ALBERT

Notes: Fully-coherent Si0.7Ge0.3 layers were deposited on Si(001) by gas-source molecular beam epitaxy (GS-MBE) from Ge2H6/Si2H6 mixtures in order to probe the effect of steady-state hydrogen coverages θH on surface morphological evolution during the growth of compressively strained films. The layers are grown as a function of thickness t at temperatures, Ts=450–550 °C, for which strain-induced roughening is observed during solid-source MBE (SS-MBE) and deposition from hyperthermal beams. With GS-MBE, we obtain three-dimensional (3D) strain-induced growth mounds in samples deposited at Ts=550 °C for which θH is small, 0.11 monolayer (ML). However, mound formation is dramatically suppressed at 500 °C (θH=0.26 ML) and completely eliminated at 450 °C (θH=0.52 ML). We attribute these large differences in surface morphological evolution primarily to θH(Ts)-induced effects on film growth rates R, adatom diffusion rates Ds, and ascending step-crossing probabilities. GS-MBE Si0.7Ge0.3(001) growth at 450 °C remains two dimensional, with a surface width 〈w〉〈0.15 nm, at all film thicknesses t=11–80 nm, since both R and the rate of mass transport across ascending steps are low. Raising Ts to 500 °C increases R faster than Ds leading to shorter mean surface diffusion lengths and the formation of extremely shallow, rounded growth mounds for which 〈w〉 remains essentially constant at (similar, equals)0.2 nm while the in-plane coherence length 〈d〉 increases from (similar, equals)70 nm at t=14 nm to 162 nm with t=75 nm. The low ascending step crossing probability at 500 °C results in mounds that spread laterally, rather than vertically, due to preferential attachment at the mound edges. At Ts=550 °C, the ascending step crossing probability increases due to both higher thermal activation and lower hydrogen coverages. 〈w〉(t) increases by more than a factor of 10, from 0.13 nm at t=15 nm to 1.9 nm at t=105 nm, while the in-plane coherence length 〈d〉 remains constant at (similar, equals)85 nm. This leads, under the strain driving force, to the formation of self-organized 3D {105}-faceted pyramids at 550 °C which are very similar to those observed during SS-MBE. © 2002 American Institute of Physics.

Notes: Electromagnetic properties of an Ag2O doped and an undoped BiPbSrCaCuO superconductor were evaluated to investigate the effect of the pinning center on the magnetic shielding and suspension/levitation phenomena. The residual magnetization M=M+–M− increased with the dopant concentration, a maximum for 2% doping, wherein a fine uniform dispersion of Ag particles was observed. The fine Ag particles form a cluster with increasing dopant as the particles condense with each other and grow, consequently does the number of flux passing through decreases, so the magnetization M decreases. This result indicates that M is proportional to the number of magnetic flux lines passing through the sample, because the smaller the particle size the larger the ratio of the surface area to the volume. Magnetic shielding was evaluated by measuring the induced voltage in the secondary coil by placing the sample in between the primary coil. The voltage was initially set to 0.5 V, and decreased to 0.17 and 0.28 V, respectively, for the undoped and 2% Ag2O doped samples. The much less change in the induced voltage for the 2% doped sample is attributed to increased flux shielding by shielding the vortex current. Simultaneous stable levitation and suspension of 2% Ag2O doped disk samples weighing 0.3 g were observed, respectively, above (3 mm) and beneath (2 mm) a toroidal permanent magnet under a field cooled condition. The role of flux pinning is discussed to account for the phenomena by considering the hysteretic force function. © 2001 American Institute of Physics.

Notes: Si ions were implanted into 100-nm-thick SiO2 layer thermally grown on crystalline Si at an energy of 55 keV with various doses ranging from 1×1014 to 1×1017 Si/cm2 at room temperature. Si ions go through the interface between SiO2 layer and Si substrate generating defects in SiO2 layer and Si substrate as well. Defect-related phenomena were characterized by photoluminescence (PL) and electron spin resonance (ESR) measurements. The PL experiment shows that there exists a dose window for a maximum intensity of luminescence related to radiative defects, while the ESR exhibits that nonradiative defects change from E′ centers to Pb centers as the dose increases. It is considered that the intensity is controlled by the density ratio of radiative to nonradiative defects induced by ion implantation. © 2000 American Institute of Physics.

Notes: The effect of surface passivation of undoped p-CdTe(100) by (NH4)2Sx treatment was investigated by using photoluminescence (PL), photoconductivity (PC), and x-ray photoelectron spectroscopy (XPS). After sulfur treatment for 2 min, the acceptor bound exciton (A0, X) peak increases greatly in the PL spectrum, and the minority-carrier lifetime of CdTe becomes the longest value in the PC measurement. The XPS spectrum for untreated CdTe shows the additional peaks on the right side of two main Te peaks corresponding to the Te 3d core levels, and these additional peaks are related to TeO3 with binding energies of 576.2 and 586.5 eV. After sulfur treatment, while the intensities of the Te 3d core levels decreased gradually, those of the TeO3 peaks disappear. In addition, the S 2p core-level spectra for sulfur-treated CdTe show the peaks at the 161.7 and 162.8 eV, which are attributed to a CdS formation at the surface of CdTe. These results indicate the sulfur effectively dissociates the native oxides from and neutralizes the dangling bonds at the surface of CdTe. © 2000 American Institute of Physics.

Notes: The effects of As doping, at concentrations CAs≤4.8×1018 cm−3, on the growth kinetics of Si(001):As layers deposited at temperatures Ts=575–900 °C by gas-source molecular-beam epitaxy from Si2H6 and AsH3 have been investigated. With constant AsH3 and Si2H6 fluxes, film deposition rates RSi increase while CAs decreases with increasing Ts. All incorporated As resides at substitutional electrically active sites for CAs up to 3.8×1018 cm−3 (Ts=800 °C), the highest value yet reported for Si(001):As growth from hydride source gases. Immediately following film growth or partial-monolayer As adsorption on clean Si(001), the samples were quenched to 300 °C and exposed to atomic deuterium (D) until saturation coverage. In situ D2 temperature-programmed desorption (TPD) spectra from both as-deposited Si(001):As and As-adsorbed Si(001) layers are composed of β1 and β2 peaks, due to D2 desorption from Si monodeuteride and dideuteride surface phases, together with a new peak β3 which we attribute to desorption from Si–As mixed dimers. Analyses of the TPD spectra show that, because of the lone-pair electrons associated with each As surface atom, the total dangling-bond coverage, and hence RSi, decreases with increasing incoming flux JAsH3 at constant Ts. From measurements of the steady-state As surface coverage θAs vs CAs and Ts, we obtain an As surface segregation enthalpy ΔHs=−0.92 eV. Dissociative AsH3 adsorption on Si(001) was found to follow second-order kinetics with a relatively Ts-independent reactive sticking probability of 0.3. Associative As2 desorption is also second order with a rate constant kd,As2=1×1013 exp(−3.0 eV/kTs). From the combined set of results, we develop a predictive model with no fitting parameters for CAs vs JAsH3, JSi2H6, and Ts. © 2000 American Institute of Physics.

Notes: Induced anisotropy with a large energy of 6×104 J/m3 is formed in an amorphous Sm–Fe based thin film by sputtering under an applied magnetic field of 500–600 Oe. The induced anisotropy results in a large anisotropy in magnetostriction, a strain anisotropy ratio reaching as high as 35, although intrinsic magnetostriction is affected only slightly. The large strain anisotropy allows one to realize a large strain in a particular direction and, hence, it is of significant practical importance. Induced anisotropy is also found to be formed by postannealing under applied magnetic field, but the magnitude of anisotropy energy formed is very small. © 2000 American Institute of Physics.

Notes: A linear perturbation theory is developed to investigate the interface instabilities of a radially-expanding, liquid jet in cylindrical geometries. The theory is applied to rapidly spreading droplets upon collision with solid surfaces as the fundamental mechanism behind splashing. The analysis is based on the observation that the instability of the liquid sheet, i.e., the formation of the fingers at the spreading front, develops in the extremely early stages of droplet impact. The shape evolution of the interface in the very early stages of spreading is numerically simulated based on the axisymmetric solutions obtained by a theoretical model. The effects that factors such as the transient profile of an interface radius, the perturbation onset time, and the Weber number have on the analysis results are examined. This study shows that a large impact inertia, associated with a high Weber number, promotes interface instability, and prefers high wave number for maximum instability. The numbers of fingers at the spreading front of droplets predicted by the model agree well with those experimentally observed. © 2000 American Institute of Physics.

Notes: We use a combination of in situ and postdeposition experimental probes together with ab initio calculations of strain coefficients and formation energies associated with specific C configurations in the Si lattice to determine C incorporation pathways and lattice site distributions in fully coherent Si1−yCy alloy layers grown by molecular-beam epitaxy on Si(001) as a function of deposition temperature Ts (380 °C–680 °C) and C fraction y (0–0.026). Lattice strain and Raman spectroscopy measurements demonstrate that all C, irrespective of y, is incorporated into substitutional lattice sites in Si1−yCy(001) layers grown at Ts≤480 °C. Increasing Ts≥580 °C leads to strong C surface segregation, as shown by in situ angle-resolved x-ray photoelectron spectroscopy, yielding additional pathways for C incorporation. Photoluminescence measurements indicate that an increasing fraction of the incorporated C in the higher-temperature layers resides in dicarbon complexes. Reflection high-energy electron diffraction and cross sectional transmission electron microscopy reveal surface roughening at Ts≥580 °C with the formation of bulk planar structures, interconnected by {113} segments, that are periodic along [001] with a periodicity which decreases with increasing Ts. We interpret the planar structures as layers of C-rich Si1−yCy which form in the presence of excess surface C resulting from segregation. Our ab initio density functional calculations show that substitutional C arranged in an ordered Si4C phase is 0.34 eV per C atom more stable than isolated substitutional C atoms. © 2002 American Institute of Physics.

Notes: Highly oriented indium tin oxide (ITO) thin films were grown by pulsed-laser deposition (PLD) on glass and single-crystal yttria-stabilized zirconia (YSZ) substrates. The structural, electrical, and optical properties of these films were investigated as a function of oxygen partial pressure. Films were deposited at substrate temperature of 300 °C in mixed gases (12 mTorr of argon and 1–50 mTorr of oxygen) using a KrF excimer laser (248 nm and 30 ns full width at half maximum) at a fluence of 1.2 J/cm2. ITO films (300 nm thick), deposited by PLD on YSZ at 300 °C in a gas mixture of 12 mTorr of argon and 6 mTorr of oxygen, exhibit a low electrical resistivity (1.6×10−4 Ω cm) with a high transparency (∼74%) at 550 nm. ITO films deposited on both glass and YSZ substrates have been used as an anode contact in organic light-emitting diodes. A comparison of the device performance for the two substrates shows that the device fabricated on the ITO/YSZ has a higher external quantum efficiency than that of the device fabricated on the ITO glass.

Notes: Transparent conducting indium tin oxide (ITO) thin films (40–870 nm) were grown by pulsed laser deposition on amorphous substrates and the structural, electrical, and optical properties of these films were investigated. Films were deposited using a KrF excimer laser (248 nm, 30 ns FWHM) at a fluence of 2 J/cm2, at substrate temperature of 300 °C and 10 mTorr of oxygen pressure. For ITO films (30–400 nm thickness) deposited at 300 °C in 10 mTorr of oxygen, a resistivity of 1.8–2.5×10−4 Ω cm was observed and the average transmission in the visible range (400–700 nm) was about 85%–90%. The Hall mobility and carrier density for ITO films (40–870 nm thickness) were observed to be in the range of 24–27 cm2/V s and 8–13×1020 cm−3, respectively. The ITO films have been used as the anode contact in organic light emitting diodes and the effect of ITO film thickness on the device performance has been studied. The optimum thickness of the ITO anode for the maximum device efficiency was observed to be about 60–100 nm. The device with the optimum thickness of ITO anode showed an external quantum efficiency of about 0.85% at 100 A/m2. © 2000 American Institute of Physics.