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Chemisorbed states of atomic oxygen and its replacement by atomic hydrogen on the diamond (100)-(2×1) surface
Surface Science 436 (1999) 63–71 www.elsevier.nl/locate/susc
Chemisorbed states of atomic oxygen and its replacement by atomic hydrogen on the diamond (100)-(2×1) surface M.Z. Hossain a, T. Kubo a,1, T. Aruga a, N. Takagi a, T. Tsuno b, N. Fujimori b, M. Nishijima a, * a Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan b Itami Research Laboratories, Sumitomo Electric Industries Ltd., 1-1-1 Koyakita, Itami, Hyogo 664-0016, Japan Received 7 January 1999; accepted for publication 12 April 1999
Abstract The chemisorbed states of atomic oxygen and the subsequent replacement of the adsorbed O atoms by atomic hydrogen on the C(100) surface have been studied by electron energy loss spectroscopy (EELS ), thermal desorption spectroscopy ( TDS ) and low-energy electron diffraction (LEED). EELS spectra of the O-adsorbed C(100) surface at 300 K show losses at 113, 150, 215 and 261 meV. The 113 and 215 meV losses are assigned as the bending and stretching modes of the surface carbonyl (vC앞O) species (on-top O), respectively, which is formed by the dimer pand s-bond breaking. The 150 meV loss is attributed to the surface ether (CMOMC ) species (bridging O) which is also formed by the dimer bond breaking. These species are desorbed as CO at ~1100 K. The 261 meV loss indicates the existence of minority O atoms nearly triple-bonded to the substrate (which is decomposed by heating up to 800 K ). When the O-preadsorbed C(100) surface is exposed to an increasing amount of H, the 215 meV loss disappears and losses appear at 104, 152 and 362 meV which are associated with the surface hydride species. No CO and CO desorptions are observed. These results suggest that the surface O atoms are replaced by atomic H completely 2 and hydride species are formed. A model for the replacement is proposed. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Chemisorption; Diamond; Electron energy loss spectroscopy; Oxygen; Thermal desorption spectroscopy; Vibrations of adsorbed molecules
1. Introduction Diamond is a promising electronic-device material superior to Si or GaAs owing to its intrinsic properties such as the large band gap (5.5 eV ), high saturation carrier velocity and high thermal * Corresponding author. Fax: +81-75-753-4000. E-mail address: [email protected] (M. Nishijima) 1 Present address: National Institute of Materials and Chemical Research, Tsukaba, Ibaraki 305-8565, Japan.
conductivity. The recent development of the epitaxial growth technique for the high quality diamond film has stimulated a large number of investigations related to this material. Among the low index faces, the best diamond homoepitaxial growth process occurs on the (100) surface [1–3]. In the diamond homoepitaxial growth process using H /CH plasma, oxygen was usually used at 2 4 the concentration of 1–2% [4,5]. In the early days, it was thought that oxygen increased the atomic hydrogen concentration through gas phase reactions and also etched the nondiamond (graphitic
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and amorphous) carbon. Recently, oxygen-containing molecules (H O, CO, CO , etc.) have been 2 2 introduced into the growth process at much larger concentration [6–8], and it was also theoretically suggested that diamond can be grown with atomic carbon and atomic oxygen alone [9]. Hence, the role of oxygen in the diamond growth process is perhaps more complicated than initially presumed. The elucidation of the interaction of atomic O with the C(100) surface and the reaction between the adsorbed O and atomic H in the gas phase promises to yield improved mechanistic understanding of the diamond homoepitaxial growth process. Despite its obvious importance, only a handful of studies have been performed which focus on the interaction of oxygen with the diamond surface. Thomas et al. [10] have investigated the interaction of O with the C(100)-(2×1) surface using thermal desorption spectroscopy ( TDS ) and low-energy electron diffraction (LEED). They found that the adsorbed O are desorbed as CO (major) and CO (minor), and that O is able to 2 convert the (2×1) structure to ‘(1×1)’. Beerling and Helms [11] studied the electronic transition of oxygenated-diamond surface by electron energy loss spectroscopy (EELS ). Skokov et al. [12] investigated the structures of the oxygenated C(100) surface using molecular dynamics simulation, where they found that oxygen in the on-top- and bridge-configurations formed by dimer p- and s-bond breaking are energetically favorable. Ando et al. [13,14] studied polycrystalline diamond powder treated in a mixed acid solution (H SO /HNO ) by infrared (IR) techniques 2 4 3 (Fourier-transform IR photoacoustic spectroscopy and diffuse reflectance IR Fourier-transform spectroscopy), and detected peaks related to the carbonyl (vC앞O) and ether (CMOMC ) species. Since the polycrystalline diamond powder contain different crystalline faces whose structures are different from each other, the formation of carbonyl and ether species on the specific crystalline faces is not clear. In this study, we investigate the interaction of atomic oxygen with the C(100)-(2×1) surface and the replacement of adsorbed O by atomic H by the use of EELS, LEED and TDS. The EELS study in the vibrational region of the oxygenated
C(100) surface gives direct evidence of the structure of the adsorbed oxygen. The surface carbonyl (vC앞O) and ether (CMOMC ) species were found to be the dominant products. A small amount of triple-bonded CO species were also detected. The adsorbed O species were found to be completely replaced by atomic H. The mechanism of the replacement is discussed.
2. Experimental The experiments were carried out using an ultrahigh vacuum ( UHV ) system with a base pressure of <1×10−10 Torr. The vacuum chamber was equipped with a high resolution electron spectrometer for EELS, a home-made electron optics for LEED, and a quadrupole mass spectrometer (QMS ) for TDS and gas analysis. For EELS measurements, a primary energy E of 7 eV, an p energy resolution of 15–20 meV (full-width at halfmaximum, FWHM ) and an incidence angle h of i 60° from the surface normal were used. All EELS spectra were recorded at 300 K. LEED patterns were observed at E =91–97 eV. p The B-doped diamond film (thickness: 20 mm) was epitaxially grown, using the microwave plasma-assisted chemical vapor deposition (CVD) method, on the (100) surface of synthesized singlecrystalline diamond (Ib type). The growth condition is described in Ref. [15]. Two of the present authors ( T.T. and N.F.) showed, by scanning tunneling microscopy in air, that the surface has the (2×1) structure [15]. The size of the substrate was 4×4×0.3 mm3. The CVD-grown diamond surface was H-terminated, and hence, the clean surface was prepared by heating the sample up to 1400 K by electron bombardment from the rear. The surface cleanness was checked by LEED and EELS. The clean surface showed a sharp doubledomain (2×1) LEED pattern as shown in Fig. 1a, and a characteristic loss spectrum as shown later. The heating rate ( b) for the TDS measurements was 8 K s−1. The sample temperature was measured by using an alumel–chromel thermocouple attached to the sample edge. The hydrogen desorption temperature (~1250 K ) measured by this method is found to be nearly the same (±20 K ) as those of other studies available [16 ].
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Fig. 1. (a) A LEED pattern of the clean C(100) surface. The primary energy used is E =91 eV. Schematic view of the p observed LEED pattern is included. (b) Comparative EELS spectra in the specular mode of the oxygenated C(100) surface where atomic O were produced by hot (i) W- and (ii) Ir-filaments, respectively. The oxygen exposure is 4.5 and 240 L, respectively. E =7.0 eV. p
All gases were introduced into the UHV chamber through a variable leak valve. Atomic H and O were formed by the exposure of H and O to 2 2 a hot (~2000 K ) and well-outgassed W-filament located 2 cm away from the sample surface. During atomic H (O) exposure, the sample temperature was increased to 500 K by the radiation from the filament. Since the dissociation probability of H 2 (O ) is not known, the H (O) exposure is given by 2 the H (O ) exposure used for the H (O) formation. 2 2 The amount of gas exposures are given in Langmuir (1 L=1×10−6 Torr s). It is noted that the hot W-filament might pro-
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duce excited O molecules, etc., in addition to 2 atomic O. We cannot exclude this possibility. However, we believe that these give a negligible effect as the dimer p- and s-bonds are very strong as will be discussed later. It has been reported that the surface can be contaminated with the W and WO species when the W-filament is used as a x source of atomic O [17]. In order to check the existence of W and WO , a hot (~1400 K ) Ir x filament is used for the production of O following the work of Thomas et al. [10] in which no Ir-related contaminations are observed by X-ray photoemission spectroscopy ( XPS) when the C(100) surface is oxidized using the Ir filament. Fig. 1b shows EELS spectra of the oxygenated C(100) surface where O is produced by the (i) Wand (ii) Ir-filaments. Losses are observed at 90, 152, 215 and 261 meV for both spectra which are associated with the surface phonons and the adsorbed O species as will be discussed later. No losses are observed, especially in Fig. 1b(i), corresponding to WO species which gives strong losses x at 75 and 134 meV [18]. There is no essential difference between (i) and (ii) except for the much higher exposure needed for the Ir filament, which suggests that the surface is free from W-related contamination. It is noted in the case of the Ir filament that a much longer exposure time is needed, because the temperature usually used (~1400 K ) is lower (to avoid the filament breaking) and thus, the production efficiency of the atomic O is also lower. In this connection, it is also noted that the influence of the H adsorption due to the residual H cannot be neglected when 2 the Ir-filament is used. Therefore, the W-filament is used for the production not only of H but also of O. Recently, it was reported that the Ir filament is also not the pure source of atomic O [19]. Unfortunately, our UHV chamber is not equipped with Auger electron spectroscopy or XPS and thus, the influence of the Ir-related contaminations cannot be ruled out completely.
3. Results Fig. 2 shows EELS spectra of the clean C(100) surface and of the same surface subsequently
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Fig. 2. EELS spectra in the specular mode of the clean C(100) surface at 300 K and of the same surface with increasing atomic O exposure. E =7.0 eV. p
exposed to an increasing amount of atomic O produced by the W-filament. For the clean surface, the loss peaks are observed at 90 and 152 meV which are associated with the surface phonons characteristic of the reconstructed C(100)-(2×1) surface [20]. The typical count rate of the elastic peak was 8×104 c s−1. A loss appears at 215 meV when the surface is exposed to 1.5 L O. The 215 meV loss grows in intensity and the losses related to the surface phonons become less clear as the exposure of atomic O is increased. For 9 L, the 152 meV loss is replaced by the loss structure in the region of 100–150 meV and a loss appears at 261 meV. Finally, losses are observed at 113, 150, 215 and 261 meV for 30 L. The 261 meV loss is not clearly detected but its existence was confirmed by the detailed measurements. The (1×1) LEED pattern with higher background was observed for the oxygenated (30 L) surface in agreement with the work of Thomas et al. [10]. When the oxygenated C(100) surface is heated
to high temperatures, the 261 meV loss disappears at 800 K as shown in Fig. 3. The intensities of the 150 and 215 meV losses decrease nearly simultaneously by heating up to 1000 K and finally disappear at 1200 K. In the corresponding TDS spectra, a single CO desorption peak with a shoulder at the lower temperature side (~1000 K ) was observed at ~1100 K as shown in Fig. 4. The CO desorption peak at 1100 K is compatible with the EELS spectra of the oxygenated C(100) surface heated to high temperature as shown in Fig. 3. The desorption of CO from the oxygenated C(100) surface implies that etching of C atoms from the substrate surface occurs. The shoulder at 1000 K may be related to O adsorption in the defect sites. When the oxygenated C(100) surface is heated to 1200 K, the characteristic phonon spectrum is restored. The (2×1) LEED pattern was also clearly observed after two cycles of atomic O adsorption and desorption experiments. After several cycles of atomic O adsorption and desorption experiments, even the (1×1) LEED pattern was not observed. No CO desorption was observed 2 when the sample surface shows the characteristic
Fig. 3. EELS spectra in the specular model of the C(100) surface exposed to 15 L oxygen and of the same surface with increasing heating temperature. The spectra were recorded at 300 K. E =7.0 eV. p
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Fig. 4. Series of CO and CO TDS spectra from the C(100) 2 surface exposed to 3–27 L oxygen. Heating rate, b=8 K s−1.
phonon loss features in EELS as well as the (2×1) LEED pattern. The CO desorption peak temperature of 1100 K is higher than that reported in the previous study by Thomas et al. [10] where an extremely broad CO desorption peak ( FWHM = 350 K for b=20 K s−1) is observed at ~900 K and CO 2 desorption is also observed at ~800 K. A broad CO desorption peak ( FWHM ~260 K for b=0.5–14 K s−1) is also reported in the work of Frenklach et al. [21] (where the activation energy for CO desorption is estimated to be 45 kcal mol−1). We also obtained the broad CO desorption peak at 900 K and CO desorption 2 peak at 800 K from the degraded surface (not shown) which are similar to the spectra reported earlier by Thomas et al. [10] and Frenklach et al. [21]. We can extract the activation energy for desorption using the Redhead formula, E=RT [ ln(nT /b)−3.46 ], where R is the gas m m constant, T the peak temperature and n the prem exponential factor [22]. Assuming the value of n=1013 s−1, the activation energy for CO desorption from the C(100) surface is estimated to be
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68 kcal mol−1. This activation energy for desorption is comparable with the work of Matsumoto et al. [23,24] on the diamond powder, where they observed the CO desorption peak at 1070 K. For comparison, it is noted that CMC bond energy is 83 kcal mol−1 [25]. Fig. 5 shows EELS spectra of the oxygenated C(100) surface with increasing atomic H exposure. It is clearly observed that the 215 meV loss is reduced in intensity as the atomic H exposure is increased, and disappears at 54 L exposure. Instead, the loss peaks appear at 152 and 362 meV which grow in intensity as the atomic H exposure is increased. At higher exposures, a loss peak is clearly observed at 104 meV. With increasing atomic H exposure to the oxygenated surface, the signal-to-noise ratio of the spectrometer gradually decreases due to the disordering of the surface. Hence, the spectra become noisy at higher exposure. However, the losses at 270 and 300 meV are identified at 54 L exposure. The intensities of the 152 and 362 meV losses are higher for 24 L than
Fig. 5. EELS spectra in the specular mode of the C(100) surface at 300 K pre-exposed to 30 L oxygen and post-exposed to an increasing amount of atomic H. E =7.0 eV. p
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those for 54 L exposure. This may be related with the appearance of the 270 meV loss at 54 L. This surface showed the (1×1) LEED pattern with higher background.
4. Discussion The early investigation of acid-treated diamond powder by IR techniques has shown a peak at 212–220 meV which is assigned as the CMO stretching mode of carbonyl (vC앞O) species [13,14]. Furthermore, the CMO stretching energy of cyclohexanone was reported at 212 meV [26 ]. Hence, the 215 meV loss peak is assigned as the stretching mode of the surface carbonyl species (on-top O) which is formed by the dimer p- and s-bond breaking [12]. Molecular dynamics calculation on the oxygenated C(100) surface has indicated that the CMO stretching of the carbonyl species is at 248 meV, which seems to be overestimated [12]. The 113 meV loss peak is due to the bending mode of the carbonyl species. The calculated value for the CO bending mode of the oxygenated C(100) surface is 110 meV [27]. The CO bending mode of adamentanone (C H O) 10 14 (which has a structure rather similar to the diamond) is observed at 107 meV [28]. A structural model of carbonyl species is shown in Fig. 6. The 150 meV loss is assigned as the CMO stretching mode of the surface ether (CMOMC ) species (bridging O) which is also formed by the scission of the dimer bonds. The CMO stretch energy at 150 meV is in agreement with the peak observed by IR at 136–155 meV in the case of oxidized diamond powder [14]. The vibrational energies of 2-oxa-adamentane (C H O) which is 9 14 the simplest molecular analogue of an ether-type oxide on diamond are not available. The CMO stretching energies of polycyclic ether dioxabicyclo [3.2.1] octane (C H O ) are reported at 148 and 9 16 2 155 meV [29]. The CMO stretching energy of a simple ether (C H O) is 136 meV [30]. However, 2 6 this is not a good molecular analogue because the C ends are terminated by light H atoms. The theoretical calculation on the monolayer O on C(100) has shown multiple peaks at 145–155 meV which are assigned to the CMO stretching mode
Fig. 6. Structural models of oxygen atoms adsorbed on the C(100) surface.
of the surface ether species [12]. The CMO stretching mode is observed at 158 meV for ethylene oxide [30]. The 150 meV loss might be attributed to the CMO stretching mode of the surface epoxy species. This possibility cannot be ruled out completely, but it is not accepted because the vibrational energy difference is large (8 meV ). The 261 meV loss with relatively small intensity is ascribed to the presence of triply-bonded CO species loosely bound to the substrate, which disappear upon heating to 800 K (Fig. 3). The CO adsorption on the down-atom of the Si(100) surface dimer shows a loss peak at 261 meV [31]. The formation process of triple-bonded CO by the atomic O exposure on the C(100) surface is not clearly understood. Roughly speaking, the intensities of the 150 and 215 meV losses are similar for 30 L exposure as shown in Fig. 2, which may simply indicate that the amount of the ether and carbonyl species formed are nearly the same. The temperature dependence of the 150 and 215 meV losses shows similar behavior as shown in Fig. 3. The CO desorption spectrum shows a single peak around 1100 K. These results suggest that the thermodynamic stability of the two species is very close. The theoretical calculations show that the binding energy difference per O atom between the carbonyl
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and ether species is small: 0.14 eV by Zheng and Smith [32], 0.015 eV by Whitten et al. [33] and −0.19 by Skokov et al. [12]. The energy difference of 0.1–0.2 eV corresponds to the difference in the desorption peak temperature of 50–100 K using the Redhead formula, which, however, is not observed. Thus our result is compatible with the energy difference of 0.015 eV obtained by Whitten et al. [33]. The single desorption peak might be explained by considering that the desorption process proceeds through a single transition state, that is, the conversion between the carbonyl and ether species occurs prior to desorption. However, we did not get any evidence of the conversion in the EELS spectra with increasing heating temperature (Fig. 3). The effect of the heat treatment of the oxygenated C(100) surface is summarized as follows: first, the triply-bonded CO species is decomposed and CO desorption occurs at 800 K. At 1100 K, the surface carbonyl and ether species are decomposed to desorb as CO. The C atoms in the first layer are etched and the bulk-like (1×1) structure may be formed. Then, the surface C atoms, which are originally in the second layer, are paired to form dimers in order to reduce the number of unsaturated dangling bonds. The characteristic phonon spectrum and the (2×1) LEED pattern are thus restored. This suggests the possibility of the layer-by-layer etching with the adsorption of O and the subsequent heat treatment. However, it is difficult to control, for example, the surface concentration of O, and thus the degradation of the phonon spectrum and the LEED pattern occur after several cycles of experiments. In Fig. 5, the 152 and 362 meV losses are assigned as the bending and stretching modes of the CH species, respectively [20,34,35]. The 300 meV loss is attributed to the overtone of the CH bending mode. The 270 meV loss may be associated with the adsorbed H, but the origin is unknown at the present stage. The 104 meV loss does not appear when the bare C(100)-(2×1) surface is exposed to atomic H. The 104 meV loss is very close to the rocking mode (in-plane hindered rotation) of CH species theoretically pre2 dicted at 108 meV [36 ]. Although the CH species 2 on the C(100) surface is not energetically favored
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due to the steric hindrance [36 ], some theoretical calculations support the existence of CH species 2 with twisted or canted geometry [37,38]. Thus, the appearance of the 104, 152 and 362 meV losses accompanied by the disappearance of the 215 meV loss suggests that O in the carbonyl (vC앞O) species is replaced by the atomic H and possibly CH species is formed in addition to the CH 2 species. The change of the 150 meV loss associated with the ether species is not clear due to the overlap with the strong peak at 152 meV attributed to the CH bending mode. However, observation and disappearance of the CO desorption peak in the TDS spectra before and after exposure of the oxygenated C(100) surface to atomic H, respectively, suggest that both the on-top and bridging O are replaced by atomic H. It is noted that no H O and OH species were detected in the TDS 2 spectra after the oxygenated surface is exposed to atomic H. A model for the replacement process is shown schematically in Fig. 7. Initially, H atoms attack the surface carbonyl and ether species to form, perhaps, hydroxyl (OH ) species which play the role of the reaction intermediate. Then, the surface O atoms are, most probably, replaced by the desorption of H O and the unsaturated dangling 2 bonds are terminated with the H to form CH and CH species. The observed (1×1) LEED pattern 2 with higher background after exposure of atomic H to the oxygenated surface is ascribed to the random distribution of CH and CH species on 2 the surface. A theoretical study of oxygenated C(100) surface in the presence of hydrogen suggests that hydroxyl species on the surface is stable [39]. Also, it is predicted that the OMH stretch vibration should be observed at 481 meV. Recently, using the hybrid density-functional theory, Okamoto [27] found that, after the partially dissociative adsorption of a H O molecule, 2 an adsorbed hydroxyl fragment is decomposed on the surface to form predominantly carbonyl species. In the present study, we did not get any evidence of hydroxyl species on the surface. This is perhaps due to the short lifetime of hydroxyl species. It is interesting to discuss whether the replacement reaction is Eley–Rideal ( ER) or Langmuir–Hinshelwood (LH ) type. Probably, it
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Fig. 7. A model for the replacement of the surface O species on the C(100) by atomic H from the gas phase.
is classified as the ER type, but more experimental efforts are needed to clarify the details. Struck and D’Evelyn [40] have studied the water (H O) adsorption on the C(100) surface by 2 infrared multiple-internal-reflection spectroscopy. They claimed that OH species are observed when the C(100) surface at 1070 K is cooled down to 300 K in 1×10−7 Torr of H O. The existence of 2 the OH species is based on the observation of the 134 and 139 meV peaks. However, it is difficult to understand that the 134 meV peak exists even after the cooled sample is heated to 1270 K. Also, the 139 meV peak may be attributed to the CD 2 deformation (scissors) mode. During the heating and cooling processes, the possibility of the adsorption of D (from the residual D ) on the surface 2
cannot be ruled out. Hence, the existence of the OH species is questionable. Typical substrate temperature during the CVD growth of diamond is ~1100–1200 K and the TDS spectrum of the hydrogenated surface shows that the H desorption starts at ~1000 K. Hence, 2 at the CVD condition, it is likely that some CMC dimers are formed on the surface which disturb the diamond growth process. The formation of surface carbonyl and ether species on the C(100) surface by atomic O exposure is accompanied by the breaking of both the s and p bonds of the surface dimer as shown in Fig. 6. (It is noted that atomic H is hardly able to break the s bond of the surface dimer.) The unusual ability of atomic O of breaking the dimer bond may play a crucial role in the continuation of the diamond growth in the oxygen-containing environment. The formation of the carbonyl species on the C(100) surface due to atomic O exposure can structurally be viewed as the adsorption of CO on the surface. It is reported that diamond can be successfully grown using only a mixture of CO and H with a significantly higher growth rate [6 ]. 2 The atomic H interaction with the oxygenated C(100) surface which is predominantly covered by the carbonyl and ether type of CO species may give a better understanding of the diamond growth especially using a mixture of CO and H gases in 2 the CVD chamber.
5. Summary The interaction of atomic O with the C(100)(2×1) surface and the replacement of adsorbed O by atomic H have been investigated by EELS, TDS and LEED. Some of the important results are summarized as follows: 1. Atomic O breaks both the s and p bonds of the surface dimer to form predominantly carbonyl (vC앞O) species (on-top O) and ether (CMOMC ) species (bridging O) as clearly indicated by the 113, 215 and the 150 meV losses, respectively. A small amount of triple-bonded CO are also detected which are characterized by the 261 meV loss. 2. A single CO desorption peak is observed at
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~1100 K which corresponds to the desorption energy of 68 kcal mol−1. 3. After the CO desorption, carbon atoms in the second layer are found to be dimerized as indicated by the reappearance of the phonon spectrum characteristic of the clean C(100)(2×1) surface and by the observation of the (2×1) LEED pattern. 4. Chemisorbed O on the C(100) surface is readily replaced by the atomic H as clearly evidenced by the disappearance of the 215 meV loss and CO desorption. A model for the replacement is proposed as shown in Fig. 7. Acknowledgements This work was supported in part by Grants-inAid from the Ministry of Education, Science, Sports and Culture (Japan) and from Sumitomo Electric Industries Ltd.
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Chemisorbed states of atomic oxygen and its replacement by atomic hydrogen on the diamond (100)-(2×1) surface M.Z. Hossain a, T. Kubo a,1, T. Aruga a, N. Takagi a, T. Tsuno b, N. Fujimori b, M. Nishijima a, * a Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan b Itami Research Laboratories, Sumitomo Electric Industries Ltd., 1-1-1 Koyakita, Itami, Hyogo 664-0016, Japan Received 7 January 1999; accepted for publication 12 April 1999
Abstract The chemisorbed states of atomic oxygen and the subsequent replacement of the adsorbed O atoms by atomic hydrogen on the C(100) surface have been studied by electron energy loss spectroscopy (EELS ), thermal desorption spectroscopy ( TDS ) and low-energy electron diffraction (LEED). EELS spectra of the O-adsorbed C(100) surface at 300 K show losses at 113, 150, 215 and 261 meV. The 113 and 215 meV losses are assigned as the bending and stretching modes of the surface carbonyl (vC앞O) species (on-top O), respectively, which is formed by the dimer pand s-bond breaking. The 150 meV loss is attributed to the surface ether (CMOMC ) species (bridging O) which is also formed by the dimer bond breaking. These species are desorbed as CO at ~1100 K. The 261 meV loss indicates the existence of minority O atoms nearly triple-bonded to the substrate (which is decomposed by heating up to 800 K ). When the O-preadsorbed C(100) surface is exposed to an increasing amount of H, the 215 meV loss disappears and losses appear at 104, 152 and 362 meV which are associated with the surface hydride species. No CO and CO desorptions are observed. These results suggest that the surface O atoms are replaced by atomic H completely 2 and hydride species are formed. A model for the replacement is proposed. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Chemisorption; Diamond; Electron energy loss spectroscopy; Oxygen; Thermal desorption spectroscopy; Vibrations of adsorbed molecules
1. Introduction Diamond is a promising electronic-device material superior to Si or GaAs owing to its intrinsic properties such as the large band gap (5.5 eV ), high saturation carrier velocity and high thermal * Corresponding author. Fax: +81-75-753-4000. E-mail address: [email protected] (M. Nishijima) 1 Present address: National Institute of Materials and Chemical Research, Tsukaba, Ibaraki 305-8565, Japan.
conductivity. The recent development of the epitaxial growth technique for the high quality diamond film has stimulated a large number of investigations related to this material. Among the low index faces, the best diamond homoepitaxial growth process occurs on the (100) surface [1–3]. In the diamond homoepitaxial growth process using H /CH plasma, oxygen was usually used at 2 4 the concentration of 1–2% [4,5]. In the early days, it was thought that oxygen increased the atomic hydrogen concentration through gas phase reactions and also etched the nondiamond (graphitic
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and amorphous) carbon. Recently, oxygen-containing molecules (H O, CO, CO , etc.) have been 2 2 introduced into the growth process at much larger concentration [6–8], and it was also theoretically suggested that diamond can be grown with atomic carbon and atomic oxygen alone [9]. Hence, the role of oxygen in the diamond growth process is perhaps more complicated than initially presumed. The elucidation of the interaction of atomic O with the C(100) surface and the reaction between the adsorbed O and atomic H in the gas phase promises to yield improved mechanistic understanding of the diamond homoepitaxial growth process. Despite its obvious importance, only a handful of studies have been performed which focus on the interaction of oxygen with the diamond surface. Thomas et al. [10] have investigated the interaction of O with the C(100)-(2×1) surface using thermal desorption spectroscopy ( TDS ) and low-energy electron diffraction (LEED). They found that the adsorbed O are desorbed as CO (major) and CO (minor), and that O is able to 2 convert the (2×1) structure to ‘(1×1)’. Beerling and Helms [11] studied the electronic transition of oxygenated-diamond surface by electron energy loss spectroscopy (EELS ). Skokov et al. [12] investigated the structures of the oxygenated C(100) surface using molecular dynamics simulation, where they found that oxygen in the on-top- and bridge-configurations formed by dimer p- and s-bond breaking are energetically favorable. Ando et al. [13,14] studied polycrystalline diamond powder treated in a mixed acid solution (H SO /HNO ) by infrared (IR) techniques 2 4 3 (Fourier-transform IR photoacoustic spectroscopy and diffuse reflectance IR Fourier-transform spectroscopy), and detected peaks related to the carbonyl (vC앞O) and ether (CMOMC ) species. Since the polycrystalline diamond powder contain different crystalline faces whose structures are different from each other, the formation of carbonyl and ether species on the specific crystalline faces is not clear. In this study, we investigate the interaction of atomic oxygen with the C(100)-(2×1) surface and the replacement of adsorbed O by atomic H by the use of EELS, LEED and TDS. The EELS study in the vibrational region of the oxygenated
C(100) surface gives direct evidence of the structure of the adsorbed oxygen. The surface carbonyl (vC앞O) and ether (CMOMC ) species were found to be the dominant products. A small amount of triple-bonded CO species were also detected. The adsorbed O species were found to be completely replaced by atomic H. The mechanism of the replacement is discussed.
2. Experimental The experiments were carried out using an ultrahigh vacuum ( UHV ) system with a base pressure of <1×10−10 Torr. The vacuum chamber was equipped with a high resolution electron spectrometer for EELS, a home-made electron optics for LEED, and a quadrupole mass spectrometer (QMS ) for TDS and gas analysis. For EELS measurements, a primary energy E of 7 eV, an p energy resolution of 15–20 meV (full-width at halfmaximum, FWHM ) and an incidence angle h of i 60° from the surface normal were used. All EELS spectra were recorded at 300 K. LEED patterns were observed at E =91–97 eV. p The B-doped diamond film (thickness: 20 mm) was epitaxially grown, using the microwave plasma-assisted chemical vapor deposition (CVD) method, on the (100) surface of synthesized singlecrystalline diamond (Ib type). The growth condition is described in Ref. [15]. Two of the present authors ( T.T. and N.F.) showed, by scanning tunneling microscopy in air, that the surface has the (2×1) structure [15]. The size of the substrate was 4×4×0.3 mm3. The CVD-grown diamond surface was H-terminated, and hence, the clean surface was prepared by heating the sample up to 1400 K by electron bombardment from the rear. The surface cleanness was checked by LEED and EELS. The clean surface showed a sharp doubledomain (2×1) LEED pattern as shown in Fig. 1a, and a characteristic loss spectrum as shown later. The heating rate ( b) for the TDS measurements was 8 K s−1. The sample temperature was measured by using an alumel–chromel thermocouple attached to the sample edge. The hydrogen desorption temperature (~1250 K ) measured by this method is found to be nearly the same (±20 K ) as those of other studies available [16 ].
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Fig. 1. (a) A LEED pattern of the clean C(100) surface. The primary energy used is E =91 eV. Schematic view of the p observed LEED pattern is included. (b) Comparative EELS spectra in the specular mode of the oxygenated C(100) surface where atomic O were produced by hot (i) W- and (ii) Ir-filaments, respectively. The oxygen exposure is 4.5 and 240 L, respectively. E =7.0 eV. p
All gases were introduced into the UHV chamber through a variable leak valve. Atomic H and O were formed by the exposure of H and O to 2 2 a hot (~2000 K ) and well-outgassed W-filament located 2 cm away from the sample surface. During atomic H (O) exposure, the sample temperature was increased to 500 K by the radiation from the filament. Since the dissociation probability of H 2 (O ) is not known, the H (O) exposure is given by 2 the H (O ) exposure used for the H (O) formation. 2 2 The amount of gas exposures are given in Langmuir (1 L=1×10−6 Torr s). It is noted that the hot W-filament might pro-
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duce excited O molecules, etc., in addition to 2 atomic O. We cannot exclude this possibility. However, we believe that these give a negligible effect as the dimer p- and s-bonds are very strong as will be discussed later. It has been reported that the surface can be contaminated with the W and WO species when the W-filament is used as a x source of atomic O [17]. In order to check the existence of W and WO , a hot (~1400 K ) Ir x filament is used for the production of O following the work of Thomas et al. [10] in which no Ir-related contaminations are observed by X-ray photoemission spectroscopy ( XPS) when the C(100) surface is oxidized using the Ir filament. Fig. 1b shows EELS spectra of the oxygenated C(100) surface where O is produced by the (i) Wand (ii) Ir-filaments. Losses are observed at 90, 152, 215 and 261 meV for both spectra which are associated with the surface phonons and the adsorbed O species as will be discussed later. No losses are observed, especially in Fig. 1b(i), corresponding to WO species which gives strong losses x at 75 and 134 meV [18]. There is no essential difference between (i) and (ii) except for the much higher exposure needed for the Ir filament, which suggests that the surface is free from W-related contamination. It is noted in the case of the Ir filament that a much longer exposure time is needed, because the temperature usually used (~1400 K ) is lower (to avoid the filament breaking) and thus, the production efficiency of the atomic O is also lower. In this connection, it is also noted that the influence of the H adsorption due to the residual H cannot be neglected when 2 the Ir-filament is used. Therefore, the W-filament is used for the production not only of H but also of O. Recently, it was reported that the Ir filament is also not the pure source of atomic O [19]. Unfortunately, our UHV chamber is not equipped with Auger electron spectroscopy or XPS and thus, the influence of the Ir-related contaminations cannot be ruled out completely.
3. Results Fig. 2 shows EELS spectra of the clean C(100) surface and of the same surface subsequently
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Fig. 2. EELS spectra in the specular mode of the clean C(100) surface at 300 K and of the same surface with increasing atomic O exposure. E =7.0 eV. p
exposed to an increasing amount of atomic O produced by the W-filament. For the clean surface, the loss peaks are observed at 90 and 152 meV which are associated with the surface phonons characteristic of the reconstructed C(100)-(2×1) surface [20]. The typical count rate of the elastic peak was 8×104 c s−1. A loss appears at 215 meV when the surface is exposed to 1.5 L O. The 215 meV loss grows in intensity and the losses related to the surface phonons become less clear as the exposure of atomic O is increased. For 9 L, the 152 meV loss is replaced by the loss structure in the region of 100–150 meV and a loss appears at 261 meV. Finally, losses are observed at 113, 150, 215 and 261 meV for 30 L. The 261 meV loss is not clearly detected but its existence was confirmed by the detailed measurements. The (1×1) LEED pattern with higher background was observed for the oxygenated (30 L) surface in agreement with the work of Thomas et al. [10]. When the oxygenated C(100) surface is heated
to high temperatures, the 261 meV loss disappears at 800 K as shown in Fig. 3. The intensities of the 150 and 215 meV losses decrease nearly simultaneously by heating up to 1000 K and finally disappear at 1200 K. In the corresponding TDS spectra, a single CO desorption peak with a shoulder at the lower temperature side (~1000 K ) was observed at ~1100 K as shown in Fig. 4. The CO desorption peak at 1100 K is compatible with the EELS spectra of the oxygenated C(100) surface heated to high temperature as shown in Fig. 3. The desorption of CO from the oxygenated C(100) surface implies that etching of C atoms from the substrate surface occurs. The shoulder at 1000 K may be related to O adsorption in the defect sites. When the oxygenated C(100) surface is heated to 1200 K, the characteristic phonon spectrum is restored. The (2×1) LEED pattern was also clearly observed after two cycles of atomic O adsorption and desorption experiments. After several cycles of atomic O adsorption and desorption experiments, even the (1×1) LEED pattern was not observed. No CO desorption was observed 2 when the sample surface shows the characteristic
Fig. 3. EELS spectra in the specular model of the C(100) surface exposed to 15 L oxygen and of the same surface with increasing heating temperature. The spectra were recorded at 300 K. E =7.0 eV. p
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Fig. 4. Series of CO and CO TDS spectra from the C(100) 2 surface exposed to 3–27 L oxygen. Heating rate, b=8 K s−1.
phonon loss features in EELS as well as the (2×1) LEED pattern. The CO desorption peak temperature of 1100 K is higher than that reported in the previous study by Thomas et al. [10] where an extremely broad CO desorption peak ( FWHM = 350 K for b=20 K s−1) is observed at ~900 K and CO 2 desorption is also observed at ~800 K. A broad CO desorption peak ( FWHM ~260 K for b=0.5–14 K s−1) is also reported in the work of Frenklach et al. [21] (where the activation energy for CO desorption is estimated to be 45 kcal mol−1). We also obtained the broad CO desorption peak at 900 K and CO desorption 2 peak at 800 K from the degraded surface (not shown) which are similar to the spectra reported earlier by Thomas et al. [10] and Frenklach et al. [21]. We can extract the activation energy for desorption using the Redhead formula, E=RT [ ln(nT /b)−3.46 ], where R is the gas m m constant, T the peak temperature and n the prem exponential factor [22]. Assuming the value of n=1013 s−1, the activation energy for CO desorption from the C(100) surface is estimated to be
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68 kcal mol−1. This activation energy for desorption is comparable with the work of Matsumoto et al. [23,24] on the diamond powder, where they observed the CO desorption peak at 1070 K. For comparison, it is noted that CMC bond energy is 83 kcal mol−1 [25]. Fig. 5 shows EELS spectra of the oxygenated C(100) surface with increasing atomic H exposure. It is clearly observed that the 215 meV loss is reduced in intensity as the atomic H exposure is increased, and disappears at 54 L exposure. Instead, the loss peaks appear at 152 and 362 meV which grow in intensity as the atomic H exposure is increased. At higher exposures, a loss peak is clearly observed at 104 meV. With increasing atomic H exposure to the oxygenated surface, the signal-to-noise ratio of the spectrometer gradually decreases due to the disordering of the surface. Hence, the spectra become noisy at higher exposure. However, the losses at 270 and 300 meV are identified at 54 L exposure. The intensities of the 152 and 362 meV losses are higher for 24 L than
Fig. 5. EELS spectra in the specular mode of the C(100) surface at 300 K pre-exposed to 30 L oxygen and post-exposed to an increasing amount of atomic H. E =7.0 eV. p
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those for 54 L exposure. This may be related with the appearance of the 270 meV loss at 54 L. This surface showed the (1×1) LEED pattern with higher background.
4. Discussion The early investigation of acid-treated diamond powder by IR techniques has shown a peak at 212–220 meV which is assigned as the CMO stretching mode of carbonyl (vC앞O) species [13,14]. Furthermore, the CMO stretching energy of cyclohexanone was reported at 212 meV [26 ]. Hence, the 215 meV loss peak is assigned as the stretching mode of the surface carbonyl species (on-top O) which is formed by the dimer p- and s-bond breaking [12]. Molecular dynamics calculation on the oxygenated C(100) surface has indicated that the CMO stretching of the carbonyl species is at 248 meV, which seems to be overestimated [12]. The 113 meV loss peak is due to the bending mode of the carbonyl species. The calculated value for the CO bending mode of the oxygenated C(100) surface is 110 meV [27]. The CO bending mode of adamentanone (C H O) 10 14 (which has a structure rather similar to the diamond) is observed at 107 meV [28]. A structural model of carbonyl species is shown in Fig. 6. The 150 meV loss is assigned as the CMO stretching mode of the surface ether (CMOMC ) species (bridging O) which is also formed by the scission of the dimer bonds. The CMO stretch energy at 150 meV is in agreement with the peak observed by IR at 136–155 meV in the case of oxidized diamond powder [14]. The vibrational energies of 2-oxa-adamentane (C H O) which is 9 14 the simplest molecular analogue of an ether-type oxide on diamond are not available. The CMO stretching energies of polycyclic ether dioxabicyclo [3.2.1] octane (C H O ) are reported at 148 and 9 16 2 155 meV [29]. The CMO stretching energy of a simple ether (C H O) is 136 meV [30]. However, 2 6 this is not a good molecular analogue because the C ends are terminated by light H atoms. The theoretical calculation on the monolayer O on C(100) has shown multiple peaks at 145–155 meV which are assigned to the CMO stretching mode
Fig. 6. Structural models of oxygen atoms adsorbed on the C(100) surface.
of the surface ether species [12]. The CMO stretching mode is observed at 158 meV for ethylene oxide [30]. The 150 meV loss might be attributed to the CMO stretching mode of the surface epoxy species. This possibility cannot be ruled out completely, but it is not accepted because the vibrational energy difference is large (8 meV ). The 261 meV loss with relatively small intensity is ascribed to the presence of triply-bonded CO species loosely bound to the substrate, which disappear upon heating to 800 K (Fig. 3). The CO adsorption on the down-atom of the Si(100) surface dimer shows a loss peak at 261 meV [31]. The formation process of triple-bonded CO by the atomic O exposure on the C(100) surface is not clearly understood. Roughly speaking, the intensities of the 150 and 215 meV losses are similar for 30 L exposure as shown in Fig. 2, which may simply indicate that the amount of the ether and carbonyl species formed are nearly the same. The temperature dependence of the 150 and 215 meV losses shows similar behavior as shown in Fig. 3. The CO desorption spectrum shows a single peak around 1100 K. These results suggest that the thermodynamic stability of the two species is very close. The theoretical calculations show that the binding energy difference per O atom between the carbonyl
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and ether species is small: 0.14 eV by Zheng and Smith [32], 0.015 eV by Whitten et al. [33] and −0.19 by Skokov et al. [12]. The energy difference of 0.1–0.2 eV corresponds to the difference in the desorption peak temperature of 50–100 K using the Redhead formula, which, however, is not observed. Thus our result is compatible with the energy difference of 0.015 eV obtained by Whitten et al. [33]. The single desorption peak might be explained by considering that the desorption process proceeds through a single transition state, that is, the conversion between the carbonyl and ether species occurs prior to desorption. However, we did not get any evidence of the conversion in the EELS spectra with increasing heating temperature (Fig. 3). The effect of the heat treatment of the oxygenated C(100) surface is summarized as follows: first, the triply-bonded CO species is decomposed and CO desorption occurs at 800 K. At 1100 K, the surface carbonyl and ether species are decomposed to desorb as CO. The C atoms in the first layer are etched and the bulk-like (1×1) structure may be formed. Then, the surface C atoms, which are originally in the second layer, are paired to form dimers in order to reduce the number of unsaturated dangling bonds. The characteristic phonon spectrum and the (2×1) LEED pattern are thus restored. This suggests the possibility of the layer-by-layer etching with the adsorption of O and the subsequent heat treatment. However, it is difficult to control, for example, the surface concentration of O, and thus the degradation of the phonon spectrum and the LEED pattern occur after several cycles of experiments. In Fig. 5, the 152 and 362 meV losses are assigned as the bending and stretching modes of the CH species, respectively [20,34,35]. The 300 meV loss is attributed to the overtone of the CH bending mode. The 270 meV loss may be associated with the adsorbed H, but the origin is unknown at the present stage. The 104 meV loss does not appear when the bare C(100)-(2×1) surface is exposed to atomic H. The 104 meV loss is very close to the rocking mode (in-plane hindered rotation) of CH species theoretically pre2 dicted at 108 meV [36 ]. Although the CH species 2 on the C(100) surface is not energetically favored
69
due to the steric hindrance [36 ], some theoretical calculations support the existence of CH species 2 with twisted or canted geometry [37,38]. Thus, the appearance of the 104, 152 and 362 meV losses accompanied by the disappearance of the 215 meV loss suggests that O in the carbonyl (vC앞O) species is replaced by the atomic H and possibly CH species is formed in addition to the CH 2 species. The change of the 150 meV loss associated with the ether species is not clear due to the overlap with the strong peak at 152 meV attributed to the CH bending mode. However, observation and disappearance of the CO desorption peak in the TDS spectra before and after exposure of the oxygenated C(100) surface to atomic H, respectively, suggest that both the on-top and bridging O are replaced by atomic H. It is noted that no H O and OH species were detected in the TDS 2 spectra after the oxygenated surface is exposed to atomic H. A model for the replacement process is shown schematically in Fig. 7. Initially, H atoms attack the surface carbonyl and ether species to form, perhaps, hydroxyl (OH ) species which play the role of the reaction intermediate. Then, the surface O atoms are, most probably, replaced by the desorption of H O and the unsaturated dangling 2 bonds are terminated with the H to form CH and CH species. The observed (1×1) LEED pattern 2 with higher background after exposure of atomic H to the oxygenated surface is ascribed to the random distribution of CH and CH species on 2 the surface. A theoretical study of oxygenated C(100) surface in the presence of hydrogen suggests that hydroxyl species on the surface is stable [39]. Also, it is predicted that the OMH stretch vibration should be observed at 481 meV. Recently, using the hybrid density-functional theory, Okamoto [27] found that, after the partially dissociative adsorption of a H O molecule, 2 an adsorbed hydroxyl fragment is decomposed on the surface to form predominantly carbonyl species. In the present study, we did not get any evidence of hydroxyl species on the surface. This is perhaps due to the short lifetime of hydroxyl species. It is interesting to discuss whether the replacement reaction is Eley–Rideal ( ER) or Langmuir–Hinshelwood (LH ) type. Probably, it
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Fig. 7. A model for the replacement of the surface O species on the C(100) by atomic H from the gas phase.
is classified as the ER type, but more experimental efforts are needed to clarify the details. Struck and D’Evelyn [40] have studied the water (H O) adsorption on the C(100) surface by 2 infrared multiple-internal-reflection spectroscopy. They claimed that OH species are observed when the C(100) surface at 1070 K is cooled down to 300 K in 1×10−7 Torr of H O. The existence of 2 the OH species is based on the observation of the 134 and 139 meV peaks. However, it is difficult to understand that the 134 meV peak exists even after the cooled sample is heated to 1270 K. Also, the 139 meV peak may be attributed to the CD 2 deformation (scissors) mode. During the heating and cooling processes, the possibility of the adsorption of D (from the residual D ) on the surface 2
cannot be ruled out. Hence, the existence of the OH species is questionable. Typical substrate temperature during the CVD growth of diamond is ~1100–1200 K and the TDS spectrum of the hydrogenated surface shows that the H desorption starts at ~1000 K. Hence, 2 at the CVD condition, it is likely that some CMC dimers are formed on the surface which disturb the diamond growth process. The formation of surface carbonyl and ether species on the C(100) surface by atomic O exposure is accompanied by the breaking of both the s and p bonds of the surface dimer as shown in Fig. 6. (It is noted that atomic H is hardly able to break the s bond of the surface dimer.) The unusual ability of atomic O of breaking the dimer bond may play a crucial role in the continuation of the diamond growth in the oxygen-containing environment. The formation of the carbonyl species on the C(100) surface due to atomic O exposure can structurally be viewed as the adsorption of CO on the surface. It is reported that diamond can be successfully grown using only a mixture of CO and H with a significantly higher growth rate [6 ]. 2 The atomic H interaction with the oxygenated C(100) surface which is predominantly covered by the carbonyl and ether type of CO species may give a better understanding of the diamond growth especially using a mixture of CO and H gases in 2 the CVD chamber.
5. Summary The interaction of atomic O with the C(100)(2×1) surface and the replacement of adsorbed O by atomic H have been investigated by EELS, TDS and LEED. Some of the important results are summarized as follows: 1. Atomic O breaks both the s and p bonds of the surface dimer to form predominantly carbonyl (vC앞O) species (on-top O) and ether (CMOMC ) species (bridging O) as clearly indicated by the 113, 215 and the 150 meV losses, respectively. A small amount of triple-bonded CO are also detected which are characterized by the 261 meV loss. 2. A single CO desorption peak is observed at
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~1100 K which corresponds to the desorption energy of 68 kcal mol−1. 3. After the CO desorption, carbon atoms in the second layer are found to be dimerized as indicated by the reappearance of the phonon spectrum characteristic of the clean C(100)(2×1) surface and by the observation of the (2×1) LEED pattern. 4. Chemisorbed O on the C(100) surface is readily replaced by the atomic H as clearly evidenced by the disappearance of the 215 meV loss and CO desorption. A model for the replacement is proposed as shown in Fig. 7. Acknowledgements This work was supported in part by Grants-inAid from the Ministry of Education, Science, Sports and Culture (Japan) and from Sumitomo Electric Industries Ltd.
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