New results in low temperature studies of semiconductor surfaces

Applied Surface Science 33/34 (1988) 1-14 North-Holland, Amsterdam

NEW R E S U L T S IN L O W T E M P E R A T U R E S T U D I E S OF SEMICONDUCTOR SURFACES V.A. G R A Z H U L I S Institute of Solid State Physics, Academy of Sciences of the USSR, Chernogolovka, Moscow district 142432, USSR

Received 23 August 1987; accepted for publication 9 October 1987

In this paper a brief review of the latest results obtained by the LEED and Auger techniques for clean Ge(lll), Si(lll) and InSb(ll0) surfaces and for the surfaces with adsorbed Ag atoms in the low temperature interval of 8-300 K is presented. It is shown that in the low temperature region one can observe a number of new phenomena that may lead to new notions about the atomic structure and properties of clean and metal covered surfaces of semiconductors. It is found that transition from low to "elevated" temperatures can dramatically change not only the structure of clean surfaces but also, the structure of interfaces and metal films. Observed physical phenomena are discussed and interpretations are suggested.

1. Introduction As shown by earlier investigations [1-5], interesting physical p h e n o m e n a m a y arise o n s e m i c o n d u c t o r surfaces at low temperatures. I n a n u m b e r of cases the existence of these p h e n o m e n a is difficult to predict. W e shall consider the latest low t e m p e r a t u r e results o b t a i n e d b y o u r group for Ge, Si, a n d InSb at T = 8 - 4 0 0 K. We shall treat b o t h the properties of clean surfaces a n d the properties of the surfaces with adsorbed Ag atoms. W i t h o u t going into the details of the experimental technique, we shall o n l y note that we employed u l t r a - h i g h - v a c u u m E S C A L A B - 5 a n d L A Z - 6 2 0 spectrometers, i n c l u d i n g a t t a c h m e n t s e n a b l i n g us to m o n i t o r the t e m p e r a t u r e from 8 to 400 K [1]. N o w let us consider some experimental results o b t a i n e d at low temperatures for G e ( l l l ) , S i ( l l l ) a n d I n S b ( l l 0 ) .

2. Clean G e ( l l l ) and S i ( l l l ) surfaces at low temperatures I n refs. [1-5] it was f o u n d that i n U H V ( - 1 0 -1° Torr) at a cleavage t e m p e r a t u r e T~l r a n g i n g from 4 to 300 K the cleaved Si(111) surface always 0 1 6 9 - 4 3 3 2 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)

1

2

V.A. Grazhulis / Low temperature studies of semiconductor surfaces

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Fig. 1. Three types of diffraction pattern with superreflections for low temperature cleavages of Ge crystals: (a) ordered superstructure; (b,c) strongly disordered superstructures (different in the orientation of the superreflections). Dashed arrows indicate the doubling period directions in a (111 ) plane. possesses a 2 × 1 superstructure, whereas the G e ( l l l ) surface structure essentially depends on T~u, namely, at T~,1>_, 60 K the cleaved Ge(l 11 ) surface also possesses a 2 × 1 superstructure but at Ten < 40 K the L E E D picture may show no superstructural 2 × 1 spots. The latter can be interpreted as being the result of a nonreconstructed state of cleaved G e ( l l l ) - I × 1 or a strongly disordered Ge(111)-2 × 1 surface. Such " u n u s u a l " behaviour of Ge(111 ) cleaved surfaces was related to the overheating and quenching p h e n o m e n a a c c o m p a n y i n g the cleavage process at low temperatures [6]. The G e ( l l l ) surfaces cleaved at low temperatures (10 300 K) in U H V were recently investigated again with the L E E D technique by our group (for details see ref. [7]). Some new results concerning the surface atomic structures were obtained. These results suggest some important conclusions: (i) All G e ( l l l ) cleavages at Td = 6 0 - 3 0 0 K show essentially the same 2 × 1 L E E D structures with is•tropic broadening of all diffraction spots and with L -~ l ~ l 0 -~ q / ( 1 5 - 2 0 ) , see fig. la. (ii) h o w - T G e ( l l l ) cleavages, T~I < 40 K, show some spread in the L E E D data, in contrast to S i ( l l l ) ; one can observe G e ( l l l ) L E E D pictures of three types, fig. 1, with weakly or strongly b r o a d e n e d extra spots of the 2 × 1 structure or even pictures without the extra spots. (iii) Strong broadening is observed only for the extra spots. The latter means that only the topmost surface dangling b o n d (SDB) layer can be essentially disordered and thus give rise to the extra spot broadening. The atomic layers below the surface do not "feel" significantly even strong disordering of the topmost layer. (iv) The extra spot broadening can be essentially anisotropic with the strongest correlation in the surface dangling b o n d system along the [112] or [701]

V.A. Grazhulis / Low temperature studies of semiconductor surfaces

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Fig. 2. Diffraction pattern (a) and the model (b) of a superstructural Ge(111)-(2×1) cluster corresponding to this pattern. (1, 2) Position of nonequivalent lattice sites on the surface. (3) Superstructural reflections.

directions. The latter is the doubling periodicity direction in the (111) cleavage plane, see fig. 1. (v) The low-T cleavages may give rise to extremely strong disordering of the G e ( l l l ) topmost layer with correlation length ~ ( L ) ~ (3-6)a0, so that this layer may be treated as "amorphous-like". (vi) These "amorphous-like" surfaces are sensitive to the primary electron beam (giving rise to some electron beam annealing effect) whereas the thermal heating does not affect significantly these surfaces throughout the investigated range (10-300 K). A cluster model of the Ge(111) surface has been proposed on the basis of the results obtained in ref. [7]. Figs. 2 and 3 present the models of the 2 × 1 surface superstructural clusters of two types adequate to produce the two types of LEED pattern shown in fig. 1. The sizes of the clusters are determined by correlation lengths ~ which, in turn, follow from the sizes of superreflections. Note that ~(10) -- a o q / l o, ~(l) -- a o q / l , ~ ( L ) -- a o q / L , where a 0 is the lattice period before doubling, q the distance between neighbouring main spots, for high-T cleavages L ~ l-- l 0 ~ q/(15-20) [7]. Figs. 2 and 3 show the position of equivalent lattice sites within each cluster. The real atomic structure is not yet known. Note that in the cluster of the first type (fig. 2) a maximal correlation length ~(1) in the dangling-bond system must arise alon_g the [112] type direction, and in the cluster of the second type along the [101], see figs. 2 and 3. The overheating (and "melting") of the dangling bond surface in the procress of cleavage at low T [7] may explain the disordering of the dangling bond system and the superspot broadening, however, it cannot explain the

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whole variety of phenomena observed for cleaved Ge (111) surfaces. From our point of view one has to treat a more complicated cleavage process in the case of Ge(lll) rather than the common one accompanied by the formation of one d a n g l i n g b o n d p e r s u r f a c e a t o m . F o l l o w i n g ref. [8] w e a s s u m e a p o s s i b i l i t y of

=

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y yyYYyY'?" yyyyyyy YYYYYYYY YYYYYYY yyyyyyyy yyyyyyy

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Fig. 4. Top view of a A ( l l l ) surface with one dangling bond per atom (a) and of a B(III) surface with three dangling bonds per atom (b). (1) Atoms located in the first atomic plane, possessing a dangling bond normal to the plane of the figure. (2) Atoms located in the second atomic plane in (a). (b) The "second" plane after the atoms of the first plane have been removed.

V.A. Grazhulis / Low temperature studies of semiconductor surfaces

5

formation of G e ( l l l ) cleaved surfaces both with one and three dangling bonds per atom (we shall call them A and B type surfaces, respectively). Normally, the latter is not considered to be probable due to its high energy. This is obviously reasonable, at least for unreconstructed and nonrelaxed surfaces. In the rest of the cases, however, the surface state is unclear and, therefore, one has to model all the possible surfaces and to treat their energies. From that point of view, it seems useful to qualitatively consider here some hypothetical properties of (111) surfaces with three dangling bonds per atom as well (for some details see ref. [8]). A (111)-1 x I(B) surface with three dangling bonds per atom is shown in fig. 4b, which has been formed by removing the top layer shown in fig. 4a. We consider qualitatively some surface reconstructions, followed by saturation of the dangling bonds. The rotation of the neighbouring atoms (fig. 4b) may lead, in the end, to some gain in the surface energy due to an increased overlapping of the wave functions of neighbouring dangling bonds. A maximal overlap may be expected at rotation angles cp = ___30 °. In this case two dangling bonds of neighbouring atoms may be in one plane. An energy gain arises due to mutual partial saturation of dangling bonds (S = 1/2) followed by the formation of a singlet pair (S = 0). An elementary analysis shows that this method of reconstruction can give rise to two essentially different superstructures: low-symmetry chain superstructure and high-symmetry cell superstructure (fig. 5). Without the numerical calculation of the surface energies it does not seem possible to determine which of these superstructures possesses a lower energy. We shall, therefore, qualitatively discuss the two superstructures.

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6

V.A. Grazhulis / Low temperature studies oJsemieonduetor s'ur/aces

(i) Chain (111)-2 x I(B) superstructure. The atomic configuration, shown in fig. 5a, exhibit a number of interesting specific features. First of all, pairs of mutually saturated bonds ( S = 0 ) form zigzag chains oriented along the [2.11]-type direction; the configuration corresponds to the superstructure 2 x 1. The chain (111)-2 x I(B) superstructure exhibits a low symmetry (holohedral point group C~); inasmuch as the superstructure possesses only the mirrorsymmetry planes, the rotation axes of the symmetry are missing. G r o u p C~ of the superstructure is not a subgroup of the point group T d of the lattice volume since all the superstructure symmetry planes are not coincident with the symmetry planes of the volume (they are turned by _+30 ° ). This is an important specific feature of a chain (111)-2 x I(B) superstructure. As seen from fig. 5a, each atom of a chain (111)-2 x I(B) superstructure possesses one unsaturated bond (S = 1/2), inclined to the surface, and two partly saturated bonds (S = 0). (ii) Cell ( l l l ) - 3 X 3 ( B ) superstructure. The atomic configuration, shown in fig. 5b, corresponds to a high symmetry 3 × 3 surface superstructure (the period tripling along all the { l l 0 } - t y p e directions). Note that the translation symmetry in the 3 x 3 superstructure can easily be disturbed due to noncorrelated (easy) rotations of atoms in the cell centers. New types of superstructures can easily be formed at the "' background" of the 3 × 3. For example, one can construct the 3 × 6 superstructure by 60 o rotation of the atoms in the centre of each second cell, located along the [110] and [011]. To conclude this section we shall note that if the atoms in the center of each cell of 3 x 3 are removed, then the number of unsaturated bonds is decreased by a factor of three (there will be only one unsaturated bond in the center of each cell). This atomic configuration will be likely to possess a smaller surface energy, the structure still remaining 3 x 3. So, on the basis of this short qualitative analysis of B(II 1) surfaces one can expect the existence of chain superstructures of the (111)-2 x I(B) type, and. also, high symmetry superstructures of the (111)-3 x 3(B) type (and other superstructures, arising from the (111)-3 x 3(B)). We do not know of papers reporting the observation of the 3 x 3-type superstructure, therefore, we shall refer only to the (111 )-2 × I(B) superstructure. If the surface energy of this superstructure is comparable to thai of the (111)-2 x I(A) then the (111)-2 x I(B) superstructure may actually appear upon Ge crystal cleavage. If distortion of the translational symmetry for this superstructure can be easily produced then it may explain the strong disordering of the topmost layer at low-T Ge cleavages. From our point of view, taking into consideration both the B- and the A-type surfaces can probably help in understanding the whole variety of phenomena observed at low temperature cleavages of Ge crystals. Evidently, in order to elucidate the real atomic structure of the (111)-2 x 1

V.A. Grazhulis / Low temperature studies of semiconductor surfaces

7

surfaces in question a numerical calculation of the atomic structures of both A-type and B-type surfaces and additional experimental investigations are needed.

3. Low temperature studies of Ge(lll), S i ( l l l ) and InSb(ll0) surfaces with Ag atom deposition There are a lot of investigations carried out at " h i g h " T for S i ( l l l ) + Ag and G e ( l l l ) + Ag, see, for instance, refs. [10-12]. In this section we briefly discuss our latest results obtained for Si(111)+Ag, G e ( l l l ) + A g and I n S b ( l l 0 ) + Ag using Auger and L E E D techniques at low temperatures. We shall show here that at low temperatures one can observe interesting physical phenomena, which cannot be observed at high temperatures. Before discussing our experimental data let us point out that low-T adsorption enables one to obtain atomically sharp interfaces and statistically homogeneous films possessing island-free structures. The latter will be valid under the assumption that the adsorbed atom retains its prior position at the surface. Let us point out that there are some peculiarities in Auger signal behaviour in the case of a statistical distribution of adatoms [9]. In ref. [9] it was shown that in the case of a Poisson distribution of adatoms on the surface and under the assumption that in Auger experiments any adatom film of thickness d o weakens the electron beam intensity by a factor of exp(do/le), where le = le(e ) is the electron free path, e the electron energy, the Auger peak intensities of the substrate and of the adatom film can be expressed in the form: /sub CCexp( --aO[1 -- exp(-- 1~1(el))] /ad(X

1-exp{-aO[1-exp(-1/l(e2))]

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/sub ~ exp{ - a O / l ( q ) } , l a d C( 1 - - exp{ -aO/l(e2) } .

(3) (4)

It should be emphasized that it is not evident a priori that expressions (1) and (2) hold for all values of 0, since it is not clear whether one can really introduce the same parameters l(e) both for 0 < 1 and 0 > 1. This question can be answered by comparing dependences (1) and (2) with experimental data.

8

V.A. Grazhulis / Low temperature studies of semiconductor surfaces

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3.1. Experimental studies of Si(lll) + Ag [9] First of all note that in the case of S i ( 1 1 1 ) + A g we assume that one monolayer of Ag (0 = 1) corresponds to 7.8 × 1014 a t o m s / c m . Typical dependences of Isi(0 ) and lAg(0 ) for the Si (92 eV) and Ag (356 eV) Auger peaks, taken at 8 and 300 K are presented in figs. 6 and 7 [9]. One can see an essential difference between low-T and high-T data. The behaviour of the Si(111) Ag system with increasing temperature from 8 to 300 K after the 8 K deposition is markedly dependent on 0. We have found that the ratio of the Auger-peak intensities, 1ag/Isi, for 0 < 1 is invariable upon heating with the error of the measurements. However, at 0 > 1

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calculation using

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Deposition at 8K

Process of heating

Observation at room temperature after heating

0.4-0.5 _< 0 _<11 (or higher)

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Not observed

2 _< 0 _< 22 (or higher)

Si(111)-1 × 1

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0.2-0.3 _< 0 _< 2.5; most clearly seen at 0 ~ 1.2-1.3

Si(lll)-~/7 × vC7R a) (+_ 19.1 ° )

a) The structure is sensitive to vacuum conditions; not observed at p >_ 3 × 10-10 Torr. b) Is likely due to double diffraction. c) Reflections are slightly blurred.

Si(111)-2 x 1

Experimental conditions

0.7_<0_<2-4

Arises from the background as rings to be partitioned into individual reflections

Not observed

0.5 _< 0 _< 3-4

Si(lll)-3 x 1 b)

Table 1 Main experimental results: ranges of silver coverage and other conditions of the LEED pattern observations

0>__0.7

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Not observed

0 >__0.5

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Weak

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(1) Greatly increases at 0 -< 3-4, homogeneous (2) Regions of increased intensity appear at 0 >_ 3-4

Weak

Background

10

V.A. Grazhulis / Low temperature studies of semiconductor surfaces

the 1Ag//Si value begins to diminish irreversibly at T>_ 100 K. On attaining 300 K this value decreases by approximately two orders of magnitude for 0 = 15. The characteristic temperature T* at which the said changes begin to be noticeable, grows with increasing coverage. For example T* = 200 K for 0 = 4, however T* = 300 K for 0 = 15. The cause of this phenomenon remains unclear. The 0 dependences of the Si and Ag Auger-signal amplitudes at - 300 K and 0 = 0-1 are practically linear (see figs. 6 and 7 and refs. [10-12]). Note that cooling of the samples (after 300 K deposition) down to 8 K and their subsequent heating up to - 300 K do not practically affect the Augerspectra amplitudes. With Ag deposition on a cleaved S i ( l l l ) surface a great variety of L E E D patterns corresponding to structures Si(111)-2 × 1, Si(l 11)-1 × 1, Si(l 11 )- ~/7 x ~/7 R ( ± 1 9 . 1 ° ) , three-domain Si(lll)-3 × 1 and A g ( l l l ) - I × 1 was observed. With 0 = 1.2-1.3, when the samples were deposited at room temperature, we were able to observe all the above structures simultaneously. The main LEED results obtained in ref. [9] are presented in table 1. From this table one can see the regions of existence of different structures as functions of O, deposition temperatures and other experimental parameters. In ref. [9] it was found that deposition of Ag atoms at 8 K does not affect significantly the Si(111)-2 × 1 superstructure. The result is that at 8 K the adsorbed Ag atoms interact very weakly with the Si(111)-2 × 1 surface, i.e., at 8 K physical rather than chemical adsorption of the Ag atoms is likely to take place. In fact, in the process of deposition at a low temperature the Si(111)-2 × 1 LEED pattern can be observed (with primary electron beam energy e = 100 eV) from 0 = 0 to 0 = 3 4. We emphasize that with an increase of the 0 value both the superstructural and main reflections become invisible practically simultaneously. The disappearance of all the LEED reflections from the S i ( l l l ) surface at 0 = 3 implies that the Ag film with 0 = 3 is nontransparent for electrons with e = 100 eV. So, from the LEED data we can estimate the electron free path l(e ~ 100 eV) without energy loss. Clearly, l < 0 = 3. With account being taken of the coefficient d this value agrees with the electron free path value obtained from the AES experimental data. Actually, fitting of the low-T AES data, see figs. 6 and 7, to theoretical expressions (1) and (2) by varying the l value gives / ( e l ) = 1.5 +_0.5 and / ( e 2 ) = 2 ± 0 . 6 . Assuming din2.9 A one obtains d l ( e l ) = 4 . 4 ± l . 4 A and d / ( e 2 ) = 6 ± l . 7 A. The absence of LEED reflections from A g ( l l l ) with 0 _< 3 - 4 or the presence of strongly diffuse reflections with 0 > 3-4, see table, indicates that the Ag atoms, adsorbed at 8 K, form a strongly disordered structure. Knowing the sizes of the A g ( l l l ) reflections, JqAg, if any, and the Ag lattice parameter aAg in a A g ( l l l ) plane, one can estimate the correlation length ~ in the system of Ag atoms. It was found that at 10 K ~ = ( q A g / A q A g ) a A g ~ (2--3)aAg for 0 > 3 4.

V.A. Grazhulis / Low temperature studies of semiconductor surfaces

11

We thus arrive at the conclusions that at 10 K during adsorption of A g atoms on a S i ( l l l ) - 2 x 1 surface an "amorphous-like" Ag film (~ _< (2--3)aAg) is formed which interacts weakly with the substrate, and that the experimental dependences Is~ia) and I(°) "Ag fit well the statistical Poisson model with one fitting parameter l(e) for all the investigated 8 values. There are interesting phenomena related to Si crystal heating after Ag deposition at 10 K [9]. We just note here that at 8>_8* = 0 . 4 - 0 . 5 and T>_ T * = 1 0 0 K an irreversible phase transition of the S i ( l l l ) - 2 x l S i ( l l l ) - I x 1 type occurs due, probably, to a drastic increase of disordered Ag film interaction with the substrate at T > T * . It was found that at 0 > 8 " the "critical" temperature T * essentially increases with increasing 19 value. Note that the strengthening of the A g - S i interaction can be interpreted as a result of the formation of the chemical bonding which needs thermal activation and, therefore, occurs only at sufficiently high temperatures, T >_ T * . (Initially, at 10 K, Ag atoms are in a physical adsorption state.) For further details, see ref. [9].

3.2. Investigation of Ge(111) + Ag Let us note, first of all, that in the case of G e ( l l l ) + Ag one monolayer of Ag (8 = 1) corresponds to 7.2 x 1014 a t o m s / c m 2. The experimental results for G e ( l l l ) + A g at T = 10 K turned out to be very different from those of S i ( l l l ) + Ag and of G e ( l l l ) + Ag obtained at 300 K [13,14]. Namely, at 10 K the superreflections of G e ( l l l ) - 2 x 1 completely disappear already at 8 = 0.1-0.2 (note that at 300 K deposition in the case of G e ( l l l ) + Ag the superreflections disappear only at 8 = 0.5-1 [13,14]). In the range O = 1 - 2 at 10 K the basic reflections from the G e ( l l l ) substrate completely disappear (at 300 K the basic reflections from a G e ( l l l ) disappear only at 8 >_ 5 [13]). The reflections from the Ag film itself become noticeable at 0 >_ 2. The result of sample heating (after 10 K deposition) to 300 K depends on 8: (i) at 8 = 0 - 1 the L E E D pattern is retained, (ii) at 8 = 1 - 2 heating to 300 K gives rise to the A g ( l l l ) - I x 1 reflections and (iii) at 8 >_ 2 the Ag reflections arising already at 10 K become less broadened during heating. So the G e ( l l l ) - 2 x 1 + Ag system strongly differs from the S i ( l l l ) - 2 x 1 + Ag one. We observed that at 10 K already with 8---0.1-0.2 the superreflections from the G e ( l l l ) - 2 x 1 completely disappear. This probably indicates that a Ag atom at 10 K readily forms chemical bonds with G e ( l l l ) surface Ag

atoms in contrast to Si(lll). This causes the G e ( l l l ) - 2 x 1 ---, G e ( l l l ) - I x 1 transition. Hence one can conclude that the activation energy for chemical bonding of the surface Ge atoms with Ag is very low, or else it is absent. A strong chemical interaction can be the reason for the disappearance at 10 K of main L E E D reflections from a G e ( l l l ) surface at 8 -- 2. This cannot be attributed to "screening" of the substrate by the Ag film, since in the

12

V.A. Grazhulis / Low temperature studies of semiconductor sur[aces

S i ( l l l ) + Ag system (see above) obtained in the same conditions, the main LEED spots are observable up to 0 = 3-5. Therefore, a strong chemical interaction of Ge atoms with the disordered system of Ag atoms results at 10 K in an essential disturbance of the translational symmetry in a few layers at the G e ( l l l ) surface. Other details on the G e ( l l l ) + Ag system can be found in ref. [13].

3.3. Investigation of InSb(llO) + Ag This section presents some results (see, also, ref. [15]) obtained by using the L E E D technique for InSb(110) cleaved surfaces covered by Ag at low temperatures. The mean thickness of the Ag films was varied, as before, in the range of 0 = 0 - 2 0 monolayers, one monolayer ( 0 = 1) corresponds now to the number of the InSb(110) substrate atoms per unit area, i.e., 6.74 × 1014 cm 2 The crystals were cleaved at 300 K. Cooling the samples down to 10 K does not change qualitatively the L E E D pattern for a clean surface. With Ag deposition at 10 K, 0 < 0 < 0.l, a distinct LEED pattern from the InSb(ll0) substrate surface is still observed, however, with 0.1 < 0 _< 1 the background grows with increasing 0 and, simultaneously, the intensity of the InSb(ll0) reflections decreases. At 0 ~ 1 1.5 the background alone is seen, the InSb(110) reflections completely disappear, and no new reflections are formed. The "screening effect" must arise with 0 > 3, see above. Therefore, the disappearance of the InSb(ll0) reflections at 0 ~ 1 1.5 may be treated, like in the case of G e ( l l l ) + Ag, as a result of an essential disturbance in the translation symmetry of several atomic layers near the lnSb(110) surface due to a strong chemical interaction of the disordered system of Ag atoms with the substrate immediately after the adsorption at 10 K (negligible activation energy for chemical bonding). We shall now consider the behaviour of I n S b ( l l 0 ) + Ag with a further increase of 0 and T ~ 10 K. At 1 _ < 0 < 4 the I n S b ( l l 0 ) + A g surface remains strongly disordered (amorphous). It does not give any LEED reflections. However, at 0 = 0 " = 4-4.5 an interesting phenomenon is observed, namely, the transition of the Ag system to the ordered state; the LEED pattern suddenly starts showing new reflections that do not repeat the earlier vanished reflections of the InSb(ll0) substrate, see fig. 8. So, we are presumably dealing with a phase transition in the Ag film from an amorphous to crystalline state at constant temperature ( T - 10 K). The critical parameter is the Ag film thickness. Thus, there exists a critical value of 0 = 0 " = 4 4.5 at which the said transition occurs. As 0 increases the intensity of the new LEED reflections grows until 0 ~ 5; with a further increase of 0 to 0 ~ 20 (we do not deal with the cases of 0 > 20 here) the intensity of the reflections remains approximately constant. This means that the transition is "diffuse" with respect to 0 in the 0

V.A. Grazhulis / Low temperature studies of semiconductor surfaces

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[-o'" i " a

b

rag. 8. (a) InSb(110)+ Ag diffraction pattern (0 = 9), obtained by 10 K deposition and heating at 300 K, Ep = 63 eV; (b) schematic presentation to explain the diffraction pattern involved; (1) basic spots of the Ag film, corresponding to the Ag(l11) bcc lattice; (2) satellites of the basic spots of silver, (3) the substrate spot sites before the silver deposition (also revealed upon heating the deposited samples to T > 300 K.

range from a b o u t 4 to a b o u t 5. This, p r o b a b l y , is a s s o c i a t e d with large fluctuations in the film thickness, arising at a low t e m p e r a t u r e of the substrate. F r o m the diffraction p a t t e r n analysis it follows [15,16] that A g film at 0-0"~ 5 a n d T ~ 10 K forms a new a t o m i c structure, n a m e l y a bcc one i n s t e a d of the usual fcc. Let us e m p h a s i z e that this " u n u s u a l " silver grows o n l y at l o w - T deposition. So, at 0 > 0 * a n d T = 10 K a new bcc m o d i f i c a t i o n of A g (with a = 3.4 ,~ [15]) arises on the InSb(110) surface as a result of the structural transition. N o t e that the A g d e p o s i t i o n at 300 K does n o t lead to the f o r m a t i o n of this A g modification. O t h e r details of the investigations m a y b e f o u n d in refs. [15,16].

4. Conclusion The p r e s e n t e d e x p e r i m e n t a l d a t a suggest that there exists a wide t e m p e r a ture interval, which has been very p o o r l y investigated as yet, b u t within which interesting physical p h e n o m e n a m a y arise. T h e r e are g r o u n d s to believe that these studies will be d e v e l o p e d intensively a n d will l e a d to new findings.

References [1] V.Yu. Aristov, N.I. Golovko, V.A. Grazhulis and Yu.A. Ossipyan, presented at 4th European Conf. on Surface Science, 1981.

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IdA. Grazhulis / Low temperature studies of semiconductor surfaces

[2] V.Yu. Aristov, N.I. Golovko, V.A. Grazhulis, Yu.A. Ossipyan and V.I. Talyanskii, Surfac~ Sci. 117 (1982) 204. [3] V.Yu. Aristov, I.E. Batov and V.A. Grazhulis, Surface Sci. 132 (1983) 73. [4] D. Haneman and R.Z. Bachrach, J. Vacuum Sci. Technol. 21 (1982) 337. [5] D. Haneman and R.Z. Bachrach, Phys. Rev. B17 (19830 3927. [6] V.A. Grazhulis and V.F. Kuleshov, Appl. Surface Sci. 22/23 (1985) 14. [7] N.I. Golovko, V.A. Grazhulis, V.F. Kuleshov and V.I. Talyanskii, Poverkhnost 1 (1986) 76 [8] V.A. Grazhulis, Poverkhnost 6 (1986) 140. [9] V.Yu. Aristov, I.L. Bolotin, V.A. Grazhulis and V.M. Zhilin, Zh. Experim. Teor. Fiz. 91 (1986) 1411. [10] A. McKinley, R.H. Williams and A.W. Park, J. Phys. C 12 (1979) 2447. [11] D. Bolmont, Ping Chen, C.A. Sdbenne and F. Proix, Phys. Rev. B 24 (1981) 4552. [12] G. Le Lay, A. Chouvet, M. Manneville and R. Kern, Appl. Surface Sci. 9 (1981) 190. [13] V.Yu. Aristov, V.A. Grazhulis and V.M. Zhilin, Poverkhnost (1987), in press. [14] G. Le Lay, G. Quentel, J.P. Faurie and A. Masson, Thin Solid Films 35 (1976) 273. [15] V.Yu. Aristov, I.L. Bolotin and V.A. Grazhulis, Pisma Zh. Eksperim. Teor. Fiz. 45 (1987) 49 [16] V.Yu. Aristov, I.L. Bolotin and v.A. Grazhulis, in: Proc. 14th Annual Conf. on the Physic~ and Chemistry of Semiconductor Interfaces, Salt Lake City, February 1987.