Atomically thin epitaxial films of NaCl on germanium

Thin Solid Films, 172 (1989) 123-132

123

PREPARATION AND CHARACTERIZATION

A T O M I C A L L Y T H I N E P I T A X I A L FILMS OF NaC1 ON G E R M A N I U M S. FOLSCH, U. BARJENBRUCH AND M. HENZLER

Institut fiir Festkdrperphysik, AppelstraBe 2, 3000 Hannover ( F.R.G.) (Received June 14, 1988 ; revised August 11, 1988 : accepted November 25, 1988)

On clean Ge(100) and G e ( l l l ) substrates thin epitaxial NaC1 layers are generated. At sample temperatures T < 150K films with the [I00] direction perpendicular to the substrate are always formed having a minimal thickness of one full lattice constant (ao = 0.563 nm). On G e ( l l l ) a structure consisting of twinned pyramids is built only at temperatures T > 150 K. Measurements of the work function change are in accord with properties concerning film structure and dissociation of NaC1 molecules on Ge(111). In contrast to bulk alkali halide crystals, these films do not exhibit any disturbing surface charging effects during photoemission or low energy electron diffraction measurements.

1. INTRODUCTION

Most surface studies are carried out on metals or semiconductors. For both structural and electronic investigations mostly electrons are used. For reproducible measurements the surface potential has to be constant and well known. This is not the case for insulators, which may charge up because of incident or emitted electrons 1. A high energy beam (E > 100 eV) may produce a reproducible potential; simultaneously, however, many insulators are dissociated by the electron beam 2,a. It is therefore desirable to avoid charging up by an appropriate measurement set-up which works also at low electron energies. For that purpose very thin epitaxial layers have been produced in ultrahigh vacuum, so that any excess charge at the surface is compensated by tunnelling to the conducting substrate. Additionally, insulator/semiconductor heterostructures are of great technological and fundamental interest 4 and have been examined extensively 5' 6. The following study is a description of the results of molecular beam epitaxy of NaCI on different orientations of clean germanium, since both structure and lattice constant of the two materials are very close (NaC1, cubic NaCl structure with ao = 0.563 nm; germanium, cubic diamond structure with ao = 0.566 nm). 0040-6090/89/$3.50

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2. EXPERIMENTAL DETAILS

The measurements were performed in an ultrahigh vacuum system, which was equipped with low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) apparatus and a vibrating capacitor for the determination of work function changes. Polished Ge(100) and G e ( l l l ) surfaces were used as substrates which were cleaned in ultrahigh vacuum using ion b o m b a r d m e n t and heating of the sample to about 900 °C. After preparation the samples exhibited the typical (100) 2 x 1 and (111) 2 x 8 L E E D patterns respectively. The only detectable contamination was a trace of o/ carbon with a surface concentration of less than 0.5/o. NaCI (Suprapur®, Fa. Merck) was evaporated out of an A120 3 crucible at about 700°C. At this temperature only molecules are evaporated, so that stoichiometry is obtained automatically 7. During evaporation, the base pressure was about 10-8 Pa with evaporation rates of 0.3 nm min - ~for Ge(100) and 1,8 nm min - J for Ge(l 1 I) substrates (the rates are derived from results of AES measurements as will be described in the following section). The substrate temperature during deposition could be varied from 100 to 1000 K. In the case of alkali halides electron-stimulated dissociation and desorption 8 can take place. Therefore special care was taken to minimize these disturbing effects by using a low current and short exposure times. 3.

RESULTS AND DISCUSSION

3.1. Low energy electron diffraction The crystallographic structure of the generated NaC1 films was determined with four-grid L E E D optics. Figures 1-6 show typical L E E D patterns obtained using the G e ( l l l ) 2 x 8 substrate. Deposition at a sample temperature of 100 K leads to a diffuse circle (Fig. 1; thickness of the film, about 3 nm). After subsequent heating to 400-500 K the circle changes into 12 separate spots (Fig. 2). At higher primary electron energies two additional circles are visible (Figs. 3 and 4). The spots belonging to the medium circle at Eo = 118 eV are rotated by 15 ° against the spot positions of the other circles. These spot positions are easily generated by superposition of three L E E D patterns belonging to NaC1 crystallites with (100) surface orientation which are rotated by 60 ° against each other. Thus, in the case of Ge(111) 2 x 8 at T = 100 K, one finds NaCI cubes with NaCl(100) lIGe(111). The heating obviously produces an azimuthal orientation of the crystallites with respect to the threefold symmetry of the substrate, which means that NaCl[010] IfGe(211). At substrate temperatures T > 150 K during deposition a completely different kind of layer structure is observed. The patterns in Figs. 5 and 6 show facet spots, which move along the connecting lines between the normal spots of the substrate during changes in the electron energy.

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Fig. 1. L E E D pattern of NaCI film (about 3 nm) deposited o n t o clean Ge(l 11) substrate at 100 K, E = 8 5 eV. Fig. 2. Same as Fig. 1 but after heating to 4 0 0 - 5 0 0 K.

Fig. 3. Same as Fig. 2 but at E = 118 eV.

Fig. 4. Same as Fig. 2 but at E = 131 eV.

Fig. 5. Ge(111) substrate at deposition temperatures T > 150 K, E = 68 eV. Fig. 6. Same as Fig. 5 but at E = 63 eV to demonstrate the shift of the facet spots.

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The facet orientation is derived by measuring the facet spot positions for a set of different electron energies. With the help of the Ewald construction the corresponding k vectors are determined and plotted in reciprocal space together with the reciprocal rods of the substrate (Fig. 7). Here a cross-section containing the substrate spots (,21), (11), (01) and (11) is shown with the three-dimensional reciprocal lattice points. It is clearly seen that the facet spots (triangles) form rods, which cross all three-dimensional reciprocal lattice points. Two conclusions are to be drawn: the grown film is epitaxial with NaCI(111)[I Ge(111) (owing to the same reciprocal lattice as the substrate) and the facet orieintations are ( 1 0 0 ) (owing to the difference of neighbouring lattice points9). In Fig. 7 only one set of facet spots is shown. The other is rotated by 180 °, so that in total six 1-100] facets are observed. As a result, on G e ( l l l ) 2 x 8 and at T > 150K one finds the following orientations: NaCl(111)llGe(1 ll) NaCI[211] II Gel211] NaCI[211] IlGe[211] After desorption of the film at about 650 K a 3 x 1 superstructure is observed (Fig. 8). This structure is typical for alkali metals adsorbed on Si(111) and Ge(111) surfacestO 12. Obviously a dissociation of NaCI and desorption of chlorine takes place on Ge(111) before complete desorption.

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Using the Ge(100) 2 x 1 substrate, NaC1 forms a perfect epitaxial film at 100 K: NaCI(100)]] Ge(100) NaCI[010] I]Gel010] Figure 9 presents the L E E D pattern taken immediately after depositing a film of about 2 nm thickness. Additional heating to 400 K for 3 rain results in a sharpening of the L E E D spots up to instrumental resolution (Fig. 10). Since the reciprocal halfwidth of the spot reflects the diameter of well-ordered crystallites 13, the heating increases the size of perfect crystallites with an average size beyond the transfer width of the instrument (20 nm).

Fig. 9. LEED pattern of an NaCl film(about 2 nm) deposited onto Ge(100)at 100 K, E = 79 eV. Fig. I0. Sameas Fig. 9 but after heating to 400 K, E = 170 eV. (The pattern is slightly distorted, because the adjustment of the sample position was not optimum.) For higher deposition temperatures (e.g. r o o m temperature), no epitaxial film of comparable quality could be achieved. In this case, the spots remained broadened and very weak in intensity.

3.2. Auger electron spectroscopy To determine the growth mode of the NaC1 films, AES has been used, since the variation in the signals of film and substrate with film thickness may be evaluated with respect to the uniformity of the film 14. NaCI was deposited onto germanium at 100 K. Since the stability of the deposition rate has been checked, the deposited amount is proportional to the deposition time. The AES signals of chlorine and germanium have been measured after each deposition and normalized with respect to the starting germanium signal [Ge]o. An appropriate measurement of the N a K L L signal could not be achieved, because the germanium substrate possesses a couple of comparable Auger intensities in this range of energy. The results for deposition at 100 K onto Ge(100) are shown in Figs. 11 and 12 and for Ge(l 11) at 100 K in Fig. 13. All figures show a linear decrease or increase up

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to the escape depth of the Auger electrons. This description is appropriate for the measurements at least up to the indicated cross. For higher amounts the experimental accuracy is not sufficient to distinguish unambiguously between linear portions or an exponential shape. Therefore the first layer is definitely complete before the second one starts. The continuation, however, may be either random or layer by layer. The so-far qualitative evaluation does not yet indicate the thickness of the first complete layer. This thickness is derived from the decrease in the signal after completion of the first layer, since the escape depth of the Auger electrons is nearly independent of the material to be transmitted; it is, however, strongly dependent on their energy 14. The decrease (or increase) in intensity at completion of the first layer enables the determination of an escape depth, if the thickness is known. Here two cases have been checked: (i) monoatomic layer with a thickness of half a lattice constant; (ii) double layer with a thickness of a full lattice constant. Table I shows a comparison between the experimentally found values of the escape depth on the basis of cases (i) TABLE 1 C O M P A R I S O N O F E X PE R IMENTALLY F O U N D ESCAPE D E P T H S W I T H T Y P I C A L V A L U E S

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and (ii) and the typical values as expected from literature 14. Evidently the thickness of the first layer is given by a full lattice constant (d =0.563 nm).

3.3 Work Junction change A(a The influence of the growth mode on the work function has been checked using Ge(111) substrates (Fig. 14).

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Curve I represents the work function change of freshly deposited films at T = I 0 0 K (NaCI(100)[[Ge(lll), marked by squares). Instead of the thickness directly the AES chlorine signal is given, so that the same scale may be used after partial desorption of the film. After measuring a data point of curve I the film was heated up to the temperature range of desorption for a short moment. This procedure causes a destruction of the stable layer and makes it possible for NaC1 to rearrange, provided that it did not desorb completely. Corresponding to the higher temperature (about 650 K), pyramids are created (marked by triangles). Additionally, a drastic lowering of the work function takes place. The reason for that behaviour of A~b may be found in the geometric structure of an NaCI pyramid. In this case ions of the same polarity are fixed in (111) planes. These planes containing different sorts of ions are oriented in a parallel way to the substrate and arranged alternately, in contrast to the ordering of ions in the case of a (100) plane. Referring again to Fig. 14, one changes from curve I to curve II by heating the film in the described manner. Additional tempering of the pyramid structure causes a lowering of coverage owing to partial desorption, thus moving along curve II to

ATOMICALLYTHIN EPITAXIALNaC1 ON Ge

131

the left. First the film bursts (region of the bend), which is accompanied by a subsequent linear decrease in IA~bl and the additional appearance of the 3 x 1 superstructure (marked by crosses). In the range of linear decrease in rabbi, the amount of NaCl-covered substrate decreases, but for the present a changed substrate remains, producing the 3 × 1 structure. After complete desorption of the film, a brilliant 3 x 1 superstructure remains causing a work function change Aq~ = --0.5 eV. This change is a characteristic feature of sodium adsorbed on a G e ( l l l ) surface producing a 3 x 1 superstructure12. 4. SUMMARY The deposition of atomically thin NaC1 films onto Ge(100) and G e ( l l l ) substrates leads to very different morphologies. The (100) plane is always formed as the stable surface plane of NaC1. It appears as a smooth surface (NaCI(100)) on Ge(100) and G e ( l l l ) for T < 150 K, but nevertheless the formation of a rough and faceted surface structure is also possible, namely NaCI(111)Jl Ge(1 l 1) for deposition temperatures T > 150 K (Fig. 15). The crystallographic orientation of a film with a few monolayers' thickness is determined by the arrangement of the layer in the initial stage of growth which is dependent on the deposition temperature.

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According to this variety the density of surface defects, in this case dislocations, edges and twin boundaries, can be also very different: smooth epitaxial films on Ge(100) with a minimum of defects, films consisting of rotated cubes on Ge(111) with a smooth surface or rough structures of twinned pyramids on Ge(111).

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s. FOLSCH, U. BARJENBRUCH, M. HENZLER

In the low t e m p e r a t u r e range the m o d e of film g r o w t h is very similar for b o t h substrates: in the initial stage islands with the height of a full lattice c o n s t a n t (d = 0.563 nm) are formed, which grow exclusively in the lateral direction until one closed layer is formed. It w o u l d be interesting to k n o w how the a r r a n g e m e n t of the ions in the d o u b l e layers p r o d u c e s the exceptional stability of that film. A d d i t i o n a l l y , dissociation of N a C I takes place on Ge(111). It might be possible that dissociation a n d thereby f o r m a t i o n of a modified interface between N a C I a n d g e r m a n i u m is responsible for the change in structure at T > 150 K. F i n a l l y the p r o d u c e d films exhibit an a b s o l u t e l y neutral b e h a v i o u r in L E E D a n d p h o t o e m i s s i o n studies. There is no charging of the sample, in c o n t r a s t to bulk alkali halide crystals. It is therefore now possible to study electronic surface properties, which will be r e p o r t e d soon 1.s. ACKNOWLEDGMENTS T h e s u p p o r t of the investigations by Kali a n d Salz AG., Kassel a n d the D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t is gratefully a c k n o w l e d g e d . REFERENCES

l 2 3 4 5 6 7 8 9 10 11 12 13 14

P.S.P. Wei, SurL Sci., 24 ( 1971 ) 219~ T.R. Pian, Sur[i Sci., 129 (1983) 573. H. Tokutaka, Sut~/i Sci., 21 (1970) 233. H. Zogg, J. Cryst. Growth, 80 (1987) 408~ L.J. Brillson, Sur[i Sci. Rep., 2 (1982) 123. A. Munoz-Yague and C. Fontaine, Sur[~ Sci,, 168 (1986) 626. G.M. Rothberg, M. Eisenstadt and P. Kusch, J. Chem. Phys., 30 (2) (1959) 517. T.R. Pian, SurL Sci.. 128 (1983) 13. M. Henzler. in Topics in Current Physics, Vol. 4, Springer, Berlin, 1977. R.E. Weber and A. k. Johnson, J. Appl. Phys., 40 (1969) 314. H. DaimonandS. Ino, Sur/iSci.,164(1985) 320.

P.W. Palmberg and W. T. Perin, SuiJi Sci., 6 (1966) 57. M. Henzler, Su(L Sci., 73 (1978) 240. L.C. Feldmann and J. W. Mayer, Fundamentals ~?fSut;/bce and Thin Film Analysis, North-Holland, Amsterdam. 1986. 15 U. Barjenbruch, S. F61sch and M. Henzler, Surll Sci., in the press.