T. Muschik

Luminescence enhancement by electrochemical etching of SiC(6H)

Abstract We report on the enhancement of photo- and electroluminescence of porous SiC(6H) over that of bulk single-crystal material. The spectra are identical. We conclude that the increased intensity is the result of geometrical carrier confinement in the granular, porous material. There is no evidence for quantum confinement. From vibration spectra we identify the passivating internal surface layer as predominantly CH x .

The relation between the visible and the infrared luminescence bands in porous silicon. Comparison with amorphous Si alloys

Abstract The energy positions of the visible and the infrared luminescence bands in porous Si are related. Their dependence on different parameters is examined. They are both blue-shifted (from 1.0 eV to 2.1 eV for the visible and from 0.8 eV to 1.3 eV for the infrared band) due to a reduced crystallite size. However, the visible band can be red-shifted by up to 300 meV by surface effects. The observed behaviour is characteristic of neither pure quantum systems nor amorphous alloys. It is typical for porous Si containing surface states.

Limitations of interface sharpness in a-Si:H/a-SiC:H multilayers

Abstract The goal of this work is to study the sharpness of interfaces in amorphous silicon based compositional multilayers (superlattices) by a number of different techniques and to discuss their limitations. From monitoring plasma transients during glow discharge deposition of a-Si:H/a-SiC:H multilayers, a lower limit of 3 A for interface sharpness is estimated. Transmission electron microscopy (TEM) images yield an upper limit of 5–10 A. These images directly show the increase in undulation from the substrate towards the film surface. From the comparison of simulated X-ray diffraction (XRD) spectra with measurements the interface sharpness is found to be between 5 and 8 A. Using a series of multilayers with increasing number of interfaces, structural characteristics of the interfacial region can be extracted. For example, infrared absorption spectroscopy (FTIR) and elastic recoil detection (ERD) lead to an estimate of 1.1 × 1014 cm-2 additional hydrogen atoms per single interface.

Electroluminescence in a-Si1−xCx:H p-i-n structures†

Experimental investigations of the electroluminscence in the visible spectral range in a-Si1−xCx: H p-i-n structures are reported. The a-Si: C: H layers were prepared by the RF plasma deposition technique from a mixture of SiH4 and CH4 gases. Maximum emission was observed at the energy ∼1.7eV. Current-voltage and breakdown characteristics of the structures were studied. The experimental results are discussed in the framework of energy diffusion of electrons and holes between localized states.

The Visible and the Infrared Luminescence Bands as a Tool for Characterization of the Porous Silicon Bandstructure

The visible and the infrared photoluminescence bands in porous Si have been studied at low temperature for two series of samples: one in which the size of the crystallites has been varied and another in which the degree of surface degradation has been changed. It is shown that the relation of the two bands can be explored for characterization of the porous Si bandstructure. The size- and the surface dependence of the valence band and of the conduction band related states is discussed. A model is proposed for explanation.

Characterization of interface properties in a-Si:H/a-Si x C1−x:H multilayers by the constant photocurrent method

Abstract In this work the constant photocurrent method (CPM) has been extended to characterize the interface properties of a-Si:H/a-Si x C1−x :H multilayers. The experimental results show that the interface defect density (N di) and the width of the interface region (l) strongly affect the apparent Urbach energy (Ecpm u) and the apparent defect absorption coefficient (α cpm d ) of CPM spectra in multilayers. Based on the assumption that the defect density decreases exponentially away from the interfaces, the CPM spectra of the multilayers have been analytically described. The calculated results show that the CPM spectra do not follow the absorption coefficient spectra in the multilayers, which is different from the case of a-Si: H bulk material. CPM spectra not only depend on N di , but also strongly on l, in contrast to the absorption coefficient spectra. The parameters l and N di can be obtained by fitting the measured CPM spectra to our theoretical expression. With this method it is found that, as the ...

Asymmetric degradation of electron and hole μτ-products in a-Si:H/a-SiC:H multilayers under illumination

In a series of a-Si:H/a-SiC:H multilayers with different well layer thicknesses d Si we have determined the electron and hole transport properties before and after a 24 hour exposure to AM1 light. Both the well layer thickness dependence in the initial state and the degradation behaviour show an asymmetry between electrons and holes. For the annealed samples the μτ -product of electrons, ( μτ ) e decreases rapidly with decreasing d Si whereas ( μτ ) h (derived from the steady-state photocarrier grating method) changes only slightly. During light soaking ( μτ ) e changes little for small d Si , whereas for ( μτ ) h this is true for large d Si . A qualitative explanation is given by the analytical dependence of ( μτ ) e and ( μτ ) h on the density of dangling bond defects. Since the samples cover a large range of sublayer thicknesses we can estimate the degradation of bulk and interface regions separately.

Temperature dependence of radiative and non-radiative lifetimes in hydrogenated amorphous silicon

Abstract The recombination lifetimes of hydrogenated amorphous silicon (a-Si:H) are studied as a function of temperature by transient photoluminescence (TPL). The decay curves in the time regime 2 ns to ≈ 100 ms are transformed into lifetime distributions (LTD). At 10 K we observe two peaks in the LTD at ≈ 2 ms and ≈ 1 μs. Increasing the temperature results in a broadening of the LTD, showing evidence for a third peak. This component exhibits a temperature dependent peak position and dominates the LTD for T K . Since the decrease of this lifetime with temperature matches the decrease of the steady state PL signal we identify it as being representative of a mainly non-radiative recombination channel.

Non-Equilibrium Carrier Dynamics in a-Si:H/a-SiC:H Multilayers

For a series of a-Si:H/a-SiC:H quantum well structures and superlattices the diffusion coefficient for the lateral ambipolar motion of optically excited free carriers was measured using the transient grating technique. A significant dependence of the diffusion coefficient on the well layer thickness was found. With decreasing quantum well thickness the lateral mobility decreases. These observations may be explained assuming that scattering due to interface roughness is the dominant scattering process.

Small-Angle X-Ray Scattering from Light Emitting Porous Silicon and Siloxene

We studied the microstructure of two types of light emitting porous silicon (PS), as-etched and rapid thermal oxidized and of material prepared according to the siloxene recipe by Small-Angle X-Ray Scattering (SAXS). In all three types of samples we found particles with nanometer dimensions. The average particle size in as-etched PS is ingood agreement with results achieved by TEM and X-ray diffraction. Shape analysis shows, that the PS skeleton consists of cylindrical shaped particles with an average heigth of 20A and a diameter of 40 A.