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Decomposition of triethylphosphine (TEP) and tertiarybutylphosphine (TBP) on a GaP(001)–(2×4) surface studied by HREELS
Applied Surface Science 142 Ž1999. 1–6
Decomposition of triethylphosphine žTEP/ and tertiarybutylphosphine žTBP/ on a GaP ž001/ – ž2 = 4/ surface studied by HREELS G. Kaneda, N. Sanada, Y. Fukuda
)
Research Institute of Electronics, Shizuoka UniÕersity, Hamamatsu 432-8011, Japan
Abstract Adsorption and decomposition of triethylphosphine ŽTEP. and t-butylphosphine ŽTBP. on a GaPŽ001. – Ž2 = 4. surface have been studied by using high-resolution electron energy loss spectroscopy ŽHREELS.. For TEP, since the vibration modes corresponding to the ethyl group and the yŽPC. mode appear at 100 and 300 K, we conclude that TEP is adsorbed molecularly at those temperatures. All the modes are decreased in intensity upon annealing the sample above 300 K and have almost disappeared at 700 K. The yŽGa–H. mode is not found at elevated temperatures, which is in sharp contrast to the previous result that the yŽSi–H. peak appears on a SiŽ001. surface for TEP. The absence of the yŽGa-H. mode is explained as the decomposition of the ethyl group into ethylene and hydrogen through a b-hydride elimination which occurs at temperatures higher than the desorption temperature of hydrogen of Ga–H. Our HREELS data suggest that switching off the ethyl group to the surface and re-adsorption of ethylene on the surface do not occur. The latter process is important because it avoids carbon incorporation in the growth of films. For TBP, most of TBP is adsorbed molecularly at 100 K and is partially dissociated into ŽC 4 H 9 .PH Ža. and HŽa. at 300 K. All the modes corresponding to the t-butyl group are decreased in intensity above 300 K and almost disappear at 550 K. The increase in intensity of the yŽGa–H. signal at elevated temperatures is not found; this can also be explained by the reason mentioned on TEP. It is also suggested that switching of tertiarybutyl group to the surface and re-adsorption of isobutene do not occur. q 1999 Elsevier Science B.V. All rights reserved. PACS: 61.16.y d; 82.40.y g; 82.65.My; 81.15.Gh Keywords: Gallium phosphide; Low index single crystal surfaces; High-resolution electron energy loss spectroscopy; Triethylphosphine; t-Butylphosphine; Chemisorption; Surface chemical reaction
1. Introduction Decomposition of metalorganic compounds as precursors for fabrication of compound semiconductor thin films, which are produced by metalorganic )
Corresponding author. Tel.: q81-53-478-1305; Fax: q81-53474-0630; E-mail: [email protected]
vapor epitaxy ŽMOVPE. Žfor review, see Ref. w1x. and atomic layer epitaxy ŽALE. Žfor review, see Ref. w2x. on semiconductor surfaces, have been widely studied. Much effort has been especially devoted to studying alkyl-gallium compounds by using X-ray photoelectron spectroscopy ŽXPS., high-resolution electron energy loss spectroscopy ŽHREELS., and temperature-programmed desorption ŽTPD. in order
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 6 5 0 - 3
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G. Kaneda et al.r Applied Surface Science 142 (1999) 1–6
to elucidate the formation mechanism of a GaAs thin film Žfor example on SiŽ100., see Ref. w3x; for example on GaAsŽ100., see Ref. w4x.. It is generally accepted that the CH 3 group of trimethylgallium ŽTMG. is dehydrogenated into carbon through a CH 2 intermediate, resulting in a high degree of carbon incorporation in the GaAs thin film w5x. On the other hand, triethylgallium ŽTEG. is decomposed into mainly Ga, C 2 H 4 Žethylene. and hydrogen through a b-hydride elimination reaction Žin which hydrogen atoms at the b position in hydrocarbon are eliminated by substrates. w3x, leading to a lower degree of carbon incorporation in the film. Recently, much attention has been paid to alkylarsine and -phosphine as alternatives to arsine and phosphine because the latter are highly toxic, but the number of reports on alkyl-V group compounds is much smaller than alkyl-III group compounds. Decomposition of trimethylarsine ŽTMAs: ŽCH 3 . 3 As. w5x and diethylarsine ŽDEAs: ŽC 2 H 5 . 2 AsH. w6x on SiŽ100. were studied by using HREELS. It was reported that the carbon deposit from TMAs is significantly less than that from TMG. DEAs was found to be partially decomposed into ŽC 2 H 5 . 2 As and H at RT. The b-hydride elimination reaction was suggested to occur at elevated temperature. Although GaP and InP thin films have been widely fabricated by MOVPE and ALE, the decomposition mechanism of alkyl-phosphine on semiconductor substrates has not been investigated in detail. We have studied the adsorption and decomposition of triethylphosphine ŽTEP: ŽC 2 H 5 . 3 P. and tbutylphosphine ŽTBP: ŽC 4 H 9 .PH 2 . on SiŽ001. by using XPS w7x, TPD w7x and HREELS w8x, and on GaPŽ001. by using TPD w9x. The TPD data from TEP adsorbed on SiŽ001. and GaPŽ001. show the similar result; it is decomposed into C 2 H 4 and H 2 . The XPS result suggests that TEP is adsorbed molecularly on SiŽ001. at RT, which was consistent with the HREELS result. The HREELS spectra of TEP on SiŽ001. indicated that the vibration mode of Si–H increases in intensity at elevated temperature and only the peak of Si–P species remains on the surface at 1100 K, leading to the conclusion that C 2 H 4 and H 2 are formed through b-hydride elimination and that no carbon is left on the surface. For TBP, the TPD results were different on SiŽ001. and GaPŽ001.: the decomposition products were C 4 H 8 Žisobutene.
and H 2 for the former and C 4 H 9 Ž t-butyl radical., C 4 H 8 , and H 2 for the latter. It was found from the HREELS spectra on SiŽ001. that TBP is partially dissociated into C 4 H 9 PH and H between 100 and 300 K, and is also decomposed by the b-hydride elimination. However, adsorbed states of TEP and TBP on GaPŽ001. during their decomposition have not been studied in spite of the importance of elucidating their decomposition mechanisms. Since it is difficult to distinguish the chemical state of phosphorous atoms in the precursors on GaPŽ001. and the phosphorous atoms in the GaPŽ001. substrate using XPS, it is necessary to use HREELS to study the adsorbed species on the surface during the decomposition. Therefore, we have investigated the adsorption and decomposition of TEP and TBP on a clean GaPŽ001. – Ž2 = 4. surface by using HREELS to elucidate the decomposition mechanism of the precursors.
2. Experimental A GaPŽ001. sample Žn-type, carrier concentration: 10 17 cmy3 . was chemically etched using an acid solution ŽHCl:HNO 3 s 1:1. at room temperature ŽRT. for 1 min. After a rinse in deionized water it was quickly introduced into the ultrahigh vacuum chamber. The sample was clamped with tantalum strips and was resistively heated. The temperature was measured by a thermocouple attached to the tantalum strip. The GaPŽ001. – Ž2 = 4. surface, which was confirmed by low-energy electron diffraction ŽLEED., was obtained by several cycles of Ar ion bombardment Ž710 V. and annealing at 723 K. The cleanliness of the sample was also confirmed by Auger electron spectroscopy ŽAES.. Adsorption of TEP Ž10 L. and TBP Ž10 L. on the clean GaPŽ001. – Ž2 = 4. surface was carried out at 100 and 300 K. Since there is a possibility that ion pumps may crack the precursor gases, the gases were exposed to the sample while pumping the chamber with a turbo-molecular pump and a liquid nitrogen shroud. The TEP- and TBP-adsorbed samples were annealed at various elevated temperatures but HREELS spectra were all measured at RT.
G. Kaneda et al.r Applied Surface Science 142 (1999) 1–6
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The HREELS used here, which was built by ourselves, consists of double monochromators and double analyzers. The resolution was typically about 10 meV at an incident energy of 5 eV. All spectra were measured in specular direction with an incident angle of 608 to the surface normal.
3. Results and discussion Fig. 1 shows HREELS spectra of the clean and TEP-adsorbed Žat 100K. GaPŽ001. surfaces. We find phonon peaks w10x at 395, 795, and 1179 cmy1 together with a peak at 2923 cmy1 which is due to hydrocarbon contamination on the clean surface. Upon adsorption of TEP at 100K, the strong peaks at 2921, 1425, 1233, 1001, and 761 cmy1 appear which correspond to the y ŽCH 3 . q yŽCH 2 ., dŽCH 3 ., v ŽCH 2 . q t ŽCH 2 ., yŽCC. q gŽCH 3 ., and yŽPC. modes w8,11x, where y, d, v, t, and g represent stretching, deformation, wagging, twisting, and rocking vibrations, respectively. This result indicates that TEP is adsorbed molecularly on the surface. The weak peaks at 3313, 2433, 2201, and 1817 cmy1 can be ascribed to the combination modes: 2921 q 395, 1425 q 1001, 1233 q 1001, and 1425 q 395 cmy1 ,
Fig. 1. HREELS spectra of the clean Ža. and TEPŽ10 L.-adsorbed Žb. GaPŽ001. surfaces at 100 K.
Fig. 2. HREELS spectra of the clean Ža. and TEPŽ10 L.-adsorbed GaPŽ001. surfaces at 300 K Žb., and subsequently annealed at 400 K Žc., 500 K Žd., 600 K Že., and 700 K Žf..
respectively. Since the melting temperature of TEP is 185 K and the exposure of the gas is 10 L, a multilayer phase of TEP would be formed at 100 K. The yŽGa-TEP. mode is not found in Fig. 1. It is speculated that the yŽGa-TEP. peak is superimposed on the phonon or elastic peaks because of the heavy mass of TEP. HREELS spectra of the clean and TEP-adsorbed GaPŽ001. surfaces at 300 K and at various elevated temperatures are displayed in Fig. 2. We find vibration peaks at 2954, 1418, 1258, 1026, and 754 cmy1 at 300 K which can be assigned to the modes mentioned above Žsee Fig. 1. although the wave numbers are different in yŽCH 3 . q yŽCH 2 ., v ŽCH 2 . q t ŽCH 2 ., and yŽCC. q gŽCH 3 . from those in Fig. 1. The difference in these wave numbers at 100 and 300 K is attributed to the difference between a multilayer phase of TEP at 100 K and chemisorbed molecules on the surface at 300 K. Similar results were obtained for TEG on GaAsŽ001. w4x. The spectral features at 300 K are in good agreement with the result of TEP on SiŽ001. w8x. All the peaks are decreased in intensity upon annealing above 300 K and almost disappear at 700 K. This is consistent
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G. Kaneda et al.r Applied Surface Science 142 (1999) 1–6
with the result that desorption of TEP Žbetween about 323 and 523 K. and evolution of ethylene and hydrogen formed by decomposition of TEP Žbetween about 373 and 623 K. occur on GaPŽ001. w9x. We do not find a peak due to the Ga–H stretching vibration mode at elevated temperatures as shown in Fig. 2, which agrees with the result for TEAs Žtriethylarsine. on GaAsŽ001. w4x. This is in sharp contrast to the result w8x that the Si–H vibration peak appears at 2042 cmy1 for TEP on SiŽ001. annealed between 450 and 800 K. The Si–H species were reported to be formed through the b-hydride elimination during decomposition of an ethyl-group on the surface at about 593 K, leading to evolution of ethylene at about 593 K and leaving hydrogen on the surface w8x. This hydrogen was evolved at about 773 K. On the other hand, since decomposition of TEP into ethylene and hydrogen occurs on GaPŽ001. at about 473 K w9x and the decomposition temperature of Ga–H is about 473–423 K w12x, the hydrogen atoms on the surface formed by the decomposition is immediately evolved from the surface. This can explain why the vibration peak of yŽGa–H. is not found in Fig. 2. We suggest that hydrogen in the ethyl group could not be eliminated by phosphorous atoms on the surface because the Ž2 = 4. surface is stabilized by gallium atoms w13x. It may be possible that the ethyl group of TEP is switched off to the GaPŽ001. surface. Such switching was observed for TMG on SiŽ001. w5x and GaAsŽ001. w4x, and for TEG on GaAsŽ001. w4x. However, since the wave numbers of the vibration modes corresponding to the ethyl group are not changed between 300 and 700 K Žsee Fig. 2., the ethyl group of TEG is not switched off to gallium sites. This is in good agreement with the result for TEP on SiŽ001. w8x. If the ethyl group is switched off to phosphorus sites, we could not distinguish ŽC 2 H 5 . n P Ž n s 1, 2, and 3. due to the limitation of our resolution. We can ruled out re-adsorption of ethylene formed by the decomposition of TEP on the surface because the typical yŽC5C. mode at about 1200 cmy1 w14x or the peak at about 665 cmy1 w15,16x due to di-s bonded ethylene to the surface was not found. Fig. 3 shows HREELS spectra of the clean and TBP-adsorbed GaPŽ001. surfaces at 100 K and those subsequently annealed at 300 K. The peak at 397 cmy1 for the clean surface can be assigned also to
Fig. 3. HREELS spectra of the clean Ža. and TBPŽ10 L.-adsorbed GaPŽ001. surfaces at 100 K Žb. and subsequently annealed at 300 K Žc..
the surface phonon w10x. Although the intensity of the surface phonon is different between Figs. 1 and 3, the reason is not clear. We find strong peaks at 2934, 2342, 1446, 1206, 982, and 822 cmy1 which correspond to yŽCH 3 ., yŽPH 2 ., dŽCH 3 ., gŽCH 3 ., yŽCC., and yŽPC. w8,11x, respectively, where y, d, and g are the same as in Fig. 1. We speculate that the peak at 278 cmy1 is due to the yŽGa–TBP. vibration because it appears only at 100 K and has a low wave number. The broad peaks at 3230 and 1726 cmy1 are ascribed to the combination of yŽCH 3 . and dŽCH 3 . modes with a 278 cmy1 peak, respectively. Since the melting temperature of TBP is 277 K and the exposure of the gas to the surface was 10 L, the multilayer phase of TBP should exist at 100 K. Since the peaks due to yŽPH 2 . and yŽPC. modes appear at 100 K, most of TBP on the surface is adsorbed molecularly. However, some of the TBP would be partially dissociated since it seems that the peaks due to yŽPH. at 2379 cmy1 w11,17x and yŽGaH. at 1915 cmy1 w17x are superimposed on the 2342 and broad 1726 cmy1 peaks, respectively. Upon heating the sample up to 300 K, all the peaks decreased in
G. Kaneda et al.r Applied Surface Science 142 (1999) 1–6
intensity without changing wave numbers except for the 2342, 1726, and 278 cmy1 peaks. The peaks at 2379 and 1915 cmy1 became clear and the peak at 278 cmy1 disappeared although the intensity of the yŽGa–H. at 1915 cmy1 was very weak. This suggests that most of the TBP molecules on the surface are partially dissociated into C 4 H 9 PHŽa. and HŽa. at 300 K, where Ža. denotes adsorbed species. The result that the yŽGa–TBP. mode is seen at 100 K and that the yŽGa–C 4 H 9 PH. does not appear at 300 K might be due to the HREELS selection rule: dipole moments near perpendicular to the surface are active for HREELS w18x, so the latter is not observable. In this work we find a peak shift from 1446 to 1427 cmy1 for dŽCH 3 . and from 822 to 803 cmy1 for yŽPC. upon heating the sample. It is not clear whether the reason for the peak shift is due to the disappearance of the multilayer phase or partial dissociation of TBP. The spectral features at 100 and 300 K are in good agreement with the result of TBP on SiŽ001. w8x, although the decomposition products on SiŽ001. w7x and GaPŽ001. w9x were different. We speculate that this is due to the difference in adsorption sites for TBP. However, a more detailed study is needed to clarify the difference. HREELS spectra of the clean and TBP-adsorbed GaPŽ001. surfaces at 300 K and those subsequently annealed at various temperatures are shown in Fig. 4. We find that the vibration peaks at 2929, 2369 Žweak., 1915 Žweak., 1417, 1193, 969, and 809 cmy1 are similar to those in Fig. 3 at 300 K. Most of the peaks almost disappear at 550 K where most of TBP on GaPŽ001. was decomposed into isobutene with evolution from the surface w9x. The spectral changes in the vibration bands corresponding to tertiarybutyl group upon annealing are very similar to those observedfor tritertiarybutylgallium ŽŽC 4 H 9 . 3 Ga.on GaAsŽ001. w19x. We find that the yŽGa–H. peak does not increase in intensity during the decomposition Žsee Fig. 4., which is due to the same reason as mentioned in Fig. 2. At elevated temperatures there are no changes in the wave numbers for tertiarybutyl group, which suggests that it is not switched off to the surface during decomposition of TBP. The possibility of re-adsorption of isobutene produced by decomposition of TBP on the surface is ruled out because the HREELS spectra in Fig. 4 are
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Fig. 4. HREELS spectra of the clean Ža. and TBPŽ10 L.-adsorbed GaPŽ001. surfaces at 300 K Žb. and subsequently annealed at 400 K Žc., 450 K Žd., 500 K Že., and 550 K Žf..
completely different from those of isobutene on RhŽ111. w20x and the pure gas w21x. In summary, we have studied the adsorption and decomposition of TEP and TBP on the GaPŽ001. – Ž2 = 4. surface by using HREELS. For TEP, since the vibration modes corresponding to ethyl group and that of yŽPC. appear at 100 and 300 K, we conclude that TEP is adsorbed molecularly on the surface although the wave numbers of some modes are different at 100 and 300 K. We observed that all modes decrease in intensity upon annealing the sample above 300 K and almost disappear at 700 K. The yŽGa–H. mode was not found at elevated temperatures, which is in sharp contrast to the result that the yŽSi–H. peak appears for TEP on the SiŽ001. surface w8x. The absence of the yŽGa–H. mode is explained by decomposition of the ethyl group into ethylene and hydrogen through the b-hydride elimination occurring at a higher temperature than the dissociation temperature of Ga–H. Our HREELS data suggest that switching off the ethyl group of TEP to the surface and re-adsorption of ethylene on the surface do not occur. The latter process is preferred because it avoids carbon incorporation in the growth of the films Žfor example, see Ref. w22x.. For TBP, most of
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G. Kaneda et al.r Applied Surface Science 142 (1999) 1–6
TBP is adsorbed molecularly at 100 K and is partially dissociated into ŽC 4 H 9 .PH Ža. and HŽa. at 300 K. All the modes corresponding to t-butyl group are decreased in intensity above 300 K and have almost disappeared at 550 K. An increase in intensity of the yŽGa–H. at elevated temperatures is not found; this can be explained by the same mechanism mentioned for TEP. It is also suggested that switching off t-butyl group to the surface and re-adsorption of isobutene do not occur. Acknowledgements The authors gratefully acknowledge partial support from the Ministry of Education, Science, Culture, and Sports, Japan and the Tokai Foundation for Technology. References w1x M.J. Ludowise, J. Appl. Phys. 58 Ž1985. R31. w2x A. Usui, H. Watanabe, Annu. Rev. Mater. Sci. 21 Ž1991. 185. w3x F. Lee, A.L. Backman, R. Lin, T.R. Gow, R.I. Masel, Surf. Sci. 216 Ž1989. 173. w4x A.A. Aquino, T.S. Jones, Appl. Surf. Sci. 104r105 Ž1996. 304.
w5x A. Forster, H. Luth, ¨ ¨ J. Vac. Sci. Technol. B 7 Ž1989. 720. w6x R. Strumpler, R.C. Monte, H. Luth, ¨ ¨ Vacuum 41 Ž1990. 975. w7x G. Kaneda, J. Murata, T. Takeuchi, Y. Suzuki, N. Sanada, Y. Fukuda, Appl. Surf. Sci. 113r114 Ž1997. 546. w8x G. Kaneda, N. Sanada, Y. Fukuda, Surf. Sci. 377–379 Ž1997. 724. w9x G. Kanada, T. Takeuchi, J. Murata, N. Sanada, Y. Fukuda, Appl. Surf. Sci. 121r122 Ž1997. 245. w10x L.H. Dubois, G.P. Schwartz, Phys. Rev. B 26 Ž1982. 794. w11x L.J. Bellamy, The Infrared Spectra of Complex Molecules, Wiley, New York, 1966. w12x H. Qui, P.E. Gee, T. Nguyen, R.F. Hicks, Surf. Sci. 323 Ž1995. 6. w13x N. Sanada, M. Shimomura, Y. Fukuda, T. Sato, Appl. Phys. Lett. 67 Ž1995. 1432. w14x E.T. Fritzgerald, J.S. Foord, J. Phys.: Condens. Matters 3 Ž1991. S347. w15x J. Yoshinobu, H. Tsuda, M. Onchi, M. Nishijima, J. Chem Phys. 87 Ž1987. 7332. w16x W. Widda, C. Huang, G.D.A. Briggs, W.H. Weiberg, J. Electron. Spectrosc. Relat. Phenom. 64r65 Ž1993. 129. w17x Y. Chen, D.J. Frankel, J.R. Anderson, G.J. Lapeyre, J. Vac. Sci. Technol. A 6 Ž1988. 752. w18x H. Ibach, D.L. Mills, in: Electron energy loss spectroscopy and surface vibrations, Academic Press, 1982. w19x E.T. FritzGerald, D. O’Hare, A.C. Jones, J.S. Foord, Surf. Sci. 278 Ž1992. 111. w20x N.F. Brown, M.A. Barteau, J. Phys. Chem. 100 Ž1996. 2269. w21x C.M. Pathak, W.H. Fletcher, J. Mol. Spectrosc. 31 Ž1969. 32. w22x R. Lin, T.R. Gow, A.L. Backman, L.A. Cadwell, F. Lee, R.I. Masel, J. Vac. Sci. Technol. B 7 Ž1989. 725.
Decomposition of triethylphosphine žTEP/ and tertiarybutylphosphine žTBP/ on a GaP ž001/ – ž2 = 4/ surface studied by HREELS G. Kaneda, N. Sanada, Y. Fukuda
)
Research Institute of Electronics, Shizuoka UniÕersity, Hamamatsu 432-8011, Japan
Abstract Adsorption and decomposition of triethylphosphine ŽTEP. and t-butylphosphine ŽTBP. on a GaPŽ001. – Ž2 = 4. surface have been studied by using high-resolution electron energy loss spectroscopy ŽHREELS.. For TEP, since the vibration modes corresponding to the ethyl group and the yŽPC. mode appear at 100 and 300 K, we conclude that TEP is adsorbed molecularly at those temperatures. All the modes are decreased in intensity upon annealing the sample above 300 K and have almost disappeared at 700 K. The yŽGa–H. mode is not found at elevated temperatures, which is in sharp contrast to the previous result that the yŽSi–H. peak appears on a SiŽ001. surface for TEP. The absence of the yŽGa-H. mode is explained as the decomposition of the ethyl group into ethylene and hydrogen through a b-hydride elimination which occurs at temperatures higher than the desorption temperature of hydrogen of Ga–H. Our HREELS data suggest that switching off the ethyl group to the surface and re-adsorption of ethylene on the surface do not occur. The latter process is important because it avoids carbon incorporation in the growth of films. For TBP, most of TBP is adsorbed molecularly at 100 K and is partially dissociated into ŽC 4 H 9 .PH Ža. and HŽa. at 300 K. All the modes corresponding to the t-butyl group are decreased in intensity above 300 K and almost disappear at 550 K. The increase in intensity of the yŽGa–H. signal at elevated temperatures is not found; this can also be explained by the reason mentioned on TEP. It is also suggested that switching of tertiarybutyl group to the surface and re-adsorption of isobutene do not occur. q 1999 Elsevier Science B.V. All rights reserved. PACS: 61.16.y d; 82.40.y g; 82.65.My; 81.15.Gh Keywords: Gallium phosphide; Low index single crystal surfaces; High-resolution electron energy loss spectroscopy; Triethylphosphine; t-Butylphosphine; Chemisorption; Surface chemical reaction
1. Introduction Decomposition of metalorganic compounds as precursors for fabrication of compound semiconductor thin films, which are produced by metalorganic )
Corresponding author. Tel.: q81-53-478-1305; Fax: q81-53474-0630; E-mail: [email protected]
vapor epitaxy ŽMOVPE. Žfor review, see Ref. w1x. and atomic layer epitaxy ŽALE. Žfor review, see Ref. w2x. on semiconductor surfaces, have been widely studied. Much effort has been especially devoted to studying alkyl-gallium compounds by using X-ray photoelectron spectroscopy ŽXPS., high-resolution electron energy loss spectroscopy ŽHREELS., and temperature-programmed desorption ŽTPD. in order
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 6 5 0 - 3
2
G. Kaneda et al.r Applied Surface Science 142 (1999) 1–6
to elucidate the formation mechanism of a GaAs thin film Žfor example on SiŽ100., see Ref. w3x; for example on GaAsŽ100., see Ref. w4x.. It is generally accepted that the CH 3 group of trimethylgallium ŽTMG. is dehydrogenated into carbon through a CH 2 intermediate, resulting in a high degree of carbon incorporation in the GaAs thin film w5x. On the other hand, triethylgallium ŽTEG. is decomposed into mainly Ga, C 2 H 4 Žethylene. and hydrogen through a b-hydride elimination reaction Žin which hydrogen atoms at the b position in hydrocarbon are eliminated by substrates. w3x, leading to a lower degree of carbon incorporation in the film. Recently, much attention has been paid to alkylarsine and -phosphine as alternatives to arsine and phosphine because the latter are highly toxic, but the number of reports on alkyl-V group compounds is much smaller than alkyl-III group compounds. Decomposition of trimethylarsine ŽTMAs: ŽCH 3 . 3 As. w5x and diethylarsine ŽDEAs: ŽC 2 H 5 . 2 AsH. w6x on SiŽ100. were studied by using HREELS. It was reported that the carbon deposit from TMAs is significantly less than that from TMG. DEAs was found to be partially decomposed into ŽC 2 H 5 . 2 As and H at RT. The b-hydride elimination reaction was suggested to occur at elevated temperature. Although GaP and InP thin films have been widely fabricated by MOVPE and ALE, the decomposition mechanism of alkyl-phosphine on semiconductor substrates has not been investigated in detail. We have studied the adsorption and decomposition of triethylphosphine ŽTEP: ŽC 2 H 5 . 3 P. and tbutylphosphine ŽTBP: ŽC 4 H 9 .PH 2 . on SiŽ001. by using XPS w7x, TPD w7x and HREELS w8x, and on GaPŽ001. by using TPD w9x. The TPD data from TEP adsorbed on SiŽ001. and GaPŽ001. show the similar result; it is decomposed into C 2 H 4 and H 2 . The XPS result suggests that TEP is adsorbed molecularly on SiŽ001. at RT, which was consistent with the HREELS result. The HREELS spectra of TEP on SiŽ001. indicated that the vibration mode of Si–H increases in intensity at elevated temperature and only the peak of Si–P species remains on the surface at 1100 K, leading to the conclusion that C 2 H 4 and H 2 are formed through b-hydride elimination and that no carbon is left on the surface. For TBP, the TPD results were different on SiŽ001. and GaPŽ001.: the decomposition products were C 4 H 8 Žisobutene.
and H 2 for the former and C 4 H 9 Ž t-butyl radical., C 4 H 8 , and H 2 for the latter. It was found from the HREELS spectra on SiŽ001. that TBP is partially dissociated into C 4 H 9 PH and H between 100 and 300 K, and is also decomposed by the b-hydride elimination. However, adsorbed states of TEP and TBP on GaPŽ001. during their decomposition have not been studied in spite of the importance of elucidating their decomposition mechanisms. Since it is difficult to distinguish the chemical state of phosphorous atoms in the precursors on GaPŽ001. and the phosphorous atoms in the GaPŽ001. substrate using XPS, it is necessary to use HREELS to study the adsorbed species on the surface during the decomposition. Therefore, we have investigated the adsorption and decomposition of TEP and TBP on a clean GaPŽ001. – Ž2 = 4. surface by using HREELS to elucidate the decomposition mechanism of the precursors.
2. Experimental A GaPŽ001. sample Žn-type, carrier concentration: 10 17 cmy3 . was chemically etched using an acid solution ŽHCl:HNO 3 s 1:1. at room temperature ŽRT. for 1 min. After a rinse in deionized water it was quickly introduced into the ultrahigh vacuum chamber. The sample was clamped with tantalum strips and was resistively heated. The temperature was measured by a thermocouple attached to the tantalum strip. The GaPŽ001. – Ž2 = 4. surface, which was confirmed by low-energy electron diffraction ŽLEED., was obtained by several cycles of Ar ion bombardment Ž710 V. and annealing at 723 K. The cleanliness of the sample was also confirmed by Auger electron spectroscopy ŽAES.. Adsorption of TEP Ž10 L. and TBP Ž10 L. on the clean GaPŽ001. – Ž2 = 4. surface was carried out at 100 and 300 K. Since there is a possibility that ion pumps may crack the precursor gases, the gases were exposed to the sample while pumping the chamber with a turbo-molecular pump and a liquid nitrogen shroud. The TEP- and TBP-adsorbed samples were annealed at various elevated temperatures but HREELS spectra were all measured at RT.
G. Kaneda et al.r Applied Surface Science 142 (1999) 1–6
3
The HREELS used here, which was built by ourselves, consists of double monochromators and double analyzers. The resolution was typically about 10 meV at an incident energy of 5 eV. All spectra were measured in specular direction with an incident angle of 608 to the surface normal.
3. Results and discussion Fig. 1 shows HREELS spectra of the clean and TEP-adsorbed Žat 100K. GaPŽ001. surfaces. We find phonon peaks w10x at 395, 795, and 1179 cmy1 together with a peak at 2923 cmy1 which is due to hydrocarbon contamination on the clean surface. Upon adsorption of TEP at 100K, the strong peaks at 2921, 1425, 1233, 1001, and 761 cmy1 appear which correspond to the y ŽCH 3 . q yŽCH 2 ., dŽCH 3 ., v ŽCH 2 . q t ŽCH 2 ., yŽCC. q gŽCH 3 ., and yŽPC. modes w8,11x, where y, d, v, t, and g represent stretching, deformation, wagging, twisting, and rocking vibrations, respectively. This result indicates that TEP is adsorbed molecularly on the surface. The weak peaks at 3313, 2433, 2201, and 1817 cmy1 can be ascribed to the combination modes: 2921 q 395, 1425 q 1001, 1233 q 1001, and 1425 q 395 cmy1 ,
Fig. 1. HREELS spectra of the clean Ža. and TEPŽ10 L.-adsorbed Žb. GaPŽ001. surfaces at 100 K.
Fig. 2. HREELS spectra of the clean Ža. and TEPŽ10 L.-adsorbed GaPŽ001. surfaces at 300 K Žb., and subsequently annealed at 400 K Žc., 500 K Žd., 600 K Že., and 700 K Žf..
respectively. Since the melting temperature of TEP is 185 K and the exposure of the gas is 10 L, a multilayer phase of TEP would be formed at 100 K. The yŽGa-TEP. mode is not found in Fig. 1. It is speculated that the yŽGa-TEP. peak is superimposed on the phonon or elastic peaks because of the heavy mass of TEP. HREELS spectra of the clean and TEP-adsorbed GaPŽ001. surfaces at 300 K and at various elevated temperatures are displayed in Fig. 2. We find vibration peaks at 2954, 1418, 1258, 1026, and 754 cmy1 at 300 K which can be assigned to the modes mentioned above Žsee Fig. 1. although the wave numbers are different in yŽCH 3 . q yŽCH 2 ., v ŽCH 2 . q t ŽCH 2 ., and yŽCC. q gŽCH 3 . from those in Fig. 1. The difference in these wave numbers at 100 and 300 K is attributed to the difference between a multilayer phase of TEP at 100 K and chemisorbed molecules on the surface at 300 K. Similar results were obtained for TEG on GaAsŽ001. w4x. The spectral features at 300 K are in good agreement with the result of TEP on SiŽ001. w8x. All the peaks are decreased in intensity upon annealing above 300 K and almost disappear at 700 K. This is consistent
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G. Kaneda et al.r Applied Surface Science 142 (1999) 1–6
with the result that desorption of TEP Žbetween about 323 and 523 K. and evolution of ethylene and hydrogen formed by decomposition of TEP Žbetween about 373 and 623 K. occur on GaPŽ001. w9x. We do not find a peak due to the Ga–H stretching vibration mode at elevated temperatures as shown in Fig. 2, which agrees with the result for TEAs Žtriethylarsine. on GaAsŽ001. w4x. This is in sharp contrast to the result w8x that the Si–H vibration peak appears at 2042 cmy1 for TEP on SiŽ001. annealed between 450 and 800 K. The Si–H species were reported to be formed through the b-hydride elimination during decomposition of an ethyl-group on the surface at about 593 K, leading to evolution of ethylene at about 593 K and leaving hydrogen on the surface w8x. This hydrogen was evolved at about 773 K. On the other hand, since decomposition of TEP into ethylene and hydrogen occurs on GaPŽ001. at about 473 K w9x and the decomposition temperature of Ga–H is about 473–423 K w12x, the hydrogen atoms on the surface formed by the decomposition is immediately evolved from the surface. This can explain why the vibration peak of yŽGa–H. is not found in Fig. 2. We suggest that hydrogen in the ethyl group could not be eliminated by phosphorous atoms on the surface because the Ž2 = 4. surface is stabilized by gallium atoms w13x. It may be possible that the ethyl group of TEP is switched off to the GaPŽ001. surface. Such switching was observed for TMG on SiŽ001. w5x and GaAsŽ001. w4x, and for TEG on GaAsŽ001. w4x. However, since the wave numbers of the vibration modes corresponding to the ethyl group are not changed between 300 and 700 K Žsee Fig. 2., the ethyl group of TEG is not switched off to gallium sites. This is in good agreement with the result for TEP on SiŽ001. w8x. If the ethyl group is switched off to phosphorus sites, we could not distinguish ŽC 2 H 5 . n P Ž n s 1, 2, and 3. due to the limitation of our resolution. We can ruled out re-adsorption of ethylene formed by the decomposition of TEP on the surface because the typical yŽC5C. mode at about 1200 cmy1 w14x or the peak at about 665 cmy1 w15,16x due to di-s bonded ethylene to the surface was not found. Fig. 3 shows HREELS spectra of the clean and TBP-adsorbed GaPŽ001. surfaces at 100 K and those subsequently annealed at 300 K. The peak at 397 cmy1 for the clean surface can be assigned also to
Fig. 3. HREELS spectra of the clean Ža. and TBPŽ10 L.-adsorbed GaPŽ001. surfaces at 100 K Žb. and subsequently annealed at 300 K Žc..
the surface phonon w10x. Although the intensity of the surface phonon is different between Figs. 1 and 3, the reason is not clear. We find strong peaks at 2934, 2342, 1446, 1206, 982, and 822 cmy1 which correspond to yŽCH 3 ., yŽPH 2 ., dŽCH 3 ., gŽCH 3 ., yŽCC., and yŽPC. w8,11x, respectively, where y, d, and g are the same as in Fig. 1. We speculate that the peak at 278 cmy1 is due to the yŽGa–TBP. vibration because it appears only at 100 K and has a low wave number. The broad peaks at 3230 and 1726 cmy1 are ascribed to the combination of yŽCH 3 . and dŽCH 3 . modes with a 278 cmy1 peak, respectively. Since the melting temperature of TBP is 277 K and the exposure of the gas to the surface was 10 L, the multilayer phase of TBP should exist at 100 K. Since the peaks due to yŽPH 2 . and yŽPC. modes appear at 100 K, most of TBP on the surface is adsorbed molecularly. However, some of the TBP would be partially dissociated since it seems that the peaks due to yŽPH. at 2379 cmy1 w11,17x and yŽGaH. at 1915 cmy1 w17x are superimposed on the 2342 and broad 1726 cmy1 peaks, respectively. Upon heating the sample up to 300 K, all the peaks decreased in
G. Kaneda et al.r Applied Surface Science 142 (1999) 1–6
intensity without changing wave numbers except for the 2342, 1726, and 278 cmy1 peaks. The peaks at 2379 and 1915 cmy1 became clear and the peak at 278 cmy1 disappeared although the intensity of the yŽGa–H. at 1915 cmy1 was very weak. This suggests that most of the TBP molecules on the surface are partially dissociated into C 4 H 9 PHŽa. and HŽa. at 300 K, where Ža. denotes adsorbed species. The result that the yŽGa–TBP. mode is seen at 100 K and that the yŽGa–C 4 H 9 PH. does not appear at 300 K might be due to the HREELS selection rule: dipole moments near perpendicular to the surface are active for HREELS w18x, so the latter is not observable. In this work we find a peak shift from 1446 to 1427 cmy1 for dŽCH 3 . and from 822 to 803 cmy1 for yŽPC. upon heating the sample. It is not clear whether the reason for the peak shift is due to the disappearance of the multilayer phase or partial dissociation of TBP. The spectral features at 100 and 300 K are in good agreement with the result of TBP on SiŽ001. w8x, although the decomposition products on SiŽ001. w7x and GaPŽ001. w9x were different. We speculate that this is due to the difference in adsorption sites for TBP. However, a more detailed study is needed to clarify the difference. HREELS spectra of the clean and TBP-adsorbed GaPŽ001. surfaces at 300 K and those subsequently annealed at various temperatures are shown in Fig. 4. We find that the vibration peaks at 2929, 2369 Žweak., 1915 Žweak., 1417, 1193, 969, and 809 cmy1 are similar to those in Fig. 3 at 300 K. Most of the peaks almost disappear at 550 K where most of TBP on GaPŽ001. was decomposed into isobutene with evolution from the surface w9x. The spectral changes in the vibration bands corresponding to tertiarybutyl group upon annealing are very similar to those observedfor tritertiarybutylgallium ŽŽC 4 H 9 . 3 Ga.on GaAsŽ001. w19x. We find that the yŽGa–H. peak does not increase in intensity during the decomposition Žsee Fig. 4., which is due to the same reason as mentioned in Fig. 2. At elevated temperatures there are no changes in the wave numbers for tertiarybutyl group, which suggests that it is not switched off to the surface during decomposition of TBP. The possibility of re-adsorption of isobutene produced by decomposition of TBP on the surface is ruled out because the HREELS spectra in Fig. 4 are
5
Fig. 4. HREELS spectra of the clean Ža. and TBPŽ10 L.-adsorbed GaPŽ001. surfaces at 300 K Žb. and subsequently annealed at 400 K Žc., 450 K Žd., 500 K Že., and 550 K Žf..
completely different from those of isobutene on RhŽ111. w20x and the pure gas w21x. In summary, we have studied the adsorption and decomposition of TEP and TBP on the GaPŽ001. – Ž2 = 4. surface by using HREELS. For TEP, since the vibration modes corresponding to ethyl group and that of yŽPC. appear at 100 and 300 K, we conclude that TEP is adsorbed molecularly on the surface although the wave numbers of some modes are different at 100 and 300 K. We observed that all modes decrease in intensity upon annealing the sample above 300 K and almost disappear at 700 K. The yŽGa–H. mode was not found at elevated temperatures, which is in sharp contrast to the result that the yŽSi–H. peak appears for TEP on the SiŽ001. surface w8x. The absence of the yŽGa–H. mode is explained by decomposition of the ethyl group into ethylene and hydrogen through the b-hydride elimination occurring at a higher temperature than the dissociation temperature of Ga–H. Our HREELS data suggest that switching off the ethyl group of TEP to the surface and re-adsorption of ethylene on the surface do not occur. The latter process is preferred because it avoids carbon incorporation in the growth of the films Žfor example, see Ref. w22x.. For TBP, most of
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TBP is adsorbed molecularly at 100 K and is partially dissociated into ŽC 4 H 9 .PH Ža. and HŽa. at 300 K. All the modes corresponding to t-butyl group are decreased in intensity above 300 K and have almost disappeared at 550 K. An increase in intensity of the yŽGa–H. at elevated temperatures is not found; this can be explained by the same mechanism mentioned for TEP. It is also suggested that switching off t-butyl group to the surface and re-adsorption of isobutene do not occur. Acknowledgements The authors gratefully acknowledge partial support from the Ministry of Education, Science, Culture, and Sports, Japan and the Tokai Foundation for Technology. References w1x M.J. Ludowise, J. Appl. Phys. 58 Ž1985. R31. w2x A. Usui, H. Watanabe, Annu. Rev. Mater. Sci. 21 Ž1991. 185. w3x F. Lee, A.L. Backman, R. Lin, T.R. Gow, R.I. Masel, Surf. Sci. 216 Ž1989. 173. w4x A.A. Aquino, T.S. Jones, Appl. Surf. Sci. 104r105 Ž1996. 304.
w5x A. Forster, H. Luth, ¨ ¨ J. Vac. Sci. Technol. B 7 Ž1989. 720. w6x R. Strumpler, R.C. Monte, H. Luth, ¨ ¨ Vacuum 41 Ž1990. 975. w7x G. Kaneda, J. Murata, T. Takeuchi, Y. Suzuki, N. Sanada, Y. Fukuda, Appl. Surf. Sci. 113r114 Ž1997. 546. w8x G. Kaneda, N. Sanada, Y. Fukuda, Surf. Sci. 377–379 Ž1997. 724. w9x G. Kanada, T. Takeuchi, J. Murata, N. Sanada, Y. Fukuda, Appl. Surf. Sci. 121r122 Ž1997. 245. w10x L.H. Dubois, G.P. Schwartz, Phys. Rev. B 26 Ž1982. 794. w11x L.J. Bellamy, The Infrared Spectra of Complex Molecules, Wiley, New York, 1966. w12x H. Qui, P.E. Gee, T. Nguyen, R.F. Hicks, Surf. Sci. 323 Ž1995. 6. w13x N. Sanada, M. Shimomura, Y. Fukuda, T. Sato, Appl. Phys. Lett. 67 Ž1995. 1432. w14x E.T. Fritzgerald, J.S. Foord, J. Phys.: Condens. Matters 3 Ž1991. S347. w15x J. Yoshinobu, H. Tsuda, M. Onchi, M. Nishijima, J. Chem Phys. 87 Ž1987. 7332. w16x W. Widda, C. Huang, G.D.A. Briggs, W.H. Weiberg, J. Electron. Spectrosc. Relat. Phenom. 64r65 Ž1993. 129. w17x Y. Chen, D.J. Frankel, J.R. Anderson, G.J. Lapeyre, J. Vac. Sci. Technol. A 6 Ž1988. 752. w18x H. Ibach, D.L. Mills, in: Electron energy loss spectroscopy and surface vibrations, Academic Press, 1982. w19x E.T. FritzGerald, D. O’Hare, A.C. Jones, J.S. Foord, Surf. Sci. 278 Ž1992. 111. w20x N.F. Brown, M.A. Barteau, J. Phys. Chem. 100 Ž1996. 2269. w21x C.M. Pathak, W.H. Fletcher, J. Mol. Spectrosc. 31 Ž1969. 32. w22x R. Lin, T.R. Gow, A.L. Backman, L.A. Cadwell, F. Lee, R.I. Masel, J. Vac. Sci. Technol. B 7 Ž1989. 725.