Thin Anodic Oxides Research Articles

Characterization of thin anodic oxides of Ti–Nb alloys by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy was used to study the interface between the anodic oxide formed on Ti–Nb alloys with specific compositions of Ti–10wt.% Nb and Ti–20wt.% Nb and the electrolyte. The anodic oxides were grown in an acetate buffer of pH 6.0 by using cyclic voltammetry electrochemical technique in which the potential is scanned at a rate of 100mVs−1. The potential applied starts from 0V and increasing at steps of 1V till 8V which allows to study the mechanism and the kinetics involved during the oxide growth. The electrochemical impedance measurements were started prior to applying any potential so that the electronic properties of the native oxide on the Ti–Nb alloys can be determined. The electrochemical measurements were then carried out after each oxide growth so that the electronic properties of the previously grown oxide can also be determined. The variation of the capacitance of the respective oxides determined from the impedance measurements with the applied potential enables the calculation of the relative permittivity of the respective oxides on the two alloys. Moreover the semiconducting properties of the oxides were determined by using Mott–Schottky analysis. The Mott–Schottky analysis involves electrochemical impedance measurements at fixed frequency with increasing applied bias potential so that the variation of the capacitance of the space charge region with the applied potential can be followed. The oxides from both alloys showed an n-type semiconducting property with 7.5×1018cm−3 and 2.4×1019cm−3 donor concentration for Ti–10wt.% Nb and Ti–20wt.% Nb alloys respectively.

Complex dielectric functions of anodic bi-layer tantalum oxide

The optical properties and structure of anodic oxides are dependent on the anodization conditions. For tantalum oxide formed in dilute phosphoric acid, the anodic oxide forms as two chemically distinct layers, referred to as a bi-layer, where the inner layer is pure tantalum oxide and the outer layer contains incorporated phosphate. The complex dielectric functions of the individual inner and outer layers are determined using spectroscopic ellipsometry. The dielectric functions of the bi-layers are compared to mono-layer oxides formed in sodium sulfate, and the effects of hydrothermal sealing are explored. For bi-layer oxides formed to 70 V in phosphoric acid, the inner layer band gap is 4.37 ± 0.02 eV and the outer layer band gap is 3.86 ± 0.06 eV. Thin anodic oxides (∼6–15 nm) are best described by a mono-layer oxide model and exhibit higher optical absorption with a band gap of 3.98 ± 0.08 eV. This study shows that spectroscopic ellipsometry is a valuable tool in assessing processing–property relationships of multi-layer anodic films.

Morphology, Composition and Electrical properties of Thin Anodic Oxides on InP

Depending on the applied electrochemical parameters, various anodic films can be grown onto InP in aqueous media. In borate buffer at pH = 9, the formation of homogeneous and thin InPO4-like films occurred when anodic treatment is performed with a low current density, while with higher imposed current configuration porous and thicker layers are grown. In this work, AFM measurements coupled with XPS analyses and capacitance-voltage investigations have been used to study the relationship between morphology, composition and electrical properties of different anodic oxide films performed on InP surface, with current densities ranging from 1 mA.cm-2 to 100 mA.cm-2.

Morphology, Composition and Electrical properties of Thin Anodic Oxides on InP

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Composition and growth of thin anodic oxides formed on InP (100)

Thin anodic oxides (<100 Å) were formed on p-InP (100) in phosphate solution (0.3 M NH 4H 2PO 4) and in sodium tungstate solution (0.1 M Na 2WO 4·2H 2O) at different temperatures (25 and 80 °C) and potentials (1–8 V). Thickness and composition were determined by different surface-analytical techniques including Auger electron spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy and transmission electron microscopy. In general, it has been observed that double-layered films are obtained with an outer In-rich layer. The thickness of the outer layer, oxide morphology and roughness as well as the composition of the duplex structure are strongly dependent on the temperature and the composition of the electrolyte. It has been found that oxides formed in phosphate exhibit a higher stability against dissolution compared with oxides formed in tungstate. The latter contain a large amount of In 2O 3, which leads to poor electrical properties.

Thin anodic oxides on n-InP studied by photocurrent transients and surface analysis

Investigation of semiconductor/electrolyte interface modified by superficial oxides has been performed with photocurrent transients and surface analysis. Studies of the photocurrent transients before and after the modification of the semiconductor surface exhibit the electrical properties of the interface, whereas XPS analysis gives access to the chemical aspect. This work evidenced a correlation between the level of photocurrent, the surface oxidation and the texture (porosity, thickness) of the oxide layer.

Correlation Between Surface Non‐Radiative Recombination and Current Oscillation at p‐Si(100) During Electropolishing in Fluoride Solution

AbstractThe electropolishing of p‐Si(100) in aqueous NH4F solution is investigated in‐situ by photoluminescence (PL) with short N2‐laser pulses. The PL signal increases with potential and reaches a maximum when current oscillations occur. The PL signal oscillates with the same period but anticorrelated to the current. The thin anodic oxides are removed at a low cathodic potential. The respective peak of the current transient is used to monitor the relative changes of the anodic oxides. The results are discussed from the point of view of non‐radiative defect generation during electropolishing, which is controlled by the potential and the rate of oxidation.

Thickness measurements of thin anodic oxides on GaAs using atomic force microscopy, profilometry, and secondary ion mass spectrometry

Techniques to determine the thickness of thin (30–200 Å) anodic oxide films on p-GaAs (100) are reported. The layers were grown potentiostatically in 0.3 M NH4H2PO4 (pH 4.4) solution. By lithographic techniques, several series of squares were etched into the oxide and the step height was measured using both atomic force microscopy (AFM) and a stylus surface profiler (Dektak). Secondary ion mass spectrometry (SIMS) profiles of samples prior to and after the photolithographic treatment show that neither the thickness nor composition of the layers are affected by the treatment. The thickness values obtained in the investigated range show standard deviations better than ±9 Å (AFM) and ±22 Å (Dektak) and correlate well in a linear relation to SIMS sputter times to the interface.

Thin anodic oxides formed on GaAs in aqueous solutions

Thin (0–250 Å) anodic oxides were formed on p-GaAs (100) in aqueous solutions (borate buffer, pH 8.4 and 0.3 M NH4H2PO4, pH 4.4). The thickness, composition, and electrochemical behavior were characterized by x-ray photoelectron spectroscopy, secondary ion mass spectrometry, ellipsometry, and electrochemical impedance spectroscopy. In borate buffer, oxide growth results in a two-layer structure with an outer As2O3-rich layer and ion transfer is mainly diffusion controlled. From the potential dependence of the film thickness a growth rate of 25 Å/V was determined. Due to the formation of a Ga-phosphate precursor layer in NH4H2PO4, films grow under field control which results in a time dependence of oxide thickness and composition. Under pseudosteady state conditions a growth rate of 28 Å/V was obtained. Films formed in NH4H2PO4 have, except for the outer phosphate layer, a fairly uniform composition in depth with a significant enrichment of Ga2O3, and show a 40 times higher specific charge transfer resistance than films formed in borate buffer. The superior quality of the oxide formed in NH4H2PO4 is also reflected in its high stability against H2O etch. In addition, substrate pretreatment and the effects of oxide washing on composition were investigated and the importance of an appropriate preparation of samples for ex situ analysis is demonstrated.

Physical and Electrical Characterization of Thin Anodic Oxides on Si(100)

Gate oxides 4 to 50 nm thick have been grown on Si(100) by anodic oxidation at room temperature. Different concentrations of aqueous were used as the electrolyte. Growth of oxides on n‐type substrates required light illumination; however, the uniformity of the oxide thickness was not critically dependent on the uniformity of the illumination as long as light saturation conditions were maintained. The oxides on n‐type Si were slightly thicker than those on p‐type Si under the same growth conditions; nevertheless the physical properties of the oxides grown on the two types of substrates were similar. The growth mechanism was determined by secondary ion mass spectrometry with 18O labeling, and depends on the solution pH. The as‐grown and annealed oxides were characterized by Fourier transform infrared and by etch rate experiments. After appropriate annealing, simple metal‐oxide‐semiconductor capacitors exhibited promising electrical properties.

Electrical Properties of Thin Anodic Oxides Formed on Silicon in Aqueous NH 4 OH Solutions

The capacitance vs. voltage and current density vs. voltage characteristics of silicon dioxide films formed by anodization of silicon in dilute ammonium hydroxide solutions have been measured. Postoxidation annealing (POA) at temperatures up to 700°C greatly reduces the leakage currents and results in breakdown voltages in excess of 10 MV/cm. However, leakage currents are still in excess of those obtained by thermal oxidation, possibly indicating some residual structural imperfection such as a transition layer or roughness at the silicon‐silicon dioxide interface. The combination of a POA at 700°C with a postmetallization anneal in forming gas at 400°C have reduced the interface state densities to with fixed charge densities at the interface of .

Kinetics of growth of thin anodic oxides of silicon at constant voltages

Constant voltage anodization of silicon was carried out to grow thin silicon oxide layers on silicon. Four different forming solutions, i.e. ethylene glycol + 0.04(N) HNO 3, KNO 3, NH 4NO 3 and Ba(NO 3) 2 were used. Oxidation proceeds at the rate of 6 Å V −. The current decay consisted of one sharply and another slowly decaying exponential. Initially, growth is linear with time when the rate of growth of the oxide is maximum. This is followed by drastic deviation from linearity towards saturation. The ionic current efficiency is highest for the forming solution containing NH 4NO 3 and the best value for this solution is 0.88% at 80 V. The refractive index of these oxides was found to be 1.46. The oxygen evolution at the anode during oxidation and the current caused by this evolution were studied using gas chromatography. The electronic properties, viz. mobile charge, fixed charge and interface states density of these oxides were measured by capacitance-voltage techniques and the best values were obtained for the solution containing 0.04(N) NH 4NO 3.

Growth kinetics of thin anodic oxides of silicon and its dependence on phosphorus concentration in silicon

A model has been proposed to study the growth kinetics of thin (no more than 300 Å thick) oxides of silicon, anodically grown at constant voltage by using ethylene glycol +0.04 N NH 4NO 3 as the electrolyte. The thickness of oxide is found to vary linearly with applied voltage at a rate of 6 Å V −1 for a phosphorus concentration in silicon of 1 × 10 15 atoms cm −3 which decreases to 5.5 Å V -1 and 5 Å V −1 for a phosphorus concentration of 8 × 10 20 atoms cm −3 and 1.3 × 10 21 atoms cm −3 respectively. The dependence of the rate of oxidation and other rate constants on phosphorus concentration in silicon have also been studied.

Invited paper. Anodic oxides on GaAs. IV. Thin anodic oxides on GaAs

As the anodic oxides produced on GaAs become thinner, the highly resistive nature of the oxides changes rapidly, allowing large leakage or tunnelling currents through the oxide without, however, permanently changing the original properties of the oxide layer. A considerable deviation from ideal insulator properties was observed with native oxides thinner than about 250 A, and with anodic AI2O3. thinner than 100 A. In the reverse bias conditions, MOS structures employing very thin oxides (< 100 A) can allow leakage currents of a few A cm-2 with non-destructive white light emission. By limiting the minority carrier leakage currents, inversion changes can be made to accumulate near the GaAs surface and with such thin oxide GaAs MOS structures, light-enhanced currents were observed with large enhancement factors.

Properties of the native oxide on GaSb

This paper reports on the optical and electrical properties of thin (t<3000 A) anodic oxides on p‐type GaSb. The oxides were grown in a 10−1N solution of KOH in methanol at constant current. Thin film interference techniques with phase shift and reflectivity corrections are used to determine the thickness, refractive index, and dispersion of the films in the range 2500<λ<8000 A. MOS structures were prepared and the conductivity, dielectric constant (K=8.5), and breakdown strength (200 V/μm) are determined. At 77 K C–V curves show the surface is normally depleted at zero bias. The resulting C–V curves are compared with the ideal and a typical density of states of 1.0×1012 s/cm−2 eV−1 is measured.

Filamentary switching and memory action in thin anodic oxides

An experimental evidence has been described which shows that reversible switching and memory action in thin anodic oxides can be governed by one and the same mechanism — a field induce formation of a predominantly metallic filament in the oxide.