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J. Phys. Chem. B 2001, 105, 1956-1960
Etching of GaAs(100) Surfaces by Halogen Molecules: Density Functional Calculations on the Different Mechanisms Arndt Jenichen*,† Institute for Surface Modification, Permoserstrasse 15, D-04303 Leipzig, Germany
Cornelia Engler Wilhelm-Ostwald-Institute of Physical and Theoretical Chemistry, UniVersity Leipzig, Linne´ strasse 2, D-04103 Leipzig, Germany ReceiVed: August 2, 2000; In Final Form: NoVember 21, 2000
Using density functional theory, reaction energies and related barrier heights are calculated for the chemisorption/desorption of X2, Ga2, As2, GaXn, and AsXn (n ) 1-3, X ) F, Cl, Br, and I) at GaAs(100) surfaces modeled by molecular clusters. The obtained data provide different reaction mechanisms for etching by halogen molecules and allow the interpretation of experimental findings. Under low F2 exposure, AsF and GaF are formed at and desorbed from the surface. The other halogen molecules cause the desorption of GaX and As2. The rate-limiting steps are the AsF and As2 removals, respectively. The I2-exposed surface can preferentially desorb iodine and, with that, stop the etching of GaAs(100) surfaces as found experimentally. Under high halogenation, strongly bound GaX2 and AsX2 as well as weakly bound GaX3 and AsX3 are found at the surface. The volatility increases for GaX3 from F to I. The volatility of AsX3 has a maximum for chlorine. AsBr3 and AsI3 hand over halogen atoms to the remaining Ga atoms of the second layer during desorption.
1. Introduction The chemical etching of semiconductors is almost exclusively performed by halogen and halogen-containing molecules. These molecules produce halogen atoms which form relatively stable bonds with the surface atoms. Furthermore, the high electronegativity of the halogen atoms leads to a weakening of bonds between the attacked surface atom and the substrate. Both facts are essential for the etching process. The different halogen atoms can cause different reactions at the surface. The most frequently investigated etchant for GaAs is Cl2. In our previous study we listed and interpreted the experimental and theoretical findings and set up reaction mechanisms for Cl2 etching of GaAs(100) surfaces.1 For the other halogen molecules, the mechanisms of etching are largely unclear and there are only few experimental and no theoretical results for the surface processes. Freedman and Stinespring2 reported the etching of GaAs(100) and (111) Ga-rich surfaces using beams of atomic chlorine and fluorine under ultra-high-vacuum conditions. Chlorine atoms produce a disordered highly-arsenic-deficient GaClx reaction product layer. Above 320 K an ordered As-rich surface is formed. Fluorination of the GaAs(100) surface at 350 K produces a disordered GaF3 layer, which is thermally stable up to 573 K. Above this temperature, the fluorine layer desorbs, leaving behind a slightly-Ga-rich surface. McFeely et al.3 studied XeF2 adsorption on the GaAs(110) surface and observed the formation of GaF, AsF, and GaF3 on the surface at 300 K. Kummel et al.4-6 investigated the reaction of monoenergetic 0.89 eV Br2 molecules with a 300 K As-rich GaAs(001)-2×4 * To whom correspondence should be addressed. E-mail: jen@ threki4.chemie.uni-leipzig.de. Fax: 49-341-9736399. † Present address: University Leipzig.
surface. At low coverage, Br2 molecules do not react with the first-layer As dimers in the initial chemisorption stage, but react exclusively with the second-layer Ga atoms exposed in trenches or at As atomic and dimer vacancies. As a result, the strained As dimer bonds and the Ga-As bonds are significantly weakened and chemically activated by the Br atoms in neighboring GaBr via inductive effects. In high coverage, Br2 reacts with As dimers and yields new surface species. The reaction products are arranged along the original dimer rows of the clean surface. GaBr and AsBr are bridge-bonded to two Ga atoms or two As atoms. AsBr2 is bonded to an As atom or a Ga atom. These surface species can be further brominated to volatile AsBr3. Weaver et al.7-9 investigated the etching of GaAs(110) surfaces by Br2. Initially, Br2 is dissociatively chemisorbed with Br bonding to both Ga and As atoms. AsBr3 formation was found at 50-100 K, but this species desorbs at temperatures higher than 150 K. GaBr3 formation also occurred, and this species remains at the surface up to 250 K. GaBr and AsBr surface species were evident for all the considered temperatures. Desorption of GaBr and As2 is found above 600 K. Yarmoff et al.10 found, in contrast to the (111) and (110) surfaces, which are etched at room temperature, that GaAs(001) instead becomes saturated with iodine. The removal of iodine by annealing generates a clean surface terminated by As; i.e., I2 dosing and removal changes the Ga-rich surface to an Asrich surface, but leaves an initial As-rich surface unchanged. In this study the reaction mechanisms for the etching of the GaAs(100) surfaces by X2 molecules with X ) F, Cl, Br, and I are set up using reaction energies and related barrier heights for chemisorption/desorption reactions of probable chemisorption structures. The surfaces are modeled by molecular clusters.
10.1021/jp002801u CCC: $20.00 © 2001 American Chemical Society Published on Web 02/13/2001
Etching of GaAs(100) Surfaces by Halogen Molecules
J. Phys. Chem. B, Vol. 105, No. 10, 2001 1957
Figure 1. Models used with the highest investigated halogenation (X ) F, Cl, Br, I): (A) 4×2 model for the Ga-rich surface (X10Ga10As8H16); (B) 2×4 model for the As-rich surface (X10As10Ga8H16); (C) 3×1 model for the Ga-rich surface (X5Ga10As8H26); (D) 1×3 model for the As-rich surface (X9As10Ga8H26).
The energies are computed by first-principles density functional theory. The data are compared with experimentally found surface compositions and with temperature-dependent distributions of the reaction products.
1. After the removal of atoms, the remaining clusters are reoptimized in the same manner as the full clusters. Additionally, dimerization of the second-layer atoms is allowed. The artificial dangling bonds (into the bulk) are saturated by hydrogen atoms. The H atoms are situated on the imaginary Ga-As bonds.
2. Models We distinguish between low and high halogenation by reason of the following experimental findings (see, e.g., ref 11): The etch rate increases with the temperature. The highly halogenated species (such as GaCl3 and AsCl3) are desorbed under low temperatures. The nonhalogenated and lowly halogenated species (such as GaCl and As2) are preferentially desorbed under high temperatures. One can conclude that under low temperatures only a small amount of halogen molecules are consumed due to the low etch rate. Therefore, sufficient halogen molecules are available for chemisorption. The opposite is true for high temperatures. The high etch rate causes a strong consumption of X2. Therefore, a deficient halogen concentration at the surface leads to lowly halogenated surface structures. Therefore, two types of models are distinguished: The 4×2 dimer models (Figure 1A,B) are used for the investigation of Ga2, As2, X2, GaX, and AsX desorption from the free or lowly halogenated surfaces. The 4×2 structure represents a reconstructed surface and also allows the desorption from the flat surface (middle) and from a step (edge) to be considered. The 3×1 monomer models (Figure 1C,D) are used for the simple modeling of highly halogenated surface structures with GaXm and AsXm (m ) 1-3) species. These models do not allow dimers as starting structures. All the investigations are done for the Ga-rich (Figure 1A,C) and the analogous As-rich (Figure 1B,D) surfaces. Each model is subdivided into two regions: The reaction area is geometryoptimized. The environment bath is fixed at the experimental bulk geometry. Fixed atoms are marked by asterisks in Figure
3. Methods For the calculation of the desorption energies and the related barrier heights, we use the B3P86 variant12,13 of density functional theory (DFT). Dissociation energies of relevant diatomic molecules calculated by different DFT methods show that the B3P86 method provides data which agree well with experimental and other theoretical values calculated by methods of higher quality. The geometries are optimized for all the systems by the Hartree-Fock (HF) method. The electronic core potentials (ECPs) as well as the valence double-ζ basis sets of Wadt and Hay14 and for the fluorine atoms the D95 basis set15 are used for the geometry optimizations. For the energy calculations, a 6D set of polarization functions is added to the non-hydrogen atoms. The undetermined exponents are optimized in calculations of homonuclear diatomic molecules in the HF-optimized atomic distances. The following values are applied: Ga, 0.16; As, 0.27; F, 0.9; Cl, 0.6; Br, 0.32; I, 0.22. The barrier heights for chemisorption/desorption reactions of molecules are obtained by the procedure of Ohno.16 For a transition state we take the point of the highest B3P86 energy along the restricted HF-optimized reaction path. With respect to the known deficiencies of the density functional methods for calculation of transition-state energies, one has to assess these data with care. But at the moment we see no chance to use higher correlated ab initio methods for these very large systems. By reason of only partial geometry optimization vibrational
1958 J. Phys. Chem. B, Vol. 105, No. 10, 2001
Figure 2. Desorption energies (Edes) and related barrier heights (EA) of the reactions with the lowest desorption energies for completely monohalogenated Ga-rich surfaces (model A).
frequencies are not calculated. The GAUSSIAN98 program system17 is used for the (B3P86/LanL2DZ(6D)//HF/LanL2DZ) calculations. The geometries must be optimized at the HF level because the DFT optimizations are too expensive for the very many large structures. The atomic distances obtained by HF and DFT are not very different. However, the polarization functions, which are necessary for the correct calculation of dissociation energies, provide larger B3P86(6D) bond lengths than the HF method without 6D functions. As a result, the absolute energies of the B3P86(6D)/HF procedure are higher than the B3P86(6D)/ B3P86(6D) ones always. As an example the differences for models A and B (Figure 1) with X ) F are 4.9 and 4.4 kcal/ mol, respectively. These deviations are not important for the aim of this study by the following reasons. The errors of the reaction energies are largely compensated by calculation of differences. The barrier heights are reduced by the energy deviations of the reactants. The energies of the transition structures are not essentially influenced because the barrier heights are the B3P86(6D) maxima along the restricted HFoptimized reaction paths. Since for the finding of the reaction mechanisms the barrier heights referring to the same reactant structure are compared, the energy deviations have no influence on the obtained results. 4. Results and Discussion For determining the mechanisms of etching processes, the reaction with the lowest barrier must be found for each of the considered structures. At first, the reaction energies are calculated for all the probable desorption reactions. Second, the barrier height for the reaction with the lowest desorption energy is calculated. If it is smaller than the next higher desorption energy, one can assume that this reaction path is the most probable one. Otherwise, one also has to calculate the barrier height for the next higher desorption energy and so on. 4.1. Low Halogenation. We showed1 that without interaction of chlorine the desorption of Ga2 and As2 is not possible under typical etching conditions as also found by experimental studies.18 In this section we start our investigations with the monohalogenation of the surface atoms including the secondlayer atoms of steps. 4.1.1. Ga-Rich Surface. Figure 2 presents the reaction energies and some related barrier heights for the chemisorption/desorption of various species from the Ga-rich surface (model A of Figure 1). We find that the desorption energies of the GaX species
Jenichen and Engler
Figure 3. Desorption energies (Edes) and related barrier heights (EA) of the reactions with the lowest desorption energies for model A after the removal of GaX from the middle or edge* position. *Note: Only the desorption energies for GaX (middle) are presented.
Figure 4. Desorption energies (Edes) and related barrier heights (EA) of the reactions with the lowest desorption energies for completely monohalogenated As-rich surfaces (model B).
agree with the related barrier heights in all cases. However, the X2 chemisorption at the second-layer As atoms of the steps is an activated process (desorption energies smaller than barrier heights). The first reaction of the starting structure is the removal of GaX either from the middle site (42-43 kcal/mol) or from the edge position (42-46 kcal/mol). For the resulting structures, Figure 3 shows that desorption of GaX is also the preferred reaction. The second removal of GaX from the middle site (1119 kcal/mol) is easier than from the edge position (27-34 kcal/ mol). The other reactions require much higher energies. In the case of iodine the recombinative desorption of I atoms from the outside As atoms is also a probable reaction (Figure 2: 46 kcal/mol). From the resulting structure the removal of GaI requires higher energies (middle, 57 kcal/mol; edge, 58 kcal/ mol; the values are not presented in a figure) in comparison with that of iodinated outside atoms (42 and 46 kcal/mol). In summary, the removal of Ga takes place by GaX desorption for all the investigated halogens under low halogenation. 4.1.2. As-Rich Surface. Figure 4 presents the desorption energies and the related barrier heights for various species from the As-rich surface (model B of Figure 1). The desorption energies of AsX agree with the barrier heights. However, the dissociative chemisorption of X2 at the As dimers is an activated process but not at the outside Ga atoms on the steps. This fact is in agreement with the findings of Kummel et al.4-6 that Br2
Etching of GaAs(100) Surfaces by Halogen Molecules
J. Phys. Chem. B, Vol. 105, No. 10, 2001 1959
Figure 5. Desorption energies (Edes) and related barrier heights (EA) of the reactions with the lowest desorption energies for model B after the removal of AsX from the middle position.
Figure 7. Desorption energies (Edes) and related barrier heights (EA) of the reactions with the lowest desorption energies for model B after the removal of X2 from the middle and edge As dimers.
Figure 6. Desorption energies (Edes) and related barrier heights (EA) of the reactions with the lowest desorption energies for model B after the removal of X2 from the middle As dimer.
Figure 8. Desorption energies (Edes) and related barrier heights (EA) of the reactions with the lowest desorption energies for the GaXn (n ) 1-3) (model C).
molecules do not react with the first-layer As dimers in the initial chemisorption stage. Figure 4 shows that dependent upon the halogen two different mechanisms occur: For fluorine and, possibly, chlorine AsX desorption from the middle position is the preferred reaction (57 and 62 kcal/mol). The resulting structure allows AsX desorption from the edge position only as the data of Figure 5 demonstrate (58 and 56 kcal/mol). For bromine, iodine, and, possibly, chlorine X2 desorption from the middle site is the most probable reaction (Figure 4: Cl, 61 kcal/mol; Br, 47 kcal/mol; I, 38 kcal/mol). The remaining structures prefer the desorption of X2 from the edge site (Figure 6: Cl, 81kcal/mol; Br, 64 kcal/mol; I, 50 kcal/mol). The probability that halogen-free As dimers at the surface are rebuilt increases from Cl to I. The resulting structures after X2 removal can desorb, at first, As2 from the middle site (Figure 7: about 50 kcal/mol) and, finally, from the edge position (about 66 kcal/ mol; the values are not presented in a figure) independent of the type of halogen atoms bound at the outside. The last reaction is the rate-limiting step of Br2 and I2 etching under low halogenation. The As2 desorption requires a relatively high temperature (>600 K) as found for Br2 etching.9 For Cl2 etching the consideration of barriers for chemisorption shows1 that As2 desorption also is the most probable path for arsenic removal and the bottleneck of Cl2 etching, which is in agreement with experiment.11 For the iodine case the recombinative I2 desorption from the
outside Ga atoms (Figure 7: 52 kcal/mol) has to be considered. This reaction requires about the same energy as As2 desorption from the middle position (51 kcal/mol). The result is the halogen-free surface, which has high As2 desorption energies (middle, 103 kcal/mol; edge, 88 kcal/mol1). If this path is the most essential one, etching of the GaAs(100) surface by I2 would not be possible. This result is found experimentally by Yarmoff et al.10 4.2. High Halogenation. 4.2.1. Ga-Rich Surface. Figure 8 presents the desorption energies and some related barrier heights for GaXn (n ) 1-3) species from a surface position where the neighboring Ga atoms are monohalogenated (model C of Figure 1). A full trihalogenation is not possible due to the small valence electron number of Ga atoms. GaX2 is strongly bound to the surface. Therefore, its desorption is not probable. These species were not found or were found only in small concentrations experimentally in the gas phase above the surface.19 The desorption probability of GaX is much higher (39-44 kcal/mol). GaCl and GaBr desorb above 600 K.9,11,18-20 For GaX3 the desorption energies and the barrier heights are low and decrease from F to I. The relatively high energy for the removal of GaF3 is consistent with the found layer built from this molecule.2,3 GaBr3 also remains at the surface up to 250 K.7-9 4.2.2. As-Rich Surface. Figure 9 presents the reaction energies and, partially, the related barrier heights for the desorption of
1960 J. Phys. Chem. B, Vol. 105, No. 10, 2001
Jenichen and Engler TABLE 1: Reaction Products (Gas Phase) Resulting from the Computed Energies etch gas
F2
Cl2
Br2
low halogenation Ga-rich GaF GaCl GaBr As-rich AsF As2 (AsCl) As2 high halogenation Ga-rich GaF3 GaCl3 As-rich AsF3 AsCl3
I2 GaI - (As2)
GaBr3 GaI3 AsBrn (n < 3) AsIn (n < 3)
and AsX3 can be found at the surface. The volatility increases for GaX3 from F to I. The volatility of AsX3 has a maximum for chlorine. AsBr3 and AsI3 hand over halogen atoms to the remaining Ga atoms of the second layer during desorption. Table 1 summarizes the most probable reaction products resulting from the computed energies.
Figure 9. Desorption energies (Edes) and related barrier heights (EA) of the reactions with the lowest desorption energies for the AsXn (n ) 1-3) (model D). *Note: AsX at neighboring positions.
AsXn (n ) 1-3) from a surface position with equal halogenation of the surrounding As atoms (model D of Figure 1). AsX and AsX2 are relatively strongly bound to the surface. Only AsF2 has a noticeably smaller desorption energy (48 kcal/mol). These species were not found or were found only in a small portion in the gas phase above the surface.19 AsX3 shows a special behavior. The desorption energies have a minimum for chlorine. The same result is found for AsX3 surrounded by AsX species, as can also be seen in Figure 9. The temperature-programmed desorption experiments of French et al.19 provide an activation energy smaller than 10 kcal/mol for AsCl3 desorption, which is in agreement with the calculated barrier height of 4 kcal/ mol. During the computation of the potential energy curves for desorption, AsBr3 and AsI3 hand over halogen atoms to the Ga atoms of the remaining second layer. That means a reaction path with a lower barrier exists, which leads to the desorption of AsXn with n smaller than 3 for X ) Br and I. 5. Summary Using density functional theory, reaction energies and, partially, related barrier heights are calculated for the chemisorption/desorption of X2, Ga2, As2, GaXn, and AsXn (n ) 1-3, X ) halogen atoms) from the halogenated GaAs(100) surfaces modeled by molecular clusters. The obtained data provide reaction mechanisms for etching by halogen molecules and allow the interpretation of experimental findings. Under low F2 exposure, AsF and GaF are formed at and desorbed from the surface. The other halogen molecules cause the desorption of GaX and As2. The rate-limiting steps are the AsF and As2 removals, respectively. The I2-exposed surface can preferentially desorb iodine and, with that, stop the etching of GaAs(100) surfaces as found experimentally. Under high halogenation, strongly bound GaX2 and AsX2 as well as weakly bound GaX3
Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. References and Notes (1) Jenichen, A.; Engler, C. J. Phys. Chem. B 2000, 104, 8210. (2) Freedman, A.; Stinespring, C. D. J. Phys. Chem. 1992, 96, 2253. (3) McLeen, A. B.; Terminello, L. J.; McFeely, F. R. Phys. ReV. B 1989, 40, 11778. (4) Liu, Y.; Komrowski, A. J.; Kummel, A. C. Phys. ReV. Lett. 1998, 81, 413. (5) Liu, Y.; Komrowski, A. J.; Kummel, A. C. J. Chem. Phys. 1999, 110, 4608. (6) Liu, Y.; Komrowski, A. J.; Kummel, A. C. Surf. Sci. 1999, 439, 29. (7) Patrin, J. C.; Weaver, J. H. Phys. ReV. B 1993, 48, 17913. (8) Gu, C.; Chen, Y.; Ohno, T. R.; Weaver, J. H. Phys. ReV. B 1992, 46, 10197. (9) Cha, C. Y.; Brake, J.; Han, B. Y.; Owens, D. W.; Weaver, J. H. J. Vac. Sci. Technol., B 1997, 15, 605. (10) Varekamp, P. R.; Hakansson, M. C.; Kanski, J.; Shuh, D. K.; Bjo¨rkqvist, M.; Gothelid, M.; Simpson, W. C.; Karlsson, U. O.; Yarmoff, J. A. Phys. ReV. B 1996, 54, 2101. (11) Su, C.; Hou, H.; Lee, G. H.; Dai, Z.; Luo, W.; Vernon, M. F.; Bent, B. E. J. Vac. Sci. Technol., B 1993, 11, 1222. (12) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (13) Perdew, J. P. Phys. ReV. B 1986, 33, 8822. (14) Wadt, W. R., Hay, P. J. J. Chem. Phys. 1985, 82, 284. (15) Dunning, T. H.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1976. (16) Ohno, T. Surf. Sci. 1996, 357-8, 322. (17) Gaussian 98, Revision A.7: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A., Gaussian, Inc., Pittsburgh, PA, 1998. (18) Ludviksson, A.; Xu, M.; Martin, R. M. Surf. Sci. 1992, 277, 282. (19) Bond, P.; Brier, P. N.; Fletcher, J.; Jia, W. J.; Price, H.; Gorry, P. A. Surf. Sci. 1998, 418, 181. (20) French, C. L.; Balch, W. S., Foord, J. S. J. Phys.: Condens. Matter 1991, 3, S351.
