Higher Doped Samples Research Articles

Electron energy-loss spectroscopy of Sr 2CuO 2+ δCl 2− y: Overdoping and role of apical oxygen for high- Tc superconductivity

By measuring O 1s and Cu 2p 3/2 absorption edges of Sr 2CuO 2+ δ Cl 2− y using orientation-dependent high-energy electron energy-loss spectroscopy, the changes of the unoccupied electronic structure close to the Fermi level induced by stoichiometry variations have been explored. An interesting hole redistribution between the in-plane O 2p orbitals and apical O 2p orbitals induced by the doping level has been revealed in this system. For the low-doped samples in which superconductivity is absent, the holes reside predominantly on the oxygen sites of the CuO 2 plane, while for the high-doped samples in which superconductivity appears, considerable holes are found to enter the apical oxygen sites. This result reveals clearly that the apical O 2p holes play an important role in the occurrence of superconductivity in the ‘apical oxygen doped’ Sr 2CuO 2+ δ Cl 2− y system.

Evolution of hydrogen induced defects during annealing of plasma treated Czochralski silicon

Transmission electron microscopy (TEM) and atomic force microscopy (AFM) are used to study the temperature evolution of hydrogen plasma induced defects in silicon. Hydrogen plasma treated n- and p-doped Czochralski silicon samples were annealed at temperatures between 200 and 1000°C for up to 10h in air. Platelets are formed on {111} and {100} crystallographic planes. The structural defects are similar in n-type and p-type material. Small defects at the surface anneal out at temperatures above 400°C and {111} platelets start to dissolve above 500°C, except in highly p-doped samples where the platelets are stable up to 600°C. The hydrogen penetrates deeper into a low doped than a high doped sample, resulting in platelet formation deeper into the sample. At annealing temperatures above 800°C, an amorphous oxide layer forms at the surface of the sample. New platelets form after 1h annealing at 1000°C. The roughest surface is found in highly n-doped samples, hydrogenated at high plasma frequency for long exposure times. For high annealing temperatures the roughness decreases.

Effect of Ru doping inLa0.5Sr0.5MnO3andLa0.45Sr0.55MnO3

We investigated the structural, magnetic, and transport properties of polycrystalline doping systems ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x}{\mathrm{O}}_{3}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.15)$ with ferromagnetic matrix and ${\mathrm{La}}_{0.45}{\mathrm{Sr}}_{0.55}{\mathrm{Mn}}_{1\ensuremath{-}y}{\mathrm{Ru}}_{y}{\mathrm{O}}_{3}$ $(0\ensuremath{\leqslant}y\ensuremath{\leqslant}0.5)$ with antiferromagnetic matrix. X-ray photoelectron spectroscopy shows that Ru ions exist mainly in the form of ${\mathrm{Ru}}^{4+}$ while there also exist a small quantity of ${\mathrm{Ru}}^{5+}$. For the low doping samples $[0.05\ensuremath{\leqslant}x(y)\ensuremath{\leqslant}0.15(0.2)]$, the ferromagnetism is enhanced and Curie temperature ${T}_{C}$ rises with increasing Ru doping level, but ${T}_{C}$ is much higher than the temperature ${T}_{\mathrm{IM}}$ corresponding to insulating-metallic transition. For the high doping samples $(0.3\ensuremath{\leqslant}y\ensuremath{\leqslant}0.5)$, oppositely the ferromagnetism is suppressed and the insulating property is enhanced with further increasing Ru doping level. These results show that there is an exchange interaction between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Ru}}^{4+}({\mathrm{Ru}}^{5+})$. The electron spin resonance spectra clearly and directly inspect that the interaction between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Ru}}^{4+}({\mathrm{Ru}}^{5+})$ is ferromagnetic. Furthermore, the electron spin resonance spectra also show that a Ru-Ru antiferromagnetic interaction takes place in the high doping region $(y=0.4,0.5)$. We explained the disagreement between ${T}_{C}$ and ${T}_{\mathrm{IM}}$.

Thermal expansion and Vickers hardness measurements on Bi 1.6Pb 0.4Sr 2Ca 2− xSm xCu 3O y superconductors

In this work, Bi 1.6Pb 0.4Sr 2Ca 2− x Sm x Cu 3O y ( x = 0.0000, 0.0005, 0.0010, 0.0050, 0.0100, 0.1000, 0.5000, 1.0000, 1.5000) superconductors were prepared by conventional solid-state reaction method. The effect of the substitution of Sm for Ca in the high- T c phase Bi-2223 was investigated by DTA, thermal expansion coefficient, Vickers microhardness, XRD and SEM analysis. The lattice constants and volume fractions are estimated from XRD analysis. The thermal expansion coefficient, the microhardness, the volume fraction, and the lattice constants strongly depend on the Sm content of the samples. XRD data show that the high- T c phase decreases and the low- T c phase increase as the Sm substitution changes from x = 0.0–1.5. The coefficients of thermal expansion (in the range of 30–800 °C) of the pure ( x = 0.0) and high doped ( x = 1.5) samples were 7.90 × 10 −7 and 5.70 × 10 −7 °C −1, respectively. The results of Vickers hardness measurements varied between 0.254 and 0.623 GPa as increasing the Sm concentration.

Ferromagnetism in manganese and cobalt co-doped ZnO bulk samples

The polycrystalline bulk samples with nominal composition Zn 1− x (Mn 1/2Co 1/2) x O ( x = 10%, 15%, 30%) were fabricated at 600 °C by the standard solid-state reaction method. The structural analysis shows that the lower doped samples ( x < 15%) are single phase of wurtzite structure, while for the high-doped sample ( x = 30%), beside the main peaks of wurtzite structure, the peaks of secondary phase are visible. Magnetization measurement indicates that all the samples show ferromagnetic ordering with T c > 300 K. Based on the structural analyses, the experimental observation is discussed.

Investigation of Degradation of Inversion Channel Mobility of SiC MOSFET due to the Increase of Channel Doping

This paper reports on the degradation of inversion channel mobility of SiC MOSFET caused by the increase of channel doping. SiC MOSFETs were fabricated on three wafers, the doping concentrations of the epitaxial layer of which were 16 10 2× cm-3 (sample A), 17 10 2× cm-3 (sample B) and 17 10 4× cm-3 (sample C). The field effect mobility sharply decreases as the doping concentration increases. Hall mobility measurements have been done to investigate the degradation of the mobility due to doping. The measurement of sample A shows that, as a consequence of the decrease of the free carrier density due to MOS interface traps, the Hall mobility is as much as a factor of ten higher than the field effect mobility. In contrast, in regard to the measurement of sample B and sample C, we encountered unstable Hall voltage and could not obtain reproducible results. This implies that such high-density traps are generated that a channel disappears in the higher-doping samples.

Superconducting gap and pseudogap behavior in high- Tc cuprates

We report the low-temperature energy gap 2 Δ 0 of Bi 2Sr 2CaCu 2O 8+ δ (Bi2212) and La 2− x Sr x CuO 4 (La214) studied over a wide range of doping levels p, by using STS, break junction tunneling and Raman scattering spectroscopy. We also report the electronic specific heat ( C el) measured in both the superconducting and normal states of La214. First, it is pointed out that the gap magnitude (2 Δ 0) at T≪ T c is closely related to the crossover temperature ( T ∗) of a small pseudogap in Bi2212, T ∗∼2Δ 0/4∼5 k B (≡T co ), and the resistivity ρ and magnetic susceptibility χ tend to be slightly suppressed below T ∗(∼T co ). A similar suppression appears in both ρ and χ of La214 at T< T co. The experimental results of C el indicate that pseudogap behavior appears in La214 below ∼ T co. It is also pointed out that the gap magnitude 2 Δ 0 is in almost linear proportion to k B T max, where T max (≫T ∗) is the temperature exhibiting a broad peak in the χ– T curve. Finally, we report that there exists a novel relation between T c and Δ 0, T c∝ pΔ 0, in both Bi2212 and La214 systems except in highly-doped samples, where T c∝2 Δ o .

The individual subband densities and mobilities in delta -doped GaAs at different temperatures

The transport properties of delta -GaAs (Si) at high temperatures were studied using the 'mobility spectrum' technique. Strong changes in the subband occupancies were observed in a low-doped sample (ND=1.5*1012 cm-2) as the temperature increased from 77 K to 300 K while in a high-doped sample (ND=5.0*1012 cm-2) these changes were small. For an explanation of the observed behaviour with temperature Poisson's and Schrodinger's equations were solved self-consistently. The mobility of the lowest subband (i=0) was found to be relatively independent of temperature while the mobilities of the higher subbands (i>0) decreased with increasing temperature in both low- and high-doped samples. At close to room temperature, the conductivity of the delta -structures can be described well by a two-carrier model where the first group consists of the electrons of all the higher subbands having approximately equal mobilities.

Detection of hot electrons at a p-type indium phosphide/platinum rotating ring/disk photoelectrode

Acetonitrile solutions containing the reversible redox couple decamethylferrocene(1+/0) (DFER, E o' =-0.40 V) and (dibromoethyl)benzene (DBEB, E 1/2 =-1.12 V) were used to detect steady-state photocurrent attributable to hot electrons at p-InP photocathodes. Reduction of DBEB at illuminated p-InP disk electrodes produces bromide ions which were quantitatively determined through their oxidation at a Pt ring electrode. Both high-doped and low-doped p-InP electrodes yielded high internal quantum yields for photoconversion (80% and 90%, respectively); however, illumination of high-doped samples invariably produced greater percentages (>30%) of DBEB reduction

Metal-insulator transition in doped conducting polymers. Disappearance of the electronic gap with persisting bond alternation in the system with site-type impurities

Electronic structures in doped conducting polymers are investigated using the Su-Schrieffer-Heeger model with acceptor doping. Lattice configurations and electronic wavefunctions in a finite system are determined for each random distribution of impurities. Stationary lattice configurations are composed of pinned solitons around impurities and almost-free solitons in impurity-free regions. After a sample average, we find that the averaged order parameter remains larger than that by Mele-Rice, as the disorder increases. A phase diagram of the electronic structure is given. The gapless state takes place at a concentration of several percent, when the impurity strength is sufficiently strong. The present results well explain the appearance of the metallic Pauli susceptibility, its magnitude, and the critical concentration, characteristic to the metal-insulator transition observed in experiments. Persistent of dimerization is also consistent with X-ray studies of highly-doped samples. Disorder effects on wavefunctions are studied. The impurities have stronger effects on the valence band states than on the conduction band states.

Surface Accumulation of Boron During Si Molecular Beam Epitaxy

ABSTRACTA high temperature, elemental boron evaporation source has been used for the study of the boron doping behaviour in Si as a function of growth temperature and doping level. Significant profile smearing of boron at doping levels below 5×1018cm−3 is observed. Profile smearing is more severe in higher doped samples for growth temperatures above 600°C at a growth rate of 0.28 nms−1. This is interpreted as arising from the formation of a surface phase of boron at higher doping levels. The marked improvement in profile abruptness at low temperatures suggests significant benefits associated with the use of an elemental boron source for the growth of high resolution Si/Si1−xGex device structures.

Growth-sector dependence of optical features in large synthetic diamonds

Infrared, ultraviolet and cathodoluminescence measurements have been made on five synthetic diamonds which were grown using the temperature-gradient technique. The samples, which were a few millimeters in size, were chosen by virtue of having relatively well-developed {110} and {113} minor facets as well as {100} and {111} major faces. The samples, labelled I–V, were prepared in the form of polished plates. Sample I was a standard type Ib synthetic diamond. Infrared and ultraviolet absorption measurements were used to determine the concentration of isolated substitutional nitrogen present in the various types of growth sector. The {111} sectors contained about 100 ppm of nitrogen; the {100} sectors contained slightly less than 50 ppm; the {113} sectors contained of the order of 10 ppm and the {110} sectors contained approximately 1 ppm. For the remaining samples, II–V, the concentration of nitrogen incorporated during growth was greatly reduced by the introduction of nitrogen getters into the synthesis capsule. Even so, concentrations of a few ppm of nitrogen could generally be detected in {111} sectors. Sample II contained a concentration of 0.27 ppm of neutral boron acceptors in the (11¯0) sector despite the fact that boron had not been deliberately introduced during synthesis. The remaining three samples were deliberately doped with boron, the amount being less for sample III than for the samples IV and V. For sample III, the uncompensated acceptor concentration was highest in the {110} sectors, being approximately 3 ppm, approximately 0.9 ppm in the {113} sectors, 0.5 ppm in the {100} sectors and too small to be measured in the {111} sectors. For the highly-doped samples, IV and V, however, the uncompensated boron concentration was highest for the {111} sectors. Acceptor-centre zero-phonon lines are broadened out in {111} sectors, presumably as a result of higher defect concentrations in these sectors. Cathodoluminescence measurements at a temperature of approximately 90 K have revealed a new vibronic system with a zero-phonon line at 2.296 eV (539.9 nm). This system was observed only in the (11¯0) sector of sample I and in non-octahedral sectors of the other samples. The 2.56 eV (484 nm) cathodoluminescence system was present with high intensity in all {111} sectors of sample I but only in one {111} sector of sample II. It was absent from sample III and weak in sample IV. All samples exhibited spectrally-broad cathodoluminescence which consisted, in general, of two components, a blue band with a peak at 420 nm and a broader green band with a peak occuring between 490 and 540 nm.

Hubbard's sub-band conduction in Si poly-SiO 2 transition layers

Conductivity and Hall coefficients were measured between 20 K and 380 K in boron-doped polysilicon films. The highly-doped samples were degenerate but unintentionally doped samples exhibited unusual behavior for polycrystalline film. To interpret the electrical properties of these samples we assumed the existence of a fine-grained non-degenerate transition layer near the Si poly-SiO 2 interface. Cross-sectional transmission electron microscopy and SIMS analysis confirmed the existence of this interface layer, which has a residual boron concentration resulting in conductivity with two activation energies, ε 1 at high temperatures and ε 2 at low temperatures in accordance with Hubbard's sub-band model.

Temperature dependence of the tunable luminescence of GaAs doping superlattices

Doping superlattices show tunable optical and electrical properties due to the space charge induced separation of photoexcited or electrically injected carriers. We have investigated the tunable luminescence in GaAs doping superlattices of doping levels n=1×10 18cm −3 and n=4×10 18cm −3 as function of excitation density and sample temperature. The temperature dependence of the tunability was investigated in the range between 4 and 700K, and we found the critical transition temperatures T 0 at 90 and 460K for the low and high doped samples, respectively. The results verify the theoretical prediction concerning the transition temperature at which the luminescence changes from full to zero tunability.

Long-term conductivity decrease in polyacetylene samples

Investigations of the conductivity decrease of polyacetylene samples doped with iodine, AsF 5 and FeCl 3, respectively, and stored under various conditions (laboratory atmosphere, dry and humid air, vacuum, argon, argon at −30 °C) are reported. In laboratory atmosphere the conductivity drops by several orders of magnitude within a few weeks in samples prepared by the conventional Shirakawa technique. In argon atmosphere at −30 °C, after an initial drop by a factor of about two within a few days, the samples are very stable. With the exception of iodine doping, highly-doped samples are less stable in laboratory atmosphere or humid air than moderately-doped ones. Samples prepared by a new method from a polyacetylene suspension turn out to be much more stable than Shirakawa samples.

N-lines and chromium-pairs in the luminescence spectra of the spinels ZnAl 2O 4:Cr 3+ and MgAl 2O 4:Cr 3+

It is shown that the N-lines in the luminescence spectra of the two spinels ZnAl 2O 4:Cr 3+ and MgAl 2O 4:Cr 3+ exhibit quite similar dependencies on chromium concentration, excitation frequency, and thermal treatment of the samples. While most of these lines are structure dependent, the line N 4 at ν R − ν ≈ 400 cm −1 and two very weak lines are in both cases due to chromium-pairs. The exchange Hamiltonian H ex = J S 1 · S 2 + j( S 1 · S 2) 2 used for the ground-state splitting is fitted by the parameters J = 40.9 cm −1, j = 1.5 cm −1 and J = 45.6 cm −1, j = 2.0 cm −1 for ZnAl-spinel and MgAl-spinel, respectively. The differences between the spectra of low-doped and high-doped samples are in both cases caused by the existence of a phonon sideband of the N 4-line, which is in many respects similar to the well-known phonon side band of the R-line.

Phonon scattering of point defect aggregates of Mg in LiF

The quantitative analysis of thermal conductivity for a large temperature interval is shown to be a method very well adapted to the study of resonant scattering of phonons by defects of size comparable with the phonon wavelength. The state of Mg in LiF has been studied by thermal conductivity measurements between 0.45 and 100° K as a function of concentration (between 6×10−5 and 2.3×10−3) and thermal treatments. In the case of low concentrations (C<2×10−4) the dipoles Mg2+-cation vacancy aggregated in trimers act as point defects. There exists no resonant mode of the cation vacancy, because of the tight binding between Mg2+ and the cation vacancy. The process of precipitation of either the metastable phase Li6MgF8 or MgF2 from a solid solution has been studied in the case of the sample withC=2.3×10−4 Mg. The numerical analysis has allowed us to calculate the size and the number of precipitates after each annealing. This size is remarkably well defined and depends only on the annealing temperature. The quantity of Mg is always very small. In the higher doped samples (C=8.4×10−4 and 2.3×10−3) the influence of dislocations and grain boundaries is predominant.