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Influence of oxygen on the thermal desorption of Cs from Si(100)
surface science ELSEVIER
Surface Science 399 (1998) 284 296
Influence of oxygen on the thermal desorption of Cs from Si(100) R o n K r o o n *, A r n o l d S i n k e Philips Research Laboratories (WY-51), Prql~ Holstlaan 4. 56_56 AA Eindhoren, The Netherlands' Received 3 July 1997; accepted for publication 15 October 1997
Abstract
This paper describes the thermal desorption of Cs from oxidized Si(100), measured as a function of sample temperature and oxygen coverage. The desorption process is found to be governed by a broad distribution of the activation energy for thermal desorption of the Cs atoms, which reflects the considerable inhomogeneity of the Si surfaces. The weight of the distribution is found to shift to higher activation energy with increasing oxygen coverage of the Si substrates, g, 1998 Elsevier Science B.V.
Keywords." Alkali metals: Silicon: Silicon oxides: Thermal desorption; X-ray photoelectron spectroscopy
1. Introduction
The behaviour of alkali metals on semiconductor surfaces has been the subject of a great many studies in recent years [1-15]. However, the vast majority of surfaces under study are well-defined reconstructed surfaces. In industrial applications, the semiconductor surfaces may suffer modest levels of contamination and/or defects, it is of considerable interest to study the effects of such deviations from ideal surfaces. In this paper we describe the thermal desorption of Cs from Si(100). We chose to study Cs because of its importance to negative electron affinity (NEA) devices. Prior to the deposition of Cs the Si substrates are cleaned with ozone, etched with moist hydrogen fluoride gas and exposed to air, resulting in microscopically rough surfaces (rough-
* Corresponding author. Fax: (+ 31 ) 40 2744335: e-mail: [email protected]
0039-6028/98/$19.00 ~ 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 9 - 6 0 2 8 ( 9 7 ) 0 0 8 2 8 - 5
ness ~ 0 . 5 n m [16,17]) with different levels of oxidation. Thermal desorption of Cs is measured as a function of sample temperature and oxygen coverage. The observed Cs desorption is accurately described in a model which takes into account a distribution of the activation energy for thermal desorption. The distribution is found to be quite broad, an indication of the inhomogeneous nature of the Si surfaces which originates from a distribution of the density of oxygen on the Si surface, added with effects of surface roughness. The weight of the distribution is found to shift to a higher activation energy with increasing oxidation of the Si surface. The Cs desorption was monitored with X-ray photoelectron spectroscopy (XPS). The XPS spectra show evidence of vibrational broadening of the Cs 3d5/2 peak. This paper is organized as follows. The experimental apparatus is described in Section 2, and experimental results are presented in Section 3. A discussion of the experimental
R. Kroon, A. Sinke / SmJctce Science 399 (1998) 284 296
results, aided by results of model calculations of the desorption process, is presented in Section 4.
2. Experimental The thermal desorption experiments are performed in a VG ESCALAB Mk-II. The ESCALAB is equipped with a fast-entry load lock, which allows samples to be placed in the U H V chamber without breaking the UHV. Base pressure in the fast-entry load lock is 5 x 10 -8 mbar, base pressure in the U H V chamber is 5 x 10 - H mbar. The Si(100) substrates that are employed in the experiments are doped with 8 x 1019m 3 As. A substrate is, firstly, cleaned with ozone to remove any carbon contamination. The ozone is made in a silent discharge in an oxygen gas flow. It is led over the substrate which is heated in an oven to a temperature of 18OC. Secondly, the substrate is etched in a room-temperature gas flow consisting of gaseous H F and water vapour, to remove the silicon oxide layer present on the substrate. This etching flow is made by leading a N2 gas flow over a 10% solution of H F in water. The quality of the 03- and HF-etching is monitored in the E S C A I A B with X-ray photoelectron spectroscopy (XPS, the basic principles of this technique are described in Ref. [181). In Appendix A we describe the determination of the surface composition from the sample's XPS spectrum. U p o n placing the etched substrate in the fast-entry load lock it is in contact with the ambient for a period less then a minute. A minor oxidation of the substrate may result, determined to be < 0 . 0 5 M E , where 1 M L is defined as one physical layer of adatoms. Typical coverage values observed directly after 03- and HF-etching are (Po=0.15 ME (of which >0.1 M L is due to oxide left after etching, and <0.05 M L is due to oxidation during sample transfer), ~0c=0.07 M L and ~0v=0.15 ML. In our experiments we investigate the influence of the oxygen coverage of a substrate on the thermal desorption of Cs. We oxidize a HF-etched substrate by exposure to the laboratory ambient (relative humidity 50-60%; temperature 19:C) until a specific oxygen coverage is reached, as determined with XPS. Subsequently, the substrate
285
and substrate holder are thoroughly outgassed in the ESCALAB in order to prevent the release of oxidizing gases when heating the sample in the actual Cs desorption experiment. Outgassing is done by slowly (overnight) raising the sample temperature on the resistively heated substrate holder to 35OC. The pressure in the U H V chamber is kept below 10 -1° mbar. In this case the sample does not oxidize during outgassing. We observed that a rapid temperature rise during outgassing can lead to oxidation of the sample since in that case the oxidising gases (mainly water) that are released are not pumped away sufficiently fast to prevent oxidation. In the desorption experiments we ramp the substrate temperature from room temperature to a set value in 3 min, after which it is kept constant within _+I'C. Substrate temperature is calibrated by using a Si substrate in which a thermocouple is placed. Therefore, the measurement of the absolute substrate temperature is estimated to be correct within _+ 10'C. After outgassing the substrate is cooled to room temperature. Cs is subsequently deposited on the substrate from a home-built source. The source is essentially an A1Si wire, containing 3%wt. Si, into which Cs has been diffused at ~ 5 0 0 C . The total Cs content is 2 3%wt. The wire is mounted inside a narrow nickel tube. When resistively heating the tube the wire releases pure atomic Cs at temperatures > 4 5 0 ( 2 . We employ a Cs deposition rate of ~2.1 M L h -1. Prior to use, the Cs source is thoroughly outgassed. During deposition the Cs coverage is monitored with XPS. The deposition is continued until the Cs coverage reaches a saturation value (which we observe to be ~0.95 ME, depending on the Si surface quality and the amount of oxygen present on the surface), a result of the Cs sticking coefficient becoming negligible at room temperature when ~0cs reaches ~1 M L [19]. Base pressure in the U H V chamber during Cs deposition is < 10-1° mbar. Following deposition, the sample is heated to a specific temperature. The decreasing Cs coverage of the Si surface in time is monitored with XPS. We also monitor the oxygen coverage, both during deposition and desorption, because the presence of Cs can significantly enhance the oxidation rate of the Si substrate [20-22]. Due to the very low
286
R. Kroon, A. Sinke / Su#Jktce Science 399 (1998) 284 296
base pressure in our U H V chamber the Si surfaces show negligible oxidation during the experiments. Finally, we observe ~0c = 0.07 _+0.02 M L and ~0v=0.15_+0.05 M L to be reproducible during the measurements, and between substrates.
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3. Experimental results Thermal desorption of Cs from Si(100) substrates which are oxidized to ~0o=0.65_+0.1 ML, is shown in Fig. 1. The experiment is repeated at various sample temperatures. The symbols denote the experimental results whereas the lines are the result of a model calculation, as described in Section 4. The values of q)c~ are corrected for the influence of ~o on the Si XPS signal (see Appendix A). This correction is done for all q)cs presented in this paper. In Fig. 1, one observes a very rapid initial decay of ~0c~(t), which is the more rapid with increasing sample temperature. The decay slows down considerably at longer times. ~oo is constant during the desorption of Cs, indicating that Cs desorbs at our sample temperatures but oxygen does not. Fig. 2 shows the thermal desorption of Cs in time at fixed sample temperature (200~C), but at different levels of Si(100) substrate oxidation. The symbols denote the experimental results, the lines
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are the result of a model calculation described in Section 4. The Cs desorption rate is observed to increase with a decreasing level of substrate oxidation. The general shape of the desorption curves is comparable to that observed in Fig. 1. Slightly hidden in Fig. 2 is the fact that (Pcs(t = 0 ) is different for the different curves. When depositing Cs to a saturation value we find this value to increase with increasing substrate oxidation, as depicted in Fig. 3. The effect is observed
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to saturate for large g'o. The results of Fig. 3 are in agreement with data published in Ref. [23]. Finally, to test our model calculation on a substrate with ~0o=0.15 ML over a wide range of temperature we performed an experiment in which we stepwise increased the sample temperature whilst monitoring ~0Cs. The results of this experiment are shown in Fig. 4, where the symbols denote the experimental results and the solid line is the result of the model calculation, q)o remained constant during the experiment. The stepwise increase of the sample temperature is depicted in the inset in Fig. 4.
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Fig. 5. The activation energy for thermal desorption Edes of Cs on oxidized Si(100) 0:o-0.65_+0.1 ML) versus Cs coverage• The Ea~~ values are obtained by applying Eq. ( 1 ) to the experimental results of Fig. 1. The different symbols reflect the different temperatures at which desorption was measured. The dashed line is a fit with a slope of 1.7 eV/ML.
(1)
We observe a strong dependence of Eaes on ~0cs: - 1.7 eV ML ~ (indicated by the dashed line). It is much stronger than the effect induced by the well-known repulsive interaction between the positively charged Cs atoms, which has been measured to be - 0 . 6 e V M L 1 on S i ( 1 0 0 ) 2 x l [26,27]. 1 This effect will be of similar magnitude on oxidized Si(100) since the fractional charge of a Cs atom is similar on Si(100)2 x 1 and oxidized Si(100), as we have determined from work function measurements on HF-etched Si(100) oxidized to ~0o=0.4 M L [28]. Thus, the shape of the desorption curves in Fig. 1 is not compatible with the repulsive-interaction mechanism. The shape of the desorption curves does imply the presence of a distribution of the activation energy for thermal desorption, which is not unexpected in regard of the likely inhomogeneity of the microscopically rough (and oxidized) Si surfaces. One assumes an ensemble of Cs atoms which is composed of subensembles with a specific activa-
where v is an attempt frequency (set equal to 1013 S-1 [24-27]), k is Boltzmann's constant, and Eden, the activation energy of the desorbing atoms, is a function of the surface coverage ~0cs. When we apply Eq. ( 1 ) to the experimental results of Fig. 1, the obtained values of Ed~ are shown in Fig. 5.
t Note that our definition of surface coverage: 1 ML Cs is one physical layer of Cs atoms, differs from the definition used by Milne et ah [26,27], where 1 ML Cs is defined as one Cs atom per substratc atom (which, on Si( 100)2 x 1, corresponds to two physical Cs layers). In the comparison between our results and Milne's, all data are expressed according to our definition of surface coverage.
4. Discussion
Thermal desorption of atomic Cs at temperature T is assumed to be governed by the first-order rate equation: dt
-r~0csexp
-
,
R. Kroon, A. Sinke / Smjhce Science 399 (1998) 284 296
288
tion energy. The subensembles desorb according to Eq. (1), but their activation energy for thermal desorption is now taken to be independent of ~Ocs, which for subensemble i yields ~0c~,i(t)=~0c~,i(0 ) exp
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A good fit of the model to the experimental results of Fig. 1, as depicted by the solid lines in that figure, yields the activation-energy distribution of Fig. 6 (in the fit we have used N = 18, this number proved necessary to obtain a good-quality fit for all the curves in Fig. 1 ). The Ea~s distribution is observed to be quite broad. Its width was estimated by extrapolation of the Edes values of Fig. 5 to q~c~= 0. Its physical origin is considered to be a distribution of the density of oxygen on the Si surface, added with effects of surface roughness and a contribution of the small amounts of C and F that are present on the surface. The influence of oxygen is apparent from the experimental results of Fig. 2; with increasing ~0o we observe a strong reduction of the thermal desorp-
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tion rate of Cs, i.e. an increasing activation energy for thermal desorption. The increasing binding strength of the Cs atoms with increasing ~0o is also reflected in the results of Fig. 3. The maximum value of the Ede s distribution is tbund to compare favourably with the energy of the Cs O chemical bond of 3.08 eV [29]. The minimum value can be compared to the activation energy at saturation coverage on clean S i ( 1 0 0 ) 2 × l of 1.2 1.3eV [26,27,30]. The strong inhomogeneity of a HFetched-and-oxidized Si surface is also reflected in the width of the O l s line in the XPS spectrum (full width at half maximum, F W H M ): on slightly oxidized Si(100)2x 1 (9)o<0.05 ML) it is 0.63_+ 0.03 eV, whereas on the HF-etched samples it is 2.0_+ 0.1 eV (as corrected for the Gaussian instrumental response function). A correlation of the O ls linewidth with ~0o is not observed. Earlier work on Cs thermal desorption from ion-bombarded Si surfaces [30] also indicates surface inhomogeneity, albeit that the thermal desorption spectra were not analysed in a quantitative manner. In the activation-energy distribution model the repulsive interaction between the Cs atoms is not explicitly described since it is a relatively small contribution to the Ede~ distribution. By fitting our model to the experimental desorption data of Fig. 2, as indicated by the solid lines in that figure, we obtain the Eae s distributions for the Si(100) substrates with different levels of oxidation. They are shown in Fig. 7. It is observed that the weight of the distribution shifts from low Eaes to high Eaes as ~0o increases. This reflects the contribution of the distribution of oxygen on the Si surface to the Eae~ distribution. The ensemble average Eae s varies fairly little with ~0o, from 1.83 eV at ~0o=0.15 M L to 2.19 eV at ~0o= 1.15 ML. At low ~0o the Ea~s distribution is still quite broad. This is a result of the surface roughness, added with the contribution of C and F present on the sample surface. The contribution of C and F to the Eae~ distribution will probably be small in regard of the small amounts of C and F present on the sample surfaces (~0c= 0.07 _+0.02 ML, q)v = 0.15 _+0.05 ML). Since the C- and F-coverages are very reproducible in the experiments, their contribution will be relatively constant.
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The activation-energy distributions of Fig. 7 at ~0o=0.15, 0.35 and 1.15 M L are obtained by fitting the Edes-distribution model to the experimental results of Fig. 2. This means they are obtained from measurements taken at one temperature only, so they are not as well-tested as the Ede~ distribution at 9)o=0.65 ML, which was tested at 100, 150, 200 and 300°C (cf. Fig. 1). To investigate the validity of the results of Fig. 7 over a wide range of temperature we performed a Cs desorption experiment on a substrate with (po=0.15 ML, in which we step-wise increased the sample temperature whilst monitoring ~0Cs. The result of the model calculation using the Eaes distribution of Fig. 7 for ~0o = 0.15 M L is in fair agreement with the experimental results, as shown in Fig. 4, thereby validating the Eo~ distribution. A surprising observation in Fig. 7 is the presence of a peak at Eaes = 1.4 eV in the Ede~ distribution for (po = 1.15 ML. This peak reflects the initial decay of ~0cs (cf. Fig. 2) by a relatively weakly bound fraction of the Cs coverage. Since the highly oxidized Si surface will not display sites with little oxygen coverage, which would be sites with a low Ed~s, we expect the weakly bound fraction of Cs to be positioned on top of the first layer of Cs atoms. The " a t o m s on t o p " are stabilized at room temperature by the strong binding of Cs to loca-
289
tions with a high density of oxygen. This effect should, to a lesser degree, also occur on the samples with ~0o<1.15 ML, which explains the observed increase of the Cs saturation coverage with increasing substrate oxidation, as depicted in Fig. 3. The "atoms on top" are not observed as a separate peak in the Edes distributions for ~0o<1.15 M L because at these oxygen coverages the surface is not fully covered with oxygen, so low-Eae s sites will be numerous due to the presence of oxygen-deficient sites. A surprising observation in Fig. 3 is the fact that the saturation value of ~0cs on HF-etched Si(100) with ~0o=0.15 M L is lower than the saturation value of ~0cs on clean Si(100)2 x 1, namely 0.85 with respect to 1.0 ML. it may be caused by a reduction of the number of available binding sites for Cs on the microscopically rough HF-etched Si(100) surface, with respect to the Si(100)2 x 1 surface where the Cs atoms are densily packed in neat rows [19]. Using glancing incidence X-ray analysis (GIXA, see Ref. [31 ]) we measure the RMS roughness of our HF-etchedand-oxidized samples to be 0.5_+0.1 nm ( ~ 2 Si layers) in agreement with Refs. [16, 17]. The correlation length of the roughness is 5 0 + 10 nm. This shows that our sample surfaces, although rough on an atomic scale, are indeed very flat, so that the effective sample surface is not really increased by the surface roughness. This indicates, when compared to the Van der Waals radius of the Cs atoms of 0.267 nm, that the roughness will cause a mild disordering of the close packing of the Cs-atoms, which will result in a moderate lowering of the Cs coverage of HF-etched Si(100) compared to Si( 100)2 x 1 (whereas an increase in the effective sample surface, thus a higher number of available binding sites, would be the case had the surface roughness been much larger than the diameter of the Cs atoms, which is probably the case for the Ar-ion bombarded Si(100) samples in Ref. [30]). The surface-roughness effect is dominant over the Cs-binding effect of the small amounts of O and F (0.15 ML) on the HF-etched sample. The small amount of C (0.07 ML) will hardly influence the Cs bonding. With increasing ~0o the strong binding of Cs by the surface oxygen causes the saturation
290
R. Kroon, A. Sillke ," Sur/ace Science 399 (1998) 284 296
value of ~Ocs to increase. This behaviour should saturate for q~o> 1 ML, as is indeed observed. Using the obtained activation-energy distributions of Figs. 6 and 7 one can calculate the time behaviour of the Cs coverage on the Si(100) substrates at specific temperatures. It is illustrative to observe the change of the Edes distributions, as shown in Fig. 8a and b for the distributions of q)o=0.65 and 0.15 ML, respectively. One observes specific subensembles to desorb with time and temperature. The reader can easily evaluate the
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behaviour of the distributions at ~0o=0.35 and 1.15 ML, by drawing the appropriate envelope around Fig. 8a and b. The 1oW-Edes side of the distributions of Fig. 8a and b denotes the energy of the desorbing Cs atoms at a specific point in time at a fixed temperature, and at a specific temperature at a fixed point in time. respectively. For example, the Ede s distributions for q)o0.65 M L in Fig. 8a show the energy of the desorbing Cs atoms to be ~1.75 eV at t = 1 0 0 h. Fig. 1 shows ~cs=0.62 M L at t = 1 0 0 h , thus, at ~0Cs= 0.62 ML the energy of the desorbing particles is v l . 7 5 e V . The analysis which yields Fig. 5 should give the same result. Indeed, in Fig. 5 good agreement is observed. We have repeated this procedure for the Edes distributions at different times and temperatures, with all results yielding good agreement. We end this section with a discussion of some interesting observations from the XPS spectra taken in this study: upon deposition of Cs on the oxidized Si samples the O ls peak is observed to generate a shoulder on the low binding energy (BE) side, as shown in Fig. 9a for saturated Cs desorption on a sample with ~0o= 1.15 M L. The line shape is adequately fitted by two peaks, one is positioned at the BE of the O ls peak before Cs deposition, the other is shifted to lower BE by ~ 2 eV. Following Michel et al. [32] and Schaefer et al. [33], the high-BE peak can be attributed to O Si, the low-BE peak to an O Cs interaction. This is confirmed by the decreasing ratio of the low-BE peak intensity to the total O ls peak intensity, and the simultaneous increase of the ratio of the high-BE peak intensity to the total O ls peak intensity, with increasing q)o, as shown in Fig. 9b. This is a geometrical effect: since Cs atoms are much larger than O or Si atoms (cf. Table A.1), with increasing q)o an increasing number of O atoms interacts with an almost unchanging number of Cs atoms (we are at Cs saturation coverage): the number of bound Cs atoms per oxygen atom effectively decreases, whereas the increasing number of O atoms interacts with a similarly increasing number of Si atoms: the number of bound Si atoms per oxygen atom remains essentially the same. The contribution of the high-BE O Si peak to the O ls peak
R. Kroon, A. Sinke ,; Smjbce Science 399 (1998)284 296
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Fig. 9. (a) After Cs deposition the XPS O Is peak of oxidized Si(100) contains a shoulder on the low binding energy side, in contrast to the spectrum before Cs deposition (which is shown in the inset). (b) ( E ) ratio of the intensity of the low-BE peak to the total O ls peak intensity: (C;) ratio of the intensity of the high-BE peak to the total O Is peak intensity, as a function of the oxygen coverage of the Si substrate. Cs was deposited to saturation level.
Fig. 10. (a) Measured binding energy of the XPS Cs 3ds, 2 peak as a function of the oxygen coverage of the Si substrate. Cs was deposited to saturation level. (b) Full width at half maximum ( F W H M ) of the XPS Cs 3ds,e peak (corrected for the gaussian instrumental response function) as a function of the oxygen coverage of the Si substrate.
will therefore increase with respect to the lowBE O Cs peak. Both the binding energy and the linewidth {full width at half maximum, F W H M ) of the Cs 3ds/2 peak show a clear correlation with ~0o, which is shown in Figs. 10a and b, respectively. This is considered to result from vibrational excitation in the XPS process. (Citrin et al. have shown that effects of vibrational excitation can have a substantial influence on XPS line shapes of inorganic solids [34-36]: an increasing linewidth with increasing oxidation number was observed in the oxides and fluorides of AI, Mg and Cu, and also
in the halogenides of K. This was ascribed to vibrational excitation.) If the potential energy surfaces of the initial and final states of an XPS transition are displaced, vibrational excitation will accompany the XPS process as a result of the F r a n k - C o n d o n principle. The spectral width of the excited vibrational manifold, and hence of the XPS spectral line, increases with increasing binding strength of the adatom to the substrate [34 36]. In our case the binding strength of Cs increases with ~Oo, which explains the increasing linewidth of the Cs 3dsn peak with ~0o. The energy of the excited vibrational manifold also increases with increasing binding strength, which reduces the
292
R. Kroon. A. Sinke
,; SutJ~we Science 399 (1998) 284 296
kinetic energy of the photoelectrons. This explains the increasing BE of the Cs 3d5/2 peak with (Po (in alkali metals, vibrational excitation can induce a BE increase of several tenths o f e V [37]). A more common explanation of an increasing BE with increasing oxidation number (read: (Po) is in terms of an increasing charge transfer from Cs to the substrate [18]). However, as was mentioned earlier in this section, work function measurements [28] revealed the fractional charge of the Cs atoms to be (to a large extent) independent of substrate oxidation, so we believe that the charge-transfer mechanism will not substantially contribute to the observed shift in BE of the Cs 3d5/2 peak. In conclusion it is found that thermal desorption of Cs from oxidized Si(100) is governed by a broad distribution of the activation energy for thermal desorption. This reflects the considerable inhomogeneity of the microscopically rough Si surfaces. The weight of the distribution is found to shift to higher activation energy with increasing oxygen coverage of the Si substrates. The Cs desorption was monitored with X-ray photoelectron spectroscopy (XPS). The XPS spectra show evidence of vibrational broadening of the Cs 3d5/2 peak.
Acknowledgements We thank Peer Zalm for stimulating discussions. We thank the referee for his/her detailed and helpful comments on our manuscript. Appendix A: Determination and calibration of surface coverage with XPS In the experiments described in this paper the coverage of a Si substrate with cesium, oxygen, carbon and fluorine is determined with X-ray photoelectron spectroscopy. The basic principles of this technique are described in Ref. [18]. In this appendix we describe the determination of the surface coverage of Si from the XPS spectrum. We also describe its calibration for the case of Cs. A typical XPS-spectrum of a Si sample is shown in Fig. 1 l a, where we observe peaks due to Cs, O, C and Si. The small ls peak of fluorine, a residue of the hydrogen fluoride etching of the sample, is
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6;0
2;0
o
BindTng Energy (eV) (a)
A
\
•
~
aA
(b) Fig. I 1. (a) Typical example of an XPS spectrum of a Si substrate. One observes peaks due to coverage with Cs, O and C. In this case ¢cs= 0.47 ML, ~0o=0.43 ML and ~0c-0.09 ML. (b) Simple model of a substrate B covered by element A.
made invisible by the strong C s ( M N N ) Auger peaks (the fluorine peaks were measured before depositing Cs on the sample). The total area under a spectral line is proportional to the surface coverage by that specific element. A simple model of a substrate B covered with element A is shown in Fig. 1 lb. The figure depicts the situation @A< 1 ME, where fPA is the surface coverage in terms of actual physical layer of element A, i.e. the fraction of the surface area covered by this element. Photoelectrons generated in the substrate have to pass the (partial) layer of adatom A, which will thus attenuate the signal strength from the substrate. Taking this extinction exponential in the layer thickness of A [18], the signal of substrate B (area of the spectral line) is given by
IB=I~(1--(OA)+I~OAexp
[
2A(EB) COS :~ ' (A.1)
R. Kroon, A. Sinke / SutJ~we Science 399 (1998) 284 296
293
Table AI Values of variables relevant to determine the surface coverage of Cs, O, C and F on a Si substrate Variable
Cs 3ds, 2
C ls
Si 2p
F ls
O ls
Remarks
a [nm] E~ [eV]
0.534 529 22.93 3.68 5.43 9.21
0.182 969 1.00 0.99 1.08 1.99
0.264 1154 0.865 1.89 1.89 1.00
0.270 567 4.26 1.37 1.95 4.76
0.280 721 2.85 1.63 2.06 3.01
2 x Van der Waals radius MgK~ X-ray source Ref. [43] Ref. [38] Ref. [38] Eq. (A.5)
CrA(Ky.)/rrc(K9;} •;.A(EA ) [ n m ]
2A(Esi) [rim]
I'A~/I~2 p
where I ~ is the signal strength of a perfectly clean and infinitly thick sample B, aA is the diameter of adatom A, 2A(Eu) is the attenuation length of a photoelectron at kinetic energy EB in element A, and :~ is the angle of exit of the photoelectron with the normal to the surface. The signal of element A is described by Ig =IA PA - - I ~ 0 a exp
2A(EA) COS C~
Dividing Eq. (A.1) by Eq. (A.2) yields ~oa =
1 - exp 2 A ( E A ) COS ~
I~ I~
Ig I~°
a A
(A.3)
Concerning the variables in Eq. (A.3), Ia and IB are measured in the experiment whereas the values of the other variables are listed in Table 1. The attenuation length 2A(EB) is derived from the empirical formula of Seah and Dench [38]: ).A(EB)=O.41alA'5~B"5, where 2A and a a are expressed in nm, and EB is expressed in eV (we do not use the TTP-2M equations [39-42] to calculate 2A(EB), this will be discussed at the end of this appendix). The ratio of I ~ and I ~ is given by [18] I~ --
I~
a photoelectron from the relevant inner shell of atom A by a photon of energy hvJ. For ag we use the values given by Scofield for Mg K~ X-rays [43]. Since the XPS spectra are collected with the electron analyser in the so-called constant analyser energy (CAE) mode, all electrons strike the electron detector with the same kinetic energy which results in an identical value for D(EA) over the entire spectrum. Thus, D(EA) and D(EB) cancel in Eq. (A.4). Furthermore, nAocaT, 3 and T(EA)OCEA°'s (for our ESCAlab system), which leads to
a~)~A(EA)~aA(KZ) a3 2B(EB)~AACrB(K:O
Taking our experimental situation (:~=0) and filling the numbers of Table 1 into Eqs. (A.3) and (A.5) yields for Si(100): 1
~0cs=
1.243(]Si2p/ICs3dS;2)+ 0.0937'
(A.6)
1
=
~°° 0.476(isizp/ims)+O.127'
(A.7)
1
~°c= 0.334(Isizp/Icls) + 0.155'
nA2A(EA)T(EA)D(EA)rTA(hOJ ) =
(h.5)
(A.8)
1 ,
(A.4)
nB)~B(EB)T(EB)D(EB)6B(hOo )
where ng is the number density of material A (number of atoms per unit volume), T(EA) is the transmission of the electron analyser for electrons at kinetic energy EA, D(EA) is the sensitivity of the electron detector for electrons at kinetic energy EA, and aA is the cross-section for the emission of
-
~aF 0.852(isi2p/ivls)+O.129
,
(A.9)
in units of monolayer (ML). In the case of Cs we can actually calibrate the semi-emperical formula Eq. (A.6). To do this one needs to measure the XPS spectrum of a Si substrate with a welldefined Cs coverage. We fabricated such a sample by heating a HF-etched Si(100) substrate to a
R. Kroon, A. Sinke ,' Sut;jhce Science 399 (1998) 284 296
294
cq
. . . . . . . DD
1.00
o
hr3
Si
r-o 0.75 LD
[]
[]
[]
0.50 o
o © k_ o
o
0.00
o o []
0
0.25
Cs 0
oOOOOb i
o ° []
0
oO
0.95 b~
l
,40
D
l i
[]
0.90
0.85
r
o
(N
i 11ML Cs J
[]
1.00
0 (D L_ o
I
O0
[]
D
i
i
i
10
20
30
deposition
time
DDD[] i
40
[]
D [] i
0.80
50
(rain)
Fig. 12. Variation of the area of the Cs 3ds, 2 and Si 2p spectral lines during deposition of Cs on Si( 100)2 x 1. Signals are normalized to their maximum value. The variation of the Cs signal is indicative of the deposition process: at 1<6 min we observe the Cs source heating up, 6 < t < 2 6 rain shows a linear increase of Cs coverage in time at a deposition rate of 2.1 M L/h, whereas at t > 2 6 min the Cs coverage saturates at 1 ME (Ocs 0.5).
temperature of 800 900°C by direct resistive heating. At these temperatures the surface oxides evaporate (as SiO), leaving an oxygen-free 2 x 1-reconstructed Si(100) surface [20]. The reconstructed surface was preserved by cooling down at a rate of < 1 C s - t . Residual carbon was subsequently removed by ion milling with Ar ÷ after which the sample was again heated to T > 8 0 0 C and slowly cooled down. The resulting S i ( 1 0 0 ) 2 x l surface showed ~0o<0.01ML and (pc<0.01 ML. During the subsequent deposition of Cs these numbers did not change. At the S i ( 1 0 0 ) 2 x l surface the Si atoms are ordered in parallel dimer rows [44,45]. The Cs atoms deposited on this surface position themselves on top of the dimer rows until a saturation coverage of 0c~=0.5 is reached; one Cs atom per two substrate Si atoms [19], i.e. 3.39x1014Cs atoms cm 2 (the presence of Cs does not significantly alter the positions of the Si surface atoms [46,47]). Prolonged deposition of Cs, with the sample at room temperature, does not lead to an increase in Cs coverage, a result of the Cs sticking coefficient becoming negligible at room temperature when ~Ocsreaches 1 ML [19]. This proces is depicted in Fig. 12, where we plot the XPS signal of Cs on the Si(100)2 × 1 substrate as a function of deposition time. The Cs 3dsn signal is observed
to saturate after v 4 0 min, clearly the saturation coverage has been reached. This is also indicated in the same figure by the Si2p signal which decreases during deposition, due to attenuation of the photoelectrons fl'om the Si substrate by the increasing Cs coverage, but is found to saturate after .~:40 min of Cs deposition. In the literature it is debated wether the Cs atoms position themselves on top of the dimer rows (single layer model [19], 0cs=0.5) or also in between the dimer rows (double layer model [48,49], 0cs= 1.0), a discrepancy which could give rise to an error of factor 2 in our calibration. One can, however, experimentally verify wether a single or a double layer is deposited. As shown in Refs [21,30,50], during rapid Cs deposition (the Cs coverage saturates within a few minutes) on Si( 100)2 x 1 the work function first passes through a minimum at 0Cs=0.5, then it increases again until it saturates at 0cs= 1. We have verified that during our slow Cs deposition (of. Fig. 12) the work function saturates at a minimum value, together with the saturation of the Cs coverage (single layer). When we enhance the temperature of our Cs source, i.e. increase the Cs flux incident on our sample surface, we can indeed deposit more Cs on the surface than with the lower flux and the work function is observed to pass through a minimum, just as described in Refs [21,30,50]. The difference in behaviour lies in the well-known fact that the Cs atoms in between the dimer rows are less stable than those on top of the dime. rows [19,26,27]. When rapidly depositing Cs both the sites on top and in between the dime," rows will be occupied. Our slow deposition ensures that we only form the single layer of Cs atoms on top of the dimer rows. Thus, in the experiments described in this paper, we deposit a single layer of Cs. This can also be inferred from a comparison of Fig. 5 of this paper with Fig. 2 in Ref. [27]. The latter figure shows that EdeS decreases with increasing Cs coverage up to slightly more than single layer coverage, but then remains constant at higher coverage. Fig. 5 clearly shows that we are in the regime of single layer coverage. The relation between 0cs and ~0cs is given by OC s = ~)Cs[)Si(IOO' "I 2 CS]
1"
(A.10)
R. Kroon. A. S&ke Suffdce Science 399 (1998)284 296 where )'Si(lOO) is the n u m b e r density of the S i ( 1 0 0 ) 2 × l surface, i.e. the n u m b e r of surface atoms per unit area. ),si(loo1=6.78 x 1014 cm 2 [51 ]. Filling the saturation values o f Ics3as,,2 and Isi2p into Eqs. (A.3), (A.10), with ~=0 and Ocs=-0.5 ML, yields Ic~s3ds,a/Is~f2p =9.58. This is in very g o o d agreement with the semi-emperical value of 9.21, as obtained from Eq. (A.5) (cf. Table 1), indicating that the a p p r o a c h leading to Eqs. (A.6) (A.9) is reliable. Using the experimental value o f oc / vo Ics3d5:2: Isi2p : 9.58, Eq. (A.6) becomes 1 ~pcs=
.
(A.11)
1.294(Isi2p/Ic~3ds., ) + 0.093 7 In this paper we use Eq. ( A . I 1 ) to determine the cesium coverage of the HF-etched samples from the XPS spectrum. In the model described above, we use the attenuation length (AL))~A(E~), as calculated from the empirical formula o f Seah and Dench [38], to account for the extinction o f substrate photoelectrons by the (partial) layer o f adatoms. We do not use the, more recently developed, inelastic mean free path ( I M F P ) which is calculated from the T T P - 2 M equations of T a m u r a et al. [39-42]. The reason for this is twofold. Firstly, when using the A L the result of the Cs-calibration experiment, oo / oo Ic~3a~._,/1 Si2p = 9.58, is in very g o o d agreement with the value calculated from Eq. (A.5), which is 9.21. This is not the case when using the I M P F : 2 the calibration experiment then yields Ic~s3d~.~,,' IS~2v = 11.23, whereas Eq. (A.5) yields a value of 7.33. Secondly, not only is the Cs coverage correctly determined when using the AL, but also the oxygen coverage. This can be inferred from the experimental result shown in Fig. 3: the observed saturation o f ~0c~should occur at an oxygen coverage o f ~0o= 1 M L (as explained in the discussion section). This is indeed observed, within experimental accuracy. Strictly speaking, to determine ~0o, ~0c, and q)v (which are determined prior to Cs deposition) the model described above is valid in case the total 21MPF-values are IMPFc~(Ecs)-4.23 nm and lMPFc~(Esi)=7.79nm (calculated from TPP-2M using the following data for Cs: p=l.873gcm 3, N v - l , Eg=0, and M= 132.9054). and 1MPFsi(Esi)=2.73 nm [39].
295
surface coverage ~o= ~0o + (Pc + ~0v< 1 ML, within the situation that all a d a t o m s are lying alongside each other. The p < 1 M L-condition is met on all substrates, with the exception of the most strongly oxidized substrate (there ~ 0 = l . 3 7 M L , which induces a mild inaccuracy). In addition, on the oxidized Si surfaces described in this paper there may be some locations where the surface coverage is thicker than one atomic layer. Surface roughness will induce a slight inaccuracy (the surface roughness is ~ t w o Si layers; ~ 0 . 5 rim). Furthermore, the semi-emperical Eqs. ( A . 7 ) - ( A . 9 ) are not calibrated, but as stated above, the value of ~0o is correctly determined. We estimate the absolute accuracy of q)o, (Pc and Pv to be _+ 10%. The accuracy of ~0cs is estimated + 3 % , as a result of the calibration. In the simple model the Cs atoms form a single overlayer on the oxidized samples, with the oxides lying in between the Cs overlayer and the Si substrate. O n the oxidized surfaces we therefore correct ~0cs as determined from Eq. (A.11) for the attenuation of the substrate photoelectrons by the oxygen coverage. This is done by dividing Isizp in E q . ( A . 1 1 ) by the extinction factor exp[-do/)oo(Esi)], where do is the oxide thickness in nm and 2o(Esi)=2.06 n m (cf. Table A1 ). For the most strongly oxidized sample (~0o-- 1.15 ME), ~0Csas determined from Eq. (A.I 1 ) is corrected by - 15%.
References [1] 1.P. Batra (Ed.), Metallization and Metal/Semiconductor Interfaces, NATO Advanced Study Institute Series B 195. 1989. [2] M. Tikhov. G. Boishin, L. Surnev. Surf. Sci. 241 (19911 /03. [3] R. Storch, H. Stolz, H.-W. Wassmuth, Ann. Physik 1 (1992) 315. [4] P. Soukiassian et al., Phys. Rev. B 46 (1992) 13471. [5] K. Yamada, S. Nishigaki, Surf. Sci. 293 (1993) L893. [6] W.C. Fan, A. Ignatiev, Surf. Sci. 296 (1993) 352. [7] P. Soukiassian et al., J. Vac. Sci. Technol. B 11 (1993) 1431. [8] C.Y. Kim et al., Surf. Sci. 324 (1995) 8. [9] N. Minami et al., Surf. Sci. 325 (1995) 1i. [10] Th. Chass6 et al.. Surf. Sci. 331 (1995) 528.
296
R. Kroon, A. Sinke / Suljitce Science 399 (1998) 284 296
[11] G.V. Benemanskaya, G.E. Frank-Kamenetskaya, M.N. Lapushkin, Surf. Sci. 331 (1995) 552. [12] L. Stauffer, C. Minor, Surf. Sci. 331 (1995) 606. [13] A. Namiki et al., Surf. Sci. 337 (1995) 8. [14] R.Z. Bakhtizin et al., Appl. Surf. Sci. 87/88 (1995) 347. [15] S. Pastuszka, A.S. Terekhov, A. Wolf, Appl. Surf. Sci. 99 (1996) 361. [16] Y.J. Chabal et al,, J. Vac. Sci. Technol. A 7 (1989) 2104. [17] M. Morita et al., J. Appl. Phys. 68 (1990) 1272. [18] D. Briggs, M.P. Seah, Practical Surface Analysis, vol. 1, 2nd edn, Wiley, Chichester, 1990. [19] J.D. Levine, Surf, Sci. 34 (1973) 90. [20] D. Edwards, W.T. Peria, Appl. Surf. Sci. 1 (1978)419. [21] J.E. Ortega et al., Phys. Rev. B 36 (1987) 6213. [22] H.-J. Ernst. M.L. Yu, Phys. Rev. B 41 (1990) 12953. [23] K. Yamada, S. Nishigaki, Appl. Surf. Sci. 99 (1996) 21. [24] P.A. Redhead, Vacuum 12 (I962) 203. [25] K.J. Rawlings et al., Surf. Sci. 118 (1982) 47. [26] R.H. Milne et al., Phys. Rev. Lett. 73 (1994) 1396. [27] R.H. Milne et al.. Surf. Sci. 336 (1995) 63. [28] P.J.W. Weijs, Unpublished results. [29] Handbook of Chemistry and Physics, CRC, Boca Raton, FL, 1980. [30] C.A. Papageorgopoulos, M. Kamaratos, Surf. Sci. 221 ( 1989 ) 263. [31] D.K.G. de Boer, Phys. Rev. B 49 (1994) 5817. [32] E.G. Michel et al., Phys. Rev. B 38 (1988) 13399. [33] J.A. Schaefer et al., Surf. Sci. 211/212 (1988) 1075.
[34] P.H. Citrin et al., Phys. Rev. B 10 (1974) 1762. [35] P.H. Citrin, G.K. Wertheim, Y. Baer, Phys. Rev. B 16 (1977) 4256. [36] P.H. Citrin, P. Eisenberger, D.R. Hamann, Phys. Rev. Lett. 33 (1974) 965. [37] W.F. Egelhoft'. Surf. Sci. Rep. 6 (1987) 253. [38] M.P. Seah, W.A. Dench, Surf. Interface Anal. 1 (1979) 2. [39] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal. 17 (1991) 911. [40] S. Tanuma, C.J. Powell, D.R. Penn. Surf. Interface Anal. 17 (1991) 927. [41] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal. 20 (1993) 77. [42] S. Tanuma, C.J. Powell, D.R. Penn, Surf. lntert:ace Anal. 21 (1993) 165. [43] J.H. Scofield, J. Electron Spectrosc. 8 (1976) 129. [44] R.E. Schlier, H.E. Farnsworth, J. Chem. Phys. 30 (1959) 917. [45] R.M. Tromp, R.J. Hamers, J.E. Demuth, Phys. Rev. Lett. 55 (1985) 1303. [46] S.T. Kim et al., Phys. Rev. B 44 (1991) 5622. [47] P.S. Mangat, P. Soukiassian, Phys. Rev. B 52 (1995) 12020. [48] T. Abukawa. S. Kono, Surf. Sci. 214 (1989) 141. [49] T. Abukawa, T. Okane, S. Kono, Surf. Sci. 256 ( 1991 ) 370. [50] Y.-C. Chao, L.S.O. Johansson, R.I.G. Uhrberg, Phys. Rev. B 54 (1996) 590l. [51] R. Lindsay et al., Phys. Rev. B 51 ( 1995 ) 11140.
Surface Science 399 (1998) 284 296
Influence of oxygen on the thermal desorption of Cs from Si(100) R o n K r o o n *, A r n o l d S i n k e Philips Research Laboratories (WY-51), Prql~ Holstlaan 4. 56_56 AA Eindhoren, The Netherlands' Received 3 July 1997; accepted for publication 15 October 1997
Abstract
This paper describes the thermal desorption of Cs from oxidized Si(100), measured as a function of sample temperature and oxygen coverage. The desorption process is found to be governed by a broad distribution of the activation energy for thermal desorption of the Cs atoms, which reflects the considerable inhomogeneity of the Si surfaces. The weight of the distribution is found to shift to higher activation energy with increasing oxygen coverage of the Si substrates, g, 1998 Elsevier Science B.V.
Keywords." Alkali metals: Silicon: Silicon oxides: Thermal desorption; X-ray photoelectron spectroscopy
1. Introduction
The behaviour of alkali metals on semiconductor surfaces has been the subject of a great many studies in recent years [1-15]. However, the vast majority of surfaces under study are well-defined reconstructed surfaces. In industrial applications, the semiconductor surfaces may suffer modest levels of contamination and/or defects, it is of considerable interest to study the effects of such deviations from ideal surfaces. In this paper we describe the thermal desorption of Cs from Si(100). We chose to study Cs because of its importance to negative electron affinity (NEA) devices. Prior to the deposition of Cs the Si substrates are cleaned with ozone, etched with moist hydrogen fluoride gas and exposed to air, resulting in microscopically rough surfaces (rough-
* Corresponding author. Fax: (+ 31 ) 40 2744335: e-mail: [email protected]
0039-6028/98/$19.00 ~ 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 9 - 6 0 2 8 ( 9 7 ) 0 0 8 2 8 - 5
ness ~ 0 . 5 n m [16,17]) with different levels of oxidation. Thermal desorption of Cs is measured as a function of sample temperature and oxygen coverage. The observed Cs desorption is accurately described in a model which takes into account a distribution of the activation energy for thermal desorption. The distribution is found to be quite broad, an indication of the inhomogeneous nature of the Si surfaces which originates from a distribution of the density of oxygen on the Si surface, added with effects of surface roughness. The weight of the distribution is found to shift to a higher activation energy with increasing oxidation of the Si surface. The Cs desorption was monitored with X-ray photoelectron spectroscopy (XPS). The XPS spectra show evidence of vibrational broadening of the Cs 3d5/2 peak. This paper is organized as follows. The experimental apparatus is described in Section 2, and experimental results are presented in Section 3. A discussion of the experimental
R. Kroon, A. Sinke / SmJctce Science 399 (1998) 284 296
results, aided by results of model calculations of the desorption process, is presented in Section 4.
2. Experimental The thermal desorption experiments are performed in a VG ESCALAB Mk-II. The ESCALAB is equipped with a fast-entry load lock, which allows samples to be placed in the U H V chamber without breaking the UHV. Base pressure in the fast-entry load lock is 5 x 10 -8 mbar, base pressure in the U H V chamber is 5 x 10 - H mbar. The Si(100) substrates that are employed in the experiments are doped with 8 x 1019m 3 As. A substrate is, firstly, cleaned with ozone to remove any carbon contamination. The ozone is made in a silent discharge in an oxygen gas flow. It is led over the substrate which is heated in an oven to a temperature of 18OC. Secondly, the substrate is etched in a room-temperature gas flow consisting of gaseous H F and water vapour, to remove the silicon oxide layer present on the substrate. This etching flow is made by leading a N2 gas flow over a 10% solution of H F in water. The quality of the 03- and HF-etching is monitored in the E S C A I A B with X-ray photoelectron spectroscopy (XPS, the basic principles of this technique are described in Ref. [181). In Appendix A we describe the determination of the surface composition from the sample's XPS spectrum. U p o n placing the etched substrate in the fast-entry load lock it is in contact with the ambient for a period less then a minute. A minor oxidation of the substrate may result, determined to be < 0 . 0 5 M E , where 1 M L is defined as one physical layer of adatoms. Typical coverage values observed directly after 03- and HF-etching are (Po=0.15 ME (of which >0.1 M L is due to oxide left after etching, and <0.05 M L is due to oxidation during sample transfer), ~0c=0.07 M L and ~0v=0.15 ML. In our experiments we investigate the influence of the oxygen coverage of a substrate on the thermal desorption of Cs. We oxidize a HF-etched substrate by exposure to the laboratory ambient (relative humidity 50-60%; temperature 19:C) until a specific oxygen coverage is reached, as determined with XPS. Subsequently, the substrate
285
and substrate holder are thoroughly outgassed in the ESCALAB in order to prevent the release of oxidizing gases when heating the sample in the actual Cs desorption experiment. Outgassing is done by slowly (overnight) raising the sample temperature on the resistively heated substrate holder to 35OC. The pressure in the U H V chamber is kept below 10 -1° mbar. In this case the sample does not oxidize during outgassing. We observed that a rapid temperature rise during outgassing can lead to oxidation of the sample since in that case the oxidising gases (mainly water) that are released are not pumped away sufficiently fast to prevent oxidation. In the desorption experiments we ramp the substrate temperature from room temperature to a set value in 3 min, after which it is kept constant within _+I'C. Substrate temperature is calibrated by using a Si substrate in which a thermocouple is placed. Therefore, the measurement of the absolute substrate temperature is estimated to be correct within _+ 10'C. After outgassing the substrate is cooled to room temperature. Cs is subsequently deposited on the substrate from a home-built source. The source is essentially an A1Si wire, containing 3%wt. Si, into which Cs has been diffused at ~ 5 0 0 C . The total Cs content is 2 3%wt. The wire is mounted inside a narrow nickel tube. When resistively heating the tube the wire releases pure atomic Cs at temperatures > 4 5 0 ( 2 . We employ a Cs deposition rate of ~2.1 M L h -1. Prior to use, the Cs source is thoroughly outgassed. During deposition the Cs coverage is monitored with XPS. The deposition is continued until the Cs coverage reaches a saturation value (which we observe to be ~0.95 ME, depending on the Si surface quality and the amount of oxygen present on the surface), a result of the Cs sticking coefficient becoming negligible at room temperature when ~0cs reaches ~1 M L [19]. Base pressure in the U H V chamber during Cs deposition is < 10-1° mbar. Following deposition, the sample is heated to a specific temperature. The decreasing Cs coverage of the Si surface in time is monitored with XPS. We also monitor the oxygen coverage, both during deposition and desorption, because the presence of Cs can significantly enhance the oxidation rate of the Si substrate [20-22]. Due to the very low
286
R. Kroon, A. Sinke / Su#Jktce Science 399 (1998) 284 296
base pressure in our U H V chamber the Si surfaces show negligible oxidation during the experiments. Finally, we observe ~0c = 0.07 _+0.02 M L and ~0v=0.15_+0.05 M L to be reproducible during the measurements, and between substrates.
1.0 T=200 oC
G" z
,~__¢,__~
0
0.6
3. Experimental results Thermal desorption of Cs from Si(100) substrates which are oxidized to ~0o=0.65_+0.1 ML, is shown in Fig. 1. The experiment is repeated at various sample temperatures. The symbols denote the experimental results whereas the lines are the result of a model calculation, as described in Section 4. The values of q)c~ are corrected for the influence of ~o on the Si XPS signal (see Appendix A). This correction is done for all q)cs presented in this paper. In Fig. 1, one observes a very rapid initial decay of ~0c~(t), which is the more rapid with increasing sample temperature. The decay slows down considerably at longer times. ~oo is constant during the desorption of Cs, indicating that Cs desorbs at our sample temperatures but oxygen does not. Fig. 2 shows the thermal desorption of Cs in time at fixed sample temperature (200~C), but at different levels of Si(100) substrate oxidation. The symbols denote the experimental results, the lines
V
0.4-
$
V
0.1 5 ML
1'0
b
2'0 5'0 40
75
25
time (h) Fig. 2. Symbols: experimentally observed time decay of the Cs coverage on Si(100) substrates showing different levels of oxidation. Sample temperature was fixed at 200C. Lines: result of a model calculation, cf. Section 4.
1.00 >
8
si@oo)2xl T
~ @
0.96
90.92
0.88
0.84 0.0,
.~
.1 5 ML
5ML
'oi
o.;o o.%
99o(tML)
1.0 ~j_
ocpo =
0.8
~oo=0.65 ML
~ o o o c
Fig. 3. Circles: saturation value of q)cs at room temperature as a function of Si(100) substrate oxidation. The solid line is a guide to the eye. Square: saturation coverage of clean
O
Si(100)2 x 1 [19]. 0.6
200 oC ~ . 00
0,._
~ o"tssea_~ o ^ o
300 oC
O.4 i
10
,
2'0
3'0
40
i
75
,
i
,
125
time (h) Fig. 1. Symbols: experimentally observed decay' in time of the Cs coverage on oxidized Si(100) substrates (¢oo=0.65+0.1 M L) at different sample temperatures. Lines: result of a model calculation, cf. Section 4.
are the result of a model calculation described in Section 4. The Cs desorption rate is observed to increase with a decreasing level of substrate oxidation. The general shape of the desorption curves is comparable to that observed in Fig. 1. Slightly hidden in Fig. 2 is the fact that (Pcs(t = 0 ) is different for the different curves. When depositing Cs to a saturation value we find this value to increase with increasing substrate oxidation, as depicted in Fig. 3. The effect is observed
R. Kroon, A. Sinke ," Surliwe Science 399 (1998) 284 296
287
3.0 ~
0.8
¢ o = 0.1 5 ML
"J
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~
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o12'o14
1 60
time ( m i r ' ) Fig. 4. Syl'nbols: measured decay of the Cs coverage on a HF-etched Si(100) substrate ((/)o-0.15 M / ) under stepwise increase of the sample temperature (shown in the inset). Solid line: result of a model calculation, cf. Section 4.
to saturate for large g'o. The results of Fig. 3 are in agreement with data published in Ref. [23]. Finally, to test our model calculation on a substrate with ~0o=0.15 ML over a wide range of temperature we performed an experiment in which we stepwise increased the sample temperature whilst monitoring ~0Cs. The results of this experiment are shown in Fig. 4, where the symbols denote the experimental results and the solid line is the result of the model calculation, q)o remained constant during the experiment. The stepwise increase of the sample temperature is depicted in the inset in Fig. 4.
.0
Fig. 5. The activation energy for thermal desorption Edes of Cs on oxidized Si(100) 0:o-0.65_+0.1 ML) versus Cs coverage• The Ea~~ values are obtained by applying Eq. ( 1 ) to the experimental results of Fig. 1. The different symbols reflect the different temperatures at which desorption was measured. The dashed line is a fit with a slope of 1.7 eV/ML.
(1)
We observe a strong dependence of Eaes on ~0cs: - 1.7 eV ML ~ (indicated by the dashed line). It is much stronger than the effect induced by the well-known repulsive interaction between the positively charged Cs atoms, which has been measured to be - 0 . 6 e V M L 1 on S i ( 1 0 0 ) 2 x l [26,27]. 1 This effect will be of similar magnitude on oxidized Si(100) since the fractional charge of a Cs atom is similar on Si(100)2 x 1 and oxidized Si(100), as we have determined from work function measurements on HF-etched Si(100) oxidized to ~0o=0.4 M L [28]. Thus, the shape of the desorption curves in Fig. 1 is not compatible with the repulsive-interaction mechanism. The shape of the desorption curves does imply the presence of a distribution of the activation energy for thermal desorption, which is not unexpected in regard of the likely inhomogeneity of the microscopically rough (and oxidized) Si surfaces. One assumes an ensemble of Cs atoms which is composed of subensembles with a specific activa-
where v is an attempt frequency (set equal to 1013 S-1 [24-27]), k is Boltzmann's constant, and Eden, the activation energy of the desorbing atoms, is a function of the surface coverage ~0cs. When we apply Eq. ( 1 ) to the experimental results of Fig. 1, the obtained values of Ed~ are shown in Fig. 5.
t Note that our definition of surface coverage: 1 ML Cs is one physical layer of Cs atoms, differs from the definition used by Milne et ah [26,27], where 1 ML Cs is defined as one Cs atom per substratc atom (which, on Si( 100)2 x 1, corresponds to two physical Cs layers). In the comparison between our results and Milne's, all data are expressed according to our definition of surface coverage.
4. Discussion
Thermal desorption of atomic Cs at temperature T is assumed to be governed by the first-order rate equation: dt
-r~0csexp
-
,
R. Kroon, A. Sinke / Smjhce Science 399 (1998) 284 296
288
tion energy. The subensembles desorb according to Eq. (1), but their activation energy for thermal desorption is now taken to be independent of ~Ocs, which for subensemble i yields ~0c~,i(t)=~0c~,i(0 ) exp
- v t exp
kT
.
(2)
The total Cs coverage behaves as the sum of the subensembles: N
oc U)= E
(3)
i-1
A good fit of the model to the experimental results of Fig. 1, as depicted by the solid lines in that figure, yields the activation-energy distribution of Fig. 6 (in the fit we have used N = 18, this number proved necessary to obtain a good-quality fit for all the curves in Fig. 1 ). The Ea~s distribution is observed to be quite broad. Its width was estimated by extrapolation of the Edes values of Fig. 5 to q~c~= 0. Its physical origin is considered to be a distribution of the density of oxygen on the Si surface, added with effects of surface roughness and a contribution of the small amounts of C and F that are present on the surface. The influence of oxygen is apparent from the experimental results of Fig. 2; with increasing ~0o we observe a strong reduction of the thermal desorp-
0.08
"~ 0
0.06
II 0.04
8 13- 0.02
0.00
11o
1:8
2:0
2}s
31o
(ev) Fig. 6. The distribution of the activation energy for thermal desorption of Cs atoms at saturation coverage on a room temperature Si(100) substrate previously oxidized to ~0o=0.65_+0.1 M L as determined by applying Eqs. (2) and (3) to the experimental results of Fig. 1.
tion rate of Cs, i.e. an increasing activation energy for thermal desorption. The increasing binding strength of the Cs atoms with increasing ~0o is also reflected in the results of Fig. 3. The maximum value of the Ede s distribution is tbund to compare favourably with the energy of the Cs O chemical bond of 3.08 eV [29]. The minimum value can be compared to the activation energy at saturation coverage on clean S i ( 1 0 0 ) 2 × l of 1.2 1.3eV [26,27,30]. The strong inhomogeneity of a HFetched-and-oxidized Si surface is also reflected in the width of the O l s line in the XPS spectrum (full width at half maximum, F W H M ): on slightly oxidized Si(100)2x 1 (9)o<0.05 ML) it is 0.63_+ 0.03 eV, whereas on the HF-etched samples it is 2.0_+ 0.1 eV (as corrected for the Gaussian instrumental response function). A correlation of the O ls linewidth with ~0o is not observed. Earlier work on Cs thermal desorption from ion-bombarded Si surfaces [30] also indicates surface inhomogeneity, albeit that the thermal desorption spectra were not analysed in a quantitative manner. In the activation-energy distribution model the repulsive interaction between the Cs atoms is not explicitly described since it is a relatively small contribution to the Ede~ distribution. By fitting our model to the experimental desorption data of Fig. 2, as indicated by the solid lines in that figure, we obtain the Eae s distributions for the Si(100) substrates with different levels of oxidation. They are shown in Fig. 7. It is observed that the weight of the distribution shifts from low Eaes to high Eaes as ~0o increases. This reflects the contribution of the distribution of oxygen on the Si surface to the Eae~ distribution. The ensemble average Eae s varies fairly little with ~0o, from 1.83 eV at ~0o=0.15 M L to 2.19 eV at ~0o= 1.15 ML. At low ~0o the Ea~s distribution is still quite broad. This is a result of the surface roughness, added with the contribution of C and F present on the sample surface. The contribution of C and F to the Eae~ distribution will probably be small in regard of the small amounts of C and F present on the sample surfaces (~0c= 0.07 _+0.02 ML, q)v = 0.15 _+0.05 ML). Since the C- and F-coverages are very reproducible in the experiments, their contribution will be relatively constant.
R. Kroon, A. Sinke /' SmJace Science 399 (1998) 284 296 0.12
0.10 0.08
,,,,
C)
II
\
'
~: "
0.06
~? 0.04
13-
0.02
0.00
1.'0
1.5
21o
£o
Edes (eV) Fig. 7. The Ed~s distributions of Cs at saturation coverage on room temperature Si(100) substrates with different oxygen coverage, as determined from the experimental results of Fig. 2. Solid line, q)o=0.I5 ML; long-dashed line, q)o=0.35 ML; shortdashed line, ~po=0.65 ML: dotted line, (#o = 1.15 ML.
The activation-energy distributions of Fig. 7 at ~0o=0.15, 0.35 and 1.15 M L are obtained by fitting the Edes-distribution model to the experimental results of Fig. 2. This means they are obtained from measurements taken at one temperature only, so they are not as well-tested as the Ede~ distribution at 9)o=0.65 ML, which was tested at 100, 150, 200 and 300°C (cf. Fig. 1). To investigate the validity of the results of Fig. 7 over a wide range of temperature we performed a Cs desorption experiment on a substrate with (po=0.15 ML, in which we step-wise increased the sample temperature whilst monitoring ~0Cs. The result of the model calculation using the Eaes distribution of Fig. 7 for ~0o = 0.15 M L is in fair agreement with the experimental results, as shown in Fig. 4, thereby validating the Eo~ distribution. A surprising observation in Fig. 7 is the presence of a peak at Eaes = 1.4 eV in the Ede~ distribution for (po = 1.15 ML. This peak reflects the initial decay of ~0cs (cf. Fig. 2) by a relatively weakly bound fraction of the Cs coverage. Since the highly oxidized Si surface will not display sites with little oxygen coverage, which would be sites with a low Ed~s, we expect the weakly bound fraction of Cs to be positioned on top of the first layer of Cs atoms. The " a t o m s on t o p " are stabilized at room temperature by the strong binding of Cs to loca-
289
tions with a high density of oxygen. This effect should, to a lesser degree, also occur on the samples with ~0o<1.15 ML, which explains the observed increase of the Cs saturation coverage with increasing substrate oxidation, as depicted in Fig. 3. The "atoms on top" are not observed as a separate peak in the Edes distributions for ~0o<1.15 M L because at these oxygen coverages the surface is not fully covered with oxygen, so low-Eae s sites will be numerous due to the presence of oxygen-deficient sites. A surprising observation in Fig. 3 is the fact that the saturation value of ~0cs on HF-etched Si(100) with ~0o=0.15 M L is lower than the saturation value of ~0cs on clean Si(100)2 x 1, namely 0.85 with respect to 1.0 ML. it may be caused by a reduction of the number of available binding sites for Cs on the microscopically rough HF-etched Si(100) surface, with respect to the Si(100)2 x 1 surface where the Cs atoms are densily packed in neat rows [19]. Using glancing incidence X-ray analysis (GIXA, see Ref. [31 ]) we measure the RMS roughness of our HF-etchedand-oxidized samples to be 0.5_+0.1 nm ( ~ 2 Si layers) in agreement with Refs. [16, 17]. The correlation length of the roughness is 5 0 + 10 nm. This shows that our sample surfaces, although rough on an atomic scale, are indeed very flat, so that the effective sample surface is not really increased by the surface roughness. This indicates, when compared to the Van der Waals radius of the Cs atoms of 0.267 nm, that the roughness will cause a mild disordering of the close packing of the Cs-atoms, which will result in a moderate lowering of the Cs coverage of HF-etched Si(100) compared to Si( 100)2 x 1 (whereas an increase in the effective sample surface, thus a higher number of available binding sites, would be the case had the surface roughness been much larger than the diameter of the Cs atoms, which is probably the case for the Ar-ion bombarded Si(100) samples in Ref. [30]). The surface-roughness effect is dominant over the Cs-binding effect of the small amounts of O and F (0.15 ML) on the HF-etched sample. The small amount of C (0.07 ML) will hardly influence the Cs bonding. With increasing ~0o the strong binding of Cs by the surface oxygen causes the saturation
290
R. Kroon, A. Sillke ," Sur/ace Science 399 (1998) 284 296
value of ~Ocs to increase. This behaviour should saturate for q~o> 1 ML, as is indeed observed. Using the obtained activation-energy distributions of Figs. 6 and 7 one can calculate the time behaviour of the Cs coverage on the Si(100) substrates at specific temperatures. It is illustrative to observe the change of the Edes distributions, as shown in Fig. 8a and b for the distributions of q)o=0.65 and 0.15 ML, respectively. One observes specific subensembles to desorb with time and temperature. The reader can easily evaluate the
0.08
"-~
0.06
(~
0.04
A i~\ At '~//f',k / iI ~//'I '~,i
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T=200oC 10o = 0.65 ML
//\
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,'I %v/
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/I
//
0.00
11o
'
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215
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(b) Fig. 8. (a) Calculated time dependence of the activation-energy distribution of Cs on Si(100) (10o-0.65 ML) at T=200 C. Distributions are shown at t = 0 , 5rain, 10mira l h, 10h, 102 h, 103 h, 104 h and l0 s h. (b) Calculated activation-energy distribution of Cs on oxidized Si(100) ((po-0.15ML) at : - 10211 as a function of temperature. Distributions are shown at T = 60.75, 100, 150, 200 and 300C. Also shown is the distribution at t = 0 .
behaviour of the distributions at ~0o=0.35 and 1.15 ML, by drawing the appropriate envelope around Fig. 8a and b. The 1oW-Edes side of the distributions of Fig. 8a and b denotes the energy of the desorbing Cs atoms at a specific point in time at a fixed temperature, and at a specific temperature at a fixed point in time. respectively. For example, the Ede s distributions for q)o0.65 M L in Fig. 8a show the energy of the desorbing Cs atoms to be ~1.75 eV at t = 1 0 0 h. Fig. 1 shows ~cs=0.62 M L at t = 1 0 0 h , thus, at ~0Cs= 0.62 ML the energy of the desorbing particles is v l . 7 5 e V . The analysis which yields Fig. 5 should give the same result. Indeed, in Fig. 5 good agreement is observed. We have repeated this procedure for the Edes distributions at different times and temperatures, with all results yielding good agreement. We end this section with a discussion of some interesting observations from the XPS spectra taken in this study: upon deposition of Cs on the oxidized Si samples the O ls peak is observed to generate a shoulder on the low binding energy (BE) side, as shown in Fig. 9a for saturated Cs desorption on a sample with ~0o= 1.15 M L. The line shape is adequately fitted by two peaks, one is positioned at the BE of the O ls peak before Cs deposition, the other is shifted to lower BE by ~ 2 eV. Following Michel et al. [32] and Schaefer et al. [33], the high-BE peak can be attributed to O Si, the low-BE peak to an O Cs interaction. This is confirmed by the decreasing ratio of the low-BE peak intensity to the total O ls peak intensity, and the simultaneous increase of the ratio of the high-BE peak intensity to the total O ls peak intensity, with increasing q)o, as shown in Fig. 9b. This is a geometrical effect: since Cs atoms are much larger than O or Si atoms (cf. Table A.1), with increasing q)o an increasing number of O atoms interacts with an almost unchanging number of Cs atoms (we are at Cs saturation coverage): the number of bound Cs atoms per oxygen atom effectively decreases, whereas the increasing number of O atoms interacts with a similarly increasing number of Si atoms: the number of bound Si atoms per oxygen atom remains essentially the same. The contribution of the high-BE O Si peak to the O ls peak
R. Kroon, A. Sinke ,; Smjbce Science 399 (1998)284 296
291
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Fig. 9. (a) After Cs deposition the XPS O Is peak of oxidized Si(100) contains a shoulder on the low binding energy side, in contrast to the spectrum before Cs deposition (which is shown in the inset). (b) ( E ) ratio of the intensity of the low-BE peak to the total O ls peak intensity: (C;) ratio of the intensity of the high-BE peak to the total O Is peak intensity, as a function of the oxygen coverage of the Si substrate. Cs was deposited to saturation level.
Fig. 10. (a) Measured binding energy of the XPS Cs 3ds, 2 peak as a function of the oxygen coverage of the Si substrate. Cs was deposited to saturation level. (b) Full width at half maximum ( F W H M ) of the XPS Cs 3ds,e peak (corrected for the gaussian instrumental response function) as a function of the oxygen coverage of the Si substrate.
will therefore increase with respect to the lowBE O Cs peak. Both the binding energy and the linewidth {full width at half maximum, F W H M ) of the Cs 3ds/2 peak show a clear correlation with ~0o, which is shown in Figs. 10a and b, respectively. This is considered to result from vibrational excitation in the XPS process. (Citrin et al. have shown that effects of vibrational excitation can have a substantial influence on XPS line shapes of inorganic solids [34-36]: an increasing linewidth with increasing oxidation number was observed in the oxides and fluorides of AI, Mg and Cu, and also
in the halogenides of K. This was ascribed to vibrational excitation.) If the potential energy surfaces of the initial and final states of an XPS transition are displaced, vibrational excitation will accompany the XPS process as a result of the F r a n k - C o n d o n principle. The spectral width of the excited vibrational manifold, and hence of the XPS spectral line, increases with increasing binding strength of the adatom to the substrate [34 36]. In our case the binding strength of Cs increases with ~Oo, which explains the increasing linewidth of the Cs 3dsn peak with ~0o. The energy of the excited vibrational manifold also increases with increasing binding strength, which reduces the
292
R. Kroon. A. Sinke
,; SutJ~we Science 399 (1998) 284 296
kinetic energy of the photoelectrons. This explains the increasing BE of the Cs 3d5/2 peak with (Po (in alkali metals, vibrational excitation can induce a BE increase of several tenths o f e V [37]). A more common explanation of an increasing BE with increasing oxidation number (read: (Po) is in terms of an increasing charge transfer from Cs to the substrate [18]). However, as was mentioned earlier in this section, work function measurements [28] revealed the fractional charge of the Cs atoms to be (to a large extent) independent of substrate oxidation, so we believe that the charge-transfer mechanism will not substantially contribute to the observed shift in BE of the Cs 3d5/2 peak. In conclusion it is found that thermal desorption of Cs from oxidized Si(100) is governed by a broad distribution of the activation energy for thermal desorption. This reflects the considerable inhomogeneity of the microscopically rough Si surfaces. The weight of the distribution is found to shift to higher activation energy with increasing oxygen coverage of the Si substrates. The Cs desorption was monitored with X-ray photoelectron spectroscopy (XPS). The XPS spectra show evidence of vibrational broadening of the Cs 3d5/2 peak.
Acknowledgements We thank Peer Zalm for stimulating discussions. We thank the referee for his/her detailed and helpful comments on our manuscript. Appendix A: Determination and calibration of surface coverage with XPS In the experiments described in this paper the coverage of a Si substrate with cesium, oxygen, carbon and fluorine is determined with X-ray photoelectron spectroscopy. The basic principles of this technique are described in Ref. [18]. In this appendix we describe the determination of the surface coverage of Si from the XPS spectrum. We also describe its calibration for the case of Cs. A typical XPS-spectrum of a Si sample is shown in Fig. 1 l a, where we observe peaks due to Cs, O, C and Si. The small ls peak of fluorine, a residue of the hydrogen fluoride etching of the sample, is
6000
Cs 3d3/2 si 2s
~f) £z
Si 2p
Cs 5d5/2
5000
©
~"
4000
c
3000
Cs(MNN) 01s
C1:
2000
looo I 0
' 800
6;0
2;0
o
BindTng Energy (eV) (a)
A
\
•
~
aA
(b) Fig. I 1. (a) Typical example of an XPS spectrum of a Si substrate. One observes peaks due to coverage with Cs, O and C. In this case ¢cs= 0.47 ML, ~0o=0.43 ML and ~0c-0.09 ML. (b) Simple model of a substrate B covered by element A.
made invisible by the strong C s ( M N N ) Auger peaks (the fluorine peaks were measured before depositing Cs on the sample). The total area under a spectral line is proportional to the surface coverage by that specific element. A simple model of a substrate B covered with element A is shown in Fig. 1 lb. The figure depicts the situation @A< 1 ME, where fPA is the surface coverage in terms of actual physical layer of element A, i.e. the fraction of the surface area covered by this element. Photoelectrons generated in the substrate have to pass the (partial) layer of adatom A, which will thus attenuate the signal strength from the substrate. Taking this extinction exponential in the layer thickness of A [18], the signal of substrate B (area of the spectral line) is given by
IB=I~(1--(OA)+I~OAexp
[
2A(EB) COS :~ ' (A.1)
R. Kroon, A. Sinke / SutJ~we Science 399 (1998) 284 296
293
Table AI Values of variables relevant to determine the surface coverage of Cs, O, C and F on a Si substrate Variable
Cs 3ds, 2
C ls
Si 2p
F ls
O ls
Remarks
a [nm] E~ [eV]
0.534 529 22.93 3.68 5.43 9.21
0.182 969 1.00 0.99 1.08 1.99
0.264 1154 0.865 1.89 1.89 1.00
0.270 567 4.26 1.37 1.95 4.76
0.280 721 2.85 1.63 2.06 3.01
2 x Van der Waals radius MgK~ X-ray source Ref. [43] Ref. [38] Ref. [38] Eq. (A.5)
CrA(Ky.)/rrc(K9;} •;.A(EA ) [ n m ]
2A(Esi) [rim]
I'A~/I~2 p
where I ~ is the signal strength of a perfectly clean and infinitly thick sample B, aA is the diameter of adatom A, 2A(Eu) is the attenuation length of a photoelectron at kinetic energy EB in element A, and :~ is the angle of exit of the photoelectron with the normal to the surface. The signal of element A is described by Ig =IA PA - - I ~ 0 a exp
2A(EA) COS C~
Dividing Eq. (A.1) by Eq. (A.2) yields ~oa =
1 - exp 2 A ( E A ) COS ~
I~ I~
Ig I~°
a A
(A.3)
Concerning the variables in Eq. (A.3), Ia and IB are measured in the experiment whereas the values of the other variables are listed in Table 1. The attenuation length 2A(EB) is derived from the empirical formula of Seah and Dench [38]: ).A(EB)=O.41alA'5~B"5, where 2A and a a are expressed in nm, and EB is expressed in eV (we do not use the TTP-2M equations [39-42] to calculate 2A(EB), this will be discussed at the end of this appendix). The ratio of I ~ and I ~ is given by [18] I~ --
I~
a photoelectron from the relevant inner shell of atom A by a photon of energy hvJ. For ag we use the values given by Scofield for Mg K~ X-rays [43]. Since the XPS spectra are collected with the electron analyser in the so-called constant analyser energy (CAE) mode, all electrons strike the electron detector with the same kinetic energy which results in an identical value for D(EA) over the entire spectrum. Thus, D(EA) and D(EB) cancel in Eq. (A.4). Furthermore, nAocaT, 3 and T(EA)OCEA°'s (for our ESCAlab system), which leads to
a~)~A(EA)~aA(KZ) a3 2B(EB)~AACrB(K:O
Taking our experimental situation (:~=0) and filling the numbers of Table 1 into Eqs. (A.3) and (A.5) yields for Si(100): 1
~0cs=
1.243(]Si2p/ICs3dS;2)+ 0.0937'
(A.6)
1
=
~°° 0.476(isizp/ims)+O.127'
(A.7)
1
~°c= 0.334(Isizp/Icls) + 0.155'
nA2A(EA)T(EA)D(EA)rTA(hOJ ) =
(h.5)
(A.8)
1 ,
(A.4)
nB)~B(EB)T(EB)D(EB)6B(hOo )
where ng is the number density of material A (number of atoms per unit volume), T(EA) is the transmission of the electron analyser for electrons at kinetic energy EA, D(EA) is the sensitivity of the electron detector for electrons at kinetic energy EA, and aA is the cross-section for the emission of
-
~aF 0.852(isi2p/ivls)+O.129
,
(A.9)
in units of monolayer (ML). In the case of Cs we can actually calibrate the semi-emperical formula Eq. (A.6). To do this one needs to measure the XPS spectrum of a Si substrate with a welldefined Cs coverage. We fabricated such a sample by heating a HF-etched Si(100) substrate to a
R. Kroon, A. Sinke ,' Sut;jhce Science 399 (1998) 284 296
294
cq
. . . . . . . DD
1.00
o
hr3
Si
r-o 0.75 LD
[]
[]
[]
0.50 o
o © k_ o
o
0.00
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0
0.25
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l
,40
D
l i
[]
0.90
0.85
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o
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i 11ML Cs J
[]
1.00
0 (D L_ o
I
O0
[]
D
i
i
i
10
20
30
deposition
time
DDD[] i
40
[]
D [] i
0.80
50
(rain)
Fig. 12. Variation of the area of the Cs 3ds, 2 and Si 2p spectral lines during deposition of Cs on Si( 100)2 x 1. Signals are normalized to their maximum value. The variation of the Cs signal is indicative of the deposition process: at 1<6 min we observe the Cs source heating up, 6 < t < 2 6 rain shows a linear increase of Cs coverage in time at a deposition rate of 2.1 M L/h, whereas at t > 2 6 min the Cs coverage saturates at 1 ME (Ocs 0.5).
temperature of 800 900°C by direct resistive heating. At these temperatures the surface oxides evaporate (as SiO), leaving an oxygen-free 2 x 1-reconstructed Si(100) surface [20]. The reconstructed surface was preserved by cooling down at a rate of < 1 C s - t . Residual carbon was subsequently removed by ion milling with Ar ÷ after which the sample was again heated to T > 8 0 0 C and slowly cooled down. The resulting S i ( 1 0 0 ) 2 x l surface showed ~0o<0.01ML and (pc<0.01 ML. During the subsequent deposition of Cs these numbers did not change. At the S i ( 1 0 0 ) 2 x l surface the Si atoms are ordered in parallel dimer rows [44,45]. The Cs atoms deposited on this surface position themselves on top of the dimer rows until a saturation coverage of 0c~=0.5 is reached; one Cs atom per two substrate Si atoms [19], i.e. 3.39x1014Cs atoms cm 2 (the presence of Cs does not significantly alter the positions of the Si surface atoms [46,47]). Prolonged deposition of Cs, with the sample at room temperature, does not lead to an increase in Cs coverage, a result of the Cs sticking coefficient becoming negligible at room temperature when ~Ocsreaches 1 ML [19]. This proces is depicted in Fig. 12, where we plot the XPS signal of Cs on the Si(100)2 × 1 substrate as a function of deposition time. The Cs 3dsn signal is observed
to saturate after v 4 0 min, clearly the saturation coverage has been reached. This is also indicated in the same figure by the Si2p signal which decreases during deposition, due to attenuation of the photoelectrons fl'om the Si substrate by the increasing Cs coverage, but is found to saturate after .~:40 min of Cs deposition. In the literature it is debated wether the Cs atoms position themselves on top of the dimer rows (single layer model [19], 0cs=0.5) or also in between the dimer rows (double layer model [48,49], 0cs= 1.0), a discrepancy which could give rise to an error of factor 2 in our calibration. One can, however, experimentally verify wether a single or a double layer is deposited. As shown in Refs [21,30,50], during rapid Cs deposition (the Cs coverage saturates within a few minutes) on Si( 100)2 x 1 the work function first passes through a minimum at 0Cs=0.5, then it increases again until it saturates at 0cs= 1. We have verified that during our slow Cs deposition (of. Fig. 12) the work function saturates at a minimum value, together with the saturation of the Cs coverage (single layer). When we enhance the temperature of our Cs source, i.e. increase the Cs flux incident on our sample surface, we can indeed deposit more Cs on the surface than with the lower flux and the work function is observed to pass through a minimum, just as described in Refs [21,30,50]. The difference in behaviour lies in the well-known fact that the Cs atoms in between the dimer rows are less stable than those on top of the dime. rows [19,26,27]. When rapidly depositing Cs both the sites on top and in between the dime," rows will be occupied. Our slow deposition ensures that we only form the single layer of Cs atoms on top of the dimer rows. Thus, in the experiments described in this paper, we deposit a single layer of Cs. This can also be inferred from a comparison of Fig. 5 of this paper with Fig. 2 in Ref. [27]. The latter figure shows that EdeS decreases with increasing Cs coverage up to slightly more than single layer coverage, but then remains constant at higher coverage. Fig. 5 clearly shows that we are in the regime of single layer coverage. The relation between 0cs and ~0cs is given by OC s = ~)Cs[)Si(IOO' "I 2 CS]
1"
(A.10)
R. Kroon. A. S&ke Suffdce Science 399 (1998)284 296 where )'Si(lOO) is the n u m b e r density of the S i ( 1 0 0 ) 2 × l surface, i.e. the n u m b e r of surface atoms per unit area. ),si(loo1=6.78 x 1014 cm 2 [51 ]. Filling the saturation values o f Ics3as,,2 and Isi2p into Eqs. (A.3), (A.10), with ~=0 and Ocs=-0.5 ML, yields Ic~s3ds,a/Is~f2p =9.58. This is in very g o o d agreement with the semi-emperical value of 9.21, as obtained from Eq. (A.5) (cf. Table 1), indicating that the a p p r o a c h leading to Eqs. (A.6) (A.9) is reliable. Using the experimental value o f oc / vo Ics3d5:2: Isi2p : 9.58, Eq. (A.6) becomes 1 ~pcs=
.
(A.11)
1.294(Isi2p/Ic~3ds., ) + 0.093 7 In this paper we use Eq. ( A . I 1 ) to determine the cesium coverage of the HF-etched samples from the XPS spectrum. In the model described above, we use the attenuation length (AL))~A(E~), as calculated from the empirical formula o f Seah and Dench [38], to account for the extinction o f substrate photoelectrons by the (partial) layer o f adatoms. We do not use the, more recently developed, inelastic mean free path ( I M F P ) which is calculated from the T T P - 2 M equations of T a m u r a et al. [39-42]. The reason for this is twofold. Firstly, when using the A L the result of the Cs-calibration experiment, oo / oo Ic~3a~._,/1 Si2p = 9.58, is in very g o o d agreement with the value calculated from Eq. (A.5), which is 9.21. This is not the case when using the I M P F : 2 the calibration experiment then yields Ic~s3d~.~,,' IS~2v = 11.23, whereas Eq. (A.5) yields a value of 7.33. Secondly, not only is the Cs coverage correctly determined when using the AL, but also the oxygen coverage. This can be inferred from the experimental result shown in Fig. 3: the observed saturation o f ~0c~should occur at an oxygen coverage o f ~0o= 1 M L (as explained in the discussion section). This is indeed observed, within experimental accuracy. Strictly speaking, to determine ~0o, ~0c, and q)v (which are determined prior to Cs deposition) the model described above is valid in case the total 21MPF-values are IMPFc~(Ecs)-4.23 nm and lMPFc~(Esi)=7.79nm (calculated from TPP-2M using the following data for Cs: p=l.873gcm 3, N v - l , Eg=0, and M= 132.9054). and 1MPFsi(Esi)=2.73 nm [39].
295
surface coverage ~o= ~0o + (Pc + ~0v< 1 ML, within the situation that all a d a t o m s are lying alongside each other. The p < 1 M L-condition is met on all substrates, with the exception of the most strongly oxidized substrate (there ~ 0 = l . 3 7 M L , which induces a mild inaccuracy). In addition, on the oxidized Si surfaces described in this paper there may be some locations where the surface coverage is thicker than one atomic layer. Surface roughness will induce a slight inaccuracy (the surface roughness is ~ t w o Si layers; ~ 0 . 5 rim). Furthermore, the semi-emperical Eqs. ( A . 7 ) - ( A . 9 ) are not calibrated, but as stated above, the value of ~0o is correctly determined. We estimate the absolute accuracy of q)o, (Pc and Pv to be _+ 10%. The accuracy of ~0cs is estimated + 3 % , as a result of the calibration. In the simple model the Cs atoms form a single overlayer on the oxidized samples, with the oxides lying in between the Cs overlayer and the Si substrate. O n the oxidized surfaces we therefore correct ~0cs as determined from Eq. (A.11) for the attenuation of the substrate photoelectrons by the oxygen coverage. This is done by dividing Isizp in E q . ( A . 1 1 ) by the extinction factor exp[-do/)oo(Esi)], where do is the oxide thickness in nm and 2o(Esi)=2.06 n m (cf. Table A1 ). For the most strongly oxidized sample (~0o-- 1.15 ME), ~0Csas determined from Eq. (A.I 1 ) is corrected by - 15%.
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