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Defect formation on the GaSb (001) surface induced by hydrogen atom adsorption
Solid State Communications 211 (2015) 10–15
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Defect formation on the GaSb (001) surface induced by hydrogen atom adsorption V.M. Bermudez 1 Electronics Science and Technology Division, Naval Research Laboratory, Washington, DC 20375, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 26 December 2014 Received in revised form 3 March 2015 Accepted 11 March 2015 Communicated by J.R. Chelikowsky Available online 20 March 2015
Density functional theory has been used to characterize the effects of adsorbed H on the electronic structure of the GaSb (001)-α(4 3) surface, which consists of a combination of Ga-Sb and Sb-Sb dimers. Adsorption of two H atoms at a Ga-Sb adatom dimer either has little effect on surface states above the bulk valence band maximum (VBM) or else eliminates them, depending on the mode of adsorption. However, adsorption at the Sb-Sb dimer in the terminating layer produces a state farther into the gap at 0.10 eV above the clean-surface VBM. Relaxation accompanying the breaking of the Sb-Sb dimer bond leads to increased interactions involving three-fold-coordinated Sb sites in the terminating layer, which in turn raises the energies of the non-bonding lone-pair orbitals. This defect state, which appears to be unique to the reconstructed GaSb (001) surface, could potentially function as a hole trap on the surface of p-type GaSb. Published by Elsevier Ltd.
Keywords: A. Hydrogen D. Defects E. Density functional theory GaSb
1. Introduction There is growing interest in the GaSb (001) surface as a substrate in electronic devices for high-speed, low-power digital and analog applications [1,2]. Hence, the formation of electricallyactive defects on surfaces and at interfaces is an important issue. Hydrogen occurs frequently as an adsorbate introduced either intentionally or inadvertently during processing. Although the effects of bulk H on GaSb electronic structure and properties have been studied intensively [3–12], less attention [10–12] has been devoted to the role of surface H, and such work has focused largely on the effects of H plasmas and the avoidance or mitigation of surface damage. Fig. 1a shows the α-(4 3) reconstruction [13], which is the most stable GaSb (001) structure [14,15] except under Sb-rich conditions. This comprises 4 Ga-Sb adatom dimers per surface unit cell on the Sb terminating layer of the bulk lattice, which itself incorporates 1 Sb-Sb dimer. The surface is non-metallic [14] since all Ga (Sb) dangling bonds are empty (doubly occupied). The nonbonding lone-pair (NBLP) orbitals on the sp3-hybridized Sb atoms form a band of surface states near the bulk valence band maximum (VBM), and the empty Ga sp2 orbitals produce a band of states near the conduction band minimum (CBM). A frequentlyobserved variant is the pseudo-(1 3), resulting from disorder in the [110] direction [16], which occurs after cleaning the surface in
E-mail address: [email protected] Retired.
1
http://dx.doi.org/10.1016/j.ssc.2015.03.011 0038-1098/Published by Elsevier Ltd.
an ultra-high-vacuum environment. Other structures include the c (2 6) that appears after desorption of an Sb capping layer from a sample grown by molecular beam epitaxy (MBE). All of the several possible GaSb (001) surface reconstructions [13-17] involve some combination of Ga-Sb and Sb-Sb dimers, and Sb-rich structures such as the β-(4 3) and c(2 6) consist mainly of the latter. In contrast to other III-Vs [18,19], there has been little work on the adsorption of H on GaSb (001). A preliminary ab initio study [20] found that adsorption of 2H atoms to form a monohydride dimer (MD, Fig. 1b) is exothermic by 2.08 eV per H and involves a pronounced relaxation consisting mainly of a reversal in the updown buckling of the dimer. However the corresponding dihydride, with adjacent GaH2 and SbH2 sites and a broken Ga-Sb dimer bond, is unstable towards desorption of H2 and restoration of the MD. These results pertain to dimer #2, the shortest of the adatom dimers. The Ga-Sb distance is computed to be 2.67, 2.70 and 2.77 Å respectively for dimer #2, for the symmetricallyequivalent dimers #1 and #3 and for dimer #4. The present work considers the effects of adsorbed H on the GaSb (001) surface electronic structure. While adsorption at a GaSb dimer either has little effect on surface states just above the bulk VBM or else removes them, reaction with the Sb-Sb dimer will be seen to yield a defect state lying well above the cleansurface VBM. The focus here is on chemically-stable surfaces exhibiting electrically-active defects; hence, adsorption of a single H atom is not considered. Such a paramagnetic species, with a defect state at the Fermi level, is not expected to be stable on a practical surface exposed to room air.
V.M. Bermudez / Solid State Communications 211 (2015) 10–15
11
Sb Ga
adatom Ga-Sb dimers terminating layer (Sb) st
1 underlayer (Ga)
1
2
3 4 1
[110] [110]
h i Fig. 1. (Color online) (a) The GaSb (001)-α(4 3) surface structure. The upper panel shows a view in approximately the 110 direction, and the lower panel shows a view along the surface normal. Dimer bonds are emphasized in red, and Ga-Sb adatom dimers are numbered for reference. The dashed line shows the surface unit cell. (b) Same structure with a single MD at dimer #2 (Ref. [20]) with H atoms in yellow. For clarity, layers below the first Ga underlayer are omitted. Note the presence of three-foldcoordinated Sb atoms in the terminating layer (other than those in the Sb-Sb dimer). Two are bonded to the dimer #4 Ga and one each is bonded to the dimer #1 and #3 Ga.
2. Computational methods
3. Results
All calculations were performed using density functional theory (DFT), including a semi-empirical treatment of dispersion, as described in detail previously [20,21]. Briefly, the model consists of a two-dimensionally-periodic (2DP) slab with 4 Ga-Sb bilayers. The bottom (Ga) layer is terminated with 2 pseudo-hydrogens (PHs) per Ga, each with a nuclear charge of 1.25 |e|. With the Ga-Sb adatom dimers bonded to the top (Sb) layer this gives a total of 5 Ga-Sb bilayers. The bottom bilayer and the PHs were fixed during relaxation, and a dipolar field was added to eliminate the spurious electric field in the vacuum space (30 Å wide). The Ga 3d and Sb 4d orbitals were included in the valence states, and scalarrelativistic effects were included in the PW91 pseudopotentials. The Sb spin–orbit coupling was neglected since a fully-relativistic treatment [9] of GaSb greatly reduces the band gap (in the local density approximation), which would complicate the task of identifying gap states. A (3 4 1) Monkhorst-Pack grid was used in geometry optimization and a (6 8 1) grid in computing the density of states (DOS). To facilitate comparison with possible future experimental results, a Gaussian broadening of 68 meV (5 milli-Rydberg (mRy)) was applied to the DOS in some cases in order to simulate the resolution attainable in synchrotron photoemission data. All results were obtained with identical computational parameters, which allows a reliable comparison between different modes of adsorption.
The investigation begins with an extension of previous work (Fig. 1b) to other Ga-Sb dimers. Fig. 2 shows the structures that can form when 2H atoms react with dimer #4, in which the Ga is backbonded to two three-fold-coordinated (TFC) Sb atoms. Bonding of H to a terminating-layer Sb is more exothermic (2.14 eV per H) than to the adatom Sb (1.94 eV per H). In the latter case the Ga-Sb dimer bond breaks, which may be facilitated by the fact that the terminating-layer Sb atoms bonded to the Ga are more free to relax than in the case of dimer #2. The Ga-Sb distance (2.99 Å) is only somewhat larger than before adsorption (2.77 Å), which suggests a possible weak residual bonding interaction. This site may also be favorable for dihydride formation, but this was not investigated. Similar results (not shown) were obtained for dimer #1, for which bonding of H to the TFC Sb in the terminating layer is more favorable (2.27 eV per H) than to the dimer Sb (2.03 eV per H). In the latter case the Ga-Sb dimer remains intact with a distance of 2.83 Å and a reversal of the buckling as for dimer #2. Fig. 3 shows the structures that can form when 2H atoms react with the Sb-Sb dimer. Each is energetically favorable, with exothermic adsorption energies of 2.02–2.04 eV per H. Due to the large dissociation energy of H2 the corresponding reaction with H2 is endothermic by 0.19 eV per H. In all cases the Sb-H bonds lie approximately in the surface plane. Using the HSb SbH structure (Fig. 3a) as an example, the Sb-H bond length is 1.74 Å,
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V.M. Bermudez / Solid State Communications 211 (2015) 10–15
Fig. 2. (Color online) Similar to Fig. 1b but showing results for reaction of 2H atoms with Ga-Sb dimer #4.
and the distance between the two Sb atoms in the broken dimer is 3.81 Å. The distance between H and the adjacent TFC Sb (to which the dimer #4 Ga is back-bonded) is 2.49 Å, and the Ga-Sb-H bond angle at the Sb-H site is 89.11. These values are representative of the other structures, except that the Sb-Sb distances are larger (4.26 and 4.80 Å for SbH SbH and SbH HSb respectively) due to the intervening H atom(s). Inspection of Figs. 1 and 3 shows that, when the Sb-Sb dimer bond breaks, the two Sb atoms relax away from each other and toward their in-plane TFC Sb nearest neighbors, which relieves the strain in the back-bonds to the firstunderlayer Ga atoms. Fig. 4 gives the DOS for the bare 2DP slab, which shows an apparent band gap (Eg) of 0.56 eV. This is a lower limit since the apparent Eg is affected by surface states (see below) lying near the VBM [14]. The computed bulk Eg of 0.21 eV [21] underestimates the experimental value [1] of 0.82 eV at T¼0 K as is typical in conventional DFT calculations. The larger Eg obtained for the 2DP slab results from quantum confinement [22]. For all DOS plots, E¼0 corresponds to the Fermi energy (EF), which coincides with the CBM. The energy of the vacuum level relative to EF is essentially constant for all structures, varying over a range of about 760 meV. Upon formation of an MD at dimer #2 (Fig. 1b) the DOS near the band edges is almost unaffected, which indicates that dimer #2 makes no signficant contribution to states in the gap. The MD may be viewed [19] as H (H þ ) bonded to Ga (Sb), which is consistent with a previous observation [20] that dissociative adsorption of H2O favors Ga-OH and Sb-H over the reverse (Ga-H and Sb-OH). The DOS results have also been obtained for different modes of adsorption of 2H atoms at Ga-Sb dimers #1 and #4, most of which (not shown) exhibit only small shifts in the surface VBM similar to that for dimer #2. For the bare α-(4 3), the surface VBM at the Γ-point comprises relatively
large contributions from the NBLP orbitals on the TFC terminatinglayer Sb sites bonded to the dimer #1 and #3 Ga atoms, in qualitative agreement with previous work [14]. Adsorption of H on one of these sites eliminates this surface state and shifts the surface VBM lower in energy, by 0.23 eV, to the bulk VBM (Fig. 4). It is noteworthy that angle-resolved photoemission data [23] for the Sb-rich c(2 6) surface show several surface-state bands, only one of which lies at the bulk VBM, which is consistent with results obtained here and in Ref. [14] showing that the surface state in the gap originates with Sb NBLP orbitals. In summary, the modes of adsorption considered thus far either have little effect or else passivate the surface by removing surface states near the VBM. When 2H atoms react with the Sb-Sb dimer, to form the structures shown in Fig. 3, there is little or no change relative to the bare surface near the CBM (Fig. 4). However, more-noticeable effects occur near the VBM, the most significant being the appearance of a state farther into the gap, at 0.12 eV above the clean-surface VBM. Fig. 5 summarizes the DOS results for all structures in Fig. 3, which are nearly identical near the band edges with the energy of the gap state being 0.12 (0.09) eV above the clean-surface VBM for SbH HSb (SbH SbH). The adsorption can be envisioned as neutral H atoms bonding to the singly-occupied dangling orbitals that result when the Sb-Sb bond breaks, with charge redistribution between the singly- and doubly-occupied Sb dangling orbitals leading to the different structures in Fig. 3. Fig. 6 shows the partial density of states (PDOS) near the band edges using the HSb SbH structure as an example. Fig. 6b shows that the hydrogenated site itself makes only a small contribution near the VBM. Since the H 1s orbitals make a negligible contribution to the near-edge DOS, the peak at about -1.5 eV in Fig. 6b can be assigned to the NBLP orbitals at the Sb-H sites. A
V.M. Bermudez / Solid State Communications 211 (2015) 10–15
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Fig. 3. (Color online) Similar to Fig. 1b but showing the three different configurations for H atoms on the Sb-Sb terminating-layer dimer. (a) HSb SbH; (b) SbH SbH; and (c) SbH HSb. For clarity, the diagrams are rotated 901 clockwise about the surface normal relative to Fig. 1b, and the dimers in (a) are marked in red and numbered as in Fig. 1a.
larger contribution (Fig. 6c) results from the TFC Sb sites in the terminating layer to which the dimer #4 Ga is back-bonded. These are the in-plane nearest neighbors of the Sb-H sites. Fig. 7 shows a charge density plot for the defect state, using the Sb-H Sb-H structure as an example, which is consistent with the results in Fig. 6. Similar results (not shown) are found for the other structures in Fig. 3. Most of the density is localized on surface TFC Sb atoms with the largest contribution being associated with those in the terminating layer to which the dimer #4 Ga is back-bonded. Smaller contributions also derive from the TFC Sb atoms to which the Ga atoms in dimers #1 and #3 are back-bonded and from a variety of underlayer Sb sites, all of which are affected to some extent by the relaxation that results from hydrogenation of the Sb-Sb dimer.
These results suggest that the defect state arises mainly from interactions involving TFC Sb sites that are increased, relative to the pristine α-(4 3) surface, due to the relaxation that occurs when hydrogenation breaks the Sb-Sb bond. This has the effect of shifting some of the Sb NBLP orbitals farther in energy above the clean-surface VBM. A similar mechanism may account for the smaller shifts that result from MD formation. The TFC Sb atoms bonded to H do not themselves contribute to the defect state, which suggests that the NBLP orbitals on these sites are stabilized by relaxation and by the higher electronegativity of H (in the Sb-H bond) vs. Sb (in the original Sb-Sb bond). The H-induced defect state appears to be unique to the reconstructed GaSb (001) surface since a similar effect has not been reported for stable forms of adsorbed H on other III–V surfaces [18].
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V.M. Bermudez / Solid State Communications 211 (2015) 10–15
Fig. 7. (Color online) Charge density plot (in green) for the gap state of the SbH SbH structure. For clarity, the Sb atom in each of the Ga-Sb adatom dimers is numbered as in Fig. 1a.
Fig. 4. (Color online) DOS results for the bare α-(4 3) surface and for the same surface with an HGa-SbH MD on dimer #2 (as in Fig. 1(b)), on dimer #1 or with the HSb SbH structure. For dimer #1 the Sb-H bond is in the terminating layer. The zero of energy is at the Fermi level, and only contributions near the band edges from the adlayer, the Sb terminating layer and the Ga first-underlayer are shown. “VBM” and “CBM” indicate band edges at the bare surface. A Gaussian broadening of 68 meV (5 mRy) has been applied in order to simulate photoemission data. The inset shows the same DOS results but with a smaller broadening (27 meV ¼ 2 mRy).
dimers in the terminating layer. This might be important in, for example, the formation of III-V heterostructures [24] via MBE. Unintentionally-doped GaSb is p-type, which has been suggested [9] to result from bulk H. An accumulation of H at the GaSb surface could potentially lead to a partially-hydrogenated interface and the presence of additional hole traps.
4. Summary and conclusions
Fig. 5. (Color online) Similar to Fig. 4 but showing results for each of the three structures in Fig. 3. “VBM” and “CBM” have the same meaning as in Fig. 4.
Density functional theory has been applied to a determination of the effects of atomic H adsorption on the electronic structure of the GaSb (001)-α(4 3) surface. Adsorption at a Ga-Sb adatom dimer, to form a MD, either has little effect on surface states above the bulk VBM or else eliminates them. On the other hand, adsorption at the Sb-Sb dimer in the terminating layer leads to a state farther into the gap, at 0.10 eV above the clean-surface VBM. It is suggested that relaxation accompanying the breaking of the Sb-Sb dimer bond leads to increased interactions involving TFC Sb sites, which in turn raises the energies of some non-bonding lone-pair orbitals above the clean-surface VBM. This H-induced gap state is potentially able to function as a hole trap on the surface of p-type GaSb.
Acknowledgments This work was supported by the Office of Naval Research. Pratibha Dev is thanked for a critical reading of the manuscript and for helpful suggestions. References
Fig. 6. (Color online) PDOS plots showing the contribution from various surface sites to the total DOS for the HSb SbH structure. (a) Total DOS for the adlayer and uppermost 2 layers of the bulk lattice. This is the same trace as in Figs. 4 and 5. The other traces show contributions from (b) the 2 Sb-H groups alone; (c) the Sb-H groups and the 3-fold-coordinated Sb atoms adjacent to the H; and (d) the 2 H atoms and all 3-fold-coordinated Sb sites in the terminating layer. “VBM” and “CBM” have the same meaning as in Fig. 4.
It is expected that, in addition to any intrinsic surface states above the VBM that survive hydrogenation, this H-induced defect will be effective as a hole trap on p-type GaSb surfaces with Sb–Sb
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[17] C. Hogan, R. Magri, R. Del Sole, Phys. Rev. Lett. 104 (2010) 157402. [18] S. Nannarone, M. Pedio, Surf. Sci. Rep. 51 (2003) 1. [19] C.W.M. Castleton, A. Höglund, M. Göthelid, M.C. Qian, S. Mirbt, Phys. Rev. B 88 (2013) 045319. [20] V.M. Bermudez, J. Appl. Phys. 113 (2013) 184906. [21] V.M. Bermudez, Appl. Phys. Lett. 104 (2014) 141605.
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[22] B. Delley, E.F. Steigmeier, Appl. Phys. Lett. 67 (1995) 2370. [23] J. Olde, K.-M. Behrens, H.-P. Barnscheidt, R. Manzke, M. Skibowski, J. Henk, W. Schattke, Phys. Rev. B 44 (1991) 6312. [24] G.J. Sullivan, A. Ikhlassi, J. Bergman, R.E. DeWames, J.R. Waldrop, C. Grein, M. Flatté, K. Mahalingam, H. Yang, M. Zhong, M. Weimer, J. Vac. Sci. Technol. B 23 (2005) 1144.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Defect formation on the GaSb (001) surface induced by hydrogen atom adsorption V.M. Bermudez 1 Electronics Science and Technology Division, Naval Research Laboratory, Washington, DC 20375, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 26 December 2014 Received in revised form 3 March 2015 Accepted 11 March 2015 Communicated by J.R. Chelikowsky Available online 20 March 2015
Density functional theory has been used to characterize the effects of adsorbed H on the electronic structure of the GaSb (001)-α(4 3) surface, which consists of a combination of Ga-Sb and Sb-Sb dimers. Adsorption of two H atoms at a Ga-Sb adatom dimer either has little effect on surface states above the bulk valence band maximum (VBM) or else eliminates them, depending on the mode of adsorption. However, adsorption at the Sb-Sb dimer in the terminating layer produces a state farther into the gap at 0.10 eV above the clean-surface VBM. Relaxation accompanying the breaking of the Sb-Sb dimer bond leads to increased interactions involving three-fold-coordinated Sb sites in the terminating layer, which in turn raises the energies of the non-bonding lone-pair orbitals. This defect state, which appears to be unique to the reconstructed GaSb (001) surface, could potentially function as a hole trap on the surface of p-type GaSb. Published by Elsevier Ltd.
Keywords: A. Hydrogen D. Defects E. Density functional theory GaSb
1. Introduction There is growing interest in the GaSb (001) surface as a substrate in electronic devices for high-speed, low-power digital and analog applications [1,2]. Hence, the formation of electricallyactive defects on surfaces and at interfaces is an important issue. Hydrogen occurs frequently as an adsorbate introduced either intentionally or inadvertently during processing. Although the effects of bulk H on GaSb electronic structure and properties have been studied intensively [3–12], less attention [10–12] has been devoted to the role of surface H, and such work has focused largely on the effects of H plasmas and the avoidance or mitigation of surface damage. Fig. 1a shows the α-(4 3) reconstruction [13], which is the most stable GaSb (001) structure [14,15] except under Sb-rich conditions. This comprises 4 Ga-Sb adatom dimers per surface unit cell on the Sb terminating layer of the bulk lattice, which itself incorporates 1 Sb-Sb dimer. The surface is non-metallic [14] since all Ga (Sb) dangling bonds are empty (doubly occupied). The nonbonding lone-pair (NBLP) orbitals on the sp3-hybridized Sb atoms form a band of surface states near the bulk valence band maximum (VBM), and the empty Ga sp2 orbitals produce a band of states near the conduction band minimum (CBM). A frequentlyobserved variant is the pseudo-(1 3), resulting from disorder in the [110] direction [16], which occurs after cleaning the surface in
E-mail address: [email protected] Retired.
1
http://dx.doi.org/10.1016/j.ssc.2015.03.011 0038-1098/Published by Elsevier Ltd.
an ultra-high-vacuum environment. Other structures include the c (2 6) that appears after desorption of an Sb capping layer from a sample grown by molecular beam epitaxy (MBE). All of the several possible GaSb (001) surface reconstructions [13-17] involve some combination of Ga-Sb and Sb-Sb dimers, and Sb-rich structures such as the β-(4 3) and c(2 6) consist mainly of the latter. In contrast to other III-Vs [18,19], there has been little work on the adsorption of H on GaSb (001). A preliminary ab initio study [20] found that adsorption of 2H atoms to form a monohydride dimer (MD, Fig. 1b) is exothermic by 2.08 eV per H and involves a pronounced relaxation consisting mainly of a reversal in the updown buckling of the dimer. However the corresponding dihydride, with adjacent GaH2 and SbH2 sites and a broken Ga-Sb dimer bond, is unstable towards desorption of H2 and restoration of the MD. These results pertain to dimer #2, the shortest of the adatom dimers. The Ga-Sb distance is computed to be 2.67, 2.70 and 2.77 Å respectively for dimer #2, for the symmetricallyequivalent dimers #1 and #3 and for dimer #4. The present work considers the effects of adsorbed H on the GaSb (001) surface electronic structure. While adsorption at a GaSb dimer either has little effect on surface states just above the bulk VBM or else removes them, reaction with the Sb-Sb dimer will be seen to yield a defect state lying well above the cleansurface VBM. The focus here is on chemically-stable surfaces exhibiting electrically-active defects; hence, adsorption of a single H atom is not considered. Such a paramagnetic species, with a defect state at the Fermi level, is not expected to be stable on a practical surface exposed to room air.
V.M. Bermudez / Solid State Communications 211 (2015) 10–15
11
Sb Ga
adatom Ga-Sb dimers terminating layer (Sb) st
1 underlayer (Ga)
1
2
3 4 1
[110] [110]
h i Fig. 1. (Color online) (a) The GaSb (001)-α(4 3) surface structure. The upper panel shows a view in approximately the 110 direction, and the lower panel shows a view along the surface normal. Dimer bonds are emphasized in red, and Ga-Sb adatom dimers are numbered for reference. The dashed line shows the surface unit cell. (b) Same structure with a single MD at dimer #2 (Ref. [20]) with H atoms in yellow. For clarity, layers below the first Ga underlayer are omitted. Note the presence of three-foldcoordinated Sb atoms in the terminating layer (other than those in the Sb-Sb dimer). Two are bonded to the dimer #4 Ga and one each is bonded to the dimer #1 and #3 Ga.
2. Computational methods
3. Results
All calculations were performed using density functional theory (DFT), including a semi-empirical treatment of dispersion, as described in detail previously [20,21]. Briefly, the model consists of a two-dimensionally-periodic (2DP) slab with 4 Ga-Sb bilayers. The bottom (Ga) layer is terminated with 2 pseudo-hydrogens (PHs) per Ga, each with a nuclear charge of 1.25 |e|. With the Ga-Sb adatom dimers bonded to the top (Sb) layer this gives a total of 5 Ga-Sb bilayers. The bottom bilayer and the PHs were fixed during relaxation, and a dipolar field was added to eliminate the spurious electric field in the vacuum space (30 Å wide). The Ga 3d and Sb 4d orbitals were included in the valence states, and scalarrelativistic effects were included in the PW91 pseudopotentials. The Sb spin–orbit coupling was neglected since a fully-relativistic treatment [9] of GaSb greatly reduces the band gap (in the local density approximation), which would complicate the task of identifying gap states. A (3 4 1) Monkhorst-Pack grid was used in geometry optimization and a (6 8 1) grid in computing the density of states (DOS). To facilitate comparison with possible future experimental results, a Gaussian broadening of 68 meV (5 milli-Rydberg (mRy)) was applied to the DOS in some cases in order to simulate the resolution attainable in synchrotron photoemission data. All results were obtained with identical computational parameters, which allows a reliable comparison between different modes of adsorption.
The investigation begins with an extension of previous work (Fig. 1b) to other Ga-Sb dimers. Fig. 2 shows the structures that can form when 2H atoms react with dimer #4, in which the Ga is backbonded to two three-fold-coordinated (TFC) Sb atoms. Bonding of H to a terminating-layer Sb is more exothermic (2.14 eV per H) than to the adatom Sb (1.94 eV per H). In the latter case the Ga-Sb dimer bond breaks, which may be facilitated by the fact that the terminating-layer Sb atoms bonded to the Ga are more free to relax than in the case of dimer #2. The Ga-Sb distance (2.99 Å) is only somewhat larger than before adsorption (2.77 Å), which suggests a possible weak residual bonding interaction. This site may also be favorable for dihydride formation, but this was not investigated. Similar results (not shown) were obtained for dimer #1, for which bonding of H to the TFC Sb in the terminating layer is more favorable (2.27 eV per H) than to the dimer Sb (2.03 eV per H). In the latter case the Ga-Sb dimer remains intact with a distance of 2.83 Å and a reversal of the buckling as for dimer #2. Fig. 3 shows the structures that can form when 2H atoms react with the Sb-Sb dimer. Each is energetically favorable, with exothermic adsorption energies of 2.02–2.04 eV per H. Due to the large dissociation energy of H2 the corresponding reaction with H2 is endothermic by 0.19 eV per H. In all cases the Sb-H bonds lie approximately in the surface plane. Using the HSb SbH structure (Fig. 3a) as an example, the Sb-H bond length is 1.74 Å,
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Fig. 2. (Color online) Similar to Fig. 1b but showing results for reaction of 2H atoms with Ga-Sb dimer #4.
and the distance between the two Sb atoms in the broken dimer is 3.81 Å. The distance between H and the adjacent TFC Sb (to which the dimer #4 Ga is back-bonded) is 2.49 Å, and the Ga-Sb-H bond angle at the Sb-H site is 89.11. These values are representative of the other structures, except that the Sb-Sb distances are larger (4.26 and 4.80 Å for SbH SbH and SbH HSb respectively) due to the intervening H atom(s). Inspection of Figs. 1 and 3 shows that, when the Sb-Sb dimer bond breaks, the two Sb atoms relax away from each other and toward their in-plane TFC Sb nearest neighbors, which relieves the strain in the back-bonds to the firstunderlayer Ga atoms. Fig. 4 gives the DOS for the bare 2DP slab, which shows an apparent band gap (Eg) of 0.56 eV. This is a lower limit since the apparent Eg is affected by surface states (see below) lying near the VBM [14]. The computed bulk Eg of 0.21 eV [21] underestimates the experimental value [1] of 0.82 eV at T¼0 K as is typical in conventional DFT calculations. The larger Eg obtained for the 2DP slab results from quantum confinement [22]. For all DOS plots, E¼0 corresponds to the Fermi energy (EF), which coincides with the CBM. The energy of the vacuum level relative to EF is essentially constant for all structures, varying over a range of about 760 meV. Upon formation of an MD at dimer #2 (Fig. 1b) the DOS near the band edges is almost unaffected, which indicates that dimer #2 makes no signficant contribution to states in the gap. The MD may be viewed [19] as H (H þ ) bonded to Ga (Sb), which is consistent with a previous observation [20] that dissociative adsorption of H2O favors Ga-OH and Sb-H over the reverse (Ga-H and Sb-OH). The DOS results have also been obtained for different modes of adsorption of 2H atoms at Ga-Sb dimers #1 and #4, most of which (not shown) exhibit only small shifts in the surface VBM similar to that for dimer #2. For the bare α-(4 3), the surface VBM at the Γ-point comprises relatively
large contributions from the NBLP orbitals on the TFC terminatinglayer Sb sites bonded to the dimer #1 and #3 Ga atoms, in qualitative agreement with previous work [14]. Adsorption of H on one of these sites eliminates this surface state and shifts the surface VBM lower in energy, by 0.23 eV, to the bulk VBM (Fig. 4). It is noteworthy that angle-resolved photoemission data [23] for the Sb-rich c(2 6) surface show several surface-state bands, only one of which lies at the bulk VBM, which is consistent with results obtained here and in Ref. [14] showing that the surface state in the gap originates with Sb NBLP orbitals. In summary, the modes of adsorption considered thus far either have little effect or else passivate the surface by removing surface states near the VBM. When 2H atoms react with the Sb-Sb dimer, to form the structures shown in Fig. 3, there is little or no change relative to the bare surface near the CBM (Fig. 4). However, more-noticeable effects occur near the VBM, the most significant being the appearance of a state farther into the gap, at 0.12 eV above the clean-surface VBM. Fig. 5 summarizes the DOS results for all structures in Fig. 3, which are nearly identical near the band edges with the energy of the gap state being 0.12 (0.09) eV above the clean-surface VBM for SbH HSb (SbH SbH). The adsorption can be envisioned as neutral H atoms bonding to the singly-occupied dangling orbitals that result when the Sb-Sb bond breaks, with charge redistribution between the singly- and doubly-occupied Sb dangling orbitals leading to the different structures in Fig. 3. Fig. 6 shows the partial density of states (PDOS) near the band edges using the HSb SbH structure as an example. Fig. 6b shows that the hydrogenated site itself makes only a small contribution near the VBM. Since the H 1s orbitals make a negligible contribution to the near-edge DOS, the peak at about -1.5 eV in Fig. 6b can be assigned to the NBLP orbitals at the Sb-H sites. A
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Fig. 3. (Color online) Similar to Fig. 1b but showing the three different configurations for H atoms on the Sb-Sb terminating-layer dimer. (a) HSb SbH; (b) SbH SbH; and (c) SbH HSb. For clarity, the diagrams are rotated 901 clockwise about the surface normal relative to Fig. 1b, and the dimers in (a) are marked in red and numbered as in Fig. 1a.
larger contribution (Fig. 6c) results from the TFC Sb sites in the terminating layer to which the dimer #4 Ga is back-bonded. These are the in-plane nearest neighbors of the Sb-H sites. Fig. 7 shows a charge density plot for the defect state, using the Sb-H Sb-H structure as an example, which is consistent with the results in Fig. 6. Similar results (not shown) are found for the other structures in Fig. 3. Most of the density is localized on surface TFC Sb atoms with the largest contribution being associated with those in the terminating layer to which the dimer #4 Ga is back-bonded. Smaller contributions also derive from the TFC Sb atoms to which the Ga atoms in dimers #1 and #3 are back-bonded and from a variety of underlayer Sb sites, all of which are affected to some extent by the relaxation that results from hydrogenation of the Sb-Sb dimer.
These results suggest that the defect state arises mainly from interactions involving TFC Sb sites that are increased, relative to the pristine α-(4 3) surface, due to the relaxation that occurs when hydrogenation breaks the Sb-Sb bond. This has the effect of shifting some of the Sb NBLP orbitals farther in energy above the clean-surface VBM. A similar mechanism may account for the smaller shifts that result from MD formation. The TFC Sb atoms bonded to H do not themselves contribute to the defect state, which suggests that the NBLP orbitals on these sites are stabilized by relaxation and by the higher electronegativity of H (in the Sb-H bond) vs. Sb (in the original Sb-Sb bond). The H-induced defect state appears to be unique to the reconstructed GaSb (001) surface since a similar effect has not been reported for stable forms of adsorbed H on other III–V surfaces [18].
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Fig. 7. (Color online) Charge density plot (in green) for the gap state of the SbH SbH structure. For clarity, the Sb atom in each of the Ga-Sb adatom dimers is numbered as in Fig. 1a.
Fig. 4. (Color online) DOS results for the bare α-(4 3) surface and for the same surface with an HGa-SbH MD on dimer #2 (as in Fig. 1(b)), on dimer #1 or with the HSb SbH structure. For dimer #1 the Sb-H bond is in the terminating layer. The zero of energy is at the Fermi level, and only contributions near the band edges from the adlayer, the Sb terminating layer and the Ga first-underlayer are shown. “VBM” and “CBM” indicate band edges at the bare surface. A Gaussian broadening of 68 meV (5 mRy) has been applied in order to simulate photoemission data. The inset shows the same DOS results but with a smaller broadening (27 meV ¼ 2 mRy).
dimers in the terminating layer. This might be important in, for example, the formation of III-V heterostructures [24] via MBE. Unintentionally-doped GaSb is p-type, which has been suggested [9] to result from bulk H. An accumulation of H at the GaSb surface could potentially lead to a partially-hydrogenated interface and the presence of additional hole traps.
4. Summary and conclusions
Fig. 5. (Color online) Similar to Fig. 4 but showing results for each of the three structures in Fig. 3. “VBM” and “CBM” have the same meaning as in Fig. 4.
Density functional theory has been applied to a determination of the effects of atomic H adsorption on the electronic structure of the GaSb (001)-α(4 3) surface. Adsorption at a Ga-Sb adatom dimer, to form a MD, either has little effect on surface states above the bulk VBM or else eliminates them. On the other hand, adsorption at the Sb-Sb dimer in the terminating layer leads to a state farther into the gap, at 0.10 eV above the clean-surface VBM. It is suggested that relaxation accompanying the breaking of the Sb-Sb dimer bond leads to increased interactions involving TFC Sb sites, which in turn raises the energies of some non-bonding lone-pair orbitals above the clean-surface VBM. This H-induced gap state is potentially able to function as a hole trap on the surface of p-type GaSb.
Acknowledgments This work was supported by the Office of Naval Research. Pratibha Dev is thanked for a critical reading of the manuscript and for helpful suggestions. References
Fig. 6. (Color online) PDOS plots showing the contribution from various surface sites to the total DOS for the HSb SbH structure. (a) Total DOS for the adlayer and uppermost 2 layers of the bulk lattice. This is the same trace as in Figs. 4 and 5. The other traces show contributions from (b) the 2 Sb-H groups alone; (c) the Sb-H groups and the 3-fold-coordinated Sb atoms adjacent to the H; and (d) the 2 H atoms and all 3-fold-coordinated Sb sites in the terminating layer. “VBM” and “CBM” have the same meaning as in Fig. 4.
It is expected that, in addition to any intrinsic surface states above the VBM that survive hydrogenation, this H-induced defect will be effective as a hole trap on p-type GaSb surfaces with Sb–Sb
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