Carbon-doped Samples Research Articles

Energy gaps in doped MgB 2

Despite early expectations the two-gap superconductivity is retained even upon the heavy carbon doping in MgB2 which leads to suppression of Tc down to 22 K. We show by the point-contact spectroscopy that this strong suppression of Tc is not due to an assumed large interband scattering which would eventually lead to the merging of the gaps. The systematic studies of the carbon doped samples with 0, 2.1, 3.8 and 10% C shows that the large gap on the ω-band is decreased in an essentially linear fashion with increasing the carbon concentrations. The changes in the small gap Δπ up to 3.8 % C are smaller but for the heavily doped sample with 10 % C and Tc = 22 K both gaps are significantly reduced, consistently with a strong suppression of Tc. Strong enhancement of the upper critical field is achieved by the carbon doping up to 3.8% C. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical and optical properties of carbon-doped GaSb

We report low-temperature Hall-effect and photoluminescence measurements on samples of GaSb lightly doped with carbon in the range of ${10}^{16}--\mathrm{}5\ifmmode\times\else\texttimes\fi{}{10}^{17}{\mathrm{cm}}^{\ensuremath{-}3}.$ Temperature, excitation intensity, and impurity concentration dependent photoluminescence (PL) measurements were used to determine a shallow acceptor binding energy for carbon in GaSb of $12.9\ifmmode\pm\else\textpm\fi{}1\mathrm{meV}.$ The carbon acceptor PL transition was determined to be due to a combination of donor-acceptor pair and free-to-bound processes. Temperature-dependent Hall-effect analysis of hole concentration and mobility were performed on nominally undoped and lightly carbon-doped epilayers and on bulk undoped Czochralski-grown GaSb. Experimental hole concentrations of the carbon-doped epilayers were fitted to a two acceptor model that included the deep native acceptor and shallow carbon acceptor levels. Activation energies of 8--11 meV were determined for the lightest carbon-doped samples through least-squares fitting of the experimental hole concentration versus temperature. Carbon-doped epilayers showed significantly higher impurity band conduction at low temperatures than Czochralski-grown GaSb due to the shallower nature of the carbon acceptor levels. Hall mobility data confirmed the presence of impurity band conduction below 10 K, in addition to the usual scattering mechanisms.

P-type carbon doping of GaSb

The growth of carbon-doped GaSb by MOVPE has never been reported to our knowledge, despite increasing interest in carbon-doped GaAsSb alloys for heterojunction bipolar transistor applications. In this work, we report the use of carbon tetrachloride (CCl4) in conjunction with triethylgallium (TEGa) and trimethylantimony (TMSb) to achieve p-type doping levels in GaSb from 5 1016 cm−3 to greater than 1019 cm−3. High resolution x-ray diffraction measurements confirm that the effect of carbon on the lattice parameter is significant for hole concentrations above 1 1019 cm−3 as in the case of GaAs. By introducing controlled low doping levels of carbon into thick homoepitaxial samples, we have succeeded in identifying a carbon-related low temperature photoluminescence band at 795 meV, which we ascribe to band-to-acceptor transitions of carbon acceptors. Temperature-dependent Hall measurements on lightly carbon-doped samples yield somewhat lower binding energies than the spectroscopic data due to impurity banding in the acceptor excited states.

Photoluminescence and Raman characterization of heavily doped Al 0.3Ga 0.7As grown by solid-source molecular beam epitaxy using carbon tetrabromide

Carbon incorporation in Al 0.3Ga 0.7As is systematically studied in solid source molecular beam epitaxy as a function of carbon tetrabromide pressure and substrate temperature. The maximum free carrier concentration attained was 1.5×10 20 cm −3. No effect of substrate temperature on doping density was observed for temperatures in the range 560–700°C. The carbon-induced lattice contraction in AlGaAs determined by X-ray diffraction was found to be proportional to hole density and similar to the contraction in GaAs. Carbon-doped samples exhibited anomalously bright room temperature photoluminescence at low excitation density. The increase in full-width at half-maximum of the 4 K photoluminescence peak and in the TO-to-LO GaAs-like mode intensity ratio in Raman spectroscopy shows a reduction in material quality for the samples grown in the “forbidden temperature range” between 600 and 640°C.

Heavily doped p-GaAs grown by low-pressure organometallic vapor phase epitaxy using liquid CCl4

A hole concentration greater than 1020 cm−3 in GaAs has been achieved using a liquid CCl4 source for carbon in a low-pressure organometallic vapor phase epitaxy system. The resistivity and hole mobility measured at 300 K for a heavily carbon-doped (1.2×1020 cm−3) Hall sample made from a thin (180 nm) epitaxial layer were 8.0×10−4 Ω cm and 65 cm2/V s, respectively. Carbon-doped samples with excellent surface morphology were achieved using a V/III ratio of 22, and growth pressure and temperature of 80 Torr and 600 °C, respectively. A novel photoluminescence technique, based on band-gap shrinkage of heavily doped p+-GaAs, has been shown to be useful for nondestructive measurement of the hole concentration in submicrometer layers.

Submicrostructural and microstructural evolution of pure and oxygen contaminated vacuum melt-spun 99.9999% Al and Al:90 at p.p.m. C

Positron lifetime (PLS) and TEM studies have been completed on a series of pure, carbon-doped (∼90 at p.p.m.) and oxygen contaminated (340–670 at p.p.m.) vacuum melt-spun (VacMS) aluminium samples. Both TEM and PLS data have shown pure and carbon-doped samples to be fully annealed at ∼473 K. Oxygen-contaminated samples were found to contain defects including dislocations (TEM) and small vacancy clusters (PLS) up to the highest annealing temperatures (853 K) studied. The results show that the presence of oxygen completely dominates the evolution of both microstructure and submicrostructure in VacMS aluminium.

Blocking of hydrogen-dislocation interactions in palladium by interstitial carbon (boron) solutes

It has been shown that the segregation of hydrogen to dislocations at 323 K decreases markedly in palladium doped with interstitital carbon or boron after a relatively low temperature annealing, 423 K. By contrast, after a similar treatment, pure palladium continues to show a large segregation of hydrogen to dislocations. TEM studies have shown, however, that the dislocation densities are large in both the pure palladium and the carbon doped samples after annealing at 423 K. These results are consistent with the explanation that the heavy interstitial atoms, carbon or boron, migrate to dislocations at 423 K and block these dislocations for occupation by hydrogen.

Investigation of the magnetic properties of β-boron doped with carbon

The results of an investigation of the magnetic properties of carbondoped boron crystals (carbon content, 0.5–3.5 at.%) in the temperature range 77–300 K are presented. The temperature and field dependence of magnetic susceptibility were investigated using electron paramagnetic resonance (EPR) and magnetoresistance methods. It was found that carbon atoms are donors substituting boron atoms in the lattice. The EPR signal g = 4.28 is due to Fe 3+ ions in a glass-type phase. The magnetoresistance of carbon-doped samples decreases at 0.6–0.8 T and the mechanism seems to be the same as for negative magnetoresistance.

Confirmation of the one-interstitial model for α-iron from positron annihilation experiments in thermal equilibrium on pure and carbon doped samples

The value of the vacancy migration enthalpy in α-iron has been the subject of a long standing controversy. In the one-interstitial model, the value of 0.55 eV has been put forward, whereas in the two-interstitial model a value of about 1.3 eV is given. In a review paper l, it was concluded that the majority of experimental data is in favour of the one-interstitial model. It was also pointed out that, if one accepts this model, an important problem remained to be solved: the vacancy migration enthalpy that can be obtained by subtracting the vacancy formation enthalpy from the self diffusion enthalpy (provided that self-diffusion is driven by a vacancy mechanism), was more than twice the vacancy migration enthalpy of the one-interstitial model. We report here on research we performed concerning this problem. The values of the enthalpies of vacancy formation and self diffusion were carefully re-investigated, taking into account: 1) the influence of ferromagnetic ordering, and 2) the influence of the strong interaction between residual carbon atoms and vacancies. To determine the ferromagnetic self diffusion enthalpy QS D,f, we used reliable literature data on self diffusion in iron. The main conclusion of our analysis is that only an upperlimit to QS D,f, can be given 2: QS D,f ⩽2.75 eV. We also performed positron annihilation measurements in thermal equilibrium on pure and carbon doped (50 and 750 appm) samples to determine the ferromagnetic vacancy formation enthalpy H V f,f. In very pure iron no trapping of positrons is observed below the Curie point, so that H V f,f cannot be determined. However in carbon-doped samples the carbon-vacancy pair can be used as a probe to determine H V f,f. We obtained: H V f,f = (2.0 ± 0.2) eV. This value is the first fully experimental determination of this parameter. By combining the data given above, we conclude that an upperlimit to the ferromagnetic vacancy migration enthalpy H V m,f is: H V m,f ⩽ 0.95 eV. On the other hand, it can be shown 3 that H V m,f ⩾ 0.16 eV. We therefore conclude that there is no longer an inconsistency between the one-interstitial model and the vacancy migration enthalpy derived from thermal equilibrium measurements.

Determination of the ferromagnetic vacancy-formation enthalpy in α-iron by the positron method using the carbon-vacancy pair as a probe

The 511 keV positron annihilation lineshape was measured as a function of temperature between 600 and 1183 K in extremely pure and carbon-doped (50 and 750 appm C) iron. Whereas in pure iron no trapping of positrons is observed below the Curie temperature (1043 K), trapping in carbon-vacancy pairs is observed in this temperature range in the carbon-doped samples. This allowed us to determine the ferromagnetic vacancy-formation enthalpy as 2.0 ± 0.2 eV.