Formation Of Native Oxide Research Articles

Rapid Surface Oxidation as a Source of Surface Degradation Factor for Bi2Se3

Bi2Se3 is a topological insulator with metallic surface states residing in a large bulk bandgap. It is believed that Bi2Se3 gets additional n-type doping after exposure to atmosphere, thereby reducing the relative contribution of surface states in total conductivity. In this letter, transport measurements on Bi2Se3 nanoribbons provide additional evidence of such environmental doping process. Systematic surface composition analyses by X-ray photoelectron spectroscopy reveal fast formation and continuous growth of native oxide on Bi2Se3 under ambient conditions. In addition to n-type doping at the surface, such surface oxidation is likely the material origin of the degradation of topological surface states. Appropriate surface passivation or encapsulation may be required to probe topological surface states of Bi2Se3 by transport measurements.

GaAs metal–oxide–semiconductor device with titanium dioxide as dielectric layer: effect of oxide thickness on the device performance

GaAs-based metal–oxide–semiconductor devices were fabricated with titanium dioxide (TiO2) as an oxide layer having three different thicknesses of 33, 54 and 71 nm. The phase purity of TiO2 was confirmed by Raman spectroscopy. Sulfur passivation on GaAs surface prior to TiO2 deposition was proposed to suppress poor native oxide formation on GaAs. Frequency-dependent studies on the capacitance–voltage were made in the frequency range 10 kHz–1 MHz. It was found that within this frequency range dispersion of the oxide capacitance was 4.9% per decade. Electrical measurements exhibited low leakage current, a dielectric constant of ≈27 and an interface trap density (Dit) of ∼1011 cm−2 eV−1. Two orders of magnitude reduction in current density were observed as the temperature went down from 290 to 100 K. Flatband voltage (VFB), dielectric loss tangent (tan φ) and ac conductivity (σac) were determined from the capacitance–voltage (C–V) and conductance–voltage (GP/ω–V) characteristics as a function of oxide thickness.

Enhanced spin signals due to native oxide formation in Ni80Fe20/Ag lateral spin valves

Large nonlocal spin valve signals are reported in mesoscopic Ni80Fe20/Ag lateral spin valves upon exposing them to air. Magnetotransport measurements combined with transmission electron microscopy show that the formation of a native oxide layer at the Ni80Fe20/Ag interface is responsible for the large signals. The results indicate that lateral spin valves with superior performance to those based on high-resistance tunnel barriers can be achieved via controllable growth of native permalloy oxides.

Ambient-Processed Colloidal Quantum Dot Solar Cells via Individual Pre-Encapsulation of Nanoparticles

We report colloidal quantum dot solar cells fabricated under ambient atmosphere with an active area of 2.9 mm(2) that exhibit 3.6% solar power conversion efficiency. The devices are based on PbS tuned via the quantum size effect to have a first excitonic peak at 950 nm. Because the formation of native oxides and sulfates on PbS leads to p-type doping and deep trap formation and because such dopants and traps dramatically influence device performance, prior reports of colloidal quantum dot solar cells have insisted on processing under an inert atmosphere. Here we report a novel ligand strategy in which we first encapsulate the quantum dots in the solution phase with the aid of a strongly bound N-2,4,6-trimethylphenyl-N-methyldithiocarbamate ligand. This allows us to carry out film formation and all subsequent device fabrication under an air atmosphere.

Effects of (NH4)2Sx treatment on indium nitride surfaces

Indium nitride (InN) surfaces treated with ammonium sulfide [(NH4)2Sx] are investigated using Hall effect measurement, x-ray photoelectron spectroscopy (XPS), and scanning Kelvin probe microscopy (SKPM). Upon the (NH4)2Sx treatment, the sheet carrier density is reduced by (0.8–0.9)×1013 cm−2, leading to an increase in the sheet resistance. By numerically solving the Poisson’s equation, the associated upward shift of the surface band bending is derived to be 0.3 eV. XPS characterization shows, on the (NH4)2Sx treated InN surface, the formation of native oxide is effectively suppressed and a covalently bonded sulfur layer with surface In atoms is formed. This surface In–S dipole layer results in an increase in the electron affinity, thus giving rise to a lower surface bending shift (0.2 eV) observed in XPS. The electron affinity increase of 0.1 eV can be deduced, which is consistent with the result obtained by SKPM. Thus, the (NH4)2Sx treatment has been demonstrated to be an effective method for reducing the surface band bending for InN.

Electrochemical and Thermal Grafting of Alkyl Grignard Reagents onto (100) Silicon Surfaces

Passivation of (100) silicon surfaces using alkyl Grignard reagents is explored via electrochemical and thermal grafting methods. The electrochemical behavior of silicon in methyl or ethyl Grignard reagents in tetrahydrofuran is investigated using cyclic voltammetry. Surface morphology and chemistry are investigated using atomic force microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS). Results show that electrochemical pathways provide an efficient and more uniform passivation method relative to thermal methods, and XPS results demonstrate that electrografted terminations are effective at limiting native oxide formation for more than 55 days in ambient conditions. A two-electron per silicon mechanism is proposed for electrografting a single (1:1) alkyl group per (100) silicon atom. The mechanism includes oxidation of two Grignard species and subsequent hydrogen abstraction and alkylation reaction resulting in a covalent attachment of alkyl groups with silicon.

The formation, imaging, and application of thin silicon-dioxide membrane

We have successfully formed and separated a very thin silicon-dioxide (SiO 2) film with a thickness of approx. 1 nm. The thin SiO 2 film was chemically grown on Si using a wet chemical treatment which is commonly used in semiconductor manufacturing. Highly selective etching of the underlying Si, through the native oxide, can be achieved using chlorine (Cl 2) gas. Such a unique phenomenon allows the formation and separation of native oxide from the Si surface. The native oxide film, which is suspended at its edges by thick-SiO 2, remains at the original level of the Si surface even after etching. The suspended Si-native oxide membrane is imaged using scanning electron microscopy (SEM). It is presumably the thinnest separated film ever recorded. The ∼1-nm-thickness SiO 2 membrane suspended by thick-SiO 2 can support subsequent film deposition. The sequential processes of native oxide formation on patterned Si, etching the Si by Cl 2 through the native oxide, and film deposition onto the native oxide membrane, can make a cavity in the solid-state substrate. This novel technique of creating a cavity by using a membrane of Si-native oxide can be applied to fabricate 3D micro-systems like pipes, diaphragms, tubes, and wave-guides on the solid-state circuits.

Effects of Surface Oxidation of AlGaN on DC Characteristics of AlGaN/GaN High-Electron-Mobility Transistors

We have investigated the effects of thin native oxide layers on the AlGaN surface on the DC characteristics of AlGaN/GaN high electron mobility transistors (HEMTs). After HEMT fabrication, the AlGaN surface between the electrodes was intentionally oxidized using either O2 or N2O plasma. X-ray photoelectron spectroscopy (XPS) analysis showed that both methods produced a native oxide of AlGaN with a thickness of about 1 nm and that an N–O chemical bond was present in the N2O plasma oxide. We observed pronounced degradation in the DC characteristics and current collapse in the HEMT with O2 plasma oxidation. In contrast, the formation of native oxide by the N2O plasma had no effect on the DC characteristics or current stability of the AlGaN/GaN HEMT. Possible mechanisms for device degradation were discussed in terms of stress and deep levels in the AlGaN layer induced by oxide formation or oxygen incorporation.

Degenerate Si as an Electrode Material for Electrochemical Biosensors

The use of degenerate (heavily doped) Si as an electrode material for electrochemical biosensors is demonstrated by a series of voltammetry and impedance studies. Cyclic voltammetry in solutions of and exhibits reversible electrochemistry without illumination, which to the best of the authors’ knowledge has not been previously reported for Si electrodes in an aqueous environment. The presence of prevents formation of the Si native oxide, while the use of a degenerate Si substrate makes its electrochemistry more like that of a metal. In addition, impedance detection of peanut protein Ara h 1 is demonstrated on a degenerate Si electrode following immobilization of the monoclonal mouse antibody to this protein. The variation in charge-transfer resistance with protein concentration is employed to estimate the dissociation constant of the surface-immobilized antibody-antigen complex as . The advantages of degenerate Si relative to n- and p-type Si include the lack of photoeffects, a simpler equivalent circuit, easier electrical connection to the working electrode, and knowledge that the applied potential applied is dropped within the electrical double layer, not within the semiconductor space-charge layer. Most important, degenerate Si electrodes are easily incorporated into Si-based semiconductor devices.

Stress development and impurity segregation during oxidation of the Si(1 0 0) surface

We have studied the segregation of P and B impurities during oxidation of the Si(1 0 0) surface by means of combined static and dynamical first-principles simulations based on density functional theory. In the bare surface, dopants segregate to chemically stable surface sites or to locally compressed subsurface sites. Surface oxidation is accompanied by development of tensile surface stress up to 2.9 Nm −1 at a coverage of 1.5 monolayers of oxygen and by formation of oxidised Si species with charges increasing approximately linearly with the number of neighbouring oxygen atoms. Substitutional P and B defects are energetically unstable within the native oxide layer, and are preferentially located at or beneath the Si/SiO x interface. Consistently, first-principles molecular dynamics simulations of native oxide formation on doped surfaces reveal that dopants avoid the formation of P–O and B–O bonds, suggesting a surface oxidation mechanism whereby impurities remain trapped at the Si/SiO x interface. This seems to preclude a direct influence of impurities on the surface electrostatics and, hence, on the interactions with an external environment.

The effect of ammonium sulfide treatment on interfacial properties in ZnS/HgCdTe heterostructure

The semiconductor-passivating layer interface, as well as the dielectric properties of the passivants, plays an important role in HgCdTe based photodiodes. ZnS is a commonly uses surface passivant for HgCdTe. The effects of sulfidation on the HgCdTe surface and interfacial characteristics of metal/ZnS/HgCdTe structures are studied. The ZnS layer was deposited by thermal evaporation after sulfidation of HgCdTe substrates. The interfacial properties of the metal insulator semiconductor (MIS) structures were determined by capacitance-voltage (C-V) measurements. A comparison of an untreated and a sulfide treated MIS capacitor showed that the fixed charge density and slow state density were 2 and 7 times lower in the sulfide treated specimens than in the untreated specimens. Sulfidation results in a decrease in the concentration of contaminants originating from the native oxide-covered HgCdTe substrates. This reduction may be due to the formation of S-S or II-S bonds at the surface layer. These bonds might act as barriers against native oxide formation when (NH4)2Sx treated HgCdTe substrates are exposed to air.

Conduction anisotropy in porous thin films with chevron microstructures

Electrical conductivity measurements were performed on structurally anisotropic thin films deposited using the glancing angle deposition apparatus [K. Robbie and M. J. Brett, J. Vac. Sci. Technol. A 15, 1460 (1997); K. Robbie, J. Sit, and M. J. Brett, J. Vac. Sci. Technol. B 16, 1115 (1998); K. Robbie and M. J. Brett, US Patent No. 5,866,204 (2 February 1999)]. The films were comprised of bilayers of titanium over silica, engineered as a chevron morphology. Samples were evaporated at various incident vapor deposition angles α, in order to investigate the effects of morphology and voiding on the behavior of conductivity. A rapid decline in the conductivity, accompanied by an increase in conduction anisotropy in the plane of the substrate, was observed with increasing α. A random walk model was developed to model the transport properties of the films, and applied to microstructures predicted by a three-dimensional ballistic thin film simulator. In order to generate reasonable agreement between the modeling and measurement, it was necessary to incorporate the effect of native oxide formation on the exposed surfaces of the titanium layer.

Ammonium sulfide treatment of HgCdTe substrate and its effects on electrical properties of ZnS/HgCdTe heterostructure

The semiconductor–passivating layer interface, as well as the dielectric properties of the passivants, plays an important role in HgCdTe based photodiodes. ZnS is a commonly used surface passivant for HgCdTe. This study examined the effects of sulfidation on the HgCdTe surface and interfacial characteristics of metal/ZnS/HgCdTe structures. The ZnS layer was deposited by thermal evaporation after sulfidation. The interfacial properties of the metal insulator semiconductor (MIS) structures were determined. A comparison of an untreated capacitor and a sulfide treated MIS capacitor showed that the fixed charge density (untreated 6.37×10 11, treated 3.2×10 11 cm −2) and slow state density (untreated 5.5×10 11, treated 7.5×10 10 cm −2) were 2 and 7 times lower in the treated than in the untreated specimens. Sulfidation results in a decrease in the concentration of contaminants originating from the native oxide-covered HgCdTe substrates. This reduction may be due to the formation of S–S or II–S bonds at the surface layer. These bonds might act as barriers against native oxide formation when (NH 4) 2S x-treated HgCdTe substrates are exposed to air.

Glancing Incidence X-Ray Diffraction of Polycrystalline Thin Films

AbstractGlancing incidence x-ray diffraction (GIXRD) is a powerful diagnostic tool for nondestructive analysis of thin film materials and structures. GIXRD fundamentals and experimental techniques and results are presented with respect to polycrystalline thin-films. The refraction index and critical incident beam angle are calculated for selected elemental and inorganic compounds. Quantitative GIXRD analysis with Cu Kα radiation is applied to several problems: recrystallization of ultra-thin chemical surface deposited CdS films; compositional grading in CuIn1-xGaxSe2films; formation of native oxides on CdTe films; interdiffusion between CdTe and CdS; Te film thickness determination on etched CdTe; and correlation of inplane residual stress with deposition conditions for sputtered Mo films.

The effects of treatment of Si(1 1 1) surfaces with NH 4F solution on Schottky diode parameters

A study on Al/p-Si(1 1 1) Schottky barrier diodes (SBDs) parameters with and without a thin chemical oxide layer on p-type Si has been performed. The oxide layer has been formed by a wet chemical oxidation in HNO 3 solution. In order to avoid native oxide formation on the sample surface, the silicon dangling bonds have been passivated with hydrogen atoms by dipping in a NH 4F solution. It is shown that these treatments give almost the same barrier height values. In addition, it has been found that the treatment of the sample back surface in NH 4F solution reduces the reverse current and the series resistance.

Limiting Native Oxide Regrowth for High-k Gate Dielectrics

AbstractA cleaning process resulting in atomically smooth, hydrogen-terminated, silicon surface that would inhibit formation of native silicon oxide is needed for high-k gate dielectric deposition. Various cleaning methods thus need to be tested in terms of resistance to native oxide formation. Native oxide re-growth is studied as a function of exposure time to atmospheric ambient using ellipsometry. Hafnium dioxide film (k ~23) is deposited on the as-cleaned substrates by electron beam evaporation and subsequently annealed in hydrogen. The difference in the effective oxide thickness re-grown on surfaces treated with the conventional RCA and modified Shiraki cleaning methods, after one-hour exposure, can be as large as 2 Å. This is significant in device applications demanding equivalent oxide thickness less than 20 Å. The degree of hydrogen passivation, surface micro-roughness and organic removal capability are considered to be the main factors that explain the differences between the cleaning methods. Data derived from capacitance-voltage analysis of test capacitors verified the trend observed in the native oxide thickness measurements. An increase of 10~15 % in accumulation capacitance is observed in the samples treated by the new cleaning method.

Epitaxial growth of CeO 2 on (100) InP using reactive r.f. magnetron sputtering

The epitaxial growth of CeO2 thin films has been realized on (100) InP substrates using reactive r.f. magnetron sputtering. Oxide films were nucleated in the presence of molecular hydrogen (4% H2/Ar sputtering gas) in order to reduce the native oxide formation on the InP surface, which interferes with CeO2 epitaxy. A metal cerium target was used as the cation source, with water vapor serving as the oxidizing species. Epitaxial films were sputter-deposited at a substrate temperature of 550 °C in a H2O vapor pressure of approximately 10-3 Torr. Crystallinity of the oxide films was examined using θ–2θ X-ray diffraction, ω-rocking curves, and in-plane φ-scans. The best results were obtained when the initial nucleation layer was deposited with P(H2O)<10-5 Torr, followed by deposition at P(H2O)=10-3 Torr. The epitaxial growth of CeO2 on InP could prove enabling in efforts to integrate functional oxides with InP-based optoelectronic and microwave technologies.

Investigation of surface structure and composition ofin situannealed Fe–Mn binary alloy samples using RHEED and electron spectroscopy

AbstractThe formation of oxides at the surface of Fe–1.5%Mn and Fe–0.6%Mn binary alloys was investigated as a function of the conditions of the heat treatments. Both the influence of temperature and the atmosphere under which the experiments were performed were studied. The range of annealing temperatures was adjusted to 800°C. The atmosphere consisted of a mixture of N2–5%H2and traces of water vapour, with different fixed dew points ranging from −10°C to −30°C. The state of the annealed surfaces was determined usingin situanalytical devices attached to the annealing reactor in order to avoid surface contamination or the formation of native oxides after the experiments due to contact with air. The structure and composition of the surfaces were determined by reflection high‐energy electron diffraction (RHEED) and electron spectroscopy (XPS, AES). Copyright © 2002 John Wiley &amp; Sons, Ltd.

Wetting Behaviour of GaN Surfaces with Ga- or N-Face Polarity

The wetting behaviour of GaN surfaces with N-face and Ga-face polarity and the influence of different surface treatments is studied by measuring the wetting angle of highly purified water by microscopic imaging. We found that wet thermal oxidation of the surface leads to a decreased wetting angle indicating an improved wetting behaviour. The presence of Al in AIN or AlGaN leads to a further reduction of the wetting angle, which is attributed to the presence of Al 2 O 3 on the surface. In addition the comparison of Ga- and N-face material revealed a lower wetting angle for all N-face samples. XPS analysis showed the enhanced formation of native oxide on the surface with N-face polarity.

The chemical composition changes of silicon and phosphorus in the process of native oxide formation of heavily phosphorus doped silicon

The chemical composition changes of silicon and phosphorus in the process of native oxide formation of both heavily phosphorus-doped Si(1 0 0) and polycrystalline Si prepared by HF-treatment were monitored for a period of 1 year, using angle dependent X-ray photoelectron spectroscopy (XPS). Right after HF-treatment, significant amounts of unoxidized-P (P ∗) and a new chemical state of Si (Si ∗) having an unstable bond were detected in the surface region of heavily P-doped Si. With decreasing depth, the concentration of P ∗ increased, whereas the Si ∗ decreased. The surface segregated P ∗ was larger for P-doped Si(1 0 0) than for P-doped poly-Si. During the growth of native oxide, the distributions of P ∗ and Si ∗ in the underlying Si surface did not change largely with oxide thickness. A part of P atoms also reacted with oxygen and incorporated into Si–O–P network in the native oxide, and the amount of P-oxide increased with the progress of oxidation. The decomposed Si 2p spectra of growing oxide films on both heavily P-doped Si showed that the dominant component was not the Si 4+, but Si 3+ state until the oxide thickness went up to over about 1.5 nm, while that for the low P-content Si was Si 4+, except at the very narrow interface.