CdSe Substrates Research Articles

Design of ZnS nanospheres antireflective structure for mid and far infrared applications

Developing high-performance mid and far infrared antireflection films is a significant requirement in the field of infrared optics. This study successfully prepared AR films composed of ZnS nanospheres for mid and far infrared bands using a combination of hydrothermal and sol-gel methods. Initially, ZnS nanospheres with uniform size and good monodispersity were synthesized via the hydrothermal method and subsequently coated on CdSe substrate through dip coating. The refractive index of the ZnS film is adjusted between 1.36 and 1.52. A significant AR effect was observed in ZnS nanospheres film on CdSe substrate at 3–5 μm and 8–14 μm, and the reflectance is approximately 3 % which is about 25 % lower than that of the uncoated sample. Upon coating with a TiO2 protective layer, the reflectivity further decreased to about 0.6 % and the film demonstrated excellent weather resistance and mechanical properties. This work provides a novel approach for the preparation and regulation of mid and far infrared AR film, significantly contributing to the development of optical film.

Preparation of double-layer two wavelength infrared antireflective coating on CdSe substrate

Designing and fabrication of the infrared antireflection coatings are quite important for the application of infrared optical elements. Herein, double-layer two wavelength infrared antireflective (AR) coating was prepared on CdSe substrate by sol-gel method, in which the bottom layer is porous TiO2 film and the top layer is SiO2 nanoparticle film containing a small amount of TiO2 (molar ratio of Si/Ti is 7.7). The reflectivity of the coated sample is about 2.1% at 2000 nm and 2.8% at 2500 nm, respectively, and the average reflectivity of the whole wide band from 1800 nm to 2600 nm is about 2.6%, which is about 27% lower than that of uncoated CdSe substrate. By adding reasonable additives and designing the film structure, the problem that thick infrared film is not resistant to high temperature annealing was solved. Moreover, the AR coating shows a super hydrophilicity, which can avoid the influence of water mist condensation on the transmittance. Importantly, this AR coating exhibits a good thermal shock resistance, and the reflection spectrum of the film did not change after ten thermal shock cycles at 400 °C, which meaning this film can be expected to meet the requirements of high temperature irradiation environment. This work provides important guidance for the preparation of infrared AR film with high thermal stability.

Luminescence of broad-band compounds of elements of groups II–VI with a tin impurity

This paper discusses the luminescence properties of films of cadmium and zinc chalcogenides obtained by the diffusion of tin from the vapor phase in a closed volume. It is established that the doping of ZnSe, CdS, and CdSe substrates increases the efficiency of the edge emission, while that of ZnTe causes it to disappear and causes a new band to appear in the energy region 1.8–2.1 eV. The absence of luminescence of CdTe : Sn films in the 0.6–1.6-eV range is explained by band corrugation.

Effective photoelectric converters of ultraviolet radiation with graded-gap ZnS-based layers

The use of ultrathin (∼10 nm) stable p-Cu1.8S films as a transparent component of the p-Cu1.8S-n-ZnS heterojunction as well as of the graded-gap layers made it possible to obtain effective photoconverters of ultraviolet radiation. The results of examination of the properties of photoactive Cu1.8S-ZnS junctions grown on the CdS or CdSe substrates with intermediate graded-gap layers CdS-ZnxCd1 − xS or CdSe-(ZnS)x(CdSe)1 − x, respectively, are presented. With the correct selection of parameters of the substrates, the graded-gap layers allows one to attain the optimal characteristics of the p—n junction, to realize high electric fields at the Cu1.8S-ZnS contact, and to solve the problem of fabrication of the back ohmic contact to ZnS without additional doping of all components of the heterostructure with a foreign impurity. Varying the thickness of a thin ZnS layer, it is possible to control the extension of the space charge in the graded-gap layer and thereby to control the long-wavelength edge of photoconverter sensitivity.

Ordered CdS micro/nanostructures on CdSe nanostructures

Composite structures of aligned and orientation-ordered quasi-one-dimensional CdSmicro/nanostructures on CdSe substrates of different shaped nanostructures have beensynthesized by using two-step thermal evaporation processes. The CdSe substratecrystalline orientations and local temperatures play their roles in the CdS nanostructuregrowth step, which is in some contrast with the vapor–liquid–solid (VLS) growthmechanism. Micro-photoluminescence measurements show strong luminescence responseson the six-fold symmetrical CdSe/CdS nanostructure. Controllable growth onvarious shaped substrates may find applications in obtaining many other alignedorientation-ordered hetero-nano/microstructure materials.

Electrodeposition of polycrystalline ZnTe from simple and citrate-complexed acidic aqueous solutions

The cathodic electrodeposition of polycrystalline zinc telluride from aqueous acidic solutions of zinc sulphate and tellurite, under constant or pulsing voltage, is described. Various compositions of the bath were tested with respect to the Zn/Te precursor ratio, including addition of citrate buffer as a complexation agent. Metallic (Ni and Ti) and semiconductor (CdTe and CdSe) electrodes were used as cathode-substrates. The deposits were characterized by XRD, SEM-EDX and FTIR techniques. The formation of compact barrier layers of zinc blend, stoichiometric ZnTe or mixed (Zn,Cd)Te was attained. Pulse plating in a citrate-free bath and constant potential plating from a citrate bath were seen to have the most beneficial effect on the properties of the electrodeposits, as leading to improved microstructure of the ZnTe films, in terms of crystallinity and stoichiometry. Further, the employment of [1 1 1]-oriented CdTe or CdSe substrates featured an epitaxial growth of polycrystalline ZnTe films as well as an improved Zn/Te atomic ratio compared to the metallic substrates.

Pore Etching in III-V and II-VI Semiconductor Compounds in Neutral Electrolyte

We propose to use a neutral electrolyte based on an aqueous solution of NaCl instead of commonly used aggressive acids or alkaline electrolytes for the purpose of electrochemical nanostructuring of GaAs and CdSe substrates. It is shown that the process of material porosification can be controlled by the conditions of anodic etching. A photoluminescence analysis of the porous structures obtained and referenced to the as-grown substrate demonstrates that an effective passivation of the surface occurs during anodization in this electrolyte. The results obtained pave the way for the development of optoelectronic devices based on electrochemically nanostructured GaAs and CdSe compounds, particularly for high-efficiency solar cells.

SYNTHESIS AND CHARACTERIZATION OF CdSe/ZnO CORE/SHELL NANOCRYSTALS

Highly luminescent monodispersed CdSe nanocrystals were prepared from CdO and Se powder in a three-component hexadecylamine-trioctylphosphine oxide-trioctylphosphine (HAD-TOPO-TOP) mixture under high temperature and nitrogen protection. CdSe/ZnO core/shell nanocrystals were produced by adding Zn(Ac) 2 and LiOH into CdSe nanocrystals solution under ultrasonic at low temperature. ZnO shell grows heterogeneously on the CdSe substrate. The structure and fluorescence have been characterized by TEM (transmission electron micrograph), XRD (X-ray diffraction), UV-Vis and PL spectra. It can be found that after the surface passivation with ZnO shell, electrons were confined in the CdSe core due to the wide band-gap effect of ZnO , which resulted in the obvious increasing of the fluorescent intensity.

Aspects of ZnSe electrosynthesis from selenite and selenosulfite aqueous solutions

Cathodic electrodeposition of ZnSe is studied in aqueous acidic selenite (Se(IV)O2) baths of free Zn2+ ions and alkaline selenosulfite (Se0SO32−) baths of zinc complex ions. The synthesis of polycrystalline, cubic ZnSe by Se(IV)-diffusion-controlled electrodeposition from acidic solutions is optimized in terms of selenite concentration, while the effect of Ti, Ni, anodized Ti and CdSe substrates on the obtained layers’ growth and structural properties is investigated. Furthermore, the production of stoichiometric though amorphous ZnSe is demonstrated using selenosulfite solutions. The results are valuated by means of X-ray diffraction, scanning electron microscopy and reflectance spectroscopy techniques.

Photoluminescent Properties of Cadmium Selenide in Contact with Solutions and Films of Metalloporphyrins: Nitric Oxide Sensing and Evidence for the Aversion of an Analyte to a Buried Semiconductor−Film Interface

The band-edge photoluminescence (PL) intensity of etched n-CdSe single crystals is quenched reversibly by adsorption of the trivalent metalloporphyrins, MTPPCl (TPP = tetraphenylporphyrin; M = Mn, Fe, Co) in nitrogen-saturated methylene chloride solution. The PL responses are concentration dependent and can be fit to the Langmuir adsorption isotherm model to yield binding constants of ∼103−104 M-1. The MTPPCl compounds react irreversibly with NO in solution to form nitrosyl adducts, and these compounds reversibly enhance the CdSe PL intensity when adsorbed onto the semiconductor surface, also with binding constants of ∼103−104 M-1. Films of MTPPCl were prepared on CdSe substrates by solvent evaporation. These coatings serve as transducers for NO detection: while the bare CdSe surface shows no response to NO gas relative to N2, the coated surfaces reversibly enhance the PL intensity (CoTPPCl) or quench it (MnTPPCl and FeTPPCl), with binding constants on the order of ∼1 atm-1. In contrast to the PL results...

Photoluminescent Properties of Cadmium Selenide in Contact with Solutions and Films of Metalloporphyrins. Evidence for Semiconductor-Mediated Adduct Formation of Oxygen with Metalloporphyrins at Room Temperature

The band-edge photoluminescence (PL) intensity of etched n-CdSe single crystals is modulated through adsorption of 17 porphyrins and metalloporphyrins from methylene chloride solution. In nitrogen-saturated solution, the PL intensity is reversibly enhanced through adduct formation with octaethylporphyrin (OEP), tetraphenylporphyrin (TPP), their divalent Co, Cu, and Cd derivatives, and with tetra(pentafluorophenyl)porphyrin (TFP), tetra-(4-tolyl)porphyrin (TTP), and tetra-(4-pyridyl)porphyrin (TPyP); in contrast, the PL intensity is reversibly quenched by exposure to divalent Mg, Ni, and Zn complexes of OEP and TPP. The concentration dependence of the PL changes can be fit to the Langmuir adsorption isotherm model to yield binding constants that range from ∼103 to 105 M-1. Saturating methylene chloride with oxygen rather than nitrogen does not affect the CdSe PL nor does oxygen affect the (metallo)porphyrin absorption spectra in methylene chloride solution. However, many of the metalloporphyrin-induced PL responses and binding constants are substantially affected by the change in dissolved gas ambient, evidencing surface-mediated adduct formation between oxygen and these metalloporphyrins at room temperature. Particularly large effects are observed for the Mg, Ni, and Zn OEP and TPP complexes, which produce PL enhancements in oxygen-saturated solution rather than PL quenching in nitrogen-saturated solution. When a thin film of each metalloporphyrin is deposited onto CdSe substrates by solvent evaporation, oxygen can be detected through reversible PL enhancement (NiTPP and ZnTPP) or quenching (all other metalloporphyrins examined save for CdOEP and CdTPP, which gave no response) relative to a gaseous nitrogen ambient. Binding constants obtained were typically in the range ∼1−10 atm-1. Steric and electronic contributions to these effects are discussed.

Adsorption of Alkanolamines onto Semiconductor Surfaces: Cadmium Selenide Photoluminescence as a Probe of Binding and Film Reactivity toward Carbon Dioxide

Films of ethanolamine, 3-aminopropanol, and 4-aminobutanol, 1−3, serve as transducers for the detection of CO2 when deposited onto emissive CdSe substrates: the band-gap photoluminescence (PL) intensity of the semiconductor is reversibly quenched by exposure to CO2, while uncoated CdSe samples display no response relative to a nitrogen reference ambient. Infrared (IR) spectra obtained with films of these alkanolamines and related alcohols and amines, 4−13, are consistent with selective reaction of the amine functionality in these films to form carbamates. The direction of the PL response corresponds to an enhancement of Lewis acidity accompanying the reaction with CO2, and use of a dead-layer model permits an estimation of the adduct-induced expansion of the depletion width as being roughly a few hundred Angstroms. Both IR and PL data reveal that the film response to CO2 begins at ∼0.1 atm and saturates by ∼0.3 atm, and their similarity indicates that the partition coefficient for binding CO2 to the bulk...

Electronic properties of clean CdSe surfaces upon Fe adsorption

Clean surfaces of n-type CdSe single crystals have been prepared by cleavage along the c-axis under ultrahigh vacuum. They were exposed to increasing doses of Fe vapor and received from 10 −1 to 10 2 monolayer (ML) deposits. At each step they are studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS). The results show that Fe dissociates the CdSe top layers. It forms an Fe Se compound on top of a Cd layer up to 2.5 ML deposit beyond which no more Se is extracted from the CdSe substrate. At that stage further Fe dilutes the Se atoms in the top alloyed layer to which the d electrons of Fe bring already a metallic character after 1 ML deposition.

Electrochemical characteristics of silver treated cadmium selenide single crystal electrodes

CdSe electrodes dip-treated in 10 mM AgNO 3 solution behave almost as a silver electrode because the CdSe/Ag interface has ohmic characteristics. On the other hand, CdSe electrodes modified by vacuum evaporation of silver and then heat-treated in a He atmosphere showed negligible dark currents in the anodic potential region. Cathodic as well as anodic photocurrents were observed at this electrode. The electrode was relatively stable and although the anodic photocurrent at this electrode was initially much smaller than the anodic photocurrent at the unmodified CdSe electrodes, it became larger than the anodic photocurrent observed at the unmodified electrode after illumination for 1000 s. A quite unusual quantum efficiency action spectrum was obtained at this electrode. A very sharp peak around 725 nm, a low but constant quantum efficiency in the shorter wavelength region and an extended response to sub-band illumination were observed. These results suggest the formation of a surface layer with many impurity levels. AES and XPS depth profiling show that Ag diffuses deeper into the CdSe substrate than several tens of nanometers. The XPS study also suggests that Ag exists as Ag(0) in this layer. The junction characteristics of Au/CdSe and Au/CdSe(Ag) show that this layer has a very high resistance. PL measurements proved the introduction of high density impurity levels within this layer. On the basis of these results, a band model is presented for this electrode.

Production and characterization of diodes made in thin p-Se layers on top of n-CdSe

Se layers were grown on top of single crystals of n-CdSe in view of their possible use in a solid-state solar cell. The layers were produced either by electrodeposition from SeO 2 in a pH 5 buffer at room temperature (298 K) and at 353 K, or by photocorrosion in a partially stabilizing solution containing various amounts of K 4[Fe(CN) 6] and 1 M KCl. The photoelectrochemical behaviour of the CdSe/Se structures was checked in phthalate buffer (pH 5) with and without the redox species (K 3[Fe(CN) 6], (K 4[Fe(CN) 6], and SeO 2). The difference in the Se and CdSe light absorption coefficient was used to excite charge carriers in either the Se layer or the CdSe substrate (λ = 500 or 700 nm monochromatic light, respectively). The behaviour of the layers deposited at 298 and 353 K differs: the higher temperature induces the formation of the trigonal modification of Se, which has a higher photoactivity than that of the amorphous Se deposited at 298 K. It is concluded that Se electrodeposited at 353 K forms p-n heterojunctions in which both Se and CdSe are active. Both photoanodic and photocathodic currents are observed and the i- Vcurves show the existence of more than one “flat-band potential”. The Se layers formed by decomposition are porous and thus no barrier is present.

Mapping the efficiency of electron-hole pair separation for a semiconductor electrode. Luminescent properties of graded cadmium sulfoselenide electrodes

Abstract : Inhomogeneous samples of n-type CdS(X)Se(1-X) (0 or = X or = 1) have been prepared by vapor-phase diffusion of S into a single-crystal CdSe Substrate. Characterization by Auger electron spectroscopy (AES) in conjunction with Ar ion sputter etching reveals that S has substituted for Se in the lattice to produce a graded region. The depth profile analysis indicates a composition of CdS over the first approx. 0.05 micron from the surface; over the next approx. 2 micron from the surface, the composition monotonically decreases in X, corresponding to a decline in band gap energy from approx. 2.4 eV for CdS to approx. 1.7 eV for CdSe. Photoluminescence (PL) and electroluminescence (EL) from the graded material appear to derive from the luminescence of the CdS(X)Se(1-x) compositions which comprise the graded region: Emission from approx. 470-750 nm matches the spectral region spanned by PL and EL from homogeneous, single-crystal CdS(X)Se(1-X) samples which emit near their band gap energies. This assignment is also supported by chemical etching experiments in which the removal of S-rich, near-surface strata progressively red-shifts the PL spectrum.

Photoluminescence and electroluminescence in graded cadmium sulfoselenide electrodes: Applications to photoelectrochemical cells

Inhomogeneous samples of n-type CdS x Se 1− x (0 ≤ x < 1) were prepared by vapor-phase diffusion of S into a single-crystal CdSe substrate. Characterization of the samples by Auger electron spectroscopy (AES) Ar ion sputter etching reveals that S has substituted for Se in the lattice to produce a graded region: The depth profile analysis indicates that from a composition with x nearly unity at the surface, x monotonically declines to zero over a distance of ∼1 μm. Correspondingly, the band gap energy diminishes from ∼2.4 eV for the CdS-like composition to ∼1.7 eV for CdSe. Photoluminescence (PL) and electroluminescence (EL) from the graded material appear to derive from the luminescence of the CdS x Se 1− x compositions which make up the graded region: Emission from ∼500–750 nm matches the spectral region spanned by PL and EL from homogeneous, single-crystal CdS x Se 1− x samples which emit near their band gap energies. A previously established linear correlation between emission maxima (nm) and composition in homogeneous CdS x Se 1− x samples provides a spatial probe of electronhole ( e −- h +) pair recombination in the inhomogeneous material: Regions from which PL and EL originate can be inferred from their spectral distribution in combination with the AES/depth profile data. PL spectra are thus shown to be dependent on excitation wavelength in a manner consistent with relative optical penetration depth. EL spectra are potential dependent and provide evidence that increasingly cathodic potentials shift the spatial origin of EL, on average, nearer to the semiconductor surface. The inhomogeneous samples can be used as photoanodes of photoelectrochemical cells employing aqueous (poly)sulfide electrolyte. Photoaction spectra exhibit their principal onset at ∼560 nm, indicating that the S-rich, near-surface region is primarily responsible for photocurrent. This spatial origin of photocurrent is also reflected in the nonuniform quenching of PL accompanying passage of photocurrent from 457.9-nm excitation. With certain assumptions, these quenching properties provide a crude map relating the efficiency of e −- h + pair separation to distance from the semiconductor-electrolyte interface; the correlation indicates that negligible separation occurs beyond ∼0.2 μm. Comparisons of these PL and EL properties with those of related graded materials and with homogeneous CdS x Se 1− x samples are discussed.

Lattice-matched heterostructures as Schottky barriers: HgSe/CdSe

A novel structure, which is both a lattice-matched heterostructure and a Schottky barrier, is fabricated by epitaxial growth of (111) oriented HgSe on (0001) oriented CdSe. Hydrogen transport CVD is used, with a HgSe source temperature of 420°C and a CdSe substrate temperature of 330°C. Single crystal growth was obtained. Of the two orientations possible for the crystal symmetries involved, only one was observed for a given specimen. The ’’Schottky barrier’’ height for this lattice-matched heterostructure is 0.73±0.02 eV as measured by the photoresponse method. This uncertainty is substantially less than is usual for Schottkies. The magnitude is greater by about one-quarter volt than is achievable with the most electronegative elemental metal, Au, in qualitative agreement with work function arguments.

The Process of Formation of Epitaxial Cadmium and Zinc Oxide Films by Interaction of Single‐Crystal AIIBVI Layers with Oxygen

AbstractThe results of electron microscopic and electron diffraction investigation of the surface during annealing of single‐crystal AIIBVI (CdS, CdSe, ZnSe, ZnS) films in the presence of oxygen are reported.It is shown that the behaviour of islands of cadmium oxide formed by chemical interaction between O2 and the CdS, CdSe substrate in the process of growth resembles that of the nuclei in film condensation. CdO films become “continuous” more quickly, if the size of blocks of substrate‐epitaxial AIIBVI film is larger. Epitaxial ZnO films formed on the surface of epitaxial ZnS and ZnSe layers are of a wurtzite structure or a cubic structure with the parameter a = 4.5 Å.