An Insight Into What Superconducts in Polycrystalline Boron-Doped Diamonds Based on Investigations of Microstructure

An insight into what superconducts in polycrystalline boron-doped diamonds based on investigations of microstructure

N. Dubrovinskaia†‡§, R. Wirth¶, J. Wosnitzaʈ, T. Papageorgiouʈ, H. F. Braun††, N. Miyajima‡‡, and L. Dubrovinsky††

†Mineralphysik und Strukturforschung, Mineralogisches Institut, Universita¨t Heidelberg, D-69120 Heidelberg, Germany; ‡Lehrstuhl fu¨r Kristallographie, ††Physikalisches Institut, and ‡‡Bayerisches Geoinstitut, Universita¨t Bayreuth, D-95440 Bayreuth, Germany; ¶GeoForschungsZentrum Potsdam, Experimental Geochemistry and Mineral Physics, 14473 Potsdam, Germany; and ʈHochfeld-Magnetlabor Dresden, Forschungszentrum Dresden-Rossendorf, D-01314 Dresden, Germany

Edited by Ho-kwang Mao, Carnegie Institution of Washington, Washington, DC, and approved June 22, 2008 (received for review February 20, 2008) The discovery of superconductivity in polycrystalline boron-doped down to 0.5 K (15, 16). This is at odds with the results for the diamond (BDD) synthesized under high pressure and high temper- polycrystalline materials and contrasts especially with one report atures [Ekimov, et al. (2004) Nature 428:542–545] has raised a (3), where superconducting onset temperatures as high as 7.4 K number of questions on the origin of the superconducting state. It were observed for CVD BDD samples with even smaller boron was suggested that the heavy boron doping of diamond eventu- concentrations [9.4 ϫ 1020 cmϪ3 (0.53 at %)] (3). ally leads to superconductivity. To justify such statements more We investigated polycrystalline BDD samples that were syn- detailed information on the microstructure of the composite ma- thesised at 20 (1) GPa and 2,300 (50) K (four samples) as well terials and on the exact boron content in the diamond grains is as at 9.0 (5) GPa and 2,500 (50) K (one sample) in a 5,000-tonne needed. For that we used high-resolution transmission electron press by using a HPHT technique as described in refs. 10 and 12. microscopy and electron energy loss spectroscopy. For the studied A mixture of graphite (referred further to as 12C) or isotopically 13 superconducting BDD samples synthesized at high pressures and pure amorphous carbon C and B4C in a ratio C:B ϭ 13:1 (Ϸ7 high temperatures the diamond grain sizes are Ϸ1–2 ␮m with a at % B) were used as starting materials. Synchrotron and SCIENCES

boron content between 0.2 (2) and 0.5 (1) at %. The grains are in-house x-ray diffraction investigations revealed for all five APPLIED PHYSICAL separated by 10- to 20-nm-thick layers and triangular-shaped samples studied that their main crystalline component (Ͼ99%) pockets of predominantly (at least 95 at %) amorphous boron. is diamond with a small amount of boron carbide B50C2. In some These results render superconductivity caused by the heavy boron samples residuals of the starting material B4C were found (10, doping in diamond highly unlikely. 12). Microprobe analysis of the samples revealed a boron concentration of 2.6 at % (4.6 ϫ 1021 cmϪ3) in the samples, superconductivity ͉ transmission electron microscopy whereas Hall-coefficient measurements gave a charge-carrier concentration of 1.4 ϫ 1021 cmϪ3, that is, three times as low as fter the discovery of superconductivity in boron-doped the apparent B concentration in the material (supporting infor- Adiamond (BDD) (1), numerous theoretical and experimen- mation (SI) Fig. S1). Both microprobe analysis and energy- tal studies (2–10) confirmed the phenomenon and went along dispersive x-ray (EDX) spectra did not show the presence of with its explanation. Superconductivity in group IV semicon- other elements than carbon and boron in BDD samples. ductors with diamond structure—such as silicon, germanium, The superconducting transition temperatures were deter- and their alloys—was already predicted in the early 1960s to mined by means of electrical transport and specific heat mea- occur at very low temperatures (11). Except for diamond (1–3, surements (Fig. S2 and Fig. S3), and were found to be in good 8, 10), there was a report on experimentally observed supercon- agreement with earlier reports (1, 7, 10). ductivity in boron-doped silicon (9). For diamond, Ekimov et al. The boron content in our samples was characterized by (1) suggested that the superconducting transition temperature measurements of diamond lattice parameters, Raman spectros- (Tc) increases with heavy boron doping (Tc Ϸ 4 K at 2.6 at % B). copy, Hall-effect measurements, scanning electron microscopy, However, further investigations of superconducting BDD pre- and microprobe analysis (10, 12). All of the methods mentioned pared by the high-pressure–high-temperature (HPHT) tech- above give only sample-averaged results. In particular, no infor- nique and by chemical vapor deposition (CVD) revealed strong mation on the spatial distribution of boron on the submicron inhomogeneities in these materials (1, 3, 7, 10, 12). In particular, level in the diamond samples was available. To investigate the B-rich phases [such as B4C (1, 7, 10, 12), used as a reactant, and microstructure and the exact boron location in our supercon- B50C2 (10, 12)] were found in HPHT BDD samples. In BDD ducting BDDs we used transmission electron microscopy prepared by CVD sp2-bonded carbon is unavoidable (3), and one (TEM). Electron transparent foils of BDD samples were pre- cannot exclude that sp2-bonded amorphous or graphite-like pared by means of focused ion beam (FIB) techniques (for carbon accumulates some amount of boron. The presence of extra phases and large discrepancies in the B content determined by various methods (1–3, 7–10, 13, 14) (such as secondary Author contributions: N.D., H.F.B., and L.D. designed research; N.D., R.W., J.W., T.P., H.F.B., ion-mass spectroscopy, Hall-effect measurements, correlations N.M., and L.D. performed research; N.D., R.W., J.W., T.P., and N.M. contributed new reagents/analytic tools; N.D., R.W., J.W., H.F.B., N.M., and L.D. analyzed data; and N.D. and with diamond lattice parameters, infrared spectroscopy, and L.D. wrote the paper. microprobe analysis) and the absence of a clear correlation The authors declare no conﬂict of interest. between Tc and the B concentration (10) raise the question of This article is a PNAS Direct Submission. how much boron indeed is incorporated into the diamond §To whom correspondence should be addressed at: Mineralphysik und Strukturforschung, structure of superconducting samples. It is remarkable that, Mineralogisches Institut, University of Heidelberg, Im Neuenheimer Feld 236, D-69120 although measurements of the temperature dependence of the Heidelberg, Germany. E-mail: [email protected]. resistance have been conducted on BDD single crystals with This article contains supporting information online at www.pnas.org/cgi/content/full/ Ϫ boron content ranging from Ϸ1019 to 2.7 ϫ 1021 cm 3 (i.e., up 0801520105/DCSupplemental. to 1.53 at % B), no evidence of superconductivity was found © 2008 by The National Academy of Sciences of the USA

www.pnas.org͞cgi͞doi͞10.1073͞pnas.0801520105 PNAS ͉ August 19, 2008 ͉ vol. 105 ͉ no. 33 ͉ 11619–11622 Downloaded by guest on September 28, 2021 Fig. 1. Bright-ﬁeld TEM images of a polycrystalline BDD sample. (A) A micrometer-sized grain is separated from other grains by a clearly visible straight boron-rich boundary. (B) Twin boundaries and dislocations observed within a grain.

details, see Materials and Methods). Fig. 1 shows two bright-field To probe the chemical nature of the inter- and intragranular TEM images of our polycrystalline sample, which consists of material in our samples, we measured the electron energy-loss micrometer-size carbon grains with predominantly diamond spectra (EELS) at boron K and carbon K ionization edges for structure as revealed by x-ray and confirmed by electron- numerous pockets and grains (Fig. 2B). The EELS spectra (Fig. diffraction data. A number of grains from different samples were 2B Lower) revealed that the pocket material contains Ϸ95% investigated by using high-resolution TEM (HRTEM) (SI Text, boron with a small amount of carbon (Ϸ5–6%, varying for the Fig. S4). All investigated grains are separated by layers of different pockets). The appearance of some amorphous boron amorphous material along straight boundaries with thicknesses material under high-pressure, high-temperature syntheses in the of 10–20 nm. The material constituting the layers along the grain B–C–N–O system was reported in refs. 17 and 18. The authors boundaries also fills triangular-shaped pockets at the grain (17, 18) provided a detailed investigation of boron K edges from junctions (Fig. 2A). With side lengths reaching 0.5 ␮m some of several ␣-rh B-bearing materials. They demonstrated that the B the triangular pockets have sizes comparable to some diamond K edge energy-loss near-edge structure (ELNES) of ␣-rh B and grains. The complete absence of any TEM diffraction contrast the products with the ␣-rh B structure exhibit an edge rich in (uniform light-gray contrast during sample tilting) indicates the features (17). The ELNES for the B K edge from ␣-rh B and amorphous state of the filler material (Fig. 2A). structurally related materials was divided into the ␲*(Ͻ195 eV)

Fig. 2. The results of TEM and EELS investigations. (A) Bright-ﬁeld TEM image of a BDD sample. The amorphous material constituting the layers along the grain boundaries also ﬁlls triangular-shaped pockets at the grain junctions. (B) Electron energy-loss spectra (EELS) at the boron K and carbon K ionization edges from the pockets (Lower) and the diamond grains (Upper) reveal that the pocket material consists of Ϸ95 at % boron with a small amount (Ϸ5–6 at %, varying in different pockets) of carbon. The amount of boron in the diamond grains does not exceed 0.5%. The ␲* peak due to sp2-bonded carbon is very common for the pure diamond investigated by TEM on carbon grid. A smaller beam size was selected to measure the intergranular pocket, thus resulting in lower intensity and a more ‘‘noisy’’ spectrum (Lower).

11620 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0801520105 Dubrovinskaia et al. Downloaded by guest on September 28, 2021 Our EELS data show as well that there are a few small (5–50 nm) isolated spots with very high B concentration within the diamond grains. These spots have platelet-like shapes (Fig. S5). HRTEM revealed their crystalline structure and the electron diffraction patterns showed reflections at 4.36 Å and 3.92 Å characteristic for B50C2, in agreement with synchrotron diffraction data obtained for this sample (12). Our observations render it highly unlikely that the observed superconductivity in BDD synthesized at high pressures and high temperatures is actually related to the boron incorporated in the diamond. If so, then even Ͻ0.5 at % B would be sufficient to induce superconductivity in diamond with onset temperatures of Ϸ2.4 K and transition widths of Ͻ1.4 K. Our heat capacity measurements confirm the bulk nature of the superconductivity. However, the total amount of material converted to the superconducting state in our samples is of the order of 20% or less, consistent with other reports (7) (Fig. S3). If boron doping at a level of 0.5 at % would actually be enough to generate superconductivity in diamond, then, based on our observation of a homogeneous doping of all diamond grains, one would expect the majority of the sample to become superconducting (not only Ͻ20%). Moreover, as mentioned, presently available data for single-crystalline BDD contradict the results obtained for polycrystalline samples. Indeed, superconductivity has not been observed resistively in single-crystalline diamonds with B con- tents up to 1.53 at % (15, 16). However, the diamond grains in our superconducting HPHT BDD samples are completely iso- SCIENCES

lated from each other by a boron-rich amorphous phase. Con- APPLIED PHYSICAL sequently, this phase controls the superconducting transition at Fig. 3. Elemental B map obtained by using EELS and consisting of four least in the electrical transport data. images of an intergranular area. It shows four diamond grains, five grain Besides the investigation of BDD produced by the HPHT boundaries, and two pockets. The vast majority of boron is concentrated in the technique we were able to study the microstructure of a super- grain boundaries and pockets. Boron is quite homogeneously dispersed in a conducting (Tc ϭ 5.2 K) boron-doped thin film deposited on a very small amount within the diamond grains. single-crystalline diamond (111) surface by use of the microwave plasma-assisted chemical vapor deposition (MPCVD) method (22, 23). The sample was kindly supplied by Dr. Y. Takano and ␴* (195–210 eV) regions. The B K edge shapes from the ␣-rh ␣ (National Institute for Materials Science, Tsukuba, Japan). Our B-bearing materials could be divided into three types (17): -rh HRTEM analysis surprisingly revealed, instead of the expected B, B6O, and B4C. However, the EELS spectra we obtained from diamond morphology, a graphite-like structure (24) with a the carbon-doped amorphous boron phase (with sharp features Ϸ ␲ ␴ homogenous B distribution within the film according to EELS at 189 eV and 199 eV in the * and * regions, respectively; mapping (Fig. S6).§§ Consequently, also for this sample, no Fig. 2B Lower)attheBK edge are different from that known for evidence for superconductivity in BDD was found. crystalline boron, boron carbides, nitrides, oxides, and boron Based on the results of our research, we can suggest a few carbooxinitrides (17–20). This suggests that the intergranular hypotheses for the explanation of the nature of superconductiv- boron does not form B12 icosahedra typical for the structure of ity in the investigated samples: (i) Undistorted diamond becomes pure boron and boron carbides. The observed EELS spectra are superconducting at B concentrations much lower than reported, also different from the spectrum of amorphous boron carbide but that strongly contradicts the results on single-crystal BDD (21). An example for an elemental mapping using EELS is shown samples (15, 16). This also does not agree with the experimental in Fig. 3. Practically all boron is concentrated in the grain heat capacity data showing that only a small fraction of the boundaries and pockets. sample becomes superconducting, although diamond forms its The accurate investigation of the interior of the diamond- major part. (ii) The graphite-like regions with comparatively structured grains (Fig. 3) shows that boron is quite homogeneously moderate B concentrations inside the grains become supercon- dispersed in a very small amount within the carbon grains. Boron ducting. However, these innermost parts of diamond grains are mapping did not show any directional dependence in B concentra- not connected to each other; this hypothesis cannot explain tion within the diamond grains. To quantify the amount of boron superconductivity observed in resistivity measurements. (iii) The in the well crystallized diamond part of the grains, we measured a intergranular boron-rich material becomes superconducting. In number of EELS spectra at various points (Fig. 2B Upper) Our our opinion, the third hypothesis is most probable, because results show consistently that the true amount of boron incorpo- carbon-doped amorphous boron phase, which is filling all of the rated into the diamond structure is between 0.2 (2) and 0.5 (1) at intergranular space in the polycrystalline samples, forms a % (see details of quantitative EELS analysis in Materials and continuous net through the whole sample and its amount agrees Methods). That is considerably less than the 0.8 at % estimated from the number of charge carriers determined by the Hall-effect measurements, and almost one order of magnitude less than the §§Besides our HRTEM investigations, we observed graphite reflections at 3.34 Å (002), 2.04 overall amount of B (2.6 at %) as determined by electron micro- Å (101), and 1.79 Å (102) in diffraction patterns obtained by use of high-brilliance probe analysis (10, 12). This result is consistent with our observation in-house and synchrotron x-ray diffraction (XRD). Hoesch et al. (24) as well studied BDD thin films and observed a (002) reflection (d Ϸ 1.79 Å) forbidden for diamond. This hints that boron is mainly localized in intergranular noncrystalline layers at the presence of graphite in their samples because the (102) graphite reflection appears and pockets. at d Ϸ 1.79 Å.

Dubrovinskaia et al. PNAS ͉ August 19, 2008 ͉ vol. 105 ͉ no. 33 ͉ 11621 Downloaded by guest on September 28, 2021 with the estimations of a superconducting part of the sample energy-filtered images applying a 20-eV window to the zero-loss peak. EELS based on the heat capacity experimental data. spectra of the different K edges (B, C) were acquired in the diffraction mode To prove that superconductivity in diamonds is caused by a with a dispersion of 0.1 eV per channel and an entrance aperture of 2 mm. The resolution of the filter was 0.9 eV at half-width of the full maximum of the certain amount of boron doping one would need to measure well zero-loss peak. The acquisition time was 1 s. Processing of the spectra (back- characterized single-crystalline BDD. Otherwise, the presence ground subtraction, removal of multiscattering, and quantification) was per- of other complex phases in the available composite materials formed by using the DigitalMicrograph software package. hampers any definite conclusion. Details of Quantitative EELS Analysis. Removal of plural scattering by Fourier- Materials and Methods ratio deconvolution by using the DM software package; t ϫ lambda was in the Details of the samples synthesis and preparation are described in refs. 10 range of 0.7 to 0.8 (thickness ϫ electron mean free path); acceptance semi- and 12. angle used was 1.8 mrad; EELS quantification occurred by using the DM software package (EELS Quantification); cross-sections were calculated by Transmission Electron Microscopy (TEM). Electron transparent foils were pre- using the Hartree–Slater model; background model was Power Law; for background and signal windows, we used default values of the software. pared by focused ion beam (FIB) techniques. This allows the preparation of From our long-standing experience with the FIB technique we can exclude site-specific TEM foils with typical dimensions of 15–20 ␮m width, Ϸ10 ␮m that the observed noncrystalline layers and pockets are produced during the length, and Ϸ0.150 ␮m thickness. Focused Ga ions have been used to sputter FIB milling. Irradiation damage as a cause of the noncrystallinity can be material off the sample. A special device, called a selected carbon mill (SCM) eliminated as well, because after insertion of the sample into the TEM electron significantly enhanced the sputtering process of diamond. In this process beam focusing was avoided until the tilt experiments had been carried out. OH-containing MgSO was used, which was heated inside the SCM device. 4 EDX spectra were obtained in the scanning transmission mode (STEM) by Water vapor was brought close to the ion beam by inserting a needle close to using the TIA software package of the TEM. Significant mass loss during the location where the foil was cut. The H2O decomposed by the ion beam and analysis was avoided by scanning the beam in a preselected window (20 ϫ 20 oxygen oxidized the diamond. Details of the FIB technique and the use of a nm or larger). The spot size was Ϸ0.1 nm, and the acquisition time was 60 s at SCM are described elsewhere (25). an average count rate of 60–80 cps. This resulted in a counting error of about TEM investigations were performed with a TECNAI F20 XTWIN transmission 4–5% at a 3␴ level. electron microscope operating at 200 kV with a field emission gun electron source. The TEM is equipped with a Gatan Tridiem filter, an EDAX Genesis x-ray ACKNOWLEDGMENTS. We thank G. Eska and P. McMillan for useful discus- analyzer with an ultrathin window and a Fishione high-angle annular dark- sions. Work at Bayreuth was supported by Deutsche Forschungsgemeinschaft field detector (HAADF). The Tridiem filter was used for the acquisition of (DFG) through DFG Priority Program 1236.

1. Ekimov EA, et al. (2004) Superconductivity in diamond. Nature 428:542–545. 15. Vishnevskii AS, Gontar’ AG, Torishnii VI, Shul’zhenko AA (1981) Electrical conductivity 2. Bustarret E, et al. (2004) Dependence of the superconducting transition temperature of heavily doped. p-type diamond. Sov Phys Semicond 15:659–661. on the doping level in single-crystalline diamond films. Phys Rev Lett 93:237005. 16. Blank VD, et al. (2007) Low-temperature electrical conductivity of heavily boron-doped 3. Takano Y, et al. (2004) Superconductivity in diamond thin films well above liquid diamond single crystals. Phys Stat Sol (B) 244:413–417. helium temperature. Appl Phys Lett 85(14):2851. 17. Garvie LAJ, Hubert H, Petuskey WT, McMillan PF, Buseck PR (1997) High-pressure, 4. Lee KW, Pickett WE (2006) Boron spectral density and disorder broadening in B-doped high-temperature syntheses in the B–C–N–O system II. Electron energy-loss spectros- diamond. Phys Rev B 73:075105. copy (EELS). J Solid State Chem 133:365–371. 5. Xiang HJ, Li Z, Yang J, Hou JG, Zhu Q (2004) Electron-phonon coupling in a boron- 18. Hubert H, et al. (1998) High-pressure, high-temperature syntheses and characteriza- doped diamond superconductor. Phys Rev B 70:212504. tion of boron suboxide (B6O). Chem Mater 10:1530–1537. 6. Boeri L, Kortus J, Andersen OK (2004) Three-dimensional MgB2-type superconductivity 19. Ahn CC, Krivanek OL, Burgner RP, Disko MM, Swann PR (1983) EELS Atlas: A Reference in hole-doped diamond. Phys Rev Lett 93:237002. Guide of Electron Energy Loss Spectra Covering All Stable Elements (Gatan, Inc., 7. Sidorov VA, Ekimov EA, Stishov SM, Bauer ED, Thompson JD (2005) Superconducting Warrendale, PA). and normal-state properties of heavily hole-doped diamond. Phys Rev B 71:060502. 20. Langenhorst F, Solozhenko VL (2002) ATEM-EELS study of new diamond-like phases in 8. Klein T, et al. (2007) Metal-insulator transition and superconductivity in boron-doped the B-C-N system. Phys Chem Chem Phys 4:5183–5188. diamond. Phys Rev B 75:165313. 21. Ge D, Domnich V, Juliano T, Stach EA, Gogots Y (2004) Structural damage in boron 9. Bustarret E, et al. (2006) Superconductivity in doped cubic silicon. Nature 444:465–468. carbide under contact loading. Acta Mater 52:3921–3927. 10. Dubrovinskaia N, Eska G, Sheshin GA, Braun HF (2006) Superconductivity in polycrystalline boron-doped diamond synthesized at 20 GPa and 2700 K. J Appl Phys 99:033903. 22. Takano Y, et al. (2005) Superconductivity in polycrystalline diamond thin films. Dia- 11. Cohen ML (1964) Superconductivity in many-valley semiconductors and in semimetals. mond Relat Mater 14:1936–1938. Phys Rev 134:A511–A521. 23. Takano Y, et al. (2007) Superconducting properties of homoepitaxial CVD diamond. 12. Dubrovinskaia N, et al. (2006) High-pressure high-temperature synthesis and charac- Diamond Relat Mater 16:911–914. terization of boron-doped diamond. Z Naturforsch 61b:1561–1565. 24. Hoesch M, et al.(2006) Acoustic and optical phonons in metallic diamond. Sci Technol 13. Bustarret E, Gheeraert E, Watanabe K (2003) Optical and electronic properties of Adv Mater 7:S31–S36. heavily boron-doped homo-epitaxial diamond. Phys Stat Sol (A) 19:9–18. 25. Wirth R (2004) Focused Ion Beam (FIB): A novel technology for advanced application of 14. Brunet F, et al. (1998) The effect of boron doping on the lattice parameter of micro- and nanoanalysis in geosciences and applied mineralogy. Eur J Mineral 16:863– homoepitaxial diamond films. Diamond Relat Mater 7:869–873. 877.

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