ULTRATHIN IONIC FILMS EPITAXIALLY GROWN ON ...

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Abstract. Ultrathin films (> 0.3 ML) of NaCI and KBr have been grown epitaxially on GaAs (001) and InSb (001) surfaces, respectively. Scanning tunnelling (STM) ...
ULTRATHIN IONIC FILMS EPITAXIALLY GROWN ON 111-V SEMICONDUCTORS STUDIED WITH ATOMIC RESOLUTION

M. SZYMONSKI, J. J. KOLODZIEJ, B. SUCH, P. CZUBA AND P. PIATKOWSKI

Institute of Physics, Jagiellonian University, ul. Reymonta 30-059 Krakow, Poland

4,

AND F. KROK

Regional Laboratory for Physicochemical Analysis and Structural Research, ul. Ingardena 3, 30-060 Krakow, Poland Abstract. Ultrathin films (> 0.3 ML) of NaCI and KBr have been grown epitaxially on GaAs (001) and InSb (001) surfaces, respectively. Scanning tunnelling (STM) and non-contact atomic force (NC-AFM) microscopies in ultrahigh vacuum were used to study surface structureS generated by growth. It was found that initially islands of monatomic thickness are formed. These islands are often cut along (110) crystallographic directions and the distribution of these islands on the substrate surface is anisotropic, which reflects the anisotropic diffusion of KBr molecules during growth. We argue that the KBrflnSb interface is stabilized by a bond between the halide ion and AlII atoms arranged in chains on (4x1) InSb. At 1-1.5 ML, a wetting single-atomic KBr film is formed and material in excess of 1 ML forms rectangular islands with edges oriented along (lOO) and (OlO) crystallographic directions. For multilayer KBr coverages, the growth is basically a layer-by-Iayer type, but due to slow diffusion of KBr molecules down across steps, the (n + 1)th layer starts to grow before the completion of the nth one. As a result, pyramidal structures of rectangular bases are formed on the surface. These rough films can be, with thermal annealing, converted to flat films exposing large (> 0.1 J.Lm) atomically flat (001) terraces. Experiments on nanoscale modification of such terraces by electron excitation are also described. 499

M. Kotrla et aL (eds.). Atomistic Aspects of EpitQJCial Growth, 499--509. e 2002 Kluwer Academic Publishers.

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1. Introduction The insulator-semiconductor interface is a basic element in contemporary electronic technology. Several ionic insulators, i.e. alkali and alkaline-earth halides have become attractive, since they may be used to construct epitaxial heterostructures, based on 111-V compounds, with functionality analogous to that of the silicon-oxide/silicon junction [1,2]. It has been found that several alkali and alkaline earth halides can be successfully grown epitaxiallyon single crystal semiconductors as Si, Ge, and GaAs [1-6]. Saiki et ai. have found that NaCI can be grown epitaxially on GaAs(OOl) substrate provided that the substrate temperature has been properly chosen [5,6]. This is due to the fact that the lattice constants of these crystals, both having cubic lattices, differ only by 0.4% at room temperature. NaCI films grown on the (001) surface, have a regular flat surface and the epitaxial orientations are: (OOl)NaCI II (001)GaAs and (010)NaCI II (OlO)GaAs. Photoemission studies have suggested that the dominant bonding at the interface is that of CI to Ga. More recently, this finding has been confirmed using Kikuchi electron holography [7]. On the other hand, our recent studies [8-10] have revealed that alkali halide surfaces can be modified in controlled way by the electron beam. NC-AFM images indicate that KBr(OOl) surface subjected to the electron bombardment develops a network of very regular rectangular pits of singlemonolayer depth. The average size of pits and their surface density can be controlled adjusting radiation dose and the temperature. The mechanism of such surface modification due to electron excitation has been analyzed in Refs. [9,10]. Such modified ionic surfaces could possibly be used as templates for growth of nanostructures. In the present paper we investigate the morphology of alkali-halide films grown from molecular beam (MBE) on 111-V substrates and the modification of surfaces of such films by the electron beam. 2. Experimental

Our experimental system consists of three interconnected parts, allowing for sample processing in the preparation chamber, sample diagnostics and modification in the analytical chamber and scanning probe microscopy in the microscope chamber. All steps of the experiment could be performed in ultra-high vacuum and the exchange of samples between chambers is facilitated by magnetically coupled transfers. The base pressure of the system is 1 x 10- 10 Torr. A schematic diagram of the experimental set-up is given in Fig.!. We used doped GaAs(001) and InSb(OOl) wafers as the substrates (conductivities 2 x 10- 3 Ocm and 3 x 10-4 Ocm, respectively). The samples were

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Time-of-f1ight

Magnetic transfer

Wobblestick

Figure 1.

Figure 2. luSb.

Schematic diagram of the experimental set-up.

LEED patterns of (a) InSb c(8x2) surface, and (b) 10 ML film of KBr on

heated. up to 900 K and 700 K for GaAs and InSb respectively and then sputter cleaned with 0.7 keY Ar+ bombardment (1 /-LAjcm2j incident angles from 45°-60°) until clear c(8x2) LEED (low energy electron diffraction) pattern was obtained [see Fig. 2(a)]. Such pattern is characteristic of longrange order established on reconstructed group-III-element-terminated surfaces of II1-V compounds [11]. After such a procedure no traces of carbon or oxygen could be found on the surface, as examined with Auger electron spectroscopy. Further characterization of the substrates with STM and NCAFM shows clearly the reconstruction rows [see Fig. 3(a,b)], as predicted by recent models [12,13]. Notice that nanoscale resolution for the NC-AFM image of III-V semiconductor surface has been obtained..

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Figure 9. 175 Ax 175 A images of InSb c(8x2) surface. (a) STM of empty states (U V, 1= 0.76 nA), (b) NC-AFM image taken with detuning equal to 200 Hz.

=1

NaCl and KBr molecular fluXes for deposition were obtained from the effusion cells heated above 700 K (monomer/dimer ratio at this temperature ~ 102 [14]). The deposition rate was monitored with a quartz crystal microbalance (QCM) placed in an equivalent position with respect to the effusion cell as the substrate. Typical deposition rates were equivalent to the growth rate of 2-4 monoiayers (ML) of the perfect crystal per minute. Those growth rates were independently calibrated with the readings on our scanning probe microscope, which could be done accurately for initial stages of the epitaxial growth. Long-range order for thick epitaxial layer was verified with LEED [see Fig. 2(b)J. High-resolution imaging of our samples was performed with Park Scientific Instruments VP2 STM/AFM DHV microscope. For ultrathin ionic overlayers (up to 4 ML) the tunnelling current could be measured with a cut tungsten wire tip and a low positive sample bias (1 +- 1.5 V) for the empty state imaging. For imaging of the occupied states a negative bias of 1 +- 1.5 V was used. For AFM imaging commercial piezoresistive cantilevers were used with typical resonant frequencies of 100 kHz. All AFM images were taken in a non-contact mode with the help of the Nanosurf "easyPLL" demodulator.

3. Growth and Characterization of Ultrathin Epitaxial Layers An STM image of an InSb(OOl) surface covered with an amount of KBr equivalent to 0.3 ML is seen in Fig. 4. It is seen that the substrate is covered with 2-D islands of deposited material with an average size of 100200 A. It is striking that the islands are frequently limited by straight edges

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Figure 4. Filled state STM images of 0.3 ML of KBr on InSb: (a) 200 x200 run; (b) 100 x 100 nm.

Figure 5. Filled state STM images of 1 ML coverage of KBr on InSb: (a) 200 x 200 nm; (b) 100 x 100 nm.

along (110) direction. This strongly indicates that deposited KBr molecules easily diffuse and aggregate along the substrate dimer rows (which extend along the (110)) while the migration along (110) encounters barriers (the reconstruction rows). This results in the characteristic shape of the islands as seen in Fig.4. At a coverage of about one ML, the KBr film covers the substrate uniformly as seen in Fig. 5. The film is sharply discontinued at the substrate steps. This behaviour is quite different from that observed for alkali halide films grown on metal substrates, where the so-called carpet-mode of the growth over steps has been seen [15-17]. Details of the film reconstruction are seen on the STM image (see Fig. 6). The structure of the film is seen

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Figure 6. Filled state STM images of average coverage of 1.5 ML of KBr on InSb. Horizontal lines visible in the images are formed by reconstruction rows on InSb surface: (a) 200 x 200 nm; (b) 100 x 100 nm; (c) 50 x 50.nm; (d) 7.5 x 7.5 nm taken in the region where a monolayer film of KBr was present.

with atomic resolution. The (4xl) reconstruction rows, running along the (110) direction, are seen. This image is very different from STM images of the reconstructed InSb(OOl) surface, known from the literature, indicating that the atomic scale corrugations in Fig. 6 retlect the structure of the ionic film surface rather than the interface. Let us consider how the ionic-covalent interface for alkali halide AmBv system is constructed. As indicated by photoemission studies [18J the dominant bonding is that of the halogen ion and the Am atom on the surface. Thus, the KBr film is stabilized mostly by Br ions bonded to indium atoms at the interface. Our previous study of NaCI/GaAs interface by means of Kikuchi electron holography [7J has indicated that the halogen ion is situated on top of Am atom rather than on the bridge site as suggested in Ref. [18]. In a recent paper [19J, a model of the KBr/InSb interface was proposed where we argue that almost flat (4 xl) ionic overlayer can be constructed in agreement with the most recent AmBv c(8x2) surface reconstruction model [l1J. This interface model can account for all properties of the KBr/InSb system discussed above, Le. the principal bonding is that between Br and In, the film is composed of ionic rows running along (110) which differ slightly in height, the film is continuous along (1, I, 0), the film surface symmetry is basically (4 xl), but there is c(8 x 2) symmetry buried underneath the film, and the majority (75%) of Br ions appear in top positions and only 25% in the bridge positions.

4. Growth and Characterization of Multilayer Structures Since the first atomic layer of KBr is properly aligned and complete, the formation of further atomic layers is basically homoepitaxial growth. Islands growing on the wetting KBr film (4 ML) are rectangular with edges aligned along (010) and (100) (Fig. 7). For rock-salt ionic crystals, such edges (steps) are electrically neutral and provide the minimum crystal energy configuration. Figure 7(a) is an NC-AFM image of a KBr film at a

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'~ o

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400 nm

Figure 7. 500 x 500 nm images of 4 ML of KBr: (a) NC-AFM image; (b) filled-state (U = -1, 1= 0.7 nA) STM image.

coverage of 4 ML. The surface is covered with multilevel rectangular features (islands and bridges) with edges oriented along (100) and (010). The amount of material associated with particular level decreases with increasing level. The film is composed of three almost complete atomic layers (note that only very few dark, zigzag shape features, which are due to breaks in the film, are seen), and two incomplete levels. For comparison, STM image of the 4 ML film covering the InSb surface is shown [Fig. 7(b)J. The features on top of the film give virtually no contrast. The tunnelling current is then predominantly due to substrate or interface states and the STM is imaging the buried interface, but not the surface of our sample. No STM for coverages greater than 4 ML could be measured. Figure 8 shows a NC-AFM image for a coverage of 120 ML. There is a tendency to form pyramidal structures with similar width of neighboring terraces and rectangular bases oriented along (100)-type directions. We think that this is the result of a self-regulating process, Le. larger terraces adsorb a larger number of KBr molecules that are confined to the terrace. During growth the area of a given terrace changes proportionally to the difference in widths of the lower terrace and the given terrace. That, after a sufficiently long time, must lead to a normalization of terrace sizes. Indeed, it can be seen that pyramids built on the 120 ML film are strikingly regular. On the other hand, typical terrace sizes found from our images have sizes of several tens of nanometers, which indicates efficient diffusion of KBr molecules on terraces (Figs. 8 and 9). Thus, the formation of structures observed in our experiment can be described through the known processes of condensation of alkali halide vapors in terms of the terrace-ledge-kink (TLK) model [20]. According to this concept, alkali halide molecules are adsorbed on the crystal surface at random positions, typically on a (100)

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A 20 10

o o Figure 8.

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400nm

A 500 x 500 nm NC-AFM image of a 120 ML KBr film.

terrace. Due to its high diffusivity, it migrates thermally until it is caught at the ledge running along (100) or (010) directions. Then it walks along the ledge until it is caught at a kink, which is a stable configuration of the system. This scheme leads to formation of large islands and terraces with (100)-type edges. However, as seen in Figs. 8 and 9(a), the film does not grow atomically flat. Often the next-level, islets aggregate on top of already formed islands. This is unexpected because the diffusion coefficients are, for alkali halide molecules, of the order of 10- 1 cm 2 js at 370 K [11]. In our experiment, on a 400 A terrace, a single molecule falls every rv 10- 5 s. During that time the diffusion length is greater than 104 A and the molecule always should reach a ledge before it can aggregate with other molecules on the terrace. Recently, there has been increasing interest in film growth when the surface diffusion of impinging atoms or molecules is restricted by barriers across steps [Ehrlich-Schwoebel barriers (ESB)] [21,22]. Studies of adsorption systems with ESB have been carried out with STM and, therefore, limited to conducting surfaces (see, for example, Refs. [23-25]). However, we are able to study such phenomena on insulating surfaces by means of NC-AFM. The discrepancy between the KBr molecule diffusion length, the average terrace size, and the formation of pyramidal structures can be explained by the presence of ES barriers for the diffusion of a KBr molecule down (100) steps. It is interesting to note that the height of the pyramids is only a fraction of average film thickness (rv for a 120 ML film). One of the objectives of the present work was the preparation of atomically flat KBr films for surface modification at nonoscales. Freshly prepared films have been annealed at 520 K. Figure 9(b) shows a NC-AFM image of the annealed film of initial coverage 60 ML. Thermal treatment resulted in removal of pyramidal forms from the surface and the formation of large

k

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:? 65000 c

! 60000 ii &55000 'iii ~

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150 100 time [s]

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Figure 9. A 60 ML film of KBr; (a) a 500 x 500 NC-AFM image of the film as grown; (b) a 500 x 500 NC-AFN image of the film after annealing; (c) a 500 x 500 NC-AFM image of the film after irradiation to the first maximum on oscillations; (d) oscillations of desorption yields of atoms emitted from the film during electron irradiation. The arrow marks the stage of the process shown in (c).

(0.1-0.2 J.1,m) atomically fiat terraces. These annealed films were subjected to electron bombardment in order to monitor the dose-dependent modification of the surface. A NC-AFM image of modified KBr film is shown in Fig. 9(c). A network of rectangular pits of mono-molecular depth is clearly seen. In our previous study [9] for bulk KBr, we found that the surface topography of electron bombarded alkali halides varies from atomically fiat to rough (i.e. covered with pits) periodically as a function of the irradiation dose constituting the layer-by-layer desorption regime. The desorption rate is coupled to the density of steps on the surface because the steps act as the primary desorption sites. The preliminary results shown in Fig. 9(d) indicate that such periodic oscillations of the desorption yield could also be observed for our thin epitaxial films. Thus, we expect that such films could be used for nanostructure formation of halides on semiconductor systems.

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5. Conclusions For the alkali halideflII-V semiconductor epitaxial system, initially a wetting, single-molecular alkali halide overlayer is formed. This layer does not destroy the overall structural characteristics of the substrate since, e.g., the indium reconstruction at the KBrflnSb interface remains unchanged under the overlayer. The growth and the structure of the first atomic layer can be explained consistently with the most recent model of the (100) AmBv c(8x2) surface reconstruction. After the first atomic layer is completed, rectangular islands exposing (100) terraces with edges oriented along the (OOl)-type directions are formed on the surface of the film. Further growth proceeds in a quasi-two-dimensional mode, i.e. basically layer-by-Iayer, but the (n + l)th layer starts to grow before the completion of nth layer. This form of growth can be interpreted in terms of interactions between ionic molecules and (100) ionic crystal terraces, steps, and ledges. The observed surface structures in epitaxial KBr arise due to fast diffusion of the KBr molecule on the (100) ionic crystal terrace and along the terrace edge, and to strong binding at ledges and Ehrlich-Schwoebel barriers inhibiting diffusion down across steps. Heating of as-grown films close to their sublimation temperature results in the formation of large, atomically flat terraces. The sizes of these terraces are similar to the typical sizes of terraces on cleaved surfaces. The epitaxial films were modified with electron bombardment, showing surface topography periodically varying with an electron dose.

Acknowledgements. The financial support for this work has been provided by the Polish Committee for Scientific Research under Projects No. 2P03B 13717 and No. 5P03B 05020. References 1.

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