Boron-doped Samples Research Articles

Formation of oxygen complexes in controlled atmosphere at surface of doped glassy carbon

The effects of boron and phosphorus incorporation in phenolic resin precursor to the oxidation resistance of glassy carbon have been studied. In order to reveal the nature and composition of the oxygen complexes formed at the surface of doped glassy carbon, under controlled atmosphere, the surface of the samples was cleaned under vacuum up to 1273 K. Specific functional groups, subsequently formed under dry CO2 or O2 atmosphere on the surface of boron-doped and phosphorus-doped glassy carbon samples, were examined using the temperature-programmed desorption method combined with mass spectrometric analysis. Characterization of surface properties of undoped and doped samples has shown that in the presence of either boron or phosphorus heteroatoms, a lower amount of oxygen complexes formed after CO2 exposure, while, typically, higher amount of oxygen complexes formed after O2 exposure. It has been concluded that the surface of undoped glassy carbon has a greater affinity towards CO2, while in the presence of either boron or phosphorus heteroatoms, the glassy carbon surface affinity becomes greater towards O2, under experimental conditions.

Structural, Microstructural and Magnetic Properties of Ball Milled Boron-Substituted Sm(Co,Fe,Cu,Zr)<sub>7.5</sub> Compounds

The powders in the micrometer scale of Sm(Co0.86Fe0.1B0.04)7.5, Sm(Co0.82Fe0.1Zr0.04B0.04)7.5, Sm(Co0.715Fe0.1Cu0.12Zr0.04B0.025)7.5, Sm(Co0.69Fe0.1Cu0.12Zr0.04B0.05)7.5 compounds have been prepared by using a high-energy planetary ball miller. The X-ray diffraction patterns have an amorphous state after 42 hours milling while the average grain size is in the range of 0.5-10 μm, as measured by using a scanning electron microscope. Amorphization has been achieved at shorter milling times for samples with higher boron content. Depending on the annealing temperature, the major phases are 1:7, 2:17 hexagonal or fcc-Co as revealed from the Xray patterns. The annealing at temperature over 900 oC for 1h helps the formation of the fcc-Co which becomes the dominant phase, mostly on the higher boron doped samples. From the hysteresis curve of Sm(Co0.715Fe0.1Cu0.12Zr0.04B0.025)7.5, a coercive field of ~9 kOe has been determined. For this sample there is difference between the two coercive fields, Ηc(χirr=max) and Ηc(M=0). The zirconium doped sample has higher coercive field than the Sm(Co0.86Fe0.1B0.04)7.5 one. The coercive field is significant increases to the copper doped samples. In the thermomagnetic curves of the asmilled powders, it has been evidenced that there is crystallization, connected to the increase of the magnetization under the constant applied field. The initial and demagnetization curves exhibit slope variations that are connected to the soft fcc-Co phase. Almost zero hydrogen absorption was found for the Cu-doped samples at room temperature.

The superconductivity in boron-doped polycrystalline diamond thick films

Boron-doped polycrystalline diamond thick film was prepared by a hot filament chemical vapor deposition (HFCVD) method. The morphology and structure of the diamond were evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and micro-Raman spectroscopy. The carrier concentration of the boron-doped diamond was 7.3 × 10 20 cm −3, determined by a Hall measurement system. The transport measurements show that the boron-doped diamond thick film is superconductive and the superconducting transition temperatures are 10 K for T c onset and 8.3 K for zero resistance, and there is a strong diamagnetic response in the alternating current (AC) magnetic susceptibility of the boron-doped diamond sample below 8.9 K. Such a high T c value can be attributed to the higher efficiency of doping, contraction of the reconstructed bonds and two-dimensional nature of the surface states for diamond thick films, all together inducing a stronger electron–phonon coupling.

Extended defects in Si films epitaxially grown by low-temperature ECRCVD

Si films grown epitaxially by low-temperature electron–cyclotron resonance chemical-vapor deposition (ECRCVD) were investigated by analyzing extended defects in these films. Extended defects were made visible by Secco etching and scanning (SEM) and transmission (TEM) electron microscopy were used to analyze them in detail. Films grown on Si(1 0 0) wafers show different kinds of etch pits with characteristic shapes, sizes and crystallographic alignments. Their correlation to crystallographic defects is discussed. The typical total etch pit density for boron-doped samples grown at 580 °C on Si wafers is in the range of 10 8 cm −2. Films grown on poly-Si seed layers on glass are strongly affected by Secco etch due to growth regions of different structural qualities and a high density of extended defects. Solar cell test structures were prepared from both types of samples and their results discussed in view to the density of extended defects.

Dopant and defect interactions in polycrystalline silicon thin-film transistors

The influence of channel doping on the threshold voltage of polycrystalline silicon thin-film transistors (TFTs) has been investigated using both boron and phosphorus implants. For each dopant type, asymmetric TFT behavior was seen between enhancement and depletion mode TFTs, with the enhancement mode TFTs always showing a greater shift in threshold voltage for a given dose. From a detailed analysis of the boron-doped samples, using a two-dimensional device simulator, it was demonstrated that the results could be best explained in terms of an asymmetric density of states, in which the deep trapping state density in the lower half of the band gap increased with increasing boron concentration, and the effect of the dopant on threshold voltage was itself determined by this local defect concentration.

Morphology, Defect Spectrum, and Photoluminescence of Thin Si0.7Ge0.3 Layers: I. Structural and Morphological Characterization

Morphology and atomic structure of thin ⟨100⟩-oriented nominally undoped and boron-doped layers, deposited by atmospheric-pressure chemical vapor deposition (APCVD) at on silicon wafers are studied in the first part of this paper. Nominal concentration of boron was for all doped samples. Local strain in the layers was estimated through X-ray diffraction data and depends on the layer thickness and presence of the dopant. Submicrometer scale morphology has been observed on the surface of the layers by atomic force microscopy. Anisotropic grain shape is clearly observed in all undoped samples, whereas round-shaped grains are found on boron-doped layers. Transverse sizes of the nonhomogeneities and surface roughness depend on the thickness and presence of dopant in the layers. For all studied samples, the average surface roughness is equal to or even higher than the nominal layer thickness, indicating that the layers are actually formed with spatially isolated submicrometer-scaled grains. Boron-doped samples have higher average surface roughness compared to nominally undoped layers with the same nominal thickness that is explained within the framework of kinetic approach of morphology formation [ J. Tersoff and F. K. LeGoues , Phys. Rev. Lett. , 72 , 3570 (1994) ] via significant lowering of potential barrier for surface roughening in the boron-doped layers.

Channel doping effects in poly-Si thin film transistors

The influence of channel doping on the threshold voltage of poly-Si thin film transistors (TFTs) has been investigated using both boron and phosphorus implants. For each dopant type, asymmetric TFT behaviour was seen between enhancement and depletion mode TFTs, with the enhancement mode TFTs always showing a greater shift in threshold voltage for a given dose. From a detailed analysis of the boron-doped samples, using a two-dimensional device simulator, it was demonstrated that the results could be best explained in terms of a non-symmetric density of states, in which the deep trapping state density in the lower half of the band-gap increased with increasing boron concentration. This is the first reported observation in poly-Si of shallow level dopants also introducing deep levels in the majority carrier half band-gap.

High rate growth and electrical/optical properties of high-quality homoepitaxial diamond (100) films

Undoped and boron doped homoepitaxial diamond films were grown by high-power microwave plasma chemical vapor deposition (MPCVD). Growth rate was 2.5 μm/h under optimized growth conditions. The grown undoped film thus grown showed large diffusion length of 10.2 μm while short lifetime of 15 ns for electron. The growth rate increased up to 25 μm/h with increasing methane concentration, while free-exciton recombination emission was observed at room temperature. We found the surface became smooth and growth rate increased with increasing substrate temperature. Hall measurement for boron-doped sample revealed high hole mobility of 1500 cm 2/Vs at 290 K for relatively high acceptor density of 1.4×10 18 cm –3. The high-power MPCVD is, thus, indispensable for high-rate growth of diamond films for electronic devices.

Photosensitization of Boron-Doped Diamond by Surface Grafting of Pyrene Groups

Boron-doped diamond samples were exposed successively to singlet oxygen and pyrene-alkylcarboxylic acid molecules. The resulting samples were studied by photoelectrochemistry. It is shown that the expected esterification reaction indeed allowed the grafting of pyrene groups onto the diamond surface. Cathodic photocurrents have been obtained with quantum yields as high as η=0.29% (λ=351 nm, U=-0.38 V vs. a saturated calomel electrode) in oxygen saturated electrolyte. The pyrene coverage is estimated from both photocurrent and photopotential measurements. The role of dissolved oxygen in the photocurrent generation is discussed.

Effects of hydrogen on electronic properties of doped diamond

We have investigated the effects of hydrogen on the electronic structure of diamond doped boron and sulfur using cluster model method within the frame of ab initio density functional theory (DFT). The results show that the presence of hydrogen results in a deep donor level with no change of conductivity type in sulfur-doped diamond samples and the formation of the multiple hydrogen–boron complexes may cause a conductivity type transition in the hydrogen-rich boron-doped diamond samples.

Superconductivity in boron-doped diamond

Here we report the experimental study of a new superconductor, B-doped diamond. Subjecting a mixture of amorphous boron with graphite to a high-temperature and high-pressure treatment, we synthesized polycrystalline boron-doped diamond samples, whose sizes were suitable for specific heat measurements. For this kind of sample, we detected two specific heat anomalies at temperatures (∼4 K) closely corresponding to temperatures at which the resistance and magnetic susceptibility anomalies take place. The most probable origin of these specific heat anomalies is a bimodal B distribution in polycrystalline diamond. The correlation of physical properties, such as specific heat, electrical resistivity, and magnetic susceptibility, near the superconducting transition temperature is clear evidence of bulk superconductivity in these boron-doped diamond samples. Specific features in the low frequency range of Raman spectra correlate with superconducting properties of diamond. These features help to select heavily boron-doped CVD diamond films (DF) grown on (111) natural diamond, which exhibit superconductivity.

FeB and CrB pair reassociation kinetics in imperfect Si controlled by contactless lifetime scan maps

The kinetics of metal-acceptor pairing in boron-doped multicrystalline samples are investigated by means of contactless lifetime measurement. The case of FeB and CrB pairs is discussed and the influence of extended defect is evidenced for FeB. It is found that the reassociation of the metal-acceptor pairs is fast immediately after the dissociation and then slows down for several days. The results allow the identification of mixed contamination by Cr and Fe.

Mechanism of p-type-to-n-type conductivity conversion in boron-doped diamond

We report on the role of two boron–hydrogen complexes in the conductivity conversion from p-type to n-type in boron-doped diamond samples. The calculated electronic structures of the simulated clusters show that the boron–hydrogen complex of hydrogen–boron pairs (BH1) is characteristic of p-type conductivity, and that of two hydrogen–boron centers (BH2) or three-hydrogen-boron centers (BH3) have a character of n-type conductivity. The phenomenon of conversion from p-type-to-n-type conductivity through deuteration may be explained by the transformation from the BH1 into BH2 and/or BH3 complexes.

Influence of hydrogen plasma treatment on electrochemical behavior of moderately and highly boron doped diamond electrodes

This article is concerned with the influence of both, the boron dopant concentration and hydrogen plasma treatment, on the electrochemical properties of polycrystalline diamond. The material characterizations, given by SIMS and XPS analysis, allowed us to understand the electrochemical behaviors ( I– V curves and capacitance–voltage measurements) of moderately and highly boron doped samples which have been hydrogenated or not, in 2 M H 2SO 4 medium containing the Ce 4+/Ce 3+ redox couple as an electrochemical probe. Moderately boron doped diamond electrodes are the most affected by hydrogen treatment, becoming insulating electrodes. For highly boron doped diamond electrodes, the charge transfer at the interface seems to be undisturbed by hydrogen treatment.

Surface resistance and field emission current measurements on chemically vapour depositedpolycrystalline diamond measured by scanning probe methods

Scanning tunnelling microscopy (STM) and current imaging tunnelling currentspectroscopy (CITS) methods were performed on polycrystalline diamond filmsgrown on silicon substrates grown by microwave plasma-enhanced chemical vapourdeposition. Large tunnelling currents were observed at some grain boundariesand crystal surfaces with secondary grains. Following atomic force microscopy(AFM) measurements, we performed scanning probe contact current (SPCC)measurements to investigate the spatial variation of electrical resistance on the surface byusing an AFM cantilever in contact mode. The conducting grain boundaries andfacets were observed on both boron-doped and undoped samples. For microscalecharacterization of the field emission properties, we performed scanning probefield emission current (SPFEC) measurements. From the results of STM/CITS,AFM/SPCC and SPFEC, it is concluded that the specific grain boundaries andfacets on polycrystalline diamonds work as initial points of electron emission andcause high field emission current through a conducting pass formed in the bulk.

Charge-sensitive deep level transient spectroscopy of boron-doped and gamma-irradiated mono- and polycrystalline diamond

Electrically active defects in the boron-doped chemical-vapor deposited polycrystalline diamond films and single-crystal diamonds synthesized by high-pressure high-temperature technique were investigated. Isothermal charge-sensitive deep level transient spectroscopy was applied to study the density, activation energy and capture cross-section of native and boron-induced defects in diamond samples. It was shown that defects in the boron-doped polycrystalline diamond samples have a continuous energy spectrum with two peaks that correspond to discrete energy levels in the forbidden band of diamond, while in boron-doped homoepitaxial diamond films and single-crystal diamonds only one boron-induced deep level was displayed. The activation energy of the boron-induced levels decreases and the defect density increases with increasing boron concentration. It was found that gamma irradiation changes the parameters of boron-induced levels and increases the density of point defects with a continuous energy spectrum. The difference in activation energy of the boron-induced level into (1 1 1) and (1 0 0) faces of single-crystal diamond was observed as well.

Effect of thermal neutron irradiation in boron-doped melt-textured YBCO superconductors

Y1.6Ba2.3Cu3.3Ox superconductors with different amounts of boron doping have been synthesized using the MPMG technique. Undoped and boron-doped samples were irradiated with thermal neutrons to study the effect of defects produced by the fission reaction, B(n,α)Li, on the pinning and the critical current. We observed that the pinning and the critical current density were improved as a result of thermal neutron irradiation. This improvement was slightly stronger for the boron-doped samples compared to the undoped ones.

Real and apparent grain sizes in chemical vapor deposited diamond

A boron-doped, chemical vapor deposited (CVD) diamond, with a doping level of approximately 10 18 cm −3, is used to demonstrate the very different grain sizes deduced from the application of various scanning electron microscopy (SEM) techniques to a polished polycrystalline surface. The boron-doped sample was chosen for this investigation because of the very different boron up-take on the {111} and {100} growth sectors and the consequent changes on the physical properties of the underlying crystal. It is shown that SEM contrast resulting from electron channeling gives a gross over-estimate of the grain size, while luminescence microscopy and surface enhanced secondary emission give a better indication, although they can give slight underestimates. The results are consistent with recently published transmission electron microscopy (TEM) observations which show that the grains in thick polycrystalline CVD diamond form clusters with common growth directions [Philos. Mag., A 82 (2002) 1741]. The individual grains within a cluster are related by various twinning operations. This behavior is characteristic of columnar CVD growth and does not depend on boron doping.

Current image tunneling spectroscopy of chemical vapor deposited diamond films

The effects of boron doping on the electron field emission properties of diamond films were examined using current image tunneling spectroscopy in atomic force microscopy (AFM). Tunneling current–voltage (It–V) characteristics measured by AFM indicate that incorporation of boron species induces the presence of the impurity state. The films doped with 3 sccm boron (B3) possess the smallest emission barrier (Eb=1.65 eV) and the largest emission ratio (η=38%) among the boron-doped diamond samples. Therefore, B3 diamond films can be turned on at the smallest electric field (E0=8.8 V/μm), exhibiting largest field emission capacity (Je=250 μA/cm2). Restated, the electron field emission behavior of diamond films is closely related to the formation of impurity states due to boron doping.

Investigating oxygen flooding at oblique 2 and 1 keV oxygen sputtering for microelectronics support applications

Low energy (2.1 and 1 keV) oxygen bombardment at oblique incidence in combination with oxygen flooding was applied to two kinds of epitaxially grown, boron-doped samples. The angles of incidence ranged between 54° and 44°, realized in a Cameca 6f. At surface saturation conditions, no indication of ripples formation was found up to at least 700 nm depth. In comparison to measurements at perpendicular oxygen beam incidence (no flooding), the boron decay lengths were about equal, whereas the apparent profile shift was clearly lower (roughly half) in the flooding case. Thus, no disadvantage is found for these oxygen flooding conditions as compared to perpendicular oxygen bombardment conditions.