A channeled ion energy loss study of the surfactant-mediated growth of Ge on Si(100)

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Surface Science348 (1996) L75-L81

Surface Science Letters

A channeled ion energy loss study of the surfactant-mediated growth of Ge on Si(100) M.A. Boshart *, A.A. Bailes III, L.E. Seiberling Department of Physics. University of Florida. P.O. Box 118440, Gaines:rile. FL 32611-8440. USA

Received 21 September 1995; accepted for publication 18 October 1995

Abstract

Scattered ion energy distribution for the system Sb/Ge/Si(100) are studied using transmission ion channeling. One monolayer (ML) of Sb was deposited on the clean Si(100) surface prior to deposition of one ML of Ge at 350°C. Experimental energy distributions for the (100), {110}, and "'random" directions axe compared with simulated energy distributions obtained by overlapping trial adsorbate positions (relative to bulk positions) with ion positions in the channel at the beam-exit surface. Ion positions and energies are calculated via a Monte Carlo simulation of channeling that incorporates a model for channeled ion energy loss. We find that the energy distributions clearly show that the surfactant, Sb, moves to the surface upon Ge deposition at 350°(?. Further, ore- results are cousistent with the sites recently reported by Grant et al. [Surf. Sci. 316 (1994) L!088], for Sb deposited on Ge/Si(100), namely, tilted Sb dimers on C-e asymmetrically displaced from bulk sites. Keywords: Adatoms; Computer simulations; Epitaxy; High energy ion scanering (HEIS); Ion-solid interactions, scattering, channeling;

Semiconductingsurfaces; Single crystal epitaxy; Surface structure

The use of surfactants for generating high quality heteroepitaxial films (i.e. G e / S i heterostructures) has been of interest for several years for both the interesting physics involved as well as applications in the semiconductor industry [1-3]. The success of surfactants in promoting the layer-by-layer growth of an adsorbate is thought to be due to an inhibition of surface diffusion, which reduces adsorbate islanding. This is thought to occur because of the low surface free energy of the surfactant, causing it to float to the

• Corresponding author. Fax: + 1 904 392 0524; E-mail: [email protected].

surface, rapidly incorporating the adsorbate into the bulk [4-7]. In this Letter, we examine surfactant-mediated growth through a study of the microscopic structure of the system S b / G e / S i ( 1 0 0 ) ( S b and Ge coverages ~ 1 ML), where Sb is deposited first followed by Ge deposition at 350°C. The same system (but Ge deposited prior to Sb) was studied previously by Grant et al. [8], where adsorbate sites were identified using transmission ion chanp.e!ing [9-13] with angular scans and data were compared to a calculation based on the continuum model [14]. Here, we introduce the use of scattered ion energy distributions for site determination in a complex system involving multi-

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M.A. Boshart et aL / Sur.tk~ceScience 348 (1996) L75-L81

low background. The sample was next placed in front of a Ge effusion cell. The sample temperamn: during the Ge evaporation was held at 350°C to ensure that the G e / S b layer attained its minimum energy configuration, without promoting Ge diffusion further into the bulk, as seen at higher temperatures [24). The LEED pattern after Ge deposition was unchanged except for a slightly higher background. The Sb and Ge coverages were 0.83 and 0.82 ML respectively, determined by Rutherford backscattering (RBS) and accurate to + 10%. The sample was transferred under vacuum to a UHV ion scattering chamber (base pressure of ~ 5 x 10- ~t Torr) and placed on a two axis goniometer. The scattering chamber is connected via differential pumping to a 3.5 MV Van de Graaff accelerator. A beam of 2.5 MeV He + ions was produced and collimated to an angular divergence of 0.03 ° , resulting in a beant spot on the sample of about 1 X 1 mm 2. Crystal alignment was performed with the G e / S b on the beam-exit side of the sample, as shown in Fig. I. In the transmission geometry, the low energy side of the Si peak corresponds to scattering from the beam-entrance side of the Si window. Thus, the Si surface peak in Fig. 1 occurs on the low energy side of the Si peak. Further, Sb can deposit

~ r b a t e s and sites. This new technique utilizes the fact that an adatom occupying a specific site 'within the channel interacts only with ions having particular trajectories, giving rise to particular energy losses of those ions, as they traverse the substrate crystal. Experimental energy distributions are thus compared to a Monte Carlo simulation of channeling [15,16] that includes a model for channeled ion energy loss [17-19]. Our data are consistent with the sites found by Grant et ai. A "'naive" model of symmetric Sb dimers and substitutional Ge does not fit our data well. A detailed description of the experimental set-up is found elsewhere [20]. Thin single-crystal silicon windows ( ~ 0.18 /xm in thickness) within a thicker Si frame were produced using a dopant-selective etch [21]. A clean Si(100)-2 × 1 surface was obtained in ultrahigh vacuum (UHV) by desorbing a "Shiraki oxide" [22]. After desorbing the oxide, Sb was deposited on the sample from a boron nitride effusion cell [i3]. The sample was held at ~ 500°C and exposed to several monolayers of Sb to citsure saturation of the surface, ~ 0.85 ML [23] (1 ML = 6.78 × 1014 atoms cm-Z). The low energy electron diffraction (LEED) pattern following Sb deposition was (1 × 1) with weak, diffuse half-order spots and a 300 • --

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M.A. Boshart et al./ Surface Science 348 ~1996) L75-L81

on both sides of the thin Si window during evaporation, resulting in the two Sb peaks in Fig. 1. The submonolayer coverage of SI~ on the beam-entrance side of the crystal (again, the low energy peak) does not affect the channeling of the beam, however. Channeling directions involved in the experiment were found by viewing the transmitted beam on a piece of quartz at the end of the beamline. By monitoring the silicon yield and comparing with simulation, the accuracy of this procedure is estimated to be 0.02-0.05 ° . Channeling directions of interest can thus be found in a couple of minutes. Good approximations to "'random" directions (necessary for adsorbate coverages, Si thickness, and overall energy resolution) were identified by simulation [16,18], and the methodology of Ref. [25] was used to locate these directions in the lab, making use of the reference directions found visually. Experimental energy distributions were obtained at three different crystallographic directions, (O,~b)= (0,0), (O,~b) = (6,45), and (O,~b) = (6,30), where 0 = tilt from the (100) and c,b=azimuth relative to the {100}. These directions are, respectively, the (100) axis, the {110) plane, and a direction found to be a good approximation to "random" incidence (directions where energy losses are close to those in amorphous media [26]). Scattered ions were detected and energy-analyzed by three ion-implanted, passivated solid state detectors located at 70, 79, and 150°. Energy distributions after scattering (the kinematic factor was not divided out) for the G e / S b signal were thus obtained. The energy distributions were each collected for 24 p,C of the integrated beam current (high symmetry directions taken first to minimize irradiation damage) and all three were taken on the same beam spot to minimize adsorbate coverage and Si thickness variation. Finally, the energy distributions were normalized to unit area. The Monte Carlo simulation program and how it can be used to create adsorbate site energy distributions (as well as advantages of using energy distributions) is described elsewhere [20]. Briefly, a large number of ion trajectories are followed by the Monte Carlo simulation, typically 20000 for a given set of crystal coordinates corresponding to the experimental conditions. In order to obtain the exit energies of the ion,::, a model for :hanneled ion energy loss is

required. For this, we made the usual assumption ~ 1 that core and valence electrons contribute independently to the stopping. For the stopping due to core electrons, we employed the semi-classical approximation, and for stopping due to valence electrans, we used the two-component free electron gas model [17-19]. The position in the channel and e n e r g y of each ion after it has traversed the crystal are then outputted. An adatom site can now be investigated by choosing a location in the channel. A 2D vibrational amplitude, p. must also be assigned to the adatom ( p = ¢'2u, where u is the ID vibrational amplitude). For a given set of crystal coordinates, the distance from the trial site to all of the outputted ions is calculated. If an ion is within two 2D vibrational amplitudes of the trial site, the gaussian centered at the trial site with a standard deviation equal to p / ¢ ' 2 is evaluated for the ion distance and added to ',he scattering yield. This value is further scaled to take into account the difference between and energy dependence of the H e - G e / S b scattering cross sections. Simulated energy distributions are generated by binning the scattering yield value calculated above based on the energy of the ion after scattering (the energy of the ion is multiplied b~ the kinematic factor of G e / S b for a given scattering angle). This is repeated for all sites (possibly involving several different adsorbates) under investigation. The resulting simulated energy distributions are smoothed to include the experimental energy resolution and straggling. This is done for all of the crystal coordinates investigated by experiment, resulting in a set of simulated energy distributions (each normalized to an area of unity) for a particular set of adsorbate sites and 2D vibrational amplitudes. However, based on previous work where adsorbate vibrations were found to be only slightly enhanced from bulk vibrations in the surface plane [20], p was set to 0.15 P, for both adsorbate species and not varied. Fig. 2 gives a good idea of how the procedure werks. Shown are four (100) channels (the large black dots indicate the Si atom rows that define the channels and the different sizes give an indication of how close the top Si atom in that row is to the surface). The small dots within the channels are ion positions after penetrating the Si crystal aligned with the (100) axis. Fig. 2a shows ions that have retained

M.A. Boshart et at. / Surface Science 348 (1996) L75-L81

(by > 30%) than that expected if the crystal had been aligned at "random" incidence with-the same crystal thickness ( ~ 1800 ,~). These ions are clearly very well channeled. Fig. 2b shows all other ions, and some channeling is still evident, but these ions are able to penetrate close to the atom rows (which is responsible for their higher rate of energy loss). Thus, an adatom site encounters a very different range of energy and number of ions de-

pending on where it is iocated within the channel (the greatest sensitivity is achieved near the middle of the channel, where fluxes are changing rapidly with channel position). The experimental energy distribution as well as overall yield (with respect to random incidence) typically give a resolution of ~ 0.1 ,~, for the adatom site [20]. This resolution is achieved by comparing each simulated energy distribution with the corresponding

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Fig. 2. View of the Si(100) surface from above. Large black dots are Si atom rows and small dots are ion positions when exiting the Si crystal. (a) Shows well-channe~':d low energy loss ions (see tcx0, while (b) shows ions with higher energy loss. Open and cross-hatched c:.rcles are Ge and Sb position,o a-cording to Ref. [8]. Proposed sites are evaluated by overlapping the site with the ion flux and comparing with experimental data. Higher ei ,-rgy loss ions penetrating close to atom rows are sensitive to near-bulk sims.

M.A. Boshart et al./ Surface Science 348 (1996) L75-L81

experimental distribution and a X 2 value of the fit is obtained. By varying the adatom positions, a best fit and most likely position for the adsorbates can be found. However, distributions near the (100) axis taken in this work are only sensitive to displacements from bulk positions in the surface plane as the (100) axis is along the sample normal. Thus, the goal of this work is to generate energy distributions of G e / S b sites found in the literature and to compare them with the experimental data. For example, Fig. 2 also shows the positions of Ge and Sb found by Grant et al. [8]. The Ge atoms are represented by the hollow circles occupying near-substitutional sites (half are substitutional). The overlying Sb dimer is represented by the crosshatched circles. It is immediately noticed that low energy loss, well-channeled ions are not sensitive to near-substitutional positions (Fig. 2a). However, a substantial increase in overlapping ions is evident for these sites in Fig. 2b, with the higher energy loss ions penetrating close to the atom rows. This result~ in a large yield difference ( ~ 25%) as well as a difference in energy of overlapping ions (resulting in a peak shift in the energy distribution that depends on the sample thickness) between completely substitutional sites and those displaced by as little as 0. ! ,~ from substitutional. This sensitivity to near-bulk positions using a Monte Carlo simulation with channeled ion energy loss is enhanced over previous transmission cha, meling studies that employ the continuum model. The continuum model excludes ions that penetrate close to atom rows and displacements of a minimum of 0.1 ,~ (the size of bulk vibrations) from bulk sites are necessary for sensitivity to those displacements [ 14]. Fig. 3 summarizes the findings in the present work. The experimental data (dots) are compared with two models for the G e / S b system for the three energy distributions taken. It should be noted that the data are inconsistent with the Ge occupying any positions other than near-substitutional. This provides immediate evidence that the Sb has indeed floated to the surface, allowing the Ge to occupy near-bulk Si lattice positions underneath (i.e. allowing layer-by-layer growth). Thus, both models under consideration assume that the Sb has returned to the surface and that the Ge occupies near-substitutional positions underneath. The first model, a "'naive"

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ENERGY(keV) Fig. 3. Monte Carlo simulated energy disttibution~(l~ne~ for possibleSb and Ge sites for the system Sb/Ge/Si(100)(see text) comparedwith experimentaldata (dots) for di~ctions close to the 000). Note that all spectraarc normalizedto have the same area under the cur~e.

approach to the system, assumes the Ge has taken exactly substitutional positions and the Sb has dimerized symmetrically as if it were on the Si(100) surface (dashed line) [12]. The second model is taken from Grant et al. [8] and places the Ge in asymmetric near-substitutional positions, while finding that the Sb dimer is also asymmetric with a tilt of about 7 °. This site is illustrated in Fig. 2 (viewed along the sample normal). It can be seen from Fig. 3 that ~ e " n a i v e " model is not well reproduced by the experimental data. The Ge peak in this model is suppressed and shifted to t o o low an energy when aligned with the (100) axis. However, the energy distribution shape for the Grant

M.A. Boshart et al. / SurJhce Science 348 (1996) L75-t.81

oduces the experimental data quite well when aligned with the (100) axis as well as aligned with the {110} plane. Although a complete site determination of this system using energy distributions would require more data (i.e. off normal axes would have to be investigated to get adatom displacements along the sample normal), much can be inferred from the energy distributions in Fig. 3 alone. Namely, at least some of the Ge adatoms are located in sites slightly displaced from bulk ( ~ 0.4 ,~) with the Sb dimer residing above. This is perhaps not too surprising as subsurface reconstntctions (often difficult to measure due to interference from the bulk si[gnal) have been calculated to be from ~ 0.1 to 0.4 A for Si subsurface layers [27-29], and Ge introduces some additional strain with its lattice mismatch. Further, the fact that the present data are consistent with the findings of Grant et ai. is interesting in that in the present experiment, the Sb floated to the surface, as opposed to being deposited on top of the Ge. We thus find that the order of deposition under the present conditions does not significantly affect the preferred final sites of the adsorbates. A study of Ge deposited at r o o m t e m p e r a t u r e on one ML of Sb on Si(100), for which the lowest energy state of the system might not be attained, should lead to insight on the mechanism by which Ge and Sb exchange positions [30-33]. This work will be presented in a separate publication. In conclusion, the surfactant-meditated growth of the system S b / G e / S i ( 1 0 0 ) has been studied using transmission ion channeling. The microscopic structure of this system was investigated using energy distributions obtained for the Sb and Ge (Sb deposited first) along major channeling directions in silicon and ¢,',.mparing with Monte Carlo simulated energy distributions for a few trial models. From these comparisons, it is evident that the Ge atoms occupy near-substitutional ( ~ 0,4 A, from substitutional) sites, giving clear indication that the surfactant again resides on the surface. Further, the location of all adatoms is in agreement with Grant et al., indicating that depositing Sb before or after Ge does not greatly affect the final geometry. W e gratefully acknowledge the technical assistance of R. Johns and D. McNeill. This work was

supported by the N S F (DMR) and the University of Florida Division of Sponsored Research.

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