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Al2O3(11̄ 02) interface
159
Surface Science 249 (1991) 159-170 North-Holland
Submonolayer
cluster formation at the @/Al
2O3(liO2) interface
Geoffrey P. Malafsky *J Electronics Science and Technology Division, Naval Research Laboratoty, Washingtan, DC 20375, USA Received 11 October 1990; accepted for publication 20 December 1990
The nucleation of clusters at the @m-substrate interface is the first step in film growth. The three principal growth modes, layer growth, &an&i-Krastanov, and Volmer-Weber, describe growth kinetics which are different regimes of tk competition between the surface kinetic processes of adsorption, diffusion, adatom captufe by a cluster, and desorption. This study probes the growth of interfacial clusters during molecular beam epitaxy of Ge on Al,4(1102). X-ray photoekctron spectroscopy (XPS) is used to monitor the increase in cluster size at submonolayer surface coverages of Ge for growth temperatures of 300 and 900 K. Ge has two binding states, Cre-Ge and Ge-A1,03, at all coverages for both temperatures. The Ge-Ge peak area reflects the concentration of surface clusters while the binding energy shift is r&ted to the average coordination of the atoms within the cluster. Clusters form from the onset of deposition at coverages of O.Ol-0.20 monolayers and increase in size rapidly with coverage, even at a growth temperatwe of 300 K. This low-coverage cluster formation is shown to be the product of the balance between the different surface kinetic processes.
The deposition of thin solid films is used in the fabrication of many applications such electronic devices, optical coatings and protective coatings [1,2]. The films can be deposited by several techniques such as chemical vapor deposition (CVD), molecular beam epitaxy (MBE), sputter deposition and evaporation [3]. ,Although each technique has certain technological advantages, the properties of the deposited film, e.g., morphology, crystallinity, defect density, and impurity ~~~ntr~tion, are determined by the growth chemistry. The growth mode of many systems can be described by three models of the kinetics of nucleation of adsorbate clusters and growth of the material film [4-61. These models differ by the relative bond strengths of the adsorbate-substrate interaction (adhesion) and the adsorbate-adsorbate interaction (cohesion). The layer-by-layer growth (LG) mode occurs when adhesion is * ONT/NRL Postdoctoral Fellow_ ’ Present address: Chemistry Division, Code 5110, Naval I&search Laboratory, Washington, DC, USA. ~39.~28/91/$03.5~
stronger than cohesion and proceeds by the completion of each monolayer (ML) before another layer begins. In contrast, the Volmer-Weber (VW) mode is characterized by larger cohesive forces than adhesive forces with three-dimensional adsorbate islands forming on the substrate. The third mode, Stranski-Krastanov (SK), is a combination of the other two since the first few layers grow in a layer-by-layer fashion while subsequent layers grow by three-dimensional islands. The actual growth mode depends upon the experimental parameters with the general trend being towards island growth for high temperatures and low deposition fluxes. However, the growth mode is also affected by perturbing factors such as surface defects, lattice strain, and impurities. The three growth models describe the macroscopic growth process. Ultimately, we would like to have an atom&tic model of the nucleation and growth mechanisms and the effect of the growth parameters and perturbing forces on these mochanisms. The growth mode is a product of the competition between surface kinetic reactions such as adsorption of the impineg atom at the variety of available surface sites, surface diffusion, nuclea-
0 19% - Ekevier Science Publishers B.V. (worth-boiled)
tion of a cluster, capture of the diffusing adatom by an existing cluster, and desorption of the adatom into the vacuum [4,5]. The growth parameters (eg., temperature, flux) and the perturbing factors (e.g., defects, strain, impurities) shift the balance of the kinetic processes and thereby alter the growth mode. The first step in film growth is the nucleation of clusters at the film-substrate interface by the agglomeration of adatoms diffusing across the surface. Therefore, the probability that au existing cluster will enlarge versus the formation of a new nucleation center depends upon the surface mobility of the adatom which is greater for high substrate temperatures and weak adsorbate-substrate bonds. Far example, the surface island density of the Au/N&l system is lower at high growth temperatures but the average island size is larger [7]. Elucidating the behavior of clusters at the interface is a fundamental step towards understanding nucleation and growth phenomena. In this study, I examined the growth of clusters at the interface during deposition of Ge on Al@, (liO2). f used X-ray photoelectron spectroscopy (XPS) to monitor the growth of clusters and the effect of the substrate temperature on the cluster growth. The Ge/Al,O, system is an example of the technologically important semiconductor on insulator (SOI) structure [S]. An insulating substrate provides several benefits to electronic devices such as a decrease in the parasitic capacitance and circuit cross-talk, and an increase in radiation hardness. Epitaxial films of Si f9-11 fF Ge [12], and InSb 1131 have been grown on sapphire substrates although these films have a high density of defects near the interface which is typical of hetero-epitaxial growth because of the disparate chemical and structural properties of the adsorbate and the substrate [14]. The prevailing growth mode on sapphire appears to be island growth by studies of Si(lOO)/Al,O, (liO2) [15] and Ge(llO)/AI,O, (1102) [16] by transmission electron microscopy (TEM) and Cu/polycrystalline Al@, by Auger electron spectroscopy (AES) f17f. However, the initial growth of dusters at sub-monolayer coverage was not addressed in these investigations. In this study, I utilized the high photoemission cross-section of Ge (approximately
fifty times greater than Si or Al [lg]) to follow the interfaeial cluster growth with XPS.
2. Experimental The experiments are conducted in a VG V80 Si MBE system consisting of a preparation chamber, a deposition chamber, and an analysis chamber 119]_The three chambers are maintained at ultrahigh vacuum (UHV) and are connected with a sample transport track which allows the sampfe to be transferred between the chambers under UHV. The base pressure of each chamber is less than 3 x lo- lo Torr and the base pressure of the deposition chamber falls to 5 x lO_” Torr when the liquid nitrogen shrouds are filled. The 75 mm diameter sapphire wafers are cut and polished by Union Carbide to within f 1” of the (1102) surface plane. The samples are chemically cleaned by a series of hot acid baths to remove organic and inorganic contamination (table 1) fl2]. After cleaning, the XPS spectra show F to be the only contaminant on the sapphire surface. The samples are annealed in the preparation chamber for thirty minutes at 1650 K by radiative heating. This treatment produces a surface with a sharp LEED pattern [10,20] and the lack of impurity peaks in the XPS spectra. The Ge is evaporated from an electron beam heated crucible at an approximate rate of 0.05 A/s
for each experiment. This rate is too low to
T&k 2 The chemical cleaning procedure of the sapphire wafers step
Solution
Temperature W
Time Win)
1 2 3 4 S 6 7 8 9 10 11
acetone methanol deionized water 1: 1 E&q, : HISO, DI water 4:I:I H20:HCI:H,0, DI water fO:l H,O:HF DI water 4:l:l H,o:HQrK,ui DI water
325 325 300 300 300 355 300 300 300 355 300
5 5 rinse I5 5 15 S 30 5 IS 5
G. P. Malafsky
/ Submonolayer cluter formation
be directly monitored by the quartz crystal microbalance (QCM) so the Ge deposition rate is determined from the accumulated thickness on the QCM over a period of four minutes. The Ge is deposited at growth temperatures of 300 and 900 K and the sample temperature is equilibrated for at least 15 minutes prior to deposition. The 900 K sample temperature is not a direct measurement since the thermocouple is connected to the heater block and not the sample. Consequently, the sample temperature is lower than the thermocouple reading and this difference is estimated to be 150 K (thermocouple reading of 1050 K). A new sample is used for each deposition at 900 K while several depositions are performed on each sample at 300 K. The reliability of the successive depositions at 300 K was checked by repeating several of the depositions directly onto a fresh sample. There was no significant difference between the XPS spectra of the two cases. The XPS spectra are acquired using AlKcw radiation (1486.6 eV) with an approximate anode power level of 450 W. The X-ray source is located 6 mm from the sample surface in order to maxi-
at the Ge/A120,(I
161
702) interface
mize the X-ray flux to the sample and to remove spurious signals originating from the Ta sample holder. The photoelectrons are collected in the direction normal to the sample surface and are focussed to the entrance of the hemispherical energy analyzer. The pass energy of the energy analyzer is 10 eV which yields a FWHM of the Ag 3d 5,2 line of 0.88 eV. The XPS spectra are taken in 0.1 eV steps and are signal averaged over five scans. The binding energy resolution is estimated to be f0.2 eV.
3. Results and discussion The Ge XPS spectra are shown in figs. 1 and 2 for substrate temperatures (Tub) of 300 and 900 K. The Ge2p,,, core level region and the LMM Auger transition region are displayed as the Ge coverage increases from 0.07 to 2.1 monolayers (ML) as measured by the QCM (e,,). 1 ML is taken to be equal to the Ge(ll0) interlayer spacing of 1.98 A [21]. The insulating sample electrically charges during the analysis causing the entire XPS
(b)
Ge-Ge
I
.I8
1215
1220 &ding
Energy
1225 (eV)
1230
330
335
340
Binding
345
Energy
350
355
360
(eV)
Fig. 1. The Ge XPS spectra for increasing surface coverages of Ge as measured by the QCM (e,,). The substrate temperature is 300 K. Each spectrum is normalized to the same (peak-baseline) height and is offset along the vertical axis. The peaks are labeled according to the probable bonding state of Ge. (a) 2p,,, region (b) LMM Auger region.
1210
1215
1220
Brndmg Energy
1225
1230
k%Vk
330
335
340
345
Brndrng Energy
350
360
(eVi
Fig. 2. Same as for fig. 1 except that the substrate temperature is 900 K. (a) 2~,,~, region (b) LMM Auger
spectrum to shift by g-fl V. Therefore, the binding energy scale is referenced to the sapphire AI 2p and the 01s peaks (74.7 and 531.6 eV, respectively [22]). The Al and 0 photoelectrons are not appreciably attenuated by the Ge overlayer because of their long attenuation lengths (15 and 8 A, respectively [23]). In addition, there is no difference in the shape nor the separation of the Al 2p and 0 1 s peaks between a clean sample and a &-coated sampfe. Consequently, the substrate peaks provide a stable reference value for the Ge peak positions. In fig. la, the Ge2p,,,, region (CJ1, = 300 K) exhibits two Ge binding states at ail coverages. The high binding energy peak position is 1220.7 eV and does not change with the Ge surface coverage. This peak position is 3.5 eV bigber than the bulk Ge peak position of 1217.2 eV [22] indicating that the oxidation state is Ge4+ [24) from GeO* on the A1,OX surface. This is the same bonding configuration proposed for the S~/A~~~~(Ii~2~ heteroepitaxial system on the basis of the greater bond strength of Si-0 relative to Si-AI 1251 and the proper epitaxiai arrangement which occurs when Si is substituted for Af in the A1,0, surface lattice ]9,26]. In addition, prefer-
355
region.
entiaf bonding tu oxygen on the sapphire surface was observed in the Nb/Al,U, (OOOI) system by XPS [27]. The peaks associated with the high binding energy state are labeled as Ge-sapphire in the figures. The Ge-sapphire Auger peak position of 348.8 eV (fig. lb) yields an Auger parameter (binding energy of core level + kinetic energy of Auger transition [28]) of 2358.5 eV. This vaiue of the Auger parameter agrees with the Auger parameter of GeO* of 2358.3 eV [29] and is a better indicator of the chemical environment than the core level binding energy because the Auger parameter cancels the shift from electrical charging and incorporates the additional binding energy shift of the Auger transition. The second Ge peak is from Ge-Ge bonding in surface clusters and has a position in the range 1218.1-1217.3 eV for the 2p,,, core level and 343.3-341.9 eV for the LMM transition. The electronic interaction of the adatoms with the surface causes the peak position to shift to higher binding energy at low surface coverages. This is typical of the growth of surface clusters [28] and will be discussed in more detail in a later section. The Ge-sapphire peak is less pronounced in the 2~,,~ spectra for the 900 K sub-
G. P. hfalafsky / Submonolayer cluster formation at the Ge/AI,O,(liO2)
strate temperature (fig. 2a) appearing only as a shoulder on the Ge-Ge peak and is completely obscured in the LMM spectra (fig. 2b). The growth of the surface clusters can be followed by the increase in the area of the Ge-Ge peak. Fig. 3 shows the change in the normalized peak area with surface coverage. The Ge coverage is given in terms of ML measured by the QCM and as an estimate of the actual surface coverage (&t). The two values differ as a result of the different sticking coefficients of Ge on the sapphire substrate and on the thick Ge film coating the QCM. esurl is calculated from the peak areas and the photoemission cross-sections of the Ge2p,,, and A12p core levels. The calculation procedure is described in the appendix. The Ge2p,,, peak areas are computed with a computer peak synthesis routine using a 75%Gaus-
sian/25%-Lorentzian lineshape for both the GeGe and the Ge-sapphire peaks. The peak areas are normalized to the Al2p peak area in each spectrum since the Al signal is negligibly attenuated by the sub-monolayer Ge film. The values of the relative sticking coefficients, 0,,,/13,,, are 0.051 and 0.013 for substrate temperatures of 300 and 900 K, respectively. The presence of the Ge-Ge peak at all coverages shows that the Ge clusters are forming at low surface coverage. The cluster signal increases with surface coverage for both substrate temperatures and the linear increase of the cluster signal at a Tsub of 300 K indicates that the sticking coefficient is not significantly changing within this range of eSurf. However, there is an increase in the slope of the Ge-Ge peak area versus Bsurf curve at a Tsub of 900 K suggesting that the sticking coefficient is
8 surf 0.000
0.025
0.050
0.075
163
interface
8 surf 0.100
0.125
0.000
0.025
0.050
0.075
0
300K
0
900K
0.100
0.125
2.0
2.5
2.5
0.0
0.5
Ge
1.0
1.5
Coverage ‘QCM
2.0
(ML)
2.5
0.0
0.5
Ge
1 .o
1.5
Coverage %a4
(ML)
Fig. 3. The Ge 2p,,, normalized peak area for the two substrate temperatures. The Ge surface coverage is given in ML measured by the QCM (f&,-,) and the calculated actual surface coverage (I&~). The coverage values for the 900 K substrate temperature are corrected for the difference in the sticking coefficients at 300 and 900 K (see appendix). The horizontal error bars represent the experimental uncertainty in the coverage and are shown for only one curve. The vertical error bars represent the uncertainty in the experimental measurement of the peak intensity and in the normalization procedure. (a) Ge-Ge bonding; (b) Ge-sapphire bonding.
increasing with Bsurf at this temperature. In contrast, fig. 3b shows that the Ge-sapphire peak area reaches a saturation value at low coverage ( @surfof approximately 0.03 and 0.01 ML for a Gub of 300 and 900 K, respectively). The Ge-sapphire peak area reflects the number of Ge adatoms which have a strong interaction with the sapphire surface. These strongly bound adatoms may be populating special surface sites with an enhanced adsorption energy such as step, kink, and ledge sites (i.e., defects) 141. In fact, there are a large number of defect sites on the annealed Al,O, surface [30,31]. The Ge-sapphire peak area saturates as these sites are filled with Ge adatoms. The relative amount of Ge adatoms agglomerated into clusters is given by the fraction of the Ge-Ge peak area to the total Ge2p,,, signal (fOc_oe). The Ge cluster fraction rises rapidly with increasing Ge coverage for both substrate temperatures (fig. 4). In fact, most ( fGe_ oe > 0.5) of the Ge adatoms are in clusters at a surface coverage (@,,,,) of less than 0.02 ML. This degree of clustering is not expected at such a low surface coverage, particularly for a substrate temperature of 300 K. The diffusion length of even a weakly chemisorbed adatom (activation energy of diffusion of 0.1 eV/atom) at 300 K is only a few atomic distances [4] and there is a low probability of adatoms encountering one another through thermally activated surface diffusion at such a low coverage. However, clustering at sub-monolayer coverage and low temperature has been observed in several systems with a range of adsorbate-substrate bond strengths, such as Si/Si [32], Cu/Si [33], and Au/GaAs f34]. It is possible that the diffusion length will be larger if the adatoms initially adsorb into a physisorbed precursor state. A weakly bound precursor is a transient intermediate state which explains anomalously low sticking coefficients in chemisorbed systems [35]. In addition, the weak adatom-substrate interaction of the precursor state can lead to an incomplete accommodation of the impinging atom’s kinetic energy upon adsorption resulting in a higher surface mobility. The cluster fo~ation can be explained without requiring the adatom to adsorb into a precursor state. The clusters form through the balance of
a surf 0.000
0.025
0.050
0.40
0.00
0.075
0
l-.AA-A 0.0
t 0.5
’
Ge
g ’ 1.0
‘ ’
0.100
300K
’
’
1.5
Coveraw
0.125
am QCM
’ ’ 2.0
r * 2.5
(ML1
Fig. 4. The fraction of the Ge-Ge peak area to the totai Ge2pj,* area, i.e., Ge-Ge peak area +Ge-sapphire peak area. The coverage values for the 900 K substrate temperature are corrected for the difference in the sticking coefficients at 300 and 900 K (see appendix). The error bars indicate the estimated uncertainty and are shown for only one curve.
surface kinetic mechanisms [4-71. The adsorbing atom diffuses across the surface until it finds either a stable bonding site on the substrate (nucleation site) or an existing cluster to which it can bond. The adatom will desorb back into the vacuum if it does not form a stable surface bond within its residence time on the surface. It is the interplay between these processes which determines the extent of interfacial clustering and the overall growth mode. The importance of desorption is demonstrated by the very low sticking coefficient of Ge on the sapphire surface at both substrate temperatures. The formation of clusters at low coverage is evident in numerical simulations of the growth kinetics of the Ag,/NaCl [4] and Au/NaCl [36] systems. In these simulations, the coupled kinetic equations are integrated and
the surface concentrations of clusters and monomers cakulated at each time step for several substrate temperatures. Even at a substrate temperature of 300 K, the surface cluster concentration surpasses the monomer concentration at a very low surface coverage (approximately JOY4 ML) even with an energy barrier of diffusion of 0.3 eV/atom. The balance of the kinetic processes is a function of the substrate temperature, the adatom-adatom interaction, and the adatom-substrate intera&ion. For example, raising the substrate temperature increases both the surface mobility and the desorption rate. The net result is the predominance of the surface cluster concentration at a lower total surface coverage. This effect occurs in both the simulation results and in this study through the more rapid increase of foe_oe with Bsurfat a substrate temperature of 900 K (fig. 4). The presence of surface defects will change the
kinetic balance (i.e., growth mode) by providing a greater concentration of stable nucleation sites [6,7]. In fact, deposition preferentially occurs at step sites in several systems [5-7,31,32]. The simulations assume that clusters of two atoms or more are stable and immobile although a recent model suggests that cluster mobility plays an important role in the growth kinetics [6]. Although the simulation results show the low coverage and low temperature clustering without invoking a precursor state, there is evidence of a precursor state in the SifSi system from the temperature dependence of the diffusion length measured with a scanning tunneling microscope @TM) 1321. The position of the Ge-Ge 2p,,, and LMM peaks shift to lower binding energy with incrcasing surface coverage (fig. 5). The peak positions approach the bulk Ge values with the 2p,,, peak attaining the bulk vahre for a substrate tempera-
0 surf 0.000
1219.0
0.025
5.050
0.075
0 surf 0.100
0.125
0.000 344.0
~.r.~~.~.l~~~*l~~‘~~“~’ .
(a)
-0 0
0.025 ’ * ’ 1 1
.
0.075 . ’ ’
0.050 1 ’
: (b)
3QOK WOK
0.100 / , ’ * ’ ’
0
0.3 25
300K
343.5
12 18.5 T 3
$ t
5
/TT
?j
343.0
6 1218.0
5
5
342.5
-2
-2
f
ia
342.0
341.5
Bulk
Bulk 341.0 1.0
5.5 Ge
1.5
Ccwefage
2.0
&&I
8
Fig- 5. The binding energy of the Ge-Ge
peak versus the Ge surface coverage. The coverage values for the 900 K substrate temperature are corrected Fez the difference in the sti&iug coefficieuts at 300 and 900 K (see appendix). The dashed line shows the position of the bulk Ge binding energy. (a) GeZp,,, peak; (b) Ge LMM Auger peak.
ture af 900 K. The total peak pasition shift is large and the LMM peaks (apfor both the 2p,,, proximately 1 eV) with the LMM peak shift larger because of the additional extra-atomic relaxation involved in the Auger transition [28]. This binding energy shift is typical of cluster growth and is caused by the change in the elecfronic environment of the atoms as the cluster size increases [28,37-40). The atoms experience electronic perturbations from the interaction with the substrate and their reduced coordination compared to bulk atams. The measured peak position is an average of the different electronic environments of the atoms in the cluster. For example, the binding energy of the photoelectrons will differ for surface atoms, edge atoms, and multilayer atoms [39]. As the cluster size increases, the influence of the low coordination atoms (e.g., edge atoms) diminishes and for muItilayered clusters the influence of surface atoms will diminish as the non-surface layers grow. The binding energy shift can be a result of either an initial or a final state effect [28,37]. The initial state effect refers to the change in the atom’s electr5nic structure prior to photoemission because of the different chemical environment. The final state effects include screening of the core hole remaining after photoemission and reflects the fact that the photoelectron energy is the difference between the final and the initial state energy levels and is not simply the energy of the electron state prior to photoemission. The extra-atomic relaxation energy from screening of the core hde is sensitive t5 the orbital interactions of the atom with its neighb5rs and is usually the only final state effect cctnsidered and can actually be Iarger than the chemical shift [28]. As the cluster size increases, the efficacy of the screening increases causing the binding energy to decrease. The extra-atomic relaxation energy can be found from the change in the Auger parameter as the cluster size increases and is given by 2 A&, = AE, -t- AE, = Aru,
(11
where E,, is the extra-atomic relaxation energy, is the kinetic energy of the Auger electron, E, is the binding energy of the core level elect+on, and QIis the Auger parameter. The Auger parameter is shown in fig. 6 as a function of surface E,
8 0.000 23630
7
0.025
0.050
surf
0.075 0
0.100
0.125
300K
1
Fig. 6. The Auger parameter of the Ge-Ge peak. The coverage values for the 900 K substrate temperature are corrected for the difference in the sticking coefficients at 300 and 900 K (see appendix). The position of the Auger parameter for bulk Ge is shown by the dashed line.
coverage. From eq. (I), the relaxation shift is 0.3 eV for a substrate temperature of 300 K in the coverage range 0.01-0.20 ML. This relaxation shift yields a chemical shift of 0.5 eV since the core ievel binding energy shift is the sum of the change in the core energy state and the relaxation energy 11281. The binding energy change with surface coverage reflects the nature of the changing electronic environment of the cluster and has been reported to be a linear relationship for several systems [28,37,40]. The binding energy shift of systems with a weak adatom-substrate interaction will be primarily determined by the adatom-adatom orbital interactions and will be proportianal to the average coordination number f37f. In contrast, the binding energy shift will be m5re c5mplex if there is a strong interaction between the orbitals of the
adatom and the substrate. The Ge-Gc Zp,, binding energy decreases rapidly with increasing coverage (fig. 5) reaching the bulk value for a cut, of 900 K at a esurfof 0.02 ML and neatly attaining the bulk value for a Tsubof 300 K at a 19~~~~ of 0.12 ML. The Ge orbitals may be strongly interacting with the Al,O, orbitals or the average coordination of the Ge atoms may be increasing at a faster rate than the surface coverage. Iiowever, it is unlikely that there is a strong Ge-Al@, interaction considering the indications of a weak chemisorbed state, i.e., high surface mobility and low sticking coefficient at a Tsubof 300 K. Consequently, the average coordination is increasing rapidly at low coverage even at a substrate temperature of 300 K. The decrease in the binding energy to the bulk value suggests that the clusters are multi-layered (3D) since the bulk value can only be reached after at least one ML is formed f37,39]. However, neither the LMM peak position (fig. 5b) nor the Auger parameter (fig. 6) attain the buIk value. This does not necessarily rule out the existence of 3D islands because the bulk band structure may not form until the average cluster size is very large [41,42]. The binding energy of the Ge-Ge 2p3,a peak is displayed in fig. 7 as a function of fGe_oe. In contrast to the nonlinear decrease of the binding energy with BsUrr,this figure shows a linear relationship indicating that the average coordination of the adatoms in the clusters is varying linearly with the fractional number of surface adatoms in clusters. In addition, the curves for the two temperatures overlap. ~n~uently~ the average coordination number depends upon the fractional cluster adatom concentration and not the surface coverage since S,,, , hence the surface cluster adatom concentration, differs by a factor of 4 at the two growth temperatures for the same value of fGe_oe (see the appendix). The relationship of the average cluster size (atoms/cluster) to the average coordination number is a function of the shape of the cluster which may not be constant throughout the range of f& if multilayered clusters form. However, the clusters arc larger but there arc fewer of them at a high temperature growth compared to a low grow& temperature [71=An equivalent value of fce_oe does not require the clusters
6
1.218.2
i
g
1217.8
6
1217.4
1217.0 0.00
0.25 Ge-Ge
Fig. 7. The binding
0.50
0.75
1.oo
Peak Area Fraction
energy versus the fractional ~--t=%/Jz
area of tile
peak.
to be the same size or to have the same coordination number since it only indicates that the number of adatoms in all clusters is the same. The independence of the average cluster coordination from the growth temperature may be a result of cluster dynamics (fragmentation, diffusion) during growth. In fact, a recent study of the Au/NaCI system proposed that the clusters have a significant mobility which varies with cluster size and that the surface diffusion of clusters plays a pivotal role in the growth kinetics f6f. Consequently, cluster dynamics may be influencing the growth kinetics by maintaining the most stable cluster size and average coordination number for each value of fo+oe during growth. It is interesting whether the kinetic simulations [4,35] can account for the correlation of the cluster size with fGe_oe at the two temperatures without invoking cluster dynamics. Also, it is worthwhile to note that there are other kinetic effects which may be important such as the forces controlling epitaxial growth since this clearly differs with growth temperature as can be seen by the ability to grow a single crystalline Ge epilayer at 900 K but not at 300 K 112).
4. Conclusion
Appendix
The MBE growth of Ge on A1,0,(1?02) proceeds by the formation of Ge clusters at submonolayer surface coverage. These clusters form at the low coverage of approximately 0.02 ML even at a growth temperature of 300 K. The XPS spectra show two binding states of Ge at all Ge-Ge bonding and Ge-sapphire coverages, bonding. The Ge-Ge peak is from Ge clusters while the Ge-sapphire peak is from GeO, surface bonding and may occur at defect sites which have an enhanced adsorption energy. The cluster formation is revealed by the increase in the area of the Ge-Ge peak relative to the Ge-sapphire peak area. The low coverage and low temperature agglomeration is a result of the balance of the surface kinetic processes of adsorption, diffusion, agglomeration, and desorption. The binding energy of the Ge-Ge peak shifts to lower binding energy as the surface coverage increases. This shift towards the bulk Ge value occurs as the average ordination number of the adatoms in the clusters increases and the screening of the core hole becomes more effective. The binding energy shift correlates with the relative fraction of the Ge-Ge peak to the total Ge signal irrespective of the growth temperature. The fractional area of the Ge-Ge peak is proportional to the number of adatoms in clusters and not necessarily to the average coordination number or the cluster size. However, the independence of the correlation from the growth temperature indicates that the average cluster coordination (and indirectly the cluster size) is proportional to the number of adatoms in clusters despite the difference in the kinetic processes at the two growth temperatures.
The actual surface coverage of Ge differs from the coverage measured by the QCM because of the different sticking coefficients of Ge on the two surfaces. The QCM is coated with a thick Ge film from prior experiments. An estimate of the true surface coverage ( Bsu,r) can be computed using the expertmental Ge 2p,,, and A12p peak areas and the theoretical phot~~ssion cross-sections of the orbitals. The intensity of the Ge peak is given by
Acknowledgements I am grateful for the financial assistance by the Office of Naval Technology through a Postdoctoral Fellows~p and the Naval Research Laboratory. Also, I would like to thank D.J. Godbey, ME. Twigg and H.L. Hughes for their support and insights into my work.
1431 I,, = f T( E, &?)a,, : c, e-ii-l)s’h, I=1
(A.11
where f is the incident flux of X-rays, T( E, Sz) is the instrument transmission function for electrons with a kinetic energy E and collected in a solid for photoemisangle Sz, ace is the cross-section sion, cr is the concentration of Ge in layer I, s is the interlayer spacing, X is the attenuation length of the photoelectrons, and the summation is over N monolayers of Ge. I only use sub-monolayer and therefore values of eocM in the calculation limit the summation to one ML which eliminates the exponential attenuation term. Thus, the Ge signal is given by Zoe =f
(A.2)
T(Ev fibwe_-
Similarly,
the Al intensity
is given by (A.31
/=I
where the parameters have the same meaning as in eq. (2) and the summation is over D layers from which the photoelectrons are emitted (a depth of several microns). The attenuation of the Al photoelectrons by the single Ge layer is small and is neglected. The surface concentration of Ge can be expressed as 'Ge
Ic;eTtEGe)aGe D = LIwA,)~A, f_lcA1*i c e-(/-l)s/h
(A-4)
The ratio of the cross-sections, uA,/ooe, is equal to 0.014 [18f. The transmission factor is proportional to E--1/z 144,451 yielding a value for T( EGe)/ T( EA,) of 0.43. The interlayer spacing of Al,O,
G. P. Malafs@
/ Submonolayer cluster formotion
Table 2 The calcuhued actual Ge surface coverage (&,,r,) versus the surface coverage measured by the QCM (BQCM)1 ML of Ge is assumed to contain 4X lOI atoms/c& (The relative sticking coefficient, s, is also fisted) @QCM (ML)
@,, (ML)
s = esUrl/flPCN
0.07 0.18 0.35 0.52
0.003 0.011 0.018 0.025
0.043 0.061 0.051 0.048 d = 0.051 f 0.008
(1102) is 2.23 A [21] and the attenuation length of the AI photoelectrons in A&O, is appro~mately 15 A [23f. The atomic layer concentration of A1203 (iio2) is approximately 4 x 1014 atoms/cm2 191. However, the value of the summation depends upon the assumed layer stoichiometry. For exam-
at the Ge/ A1,03(1 iO2) interface
169
for a layer pie, c,, is 1.6 x 1014 atoms/cm2 stoic~omet~ of A1203_ In contrast, the crystallographic layer sequence is Al-O-Al-O-O-Al[9] in which case c,, is either 4 x 1014 or 0 atoms/cm2. The summation value is nearly the same for both cases; the values are 1.1 X 1015 atoms/c& and 1.3 X 1015 atoms/cm*, respectively. Consequently, I use an average value of 1.2 X 1015 atoms/c&. The surface Ge concentration is calculated for a substrate temperature of 300 K and is listed in table 2. The average relative sticking coefficient, 0,,,,/@,, , is 0.051. The sticking coefficient at a I&,, of 900 K is found from the relative Ge2p signal at 900 K versus 300 K. Fig. 8 shows the normalized Ge2p,,,, signal for each temperature as @,, increases. The curves are fitted with linear least-squares lines with a slope ratio (m,O/m,,O) of 0.25. Therefore, the average sticking coefficient at a substrate temperature of 900 K is 0.013.
References
0.0
0.5
1.0
1.5
Ge Coverage
2.0
2.5
(ML)
%CM
Fig. 8. The normalized total Ge2p,,, peak area versus the surface coverage measured by the QCM ( BqcM). The horizontal error bars indicate the experimental uncertainty in the coverage measurement while the vertical error bars indicate the uncertainty from the experimental measurement of the intensity and in the normahxation procedure. The solid lines are least-squares lines fit to the data.
[l] CF. Powell, J.H. Oxfey and J.M. Blocher, Jr., Eds., Vapor Deposition (Wiley, New York, 1986). [2] Deposition Te&nologies for Films and Coatings, Eds. R.F. Buushab et al. (Noyes Publications, Park Ridge, NJ, 1982). [3] Semiconductor Materials and Process Technology Handbook, Ed. G.E. McGuire (Noyes Publications, P&k Ridge, NJ, 1988). 141 K. Reichfelt, Vacuum 38 (1988) 1083. 151 J.A. Venables and G.D.T. Spiller, in: Surface Mobilities on Solid Materials, Ed. V.T. Bit& NATO AS1 Series B 86 (Plenum Press, New York, 1983) p. 341. 161J.L. Robins, Appl. Surf. Sci. 33/34 (1988) 379. 171A.D. Gates and J.L. Robins, Surf. Sci. 191 (1987) 499. Is1 P.K. Vasudev, in: EpitaxiaI Silicon Technology, Ed. B.J. Baliga (Academic Press, New York, 1986) p. 233. A R. Nodder and I. Cadoff, Trans. Met. Sot. AIME, 233 (1965) 549. [LOI CC. Chang, J. Vat. Sci. Technot. 8 (1971) 500. [ill M.S. Abrahams, J.L. Hutch&on and G.R. Booker, Phys. Status Sohdi (a) 63 (1981) K3. WI D.J. Godbey, S.B. Quadri, M.E. Twigg and ED. Richmond, Appl. Phys. Lett. 54 (1989) 2449. I131 K.D. Jam&on, A. Bensaoula, A. Ignatiev, C.F. Huang and W.S. Chen, Appl. Phys. Lett. 54 (1989) 1916. The Technology and Physics of Molecular Beam Epitaxy, 1141 Ed. E.H.C. Parker (Plenum, New York, 1985). WI J.G. Pelligrino, E.D. Richmond and ME. Twigg, Proc. MRS Reno, Spring 1988.
170
G. P. Malafsky
/ Submonolayer cluster formation at the Ge/AI,O,(I
[16] D.J. Godbey and M.E. Twigg, in: Layered Structures-Heteroepitaxy, Superlattices, Strain and Metastability, Proc. MRS Fall 1989, 160 (1990) 505. (17) V. DiCastro and G. Polzonetti, Surf. Sci. 189/190 (1987) 1085. [18] J.H. Scofield, J. Electron. Spectrosc. Relat. Phenom. 8 (1976) 129. 1191 Vacuum Generators, Danvers, MA. 1201 W.J. Gignac, R.S. Williams and S.P. Kowalczyk, Phys. Rev. B 32 (1985) 237. [21] W.E. Lee and K.P.D. Lagerloff, J. Electron. Microsc. Technol. 2 (1985) 247. [22] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy (Perkin-Elmer, Eden Prairie, MN 1979). (231 F.L. Battye, J.G. Jenkin, J. Liesegang and R.C.G. Leckey, Phys. Rev. B 9 (1974) 2887. [24] D. Schmeisser, R.D. Schnell, A. Bogen, F.J. Himpsel, D. Rieger, G. Landgren and J.F. Morar, Surf. Sci. 172 (1986) 455. [25] CRC Handbook of Chemistry and Physics, Ed. R.C. Weast (CRC Press, Boca Raton, FL, 1980). 126) F.A. Ponce, Appl. Phys. Lett. 41 (1982) 371. (271 F.S. Ohuchi, J. Mater. Sci. Lett. 8 (1989) 1427. [28] W.F. Egelhoff Jr., Surf. Sci. Rep. 6 (1987) 253. [29] C.D. Wagner, Faraday Discuss. Chem. Sot. 60 (1975) 291. [30] R.C. Barrett and C.F. Quate, J. Vat. Sci. Technol. A 8 (1990) 400. [31] G.C. Ndubuisi, J. Liu and J.M. Cowley, in: Proc. 47th
102) interface
Meeting Electron Microscopy Society of America, Ed. G.W. Bailey (San Francisco Press, San Francisco, 1989) p. 544. [32] Y.W. MO, R. Kariotis, B.S. Swartzentraber, M.B. Webb and M.G. Lagally, J. Vat. Sci. Technol. A 8 (1990) 201. [33] St. Tosch and H. Neddermeyer, Surf. Sci. 211/212 (1989) 133. 1341 R.M. Feenstra, J. Vat. Sci. Technol. B 7 (1989) 925. [35] D.A. King, in: Chemistry and Physics of Solid Surfaces, Vol II, Ed. R. Vanselow (CRC Press, Boca Raton, FL, 1979). [36] K. Reichelt, B. Lampert and H.P. Siegers, Surf. Sci. 93 (1980) 159. [37] M.G. Mason, Phys. Rev. B 27 (1983) 748. [38] T.T.P. Cheung, Surf. Sci. 40 (1984) 151. [39] M. Salmeron, S. Ferrer, M. Jazzar and G.A. Somorjai, Phys. Rev. B 28 (1983) 1158. [40] A. Stenborg, 0. Bjomeholm, A. Nilsson, N. Martensson, J.N. Andersen and C. Wigren, Surf. Sci. 211/212 (1989) 470. [41] S.T. Lee, G. Apai, M.G. Mason, R. Benbow and Z. Hurych, Phys. Rev. B 23 (1981) 505. [42] H. Roulet, J.M. Mariot, G. Dufour and C.F. Hague, J. Phys. F 10 (1980) 1025. [43] M.P. Seah, in: Practical Surface Analysis, Eds. D. Briggs and M.P. Seah (Wiley, New York, 1983) p. 181. [44] A.E. Hughes and C.C. Phillips, Surf. Interface Anal. 5 (1982) 220. [45] M.P. Seah, Surf. interface Anal. 2 (1980) 222.
Surface Science 249 (1991) 159-170 North-Holland
Submonolayer
cluster formation at the @/Al
2O3(liO2) interface
Geoffrey P. Malafsky *J Electronics Science and Technology Division, Naval Research Laboratoty, Washingtan, DC 20375, USA Received 11 October 1990; accepted for publication 20 December 1990
The nucleation of clusters at the @m-substrate interface is the first step in film growth. The three principal growth modes, layer growth, &an&i-Krastanov, and Volmer-Weber, describe growth kinetics which are different regimes of tk competition between the surface kinetic processes of adsorption, diffusion, adatom captufe by a cluster, and desorption. This study probes the growth of interfacial clusters during molecular beam epitaxy of Ge on Al,4(1102). X-ray photoekctron spectroscopy (XPS) is used to monitor the increase in cluster size at submonolayer surface coverages of Ge for growth temperatures of 300 and 900 K. Ge has two binding states, Cre-Ge and Ge-A1,03, at all coverages for both temperatures. The Ge-Ge peak area reflects the concentration of surface clusters while the binding energy shift is r&ted to the average coordination of the atoms within the cluster. Clusters form from the onset of deposition at coverages of O.Ol-0.20 monolayers and increase in size rapidly with coverage, even at a growth temperatwe of 300 K. This low-coverage cluster formation is shown to be the product of the balance between the different surface kinetic processes.
The deposition of thin solid films is used in the fabrication of many applications such electronic devices, optical coatings and protective coatings [1,2]. The films can be deposited by several techniques such as chemical vapor deposition (CVD), molecular beam epitaxy (MBE), sputter deposition and evaporation [3]. ,Although each technique has certain technological advantages, the properties of the deposited film, e.g., morphology, crystallinity, defect density, and impurity ~~~ntr~tion, are determined by the growth chemistry. The growth mode of many systems can be described by three models of the kinetics of nucleation of adsorbate clusters and growth of the material film [4-61. These models differ by the relative bond strengths of the adsorbate-substrate interaction (adhesion) and the adsorbate-adsorbate interaction (cohesion). The layer-by-layer growth (LG) mode occurs when adhesion is * ONT/NRL Postdoctoral Fellow_ ’ Present address: Chemistry Division, Code 5110, Naval I&search Laboratory, Washington, DC, USA. ~39.~28/91/$03.5~
stronger than cohesion and proceeds by the completion of each monolayer (ML) before another layer begins. In contrast, the Volmer-Weber (VW) mode is characterized by larger cohesive forces than adhesive forces with three-dimensional adsorbate islands forming on the substrate. The third mode, Stranski-Krastanov (SK), is a combination of the other two since the first few layers grow in a layer-by-layer fashion while subsequent layers grow by three-dimensional islands. The actual growth mode depends upon the experimental parameters with the general trend being towards island growth for high temperatures and low deposition fluxes. However, the growth mode is also affected by perturbing factors such as surface defects, lattice strain, and impurities. The three growth models describe the macroscopic growth process. Ultimately, we would like to have an atom&tic model of the nucleation and growth mechanisms and the effect of the growth parameters and perturbing forces on these mochanisms. The growth mode is a product of the competition between surface kinetic reactions such as adsorption of the impineg atom at the variety of available surface sites, surface diffusion, nuclea-
0 19% - Ekevier Science Publishers B.V. (worth-boiled)
tion of a cluster, capture of the diffusing adatom by an existing cluster, and desorption of the adatom into the vacuum [4,5]. The growth parameters (eg., temperature, flux) and the perturbing factors (e.g., defects, strain, impurities) shift the balance of the kinetic processes and thereby alter the growth mode. The first step in film growth is the nucleation of clusters at the film-substrate interface by the agglomeration of adatoms diffusing across the surface. Therefore, the probability that au existing cluster will enlarge versus the formation of a new nucleation center depends upon the surface mobility of the adatom which is greater for high substrate temperatures and weak adsorbate-substrate bonds. Far example, the surface island density of the Au/N&l system is lower at high growth temperatures but the average island size is larger [7]. Elucidating the behavior of clusters at the interface is a fundamental step towards understanding nucleation and growth phenomena. In this study, I examined the growth of clusters at the interface during deposition of Ge on Al@, (liO2). f used X-ray photoelectron spectroscopy (XPS) to monitor the growth of clusters and the effect of the substrate temperature on the cluster growth. The Ge/Al,O, system is an example of the technologically important semiconductor on insulator (SOI) structure [S]. An insulating substrate provides several benefits to electronic devices such as a decrease in the parasitic capacitance and circuit cross-talk, and an increase in radiation hardness. Epitaxial films of Si f9-11 fF Ge [12], and InSb 1131 have been grown on sapphire substrates although these films have a high density of defects near the interface which is typical of hetero-epitaxial growth because of the disparate chemical and structural properties of the adsorbate and the substrate [14]. The prevailing growth mode on sapphire appears to be island growth by studies of Si(lOO)/Al,O, (liO2) [15] and Ge(llO)/AI,O, (1102) [16] by transmission electron microscopy (TEM) and Cu/polycrystalline Al@, by Auger electron spectroscopy (AES) f17f. However, the initial growth of dusters at sub-monolayer coverage was not addressed in these investigations. In this study, I utilized the high photoemission cross-section of Ge (approximately
fifty times greater than Si or Al [lg]) to follow the interfaeial cluster growth with XPS.
2. Experimental The experiments are conducted in a VG V80 Si MBE system consisting of a preparation chamber, a deposition chamber, and an analysis chamber 119]_The three chambers are maintained at ultrahigh vacuum (UHV) and are connected with a sample transport track which allows the sampfe to be transferred between the chambers under UHV. The base pressure of each chamber is less than 3 x lo- lo Torr and the base pressure of the deposition chamber falls to 5 x lO_” Torr when the liquid nitrogen shrouds are filled. The 75 mm diameter sapphire wafers are cut and polished by Union Carbide to within f 1” of the (1102) surface plane. The samples are chemically cleaned by a series of hot acid baths to remove organic and inorganic contamination (table 1) fl2]. After cleaning, the XPS spectra show F to be the only contaminant on the sapphire surface. The samples are annealed in the preparation chamber for thirty minutes at 1650 K by radiative heating. This treatment produces a surface with a sharp LEED pattern [10,20] and the lack of impurity peaks in the XPS spectra. The Ge is evaporated from an electron beam heated crucible at an approximate rate of 0.05 A/s
for each experiment. This rate is too low to
T&k 2 The chemical cleaning procedure of the sapphire wafers step
Solution
Temperature W
Time Win)
1 2 3 4 S 6 7 8 9 10 11
acetone methanol deionized water 1: 1 E&q, : HISO, DI water 4:I:I H20:HCI:H,0, DI water fO:l H,O:HF DI water 4:l:l H,o:HQrK,ui DI water
325 325 300 300 300 355 300 300 300 355 300
5 5 rinse I5 5 15 S 30 5 IS 5
G. P. Malafsky
/ Submonolayer cluter formation
be directly monitored by the quartz crystal microbalance (QCM) so the Ge deposition rate is determined from the accumulated thickness on the QCM over a period of four minutes. The Ge is deposited at growth temperatures of 300 and 900 K and the sample temperature is equilibrated for at least 15 minutes prior to deposition. The 900 K sample temperature is not a direct measurement since the thermocouple is connected to the heater block and not the sample. Consequently, the sample temperature is lower than the thermocouple reading and this difference is estimated to be 150 K (thermocouple reading of 1050 K). A new sample is used for each deposition at 900 K while several depositions are performed on each sample at 300 K. The reliability of the successive depositions at 300 K was checked by repeating several of the depositions directly onto a fresh sample. There was no significant difference between the XPS spectra of the two cases. The XPS spectra are acquired using AlKcw radiation (1486.6 eV) with an approximate anode power level of 450 W. The X-ray source is located 6 mm from the sample surface in order to maxi-
at the Ge/A120,(I
161
702) interface
mize the X-ray flux to the sample and to remove spurious signals originating from the Ta sample holder. The photoelectrons are collected in the direction normal to the sample surface and are focussed to the entrance of the hemispherical energy analyzer. The pass energy of the energy analyzer is 10 eV which yields a FWHM of the Ag 3d 5,2 line of 0.88 eV. The XPS spectra are taken in 0.1 eV steps and are signal averaged over five scans. The binding energy resolution is estimated to be f0.2 eV.
3. Results and discussion The Ge XPS spectra are shown in figs. 1 and 2 for substrate temperatures (Tub) of 300 and 900 K. The Ge2p,,, core level region and the LMM Auger transition region are displayed as the Ge coverage increases from 0.07 to 2.1 monolayers (ML) as measured by the QCM (e,,). 1 ML is taken to be equal to the Ge(ll0) interlayer spacing of 1.98 A [21]. The insulating sample electrically charges during the analysis causing the entire XPS
(b)
Ge-Ge
I
.I8
1215
1220 &ding
Energy
1225 (eV)
1230
330
335
340
Binding
345
Energy
350
355
360
(eV)
Fig. 1. The Ge XPS spectra for increasing surface coverages of Ge as measured by the QCM (e,,). The substrate temperature is 300 K. Each spectrum is normalized to the same (peak-baseline) height and is offset along the vertical axis. The peaks are labeled according to the probable bonding state of Ge. (a) 2p,,, region (b) LMM Auger region.
1210
1215
1220
Brndmg Energy
1225
1230
k%Vk
330
335
340
345
Brndrng Energy
350
360
(eVi
Fig. 2. Same as for fig. 1 except that the substrate temperature is 900 K. (a) 2~,,~, region (b) LMM Auger
spectrum to shift by g-fl V. Therefore, the binding energy scale is referenced to the sapphire AI 2p and the 01s peaks (74.7 and 531.6 eV, respectively [22]). The Al and 0 photoelectrons are not appreciably attenuated by the Ge overlayer because of their long attenuation lengths (15 and 8 A, respectively [23]). In addition, there is no difference in the shape nor the separation of the Al 2p and 0 1 s peaks between a clean sample and a &-coated sampfe. Consequently, the substrate peaks provide a stable reference value for the Ge peak positions. In fig. la, the Ge2p,,,, region (CJ1, = 300 K) exhibits two Ge binding states at ail coverages. The high binding energy peak position is 1220.7 eV and does not change with the Ge surface coverage. This peak position is 3.5 eV bigber than the bulk Ge peak position of 1217.2 eV [22] indicating that the oxidation state is Ge4+ [24) from GeO* on the A1,OX surface. This is the same bonding configuration proposed for the S~/A~~~~(Ii~2~ heteroepitaxial system on the basis of the greater bond strength of Si-0 relative to Si-AI 1251 and the proper epitaxiai arrangement which occurs when Si is substituted for Af in the A1,0, surface lattice ]9,26]. In addition, prefer-
355
region.
entiaf bonding tu oxygen on the sapphire surface was observed in the Nb/Al,U, (OOOI) system by XPS [27]. The peaks associated with the high binding energy state are labeled as Ge-sapphire in the figures. The Ge-sapphire Auger peak position of 348.8 eV (fig. lb) yields an Auger parameter (binding energy of core level + kinetic energy of Auger transition [28]) of 2358.5 eV. This vaiue of the Auger parameter agrees with the Auger parameter of GeO* of 2358.3 eV [29] and is a better indicator of the chemical environment than the core level binding energy because the Auger parameter cancels the shift from electrical charging and incorporates the additional binding energy shift of the Auger transition. The second Ge peak is from Ge-Ge bonding in surface clusters and has a position in the range 1218.1-1217.3 eV for the 2p,,, core level and 343.3-341.9 eV for the LMM transition. The electronic interaction of the adatoms with the surface causes the peak position to shift to higher binding energy at low surface coverages. This is typical of the growth of surface clusters [28] and will be discussed in more detail in a later section. The Ge-sapphire peak is less pronounced in the 2~,,~ spectra for the 900 K sub-
G. P. hfalafsky / Submonolayer cluster formation at the Ge/AI,O,(liO2)
strate temperature (fig. 2a) appearing only as a shoulder on the Ge-Ge peak and is completely obscured in the LMM spectra (fig. 2b). The growth of the surface clusters can be followed by the increase in the area of the Ge-Ge peak. Fig. 3 shows the change in the normalized peak area with surface coverage. The Ge coverage is given in terms of ML measured by the QCM and as an estimate of the actual surface coverage (&t). The two values differ as a result of the different sticking coefficients of Ge on the sapphire substrate and on the thick Ge film coating the QCM. esurl is calculated from the peak areas and the photoemission cross-sections of the Ge2p,,, and A12p core levels. The calculation procedure is described in the appendix. The Ge2p,,, peak areas are computed with a computer peak synthesis routine using a 75%Gaus-
sian/25%-Lorentzian lineshape for both the GeGe and the Ge-sapphire peaks. The peak areas are normalized to the Al2p peak area in each spectrum since the Al signal is negligibly attenuated by the sub-monolayer Ge film. The values of the relative sticking coefficients, 0,,,/13,,, are 0.051 and 0.013 for substrate temperatures of 300 and 900 K, respectively. The presence of the Ge-Ge peak at all coverages shows that the Ge clusters are forming at low surface coverage. The cluster signal increases with surface coverage for both substrate temperatures and the linear increase of the cluster signal at a Tsub of 300 K indicates that the sticking coefficient is not significantly changing within this range of eSurf. However, there is an increase in the slope of the Ge-Ge peak area versus Bsurf curve at a Tsub of 900 K suggesting that the sticking coefficient is
8 surf 0.000
0.025
0.050
0.075
163
interface
8 surf 0.100
0.125
0.000
0.025
0.050
0.075
0
300K
0
900K
0.100
0.125
2.0
2.5
2.5
0.0
0.5
Ge
1.0
1.5
Coverage ‘QCM
2.0
(ML)
2.5
0.0
0.5
Ge
1 .o
1.5
Coverage %a4
(ML)
Fig. 3. The Ge 2p,,, normalized peak area for the two substrate temperatures. The Ge surface coverage is given in ML measured by the QCM (f&,-,) and the calculated actual surface coverage (I&~). The coverage values for the 900 K substrate temperature are corrected for the difference in the sticking coefficients at 300 and 900 K (see appendix). The horizontal error bars represent the experimental uncertainty in the coverage and are shown for only one curve. The vertical error bars represent the uncertainty in the experimental measurement of the peak intensity and in the normalization procedure. (a) Ge-Ge bonding; (b) Ge-sapphire bonding.
increasing with Bsurf at this temperature. In contrast, fig. 3b shows that the Ge-sapphire peak area reaches a saturation value at low coverage ( @surfof approximately 0.03 and 0.01 ML for a Gub of 300 and 900 K, respectively). The Ge-sapphire peak area reflects the number of Ge adatoms which have a strong interaction with the sapphire surface. These strongly bound adatoms may be populating special surface sites with an enhanced adsorption energy such as step, kink, and ledge sites (i.e., defects) 141. In fact, there are a large number of defect sites on the annealed Al,O, surface [30,31]. The Ge-sapphire peak area saturates as these sites are filled with Ge adatoms. The relative amount of Ge adatoms agglomerated into clusters is given by the fraction of the Ge-Ge peak area to the total Ge2p,,, signal (fOc_oe). The Ge cluster fraction rises rapidly with increasing Ge coverage for both substrate temperatures (fig. 4). In fact, most ( fGe_ oe > 0.5) of the Ge adatoms are in clusters at a surface coverage (@,,,,) of less than 0.02 ML. This degree of clustering is not expected at such a low surface coverage, particularly for a substrate temperature of 300 K. The diffusion length of even a weakly chemisorbed adatom (activation energy of diffusion of 0.1 eV/atom) at 300 K is only a few atomic distances [4] and there is a low probability of adatoms encountering one another through thermally activated surface diffusion at such a low coverage. However, clustering at sub-monolayer coverage and low temperature has been observed in several systems with a range of adsorbate-substrate bond strengths, such as Si/Si [32], Cu/Si [33], and Au/GaAs f34]. It is possible that the diffusion length will be larger if the adatoms initially adsorb into a physisorbed precursor state. A weakly bound precursor is a transient intermediate state which explains anomalously low sticking coefficients in chemisorbed systems [35]. In addition, the weak adatom-substrate interaction of the precursor state can lead to an incomplete accommodation of the impinging atom’s kinetic energy upon adsorption resulting in a higher surface mobility. The cluster fo~ation can be explained without requiring the adatom to adsorb into a precursor state. The clusters form through the balance of
a surf 0.000
0.025
0.050
0.40
0.00
0.075
0
l-.AA-A 0.0
t 0.5
’
Ge
g ’ 1.0
‘ ’
0.100
300K
’
’
1.5
Coveraw
0.125
am QCM
’ ’ 2.0
r * 2.5
(ML1
Fig. 4. The fraction of the Ge-Ge peak area to the totai Ge2pj,* area, i.e., Ge-Ge peak area +Ge-sapphire peak area. The coverage values for the 900 K substrate temperature are corrected for the difference in the sticking coefficients at 300 and 900 K (see appendix). The error bars indicate the estimated uncertainty and are shown for only one curve.
surface kinetic mechanisms [4-71. The adsorbing atom diffuses across the surface until it finds either a stable bonding site on the substrate (nucleation site) or an existing cluster to which it can bond. The adatom will desorb back into the vacuum if it does not form a stable surface bond within its residence time on the surface. It is the interplay between these processes which determines the extent of interfacial clustering and the overall growth mode. The importance of desorption is demonstrated by the very low sticking coefficient of Ge on the sapphire surface at both substrate temperatures. The formation of clusters at low coverage is evident in numerical simulations of the growth kinetics of the Ag,/NaCl [4] and Au/NaCl [36] systems. In these simulations, the coupled kinetic equations are integrated and
the surface concentrations of clusters and monomers cakulated at each time step for several substrate temperatures. Even at a substrate temperature of 300 K, the surface cluster concentration surpasses the monomer concentration at a very low surface coverage (approximately JOY4 ML) even with an energy barrier of diffusion of 0.3 eV/atom. The balance of the kinetic processes is a function of the substrate temperature, the adatom-adatom interaction, and the adatom-substrate intera&ion. For example, raising the substrate temperature increases both the surface mobility and the desorption rate. The net result is the predominance of the surface cluster concentration at a lower total surface coverage. This effect occurs in both the simulation results and in this study through the more rapid increase of foe_oe with Bsurfat a substrate temperature of 900 K (fig. 4). The presence of surface defects will change the
kinetic balance (i.e., growth mode) by providing a greater concentration of stable nucleation sites [6,7]. In fact, deposition preferentially occurs at step sites in several systems [5-7,31,32]. The simulations assume that clusters of two atoms or more are stable and immobile although a recent model suggests that cluster mobility plays an important role in the growth kinetics [6]. Although the simulation results show the low coverage and low temperature clustering without invoking a precursor state, there is evidence of a precursor state in the SifSi system from the temperature dependence of the diffusion length measured with a scanning tunneling microscope @TM) 1321. The position of the Ge-Ge 2p,,, and LMM peaks shift to lower binding energy with incrcasing surface coverage (fig. 5). The peak positions approach the bulk Ge values with the 2p,,, peak attaining the bulk vahre for a substrate tempera-
0 surf 0.000
1219.0
0.025
5.050
0.075
0 surf 0.100
0.125
0.000 344.0
~.r.~~.~.l~~~*l~~‘~~“~’ .
(a)
-0 0
0.025 ’ * ’ 1 1
.
0.075 . ’ ’
0.050 1 ’
: (b)
3QOK WOK
0.100 / , ’ * ’ ’
0
0.3 25
300K
343.5
12 18.5 T 3
$ t
5
/TT
?j
343.0
6 1218.0
5
5
342.5
-2
-2
f
ia
342.0
341.5
Bulk
Bulk 341.0 1.0
5.5 Ge
1.5
Ccwefage
2.0
&&I
8
Fig- 5. The binding energy of the Ge-Ge
peak versus the Ge surface coverage. The coverage values for the 900 K substrate temperature are corrected Fez the difference in the sti&iug coefficieuts at 300 and 900 K (see appendix). The dashed line shows the position of the bulk Ge binding energy. (a) GeZp,,, peak; (b) Ge LMM Auger peak.
ture af 900 K. The total peak pasition shift is large and the LMM peaks (apfor both the 2p,,, proximately 1 eV) with the LMM peak shift larger because of the additional extra-atomic relaxation involved in the Auger transition [28]. This binding energy shift is typical of cluster growth and is caused by the change in the elecfronic environment of the atoms as the cluster size increases [28,37-40). The atoms experience electronic perturbations from the interaction with the substrate and their reduced coordination compared to bulk atams. The measured peak position is an average of the different electronic environments of the atoms in the cluster. For example, the binding energy of the photoelectrons will differ for surface atoms, edge atoms, and multilayer atoms [39]. As the cluster size increases, the influence of the low coordination atoms (e.g., edge atoms) diminishes and for muItilayered clusters the influence of surface atoms will diminish as the non-surface layers grow. The binding energy shift can be a result of either an initial or a final state effect [28,37]. The initial state effect refers to the change in the atom’s electr5nic structure prior to photoemission because of the different chemical environment. The final state effects include screening of the core hole remaining after photoemission and reflects the fact that the photoelectron energy is the difference between the final and the initial state energy levels and is not simply the energy of the electron state prior to photoemission. The extra-atomic relaxation energy from screening of the core hde is sensitive t5 the orbital interactions of the atom with its neighb5rs and is usually the only final state effect cctnsidered and can actually be Iarger than the chemical shift [28]. As the cluster size increases, the efficacy of the screening increases causing the binding energy to decrease. The extra-atomic relaxation energy can be found from the change in the Auger parameter as the cluster size increases and is given by 2 A&, = AE, -t- AE, = Aru,
(11
where E,, is the extra-atomic relaxation energy, is the kinetic energy of the Auger electron, E, is the binding energy of the core level elect+on, and QIis the Auger parameter. The Auger parameter is shown in fig. 6 as a function of surface E,
8 0.000 23630
7
0.025
0.050
surf
0.075 0
0.100
0.125
300K
1
Fig. 6. The Auger parameter of the Ge-Ge peak. The coverage values for the 900 K substrate temperature are corrected for the difference in the sticking coefficients at 300 and 900 K (see appendix). The position of the Auger parameter for bulk Ge is shown by the dashed line.
coverage. From eq. (I), the relaxation shift is 0.3 eV for a substrate temperature of 300 K in the coverage range 0.01-0.20 ML. This relaxation shift yields a chemical shift of 0.5 eV since the core ievel binding energy shift is the sum of the change in the core energy state and the relaxation energy 11281. The binding energy change with surface coverage reflects the nature of the changing electronic environment of the cluster and has been reported to be a linear relationship for several systems [28,37,40]. The binding energy shift of systems with a weak adatom-substrate interaction will be primarily determined by the adatom-adatom orbital interactions and will be proportianal to the average coordination number f37f. In contrast, the binding energy shift will be m5re c5mplex if there is a strong interaction between the orbitals of the
adatom and the substrate. The Ge-Gc Zp,, binding energy decreases rapidly with increasing coverage (fig. 5) reaching the bulk value for a cut, of 900 K at a esurfof 0.02 ML and neatly attaining the bulk value for a Tsubof 300 K at a 19~~~~ of 0.12 ML. The Ge orbitals may be strongly interacting with the Al,O, orbitals or the average coordination of the Ge atoms may be increasing at a faster rate than the surface coverage. Iiowever, it is unlikely that there is a strong Ge-Al@, interaction considering the indications of a weak chemisorbed state, i.e., high surface mobility and low sticking coefficient at a Tsubof 300 K. Consequently, the average coordination is increasing rapidly at low coverage even at a substrate temperature of 300 K. The decrease in the binding energy to the bulk value suggests that the clusters are multi-layered (3D) since the bulk value can only be reached after at least one ML is formed f37,39]. However, neither the LMM peak position (fig. 5b) nor the Auger parameter (fig. 6) attain the buIk value. This does not necessarily rule out the existence of 3D islands because the bulk band structure may not form until the average cluster size is very large [41,42]. The binding energy of the Ge-Ge 2p3,a peak is displayed in fig. 7 as a function of fGe_oe. In contrast to the nonlinear decrease of the binding energy with BsUrr,this figure shows a linear relationship indicating that the average coordination of the adatoms in the clusters is varying linearly with the fractional number of surface adatoms in clusters. In addition, the curves for the two temperatures overlap. ~n~uently~ the average coordination number depends upon the fractional cluster adatom concentration and not the surface coverage since S,,, , hence the surface cluster adatom concentration, differs by a factor of 4 at the two growth temperatures for the same value of fGe_oe (see the appendix). The relationship of the average cluster size (atoms/cluster) to the average coordination number is a function of the shape of the cluster which may not be constant throughout the range of f& if multilayered clusters form. However, the clusters arc larger but there arc fewer of them at a high temperature growth compared to a low grow& temperature [71=An equivalent value of fce_oe does not require the clusters
6
1.218.2
i
g
1217.8
6
1217.4
1217.0 0.00
0.25 Ge-Ge
Fig. 7. The binding
0.50
0.75
1.oo
Peak Area Fraction
energy versus the fractional ~--t=%/Jz
area of tile
peak.
to be the same size or to have the same coordination number since it only indicates that the number of adatoms in all clusters is the same. The independence of the average cluster coordination from the growth temperature may be a result of cluster dynamics (fragmentation, diffusion) during growth. In fact, a recent study of the Au/NaCI system proposed that the clusters have a significant mobility which varies with cluster size and that the surface diffusion of clusters plays a pivotal role in the growth kinetics f6f. Consequently, cluster dynamics may be influencing the growth kinetics by maintaining the most stable cluster size and average coordination number for each value of fo+oe during growth. It is interesting whether the kinetic simulations [4,35] can account for the correlation of the cluster size with fGe_oe at the two temperatures without invoking cluster dynamics. Also, it is worthwhile to note that there are other kinetic effects which may be important such as the forces controlling epitaxial growth since this clearly differs with growth temperature as can be seen by the ability to grow a single crystalline Ge epilayer at 900 K but not at 300 K 112).
4. Conclusion
Appendix
The MBE growth of Ge on A1,0,(1?02) proceeds by the formation of Ge clusters at submonolayer surface coverage. These clusters form at the low coverage of approximately 0.02 ML even at a growth temperature of 300 K. The XPS spectra show two binding states of Ge at all Ge-Ge bonding and Ge-sapphire coverages, bonding. The Ge-Ge peak is from Ge clusters while the Ge-sapphire peak is from GeO, surface bonding and may occur at defect sites which have an enhanced adsorption energy. The cluster formation is revealed by the increase in the area of the Ge-Ge peak relative to the Ge-sapphire peak area. The low coverage and low temperature agglomeration is a result of the balance of the surface kinetic processes of adsorption, diffusion, agglomeration, and desorption. The binding energy of the Ge-Ge peak shifts to lower binding energy as the surface coverage increases. This shift towards the bulk Ge value occurs as the average ordination number of the adatoms in the clusters increases and the screening of the core hole becomes more effective. The binding energy shift correlates with the relative fraction of the Ge-Ge peak to the total Ge signal irrespective of the growth temperature. The fractional area of the Ge-Ge peak is proportional to the number of adatoms in clusters and not necessarily to the average coordination number or the cluster size. However, the independence of the correlation from the growth temperature indicates that the average cluster coordination (and indirectly the cluster size) is proportional to the number of adatoms in clusters despite the difference in the kinetic processes at the two growth temperatures.
The actual surface coverage of Ge differs from the coverage measured by the QCM because of the different sticking coefficients of Ge on the two surfaces. The QCM is coated with a thick Ge film from prior experiments. An estimate of the true surface coverage ( Bsu,r) can be computed using the expertmental Ge 2p,,, and A12p peak areas and the theoretical phot~~ssion cross-sections of the orbitals. The intensity of the Ge peak is given by
Acknowledgements I am grateful for the financial assistance by the Office of Naval Technology through a Postdoctoral Fellows~p and the Naval Research Laboratory. Also, I would like to thank D.J. Godbey, ME. Twigg and H.L. Hughes for their support and insights into my work.
1431 I,, = f T( E, &?)a,, : c, e-ii-l)s’h, I=1
(A.11
where f is the incident flux of X-rays, T( E, Sz) is the instrument transmission function for electrons with a kinetic energy E and collected in a solid for photoemisangle Sz, ace is the cross-section sion, cr is the concentration of Ge in layer I, s is the interlayer spacing, X is the attenuation length of the photoelectrons, and the summation is over N monolayers of Ge. I only use sub-monolayer and therefore values of eocM in the calculation limit the summation to one ML which eliminates the exponential attenuation term. Thus, the Ge signal is given by Zoe =f
(A.2)
T(Ev fibwe_-
Similarly,
the Al intensity
is given by (A.31
/=I
where the parameters have the same meaning as in eq. (2) and the summation is over D layers from which the photoelectrons are emitted (a depth of several microns). The attenuation of the Al photoelectrons by the single Ge layer is small and is neglected. The surface concentration of Ge can be expressed as 'Ge
Ic;eTtEGe)aGe D = LIwA,)~A, f_lcA1*i c e-(/-l)s/h
(A-4)
The ratio of the cross-sections, uA,/ooe, is equal to 0.014 [18f. The transmission factor is proportional to E--1/z 144,451 yielding a value for T( EGe)/ T( EA,) of 0.43. The interlayer spacing of Al,O,
G. P. Malafs@
/ Submonolayer cluster formotion
Table 2 The calcuhued actual Ge surface coverage (&,,r,) versus the surface coverage measured by the QCM (BQCM)1 ML of Ge is assumed to contain 4X lOI atoms/c& (The relative sticking coefficient, s, is also fisted) @QCM (ML)
@,, (ML)
s = esUrl/flPCN
0.07 0.18 0.35 0.52
0.003 0.011 0.018 0.025
0.043 0.061 0.051 0.048 d = 0.051 f 0.008
(1102) is 2.23 A [21] and the attenuation length of the AI photoelectrons in A&O, is appro~mately 15 A [23f. The atomic layer concentration of A1203 (iio2) is approximately 4 x 1014 atoms/cm2 191. However, the value of the summation depends upon the assumed layer stoichiometry. For exam-
at the Ge/ A1,03(1 iO2) interface
169
for a layer pie, c,, is 1.6 x 1014 atoms/cm2 stoic~omet~ of A1203_ In contrast, the crystallographic layer sequence is Al-O-Al-O-O-Al[9] in which case c,, is either 4 x 1014 or 0 atoms/cm2. The summation value is nearly the same for both cases; the values are 1.1 X 1015 atoms/c& and 1.3 X 1015 atoms/cm*, respectively. Consequently, I use an average value of 1.2 X 1015 atoms/c&. The surface Ge concentration is calculated for a substrate temperature of 300 K and is listed in table 2. The average relative sticking coefficient, 0,,,,/@,, , is 0.051. The sticking coefficient at a I&,, of 900 K is found from the relative Ge2p signal at 900 K versus 300 K. Fig. 8 shows the normalized Ge2p,,,, signal for each temperature as @,, increases. The curves are fitted with linear least-squares lines with a slope ratio (m,O/m,,O) of 0.25. Therefore, the average sticking coefficient at a substrate temperature of 900 K is 0.013.
References
0.0
0.5
1.0
1.5
Ge Coverage
2.0
2.5
(ML)
%CM
Fig. 8. The normalized total Ge2p,,, peak area versus the surface coverage measured by the QCM ( BqcM). The horizontal error bars indicate the experimental uncertainty in the coverage measurement while the vertical error bars indicate the uncertainty from the experimental measurement of the intensity and in the normahxation procedure. The solid lines are least-squares lines fit to the data.
[l] CF. Powell, J.H. Oxfey and J.M. Blocher, Jr., Eds., Vapor Deposition (Wiley, New York, 1986). [2] Deposition Te&nologies for Films and Coatings, Eds. R.F. Buushab et al. (Noyes Publications, Park Ridge, NJ, 1982). [3] Semiconductor Materials and Process Technology Handbook, Ed. G.E. McGuire (Noyes Publications, P&k Ridge, NJ, 1988). 141 K. Reichfelt, Vacuum 38 (1988) 1083. 151 J.A. Venables and G.D.T. Spiller, in: Surface Mobilities on Solid Materials, Ed. V.T. Bit& NATO AS1 Series B 86 (Plenum Press, New York, 1983) p. 341. 161J.L. Robins, Appl. Surf. Sci. 33/34 (1988) 379. 171A.D. Gates and J.L. Robins, Surf. Sci. 191 (1987) 499. Is1 P.K. Vasudev, in: EpitaxiaI Silicon Technology, Ed. B.J. Baliga (Academic Press, New York, 1986) p. 233. A R. Nodder and I. Cadoff, Trans. Met. Sot. AIME, 233 (1965) 549. [LOI CC. Chang, J. Vat. Sci. Technot. 8 (1971) 500. [ill M.S. Abrahams, J.L. Hutch&on and G.R. Booker, Phys. Status Sohdi (a) 63 (1981) K3. WI D.J. Godbey, S.B. Quadri, M.E. Twigg and ED. Richmond, Appl. Phys. Lett. 54 (1989) 2449. I131 K.D. Jam&on, A. Bensaoula, A. Ignatiev, C.F. Huang and W.S. Chen, Appl. Phys. Lett. 54 (1989) 1916. The Technology and Physics of Molecular Beam Epitaxy, 1141 Ed. E.H.C. Parker (Plenum, New York, 1985). WI J.G. Pelligrino, E.D. Richmond and ME. Twigg, Proc. MRS Reno, Spring 1988.
170
G. P. Malafsky
/ Submonolayer cluster formation at the Ge/AI,O,(I
[16] D.J. Godbey and M.E. Twigg, in: Layered Structures-Heteroepitaxy, Superlattices, Strain and Metastability, Proc. MRS Fall 1989, 160 (1990) 505. (17) V. DiCastro and G. Polzonetti, Surf. Sci. 189/190 (1987) 1085. [18] J.H. Scofield, J. Electron. Spectrosc. Relat. Phenom. 8 (1976) 129. 1191 Vacuum Generators, Danvers, MA. 1201 W.J. Gignac, R.S. Williams and S.P. Kowalczyk, Phys. Rev. B 32 (1985) 237. [21] W.E. Lee and K.P.D. Lagerloff, J. Electron. Microsc. Technol. 2 (1985) 247. [22] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy (Perkin-Elmer, Eden Prairie, MN 1979). (231 F.L. Battye, J.G. Jenkin, J. Liesegang and R.C.G. Leckey, Phys. Rev. B 9 (1974) 2887. [24] D. Schmeisser, R.D. Schnell, A. Bogen, F.J. Himpsel, D. Rieger, G. Landgren and J.F. Morar, Surf. Sci. 172 (1986) 455. [25] CRC Handbook of Chemistry and Physics, Ed. R.C. Weast (CRC Press, Boca Raton, FL, 1980). 126) F.A. Ponce, Appl. Phys. Lett. 41 (1982) 371. (271 F.S. Ohuchi, J. Mater. Sci. Lett. 8 (1989) 1427. [28] W.F. Egelhoff Jr., Surf. Sci. Rep. 6 (1987) 253. [29] C.D. Wagner, Faraday Discuss. Chem. Sot. 60 (1975) 291. [30] R.C. Barrett and C.F. Quate, J. Vat. Sci. Technol. A 8 (1990) 400. [31] G.C. Ndubuisi, J. Liu and J.M. Cowley, in: Proc. 47th
102) interface
Meeting Electron Microscopy Society of America, Ed. G.W. Bailey (San Francisco Press, San Francisco, 1989) p. 544. [32] Y.W. MO, R. Kariotis, B.S. Swartzentraber, M.B. Webb and M.G. Lagally, J. Vat. Sci. Technol. A 8 (1990) 201. [33] St. Tosch and H. Neddermeyer, Surf. Sci. 211/212 (1989) 133. 1341 R.M. Feenstra, J. Vat. Sci. Technol. B 7 (1989) 925. [35] D.A. King, in: Chemistry and Physics of Solid Surfaces, Vol II, Ed. R. Vanselow (CRC Press, Boca Raton, FL, 1979). [36] K. Reichelt, B. Lampert and H.P. Siegers, Surf. Sci. 93 (1980) 159. [37] M.G. Mason, Phys. Rev. B 27 (1983) 748. [38] T.T.P. Cheung, Surf. Sci. 40 (1984) 151. [39] M. Salmeron, S. Ferrer, M. Jazzar and G.A. Somorjai, Phys. Rev. B 28 (1983) 1158. [40] A. Stenborg, 0. Bjomeholm, A. Nilsson, N. Martensson, J.N. Andersen and C. Wigren, Surf. Sci. 211/212 (1989) 470. [41] S.T. Lee, G. Apai, M.G. Mason, R. Benbow and Z. Hurych, Phys. Rev. B 23 (1981) 505. [42] H. Roulet, J.M. Mariot, G. Dufour and C.F. Hague, J. Phys. F 10 (1980) 1025. [43] M.P. Seah, in: Practical Surface Analysis, Eds. D. Briggs and M.P. Seah (Wiley, New York, 1983) p. 181. [44] A.E. Hughes and C.C. Phillips, Surf. Interface Anal. 5 (1982) 220. [45] M.P. Seah, Surf. interface Anal. 2 (1980) 222.