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Adsorption of disilane on Si(111)-(7×7) and initial stages of CVD growth
Surface Science 416 (1998) 226–239
Adsorption of disilane on Si(111)-(7×7) and initial stages of CVD growth J. Braun, H. Rauscher *, R.J. Behm Abteilung Oberfla¨chenchemie und Katalyse, Universita¨t Ulm, D-89069 Ulm, Germany Received 11 March 1998; accepted for publication 9 July 1998
Abstract Adsorption and reaction of disilane (Si H ) on Si(111)-(7×7) surfaces have been studied in the temperature range 300–760 K by 2 6 scanning tunneling microscopy. In the entire temperature range the interaction of Si H with Si(111)-(7×7) is dissociative. At room 2 6 temperature it leads to continuous reactions of adatom dangling bonds with hydrogen atoms, which are released in the decomposition process. Protruding surface species observed after prolonged Si H exposition are assigned to deposited SiH fragments. After 2 6 x exposures to 120 L at temperatures between 400 and 690 K a variety of surface structures, such as Si H clusters, hydrogenx y terminated adatoms and disordered regions, are observed. At 730–760 K growth leads to the formation of triangular islands without stacking faults and by incorporation of Si at steps with the grown areas exhibiting a defective adatom structure. Re-annealing to 800 K restores the DAS structure and allows the amount of silicon deposited to be determined. The initial sticking coefficient for dissociative Si H adsorption at 300 K is determined as S =8.5×10−5. Silicon deposition proceeds via an activated adsorption 2 6 0 process. The value of the activation energy is determined as #0.1 eV between 300 K and 650 K, while it is 1.1 eV between 650 K and 760 K, pointing toward an additional reaction channel at temperatures above 650 K. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Chemical vapor deposition; Chemisorption; Disilane; Scanning tunneling microscopy; Silicon; Surface reactions
1. Introduction Disilane (Si H ) has received much attention 2 6 during recent years as a precursor gas for the growth of thin epitaxial silicon layers on silicon surfaces by chemical vapor deposition (CVD) because its handling is safer than that of the commonly used SiH precursor and because the 4 additional Si–Si bond may be broken more easily during adsorption. The latter would lead to a higher dissociation probability of the molecules on * Corresponding author. Fax: +49 731 5025452; e-mail: [email protected]
the surface and hence to a more facile growth of epitaxial Si layers [1]. Most of the studies on the interaction of Si H with Si(111) surfaces concen2 6 trate on the temperature region above 700 K, where hydrogen which is released in the course of disilane decomposition on the surface, desorbs readily and Si layers grow continuously. In this growth regime, detailed information on the structures formed during layer formation and the associated kinetics is available [2–7]. Much less is known on the dissociative adsorption of disilane on Si(111)-(7×7) at lower temperatures (≤700 K ). Multiple internal reflection infrared (MIRIS ) [8,9] and electron energy loss spectroscopy ( EELS ) [10] measurements showed
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that disilane molecules are no longer stable on the surface at 300 K, and there is general agreement that below 760 K dissociative disilane adsorption proceeds preferentially via Si–Si bond scission [9– 12]. From EELS and ultraviolet photoelectron spectroscopy ( UPS ) measurements after disilane adsorption at 200 K [8] it has been concluded that SiH is the first dissociation product on the surface 3 and that this species is preferentially adsorbed on rest atom dangling bonds [8]. In those experiments it was found that the adsorbed SiH species are 3 highly strained owing to the influence of the surrounding adatoms [8]. From the preferential quenching of the electronic S state (a rest atom 2 dangling bond state) observed with UPS [12,13] it was also concluded that the Si H molecules react 2 6 preferentially at the rest atom sites and that the reaction involves Si–Si bond cleavage in the molecule. The primary dissociation fragment SiH is 3 not stable on Si(111)-(7×7) at 300 K. It decomposes quickly into adsorbed SiH and H, as 2 concluded from EELS measurements [10] and in agreement with MIRIS [8,9] and UPS [12,13] data. It has been proposed earlier that in the temperature region around 700–750 K there should be a fundamental change in the dissociative Si H 2 6 adsorption mechanism [2,3]. Such a change in the dominant adsorption mechanism could dramatically influence the reactive sticking coefficient. From measurements of the deposited H coverage via temperature programmed desorption (TPD) it was concluded that the dissociative adsorption of Si H in the low temperature range is associated 2 6 with a negative effective activation energy [11]. On the other hand, a positive activation energy of 0.8 eV has recently been determined for dissociative adsorption of disilane between 750 and 810 K from measurements of the deposited amount of Si [7]. In this paper we report results of an ongoing scanning tunneling microscopy (STM ) study on the interaction of Si H with Si(111)-(7×7) which 2 6 aims at a detailed microscopic understanding of the Si H adsorption process in the temperature 2 6 regime between 300 and 760 K. We first discuss in detail the adsorption behavior at room temperature and specific differences to SiH adsorption. 4
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Recent STM results on the adsorption behavior of Si H on Si(111)-(7×7) [14] at room temper2 6 ature have indicated a reactive sticking coefficient which is much lower than that assumed previously. The reactive coefficient is defined as the probability for a particle to adsorb and undergo decomposition. Furthermore, clear differences in the resulting surface structures were observed between the first stages of SiH [15] and Si H [14] adsorption at 4 2 6 300 K. These points will be addressed here in detail. In the second part of the paper we concentrate on the adsorption behavior at elevated temperatures and report measurements of the reactive sticking coefficient by direct determination of the amount of deposited silicon. These experiments indicate an increasing reaction probability with increasing temperatures. In particular they show a drastic change in the reaction probability above 650 K, which is correlated with earlier proposals [2,3] for a change in the adsorption mechanism in this temperature region.
2. Experimental The experiments were performed in two different stainless steel ultrahigh vacuum ( UHV ) chambers with base pressures of 1×10−10 mbar, equipped with a home-built pocket size and a home-built beetle-type scanning tunneling microscope, respectively. Imaging was performed in the constant current mode with typical tunnel currents of 30 pA or 0.1 nA (tunnel voltages are given in the text). Heating of the samples was achieved by electron impact from the back and controlled by the filament emission current, which was calibrated by a thermocouple directly attached to the sample. The samples are cut from p-doped Si(111) wafers with a resistivity of 1–20 V cm ( Wacker Chemitronic) and cleaned using a standard cleaning and preparation procedure. This includes two supersonic cleaning cycles in an acetone and methanol bath, respectively, after which the sample is introduced into the UHV system. There it is degassed for 12–16 h at a temperature below 1000 K. Finally, the native oxide is removed by several flash heating steps to 1400 K. In the final heating step, this temperature is maintained for 2 min, after which
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it is slowly (#1 K s−1) cooled to room temperature. The defect density of the resulting (7×7) reconstruction was between 1 and 2% of an adatom layer. Disilane (purchased from Linde, purity 99.994%) was introduced into the vacuum via a precision leak valve and frequently checked with a quadrupole mass spectrometer for purity. During Si H exposure the ion pumps in the STM 2 6 chambers were switched off to avoid dissociation of Si H . The pressure was measured by a distant 2 6 ion gauge, not within line of sight of the sample. During exposure the sample was taken out of the STM to avoid shadowing or other tip effects on the adsorption process. Exposures given in the text are corrected for the specific sensitivity of the ion gauge towards Si H (factor 2.4) [10]. 2 6 3. Results and discussion 3.1. Room temperature exposure Information on topographic changes of the Si(111)-(7×7) surface induced by interaction with Si H can be gained from the STM images in 2 6 Fig. 1, which were recorded after increasing exposures of the surface to disilane at 300 K, to 120, 240, 480 and 4800 L. Exposures of less than 500 L (1 L=1.33×10−6 mbar s) lead only to a change in the appearance of a fraction of initially bright adatoms in the STM images, which apparently become darker. The difference in brightness between the ‘‘darkened’’ and ‘‘bright’’ sites decreases if the tunneling voltage is increased to values between 2 and 3 V. This change in adatom appearance, which is well known from other STM studies on the interaction of silicon CVD precursors such as SiH [15–17] with Si(111)-(7×7) and 4 also from hydrogen adsorption on that surface [18,19], is characteristic for adatoms whose dangling bonds have reacted with hydrogen atoms whereupon the dangling bond states are quenched. The resulting lower local density of electronic states around the Fermi level above these reacted, hydrogen-terminated adatoms forces the tip closer to the surface in order to maintain a constant current which causes their darker (i.e. topographically deeper) appearance in the STM images. All
adatoms are still at their original positions and reacted sites appear just less prominent than unreacted ones, which can be seen most clearly in Fig. 1d. Hence we assign these darker sites to adatoms which have reacted with hydrogen atoms released as dissociation fragments from decomposing disilane molecules during the adsorption process. After exposure to 120 L ( Fig. 1a) 2.8% of the adatom sites are hydrogen saturated. With increasing Si H exposure this reaction proceeds to 5.7% 2 6 after 240 L and then to 13.9% after 480 L. There is no preferential reaction of center or corner adatoms as compared with the respective other adatom type: the ratio between reacted corner and center adatoms is nearly 1:1. This finding holds true also for higher exposures: in experiments with disilane exposures up to 4800 L we consistently found that center and corner adatoms are involved in an equal ratio in the adsorption process of the precursor. In addition to the reaction of the adatoms with hydrogen we observed protruding features on top of the adatom layer. We expect that unsaturated SiH species (SiH, SiH ) or SiH clusters located x 2 x on top of the adatom layer would show up as protrusions in the STM images under our usual tunneling conditions (U =+1.7 V ) since they sample would contain one or more Si atom(s) in an unsaturated configuration with at least one dangling bond remaining. In fact, we observed such protrusions only very rarely for exposures of less than 500 L. A few examples are found in the middle right region of Fig. 1b. Hence we conclude that most of the silicon-containing fragments deposited by Si H dissociation are bound to the 2 6 rest atom layer in a geometry so far unknown, and they remain invisible under our tunneling conditions, i.e. for sample voltages of up to 3 V. The nature of the SiH species deposited by x Si H adsorption at room temperature can be 2 6 clarified by drawing on earlier work involving UPS [12,13], MIRIS [9] and TPD [11] measurements. From those results it has been concluded that adsorption of the molecule takes place under scission of the Si–Si bond and the two evolving SiH 3 fragments react mainly with rest atom dangling bonds of the surface. These SiH groups dissociate 3
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(a)
(b)
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˚ ×300 A ˚ ), (b) 240 L (450 A ˚ ×450 A ˚ ), (c) 480 L (390 A ˚ ×390 A ˚ ) and Fig. 1. STM images recorded after exposures to (a) 120 L (300 A ˚ ×360 A ˚ ) Si H of a Si(111)-(7×7) surface at 300 K. In (d ) some of the protrusions which are discussed in the (d) 4800 L (360 A 2 6 text are marked by arrows.
quickly at room temperature [3,12,13], which leads to the coexistence of adsorbed hydrogen (forming Si–H groups) and adsorbed SiH species 2 on the surface [10] at 300 K. It was also proposed
that above 700 K additional adsorption mechanisms become effective, one of which includes the ejection of SiH into the gas phase while the 4 remaining SiH fragment is accommodated on the 2
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surface [3]. An alternative reaction mechanism, which includes dissociative Si H adsorption by 2 6 Si–H bond activation and formation of an intermediate silasilyl (Si H ) species, was proposed by Xia 2 5 et al. [20], based upon supersonic molecular beam scattering measurements between 600 and 950°C. This mechanism would also lead to adsorbed H, SiH and SiH (the latter dissociate further into 2 3 SiH , SiH and H ) and/or to gaseous SiH in 2 4 subsequent dissociation steps [20]. From the available literature it can therefore be concluded that at room temperature the only stable adsorbates evolving form Si H decomposition on Si(111)2 6 (7×7) are SiH and H, regardless of the initial 2 dissociative adsorption step. The latter species leads to a SiH group, by reaction with a surface dangling bond. Our STM measurements show that adsorbed hydrogen atoms released by Si H decomposition 2 6 preferentially adsorb on adatoms, similar to previous findings for SiH decomposition [15,16 ]. This 4 means also that the remaining SiH fragments x (from the literature results summarized above it can be concluded that x=2) are adsorbed on the rest atom layer, otherwise they should show up as protrusions in the STM images. For low exposures this is only observed in a few exceptions (Fig. 1a–c). For higher exposures and Si H fragment cover2 6 ages the situation changes drastically. Homogeneously distributed protrusions – assigned to SiH fragments or small Si H clusters – are found x y x more frequently on the surface, as shown in Fig. 1d after 4800 L (some are marked by arrows). At this stage of the reaction 31.1% of the adatoms are saturated by hydrogen. To verify that particularly at low exposures (Fig. 1a–c) most of the deposited Si is present as SiH species not resolved in STM images and 2 hence to confirm the adsorption mechanism discussed above we need to know a correlation between the number of reacted adatoms and the amount of deposited Si after disilane exposure at 300 K. For this purpose the deposited Si has to be made visible in the STM topographs, which was achieved by a subsequent annealing step to about 690 K. This annealing procedure is sufficient to decompose all SiH species except for H-termix
nated Si adatoms. In addition, Si/SiH species become mobile and condense into islands. More specifically, heating to 690 K removes hydrogen from the b state, which was earlier attributed to 2 surface dihydride decomposition [21]. The resulting surface can be seen in the STM topographs of Fig. 2a–d, which were recorded on the same samples as Fig. 1a–d, but after an additional annealing step to 690 K for 2 min subsequent to room temperature Si H exposure. After 2 6 the annealing step we find considerably more bright protrusions than after adsorption at 300 K. These protrusions are homogeneously distributed over the surface and their number increases with disilane exposure. Their appearance is similar to that of the protrusions which were already observed at room temperature after prolonged disilane exposure (Fig. 1d). We assign them to (probably hydrated ) silicon clusters on top of the (7×7) reconstruction formed in the course of surface dihydride decomposition. Occasionally, small patches of (앀3×앀3)R30° reconstructions can be found after the annealing step (see inset to Fig. 2d). A similar rearrangement of the adatoms during the hydride decomposition and H desorp2 tion process has been observed in situ at temperatures around 760–805 K by Morita et al. [22] during hydrogen desorption from Si(111). However, the metastable (앀3×앀3)R30° reconstruction disappeared completely in those experiments after cooling to 300 K [22]. Although it was not possible to resolve SiH x species adsorbed on the rest atom dangling bonds using STM, the appearance of the Si clusters after annealing the surface to 690 K provides direct evidence for silicon deposition by disilane exposure at 300 K. It also proves that after room temperature adsorption the deposited SiH fragments x must be higher hydrides (at least SiH ), since –SiH 2 species, including both adsorbed SiH groups and adsorbed H would not decompose upon annealing to 690 K. Our results are in very good agreement with the UPS results of Avouris et al. [12] who equally found silicon dihydrides (and no higher hydrides) upon disilane exposure at room temperature. They also agree perfectly with the adsorption model
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(b)
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Fig. 2. STM images recorded after exposure of (a) 120 L, (b) 240 L, (c) 480 L and (d) 4800 L Si H to a Si(111)-(7×7) surface at 6 ˚ ×1000 A ˚ . The inset of2 Fig. 300 K and subsequent annealing to 690 K for 2 min. Image sizes are 1000 A 2(d) shows a small island of a (앀3×앀3)R30° reconstruction which is occasionally observed after the annealing step.
proposed in the literature [9–12] according to which at temperatures below 760 K disilane chemisorption proceeds via Si–Si bond scission. MIRIS measurements by Uram et al. [8] showed that
SiH groups, which are probably adsorbed on the 3 rest atom dangling bonds as an initial dissociation fragment, are highly strained at 200 K and that these silyl species decompose below room temper-
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ature into SiH and H. The latter of these two 2 species bonds to an unsaturated Si atom which leads to the formation of surface SiH groups. Our findings in combination with previous data lead to the following scenario for dissociative disilane adsorption at room temperature. After dissociation of the disilane molecule into two SiH groups these species decompose further into 3 an SiH group and a hydrogen atom. The SiH 2 2 group is accommodated by reaction with the rest atom layer while the H-atom reacts with an Si adatom and saturates its dangling bond. This means that four dangling bonds (two rest atom and two adatom dangling bonds) are saturated for each adsorbed disilane molecule. The two types of adatoms (corner or center adatoms) are hydrogen terminated with equal probability upon SiH disso3 ciation, as discussed above. There is no indication for a preferential activity of certain adatoms or adatom–rest atom/corner hole combinations as observed recently for the dissociative adsorption of SiH [15] or SiH Cl [23] on Si(111)-(7×7). 4 2 2 Our results rather indicate that after Si–Si bond scission of the disilane molecules the two SiH 3 groups dissociate independently from each other and that the energetics of the transition state involved in the formation of SiH and SiH in this 2 four-center reaction is not (or only weakly) dependent on the type of the adatom in which the splitoff hydrogen atom is finally adsorbed. Based on this adsorption model the number of hydrogen-saturated adatoms is a precise measure for the uptake of Si H , which allows the direct 2 6 determination of the Si H reactive sticking 2 6 coefficient under these conditions, at 300 K (see Fig. 1a–d). By counting the reacted adatoms and assuming a constant reaction probability in the low coverage region the initial reactive sticking coefficient for disilane at room temperature on Si(111)-(7×7) is calculated to S =8.5×10−5. 0 From the Si coverages derived by the above method the Si clusters produced by the 690 K annealing step should contain about five Si atoms on average, provided that all Si contained in the SiH groups is incorporated in these clusters after 2 annealing. Judging from the size of the clusters this appears reasonable, supporting our coverage estimate. If not all of the Si atoms are incorporated
in these clusters, which may well be the case because of the moderate annealing temperature, then this value represents an upper limit for the average number of Si atoms per cluster. The value of the reactive sticking coefficient at 300 K of S =8.5×10−5 obtained from our meas0 urements differs markedly from the values of S =0.3±0.1 [10] and S =0.47±0.1 [11] reported 0 0 earlier. It contrasts with the general assumption that at room temperature the reactive sticking coefficient of disilane is about three orders of magnitude larger than that for monosilane (SiH ) 4 adsorption [17], where the latter is in the region of 10−5 for interaction with Si(111)-(7×7) [24]. The exact reason for this difference from the earlier results [10,11] is so far unknown, but it should be kept in mind that disilane is extremely sensitive to electron-induced fragmentation caused, e.g., by hot filaments or active ion pumps in the UHV system [25,26 ]. Such effects may easily simulate an apparently high initial reactive sticking coefficient for Si H adsorption [26 ]. From our topo2 6 graphic measurements a high reactive sticking coefficient at 300 K can definitely be excluded. Hence, at room temperature the reactive sticking coefficient for Si H is of the same order of magni2 6 tude as that for SiH [24]. We should like to note 4 here that these results on the magnitude of the reactive sticking coefficient at room temperature agree excellently with recent findings [25]. 3.2. Si H exposure between 400 and 760 K 2 6 In the second part we investigated disilane adsorption at elevated temperatures. From a mechanistic point of view the temperature regime between 400 and 760 K is particularly interesting since there are controversies in the literature on the disilane adsorption kinetics in this region. Although it is generally agreed that disilane chemisorbs dissociatively by Si–Si bond scission, data published by Gates [11] point towards a negative value for the activation energy and have been interpreted as being indicative of precursormediated adsorption in the temperature region 400–735 K. On the other hand, Andersohn et al. [7] concluded from their data that disilane adsorption is activated in the temperature region
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750–810 K and calculated an activation energy of #0.8 eV. Different experimental methods were used in those two studies: Gates [11] determined the reactive sticking coefficient from the hydrogen coverage of the substrate after disilane exposure, while Andersohn et al. [7] measured the amount of deposited Si by STM. Since these temperaturedependent measurements cover different, nonoverlapping temperature ranges we performed similar deposition/annealing experiments as described above or in Ref. [7] in order to determine the Si coverage by STM after deposition at different temperatures, up to 760 K. The Si surfaces obtained immediately after constant Si H exposure of 120 L at increasing temper2 6 atures (400, 560, 730 and 760 K ) are shown in the STM images of Fig. 3a–d. These surface topographies show very different structures which depend on temperature, hydrogen coverage and the amount of deposited silicon. This and the high degree of disorder on the surface make a correct and exact determination of the grown Si extremely difficult. For example, after exposure at 400 K the DAS structure is disturbed by hydrogen-terminated and by missing adatoms and probably also by Si H clusters ( Fig. 3a). After 560 K exposure x y the surface contains a variety of structures, including Si H clusters, hydrogen-terminated adatoms, x y remnants of DAS structures and disordered areas at the steps ( Fig. 3b). At higher adsorption temperatures, e.g. at 730 K or 760 K, triangular islands are formed, which are covered by a layer of disordered adatoms (Fig. 3c and 3d). Furthermore, the defect concentration within these islands is not known. To eliminate the resulting uncertainties in the determination of deposited Si the samples were annealed to about 800 K for 2 min after exposure to desorb the hydrogen and to aggregate all deposited Si into clusters or islands (see Section 3.1). The Si surfaces resulting after this annealing step are shown in the STM images of Fig. 3e–h. Since the islands observed after the 800 K annealing step all show well-resolved DAS structures with (7×7) and (5×5) reconstruction (see high resolution STM image in Fig. 4), we do not need to make any model assumption to determine the number of deposited silicon atoms. By measuring the area
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of the nucleated silicon islands or, for small islands, counting the adatoms or DAS unit cells, we can directly determine the amount of silicon deposited during the exposure at each temperature. Only surface areas in the middle of large terraces, such as those in Fig. 3e–g, far away from the steps, were used for the evaluation of the deposited amount of Si. This eliminates the influence of Si incorporation at the steps on these results. Additionally, to check for the influence of mobile SiH species on the Si coverage determinax tion, we analyzed the defect concentration on the surface after the 800 K annealing step and compared it with that of the initial surface. During the re-annealing Si may be removed from the substrate and transported to the islands via mobile Si H x y species, which could add material to the islands. To determine the net amount of deposited Si, the concentration of defects persisting on the surface after annealing must therefore by subtracted from the new islands. This additional defect density after the annealing step on surfaces such as those presented in Fig. 3e–h was always below 2% of the adatoms (cf. Fig. 4). This means that material etched from the initial surface as SiH and incorpox rated into the new islands makes up less than 0.24% of a bilayer (1 bilayer=1.56×1015 atoms cm−2). For the images in Fig. 3e–h this amounts to 12%, 6.9%, 0.6% and 0.3% of the grown islands. The remainder must therefore be Si deposited by Si H decomposition. 2 6 The results of the quantitative evaluation of the reactive sticking coefficient (now for increasing coverages) are shown in Fig. 5, together with results of previous measurements by other groups [7,11]. In the temperature region between 400 and 650 K we find the reactive sticking coefficient to increase with higher temperature. These data can be evaluated in terms of an Arrhenius behavior, provided that within the present coverage regime the sticking coefficient does not depend on the coverage. This was indeed demonstrated by Gates [11] for coverages below #2.4×1014 H atoms cm−2 (4×1013 adsorbed disilane molecules per cm2), i.e. for the coverages obtained in our measurements up to 650 K. Here, the amount of adsorbed hydrogen was determined from the deposited Si which is between 0.02 and 0.05
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(a)
(b)
(e)
(f)
Fig. 3. Upper row: STM images recorded after exposure of a Si(111)-(7×7) surface to 120 L Si H at (a) 400 K, (b) 560 K, (c) 2 6 730 K and (d) 760 K. Lower row: STM images recorded after exposure of a Si(111)-(7×7) surface to 120 L Si H at (e) 400 K, (f ) 2 6 560 K, (g) 730 K and (h) 760 K and an additional annealing step to 800 K for 2 min. Grown silicon in bilayers (BL, 1 BL=1.56×1015 ˚ ˚ ˚ ×2150 A ˚ (c) atoms cm−2): (e) 0.02 BL, (f ) 0.035 BL, (g) 0.38 BL, (h) 0.74 BL. Image sizes are (650 A×650 A) (a, b and d), 2150 A ˚ ×6500 A ˚ (e–h). and 6500 A
bilayers, corresponding to a maximum coverage of (0.9–2.3)×1014 H atoms per cm2 in our experiments. From the slope in the Arrhenius plot it can
be seen that the adsorption is activated and an activation energy of about 0.1 eV can be determined. This finding is in contrast to the negative
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(c)
(d)
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Fig. 3. (continued)
activation energy deduced from TPD measurements [11], but it does not exclude a weakly adsorbed precursor if the energy barrier from the precursor state to the chemisorbed state is lower than that for desorption from the precursor. Our results are on the other hand in qualitative
agreement with the general trend of increasing layer growth rate found at higher temperatures [7,27], which was determined from the deposited amount of silicon [7,27]. The quantitative differences to the reactive sticking coefficient deduced from the data of Andersohn
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˚ ×400 A ˚ ) recorded after Fig. 4. Detail of a STM image (400 A exposure of a Si(111)-(7×7) surface to 120 L Si H at 560 K 2 6 and subsequent annealing to 800 K for 2 min. After the annealing step the two grown islands in the image exhibit (7×7) and (5×5) DAS reconstructions, respectively, so that the amount of grown Si can be precisely determined.
et al. [7] as compared with ours may be due to differences in the calibration and measurement of the growth temperature between Ref. [7] and ourselves. The difference from the results in Ref. [11], where the reactive sticking coefficient and the activation energy were determined after a fixed disilane exposure of 4.2×1013 cm−2, indicates, however, that the hydrogen coverage alone, without further information on the deposited Si amount, is not in general a good quantity to determine the amount of adsorbed Si, as was already pointed out in Ref. [7]. The amount of deposited silicon can be determined directly from the STM images even at temperatures where for our terrace widths the growth mode changes from island growth to step flow (Fig. 6). For example, at T=760 K and a pressure of p(Si H )=4.2×10−7 mbar the depos2 6 ited silicon and SiH fragments are sufficiently x mobile to diffuse across the terraces, at least over ˚ and to reach nearby steps where they #1500 A are incorporated. Nevertheless, the newly grown areas at the steps can be clearly distinguished from the original substrate, since the latter exhibits an almost perfect (7×7) structure, while the former
Fig. 5. Arrhenius plot of the reactive sticking coefficient S of Si H on Si(111)-(7×7) directly determined from the amount of 2 6 deposited Si and averaged over a variable H coverage (see text). Data from Refs. [7,11] are also included.
J. Braun et al. / Surface Science 416 (1998) 226–239
˚ ×6500 A ˚ ) of a Si(111)-(7×7) surFig. 6. STM image (6500 A face after exposure to 120 L Si H at 760 K. The contrast 2 6 betwen the original substrate and the newly grown material grown at the steps is obvious and allows to determnine the amount of deposited Si.
does not show a perfect DAS structure. More specifically, these areas consist of a relatively disordered adatom phase with the same apparent height as the original substrate terraces. The difference in crystallinity leads to a contrast between the substrate and the new areas in the STM image and allows one to precisely distinguish between them. In this way, the amount of Si deposited is determined as 0.74 bilayers from Fig. 6. The uncertainty in the amount of deposited Si in these measurements is determined by the difference in the adatom density between the well-ordered (7×7) and this phase, which is not known exactly. In addition to growth at steps, triangular islands are observed on ˚ ). These larger terraces (terrace width>1500 A islands are similar to those observed earlier after interaction of the Si(111)-(7×7) surface with SiH in the same temperature regime [28]. The 4 islands are one bilayer high and exhibit defective adatom structures (see Fig. 3d). They were created by homogeneous nucleation, and most of them are oriented in the same way, i.e. their vertices point towards [112: ] and equivalent directions, similar to
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findings reported in Ref. [28]. Hence, these islands have grown on areas without stacking fault. A closer look at the newly grown structures in Fig. 6 reveals some more interesting features, concerning the growth of second Si bilayer islands. Owing to the miscut of the wafer, step edges with different orientations are formed on the initial surface; both steps with their outward normals pointing towards 112: directions (denoted
112: steps) and other, higher indexed steps, rotated by an angle of about 150° to the 112: steps, are observed [28]. Some 112: steps are marked in Fig. 6. Interestingly, the border between the original true 112: step and the newly grown first layer is nearly always free of second layer material, while second layer growth is observed at the border between steps running along other directions and newly grown material. This confirms previous results from the same Si H /Si(111)-(7×7) system reported in Ref. [29], 2 6 where it was found by in situ STM observations between 720 and 760 K that nucleation takes place at the borders between the (7×7) reconstructed and the unreconstructed regions and is more favorable at 1: 1: 2-type step edges than at 112: steps. The dramatic increase in the reactive sticking coefficient between 650 and 760 K by about two orders of magnitude supports the conclusion in Refs. [2,3] that the mechanism for disilane adsorption changes in this temperature region. Activated H desorption alone, which also sets in in this 2 temperature region, could not account for this strong increase, as shown in the following. A fixed disilane exposure of 120 L at T≥650 K would lead to a H coverage clearly beyond the region for ad constant Si H sticking [11], if no H desorption is 2 6 taking place, and should thus result in a lower sticking coefficient for disilane. On the other hand, H desorption alone could lift the sticking coeffi2 cient at most to the value of the initial sticking coefficient at that temperature. The strong increase of the sticking coefficient observed experimentally must therefore be interpreted as the opening of an additional adsorption mechanism. Evidence for additional Si H adsorption chan2 6 nels on Si(111)-(7×7) at elevated temperatures was also gained in earlier studies. In Ref. [2] it
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was proposed that at surface temperatures between 770 and 1170 K the main channel for Si growth from Si H CVD involves decomposition of disi2 6 lane with SiH and SiH as products, where the 4 2 former desorbs instantaneously and the latter decomposes further. Two reaction mechanisms for this channel were proposed in Ref. [3]: Si H (g)+2dbSiH (g)+SiH(ad)+H(ad) 2 6 4
(1)
and Si H (g)+2SiH(ad)2SiH (g)+2Si(ad). 2 6 4
(2)
Another mechanism for the reaction of disilane, involving Si–H bond activation and silasilyl formation, was proposed in Ref. [20]: Si H (g)+2dbSi H (ad)+H(ad). 2 6 2 5
(3)
The second of these reactions effectively removes hydrogen from the surface. The overall sticking coefficient depends on the contribution from each of these reactions. The former two additional mechanisms were made responsible for the steep increase in the reactive sticking coefficient at T>650 K [2,3]. Activation energies of 5±1 kcal mol−1 (21±4.2 kJ mol−1) and 25± 6 kcal mol−1 (105±25.2 kJ mol−1; 1.08±0.26 eV per particle), respectively, were reported in Ref. [2] for reactions (1) and (2). From the slope of our Arrhenius plot in Fig. 5 an activation energy of 1.1 eV can be deduced in the high temperature part (above 650 K ), close to the value of 0.8 eV obtained above 770 K in Ref. [7]. This activation energy is also in perfect agreement with the activation energy of 105±25.2 kJ mol−1 reported in Ref. [2] for reaction (2). These findings also support the previous proposals of at least one additional dissociative adsorption channel for Si H 2 6 adsorption on Si(111)-(7×7) at higher temperatures. This additional mechanism for molecular decomposition, which gains importance at elevated temperatures is the reason for the higher Si growth rates using the precursor Si H as compared with 2 6 SiH . This means that the pre-exponential factor 4 for this additional adsorption channel must be considerably greater than that for the reaction which is effective at lower temperatures.
4. Conclusions In summary, we have shown that the room temperature adsorption of disilane on Si(111)(7×7) proceeds with an initial reactive sticking coefficient of S =8.5×10−5, a value which is by 0 several orders of magnitude lower than that assumed earlier. The initial reaction involves scission of the Si H Si–Si bond, followed by further 2 6 decomposition of the evolving SiH species into 3 SiH , which adsorbs at rest atoms, and H which 2 preferentially adsorbs at adatoms. In this fourcenter reaction, corner and center adatoms become hydrogen terminated with equal probability. For exposures at temperatures up to 690 K a variety of surface structures are formed, including Si H x y clusters, and hydrogen-terminated adatoms. At 730–760 K growth proceeds by homogeneous nucleation and growth of triangular Si bilayer islands without stacking faults and also by condensation of material at steps. Heterogeneous nucleation of second layer islands is unfavorable at the borderline between (7×7) and first layer areas grown at 112: steps as compared with other step directions and also to homogeneous nucleation. By directly determining the amount of deposited Si it is found that between 400 K and 650 K the dissociative adsorption of Si H on Si(111)-(7×7) 2 6 is activated with an apparent activation energy of 0.1 eV. Above 650 K the reactive sticking coefficient increases drastically, indicating that at least one additional adsorption mechanism with an effective activation energy of #1.1 eV becomes effective.
Acknowledgements This work was supported by the Stiftung Volkswagenwerk under Grant I/72162. The Si wafers were kindly provided by Wacker Chemitronic.
References [1] J.M. Jasinski, S.M. Gates, Acc. Chem. Res. 24 (1991) 9. [2] S.K. Kulkarni, S.M. Gates, B.A. Scott, H.H. Sawin, Surf. Sci. 239 (1990) 13.
J. Braun et al. / Surface Science 416 (1998) 226–239 [3] S.K. Kulkarni, S.M. Gates, C.M. Greenlief, H.H. Sawin, Surf. Sci. 239 (1990) 26. [4] S.K. Kulkarni, S.M. Gates, C.M. Greenlief, H.H. Sawin, J. Vac. Sci. Technol. A 8 (1990) 2956. [5] S.M. Mokler, W.K. Liu, N. Ohtani, B.A. Joyce, Appl. Surf. Sci. 60/61 (1992) 92. [6 ] U. Ko¨hler, L. Andersohn, B. Dahleimer, Appl. Phys. A 57 (1993) 491. [7] L. Andersohn, Th. Berke, U. Ko¨hler, B. Voigtla¨nder, J. Vac. Sci. Technol. A 14 (1996) 312. [8] K.J. Uram, U. Jansson, J. Vac. Sci. Technol. B 7 (1989) 1176. [9] K.J. Uram, U. Jansson, Surf. Sci. 249 (1991) 105. [10] R. Imbihl, J.E. Demuth, S.M. Gates, B.A. Scott, Phys. Rev. B 39 (1989) 5222. [11] S.M. Gates, Surf. Sci. 195 (1988) 307. [12] Ph. Avouris, F. Bozso, J. Phys. Chem. 94 (1990) 2243. [13] J.J. Koulman, F. Ringeisen, M. Alaoui, D. Bolmont, Phys. Rev. B 41 (1990) 3878. [14] J. Braun, H. Rauscher, R.J. Behm, Surf. Sci. 409 (1998) L715. [15] U. Memmert, A. Berko´, R.J. Behm, Surf. Sci. 325 (1995) L441. [16 ] L. Bolotov, H. Rauscher, R.J. Behm, Chem. Phys. Lett. 243 (1995) 445.
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[17] D. Albertini, F. Thibaudau, L. Masson, F. Salvan, Surf. Sci. 350 (1996) L216. [18] J.J. Boland, Surf. Sci. 244 (1991) 1. [19] T. Sakurai, Y. Hasegawa, T. Hashizume, I. Kamiya, T. Die, I. Sumita, H.W. Pickering, S. Hyodo, J. Vac. Technol. A 8 (1990) 259. [20] L.-Q. Xia, M.E. Jones, N. Maity, J.R. Engstrom, J. Chem. Phys. 103 (1995) 1691. [21] G. Schulze, M. Henzler, Surf. Sci. 124 (1983) 337. [22] Y. Morita, K. Miki, H. Tokumoto, Surf. Sci. 298 (1993) L163. [23] M. Fehrenbacher, H. Rauscher, R.J. Behm, Z. Phys. Chem. 198 (1997) 205. [24] M. Fehrenbacher, U. Memmert, H. Rauscher, R.J. Behm, J. Vac. Sci. Technol. A 14 (1996) 1499. [25] U. Ko¨hler, personal communication. [26 ] R. Imbihl, personal communication. [27] R.J. Buss, P. Ho, W.G. Breiland, M.E. Coltrin, J. Appl. Phys. 63 (1988) 2808. [28] M. Fehrenbacher, H. Rauscher, U. Memmert, R.J. Behm, Surf. Sci. 385 (1997) 123. [29] K. Fujita, Y. Kusumi, M. Ichikawa, Surf. Sci. 380 (1997) 66.
Adsorption of disilane on Si(111)-(7×7) and initial stages of CVD growth J. Braun, H. Rauscher *, R.J. Behm Abteilung Oberfla¨chenchemie und Katalyse, Universita¨t Ulm, D-89069 Ulm, Germany Received 11 March 1998; accepted for publication 9 July 1998
Abstract Adsorption and reaction of disilane (Si H ) on Si(111)-(7×7) surfaces have been studied in the temperature range 300–760 K by 2 6 scanning tunneling microscopy. In the entire temperature range the interaction of Si H with Si(111)-(7×7) is dissociative. At room 2 6 temperature it leads to continuous reactions of adatom dangling bonds with hydrogen atoms, which are released in the decomposition process. Protruding surface species observed after prolonged Si H exposition are assigned to deposited SiH fragments. After 2 6 x exposures to 120 L at temperatures between 400 and 690 K a variety of surface structures, such as Si H clusters, hydrogenx y terminated adatoms and disordered regions, are observed. At 730–760 K growth leads to the formation of triangular islands without stacking faults and by incorporation of Si at steps with the grown areas exhibiting a defective adatom structure. Re-annealing to 800 K restores the DAS structure and allows the amount of silicon deposited to be determined. The initial sticking coefficient for dissociative Si H adsorption at 300 K is determined as S =8.5×10−5. Silicon deposition proceeds via an activated adsorption 2 6 0 process. The value of the activation energy is determined as #0.1 eV between 300 K and 650 K, while it is 1.1 eV between 650 K and 760 K, pointing toward an additional reaction channel at temperatures above 650 K. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Chemical vapor deposition; Chemisorption; Disilane; Scanning tunneling microscopy; Silicon; Surface reactions
1. Introduction Disilane (Si H ) has received much attention 2 6 during recent years as a precursor gas for the growth of thin epitaxial silicon layers on silicon surfaces by chemical vapor deposition (CVD) because its handling is safer than that of the commonly used SiH precursor and because the 4 additional Si–Si bond may be broken more easily during adsorption. The latter would lead to a higher dissociation probability of the molecules on * Corresponding author. Fax: +49 731 5025452; e-mail: [email protected]
the surface and hence to a more facile growth of epitaxial Si layers [1]. Most of the studies on the interaction of Si H with Si(111) surfaces concen2 6 trate on the temperature region above 700 K, where hydrogen which is released in the course of disilane decomposition on the surface, desorbs readily and Si layers grow continuously. In this growth regime, detailed information on the structures formed during layer formation and the associated kinetics is available [2–7]. Much less is known on the dissociative adsorption of disilane on Si(111)-(7×7) at lower temperatures (≤700 K ). Multiple internal reflection infrared (MIRIS ) [8,9] and electron energy loss spectroscopy ( EELS ) [10] measurements showed
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that disilane molecules are no longer stable on the surface at 300 K, and there is general agreement that below 760 K dissociative disilane adsorption proceeds preferentially via Si–Si bond scission [9– 12]. From EELS and ultraviolet photoelectron spectroscopy ( UPS ) measurements after disilane adsorption at 200 K [8] it has been concluded that SiH is the first dissociation product on the surface 3 and that this species is preferentially adsorbed on rest atom dangling bonds [8]. In those experiments it was found that the adsorbed SiH species are 3 highly strained owing to the influence of the surrounding adatoms [8]. From the preferential quenching of the electronic S state (a rest atom 2 dangling bond state) observed with UPS [12,13] it was also concluded that the Si H molecules react 2 6 preferentially at the rest atom sites and that the reaction involves Si–Si bond cleavage in the molecule. The primary dissociation fragment SiH is 3 not stable on Si(111)-(7×7) at 300 K. It decomposes quickly into adsorbed SiH and H, as 2 concluded from EELS measurements [10] and in agreement with MIRIS [8,9] and UPS [12,13] data. It has been proposed earlier that in the temperature region around 700–750 K there should be a fundamental change in the dissociative Si H 2 6 adsorption mechanism [2,3]. Such a change in the dominant adsorption mechanism could dramatically influence the reactive sticking coefficient. From measurements of the deposited H coverage via temperature programmed desorption (TPD) it was concluded that the dissociative adsorption of Si H in the low temperature range is associated 2 6 with a negative effective activation energy [11]. On the other hand, a positive activation energy of 0.8 eV has recently been determined for dissociative adsorption of disilane between 750 and 810 K from measurements of the deposited amount of Si [7]. In this paper we report results of an ongoing scanning tunneling microscopy (STM ) study on the interaction of Si H with Si(111)-(7×7) which 2 6 aims at a detailed microscopic understanding of the Si H adsorption process in the temperature 2 6 regime between 300 and 760 K. We first discuss in detail the adsorption behavior at room temperature and specific differences to SiH adsorption. 4
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Recent STM results on the adsorption behavior of Si H on Si(111)-(7×7) [14] at room temper2 6 ature have indicated a reactive sticking coefficient which is much lower than that assumed previously. The reactive coefficient is defined as the probability for a particle to adsorb and undergo decomposition. Furthermore, clear differences in the resulting surface structures were observed between the first stages of SiH [15] and Si H [14] adsorption at 4 2 6 300 K. These points will be addressed here in detail. In the second part of the paper we concentrate on the adsorption behavior at elevated temperatures and report measurements of the reactive sticking coefficient by direct determination of the amount of deposited silicon. These experiments indicate an increasing reaction probability with increasing temperatures. In particular they show a drastic change in the reaction probability above 650 K, which is correlated with earlier proposals [2,3] for a change in the adsorption mechanism in this temperature region.
2. Experimental The experiments were performed in two different stainless steel ultrahigh vacuum ( UHV ) chambers with base pressures of 1×10−10 mbar, equipped with a home-built pocket size and a home-built beetle-type scanning tunneling microscope, respectively. Imaging was performed in the constant current mode with typical tunnel currents of 30 pA or 0.1 nA (tunnel voltages are given in the text). Heating of the samples was achieved by electron impact from the back and controlled by the filament emission current, which was calibrated by a thermocouple directly attached to the sample. The samples are cut from p-doped Si(111) wafers with a resistivity of 1–20 V cm ( Wacker Chemitronic) and cleaned using a standard cleaning and preparation procedure. This includes two supersonic cleaning cycles in an acetone and methanol bath, respectively, after which the sample is introduced into the UHV system. There it is degassed for 12–16 h at a temperature below 1000 K. Finally, the native oxide is removed by several flash heating steps to 1400 K. In the final heating step, this temperature is maintained for 2 min, after which
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it is slowly (#1 K s−1) cooled to room temperature. The defect density of the resulting (7×7) reconstruction was between 1 and 2% of an adatom layer. Disilane (purchased from Linde, purity 99.994%) was introduced into the vacuum via a precision leak valve and frequently checked with a quadrupole mass spectrometer for purity. During Si H exposure the ion pumps in the STM 2 6 chambers were switched off to avoid dissociation of Si H . The pressure was measured by a distant 2 6 ion gauge, not within line of sight of the sample. During exposure the sample was taken out of the STM to avoid shadowing or other tip effects on the adsorption process. Exposures given in the text are corrected for the specific sensitivity of the ion gauge towards Si H (factor 2.4) [10]. 2 6 3. Results and discussion 3.1. Room temperature exposure Information on topographic changes of the Si(111)-(7×7) surface induced by interaction with Si H can be gained from the STM images in 2 6 Fig. 1, which were recorded after increasing exposures of the surface to disilane at 300 K, to 120, 240, 480 and 4800 L. Exposures of less than 500 L (1 L=1.33×10−6 mbar s) lead only to a change in the appearance of a fraction of initially bright adatoms in the STM images, which apparently become darker. The difference in brightness between the ‘‘darkened’’ and ‘‘bright’’ sites decreases if the tunneling voltage is increased to values between 2 and 3 V. This change in adatom appearance, which is well known from other STM studies on the interaction of silicon CVD precursors such as SiH [15–17] with Si(111)-(7×7) and 4 also from hydrogen adsorption on that surface [18,19], is characteristic for adatoms whose dangling bonds have reacted with hydrogen atoms whereupon the dangling bond states are quenched. The resulting lower local density of electronic states around the Fermi level above these reacted, hydrogen-terminated adatoms forces the tip closer to the surface in order to maintain a constant current which causes their darker (i.e. topographically deeper) appearance in the STM images. All
adatoms are still at their original positions and reacted sites appear just less prominent than unreacted ones, which can be seen most clearly in Fig. 1d. Hence we assign these darker sites to adatoms which have reacted with hydrogen atoms released as dissociation fragments from decomposing disilane molecules during the adsorption process. After exposure to 120 L ( Fig. 1a) 2.8% of the adatom sites are hydrogen saturated. With increasing Si H exposure this reaction proceeds to 5.7% 2 6 after 240 L and then to 13.9% after 480 L. There is no preferential reaction of center or corner adatoms as compared with the respective other adatom type: the ratio between reacted corner and center adatoms is nearly 1:1. This finding holds true also for higher exposures: in experiments with disilane exposures up to 4800 L we consistently found that center and corner adatoms are involved in an equal ratio in the adsorption process of the precursor. In addition to the reaction of the adatoms with hydrogen we observed protruding features on top of the adatom layer. We expect that unsaturated SiH species (SiH, SiH ) or SiH clusters located x 2 x on top of the adatom layer would show up as protrusions in the STM images under our usual tunneling conditions (U =+1.7 V ) since they sample would contain one or more Si atom(s) in an unsaturated configuration with at least one dangling bond remaining. In fact, we observed such protrusions only very rarely for exposures of less than 500 L. A few examples are found in the middle right region of Fig. 1b. Hence we conclude that most of the silicon-containing fragments deposited by Si H dissociation are bound to the 2 6 rest atom layer in a geometry so far unknown, and they remain invisible under our tunneling conditions, i.e. for sample voltages of up to 3 V. The nature of the SiH species deposited by x Si H adsorption at room temperature can be 2 6 clarified by drawing on earlier work involving UPS [12,13], MIRIS [9] and TPD [11] measurements. From those results it has been concluded that adsorption of the molecule takes place under scission of the Si–Si bond and the two evolving SiH 3 fragments react mainly with rest atom dangling bonds of the surface. These SiH groups dissociate 3
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(b)
(c)
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˚ ×300 A ˚ ), (b) 240 L (450 A ˚ ×450 A ˚ ), (c) 480 L (390 A ˚ ×390 A ˚ ) and Fig. 1. STM images recorded after exposures to (a) 120 L (300 A ˚ ×360 A ˚ ) Si H of a Si(111)-(7×7) surface at 300 K. In (d ) some of the protrusions which are discussed in the (d) 4800 L (360 A 2 6 text are marked by arrows.
quickly at room temperature [3,12,13], which leads to the coexistence of adsorbed hydrogen (forming Si–H groups) and adsorbed SiH species 2 on the surface [10] at 300 K. It was also proposed
that above 700 K additional adsorption mechanisms become effective, one of which includes the ejection of SiH into the gas phase while the 4 remaining SiH fragment is accommodated on the 2
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surface [3]. An alternative reaction mechanism, which includes dissociative Si H adsorption by 2 6 Si–H bond activation and formation of an intermediate silasilyl (Si H ) species, was proposed by Xia 2 5 et al. [20], based upon supersonic molecular beam scattering measurements between 600 and 950°C. This mechanism would also lead to adsorbed H, SiH and SiH (the latter dissociate further into 2 3 SiH , SiH and H ) and/or to gaseous SiH in 2 4 subsequent dissociation steps [20]. From the available literature it can therefore be concluded that at room temperature the only stable adsorbates evolving form Si H decomposition on Si(111)2 6 (7×7) are SiH and H, regardless of the initial 2 dissociative adsorption step. The latter species leads to a SiH group, by reaction with a surface dangling bond. Our STM measurements show that adsorbed hydrogen atoms released by Si H decomposition 2 6 preferentially adsorb on adatoms, similar to previous findings for SiH decomposition [15,16 ]. This 4 means also that the remaining SiH fragments x (from the literature results summarized above it can be concluded that x=2) are adsorbed on the rest atom layer, otherwise they should show up as protrusions in the STM images. For low exposures this is only observed in a few exceptions (Fig. 1a–c). For higher exposures and Si H fragment cover2 6 ages the situation changes drastically. Homogeneously distributed protrusions – assigned to SiH fragments or small Si H clusters – are found x y x more frequently on the surface, as shown in Fig. 1d after 4800 L (some are marked by arrows). At this stage of the reaction 31.1% of the adatoms are saturated by hydrogen. To verify that particularly at low exposures (Fig. 1a–c) most of the deposited Si is present as SiH species not resolved in STM images and 2 hence to confirm the adsorption mechanism discussed above we need to know a correlation between the number of reacted adatoms and the amount of deposited Si after disilane exposure at 300 K. For this purpose the deposited Si has to be made visible in the STM topographs, which was achieved by a subsequent annealing step to about 690 K. This annealing procedure is sufficient to decompose all SiH species except for H-termix
nated Si adatoms. In addition, Si/SiH species become mobile and condense into islands. More specifically, heating to 690 K removes hydrogen from the b state, which was earlier attributed to 2 surface dihydride decomposition [21]. The resulting surface can be seen in the STM topographs of Fig. 2a–d, which were recorded on the same samples as Fig. 1a–d, but after an additional annealing step to 690 K for 2 min subsequent to room temperature Si H exposure. After 2 6 the annealing step we find considerably more bright protrusions than after adsorption at 300 K. These protrusions are homogeneously distributed over the surface and their number increases with disilane exposure. Their appearance is similar to that of the protrusions which were already observed at room temperature after prolonged disilane exposure (Fig. 1d). We assign them to (probably hydrated ) silicon clusters on top of the (7×7) reconstruction formed in the course of surface dihydride decomposition. Occasionally, small patches of (앀3×앀3)R30° reconstructions can be found after the annealing step (see inset to Fig. 2d). A similar rearrangement of the adatoms during the hydride decomposition and H desorp2 tion process has been observed in situ at temperatures around 760–805 K by Morita et al. [22] during hydrogen desorption from Si(111). However, the metastable (앀3×앀3)R30° reconstruction disappeared completely in those experiments after cooling to 300 K [22]. Although it was not possible to resolve SiH x species adsorbed on the rest atom dangling bonds using STM, the appearance of the Si clusters after annealing the surface to 690 K provides direct evidence for silicon deposition by disilane exposure at 300 K. It also proves that after room temperature adsorption the deposited SiH fragments x must be higher hydrides (at least SiH ), since –SiH 2 species, including both adsorbed SiH groups and adsorbed H would not decompose upon annealing to 690 K. Our results are in very good agreement with the UPS results of Avouris et al. [12] who equally found silicon dihydrides (and no higher hydrides) upon disilane exposure at room temperature. They also agree perfectly with the adsorption model
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Fig. 2. STM images recorded after exposure of (a) 120 L, (b) 240 L, (c) 480 L and (d) 4800 L Si H to a Si(111)-(7×7) surface at 6 ˚ ×1000 A ˚ . The inset of2 Fig. 300 K and subsequent annealing to 690 K for 2 min. Image sizes are 1000 A 2(d) shows a small island of a (앀3×앀3)R30° reconstruction which is occasionally observed after the annealing step.
proposed in the literature [9–12] according to which at temperatures below 760 K disilane chemisorption proceeds via Si–Si bond scission. MIRIS measurements by Uram et al. [8] showed that
SiH groups, which are probably adsorbed on the 3 rest atom dangling bonds as an initial dissociation fragment, are highly strained at 200 K and that these silyl species decompose below room temper-
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ature into SiH and H. The latter of these two 2 species bonds to an unsaturated Si atom which leads to the formation of surface SiH groups. Our findings in combination with previous data lead to the following scenario for dissociative disilane adsorption at room temperature. After dissociation of the disilane molecule into two SiH groups these species decompose further into 3 an SiH group and a hydrogen atom. The SiH 2 2 group is accommodated by reaction with the rest atom layer while the H-atom reacts with an Si adatom and saturates its dangling bond. This means that four dangling bonds (two rest atom and two adatom dangling bonds) are saturated for each adsorbed disilane molecule. The two types of adatoms (corner or center adatoms) are hydrogen terminated with equal probability upon SiH disso3 ciation, as discussed above. There is no indication for a preferential activity of certain adatoms or adatom–rest atom/corner hole combinations as observed recently for the dissociative adsorption of SiH [15] or SiH Cl [23] on Si(111)-(7×7). 4 2 2 Our results rather indicate that after Si–Si bond scission of the disilane molecules the two SiH 3 groups dissociate independently from each other and that the energetics of the transition state involved in the formation of SiH and SiH in this 2 four-center reaction is not (or only weakly) dependent on the type of the adatom in which the splitoff hydrogen atom is finally adsorbed. Based on this adsorption model the number of hydrogen-saturated adatoms is a precise measure for the uptake of Si H , which allows the direct 2 6 determination of the Si H reactive sticking 2 6 coefficient under these conditions, at 300 K (see Fig. 1a–d). By counting the reacted adatoms and assuming a constant reaction probability in the low coverage region the initial reactive sticking coefficient for disilane at room temperature on Si(111)-(7×7) is calculated to S =8.5×10−5. 0 From the Si coverages derived by the above method the Si clusters produced by the 690 K annealing step should contain about five Si atoms on average, provided that all Si contained in the SiH groups is incorporated in these clusters after 2 annealing. Judging from the size of the clusters this appears reasonable, supporting our coverage estimate. If not all of the Si atoms are incorporated
in these clusters, which may well be the case because of the moderate annealing temperature, then this value represents an upper limit for the average number of Si atoms per cluster. The value of the reactive sticking coefficient at 300 K of S =8.5×10−5 obtained from our meas0 urements differs markedly from the values of S =0.3±0.1 [10] and S =0.47±0.1 [11] reported 0 0 earlier. It contrasts with the general assumption that at room temperature the reactive sticking coefficient of disilane is about three orders of magnitude larger than that for monosilane (SiH ) 4 adsorption [17], where the latter is in the region of 10−5 for interaction with Si(111)-(7×7) [24]. The exact reason for this difference from the earlier results [10,11] is so far unknown, but it should be kept in mind that disilane is extremely sensitive to electron-induced fragmentation caused, e.g., by hot filaments or active ion pumps in the UHV system [25,26 ]. Such effects may easily simulate an apparently high initial reactive sticking coefficient for Si H adsorption [26 ]. From our topo2 6 graphic measurements a high reactive sticking coefficient at 300 K can definitely be excluded. Hence, at room temperature the reactive sticking coefficient for Si H is of the same order of magni2 6 tude as that for SiH [24]. We should like to note 4 here that these results on the magnitude of the reactive sticking coefficient at room temperature agree excellently with recent findings [25]. 3.2. Si H exposure between 400 and 760 K 2 6 In the second part we investigated disilane adsorption at elevated temperatures. From a mechanistic point of view the temperature regime between 400 and 760 K is particularly interesting since there are controversies in the literature on the disilane adsorption kinetics in this region. Although it is generally agreed that disilane chemisorbs dissociatively by Si–Si bond scission, data published by Gates [11] point towards a negative value for the activation energy and have been interpreted as being indicative of precursormediated adsorption in the temperature region 400–735 K. On the other hand, Andersohn et al. [7] concluded from their data that disilane adsorption is activated in the temperature region
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750–810 K and calculated an activation energy of #0.8 eV. Different experimental methods were used in those two studies: Gates [11] determined the reactive sticking coefficient from the hydrogen coverage of the substrate after disilane exposure, while Andersohn et al. [7] measured the amount of deposited Si by STM. Since these temperaturedependent measurements cover different, nonoverlapping temperature ranges we performed similar deposition/annealing experiments as described above or in Ref. [7] in order to determine the Si coverage by STM after deposition at different temperatures, up to 760 K. The Si surfaces obtained immediately after constant Si H exposure of 120 L at increasing temper2 6 atures (400, 560, 730 and 760 K ) are shown in the STM images of Fig. 3a–d. These surface topographies show very different structures which depend on temperature, hydrogen coverage and the amount of deposited silicon. This and the high degree of disorder on the surface make a correct and exact determination of the grown Si extremely difficult. For example, after exposure at 400 K the DAS structure is disturbed by hydrogen-terminated and by missing adatoms and probably also by Si H clusters ( Fig. 3a). After 560 K exposure x y the surface contains a variety of structures, including Si H clusters, hydrogen-terminated adatoms, x y remnants of DAS structures and disordered areas at the steps ( Fig. 3b). At higher adsorption temperatures, e.g. at 730 K or 760 K, triangular islands are formed, which are covered by a layer of disordered adatoms (Fig. 3c and 3d). Furthermore, the defect concentration within these islands is not known. To eliminate the resulting uncertainties in the determination of deposited Si the samples were annealed to about 800 K for 2 min after exposure to desorb the hydrogen and to aggregate all deposited Si into clusters or islands (see Section 3.1). The Si surfaces resulting after this annealing step are shown in the STM images of Fig. 3e–h. Since the islands observed after the 800 K annealing step all show well-resolved DAS structures with (7×7) and (5×5) reconstruction (see high resolution STM image in Fig. 4), we do not need to make any model assumption to determine the number of deposited silicon atoms. By measuring the area
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of the nucleated silicon islands or, for small islands, counting the adatoms or DAS unit cells, we can directly determine the amount of silicon deposited during the exposure at each temperature. Only surface areas in the middle of large terraces, such as those in Fig. 3e–g, far away from the steps, were used for the evaluation of the deposited amount of Si. This eliminates the influence of Si incorporation at the steps on these results. Additionally, to check for the influence of mobile SiH species on the Si coverage determinax tion, we analyzed the defect concentration on the surface after the 800 K annealing step and compared it with that of the initial surface. During the re-annealing Si may be removed from the substrate and transported to the islands via mobile Si H x y species, which could add material to the islands. To determine the net amount of deposited Si, the concentration of defects persisting on the surface after annealing must therefore by subtracted from the new islands. This additional defect density after the annealing step on surfaces such as those presented in Fig. 3e–h was always below 2% of the adatoms (cf. Fig. 4). This means that material etched from the initial surface as SiH and incorpox rated into the new islands makes up less than 0.24% of a bilayer (1 bilayer=1.56×1015 atoms cm−2). For the images in Fig. 3e–h this amounts to 12%, 6.9%, 0.6% and 0.3% of the grown islands. The remainder must therefore be Si deposited by Si H decomposition. 2 6 The results of the quantitative evaluation of the reactive sticking coefficient (now for increasing coverages) are shown in Fig. 5, together with results of previous measurements by other groups [7,11]. In the temperature region between 400 and 650 K we find the reactive sticking coefficient to increase with higher temperature. These data can be evaluated in terms of an Arrhenius behavior, provided that within the present coverage regime the sticking coefficient does not depend on the coverage. This was indeed demonstrated by Gates [11] for coverages below #2.4×1014 H atoms cm−2 (4×1013 adsorbed disilane molecules per cm2), i.e. for the coverages obtained in our measurements up to 650 K. Here, the amount of adsorbed hydrogen was determined from the deposited Si which is between 0.02 and 0.05
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(e)
(f)
Fig. 3. Upper row: STM images recorded after exposure of a Si(111)-(7×7) surface to 120 L Si H at (a) 400 K, (b) 560 K, (c) 2 6 730 K and (d) 760 K. Lower row: STM images recorded after exposure of a Si(111)-(7×7) surface to 120 L Si H at (e) 400 K, (f ) 2 6 560 K, (g) 730 K and (h) 760 K and an additional annealing step to 800 K for 2 min. Grown silicon in bilayers (BL, 1 BL=1.56×1015 ˚ ˚ ˚ ×2150 A ˚ (c) atoms cm−2): (e) 0.02 BL, (f ) 0.035 BL, (g) 0.38 BL, (h) 0.74 BL. Image sizes are (650 A×650 A) (a, b and d), 2150 A ˚ ×6500 A ˚ (e–h). and 6500 A
bilayers, corresponding to a maximum coverage of (0.9–2.3)×1014 H atoms per cm2 in our experiments. From the slope in the Arrhenius plot it can
be seen that the adsorption is activated and an activation energy of about 0.1 eV can be determined. This finding is in contrast to the negative
J. Braun et al. / Surface Science 416 (1998) 226–239
(c)
(d)
(g)
(h)
235
Fig. 3. (continued)
activation energy deduced from TPD measurements [11], but it does not exclude a weakly adsorbed precursor if the energy barrier from the precursor state to the chemisorbed state is lower than that for desorption from the precursor. Our results are on the other hand in qualitative
agreement with the general trend of increasing layer growth rate found at higher temperatures [7,27], which was determined from the deposited amount of silicon [7,27]. The quantitative differences to the reactive sticking coefficient deduced from the data of Andersohn
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J. Braun et al. / Surface Science 416 (1998) 226–239
˚ ×400 A ˚ ) recorded after Fig. 4. Detail of a STM image (400 A exposure of a Si(111)-(7×7) surface to 120 L Si H at 560 K 2 6 and subsequent annealing to 800 K for 2 min. After the annealing step the two grown islands in the image exhibit (7×7) and (5×5) DAS reconstructions, respectively, so that the amount of grown Si can be precisely determined.
et al. [7] as compared with ours may be due to differences in the calibration and measurement of the growth temperature between Ref. [7] and ourselves. The difference from the results in Ref. [11], where the reactive sticking coefficient and the activation energy were determined after a fixed disilane exposure of 4.2×1013 cm−2, indicates, however, that the hydrogen coverage alone, without further information on the deposited Si amount, is not in general a good quantity to determine the amount of adsorbed Si, as was already pointed out in Ref. [7]. The amount of deposited silicon can be determined directly from the STM images even at temperatures where for our terrace widths the growth mode changes from island growth to step flow (Fig. 6). For example, at T=760 K and a pressure of p(Si H )=4.2×10−7 mbar the depos2 6 ited silicon and SiH fragments are sufficiently x mobile to diffuse across the terraces, at least over ˚ and to reach nearby steps where they #1500 A are incorporated. Nevertheless, the newly grown areas at the steps can be clearly distinguished from the original substrate, since the latter exhibits an almost perfect (7×7) structure, while the former
Fig. 5. Arrhenius plot of the reactive sticking coefficient S of Si H on Si(111)-(7×7) directly determined from the amount of 2 6 deposited Si and averaged over a variable H coverage (see text). Data from Refs. [7,11] are also included.
J. Braun et al. / Surface Science 416 (1998) 226–239
˚ ×6500 A ˚ ) of a Si(111)-(7×7) surFig. 6. STM image (6500 A face after exposure to 120 L Si H at 760 K. The contrast 2 6 betwen the original substrate and the newly grown material grown at the steps is obvious and allows to determnine the amount of deposited Si.
does not show a perfect DAS structure. More specifically, these areas consist of a relatively disordered adatom phase with the same apparent height as the original substrate terraces. The difference in crystallinity leads to a contrast between the substrate and the new areas in the STM image and allows one to precisely distinguish between them. In this way, the amount of Si deposited is determined as 0.74 bilayers from Fig. 6. The uncertainty in the amount of deposited Si in these measurements is determined by the difference in the adatom density between the well-ordered (7×7) and this phase, which is not known exactly. In addition to growth at steps, triangular islands are observed on ˚ ). These larger terraces (terrace width>1500 A islands are similar to those observed earlier after interaction of the Si(111)-(7×7) surface with SiH in the same temperature regime [28]. The 4 islands are one bilayer high and exhibit defective adatom structures (see Fig. 3d). They were created by homogeneous nucleation, and most of them are oriented in the same way, i.e. their vertices point towards [112: ] and equivalent directions, similar to
237
findings reported in Ref. [28]. Hence, these islands have grown on areas without stacking fault. A closer look at the newly grown structures in Fig. 6 reveals some more interesting features, concerning the growth of second Si bilayer islands. Owing to the miscut of the wafer, step edges with different orientations are formed on the initial surface; both steps with their outward normals pointing towards 112: directions (denoted
112: steps) and other, higher indexed steps, rotated by an angle of about 150° to the 112: steps, are observed [28]. Some 112: steps are marked in Fig. 6. Interestingly, the border between the original true 112: step and the newly grown first layer is nearly always free of second layer material, while second layer growth is observed at the border between steps running along other directions and newly grown material. This confirms previous results from the same Si H /Si(111)-(7×7) system reported in Ref. [29], 2 6 where it was found by in situ STM observations between 720 and 760 K that nucleation takes place at the borders between the (7×7) reconstructed and the unreconstructed regions and is more favorable at 1: 1: 2-type step edges than at 112: steps. The dramatic increase in the reactive sticking coefficient between 650 and 760 K by about two orders of magnitude supports the conclusion in Refs. [2,3] that the mechanism for disilane adsorption changes in this temperature region. Activated H desorption alone, which also sets in in this 2 temperature region, could not account for this strong increase, as shown in the following. A fixed disilane exposure of 120 L at T≥650 K would lead to a H coverage clearly beyond the region for ad constant Si H sticking [11], if no H desorption is 2 6 taking place, and should thus result in a lower sticking coefficient for disilane. On the other hand, H desorption alone could lift the sticking coeffi2 cient at most to the value of the initial sticking coefficient at that temperature. The strong increase of the sticking coefficient observed experimentally must therefore be interpreted as the opening of an additional adsorption mechanism. Evidence for additional Si H adsorption chan2 6 nels on Si(111)-(7×7) at elevated temperatures was also gained in earlier studies. In Ref. [2] it
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was proposed that at surface temperatures between 770 and 1170 K the main channel for Si growth from Si H CVD involves decomposition of disi2 6 lane with SiH and SiH as products, where the 4 2 former desorbs instantaneously and the latter decomposes further. Two reaction mechanisms for this channel were proposed in Ref. [3]: Si H (g)+2dbSiH (g)+SiH(ad)+H(ad) 2 6 4
(1)
and Si H (g)+2SiH(ad)2SiH (g)+2Si(ad). 2 6 4
(2)
Another mechanism for the reaction of disilane, involving Si–H bond activation and silasilyl formation, was proposed in Ref. [20]: Si H (g)+2dbSi H (ad)+H(ad). 2 6 2 5
(3)
The second of these reactions effectively removes hydrogen from the surface. The overall sticking coefficient depends on the contribution from each of these reactions. The former two additional mechanisms were made responsible for the steep increase in the reactive sticking coefficient at T>650 K [2,3]. Activation energies of 5±1 kcal mol−1 (21±4.2 kJ mol−1) and 25± 6 kcal mol−1 (105±25.2 kJ mol−1; 1.08±0.26 eV per particle), respectively, were reported in Ref. [2] for reactions (1) and (2). From the slope of our Arrhenius plot in Fig. 5 an activation energy of 1.1 eV can be deduced in the high temperature part (above 650 K ), close to the value of 0.8 eV obtained above 770 K in Ref. [7]. This activation energy is also in perfect agreement with the activation energy of 105±25.2 kJ mol−1 reported in Ref. [2] for reaction (2). These findings also support the previous proposals of at least one additional dissociative adsorption channel for Si H 2 6 adsorption on Si(111)-(7×7) at higher temperatures. This additional mechanism for molecular decomposition, which gains importance at elevated temperatures is the reason for the higher Si growth rates using the precursor Si H as compared with 2 6 SiH . This means that the pre-exponential factor 4 for this additional adsorption channel must be considerably greater than that for the reaction which is effective at lower temperatures.
4. Conclusions In summary, we have shown that the room temperature adsorption of disilane on Si(111)(7×7) proceeds with an initial reactive sticking coefficient of S =8.5×10−5, a value which is by 0 several orders of magnitude lower than that assumed earlier. The initial reaction involves scission of the Si H Si–Si bond, followed by further 2 6 decomposition of the evolving SiH species into 3 SiH , which adsorbs at rest atoms, and H which 2 preferentially adsorbs at adatoms. In this fourcenter reaction, corner and center adatoms become hydrogen terminated with equal probability. For exposures at temperatures up to 690 K a variety of surface structures are formed, including Si H x y clusters, and hydrogen-terminated adatoms. At 730–760 K growth proceeds by homogeneous nucleation and growth of triangular Si bilayer islands without stacking faults and also by condensation of material at steps. Heterogeneous nucleation of second layer islands is unfavorable at the borderline between (7×7) and first layer areas grown at 112: steps as compared with other step directions and also to homogeneous nucleation. By directly determining the amount of deposited Si it is found that between 400 K and 650 K the dissociative adsorption of Si H on Si(111)-(7×7) 2 6 is activated with an apparent activation energy of 0.1 eV. Above 650 K the reactive sticking coefficient increases drastically, indicating that at least one additional adsorption mechanism with an effective activation energy of #1.1 eV becomes effective.
Acknowledgements This work was supported by the Stiftung Volkswagenwerk under Grant I/72162. The Si wafers were kindly provided by Wacker Chemitronic.
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