Nominal Doping Level Research Articles

Room temperature ferromagnetism in bulk Mn-Doped Cu2O

Bulk Mn-doped Cu2O samples were produced by reacting Cu2O and Mn2O3 powders in Ar gas at 650 and 800°C to give a nominal composition of 1.7at.% Mn-doped Cu2O. From x-ray energy dispersive spectrum analysis, the actual doping level was lower at 0.3–0.5at.% Mn. Room temperature ferromagnetism with a coercive field of 50Oe was found in the 650°C samples. The Curie temperature (TC) of samples sintered at 650°C was above 300K, whereas for 800°C samples it was 215±5K. Using the nominal doping level, the magnetization saturation value was calculated to be ∼0.4μB∕Mn at 10K.

Raman study of As outgassing and damage induced by ion implantation in Zn-doped GaAs

Room temperature micro-Raman investigations of LO phonon and LO phonon-plasmon coupling is used to study the As outgassing mechanism and the disordering effects induced by ion implantation in Zn-doped GaAs with nominal doping level p=7×1018cm−3. The relative intensity of these two peaks is measured right after rapid vacuum thermal annealings (RVTA) between 200 and 450°C, or after ion implantations carried out at energies of 40keV with P+, and at 90 and 170keV with As+. These intensities provide information regarding the Schottky barrier formation near the sample surface. Namely, the Raman signature of the depletion layer formation resulting from As desorption is clearly observed in samples submitted to RVTA above 300°C, and the depletion layer depths measured in ion implanted GaAs:Zn are consistent with the damage profiles obtained through Monte Carlo simulations. Ion channeling effects, maximized for a tilt angle set to 45° during implantation, are also investigated. These results show that the Raman spectroscopy is a versatile tool to study the defects induced by postgrowth processes in multilayered heterostructures, with probing range of about 100nm in GaAs-based materials.

Non-Contact Determination of Free Carrier Concentration in n-GaInAsSb

ABSTRACTGaSb-based semiconductors are of interest for mid-infrared optoelectronic and high-speed electronic devices. Accurate determination of electrical properties is essential for optimizing the performance of these devices. However, electrical characterization of these semiconductors is not straightforward since semi-insulating (SI) GaSb substrates for Hall measurements are not available. In this work, the capability of Raman spectroscopy for determination of the majority carrier concentration in n-GaInAsSb epilayers was investigated. Raman spectroscopy offers the advantage of being non-contact and spatially resolved. Furthermore, the type of substrate used for the epilayer does not affect the measurement. However, for antimonide-based materials, traditionally employed Raman laser sources and detectors are not optimized for the analysis wavelength range dictated by the narrow band gap of these materials. Therefore, a near-infrared Raman spectroscopic system, optimized for antimonide-based materials, was developed.Ga0.85In0.15As0.13Sb0.87 epilayers were grown by organometallic vapor phase epitaxy with doping levels in the range 2 to 80 × 1017 cm-3, as measured by secondary ion mass spectrometry. For a particular nominal doping level, epilayers were grown both lattice matched to n-GaSb substrates and lattice-mismatched to SI GaAs substrates under nominally identical conditions. Single magnetic field Hall measurements were performed on the epilayers grown on SI GaAs substrates, while Raman spectroscopy was used to measure the carrier concentration of epilayers grown on GaSb and the corresponding SI GaAs substrates. Contrary to Hall measurements, Raman spectra indicated that the GaInAsSb epilayers grown on GaSb substrates have higher free carrier concentrations than the corresponding epilayers grown on SI GaAs substrates under nominally identical conditions. This is contrary to the assumption that for nominally identical growth conditions, the resulting carrier concentration is independent of substrate, and possible mechanisms will be discussed.

Reentrant spin glass behavior in layered manganite La1.2Sr1.8Mn2O7 single crystals

We report a detailed study of ac/dc magnetization and longitudinal/transverse transport properties of La1.2Sr1.8Mn2O7 single crystals below Tc=118 K. We find that the resistivity minimum at 40 K is related to the reentrant spin glass phase at the same temperature, accompanied by additional anomalous Hall effects. The carrier concentration from the ordinary Hall effects remains constant during the transition and is close to the nominal doping level (0.4 holes/Mn). The spin glass behavior comes from the competition between ferromagnetic double exchange and antiferromagnetic superexchange interactions, which leads to phase separation, i.e., a mixture of ferromagnetic and antiferromagnetic clusters, representing the canted antiferromagnetic state.

MOVPE growth and characterization of Mg-doped GaN

Mg-doping of GaN films has been investigated as a function of growth rate, reactor pressure and N to Ga flow ratio (V/III ratio). The nominal doping level, determined by the Mg to Ga flow ratio, was kept constant. Secondary-ion-mass-spectroscopy (SIMS) measurements indicate that more Mg is incorporated at low pressure; the incorporation efficiency is independent of the other variables. The electrical properties of the films grown at atmospheric pressure appear independent of growth rate and V/III ratio, displaying an average hole concentration of 7.4×10 17 cm −3 and mobility of 9.3 cm 2/V s. However, films grown at a reduced pressure of 76 Torr exhibit a strong dependence on V/III ratio; the conductivity is observed to improve by an order of magnitude when the V/III ratio is raised by a factor of four. We observe a simultaneous increase in hole concentration and mobility in these films suggesting that a high degree of compensation, probably due to nitrogen vacancies, is present in the films grown at low V/III ratios. Finally, room-temperature photoluminescence (PL) measurements were performed on the samples. The PL spectra are dominated by a broad peak at 2.8 eV; the peak intensity is found to increase with increasing V/III ratio.

Structural study of the charge-density-wave modulation of isoelectronically doped

We present a neutron and x-ray diffraction study of the charge-density-wave modulation wave vector of with isoelectronic Nb doping. In contrast to other charge-density-wave materials, like and , Nb-doped reveals a change of the modulation wave vector with doping. This change corresponds to a shift of the satellite position towards the Brillouin-zone centre on a nominal doping level of 0.8% Nb being exceeded. On a 1.2% level of Nb doping being reached, strong satellite broadening is observed, indicating the onset of short-range charge-density-wave correlation. A recently derived phenomenological model of the phase transition in , which accounts for changes in the modulation wave vector upon doping, is briefly discussed.

Plasmon excitations and the effects of surface preparation in n-type InAs(001) studied by electron energy loss spectroscopy

High resolution electron energy loss spectroscopy (HREELS) has been used to study the surface plasmon excitations on n-type InAs(001) surfaces prepared either by decapping a protective As layer or by argon ion bombardment and annealing (IBA) procedures. The plasmon frequency for heavily doped decapped samples indicates a free carrier concentration in good agreement with the nominal bulk doping level ( n ∼ 5.0 × 10 18 cm −3). Subjecting the decapped sample to IBA results in a much higher carrier concentration (∼ 10 19 cm −3). Measurements from a lower doped InAs(001) sample ( n ∼ 2.0 × 10 16 cm −3) prepared by IBA also show an increased carrier concentration (∼ 10 18 cm −3), suggesting the presence of additional free carriers as a consequence of the structural damage induced by the sputtering process. For both surface preparations, the plasmon frequency does not vary as a function of the incident electron beam energy and suggests a homogeneous free carrier profile. Measurements of the plasmon frequency as a function of the incident electron beam energy indicate that the depth of electronic damage extends at least 400 Å into the material. The frequency of the surface plasmon is also strongly affected by the ion bombardment and annealing conditions. In particular, the use of high ion beam energies (> 2 keV) directed along the surface normal, gives rise to the highest residual carrier concentrations, consistent with the generation of higher degrees of structural damage in the material.

EELS and photoemission study of antimony-doped Cd2SnO4 ceramics

Ceramic samples of antimony-doped dicadmium stannate Cd2Sn1 –xSbxO4 prepared in air by standard solid-state synthetic procedures have been studied over the range 0 ⩽x⩽ 0.05 by electron energy loss spectroscopy (EELS) and ultraviolet and X-ray photoemission spectroscopy (UPS and XPS). The maximum plasmon energy for doped samples is about 0.6 eV, corresponding to a carrier concentration of 2.4 × 1020 cm–3. The carrier concentration probed by EELS always exceeds the nominal doping level by about 4.0 × 1019 cm–3. UPS shows a well defined conduction band feature, whose intensity increases with antimony concentration. The shape of the conduction band conforms tolerably well to a parabolic free-electron-like profile. The width of the conduction band of highly doped samples is in good agreement with that expected from a free electron model. A pronounced segregation of Sb to surface is evident from XPS and is discussed in detail.

NEW SET OF LOW CONCENTRATION STANDARDS FOR LA, CE, SM, YB, LU, Y, SC, V, NB AND TA IN SILICATES

We have developed a set of four synthetic standards for the rare earth and high field strength elements designed for use in the determination of those elements in silicates. The base material is a glass nominally at the eutectic of the MgO‐Al2O3‐SiO2 ternary system. The nominal doping level was 2 wt% of the elements as oxides. To avoid problems associated with peak interference, the elements were doped in four separate glasses.

A photoemission study of Sb-doped TiO 2

The influence of Sb-doping on the electronic structure of TiO 2 (rutile) has been investigated by core and valence level photoemission spectroscopy, supplemented by high resolution electron microscopy, SQUID magnetometry and infrared reflectance spectroscopy. Pronounced segregation of Sb to polycrystalline TiO 2 surfaces is evident from core level XPS, with a surface Sb concentration that increases only weakly with nominal Sb doping level in the range between 1% and 10%. Segregated Sb ions may be imaged directly in high resolution transmission electron microscopy. Ultraviolet photoemission spectra reveal the presence of an Sb-induced state toward the bottom of the bulk bandgap. The experimental data are discussed in terms of a model involving bulk substitution of Sb(V) ions onto Ti(IV) sites, largely compensated by cation vacancies or anion interstitials. Electrons are trapped by surface Sb ions to generate an Sb(III) lone-pair-like surface state. The influence of Sb doping on the electronic structure of TiO 2 is contrasted with the behaviour of Sb in SnO 2.

Observation of a 1.2 eV defect photoluminescence peak in heavily planar-doped n-type GaAs grown by molecular- beam epitaxy

In this work, the results of a study on the 10 K and room temperature photoluminescence (PL) response of single and multiple silicon planar- (also known as delta) doped GaAs are reported. These PL results were compared to the PL results from a comprehensive set of 24 uniformly silicon-doped n-type samples with nominal doping levels from 1015 to 1019 cm−3. The silicon planar-doped GaAs samples consisted of a set of seven samples and were grown in sequential order so as to minimize any systematic variations or errors. All of the planar-doped samples were cladded with AlGaAs to increase the PL efficiency. For all cases investigated, both the free carrier density and Hall mobility monotonically decrease as the nominal effective doping increases. Strong peak broadening and shifting was observed indicative of the Moss–Burstein effect in both the uniformly and planar-doped samples having an effective nominal volume doping density in the range of 1018 cm−3 or higher. The most striking feature observed in the PL spectra of the high integrated sheet charge density planar doped samples was a strong peak which occurred at a photon energy of ∼1.2 eV. Also this 1.2 eV peak was observed in all uniformly doped samples which had a nominal doping density greater than ∼6×1018 cm−3.

Influence of the impurity concentration on the microstructure of compound semiconductors - the example of ZnS:Cr optical spectra

ZnS:Cr crystals with nominal doping levels from 30 to 10000 ppm have been studied at cryotemperatures with respect to fine structure of optical no-phonon transitions in absorption, emission and excitation spectra and the overall shape of their 5E(D) → 5T 2(D) Cr 2+ (d 4) photoluminescence band. Various features can be interpreted by transitions at micropolytypic centres in stacking-faulted environments. The gross shape of the internal Cr 2+ emission band mentioned turns out to be largely determined by concentration-dependent structural changes in the impurity environment.

The structure of the 44 K superconductor (Y 1− xCa x)Ba 2Cu 3O 6+δ (δ ≤ 0.2)

Replacing Y( III) by Ca( II) in (Y 1− x Ca x )Ba 2Cu 3O 6 induces superconductivity. A solid-solution limit exists, as is evidenced by the lack of variation in unit cell parameters for values of x > 0.3. At low nominal doping levels ( x < 0.3), materials prepared by solid-state techniques consist of a complex intergrowth of Ca-rich and Ca-poor 123-type phases, as well as YBa 2Cu 3O 6 and BaCuO 2. At higher nominal doping levels the Ca content of the 123-type phase becomes constant ( x ∼ 0.30). Structural refinements using data collected by X-ray and neutron powder diffractometry are consistent with Ca-doping causing oxidation only of Cu in the sheet, leaving the copper in the so-called chain site as Cu(I). Many of the structural changes occurring in this system upon oxidation, such as a flattening of the CuO 2 sheets, a contraction of the in-plane CuO distance, and a decrease in the CuCu distance between planes, also occur in the structurally related superconducting system, Pb 2Sr 2(Y 1− x Ca x )Cu 3O 8.

Numerical study of an AlGaAs/GaAs heterostructure bipolar transistor: Emitter design and compositional grading

The effect of various forms of compositional grading of Al in the emitter of a HBT is studied via numerical simulation. The governing drift and diffusion equations and Poisson's equation are solved in two dimensions. For the nominal doping levels and layer widths considered, linear grading over a 300 Å region provides the best overall performance in terms of collector current and high frequency characteristics. When linear grading is introduced over a 300 Å-wide undoped region (spacer layer) the turn-on voltage is found to decrease; the f T is also higher than for other forms of grading at I c below 3000 A/cm 2. When a parabolic grading profile is implemented in the emitter, the resulting performance is not significantly different from linearly graded devices to warrant complicated process steps.

Recombination in solar-grade polycrystalline silicon

Deep level transient spectroscopy (DLTS) and scanning electron microscope-electron beam induced current techniques were used to study recombination in p-type polycrystalline silicon, grown by a casting technique to form ingots with a nominal doping level of 1016 acceptors/cm3. Both Schottky diodes and n+p mesa structures were used for the measurements. In general, very complex DLTS spectra were obtained, and several hole and electron traps were identified. It was found that the hole traps were grain-boundary related. By contrast, the electron traps were found to exist in all regions of the ingot, they were more concentrated in areas near the crucible and at the bottom of the ingot, and they appeared to be growth related.

Dolts of Polysilicon for Solar Cell Applications

AbstractDLTS was applied to p—type polycrystalline silicon, grown by a casting technique to form ingots with a nominal doping level of ∼1016 acceptors/cmg. Both Schottky diodes and n+p mesa structures were used for the measurements. Very complex DLTS spectra were obtained from diodes that contained electrically active grain boundaries, whereas no traps were detected in areas that did not contain electrically active grain boundaries. Several electron and hole traps were resolved.