Ellipsometric investigation of the growth mode of antimony overlayers on GaAs(110)

Thin Solid Films, 177 (1989).287-293 PREPARATIONAND CHARACTERIZATION

287

ELLIPSOMETRIC INVESTIGATION OF THE GROWTH MODE OF ANTIMONY OVERLAYERS ON GaAs(110) R.STRLIMPLERAND H. LUTH 2. Physikalisches lnstitut der Rheinisch- Westfiilischen Technischen Hochschule Aachen, D-5100 Aachen, (F.R.G.), and lnstitut fiir Schicht und lonentechnik der Kernforschungsanlage Jiilich, D-5170 Jiilich (F.R.G.)

(ReceivedFebruary27, 1989;acceptedMay 5, 1989)

Antimony overlayers deposited on GaAs(110) surfaces prepared by cleavage in ultrahigh vacuum have been investigated by ellipsometric spectroscopy. Antimony films in the submonolayer coverage range exhibit spectral features arising from interband transitions. Films of up to 10ML (where ML denotes monolayer) thickness are of inhomogeneous consistency as demonstrated by comparison with calculations based on the Maxwell Garnett model. Slight annealing causes a smoothing of the films. Thicker films exhibit an optical behaviour like that of polycrystalline antimony.

1. INTRODUCTION The deposition of group V elements on surfaces of III-V semiconductors has attracted considerable attention recently 1-6. From the view point of practical applications this might be of interest in relation to problems of III-V layer heteroepitaxy. From a more fundamental standpoint the morphology of the film and its effect on the interfacial electronic structure are of general interest, in particular with respect to the formation of Schottky barriers. Within the variety of possible systems, antimony on GaAs(1 i0) in particular was studied more in detail. An interesting aspect in this case is the formation of a well-ordered monolayer of antimony atoms on the clean cleaved GaAs surface, even at substrate temperatures around 300 K. Further deposition of antimony on top of this first adlayer leads to film growth without long-range order 7. Since the formation of Schottky barriers on the GaAs(110)-Sb system seems to be affected also by the structure and/or morphology of these "thicker" overlayers8, a deeper insight into the growth mode and the film morphology is important. The present paper is intended to contribute to this question. It furthermore demonstrates that ellipsometric spectroscopy can provide useful information about film growth in general.

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2. EXPERIMENTALDETAILS The measurements have been performed on GaAs(110) surfaces of chromiumdoped semi-insulating and n-type (tellurium doped (2.5-5)x 101T cm -3) material. The surfaces were prepared by cleavage in ultrahigh vacuum (p < 10- s Pa) using the double-wedge technique. The semi-insulating samples could be heated indirectly by mounting them onto a ZnO crystal which was heated by direct electrical current flow. The sample temperature was measured by an Ni-NiCr thermocouple fixed between the ZnO and the GaAs crystal. High purity antimony was evaporated from an electrically heated molybdenum basket with an arrival rate of antimony atoms at the crystal surface of about 0.6 M L m i n - 1 (where ML denotes monolayer). 1 M L contains 8.85 x 1014 atoms c m - 2 referred to the GaAs(110) surface. The evaporation rate was controlled by a quartz balance which was calibrated by measuring a thick film using a Tolansky microscope. In ellipsometry the change in polarization on reflection is measured in terms of two ellipsometric angles A and ~k,where p = --rll= tan ~kexp(iA) r±

(1)

is the ratio of the two complex reflection coefficients r Hand r± for light polarized parallel and perpendicular to the plane of incidence. The ellipsometric spectra were obtained with a manual null ellipsometer outside the vacuum chamber which was equipped with quartz windows. It consists of two rotatable polarizers (in the incident and reflected light beams) and a fixed quarter-wave plate. From component settings at extinction, A and ~b were determined for the clean surface and after the deposition of antimony such that in the difference measurements errors due to the ultrahigh vacuum windows were minimized. The spectra were recorded between 1.4 and 4.1 eV in steps of 0.1 eV photon energy. More experimental details are given elsewhere 9. 3. RESULTS Apart from the in situ measurements on antimony overlayers deposited onto the GaAs surfaces in ultrahigh vacuum the optical constants of mechanically polished polycrystalline antimony were determined in air (Fig. 1). They exhibit the typical behaviour of a semimetal. Starting from low photon energies Re e decreases slightly and begins to increase at 2 eV. Im e decreases rapidly between 1 and 2 eV and reaches zero for higher energies. Similar ellipsometric data obtained by Aspnes 1° and results from a Kramers-Kronig analysis of reflectivity measurements by Cardona and Greenaway 11 are shown for comparison. The dielectric functions Re e and Im e of a series of thin antimony films on GaAs(110) are shown in Fig. 2. They were calculated using a "one-layer" model 12. Submonolayer coverages exhibit maxima in Im e near 2.6, 3 and 3.7 eV. For 1.5 M L only a broad maximum between 2.5 and 3 eV can be seen. It disappears with increasing thickness until Im e reveals semimetallic character for a 20 M L deposit. Similarly, the prominent structures in Re e for 0.5 M L disappear with increasing thickness.

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In order to obtain a better insight into the growth mechanism we deposited antimony onto the hot substrate and performed annealing experiments. Figures 3(a) and 3(b) (full circles) show the dielectric functions of 1 M L of antimony deposited at a substrate temperature of 200 °C which was maintained for 15 min. The main features of these curves, especially the maxima of Im e near 2.6, 2.95 and 3.25 eV, resemble those of 1 M L of antimony deposited at room temperature (RT) 13. Further deposition of antimony up to 10ML at 200°C yields dielectric functions Re e and Im e as shown in Figs. 3(c) and 3(d) respectively (open squares). Compared with 10 M L deposited at RT (open circles), Re e exhibits lower values over the whole energy range. In Im e the broad maximum between 2 and 2.5 eV has disappeared. Instead, Im e decreases monotonically, indicating the more metallic character of the film. Similar spectra are observed if the 10 ML film (RT deposited) is annealed to 200 °C for 15 min (Figs. 3(c) and 3(d), open triangles). The absolute values of Re e are even lower than those resulting from deposition at 200 °C. In addition a minimum near 2.8 eV appears such that this spectrum is similar to Re e of a 20 ML antimony film (Fig. 2(d)). In Im ~ a broad shoulder remains but the curve is bent upwards, suggesting a more metallic character of the film. In a further annealing experiment the 10 M L antimony film deposited onto the 200 °C hot substrate is annealed to 375 °C for 15 min. Re e and Im e of the resulting film are shown in Figs. 3(a) and 3(b) respectively (open triangles). They appear to be identical with those o f a 1 M L film. Only the maximum in Im e at 2.95 eV is missing.

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Obviously most of the antimony has been evaporated but a coverage of about 1 ML remains on the surface. This is in good agreement with results obtained using Auger electron spectroscopy from previous annealing experiments 7' ~3. 4. DISCUSSION Regarding the elipsometric spectra of antimony films it is useful to divide the growth of antimony on GaAs into three steps. First, in the submonolayer and monolayer coverage regime the dielectric functions of the films are determined by sharp structures that are probably related to the electronic structure of the interface. After a correction of the spectra for the Franz-Keldysh effect 14 as previously discussed 15, the remaining maxima of Im e at 2.65, 3.2 and 3.7 eV are interpreted as interband transitions. Second, for films with thicknesses between 1.5 and 10 ML the optical properties are dominated by the film morphology. They do not exhibit the semimetallic character of antimony or the sharp features of submonolayer films. In the third stage of growth we consider "thick" films with thicknesses more than 20 ML. They exhibit the same optical behaviour as polycrystalline antimony. To discuss the optical behaviour of medium thick films we use the Maxwell Garnett model for the description of an inhomogeneous growth mode 16. In this approach a medium with dielectric constant emis assumed which contains a volume fraction f of homogeneously distributed spheres of optically absorbing material with dielectric function ei(co)(Fig. 4). This system is polarized by an external electric

ELLIPSOMETRYOF Sb GROWTHON GaAs(110)

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field E o resulting in a mean field E within the layer. Under the assumption that the fields Ej inside the spheres and E outside the spheres are homogeneous and that the spheres are small compared with the wavelength of the external field Eo an effective dielectric function e(og,f ) can be derived17:

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Even with the restrictions made in the model this formula is adequate for describing inhomogeneities of thin layers. Using the measured dielectric functions of polycrystalline antimony for el(O) and em = 0.5(l'+eGaAs) (proposed by Bedeaux and Vlieger 18) the effective dielectric functions for several volume fractions are calculated as shown in Figs. 5(c) and 5(d). With increasing f the absolute value of Re e decreases more and more while the minimum at 2.6 eV for f = 0.3 shifts to lower energies. In Ime two maxima near 2.4 and 3.1 eV appear for small volume fractions. With increasing f the first half of the curve bends up and the maxima disappear. For comparison the measured dielectric functions of a 5 M L antimony film are shown in Figs. 5(a) and 5(b). The experimental data can be fitted quite well with f = 0.3. The volume fraction f should be interpreted only in a qualitative manner because of the strong dependence of the calculation on the choice of era. Nevertheless, the inhomogeneity, i.e. the three-dimensional growth, of a 5 M L film as well as of a 10 M L film (Fig. 2(c)) is demonstrated quite clearly. The spectra of the annealed films in Figs. 3(c) and 3(d) can also be interpreted in terms of the Maxwell Garnett model. A 10 ML film after annealing to 200 °C exhibits a decrease in the values of Re e and a bending up of the first part o f I m e. Compared with the results of the Maxwell Garnett model this means a smoothing of the rough film. For deposition at 200°C the tendency is the same. The film seems to be more homogeneous because of a higher surface mobility of the impinging antimony atoms.

Fig. 4. Schematicrepresentationof the MaxwellGarnett model.Sphereswithdielectricfunctionei(~o)are embedded in a non-absorbing medium (of dielectricfunction em).Eo is the external electric field. The electricfieldsinsidethe spheresand in the non-absorbingmediumare denoted by Ei and Emrespectively. E is the electricfieldaveragedover a volumecontainingmany spheres.

292

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To summarize, ellipsometry confirms the adequacy of a monolayer + simultaneous multilayers (MSM) model to describe the growth of antimony films on GaAs(110). In this approach the formation of a closed first adlayer followed by a corrugated overlayer resulting from the simultaneous formation of several layers is assumed13. For thin films the roughness of the overlayer is seen in ellipsometry. For thicker films the first layers of the simultaneously grown multilayers are closed; thus films thicker than 20 ML seem to be optically homogeneous. In the model of simultaneous multilayers the assumption is made that impinging atoms do not migrate on the surface 19. In this context the annealing experiments confirm the assumptions of the MSM model, since a higher surface mobility arising from elevated substrate temperature yields more homogeneous films. In general, this paper demonstrates that ellipsometric spectroscopy can yield useful information about the growth mode of overlayers deposited on semiconductor surfaces.

ELLIPSOMETRY OF S b GROWTH ON G a A s ( 1 1 0 )

293

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