SURFACE AND INTERFACE ANALYSIS, VOL. 26, 105È108 (1998) Electron-stimulated Desorption Total Cross-section Determination for Digermane on Si(100) A. F. Aguilera,1,2 J. H. Campbell,3 J. H. Craig, Jr.2,3,* and K. H. Pannell1,2 1 Department of Chemistry, 500 W. University Ave., University of Texas at El Paso, El Paso, TX 79968-0513, USA 2 Materials Research Institute, 500 W. University Ave., University of Texas at El Paso, El Paso, TX 79968-6664, USA 3 Department of Physics, 500 W. University Ave., University of Texas at El Paso, El Paso, TX 79968-0515, USA We have studied digermane-covered Si(100) using electron-stimulated desorption (ESD). Estimates are presented for the total H(a) ESD removal cross-section for digermane-exposed Si(100) substrates at 85 K using electrons incident at 150 eV energy. It is found that electron-enhanced deposition of Ge occurs only when physisorbed digermane is present. Auger electron spectroscopy provided the means for determining the relative amounts of germanium adsorbed on the Si(100) surface following digermane exposures, electron irradiation and surface reconstruction. It is found that two coverage regimes are important : initial dosing of digermane on Si(100) at 85 K results in overlayers consisting of both physisorbed digermane and chemisorbed GeH (a) (x = 1, 2 or 3) species ; x and short anneals to 200 K following exposure of the Si(100) surface at 85 K lead to the presence of only chemisorbed GeH (a). The two coverage regimes exhibit di†erent ESD behavior. Two kinetic energy distribution (KED) x peaks are seen when physisorbed digermane is present, and only one when it is absent. The ESD signal decay curves obtained from the two surfaces are also di†erent : the presence of physisorbed digermane results in a twocomponent exponential signal decay ; the absence of the physisorbed species results in a single-exponential decay. The total H removal cross-section from the physisorbed digermane overlayer was determined to be r ¿ 1.4 Â 10—15 cm2, while that from Si(100) with only adsorbed GeH present was found to be r ¿ 2.6 Â 10—16 x cm2. Our results suggest that adsorbed GeH (a) species remain intact on the surface even when the Si(100) subx strate is annealed to 200 K, indicating that hydrogen migration from surface GeH (a) to Si surface sites does not x occur at 200 K. ( 1998 John Wiley & Sons, Ltd. Surf. Interface Anal. 26, 105È108 (1998) KEYWORDS : ESD ; Si(100) ; electron-stimulated desorption ; silicon INTRODUCTION The e†ects of an electron beam on digermane overlayers on Si(100) surfaces are interesting in part because enhanced Ge deposition has been observed.1 The observation of enhanced Ge deposition and e†orts to characterize this phenomenon may lead to a better understanding of Ge H adsorption and decomposition 2 6 Although we have examined mechanisms on Si(100). Ge H on Si(100) previously, and have demonstrated 6 deposition is enhanced under electron irradiathat2 Ge tion, further studies are needed to better characterize the interaction of the electron beam with adsorbed Ge H . In this work, we report the H(a) removal cross2 6 from Ge H -covered Si(100) surfaces using an section 2 6 upon the surface at 70¡ o† the electron beam incident surface normal and 150 eV kinetic energy. Samples of p-silicon(100) with a resistivity of D0.1 ) cm were prepared and placed into an ultrahigh vacuum system operating at D2 ] 10~10 Torr. Auger electron * Correspondence to : J. H. Craig, Materials Research Institute, 500 W. University Ave., University of Texas at El Paso, El Paso, TX 79968-6664, USA. E-mail : jcraig=utep.edu. Contract grant sponsor : National Science Foundation ; grant no. : CHE8920120. CCC 0142È2421/98/020105È04 $17.50 ( 1998 John Wiley & Sons, Ltd. spectroscopy (AES) analysis conÐrmed contaminantfree initial surface conditions. First, electron-stimulated desorption (ESD) kinetic energy distributions (KEDs) for positive hydrogen ions from digermane-dosed Si(100) surfaces held at 85 K were obtained by methods described in detail elsewhere.2 The H` ESD KEDs exhibited bimodal peak shapes that were Ðtted by the least-squares method to a model based upon the work of Nishijima and Propst,3 as shown in Fig. 1. The bimodal peak in Fig. 1(a) is suggestive of the presence of two distinct binding states from which the desorbing positive hydrogen ions originate,2 and is consistent with the fact that no annealing step was used (i.e. physisorbed digermane was present on surface). Previous temperature-programmed desorption (TPD) results indicated the presence of physisorbed digermane even for low exposures on Si(100) at 85 K. Work done elsewhere4 indicates that for Ge H adsorption at 85 K 2 GeH 6 there is at least one chemisorbed state (x \ 1, 2 or 3) on Si(100) that produces H at 590x K during TPD experiments. Thus, we conclude2 that our bimodal H` ESD KEDs arise from hydrogen ions desorbing via ESD from molecularly adsorbed (physisorbed) Ge H and directly from adsorbed germyl fragments (GeH 2) on6 x the Si(100) surface. In a separate set of experiments, ESD KEDs from adsorbed GeH fragments on Si(100) x were obtained after Ñashing the Si(100) sample to 200 K to remove molecularly adsorbed (physisorbed) Received 20 May 1997 Accepted 29 August 1997 106 A. F. AGUILERA, J. H. CAMPBELL, J. H. CRAIG, JR. AND K. H. PANNELL Figure 1. (a) An ESD KED profile showing two distinct states. (b) An ESD KED obtained after a 200 K anneal to remove physisorbed Ge H , which exhibits only one state. The insets in (a) and (b) show fits of our decay data using Eqn (1). The inset to (a) shows a 2 6 double-exponential fit to data, obtained from a Si(100) substrate on which Ge H (a) and GeH (a) are adsorbed. The inset to (b) required 2 6 x only a single-exponential fit. digermane. The H` ESD KEDs from Si(100) having only adsorbed GeH (a) exhibit only one ESD KED peak, as seen in Fig.x 1(b). The TPD results conÐrmed that no physisorbed digermane molecules were present when performing this second set of ESD measurements. In this work we directly measure the H` ESD signal decays by setting the energy window (pass energy) of our Bessel Box energy analyzer5 to a constant energy (the Bessel box energy resolution is D1 eV). A single exponential decay, given by Eqn (1) C I(t) \ I ] a b D 1 [ exp([bt) bt (1) can be used to Ðt the experimentally obtained H` ESD signal decays as described elsewhere.6 In Eqn (1), I(t) is the ion current at time t, I is the background signal, a b is a Ðtted amplitude parameter and b is the decay conSURFACE AND INTERFACE ANALYSIS, VOL. 26, 105È108 (1998) stant (b \ pJ/e). We note that Eqn (1) was derived in a manner that included the spatial dependence of the electron beam current density proÐle within the beam spot.6 A Faraday cup was used to obtain electron beam current density proÐles. The experimentally determined electron beam current density proÐle was found to be well approximated by J(r) \ J exp([r2/a2) (2) 0 where J(r) is the spatially dependent current density, J is the current density at the center of the electron-beam0 spot, r is the independent variable and a is the Gaussian width of the spot. From the Ðts of the exponential decay in Eqn (1) to our data (see insets to Fig. 1), it is quite straightforward to extract the decay constant. Provided that the relation eb \ J p holds true, we may then plot eb vs. J to obtain a line0 of slope p, which in this case is the total0 H(a) removal cross-section for the process. ( 1998 John Wiley & Sons, Ltd. ESD OF DIGERMANE-EXPOSED Si(100) We obtained experimental H` ESD signal decay curves from Si(100) surfaces both with and without a molecularly adsorbed Ge H overlayer, as shown in the 2 6 insets to Fig. 1. From single-exponential decay Ðts to the experimental decays at various J , plots of eb vs. J 0 0 indicate that the H(a) removal cross-section for surfaces with physisorbed digermane present are a factor of two greater than for surfaces with no physisorbed digermane present on the surface. However, it is known that molecularly adsorbed Ge H coexists with GeH (a) on 2 K 6 even for relatively x low the Si(100) surface at 85 Ge H (g) exposures. Thus, in the ESD experiments 2 6 where no e†ort is made to remove physisorbed Ge H , 2 6 our Si(100) surface is covered with a mixture of strongly bound (chemisorbed) GeH species and more weakly x bound (physisorbed) Ge H molecules. Intuition sug2 gests that the ESD signal 6 would have contributions from both GeH (a) and Ge H (a), and bimodal ESD KEDs such as xthat shown 2 in6 Fig. 1(a) conÐrm this suspicion. With two surface states contributing to the ESD signal, it is appropriate to Ðt the experimental ESD signal decay curves obtained in the presence of physisorbed digermane with double-exponential decay curves. Double-exponential decay Ðts were used for the experimental decays obtained from surfaces having physisorbed digermane, and an example of such a Ðt is shown in the inset to Fig. 1(a), where the dashed lines represent the component single-exponential decay curves that are summed to obtain the resultant best-Ðt double-exponential decay. Plots of eb vs. J are made using the decay constants b obtained from0 each component decay curve, and arei shown as the squares and triangles in Fig. 2. Cross-sections deduced from the best-Ðt straight lines to the points in Fig. 2 indicate that p \ 1.4 ^ 0.45 ] 10~15 cm2 and p \ 2.3 ^ 1.5 1 10~16 cm2 are the values of the component 2 ] H(a) removal cross-sections. We note that the value for p is 1 an unusually large cross-section, of the order of the geometrical cross-section. However, we have previously observed that physisorbed digermane is very susceptible to electron beam-induced decomposition.1 Figure 2. Values of e b obtained from exponential decay fits to the experimental signal decays are plotted vs . the J value used to obtain the experimental decay. Single-exponential0 decays were used to fit ESD signal decays obtained following a short anneal to 200 K, and the resulting data are shown as circles. Squares and triangles represent data obtained from double-exponential fits to the ESD signal decays obtained with physisorbed digermane present on the surface. ( 1998 John Wiley & Sons, Ltd. 107 We obtained several H` ESD signal decays following Ñashing of the Si(100) to 200 K to remove the weakly bound Ge H species, and found that a single2 6 exponential decay Ðt the experimental data quite nicely. An example of such a Ðt to the data is shown in the inset to Fig. 1(b). Using the relation eb \ J p and the 0 same analysis as was used above, we obtained the data represented by circles in Fig. 2, for which the best-Ðt least-squares straight line gives a slope of p \ 2.8 ^ 0.4 ] 10~16 cm2. The observation made above that ESD KEDs for GeH /Si(100) exhibit only x one ESD desorption state supports the use of the singleexponential Ðt to the decays in Fig. 1(b). Our results indicate that there is an ESD process occurring with high cross-section when physisorbed digermane is present on the surface. The high crosssection process correlates well with the presence of physisorbed digermane, so it is reasonable to attribute this H` ESD signal to removal of H(a) from physisorbed Ge H species. The high cross-section process only 2 6 when the high-energy (D5 eV) KED peak is occurs present, so we assign the high-energy KED peak as being due to H` desorbed from physisorbed digermane. Just as the low-energy KED peak at D3.1 eV is always present, so is the H` ESD process having a crosssection of p D 2.6 ] 10~16 cm2, where the two low cross-section values are essentially the same within experimental error and have thus been averaged. The origin of the H` ESD signal decay having a cross-section of p D 2.6 ] 10~16 cm2 is less certain. Possible surface states from which this H` ESD signal might arise include GeH (a) as well as various surface silicon hydride species.7x The primary question is whether hydrogen atoms associated with surface germyl species migrate to silicon surface sites during the 200 K anneal used to remove physisorbed digermane. We estimate that the 200 K anneal supplies up to 12 kcal mol~1 towards the activation of surface processes such as di†usion of hydrogen from GeH (a) species to neighx boring silicon surface sites. If the activation barrier is as low as 9 kcal mol~1, as recently postulated by Russell and Eckerdt,8 such di†usion would be expected to occur and the post-annealed samples would actually be exhibiting H(a) removal cross-sections for removal of H(a) from surface silicon hydrides. Alternatively, if the activation barrier is higher, or the efficiency of energy transfer from the surface to the adsorbate is lower than we estimate, the low-energy KED peak would originate from electron-stimulated removal of H(a) from surface GeH (a) species. x ESD data suggest that the GeH (a) species Our remains intact on the surface during the 200x K anneal. Support for this conclusion arises from several observations. The low-energy ESD KED peaks in Fig. 1 appear at the same energy within Ðtting errors both before and after the 200 K anneal, and the low-energy KED peaks reported here appear at slightly higher energies than similar peaks reported for ESD from Si H /Si(100).7 Additionally, the cross-section for the 2 6 low-energy KED peak found in this work (p D 2.6 ] 10~16 cm2) is somewhat larger than the cross-section for the low-energy (dihydride) KED peak found in the work done with disilane adsorbed on Si(100).7 Finally, the monohydride state would be expected to form before the dihydride, and the ESD SURFACE AND INTERFACE ANALYSIS, VOL. 26, 105È108 (1998) 108 A. F. AGUILERA, J. H. CAMPBELL, J. H. CRAIG, JR. AND K. H. PANNELL KED peak from the monohydride occurs near 5 eV and is not present in our post-anneal ESD KEDs. Thus, ESD from surface silicon hydrides does not appear to be occurring. In summary, previous work in our laboratory has shown that molecularly adsorbed (physisorbed) Ge H 2 6 must be present on the Si(100) surface for electron irradiation to signiÐcantly enhance Ge atom deposition. The work reported here shows that enhanced Ge deposition is the result of electron-induced dissociation of physisorbed digermane. Following exposure of the Si(100) surface at 85 K to digermane gas, our ESD KED data strongly suggest that two distinct Ge-H states exist on the surface, which we ascribe to GeH (a) x and Ge H (a). Finally, after annealing of Si(100) sur2 6 faces covered with digermane to 200 K, our ESD data suggest that the only adsorbate remaining on the surface is GeH (a). x Acknowledgements This work was supported in part by the Science and Technology Program of the National Science Foundation, grant no. CHE8920120. The authors also wish to acknowledge support received from the NSF Materials Research Center of Excellence at the university of Texas at El Paso, cooperative agreement No. HRD-9353547. A.F.A. acknowledges support from the Fulbright Institute of International Education and Consejo Nacional de Ciencia y Tecnologia (Mexico). REFERENCES 1. J. H. Campbell, J. Lozano, A. F. Aguilera, J. H. Craig, Jr. and K. H. Pannell, Appl . Surf . Sci . 108, 345 (1997). 2. J. H. Campbell, M. V. Ascherl and J. H. Craig, Jr., J . Vac . Sci . Technol . A 12, 2128 (1994). 3. M. Nishijima and F. M. Propst, Phys . Rev . B 2, 2368 (1970). 4. B. M. H. Ning and J. E. Crowell, Surf . Sci . 295, 79 (1993). SURFACE AND INTERFACE ANALYSIS, VOL. 26, 105È108 (1998) 5. J. H. Craig, Jr. and W. G. Durrer, J . Vac . Sci . Technol . A 7, 3337 (1989). 6. B. Xia and S. C. Fain, Jr., Phys . Rev . B 50, 14565 (1994). 7. J. Lozano, J. H. Craig, Jr., J. H. Campbell and M. V. Ascherl, Nucl . Instrum . Methods Phys . Res . B 100, 407 (1995). 8. N. M. Russell and J. G. Eckerdt, Surf . Sci . 369, 51 (1996). ( 1998 John Wiley & Sons, Ltd.