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Heteroepitaxial growth of erbium carbide on boron doped homoepitaxial diamond (100) films
Diamond and Related Materials 11 (2002) 1332–1336
Heteroepitaxial growth of erbium carbide on boron doped homoepitaxial diamond (100) films C. Sabya,*, P. Mureta, F. Pruvosta, G. Patratb a
´ ´ Electroniques des Solides, C.N.R.S., B.P. 166, 38042 Grenoble Cedex 9, France Laboratoire d’Etude des Proprietes b Laboratoire de Cristallographie, C.N.R.S., B.P. 166, 38042 Grenoble Cedex 9, France Received 3 April 2001; received in revised form 12 June 2001; accepted 10 October 2001
Abstract The erbium carbide formation on homoepitaxial C(100) diamond thin films has been studied by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), grazing-incidence X-ray diffraction (GIXD) and Raman spectroscopy. Diamond films, 2.5 mm thick, are grown by microwave plasma chemical vapor deposition (MWCVD) and p-type doped (1017 Bycm3) in the vapor phase. Carbide formation is monitored on two different films: either erbium is evaporated under ultra high vacuum (UHV) on a free diamond surface and this step is followed by an anneal; or a reactive deposition is performed at 750 8C under 5=10y6 mbar of CH4 on a hydrogenated diamond surface. After annealing up to 850 8C, the reaction is not complete in the first deposit and only a fraction of erbium atoms forms carbide. On the contrary, a heteroepitaxial dicarbide layer is grown when methane pressure is used. GIXD measurements performed on this latter type of film show that the (100) plane ErC2 is parallel to the C(100) plane but the carbide lattice is rotated 458 in comparison with the diamond lattice, corresponding to a w110x ErC2 y yw100x C epitaxial relationship. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: CVD diamond films; Carbide; Heteroepitaxy; Schottky diode
1. Introduction Homoepitaxial (100)C diamond films grown by chemical vapor deposition (CVD) and doped with boron (p-type) are now available with good electrical quality and a large range of boron concentrations w1x. One issue of high temperature electronics on p-type diamond consists in achieving Schottky contacts with large potential barrier heights. The metallic contact must show good adhesion on the diamond substrate, chemical inertness and stability at elevated temperatures, as well as low work function. Also, the structural damage near the metalydiamond interface has to be avoided because it would induce localized states near the interface. To fulfill these conditions, erbium appears to be a good candidate. It forms a dicarbide a-ErC2 which has the tetragonal CaC2 structure w2x and a lattice mismatch with C(100) of less than 1%, a favorable case for *Corresponding author. Tel.: q33-4-76-88-7451; fax: q33-4-7688-7988. E-mail address: [email protected] (C. Saby).
epitaxy. Finally, its work function is low (2.9–3 eV) and gives a Schottky barrier on p-type diamond of 1.9 eV w3x. The purpose of this paper is to show that the erbium carbide heteroepitaxy on the hydrogenated diamond (100) surface is feasible. This type of surface is devoid of oxygen and this fact favors the formation of erbium carbide w4x whereas oxygen would cause oxidation. The reaction at 750 8C with erbium evaporated either in ultra vacuum or under 5=10y6 mbar of methane was studied by low-energy electron diffraction (LEED), photoelectron spectroscopy (XPS), grazing-incidence Xray diffraction (GIXD) and Raman spectroscopy. 2. Experiments The diamond films were grown by microwave plasma assisted CVD in a NIRIM type reactor at 830 8C, 30 torr and 4% methane in hydrogen. The substrate was (100) type Ib synthetic diamond, 3=3=0.5 mm3, containing 1019 cmy3 nitrogen. A buffer layer of 0.5 mm of undoped diamond is grown before the 2-mm-thick
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 6 5 2 - 5
C. Saby et al. / Diamond and Related Materials 11 (2002) 1332–1336
doped film. The film was doped during the growth by diborane with wBx y wCxs0.1 ppm. The boron concentration in the solid phase, measured by infrared absorption spectroscopy and calibrated by SIMS w1x was 1=1017 cmy3. The activation energy of the conductivity between room temperature and 500 8C was 0.38 eV w5x, characteristic of boron doped diamond. The sample was hydrogenated in a microwave plasma at 830 8C during 15 min (no oxygen on the surface). It was then pasted on a 1 cm2 square silicon wafer and mounted on a resistively heated holder. Electrical contact was provided between the film and the silicon wafer by means of silver paste. An in situ annealing at 900 8C under ultra high vacuum (UHV) is performed for hydrogen desorption of the subsurface ‘H doped’ layer obtained after the growth in the hydrogen microwave plasma. Erbium is then deposited using a UHV electron-gun evaporator which heats an Er wire by electronic bombardment w6x. Two types of deposit were investigated. The first method consists in evaporating erbium under UHV followed by annealing at 750 or 850 8C. In the second one, the diamond film was held at 750 8C during the deposit under 5=10y6 mbar of CH4. One monolayer of erbium is defined as a compact layer of erbium atoms, i.e. 3.5 ˚ thick, the atomic diameter of erbium. The electron A spectrometer was calibrated with reference to the Ag 3d5y2 line (368.3 eV) and the Fermi level was determined from a silver sample. For the GIXD mapping in the reciprocal space, the samples were excited by the Cu Ka X-ray line at grazing incidence. The Raman spectra were obtained with a DILOR Infinity MicroRaman spectrometer, the films being excited by the 633nm line of a He–Ne laser. Molybdenum carbide dots serve as ohmic contacts on the top surface w3x and erbium carbide contacts are delineated as square dots 0.2 mm wide, by means of electron-beam lithography.
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Fig. 1. C1s core-level excited by the Mg Ka X-ray. Hydrogenated diamond after a 30-min anneal at 900 8C in UHV (curve a); the same as (a) with a subsequent deposit of 10 monolayers of erbium (curve b); the same as (b) annealed at 750 8C during 15 min (curve c); the same as (a) with a subsequent deposit of 10 monolayers of erbium evaporated on a diamond surface heated at 750 8C under 5=10y6 mbar of CH4 (curve d). Binding energy is referenced to the Fermi level EF.
3. Results and discussion The as-grown surface exhibits a C(100) (2=1) LEED pattern, and no modification of this surface arrangement is observed after annealing at 900 8C in UHV. XPS check scans performed on the hydrogenated sample prior to and after annealing indicate that only carbon is present, hydrogen not being detectable by XPS. Fig. 1 displays the C1s core levels measured on the annealed sample without and with erbium deposit. After deposition of 10 monolayers in UHV (curve b), the intensity of the diamond C1s is decreased and the peak is shifted (q0.5 eV), due to surface band bending. Erbium does not react at room temperature with the diamond surface. The LEED pattern remains smooth during the deposit and after the anneal at 750 8C. However, after the erbium layer annealing, two smaller C1s peaks appear (Fig. 1, curve c). The chemical shifts show the sign and magnitude characteristic of carbides w7x, especially the
peak displacement in the direction opposite to those of all other carbon compounds. The curve can be decomposed into three components (Fig. 2, curve a). The small intensity of these shoulders wFig. 2, curve a, (1), (2)x indicates that the reaction is not complete; only a fraction of the erbium atoms reacts with carbon. Carbon is strongly bound in diamond, which makes the reaction very difficult. To solve this problem, a second method is applied in which a reactive deposit is performed under methane pressure (5=10y6 mbar), at 750 8C. The decomposition of CH4 provides the missing carbon atoms to form the carbide. After the evaporation of 10 monolayers, the two same peaks appear wFig. 1, curve d, (3), (4)x but contrary to the first deposit, their relative intensities and shapes are different. The peak located at 283.2 eV (Fig. 2, curve b) is bigger than the other peaks. Its large intensity indicates an important carbide formation and therefore shows that erbium has a cata-
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Fig. 2. C1s core-level excited by the Mg Ka X-ray. Spectral decomposition in three Gaussian peaks in hydrogenated diamond first annealed at 900 8C in UHV with a subsequent deposit of 10 monolayers of erbium annealed at 750 8C during 15 min (curve a); hydrogenated diamond first annealed at 900 8C in UHV with a subsequent deposit of 10 monolayers of erbium evaporated on a diamond surface heated at 750 8C under 5=10y6 mbar of CH4 (curve b).
lytic effect for the decomposition of methane because the normal decomposition temperature of CH4 is higher than 900 8C w8x without any catalysis. The presence of two peaks in the XPS C1s energy range of carbides can be explained by the existence of two carbides. In fact, all carbon atoms in dicarbide ErC2 have the same coordination environment: on the one hand, a single bond with one erbium atom and on the other hand a triple bond with another carbon atom w2x. As the variation in the elemental binding energies arises from the difference between the chemical potentials of compounds, we can conclude that two carbides have been formed, i.e. ErC2, and maybe Er3C, not exceeding 10% of the ErC2 quantity. Moreover, the dominant carbide becomes ErC2, as demonstrated below, in contrast to the first preparation method. Carbon C1s peak is located at 285.0 eV. After the second preparation procedure (Fig. 2, curve b), the peak is wider than the initial C1s diamond peak (Fig. 1, curve a), with FWHM equal to 2.0 eV in contrast to
1.1 eV for the hydrogenated sample. There are two possible reasons for this. The C1s peak may result only from the contribution of amorphous or graphitic phases which indeed show a C1s energy position near to the diamond C1s one w7x. Such a signal could be due to an excess of carbon provided by the methane catalytic decomposition and the carbide growth would be twodimensional. Alternatively, the widening of the C1s signal could come from its composite nature, originating both from the simultaneous presence of diamond and graphite. In this case, the diamond layer would be only partly covered by carbide crystallites and the growth would be a 3D growth. The more probable situation is the first for the following reasons. The deposited layer (10 ML) does not exhibit any LEED pattern. However, this technique is only sensitive to a few monolayers and the result is compatible with the presence of a topmost amorphous carbon layer. Grazing incidence X-ray diffraction (GIXD) provides information about the nature of the compound and its crystallographic structure. The cartography (K, P) of the sample is reported in Fig. 3, where 2K is the angle formed by the incident beam and the diffracted beam, and P the rotation angle of the sample around the axis normal to its surface. The spot situated at (Ks37.58, Ps258) is the (220) spot of the diamond substrate w9x. The four others spots (Fig. 3) come from the dicarbide ErC2 w10x, in registry with the reciprocal lattice of the susbstrate. This result is in accordance with the XPS spectra and demonstrates the formation of dicarbide ErC2, more likely as a 2D layer because scattered 3D crystallites would hardly display a GIXD pattern. The four spots coming from ErC2 indicates that the com-
Fig. 3. GIXD (Cu Ka X-ray) mapping (K, P) of hydrogenated diamond annealed at 900 8C in UHV with a subsequent deposit of 10 monolayers of erbium evaporated at 750 8C under 5=10y6 mbar of CH4.
C. Saby et al. / Diamond and Related Materials 11 (2002) 1332–1336
Fig. 4. Room temperature Raman spectrum of the same sample as in Fig. 3, recorded at a laser exciting wavelength of 633 nm.
pound is crystalline with the c-axis normal to the surface. Therefore heteroepitaxy has been realized. But, the (220) and (110) spots of the carbide are at 458 (P angle) to the (220) diamond spots, which means that the carbide lattice is rotated 458 in comparison with the diamond lattice, in a w110x ErC2 yy w100x C alignment. This rotation is surprising since the ErC2 structure is tetragonal and the lattice mismatch with C(100) is less than 1% w2x. This may be explained by the number of dangling bonds of each material. Indeed, the surface carbon atoms have two dangling bonds along the w110x axis whereas the carbide comprises either one bond parallel to the c-axis of ErC2 when the carbide lattice is terminated by an erbium atom or three dangling bonds when terminated by a carbon atom. In such a situation, the w100x ErC2 yy w100x C alignment does not seem to be the most favorable. Another confirmation of the ErC2 formation was obtained from Raman spectroscopy. The triple bond of carbon has a characteristic stretching mode frequency located between 2230 and 2300 cmy1 w11x. The Raman spectrum (Fig. 4) shows a peak situated at 1332 cmy1, which is the diamond peak of the substrate. The other structure located at 2250 cmy1 is attributed to the triple
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bond of carbon in ErC2. Indeed, this frequency is in the frequency range of the carbon triple bond stretch mode w11x. Contrary to the first deposition method which led to rectifying erbium carbideydiamond (p-type) contacts w3x, the electrical characteristics of the ErC2 yC contacts obtained by the second procedure are different: no rectification occurs and the contact is always ohmic despite the presence of an ordered erbium compound. For a perfect interface, this result is unexpected because rare earth compounds such as silicides generally show barrier heights close to that of the metal alone on the same semiconductor. But here, interface defects are probably induced by the imperfect matching between the ErC2 and the diamond broken bonds, and they involve interface states which pin the Fermi level too close to the valence band maximum of diamond. Moreover, since the 281.2-eV XPS peak, related to another carbide than ErC2, does not differ significantly in the two preparation procedures, this metal-reach carbide has likely been formed at the Erydiamond interface and even if it does not cover the whole diamond surface, it would at least disturb the perfect matching between the ErC2 and diamond lattices, generating additional defects. Alternative or complementary reasons for the ohmic behavior linked to the presence of graphitic phases andy or hydrogen may be invoked. It is reasonable to speculate on the possible presence of one of these species or both at the interface, due to methane decomposition. In this case, the ohmic behavior may result from the high work function of carbon, from the barrier height lowering effect of hydrogen w12x or from large tunneling currents due to the strong p-type doping caused by the sub-surface hydrogen. 4. Conclusions Surface sensitive techniques have been used to investigate the formation of erbium carbide on homoepitaxial p-doped diamond (100) films by two processes. Evaporation of erbium under UHV leads to a partial reaction of erbium with diamond, only after annealing at 750 8C. Under methane pressure (5=10y6 mbar) at 750 8C, evaporated erbium reacts with the carbon produced by the decomposition of CH4, and a heteroepitaxial dicarbide ErC2 layer is grown. The evidence of triple bonds existing only in the ErC2 carbide with a CaC2 structure comes from the Raman spectrum. The c-axis of the tetragonal ErC2 is perpendicular to the (100) diamond surface but the carbide lattice is rotated 458 in comparison with the diamond lattice in the surface plane as shown by X-ray diffraction at grazing incidence. The junction is rectifying in the first case but ohmic in the second case, probably because of the disorder and the resulting interface states induced by the imperfect matching of the interfaces. But the presence of hydrogen
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andyor graphitic phases provided by the methane decomposition cannot be ruled out. References w1x E. Gheeraert, A. Deneuville, J. Mambou, Diamond Relat. Mater. 7 (1998) 1509. w2x G.-Y. Adachi, N. Imanaka, Z. Fuzhong, in: K.A. Gschneider, L. Eyrin (Eds.), Handbook on the Physics and Chemistry of Rare Earths, 15, Elseiver Science, 1991. w3x P. Muret, F. Pruvost, C. Saby, et al., Diamond Relat. Mater. 8 (1999) 961. w4x C. Saby, T.A. Nguyen Tan, F. Pruvost, P. Muret, Appl. Surf. Sci. 166 (2000) 119.
w5x J.-P. Lagrange, A. Deneuville, E. Gheeraert, Diamond Relat. Mater. 7 (1998) 1390. w6x D. Lollman, T.A. Nguyen Tan, J.Y. Veuillen, et al., Appl. Surf. Sci. 65y66 (1993) 725. w7x J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben (Eds.), Handbook of X-ray Electron Spectroscopy, Physical Electronics Inc, USA, 1995. w8x A. Aoki, T. Shimizu, Y. Kitayama, T. Kodama, J. Phys. IV France 9 (1999) Pr3–Pr337. w9x P.D. Ownby, X. Yang, J. Liu, J. Am. Ceram. Soc. 75 (1992) 1867. w10x M. Atoji, J. Chem. Phys. 57 (1972) 2410. w11x J.W. Robinson, Handbook of Spectroscopy, 2,, CRC Press, 1974. w12x W. Monch, ¨ Europhys. Lett. 27 (1994) 479.
Heteroepitaxial growth of erbium carbide on boron doped homoepitaxial diamond (100) films C. Sabya,*, P. Mureta, F. Pruvosta, G. Patratb a
´ ´ Electroniques des Solides, C.N.R.S., B.P. 166, 38042 Grenoble Cedex 9, France Laboratoire d’Etude des Proprietes b Laboratoire de Cristallographie, C.N.R.S., B.P. 166, 38042 Grenoble Cedex 9, France Received 3 April 2001; received in revised form 12 June 2001; accepted 10 October 2001
Abstract The erbium carbide formation on homoepitaxial C(100) diamond thin films has been studied by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), grazing-incidence X-ray diffraction (GIXD) and Raman spectroscopy. Diamond films, 2.5 mm thick, are grown by microwave plasma chemical vapor deposition (MWCVD) and p-type doped (1017 Bycm3) in the vapor phase. Carbide formation is monitored on two different films: either erbium is evaporated under ultra high vacuum (UHV) on a free diamond surface and this step is followed by an anneal; or a reactive deposition is performed at 750 8C under 5=10y6 mbar of CH4 on a hydrogenated diamond surface. After annealing up to 850 8C, the reaction is not complete in the first deposit and only a fraction of erbium atoms forms carbide. On the contrary, a heteroepitaxial dicarbide layer is grown when methane pressure is used. GIXD measurements performed on this latter type of film show that the (100) plane ErC2 is parallel to the C(100) plane but the carbide lattice is rotated 458 in comparison with the diamond lattice, corresponding to a w110x ErC2 y yw100x C epitaxial relationship. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: CVD diamond films; Carbide; Heteroepitaxy; Schottky diode
1. Introduction Homoepitaxial (100)C diamond films grown by chemical vapor deposition (CVD) and doped with boron (p-type) are now available with good electrical quality and a large range of boron concentrations w1x. One issue of high temperature electronics on p-type diamond consists in achieving Schottky contacts with large potential barrier heights. The metallic contact must show good adhesion on the diamond substrate, chemical inertness and stability at elevated temperatures, as well as low work function. Also, the structural damage near the metalydiamond interface has to be avoided because it would induce localized states near the interface. To fulfill these conditions, erbium appears to be a good candidate. It forms a dicarbide a-ErC2 which has the tetragonal CaC2 structure w2x and a lattice mismatch with C(100) of less than 1%, a favorable case for *Corresponding author. Tel.: q33-4-76-88-7451; fax: q33-4-7688-7988. E-mail address: [email protected] (C. Saby).
epitaxy. Finally, its work function is low (2.9–3 eV) and gives a Schottky barrier on p-type diamond of 1.9 eV w3x. The purpose of this paper is to show that the erbium carbide heteroepitaxy on the hydrogenated diamond (100) surface is feasible. This type of surface is devoid of oxygen and this fact favors the formation of erbium carbide w4x whereas oxygen would cause oxidation. The reaction at 750 8C with erbium evaporated either in ultra vacuum or under 5=10y6 mbar of methane was studied by low-energy electron diffraction (LEED), photoelectron spectroscopy (XPS), grazing-incidence Xray diffraction (GIXD) and Raman spectroscopy. 2. Experiments The diamond films were grown by microwave plasma assisted CVD in a NIRIM type reactor at 830 8C, 30 torr and 4% methane in hydrogen. The substrate was (100) type Ib synthetic diamond, 3=3=0.5 mm3, containing 1019 cmy3 nitrogen. A buffer layer of 0.5 mm of undoped diamond is grown before the 2-mm-thick
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 6 5 2 - 5
C. Saby et al. / Diamond and Related Materials 11 (2002) 1332–1336
doped film. The film was doped during the growth by diborane with wBx y wCxs0.1 ppm. The boron concentration in the solid phase, measured by infrared absorption spectroscopy and calibrated by SIMS w1x was 1=1017 cmy3. The activation energy of the conductivity between room temperature and 500 8C was 0.38 eV w5x, characteristic of boron doped diamond. The sample was hydrogenated in a microwave plasma at 830 8C during 15 min (no oxygen on the surface). It was then pasted on a 1 cm2 square silicon wafer and mounted on a resistively heated holder. Electrical contact was provided between the film and the silicon wafer by means of silver paste. An in situ annealing at 900 8C under ultra high vacuum (UHV) is performed for hydrogen desorption of the subsurface ‘H doped’ layer obtained after the growth in the hydrogen microwave plasma. Erbium is then deposited using a UHV electron-gun evaporator which heats an Er wire by electronic bombardment w6x. Two types of deposit were investigated. The first method consists in evaporating erbium under UHV followed by annealing at 750 or 850 8C. In the second one, the diamond film was held at 750 8C during the deposit under 5=10y6 mbar of CH4. One monolayer of erbium is defined as a compact layer of erbium atoms, i.e. 3.5 ˚ thick, the atomic diameter of erbium. The electron A spectrometer was calibrated with reference to the Ag 3d5y2 line (368.3 eV) and the Fermi level was determined from a silver sample. For the GIXD mapping in the reciprocal space, the samples were excited by the Cu Ka X-ray line at grazing incidence. The Raman spectra were obtained with a DILOR Infinity MicroRaman spectrometer, the films being excited by the 633nm line of a He–Ne laser. Molybdenum carbide dots serve as ohmic contacts on the top surface w3x and erbium carbide contacts are delineated as square dots 0.2 mm wide, by means of electron-beam lithography.
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Fig. 1. C1s core-level excited by the Mg Ka X-ray. Hydrogenated diamond after a 30-min anneal at 900 8C in UHV (curve a); the same as (a) with a subsequent deposit of 10 monolayers of erbium (curve b); the same as (b) annealed at 750 8C during 15 min (curve c); the same as (a) with a subsequent deposit of 10 monolayers of erbium evaporated on a diamond surface heated at 750 8C under 5=10y6 mbar of CH4 (curve d). Binding energy is referenced to the Fermi level EF.
3. Results and discussion The as-grown surface exhibits a C(100) (2=1) LEED pattern, and no modification of this surface arrangement is observed after annealing at 900 8C in UHV. XPS check scans performed on the hydrogenated sample prior to and after annealing indicate that only carbon is present, hydrogen not being detectable by XPS. Fig. 1 displays the C1s core levels measured on the annealed sample without and with erbium deposit. After deposition of 10 monolayers in UHV (curve b), the intensity of the diamond C1s is decreased and the peak is shifted (q0.5 eV), due to surface band bending. Erbium does not react at room temperature with the diamond surface. The LEED pattern remains smooth during the deposit and after the anneal at 750 8C. However, after the erbium layer annealing, two smaller C1s peaks appear (Fig. 1, curve c). The chemical shifts show the sign and magnitude characteristic of carbides w7x, especially the
peak displacement in the direction opposite to those of all other carbon compounds. The curve can be decomposed into three components (Fig. 2, curve a). The small intensity of these shoulders wFig. 2, curve a, (1), (2)x indicates that the reaction is not complete; only a fraction of the erbium atoms reacts with carbon. Carbon is strongly bound in diamond, which makes the reaction very difficult. To solve this problem, a second method is applied in which a reactive deposit is performed under methane pressure (5=10y6 mbar), at 750 8C. The decomposition of CH4 provides the missing carbon atoms to form the carbide. After the evaporation of 10 monolayers, the two same peaks appear wFig. 1, curve d, (3), (4)x but contrary to the first deposit, their relative intensities and shapes are different. The peak located at 283.2 eV (Fig. 2, curve b) is bigger than the other peaks. Its large intensity indicates an important carbide formation and therefore shows that erbium has a cata-
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Fig. 2. C1s core-level excited by the Mg Ka X-ray. Spectral decomposition in three Gaussian peaks in hydrogenated diamond first annealed at 900 8C in UHV with a subsequent deposit of 10 monolayers of erbium annealed at 750 8C during 15 min (curve a); hydrogenated diamond first annealed at 900 8C in UHV with a subsequent deposit of 10 monolayers of erbium evaporated on a diamond surface heated at 750 8C under 5=10y6 mbar of CH4 (curve b).
lytic effect for the decomposition of methane because the normal decomposition temperature of CH4 is higher than 900 8C w8x without any catalysis. The presence of two peaks in the XPS C1s energy range of carbides can be explained by the existence of two carbides. In fact, all carbon atoms in dicarbide ErC2 have the same coordination environment: on the one hand, a single bond with one erbium atom and on the other hand a triple bond with another carbon atom w2x. As the variation in the elemental binding energies arises from the difference between the chemical potentials of compounds, we can conclude that two carbides have been formed, i.e. ErC2, and maybe Er3C, not exceeding 10% of the ErC2 quantity. Moreover, the dominant carbide becomes ErC2, as demonstrated below, in contrast to the first preparation method. Carbon C1s peak is located at 285.0 eV. After the second preparation procedure (Fig. 2, curve b), the peak is wider than the initial C1s diamond peak (Fig. 1, curve a), with FWHM equal to 2.0 eV in contrast to
1.1 eV for the hydrogenated sample. There are two possible reasons for this. The C1s peak may result only from the contribution of amorphous or graphitic phases which indeed show a C1s energy position near to the diamond C1s one w7x. Such a signal could be due to an excess of carbon provided by the methane catalytic decomposition and the carbide growth would be twodimensional. Alternatively, the widening of the C1s signal could come from its composite nature, originating both from the simultaneous presence of diamond and graphite. In this case, the diamond layer would be only partly covered by carbide crystallites and the growth would be a 3D growth. The more probable situation is the first for the following reasons. The deposited layer (10 ML) does not exhibit any LEED pattern. However, this technique is only sensitive to a few monolayers and the result is compatible with the presence of a topmost amorphous carbon layer. Grazing incidence X-ray diffraction (GIXD) provides information about the nature of the compound and its crystallographic structure. The cartography (K, P) of the sample is reported in Fig. 3, where 2K is the angle formed by the incident beam and the diffracted beam, and P the rotation angle of the sample around the axis normal to its surface. The spot situated at (Ks37.58, Ps258) is the (220) spot of the diamond substrate w9x. The four others spots (Fig. 3) come from the dicarbide ErC2 w10x, in registry with the reciprocal lattice of the susbstrate. This result is in accordance with the XPS spectra and demonstrates the formation of dicarbide ErC2, more likely as a 2D layer because scattered 3D crystallites would hardly display a GIXD pattern. The four spots coming from ErC2 indicates that the com-
Fig. 3. GIXD (Cu Ka X-ray) mapping (K, P) of hydrogenated diamond annealed at 900 8C in UHV with a subsequent deposit of 10 monolayers of erbium evaporated at 750 8C under 5=10y6 mbar of CH4.
C. Saby et al. / Diamond and Related Materials 11 (2002) 1332–1336
Fig. 4. Room temperature Raman spectrum of the same sample as in Fig. 3, recorded at a laser exciting wavelength of 633 nm.
pound is crystalline with the c-axis normal to the surface. Therefore heteroepitaxy has been realized. But, the (220) and (110) spots of the carbide are at 458 (P angle) to the (220) diamond spots, which means that the carbide lattice is rotated 458 in comparison with the diamond lattice, in a w110x ErC2 yy w100x C alignment. This rotation is surprising since the ErC2 structure is tetragonal and the lattice mismatch with C(100) is less than 1% w2x. This may be explained by the number of dangling bonds of each material. Indeed, the surface carbon atoms have two dangling bonds along the w110x axis whereas the carbide comprises either one bond parallel to the c-axis of ErC2 when the carbide lattice is terminated by an erbium atom or three dangling bonds when terminated by a carbon atom. In such a situation, the w100x ErC2 yy w100x C alignment does not seem to be the most favorable. Another confirmation of the ErC2 formation was obtained from Raman spectroscopy. The triple bond of carbon has a characteristic stretching mode frequency located between 2230 and 2300 cmy1 w11x. The Raman spectrum (Fig. 4) shows a peak situated at 1332 cmy1, which is the diamond peak of the substrate. The other structure located at 2250 cmy1 is attributed to the triple
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bond of carbon in ErC2. Indeed, this frequency is in the frequency range of the carbon triple bond stretch mode w11x. Contrary to the first deposition method which led to rectifying erbium carbideydiamond (p-type) contacts w3x, the electrical characteristics of the ErC2 yC contacts obtained by the second procedure are different: no rectification occurs and the contact is always ohmic despite the presence of an ordered erbium compound. For a perfect interface, this result is unexpected because rare earth compounds such as silicides generally show barrier heights close to that of the metal alone on the same semiconductor. But here, interface defects are probably induced by the imperfect matching between the ErC2 and the diamond broken bonds, and they involve interface states which pin the Fermi level too close to the valence band maximum of diamond. Moreover, since the 281.2-eV XPS peak, related to another carbide than ErC2, does not differ significantly in the two preparation procedures, this metal-reach carbide has likely been formed at the Erydiamond interface and even if it does not cover the whole diamond surface, it would at least disturb the perfect matching between the ErC2 and diamond lattices, generating additional defects. Alternative or complementary reasons for the ohmic behavior linked to the presence of graphitic phases andy or hydrogen may be invoked. It is reasonable to speculate on the possible presence of one of these species or both at the interface, due to methane decomposition. In this case, the ohmic behavior may result from the high work function of carbon, from the barrier height lowering effect of hydrogen w12x or from large tunneling currents due to the strong p-type doping caused by the sub-surface hydrogen. 4. Conclusions Surface sensitive techniques have been used to investigate the formation of erbium carbide on homoepitaxial p-doped diamond (100) films by two processes. Evaporation of erbium under UHV leads to a partial reaction of erbium with diamond, only after annealing at 750 8C. Under methane pressure (5=10y6 mbar) at 750 8C, evaporated erbium reacts with the carbon produced by the decomposition of CH4, and a heteroepitaxial dicarbide ErC2 layer is grown. The evidence of triple bonds existing only in the ErC2 carbide with a CaC2 structure comes from the Raman spectrum. The c-axis of the tetragonal ErC2 is perpendicular to the (100) diamond surface but the carbide lattice is rotated 458 in comparison with the diamond lattice in the surface plane as shown by X-ray diffraction at grazing incidence. The junction is rectifying in the first case but ohmic in the second case, probably because of the disorder and the resulting interface states induced by the imperfect matching of the interfaces. But the presence of hydrogen
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andyor graphitic phases provided by the methane decomposition cannot be ruled out. References w1x E. Gheeraert, A. Deneuville, J. Mambou, Diamond Relat. Mater. 7 (1998) 1509. w2x G.-Y. Adachi, N. Imanaka, Z. Fuzhong, in: K.A. Gschneider, L. Eyrin (Eds.), Handbook on the Physics and Chemistry of Rare Earths, 15, Elseiver Science, 1991. w3x P. Muret, F. Pruvost, C. Saby, et al., Diamond Relat. Mater. 8 (1999) 961. w4x C. Saby, T.A. Nguyen Tan, F. Pruvost, P. Muret, Appl. Surf. Sci. 166 (2000) 119.
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