SciELO - Scientific Electronic Library Online

 
vol.29 issue4Ab initio theoretical studies of atomic and electronic structures of III-nitride (110) surfacesVacancy diffusion in silicon: analysis of transition state theory author indexsubject indexarticles search
Home Pagealphabetic serial listing  

Services on Demand

Journal

Article

Indicators

Related links

Share


Brazilian Journal of Physics

Print version ISSN 0103-9733On-line version ISSN 1678-4448

Braz. J. Phys. vol.29 no.4 São Paulo Dec. 1999

https://doi.org/10.1590/S0103-97331999000400045 

Tellurium - Modified Surface States of GaAs(001) and InAs(001)

 

R. Claudino da Silva
Instituto de Ciências, Escola Federal de Engenharia de Itajubá,
C.P. 50, 37500-000 Itajubá-MG

and A. C. Ferraz
Departamento de Física dos Materiais, Instituto de Física,
Universidade de São Paulo, C.P.66318, 05315-970 São Paulo-SP

 

Received February 8, 1999

 

 

We investigate the stability and electronic structure of  1×2 and 2×2 GaAs(001):Te and InAs(001):Te surfaces with different degrees of tellurium coverage (qTe = a4414.gif (106 bytes), a4412.gif (100 bytes), a4434.gif (104 bytes), 1), by means of rst-principles pseudopotential calculations within density-functional theory. The adsorption stability decreases as the tellurium coverage increases. The adsorption on InAs(001) is more stable than on GaAs(001). As-Ga (or As-In) bonds for the uppermost As atoms are s2p2 -like (nearly planar), while the Te bonds at the surfaces are sp3-like. The hetero-dimers in qTe = a4412.gif (100 bytes) modify the character of GaAs(001):Te and InAs(001):Te surfaces resulting in a semiconductor structure.

 

 

I  Introduction

There are observation [1] of the surfactant action of Te in the heteroepitaxial growth of large mismatch systems such as InAs/GaAs by addition of Te over GaAs(001). In ref. [2] the authors propose that the 2D growth is stabilized by half a monolayer of Te atoms that remain on the interface. This model is supported by the experimental indication that only the excess of half a monolayer of Te atoms floats at the InAs surface [3] and by a total energy calculation showing a reduction of the GaAs/InAs interface strain energy by a Te interlayer [4]. Our previous theoretical calculations with adsorbed atoms at bridge positions [5, 6] determined a pattern of atomic relaxation with dimerization in (-110) direction in agreement with experimental results [7], indicating that the GaAs(001):Te-2×2 (qTe = 1) surface has two dimers geometries at the lowest configuration. Also, for the concentration (qTe = a4412.gif (100 bytes)) Te atoms prefers to be adsorbed on the sites off-chain, indicating a monoatomic adsorption. We report an investigation of stability and electronic structure of 1×2 and 2×2 GaAs(001):Te, Ga terminated, and InAs(001):Te, In terminated surfaces, covered by Te and As atoms with concentration qTe = a4414.gif (106 bytes), a4412.gif (100 bytes), a4434.gif (104 bytes) and 1.

 

II  Calculation

Our calculations to simulate the systems are based on density-functional theory within the local-density approximation (DFT-LDA). We consider an artificial periodic slab geometry along the (001) direction. The unit cell includes an atomic slab with six atomic GaAs(001), or InAs (001) layers for Ga (or In) terminated surfaces, covered by Te or As adlayer and a vacuum region equivalent in thickness to a six layers. The dangling bonds on the back-side of the slab (As surface) were satured H atoms. The tree uppermost layers and surface are allowed to relax. The eletric field resulting from the inequivalence of the two surfaces is corrected by a dipole calculated self-consistently [8]. The electron-ion interaction is treated by using fully separable, norm-conserving pseudopotentials [9]. For the electron-electron interaction we employ the exchange correlation potential of Ceperlay-Alder [10] as parameterized by Perdew and Zunger [11]. Single particle orbitals are expanded into plane waves up to a cut-off energy of 12 Ry. kvec.gif (103 bytes)-space integrations are replaced by a sum over eight (1×2 cells) or four (2×2 cells) special points in the irreducible part of the surface Brillouin zone. The minimum of the total-energy functional with respect to both the electronic and atomic degrees of freedom is found by means of a molecular-dynamical approach [12].

 

III  Results and discussion

The stability of the Te and As adsorption on GaAs(001) e InAs(001) surfaces are investigated for 1×2 and 2×2 cells. To analyze the stability we consider the formation energies (grand canonical potentials) defined as [13]:

where Etot is the total energy of relaxed structure, EGa(In) is the total energy for full relaxed clean surface GaAs(InAs), nTe and nAs are the number of adsorbed tellurium and arsenic atoms at the surface, and mTe and mAs the chemical potentials. This a relative formation energy and we can take it as a function of the tellurium chemical potential, fixing mAs from the As bulk value. The maximum value for mTe corresponds to the Te bulk elemental phase and the lower limit was taken as the chemical potencial of Te atoms adsorbed on GaAs(001) or InAs(001) surface, ugate.gif (261 bytes):

Our calculated value for the chemical potencial of Te atoms adsorbed on the surfaces, relative to utebulk.gif (165 bytes), is -1.40 eV/atom for GaAs(001), and -1.49 eV/atom for the InAs(001). These values define the range for the different coverages at the surfaces. Fig. 1 shows the formation energies with qTe = a4414.gif (106 bytes), a4412.gif (100 bytes), a4434.gif (104 bytes)  and 1 for (a)GaAs(001):Te and (b)InAs(001):Te. The formation energies in units eV/1x1 surface, for the adsorptions on GaAs(001) are Eform = -3.75 - mTe for qTe = a4414.gif (106 bytes), Eform = -2.34 - mTe for qTe = a4412.gif (100 bytes), Eform = -1.72 - mTe for qTe = a4434.gif (104 bytes) and Eform = -1.40 - mTe for a full coverage of Te (qTe = 1). We also have considered three possible geometries for the qTe = a4412.gif (100 bytes) in a 2×2 cell with Te-Te, Te-As and As-As dimers at the surface. As their surface energies differ by less than 0.05 eV/cell, the 1×2 reconstruction with hetero dimers at the surface should be the more stable structure, in agreement with STM observations [7]. The Te fully coverage has the same formation energy for 1×2 and 2×2 reconstructions. For adsorption on InAs(001) surfaces, we have obtained the formation energies in units eV/1x1 surface as: Eform = -4.16 - mTe for qTe = a4414.gif (106 bytes), Eform = -2.56 - mTe for qTe = a4412.gif (100 bytes), Eform = -1.86 - mTe for qTe = a4434.gif (104 bytes) and Eform = -1.49 - mTe for a Te fully coverage (qTe = 1). Also here we have the same formation energy for 1×2 or 2×2 systems. As found for GaAs, the three geometries with qTe = a4412.gif (100 bytes) present the more stable structure with a 1×2 reconstruction. In Fig. 1, we can observe that the stability decrease as the tellurium concentration increase at the coverage, and the adsorption on InAs(001) (Fig. 1b) is more stable than on GaAs(001) (Fig. 1a).

 

a44f1.gif (12838 bytes)

Figure 1. Formation energies for (a) GaAs(001):Te; and (b) InAs(001):Te.

 

We verified that As-Ga (or As-In) bonds for the uppermost As atoms are s2p2-like (nearly planar) while the Te-Ga (or Te-In) at the surfaces are sp3-like for every coverage and reconstruction. The surface band structure presents a semi-metalic character for every coverages with qTe ¹ a4412.gif (100 bytes), at the GaAs(001) or InAs(001). For the qTe = a4412.gif (100 bytes) concentration we verified a 1×2 reconstruction, with the hetero dimers modifying the structure to a semiconductor character.

In Fig. 2 we present the electronic band structure of the 1×2 semiconductor surface: (a)GaAs(001):Te and (b)InAs(001):Te. These systems have five surface states at the hetero dimers. The surface states charge densities are plotted in Fig. 3 for GaAs that have the same behavior for InAs. The first unoccupied state C1 (Fig. 2 and 3-a) is about 0.6 eV below the conduction band along the ktraco.gif (106 bytes) - jtraco.gif (98 bytes)¢ - p824.gif (100 bytes) - ktraco.gif (106 bytes) directions for GaAs and 0.8eV for InAs. This state are s* like. The V1 and V2 surface states (Fig. 2 and 3-b, 3-c) are antibonding p* - combinations of pz orbitals localized at As and Te surface atoms. The V2 state is well localized around the ktraco.gif (106 bytes) point and 0.44 eV above the valence band for GaAs and 0.4 eV for InAs. For GaAs(001):Te, V1 has a dispersion of 0.82 eV along jtraco.gif (98 bytes) - ktraco.gif (106 bytes) - jtraco.gif (98 bytes)¢ - p824.gif (100 bytes), while InAs(001):Te has a dispersion of 0.71 eV at the same directions. The surface state V3 (Fig. 2 and 3-d) corresponds to the interaction between the second layer, the surface, and the third layer atoms. At the internal gap, we verify the surface state V4 with s-state combinations localized at the As surface atoms. Finally, the V5 state at the botton of the valence band are s - like, with combinations of Te and As s-orbitals at the surface. This V5 state is well localized around the ktraco.gif (106 bytes) point at the surface Brillouin zone for GaAs and spread along jtraco.gif (98 bytes) - ktraco.gif (106 bytes) - jtraco.gif (98 bytes)¢ for InAs with a maximum dispersion of 0.6eV.

 

Figure 2. Electronic band structure for (a) GaAs(001):Te-(1×2); and (b) InAs(001):Te-(1 ×2) with qTe = a4412.gif (100 bytes).

 

 

Figure 3. Energy-resolved charge density contour plots for the GaAs(001):Te-(1×2) with qTe = a4412.gif (100 bytes).

 

IV  Conclusion

We observed that the adsorption stability decreases as the tellurium coverage increases. The formation energies indicate that the tellurium adsorption on InAs(001) is more stable than on GaAs(001). The uppermost As atoms have s2p2-like bonds while the Te at the surfaces have sp3-like bonds. The surface states at the fundamental gap are antibonding p*-combinations of pz orbitals localized at the surface atoms (Te or As) and p-bonding combinations of As-As, Te-Te dimers and As-Te hetero-dimers. The GaAs(001):Te and InAs(001):Te surfaces with qTe = a4412.gif (100 bytes) exibit a s-type surface state below the valence band localized at the As-Te hetero-dimer. The hetero-dimers in qTe = a4412.gif (100 bytes)  modify the character of GaAs(001):Te and InAs(001):Te surfaces resulting in a semiconductor structure.

 

Acknowledgments

The authors would like to thank CENAPAD-SP and LCCA-USP for computational support which enable this work.

 

References

[1] N. Grandjean, J. Massies and V. H. Etgens, Phys. Rev. Lett. 64, 799 (1992).         [ Links ]

[2] N. Grandjean and J. Massies, Phys. Rev. B 53, 13231 (1996).         [ Links ]

[3] W. N. Rodrigues, V. H. Etgens, M. Sauvage-Simkin, G. Rossi, F. Sirotti, R. Pinchaux and F. Rochet, Sol. State. Comm. 95, 873 (1995).         [ Links ]

[4] R. H. Miwa, A. C. Ferraz, W. N. Rodrigues and H. Chacham, Surf. Sci. 415, 20 (1998).         [ Links ]

[5] A. C. Ferraz and R. Claudino da Silva, Surf. Sci. 352-354, 379 (1996).         [ Links ]

[6] R. Claudino da Silva and A. C. Ferraz, Braz. J. Phys. 27/A, 234 (1997).         [ Links ]

[7] D. K. Biegelsen, R. D. Bringans, J. E. Northrup and L. E. Swartz, Phys. Rev. B 41, 5701 (1990).         [ Links ]

[8] J. Neugebauer and M. Scheffler, Phys. Rev. B46, 16067 (1992).         [ Links ]

[9] X. Gonze, R. Stumpf and M. Scheffler, Phys. Rev. B 44, 8503 (1991).         [ Links ]

[10] P. M. Ceperlay and B. J. Alder, Phys. Rev. Lett. 45, 566 (1980).         [ Links ]

[11] J. P. Perdew and A. Zunger, Phys. Rev. B 23, 5048 (1981).         [ Links ]

[12] R. Carr and M. Parrinello, Phys. Rev. Lett. 55, 2471 (1985).         [ Links ]

[13] A. Kley and J. Neugbauer, Phys. Rev. B 50, 8616 (1994).         [ Links ]

Creative Commons License All the contents of this journal, except where otherwise noted, is licensed under a Creative Commons Attribution License