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Surface reaction of sodium on InP(111) and its role on enhancement of water vapor adsorption
Surface Science 201 (1988) 531-539 North-Holland, Amsterdam
531
SURFACE REACTION OF SODIUM ON l n P ( l l l ) AND ITS ROLE ON ENHANCEMENT OF WATER VAPOR ADSORPTION
Guo-sheng DONG, Xun-min DING, Xiao-yuan HOU and Xun WANG Surface Physics Laboratory, Fudan University, Shanghai, People's Rep. of China Received 16 December 1987; accepted for publication 15 March 1988
A LEED, UPS and HREELS study of the coadsorption of sodium and water vapor on the InP(ili) surface has been performed. The room temperature deposition of sodium adatoms ca the lnP(iii) surface leads to a work function reduction of 2.6 eV accompanied with the exchange reaction by breaking the In-P bonds and releasing free In atoms. The adsorption of water vapor is greatly enhanced by the predeposited sodium atoms. The variation of the valence band spectra shows that the major portion of adsorbed water vapor are in the form of nondissociated molecules. However, HREELS results show that in addition to the OH stretching mode at 450 meV and HOH bending mode at 188 meV from H 2 0 molecules there are loss peaks at energies of 156 and 282 meV which are assigned to the loss mechanism of NaOH and P - H vibration modes. The reaction of water molecules with Na is suggested. No In-H vibration mode has been observ~-d.
1. Introduction
The adsorption of alkali metals on semiconductor surfaces has been of growing interest in the area of surface physics and chemistry. The motivation for such studies comes from the following reasons: First, earlier workg which were devoted to the lowering the work functions of III-V semiconductor surfaces by "alkalimetal deposition and oxygen adsorption opened the possibility of achieving negative electron affinity which has technological importance in fabricating photocathode devices [1-3]. Second, the investigation on the formation of alkal~ metal-semiconductor interfaces might give some additional insight into the overall understanding of Schottky barrier fo~ ration. Third, since the existence of a thin overlayer alkali metal on a semiconductor surface could dramatically enhance the sticking coefficient of oxygen [2]. Soukiassian and co-workers found recently that the alkali metals could be used as a catalytic promoter to realize room temperature oxidation or nitridation of semiconductors [4-8], which would be a promising application in device technology. All the works done previously were mainly focused on elemental semiconductors and nonpolar surfaces of III-V compound serrficonductor~, i.e., Si(100), Si(lll), GaAs(ll0) and InP(ll0). For the III-V polar surfaces 0039-6028/88/'$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
532
Guo-sheng Dong et al. / Coadsorption of Na and HzO on InP(111)
which might in principle behave somewhat different from the nonpolar surface due to their different surface atomic structure and chemical activity, we observed a similar enhancement of oxygen adsorption on a G a A s ( l l l ) surface by Na and Cs overlayers as on GaAs(l!0) [9]. The present work demonstrates another example of alkali metal assisted molecule species uptake on a polar III-V semiconductor surface. The system which we investigated is water vapor on the sodium predeposited I n P ( l l l ) surface. Up to now, only Montgomery and Williams studied the water adsorption upon a cleaved InP(ll0) surface without the presence of alkali metal [10]. Our experimental results show that the sodium adatoms on lnP(11"1) surface may interact with both the substrate and the water vapor adsorbate.
2. Experimented The works were performed in a VG ADES-400 electron spectrometer equipped with low energy electron diffraction (LEED), ultra-violet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss ~pectroscopy (HREELS). The base pressure in its vacuum chamber pumped by a diffusion pump and a sublimator was below 1 × 10 -8 Pa. A Sn-doped n-type InP(H1) single crystal wafer with a carrier concentration of 6 × 10~7cm-3 was loaded into the vacuum chamber after an ordinary chenfical cleaning procedure. In order to prepare a clean, ordered and well-defined surface, the sample was treated by 700 eV argon ion bombardment for 45 rain and annealing at 350°C under a phosphorus pressure of ( 2 - 5 ) × 10 -6 Pa for 10 rain, followed by annealing at 300°C without phosphorus content in vacuum for 30 min. The effect of heat annea|ing under the phosphorus ambience is to eliminate the possibly existing In islands generated by the ion sputtering on the surface [11]. Cleanliness was usually checked by XPS, although UPS and HREELS were also used as they proved quite sensitive to small amounts of certain impurities. LEED observation shows a (1 × 1) pattern which indicated that the surface was a P-terminated ordered structure [2]. Sodium deposition onto the InP(111) surface was accomplished by heating an SAES Getters source. The pressure during sodium deposition was below 1 x 10 -7 Pa. Calibration of Na coverage was done by checking the work function change of the sample (see section 3 be.low). Deposition of Na atoms did not show any ordered surface reconstruction but increased the background intensity on the LEED screen. Both He I and He II light source.~ were used ~o take valence band spectra of InP and In 4d core level spectra at an angle ot incidence of 45 ° and a photoel~tron emission angle of 0 ° or 60 °. The photoelectrons were d e t c ~ i m
m
~
Guo-shen8 Dong et al. / Coadsorption of Na and H20 on InP(TT/)
533
by a hemispherical electron energy analyzer with an energy resolution of 0.2 eV. The work function of the sample was derived from the high binding energy edge of the secondary electron distribution curve in the UP spectrum excited by He I light. In HREELS measurements, the primary beam energy was 3 eV with the angle of incidence fixed at 45 ° and the spectra were taken for specular beam direction. The water adsorption experiments were carried out at room temperature. Through a fiquid nitrogen trap, the water vapor produced by the volatilization of deionized water in a bubbler was introduced into the vacuum. The purity of exposed water vapor was monitored by a quadrupole mass spectrometer. The exposure pressure was 2.6 × 10-3 Pa. All the ionization gauges and other hot filaments were turned down during the exposure to prevent the dissociation of H 2 0 molecules and to make sure that the surface was exposed to a molecular water ambience.
3. Results m
m
m
Submonolayer coverages of Na on InP(111) resulted in a reduction of the work function as shown in fig. 1, where the abscissa, which is supposed to be proportional to the amount of Na deposited on the surface, is now scaled by the relative intensity (peak area) of the Na ls XPS signal instead of the Na evaporation time so that the influence of nonstability of the evaporation rate could be avoided. The work function decreases monotonically with the coverage of Na and saturates at a value of 2.9 eV. Although the variation of the 0 O2.
O.4
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I,O
t O o O O O
4
Q Q Q O
Q O O
l~laflve ~ n s l ~ of No Is ~
@
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S~na!
Fig. ~. Work function change versus Na coverage on I n P ( l l l ) .
534
Guo.sheng Dong et al. / Coadsorption of Na and HzO on InP(l l l )
| ill z
20
19
18
I'/'
HS
Binding Energy (eV) Fig. 2. In 4d core level UPS at different coverages of N a on InP (111).
work function is larger in the present case, as compared with the work function reduction of a Na-covered G a A s ( l l l ) surface [9], the ultimate work [ur~ctions at higher Na coverages are almost the same in both cases. It was found that the relative intensity of the Na signal did net show any further increase beyond the value of 100 in fig. 1, even though the SAES Getters source s t ~ provided Na flux onto the sample surface. The saturation of the XPS intensity implies that the sticking coefficient of N a atoms decreases and approaches to zero as its coverage increases. Spicer's group found that for Cs on GaAs(ll0), the Cs 4d photoemission intensity saturated at a point of one monolayer coverage. Beyond this coverage, additional Cs deposited will be pumped away from the surface [13]. The same consideration might be applied to our N a / I n P ( l l l ) system, i.e. the saturation of N a ls intensity corresponds to 1 ML coverage. The upper scale of fig. 1 shows the estimated coverages of Na as determined from the Na XPS intensities. The work function minimum occurs at a coverage of - 0 . 7 ML, which is comparable ~ t h the results of previo.~o works, e.g., 0.6 ML Cs on GaAs(ll0) or 0.75 ML Cs on G a A s ( l l l ) [14] and also with the result of Rodway [15]. Fig. 2 shows the evolution of UP spectra for In 4d core levels excited by He II light at different coverages of Na on the InP(111) surface. Above a Na a
t
o
m
Guo-sheng Dong et al. / Coadsorption of Na and H20 on InP(TIT)
535
J
Fig. 3. Valence band UPS excited by He I light: (a) clean InP(111) surface; (b) after exposure to 2×103 L H20; (c) deposited with 0.25 ML Na; (d) coadsorbed with 0.25 ML Na and 2)<103 L H20.
coverage of 0.25 ML, the In 4d 5/2 peak spreads out at the low binding energy side and gradually a new peak at 16.7 eV develops as the Na coverage increases. This low binding energy peak corresponds to the In 4d5/2 core level emission of metallic In. Meanwhile, the intensity of In 4d5/2 flom the InP substrate decreases with increasing Na coverage and drops down to about 40% of its original intensity at 1 ML coverage of Na atoms. These facts imply that the N a adatoms may react with the InP surface to break the I n - P bonds and release some free In atoms. The similar exchange reaction has been reported for the ',flkali metals deposited on I I I - V semiconductor nonpolar surfaces in the literature [16]. We believe that the big difference between the electronegativities of Na and In makes this exchange reaction possible. The behaviour of water adsorption was studied by UPS and HREELS measurements. The valence band photoelectron spectra excited by He I light are shown in fig. 3, where the position of valence band maximum (VBM) with respect to the Fermi level E F was determined by measuring the binding energies of In 4d core levels [12]. For the clean InP(111) surface., the predominate structures in the UP spectra are the peaks located several electronvolts below the VBM. In curve (a), there are a significant sharp peak at 0.1 eV below VBM and two broad bands peaked at 5.7 and 9.1 eV which are superpositioned on tl~ secondary electron background. After the surface was exposed to 2 x 103 L H 2 0 or deposited with 0.25 ML of Na, the UP spectrum changes to curve (b) and (c), respectively. No rem~kable changes could be seen as compared with curve (a) except the intensity reduction of the 0.1 eV
~
536
M
Guo-sheng Dong et al. / Coadsorption of Na and 1-120 on lnP(l l l)
•.--4,oeVq LseVF
(b)
IP [eV)
+~4~:0V~ LgeV
(a)
""--
.
.
.
.
.
E (eV] Fig. 4. Photoelectron spectra of H20: (a) difference spectrum of fig. 3, curve (d) - curve (c). (b) PE spectrum of H 2 0 in vapor phase, after ref. [17].
peak. This peak has been identified as the surface state peak induced by the aangting bonds ut phosphorus surface atoms [12], and its sensibility to the surface contamination (adsorption and adatom deposition) is understandable. The UP spec:!rum for a surface deposited with 0.25 ML Na and exposed to 2 × 103 L H 2 0 reveals a significant difference from that of a clean surface as shown by curve (d) in fig. 3, which implies that the sticking coefficient of H20 molecules on I n P ( l l l ) is greatly enhanced by the predeposited N a overlayer. The peak 0.1 eV below VBM fully disappears due to the saturation of P dangling bonds by water adsorption. The difference spectrum of curve (d) and curve tc) gives rise to three water-vapor-induced structures at 3.6, 5.5 and 9.7 eV below VBM, as shown in fig. 4a. Fig. 4b is the vapor phase photoelectron spectrum of water molecules by Turner et al. [17] versus the ionization potential IP. The three emissions Ib 1, 3al and lb 2 are the characteristic peaks of the molecular orbitals of H20. The energy seperations of these peaks in fig. 4a coincide exactly with those of fig. 4b. T)ds suggests that the major portion of water vapor adsorbed on the surface are in the form of nondissociated molecules.
Guo-sheng Dong et al. / Coadsorption of Na and II 20 on InP(-/-[l)
537
x=o~
xsa,
w
z
L.
xi@
o
~o
~x~ Energy
~o
~o
~o
Lore (meV)
Fig. 5. HREELS of clean lnP(lll) surface (a) and a surface coadsorbed with 025 ML Na and 2x103 L H20 (b).
Fig. 5 shows the results of HREELS measurements. After exposure of the clean InP(1--il) surface to 2 x 103 L H 2 0 nothing could be seen in the spectrum except for the surface optical phonon loss peak of InP at 42 meV and its overtone at 84 meV. On the surface of Na predeposited lnP(lll), the adsorption of water vapor leads to the appearance of four new loss peaks at energies of 156, 188, 282 and 450 meV, respectively. The loss peaks at 188 meV (1504 cm -1) and 450 meV (3600 cm -1) are the characteristic peaks of water molecules. They originate from the bending vibration mode of H O H and the stretching vibration mode of OH, respectively. The loss energies obtained here are quite close to the results of infrared and Raman spectroscopy measurements [18]. This also evidences that the water molecules on the InP surface are adsorbed as non-dissociated species. The 156 meV loss peak probably arises from the bending mode of OH in NaOH, which is the product of the reaction of water vapor with deposited N a atoms. The released atomic hydrogen from water molecules could combine with surface P atoms to form P - H bonds, which attribute to the 282 meV peak in the loss spectrum [19]. However, no indication of the I n - H vibratien mode at 209 meV has been found.
4. Discussion
The work function change shown m fig. 1 is believed to be a common feature of alkali metals on the surfaces of semiconductors [9,14,15,20] and is slightly different from that of alkali metals on metal surfaces [21-23]. For the
538
Guo-sheng Dong et al. / Coadsorption of Na and H20 on InP(l l l )
latter case, one typically finds a strong decrease in work function which is followed by a minimum and a slow increase until the alkali bulk value is approached. The theory of alkali adsorption has shown that the alkali adsorbs ionically at low coverage whereas for increash~g coverage the adsorbate gradually achieves charge neutrality [24]. The alkali atoms on the metal surface could easily transfer their valence electrons to the substrate and a dipole electric field formed by the alkali ions and their image cha_r_ges causes a lowering of work function. However, in the case of N a on InP(111), the highly polarized bonds between 1~ and N a contribute more likely to the surface dipoles than the image charges. Below a coverage of I ML, the N a atoms react with the InP surface to break the I n - P bonds. It is expected that the N a atoms are not in the form of free atoms but bond to surface P atoms and transfer more valence electrons towards P than In due to their small electronegativity. As a result of the surface exchange reaction, the N a atoms might be distributed in a relatively random manner leaving a certain amount of unsaturated P bonds or defects. The scattering of low energy electrons from these randomly distributed Na atoms released metallic In and the defects contributed to a strong background and a diffused pattern on the LEED screen. The effects of Na on the enhancement of wzter vapor adsorption seems somehow different from the effect of alkali ,aetals on oxygen uptake. As previous works stated, the alkali metal was fnw,nd to dissociate molecular oxygen and to cause an enhancement of its sticking coefficient [4,7]. However, unlike the oxygen adsorption, the water molecules do not necessarily undergo a dissociation step during the uptake process. The enhancement of the sticking coefficient might be clue to the creation of surface defects by Na deposition as described previously. On the other hand, the water molecules should chemicaUy react to the sodium atoms resulting in the formation of N a O H and atomic hydrogen bonded to surface P atoms as illustrated by HREELS. Additional evidence for the existence of N a O H is provided by fig. 4. Citrin measured the valence band spectrum of sodium hydroxide and found that all the molecular orbitals lbl, 3al, lb2 and l a 1 (not shown in fig. 4) had the same e n e r ~ seperations as those hi the spectrum of water vapor [25]. But the relative intensities of 3al and lb2 normalized to that of the lb~ orbital are 1.8 and 3.1, respectively, for NaOH, which ditfer from the values of 1.7 and 0.8 for H20. In our figs. 4a and 4b the intensities of the 3a~ and lb2 orbitals show a similar tendency as the result of Citrin, i.e., the intensity of 3a~ is larger than that of lb2 in curve (b) but is smaller than the latter in curve (a). TI',L~ implies that part of the adsorbed H20 molecules has been converted into Na.OH. Webb and Lichtensteiger have found on the GaAs(ll0) surface by UPS, XPS and SIMS, that G a - O H bonds could be formed due to dissociative adsorption of H 2 0 for doses exceeding 10 9 L [26]. In our case, the surface In atoms have been replaced by Na to produce free In atoms, no In dangling bonds are exposed on the surface. Moreover, if there exist O H - radicals they
Cruo.sheng Dong et al. / Coadsorption of Na and t120 oa InP('[TI)
539
may combine more easily with Na than In. So, the absence of In-OH and I n - H vibrational modes in HREELS seems reasonable.
References [1] J.J. Scheer and J. van Laar, Solid State Commun. 3 (1965) 189. [2] See, e.g., W.E. Spicer, Appl. Phys. 12 (1977) 115. [3] W.E. Spicer, L Lindau, C.Y. Su, P.W. Chye and P. Pianette, Appl. Phys. Letters 33 (1978) 934. [4] P. Soukiassian, T.M. Gentle, M.H. Bakshi and Z. Hurych, J. Appl. Phys. 60 (1986) 4339. [5] A. Franciosi, P. Soukiassian, P. Philip, S. Chang, A. Wall, A. Raisanen and N. Trovillier, Phys. Rev. B 35 (1987) 910. [6] P. Soukiassian, M.H. Bakahi, Z Hurych and T.M. Gentle, Phys. Rev. B 35 (1987) 4176. [7] P. Soukiassian, M.H. Bakshi and Z. Hurych, J. Appl. Phys. 61 (1987) 2679. [8] P. Soukiassian, M.H. Bakshi, H.I. Starnberg, Z. Huryeh, T.M. Gentle and K.P. Schuette, Phys. Rev. Letters 59 (1987) 1488. [9] Xun-min Ding, Guo-sheng Dong, Xiao-yaan Hou and Xun Wang, Solid State Commun. 61 (1987) 391. [10] V. Montgomery and R.H. Williams, J. Phys. C 15 (1982) 5887. [11] Xiao-feng Jin, Ming-ren Yu, Fu-rong Zhu and Xun Wang, Semicond. Sci. Teclmol. 1 (1986) 293. [12] Xiao-yuan Hou, Guo-sheng Dong, Xun-min Ding and Xun Wang, Surface Sci. 183 (1987) 123. [13] C.Y. Su, P.W. Chye, P. Pianetta, I. Lindau and W.E. Spicer, Surface Sci. 86 (1979) 894. [14] H.J. Clements, J. yon Wienskowski and W. Monch, Surface Sci 78 (1978) 648. [15] D. Rodway, Surface Sci 147 (1984) 103. [16] P.W. Chye, I.A. Babalola, T. Sukegawa and W.E. Spicer, Phys. Rev. B 13 (1976) 4439. [17] D.W. Turner, A.D. Baker, C. Baker and C.R. Brundle, Molecular Photoelectron Spectroscopy (Academic Press, London, 1970) [18] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds (Wiley, New York, 1978) [19] Xiao-yuan Hou, Shu Yang, Guo-sheng Dong, Xun-min Ding and Xun Wang, Phys. Rev. B 35 (1987) 8015. [20] R.E. Weber and W.T. Peria, Surface 3ci. 14 (1969) 13. [21] R.L. Gerlach and T.N. Rhodin, Surface Sci. 19 (1970) 403. [22] G. Broden and H.P. Bonzel, Surface Sci. 84 (1979) 106. [23] E.L. Garfunkel, Xun-mii~ DLag, Guo-sheng Dong, Shu Yang, Xiao-yuan Hou .~ad Xu/t Wang, Surface Sci. 164 (1985) 511. [24] N.D. Lang, Phys. Rev. B 4 (1971) 4234. [25] P.H. Citrin, Phys. Rev. B 8 (1973) 5545. [26] C. Webb and M. Lichtensteiger, J. Vacuum Sci. Technol. 21 (1982) 659.
531
SURFACE REACTION OF SODIUM ON l n P ( l l l ) AND ITS ROLE ON ENHANCEMENT OF WATER VAPOR ADSORPTION
Guo-sheng DONG, Xun-min DING, Xiao-yuan HOU and Xun WANG Surface Physics Laboratory, Fudan University, Shanghai, People's Rep. of China Received 16 December 1987; accepted for publication 15 March 1988
A LEED, UPS and HREELS study of the coadsorption of sodium and water vapor on the InP(ili) surface has been performed. The room temperature deposition of sodium adatoms ca the lnP(iii) surface leads to a work function reduction of 2.6 eV accompanied with the exchange reaction by breaking the In-P bonds and releasing free In atoms. The adsorption of water vapor is greatly enhanced by the predeposited sodium atoms. The variation of the valence band spectra shows that the major portion of adsorbed water vapor are in the form of nondissociated molecules. However, HREELS results show that in addition to the OH stretching mode at 450 meV and HOH bending mode at 188 meV from H 2 0 molecules there are loss peaks at energies of 156 and 282 meV which are assigned to the loss mechanism of NaOH and P - H vibration modes. The reaction of water molecules with Na is suggested. No In-H vibration mode has been observ~-d.
1. Introduction
The adsorption of alkali metals on semiconductor surfaces has been of growing interest in the area of surface physics and chemistry. The motivation for such studies comes from the following reasons: First, earlier workg which were devoted to the lowering the work functions of III-V semiconductor surfaces by "alkalimetal deposition and oxygen adsorption opened the possibility of achieving negative electron affinity which has technological importance in fabricating photocathode devices [1-3]. Second, the investigation on the formation of alkal~ metal-semiconductor interfaces might give some additional insight into the overall understanding of Schottky barrier fo~ ration. Third, since the existence of a thin overlayer alkali metal on a semiconductor surface could dramatically enhance the sticking coefficient of oxygen [2]. Soukiassian and co-workers found recently that the alkali metals could be used as a catalytic promoter to realize room temperature oxidation or nitridation of semiconductors [4-8], which would be a promising application in device technology. All the works done previously were mainly focused on elemental semiconductors and nonpolar surfaces of III-V compound serrficonductor~, i.e., Si(100), Si(lll), GaAs(ll0) and InP(ll0). For the III-V polar surfaces 0039-6028/88/'$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
532
Guo-sheng Dong et al. / Coadsorption of Na and HzO on InP(111)
which might in principle behave somewhat different from the nonpolar surface due to their different surface atomic structure and chemical activity, we observed a similar enhancement of oxygen adsorption on a G a A s ( l l l ) surface by Na and Cs overlayers as on GaAs(l!0) [9]. The present work demonstrates another example of alkali metal assisted molecule species uptake on a polar III-V semiconductor surface. The system which we investigated is water vapor on the sodium predeposited I n P ( l l l ) surface. Up to now, only Montgomery and Williams studied the water adsorption upon a cleaved InP(ll0) surface without the presence of alkali metal [10]. Our experimental results show that the sodium adatoms on lnP(11"1) surface may interact with both the substrate and the water vapor adsorbate.
2. Experimented The works were performed in a VG ADES-400 electron spectrometer equipped with low energy electron diffraction (LEED), ultra-violet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss ~pectroscopy (HREELS). The base pressure in its vacuum chamber pumped by a diffusion pump and a sublimator was below 1 × 10 -8 Pa. A Sn-doped n-type InP(H1) single crystal wafer with a carrier concentration of 6 × 10~7cm-3 was loaded into the vacuum chamber after an ordinary chenfical cleaning procedure. In order to prepare a clean, ordered and well-defined surface, the sample was treated by 700 eV argon ion bombardment for 45 rain and annealing at 350°C under a phosphorus pressure of ( 2 - 5 ) × 10 -6 Pa for 10 rain, followed by annealing at 300°C without phosphorus content in vacuum for 30 min. The effect of heat annea|ing under the phosphorus ambience is to eliminate the possibly existing In islands generated by the ion sputtering on the surface [11]. Cleanliness was usually checked by XPS, although UPS and HREELS were also used as they proved quite sensitive to small amounts of certain impurities. LEED observation shows a (1 × 1) pattern which indicated that the surface was a P-terminated ordered structure [2]. Sodium deposition onto the InP(111) surface was accomplished by heating an SAES Getters source. The pressure during sodium deposition was below 1 x 10 -7 Pa. Calibration of Na coverage was done by checking the work function change of the sample (see section 3 be.low). Deposition of Na atoms did not show any ordered surface reconstruction but increased the background intensity on the LEED screen. Both He I and He II light source.~ were used ~o take valence band spectra of InP and In 4d core level spectra at an angle ot incidence of 45 ° and a photoel~tron emission angle of 0 ° or 60 °. The photoelectrons were d e t c ~ i m
m
~
Guo-shen8 Dong et al. / Coadsorption of Na and H20 on InP(TT/)
533
by a hemispherical electron energy analyzer with an energy resolution of 0.2 eV. The work function of the sample was derived from the high binding energy edge of the secondary electron distribution curve in the UP spectrum excited by He I light. In HREELS measurements, the primary beam energy was 3 eV with the angle of incidence fixed at 45 ° and the spectra were taken for specular beam direction. The water adsorption experiments were carried out at room temperature. Through a fiquid nitrogen trap, the water vapor produced by the volatilization of deionized water in a bubbler was introduced into the vacuum. The purity of exposed water vapor was monitored by a quadrupole mass spectrometer. The exposure pressure was 2.6 × 10-3 Pa. All the ionization gauges and other hot filaments were turned down during the exposure to prevent the dissociation of H 2 0 molecules and to make sure that the surface was exposed to a molecular water ambience.
3. Results m
m
m
Submonolayer coverages of Na on InP(111) resulted in a reduction of the work function as shown in fig. 1, where the abscissa, which is supposed to be proportional to the amount of Na deposited on the surface, is now scaled by the relative intensity (peak area) of the Na ls XPS signal instead of the Na evaporation time so that the influence of nonstability of the evaporation rate could be avoided. The work function decreases monotonically with the coverage of Na and saturates at a value of 2.9 eV. Although the variation of the 0 O2.
O.4
O6
QS
I,O
t O o O O O
4
Q Q Q O
Q O O
l~laflve ~ n s l ~ of No Is ~
@
•
S~na!
Fig. ~. Work function change versus Na coverage on I n P ( l l l ) .
534
Guo.sheng Dong et al. / Coadsorption of Na and HzO on InP(l l l )
| ill z
20
19
18
I'/'
HS
Binding Energy (eV) Fig. 2. In 4d core level UPS at different coverages of N a on InP (111).
work function is larger in the present case, as compared with the work function reduction of a Na-covered G a A s ( l l l ) surface [9], the ultimate work [ur~ctions at higher Na coverages are almost the same in both cases. It was found that the relative intensity of the Na signal did net show any further increase beyond the value of 100 in fig. 1, even though the SAES Getters source s t ~ provided Na flux onto the sample surface. The saturation of the XPS intensity implies that the sticking coefficient of N a atoms decreases and approaches to zero as its coverage increases. Spicer's group found that for Cs on GaAs(ll0), the Cs 4d photoemission intensity saturated at a point of one monolayer coverage. Beyond this coverage, additional Cs deposited will be pumped away from the surface [13]. The same consideration might be applied to our N a / I n P ( l l l ) system, i.e. the saturation of N a ls intensity corresponds to 1 ML coverage. The upper scale of fig. 1 shows the estimated coverages of Na as determined from the Na XPS intensities. The work function minimum occurs at a coverage of - 0 . 7 ML, which is comparable ~ t h the results of previo.~o works, e.g., 0.6 ML Cs on GaAs(ll0) or 0.75 ML Cs on G a A s ( l l l ) [14] and also with the result of Rodway [15]. Fig. 2 shows the evolution of UP spectra for In 4d core levels excited by He II light at different coverages of Na on the InP(111) surface. Above a Na a
t
o
m
Guo-sheng Dong et al. / Coadsorption of Na and H20 on InP(TIT)
535
J
Fig. 3. Valence band UPS excited by He I light: (a) clean InP(111) surface; (b) after exposure to 2×103 L H20; (c) deposited with 0.25 ML Na; (d) coadsorbed with 0.25 ML Na and 2)<103 L H20.
coverage of 0.25 ML, the In 4d 5/2 peak spreads out at the low binding energy side and gradually a new peak at 16.7 eV develops as the Na coverage increases. This low binding energy peak corresponds to the In 4d5/2 core level emission of metallic In. Meanwhile, the intensity of In 4d5/2 flom the InP substrate decreases with increasing Na coverage and drops down to about 40% of its original intensity at 1 ML coverage of Na atoms. These facts imply that the N a adatoms may react with the InP surface to break the I n - P bonds and release some free In atoms. The similar exchange reaction has been reported for the ',flkali metals deposited on I I I - V semiconductor nonpolar surfaces in the literature [16]. We believe that the big difference between the electronegativities of Na and In makes this exchange reaction possible. The behaviour of water adsorption was studied by UPS and HREELS measurements. The valence band photoelectron spectra excited by He I light are shown in fig. 3, where the position of valence band maximum (VBM) with respect to the Fermi level E F was determined by measuring the binding energies of In 4d core levels [12]. For the clean InP(111) surface., the predominate structures in the UP spectra are the peaks located several electronvolts below the VBM. In curve (a), there are a significant sharp peak at 0.1 eV below VBM and two broad bands peaked at 5.7 and 9.1 eV which are superpositioned on tl~ secondary electron background. After the surface was exposed to 2 x 103 L H 2 0 or deposited with 0.25 ML of Na, the UP spectrum changes to curve (b) and (c), respectively. No rem~kable changes could be seen as compared with curve (a) except the intensity reduction of the 0.1 eV
~
536
M
Guo-sheng Dong et al. / Coadsorption of Na and 1-120 on lnP(l l l)
•.--4,oeVq LseVF
(b)
IP [eV)
+~4~:0V~ LgeV
(a)
""--
.
.
.
.
.
E (eV] Fig. 4. Photoelectron spectra of H20: (a) difference spectrum of fig. 3, curve (d) - curve (c). (b) PE spectrum of H 2 0 in vapor phase, after ref. [17].
peak. This peak has been identified as the surface state peak induced by the aangting bonds ut phosphorus surface atoms [12], and its sensibility to the surface contamination (adsorption and adatom deposition) is understandable. The UP spec:!rum for a surface deposited with 0.25 ML Na and exposed to 2 × 103 L H 2 0 reveals a significant difference from that of a clean surface as shown by curve (d) in fig. 3, which implies that the sticking coefficient of H20 molecules on I n P ( l l l ) is greatly enhanced by the predeposited N a overlayer. The peak 0.1 eV below VBM fully disappears due to the saturation of P dangling bonds by water adsorption. The difference spectrum of curve (d) and curve tc) gives rise to three water-vapor-induced structures at 3.6, 5.5 and 9.7 eV below VBM, as shown in fig. 4a. Fig. 4b is the vapor phase photoelectron spectrum of water molecules by Turner et al. [17] versus the ionization potential IP. The three emissions Ib 1, 3al and lb 2 are the characteristic peaks of the molecular orbitals of H20. The energy seperations of these peaks in fig. 4a coincide exactly with those of fig. 4b. T)ds suggests that the major portion of water vapor adsorbed on the surface are in the form of nondissociated molecules.
Guo-sheng Dong et al. / Coadsorption of Na and II 20 on InP(-/-[l)
537
x=o~
xsa,
w
z
L.
xi@
o
~o
~x~ Energy
~o
~o
~o
Lore (meV)
Fig. 5. HREELS of clean lnP(lll) surface (a) and a surface coadsorbed with 025 ML Na and 2x103 L H20 (b).
Fig. 5 shows the results of HREELS measurements. After exposure of the clean InP(1--il) surface to 2 x 103 L H 2 0 nothing could be seen in the spectrum except for the surface optical phonon loss peak of InP at 42 meV and its overtone at 84 meV. On the surface of Na predeposited lnP(lll), the adsorption of water vapor leads to the appearance of four new loss peaks at energies of 156, 188, 282 and 450 meV, respectively. The loss peaks at 188 meV (1504 cm -1) and 450 meV (3600 cm -1) are the characteristic peaks of water molecules. They originate from the bending vibration mode of H O H and the stretching vibration mode of OH, respectively. The loss energies obtained here are quite close to the results of infrared and Raman spectroscopy measurements [18]. This also evidences that the water molecules on the InP surface are adsorbed as non-dissociated species. The 156 meV loss peak probably arises from the bending mode of OH in NaOH, which is the product of the reaction of water vapor with deposited N a atoms. The released atomic hydrogen from water molecules could combine with surface P atoms to form P - H bonds, which attribute to the 282 meV peak in the loss spectrum [19]. However, no indication of the I n - H vibratien mode at 209 meV has been found.
4. Discussion
The work function change shown m fig. 1 is believed to be a common feature of alkali metals on the surfaces of semiconductors [9,14,15,20] and is slightly different from that of alkali metals on metal surfaces [21-23]. For the
538
Guo-sheng Dong et al. / Coadsorption of Na and H20 on InP(l l l )
latter case, one typically finds a strong decrease in work function which is followed by a minimum and a slow increase until the alkali bulk value is approached. The theory of alkali adsorption has shown that the alkali adsorbs ionically at low coverage whereas for increash~g coverage the adsorbate gradually achieves charge neutrality [24]. The alkali atoms on the metal surface could easily transfer their valence electrons to the substrate and a dipole electric field formed by the alkali ions and their image cha_r_ges causes a lowering of work function. However, in the case of N a on InP(111), the highly polarized bonds between 1~ and N a contribute more likely to the surface dipoles than the image charges. Below a coverage of I ML, the N a atoms react with the InP surface to break the I n - P bonds. It is expected that the N a atoms are not in the form of free atoms but bond to surface P atoms and transfer more valence electrons towards P than In due to their small electronegativity. As a result of the surface exchange reaction, the N a atoms might be distributed in a relatively random manner leaving a certain amount of unsaturated P bonds or defects. The scattering of low energy electrons from these randomly distributed Na atoms released metallic In and the defects contributed to a strong background and a diffused pattern on the LEED screen. The effects of Na on the enhancement of wzter vapor adsorption seems somehow different from the effect of alkali ,aetals on oxygen uptake. As previous works stated, the alkali metal was fnw,nd to dissociate molecular oxygen and to cause an enhancement of its sticking coefficient [4,7]. However, unlike the oxygen adsorption, the water molecules do not necessarily undergo a dissociation step during the uptake process. The enhancement of the sticking coefficient might be clue to the creation of surface defects by Na deposition as described previously. On the other hand, the water molecules should chemicaUy react to the sodium atoms resulting in the formation of N a O H and atomic hydrogen bonded to surface P atoms as illustrated by HREELS. Additional evidence for the existence of N a O H is provided by fig. 4. Citrin measured the valence band spectrum of sodium hydroxide and found that all the molecular orbitals lbl, 3al, lb2 and l a 1 (not shown in fig. 4) had the same e n e r ~ seperations as those hi the spectrum of water vapor [25]. But the relative intensities of 3al and lb2 normalized to that of the lb~ orbital are 1.8 and 3.1, respectively, for NaOH, which ditfer from the values of 1.7 and 0.8 for H20. In our figs. 4a and 4b the intensities of the 3a~ and lb2 orbitals show a similar tendency as the result of Citrin, i.e., the intensity of 3a~ is larger than that of lb2 in curve (b) but is smaller than the latter in curve (a). TI',L~ implies that part of the adsorbed H20 molecules has been converted into Na.OH. Webb and Lichtensteiger have found on the GaAs(ll0) surface by UPS, XPS and SIMS, that G a - O H bonds could be formed due to dissociative adsorption of H 2 0 for doses exceeding 10 9 L [26]. In our case, the surface In atoms have been replaced by Na to produce free In atoms, no In dangling bonds are exposed on the surface. Moreover, if there exist O H - radicals they
Cruo.sheng Dong et al. / Coadsorption of Na and t120 oa InP('[TI)
539
may combine more easily with Na than In. So, the absence of In-OH and I n - H vibrational modes in HREELS seems reasonable.
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