NUCLEATION AND GROWTH OF Cr ON STEPPED SURFACES WITH FACETS AN FEEM STUDY

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NUCLEATION AND GROWTH OF Cr ON STEPPED SURFACES WITH FACETS AN FEEM STUDY

A. Melmed, N. Shinn

To cite this version:

A. Melmed, N. Shinn. NUCLEATION AND GROWTH OF Cr ON STEPPED SURFACES WITH FACETS AN FEEM STUDY. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-33-C6-38.

�10.1051/jphyscol:1987606�. �jpa-00226809�

NUCLEATION AND G R O W T H O F Cr O N STEPPED SURFACES WITH FACETS A N FEEM STUDY

A.J. Melmed and N.D. Shinn*

National Bureau of Standards, Gaithersburg, MD 20899, U.S.A.

*~ivision 1134, Sandia National Laboratories, Albuquerque, N M 87185, U.S.A.

Rksum6.- Etude, par microscopic e f f e t d e champs, d e l a nuclkation et d e la croissance Cpitaxiale d e c r i s t a u x d e Cr, form& par dCposition d e vapeur d e C r sur d e s s u r f a c e s d e W et d e R e nettoydes thermiquement. L e s s u r f a c e s d e s s u b s t r a t s s o n t constitu6es d e t e r r a s s e s e t d e f a c e t t e s d e faibles indices, d6limitCes par d e s "marches" atomiques. Sur les s u r f a c e s d e Re, l a nucl6ation multiple conduisant h une croissance polycristalline s e m b l e in8vitable. L'Ctude plus extensive du systiime Cr/W conduit i quelques r b u l t a t s surprenants. Dans l e domaine d e t e m p k r a t u r e compris e n t r e 5 0 0 et 1050 K, l a nuclkation et la croissance cristalline n'apparait qu'aprils la formation d'une ou deux monocouches atomiques (mode Stranski-Krastanov). Cependant i 300 K et sur la f a c e (011) d u W on observe n e t t e m e n t deux modes distincts d e croissance e n fonction d e l a g k m 6 t r i e d u s y s t i m e d e d6pdt d e vapeur; l e mode Stranski-Krastanov ou l e mode Volmer-Weber.

Abstract.- T h e nucleation and epitaxial g r o w t h of C r c r y s t a l s resulting from vapor-deposition of C r o n t o thermally annealed W and R e s u r f a c e s has been studied using field e l e c t r o n emission microscopy. T h e s u b s t r a t e s u r f a c e s consisted of low-index f a c e t s and t e r r a c e s s e p a r a t e d by a t o m i c steps. Multiple nucleation, leading t o polycrystal growth, could not b e avoided on t h e R e surfaces.

T h e Cr/W s y s t e m was studied m o r e extensively and was found t o yield s o m e surprising results. In t h e t e m p e r a t u r e range of a b o u t 500-1050 K, nucleation and growth of C r crystals occured only a f t e r 1-2 monolayers of C r a t o m s had f o r m e d ( t h e Stranski-Krastanov mode). However, at - 300 K on {011[ W planes, t w o d i f f e r e n t g r o w t h modes w e r e observed consistently depending upon t h e vapor-desposition g e o m e t r y ; t h e Stranski-Krastanov o r t h e Volmer-Weber mode.

I. Introduction

Field emission microscopy (FEEM) can provide fundamental infor- mation relating directly or indirectly to nucleation and epitaxial growth on microscopic as well as macroscopic surfaces.

The recently increased effort to develop materials with unusual properties, often derived from compositional modulation or thin layer- ing, has led to an enhanced interest in learning more about thin film epitaxy. Purity, structure and interface integrify are among the para- meters to be optimized. Also, surface sclentlsts have become in- terested in materials which are often difficult or impossible to clean by conventional methods, and situ clean specimen preparation by epitaxial growth offers a suitable alternative.

The use FEEM and thermally cleaned specimens for the substrates provides surfaces with large low-index facets and terraces separated by atomic steps. This, plus the ability to observe directly surface diffusion and the microscopic details of nucleation provides a large amount of information under well-controlled experimental conditions.

11. Experimental

The apparatus and procedures used in these experiments have been

w e l l d e s c r i b e d i n t h e l i t e r a t u r e ( 1 , 2 ) . However, we f o u n d t h a t s p e c i a l c a r e h a d t o

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987606

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be exercised in the preparation and use of the Cr vapor source. High purity Cr (99.9994% pure) was thermally vaporized from a previously degassed tungsten basket supported by four W heating wires. The source was positioned below and ahead of the FEEM tip and Cr atoms impinged on the central tip region from a direction of 50-60" below normal.

Maintaining a source temperature below about 2300 K was found to be necessary in order to avoid oxygen-contamination of the Cr crystals. Interest- ingly, thermal evaporation of an oxygen-contaminated epitaxed Cr crystal on W resulted in an oxidized W surface; whereas, in the Cr/Re experiments we initially had carbon contamination from a contaminated Cr source, and the carbon was removed from the Re surface by reaction with the subsequently deposited pure Cr.

111. Results and Discussion

The Cr crystals which grew on thermally cleaned Re substrates were nucleated at the step edges of (loo), (101), (102) and (110); our specimens were oriented such that the basal planes, (001) Re, were not observed.

Fig. FEEM micrographs.

A) Clean, thermally annealed Re.

B) Polycrystalline Cr grown on Re.

C) Small Cr crystal on Re.

(S-K) mode of growth. The most often observed epitaxial relationship was (011)Crl l(101)Re or (011)Cf,/ l(100)Re with [Ill] Cr11[010]Re. Figure 1 ill- ustrates these results. It should be noted that, contrary to the results found for Eu/Re epitaxy (I), only BCC Cr was observed in the present work.

Surface diffusion of Cr,@ was examined qualitatively. It was found that at most 1-2 monolayers of Cr could be diffused over the curved parts of the W field emitter and diffusion into the (001) plane v$cinals was energetically most difficult (and this had obqervable.conseque'nces in attempts to grow large Cr crystals). If larger amounts of Cr were deposited initially, then heating with or without the presence of g n electric field resulted in the spreading of a layer of about 2 monolayers plus the agglomerization of the remainder of the Cr (see figure 2). The dosage of Cr was calibrated approximately by consider- ing that the coverage for a film with a marginally sharp boundary was about 1.5 monolayers (2). This diffusion behavior and the coverage estimates have been confirmed in a recent low-energy-electron diffraction, Auger electron spectros- copy, temperature-programmed desorption study (3).

Fig. FEEM micrograph obtained after attempted diffusion of a multi- layer Cr (deposited from below) into upper part of W surface.

Only about 2 ML migrated, as indicated by advancing Cr film at arrow; the remainder formed (brightly emitting) clusters.

Vapor deposition of Cr at rates of 0.1-1 monolayer per second and sub- strate temperatures of about 300-1050 K resulted in various nucleation and growth phenomena which we will discuss in relation to two temperature regimes;

500-1050 K and -300 K.

Nucleation and growth of Cr crystals in the 500-1050 K range generally followed the S-K mode, as in numerous earlier FEEM studies of metal/metal epitaxy (4). Nucleation occurred only after formation of 1-2 monolayers of Cr atoms. It was difficult for us to grow very large single crystals of Cr, due to a temperature (and thus flux) limitation of the Cr vapor source. Source temperatures above about 2300 K resulted in the growth of Cr crystals which were contaminated with oxygen (see figure 3). This oxygen contamination was detected easily by heating the substrate to about 1500 K, which caused evapora- tion of the Cr and left behind an oxidized W surface. We found that the edges of (011) W planes were the most energetic binding places and thus the most common sites for Cr nucleation at the highest substrate temperatures we could use. The (001) W plane in direct sight of the Cr vapor source was the next most common nucleation site. These results are seen in figure 4.

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Fig.3 FEEM micrographs of A) a clean Cr/W crystal (defects indicated by arrows) and B) an oxygen-contaminated Cr/W crystal.

Fig. FEEM micrographs of A) Cr crystallites nucleated at -700 K at 3 (011) planes and the (001) plane, B) poly- crystalline Cr and C) Cr

sometimes a Cr crystal which nucleated and grew on the (001)W plane in direct view of the Cr source grew with some additional facets which were not seen on the crystals which grew from nuclei in other parts of the W substrate (fig.

4B).

Nucleation and growth of Cr crystallites occurred also on (011) W at relatively low substrate temperatures, around 300 K, and this process was surprisingly sensitive to the angle-of-impingement of the Cr atoms onto the W surface. As a function of deposition amount, nucleation occurred first on the central (011) W plane, or its edge, which was located such that the Cr atoms impinged at an angle of about 50-60' below the surface normal, and later nucleation occurred on the lower-most (011) W plane, which faced the Cr source directly (normal incidence). The Cr flux to each of these planes was ap- proximately equal (somewhat geater to the lower plane). Nuclei were observed on the central (011) W plane for Cr depositions as small as 1/10 monolayer, whereas about 1-2 monolayers of Cr typically were deposited on the lower (011) W plane before discrete nuclei were seen. This result was consistently observed over a variation by a factor of about 10 in Cr vapor-deposition flux.

Figure 5 shows two prominent nuclei which formed and grew somewhat at the edge of the central (011) W plane, while no nuclei formed on the lowermost (011) planes.

Fig. FEEM micrograph showing two Cr nuclei at edge of central (011) W plane. 300 K experiment.

The normal incidence result, that is the result found here for Cr nuclea- tion at LEED/AES/TPD on the lower-left (011) W plane has been confirmed by the LEED/TPD work of reference 3, where crystallites were observed to grow after a Cr coverage of 1.8 monolayers.

It is clear from these results that two modes of Cr nucleation and growth occur on (011) W. The only experimental parameter different in the case of the central (011) W plane compared to the lower one is the angle-of-impingement of the Cr atoms. Thus we conclude that at -300 K the S-K mode prevails for normally incident Cr atoms, while Cr atoms impinging at about 60" below normal.

retain enough lateral energy to enable surface diffusion sufficient for nucleation to occur on an otherwise bare surface, which is the Volmer-Weber mode. Further discussion of these results has been written elsewhere (5).

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Fig. 6 FEEM micrographs of room-temperature nucleation of Cr after depositions of A) about 1/5 ML Cr, B) about 3/5 ML Cr, C) about 2 ML Cr and

D) several ML Cr.

Acknowledgement

N.D.S. acknowledges support of an NBS-NRC postdoctoral research as- sociateship (1984-1985) and partial support from the U.S.Dept. of Energy under contract DEAC 04-76 DP 00789 with Sandia National Labora- tories.

References

1. A. J. Melmed, V. Maurice, 0. Frank.and J. H. Block, J. de Phys. 45, (1984) C9-47, and J. Crystallography, in press.

2. R. Gomer, "Field Emission and Field Ionization," Harvard University Press, Cambridge, MA, 1961.

3. N. D. Shinn and P. J. Berlowitz, Surf. Sci., to be published.

4. See, for example, A. J. Melmed, J. Appl. Phys. 36, (1965) 3585.

5. A. J. Melmed and N. D. Shinn, to be published.