First-principles study of GaAs Ã4 surface oxidation and passivation with H, Cl, S, F, and GaO

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1 JOURNAL OF APPLIED PHYSICS 107, First-principles study of GaAs Ã4 surface oxidation and passivation with H, Cl, S, F, and GaO Weichao Wang, 1 Geunsik Lee, 2 Min Huang, 2 Robert M. Wallace, 1,2 and Kyeongjae Cho 1,2,a 1 Department of Materials Science and EngineeringThe University of Texas at Dallas, Richardson, Texas 75080, USA 2 Department of Physics, The University of Texas at Dallas, Richardson, Texas 75080, USA Received 7 December 2009; accepted 23 February 2010; published online 26 May 2010 The interactions of oxygen atoms on the GaAs surface and the passivation of oxidized GaAs surface were studied by density functional theory. The results indicate that oxygen atoms adsorbed at back-bond sites satisfy the bond saturation conditions and do not induce surface gap states. However, due to the oxygen replacement of an As dimer atom at a trough site or row site, the As As bond is broken, and gap states are produced leading to the Fermi level pinning because of unsaturated As dangling bonds. Atomic H, Cl, S, F, and the molecular species GaO were examined to passivate the unsaturated As dangling bond. The results show that H, Cl, F, and GaO can remove such gap states. It is also found that the interaction of S with the unsaturated As dangling bond does not remove the gap states, and new gap states are generated upon single S adsorption. A higher S coverage forms S S dimer pairs which passivate two unsaturated As atoms, and removes the As-induced gap states American Institute of Physics. doi: / I. INTRODUCTION Metal-oxide-semiconductor field effect transistors MOSFETs with high mobility channel materials are candidates for advanced complementary metal-oxidesemiconductor CMOS device structures because it is much easier to enhance device performance through device scaling compared with traditional Si CMOS. Gallium arsenide, with five times higher electron mobility compared to silicon, has a potential to achieve a III-V-based channel device. However, it is difficult to achieve a practical enhancement-mode GaAsbased MOSFET due to the poor interface quality between GaAs and gate dielectric films. 1 GaAs-based interface is much more complicated than silicon based interface. For the silicon based interface, Si interacts with oxygen interfacial atoms by forming Si O or O Si O bonds. Each Si atom contributes one electron for each silicon oxygen bond. Therefore, interfacial Si dangling bonds could be fully saturated or be fully empty easily by transferring integer number of electrons. However, in the GaAs-based interface, the interfacial Ga- or As- dangling bonds are partially saturated 2 which could hardly be fully saturated to obtain the ideal high-k/gaas interface. Furthermore, the partially saturated bonds could induce midgap states which leads to Fermi level pinning. This pinning is most likely due to the presence of As O and Ga O bonds since clean GaAs surface does not pin Fermi level. Therefore, understanding the oxidation and passivation of GaAs surface are very helpful to gain an important insight for high-k/gaas interface study. GaAs 001 surface is the most intensively studied system among III-V materials. 3 5 It consists of alternating planes of Ga and As that are separated by 1.41 Å. 5 Both a Electronic mail: kjcho@utdallas.edu. Ga-terminated and As-terminated GaAs 001 surfaces were observed to reconstruct forming As As dimers or Ga Ga dimers on the surface. 6 The oxidation and passivation of a GaAs 001 surface are important issues of GaAs-based MOSFETS, thus of great interest to researchers. Previous work 7 9 studied diverse surface oxidation models, including SiO adsorption and the replacement of As dimer atoms by two oxygen atoms on GaAs 001 surface. Their results showed that a possible mechanism of Fermi level pinning is not due to the intrinsic properties of GaAs 001 surface, but due to the specific bonding geometries resulting from the oxidation. Recently, based on the scanning tunneling spectroscopy and density functional theory DFT studies for different adsorbates bonding to GaAs 001 surface, Winn et al. 10 proposed that the Fermi level pinning mechanisms could be identified to be direct or indirect. The direct Fermi level pinning is due to the states in the band gap region directly induced by the adsorbate. On the other hand, the indirect Fermi level pinning is due to the states in the band gap region induced by the secondary effects, such as the generation of undimerized As atoms. Earlier studies 11 assumed that the unpinning of surface states for GaAs 001 surfaces in air is because of the formation of excess As due to the oxidation of GaAs. It is known that the oxidation of GaAs results in the formation of excess As, and oxides such as Ga 2 O 3 and As 2 O 3 on the surface. The As oxide and As could be photochemically removed from the surface since they are highly soluble in oxygenated water. The remaining As-free surface is passivated by the Ga oxide layer resulting in the unpinning surface states of GaAs surface. How excess As and As oxide induce surface gap states is still inaccessible at atomic understanding. Therefore, clear understanding of /2010/ /103720/10/$ , American Institute of Physics

2 Wang et al. J. Appl. Phys. 107, the mechanisms of Fermi level pinning is needed and would be helpful to create unpinned surfaces using passivation methods. To passivate GaAs surface and remove the surface states, several species such as sulfide and gallium oxide were applied to oxidized GaAs 001 surfaces. Traub et al. 12 used chlorine to passivate the oxidized surface, and the x-ray photoelectron spectroscopy revealed that GaAs 001 surface with Cl could be effectively passivated with wet chemical methods. Winn et al. 9 proposed that H could be used to passivate the undimerized As atoms induced by the adsorbates. With H passivation, the As-induced gap states were suppressed into the valence band edge region, yielding a clean unpinned surface. S interaction with GaAs 001 surface had been studied by Szucs et al. 13 with several model geometries of the GaAs 001 surface by DFT. In Ga-rich GaAs surface, the top layer formed a Ga S like monolayer, and under certain conditions the S passivated surface can also reconstruct forming a S S dimer pair. Guo-Ping and Ruda 14 also studied S passivation of GaAs 100 surfaces through ab initio molecular-orbital calculations. Their results showed that S pasivation result in the opening of surface Ga dimmers, which in turn, lowers raises the highest occupied lowest unoccupied surface states. In addition to the use of extrinsic elements to passivate GaAs 001 surface, native oxides were also studied. The origin of Ga 2 O 3 passivation mechanisms for reconstructed GaAs 001 surface was investigated by using Ga 7 As 7 O 2 H 20 cluster model. 15 The simulation showed that the reduction in the density of surface states located within the bulk energy gap derives from initial near-bridgebonded O atoms. However, this cluster models could overestimate energy gap due to the strong quantum size effect of GaAs material. Although several diverse experimental and theoretical works were done to investigate the oxidation and passivation mechanisms for GaAs 001 surface, the systematic theoretical atomic level interpretation of favored oxidation and passivation structures and properties could lead us a better and complete understanding of the mechanisms thus very useful for GaAs-based MOSFETs devices In this paper, we present systematical computational studies based on DFT to investigate the oxidation and passivation of GaAs 001 surface. The most favored geometries of oxidation and passivation were predicted, and the Fermi level pinning mechanisms were studied for GaAs 001 surface. Based on the understanding of Fermi level pinning mechanism, a series of candidates including H, Cl, F, S, and GaO are studied for the passivation of oxidized GaAs surface from the theoretical perspective. II. MODELS AND COMPUTATIOAL METHOD The present calculations are based on the DFT with the generalized gradient approximation PW91 scheme, and have used a plane wave basis E cutoff =400 ev and pseudopotentials implemented in the VASP code The pseudopotential we have used is a type of projector augmented wave PAW Refs. 19 and 20 and 4s and 4p orbitals are treated as valence shells for Ga and As. For the test of applicability and accuracy of the PAW potentials, we studied the structural properties of bulk GaAs and compared the obtained results with available experimental values. The calculated lattice constant of bulk GaAs is Å compared to the experimental value of Å and the compressibility modulus is 65.0 GPa compared to experimental value of 75.3 GPa. 21 The clean GaAs 001 surface was modeled using eightlayer and twelve-layer slabs with the bottom surface passivated with pseudohydrogen with 1.25 valence electrons to mimic bulk As bonding. To rigorously keep GaAs bulk behavior and avoid exchange-correlation induced numerical errors, we used GaAs experimental lattice parameter of Å and a vacuum thickness of 10 Å separating the slabs for the GaAs 001 surfaces. Our calculated results for surface reconstruction and stability are in good agreement with the results obtained from theoretical equilibrium lattice constant in Refs In this specific surface calculation with 2 4 surface unit cell, a mesh size of was used for the k-point sampling. Several GaAs 001 reconstructed surfaces, i.e., 2 4, 4 2, 2 4, 4 2, 2 4, 4 2, 2 2 4, 2 4 2, 2 4, and 4 2 were studied in the present work. The 2 4 reconstruction 4 2 reconstruction corresponds to four As Ga dimer pairs at the top of GaAs 001 surface in As Ga rich condition. The represents two As Ga dimers missing compared to reconstruction in As Ga rich condition. The has one As Ga dimer missed at the top surface compared to The misses two As Ga dimers at the top surface and one Ga As dimmer in the second top layer compared to the To form , one of As dimer leaves its original position and relocates at the top two As Ga dimer pairs center. And the new As Ga dimer pair is perpendicular to the rest of three As Ga dimers however parallel to the GaAs 001 surface. To find the relative stabilities among these different reconstructed surface structures, formation energies per unit area were calculated by using E F tot tot =1/A U slab N Ga Ga N As As, where U slab is the total energy of the slab, N i is the number of atoms of type i, A is the surface area, and i is the corresponding chemical potential for species i Ga or As in the slab. The Ga and As chemical potentials are not independent from each other but are constrained by Ga + As = GaAs. Here GaAs is the GaAs chemical potential. The As chemical potential is bounded by GaAs As 0. In the Ga rich conditions, the Ga chemical potential keeps the same value as in bulk Ga which is zero, and the As chemical simply equals to GaAs chemical potential. As a result, E F only depends on one variable, i.e., As, which varies from 0.78 ev to 0. In As rich limit, E F only depends on Ga. Figure 1 shows the formation energies of different GaAs 001 surface reconstruction using eight GaAs atomic layer models versus As chemical potentials. It is clear that GaAs is the most stable surface over a large arsenic chemical potential range which is consistent with other reports. 22 It is important to note that there is a strong quantum confinement effect for a finite GaAs slab thickness used in the surface modeling study. To examine this specific effect,

3 Wang et al. J. Appl. Phys. 107, FIG. 1. Formation energies of different GaAs 001 surface reconstructions eight GaAs atomic layers vs As chemical potentials. Maximum As chemical potential corresponds to As rich conditions, on the contrary, minimum As chemical indicates Ga rich conditions. GaAs slabs with different layers 4 to 40 atomic layers with both sides passivated with pseudohydrogens to remove surface states were tested for the energy gap changes. Figure 2 shows that the GaAs :H surface energy gap decreases as the slab thickness e.g., number of atomic layers increases. In Fig. 2, the green filled squares represent the calculated energy gaps, and the blue curve is the fitted equation based on quantum confinement effect, i.e., 1/t 2. The red dash line is the calculated GaAs bulk energy gap 0.68 ev which is smaller than the experimental band gap of 1.42 ev due to the well-known DFT band gap underestimation. From the diagram, strong quantum confinement can be seen for thin slabs with less than 20 atomic layers t=2.8 nm. The energy gaps decreased from 2.6 ev to 0.68 as atomic layers increasing from 4 to 40 layers. Furthermore, the electronic structures indicated that 12-layer slab corresponding to E g =1.20 ev was reasonably thick enough to avoid the strong quantum confinement effect. One should notice that there is a diverging region between the calculated energy gap values and the inverse square fit. From the E-k relationship, i.e., E = k 2 /2m, energy gap values depend on the effective mass of conducting and valence band edges as well as the size of the system. However, the fit equation only considers one variable, i.e., size of the system, which induces the derivation between the calculated gap values from the fit data. Moreover, one should be careful in the interpretation of the near edge states close to the valance and conduction band edges. For the present work, the DFT calculation predicts an FIG. 3. Top a and side b view of GaAs surface. Large small filled circles indicate top- third- layer As atoms, whereas large small empty circles represent second- fourth- layer Ga atoms. energy gap of 0.68 ev for the bulk GaAs, which underestimates the experimental value of 1.42 ev by 0.74 ev. However, the existence of midgap states does not change significantly relative to the band edges when the band gap underestimation is corrected by the GW approximation. 26 Therefore, the underestimation of band gap would not change the overall analysis on the Fermi energy pinning mechanisms study. III. RESULTS AND DISCUSSION A. GaAs Ã4 clean surface The GaAs surface was first proposed by Chadi 23 in 1987, and is considered as the standard model for 2 4 reconstruction. Previous experimental and theoretical work 24,25 revealed that the GaAs 001 phase, which is stable over a broad chemical potential range, has a structure containing two As dimers at the top-most surface layer and a third dimer located two layers below shown in Fig. 3. In this model, the surface is terminated with As atoms, and dangling bonds are easy to buckle together to form the As dimers. In Fig. 4 we show the projected band structure of GaAs bulk presented with dots, together with the bound surface states for GaAs in the energy region of the fundamental gap. In the present work, we found that the FIG. 2. Color online GaAs surface energy gaps vs atomic layers used in surface slab. FIG. 4. Color online Left panel represents band structure bound states for GaAs surface plotted over the projected bulk band structure dot regions. Right panel is the corresponding total density of states of the clean surface.

4 Wang et al. J. Appl. Phys. 107, dotted bulk regions in Fig. 4 are contributed by two bottom Ga and As layers which show GaAs bulk behaviors. For the clean GaAs surface, the gap is essentially free of surface states. Five valence bands labeled as V1-V5 in Fig. 4 appear above the bulk band edge at K point in the present work rather than four valence bands in Schmidt s work. 27 Slightly above the bulk valence band edge at K point, we found the two highest occupied states V1 and V2 lie at 0.41 ev and 0.32 ev, respectively. They correspond to the combinations of antibonding and p z orbitals of As dimers located at the third-layer V1 and top-layer V2 as shown in Fig. 5. The bonding of As dimers at the thirdlayer give rise to V3 and V4. V5 is composed of the combinations of bonding and p orbitals for Ga and top As atoms located at second top layer center. Compared to other GaAs 001 surface reconstructions, we found that the states localized at the top-layer dimers show nearly identical charge distribution due to the symmetry of surface geometry. The lowest unoccupied state C1 is a combination of antibonding and in-plane p orbitals of the top-layer As dimers see Fig. 5. C2 see Fig. 5 is related to the threefold-coordinated Ga atoms located at the second-layer. This state is almost entirely localized at the Ga atoms on one side of the dimer block close to the third-layer As dimer. Based on the analysis, we found that all the As dangling bonds are fully saturated and contribute to valence band edge states. Moreover, four completely empty Ga dangling bonds threefoldcoordinated second-layer Ga atoms contribute conduction band edge states. From the charge transfer point of view, the threefoldcoordinated Ga atoms transfer total three electrons to six As dimer atoms so that all the As dangling bonds are fully saturated, thus opening a surface gap shown in Fig. 4. However, when a high-k oxide material grows on this specific reconstruction structure, Ga O and As O bonds could be formed. As a result, three specific As dimmer pairs are broken so that this specific surface electron distribution balance is destroyed; surface electronic behavior should change accordingly. Since As O and Ga O bonds are partially saturated due to charge mismatch, the gap states induced by these partially saturated bonds could generate Fermi level pinning. B. GaAs Ã4 surface oxidation: Atomic structures and electronic structures In reality, the modeling of an oxidized GaAs surface is an important and difficult issue due to the complexity of possible oxidation structures. In the following, oxygen atoms interacting with GaAs surface will be systematically studied for this purpose. FIG. 5. Contour plots of the charge distribution at K point for surface localized states of the GaAs surface. The contour spacing is eå 3. All plots are drawn parallel to the surface normal. C2 and V5 are plotted along a plane parallel to the 2 4 direction cutting through the bonds between first- and second-layer anions and through dimer 3 see Fig. 3. C1, V2 are localized at both dimers 1 and 2 and both dimers have nearly identical charge distributions. V1 and V3 are localized at dimer Atomic structures for GaAs surface oxidation For a single oxygen atom interaction with the surface, we consider two different oxide structures, viz. adsorption and replacement. For computational efficiency, slabs with only eight atom layers passivated by one pseudohydrogen atom layer were considered for the oxidation discussion. However, for the passivation work, more accurate electronic structures are required so that slabs with 12 atomic layers were used in the passviation part. Experimental work 28 showed the presence of native oxides including Ga 2 O, Ga 2 O 3,As 2 O 3, etc. in GaAs surface. These native oxides are critical factors influencing the quality of GaAs surface or interface. However, the Ga O and As O bonds essentially originate from one or two oxygen atoms interacting with GaAs pure surface or their combination. Therefore, the interactions between one and two oxygen atoms and GaAs surface would be symmetrically studied in the following. To examine general stabilities of different oxidized surfaces, formation energy versus arsenic chemical potential was calculated. Similar to the study of different reconstructed GaAs 001 surface stabilities in Fig. 1, formation tot energies are defined as: E F =1/A U slab N Ga Ga N As As tot N O O, where U slab is the total energy of the slab, N i is the number of atoms of type i, A is the surface area, and i is the corresponding chemical potential for species i Ga or As in the slab. O chemical potential is constrained by O 2 gas without considering native oxides, i.e., Ga 2 O 3, As 2 O 3, and As 2 O 5. This is because the initial GaAs surface is a clean one rather than an oxidized one. So it is reasonable to consider the ambient O is under rich condition. Consequently, As chemical potential is the only one variable which varies from 0.78 ev to 0, and the negative slope reflects the As richness. For one oxygen adsorption cases shown in the first row of the Fig. 6, oxygen adsorbed on the bridge site of two Ga atoms Fig. 6 1a shows its instability compared to the other three structures Figs. 6 1b 6 1d from the formation energy shown in Fig. 7. Oxygen adsorption on the back-bond site shown in Fig. 6 1b exhibits a high stability based on its

5 Wang et al. J. Appl. Phys. 107, FIG. 6. Color online Side view of one oxygen atom adsorption 1a 1d in the first row and replacement 2a 2d in the second row on GaAs surface. 3a 3d indicate the two oxygen atom adsorption 3a and replacement 3b, 3c, and 3d on GaAs surface. Big black white balls indicate surface As Ga atoms and small black white balls represent As Ga atoms in the sublayer. O is specified in each figure. low formation energy. The trough site As dimer Fig. 1 d site is slightly more stable than the Fig. 6 1c structure. In the second row of Fig. 6, we show four possible configurations for the replacement of one surface atom by oxygen atom. In Fig. 6 2a, one of the second top layer Ga is substituted by one oxygen atom forming two As O bonds. From the high formation energy shown in Fig. 7, we found this is not an energetically favorable structure. In Fig. 6 2b, oxygen replaces one of the row site As dimer atoms and forms three bonds, i.e., two Ga O bonds and one O As dimer bond. In Fig. 6 2c, row site As atom is replaced by one oxygen atom, but the O atom only forms two Ga O bonds compared to three bonds in Fig. 6 2b. The total energies of structure in Figs. 6 2b 6 2d can be directly compared since they have the same number of Ga, As, and O atoms. Our calculation exhibits the energy difference of 0.27 ev between Figs. 6 2b and 6 2c, and the structure in Figs. 6 2d is 1.43 ev and 1.16 ev more stable than structure in Figs. 6 2b and 6 2c, respectively. Among the four possible replacement sites, the structure in Fig. 6 2d has the lowest formation energy. In this case, one of trench site As dimer atoms was replaced by one oxygen atom. Compared to the first and second row oxidized surfaces, it is easy to find that one oxygen tends to replace surface As atoms rather than Ga atoms. For two oxygen atoms interacting with the surface shown in third row of Fig. 6, a similar study was done to determine the possible stable structures. Two oxygen atoms prefer to stay in the two back-bond sites see Fig. 6 3a. We found that in two oxygen replacement cases, the formation energy is lower than that of Fig. 6 3a, and this lower energy indicates that two oxygen atoms prefer to replace surface As atoms rather than adsorb on the surface. This finding is similar to the case of one oxygen atom interacting with the specific surface. In the case of replacement, two oxygen atoms prefer to replace the trench sites As dimer atoms and form two Ga O Ga bonds Fig. 6 3c. Nevertheless, the structure with O row shown in Fig. 6 3b indicates the O 2 adsorption site according to the scanning tunneling microscope STM study by Hale et al., 7 so it was studied to verify the Fermi energy pinning mechanism, which is shown in Sec. III B 2. As shown in Fig. 6 3b, two As dimer atoms are replaced by two oxygen atoms and two dangling bonds associated with FIG. 7. Different oxidized surface formation energies vs As chemical potentials. Maximum As chemical potential corresponds to As rich conditions, on the contrary, minimum As chemical indicates Ga rich conditions. The notation of each line corresponds to that in Fig. 6, respectively.

6 Wang et al. J. Appl. Phys. 107, the top As atoms were kept. Other possible structures for two O atoms replacing surface As atoms, such as one trench site As dimer atom and one row site As dimer atom replaced by O atoms or structure shown in Fig. 6 3d, were also studied. The energies of these structures are only less than 5 mev lower than that of the structure in Fig. 6 3b. Therefore, our study focuses on Fig. 6 3b. From these data we can make a general statement that clean surface is easy to be oxidized since most of oxidized surface formation energies are lower than clean surface. In addition, replacement cases always show higher stabilities than adsorption cases. It is also found that the more oxygen atoms interacting with surface, the more possibilities to be stabilized on the surface. 2. Electronic structures for GaAs surface oxidation To further analyze the surface properties and possible Fermi level pinning mechanisms of GaAs 001 surfaces, the electronic structures of oxidized GaAs 001 surfaces were investigated. For the electronic structure of an oxygen atom adsorption on the back-bond site on GaAs surface shown in Fig. 6 1b, the corresponding band structure Fig. 8 a indicates no defect states in the band gap region. In the case of charge distribution, the specific back-bonds Ga O As are saturated electrons and 1.25 electrons transfer from adjacent Ga atom and As atoms to this oxygen, respectively. At the K point of this specific oxidized surface Brillouin zone, surface bands are contributed by the empty bonds of four three-coordinated Ga atoms at second-layer. The oxygen replacement of an As dimer atom in the trough site shown in Fig. 6 2d behaves differently from the oxygen adsorption on the back-bond site of Fig. 6 1b. There is one band crossing the Fermi level which leads to Fermi energy pinning according to Tersoff s pinning model. 29 This specific band contributes one peak in the total density of states gap region shown in Fig. 8 b right panel. Charge density analysis Fig. 9 a on the substitution case shows that this specific band corresponds to the As half-saturated dangling bond in the remaining undimerized As atom. And this specific As atom p orbital mainly contributes the gap states as shown from further local density of states study. For the two oxygen atoms interacting with the GaAs surface, two kinds of oxidized surfaces, viz. adsorption and replacement, were also studied. Figure 6 3a shows that the two oxygen atoms adsorb on the back-bonds sites forming one Ga O bond and one As O bond. The corresponding band structure shown in Fig. 8 c indicates no gap states like one O atom adsorption case and 0.07 ev above the top of valance band of the clean surface at K point, there are two valence bands V1 and V2 shown in Fig. 8 c. These two bands are mainly contributed by two As O bonds similar to one oxygen adsorption at the back-bonds sites. V1 indicates the combinations of antibonding and p orbitals of top As dimmer atoms, and V2 is related to the oxygen contribution. The corresponding charge plot is shown in Fig. 9 b. Two configurations for the replacement type were studied as shown in Figs. 6 3b and 6 3c. The more stable surface Fig. 6 c exhibits no defect FIG. 8. Color online a, b, c, and d left panels represent band structures for one oxygen adsorption, one oxygen replacement, two oxygen adsorption, and two oxygen replacement shown as Fig. 6 1b, Fig. 6 2c, Fig. 6 3a, and Fig. 6 3c, respectively. The right panel indicates the corresponding oxidized total density of states. The dot region indicates the projected GaAs bulk bands. level shown in Fig. 8 c in the gap region since two As atoms of the As dimer were replaced by two oxygen atoms and formed two Ga O bonds without any partially occupied As dangling bonds. The structure in Fig. 6 3b shows three bands in the gap region which is 0.750, 0.82, and 1.57 ev above the top of valance band clean surface band edge at K point. These three bands correspond to one main peak and several shoulder peaks in the total density of state gap region Fig. 8 d right panel. Figure 9 c reveals that C1 and C2 are combinations of antibonding and p orbitals of O As bonds. In case of C3, combination of p orbitals of Ga and O gives rise to C3. In the case of the specific oxidized surface shown in Fig. 6 3b, our result is qualitatively different from the work

7 Wang et al. J. Appl. Phys. 107, reported by Hale et al. 7 in In that work, it was argued that the Fermi energy pining was due to the Ga atom which was in the second top layer and bonded to two oxygen atoms. Whereas, more recent and refined work by Winn et al. 10 in 2007 revealed that the Fermi energy pinning is due to the undimerized As atom rather than the specific Ga atom, which agrees well with the results reported here. The atomic and electronic structures for multiple oxygen atoms, i.e., three, four, five, and six oxygen atoms, interactions with GaAs surface were also investigated in the present work. Multiple oxygen atoms oxidized surfaces are found to be the combination of one and two oxidized surfaces. The present results again reveal that surface states are from unsaturated surface As atoms rather than other mechanisms. FIG. 9. Color online a, b, and c present contour plots of the charge distribution at K point for surface localized states of the oxidized GaAs surface. The contour spacing is eå 3.All plots are drawn parallel to the surface normal. Red balls and black balls indicate oxygen and Arsenic atoms. a is the charge plot along O and the undimerzied As atoms. b is the charge plot of two oxygen adsorption on the back-bonds sites shown in Figs. 6 a. c corresponds to charge plot of two oxygen replacement of top As dimer atoms shown in Figs. 6 b. C. Atomic structures and electronic structures of passivation of oxidized GaAs Ã4 surface 1. Atomic structures of passivation of oxidized GaAs surface In this section, we studied the passivation of oxidized GaAs by applying several candidates including H, Cl, F, S, and GaO to the oxidized surfaces shown in Figs. 6 2c and 6 3b. Figure 10 a presents the energetically favorable H position on the Fig. 6 2c structure obtained after examining possible adsorption sites. H prefers to stay 0.28 Å above the top As atomic layer to form a H As bond of 1.55 Å with dangling As atom. For F case shown in Fig. 10 b, the favorable site is 0.90 Å above the surface top layer with a F As bond length of 1.82 Å. Similar result was obtained for a Cl interaction with the specific oxidized surface Fig. 10 c and the As Cl bond length is 2.25 Å. To compare the stabilities of these three bonds, a binding energy analysis was conducted. The binding energy of passivation is defined as E b =E oxd +E pasv E tot, where E b is the binding energy, E tot is the total energy of H, F, Cl, and S interacting with the oxidized surface, E oxd is the energy of oxidized surface. In the case of H, Cl, F, and S passivation, E pasv could be defined as E pasv = E mol n E atm /n, where E mol are total energies for H 2, Cl 2,F 2, and S 8 Ref. 30 and E atm are the energies of the isolated H, F, Cl, and S atom. n represents the number of atoms in each molecule. In the case of GaO, E pasv was obtained by calculating the total energy of an isolate GaO molecule. Present calculation results of H 2 bond energy is 4.50 ev compared to experimental value of 4.53 ev. 31 Based on the parameters, we found E b =2.15 ev for H passivation. F, Cl, S, 2S, and GaO binding energies were 3.88 ev, 2.22 ev, 0.03 ev, 2.52 ev, and 3.84 ev, respectively. For S passivation, two different oxidized surface configurations are considered in this work. Figure 10 d presents two S atoms form two bonds with two dangling As atoms. And the two S atoms form a S S dimer pair with a dimer bond length of 2.11 Å. With the presence of S S dimer pairs and S As bonds, our surface model is consistent with experiment observations. 32 Fig. 10 e indicates one S atom interacting with the specific As atom with dangling bond. It sits 0.73 Å above the top layer of the oxidized surface. Figure

8 Wang et al. J. Appl. Phys. 107, FIG. 10. Color online Side view of H, Cl, F, S, and GaO adsorption on oxidized GaAs surface see Fig. 6 2c. For 2S case, it is for Fig. 6 3b. Big black white balls indicate As Ga atoms and small black white balls represent As Ga atoms in the sublayer. O and passivation species are specified in each figure. 10 f displays the molecular species GaO cluster adsorption on the GaAs oxidized surface. The GaO see Fig. 10 f acts as a bridge to connect the dangling As atom and oxygen atom. 2. Electronic structures of passivation of oxidized GaAs surface Hydrogen is known to be very effective at passivating silicon. 33,34 In the case of GaAs, the H atom transfers 0.16 electrons to the specific adjacent As atom based on Bader charge analysis 35 so that the dangling As bond is partially saturated. The gap bands induced by dangling bond are suppressed into the valence bands region, thus opening a surface gap like the clean surface. This finding agrees with that of Winn s work. 9 For the F and Cl cases, F and Cl obtain 0.61 and 0.46 electrons from the adjacent undimerized As atom, respectively. Therefore, F and Cl help to compress the gap bands induced by specific As dangling bonds into the conducting region. So F and Cl could unpin the Fermi level. In these two cases, p orbital surface states move to valence bands region and form bulklike new p orbital states since the As half-saturated dangling bond is saturated. Comparing F passivating Si-Based gate stacks, 36 F is also effective to passivate GaAs-based device due to F As strong bond and F-induced unpin GaAs oxidized surface. Sulfur plays dual roles in its interaction with the oxidized GaAs surface. One S atom forms a S As bond see Fig. 10 e with one oxygen atom oxidized surface. Band structure Fig. 11 a shows one band crossing the Fermi level. And this specific S-band contributes one peak in the total density of states gap region shown in Fig. 11 a right panel. Moreover, 0.57 ev above the bulk band at K point, there is a second band. Antibonding shown in Fig. 12 a indicates band one is related to the specific undimerized As and S atoms. From charge transfer perspective, there is only one extra electron in the oxidized surface compared to clean GaAs surface. Nevertheless, S atom needs two more electrons to form a closed electron shell when S interacts with oxidized surface. Therefore, the S As bond is not fully saturated and leads to high density surface states trapping Fermi level. When two S atoms interact with two undimerized As surface, charge transfer becomes more complex than one S case. Among three oxidized gap bands shown in Fig. 8 d, i.e., C1, C2, and C3, C1 and C2 are related to undimerized As bonds and C3 is induced by O Ga O unsaturated bonds. Each of two S atoms gets 0.17 electrons from its own adjacent As atom and form a S S dimer pair. Meanwhile, 0.2 more electrons from Ga connected two oxygen atoms compared to corresponding oxidized case transfer to As S bonds. Finally, C1, C2, and C3 were pushed into conducting band region and open a clean gap shown in Fig. 11 b. In addition, the specific dimer bond is the antibonding combination of p orbitals of two S atoms shown in Fig. 12 b. In the case of oxide GaO molecule adsorption on an oxidized GaAs surface, a clean gap was obtained which indicated As dangling bonds are fully saturated by obtaining a electron from GaO species. FIG. 11. Color online a and b left panel represent band structures for passivation configurations, i.e., S and 2S, respectively. The right panel indicates the corresponding passivated surface total density of states. The dot region indicates the projected GaAs bulk bands.

9 Wang et al. J. Appl. Phys. 107, To remove the Fermi level pinning, H, Cl, F, GaO, and S interaction with oxidized GaAs surface were investigated. H, Cl, or F could successfully eliminate unsaturated As dangling bonds by forming strong bonds with the specific undimerized As atom. One S atom forms a bond with As dangling atom, however, it does not remove As gap states since S itself generates new surface states due to the unsaturated dangling S bond. Two S atoms form one S S dimer pair and remove two undimerized As gap states. Native oxide cluster, i.e., GaO, could completely clean the gap region states and therefore become good passivation candidates. ACKNOWLEDGMENTS FIG. 12. Color online a and b present contour plots of the charge distribution at K point for surface localized states of the passivated GaAs surface. The contour spacing is eå 3.All plots are drawn parallel to the surface normal. Solid balls indicate As atoms. a indicates charge plot along S and As bonds. b is the charge plot along S-S pair. Based on the present analysis, H, Cl, F, and GaO could be used to passivate any system that has undimerized As atoms. However, once these candidates have been used to passivate the undimerized As atoms, any states left in the band gap region should be from the extrinsic adsorbate bonding with the GaAs surface. For the S case, if S bonds are saturated, it would also help to eliminate the surface state density. On the other hand, if there remain S dangling bonds, the generation of new states within the gap region occurs, leading to Fermi energy pinning. IV. CONCLUSION First principles calculations were performed to study oxygen atoms interaction with the GaAs surface. For one oxygen atom interaction case, we found a preference to absorb in the back-bond sites, and also oxygen prefers to replace one of the trough site As dimer atoms. For an interaction of two oxygen atoms with this GaAs surface, back-bond sites were found to be the preferable absorption sites and the trench site As dimer were easily replaced by two oxygen atoms. The present results show that the Fermi energy pinning is extrinsic rather than intrinsic to the GaAs 001 surface. The present work also predicts that the additional oxygen atoms adsorption on the back-bond site satisfies the bond saturation condition leading to no effect on the surface gap states. However, for the oxygen replacement of an As dimer atom of a trough site and two oxygen replacement of a row site As dimer, midgap states are produced leading to the Fermi level pinning due to the unsaturated As dangling bonds. These results confirm that control of oxygen on the GaAs surface is critical to the control of states leading to the Fermi level pinning. This research is supported by the FUSION/COSAR project and the MSD Focus Center Research Program. 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