Anodic Sulfide Research Articles

The formation of anodic sulfide films on tin electrodes

The electrodeposition of tin sulfide on polycrystaline tin from alkaline solution has been investigated using cyclic voltammetry. The first anodic peak in the cyclic voltammogram is associated with the deposition of a Sn S monolayer from the adsorption of hydrosulfide on the electrode surface. At more positive potentials there is a second anodic peak related to a SnS 2 film over the electrode. The growth of the anodic film has been interpreted by the hopping-motion model for a high field assisted ionic migration process with the rate limiting energy barrier located at the metal/film interface.

Aqueous electrochemical growth of anodic sulfide films on mercury cadmium telluride

We present here the first demonstration that oxide-free anodic sulfide layers can be grown on HgCdTe from aqueous electrolytic solutions. Previous work has shown that anodic sulfide films grown from nonaqueous solutions have great potential as passivating layers for HgCdTe. In this work Auger electron spectroscopy depth profiles are used to show that little or no oxide is left at the HgCdTe/CdS interface even when an aqueous growth electrolyte is utilized. Capacitance-voltage data on metal-insulator-semiconductor structures show that the temperature stability of the aqueous sulfide films may be superior to those grown from nonaqueous electrolytes.

The interface chemistry of HgCdTe passivated with native sulfide layers grown from nonaqueous and aqueous polysulfide solutions

We have grown anodic sulfide passivation layers on HgCdTe from both nonaqueous and aqueous polysulfide solutions. In both cases CdS layers are nominally obtained. The growth behavior of the sulfide layers is found to be quite different in the different electrolytes. Capacitance–voltage measurements on metal-insulator semiconductor device structures that incorporated a ZnS cap and Pd gate metal over the CdS have been used to compare the electrical properties of the interfaces produced. Preliminary evidence suggests that aqueous anodic sulfide layers may be more stable than nonaqueous ones, but contain more positive fixed charge and mobile ion charge. Auger electron spectroscopy depth profiles indicate that the aqueous anodic sulfides are oxide free, but appear to contain more Hg and Te contamination than nonaqueous anodic sulfides.

Passivation of mercury cadmium telluride surfaces

The practical aspects of HgCdTe surface passivation, as related to devices and applications, are reviewed. Currently used technologies are classified in two categories: thick deposited dielectrics and a two-layer combination of a thin native film with a thick deposited dielectric film. The first category includes ZnS, low-temperature photochemical vapor deposited SiO2 (photox), and electron cyclotron resonance plasma deposited SiNx. The second category includes anodic oxide, plasma oxide, photochemical native oxide, anodic sulfide, and anodic fluorides. The unique behavior of HgCdTe surfaces, metal-insulator semicoductor structures, and gate-controlled p–n junctions are reviewed. The surface requirements of HgCdTe devices of interest for present and next-generation focal plane arrays are discussed in terms of the reviewed technologies.

Surface recombination velocity of anodic sulfide and ZnS coated p-HgCdTe

The surface recombination velocity s has been determined for Hg1−xCdxTe (x∼0.2) for two different surface passivations: (i) anodic sulfide with an overcoating of ZnS and (ii) ZnS coating on freshly etched samples. The method used was photoelectromagnetic effect, in which a magnetic field is applied perpendicular to the diffusion current of optically generated injected carriers. Analysis of the magnetic field dependence of the resulting current can yield s, as well as carrier mobility, lifetime, etc. The temperature dependence of s is very similar for the two passivations at temperatures higher than 50 K, and is increasing with decreasing temperature. At low temperatures s continues to rise for ZnS passivation and stays flat for the native anodic sulfide. Two activation energies are determined: 12.5±1.5 meV at temperatures higher than 60 K, and 2.3±0.2 meV at temperatures lower than 30 K. The high-temperature activation energy is identical for both passivations. It is concluded that the same surface traps control the surface recombination rate for the two passivations, and their concentration is equal in both cases. The traps are apparently ‘‘intrinsic,’’ and can be related to lattice defects, most probably vacancies. Their concentration at the surface is similar to that in the bulk.

The temperature dependence of the anomalous Hall effects in p-type HgCdTe

The anomalous Hall effects of narrow-band-gap p-type HgCdTe, which manifest themselves as negative Hall coefficients at low temperatures, have caused serious problems in material characterization in the past two decades. These phenomena are now widely recognized as being caused by the inverted surface effect describable by Petritz’s two-layer model [Phys. Rev. 110, 1254 (1958)]. We report results from variable-temperature Hall measurements on p-type HgCdTe liquid-phase epitaxy and bulk slices with bare, anodically sulfidized, and anodically oxidized surfaces. We show that the Hall anomalies can be eliminated by depositing an anodic sulfide layer on the surface and subsequently can be restored by removing the sulfide layer. Based on Petritz’s two-layer model, we were able to use the same set of bulk transport parameters and different sets of surface transport parameters to fit the experimental temperature-dependent Hall coefficient and Hall mobility curves of the same sample with surfaces that have been subjected to different chemical treatments. It was demonstrated that the Hall anomalies occurred when the n-type surface conductivity increased relative to the p-type bulk conductivity. The surface conductivity is mainly controlled by the density of fixed positive surface charges which was found to be larger on an anodically oxidized surface and smaller on an anodically sulfidized surface than on a bare surface.

The Effect of Surface Layers in Epitaxial N-Type Hg1−xCdxTe

ABSTRACTThe presence of low mobility surface electrons on n-Hg1−1CdxTe is generally not apparent in the temperature dependence of the Hall Coefficient (RH) or the Hall mobility (µH) of the layer. However, its influence is clearly seen in the magnetic field (B-field) dependence of RH. The B-field dependence of RH can be analyzed to extract the bulk and surface carrier concentrations and their respective mobilities.This diagnostic technique has been used for evaluating epitaxial Hg1−xCdxTe layers grown by organometallic vapor phase epitaxy (OMVPE), which have been converted to n-type by annealing in Hg overpressure. In addition, the effect of anodic sulfide passivation on the B-field dependence of Hall coefficient is also outlined.

Hg content and thermal stability of the anodic sulfide films on Hg1−xCdxTe investigated by 30–40-MeV O5+ ion backscattering

The Hg content of anodic sulfide films on Hg1−xCdxTe (MCT) were investigated for as-grown and heat-treated films by using 30–40-MeV O5+ ion backscattering. Heat treatments were performed in a vacuum for 60 min at various temperatures up to 300 °C for the samples with x=0.20 and up to 350 °C for the samples with x=0.30 and 0.35. A significant Hg content of about 15 at. % of Cd was found in the as-grown anodic sulfide film on MCT samples with x=0.20, 0.30, and 0.35. After heat treatment above 260 °C for x=0.30 and 0.35 and above 210–240 °C for x=0.20, the Hg atoms were removed from the films, and Hg deficient layers were formed near the interface region of the MCT crystal. This suggests that the residual Hg in the anodic sulfide films introduced during the sulfidization process plays an important role toward the degradation of the passivation properties of the sulfide film on MCT.

Inverted surface effect of p-type HgCdTe

By surface passivation using anodic sulfidization, we demonstrated that the inverted surface effect which gives rise to negative Hall coefficients commonly measured in p-type HgCdTe at low temperatures can be eliminated. Our results of Hall measurements as a function of magnetic field at 77 K and computer simulations allow us to distinguish two different existing models (with shunting or nonshunting n-type inversion layer) of the inverted surface effect. Bulk and surface transport parameters such as hole concentration, hole mobility, surface electron concentration, and surface electron mobility have been derived from computer best fits of experimental Hall coefficient curves.

Characterization of anodic sulfide films on Hg0.78Cd0.22Te

Anodic sulfide films were grown on HgCdTe and studied by Auger electron spectroscopy, surface analysis by laser ionization, Rutherford backscattering spectroscopy, Raman and photoluminescence spectroscopy, and high-resolution transmission electron microscopy to determine their elemental composition and their chemical and physical structure. This information, when coupled with a knowledge of the possible chemical reactions at the interface between the film and the HgCdTe substrate, explains why anodic surface films are more thermally stable than anodic oxide films.

Ionizing Radiation Effects in N-Channel (Hg,Cd) Te MISFETs with Anodic Sulfide Passivation

The effect of ionizing radiation, up to a dose of 1.6x105 rad(ZnS)**, on n-channel (Hg,Cd)Te MISFETs, fabricated using anodic sulfide/ZnS insulators, has been investigated. Devices were irradiated under DC bias at 78K. Radiation induced threshold voltage shifts, degraded transconductance, and decreased subthreshold current-gate voltage slopes were observed. For all but the most negative biases during irradiations threshold voltage shifts (a maximum of -6.0 V at 1.6×105 rad(ZnS)) are associated with net hole trapping, with evidence for trapping near the interface for zero applied field during irradiation. Evidence of electron trapping was observed for large negative biases during irradiation. The radiation induced degradation in transconductance is attributed to surface mobility reduction (30% after 1.6×105 rad(ZnS)) associated with scattering from interface charge. The decreassd subthreshold slope (82 mV/decade at 50K after 1.0×105 rad(ZnS)) is attributed to modification of the pre-irradiation gate voltage-surface potential relationship by radiation induced interface traps.

The electrochemical behaviour of copper in alkaline solutions containing sodium sulphide

The electrochemical behaviour of copper in NaHCO3 solution (pH 9) and NaOH solutions (pH 14) in the presence of sodium sulphide (10−3 to 5×10−2m) was investigated by using rotating disc electrode and rotating ring-disc techniques, triangular potential scanning voltammetry and potentiostatic steps. When the potential increases from −1.2V upwards, copper sulphide layers are firstly formed at potentials close to the equilibrium potentials of the Cu/Cu2S and Cu/CuS reversible electrodes. When the potential exceeds 0.0 V (NHE), the copper oxide layer is electroformed. Pitting corrosion of copper is observed at potentials greater than −0.3 V. The charateristics of copper pitting are also determined through SEM optical microscopy and EDAX analysis. There are two main effects in the presence of sodium sulphide, namely, the delay in the cuprous oxide formation by the presence of the previously formed copper sulphide layer and the remarkable increase of copper electrodissolution when the potential exceeds the cuprous oxide electroformation threshold potential. These results are interpreted on the basis of a complex structured anodic sulphide layer and on a weakening of the metal-metal bond by the presence of adsorbed sulphur on the copper surface.

Interface of p-type Hg1−xCdxTe passivated with native sulfides

The process for forming native sulfide films on p-type Hg1−xCdxTe with x≂0.2–0.3 by anodic sulfidization is described and studied. The analytical, optical, and electrical properties of the resulting interface are reported. The results of Auger electron spectroscopy analysis indicate that homogeneous CdS films with an abrupt interfacial transition are formed on Hg1−xCdxTe with x=0.215–0.290. The measured dispersion of the index of refraction and the dielectric constant of the native sulfide films are consistent with the reported data for bulk CdS. The interface between Hg1−xCdxTe and its native sulfide in combination with deposited ZnS has excellent electrical properties. A low fixed surface state density of the order of Nss ≂5×1010 cm−2, a low concentration of fast surface state density of the order of 5×109 cm−2 eV−1, and a small amount of trapping effects in the sulfide are observed. The MIS capacitors exhibit thermal stability up to 95 °C anneal in vacuum. The main features of the interface of p-HgCdTe utilizing native sulfides are the low fixed surface states and the practically flatband conditions which are imposed for a wide range of composition x and doping level of the substrates. It contrasts with the high fixed positive surface-state density which is observed on interfaces utilizing native oxides of HgCdTe and which invariably invert the surfaces of p-Hg1−xCdxTe. The properties of the interface between Hg1−xCdxTe and its native sulfide suggest potential applications for improving the surface passivation of photovoltaic diodes implemented on p-type Hg1−xCdxTe.

XPS and magneto-transport studies on the surface of CdHgTe

X-ray photoemission spectroscopy (XPS) measurements linked with an argon ion sputtering technique have been performed to characterize surface chemical and physical properties of p-type Cd 0.25Hg 0.75Te. The results acquired under dry oxidation conditions suggest that the production of Hg vacancies and Cd migration play an important role for the oxidation process, and CdTeO 3 is considered to be a major constituent on the oxide surfaces. Transverse magneto-resistance measurement was made of p-type samples with anodic films at 4.2 K up to 5 T. Shubnikov-De Haas oscillation was observed in the samples with anodic oxide film, but not in the case with anodic sulfide film. The effect of the interface on the magneto-transport properties is discussed.

Formation and Reduction of Anodic Sulfide Films on Antimony Electrodes

The growth and reduction of anodic films on antimony electrodes in alkaline sulfide solutions has been investigated using potentiodynamic, galvanostatic, and potentiostatic techniques. The sulfidation mechanism and the efficiency of charge storage in the sulfide film are dependent on the electrode potential and current density. The initial stages of film formation follow a high field growth process involving ionic transport through the sulfide film. The growth of thicker films is controlled by coupled electrochemical and chemical reactions involving mass transport and nucleation. Metal dissolution accompanies film formation under most growth conditions. The anodic and cathodic behavior of these films is consistent with their rectification or n‐type semiconductor properties.

Anodic sulfide films on Hg1−xCdxTe

A novel anodic sulfidization process for forming native sulfide films on Hg1−xCdxTe is described. In the new process native sulfide films rather than native oxides terminate the lattice and passivate the surface. The results of Auger electron spectroscopy analysis indicate that native homogeneous CdS films are formed with an abrupt interfacial transition. The measured capacitance-voltage characteristics of metal-insulator-semiconductor devices indicate that the films have a low negative fixed surface charge density of the order of –1×1011 e cm−2 and a relatively low concentration of fast surface charge density. The native sulfide films leave the surface of p-type Hg1−xCdxTe practically at flat band and in this respect are superior to native oxide films which invert the surface of p-type material. The new surface passivation is in particular suitable for photovoltaic diodes implemented on p-type Hg1−xCdxTe.

Semiconductor liquid junction solar cells based on anodic sulphide films

ALTHOUGH good performance has been achieved with established solid state junction solar cells, their cost remains well above that required for large scale terrestrial applications. New concepts1,2 for photovoltaic energy conversion have been introduced which involve the junction between a semiconductor and an electrolyte. These schemes may lead to new possibilities of considerably reduced materials and fabrication costs, providing that many technical problems could be solved. We report here metal–semiconductor–liquid electrolyte junctions in which an n-type sulphide semiconductor is anodically formed in situ on its metal and, under photoexcitation, drives a sulphide–polysulphide redox couple when connected to a suitable cathode. The original contribution in this area was made by Gerischer2 who demonstrated that the cell (single crystal n-CdS/Fe(CN)64−, Fe(CN)63−/SnO2) has an initial conversion efficiency > 5% (ref. 3) for sunlight to electric power, in spite of the 2.4-eV band gap of CdS, which implies that wavelengths > 550 nm (the greater part of the incident solar photons) are ineffective.

Analytik der polyacrylnitrilfasern : V über fine abgewandete reduktionsmethode zur bestimmung geringer schwefelmengen in acrylnitrilpolymerisaten

Sulphur contents (0.01–0.2% S) in polyacrylonitriles (50–100 mg samples) can be estimated by decomposition of the polymer by heating with KOH, reduction of the sulphate formed (with acetic acid, hydriodie acid, and hypophosphite) and absorption of hydrogen sulphide in 1 N sodium hydroxide. Sulphide is then determined polarographically utilizing the anodic sulphide wave, or potentiometrically with 0.001 M mercuric chloride as titrant and amalgamated silver wire and stainless steel wire as electrodes. The whole procedure is done easily in a stream of nitrogen and can therefore be applied even when the atmosphere is contaminated with sulphur-containing substances.