T. Fischer

Depth microscopy at interfaces

Abstract High resolution depth microscopy with swift heavy ions is performed at the Munich tandem accelerator and its Q3D magnetic spectrograph using the elastic recoil detection technique. The method gives the possibility to analyze elemental and isotopic concentration profiles of light and medium heavy atoms in thin films and at interfaces. A monitor detector has been installed in the scattering chamber of the Q3D in order to get the relative elemental content for the high resolution depth profiles without absolute knowledge of beam current and charge yields. It is a detector telescope which consists of an ionisation chamber for energy loss and a silicon detector for a residual energy measurement in order to obtain the nuclear charge of the detected target atoms. Depth microscopy was applied for quantitative analysis of concentration profiles at the interfaces between a-Si : H and a-SiC : H layers. Hydrogen profiles can directly be compared to carbon and oxygen profiles showing an excess hydrogen content at the interface between the two layers.

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.

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.

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.

Improvement of the Luminescing Behaviour of Simmplanted SiO 2 Films

In previous work we reported on the observation of continuously tunable photoluminescence in Si + -implanted SiO 2 -films with moderate intensities. In this paper we demonstrate improved performance of such samples. The photoluminescence intensity increases abruptly up to two orders of magnitude when the anneal temperature is elevated to values higher than 1000…1100°C. This strong photoluminescence degrades less than that of porous silicon. Very fine tunability in the spectral range from 2.1 eV to 1.3 eV is achieved in samples implanted with a graded dose. In the analysis of the results we try to distinguish between the contributions of the Si-nanocrystals and of the oxide related defects.

Properties and stability of hydrogenated amorphous silicon films with a low hydrogen content prepared by cyclic chemical vapour deposition and hydrogenation

Abstract Highly photoconductive hydrogenated amorphous silicon films (a-Si:H) have been deposited by a new cyclic deposition method involving thermal chemical vapour deposition at substrate temperatures of around 500°C and intermittent hydrogen plasma treatment steps. The main features of the films are the low hydrogen content of less than 5at.% and the high photoconductivity-to-dark conductivity ratio of around 10 5 . By simply varying either the plasma power or the duration of the hydrogenation time a transition from amorphous to microcrystalline films is observed. The electronic quality and stability under illumination of the best amorphous films obtained so far by the cyclic method were comparable with those of films deposited by conventional glow discharge.

Response time measurements in microcrystalline silicon

Abstract The response time, τR, measured by the small-signal photocurrent decay in hydrogenated microcrystalline silicon films (μc-Si:H), which were prepared by three different deposition techniques, reaches values of about 20 μs for the initial decay and more than 100 μs at longer times whereas it is typically 1 μs in hydrogenated amorphous silicon. We have correlated τR with the dark conductivity activation energy since it is related to the Fermi level position. Quite similar to n-type amorphous films, τR increases in μc-Si:H films when the Fermi level moves towards the conduction band edge. We studied this behaviour numerically and discuss to what extent the analysis applies to both amorphous and microcrystalline films.

Effect of the barrier thickness on interface defect density in amorphous‐Si:H/amorphous‐Si1−xCx:H multilayers

The constant photocurrent spectra (CPM) have been measured in a series of undoped a‐Si:H/a‐Si1−xCx:H multilayers with different thickness (db) of the a‐Si1−xCx:H barrier layers from 5 to 100 A. It is found that the apparent defect absorption and the apparent Urbach energy in the CPM spectra increase monotonically with db. These results show that the interface properties are strongly affected by the barrier layer thickness. The interface defect density, as evaluated from the CPM spectra, increases from 5×109 to 9×1011 cm−2 with db.

Interdiffusion in amorphous Si/SiC multilayers

Abstract Measurements are reported of the interdiffusion coefficient D of hydrogenated amorphous Siue5f8C (a-Si1−Cx:H) films using X-ray diffraction to monitor the time dependence of the decrease in composition modulation in annealed multilayers. For a multilayer with a carbon fraction of 0.38 the diffusion coefficient is below 10−18 cm2 s−1 in the temperature range from 550 to 650°C with an activation energy of 1.43 eV. The crystallization temperature is above 925°C.