Anodic Fluoride Research Articles

Composition and growth mechanism of nanoporous anodic fluoride films on stainless steel

Anodizing of 304L stainless steel performed in ethylene glycol solution containing 0.1 M NH4F and 0.1 M H2O at constant voltage under static conditions at 5 °C results in the formation of porous anodic films. Several analysis techniques revealed a rather complex composition of the anodic layer for stainless steel compared to that reported in the literature for iron in the same anodizing conditions. Contrary to what might be expected, the anodic layers consist mainly of iron and chromium fluorides rather than oxides. Furthermore, the multilayer fitting of the Rutherford Backscattered spectroscopy shows a decreasing content of chromium and nickel fluorides from the outermost layer to the innermost layer at the metal/film interface, which is composed only of iron fluoride. Film-assisted dissolution mechanisms and the Gibbs-free energy appear to be responsible for the cation distribution and compounds formed throughout the anodic film. In addition, the thickness and final composition of the anodic layer appear to be dependent on the cleaning process carried out after the anodizing.Graphical abstract

Anodic fluoride passivation of type II InAs/GaSb superlattice for short-wavelength infrared detector

One of the major challenges of antimonide-based devices arises owing to the large number of surface states generated during fabrication processes. Surface passivation and subsequent capping of the surfaces are absolutely essential for any practical applicability of this material system. In this paper, we proposed a new passivation method (zinc sulfide coating after anodic fluoride) for InAs/GaSb superlattice infrared detectors. InAs/GaSb superlattice short-wavelength infrared materials were grown by molecular beam epitaxy on GaSb (100) substrates. A GaSb buffer layer, which can decrease the occurrence of defects with similar pyramidal structure, was grown for optimized superlattice growth condition. High resolution X-ray diffraction indicated that the period of the superlattice corresponding to fourth satellite peak was 39.77 A. The atomic force microscopy images show the roughness was below 1.7 nm. The result of photoresponse spectra shows that the cutoff wavelength was 3.05 μm at 300 K.

ChemInform Abstract: Composition, Growth Mechanism, and Oxidation of Anodic Fluoride Films on Hg1-xCdxTe (x ≅0.2).

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A study of galvanomagnetic phenomena in MBE-grown n-CdxHg1−x Te films

The magnetic-field dependences of the Hall coefficient and the conductivity of n-type CdxHg1−xTe epitaxial structures were measured at 77 K. The structures were grown by molecular-beam epitaxy with a prescribed solid solution composition profile across the thickness. A specific feature of the obtained dependences is that the conductivity and the absolute value of the Hall coefficient decrease with an increasing magnetic field. The obtained experimental dependences can only be described in terms of a model including low-mobility electrons. It is shown that anodic oxide deposited onto the CdxHg1−xTe film surface makes the concentration of low-mobility electrons higher and that of anodic fluoride lower. The possible reasons for the appearance of low-mobility electrons are discussed. The most probable sources of such electrons are surface layers and electrical microheterogeneities in CdxHg1−xTe films.

Anodic fluoride on HgMnTe

The formation of anodic fluoride on HgMnTe has been studied and the composition of the interface and the chemical depth profiles have been characterized by x-ray photoelectron spectroscopy technique. Comparison results between HgMnTe and HgCdTe were given. Metal–insulator–semiconductor devices containing an anodic fluoride layer and a ZnS coating were fabricated and characterized. The mixture of anodic fluoride and thin native oxide layer may create a good interface for HgMnTe passivation.

Chemical effects of annealing ZnS-capped anodic films on Hg 1− xCd xTe( x≈0.2)

The chemical effects of annealing two cases of three-phase structures: ZnS-anodic-oxide-Hg 1− x Cd x Te and ZnS-anodic-fluoride-Hg 1− x Cd x Te used in infrared detector devices was studied using Auger electron spectroscopy. The presence of the three-element stable ion TeO 3 2− in the Hg Cd Te O system and the absence of an analog three-element ion in the Hg Cd Te F system lead to completely different degradation mechanisms in the two structures studied. While the anodic oxide reacts readily with the underlying semiconductor, forming CdTeO 3, the anodic fluoride is relatively stable in contact with the Hg 1 − x Cd x Te. On the other hand, the anodic fluoride reacts with the ZnS overlayer forming the stable ZnF 2 and ZnTeO 3 compounds and Te or HgTe. Electrical devices fabricated on Hg 1 − x Cd x Te using the two-layer ZnS-anodic-fluoride passivation process have a higher thermal stability than devices with the anodic oxide since the strong chemical interaction occurs at the ZnS-anodic-fluoride rather than at the critical fluoride-Hg 1 − x Cd x Te interface.

Chemical effects of annealing ZnS capped anodic fluoride films on Hg1−xCdxTe (x∼0.2)

The chemical effects of annealing a three-phase structure: ZnS–anodic fluoride–Hg1−xCdxTe used in infrared detector devices were studied using Auger electron spectroscopy (AES). The ZnS cap prevents the oxidation of the anodic fluoride upon annealing. The anodic fluoride is unstable in contact with the ZnS and a reaction takes place at their interface forming the stable ZnF2 compound and Te or HgTe. Nevertheless, the annealing does not affect the film–substrate interface as long as the fluoride has not reacted completely. This explains the relative thermal stability of electrical devices fabricated using this passivation scheme.

Properties of anodic fluoride films on Hg1−xCdtxTe

Abstract We present results concerning the properties of anodic fluoride films grown on Hg 1−x Cd x Te with values of x between 0.2 and 0.3. Analysis of the growth rate indicated that the film was only partially fluoridized, with a relatively high amount of HgTe and/or elemental tellurium present. X-ray photoelectron spectroscopy confirmed the presence of two different tellurium peaks, one corresponding to tellurium bonded fluorine, and the other to tellurium bonded mercury or to elemental tellurium. The films were characterized by optical properties such as refractive index and band gap, electrical properties such as dielectric constant and resistivity, and chemical properties such as their reactivity in various solvents. It is shown that the anodic fluoride layers behave as an insulator, with a strong tendency to become oxidized. The electrical characteristics of the anodic fluoride-HgCdTe interface were determined from capacitance-voltage measurements of metal/insulator/semiconductor devices.

Composition, Growth Mechanism, and Oxidation of Anodic Fluoride Films on Hg1 − xCdxTe ( x ∼ 0.2 )

Fluoridic films produced by the anodization of surfaces in nonaqueous solutions were characterized by Auger electron spectroscopy, x‐ray photoelectron spectroscopy, and electron microprobe analysis. The films consist of three distinct regions. A thick uniform region, containing the fluorides of cadmium, mercury, and tellurium, and, is covered by a thin layer. The third region—the film‐substrate interface—poor in mercury, consists mainly of and . Using ultra‐thin Pd marker layers and growth from baths saturated in it is shown that these anodic films are grown by two mechanisms: the dominant one occurs by motion of the film‐substrate interface into the semiconductor, consuming the original surface. There is, however, some growth at the film‐electrolyte interface which forms the thin layer on top of the structure. This second mechanism has a crucial effect on the stability of the film against oxidation. The rich region acts as a diffusion barrier for the in‐diffusion of oxidizing species. Tellurium ions, on the other hand, diffuse to the outer surface to be oxidized there.

The characterization of anodic fluoride films on Hg1−xCdxTe and their interfaces

A novel anodic fluoridization process for forming native fluoride films on Hg1−xCdxTe is described. As in the case of the anodic oxide, the anodic fluoride grows in two steps: a dissolution–precipitation step is followed by a bulk growth. Anodic fluorides free of oxygen are grown from nonaqueous solutions. The presence of water causes the growth of films containing both fluorine and oxygen, whereas a low hydroxyl ion concentration causes the growth of anodic oxide alone. However, its electrical behavior is different from that of anodic oxide grown from fluoride‐free baths. The results of Auger electron spectroscopy analysis indicate that the anodic fluoride is composed of cadmium fluoride in a matrix of tellurium, cadmium, and mercury. The fluoride is homogeneous and has a relatively abrupt dielectric–semiconductor interfacial transition, free of contaminants. Metal–insulator–semiconductor devices containing a fluoride film and evaporated ZnS have been fabricated and characterized. By choosing an appropriate ratio of fluoride to hydroxyl ions in the anodization bath the band bending at the n‐type Hg1−xCdxTe surface can be adjusted from near flatband conditions to strong accumulation. Anodization from a nonaqueous solution results in a p‐type Hg1−xCdxTe surface that is slightly depleted due to a fixed charge density of ∼5×1010 cm−2. There is only a negligible density of both slow and fast surface states. Low and reproducible surface state densities are achieved by a short dissolution–precipitation step. Such interfaces are stable up to 105 °C.