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A low energy electron diffraction study of the PH3 adsorption on the Si (111) surface
SURFACE
SCIENCE 8 (1967) 381-398 o North-Holland
A LOW ENERGY
ELECTRON
PH, ADSORPTION
Publishing Co., Amsterdam
DIFFRACTION
STUDY
OF THE
ON THE Si (111) SURFACE
A. J. VAN BOMMEL and F. MEYER Philips Research Laboratories, N. V. Philips’ Gloeilampenfabrieken, Eindhoven, The Netherlands Received 18 April 1967 The reaction of PHa with a clean silicon (111) surface was studied by low energy electron diffraction (LEED). A number of diffraction patterns were observed in the temperature range of 251000°C using PHs pressures of 5 x 10-s to 1O-5 Torr. The LEED pattern of one of them, the P-6 2/3 structure, could be interpreted by multiple diffraction. The structure appears to be a densely packed phosphorus layer on top of the surface. The activation energy for desorption of the layer was calculated. For the other patterns no structure models could be proposed with certainty. The correlation with adsorption measurements on powders is discussed and the LEED results of reactions of other hydrides with silicon and germanium (111) surfaces are mentioned.
1. Introduction
Very recently Boonstra reported the results of gas adsorption of hydrides on clean germaniumr) and silicons)powders using volumetric measurements. The hydrides were of the general formula H,A, e.g. HCI, HBr; H20, H,S, H,Se; NH,, PH,, ASH,. At room temperature a fast adsorption took place to an extent of one molecule H,A per 2x surface atoms, which amount could not be pumped off. Heating in vacuum to 300 “C for germanium or, to 500 “C for silicon removed all the hydrogen from the adsorbent. Cooling to room temperature and new exposure to the gas resulted in adsorption of approximately half the original amount. Extrapolation of these results will give a final coverage of one atom A per x surface atoms after repeated cycles. It was suggested that a dissociative adsorption of H,A takes place, where each hydrogen atom is separately bound to a surface atom and A to a number of surface atoms according to its valency. This model gives a complete compensation of the free bonds because the exposed surface of the powders, which were prepared by crushing a single crystal, consists mainly of (111) surfaces a) having one free bond per surface atom. If we assume that the adsorption characteristics for a large single crystal surface are the same as for a powder, then we can use these data on 381
382 powders
A. J. VAN
for the interpretation
BOMMEL
AND
F. MEYER
of low energy
electron
diffraction
(LEED)
patterns hydride
which are obtained after gas adsorption on the (111) surfaces. The adsorption is interesting from a LEED point of view, because it
appears
to be a convenient
way to deposit certain atoms, e.g. S, Se, N, P, As
on the surface. We reported a study of H,S and H,Se adsorption on Ge (11 1)4), where a model was proposed in accordance with the results of adsorption measurements on powders. In this paper we wish to report on the reaction of PH3 with the Si (111) surface. Measurements on powders indicate that the hydrogen is completely removed at temperatures higher than 500 “C and this study is mainly concerned with the surface structures of phosphorus on Si (111). LEED studies of phosphorus adsorption on Si (111)s) and diamond (111)s) surfaces have been reported by Lander and Morrison.
2. Experimental The silicon samples used in this study were cut from a high-ohmic crystal. They were ground parallel to the (111) plane (deviation less than 2’) mechanically polished and finally etched in a solution of 400 mg KMnO, in 25 cm3 HF 50%. The dimensions of the samples were approximately 15 x 6 x 0.6 mm. The samples were clamped in tantalum strips and cleaned in high vacuum (< 10m9 Torr) by heating to temperatures up to 1200 “C. Heating was achieved by passing a current through the crystal. Crystal temperatures higher than 700 “C were measured by means of an optical pyrometer. The emission coefficients for silicon are known from the work of Allen’). An I.R. pyrometer, which was calibrated against the normal optical pyrometer in the overlap region, was used for temperatures between 500 “C and 700 “C. Temperatures below 500 “C measured with the I.R. pyrometer cannot be trusted, as was shown recently by Jonas). LEED measurements were taken with a Varian post-acceleration type apparatus and spot-intensities
were measured
with a spot-photometer.
3. Results 3.1. CLEAN Si (111) SURFACE The clean annealed Si (111) surface has been studied extensively by LEEDS? 9, la). Si (11 l)-7 appeared to be representative for the clean surface, and this structure was the starting point for our investigations. It should be noted here that we have observed also the Si (11 l)-,/19 structure. This structure, observed already by Schlier and Farnsworthg) in 1959, was originally interpreted as a clean-surface structure (indeed giving the same
PHI
ADSORPTION
ON
Si (111)
383
SURFACE
reactions with PH3 at temperatures higher than 500 “C). Closer examination, however, proved that the ,/19 structure is due tonickel contaminationll). 3.2. PHJ
ADSORPTION
As a result of the reaction of PHI, with the Si (111) surface at different temperatures, a number of surface structures was observed. Of all these structures the intensities (I) of three integral spots, the 00, 10 and 01 were measured as a function of electron voltage (V). We have chosen the 10 beam in the crystallographic [l 121 azimuth and the 01 beam in the [i2i] azimuth. Because of the threefold symmetry of the LEED patterns, the 10 and 01 represent the first hexagon of integral spots around the 00. We did not try to resolve the surface structures from these data, but are merely using them as “fingerprints”. In general it may be said that different Zvs. Vplots indicate different structures although the reverse is not necessarily true. The clean annealed Si (11 l)-7 surface was exposed at room temperature to PH3 pressures of lo-’ to 10V5 Torr. Within a few minutes the intensity distribution of the non-integral beams changed notably. Photographs of the LEED patterns are given in fig. 1. This can be interpreted as an ordered adsorption of PHJ, the Si (111)-P-7 structure. The Z vs. V plots for the 00, 01 and 10 were only slightly different from those of the clean structure. The P-7 structure was stable in vacuum at room temperature. Heating
(4
Fig. 1.
0.1
-
@I
LEED patterns of Si (111) surface structures. (a) Si (11 I)-7-clean at 59 V. (b) Si (111)-P-7 at 59 V.
384
A. J. VAN
BOMMEL
AND
F. MEYER
to 500 “C showed a sudden pressure rise in the LEED chamber, which is probably due to desorbed hydrogen. All non-integral beams disappeared, but above 80 V, the pattern showed vague streaks, which appeared to be remains of the 7-structure. The Z vs. V plots of this Si (111)-P-l pattern differed in some respects from those of the clean and P-7 patterns. Two more cycles of adsorption at room temperature, followed by heating in vacuum gave a P-l pattern without any streaks. Examination of the Zvs. V plots showed that the differences with the 7-patterns had become more pronounced and a new peak in the 10 beam at 80 V became much more prominent at higher phosphorus coverages. Photographs of these Si (11 l)P-l LEED patterns are given in fig. 2. Heating the crystal in PH, at 500 “C also gave a P-l pattern and the Z vs. V plots were very similar to those of
(4
\
(b)
Fig. 2. LEED patterns of Si (111) surface structures. (a) Si (11 I)-7-clean at 80 V. (b) Si (111)-P-l at 80 V. Low phosphorus coverage. (c) Si (111)-P-l at 80 V. High phosphorus coverage.
PH3
ADSORPTION
ON
Si(l11)
385
SURFACE
a P-l pattern obtained by a few cycles of adsorption at room temperature followed by heating in vacuum. From a LEED point of view, a complete l-structure can only be explained by an integral number of adsorbed atoms per surface atom and the simplest solution is to assume that one monolayer of phosphorus atoms replaces the surface silicon atoms in their trivalent positions. The monolayer is defined here as a coverage of one adsorbed atom per surface atom. Powder measurements, however, indicate a coverage of + of a monolayer under what seems to be the same experimental conditions. In such a case one would expect a surface structure with a larger unit mesh, for instance a P-,/3 structure, as was observed for phosphorus adsorption on diamond (111)s).
(a)
(‘4 \
(4
(4
Fig. 3. LEED patterns of Si (111) surface structures. (a) Si (111)-P-6 d3 at 40 V. (b) Si (111)-P-6 43 at 40 V. Crystal rotated over 15”. (c) Si (111)-P-6 2/3 at 80 V. (d) Si (11 l)P-2 2/3 at 45 V.
386
A. J. VAN
BOMMEL
AND
F. MEYER
The reason why such a surface structure is not observed on silicon must be sought in a disorder in the phosphorus coverage, although there is no support for this from background intensity. If the l-structure corresponds to a monolayer of phosphorus, then one would have to assume that the phosphine does not desorb completely from the apparatus walls and crystal supports, even after two days of pumping, and that, during the short crystal heating, adsorption of phosphorus takes place by decomposition of this desorbed phosphine. It is not possible to decide from our data which view is the more likely. 3.3. p-T DIAGRAM OF THE REACTION OF PH3 WITH Si(111) When the crystal is heated in a phosphine atmosphere a number of surface structures is formed each in a certain pressure-temperature range. Photographs of the LEED patterns are reproduced in figs. l-3. The Si (111)-P-6 J3 structure is the same as the one reported by Lander and Morrisons) for phosphorus adsorption on Si (111). The interpretation of the patterns will be discussed in section 4. Fig. 4 shows the pressure-temperature ranges of the different surface structures. The pressures are readings of an ionization gauge calibrated for N, and are not corrected for the difference in ionization cross-sections. The actual PH, pressures are probably lower by a factor of 3 to 4. The dashed curves in fig. 4 represent irreversible reactions, whereas the drawn curves correspond to stationary states or equilibria. The dotted curve indicates a change in the P-l structure as will be discussed in section 4.2. The relevant chemical reactions at the silicon surface, for the formation of the surface structure 4, can be given as a) PHs,., d PH3ads.q b) PH3,a..y4’ac43 Hads.q c) 2 Hads.q + Hzsan d) Pads.q -+ Pdes. (e.g. as P, or Sip,,, or as P dissolved in bulk silicon) (ads. q denotes the adsorption state in the corresponding surface formation). At a PH3 pressure p the formation rate of a surface structure from the gas-phase can in a first approximation be given by: kA = fJqP exP (-
EA/RT),
(1)
where bq is the condensation coefficient of PH, during the formation of surface structure q, and EA is the activation energy of the rate determining reaction step. Above 500 “C the formation rate of the P-l from the P-7 structure is found to be temperature independent and linear with pressure. At a pressure of lo-’ Torr the formation took 4 min and the experimental points in fig. 4
PHs
ADSORPTION
ON
si(ll1)
SURFACE
l
Fig. 4. Pressure-temperature diagram of the reaction between PH3 and the silicon (111) surface, showing the different surface structures. (------) irreversible reactions. (reversible transitions under influence of PH3. (*-*+*.) change in the phosphorus conten! of the P-l structure,
A. J. VAN BOMMEL
388
represent
the temperature
structure
was (lo-‘/p)
action
rate decreases
AND F. MEYER
and pressure
where
x 4 min (p pressure sharply,
the formation
in Torr).
but the temperatures
time of the
Below 500 “C the recould not be measured
accurately enough to determine the activation energy. The results of adsorption measurements on powders suggest that the reactions a and b take place already at room temperature, whereas the H, desorption starts at approximately 500 “C. Therefore it seems likely that the rate determining step at temperatures below 500 “C is reaction c, the H, desorption. Apparently, above 500 “C, the supply of PH3 becomes the rate determining step and the reaction rate is mainly determined by the condensation coefficient or_ I and the PH3 pressure p. The formation rate of the P-6 J3 from the P-l shows the same behaviour. Above 525 “C it is temperature independent and linear with pressure, whereas at lower temperatures the rate decreases strongly. At the temperature of 525 “C and a pressure of 10e7 Torr the structure is formed in 2 min. The experimental points in fig. 4 are determined in the same way as for the Pus P-7--+ P-l
reaction. As will be shown in section 4.1, the P-6 ,/3 structure probably corresponds to a densely packed phosphorus layer (average coverage 3.3 P per Si surface atom) on top of the silicon substrate. Because the Si-P-1 structures before formation at the low temperature side and after decomposition of the P-6 43 at higher temperatures are identical, the reaction kinetics will be treated as if the deposition and removal of the densely packed phosphorus layer are the only reactions occurring at the surface. The rate determining step above 525 “C must be a reaction with a very small activation energy EA and the rate is determined by the condensation coefficient cp_6 J3 and the PH, pressure p. It should be noted that the a,_,J3 is larger than the ep_l because more phosphorus is deposited in a shorter time interval. The experimental points defining the drawn curves represent the sharp, reversible, transition between two structures under influence of PH,. At these points there is a stationary state where the decomposition just cancels the formation. The decomposition rate of a surface structure can be given as: ii, = Cexp(where C is a constant action d. For the stationary
E,/RT),
and ED the activation
energy
(2) of decomposition
re-
state kA is equal to k,:
k, = k, = a,p exp (-
E,/RT)
= C exp (-
ED/RT) ,
(3)
PH3 ADSORPTION
ON
Si(ll1)
389
SURFACE
which can be put in the form of the Clausius-Clapeyron
equation. .
G2 - f
R(ln p1 - In pz) = (ED - EA) (
1>
Using this equation a value of ED- EA = 85 kcal/mol was calculated for the P-6 ,/3 structure. The activation energy for the decomposition of P-6 ,/3 can be determined directly by measuring the decomposition time (vanishing of the fractional order spots) in vacuum at different temperarures. A value ofE D= 70 &- 15 kcal/mol was obtained. Comparison of these results shows that EA must be small (lEAI < 15 kcal/mol), in agreement with the fact that the formation rate of the P-6 ,/3 above 525 “C is not notably temperature dependent. The formation of the P-2 ,/3 structure was very difficult and it generally took 3-13 hours before the fractional order spots appeared. The temperature and pressure dependence of the formation were not clear and other factors may have an influence. The experimental procedure consisted of a stepwise increase of the crystal temperature at a constant PH3 pressure and the experimental points at the low temperature side correspond to the first appearance of the P-2 43 structure. At the high-temperature side a reversible transition Pn3 P-2,/3 P P-l was observed and a value ED- EA = 115 kcal/mol was calculated using the Clausius-Clapeyron equation. 4. Discussion 4.1. THE Si (111)-P-6 J3 STRUCTURE
First we wish to discuss the Si (111)-P-6 ,/3 structure which was also reported by Lander and Morrisons). These authors suggested a structure with a large unit mesh with dimensions 6 43 x 6 ,/3 bulk unit meshes. This does not explain, however, why the fractional order spots are only present in clusters around the integral spots as can be seen from the photographs in fig. 3. We wish to give another interpretation of this cluster-pattern which does explain this phenomenon. As was first suggested by Bauerrs), multiple scattering plays an important role in LEED. This was experimentally found by Tuckeri3) and Taylor14) and recently dealt with by McRaer5) from a theoretical viewpoint. If we assume an adsorbent A with a layer B on top of it, the electron beam can be diffracted by both A and B. Several combinations are possible, as
390
A. J. VAN
BOMMEL
AND
F. MEYER
shown in fig. 5, e.g. AB, BA, ABA etc. For normal incidence of the electron beam, the direction of the multiple diffracted beam can be calculated by using the formulae: dAsin8r = A, (5) dB(sin 8, - sin 0,) = A,
etc.,
(6)
where dA and dB are the spacings in A and B respectively. It is easier, however, to find the direction of the multiply diffracted beams by addition of the relevant reciprocal lattice vectors as shown in figs. 6 and 7. Fig. 6 shows part of the reciprocal lattice of the 6 ,/3 structure. The indices of the 6 43 structure are given along with the indices of the ideal silicon surface. The silicon 11 spot coincides with the 18 0 spot of the 6 43 structure. It appears
AB
Fig. 5.
BA
ABA
Multiple diffraction by substrate A and top layer B. A is silicon and B is phosphorus.
that an excellent explanation can be given for the positions of the fractional order spots and also for the main features of their intensity vs. voltage plots, if one assumes multiple scattering of a silicon (111) substrate giving only integral diffraction beams and a phosphorus layer on top of it, which has reciprocal unit vectors 10 and 01 coincident with the 19 0 and 0 19 vectors of the 6 43 reciprocal lattice. This corresponds to a spacing of A.6 43 x Si (10) spacing. The multiple diffracted beam can only be present if the primary diffracted beam can exist. For normal incidence this can be easily calculated from dsine=ii Ild, (7)
PH3
ADSORPTION
ON
Si (111)
SURFACE
391
where V, = accelerating voltage of the electrons in volts, v = inner potential, V, = contact potential difference between cathode and silicon. (Measurement of V, gave a value of 1.5 V.) From (7) and (8) it follows that at voltages V= V,+ K + V, between 13.5 and 41 V only the 00 beam and the (IO) and (01) integral silicon beams are allowed in the silicon crystal. In this voltage range it is not possible for the 00 beam to serve as a primary beam for the phosphorus layer (except for its OO), because the phosphorus spacing is too small. Therefore the fractional order spots observed must have one of the (10) or (01) integral silicon beams as primary beam. Fig. 7 shows how the pairs of fractional order spots close to the (10) and (01) integral silicon beams are formed by siliconphosphorus double diffraction. Because of the threefold symmetry of the pattern each pair of fractional order spots thus formed will have the same intensity vs. voltage dependence as the nearest integral spot, if one assumes
IS 19~Pll 8 !S&i
0 3
/ 19 0Pt
0
o=Si I 1
Fig. 6. Reciprocal lattice of the Si (111)-P-6 43 structure. () unit mesh of the 6 43 (the small mesh). (------) unit mesh of the silicon substrate. () unit mesh of the phosphorus top layer.
392
A. J. VAN
BOMMEL AND F. MEYER
that the diffraction in the phosphorus layer does not affect the intensity distribution. This identical voltage dependence was indeed observed over the whole voltage range from 13-200 V. At electron voltages below 100 V the electron energy dependence of the P-10 intensity (the 19 0 position in the reciprocal lattice) is very similar to the 00 beam. This suggests that the P-10 beam is mainly formed by double diffraction of Si-OO+P-10 and not by single diffraction from the phosphorus layer. PO1 Siiz
lO
0
O.
0
0 l
Fig. 7. Part of the reciprocal lattice of the Si (111)-P-6 2/3 structure and graphical direct diffraction from phosphorus layer. representation of multiple diffraction. (-) (------) diffraction from phosphorus layer with an integral beam of silicon as primary beam. The lines interconnecting the fractional order spots immediately around the 00 serve only to indicate the first and second hexagon as discussed in section 4.1.
Two hexagons of fractional order beams immediately around the 00 beam are shown in fig. 7. The voltages at which the beams first appear give information about their formation and about the inner potential in the silicon crystal. The first hexagon of fractional order spots around the 00 becomes visible at I!,=9 V. They are most probably formed by Si-lO+P-iO+Si-01 triple diffraction (and the other possibilities according to symmetry). The
PHs
ADSORPTIONON si (111) SURFACE
393
sixfold symmetry of these spots follows from the sixfold symmetry of the P (10) beams and from the fact that each spot has a cont~butio~ of a 10 and a 01 beam from silicon. From the plane-grating formula (7) it appears that the Si-10 and Si-01 beams are permitted for V> 13.5 V. Combination of this value with the measured I’, = 9 V gives Vi+ V, 2 4.5 V. The second hexagon of fractional order spots around the 00 appears at I$=41 V, They have threefold symmetry. The three spots in the Si (01)” directions are most probably formed by Si-OZ-+P-1 1 +Si-Of triple di~raction. The precursors of these spots, the 77 spots in fig. 6, which are formed by Si-OZ-+P-1 1 double diffraction, have indeed the same intensity vs. voltage curve. The three spots in the Si
* Vis
in 00
Multiple diffraction
vmeas
Vcale
11
15.2
24 31 45 54 84 124
Vplot of 00 beam and from multiple diffraction data*
vi-l-vi! .-_ 4.2 10 15 10 12
34 60 94 136
given in volts; depth spacing is 3.15 A;
VI-i-vc
vi+
V, = Vea~o-
4.5
14
I&,,,.
394
A. 1. VAN
EOMMEL
AND
F. MEYER
spots will appear close to the integral beams of silicon because an increasing number of consecutive diffractions is necessary to form a beam further removed from the integral spot. The unit cell edge in an ideal Si (111) plane is 3.84 A. Therefore the unit cell edge of the layer is & J3 x 3.84=2.10 A. Because the covalent radius of phosphorus is 1.10 A, the only possible structure for this layer is a densely packed phosphorus layer. The silicon substrate in this P-6 J3 structure gives only integral beams. The observation that a silicon surface rearranges to a l-structure under influence of phosphorus, suggests strongly that in this Si-1 substrate phosphorus is also incorporated. 4.2. THE Si (111)-P-l STRUCTURES Thep-Tdiagram of fig. 4 shows three regions with a Si-(11 1)-P-l structure. It should be noted that one normally observes a P-l structure in the whole pressure-temperature range between the Si (111)-P-6 ,/3 and the Si (11 l)-7 structure, because the formation of the Si (111)-P-2 ,/3 structure is very slow. The ambiguity in the interpretation of the P-l pattern was discussed in section 3.2. It is not clear whether the structure corresponds to a coverage of one phosphorus atom per silicon surface atom as suggested by the LEED pattern, or to a disordered coverage of one phosphorus per three silicon surface atoms in analogy with adsorption measurements on powders. The P-l structure formed by decomposition of the P-6 ,/3 has identical I vs. V plots for 00, 10 and 01 beams with the first P-l structure, and both are different from those of the P-6 ,/3. At higher temperatures, represented roughly by the dotted curve in fig. 4, the I vs. V plots of the l-structure change gradually, showing up most clearly in the gradual disappearance of the 80 V peak in the 10 beam. This probably corresponds to a lowering of the phosphorus coverage at higher temperatures, in agreement with the measurements in section 3.2, where the l-structure formed by adsorption at room temperature, followed by heating in vacuum, showed an increase in the 80 V peak after each cycle. Fig. 8 gives the I vs. V curves of the 10 beams of these P-l structures. The P-l structures from the temperature regions just before and just after the P-6 ,,/3 behave in exactly the same way in vacuum, decomposing at 630 “C into a 7-structure via a gradual decrease of the 80 V peak in the 10 beam. This decomposition takes approximately one hour at this temperature. The P-2 ,,/3 structure giving the clean pattern in the same period of time at 680 “C, appears to be more stable in vacuum than the P-l structure. 4.3. THE Si (ill)-P-2J3
STRUCTURE
The formation of the P-2 ,,/3 structure from the P-l is difficult and it is
PI-b ADSORPTIONON
si(lll)
395
SURFACE
generally $ to 14 hours before the superstructure becomes visible. Its formation is in the same p-T range where the gradual decrease in phosphorus coverage in the P-l structure takes place, but its I vs. V plots are identical with those of the high coverage P-l structure. It proved to be impossible to transform the P-2 43 structure into the P-6 43 structure directly under any circumstances. It was only if the P-2 ,,/3 structure was just being formed and its fractional order spots were still
b
0
50
100
Z"" -c Y
150
Fig. 8. Intensity vs. voltage curves of the 10 beams in the Si (111)-P-l structures. (a}low phosphorus
coverage (b) high phosphorus
coverage.
rather weak, that a lowering of the temperature to the pT range where the P-6 ,/3 is stable, gave the patterns of both structures together. It is quite probable that in this case part of the surface still had a P-l structure, which readily formed the P-6 43. This suggests that the densely packed phosphorus layer of the P-6 ,/‘3 structure cannot be deposited on every substrate but has a definite bonding with the surface. The P-2 ,/3 appears to have a more
A. J. VAN
396
inert surface compensation
BOMMEL
AND
F. MEYER
than the P-l structures, which might be related to a better of the free bonds. This would be in accordance with the higher
stability of the P-2 zj3 structure in vacuum. The formation temperature of the P-2 ,/3
is in the same
temperature
range as its decomposition temperature in vacuum. The structure is very slow to form and nucleation on the surface is probably difficult. In such a case, “island formation” seems probable, and the fact that the P-6 J3 and P-2 J3 can be formed together when the P-2 43 is not yet fully developed points in this direction. It is possible to propose a model in which a layer oftrivalent P compensates the extra bonds of the silicon surface. This model, in which a coverage of 3 monolayer is needed, gives a 2 43 x 43 unit cell which is equally possible in three positions, each rotated over 120”, giving threefold symmetry as an average. If one assumes mixed layers of phosphorus and silicon, however, more structures are possible and at present it does not seem feasible to propose a model with any degree of certainty. 5. Adsorption measurements on powders compared with LEED Adsorption measurements on silicon and germanium powders 1~a) at room temperature showed that both elements behaved in exactly the same way towards 0, and hydrides of the general formula H,A. Moreover the volumes of gas adsorbed were the same for hydrides having the same value of x. After adsorption and subsequent evacuation, the hydrogen could be removed by heating to 300 “C or 500 “C for germanium and silicon respectively. This procedure was repeated with single crystal surfaces in a LEED apparatus. Gas adsorption at room temperature did not change the clean patterns very much. Small differences in the intensities of the superstructure spots of the Si-7 structure were noticed. The results of heating the gascovered surfaces in vacuum are given in table 2. The temperatures were approximately 300 “C and 500 “C for germanium and silicon respectively, and at those temperatures a pressure rise was observed probably due to hydrogen
evolution.
In general
more cycles of adsorption 2 at room temperature vacuum
at room temper-
TABLE
Surface structures
formed by adsorption
followed by heating in AsH3
HzS
HzSe
Ge (111)
2
2
1
1
1
Si (111)
_
1
1 8 at 700 “C
1
1
NH3
PHz
PHs
ADSORPTIONON
Si(ll1)
SURFACE
397
ature followed by heating in vacuum lowered the background intensity but no new fractional order spots were observed. In the case of PH, and ASH, adsorption on silicon, the intensity vs. voltage curve of the 10 beam changed notably. The Si (111)-P-l structures have been discussed in section 3.2. The ASH, adsorption is completely analogous but we failed to observe any other surface structures by heating in vacuum or in an ASH, atmosphere. In the case of NH3 adsorption further heating in vacuum resulted in a new surface structure. At 700 “C the Si(lll)-N-8 structure was formed from the lstructure. This structure is very stable and it decomposes in vacuum only at a temperature of 1000 “C. Heating in a gas atmosphere also gave the land 8-structure for NH3, but at 900 “C a new surface structure appears with very strong spots near the (0 +-) and (J+ 0) positions.
(4 \ 1.0 Fig. 9.
LEED patterns of Si (111) surface structures after reaction with NH3. (a) Si (111)-N-8 at 60 V (b) Si (111)-N at 50 V. High temperature structure.
These spots do not seem to be in positions of the 8-structure and it might be that this is the beginning of the growth of silicon nitride. Its unit mesh vectors are rotated relative to those of the substrate. Photographs of the LEED patterns of the nitrogen structures are given in fig. 9. In general one can say that the correlation between adsorption measurements on powders and LEED data is not clear cut, except perhaps in the case of H,S and H,Se adsorption on germanium4) It therefore appears that the coverage of the single crystal surface must be known directly and ellipsometry might be a suitable technique for measuring this. The possibilities of a combination of this technique and LEED will be investigated.
398
A. J. VAN BOMMEL
AND F. MEYER
6. Conclusion
The results can be summarized as follows: 1) The correlation between adsorption measurements on powders and LEED patterns is not clear. The P-l structure, which corresponds to a coverage of approximately 3 monolayer P according to powder measurements, can best be interpreted from the LEED point of view by a full monolayer. 2) A number of surface structures are obtained by heating the silicon crystal in PH,. The pressure-temperature diagram for the reaction is determined. For the reversible transitions PH3
P-6J3’zPp-1
and
P-2J3
P P-l
values for ED- EA (difference between activation energy of decomposition and adsorption) of 85 + 15 kcal/mol and 115 ) 20 kcal/mol respectively are calculated using the Clausius-Clapeyron equation. A value of 70 kcal/mol was calculated for the activation energy of decomposition of the P-6 ,/3 by measuring the temperature dependence of the decomposition rate in vacuum. 3) The P-6 ,,/3 cluster-pattern corresponds probably to a densely packed phosphorus layer on top of the surface. The fractional order spots in the LEED pattern are formed by multiple diffraction by the crystal and the phosphorous layer. From the appearance voltage of certain fractional order spots values can be calculated for the inner potential. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15)
A. H. Boonstra and J. Van Ruler, Surface Sci. 4 (1966) 141. A. H. Boonstra, Thesis, Eindhoven, 1967. A. J. Rosenberg, J. Phys. Chem. Solids 14 (1960) 175. A. J. Van Bommel and F. Meyer, Surface Sci. 6 (1967) 391. J. J. Lander and J. Morrison, J. Chem. Phys. 37 (1962) 729. J. J. Lander and J. Morrison, Surface Sci. 4 (1966) 241. F. G. Allen, J. Appl. Phys. 28 (1957) 1510. F. Jona and H. R. Wendt, J. Appl. Phys. 37 (1966) 3637. R. E. Schlier and H. E. Farnsworth, J. Chem. Phys. 30 (1959) 917. F. Jona, IBM J. Res. Develop. 9 (1965) 375. A. J. Van Bommel and F. Meyer, Surface Sci. 8 (1967) 467. E. Bauer, Phys. Rev. 213 (1961) 1206. Ch. W. Tucker, J. Appl. Phys. 35(1964) 1897 N. J. Taylor, Surface Sci. 4 (1966) 161. E. G. McRae, J. Chem. Phys. 45 (1966) 3258.
SCIENCE 8 (1967) 381-398 o North-Holland
A LOW ENERGY
ELECTRON
PH, ADSORPTION
Publishing Co., Amsterdam
DIFFRACTION
STUDY
OF THE
ON THE Si (111) SURFACE
A. J. VAN BOMMEL and F. MEYER Philips Research Laboratories, N. V. Philips’ Gloeilampenfabrieken, Eindhoven, The Netherlands Received 18 April 1967 The reaction of PHa with a clean silicon (111) surface was studied by low energy electron diffraction (LEED). A number of diffraction patterns were observed in the temperature range of 251000°C using PHs pressures of 5 x 10-s to 1O-5 Torr. The LEED pattern of one of them, the P-6 2/3 structure, could be interpreted by multiple diffraction. The structure appears to be a densely packed phosphorus layer on top of the surface. The activation energy for desorption of the layer was calculated. For the other patterns no structure models could be proposed with certainty. The correlation with adsorption measurements on powders is discussed and the LEED results of reactions of other hydrides with silicon and germanium (111) surfaces are mentioned.
1. Introduction
Very recently Boonstra reported the results of gas adsorption of hydrides on clean germaniumr) and silicons)powders using volumetric measurements. The hydrides were of the general formula H,A, e.g. HCI, HBr; H20, H,S, H,Se; NH,, PH,, ASH,. At room temperature a fast adsorption took place to an extent of one molecule H,A per 2x surface atoms, which amount could not be pumped off. Heating in vacuum to 300 “C for germanium or, to 500 “C for silicon removed all the hydrogen from the adsorbent. Cooling to room temperature and new exposure to the gas resulted in adsorption of approximately half the original amount. Extrapolation of these results will give a final coverage of one atom A per x surface atoms after repeated cycles. It was suggested that a dissociative adsorption of H,A takes place, where each hydrogen atom is separately bound to a surface atom and A to a number of surface atoms according to its valency. This model gives a complete compensation of the free bonds because the exposed surface of the powders, which were prepared by crushing a single crystal, consists mainly of (111) surfaces a) having one free bond per surface atom. If we assume that the adsorption characteristics for a large single crystal surface are the same as for a powder, then we can use these data on 381
382 powders
A. J. VAN
for the interpretation
BOMMEL
AND
F. MEYER
of low energy
electron
diffraction
(LEED)
patterns hydride
which are obtained after gas adsorption on the (111) surfaces. The adsorption is interesting from a LEED point of view, because it
appears
to be a convenient
way to deposit certain atoms, e.g. S, Se, N, P, As
on the surface. We reported a study of H,S and H,Se adsorption on Ge (11 1)4), where a model was proposed in accordance with the results of adsorption measurements on powders. In this paper we wish to report on the reaction of PH3 with the Si (111) surface. Measurements on powders indicate that the hydrogen is completely removed at temperatures higher than 500 “C and this study is mainly concerned with the surface structures of phosphorus on Si (111). LEED studies of phosphorus adsorption on Si (111)s) and diamond (111)s) surfaces have been reported by Lander and Morrison.
2. Experimental The silicon samples used in this study were cut from a high-ohmic crystal. They were ground parallel to the (111) plane (deviation less than 2’) mechanically polished and finally etched in a solution of 400 mg KMnO, in 25 cm3 HF 50%. The dimensions of the samples were approximately 15 x 6 x 0.6 mm. The samples were clamped in tantalum strips and cleaned in high vacuum (< 10m9 Torr) by heating to temperatures up to 1200 “C. Heating was achieved by passing a current through the crystal. Crystal temperatures higher than 700 “C were measured by means of an optical pyrometer. The emission coefficients for silicon are known from the work of Allen’). An I.R. pyrometer, which was calibrated against the normal optical pyrometer in the overlap region, was used for temperatures between 500 “C and 700 “C. Temperatures below 500 “C measured with the I.R. pyrometer cannot be trusted, as was shown recently by Jonas). LEED measurements were taken with a Varian post-acceleration type apparatus and spot-intensities
were measured
with a spot-photometer.
3. Results 3.1. CLEAN Si (111) SURFACE The clean annealed Si (111) surface has been studied extensively by LEEDS? 9, la). Si (11 l)-7 appeared to be representative for the clean surface, and this structure was the starting point for our investigations. It should be noted here that we have observed also the Si (11 l)-,/19 structure. This structure, observed already by Schlier and Farnsworthg) in 1959, was originally interpreted as a clean-surface structure (indeed giving the same
PHI
ADSORPTION
ON
Si (111)
383
SURFACE
reactions with PH3 at temperatures higher than 500 “C). Closer examination, however, proved that the ,/19 structure is due tonickel contaminationll). 3.2. PHJ
ADSORPTION
As a result of the reaction of PHI, with the Si (111) surface at different temperatures, a number of surface structures was observed. Of all these structures the intensities (I) of three integral spots, the 00, 10 and 01 were measured as a function of electron voltage (V). We have chosen the 10 beam in the crystallographic [l 121 azimuth and the 01 beam in the [i2i] azimuth. Because of the threefold symmetry of the LEED patterns, the 10 and 01 represent the first hexagon of integral spots around the 00. We did not try to resolve the surface structures from these data, but are merely using them as “fingerprints”. In general it may be said that different Zvs. Vplots indicate different structures although the reverse is not necessarily true. The clean annealed Si (11 l)-7 surface was exposed at room temperature to PH3 pressures of lo-’ to 10V5 Torr. Within a few minutes the intensity distribution of the non-integral beams changed notably. Photographs of the LEED patterns are given in fig. 1. This can be interpreted as an ordered adsorption of PHJ, the Si (111)-P-7 structure. The Z vs. V plots for the 00, 01 and 10 were only slightly different from those of the clean structure. The P-7 structure was stable in vacuum at room temperature. Heating
(4
Fig. 1.
0.1
-
@I
LEED patterns of Si (111) surface structures. (a) Si (11 I)-7-clean at 59 V. (b) Si (111)-P-7 at 59 V.
384
A. J. VAN
BOMMEL
AND
F. MEYER
to 500 “C showed a sudden pressure rise in the LEED chamber, which is probably due to desorbed hydrogen. All non-integral beams disappeared, but above 80 V, the pattern showed vague streaks, which appeared to be remains of the 7-structure. The Z vs. V plots of this Si (111)-P-l pattern differed in some respects from those of the clean and P-7 patterns. Two more cycles of adsorption at room temperature, followed by heating in vacuum gave a P-l pattern without any streaks. Examination of the Zvs. V plots showed that the differences with the 7-patterns had become more pronounced and a new peak in the 10 beam at 80 V became much more prominent at higher phosphorus coverages. Photographs of these Si (11 l)P-l LEED patterns are given in fig. 2. Heating the crystal in PH, at 500 “C also gave a P-l pattern and the Z vs. V plots were very similar to those of
(4
\
(b)
Fig. 2. LEED patterns of Si (111) surface structures. (a) Si (11 I)-7-clean at 80 V. (b) Si (111)-P-l at 80 V. Low phosphorus coverage. (c) Si (111)-P-l at 80 V. High phosphorus coverage.
PH3
ADSORPTION
ON
Si(l11)
385
SURFACE
a P-l pattern obtained by a few cycles of adsorption at room temperature followed by heating in vacuum. From a LEED point of view, a complete l-structure can only be explained by an integral number of adsorbed atoms per surface atom and the simplest solution is to assume that one monolayer of phosphorus atoms replaces the surface silicon atoms in their trivalent positions. The monolayer is defined here as a coverage of one adsorbed atom per surface atom. Powder measurements, however, indicate a coverage of + of a monolayer under what seems to be the same experimental conditions. In such a case one would expect a surface structure with a larger unit mesh, for instance a P-,/3 structure, as was observed for phosphorus adsorption on diamond (111)s).
(a)
(‘4 \
(4
(4
Fig. 3. LEED patterns of Si (111) surface structures. (a) Si (111)-P-6 d3 at 40 V. (b) Si (111)-P-6 43 at 40 V. Crystal rotated over 15”. (c) Si (111)-P-6 2/3 at 80 V. (d) Si (11 l)P-2 2/3 at 45 V.
386
A. J. VAN
BOMMEL
AND
F. MEYER
The reason why such a surface structure is not observed on silicon must be sought in a disorder in the phosphorus coverage, although there is no support for this from background intensity. If the l-structure corresponds to a monolayer of phosphorus, then one would have to assume that the phosphine does not desorb completely from the apparatus walls and crystal supports, even after two days of pumping, and that, during the short crystal heating, adsorption of phosphorus takes place by decomposition of this desorbed phosphine. It is not possible to decide from our data which view is the more likely. 3.3. p-T DIAGRAM OF THE REACTION OF PH3 WITH Si(111) When the crystal is heated in a phosphine atmosphere a number of surface structures is formed each in a certain pressure-temperature range. Photographs of the LEED patterns are reproduced in figs. l-3. The Si (111)-P-6 J3 structure is the same as the one reported by Lander and Morrisons) for phosphorus adsorption on Si (111). The interpretation of the patterns will be discussed in section 4. Fig. 4 shows the pressure-temperature ranges of the different surface structures. The pressures are readings of an ionization gauge calibrated for N, and are not corrected for the difference in ionization cross-sections. The actual PH, pressures are probably lower by a factor of 3 to 4. The dashed curves in fig. 4 represent irreversible reactions, whereas the drawn curves correspond to stationary states or equilibria. The dotted curve indicates a change in the P-l structure as will be discussed in section 4.2. The relevant chemical reactions at the silicon surface, for the formation of the surface structure 4, can be given as a) PHs,., d PH3ads.q b) PH3,a..y4’ac43 Hads.q c) 2 Hads.q + Hzsan d) Pads.q -+ Pdes. (e.g. as P, or Sip,,, or as P dissolved in bulk silicon) (ads. q denotes the adsorption state in the corresponding surface formation). At a PH3 pressure p the formation rate of a surface structure from the gas-phase can in a first approximation be given by: kA = fJqP exP (-
EA/RT),
(1)
where bq is the condensation coefficient of PH, during the formation of surface structure q, and EA is the activation energy of the rate determining reaction step. Above 500 “C the formation rate of the P-l from the P-7 structure is found to be temperature independent and linear with pressure. At a pressure of lo-’ Torr the formation took 4 min and the experimental points in fig. 4
PHs
ADSORPTION
ON
si(ll1)
SURFACE
l
Fig. 4. Pressure-temperature diagram of the reaction between PH3 and the silicon (111) surface, showing the different surface structures. (------) irreversible reactions. (reversible transitions under influence of PH3. (*-*+*.) change in the phosphorus conten! of the P-l structure,
A. J. VAN BOMMEL
388
represent
the temperature
structure
was (lo-‘/p)
action
rate decreases
AND F. MEYER
and pressure
where
x 4 min (p pressure sharply,
the formation
in Torr).
but the temperatures
time of the
Below 500 “C the recould not be measured
accurately enough to determine the activation energy. The results of adsorption measurements on powders suggest that the reactions a and b take place already at room temperature, whereas the H, desorption starts at approximately 500 “C. Therefore it seems likely that the rate determining step at temperatures below 500 “C is reaction c, the H, desorption. Apparently, above 500 “C, the supply of PH3 becomes the rate determining step and the reaction rate is mainly determined by the condensation coefficient or_ I and the PH3 pressure p. The formation rate of the P-6 J3 from the P-l shows the same behaviour. Above 525 “C it is temperature independent and linear with pressure, whereas at lower temperatures the rate decreases strongly. At the temperature of 525 “C and a pressure of 10e7 Torr the structure is formed in 2 min. The experimental points in fig. 4 are determined in the same way as for the Pus P-7--+ P-l
reaction. As will be shown in section 4.1, the P-6 ,/3 structure probably corresponds to a densely packed phosphorus layer (average coverage 3.3 P per Si surface atom) on top of the silicon substrate. Because the Si-P-1 structures before formation at the low temperature side and after decomposition of the P-6 43 at higher temperatures are identical, the reaction kinetics will be treated as if the deposition and removal of the densely packed phosphorus layer are the only reactions occurring at the surface. The rate determining step above 525 “C must be a reaction with a very small activation energy EA and the rate is determined by the condensation coefficient cp_6 J3 and the PH, pressure p. It should be noted that the a,_,J3 is larger than the ep_l because more phosphorus is deposited in a shorter time interval. The experimental points defining the drawn curves represent the sharp, reversible, transition between two structures under influence of PH,. At these points there is a stationary state where the decomposition just cancels the formation. The decomposition rate of a surface structure can be given as: ii, = Cexp(where C is a constant action d. For the stationary
E,/RT),
and ED the activation
energy
(2) of decomposition
re-
state kA is equal to k,:
k, = k, = a,p exp (-
E,/RT)
= C exp (-
ED/RT) ,
(3)
PH3 ADSORPTION
ON
Si(ll1)
389
SURFACE
which can be put in the form of the Clausius-Clapeyron
equation. .
G2 - f
R(ln p1 - In pz) = (ED - EA) (
1>
Using this equation a value of ED- EA = 85 kcal/mol was calculated for the P-6 ,/3 structure. The activation energy for the decomposition of P-6 ,/3 can be determined directly by measuring the decomposition time (vanishing of the fractional order spots) in vacuum at different temperarures. A value ofE D= 70 &- 15 kcal/mol was obtained. Comparison of these results shows that EA must be small (lEAI < 15 kcal/mol), in agreement with the fact that the formation rate of the P-6 ,/3 above 525 “C is not notably temperature dependent. The formation of the P-2 ,/3 structure was very difficult and it generally took 3-13 hours before the fractional order spots appeared. The temperature and pressure dependence of the formation were not clear and other factors may have an influence. The experimental procedure consisted of a stepwise increase of the crystal temperature at a constant PH3 pressure and the experimental points at the low temperature side correspond to the first appearance of the P-2 43 structure. At the high-temperature side a reversible transition Pn3 P-2,/3 P P-l was observed and a value ED- EA = 115 kcal/mol was calculated using the Clausius-Clapeyron equation. 4. Discussion 4.1. THE Si (111)-P-6 J3 STRUCTURE
First we wish to discuss the Si (111)-P-6 ,/3 structure which was also reported by Lander and Morrisons). These authors suggested a structure with a large unit mesh with dimensions 6 43 x 6 ,/3 bulk unit meshes. This does not explain, however, why the fractional order spots are only present in clusters around the integral spots as can be seen from the photographs in fig. 3. We wish to give another interpretation of this cluster-pattern which does explain this phenomenon. As was first suggested by Bauerrs), multiple scattering plays an important role in LEED. This was experimentally found by Tuckeri3) and Taylor14) and recently dealt with by McRaer5) from a theoretical viewpoint. If we assume an adsorbent A with a layer B on top of it, the electron beam can be diffracted by both A and B. Several combinations are possible, as
390
A. J. VAN
BOMMEL
AND
F. MEYER
shown in fig. 5, e.g. AB, BA, ABA etc. For normal incidence of the electron beam, the direction of the multiple diffracted beam can be calculated by using the formulae: dAsin8r = A, (5) dB(sin 8, - sin 0,) = A,
etc.,
(6)
where dA and dB are the spacings in A and B respectively. It is easier, however, to find the direction of the multiply diffracted beams by addition of the relevant reciprocal lattice vectors as shown in figs. 6 and 7. Fig. 6 shows part of the reciprocal lattice of the 6 ,/3 structure. The indices of the 6 43 structure are given along with the indices of the ideal silicon surface. The silicon 11 spot coincides with the 18 0 spot of the 6 43 structure. It appears
AB
Fig. 5.
BA
ABA
Multiple diffraction by substrate A and top layer B. A is silicon and B is phosphorus.
that an excellent explanation can be given for the positions of the fractional order spots and also for the main features of their intensity vs. voltage plots, if one assumes multiple scattering of a silicon (111) substrate giving only integral diffraction beams and a phosphorus layer on top of it, which has reciprocal unit vectors 10 and 01 coincident with the 19 0 and 0 19 vectors of the 6 43 reciprocal lattice. This corresponds to a spacing of A.6 43 x Si (10) spacing. The multiple diffracted beam can only be present if the primary diffracted beam can exist. For normal incidence this can be easily calculated from dsine=ii Ild, (7)
PH3
ADSORPTION
ON
Si (111)
SURFACE
391
where V, = accelerating voltage of the electrons in volts, v = inner potential, V, = contact potential difference between cathode and silicon. (Measurement of V, gave a value of 1.5 V.) From (7) and (8) it follows that at voltages V= V,+ K + V, between 13.5 and 41 V only the 00 beam and the (IO) and (01) integral silicon beams are allowed in the silicon crystal. In this voltage range it is not possible for the 00 beam to serve as a primary beam for the phosphorus layer (except for its OO), because the phosphorus spacing is too small. Therefore the fractional order spots observed must have one of the (10) or (01) integral silicon beams as primary beam. Fig. 7 shows how the pairs of fractional order spots close to the (10) and (01) integral silicon beams are formed by siliconphosphorus double diffraction. Because of the threefold symmetry of the pattern each pair of fractional order spots thus formed will have the same intensity vs. voltage dependence as the nearest integral spot, if one assumes
IS 19~Pll 8 !S&i
0 3
/ 19 0Pt
0
o=Si I 1
Fig. 6. Reciprocal lattice of the Si (111)-P-6 43 structure. () unit mesh of the 6 43 (the small mesh). (------) unit mesh of the silicon substrate. () unit mesh of the phosphorus top layer.
392
A. J. VAN
BOMMEL AND F. MEYER
that the diffraction in the phosphorus layer does not affect the intensity distribution. This identical voltage dependence was indeed observed over the whole voltage range from 13-200 V. At electron voltages below 100 V the electron energy dependence of the P-10 intensity (the 19 0 position in the reciprocal lattice) is very similar to the 00 beam. This suggests that the P-10 beam is mainly formed by double diffraction of Si-OO+P-10 and not by single diffraction from the phosphorus layer. PO1 Siiz
lO
0
O.
0
0 l
Fig. 7. Part of the reciprocal lattice of the Si (111)-P-6 2/3 structure and graphical direct diffraction from phosphorus layer. representation of multiple diffraction. (-) (------) diffraction from phosphorus layer with an integral beam of silicon as primary beam. The lines interconnecting the fractional order spots immediately around the 00 serve only to indicate the first and second hexagon as discussed in section 4.1.
Two hexagons of fractional order beams immediately around the 00 beam are shown in fig. 7. The voltages at which the beams first appear give information about their formation and about the inner potential in the silicon crystal. The first hexagon of fractional order spots around the 00 becomes visible at I!,=9 V. They are most probably formed by Si-lO+P-iO+Si-01 triple diffraction (and the other possibilities according to symmetry). The
PHs
ADSORPTIONON si (111) SURFACE
393
sixfold symmetry of these spots follows from the sixfold symmetry of the P (10) beams and from the fact that each spot has a cont~butio~ of a 10 and a 01 beam from silicon. From the plane-grating formula (7) it appears that the Si-10 and Si-01 beams are permitted for V> 13.5 V. Combination of this value with the measured I’, = 9 V gives Vi+ V, 2 4.5 V. The second hexagon of fractional order spots around the 00 appears at I$=41 V, They have threefold symmetry. The three spots in the Si (01)” directions are most probably formed by Si-OZ-+P-1 1 +Si-Of triple di~raction. The precursors of these spots, the 77 spots in fig. 6, which are formed by Si-OZ-+P-1 1 double diffraction, have indeed the same intensity vs. voltage curve. The three spots in the Si
* Vis
in 00
Multiple diffraction
vmeas
Vcale
11
15.2
24 31 45 54 84 124
Vplot of 00 beam and from multiple diffraction data*
vi-l-vi! .-_ 4.2 10 15 10 12
34 60 94 136
given in volts; depth spacing is 3.15 A;
VI-i-vc
vi+
V, = Vea~o-
4.5
14
I&,,,.
394
A. 1. VAN
EOMMEL
AND
F. MEYER
spots will appear close to the integral beams of silicon because an increasing number of consecutive diffractions is necessary to form a beam further removed from the integral spot. The unit cell edge in an ideal Si (111) plane is 3.84 A. Therefore the unit cell edge of the layer is & J3 x 3.84=2.10 A. Because the covalent radius of phosphorus is 1.10 A, the only possible structure for this layer is a densely packed phosphorus layer. The silicon substrate in this P-6 J3 structure gives only integral beams. The observation that a silicon surface rearranges to a l-structure under influence of phosphorus, suggests strongly that in this Si-1 substrate phosphorus is also incorporated. 4.2. THE Si (111)-P-l STRUCTURES Thep-Tdiagram of fig. 4 shows three regions with a Si-(11 1)-P-l structure. It should be noted that one normally observes a P-l structure in the whole pressure-temperature range between the Si (111)-P-6 ,/3 and the Si (11 l)-7 structure, because the formation of the Si (111)-P-2 ,/3 structure is very slow. The ambiguity in the interpretation of the P-l pattern was discussed in section 3.2. It is not clear whether the structure corresponds to a coverage of one phosphorus atom per silicon surface atom as suggested by the LEED pattern, or to a disordered coverage of one phosphorus per three silicon surface atoms in analogy with adsorption measurements on powders. The P-l structure formed by decomposition of the P-6 ,/3 has identical I vs. V plots for 00, 10 and 01 beams with the first P-l structure, and both are different from those of the P-6 ,/3. At higher temperatures, represented roughly by the dotted curve in fig. 4, the I vs. V plots of the l-structure change gradually, showing up most clearly in the gradual disappearance of the 80 V peak in the 10 beam. This probably corresponds to a lowering of the phosphorus coverage at higher temperatures, in agreement with the measurements in section 3.2, where the l-structure formed by adsorption at room temperature, followed by heating in vacuum, showed an increase in the 80 V peak after each cycle. Fig. 8 gives the I vs. V curves of the 10 beams of these P-l structures. The P-l structures from the temperature regions just before and just after the P-6 ,,/3 behave in exactly the same way in vacuum, decomposing at 630 “C into a 7-structure via a gradual decrease of the 80 V peak in the 10 beam. This decomposition takes approximately one hour at this temperature. The P-2 ,,/3 structure giving the clean pattern in the same period of time at 680 “C, appears to be more stable in vacuum than the P-l structure. 4.3. THE Si (ill)-P-2J3
STRUCTURE
The formation of the P-2 ,,/3 structure from the P-l is difficult and it is
PI-b ADSORPTIONON
si(lll)
395
SURFACE
generally $ to 14 hours before the superstructure becomes visible. Its formation is in the same p-T range where the gradual decrease in phosphorus coverage in the P-l structure takes place, but its I vs. V plots are identical with those of the high coverage P-l structure. It proved to be impossible to transform the P-2 43 structure into the P-6 43 structure directly under any circumstances. It was only if the P-2 ,,/3 structure was just being formed and its fractional order spots were still
b
0
50
100
Z"" -c Y
150
Fig. 8. Intensity vs. voltage curves of the 10 beams in the Si (111)-P-l structures. (a}low phosphorus
coverage (b) high phosphorus
coverage.
rather weak, that a lowering of the temperature to the pT range where the P-6 ,/3 is stable, gave the patterns of both structures together. It is quite probable that in this case part of the surface still had a P-l structure, which readily formed the P-6 43. This suggests that the densely packed phosphorus layer of the P-6 ,/‘3 structure cannot be deposited on every substrate but has a definite bonding with the surface. The P-2 ,/3 appears to have a more
A. J. VAN
396
inert surface compensation
BOMMEL
AND
F. MEYER
than the P-l structures, which might be related to a better of the free bonds. This would be in accordance with the higher
stability of the P-2 zj3 structure in vacuum. The formation temperature of the P-2 ,/3
is in the same
temperature
range as its decomposition temperature in vacuum. The structure is very slow to form and nucleation on the surface is probably difficult. In such a case, “island formation” seems probable, and the fact that the P-6 J3 and P-2 J3 can be formed together when the P-2 43 is not yet fully developed points in this direction. It is possible to propose a model in which a layer oftrivalent P compensates the extra bonds of the silicon surface. This model, in which a coverage of 3 monolayer is needed, gives a 2 43 x 43 unit cell which is equally possible in three positions, each rotated over 120”, giving threefold symmetry as an average. If one assumes mixed layers of phosphorus and silicon, however, more structures are possible and at present it does not seem feasible to propose a model with any degree of certainty. 5. Adsorption measurements on powders compared with LEED Adsorption measurements on silicon and germanium powders 1~a) at room temperature showed that both elements behaved in exactly the same way towards 0, and hydrides of the general formula H,A. Moreover the volumes of gas adsorbed were the same for hydrides having the same value of x. After adsorption and subsequent evacuation, the hydrogen could be removed by heating to 300 “C or 500 “C for germanium and silicon respectively. This procedure was repeated with single crystal surfaces in a LEED apparatus. Gas adsorption at room temperature did not change the clean patterns very much. Small differences in the intensities of the superstructure spots of the Si-7 structure were noticed. The results of heating the gascovered surfaces in vacuum are given in table 2. The temperatures were approximately 300 “C and 500 “C for germanium and silicon respectively, and at those temperatures a pressure rise was observed probably due to hydrogen
evolution.
In general
more cycles of adsorption 2 at room temperature vacuum
at room temper-
TABLE
Surface structures
formed by adsorption
followed by heating in AsH3
HzS
HzSe
Ge (111)
2
2
1
1
1
Si (111)
_
1
1 8 at 700 “C
1
1
NH3
PHz
PHs
ADSORPTIONON
Si(ll1)
SURFACE
397
ature followed by heating in vacuum lowered the background intensity but no new fractional order spots were observed. In the case of PH, and ASH, adsorption on silicon, the intensity vs. voltage curve of the 10 beam changed notably. The Si (111)-P-l structures have been discussed in section 3.2. The ASH, adsorption is completely analogous but we failed to observe any other surface structures by heating in vacuum or in an ASH, atmosphere. In the case of NH3 adsorption further heating in vacuum resulted in a new surface structure. At 700 “C the Si(lll)-N-8 structure was formed from the lstructure. This structure is very stable and it decomposes in vacuum only at a temperature of 1000 “C. Heating in a gas atmosphere also gave the land 8-structure for NH3, but at 900 “C a new surface structure appears with very strong spots near the (0 +-) and (J+ 0) positions.
(4 \ 1.0 Fig. 9.
LEED patterns of Si (111) surface structures after reaction with NH3. (a) Si (111)-N-8 at 60 V (b) Si (111)-N at 50 V. High temperature structure.
These spots do not seem to be in positions of the 8-structure and it might be that this is the beginning of the growth of silicon nitride. Its unit mesh vectors are rotated relative to those of the substrate. Photographs of the LEED patterns of the nitrogen structures are given in fig. 9. In general one can say that the correlation between adsorption measurements on powders and LEED data is not clear cut, except perhaps in the case of H,S and H,Se adsorption on germanium4) It therefore appears that the coverage of the single crystal surface must be known directly and ellipsometry might be a suitable technique for measuring this. The possibilities of a combination of this technique and LEED will be investigated.
398
A. J. VAN BOMMEL
AND F. MEYER
6. Conclusion
The results can be summarized as follows: 1) The correlation between adsorption measurements on powders and LEED patterns is not clear. The P-l structure, which corresponds to a coverage of approximately 3 monolayer P according to powder measurements, can best be interpreted from the LEED point of view by a full monolayer. 2) A number of surface structures are obtained by heating the silicon crystal in PH,. The pressure-temperature diagram for the reaction is determined. For the reversible transitions PH3
P-6J3’zPp-1
and
P-2J3
P P-l
values for ED- EA (difference between activation energy of decomposition and adsorption) of 85 + 15 kcal/mol and 115 ) 20 kcal/mol respectively are calculated using the Clausius-Clapeyron equation. A value of 70 kcal/mol was calculated for the activation energy of decomposition of the P-6 ,/3 by measuring the temperature dependence of the decomposition rate in vacuum. 3) The P-6 ,,/3 cluster-pattern corresponds probably to a densely packed phosphorus layer on top of the surface. The fractional order spots in the LEED pattern are formed by multiple diffraction by the crystal and the phosphorous layer. From the appearance voltage of certain fractional order spots values can be calculated for the inner potential. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15)
A. H. Boonstra and J. Van Ruler, Surface Sci. 4 (1966) 141. A. H. Boonstra, Thesis, Eindhoven, 1967. A. J. Rosenberg, J. Phys. Chem. Solids 14 (1960) 175. A. J. Van Bommel and F. Meyer, Surface Sci. 6 (1967) 391. J. J. Lander and J. Morrison, J. Chem. Phys. 37 (1962) 729. J. J. Lander and J. Morrison, Surface Sci. 4 (1966) 241. F. G. Allen, J. Appl. Phys. 28 (1957) 1510. F. Jona and H. R. Wendt, J. Appl. Phys. 37 (1966) 3637. R. E. Schlier and H. E. Farnsworth, J. Chem. Phys. 30 (1959) 917. F. Jona, IBM J. Res. Develop. 9 (1965) 375. A. J. Van Bommel and F. Meyer, Surface Sci. 8 (1967) 467. E. Bauer, Phys. Rev. 213 (1961) 1206. Ch. W. Tucker, J. Appl. Phys. 35(1964) 1897 N. J. Taylor, Surface Sci. 4 (1966) 161. E. G. McRae, J. Chem. Phys. 45 (1966) 3258.