G. Weiser

Resonant nonradiative energy transfer to erbium ions in amorphous hydrogenated silicon

The simultaneous study of absorption, luminescence, and ODMR spectra of erbium doped a-Si:H and a SiCx:H alloy reveals that Er3+ ions are pumped by a resonant but nonradiative energy transfer from electron–hole pairs excited in the host. Direct optical pumping into absorption lines of Er3+ is not observed. The emission of the Er3+ ions is strong and decreases only moderately from 77 K to room temperature. We propose an energy transfer by a Forster mechanism, based on resonant dipole coupling, which quenches efficiently the luminescence of the host in the case of large erbium concentration. Resonance of electron–hole pairs to the excited state of the rare earth ion is achieved as electrons thermalize in tail states.

Optical properties and structure of si1 − xcx: H alloys

Measurements of the absorption constant by transmission and PDS spectroscopy, of the refractive index and of the reflectivity up to 10 eV of silicon carbon alloys prepared by plasma deposition from silane methane mixtures are reported for composition up to x = 0.87. Mass density measurements up to x = 0.4 show little variation of the total density of Si and C atoms. In this range of composition the alloys are very similar to amorphous silicon with steep absorption edges, shifting to higher energy, and an e 2 peak which remains near 3.8 eV though decreasing in strength. It is concluded that these alloys are built by Si-Si bonds. Carbon contributes little to the formation of the lattice and is incorporated into the Si lattice as CH 3 and CH 2 groups. At higher carbon content the silicon based lattice is replaced by a new lattice which is not a tetrahedrally coordinated Si-C lattice. This transition is obvious by a dramatic change of the spectra in a narrow range of compositions. The absorption spectra develop an extremely shallow tail and the ge 2 peak near 3.8 eV disappears.

Optical spectra of glow-discharge-deposited silicon

Abstract The reflectance and the imaginary part e2 of the dielectric function of glowdischarge-deposited silicon (g-Si) have been investigated in the energy range between 1 and 10 eV. The spectra depend strongly on the deposition temperature but do not change much after annealing. The e2 peak, which is considerably larger in g-Si than in evaporated films, occurs also at higher energy, namely 3·55 eV. We assume that this results from an improvement of the covalent network by the incorporation of hydrogen. Decreasing deposition temperature reduces the oscillator strength of the optical transitions, particularly at high energy and this is interpreted as due to an increasing localization of the deeper-lying valence states.

Interband transitions of quantum wells and device structures containing Ga(N, As) and (Ga, In)(N, As)

The unusual N-induced band formation and band structure of Ga(N, As) and (Ga, In)(N, As) alloys are also reflected in the electronic structure of quantum wells (QWS) and device structures containing these non-amalgamation-type alloys. This review is divided into three parts. The first part deals with band structure aspects of bulk Ga(N, As) and motivates the possibility of a k · p-like parameterization of the band structure in terms of the level repulsion model between the conduction band edge of the host and a localized N-level. The second part presents experimental studies of interband transitions in Ga(N, As)/GaAs and (Ga, In)(N, As)/GaAs QW structures addressing band offsets, electron effective mass changes and an intrinsic mechanism contributing to the blueshift of the (Ga, In)(N, As) band gap on annealing. The observed interband transitions can be well described using a ten-band k · p model based on the level repulsion scheme. The third part deals with (Ga, In)(N, As)-based laser devices. The electronic structure of the active region of vertical-cavity surface-emitting laser and edge-emitter laser structures is studied by modulation spectroscopy. The gain of such structures is measured by optical methods and analysed in terms of a model combining the ten-band k · p description of the band structure and generalized Bloch equations.

Electroabsorption studies on InGaAs/InGaAsP quantum‐well laser structures

Field‐induced changes of the transmittance of quantum‐well laser structures with five ternary wells and quaternary barriers and waveguides, grown on InP substrates, have been investigated at low temperatures. The method selectively picks up states in undoped regions that are particularly sensitive to electric fields: the heavy‐hole exciton in quantum wells, responding by the quantum confined Stark effect, and the band gap of the thicker waveguides which responds by the Franz–Keldysh effect. Both effects are used to determine the transition energies and the electric field in the respective regions. It is found that in typical laser structures electrons and holes are localized to single wells as their coupling is overcome by the internal field. The internal field corresponds approximately to the ratio of the gap energy of the waveguide material and the thickness of undoped regions. Stark localization persists even if the internal field is reduced by forward bias down to a level where diffusion currents lead...

Electroabsorption studies on amorphous silicon

Abstract The electroabsorption spectra of plasma deposited and chemical vapor deposited amorphous silicon films have been studied in comparison with their absorption. In all cases a peak with 350–400 meV halfwidth is observed similar to the results on chalcogenide glasses. The peak occurs at moderate values of the absorption constant near 10 4 cm −1 and follows the shift of the absorption edge with variation of the deposition temperature. No response has been obtained in the low absorption region below 1.5 eV. The field modulated reflectance which has been calculated from the electroabsorption data is about two orders of magnitude smaller than the electroabsorption signal. The electroabsorption spectrum is interpreted as resulting from the influence of the external field on the localized states only.

Reflectivity and dielectric function of glow-discharge silicon

Abstract The reflectivity and the imaginary part of the dielectric function ϵ2 of glow discharge silicon have been studied in the energy range between 1 and 10 eV. The spectra depend strongly on the deposition temperature but change little with annealing. They differ also considerably from the spectra of evaporated films or of CVD Si. The ϵ2 peak of glow discharge Si occurs at higher energy and reaches a much larger value. These differences are interpreted as differences in the short range order in the films. It is assumed that by incorporation of hydrogen a less distorted random network is achieved where the bonds are better saturated than in films prepared by other methods.

Electroreflectance study of the charge-transfer excitons in anthracene-PMDA

Abstract Electroreflectance spectra of the charge-transfer excitons in anthracene-pyromellitic dianhydride (PMDA) have been studied for various orientations of

Variation with disorder of absorption and electroabsorption spectra of a π-conjugated polymer: 4BCMU

Abstract Absorption and electroabsorption spectra of a polydiacetylene with alkyl-urethane sidegroup (4BCMU) are compared in states of decreasing order: polymer chains diluted and oriented in a single crystal of the monomer, different types of polymer single crystals and spin cast films as a highly disordered material. The EA spectra of the most perfectly oriented chains serve as reference to determine the relevant states and to evaluate quantitatively the characteristic properties of the different field effects: the quadratic Stark effect of an exciton of Bu symmetry which dominates the absorption spectrum and the Franz-Keldysh effect of a continuum of states which is also involved in the excitonic Stark effect. The Stark effect implies that the exciton must be delocalized because its size of about 2.5 nm is much larger than the polymer repeat unit. This exciton is well separated from other states. Exciton and continuum states have different sensitivity to disorder which is used to estimate the range of order: more than 20 nm in the chains oriented in a single crystalline monomer matrix and 10 nm or less in polymer crystals. Disorder on a shorter range introduces asymmetric charge distribution resulting from boundary conditions for the plane wave part of the eigenstates which contributes an additional component to the EA spectra of second derivative. Homogeneous chain segments in these films are shorter than 5 nm but still larger than the size of the exciton.

Correlation of structure and optical properties of a-Si1−xCx:H alloys☆

Evaluation of optical properties in the UV region and of the mass density of Si1−xCx:H alloys, prepared by plasma deposition from silan/methane mixtures indicates that these films are formed primarily by SiSi bonds up to x = 0.4. Carbon is incorporated mainly as methyl- and methane-groups. At higher carbon content the covalent network and its properties change rapidly in a narrow range of composition, where the concentration of SiSi bonds falls below a critical value.