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RHEED study of the initial stages of crystallization and oxidation of lead and tin
Journal of Crystal Growth 53 (1981) 382—396 North-Holland Publishing Company
RHEED STUDY OF THE INITIAL STAGES OF CRYSTALLIZATION AND OXIDATION OF LEAD AND TIN S.K. PENEVA Department of Physical Chemistry, Chemical Faculty, University of Sofia 1, Anton Ivanov Road, Sofia 1126, Bulgaria
and K,D. DJUNEVA and E.A. TSUKEVA Department of Atomic Physics, Physical Faculty, University of Sofia, 5 Anton Ivanov Road, Sofia 1126, Bulgaria Received 10 September 1980; manuscript received in final form 20 November 1980
RIIEED investigations of the initial stages of crystallization and oxidation of massive lead and tin and their thin films on (11 flSi are discussed. Diamond cubic structures with lattice parameters around the lattice parameter of sillcon exists both in thin lead and tin films, aod in massive lead and tin, freshly solidified in vacuum of—S X 10 Torr. Experimental evidence for almost identical structures of at least three oxide types is presented, demonstrating that lead and tin behave like silicon not only in the initial stages of crystallization, but also in the initial stages of oxidation. The abeady discussed tin phase with a = 6.40 A has been identified as a tin analogue of SiO. Some data on the phase composition of thin lead films, heated to 700°C, are reported. One of the phases in heated lead films was interpreted as a lead analogue of u-Sn.
1. Introduction Recent Reflection High Energy Electron Diffraction (RHEED) and Depth Selective Mossbauer Spectroscopy (DSMS) studies on evaporated in vacuum of ‘~-‘5X 10—6 Torr, heated and unheated, ~‘S0O A and 1000 A thick tin films, deposited onto (11 l)Si, have shown that the tin has diamond cubic structure with a = 5.60 ±0.06 A, and has a very positive value (up to a certain temperature) of the isomeric shift (4.2 ± 0.34 mm 5—t) relative to Sn02 [11. The present study is RHEED proof that identical structure exists in thin lead films evaporated under the same conditions onto (11 l)Si and treated as described in ref. [1]. It is shown that this crystalline state of both metals in thin film state is one of the initial phases formed after solidification in vacuum of molten massive tin and lead, Islands of oxides were observed on thin lead and tin films, heated in the formerly mentioned conventional vacuum, and on the surface of the massive lead and tin, melted and solidified in vacuum. The present paper demonstrates the great similarity between the oxides grown initially on the surfaces of both metals
and on their thin films, and correlates the observed structures with the metastable oxide phases of tin described in ref. [21. As in ref. [2], the crystallographic analysis is carried out by accounting the structure of the known lead oxides (ASTM Table and, e.g., ref. 131), and by considering the structures of the oxides from the isomorphous series of lead, derived according to refs. [4,5]. More attention is given to lead and its oxides, since some data on tin and its oxides have already been published [I ,2]. Different coexisting crystalline states were found for lead and its oxides as well, with lattice parameters randomly varying in certain limits. They are considered, as in ref. [2] . to be an indication for three-dimentional incommensurate states (3DIS) 16]. Thin lead films are important superconductive materials and lead oxides are isolators. Oxide-free lead films are usually obtained in vacuum of not worse than I 0’~Torr. The usual structure of lead is fcc, but X-ray powder data supporting the existence of hexagonal phases in ultra thin lead films are reported by Ovadyahu [71.There are objections [8], considerlug the results of ref. [7] ambiguous, despite the fact that the interpretation in ref. [8], that it is a ease of
0022-0248/8l/0000—0000/$02.50 © North-Holland Publishing Company
S. K. Peneva et al.
/ RHEED study of initial states of crystallization of Pb and Sn
383
oxidation of the lead surface, cannot explain all the results reported in ref. [7]. Detailed analysis of the oxides formed on the surface of thin lead films can be found, e.g., in the works ofLightet al. [9] and Matthews et al. [10]. It is shown that exposure to oxygen even when the substrate temperature is 240 K leads to an ample quantity of yellow PbO (ASTM 5-0570) with bigger crystallites of red PbO (ASTM
diffractometer technique, with filtered Cu-Ka radiation. RHEED was carried out in an EF4 (Karl Zeiss) operating at 65 kV, with TlCl external standard. A point-to-point revaluation of the error 1~d/dwas performed by taking into account all specific geometrical limitatioi~isof the EF4 [11], putting the error Mid for RHEED from 1.5% up to 3% and above (if, e.g.,
5-0561) on the top, here and there on the surface. In air, freshly evaporated lead was covered with unidentified amorphous oxide [10].
Debye rings corresponding to interplanar distan~esof the order of 5 A and above are measured). Car’ was taken in the present study that the error for ~ingle crystal RHEED photographs did not exceed 2~. It ranged from 1.5% to 2%. For interplanar distances measured by X-ray technique, the error was 0.1%.
3. Experimental techniques It was aimed to obtain as many metal/Si samples as possible, since structural investigations of metastable phases require good statistics. Weighted amounts of99.999%tin or lead (Prolabo, France) were evaporated onto clean (11 l)n-Si [1] in vacuum of ~-~5X 10_6 Torr from Ta(Sn) and Mo(Pb), or Ni(Pb), evaporators. The silicon substrates were kept at the required distances, on a hemisphere, so as to obtain approximately the desired thicknesses of 200 A and 1000 ±100 A (checked by weight measurements). The rate of evaporation was arbitrary since the main structural and electrophysical results were carried out for metal/Si (MS) samples heated above the melting point of the respective metal. After evaporation the samples were removed from the sample holders and placed in a quartz resistance furnace. The heating was carried out again in the same vacuum, but there was a brake of the vacuum for about half an hour leading, according to ref. [10], to an unidentified amorphous layer on the surface of the lead samples. The oxidation of tin appears to follow similar pattern with slower rate. A chromel/alumel thermocouple was attached to the tantalum sample holder next to the MS samples to check the temperature, which was kept constant within an accuracy of ±5°C. 99.999% pure slabs of massive tin and lead were melted in the same vacuum chamber in crucibles of the same metal as the evaporators of the respective metal, and cooled with the same cooling rate as the corresponding MS samples. The initially grown phases in the massive metals were studied, for the sake of comparison, both with RHEED and standard X-ray
3. Results and discussions RHEED results are based on single crystal diffraction analysis of several tens of mainly —~l000A thick lead films on (11 1)Si obtained from about 20 independent evaporation experiments. Powder data were also considered, but their interpretation is ambiguous since the systems are multiphase with some of the structures in incommensurate state [6].
3.1. Structure of unheated ~1OOOA thick Pb films on (111)Si; comparison with the structure of unheated ~1OOO A Sn films on (111)Si and with the structure of the phases observed on massive tin and lead, freshly solidified in vacuum Freshly evaporated lead films showed greater tendency for single crystal growth than tin films [1]. Decrease of the evaporation rate increased the single crystal area of the lead films, and the polycrystalline tin film became textured. Only once (fig. 1) it was possible to observe a RHEED photograph of what is thought to be (123)RLP of orthorhombically deformed lead with a = 4.87 A, b = 4.54 A and c = 5.04 A, but this was an exception. The usually obserbed structures, both for lead and tin films on (11 1)Si, are diamond cubic with lattice parameters either close to the lattice parameter of silicon (a = 5.42 A), or to the lattice parameter of germanium a = 5.66 A), occasionally in incommensurate state [61. These structures are designated as Sn(Si) and Pb(Si). An illustration of(llO)RLP of diamond cubic
384
5. K. Peneva et al.
/ RIIL’ED study
of initial states of crystallization ot Pb and So
Fig. 1. (123) reciprocal lattice projection (RLP) of what is
= =
=
thought to be orthorhombicaily deformed lead observed on freshly evaporated =~1000A thick lead films on (ll1)Si; 2.77 A, d 331 1.09 A, and the angle between both reciprocal lattice vectors is 86 ± 1°.The polycrystailine data are given in column 5, table 1. (Ilere and further the error in measuring interplanar distances is taken at the upper linnit ~d/d 2%.)
___________________________________________
lead is shown in fig. 2a, and [100] texture of diamond cubic tin can be seen in fig. 2b. A superimposition of two RLP, one belonging to the [100] texture Sn(Si) with a 5.60 A, and the other to the single crystal region of Sn(Si) with a 5.5 1 A, is seen in fig. 2c. Single crystal areas on unheated Sn films are very rare. An interesting observation was the lack of fitness of the Pb(Si) lattice on top of the silicon lattice, observed both for the single crystal area of unheated and heated Pb films, figs. 2a and 3, respectively. Both
=
=
a
RHEED Pb(Si) ness plane. in and the photographs silicon orientation are show parallel, of both that lattices the there (11in 1)and isthe planes random(111) of structures The fact are that not diamond promoted cubic by but the Pb(Si) influence Sn(Si) of the silicon substrate, but that their existence is only stabilized in thin film state on top of the silicon was confirmed by RHEED and X-ray studies (table 2) of freshly crystallized massive lead and tin sam-
Fig. 2. (a) (110) RLP of diamond cubic lead, designated as Pb(Si), observed in ~1000 A thick lead films on (11 l)Si. The values of the mterplanar distances are d fj = dj it = 3.11 A and d002 = 2.73 A, corresponding, within the limits of error, to the lattice parameter a = 5.42 A. The diffraction image
raph was already shown in ref. [11. (e) The same as in rb), but (11 1)RLP of Sn(Si) with a = 5.51 A (d220 = 1.95 A) is superimposed on the RLP of the 11001 texture. Single crystal areas of Sn(Si) on unheated tin films are exceptions.
shows that the (ill) planes of Pb(Si) and the silicon substrate are parallel, but the 111 type reciprocal lattice points (the elongated ones belonging to Si) are not parallel, as it would have been expected by epitaxial growth. (b) 11001. texture of diamond cubic tin with a = 5.60 A obtained from =1000 A thick tin film on (111) Si substrate at slow evaporation rate. Indexing of such RHEED photog-
S.K. Peneva et al.
/ RHEED study of initial states of crystallization of Pb and Sn
385
Fig. 3. RHEED photograph of —1000 A thick lead film on (11l)Si heated for 2 h 16 mm at 700°C, in vacuum of ~ x 10~ Torr. Part of the lead has got evaporated and RLP of both lattices are visible. They show that (111) planes of Pb(Si) and Si are parallel but the 111 reciprocal lattice vectors are not parallel, as it would have been expected if the Pb(Si) grew epitaxially on the top of the silicon. (211)RLP of at least two 3DIS of Pb(Si) can be seen. One of them is witha = 5.43 A,/diii = 3.15 A and d 02~= 1.93 A.
pies. Large single crystal regions of diamond cubic metals with the same lattice parameters as the Pb(Si) and Sn(Si) in thin films on (11 l)Si were observed, and they are illustrated in figs. 4a and b for the case of massive lead, and in fig. 5 for massive tin. Fig. 5 is an illustration of the lower tendency for single crystal growth of Sn(Si) in massive tin as compared to the large single crystal regions of Pb(Si) observed on the surface of massive lead (figs. 4a and 4b). Figs. 6a and 6b show deformed single crystal regions of diamond cubic lead with one of the lattice parameters being less than 5 A. It is thought that such structures can be regarded as one of the possible ways of formation of the ordinary fcc lead in the massive metal,
3.2. Structures of oxides formed on unheated and heated, -~1OOOA thick Sn and Pb films on (111)Si; correlation with the initially formed oxide structures on the surface of massive tin and lead Tin and lead get readily oxidized. Fig. 7a shows how the [1001 textured Sn(Si) thin film, illustrated in fig. 2b, has changed after a 5 day stay in air. (1l1)RLP of deformed oxide phase, designated as “a = 6.40 A”, and discussed in ref. [2], is visible. Fig. 7b shows the surface of massive tin heated at 300°C
Fig. 4. (a) Pb(Si) large single crystal region formed on the surface of freshly solidified massive 99.999% lead heated in vacuum of —5 x 106 Torr at 400°Cfor 25 mm, and cooled at the same cooling rate as the thin lead films heated at the
same conditions for further electrophysical measurements 111. (110) RLPa of several 3DIS with lattice parameters varying around = 5.66 A are observed. One of them is with d1 j~ = 3.24 A and d002 = 2.84 A. (b) (211)RLP of another single crystal region. Here the Pb(Si) lattice is with deformations (orthorhombic or even monoclinic). The measured ~ = 3.20 A and d02~ 1.98 A a interplanar 5.45 A, bdistances c = 5.59areAdjorthorhombic deformation.
in vacuum of ~5 X 10-6 Torr for 2 h 40 mm and cooled with the same rate as the thin tin films. A similar deformed (11 l)RLP of “a = 6.40 A” superimposed on a complicated powder diffraction picture is visible. These observations: (i) Clarify the origin of the unknown phase a = 6.46 ±0.06 A, observed by heating thin tin films on (lll)Si in the temperature interval 300—800°C,and discussed in ref. [1]. (ii) Show that the initial oxidation structure of tin
386
5K. Peneva et al. /RHEED study of initial States of crystallization of Ph and Sn
Fig. 5. (211)RLP of diamond cubic tin Sn(Si) superimposed
on complicated powder diffraction picture observed from massive 99.999% tin heated at 300°C for 2h 40 mm in vacuum of —S x 106 Torr and cooled down with the same cooling rate as the Sn/Si samples described in ref. [11. For the sake of clarity, the diffraction spots of Sn(Si) are in circles. The figure is not a typical RHEED photograph. Here and in figs. 6a, 7b, 8 and ha, part of the electrons give rise to a transmission electron diffraction image, obtained from the most protruding portions of the massive metals frozen like droplets.
with the diamond cubic lattice of Si, is SnO with the lattice of SiO, and exactly with the same lattice parametef, within the experimental error, as the a = 6.40 A of SiO. If this is so, one would expect undeformed SnO designated as SnO(SiO) to belong to the Pa3 space group. The initial valence state of SnO(SiO) at room temperature is expected both from the chemistry of tin andhappens from the of lattice to 2~.However, what to type the valence states be Sn of tin in this type of lattice and its 3DIS at higher temperatures is so tar unclear. Rudarska [2] has shown that it is observed among the oxides formed on massive tin up to 600°C; and DSMS results taken at room temperature [21 of chemically vapour grown tin oxide films, where this phase and its 3DIS are 4~tin only, present in remarkable quantities, show Sn Thin lead films are covered initially with an amorphous oxide layer which does not influence the RHEED photographs if the films are studied shortly after evaporation [10]. RHEED pictures of lead films exposed to air for several days show amorphous surface structure. If the samples are kept at several Torr (the vacuum chamber of the EF) for several days, the usually observed polycrystallmne picture is altered and the films become blueish (table 1).
Fig. 6(a) Supposed transitional structures between Pb(Si) and ordinary tin. Massive lead was heated for 25 mm at 400 ( in .
...
-
-
vacuum and cooled as described In IlL’. 4a. ~‘2O2 = d 022 = 1.85 A, and d2~0= 2.06 A. If orthorhombic deformation is assumed, the lattice parameters calculated from the (Ill )RLP are a = hthe = 5.83 A, for andthee mixed = 4.79 geometry A. The of remark in fig. 5 explains reason the diffraction pIioto~,raph. (b) As in (a) but the massive lead was heated for 25 mom at 350°C. The (Ill) REP gives d 20~= d0~2= 1.66 i\ and d2~0= 1.95 A. The lattice parameters would be a = 6 = 5.50 A and c = 4.15 A if orthorhonibic deformation is assumed. RHEED photographs of heated lead thin recrystallization of the amorphous oxidefilms, layerwhere may have taken place, and of the surface of freshly solidified massive lead in vacuuni. where reaction between the lead and the adsorbed oxygen has probably taken place, show large single crystal areas of similar “a = 6.40 A” phase (fIg’s. 8 amid ba ‘~)c).The data obtained from these RHEED photographs are the same as those of the SnO(SiO) discussed above and reported in ref. [2]. This lead oxide phase is accordingly designated as PhO(SiO). PbO(SjQ has
S.K. Peneva et al.
/ RHEED study of initial states of crystallization of Pb and Sn
387
Fig. 8. (111)RLP of PbO(SiO) with a = 6.36 A, observed on the surface of massive lead treated as described in fig. 4a. The interplanar distances for the 110 type planes are 4.50 A. The random variation of the intensity of the diffraction spots is an indication of structural disorder. The mixed geometry of the diffraction photograph was explained in fig. 5. —~==
500°C (fig. 10). In ref. [2], the phase “a
I, Fig. 7. (a) RHEED photograph of the textured along [100] thin tin film on (11 1)Si and presented in fig. 2b, after a 5 day stay in air (lll)RLP of deformed phase “a = 6.40 A” [21 is visible with a = 6.27 A b = 6.54 A and C = 5.70 A. Here di ol = 4.22 A, duo = 4.53 A and d0~1 = 4.30 A. (b) Deformed (111)RLP of the same phase “a = 6.40 A” observed on the surface of freshly solidified massive tin treated as described in fig. 5. The single crystal diffraction image is superimposed on a complicated powder pattern. The observed interplanar distances dioj = 4.15 A, duo = 4.44 A and d0~1= 4.32 A define a lattice with a = 6.02 A b = 6.58 A and c = 5.73 A. ‘ In both eases the calculations were performed by assuming orthorhombic deformations only. The mixed diffraction geometry in fig. 7b was explained in the remark of fig. 5.
been observed up to 700°C, but at this high temperature it is monoclinically deformed, Two more types of lead oxide with structures similar to the structures of the tin oxides reported in ref. [2] have been observed. A regular phase transformation between a deformed PbO(SiO) and a pseudocubic phase with a = 7.65 A has been observed by heating ‘-‘lOOO A thick lead films for 40 mm at
= 7.40 A” (massive tin), or “a = 7.78 A” (thin tin oxide films) is considered as tin perovskite, a crystal chemical analogue to CdTiO 3 and CaTiO3, as all three elements belong to the isomorphous series of tin Cd and Ti
[5], and Ca [4]. By extending the same approach to the lead oxide transformation observed at 500’~C (fig. 10), the observed phase “a = 7.65 A” can be regarded as lead perovskite, a crystal chemical analogue to CaZrO3, as Ca and Zr belong to the isomorphous series of lead [5]. It was thought that in chemically vapour grown tin oxide films, the transition, discussed in relation with fig. 10, occurs from “a = 7.78 A” to “a = 6.40 ,,
..
-
-
.
-
A
[2]. This point requires further clarification, since with the lead oxides, the same transition takes place in opposite direction. Fig. 11. shows several 3DIS of unknown lead oxide phase, designated as PbO~,observed on massive lead, and treated as described in fig. 2a. Similar structures are observed up to 500°C,by oxidation of massive tin in air [11]. Following the — so far close — similarity of the initial stages of oxidation of both metals, these oxides are considered to have identical structures and are the third oxide in the row of identical oxides of tin and lead. It is to be remarked that PbO~, and the accordingly designated SnO~, were observed so far on massive metal surfaces, and
388
S.K. Peneva et al.
/ RHEED study of initial states of crystallization of Pb and
Sn
Table 1 RHEED powder diffraction data obtained from =1000 A thick lead films evaporated onto (hll)Si Oxidized thin Pb films
Unoxidized thin Pb films
~1000 A 1
2
3
4
4.83 3.94 3.57
4.58
—
—
—
—
—
3.37
3.33
—
—
3.12
3.17 2.90
3.10 —
3.11 2.87
2.80
2.84
—
2.82
2.76 2.46
—
—
—
—
2.15
—
—
—
2.07—2.02 —
—
—
1.75
—
1.75
—
—
1.67
—
1.67—1.65
—
3.17
3.14
—
2.88
—
—
2.81
2.79
2.68 2.52 2.27
—
—
2.04—1.99 —
2.39 2.22 2.03 1.88
—
220/2.00 —
1.83 1.75 1.67
1.63
311/1.70
—
222/1.63
—
1.54
1.54
1.57
1.48 1.44
1.50 1.43
1.47
1.37
1.33 1.30
—
1.53 1.49 1.42 1.37 1.30
400/1.42
—
1.45 1.43 1.38 1.31 1.28
1.26
420/1.27
-
—
—
1.31—1.29
1.29
—
—
—
1.22—1.20
—
1.13
—
—
1.11
—
—
—
—
—
—
—
—
-
—
—
1.20
1.10 1.08
—
0.93
—
—
0.92 —
-
331/1.30
—
—
—
—
220/1.92 —
220/1.75 (31) —
311/1.63 222/1.56 —
— —
—
311/1.49 (32) 222/1.43 (9) 400/1.36 -
331/1.24 420/1.21 422/1.11
400/1.24
331/1.14 (10)
(7)
422/1.01
(6) (5) (5)
511/1.00
333/1.00 53 1/0.95
440/0.96
511/0.95
531/0.92
333/0.95
—
(2)
420/1.11 511/1.04 333/1.04
—
1.02 —
—
422/1.15
—
—
—
1.71—1.66
—
-
—
—
1.64
—
200/2.71
—
1.78 1.75
—
1.50 1.46—1.43
111/2.86 (100)
200/2.48 (50) —
1.99
111/3.13
200/2.83
—
1.23 1.22
-
111/3.27
2.96
—
—
10
—
—
—
9
--
—
—
8
-
3.32
1.87
1.61
—
7 —
2.04—2.00
—
1.55
Pb-fcc, ASTM 4-0686; hkl/d(A) (‘rel)
—
2.31
1.96—1.90 1.84
6
Pb(Si), a = 5.42 A; hkl/d(A)
—
—
—
5
‘3000 A
Pb(Si), a = 5.66 A; hkl/d(A)
600/0.94 442/0.94 600/0.90 442/0.90
Columns 1 and 2 contain data from samples oxidized at several Torr. Data from unoxidized samples (investigated almost immediately after evaporation) are included in columns 3—7. The samples described in column 7 is 3000 A thick. For reference: the table contains the interplanar distances of a = 5.42 A for Pb(Si), of a = 5.66 A for Pb(Si) and of Pb from ASTM 4-0686. 1 orbidden reflections for the diamond cubic structures are also included in the table.
are as a rule in incommensurate state (fig. 10) [11]. In very thin tin oxide films grown by chemical vapour decomposition oftin chlorides [21was observed
oxide phase of the fcc type, and a = 5.42 A. It was interpreted as the disordered state of SnO with the structure of CaF 2, the ordered state being the
Table 2 X-ray powder diffraction data obtained from 99.999% pure lead slabs X-ray results of Pb: d(A)/Irel
RHEED of
Pb,
Pb from
ASTM 4-06 86 hkl/d(A) (‘rel)
Pb(Si), a = 5.42 A: hkl/d(A)
PbO(SiO), I = 6.40 A: hkl/d(A)
PbO, ASTM 5-0570: hkl/d(A) (‘rel)
5
6
7
8
Unheated
Heated
Heated
1
2
3
column 3: d(A) 4
—
—
—
—
—
3.15/w
3.12/m~
3.12/vw
3.15
—
—
—
—
—
2.82/vs a)
2.85/s a)
2.83/s a)
2.84 a)
—
—
—
—
2.45/s a)
2.47/s a) 2.37—
2.47/rn~a) 2.33/w
2.48 a) —
—
—
—
—
111/2.86 (100) —
—
111/3.13 —
200/2.71
—
002/3.20 —
021/2.86
—
—
—
—
—
—
—
—
—
200/2.475 (50)
001/5.89
(3)
—
111/3.067 (100)
002/2.956 (31) 200/2.74
(28)
—
020/2.377 (20)
2.30/vw
2.12/vw
2.12/vw
221/2.13
—
003/2.1 3 —
—
—
1.91/m
1.95/w
1.93/vw
—
—
—
2.01 1.89 1.87
1.74/s a)
1.74/s a)
1.75/s a)
—
—
—
—
1.69
—
—
—.
—
—
—
1.53/w 1.48/vs a)
l.54/m
1.54/w
1.49/vs a)
1.49/vs a)
1.57 1.48
—
—
—
—
222/1.429 (9)
400/1.238 (2)
1.44/rn a)
1.43/m a)
1.42/mi a)
—
—
—
1.31/w
—
1.31/vw
—
—
—
1.42 a) 1.36 1.29 1.27
1.23/rn a)
h.24/vw a)
1.24/w a)
1.25 a)
—
—
—
1.22
—
h.14/m—s a) l.11/m_sa)
1.14/s a) 1.11/s a)
—
—
—
1.13 a) 1.08 a) 1.07
—
1.01/rn—s a)
1.01/s a)
—
—
0.954/rn a)
0.953/s a)
0.946
—
0.951/rn
—
—
—
—
—
—
—
—
—
—
—
—
—
—
0.838/m—s a)
— — —
220/1.75 (31)
—
311/1.49 (32) —
—
—
220/1.92 — —
311/1.63 —
222/1.56 — —
—
400/1.36
—
—
—
—
331/1.24
202/2.008 003/7.963 022/1.85 220/1.797 113/1.724 311/1.64
(12) (2) (14) (14) (15) (13)
041/1.55
222/1.534
(9)
033/1.51
213/1.514 (2) 004/1.474 (11) 131/1.474 (11)
—
311/1.92 —
032/1.78 — —
—
042/1.43
Etc.
—
422/1.31 —
511/1.23 333/1.23
—
—
331/1.136 (10) 420/1.107 (7) —
422/1.011 (6)
420/1.21
—
—
422/1.11
044/1.13 053/1.097
511/1.04
062/1.01
—
—
33 3/1.04
—
—
(?) (?)
0.927 0.886
511/0.953 (5) 333/0.953 (5) — —
0.889/vw 0.8751w a)
—
—
—
—
0.865/vw
—
—
0.838/s a)
—
0.836/rn—s a)
0.826/vs a)
—
0.826/m
0.824/s
—
—
440/0.875 (1)
53 1/0.837 (9) 600/0.825 (4)
440/0.96 —
531/0.92 600/0.90 442/0.90 —
620/0.86
—
007/0.91 —
641/0.879 721/0.871 633/0.870
533/0.83
731/0.833
622/0.820
553/0.833 643/0.819
442/0.825 —
—
650/0.819 —
—
Column 1: kept for a while at several Torr after cutting. Column 2: heated in vacuum of 5 X 10~Torr at 300°Cfor 1 h 40 mm and cooled for 1 h to 20°C. Column 3: heated in vacuum of 5 X 10~Torr at 400°C for 25 mm and cooled for 1 h 30 mm until it teaches 20°C. Column 4: RHEED data from the surface of the sample discussed in column 3. The last part of the table includes theoretical interplanar distances (including the forbidden ones for the diamond Pb(Si). vs = very strong, s = strong, rn—s = medium to strong, m = medium, w = weak, vw = very weak. a) Diffraction lines of the fcc lead.
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~
lig. 10. Regular phase transition between defornied PbO(SiO and a pisase defined as the lead perovskite, following time conclusions in ref. 2, and the great similarity between lead and tin reported here, The brighter spots are of the 220 type, and arc belonging to the pernvskite. The three interplanar distances of the 110 type are 4.68, 4.64 and 4.37 A, and are defining a lattice with a = 6.23 .k, 6 = 7.09 A and c = 6.14 A (nrtlmorhombic deformation is assumed), There are several 3DIS of the PbO(SiO) an the RHEFD photograph. The three 220 type interplanar distances are alnmost equal and define an almost undeformed perovskite with a = 7.65 A.
Fig. 9. (11 1)RI.P of PbO(SiO) observed on the surface of —1000 A thick lead films, heated in vacuum of —5 X 10~ Torr, heating conditions: (a) 300°C,hh 40 mm; fb) 400°C, 1 h 30 nmin, (a) An almost undeformed PbO(SiO) lattice with a = 6.37 A (three almost equal interplanar distances of the 110 type with d = 4.51 A), but with structural disorder, (b)d 10~ = 4.64 A, duo = 4.88 A and d0~5= 4.73 A define a PbO(5iO) lattice with a = 6.75 A, 6 = 7.05 A and c = 6.39 A, again with structural disorder in the lattice, The calculations were performed by considering orthorhombie deforniation of the lattice. Here, and in figs, 10, 1 2a and 12c, the diffraction picture is not typical RIIEED, since it is obtained from the edges of the corresponding sanuples, and part of the electrons give rise to transmission electron diffraction pictures, Fig. 11, (a) RIIEED photograph of unknown lead oxide in incommensurate state observed on tIme surface of massive lead treated as described in fig. 4a. Exactly the same type of oxide has been observed in the initial stages of oxidation of tin in air [11].Sketch of tIme RHEFD photograph is shown in (b). The interplanar distances and the angles between the planes nornials are: d1 = 3.72 A, d2 = 4.47 A, d3 = 3.77 A, o = 70°,p = 5 1.5°,‘y = 58.5°.The diffraction photograph is not of pure RHEED type. See time remark in fig. 5.
d?
~=
[
~ /
/
b
jl
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391
ordinary c~-SnO.The phase observed in ref. [1], and
that case the phase a = 5.43 A in the present study,
designated as Sn(Si) with diamond cubic structure and a = 5.43 A, is not an oxide according to the MOssbauer results, and the considerably slower rate of
obtained at similar conditions where Sn(Si) exists, is
oxidation of tin as compared to lead. According to ref. [10], lead surfaces become oxidized in air, but the oxides are amorphous and do not hinder RHEED
in chemically vapour grown tin oxide films (TOF) [21, was also considered. The very thin tin oxide films, where the phase treated as SnO(CaF2) was
observations carried out shortly after evaporation. In
detected [21, were grown at 420°C,simultaneously
Pb(Si). The opposite possibility, Sn(Si) to be observed
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Fig. 12. Different RLP of Pb(Si) observed by heating in vacuum of—S X 10—6 Torr of —1000 A thick lead films on (ll1)Si. The samples are from different evaporation experiments and each of them was treated at the particular conditions listed below. For (a) and (c), the remark in fig. 9b is valid. (a) Heating temperature 300°C,heating time 33 mm. Below the melting point of lead the diffraction picture is complicated. (123)RLP of Pb(Si) is visible, corresponding to a lattice with a = 5.66 A. Here diii = 3.26 A, d 420 = 1.27 A, and the angle between both reciprocal lattice vectors is 75°.Here, the powder data correspond to Pb(Si) with the same lattice parameter as observed for the single crystal Pb(Si) region. (b) Heating temperature 350°C, heating time 29 mm. (211)RLP of Pb(Si) with a = 5,53 A is visible; d1j1 = 3.19 A, d02~=
1.94 A.
(c) Heating
temperature 400°C,heating time 25 min. (111)RLP of Pb(Si). The interplanar distances of the 220 type planes are = 5.42 A. The 110 RLP of the silicon lattice is on the left handside of the figure (transmission electron diffraction from an edge of the substrate), showing the orientation of the substrate towards the lead film,
1.92 A, and correspond to a lattice with a
(d) Heating temperature 400°C,heating time 1 h 40 mm. 3DIS of Pb(Si) are visible. (211) of one of them show dju i
=
3.09 A,
and do2l = 1.88 A, corresponding toa = 5.36 A. (e), (f) The heating temperature for (e) is 350°C,1 h heating time and for (f) 500°C,1 h heating time. (110)RLP of Pb(Si) with a = 5.42 A is visible on both figures withdu i~ = 3.12 A and d002 = 2.72 A. (g), (h) Heating temperature 500°C, lii heating time, (I l0)RLP (g) and (123)RLP (h) of Pb(Si) with a = 5.66 A are visible. = 3.29 A, d002 = 2.83 A and d420 = 1.30 A.
on (111 )Si, fused quartz and (1 00)NaCI. At these conditions, the following reaction is possible 2 SnO —~ Sn02 + Sn
inverse layer at the boundary lead/silicon [11, with some phase, defect, superstructure or crystalline state
in the film. Details of these results will be published separately, but the conclusions correlated with the
However, if the phase a = 5.42 A was tin, it would have been detected on all substrates, which was not observed. Moreover, the so far unpublished results of the Mössbauer studies on TOF, would have indicated
topic of the present paper are: (i) Ph(Si) exists up to 700°C(the highest temperatore used for heat treatment of Pb/Si samples), with
Therefore, it is concluded that the existence of
lattice parameter “a” varying in an unpredicted way between a = 5.66 A and a = 5.36 A (figs. 12a—12h), Heating below the melting point of lead (fig. 12a)
SnO(CaF2) is governed by the influence of the silicon and NaCl substrates.
leads mainly to polycrystallmne films. Occasionally single crystal diffraction pictures, superimposed on a
3.3. Structure of heated -°1000A thick lead films on (11J)Si, in the temperature interval 300—800°C
powder diffraction pattern, were observed. The tendency for single crystal growth increased considerably above the melting point of lead, actually above
Structural investigations of heated lead films are carried out to correlate eventually the existence of an
example of how the lattice of Pb(Si) fits on top of the silicon lattice. Fig. l2h shows that above 500°C
the presence of Sn(Si), which is again not the case,
350°C for the present study). Fig. 12c is a good
Fig. 13. Reciprocal lattice projections of what we consider to be a-Pb, corresponding to the diamond cubic cs-Sn (a = 6.49 A). This phase is always in incommensurate state. As in figs. 12, the samples are usually frorn different evaporation experiments, and each sample was treated at the particular conditions listed bellow. -‘-1000 A thick Pb films on (111)Si were studied. (a) 3DIS of a-Pb observed on (123)RLP. One of the incommensurate structures is with a = 6.87 A. Heating temperature 300°C, heating time 33 mm. (b) The disorder in the 3DIS cs-Pb increases by increasing the heating time. Here the heating temperature is again 300°C, but the corresponding sample was heated for 1 h 7 mm. The phase is no more cubic. One of the 3DIS is with a = 5.94 A and b = c = 6.22 A. Here d 20~= d0~2 2.15 A, d2~0= 2.20 A (orthorhombic deformation is assumed). (c) Further increase of the disorder by increasing the heating time to 1 h 40 mm at the heating temperature 300°C. One of the 3DIS is with a = b = 5.60 A, c = 6.32 A, corresponding to d0~2= d20~= 2.10 A and d2~0= 1.98 A (orthorhombic deformation
is assumed). (d) New phase obtained from a-Pb;heating temperature 350°Cheating time 56 mm. By comparing figs. 12c and 12d, one can see the geometry of the phase transformation. One of the 3DIS shows two almost perpendicular reciprocal lattice vectors, corresponding to interplanar distances 5.69 and 2.50 A. (e) Superstructure, considered to be connected with the phase transitions of the incommensurate a-Pb. Heating temperature 400°C, heating time 25 mm. Both reciprocal lattice vectors are at 90°C towards each other. The corresponding interplanar distances are 5.64 and 1.22 A.
5K. Peneva et at /RHEED study of initial states of crystallization of Ph and Sn
394
there is a considerable amorphous layer on the surface of the lead samples. RHEED investigations of “—200 A thick thin lead films on (11 l)Si, heated at the same conditions as the -“-1000 A thick lead films, showed that the (Pb(Si) lattice near the surface is expanded, and that increasing of the heating time increased the lattice deformation. For example, 50 mm heating of such ~200 A film at 400°C leads to a Pb(Si) lattice with a 5.70 A. Heating for 1 h 15 mm transformed the cubic Pb(Si) lattice to an eventually orthorhombic lattice with a 5.86 A, b 5.72 A and c 5.43 A. (ii) It is thought that one of the phases observed in heated thin lead films imitates the structure of the a-Sn, and is accordingly designated as a-Pb (figs. 13a—13e). It is always in incommensurate state, and with increase of the temperature one can see how the numerous incommensurate structures deform themselves in more primitive lattices, clearly visible by
=
=
=
part comparing, of the e.g., unexplained figs.created 13cthat RLP, and illustrated It of isphases. thought in fig. that 13e are transitions as anindications example, between (fig. are 14) incommensurate asthe a13d. result so-called similar a-Pb and There phase its derivates can exist at higher temperature but such RLP were observed relatively rarely. as well,
3,4. Powder diffraction data obtained by RHEED and X-ray diffractometer measurements of unheated “-‘-1000 A thick lead films on (111)Si and of freshly solidified massive lead The formerly described single crystal results indicate that indexing of RHEED and X-ray powder diffraction data (tables 1 and 2) is not straightforward. X-ray analysis of massive, freshly cut lead, kept at several Tort for a while, shows not only the theoretical lines of fcc lead, but also some extra lines with negligible intensity (table 2, column 1) interpreted as lines of PbO(SiO). Heated nmassive lead samples at 300°C for 1 h 40 mnin (column 2) and for 25 nun at 400°C (column 3) and cooled for 1 Is from 300 to 20°C and for I h 30 mitt from 400 to 20°C, do not show a remarkable change of the diffraction picture , If RHEED single crystal results are not accounted for, the interpretation would be misleading, since there is overlapping of the reflections even when tile phases are with the theoretical interplamiar
—
—
Fig. 14.
3DIS of intermediate phases resembling those
discussed in connection with fig. 13. The —1000 A thick lead film on (1l1)Si was heated for 50 mm at 600°C.The 000 points of time 3DIS phases are displaced over several A. (211)RLP of one 3DhS phase shows d~ d02~= 1.86 A, defining a lattice with a c = 5.26 A.
=
11 = 3.21 A and 6.37 A and h =
distances, as shown in table 2. The data from this table show that the freshly solidified lead contains Pb (fcc) (ASTM 4—0686) and textured along the [100] direction PbO(SiO) -— note the presence of 002, 003, 31], 041 and 422 reflections, Actually PbO(SiO is textured along the [100] direction even on the unheated oxidized lead sanmple (column I). Tile presence of Ph(Si) cannot he unamTmbiguously detected with tile conventional X-ray technique used in the present study. However, time intensity ol sonme lines of the I’cc lead, especially thosc with imlterplanar distances below 1.14 A, is considerably lsiglmer than expected from time theory, amid cannot he explained by ams eventual increase of tile PbO(SiO) since tile freshly cut lead has been heated iii vacuum
S.K. Peneva et al.
/ RHEED study of initial states of crystallization of Pb and Sn
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and investigated immediately after cooling. Moreover, RHEED results indicate that the oxide islands are here and there on the surface. Therefore, this unusual increase of the intensity can be attributed to the presence of Pb(Si). This conclusion is in accordance with RHEED results and if it proves, by more a sophisticated contemporary X-ray technique, to be correct, it would mean that the growth of Pb(Si) is a volume effect. The RHEED powder data obtained from the surface of the sample, described in column 3, are included for comparison in column 4. It is obvious that the surface structure is far more complex, a result already mentioned in connection with the single-crystal analysis. The extra line d = 2.32 A is explained, according to refs. [9,10] as a reflection of the orthorhombic (ASTM 5-0570). RHEED powder diffraction data of unheated “-‘-1000 A, and in one case “—3000 A, thick lead films are illustrated in table 1. Columns 1 and 2 include data from oxidized films (the first sample has remained at several Tort for one day, and the second sample for more than 10 days — columns 1 and 2, respectively). RHEED investigations of the other samples were carried out almost immediately after their removing from the evaporating system. All the samples described in table I are from independent evaporation experiments. The polycrystalline part of the lead films (unoxidized, columns 3—7) contain ordinary fcc lead and Pb(Si) with deformed lattices. The interplanar data vary from sample to sample, which is in accordance with the single crystal results. We did not dare to perform a line-to-line indexing of these multiphase systems, but by using the single crystal results, one can have a very general idea about the phase composition of the film. The data from figs. 8, 9a and 9b indicate that interplanar distances of the order of
diffraction from the 100 plane of some substructure of a-Pb, as discussed in connecting with figs. 13d, 13e and fig. 14. The unknown peak corresponding to d = 5,67 A in ref. [9] can be interpreted by either of these two possibilities. The results of the present study were compared with the data of Ovadyahu [7], but the published diffraction results, as pointed out in ref. [8], can be interpreted as corresponding to the orthorhombic PhD. However, the describtion of his results can somehow be correlated to the data of the present work, especially when he states that increase of the evaporation rate, and ours is quite high, leads to ample quantity of what he describes as double hexagonal lead. It is hoped that single crystal THEED and RHEED of films of his type may clearify this point.
4.50 A can be 110 type reflections of PbO(SiO). However, the existence of the phase PbO~(fig. 1 Ia)
These oxides are called SnO(SiO) and PbO(SiO) and are often in incommensurate state. [6]. It is con-
is not to be neglected despite the fact that so far its presence has been observed on massive lead only. Interplanar distances in the range of 5.50 A can belong to 110 reflections of deformed perovskite. Powder data of tin oxide films, where such interplanar distances have been calculated, have been interpreted in that way. However, for unoxidized lead films, such reflections can be interpreted as
sidered that the initial valence state of tin in 2”, but according to the room temperaSnO(SiO) is Sn ture DSMS results, a transformation to the Sn4” state in the same structure is not to be neglected [21. Phase transformation of PbO(SiO) into a cubic phase with a = 7.65 A has been observed. A similar phase transformation takes place with SnO(SiO [2]. It is interpreted, following refs. [4,5], and the
4. Conclusions The results of the present single crystal RHEED study can be summarized as follows: Lead with the structure and lattice parameter of silicon is formed after evaporation onto (11 l)Si. This crystalline state of lead, Pb(Si), and the already reported [I] identical crystalline state of tin on (11 1)Si, Sn(Si), were found to be one of the initial crystal structures of both metals, formed after solidification of the massive metals in vacuum of “-‘S X 10—6 Torr. Islands of different types of oxide were formed on the surface of the massive metals — tin and lead — and on their respective thin films on (II l)Si, during heating in the conventional vacuum of “—5 X lO_6 Torr. The initially formed oxide on Sn(Si) and on Pb(Si) closely resembles SiO, and has the same lattice parameter as SiO (within the limits of error).
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close siniilarity of the initial stages or crystallization and oxidation of both metals, as the lead 24Pb44O perovskite Pb 3,
The similarity of the initial stages of oxidation of both metals has been observed and by comparing the RLP of unknown tin [111 and lead oxides, designated as PbO~ and SnOt. These types of oxide were observed so far on massive metals only and are as a rule in incommensurate states The present paper includes some data on the crystalline state of Pb(Si) in thin lead films heated to 700°C. It is shown that its lattice is considerably deformed in ~‘200 A thick films. One phase in the heated thin lead flints, appearing always ill incommensurate state, closely resembles a-Sn, and for convenience is called a-Pb, It appears in a certain temperature interval, and by increasing tile heating time at any temperature in this temperature interval, it disproportionates. It is shown that phase transition of the 3DIS of a-Ph is one of the ways of substructure and (or) superlattice formation. An attemiipt has been made in the present study to use the single crystal RHEED results for interpretation of the complicated X-ray and RHEED powder data of polycrystalline lead f’ilms. The former results require a restatement of the crystal chemical approach [4,5] for structure analysis adopted for studies of multiphase metastable systems. At least for the metals of the fourth group of the Periodic Table, the initial stages of crystallization and oxidation are identical (for sonme of the phases) and closely resemble the bahaviour of silicon. It appears that the electron state of silicon, tin, and lead are
of Pb and Sn
governed by sinmilat factors for quite some time, leading to great similarity not only between the initially grown phases of both metals but also, to identical chemical behaviour (oxidation in our case). In other words, it takes time for each element to achieve its “-personality”. It has been found that the formerly discussed metastable phases get stabilized its thin films,
References [11 K.D. Djuneva, S.K. Peneva, E.A. Tsukeva and I. Batov, Thin Solid Films 67 (1980) 371. [21 5K. Peneva, R.K. Rudarska, D.D. Nihtianova and l,Z. Kostadinov, presented at 6th Intern. Conf, on Crystal Growth Moscow, 1980, and other papers given at the(ICCG-6), Conference.
[31 E.G. Rochow and E.W. Abel, in: The Chemistry of Germanium, Tin and Lead, Vol. 14 (Pergamon, Oxford, 1975) p. 119. [41 V.M. Goldschmidt, Geochemische Verteilungsgesetze, Vol. VII (1926). [5] K.A. Vlassov, The Periodic Low and the Isomorphism (Institute of Mineralogy and Geochemistry, Moscow) (in Russian).
[61 V.L. Pokrovski, Solid State Comnmun. 26 (1978) 77; L.A. Bolshov et al., Usp. Fiz. Nauk 122 (1977) 125. [71 Z. Ovadyahu, 3. Phys. F (Metal Phys.) 8 (1978) 403. [81 G,D.T, Spiller and P.J. Dobson, J. Phys. F (Metal Phys.) 8(1978) L135. [91 TB. Light, 3M. Elridge, J,W. Matthews and J.H. Greiner, 3. Appl. Phys. 46 (1975) 1489. [101 J.W. Matthews, C.J. Kircher and RE. Drake, Thin Solid Films 42 (1977) 69; 47 (1977) 95,
[11] R.K. Rudarska, private communication.
RHEED STUDY OF THE INITIAL STAGES OF CRYSTALLIZATION AND OXIDATION OF LEAD AND TIN S.K. PENEVA Department of Physical Chemistry, Chemical Faculty, University of Sofia 1, Anton Ivanov Road, Sofia 1126, Bulgaria
and K,D. DJUNEVA and E.A. TSUKEVA Department of Atomic Physics, Physical Faculty, University of Sofia, 5 Anton Ivanov Road, Sofia 1126, Bulgaria Received 10 September 1980; manuscript received in final form 20 November 1980
RIIEED investigations of the initial stages of crystallization and oxidation of massive lead and tin and their thin films on (11 flSi are discussed. Diamond cubic structures with lattice parameters around the lattice parameter of sillcon exists both in thin lead and tin films, aod in massive lead and tin, freshly solidified in vacuum of—S X 10 Torr. Experimental evidence for almost identical structures of at least three oxide types is presented, demonstrating that lead and tin behave like silicon not only in the initial stages of crystallization, but also in the initial stages of oxidation. The abeady discussed tin phase with a = 6.40 A has been identified as a tin analogue of SiO. Some data on the phase composition of thin lead films, heated to 700°C, are reported. One of the phases in heated lead films was interpreted as a lead analogue of u-Sn.
1. Introduction Recent Reflection High Energy Electron Diffraction (RHEED) and Depth Selective Mossbauer Spectroscopy (DSMS) studies on evaporated in vacuum of ‘~-‘5X 10—6 Torr, heated and unheated, ~‘S0O A and 1000 A thick tin films, deposited onto (11 l)Si, have shown that the tin has diamond cubic structure with a = 5.60 ±0.06 A, and has a very positive value (up to a certain temperature) of the isomeric shift (4.2 ± 0.34 mm 5—t) relative to Sn02 [11. The present study is RHEED proof that identical structure exists in thin lead films evaporated under the same conditions onto (11 l)Si and treated as described in ref. [1]. It is shown that this crystalline state of both metals in thin film state is one of the initial phases formed after solidification in vacuum of molten massive tin and lead, Islands of oxides were observed on thin lead and tin films, heated in the formerly mentioned conventional vacuum, and on the surface of the massive lead and tin, melted and solidified in vacuum. The present paper demonstrates the great similarity between the oxides grown initially on the surfaces of both metals
and on their thin films, and correlates the observed structures with the metastable oxide phases of tin described in ref. [21. As in ref. [2], the crystallographic analysis is carried out by accounting the structure of the known lead oxides (ASTM Table and, e.g., ref. 131), and by considering the structures of the oxides from the isomorphous series of lead, derived according to refs. [4,5]. More attention is given to lead and its oxides, since some data on tin and its oxides have already been published [I ,2]. Different coexisting crystalline states were found for lead and its oxides as well, with lattice parameters randomly varying in certain limits. They are considered, as in ref. [2] . to be an indication for three-dimentional incommensurate states (3DIS) 16]. Thin lead films are important superconductive materials and lead oxides are isolators. Oxide-free lead films are usually obtained in vacuum of not worse than I 0’~Torr. The usual structure of lead is fcc, but X-ray powder data supporting the existence of hexagonal phases in ultra thin lead films are reported by Ovadyahu [71.There are objections [8], considerlug the results of ref. [7] ambiguous, despite the fact that the interpretation in ref. [8], that it is a ease of
0022-0248/8l/0000—0000/$02.50 © North-Holland Publishing Company
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oxidation of the lead surface, cannot explain all the results reported in ref. [7]. Detailed analysis of the oxides formed on the surface of thin lead films can be found, e.g., in the works ofLightet al. [9] and Matthews et al. [10]. It is shown that exposure to oxygen even when the substrate temperature is 240 K leads to an ample quantity of yellow PbO (ASTM 5-0570) with bigger crystallites of red PbO (ASTM
diffractometer technique, with filtered Cu-Ka radiation. RHEED was carried out in an EF4 (Karl Zeiss) operating at 65 kV, with TlCl external standard. A point-to-point revaluation of the error 1~d/dwas performed by taking into account all specific geometrical limitatioi~isof the EF4 [11], putting the error Mid for RHEED from 1.5% up to 3% and above (if, e.g.,
5-0561) on the top, here and there on the surface. In air, freshly evaporated lead was covered with unidentified amorphous oxide [10].
Debye rings corresponding to interplanar distan~esof the order of 5 A and above are measured). Car’ was taken in the present study that the error for ~ingle crystal RHEED photographs did not exceed 2~. It ranged from 1.5% to 2%. For interplanar distances measured by X-ray technique, the error was 0.1%.
3. Experimental techniques It was aimed to obtain as many metal/Si samples as possible, since structural investigations of metastable phases require good statistics. Weighted amounts of99.999%tin or lead (Prolabo, France) were evaporated onto clean (11 l)n-Si [1] in vacuum of ~-~5X 10_6 Torr from Ta(Sn) and Mo(Pb), or Ni(Pb), evaporators. The silicon substrates were kept at the required distances, on a hemisphere, so as to obtain approximately the desired thicknesses of 200 A and 1000 ±100 A (checked by weight measurements). The rate of evaporation was arbitrary since the main structural and electrophysical results were carried out for metal/Si (MS) samples heated above the melting point of the respective metal. After evaporation the samples were removed from the sample holders and placed in a quartz resistance furnace. The heating was carried out again in the same vacuum, but there was a brake of the vacuum for about half an hour leading, according to ref. [10], to an unidentified amorphous layer on the surface of the lead samples. The oxidation of tin appears to follow similar pattern with slower rate. A chromel/alumel thermocouple was attached to the tantalum sample holder next to the MS samples to check the temperature, which was kept constant within an accuracy of ±5°C. 99.999% pure slabs of massive tin and lead were melted in the same vacuum chamber in crucibles of the same metal as the evaporators of the respective metal, and cooled with the same cooling rate as the corresponding MS samples. The initially grown phases in the massive metals were studied, for the sake of comparison, both with RHEED and standard X-ray
3. Results and discussions RHEED results are based on single crystal diffraction analysis of several tens of mainly —~l000A thick lead films on (11 1)Si obtained from about 20 independent evaporation experiments. Powder data were also considered, but their interpretation is ambiguous since the systems are multiphase with some of the structures in incommensurate state [6].
3.1. Structure of unheated ~1OOOA thick Pb films on (111)Si; comparison with the structure of unheated ~1OOO A Sn films on (111)Si and with the structure of the phases observed on massive tin and lead, freshly solidified in vacuum Freshly evaporated lead films showed greater tendency for single crystal growth than tin films [1]. Decrease of the evaporation rate increased the single crystal area of the lead films, and the polycrystalline tin film became textured. Only once (fig. 1) it was possible to observe a RHEED photograph of what is thought to be (123)RLP of orthorhombically deformed lead with a = 4.87 A, b = 4.54 A and c = 5.04 A, but this was an exception. The usually obserbed structures, both for lead and tin films on (11 1)Si, are diamond cubic with lattice parameters either close to the lattice parameter of silicon (a = 5.42 A), or to the lattice parameter of germanium a = 5.66 A), occasionally in incommensurate state [61. These structures are designated as Sn(Si) and Pb(Si). An illustration of(llO)RLP of diamond cubic
384
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Fig. 1. (123) reciprocal lattice projection (RLP) of what is
= =
=
thought to be orthorhombicaily deformed lead observed on freshly evaporated =~1000A thick lead films on (ll1)Si; 2.77 A, d 331 1.09 A, and the angle between both reciprocal lattice vectors is 86 ± 1°.The polycrystailine data are given in column 5, table 1. (Ilere and further the error in measuring interplanar distances is taken at the upper linnit ~d/d 2%.)
___________________________________________
lead is shown in fig. 2a, and [100] texture of diamond cubic tin can be seen in fig. 2b. A superimposition of two RLP, one belonging to the [100] texture Sn(Si) with a 5.60 A, and the other to the single crystal region of Sn(Si) with a 5.5 1 A, is seen in fig. 2c. Single crystal areas on unheated Sn films are very rare. An interesting observation was the lack of fitness of the Pb(Si) lattice on top of the silicon lattice, observed both for the single crystal area of unheated and heated Pb films, figs. 2a and 3, respectively. Both
=
=
a
RHEED Pb(Si) ness plane. in and the photographs silicon orientation are show parallel, of both that lattices the there (11in 1)and isthe planes random(111) of structures The fact are that not diamond promoted cubic by but the Pb(Si) influence Sn(Si) of the silicon substrate, but that their existence is only stabilized in thin film state on top of the silicon was confirmed by RHEED and X-ray studies (table 2) of freshly crystallized massive lead and tin sam-
Fig. 2. (a) (110) RLP of diamond cubic lead, designated as Pb(Si), observed in ~1000 A thick lead films on (11 l)Si. The values of the mterplanar distances are d fj = dj it = 3.11 A and d002 = 2.73 A, corresponding, within the limits of error, to the lattice parameter a = 5.42 A. The diffraction image
raph was already shown in ref. [11. (e) The same as in rb), but (11 1)RLP of Sn(Si) with a = 5.51 A (d220 = 1.95 A) is superimposed on the RLP of the 11001 texture. Single crystal areas of Sn(Si) on unheated tin films are exceptions.
shows that the (ill) planes of Pb(Si) and the silicon substrate are parallel, but the 111 type reciprocal lattice points (the elongated ones belonging to Si) are not parallel, as it would have been expected by epitaxial growth. (b) 11001. texture of diamond cubic tin with a = 5.60 A obtained from =1000 A thick tin film on (111) Si substrate at slow evaporation rate. Indexing of such RHEED photog-
S.K. Peneva et al.
/ RHEED study of initial states of crystallization of Pb and Sn
385
Fig. 3. RHEED photograph of —1000 A thick lead film on (11l)Si heated for 2 h 16 mm at 700°C, in vacuum of ~ x 10~ Torr. Part of the lead has got evaporated and RLP of both lattices are visible. They show that (111) planes of Pb(Si) and Si are parallel but the 111 reciprocal lattice vectors are not parallel, as it would have been expected if the Pb(Si) grew epitaxially on the top of the silicon. (211)RLP of at least two 3DIS of Pb(Si) can be seen. One of them is witha = 5.43 A,/diii = 3.15 A and d 02~= 1.93 A.
pies. Large single crystal regions of diamond cubic metals with the same lattice parameters as the Pb(Si) and Sn(Si) in thin films on (11 l)Si were observed, and they are illustrated in figs. 4a and b for the case of massive lead, and in fig. 5 for massive tin. Fig. 5 is an illustration of the lower tendency for single crystal growth of Sn(Si) in massive tin as compared to the large single crystal regions of Pb(Si) observed on the surface of massive lead (figs. 4a and 4b). Figs. 6a and 6b show deformed single crystal regions of diamond cubic lead with one of the lattice parameters being less than 5 A. It is thought that such structures can be regarded as one of the possible ways of formation of the ordinary fcc lead in the massive metal,
3.2. Structures of oxides formed on unheated and heated, -~1OOOA thick Sn and Pb films on (111)Si; correlation with the initially formed oxide structures on the surface of massive tin and lead Tin and lead get readily oxidized. Fig. 7a shows how the [1001 textured Sn(Si) thin film, illustrated in fig. 2b, has changed after a 5 day stay in air. (1l1)RLP of deformed oxide phase, designated as “a = 6.40 A”, and discussed in ref. [2], is visible. Fig. 7b shows the surface of massive tin heated at 300°C
Fig. 4. (a) Pb(Si) large single crystal region formed on the surface of freshly solidified massive 99.999% lead heated in vacuum of —5 x 106 Torr at 400°Cfor 25 mm, and cooled at the same cooling rate as the thin lead films heated at the
same conditions for further electrophysical measurements 111. (110) RLPa of several 3DIS with lattice parameters varying around = 5.66 A are observed. One of them is with d1 j~ = 3.24 A and d002 = 2.84 A. (b) (211)RLP of another single crystal region. Here the Pb(Si) lattice is with deformations (orthorhombic or even monoclinic). The measured ~ = 3.20 A and d02~ 1.98 A a interplanar 5.45 A, bdistances c = 5.59areAdjorthorhombic deformation.
in vacuum of ~5 X 10-6 Torr for 2 h 40 mm and cooled with the same rate as the thin tin films. A similar deformed (11 l)RLP of “a = 6.40 A” superimposed on a complicated powder diffraction picture is visible. These observations: (i) Clarify the origin of the unknown phase a = 6.46 ±0.06 A, observed by heating thin tin films on (lll)Si in the temperature interval 300—800°C,and discussed in ref. [1]. (ii) Show that the initial oxidation structure of tin
386
5K. Peneva et al. /RHEED study of initial States of crystallization of Ph and Sn
Fig. 5. (211)RLP of diamond cubic tin Sn(Si) superimposed
on complicated powder diffraction picture observed from massive 99.999% tin heated at 300°C for 2h 40 mm in vacuum of —S x 106 Torr and cooled down with the same cooling rate as the Sn/Si samples described in ref. [11. For the sake of clarity, the diffraction spots of Sn(Si) are in circles. The figure is not a typical RHEED photograph. Here and in figs. 6a, 7b, 8 and ha, part of the electrons give rise to a transmission electron diffraction image, obtained from the most protruding portions of the massive metals frozen like droplets.
with the diamond cubic lattice of Si, is SnO with the lattice of SiO, and exactly with the same lattice parametef, within the experimental error, as the a = 6.40 A of SiO. If this is so, one would expect undeformed SnO designated as SnO(SiO) to belong to the Pa3 space group. The initial valence state of SnO(SiO) at room temperature is expected both from the chemistry of tin andhappens from the of lattice to 2~.However, what to type the valence states be Sn of tin in this type of lattice and its 3DIS at higher temperatures is so tar unclear. Rudarska [2] has shown that it is observed among the oxides formed on massive tin up to 600°C; and DSMS results taken at room temperature [21 of chemically vapour grown tin oxide films, where this phase and its 3DIS are 4~tin only, present in remarkable quantities, show Sn Thin lead films are covered initially with an amorphous oxide layer which does not influence the RHEED photographs if the films are studied shortly after evaporation [10]. RHEED pictures of lead films exposed to air for several days show amorphous surface structure. If the samples are kept at several Torr (the vacuum chamber of the EF) for several days, the usually observed polycrystallmne picture is altered and the films become blueish (table 1).
Fig. 6(a) Supposed transitional structures between Pb(Si) and ordinary tin. Massive lead was heated for 25 mm at 400 ( in .
...
-
-
vacuum and cooled as described In IlL’. 4a. ~‘2O2 = d 022 = 1.85 A, and d2~0= 2.06 A. If orthorhombic deformation is assumed, the lattice parameters calculated from the (Ill )RLP are a = hthe = 5.83 A, for andthee mixed = 4.79 geometry A. The of remark in fig. 5 explains reason the diffraction pIioto~,raph. (b) As in (a) but the massive lead was heated for 25 mom at 350°C. The (Ill) REP gives d 20~= d0~2= 1.66 i\ and d2~0= 1.95 A. The lattice parameters would be a = 6 = 5.50 A and c = 4.15 A if orthorhonibic deformation is assumed. RHEED photographs of heated lead thin recrystallization of the amorphous oxidefilms, layerwhere may have taken place, and of the surface of freshly solidified massive lead in vacuuni. where reaction between the lead and the adsorbed oxygen has probably taken place, show large single crystal areas of similar “a = 6.40 A” phase (fIg’s. 8 amid ba ‘~)c).The data obtained from these RHEED photographs are the same as those of the SnO(SiO) discussed above and reported in ref. [2]. This lead oxide phase is accordingly designated as PhO(SiO). PbO(SjQ has
S.K. Peneva et al.
/ RHEED study of initial states of crystallization of Pb and Sn
387
Fig. 8. (111)RLP of PbO(SiO) with a = 6.36 A, observed on the surface of massive lead treated as described in fig. 4a. The interplanar distances for the 110 type planes are 4.50 A. The random variation of the intensity of the diffraction spots is an indication of structural disorder. The mixed geometry of the diffraction photograph was explained in fig. 5. —~==
500°C (fig. 10). In ref. [2], the phase “a
I, Fig. 7. (a) RHEED photograph of the textured along [100] thin tin film on (11 1)Si and presented in fig. 2b, after a 5 day stay in air (lll)RLP of deformed phase “a = 6.40 A” [21 is visible with a = 6.27 A b = 6.54 A and C = 5.70 A. Here di ol = 4.22 A, duo = 4.53 A and d0~1 = 4.30 A. (b) Deformed (111)RLP of the same phase “a = 6.40 A” observed on the surface of freshly solidified massive tin treated as described in fig. 5. The single crystal diffraction image is superimposed on a complicated powder pattern. The observed interplanar distances dioj = 4.15 A, duo = 4.44 A and d0~1= 4.32 A define a lattice with a = 6.02 A b = 6.58 A and c = 5.73 A. ‘ In both eases the calculations were performed by assuming orthorhombic deformations only. The mixed diffraction geometry in fig. 7b was explained in the remark of fig. 5.
been observed up to 700°C, but at this high temperature it is monoclinically deformed, Two more types of lead oxide with structures similar to the structures of the tin oxides reported in ref. [2] have been observed. A regular phase transformation between a deformed PbO(SiO) and a pseudocubic phase with a = 7.65 A has been observed by heating ‘-‘lOOO A thick lead films for 40 mm at
= 7.40 A” (massive tin), or “a = 7.78 A” (thin tin oxide films) is considered as tin perovskite, a crystal chemical analogue to CdTiO 3 and CaTiO3, as all three elements belong to the isomorphous series of tin Cd and Ti
[5], and Ca [4]. By extending the same approach to the lead oxide transformation observed at 500’~C (fig. 10), the observed phase “a = 7.65 A” can be regarded as lead perovskite, a crystal chemical analogue to CaZrO3, as Ca and Zr belong to the isomorphous series of lead [5]. It was thought that in chemically vapour grown tin oxide films, the transition, discussed in relation with fig. 10, occurs from “a = 7.78 A” to “a = 6.40 ,,
..
-
-
.
-
A
[2]. This point requires further clarification, since with the lead oxides, the same transition takes place in opposite direction. Fig. 11. shows several 3DIS of unknown lead oxide phase, designated as PbO~,observed on massive lead, and treated as described in fig. 2a. Similar structures are observed up to 500°C,by oxidation of massive tin in air [11]. Following the — so far close — similarity of the initial stages of oxidation of both metals, these oxides are considered to have identical structures and are the third oxide in the row of identical oxides of tin and lead. It is to be remarked that PbO~, and the accordingly designated SnO~, were observed so far on massive metal surfaces, and
388
S.K. Peneva et al.
/ RHEED study of initial states of crystallization of Pb and
Sn
Table 1 RHEED powder diffraction data obtained from =1000 A thick lead films evaporated onto (hll)Si Oxidized thin Pb films
Unoxidized thin Pb films
~1000 A 1
2
3
4
4.83 3.94 3.57
4.58
—
—
—
—
—
3.37
3.33
—
—
3.12
3.17 2.90
3.10 —
3.11 2.87
2.80
2.84
—
2.82
2.76 2.46
—
—
—
—
2.15
—
—
—
2.07—2.02 —
—
—
1.75
—
1.75
—
—
1.67
—
1.67—1.65
—
3.17
3.14
—
2.88
—
—
2.81
2.79
2.68 2.52 2.27
—
—
2.04—1.99 —
2.39 2.22 2.03 1.88
—
220/2.00 —
1.83 1.75 1.67
1.63
311/1.70
—
222/1.63
—
1.54
1.54
1.57
1.48 1.44
1.50 1.43
1.47
1.37
1.33 1.30
—
1.53 1.49 1.42 1.37 1.30
400/1.42
—
1.45 1.43 1.38 1.31 1.28
1.26
420/1.27
-
—
—
1.31—1.29
1.29
—
—
—
1.22—1.20
—
1.13
—
—
1.11
—
—
—
—
—
—
—
—
-
—
—
1.20
1.10 1.08
—
0.93
—
—
0.92 —
-
331/1.30
—
—
—
—
220/1.92 —
220/1.75 (31) —
311/1.63 222/1.56 —
— —
—
311/1.49 (32) 222/1.43 (9) 400/1.36 -
331/1.24 420/1.21 422/1.11
400/1.24
331/1.14 (10)
(7)
422/1.01
(6) (5) (5)
511/1.00
333/1.00 53 1/0.95
440/0.96
511/0.95
531/0.92
333/0.95
—
(2)
420/1.11 511/1.04 333/1.04
—
1.02 —
—
422/1.15
—
—
—
1.71—1.66
—
-
—
—
1.64
—
200/2.71
—
1.78 1.75
—
1.50 1.46—1.43
111/2.86 (100)
200/2.48 (50) —
1.99
111/3.13
200/2.83
—
1.23 1.22
-
111/3.27
2.96
—
—
10
—
—
—
9
--
—
—
8
-
3.32
1.87
1.61
—
7 —
2.04—2.00
—
1.55
Pb-fcc, ASTM 4-0686; hkl/d(A) (‘rel)
—
2.31
1.96—1.90 1.84
6
Pb(Si), a = 5.42 A; hkl/d(A)
—
—
—
5
‘3000 A
Pb(Si), a = 5.66 A; hkl/d(A)
600/0.94 442/0.94 600/0.90 442/0.90
Columns 1 and 2 contain data from samples oxidized at several Torr. Data from unoxidized samples (investigated almost immediately after evaporation) are included in columns 3—7. The samples described in column 7 is 3000 A thick. For reference: the table contains the interplanar distances of a = 5.42 A for Pb(Si), of a = 5.66 A for Pb(Si) and of Pb from ASTM 4-0686. 1 orbidden reflections for the diamond cubic structures are also included in the table.
are as a rule in incommensurate state (fig. 10) [11]. In very thin tin oxide films grown by chemical vapour decomposition oftin chlorides [21was observed
oxide phase of the fcc type, and a = 5.42 A. It was interpreted as the disordered state of SnO with the structure of CaF 2, the ordered state being the
Table 2 X-ray powder diffraction data obtained from 99.999% pure lead slabs X-ray results of Pb: d(A)/Irel
RHEED of
Pb,
Pb from
ASTM 4-06 86 hkl/d(A) (‘rel)
Pb(Si), a = 5.42 A: hkl/d(A)
PbO(SiO), I = 6.40 A: hkl/d(A)
PbO, ASTM 5-0570: hkl/d(A) (‘rel)
5
6
7
8
Unheated
Heated
Heated
1
2
3
column 3: d(A) 4
—
—
—
—
—
3.15/w
3.12/m~
3.12/vw
3.15
—
—
—
—
—
2.82/vs a)
2.85/s a)
2.83/s a)
2.84 a)
—
—
—
—
2.45/s a)
2.47/s a) 2.37—
2.47/rn~a) 2.33/w
2.48 a) —
—
—
—
—
111/2.86 (100) —
—
111/3.13 —
200/2.71
—
002/3.20 —
021/2.86
—
—
—
—
—
—
—
—
—
200/2.475 (50)
001/5.89
(3)
—
111/3.067 (100)
002/2.956 (31) 200/2.74
(28)
—
020/2.377 (20)
2.30/vw
2.12/vw
2.12/vw
221/2.13
—
003/2.1 3 —
—
—
1.91/m
1.95/w
1.93/vw
—
—
—
2.01 1.89 1.87
1.74/s a)
1.74/s a)
1.75/s a)
—
—
—
—
1.69
—
—
—.
—
—
—
1.53/w 1.48/vs a)
l.54/m
1.54/w
1.49/vs a)
1.49/vs a)
1.57 1.48
—
—
—
—
222/1.429 (9)
400/1.238 (2)
1.44/rn a)
1.43/m a)
1.42/mi a)
—
—
—
1.31/w
—
1.31/vw
—
—
—
1.42 a) 1.36 1.29 1.27
1.23/rn a)
h.24/vw a)
1.24/w a)
1.25 a)
—
—
—
1.22
—
h.14/m—s a) l.11/m_sa)
1.14/s a) 1.11/s a)
—
—
—
1.13 a) 1.08 a) 1.07
—
1.01/rn—s a)
1.01/s a)
—
—
0.954/rn a)
0.953/s a)
0.946
—
0.951/rn
—
—
—
—
—
—
—
—
—
—
—
—
—
—
0.838/m—s a)
— — —
220/1.75 (31)
—
311/1.49 (32) —
—
—
220/1.92 — —
311/1.63 —
222/1.56 — —
—
400/1.36
—
—
—
—
331/1.24
202/2.008 003/7.963 022/1.85 220/1.797 113/1.724 311/1.64
(12) (2) (14) (14) (15) (13)
041/1.55
222/1.534
(9)
033/1.51
213/1.514 (2) 004/1.474 (11) 131/1.474 (11)
—
311/1.92 —
032/1.78 — —
—
042/1.43
Etc.
—
422/1.31 —
511/1.23 333/1.23
—
—
331/1.136 (10) 420/1.107 (7) —
422/1.011 (6)
420/1.21
—
—
422/1.11
044/1.13 053/1.097
511/1.04
062/1.01
—
—
33 3/1.04
—
—
(?) (?)
0.927 0.886
511/0.953 (5) 333/0.953 (5) — —
0.889/vw 0.8751w a)
—
—
—
—
0.865/vw
—
—
0.838/s a)
—
0.836/rn—s a)
0.826/vs a)
—
0.826/m
0.824/s
—
—
440/0.875 (1)
53 1/0.837 (9) 600/0.825 (4)
440/0.96 —
531/0.92 600/0.90 442/0.90 —
620/0.86
—
007/0.91 —
641/0.879 721/0.871 633/0.870
533/0.83
731/0.833
622/0.820
553/0.833 643/0.819
442/0.825 —
—
650/0.819 —
—
Column 1: kept for a while at several Torr after cutting. Column 2: heated in vacuum of 5 X 10~Torr at 300°Cfor 1 h 40 mm and cooled for 1 h to 20°C. Column 3: heated in vacuum of 5 X 10~Torr at 400°C for 25 mm and cooled for 1 h 30 mm until it teaches 20°C. Column 4: RHEED data from the surface of the sample discussed in column 3. The last part of the table includes theoretical interplanar distances (including the forbidden ones for the diamond Pb(Si). vs = very strong, s = strong, rn—s = medium to strong, m = medium, w = weak, vw = very weak. a) Diffraction lines of the fcc lead.
390
5K. Penera ct at
/ RHI:7:’D study
of imutial states of crs’stallization of Ph amid Smm
~
lig. 10. Regular phase transition between defornied PbO(SiO and a pisase defined as the lead perovskite, following time conclusions in ref. 2, and the great similarity between lead and tin reported here, The brighter spots are of the 220 type, and arc belonging to the pernvskite. The three interplanar distances of the 110 type are 4.68, 4.64 and 4.37 A, and are defining a lattice with a = 6.23 .k, 6 = 7.09 A and c = 6.14 A (nrtlmorhombic deformation is assumed), There are several 3DIS of the PbO(SiO) an the RHEFD photograph. The three 220 type interplanar distances are alnmost equal and define an almost undeformed perovskite with a = 7.65 A.
Fig. 9. (11 1)RI.P of PbO(SiO) observed on the surface of —1000 A thick lead films, heated in vacuum of —5 X 10~ Torr, heating conditions: (a) 300°C,hh 40 mm; fb) 400°C, 1 h 30 nmin, (a) An almost undeformed PbO(SiO) lattice with a = 6.37 A (three almost equal interplanar distances of the 110 type with d = 4.51 A), but with structural disorder, (b)d 10~ = 4.64 A, duo = 4.88 A and d0~5= 4.73 A define a PbO(5iO) lattice with a = 6.75 A, 6 = 7.05 A and c = 6.39 A, again with structural disorder in the lattice, The calculations were performed by considering orthorhombie deforniation of the lattice. Here, and in figs, 10, 1 2a and 12c, the diffraction picture is not typical RIIEED, since it is obtained from the edges of the corresponding sanuples, and part of the electrons give rise to transmission electron diffraction pictures, Fig. 11, (a) RIIEED photograph of unknown lead oxide in incommensurate state observed on tIme surface of massive lead treated as described in fig. 4a. Exactly the same type of oxide has been observed in the initial stages of oxidation of tin in air [11].Sketch of tIme RHEFD photograph is shown in (b). The interplanar distances and the angles between the planes nornials are: d1 = 3.72 A, d2 = 4.47 A, d3 = 3.77 A, o = 70°,p = 5 1.5°,‘y = 58.5°.The diffraction photograph is not of pure RHEED type. See time remark in fig. 5.
d?
~=
[
~ /
/
b
jl
S.K. Peneva et al.
/ RHEED study
of initial stares of crystallization of Pb and Sn
391
ordinary c~-SnO.The phase observed in ref. [1], and
that case the phase a = 5.43 A in the present study,
designated as Sn(Si) with diamond cubic structure and a = 5.43 A, is not an oxide according to the MOssbauer results, and the considerably slower rate of
obtained at similar conditions where Sn(Si) exists, is
oxidation of tin as compared to lead. According to ref. [10], lead surfaces become oxidized in air, but the oxides are amorphous and do not hinder RHEED
in chemically vapour grown tin oxide films (TOF) [21, was also considered. The very thin tin oxide films, where the phase treated as SnO(CaF2) was
observations carried out shortly after evaporation. In
detected [21, were grown at 420°C,simultaneously
Pb(Si). The opposite possibility, Sn(Si) to be observed
392
S.K. Penem’a
Ct
al.
/ RHEEI) study
of initial states of crvstallizatiom, of Ph and Sn
Fig. 12. Different RLP of Pb(Si) observed by heating in vacuum of—S X 10—6 Torr of —1000 A thick lead films on (ll1)Si. The samples are from different evaporation experiments and each of them was treated at the particular conditions listed below. For (a) and (c), the remark in fig. 9b is valid. (a) Heating temperature 300°C,heating time 33 mm. Below the melting point of lead the diffraction picture is complicated. (123)RLP of Pb(Si) is visible, corresponding to a lattice with a = 5.66 A. Here diii = 3.26 A, d 420 = 1.27 A, and the angle between both reciprocal lattice vectors is 75°.Here, the powder data correspond to Pb(Si) with the same lattice parameter as observed for the single crystal Pb(Si) region. (b) Heating temperature 350°C, heating time 29 mm. (211)RLP of Pb(Si) with a = 5,53 A is visible; d1j1 = 3.19 A, d02~=
1.94 A.
(c) Heating
temperature 400°C,heating time 25 min. (111)RLP of Pb(Si). The interplanar distances of the 220 type planes are = 5.42 A. The 110 RLP of the silicon lattice is on the left handside of the figure (transmission electron diffraction from an edge of the substrate), showing the orientation of the substrate towards the lead film,
1.92 A, and correspond to a lattice with a
(d) Heating temperature 400°C,heating time 1 h 40 mm. 3DIS of Pb(Si) are visible. (211) of one of them show dju i
=
3.09 A,
and do2l = 1.88 A, corresponding toa = 5.36 A. (e), (f) The heating temperature for (e) is 350°C,1 h heating time and for (f) 500°C,1 h heating time. (110)RLP of Pb(Si) with a = 5.42 A is visible on both figures withdu i~ = 3.12 A and d002 = 2.72 A. (g), (h) Heating temperature 500°C, lii heating time, (I l0)RLP (g) and (123)RLP (h) of Pb(Si) with a = 5.66 A are visible. = 3.29 A, d002 = 2.83 A and d420 = 1.30 A.
on (111 )Si, fused quartz and (1 00)NaCI. At these conditions, the following reaction is possible 2 SnO —~ Sn02 + Sn
inverse layer at the boundary lead/silicon [11, with some phase, defect, superstructure or crystalline state
in the film. Details of these results will be published separately, but the conclusions correlated with the
However, if the phase a = 5.42 A was tin, it would have been detected on all substrates, which was not observed. Moreover, the so far unpublished results of the Mössbauer studies on TOF, would have indicated
topic of the present paper are: (i) Ph(Si) exists up to 700°C(the highest temperatore used for heat treatment of Pb/Si samples), with
Therefore, it is concluded that the existence of
lattice parameter “a” varying in an unpredicted way between a = 5.66 A and a = 5.36 A (figs. 12a—12h), Heating below the melting point of lead (fig. 12a)
SnO(CaF2) is governed by the influence of the silicon and NaCl substrates.
leads mainly to polycrystallmne films. Occasionally single crystal diffraction pictures, superimposed on a
3.3. Structure of heated -°1000A thick lead films on (11J)Si, in the temperature interval 300—800°C
powder diffraction pattern, were observed. The tendency for single crystal growth increased considerably above the melting point of lead, actually above
Structural investigations of heated lead films are carried out to correlate eventually the existence of an
example of how the lattice of Pb(Si) fits on top of the silicon lattice. Fig. l2h shows that above 500°C
the presence of Sn(Si), which is again not the case,
350°C for the present study). Fig. 12c is a good
Fig. 13. Reciprocal lattice projections of what we consider to be a-Pb, corresponding to the diamond cubic cs-Sn (a = 6.49 A). This phase is always in incommensurate state. As in figs. 12, the samples are usually frorn different evaporation experiments, and each sample was treated at the particular conditions listed bellow. -‘-1000 A thick Pb films on (111)Si were studied. (a) 3DIS of a-Pb observed on (123)RLP. One of the incommensurate structures is with a = 6.87 A. Heating temperature 300°C, heating time 33 mm. (b) The disorder in the 3DIS cs-Pb increases by increasing the heating time. Here the heating temperature is again 300°C, but the corresponding sample was heated for 1 h 7 mm. The phase is no more cubic. One of the 3DIS is with a = 5.94 A and b = c = 6.22 A. Here d 20~= d0~2 2.15 A, d2~0= 2.20 A (orthorhombic deformation is assumed). (c) Further increase of the disorder by increasing the heating time to 1 h 40 mm at the heating temperature 300°C. One of the 3DIS is with a = b = 5.60 A, c = 6.32 A, corresponding to d0~2= d20~= 2.10 A and d2~0= 1.98 A (orthorhombic deformation
is assumed). (d) New phase obtained from a-Pb;heating temperature 350°Cheating time 56 mm. By comparing figs. 12c and 12d, one can see the geometry of the phase transformation. One of the 3DIS shows two almost perpendicular reciprocal lattice vectors, corresponding to interplanar distances 5.69 and 2.50 A. (e) Superstructure, considered to be connected with the phase transitions of the incommensurate a-Pb. Heating temperature 400°C, heating time 25 mm. Both reciprocal lattice vectors are at 90°C towards each other. The corresponding interplanar distances are 5.64 and 1.22 A.
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there is a considerable amorphous layer on the surface of the lead samples. RHEED investigations of “—200 A thick thin lead films on (11 l)Si, heated at the same conditions as the -“-1000 A thick lead films, showed that the (Pb(Si) lattice near the surface is expanded, and that increasing of the heating time increased the lattice deformation. For example, 50 mm heating of such ~200 A film at 400°C leads to a Pb(Si) lattice with a 5.70 A. Heating for 1 h 15 mm transformed the cubic Pb(Si) lattice to an eventually orthorhombic lattice with a 5.86 A, b 5.72 A and c 5.43 A. (ii) It is thought that one of the phases observed in heated thin lead films imitates the structure of the a-Sn, and is accordingly designated as a-Pb (figs. 13a—13e). It is always in incommensurate state, and with increase of the temperature one can see how the numerous incommensurate structures deform themselves in more primitive lattices, clearly visible by
=
=
=
part comparing, of the e.g., unexplained figs.created 13cthat RLP, and illustrated It of isphases. thought in fig. that 13e are transitions as anindications example, between (fig. are 14) incommensurate asthe a13d. result so-called similar a-Pb and There phase its derivates can exist at higher temperature but such RLP were observed relatively rarely. as well,
3,4. Powder diffraction data obtained by RHEED and X-ray diffractometer measurements of unheated “-‘-1000 A thick lead films on (111)Si and of freshly solidified massive lead The formerly described single crystal results indicate that indexing of RHEED and X-ray powder diffraction data (tables 1 and 2) is not straightforward. X-ray analysis of massive, freshly cut lead, kept at several Tort for a while, shows not only the theoretical lines of fcc lead, but also some extra lines with negligible intensity (table 2, column 1) interpreted as lines of PbO(SiO). Heated nmassive lead samples at 300°C for 1 h 40 mnin (column 2) and for 25 nun at 400°C (column 3) and cooled for 1 Is from 300 to 20°C and for I h 30 mitt from 400 to 20°C, do not show a remarkable change of the diffraction picture , If RHEED single crystal results are not accounted for, the interpretation would be misleading, since there is overlapping of the reflections even when tile phases are with the theoretical interplamiar
—
—
Fig. 14.
3DIS of intermediate phases resembling those
discussed in connection with fig. 13. The —1000 A thick lead film on (1l1)Si was heated for 50 mm at 600°C.The 000 points of time 3DIS phases are displaced over several A. (211)RLP of one 3DhS phase shows d~ d02~= 1.86 A, defining a lattice with a c = 5.26 A.
=
11 = 3.21 A and 6.37 A and h =
distances, as shown in table 2. The data from this table show that the freshly solidified lead contains Pb (fcc) (ASTM 4—0686) and textured along the [100] direction PbO(SiO) -— note the presence of 002, 003, 31], 041 and 422 reflections, Actually PbO(SiO is textured along the [100] direction even on the unheated oxidized lead sanmple (column I). Tile presence of Ph(Si) cannot he unamTmbiguously detected with tile conventional X-ray technique used in the present study. However, time intensity ol sonme lines of the I’cc lead, especially thosc with imlterplanar distances below 1.14 A, is considerably lsiglmer than expected from time theory, amid cannot he explained by ams eventual increase of tile PbO(SiO) since tile freshly cut lead has been heated iii vacuum
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and investigated immediately after cooling. Moreover, RHEED results indicate that the oxide islands are here and there on the surface. Therefore, this unusual increase of the intensity can be attributed to the presence of Pb(Si). This conclusion is in accordance with RHEED results and if it proves, by more a sophisticated contemporary X-ray technique, to be correct, it would mean that the growth of Pb(Si) is a volume effect. The RHEED powder data obtained from the surface of the sample, described in column 3, are included for comparison in column 4. It is obvious that the surface structure is far more complex, a result already mentioned in connection with the single-crystal analysis. The extra line d = 2.32 A is explained, according to refs. [9,10] as a reflection of the orthorhombic (ASTM 5-0570). RHEED powder diffraction data of unheated “-‘-1000 A, and in one case “—3000 A, thick lead films are illustrated in table 1. Columns 1 and 2 include data from oxidized films (the first sample has remained at several Tort for one day, and the second sample for more than 10 days — columns 1 and 2, respectively). RHEED investigations of the other samples were carried out almost immediately after their removing from the evaporating system. All the samples described in table I are from independent evaporation experiments. The polycrystalline part of the lead films (unoxidized, columns 3—7) contain ordinary fcc lead and Pb(Si) with deformed lattices. The interplanar data vary from sample to sample, which is in accordance with the single crystal results. We did not dare to perform a line-to-line indexing of these multiphase systems, but by using the single crystal results, one can have a very general idea about the phase composition of the film. The data from figs. 8, 9a and 9b indicate that interplanar distances of the order of
diffraction from the 100 plane of some substructure of a-Pb, as discussed in connecting with figs. 13d, 13e and fig. 14. The unknown peak corresponding to d = 5,67 A in ref. [9] can be interpreted by either of these two possibilities. The results of the present study were compared with the data of Ovadyahu [7], but the published diffraction results, as pointed out in ref. [8], can be interpreted as corresponding to the orthorhombic PhD. However, the describtion of his results can somehow be correlated to the data of the present work, especially when he states that increase of the evaporation rate, and ours is quite high, leads to ample quantity of what he describes as double hexagonal lead. It is hoped that single crystal THEED and RHEED of films of his type may clearify this point.
4.50 A can be 110 type reflections of PbO(SiO). However, the existence of the phase PbO~(fig. 1 Ia)
These oxides are called SnO(SiO) and PbO(SiO) and are often in incommensurate state. [6]. It is con-
is not to be neglected despite the fact that so far its presence has been observed on massive lead only. Interplanar distances in the range of 5.50 A can belong to 110 reflections of deformed perovskite. Powder data of tin oxide films, where such interplanar distances have been calculated, have been interpreted in that way. However, for unoxidized lead films, such reflections can be interpreted as
sidered that the initial valence state of tin in 2”, but according to the room temperaSnO(SiO) is Sn ture DSMS results, a transformation to the Sn4” state in the same structure is not to be neglected [21. Phase transformation of PbO(SiO) into a cubic phase with a = 7.65 A has been observed. A similar phase transformation takes place with SnO(SiO [2]. It is interpreted, following refs. [4,5], and the
4. Conclusions The results of the present single crystal RHEED study can be summarized as follows: Lead with the structure and lattice parameter of silicon is formed after evaporation onto (11 l)Si. This crystalline state of lead, Pb(Si), and the already reported [I] identical crystalline state of tin on (11 1)Si, Sn(Si), were found to be one of the initial crystal structures of both metals, formed after solidification of the massive metals in vacuum of “-‘S X 10—6 Torr. Islands of different types of oxide were formed on the surface of the massive metals — tin and lead — and on their respective thin films on (II l)Si, during heating in the conventional vacuum of “—5 X lO_6 Torr. The initially formed oxide on Sn(Si) and on Pb(Si) closely resembles SiO, and has the same lattice parameter as SiO (within the limits of error).
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/ RHEED study of initial states of crystallization
close siniilarity of the initial stages or crystallization and oxidation of both metals, as the lead 24Pb44O perovskite Pb 3,
The similarity of the initial stages of oxidation of both metals has been observed and by comparing the RLP of unknown tin [111 and lead oxides, designated as PbO~ and SnOt. These types of oxide were observed so far on massive metals only and are as a rule in incommensurate states The present paper includes some data on the crystalline state of Pb(Si) in thin lead films heated to 700°C. It is shown that its lattice is considerably deformed in ~‘200 A thick films. One phase in the heated thin lead flints, appearing always ill incommensurate state, closely resembles a-Sn, and for convenience is called a-Pb, It appears in a certain temperature interval, and by increasing tile heating time at any temperature in this temperature interval, it disproportionates. It is shown that phase transition of the 3DIS of a-Ph is one of the ways of substructure and (or) superlattice formation. An attemiipt has been made in the present study to use the single crystal RHEED results for interpretation of the complicated X-ray and RHEED powder data of polycrystalline lead f’ilms. The former results require a restatement of the crystal chemical approach [4,5] for structure analysis adopted for studies of multiphase metastable systems. At least for the metals of the fourth group of the Periodic Table, the initial stages of crystallization and oxidation are identical (for sonme of the phases) and closely resemble the bahaviour of silicon. It appears that the electron state of silicon, tin, and lead are
of Pb and Sn
governed by sinmilat factors for quite some time, leading to great similarity not only between the initially grown phases of both metals but also, to identical chemical behaviour (oxidation in our case). In other words, it takes time for each element to achieve its “-personality”. It has been found that the formerly discussed metastable phases get stabilized its thin films,
References [11 K.D. Djuneva, S.K. Peneva, E.A. Tsukeva and I. Batov, Thin Solid Films 67 (1980) 371. [21 5K. Peneva, R.K. Rudarska, D.D. Nihtianova and l,Z. Kostadinov, presented at 6th Intern. Conf, on Crystal Growth Moscow, 1980, and other papers given at the(ICCG-6), Conference.
[31 E.G. Rochow and E.W. Abel, in: The Chemistry of Germanium, Tin and Lead, Vol. 14 (Pergamon, Oxford, 1975) p. 119. [41 V.M. Goldschmidt, Geochemische Verteilungsgesetze, Vol. VII (1926). [5] K.A. Vlassov, The Periodic Low and the Isomorphism (Institute of Mineralogy and Geochemistry, Moscow) (in Russian).
[61 V.L. Pokrovski, Solid State Comnmun. 26 (1978) 77; L.A. Bolshov et al., Usp. Fiz. Nauk 122 (1977) 125. [71 Z. Ovadyahu, 3. Phys. F (Metal Phys.) 8 (1978) 403. [81 G,D.T, Spiller and P.J. Dobson, J. Phys. F (Metal Phys.) 8(1978) L135. [91 TB. Light, 3M. Elridge, J,W. Matthews and J.H. Greiner, 3. Appl. Phys. 46 (1975) 1489. [101 J.W. Matthews, C.J. Kircher and RE. Drake, Thin Solid Films 42 (1977) 69; 47 (1977) 95,
[11] R.K. Rudarska, private communication.