Generalized stacking fault energy in FCC metals with MEAM

The second nearest-neighbor modified embedded atom method (2NN-MEAM) is used to investigate the generalized stacking fault (GSF) energy surfaces of eight FCC metals Cu, Ag, Au, Ni, Pd, Pt, Al and Pb. An offset is observed in all the metals for the displacement δ_us of unstable stacking fault energy from the geometrically symmetric displacement point . The offset value is the greatest for Al and the smallest for Ag. By analyzing the stable stacking fault energy γ_sf and unstable stacking fault energy γ_usf, it can be predicted that stacking fault is more favorable in Cu, Ag, Au, and especially in Pd than the other metals, while it is most preferred to create partial dislocation for Ag and to create full dislocation for Al.     

Molecular dynamics simulations of the interaction between 60° dislocation and self-interstitial cluster in silicon

Molecular dynamics simulations are performed to investigate the interaction between 60° shuffle dislocation and tetrainterstitial (I₄) cluster in silicon, using Stillinger-Weber (SW) potential to calculate the interatomic forces. Based on Parrinello-Rahman method, shear stress is exerted on the model to move the dislocation. Simulation results show that the I₄ cluster can bend the dislocation line and delay the dislocation movement. During the course of intersection the dislocation line sections relatively far away from the I₄ cluster accelerate first, and then decelerate. The critical shear stress unpinning the 60° dislocation from the I₄ cluster decreases as the temperature increases in the models.     

Effects of different ions implantation on yellow luminescence from GaN

The influence of C, N, O, Mg, Si and co-implants (Mg+Si) ions implantation with fluences in the wide range 10¹³-10¹⁷ cm⁻² on the yellow luminescence (YL) properties of wurtzite GaN has been studied by photoluminescence (PL) spectroscopy. Two types of n-type GaN samples grown by metal-organic chemical vapor deposition method (MOCVD) and labeled as No-1 and No-2 were studied. In their as-grown states, No-1 samples had strong YL, while No-2 samples had weak YL. Results of the frontside and backside PL measurements in one of the as-grown GaN epifilms are also presented. Comparing the intensity of YL between frontside and backside PL spectra, the backside PL spectrum shows the more intense YL intensity. This implies that most of the intrinsic defects giving rise to YL exist mainly near the interface between the epilayer and buffer layer. Our experimental results show that the intensity ratio of YL to near-band-edge UV emission (I_YL/I_UV) decreases gradually by increasing the C implantation fluence from 10¹³ to 10¹⁶ cm⁻² for No-1 samples after annealing at 900 °C. When the fluence is 10¹⁷ cm⁻², a distinct change of the I_YL/I_UV is observed, which is strongly increased after annealing. For No-2 samples, after annealing the I_YL/I_UV decreases gradually with increase in the C implantation fluence from 10¹³ to 10¹⁵ cm⁻². The I_YL/I_UV is gradually increased with increasing C fluence from 10¹⁶ to 10¹⁷ cm⁻² after annealing, while I_YL/I_UV for other ions-implanted GaN samples decreases monotonically with increase in the ions implantation fluences from 10¹³ to 10¹⁷ cm⁻² for both No-1 samples and No-2 samples. It is noted that for annealed C-implanted No-2 samples I_YL/I_UV is much higher than that of the as-grown one and other ion-implanted ones. In addition, I_YL/I_UV for the Mg, Si, and co-implants (Mg+Si) implanted No-2 samples with a fluence of 10¹³ cm⁻² after being annealed at 900 °C is higher than that of the as-grown one. Based on our experimental data and literature results reported previously, the origins of the YL band have been discussed.     

Ab initio study of I<Subscript>2</Subscript> and T<Subscript>2</Subscript> stacking faults in C14 Laves phase MgZn<Subscript>2</Subscript>

Based on the synchroshear mechanism, the formation of intrinsic stacking fault I ₂ and twin-like stacking fault T ₂ in C14 Laves phases has been modeled in detail and the generalised stacking fault energy curve of I ₂ and T ₂ for C14 Laves phase MgZn₂ has been calculated from first-principles. The results demonstrate that the unstable stacking fault energy of I ₂ by synchroshear is still very large, and the stable stacking fault energy of I ₂ is higher in comparison with pure Mg implying that the formation of I ₂ stacking fault in MgZn₂ is difficult. Starting with the I ₂ configuration, the T ₂ stacking fault can be formed by an additional synchroshear. The unstable and stable stacking fault energies of T ₂ are only slightly larger than those of I ₂, implying that the formation of T ₂ may be essentially similar to that of I ₂. From the obtained generalised stacking fault energy, the relevant deformation mechanism of MgZn₂ is also discussed. Finally, the electronic structure during synchroshear process is further studied.     

The nano-structural properties of hydrogenated a-Si and Si-C thin films alloys by GISAXS and vibrational spectroscopy

Amorphous hydrogenated silicon (a-Si:H) with high hydrogen content (10-40 at.%), and non-stehiometric silicon-carbon (Si_1−xC_x) thin films with a variation of the carbon to silicon ratio up to 0.3, were deposited by using a magnetron sputtering source. The Si_1−xC_x thin films were partially crystallised after deposition by thermal annealing up to 1050 °C.The GISAXS (Grazing Incidence Small Angle X-ray Scattering) spectra of all of the prepared specimens indicate the presence of “particles” in the “bulk” of the films. For the a-Si:H samples, “particles” are most probably voids agglomerates with a variation in size between 3 and 6 nm. The mean value of the size distribution of the “particles” increases while its width slightly decreases with the hydrogen content in the film. This indicates a better structural ordering which is consistent with the results of Raman spectroscopy that show a decrease of the ratio between intensities of transversal acoustic (TA) and transversal optic (TO) phonon peaks, I_TA/I_TO, and a narrowing of the TO peak with increasing hydrogen content. These results are discussed as a consequence of the beneficial influence of hydrogen bombardment during the film growth.For Si_1−xC_x thin films, the “particles” are assumed to be SiC nano-crystals with a size between 2 and 14 nm and they are larger in films with a higher carbon concentration. Inside each of the films, the crystals are larger closer to surface and they grow faster in the direction parallel to the surface than in that which is perpendicular to it.     

Decomposition of thin titanium deuteride films; thermal desorption kinetics studies combined with microstructure analysis

The thermal evolution of deuterium from thin titanium films, prepared under UHV conditions and deuterated in situ at room temperature, has been studied by means of thermal desorption mass spectrometry (TDMS) and a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The observed Ti film thickness dependent morphology was found to play a crucial role in the titanium deuteride (TiD_y) film formation and its decomposition at elevated temperatures. TDMS heating induced decomposition of fine-grained thin Ti films, of 10-20 nm thickness, proceeds at low temperature (maximum peak temperature T_m about 500 K) and its kinetics is dominated by a low energy desorption (E_D = 0.61 eV) of deuterium from surface and subsurface areas of the Ti film. The origin of this process is discussed as an intermediate decomposition state towards recombinative desorption of molecular deuterium. The TiD_y bulk phase decomposition becomes dominant in the kinetics of deuterium evolution from thicker TiD_y films. The dominant TDMS peak at approx. T_m = 670 K, attributed to this process, is characterized by E_D = 1.49 eV.     

Effect of deposition conditions on YBa2Cu3O7−δ thin films by inverted cylindrical magnetron sputtering and substrate effects

The dependence of YBCO thin film properties on the deposition conditions was studied for different substrates. The deposition conditions were optimized for the epitaxial growth of high quality YBCO thin films of 1500 Å thickness onto single crystal (100-oriented) SrTiO₃ (STO), MgO and LaAlO₃ (LAO) substrates by DC Inverted Cylindrical Magnetron Sputtering (ICMS). The samples were investigated in detail by means of X-ray diffraction analysis (XRD), EDX, AFM, ρ-T, magnetic susceptibility and current-voltage (I-V) characterizations. The samples show strong diamagnetic behavior and sharp transition temperatures of 89-91 K with ΔT<0.5 K. XRD of the samples exhibited highly c-axis orientation. The full width at half maximum (FWHM) values of the rocking curves were ranging typically from 0.22 to 0.28°. The samples have smooth surfaces as shown from AFM micrographs. The surface roughness, R_a, changed between 5-7 nm. I-V characteristics were obtained from the 20 μm-wide microbridges, which were patterned by a laser writing technique. The critical current densities (J_c, 1.06×10⁶ for LAO-based YBCO, 1.39×10⁶ for MgO-based YBCO, 1.67×10⁶ A/cm² for STO based YBCO) of the microbridges were evaluated from I-V curves at 77 K.     

Low-temperature oxidation of SiGe by liquid-phase deposition

Low-temperature silicon dioxide (SiO₂) films were grown on silicon germanium (SiGe) surfaces using the liquid-phase deposition (LPD) method. The growth solutions of LPD-SiO₂ are hydrofluorosilicic acid (H₂SiF₆) and boric acid (H₃BO₃). It was found that the growth rate increases with increasing temperature and concentration of H₃BO₃. The Auger electron spectroscopy profile shows that no pileup of Ge atoms occurs at the interface of SiO₂/SiGe after the LPD-SiO₂ growth. Al/LPD-SiO₂/p-SiGe MOS capacitors were prepared to determine capacitance-voltage (C-V) and current-voltage (I-V) characteristics. In our experiments, a low leakage current density of 8.69 × 10⁻⁹ A/cm² under a 2 MV/cm electric field was observed. Such a value is much smaller than those of plasma- and thermal-oxides as a result of no plasma damage and a lower growth temperature. Moreover, lower oxide charges and interface charge densities of 3.82 × 10¹⁰ cm⁻² and 1.12 × 10¹¹ eV⁻¹ cm⁻², respectively, were achieved in our LPD-SiO₂ compared to direct photochemical-vapor-deposition-SiO₂.     

45 nm CMOS technology with low temperature selective epitaxy of SiGe

This paper describes the advanced embedded silicon germanium (eSiGe) technologies to apply the 45 nm node CMOS fabrication technology. There are three key techniques as follows. The first technique is a low temperature of epitaxial growth at 550 °C to suppress staking faults in eSiGe layer. The second one is a controlling of recess shape for eSiGe. Sigma(Σ)-shaped recess is applied, because the strain force on the channel of MOSFET is increased effectively by narrowing spacing between source and drain. The third one is to apply particular surface cleaning treatment before the epitaxial growth, to get the excellent SiGe crystallinity. We demonstrated the drain current of I_on = 725 μA/μm and I_off = 100 nA/μm for PMOSFET using above these techniques.     

Reflection and vibrational spectra of SbSCl0.1I0.9 crystals in the ferroelectric phase-transition region

SbSCl_0.1I_0.9 crystals were grown from the vapor phase and reflection spectra were recorded using a Fourier-transform IR spectrometer. The optical parameters and optical functions along the c axis were calculated using an improved Kramers–Kronig technique with two confining spectral limits. The reflection spectra were analyzed using the oscillator parameter fitting technique for comparison of results. The vibrational frequencies ω_L and ω_T were evaluated. The ferroelectric phase-transition temperature was estimated as 330 K for SbSCl_0.1I_0.9 from experimental reflection measurements and theoretical investigation of the potential energy of Sb atoms for B_1u soft normal modes of SbSI and SbSCl_0.1I_0.9 crystals.     

Propagation regimes of intense circularly polarized laser beam in magnetoactive plasma

This paper presents an analytical and numerical investigation of an intense circularly polarized wave propagating along the static magnetic field parallel to oscillating magnetic field in magnetoactive plasma. In the relativistic regime such a magnetic field is created by pulse itself. The authors have studied different regimes of propagation with relativistic electron mass effect for magnetized plasma. An appropriate expression for dielectric tensor in relativistic magnetoactive plasma has been evaluated under paraxial theory. Two modes of propagation as extraordinary and ordinary exist; because of the relativistic effect, ultra-strong magnetic fields are generated which significantly influence the propagation of laser beam in plasma. The nature of propagation is characterized through the critical-divider curves in the normalized beam width with power plane For given values of normalized density (ω_p/ω) and magnetic field (ω_c/ω) the regions are namely steady divergence (SD), oscillatory divergence (OD) and self-focusing (SF). Numerical computations are performed for typical parameters of relativistic laser-plasma interaction: magnetic field B = 10-100 MG; intensity I = 10¹⁶ to 10²⁰ W/cm²; laser frequency ω = 1.1 × 10¹⁵ s⁻¹; cyclotron frequency ω_c = 1.7 × 10¹³ s⁻¹; electron density n_e = 2.18 × 10²⁰ cm⁻³. From the calculations, we confirm that a circularly polarized wave can propagate in different regimes for both the modes, and explicitly indicating enhancement in wave propagation, beam focusing/self-guiding and penetration of E-mode in presence of magnetic field.     

Shallow levels in virgin hydrothermally grown n-type ZnO studied by thermal admittance spectroscopy

Three n-type single crystal hydrothermally grown ZnO samples with resistivities of 5.1±0.6, 15±2 and 220±20 Ω cm, respectively, have been electrically characterized using thermal admittance spectroscopy (TAS). The presence of three main donors: two shallow ones D₁ and D₂ and a deeper one D₃ with activation energies of ∼30, ∼50 and ∼290 meV, respectively, are detected. In addition, the TAS spectra reveal the presence of a fourth level, D_X, with a peak amplitude in the conductance spectra that decreases with the temperature occurrence. It is shown that this anomalous behavior is consistent with D_X being a negative-U defect of donor-type. An activation energy of ∼80 meV for the ++/+ transition, a capture cross section equal to ∼3×10⁻¹⁷ cm² and an energy barrier for atomic reconfiguration of ∼0.25 eV, respectively, deduced according to the assignment of D_X to a negative-U defect. A tentative assignment of the D_X defect with oxygen vacancies is discussed.     

On a presence of SimHn clusters in a-Si:H/c-Si structures

Dominant aim of the paper was to verify the existence of the Si_mH_n clusters in a-Si:H layers. Thin layers were deposited by plasma-enhanced chemical vapor deposition (PECVD) on both glass and crystalline silicon substrates. Their IR and structural properties were investigated by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction at grazing incidence angle (XRDGI). We have found that the layer probably consists of larger structurally ordered parts corresponding to Si_mH_n clusters and separated groups of (Si-H_x)_N. The ordered parts could be identified as some of Si_mH_n clusters ranging from (10, 16) to (84, 64) represented by corresponding vibration frequencies in three following IR regions: 600-750, 830-900 and 2080-2180 cm⁻¹. XRDGI measurement indicates that diffraction maximum at around 2Θ = 28° can be attributed to an existing Si_mH_n cluster.     

Dopant diffusion and segregation in semiconductor heterostructures: Part 1. Zn and Be in III-V compound superlattices

chemical effect on the neutral species; and (ii) a Fermi-level effect on the ionized species, because, in addition to the chemical effect, the solubility of the species also has a dependence on the semiconductor Fermi-level position. For Zn and Be in GaAs and related compounds, their diffusion process is governed by the doubly-positively-charged group III element self-interstitials (I²⁺ _III), whose thermal equilibrium concentration, and hence also the diffusivity of Zn and Be, exhibit also a Fermi-level dependence, i.e., in proportion to p². A heterojunction consists of a space-charge region with an electric field, in which the hole concentration is different from those in the bulk of either of the two layers forming the junction. This local hole concentration influences the local concentrations of I²⁺ _III and of Zn^- or Be^-, which in turn influence the distribution of these ionized acceptor atoms. The process involves diffusion and segregation of holes, I²⁺ _III, Zn^-, or Be^-, and an ionized interstitial acceptor species. The junction electric field also changes with time and position. Received: 20 August 1998/Accepted: 23 September 1998     

Dopant diffusion and segregation in semiconductor heterostructures: Part 2. B in GexSi1-x/Si structures

x Si_1-x/Si heterostructures have been obtained. Here the chemical effects seem to be of less importance. The Fermi-level effect determines the ionized boron solubilities in Ge_xSi_1-x and in Si, as well as the thermal equilibrium concentration of the singly-positively-charged crystal self-interstitials I⁺ which governs the boron diffusion process. The junction carrier concentration affects the concentration of I⁺ and solubility of B in the region and hence controls B diffusion across the heterojunction. Received: 20 August 1998/Accepted: 23 September 1998     

Ab initio calculations of generalized-stacking-fault energy surfaces and surface energies for FCC metals

The ab initio calculations have been used to study the generalized-stacking-fault energy (GSFE) surfaces and surface energies for the closed-packed (1 1 1) plane in FCC metals Cu, Ag, Au, Ni, Al, Rh, Ir, Pd, Pt, and Pb. The GSFE curves along (1 1 1) direction and (1 1 1) direction, and surface energies have been calculated from first principles. Based on the translational symmetry of the GSFE surfaces, the fitted expressions have been obtained from the Fourier series. Our results of the GSFEs and surface energies agree better with experimental results. The metals Al, Pd, and Pt have low γ_us/γ_I value, so full dislocation will be observed easily; while Cu, Ag, Au, and Ni have large γ_us/γ_I value, so it is preferred to create partial dislocation. From the calculations of surface energies, it is confirmed that the VIII column elements Ni, Rh, Ir, Pd, and Pt have higher surface energies than other metals.     

The influence of the substrate temperature on the photovoltaic properties of spray-deposited CdS:In thin films

Polycrystalline and highly transparent CdS:In thin films were produced by the spray pyrolysis (SP) technique at different substrate temperatures ranging from 350 to 490 °C on glass substrates. The effect of the substrate temperature on the photovoltaic properties of the films was investigated by studying the transmittance measurements, X-ray diffraction (XRD) patterns, scanning electron microscope (SEM) observations and the I-V plots. The transmittance measurements were used to estimate the band gap energy by the linear fit of (αhν)² versus hν. The band gap energy was found to be slightly increasing with the substrate temperature. XRD diffractograms show that a phase transition from the cubic to the hexagonal phase occurs by increasing the substrate temperature, beside more orientation of crystal growth. Also they show that complex cadmium compounds are still present till T_s ≈ 460 °C after which they practically disappear. From the linear I-V plots the resistivity was estimated and found to be strongly decreasing with the substrate temperature.     

Gettering efficiencies for Cu and Ni as a function of size and density of oxygen precipitates in p/p- silicon epitaxial wafers

We have measured the gettering efficiencies for Cu and Ni in p/p-Si epitaxial wafers. The wafers were pretreated to obtain oxygen precipitates of different sizes and densities in the bulk. Gettering tests started with a reproducible spin-on spiking in the range of 10¹² atoms/cm², followed by thermal treatment to drive-in and redistribute the impurities in the wafer. Subsequently, the wafers were analyzed by a novel stratigraphical layer-by-layer etching technique in combination with inductively coupled plasma mass spectrometry. Gettering efficiencies for Ni did not depend on oxygen precipitate sizes and densities as long as ΔO_i was larger than 0.2×10¹⁷ atoms/cm³ and the bulk micro defect densities were detectable by preferential etching (10⁷ cm^-3). In these cases, gettering efficiencies were 96–99% for Ni, while wafers not containing any measurable BMDs exhibited no detectable gettering. Cu exhibited a more complex behavior because the total Cu contamination was found to be divided into two species, one mobile and the other immobile species. A dependence on BMD size and BMD density of the Cu distributions in the wafers was also detected. Gettering effects were increased with increasing BMD densities and sizes. For BMD densities <10⁹ cm^-3, Cu was not efficiently gettered by oxygen precipitates. Even for BMD densities >10¹⁰ cm^-3, gettering effects due to oxygen precipitates were one order of magnitude lower than in heavily boron-doped silicon. Received: 19 January 2001 / Accepted: 31 January 2001 / Published online: 20 June 2001     

Investigation of spectroscopic properties and thermal stability of Er/Yb-co-doped Bi2O3-B2O3-Ga2O3 glasses

A series of Er³⁺/Yb³⁺-co-doped 60Bi₂O₃-(40−x) B₂O₃ -xGa₂O₃ (BBGA x=0, 4, 8, 12, 16 mol%) glasses have been prepared. The absorption spectra, emission spectra, fluorescence lifetime of Er³⁺:⁴I_13/2 level and thermal stability were measured and investigated. Three Judd-Ofelt intensity parameters Ω_t (t=2,4,6) (Ω₂=(4.67-5.93)×10⁻²⁰ cm², Ω₄=(1.50-1.81)×10⁻²⁰ cm², Ω₆=(0.92-1.17)×10⁻²⁰ cm²) of Er³⁺ ions were calculated by Judd-Ofelt theory. It is found that the Ω₆ first increases with the increase of Ga₂O₃ content from 0 to 8 mol% and then decreases, which is mainly affected by the number of non-bridging oxygen ions of the glass network. The high peak of stimulated emission cross-section () of Er³⁺: ⁴I_13/2→⁴I_15/2 transition were obtained according to McCumber theory and broad full width at half maximum (FWHM=69-76 nm) of the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions were measured. The results indicate that these new BBGA glasses can be used as a candidate host material for potential broadband optical amplifiers.     

Influence of pH on microstructural and optical properties of electrosynthesized CdSe thin films

Cadmium selenide (CdSe) thin films were electrosynthesized onto well cleaned stainless steel and fluorine-doped tin oxide (FTO) coated glass (10-15 Ω/cm²) substrates at different pH of the electrolytic solution. X-ray diffraction study reveals a cubic structure with preferential orientation along the (1 1 1) direction. The structural parameters such as grain size (D), lattice constant (a), strain (ε), dislocation density (δ), average internal stress (S) and degree of preferred orientation (I₁₁₀/I₂₂₀) in the film are calculated and they are found to be dependent on the pH of the depositing bath. EDAX analysis confirms nearly stoichiometric composition of the film deposited at pH 2.70. AFM analysis shows uniform deposition of the film over the entire substrate surface. In optical studies, the transition of the deposited film is found to be a direct allowed transition. The optical energy gaps are found to be in the range from 1.87 to 2.04 eV depending on the pH of the depositing bath. Photoluminescence (PL) spectrum shows blue shift in PL peak position and reduction in luminescence intensity for the film deposited at pH other than 2.70.