometer. The signal output of the Raman spectrometer was calibrated using the pump laser and several neutral density filters. The Raman linewidth (full-width at half-maximum) of the LO phonons was determined to be 0.95±0.05 cm⁻¹, using the variation of the CARS signal with the wavelength of the Stokes laser. The measured powers of the CARS and NARS signals are about a factor of 5 and 1.5, respectively, smaller than those calculated from the corresponding theoretical expressions.     

Investigations of two-level tunneling systems in n-irradiated quartz by diffuse x-ray scattering

Abstract

Recent observation of the acoustic saturation and of phonon echoes in crystalline quartz after light irradiation with fast neutrons indicates that there are the same low-energy atomic tunnelling systems as in Si0₂ glass.

We report measurements of the diffuse X-ray scattering and of the lattice parameter change from neutron irradiated quartz single crystals studying the strength and symmetry of strain defects and their local correlations. For dose 3 × 10¹⁸ n/cm² the diffuse scattering close to the Bragg peaks together with the lattice parameter change shows that the neutrons create defective regions with a volume change of 29 mean atomic volumes. The distortion field of these regions shows no high symmetry. The radius of the heavily strained region is ～ 10 Å. For higher doses the regions grow a little bit but mainly new defective regions are built.

According to the various volume fractions of defective regions at various doses the first strong amorphous halo at about K≈ 1.5 A^?1 was detected. Since this amorphous scattering is isotropic and during annealing the regions disappear mainly without changing their size we are led to the conclusion that the defective regions are glass-like in nature. So the size of the tunnelling system has to be smaller than 20 Å.     

Amorphous GaP produced by ion implantation

Two types of non-crystalline states (“disordered” and “amorphous”) of GaP were produced by using ion implantation and post annealing. A structural-phase-transition-like annealing behaviour from the “disordered” state to the “amorphous” state was observed.The ion dose dependence and the annealing behaviour of the atomic structure of GaP implanted with 200 keV ? N⁺ ions were studied by using electron diffraction, backscattering and volume change measurements. The electronic structure was also investigated by measuring optical absorption and electrical conductivity.The implanted layer gradually loses the crystalline order with the increase of the nitrogen dose.The optical absorption coefficient α and electric conductivity σ of GaP crystals implanted with 200 keV?N⁺ ions of 1 × 10¹⁶ cm^?2 were expressed as αhν = C(hν ? E₀)ⁿ and log $\text{σ = A ? BT}{}^{\mathrm{<mtext>-</mtext><mtext>1</mtext><mtext>4</mtext>}}$, respectively. Moreover, the volume of the implanted layer increased about three percent and the electron diffraction pattern was diffused halo whose intensity monotonically decreases along the radial direction. These results indicate that the as-implanted layer has neither a long range order nor a short range order (“disordered state”).In the sample implanted at 1 × 10¹⁶ cm^?2, a structural phase-transition-like annealing stage was observed at around 400°C. That is, the optical absorption coefficient α abruptly fell off from 6 × 10⁴ to 7 × 10³ cm^?1 and the volume of the implanted layer decreased about 2% within an increase of less than 10 degrees in the anneal temperature. Moreover, the short range order of the lattice structure appeared in the electron diffraction pattern. According to the backscattering experiment, the heavily implanted GaP was still in the non-crystalline state even after annealing.These facts lead us to believe that heavily implanted GaP, followed by annealing at around 400°C, is in the “amorphous” state, although as-implanted Gap is not in the “amorphous” state but in the “disordered” state.     

纳米磷化镓粉体还原氮过程的Raman光谱分析

利用Raman光谱对还原氮后的纳米磷化镓(GaP)粉体进行了表征。结果表明:纳米GaP粉体表面含有Ga-O,P-O和H-O化学键。此外,进行氮还原过程后,在Raman位移约为1700~3300cm-1范围内(相当于709~800nm或1.55~1.75eV),纳米GaP的Raman光谱出现了一个宽、强荧光发射峰;而在未进行通氮处理的纳米GaPRaman光谱中,没有观察到该荧光峰的存在。本文对该荧光发射峰的起因作了初步分析。     

The GaP(001) surface and the adsorption of Cs

GaP(001) cleaned by argon-ion bombardment and annealed at 500°C showed the Ga-stabilized GaP(001)(4 × 2) structure. Only treatment in 10^?5 Torr PH₃ at 500°C gave the P-stabilized GaP(001)(1 × 2) structure. The AES peak ratio $\text{P}\text{Ga}$ is 2 for the (4 × 2) and 3.5 for the (1 × 2) structure. Cs adsorbs with a sticking probability of unity up to 5 × 10¹⁴ Cs atoms cm^?2 and a lower one at higher coverages. The photoemission measured with uv light of 3660 Å showed a maximum at the coverage of 5 × 10¹⁴ atoms cm^?2. Cs adsorbs amorphously at room temperature, but heat treatment gives ordered structures, which are thought to be reconstructed GaP(001) structures induced by Cs. The LEED patterns showed the GaP(001)(1 × 2) Cs structure formed at 180°C for 10 h with a Cs coverage of 5 × 10¹⁴ atoms cm^?2, the GaP(001)(1 × 4) Cs formed at 210°C for 10 hours with a Cs coverage of 2.7 × 10¹⁴ atoms cm^?2, the GaP(001)(7 × 1) and the high temperature GaP(001)(1 × 4), the latter two with very low Cs content. Desorption measurements show three stability regions: (a) between 25–150°C for coverages greater than 5 × 10¹⁴ atoms cm^?2, and an activation energy of 1.2 eV; (b) between 180–200°C with a coverage of 5 × 10¹⁴ atoms cm^?2, and an activation energy of 1.8 eV; (c) between 210–400°C with a coverage of 2.7 × 10¹⁴ atoms cm^?2, and an activation energy of 2.5 eV.     

Investigation of local lattice structure around Cr3+ ions at the trigonal and tetragonal sites in RbCdF3: Cr3+ crystal

The local lattice structure and EPR, optical spectra for Cr³⁺ doped in RbCdF₃ crystal have been studied by diagonalizing the complete energy matrices. The results show that the local structure of the Cr³⁺ ions in RbCdF₃ exhibits a compressed distortion at the trigonal and tetragonal sites. The compressed distortion can be ascribed to the fact that the radius of Cr³⁺ ion is smaller than that of Cd²⁺ ion, and therefore Cr³⁺ ion will draw the fluorin ligands inwards. The variational ranges of the local structural parameters for Cr³⁺ doped in RbCdF₃ crystal R =?1.9491 Å ～?1.9814 Å, θ?= 55.234° ～?55.286° at the trigonal site and R ₁ =?1.8617 Å ～?1.8928 Å, R ₂ =?1.9527 Å ～?1.9851 Å at tetragonal site are obtained respectively, and the EPR and optical spectra agree well with the experimental results.     

Sub‐micron imaging of sub‐surface nanocrystalline structure in silicon

This work reports on a surprising and abnormal increase of the Raman intensity when the probing area is moved to the edge of a mechanically cleaved Si wafer. Our detailed surface structure study based on atomic force microscope and scanning electron microscope rules out any effect from surface morphology. Systematic study of the Raman system focusing effect with a range of µm finds no focusing effect involved in our observed phenomenon. The linewidth, wavenumber, and intensity of the 521 cm⁻¹ peak are obtained and evaluated quantitatively when the testing region is moved from the center to the cleaved edge. When the grain size of crystalline silicon varies from 20 to 10 nm, the Raman intensity of the 521 cm⁻¹ peak is increasing abnormally, by about 100%. Meanwhile, both the linewidth and wavenumber change by about 2 cm⁻¹ and 4 cm⁻¹, respectively. If instrument and surface factors are well controlled/defined, the change of Raman intensity could provide a sensitive and complementary method for grain size characterization in addition to the Raman wavenumber and linewidth methods. Copyright © 2013 John Wiley & Sons, Ltd.     

Multilayer of phospholipid membranes on a hydrosol substrate

The molecular structure of a multilayer of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) adsorbed on the surface of the hydrosol of silica nanoparticles has been studied by the synchrotron radiation scattering method. According to the reflectometry data, the multilayer is formed by planar phospholipid bilayers with a thickness of (69 ± 1) Å and its total thickness is about 400 Å. Grazing incidence diffraction indicates that the bilayers are in the crystal state with an area of (41.6 ± 0.7) Ų per molecule.     

A proposed ablation laser

A new type of eximer laser is proposed; it involves the simultaneous sublimation and excitation of a frozen film by means of a relativistic electron beam. Calculated beam requirements for noble gases are ～ 10 kA/cm² of 400 kV electrons for periods of 2 to 12 ns. Predicted advantages of this laser are high gain and the elimination of wavelength limiting optical windows. Preliminary experimental fluorescence spectra have been obtained from xenon films. The emission from the solid phase consists primarily of two 100 Å wide bands centered at 1690 Å and 1730 Å the fluorescence lifetimes are 4 ± 2 ns for both bands.     

Raman scattering studies of individual polar semiconducting nanowires: phonon splitting and antenna effects

Results from Raman scattering experiments on individual crystalline GaP nanowires are presented which indicate that the shape of the nanowire, i.e., the high aspect ratio, may be responsible for two new phenomena involving optical phonons: (1) a shape-inducedsplitting of both the longitudinal optical (LO) and transverse optical (TO) phonons at the center of the Brillouin zone (q=0), and (2) a Raman scattering “antenna” effect which masks the normal Raman polarization selection rules. We suggest that (1) stems from the asymmetry in the long range dipolar sums that control the electromagnetic LO-TOsplitting, and we identify the Raman antenna effect (2) with the internal electric field created by Mie resonances in the nanowire driven by the incident laser field. Although these effects are reported here for GaP, they are expected to be general effects observable in many semiconducting nanowire systems. PACS 78.67.-n; 78.67.Lt; 78.30.-j; 78.30.Fs; 72.10.Di     

Effect of MeV electron irradiation on the free volume of polyimide

The free volume of the microvoids in the polyimide samples, irradiated with 6 MeV electrons, was measured by the positron annihilation technique. The free volume initially decreased the virgin value from ～13.70 to ～10.98 Å³ and then increased to ～18.11 Å³ with increasing the electron fluence, over the range of 5?×?10¹⁴ – 5?×?10¹⁵ e/cm². The evolution of gaseous species from the polyimide during electron irradiation was confirmed by the residual gas analysis technique. The polyimide samples irradiated with 6 MeV electrons in AgNO₃ solution were studied with the Rutherford back scattering technique. The diffusion of silver in these polyimide samples was observed for fluences >2?×?10¹⁵ e/cm², at which microvoids of size ≥3 Å are produced. Silver atoms did not diffuse in the polyimide samples, which were first irradiated with electrons and then immersed in AgNO₃ solution. These results indicate that during electron irradiation, the microvoids with size ≥3 Å were retained in the surface region through which silver atoms of size ～2.88 Å could diffuse into the polyimide. The average depth of diffusion of silver atoms in the polyimide was ～2.5 μm.