CdSe Samples Research Articles

Interband magneto-optics of CdSe and optical spectra of CdSe-based quantum dots

Band parameters of bulk CdSe have been extracted from magneto-optical measurements of superthin (0.4 μm) CdSe samples at 1.7 K. True absorption spectra for a CdSe planar QD-system in a ZnSSe matrix were obtained. A theoretical model that properly describes the energy spectra and exciton behavior in planar QDs is developed. The theoretical model predicts the structure parameters, that are necessary for proper carrier localization and increase of the exciton oscillator strength.

Optical properties of semimagnetic semiconductors studied by thermoreflectance spectroscopy

The effect of introducing the magnetic elements Ni and Co into a II–VI host lattice, and their influence on the fundamental band gap energy, have been studied by means of thermoreflectance (TR) spectroscopy, following the evolution of the E0 transition at the Γ-point of the Brillouin zone, in terms of the magnetic ion concentration, and/or the film crystallite size. Two systems have been studied: (1) Cd1−xNixTe thin films grown by r.f. supttering onto Corning glass substrates with Ni concentrations in the range: 0 < x < 0.15. The films showed a nanocrystalline structure with the grain size decreasing as the film Ni content increases. From the TR spectra it was found that the band gap shifts to higher energies as the nanocrystalline size decreases, giving us evidence of a quantum-size effect due to electron confinement; (2) Cd1−xCoxSe crystals grown by the vapor phase chemical transport technique at a temperature of 950°C, for two concentration values: x = 0.01 and 0.05. As compared to a pure CdSe sample, the crystal with a Co concentration of 1 at% shows a shift of the E0 transition of about 70 meV to lower energies. For both samples, there appear additional oscillations in the low energy part of the TR spectrum which we interpret as due to the levels introduced by the Co-3d orbitals. The shifts of the E0-point are also discussed.

Photoluminescent Properties of Cadmium Selenide in Contact with Solutions and Films of Vaska's Complex: Effects of Oxygen and Carbon Monoxide

The band-edge photoluminescence (PL) intensity of etched n-CdSe single crystals is reversibly enhanced in the presence of nitrogen-saturated toluene solutions of trans-IrCl(CO)(PPh3)2, Vaska's complex (1), relative to nitrogen-saturated toluene alone. The PL enhancement is indicative of adsorption of a Lewis base onto the n-CdSe crystals. When the toluene solution of 1 is saturated with carbon monoxide, repetition of these experiments produces larger reversible PL enhancements; when the same solution is saturated with oxygen, large reversible quenching is observed relative to toluene. These PL intensity changes are attributed to binding to the surface of 1·CO and 1·O2, the CO and O2 adducts of 1, respectively. The concentration-dependent PL changes are well fit by the Langmuir adsorption isotherm model; responses to 1 and 1·CO are characterized by similar binding constants K of nearly 105 M-1, while K for 1·O2 is 107 M-1. In the solid state, 1 is known to bind CO and O2 reversibly. When the CdSe surface is coated with thin films of 1, evaporated from toluene solution, CO and O2 gases can be detected through reversible enhancement and quenching, respectively, of CdSe PL intensity relative to a gaseous nitrogen ambient; neither gas affects the PL intensity of uncoated CdSe samples appreciably. Films of 1 coated onto NaCl plates or CdSe crystals yield binding constants for CO from IR spectral changes that are experimentally indistinguishable from those obtained by PL measurements, ∼102 M-1. In contrast, the binding constant for O2, determined from IR spectral changes to be ∼102 M-1, is roughly an order of magnitude lower than that estimated from PL changes, evidencing a roughly 10-fold enhancement in binding affinity at the semiconductor−film interface for 1 toward O2 relative to the bulk film environment. Aspects of the coordination chemistry of 1 that may account for these electrooptical effects are discussed.

Photoluminescence of heavily doped n-type CdSe.

We report on a study of the photoluminescence of a series of indium-doped n-type CdSe samples with carrier concentrations above the critical concentration for the metal-insulator transition. The photoluminescence was measured for excitation energies both greater and smaller than the optical gap, and excitation spectroscopy was also used as a probe. As in earlier results for CdS the spectra of CdSe exhibit a sharp edge at high energies corresponding to the decay of electrons from the Fermi level in the conduction or donor-impurity band to states in the valence band, and a broad tail at lower energies due to transitions from the rest of the Fermi sea. While in CdS the high-energy edge moved monotonically to higher energy with increasing carrier concentration, ${\mathit{N}}_{\mathit{D}}$-${\mathit{N}}_{\mathit{A}}$, this monotonic behavior did not hold for two of the CdSe samples. We suggest that this unexpected behavior can be attributed to the effect of different levels of compensation among the samples used in the present studies. The full results are interpreted in terms of band filling, electronic interactions, and band-gap renormalization in the presence of compensation.

Excitonic Optical Nonlinearity in II‐VI MOCVD‐Grown Crystals

AbstractUsing the method of single‐ and two‐beam laser spectroscopy, the nonlinear optical properties of high‐quality MOCVD‐grown ZnSe and CdSe samples are studied. At nitrogen temperatures the dominating mechanism of absorption nonlinearity is the plasma screening of excitons. Along with this, in the nonlinear response of ZnSe crystals the exciton‐exciton interaction causing band broadening takes place. At room temperatures, in CdSe a strong dependence is revealed of absorption spectrum deformation on the exciting radiation quanta energy which is associated with slowing down of electron‐hole plasma cooling.

The preparation and properties of cobalt-doped II–VI chalcogenides

Samples of cobalt-doped ZnS, ZnSe, CdS, and CdSe were prepared by the direct combination of the elements. The limit of solubility of cobalt in these systems was established. Magnetic measurements indicate that the antiferromagnetic interactions of the cobalt in the zinc systems were greater than those observed in the cadmium chalcogenides. Single crystals of cobalt-doped zinc chalcogenides were grown by chemical vapor transport. Their IR transmission was measured and the cobalt concentration for maximum hardness was determined.

Femtosecond Studies of Coherent Transients in Semiconductors

The coherent interaction of femtosecond laser pulses and thin CdSe and GaAs samples is investigated experimentally and theoretically. Oscillatory structures in the differential probe transmission around the exciton resonance and around the pump frequency are observed when the probe pulse precedes the pump. Comparison with theory attributes the oscillations to the coherent coupling between the light field and the electron-hole transitions in the semiconductor. For nonresonant excitation, the oscillatory structures around the exciton are identified as the early stages of the optical Stark effect.

Localization of excitons in space‐charge layers

AbstractExciton localization in near‐surface regions of semiconductors is discussed. Various mechanisms are listed responsible for the effect based on the binding of the excitons by nearsurface imperfections in crystals. One of the most important mechanisms of the localization is the Stark‐effect on excitons in space‐large layers of semiconductors. It is accounted for in the numerical computation of the exciton reflectance. Essential is a nonuniform character of the electric field. The localization is accompanied by a large enhancement of the exciton reflectance and the appearance of a low‐energy spike in the reflection lineshape. The computed lineshapes are typical of the A n = 1 exciton in a number of CdS and CdSe samples. It is shown that the increase of the bulk damping from 1 to 5 em−1 diminishes the influence of localization and transforms the enhanced lineshapes to the ideal form. The latter is typical of the B‐type excitons in the same crystals. This finding resolves the paradox of different sensitivity of the A‐ and B‐excitons to various (nondestructive) treatments of the surfaces. It explains also the anomalous behaviour of the minimal reflectance with temperature in CdS reported recently in the literature.

Photoluminescence on CdSe and CdTe under hydrostatic pressure

Photoluminescence measurements on CdSe and CdTe samples have been made under hydrostatic pressure up to the phase transition. Transition pressure values are reported. Nonlinear pressure dependence of the direct gap is observed for both materials. Pressure coefficients are in good agreement with values calculated from the dielectric theory of the chemical Bond.

Polarization effects in the luminescence of cadmium selenide electrodes

Abstract : Polarization properties of the emission from single-crystal, n-type CdSe samples have been examined. When excited with several ultrabond gap wavelengths, a-plates of n-CdSe (wurtzite structure) exhibit polarized photo-luminescence (PL) spectra consistent with the reported electronic structure of the solid. The spectra consist of two bands near the band gap energy (approximately 1.7 eV), assignable to transitions between the conduction band and a split valence band having its two highest-energy band edges separated by approximately 0.022 eV; the lower-energy band is preferentially polarized with E is perpendicular toc while the other band displays no polarization preference. Time-resolved PL data, obtained with 620-nm excitation from a N2-pulsed dye laser, were independent of polarization. Intensity-time curves yielded a decay time ( t sub l/e) of approximately 20 ns for both bands, providing evidence that the two emissive excited states are in thermal equilibrium. Emissive properties of n-CdSe, a-plate electrodes were also consistent with thermal equilibration. Polarized electroluminescence (EL) spectra, obtained using the solid as a dark cathode in aqueous peroxydisulfate electrolyte, closely resemble their PL counterparts, demonstrating attainment of similar excited-state populations in the two experiments. And when the semiconductor is used as the photoanode of a photoelectrochemical cell employing diselenide electrolyte, the two PL bands resulting from 514.5- or 632.8-nm excitation are quenched in parallel by the electric field present in the semiconductor. (Author)

X-ray photoelectron spectroscopy studies of the role of etching in improving CdSe films for photoelectrochemical solar energy conversion

Polycrystalline thin film CdSe photoelectrochemical (PEC) cells have shown a great promise for low cost terrestrial power generation in the past several years 1-5. Many details of the CdSe films, particularly the atomic levels are not understood. Controlled studies of the atomic composition of the surface and structure of films have been reported 6. It is known 7 that chemical etching has an important effect on the solar conversion efficiency of such films but the reasons are not fully understood. We report here the results of X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) measurements on the effects of etching of CdSe photoanodes. The CdSe samples used in this study were prepared in a manner similar to previous work 6, by electrolysis of HC1 solution containing CdC12 and SeO 2. A carbon plate was used as the anode and was spaced 2 cm from the titanium substrate. The potential of the substrate was maintained constant with respect to a saturated calomel electrode by a potentiostatic technique and by varying the SeO 2 concentration. The current was passed for a period of 2 h with continuous stirring. The samples were then annealed in air for 2 h above 400 °C. For each experiment, two samples were prepared together, one in the form of a strip and the other as a cylinder suitable for use with the XPS equipment. The strip was used for currentvoltage measurements to check the performance of the surfaces. The counterelectrode and the electrolyte have been described earlier 4. XPS measurements were made with a Varian lEE-15 spectrometer. Annealed unetched films showed the presence of cadmium, selenium and chlorine in the ratios 1:1:0.1 as well as the usual carbon and oxygen. Titanium was present on some samples but not on others. Cadmium and selenium peak positions were identical with those of polycrystalline CdSe (Cleveland Crystals Inc.). The etching solution (aqua regia) was a 12~o aqueous solution of 4:1 HC1-HNO 3 (by volume). The heights of various peaks were measured after each of a series of etching steps. The source of chlorine is probably the HC1 used for the deposition of the CdSe films. Even after the first 2 s of etching, a strong decrease in the chlorine peak was observed. A consistent explanation is that chlorine diffuses to the surface during annealing and is subsequently removed by etching. There was no evidence for the presence of chlorine in the surface of unannealed films. Etching increased the intensity of the titanium peak slightly, if titanium was present before etching. This may be due to the

Role of elastic exciton-defect scattering in resonant Raman and resonant Brillouin scattering in CdSe

We report a detailed resonant Brillouin and Raman study in the wurtzite semiconductor CdSe in the vicinity of its lowest-energy $A$ exciton. By using a spectrometer and laser system with a combined spectral resolution of 0.16 ${\mathrm{cm}}^{\ensuremath{-}1}$ we were able to measure the widths and line shapes of Raman and Brillouin modes. In a nominally undoped CdSe sample we found that wave vector is conserved in the light-scattering process only for incident photon frequencies below the longitudinal exciton frequency. For incident laser frequencies above the longitudinal exciton frequency, scattering processes which do not conserve wave vector became dominant. We propose that this breakdown in wave-vector conservation is due to elastic exciton-defect scattering, and our results are in qualitative agreement with the predictions of the elastic exciton-defect scattering theory of Gogolin and Rashba. In another CdSe sample doped with Li, we found that its Raman and Brillouin spectra are always dominated by wave-vector-nonconserving modes even for incident photon frequencies below the $A$-exciton frequency. From the Brillouin spectra we deduced the following parameters for the $A$ exciton in CdSe: ${M}_{\ensuremath{\perp}} (\mathrm{exciton}\mathrm{mass}\mathrm{perpendicular}\mathrm{to}\mathrm{the} c \mathrm{axis})=0.40{m}_{0}$ (free electron mass), ${M}_{\ensuremath{\parallel}}\ensuremath{\sim}1.3{m}_{0}$, ${\ensuremath{\omega}}_{T} (\mathrm{transverse}\mathrm{exciton}\mathrm{frequency})=14713$ ${\mathrm{cm}}^{\ensuremath{-}1}$ (at \ensuremath{\sim}10 K), and the splitting between longitudinal and transverse exciton frequencies is 4 ${\mathrm{cm}}^{\ensuremath{-}1}$.

Electrical conduction in CdSe near the metal-insulator transition

Abstract Strongly compensated samples of CdSe with 10l7–1018 cm−3 have been investi-gated. Two samples exhibit metallic and three non-metallic behaviour. The various regions of impurity-band conductivity have been identified from Hall and resistivity measurements. The density of states at the Fermi level has been estimated and compared with theory. A negative magnetoresistance has been observed in both metallic and non-metallic samples. A strongly temperature-dependent positive magnetoresistance in the variable-range-hopping regime is explained in terms of a simple extension of the Mikoshiba theory.

Defect structure of CdSe in selenium vapour

High temperature measurements of charge carrier densities and mobilities versus pressure of selenium vapour are performed both in p-type undoped CdSe samples and n-CdSe doped by either donors (In) or acceptors (Cu). For undoped p-CdSe crystals in the temperature range from 850 to 1000 K the neutrality equation may be approximated as p = 2[VCd−-], if the selenium pressure does not differ from the saturation pressure by one and half order of magnitude. A theoretical calculation of isothermal dependences of charge carriers on the pressure of selenium vapour is in a good agreement with experimental data. It is found that hole conductivity of CdSe crystals at room temperature is due to the cadmium vacancies with ionization energy E = Ev + (0.30 to 0.36) eV. Experimental and theoretical data for doped CdSe crystals have shown that the density of In or Cu impurities at the cadmium sites is temperature dependent and these impurities are neutral at the interstitial sites. [Russian Text Ignored].

Intensities and Doublet Separations of Forbidden Transitions in the Paramagnetic Resonance of Mn++in the II-VI Compounds

The forbidden hyperfine transitions in the paramagnetic resonance spectrum of Mn ++ are observed in single crystal and powder samples of cubic CdTe and CdSe. The data of the paramagnetic resonance observation on six chalcogenides of zinc and cadmium are summarized, and the intensity of the forbidden transition J and the separation of the forbidden doublet δ H are discussed in terms of the covalence of the compound. It is shown that \(r_{\text{L}}/\{(\sqrt{3}/4)a_{o}+\alpha\}\), where r L represents the anion radius, a o the lattice constant and α a correction term, is a useful parameter for the analysis of J and δ H .

Proposal for Beating Two Optical Masers

An experimental arrangement is proposed with which two optical masers may be made to beat to produce a far-infrared source. The beating process occurs in a semiconductor which lacks inversion symmetry, allowing the third-order process. By selecting a semiconductor with an appropriate energy gap, the energy denominators in the matrix element may be made small, enhancing the infrared intensity. Using the exciton states as the virtual intermediate states, the power output is calculated in the effective-mass approximation. For beating a ruby maser at T=−180°C (6934 Å) with one at T=20°C (6943 Å), a CdSe sample at 4.2°K is proposed because its exciton energies are just higher than the maser energies. For input power of 10 kW for each energy, a 19-cm−1 infrared beam of over a watt is expected. The competing process of two-photon absorption heats the sample to approximately 65°K during a pulse, but this effect is not disastrous.