S. Z. Weisz

EFFECTS OF AQUAREGIA TREATMENT OF INDIUM-TIN-OXIDE SUBSTRATES ON THE BEHAVIOR OF DOUBLE LAYERED ORGANIC LIGHT-EMITTING DIODES

The effects of a controlled aquaregia treatment of indium–tin–oxide (ITO) substrates on the behavior of highly efficient vacuum evaporated double layered 8-tris-hydroxyquinoline Al (Alq3)-based light-emitting diodes are described. It is found that in suitably treated devices, both current injection and the electroluminescence (EL) are significantly enhanced. The enhancement is believed to result from the greater ITO/hole transporting layer contact areas and the contact conditions. The observed dependence of I(V), the EL output, and the EL efficiency on the ITO surface morphology indicates that space-charge-limited currents dominate the behavior of the devices.

Crystalline phases at the p‐ to n‐type transition in Cu‐ternary semiconducting films

We report here a study of the Raman spectra of ternary Cu–In–S and Cu–In–Se polycrystalline film compounds as a function of the x=[In]/{Cu]+[In]} ratio. Using these spectra we were able to identify, with high resolution in x, the phases present in the films. We found that the single phase of chalcopyrite CnInSe2 exists over the fairly wide composition range of 0.48⩽x⩽0.55, and that the lattice disorder increases with the increase of In content. No such single phase range was found for the Cu–In–S films. Considering the electrical properties of these materials around x=0.5, it is concluded that the native defect model accounts for the electrical properties of the Cu–In–Se films but does not account simply for the electrical properties of the Cu–In–S films.

DEPENDENCE OF THE PHOTOTRANSPORT PROPERTIES ON THE POSITION OF THE FERMI LEVEL IN POLYCRYSTALLINE CUINS2 FILMS

We report a simultaneous study of the phototransport properties of both the majority and minority carriers in polycrystalline CuInS2 layers. This is done for n‐type as well as p‐type layers. The dependencies of these properties and their light intensity exponents on the position of the Fermi level yield a picture of the recombination levels and the recombination kinetics in these layers. We show that the simplest model which is consistent with the data is that of a symmetric two‐level system. One level is associated with donorlike recombination centers lying around 0.35 eV below the conduction band edge, and the other level is associated with acceptorlike recombination centers lying around 0.35 eV above the valence band edge. This interpretation of the results is shown to be consistent with the luminescence and transport data reported previously on single crystals of CuInS2.

Optical properties of nanocrystalline silicon within silica gel monoliths

Described herein is the incorporation of nanocrystalline silicon nc-Si from porous silicon (PSi) in a silica matrix fabricated by the sol-gel technique that yields highly photoluminescent (PL) and optically transparent monoliths with uniformly distributed nc-Si inclusions or nanoclusters. The sample monoliths were prepared with PSi-derived nanoclusters (PSi-n) with average diameters of 14–45nm. Concentrated samples of PSi-n-exhibited blueshifted orange emission bands with maximum peaks between 600 and 750nm with PL emission intensities ten times stronger than those of the original PSi, while diluted samples exhibited UV to blue (350–450nm) emission bands. The PL quantum yield of the typical PSi-n monoliths was 44% higher than the native PSi. Light absorption measurements showed a linear response to laser powers before the saturation threshold at 80mW. PL bleaching following 3h of constant laser power exposure resulted in 90% reduction of the maximum initial PL. Mechanical and thermal stability properties ...

Identification of nonambipolar transport in the application of a photocarrier grating to hydrogenated amorphous silicon

Thus far the many reports concerning the utilization of the photocarrier grating (PCG) method have assumed that ambipolar transport takes place in such a PCG when it is imposed on hydrogenated amorphous silicon (a‐Si:H). This assumption, which is decisive in the interpretation of the experimental results in terms of the ambipolar diffusion length, has not been tested thus far. In this letter a corresponding testing criterion is proposed, and it is demonstrated that whenever ambipolarity is lost, the PCG‐derived diffusion lengths may be wrong. The finding that ambipolarity is maintained in device quality a‐Si:H is shown to confirm the theoretical suggestion that, whenever observed, the ambipolarity in a‐Si:H is due to shallow trapping effects.

Auger electron spectroscopy for quantitative analysis

A simple mathematical analysis shows that unless the ratio of the instrumental resolution width to the natural Auger linewidth is less than about 0.3, the measured line intensities do not represent accurately the atomic concentrations. To overcome this difficulty, a universal curve is presented whereby the experimentally measured line intensities can be corrected so as to represent quite accurately the relative atomic concentrations in one’s sample. Unfortunately, however, the available sensitivity data required for quantification were not always measured with sufficient instrumental resolution. It is our contention that there is a need for new sensitivity measurements in which the required resolution is ensured.

Relation between electroluminescence and photoluminescence in porous silicon

Abstract We present combined measurements of electroluminescence (EL) and photoluminescence (PL) in p-type porous silicon. The EL spectra were measured using an electrolyte contact for electron injection into the porous face of the sample. Upon applying the current, the EL intensity first rises with time, reaches a maximum, and then decays to zero. (The whole process takes about half an hour.) At the same time, the peak of the EL spectrum shifts from ≈850 nm in the beginning to ≈600 nm at the end of the process. The PL, which was measured simultaneously, peaked at ≈750 nm in the beginning and was much wider than all of the EL spectra. Towards the end of the EL process, the red part of the PL spectrum practically disappears. This shifts the PL peak towards the blue, to about the same wavelength as the EL peak (≈600 nm) and the spectrum becomes much narrower, comparable to the EL spectrum. The voltage across the sample during the EL process shows a moderate increase up to the point where the EL disappears, and then the voltage rises steeply. This behavior is associated with the build-up of a thin oxide layer on the porous surface. The combined results of EL and PL, and especially the disappearance of the red part in the photoluminescence spectrum at the end of the EL process, suggest that in addition to quantum confinement, localized surface states play an important role in the luminescence process, at least in the red part of the spectrum. Such states may be associated with adsorbed species and disappear upon oxidation.

Non-parabolicity in the lowest conduction band of CdS

Abstract Pulse measurements on the CdS/electrolyte system are used to induce and study space-charge layers at the CdS surface. Such measurements show that the free-electron-like lowest conduction band in hexagonal CdS is highly non-parabolic. A simple model for the structure of this band, which assumes parabolicity up to 0.125 eV above the band edge and a linear dependence of the energy on the wavevector at higher energies, accounts well for the experimental results. The density of states function that emerges from these results is compatible with reported theoretical calculations. In addition, the measurements indicate that surface states are practically absent at the CdS surface in contact with the electrolyte. They also shed light on the process of charge leakage across the CdS/electrolyte interface, leakage that occurs mostly when strong accumulation layers are induced.

Comparative analysis of the 1.54 μm emission of Er-doped Si/SiO2 films and the size distribution of the nanostructure

Abstract Er 3+ -doped nanocrystalline-Si/SiO 2 composite films were synthesized by RF co-sputtering of bulk-Si, SiO 2 , and Er 2 O 3 targets. The visible and 1.54 μm emission bands of the samples were measured, as well as their optical transmission. We varied the relative concentrations of Si, Er, and the annealing temperature. The nanoparticles size distributions of the samples were obtained from their optical transmission spectra. We analyzed the dependence of the characteristic 1.54 μm Er-emission intensity on the size and concentration of the nanocrystalline particles and the dependence of the emission on the preparation conditions.

Accurate determination of the two carriers steady‐state mobility‐lifetime products in hydrogenated amorphous silicon

The photocarrier grating (PCG) technique is used for the first time in the high electric field regime. The results are shown to confirm previously unproven theoretical predictions for this regime. It is demonstrated that, by application of the PCG technique in the high‐field regime, accurate values for the ratio of the two carriers mobility‐lifetime products can be deduced. This is in contrast with the fact that their sum cannot be derived accurately from the measurement of photoconductivity because one cannot determine accurately the carrier generation rate. Combining the ratio, determined by the high field PCG, with the low‐field data yields accurate values for the mobility‐lifetime products of both carriers.