Fluorine and Oxygen Adsorption and Their Coadsorption on the (111

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Fluorine and Oxygen Adsorption and Their Coadsorption on the (111) Surface of InAs and GaAs Alexander V. Bakulin,*,†,‡ Svetlana E. Kulkova,†,‡ Maxim S. Aksenov,§ and Natalia A. Valisheva§ †

Institute of Strength Physics and Material Science, Siberian Branch of the Russian Academy of Sciences, 2/4 Akademichesky Avenue, Tomsk, 634055, Russia ‡ National Research Tomsk State University, 36 Lenina Avenue, Tomsk, 634050, Russia § Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, 2 Pirogova Street, Novosibirsk, 630090, Russia

ABSTRACT: Oxygen and fluorine adsorption and their coadsorption on the (111) unreconstructed surface of semiconductors InAs and GaAs were studied using the projector augmented-wave method with the generalized gradient approximation for the exchange−correlation functional and hybrid functional approach. The energetically preferable adsorbate sites on the surface were determined. It is shown that fluorine adsorption above surface cations on the AIIIBV(111)A-(1 × 1) unreconstructed surface leads to a removal of the surface state formed by cation pz-orbitals and to an unpinning of the Fermi level, whereas oxygen adsorption induces additional surface states in the band gap. The influence of fluorine and oxygen coadsorption and also fluorine concentration on the surface states in the band gap is discussed. It is shown that oxygen-induced surface states are completely or partially removed from the band gap by fluorine coadsorption if it forms bonds with cation surface atoms involved in an interaction with oxygen. The increase of fluorine concentration leads to considerable changes of the near-surface-layer structure due to the penetration of both electronegative adsorbates into the substrate and affects the electron properties of oxygen/ AIIIBV(111) interface.

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method shows that the transition region at the FAOL/AIIIBV interface consists of fluorides and oxyfluorides of semiconductor elements,3−7 and its formation can explain an elimination of interface states in the band gap. Note that the complex composition of the AOL/In(Ga)As interface transition region considerably complicates finding out the microscopic nature of interface states. The task becomes more complicated when studying the formation peculiarities of interfaces based on ternary (InGaAs, InAlAs, GaAlAs) compounds. At present, ab initio calculations within methods based on the density functional theory, which allow a detailed investigation of the electron states and chemical bonds at interfaces, are actively used to understand interface states appearance/disappearance mechanisms.8 Earlier we studied fluorine and oxygen adsorption and their coadsorption on the InAs(111)-(2 × 2) reconstructed surface, which has an indium

INTRODUCTION III V A B semiconductors, such as InAs, GaAs, and their quasibinary compounds InxGa(1−x)As, are perspective materials for practical applications due to their high charge-carrier mobility and the direct-band structure. The progress in device fabrication on their basis is determined by a special technique that provides low density of states (DOS) at insulator/AIIIBV semiconductor interfaces caused by the presence of native oxide on a semiconductor surface.1,2 Controlled growth of fluorine-containing anodic oxide layers (FAOL) in different liquid and gas media is one of the ways to modify a semiconductor surface to create metal−insulator− semiconductor (MIS) structures. For InAs, the method provides low DOS ( 0) are given in blue and green in the upper and bottom panels.

remains unclear how dispersion curves of the conduction band (CB) edge or surface states located in the band gap were shifted. Our calculations demonstrate that, in general, all electron states above the valence band maximum (VBM) are shifted toward higher energies; however, these shifts are quite different for each symmetrical point of the two-dimensional Brillouin zone. In this connection, a band gap increase in bulkprojected states within PAW-GGA calculations can lead to the occurrence of a larger number of electron states near the edge of the fundamental gap. This is seen in the electron energy spectra of the GaAs(111) surface presented in the upper panel of Figure 2c,d. Let us analyze a more detailed surface states structure for oxygen adsorption in the In−B site on the InAs(111)-(1 × 1) surface (Figure 3a). The adsorption-induced surface states are

mainly due to a strong hybridization of oxygen orbitals and those of subsurface arsenic. It is seen that the surface states located lower than the VBM and within the band gap (Figure 3b) at the Γ̅ point of the 2D BZ are mainly localized on arsenic subsurface atoms, whereas at the edge of 2D BZ (at the K̅ point) they are on oxygen. The surface states induced by oxygen hybridization with the cation of the upper layer lie mainly in the band gap near the K̅ point. It should be noted that the size of the filled circles in Figure 3b shows the degree of surface state localization on the atoms. Determining surface atom charge states was carried out using the Bader method.22 In this approach, a space is divided into atomic regions where the dividing surfaces are at a minimum in the charge density, so the Bader volume can be different from a sphere (Figure 3c). The calculations give the charge excess of 17494

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(Figure 4). It is seen that the arsenic surface states localized below the VBM are distributed over whole the 2D BZ (Figure 4a) for InAs(111), but the surface states lie near the band gap edge at the K̅ point and near the CB bottom for the GaAs(111) surface (Figure 4b). In general, the surface states localized in the conduction band lie closer to band gap edges in case of the GaAs(111) surface in comparison with those for InAs(111), but the structure of the surface states spectrum is quite similar for both semiconductor surfaces. Oxygen and Fluorine Coadsorption on the InAs(111)(1 × 1) Surface. Let us discuss the changes in the surface states spectrum of the InAs(111)-(1 × 1) surface due to oxygen and fluorine coadsorption. As was mentioned above, oxygen adsorption leads to the appearance of additional surface states in the semiconductor band gap with the structure strongly depending on the adsorbate geometry (Figure 5a−c), which

+0.24e within the Bader volume around the indium surface atom compared to its value in the bulk (+0.63e). The charge of the subsurface arsenic atom changes more considerably: arsenic accumulates −0.63e in the bulk, but only −0.06e at the surface with oxygen. The charge transfer to the oxygen adatom from the semiconductor surface is ∼1.0e, which is less than required for a complete O p-shell occupation. The distribution of charge density difference (Figure 3d) confirms the picture: the electron accumulation region near the oxygen atom and on the O−As bond is seen, whereas charge depletion occurs mainly near the subsurface arsenic atom. Thus, depletion of the occupied orbitals of subsurface arsenic atoms, which is clearly seen in Figure 3b, is the reason for the appearance of surface states directly in the band gap. A considerable increase of the first interlayer distance up to 1.12 Å at adsorbed oxygen compared to the bulk value of 0.90 Å indicates also the weakening of In−As bonding. The In−As interatomic distance increases from 2.67 Å for the clean InAs(111) surface to 2.79 Å in case of the surface with an adsorbed O atom in the In−B site. A similar picture is observed also on the GaAs(111) surface; therefore, it is not discussed. Fluorine-Induced Changes in the Electron Energy Spectrum of AIIIBV(111)-(1 × 1) Surface. The calculations of fluorine adsorption on the InAs(111)-(1 × 1) unreconstructed surface show that its largest binding energy with the surface corresponds to the In-T site above the surface indium atom (Table 1). The fluorine adsorption in all the considered sites on the surface leads to a smaller increase (less than 2.3%) of the first interlayer distance and In−As bond length compared to the case of oxygen adsorption. The calculations show that the fluorine adatom accumulates a charge of −0.60e within its Bader volume. We are reminded that fluorine needs only one electron for complete occupation of its p-shell. The charge of the indium surface atom increases to +0.97e, whereas the charge of the arsenic subsurface atom (−0.61e) does not differ practically from its bulk value. Since the charge of the arsenic subsurface atom changes less than 0.02e, this fact can explain the absence of the arsenic unoccupied surface states in the band gap in comparison with the previous case. It is well-known that arsenic states primarily dominate in the region below the VBM. It is seen in Figure 4 that the surface states, typical of a clean AIIIBV(111)-(1 × 1) unreconstructed surface, are removed from the band gap by fluorine adsorption at the In(Ga)-T site as at the (2 × 2) reconstruction (Figure 1b, d). Since the surface state in the band gap is primarily the cation state, formed by In(Ga) pz-orbitals (Figure 1a,c), it shifts to the conduction band due to the F adsorption on both semiconductor surfaces

Figure 5. Electron energy spectra of the InAs(111)A-(1 × 1) unreconstructed surface with oxygen adsorbed in As2-T (a), As4-T (b), and In−B (c) sites and those with fluorine coadsorption in the In-T site (d−f).

indicates the local character of the adsorbate−surface interaction. It is demonstrated in Figure 5d,e that fluorine coadsorption on the InAs(111)-(1 × 1) unreconstructed surface decreases the density of surface states in the band gap, just as on the reconstructed InAs(111)-(2 × 2) surface,9 though it does not remove them completely. In general, fluorine slightly shifts oxygen-induced surface states toward the conduction band. A pronounced effect of the fluorine on the surface states, which are even more shifted toward the band gap edges (Figure 6), is also observed with an increase of fluorine concentration to two atoms when oxygen is adsorbed in As2-T and As4-T sites. The surface states localization due to oxygen and fluorine coadsorption is shown in Figure 7. It is seen that the surface states that are shifted to the CB bottom are localized at the In atoms, whereas SSs near the bulk VBM at the Γ̅ point are mainly As states. The surface states localized primarily at the O atom are concentrated along the M̅ −K̅ direction of the 2D BZ. The mixed O and In surface states lie around −2 eV at the K̅ point. Note that the states, localized on the fluorine atom adsorbed in the In-T site, are located considerably lower (by 4.5

Figure 4. Electron energy spectra of InAs(111)-(1 × 1) and GaAs(111)-(1 × 1) surfaces with fluorine absorbed in the In(Ga)-T site obtained by the HSE06 approach. 17495

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shift of the As SS toward the CB is caused by a stronger F−As interaction in the case of GaAs(111) compared to InAs(111). The Changes of the Atomic Structure due to F and O Coadsorption. The atomic structure of the InAs(111)-(1 × 1) surface is considerably changed by the coadsorption of two electronegative adsorbates. It is necessary to note that the bond lengths of oxygen and fluorine with semiconductor atoms for preferential adsorption sites are equal to 2.26 Å (O−In1), 1.82 Å (O−As2), and 2.05 Å (F−In1). Since fluorine adsorbed in the In-T site shifts the indium atom by 0.68 Å toward vacuum, the oxygen atom can penetrate into the subsurface region due to the increase of the first interlayer distance. In this case, the oxygen atom is located 0.45 Å lower than the indium surface atoms, as seen in Figure 7a (upper panel). Upon adsorption of two fluorine atoms, there occurs a further oxygen penetration into the subsurface layers, where it is located 1.85 Å lower than indium surface atoms, whereas fluorine atoms lie practically in one layer with indium. The calculated interatomic distances in the surface layers for equilibrium structures with one−four fluorine atoms are summarized in Table 2. In case of three fluorine atoms per (1 × 1) surface cell, one of them, just as the oxygen atom, penetrates into the subsurface layer (Figure 7b). Thereat, oxygen diffuses deeper into the substrate and is located between arsenic of the second layer and indium of the third layer. It increases the distance between these atoms up to 3.43 Å, which is more than the sum of the covalent radii of semiconductor components. Thus, oxygen, breaking the bonds in the semiconductor near-surface layers, forms new O−As2 and O−In3 bonds. However, the angles between the bonds of the atoms nearest to oxygen remain practically the same, 109.56° (In1−As2−O) and 105.95° (O−In3−As4), as that of In−As−In in the bulk semiconductor (109.47°). Only the As2− O−In3 angle between oxygen bonds with arsenic and indium increases to 125.00°. Besides, there is a considerable increase of the first interlayer distance between indium and arsenic layers to 2.64 Å (0.90 Å in bulk) in the system with adsorbates. The formation of fluorine bonds not only with surface indium but also with subsurface arsenic is observed with an increase of its concentration (Table 2). Both adsorbates in the case of the GaAs(111) surface behave in a similar way (Figure 7c). Thus, the increase of fluorine concentration leads to a considerable change of the structure of the AIIIBV semiconductor subsurface layers, which is connected with the appearance of the fluorine-containing oxide layer forming the FAOL/AIIIBV interface with a low density of surface states.3,4 As follows from the consideration, the atomic structure of the fluorine-containing oxide layer considerably depends on the fluorine concentration. Charge Density Distribution and Local Densities of States. The calculations demonstrate the increase of the positive charge of arsenic (As2) due to oxygen and fluorine coadsorption. We think that these As atoms form the first layer of the so-called fluorine-containing anodic layer at the interface

Figure 6. Electron energy spectra of the InAs(111)A-(1 × 1) unreconstructed surface with oxygen adsorbed in As2-T (a) and As4-T (b) sites and with the two fluorine atoms adsorbed above the indium atom.

Figure 7. Atomic (upper panel) and electronic (bottom panel) structures of the InAs(111)-(1 × 1) surface with oxygen adsorbed in the In−B site and with one (a) and three (b) fluorine atoms and also of GaAs(111)-(1 × 1) with oxygen and three fluorine atoms (c).

eV) than the bulk VBM; therefore, they are not shown in Figure 7a. These surface states are mainly localized at the K̅ point. It is seen that oxygen-induced surface states are practically completely removed by the coadsorption of three fluorine atoms, and as seen in Figure 7b (bottom panel), only the states near the band gap edges remain. These states are mostly localized on arsenic and oxygen atoms, whereas the surface states at the Γ̅ point of 2D BZ, which are also localized on arsenic atoms, shift into the conduction band. The surface states shown below −1 eV are mainly fluorine ones. An almost similar picture is observed for GaAs(111)-(1 × 1). The mixed O and As surface states distributed around the M̅ and K̅ points of 2D BZ are located ∼0.3 eV lower than those for InAs(111)(1 × 1). The surface states localized on F atoms are also shifted deeper into the valence band. It should be noted that a larger

Table 2. Distances (Å) between the Near-Surface Layer Atoms for InAs(111)A with Oxygen and One to Four Fluorine Atoms per Surface cell (1 × 1) O/InAs(111)

F−In1

F−As2

O−As2

O−In3

In1−As2

As2−In3

In3−As4

As4−In5

1F 2F 3F 4F

2.05 2.20−2.28 2.20−2.28 2.19−2.23

3.60 3.02 2.20 1.82−2.23

1.82 1.72 1.72 1.70

3.99 3.58 2.13 2.17

2.70 2.82 2.77 3.70

2.75 2.77 3.43 3.54

2.67 2.67 2.66 2.66

2.71 2.69 2.69 2.71

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which indicates the strong hybridization of its states with both semiconductor components. The appearance of a small oxygen peak at −10.5 eV reflects its interaction with the s-states of arsenic (As2), which has a sharp DOS peak at the same energies. It should be emphasized that the sharp peak of O sstates is not shown in Figure 9a because it is located at −19 eV. The increase of As and In unoccupied electron states indicates a charge transfer from the semiconductor substrate to the electronegative adsorbate that is in consistent with the results of Bader charge analysis. In the case of fluorine adsorption on the InAs(111)-(1 × 1) surface, the changes in the fine DOS structure of substrate atoms are less expressed. The depletion of As and In p-states is also observed, but in the former case, it is more pronounced (Figure 9b). Since fluorine is adsorbed above the indium atom, it interacts with the arsenic atom indirectly through the hybridization of indium and arsenic electron states. The microscopic nature of such DOS changes was discussed in detail in our previous paper,15 where the interaction of semiconductor surface with electronegative halogens was considered. Besides, the fluorine p-band has a smaller width (∼5.0 eV), compared to that of oxygen (∼7.5 eV), and its sharp peak is located at −1.8 eV. The shift of the center of gravity of the F p-band, related to the corresponding one of the O pband, confirms the smaller binding energy of this adsorbate with the semiconductor substrate. Note that the fluorine valence s-band is located ∼1.5 eV deeper than the oxygen sband, and neither of these states of both elements contribute to the interaction with the semiconductor surface. The local DOSs of surface and subsurface atoms for InAs(111)-(1 × 1) with the coadsorption of three fluorine atoms and one oxygen atom are given in Figure 9c. It is seen that the p-bands of both the fluorine surface atom and the atom diffusing deeper in the semiconductor substrate are considerably different from the above considered case. These p-bands lie practically at the same energy region as the electron states of semiconductor atoms. The increase of the fluorine p-bandwidth reflects its stronger hybridization with both indium and arsenic atoms. The fine DOS structure of fluorine surface atom agrees well with that of the In1 atom, whereas the states of the fluorine subsurface atom are shifted toward negative energies as a consequence of its stronger interaction with As2 states (Figure 9c). A small peak of subsurface fluorine DOS is located exactly at the same energy (−10.3 eV), at which there is a sharp peak

with the semiconductor. It should be noted that arsenic interacts with two competing electronegative elements (fluorine and oxygen), which tend to occupy completely their p-shells. As a result, arsenic loses ∼1.8e with an increase of fluorine concentration up to three atoms on the surface unit cell, while cation loses less than ∼1.0e. The charge transfer to O or F atoms equals to 1.02e and 0.65−0.70e, respectively. Figure 8 demonstrates the distribution of charge density differences Δρ(r) in the near-surface layers of InAs (111)-(1 ×

Figure 8. Charge density difference Δρ(r) for the InAs(111) surface with one oxygen atom and three fluorine atoms shown by isosurfaces: (a) electron charge accumulation regions [Δρ(r) < 0] and (b) charge depletion regions [Δρ(r) > 0].

1) with both electronegative adsorbates. It is seen in Figure 8a that the electron charge accumulation takes place around both fluorine atoms of the surface layer and an atom which forms a bond with arsenic of the subsurface layer and also around oxygen. On the contrary, the charge depletion regions are localized around arsenic atoms of the subsurface layer and near the indium atoms of the surface layer (Figure 8b). Such a shape of the charge density distributions reflects the ionic character of the adsorbates bonding with the semiconductor substrate. The calculated partial densities of states of surface atoms of InAs(111)-(1 × 1) with adsorbed O and F atoms in the preferential sites are presented in Figure 9. It is seen that the electron states of both surface indium and subsurface arsenic atoms are considerably shifted compared to the states of the corresponding atoms of deeper layers, which indicates their depletion due to the interaction with electronegative oxygen (Figure 9a). The peaks of s,p-states of near-surface In and As atoms agree well with the corresponding peaks of O p-states,

Figure 9. Partial DOS of near-surface atoms of InAs(111)-(1 × 1) with adsorbed oxygen in the In−B site (a), with fluorine in In-T site (b), and with their coadsorption (c) when the number of fluorine atoms is equal to three atoms per surface cell. 17497

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The Journal of Physical Chemistry C of As2 s-states. In general, a strong overlapping of bands of both F and O adsorbates with those of the semiconductor atoms is observed. It is necessary to point out that the surface states do not occur within the semiconductor band gap, and the presence of electron states at the edge of this gap is conditioned by the smoothing of partial DOS curves. The chemical bonding of both adsorbates with atoms of surface and subsurface layers has mainly ionic character, which is qualitatively confirmed by the trends in the change of the partial DOS and agrees well with the charge calculations by the Bader method. The structure of DOS curves of the In3 and As4 atoms, which is located deeper from the surface (Figure 9c), differs insignificantly from the corresponding DOS of the bulk semiconductor. Increase of Fluorine Concentration. It is necessary to note that additional calculations of AIIIBV(111) surfaces with four and six fluorine atoms adsorbed onto the (1 × 1) surface cell were also performed. It is seen that, in case of four atoms, an additional fluorine atom penetrates into the substrate, forming a bond with both indium and arsenic atoms (Figure 10a). The distances between surface layer atoms are given in

Figure 11. Relative surface energies of fluorine- and oxygen-covered surface InAs(111) as a function of F chemical potential. bulk σNFF(O)/InAs = E NFF(O)/InAs − NInμInAs − NOμO − NFμF

(4)

In eq 4, μbulk InAs is the energy of bulk semiconductor, μO and μF are the chemical potentials of oxygen and fluorine, NO and NF are number of O and F atoms. The upper limit of the chemical potential of fluorine is determined by the F2 molecule, so that μF ≤ 1/2 E(F2). The calculated total energy of fluorine molecule equals 3.31 eV. More details on the calculation of such diagrams can be found in the literature.23,24 It should be noted that the adsorption of one fluorine atom decreases the InAs(111) surface energy by 3.53 J/m2, while the oxygen effect is less pronounced, 2.07 J/m2. These results are given for the most preferential configurations of adsorbed atoms on the surface. It is seen from Figure 11 that an O-covered InAs surface is found to be favored under F-poor conditions. With an increase in fluorine chemical potential, the structure with an adsorbed oxygen atom and three fluorine atoms becomes the most stable. With a further increase in fluorine chemical potential, the most stable structure is one with four fluorine atoms (Figure 10a). The adsorption of six fluorine atoms (Figure 10b) is favorable in a very narrow region under the Frich condition. The change of InAs surface energy is only 0.71 J/m2 for structure with six F atoms compared with the structure with four F atoms, which can reflect the change of the binding mechanism of atoms, which form molecules on the surface, as was mentioned above. In general, these results are in the line with trends discussed earlier. Effect of Adsorbate Concentration Decrease. Finally, several words should be said about the oxygen adsorption on the InAs(111) unreconstructed surface for its submonolayer concentration. The (2 × 2) unreconstructed surface structure was used to decrease the oxygen concentration on the surface. In general, the electron spectrum of the surface with one oxygen atom, 0.25 monolayer (ML), does not considerably differ from the spectrum with the oxygen coverage of 1 ML. In the enlarged cell, fluorine can occupy the sites both above the indium atoms nearest to oxygen and those distant from it. The calculations showed that the fluorine adsorption above the indium atoms interacting with oxygen leads to practically the same changes in the band gap region, as was noticed earlier for the (1 × 1) surface cell, whereas the fluorine adsorption above the indium atoms distant from oxygen does not practically influence the surface states spectrum. As is seen in Figure 12, the electron energy spectra of the InAs(111)-(2 × 2) unreconstructed surface with two fluorine atoms adsorbed above one indium atom nearest to oxygen, and also with those adsorbed above two indium atoms interacting with oxygen, do not practically have surface states in the band gap as well.

Figure 10. Equilibrium atomic structures of the InAs(111)-(1 × 1) surface with oxygen and four (a) and six (b) fluorine atoms per surface cell. (c) The electron energy spectrum for the system in part a.

Table 2. The interatomic distance (In1−As2) increases to 3.70 Å with the fluorine concentration growth, and the interlayer distance between the corresponding atomic layers changes by 0.17 Å, compared to the structure with three fluorine atoms. It is seen in Figure 10b that the further increase of fluorine concentration does not lead to a change of the surface layers structure because additional F atoms do not penetrate into the substrate, but they tend to form a fluorine molecule. The calculated distance between these fluorine atoms equals 1.63 Å, which exceeds the bond length in the F2 molecule only by 0.21 Å. At the same time, in connection with a further depletion of the occupied states near bulk valence band edges due to the interaction of both electronegative adsorbates with semiconductor surface, the appearance of states in the band gap is observed (Figure 10c), which agrees with experiments.4 Thus, the fluorine concentration should be controlled in order to have its positive effect on the density of states in the band gap. In addition, we calculated relative surface energies for InAs(111) with adsorbed oxygen atom as a function of fluorine chemical potential (μF) in order to understand which one of the considered structures is more stable with variation of the concentration of fluorine. The corresponding diagram is presented in Figure 11. The relative surface energy was defined as σRS = (σNFF(O)/InAs − σO/InAs)

(3)

where the surface energy of InAs(111) with adsorbed oxygen and fluorine is calculated using following equation 17498

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approach, remain the same using HSE06, the latter is necessary for a better understanding of the surface states origin near the band gap edges at adsorption. Evidently, the situation at the FAOL/InAs interface is more complicated, and it cannot be completely described by the present calculations. However, the revealed trends are consistent with the experimental findings concerning the effect of fluorine for the composition of anodic oxide layers on the InAs(111)A surface.3,4 In general, the obtained results contribute to a deeper understanding of the electron properties of AIIIBV-based materials and can be used for their prediction.

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Figure 12. Electron energy spectra of the InAs(111)-(2 × 2) unreconstructed surface with the 0.25 ML coverage of oxygen and two configurations of coadsorbed fluorine atoms on the surface: two F atoms adsorbed above one In atom interacting with oxygen (left) and two F atoms adsorbed above both In atoms that interact with oxygen (right).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +7 (3822) 28 69 52. Fax: +7 (3822) 49 15 76.

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Notes

The authors declare no competing financial interest.

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SUMMARY Thus, oxygen and fluorine adsorption and their coadsorption on the (111)-(1 × 1) unreconstructed surface for InAs and GaAs have been studied by the projector augmented-wave method within density functional theory. The indium top site was found to be the most energetically favorable for fluorine adsorption, whereas oxygen prefers to be bonded to the bridge In site. It is shown that the oxygen binding energy on the AIIIBV(111)-(1 × 1) unreconstructed surface is 0.82 eV (InAs) and 0.23 eV (GaAs) higher than that of the fluorine adatom. Besides, the fluorine and oxygen binding energies on the (111)(1 × 1) unreconstructed surface of InAs (GaAs) are 0.91 eV (0.87 eV) and 0.33 eV (0.32 eV) higher than those on the (111)-(2 × 2) reconstructed surface. The fluorine adsorption in the energetically preferable In(Ga)-T site completely removes the surface state, typical of the AIIIBV(111)-(1 × 1) unreconstructed surface that leads to the unpinning of the Fermi level. The oxygen adsorption on the (111)-(1 × 1) surface induces the appearance of additional states in the band gap whose structure depends on the adsorbate site. Similar trends revealed on both semiconductor surfaces indicate a weak influence of the cations chemical composition on the interaction of the semiconductor AIIIBV(111) surface with both electronegative adsorbates. The influence of fluorine and oxygen coadsorption and of their concentration on the surface states in the band gap was studied. It was shown that the oxygen-induced surface states are completely or partly removed from the band gap by fluorine coadsorption if it forms bonds with the indium atoms involved in an interaction with oxygen. The fluorine coadsorption and an increase of its concentration bring about appreciable structural changes in the near-surface layers due to the penetration of both oxygen and fluorine atoms into the substrate. A considerable increase of interatomic distances between substrate atoms leads to a break of In−As bonds and also to the formation of new F−As bonds. The initial stage of the fluorine-containing anodic layer formation is discussed. It is shown that the increase of fluorine concentration at more than three atoms per (1 × 1) surface cell leads to the appearance of additional states in the band gap as a consequence of their depletion near the VBM. The use of the hybrid HSE06 functional increases the semiconductor band gap width, which is very important for narrow band gap semiconductors, such as InAs. Although the trends in the adsorbates’ behavior on the semiconductor surface, revealed within the PAW-GGA

ACKNOWLEDGMENTS The work was carried out supported by RFBR (project no. 1302-98017r_a and no. 14-29-08124), also by the Tomsk State University Competitiveness Improvement Program. Numerical calculations were performed partly on the SKIF-Cyberia supercomputer at the National Research Tomsk State University, also using the resources of the Supercomputing Center of the Lomonosov Moscow State University.25

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