Anodic Sulfide Films Research Articles

Optical characterisation of anodic sulphide films on Hg 1− xCd xTe (MCT) grown by the potential step method

The growth of native sulphide films on Hg 1− x Cd x Te (MCT) by potential steps to two different electrochemical growth regions has been studied by in-situ ellipsometry, photocurrent spectroscopy and second harmonic generation (SHG) rotational anisotropy. Films grown at −0.4 V versus SCE were porous and consisted mainly of cubic close packed (ccp) CdS. The photocurrent spectrum of the film exhibited a cut-off which closely conformed to the expected bandgap of CdS. The gradual increase in SH intensity as the sulphide film thickened was due to a contribution to the overall SH signal from the CdS film itself. No evidence of any hcp species was apparent in the rotational anisotropy patterns, with the four-fold pattern expected for the vicinal MCT surface being maintained even after film growth. When the potential was stepped to −0.3 V, the film grown absorbed the ellipsometer radiation once a thickness of 86 nm was reached. Photocurrent measurements showed a tail extending into the red region of the spectrum. These phenomena have been attributed to the incorporation of ccp HgS into the sulphide film as at this potential, the electrochemical reaction of the HgTe component of MCT can occur. SHG rotational anisotropy confirmed the four-fold symmetry of the ccp surface film.

Thickness Dependence of the Anisotropy of Anodic Sulphide Films on CdxHg1?xTe Studied Using Surface Second Harmonic Generation

The growth of anodic sulphide films on vicinal Cd x Hg 1-x Te (CMT) surfaces has been investigated as a function of sulphide layer thickness by rotation anisotropy using second harmonic generation (SHG). The presence of thin CdS films on the CMT surface does not alter the fourfold pattern observed for the bare CMT surface. However, with increasing sulphide film thickness (>100 nm) and for growth at constant potential, a drastic loss in the reflected SH intensity occurs. This reduction has been attributed to the formation and incorporation of β-HgS in the layer at the more positive potentials. This compound absorbs the SH signal generated by the composite system of CMT/sulphide layer.

Thickness Dependence of the Anisotropy of Anodic Sulphide Films on CdxHg1xTe Studied Using Surface Second Harmonic Generation

The growth of anodic sulphide films on vicinal CdxHg1—xTe (CMT) surfaces has been investigated as a function of sulphide layer thickness by rotation anisotropy using second harmonic generation (SHG). The presence of thin CdS films on the CMT surface does not alter the fourfold pattern observed for the bare CMT surface. However, with increasing sulphide film thickness (>100 nm) and for growth at constant potential, a drastic loss in the reflected SH intensity occurs. This reduction has been attributed to the formation and incorporation of β-HgS in the layer at the more positive potentials. This compound absorbs the SH signal generated by the composite system of CMT/sulphide layer.

Surface second harmonic generation in the characterization of anodic sulphide and oxide films on Hg1−xCdxTe (MCT)

Rotation anisotropy by second harmonic generation (SHG) is carried out on epitaxial Hg1−xCdxTe (MCT) and oxide- and sulphide-covered MCT surfaces and shows the fourfold symmetry pattern expected from the {100} surface (C4v symmetry). The uneven nature of the four peaks confirm the vicinal surface obtained from the growth of the MCT on GaAs {100} substrate orientated 4° toward the 〈110〉 direction. The increase in the SH intensity observed for the oxide-covered MCT surface is associated with charge accumulation at the MCT/oxide interface since the oxide is centrosymmetric and cannot generate SH. The CdS layer on the other hand is strongly nonlinear active and generation here comes from a composite of one noncentrosymmetric layer on top of another. This leads to interactions in the observed SH arising from the coupling depths (∼40 nm) at the two interfaces and from the coherence length (∼1200 nm) in the CdS layer. The in-situ SHG measurements during the growth of the anodic oxide and sulphide layers would suggest that a species, most likely HgTe is embedded in the anodic layer during the initial stages and absorbs the SH radiation at 532 nm. The rotational anisotropy of the sulphide-covered MCT surface confirms that the CdS layer formed maintains the cubic closed pack symmetry of the underlying MCT.

Anodic sulfide films on HgTe and Cd0.24Hg0.76Te studied by in-situ ellipsometry

L. E. A. Berlouis, P. V. E. Elfick and H. Tarry, J. Chem. Soc., Faraday Trans., 1997, 93, 2291 DOI: 10.1039/A701067B

Growth and characterization of anodic sulphide films on Hg 0.80Cd 0.20Te grown from freshly prepared solution and stored solution

Anodic sulphide films on Hg 0.8Cd 0.2Te have been grown using a non-aqueous solution of Na 2S and ethylene glycol. Good uniform reproducible films are obtained when the pH value of the solution lies in the range 11–12. The films have been characterized using Auger electron spectroscopy, electron spectroscopy for chemical analysis and optical measurements. It has been observed that films grown from freshly prepared solution are mainly sulphide while these grown from stored solution contain both sulphides and oxides. The refractive index of the films grown using stored solution has been observed to be significantly lower than that of films grown using freshly prepared solution.

In-situ spectroscopic characterisation of passivation layers on (Cd,Hg)Te

The growth of anodic sulphide films on CMT from aqueous solutions is investigated. In-situ spectroscopic ellipsometry and photocurrent measurements are used to characterise the surface films. The initial stages of growth appears to yield barrier-type films of CdS whereas the thicker films (>100 Å) are shown to have a porous structure. Impurities are also incorporated during growth into the thicker sulphide layers which increase the light absorption below the band gap of CdS (2.42 eV).

Thermal instability of anodic sulfide films on Hg1−xCdxTe

Thermal instability of anodic sulfide films on Hg1−xCdxTe was studied. Instead of using laser ionization or ion backscattering in studying oxidic films after heat treatment, we used x-ray photoelectron spectroscopy (XPS) in analyzing nonaqueous sulfidization. In this case, XPS is feasible, simpler, and has the advantage of better resolution. Hg evaporation was investigated here by measuring Te(3d5/2) of Te oxide peak rather than Hg lines. Correlation between thermal instability and composition x was obtained.

Traps in ZnS/Anodic Sulfide Film on Mercury Cadmium Telluride

ABSTRACTWe studied the influence of visible light on the ZnS/anodic sulfide/HgCdTe interface.We measured flatband voltage of metal-insulator-semiconductor diodes with semitransparent electrodes at 77 K. In the dark (i.e., without visible light). The flatband voltage of an metal-insulator-semiconductor diode is 0 V. We scanned visible light from 400 to 800 nm and measured the flatband voltage using the photocapacitance measurement. After an initial positive shift, the flatband voltage moved negative by as the wavelength shortened. We studied the relationship between the flatband voltage shift and photon energy using Fowler plots. Results suggest one electron trap level 1.5 eV above the valence band and two hole trap levels 1.7 and 2.3 eV below the conduction band.ZnS/anodic sulfide films on mercury cadmium telluride, studied using photoluminescence spectroscopy, showed broad photoluminescence peaks at 1.5, 1.7, and 2.3 eV, and the edge emission of the anodic sulfide.These results suggest that photoionization of the electron trap and two hole traps in the ZnS/anodic sulfide film causes the flatband shift.

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.

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.

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.

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.

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.