InAs Interface by Fluorine

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Passivation Mechanism of the Native Oxide/InAs Interface by Fluorine Natalia A. Valisheva, Alexander V. Bakulin, Maxim S. Aksenov, Sayana E. Khandarkhaeva, and Svetlana E. Kulkova J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03757 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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The Journal of Physical Chemistry

Passivation Mechanism of the Native Oxide/InAs Interface by Fluorine N.A. Valisheva1, A.V. Bakulin3,4, M.S. Aksenov1,2*, S.E. Khandarkhaeva1,2, and S.E. Kulkova3,4 1

Rzhanov Institute of Semiconductor Physics SB RAS, 13 Lavrentiev Avenue, Novosibirsk, 630090,

Russian Federation 2

Novosibirsk State University, 2 Pirogov Street, Novosibirsk, 630090, Russian Federation

3

Institute of Strength Physics and Materials Science SB RAS, 2/4 Akademichesky Avenue, Tomsk,

634055, Russian Federation 4

Tomsk State University, 36 Lenin Avenue, Tomsk, 634050, Russian Federation

E-mail: [email protected]

ABSTRACT The comparative experimental and theoretical studies of the fluorine/oxygen ratio influence on the structural and electronic properties of anodic layer (AL)/InAs interface by XPS, HRTEM, C-V (77K) measurements and ab initio calculation of fluorine and oxygen adsorption on the InAs(111)A-(1×1) unreconstructed surface were performed. The well-ordered transition region (TR), composed of indium and arsenic oxyfluorides, and extension of the interplanar distance at the fluorinated anodic layer (FAL)/InAs interface were experimentally revealed. The theoretical modeling of AL/InAs and FAL/InAs interfaces showed that the fluorinated TR formation removes the InAs surface distortion, whereas the In(InAs)-F-As(FAL) and In(InAs)-O-As(FAL) bonds formation is a reason for the interplanar distance increase between FAL and the InAs surface. The decrease of the interface states density in the InAs bandgap and the Fermi level unpinning at the FAL/InAs interface result from the positive charge increase on FAL arsenic atoms near the InAs surface during the As-F bonds formation, while the electron accumulation on oxygen atoms and InAs subsurface arsenic atoms is the reason for the states appearance in the InAs bandgap at the anodic (native) oxide/InAs interface.

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INTRODUCTION АIIIBV semiconductors are promising materials for various device applications due to their superior III

electron mobility, as compared to silicon. At the same time, the formation of an insulator/А As

interface with a low density of interface states (Dit) is a key problem for the device fabrication based on metal-insulator-semiconductor (MIS) structures. This is also relevant for devices in which the passivation of mesa structure side wall is required. It is especially important for multilayer heterostructures based on ternary alloy (InAlAs, InGaAs, etc.) due to a more complex composition of the native oxide on a semiconductor surface. The various passivation techniques, based on reduction or complete removal of the native oxide from a semiconductor surface, as well as its modification, have been developed.1 The Dit of more than 1012 eV-1cm-2 at 300 K were

obtained by the atomic layer deposition (ALD) of high-k dielectrics (Al2O3, HfO2, TiO2 and other) on InAs surface. Despite the so-called “self-cleaning” effect and a various chemical treatments of the surface, the native oxide at the interface is not completely removed by this method.2-4 The models of defect states at dielectric/III-V semiconductor interfaces have shown that the Dit value and Fermi level pinning are determined by the semiconductor surface, as well as native oxides defects near the interface.5 Thus, to study the interface formation features, clarification of its atomic structure is required. Ab initio calculations, in conjunction with experimental methods, allow one to trace in detail the mechanisms of the interface traps appearance/elimination .5 The most adequate information can be obtained by the study of a significant effect of interface trap reduction, for example, as it occurs at Gd2O3/Ga2O3/GaAs interface (Dit~5·1010 eV-1cm-2) grown by the molecular beam epitaxy (MBE) method.6 Earlier we realized the fluorinated anodic layer (FAL)/InAs interface with minimum Dit ~ (2÷5)×1010 cm-2eV-1 at 77K by anodic oxidation in liquid and dry fluorine-containing mediums. The medical cooled CID-camera based on In2O3/SiO2/FAL/InAs MIS-structures with the highcontrast images of two-dimensional thermal patterns with a resolution up to 7 mK was fabricated.7,8 The sharp influence of fluorine on the Dit of the AL/InAs interface was established.9 The FAL chemical composition, the morphology of InAs surface and FAL/InAs interface, as well as electrophysical parameters of Au/FAL/InAs metal-oxide-semiconductor (MOS) structures, were studied.10-12 The Dit and the composition of FAL dependences on fluorine concentration were revealed. However, the influence of the composition and morphology of FAL/InAs interface on the Dit at the microscopic level remains not fully understood. Particularly, in spite of the technological success in the interface formation with low 2 ACS Paragon Plus Environment

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Dit and creation a some devices, the mechanism of passivation of anodic oxide/InAs(111)A interface by fluorine is still unknown. In this work, the investigation of states formation peculiarity at the FAL/InAs interface was continued. The study of fluorine/oxygen ratio influence on the chemical composition and morphology of the FAL/InAs interface, fabricated by various methods, and also on the interface states density of the corresponding Au/FAL/InAs MOS-structures were studied by X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and the capacitance-voltage (C-V) characteristic methods. Ab initio calculations were performed to estimate the sequence effect of fluorine and oxygen adsorption on the atomic and electronic structure of the InAs(111)A surface.

EXPERIMENTAL AND THEORETICAL DETAILS The anodic layer (AL) was grown on the n-InAs (2÷5×1015 cm-3) epitaxial layers formed on the n+-InAs(111)A substrates. Before anodic oxidation, the samples were degreased in toluene, treated in the solution of monoethanolamine with hydrogen peroxide at volume ratio 1:1, washed in deionized water (5 min.) and dried in pure argon. The AL growth was carried out in two ways. First, the electrolytic oxidation at room temperature in a two-electrode cell (galvanostatic mode at current density 0.1 mA/cm2) in acid (H3PO4:isopropanol:glycerin = 5:65:30) and alkaline (25% NH4OH:ethylene glycol = 1:5) electrolytes, containing different amount of fluorine (NH4F), was carried out.13 Second, the dry oxidation in a low-current (j = 1030 A/cm2) Townsend gas discharge (TGD) plasma using the parallel-plate geometry and an O2:CF4:Ar gas mixture after the HCl-isopropanol solution treatment was made.10 In both cases, the anodic layer thicknesses were set by appropriate growth constants and controlled by the "Microscan" scanning and the LEF-3M ellipsometers with He-Ne lasers (632.8 nm wavelength) at the incidence angles 60° and 55°, respectively. It was calculated within the uniform isotropic film model on a substrate with complex refractive index Nsub = 3.898–0.683j. The AL (15-20 nm) chemical composition was studied by XPS using an SPECS spectrometer with a PHOIBOS-150-MCD-9 hemispherical energy analyzer and X-ray monochromator FOCUS-500 (AlKa irradiation, hν = 1486.74 eV, 200 W). The binding energy scale was preliminarily calibrated by the peaks position of Au 4f7/2 (84.0 eV) and Cu 2p3/2 (932.67 eV) core levels. The In4d, As3d, O1s and F1s core levels were measured during layerwise argon ion etching performed by using an argon ion gun (SPECS model IQE 11/35). The energy of Ar+ ions for the depth profiling, the current density and the angle of sputter

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erosion were 1.2 keV, 2 A/cm2 and 45°, respectively. The AL etching time before reaching the InAs volume was 10-15 minutes. The HRTEM images of the AL(FAL)/InAs(111)A interface were obtained with Cscorrected transmission electron microscope Jeol JEM-2200FS. The microscope magnification was calibrated using FFT-spectra by InAs{111} reflections. The MOS-structures were fabricated by the Au (200 nm) evaporation on the anodic layer through a mask sized 2×10-3 cm2. To form an ohmic contact, indium (0.2 µm) was deposited on the back of wafers. The multi-frequency C-V and G-V characteristics of the MOS-structures were performed at temperature 77 K using a WK analyzer 6440B. The Dit was calculated by Terman method.14,15 The atomic and electronic structures of the InAs(111)A-(1×1) unreconstructed surface were calculated using the projector augmented-wave (PAW) method implemented in the VASP code.16,17 The PAW data sets for semiconductor components from the proprietary VASP library version 5.2 were accepted. The elements valence configurations, including As – 4s24p3 and In – 5s25p1, were considered. The generalized gradient approximation (GGA)18 for the exchangecorrelation functional, as well as hybrid functional HSE06,19,20 were employed. The InAs(111)A-(1×1) unreconstructed surface was modeled by eight-layer films separated by a vacuum gap ~10 Å. The As-terminated surface saturated with pseudo-hydrogen atoms (one H atom per As atom) with the electrons of 0.75 valence, and thus, one of the film surfaces had a bulk-like view. Four topmost atomic layers from the In-terminated surface were optimized, while two indium and arsenic layers from the opposite side were fixed in the bulk values. The relaxation of atomic positions was preformed until the forces at atoms were smaller than 0.01 eV/Å using Newtonian dynamics. The integration over the Brillouin zone was carried out using a 7×7×1 grid of the k-points obtained by the Monkhorst-Pack scheme.21 The charge states of atoms were estimated by the Bader charge analysis.22 In this approach, a space is divided into atomic regions where the dividing surfaces are at a minimum in the charge density. The charge density difference ∆ρ(r) was used for the visualization of charge density redistribution. More details about the calculation can be found in our previous article.23

RESULTS AND DISCUSSION

The experimental multi-frequency C-V and corresponding G/ω-V dependencies of the Au/AL/InAs(111)A with the oxide films grown in oxygen TGD plasma (a) and in fluorinecontaining (CF4/O2=0.75:1) TGD plasma (b) at 77 K are shown in Figure 1a, b. For the MOS4 ACS Paragon Plus Environment

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structure with non-fluorinated (native) AL the C-V curves demonstrate a significant frequency dispersion, including the dispersion in accumulation (~15 %) associated with the influence of boundary traps.24 There are also characteristic peaks associated with charge exchange of interface traps on the G/ω-V curves for frequencies less than 100 kHz. The Dit calculation carried out according to the technique described in Ref. [15] showed that the traps are uniformly distributed over the band gap with a density of about 1012 eV-1cm-2. The introduction of fluorine into the oxidizing medium always leads to an almost complete elimination of the frequency dispersion coupled with a sharp decrease in Dit value (Figure 1b, c). The absence of any frequency dispersion in the accumulation mode indicates the complete elimination of border traps.25 It was shown that this result is practically independent of the thickness of the oxide,15 as well as of the oxidation method.11,12 The influence of the fluorine concentration in the oxidation medium on the Dit (77K) near Ef of Au/FAL/InAs MOS-structures for two oxidation techniques is shown in Figure 1d. The anodic layers, forming the AL/InAs interface with the high Dit (77 K), have the same composition independently from the oxidation medium composition. They mainly consist of In2 O3 and As2 O3 uniformly distributed through the anodic film and at the interface (Figure 2a). Dit decreases by more than one order of magnitude at the low fluorine concentration and changes insignificantly with a further fluorine concentration increase (Figure 1d). Besides, for other oxidation conditions, a different fluorine amount is required to form the FAL/InAs interface with low Dit (77K). The NH4F concentration for the InAs oxidation in an alkaline electrolyte (it is not presented in Figure 1d) should be approximately hundred times (12-24 g/l) higher than that for the oxidation in an acid electrolyte (0.1-2.5 g/l), though, in both cases, the FALs have the same composition presented in Figure 2b. The analysis of the FAL XPS results reveals the presence of indium and arsenic oxyfluorides (InxOyFz, AsxOyFz) with a different F/O ratio distribution over films and at the FAL/InAs interfaces (Figure 2b,c). The fluorine distribution has a dome-shaped dependence with its maximum near the middle of the films. In general, the F/O ratio over FALs films increases with the NH4F concentration increase in electrolytes (Figure 2b,c). It should be noted that similar XPS results were obtained for the FAL formed in TGD plasma.12 Oxygen is always present in FALs and at the interface, but a significant difference of the F/O ratio has a negligible effect on the Dit (77 K) value with the increasing fluorine concentration higher than the optimal one (Figure 1d). Oxygen is not detected only at the FAL/InAs interface formed at the highest (15 g/l) NH4F concentration in the acid electrolyte, when a uniform film can be grown on all the sample surface (Figure 2c). In this case, it mainly consists of indium, arsenic and fluorine at ratio 1:1:3. Taking into account the observed chemical shifts of the elements and its 5 ACS Paragon Plus Environment


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concentration, it is possible to conclude that the FAL/InAs interface consists of indium fluoride (InFx, x~3) and arsenic at the ratio of InF3:As = 1:1.11 Studying the influence of fluorine concentration on the FAL/InAs(111)A interface morphology revealed a well-ordered transition layer (TL) between the crystalline InAs surface and amorphous anodic films. This region can be seen in the HRTEM images as the light-colored areas caused by the accumulation of the light-weighed fluorine atoms (Figure 3, left panel). The peaks of the laterally average optical density (Figure 3, right panel) also demonstrate a wellordered structure of the transition layer. At the interface without fluorine (Figure 3a), the transition region is always about 2-3 monolayers independently from the oxidation condition with the interplanar distances shorter than the interplanar distance for the (111) planes of the InAs bulk (3.5 Å).26 The oxygen/InAs interface fluorination leads to a widening of the TL region to 4-6 ML (Figure 3b-d) due to the well-ordered structure formation with the interplanar distances close to the InAs bulk. The interface obtained by electrolytic oxidation in the alkaline electrolyte (Figure 3b) has a smaller interface roughness and transition layer width than that formed in the acid electrolyte (Figure 3c) or by the dry oxidation in TGD plasma (Figure 3d). The change of interplanar distances through TL with an increase of fluorine/oxygen ratio in fluorinated layers clearly shows the extension of the first interplanar distance near the InAs surface (Figure 4a,b). At the same time, the significant change of the interplanar distance between the next monolayers is not observed within the measurements precision (±0.1 Å). Further, the well-ordered structure breaks down in the amorphous oxide (Figure 4). At high fluorine concentrations in oxidation mediums, the first interplanar distance in TL is close to the InF3 one (d012 = 3.8 Å),26 which was detected by XPS at the FAL/InAs interface formed by electrolytic oxidation in the acid electrolyte with the highest NH4F concentrations (Figure 2c). The revealed crystalline region at the FAL/InAs interface allows us to carry out the theoretical modeling of the atomic structure of this region and to investigate its influence on the InAs(111)A surface electronic properties. The calculation demonstrated that the oxygen interaction with the InAs(111)A-(1×1) unreconstructed surface significantly changes the atomic structure of the InAs surface, and the states are induced in the InAs bandgap (Figure 5a,b). The atomic and electronic structures depend on the oxygen amount, as well as on the adsorption site.23 In a more energetically preferable oxygen adsorption position between two indium atoms (In-B), the oxygen atom bonds form not only with these atoms, but with the arsenic atoms of the second layer (Figure 5a). In this case, the oxygen atom is located above the surface indium atoms (In1) shifted towards 6 ACS Paragon Plus Environment

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vacuum by 0.09-0.15 Å, while the subsurface arsenic atoms (As2) are shifted by 0.14-0.18 Å towards the InAs bulk. In the case of three oxygen atoms adsorption on the InAs (111)-(1×1) surface, the penetration of all oxygen atoms into the subsurface region (1.47 Å below the indium surface atoms) is observed (Figure 5b, top). This is accompanied by a significant increase of electronic states density in the bandgap (Figure 5b, bottom). Modeling the real FAL/InAs interface by the calculation of fluorine co-adsorption on the surface with oxygen adsorbed in preferential sites results in appreciable structural changes in two InAs near-surface layers. This is due to the adsorbates penetration into the substrate when the amount of fluorine atoms on one In atom increases up to three. The oxygen-induced surface states are almost completely removed due to the simultaneous fluorine and oxygen interaction. Only the states near the bandgap edges are observed (Figure 5c, bottom panel). The various succession of fluorine and oxygen co-adsorption at ratio F/O = 3 does not change the electronic band structures (Figure 5c-d, bottom panel). There is only a significant difference in the atomic structure of the InAs surface. The oxygen penetrates between the arsenic atoms of the second layer (As2) and the indium atoms of the third layer (In3) when fluorine atoms are adsorbed after oxygen adsorption (Figure 5c). The fluorine incorporation between these layers takes place when the reverse adsorption sequence is used (Figure 5d). The fluorine atoms also lie between the second and third InAs layers if the oxygen atom adsorbs on the surface with two fluorine atoms and, after that, one more fluorine atom adsorbs, too (Figure 5e). The atomic structures in Figure 5c-e can be considered as the models of the FAL/InAs(111)A interface with a low states density. The third layer (In3) of indium atoms (under oxygen or fluorine) is the InAs(111)A-(1×1) surface layer in this case. The structures located over these In3 atoms and formed by In1 and As2 atoms, together with oxygen and fluorine atoms, are the FAL transitional region near the InAs surface. The calculated structures show that the fluorinated oxide layers do not distort the atomic structure of InAs near-surface layers. Note that a similar phenomenon was observed at the fluorine adsorption in the In-T position on the InAs(111)-(1×1) surface. That resulted in the elimination of the surface states, conditioned by the pz-indium orbitals, and in the Fermi level unpinning.23 In all cases, the atomic structure of the InAs surface layer does not differ from the InAs bulk structure. The interatomic distance in the complex structure and the composition of the FAL transition layer is determined by the fluorine amount. The presence of oxygen and/or fluorine between the InAs surface and transitional layer at the FAL/InAs interface leads to a significant increase of the interface interatomic distance, that corresponds to the experimentally revealed results obtained by HRTEM measurements (Figure 4). These results, in conjunction 7 ACS Paragon Plus Environment


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with Dit experimental data, correlate with theoretical work [27], in which the authors demonstrate that the distortion should be the reason for high Dit at the oxygen/InAs interface due to the lattice mismatch and the tensile strain. The analysis of the obtained structural and electronic characteristics shows that the oxygen interaction with the InAs surface results in a charge accumulation near the oxygen atoms and on the O-As bond (Figure 6a, blue region), and the charge depletion near the arsenic subsurface atoms (Figure 6a, green region). The charge of As2 atoms increases to -0.06e and +1.54e during the interaction with one and three oxygen atoms, respectively, while the value of the As bulk atom charge is -0.60e. The charge state of indium surface atoms (In1) decreases less (from +0.50e on a clean surface to +0.87e and +0.76e during the one and three oxygen atoms adsorption, respectively). The major contribution to the states in the InAs bandgap is given by the indium and arsenic atoms of two surface layers at one oxygen atom adsorption. As compared to the adsorption of one oxygen atom, when the charge of the deep-lying atoms remains practically unchanged, the hybridization between the states of deeper layers (As2, In3, As4) and adsorbates states takes place at three oxygen atoms adsorption. It leads to the appearance of the peaks on the DOS of In3 and As4 atoms coinciding with oxygen DOS peaks in energy (Figure 6b). The In3 and As4 atoms give the main contribution to the bandgap states in this case. At the FAL/InAs interface with a low states density, the strong hybridization of fluorine and oxygen bands with the semiconductor atoms in the FAL transitional layer, as well as the fluorine tendency to occupy completely its p-shells, results in the fluorine bonding not only with indium, but also with the transitional region arsenic atoms lying near the In-terminated InAs surface. This takes place when the F/O ratio is equal to three only. The charge density differences, calculated for the FAL/InAs structure with oxygen at the interface (Figure 5c), clearly demonstrate that the charge is accumulated on fluorine-arsenic bonds and also on oxygen atoms (Figure 6c, blue region). While the indium atoms charge states reduce insignificantly (to +1.0e), the arsenic atoms lose approximately 1.8e (Figure 6c, green region). The charge transfer to O and F atoms equals 1.02e and 0.65-0.70e, respectively. The chemical bonding at the interface and in the transitional layer near the FAL/InAs interface is mainly of ionic character. This is qualitatively confirmed by the trends in the change of partial DOS (Figure 6d), and agrees well with the charge density difference distribution (Figure 6c) and FAL chemical composition (Figure 2b,c). Moreover, it is well known that metal fluorides have their ionic contribution to these chemical bonds at a higher degree, as well as fluorinated amorphous oxide glasses are more ordered, as compared to those of metal oxides.28, 29 The DOS curves structure of the InAs surface layers at the FAL/InAs interface (In3 and As4 layers in Figure 6c) differs insignificantly 8 ACS Paragon Plus Environment

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from the corresponding DOS of the bulk semiconductor atoms. It is shown that the atomic and electronic structures of these InAs surface layers have the same character as in the case of only fluorine atoms interaction with the InAs surface. The charge of the surface In3 atoms increases up to +1.03e, whereas the subsurface As4 atom charge is almost equal to the As bulk value (0.60 e).

SUMMARY The theoretical simulation of the atomic structures and electronic properties of oxygen/InAs (high Dit) and fluorine-oxygen/InAs (low Dit) interfaces with a different F/O ratio within the density functional theory was carried out. The obtained trends well correlate with the experimental data of the chemical composition (XPS), morphology (HRTEM) and the interface states density (C-V measurements) at the real fluorinated anodic layer/InAs interface. The calculations show that the fluorination of indium (In2O3) and arsenic (As2O3) oxides, forming the anodic (native) layer at the AL/InAs interface leads to radical changes of the atomic and electronic structures of the semiconductor near-surface layers and the oxide layer adjacent to the InAs surface. The formation of the well-ordered fluorinated transitional layer at the FAL/InAs interface, consisting of indium and arsenic oxyfluorides, removes the distortion of the InAs surface structure. The In(InAs)-F-As(FAL) and In(InAs)-O-As(FAL) bonds formation is the reason for the extending interplanar distance in the FAL/InAs interface revealed experimentally by HRTEM. There is an optimal F/O ratio in FALs when the states are eliminated from the bandgap and differences in the structure (chemical composition) of the wellordered fluorine-containing transition region do not affect the appearance of the states in the InAs bandgap at the FAL/InAs interface. The states removal occurs due to the increase of the positive charge on arsenic atoms of the fluorinated layer near the InAs surface during the formation of As-F bonds. The Fermi level unpinning at the FAL/InAs interface is conditioned by the removal of the interfacial states localized mainly on the InAs near-surface arsenic atoms, as well as on the oxygen ones at the anodic layer/InAs interface when one indium atom bonds with three fluorine atoms.

ACKNOWLEDGMENTS The authors acknowledge prof. O.E. Tereshchenko for his useful discussions of HRTEM results. The work was carried out supported by RFBR (project № 14-29-08124) and also by the Tomsk State University Competitiveness Improvement Program. Some experiments were carried 9 ACS Paragon Plus Environment


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out using the equipment of CCP "Nanostructures". 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.30 REFERENCES

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Figure 1. Experimental C–V and G/ω–V measured at 1 kHz, 10 kHz, 100 kHz, 500 kHz , and 1 MHz of MOS-structures with AL formed in TGD plasma (a) without CF4 and (b) with CF4. All the data were measured at 77 K in the dark using a small ac signal of 25 mV. (c) Dit distribution in the InAs gap for the above two samples obtained by Terman method. (d) Dependences of the Dit (77 K) near the Ef in the Au/AL/InAs MOS-structures on the fluorine amount in the oxidation medium during the AL in the acid electrolyte (1) and TGD plasma (2).



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Figure 2. The XPS component profiles of the ALs formed by the InAs oxidation in the acid electrolyte with 0 (a), 2.5 (b) and 15 g/l (c) NH4F.


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Figure 3. HRTEM cross-sectional images (left panel) and the corresponding laterally averaged optical density histograms (right panel) of the AL/InAs(111)A interface formed in the alkaline electrolyte without (a) and with 24 g/l NH4F (b), acid electrolyte with 15 g/l NH4F (c) and TGD plasma with CF4/O2 = 0,5 (d).



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Figure 4. The dependences of the TL interplanar distances on the fluorine amount at the AL/InAs interface formed by the electrolytic oxidation in alkaline (curves 1,4), acid (curves 2,4) electrolytes with different NH4F concentrations (a) and by TGD plasma with a different CF4/O2 ratio (b).


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Figure 5. The atomic structures (upper panel) and the electronic band structure along the twodimensional Brillouin zone high-symmetry directions (bottom panel) of the near-surface atoms of InAs(111)A-(1×1) surface with adsorbed one (a) and three (b) oxygen atoms, different variants of the fluorine and oxygen coabsorption sequence in the preferential sites at ratio F:O = 3:1(c-e). The circles color on the electron band structure coincides with the atoms color, where the states are localized: blue, red, yellow and grey are F, O, As and In atoms, respectively. The circles size represents the localization degree.



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Figure 6. The charge density difference ∆ρ(r) (a,c) and the partial DOS (b,d) of the near-surface atoms of the InAs(111)A-(1×1) surface with adsorbed three oxygen atoms (a,b) and coadsorbed fluorine and oxygen atoms at ratio 3:1 (c,d) in preferential sites. The ∆ρ(r) shown by isosurfaces: blue and green regions are charge accumulation (∆ρ(r)0) regions, respectively.


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InAs Interface by Fluorine

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