Frank Wünsch

Influence of a space charge region on charge carrier kinetics in silicon wafers

p-type Si wafers provided with silicon nitride films at the surface are investigated by contactless transient photoconductivity measurements. Comparing wafers of different resistivities and wafers at one side and at two sides provided with nitride films shows that separation and storage of excess charge carriers in the space charge region is the main kinetic process at low injection level. Fitting of the experimental data with a simple model leads to the conclusion that the electron mobility in the interface is strongly reduced, a value 250(−+20%) cm2 V−1 s−1 is determined, independent of the wafer resistivity. This confirms the reduction of the electron mobility reported in the literature on the basis of completely different measurements.

Amorphous silicon/crystalline silicon heterojunctions for solar cells

Amorphous silicon/crystalline silicon (c-Si) heterojunctions are studied by contactless transient photoconductivity measurements at the microwave frequency range. Excellent passivation of the n-type crystalline silicon (n c-Si) surface by n-type, hydrogenated amorphous silicon (n a-Si:H) films is observed. The influence of band offsets is discussed. It is shown that energy converting junctions can be characterized accurately by these measurements. Charge carrier kinetics in the junctions are controlled by charge carrier separation determined by band bending and by interface recombination.

Optoelectronic properties of microcrystalline silicon films

The optoelectronic properties of microcrystalline silicon (μc-Si) films were determined by contactless transient photoconductivity measurements in the microwave frequency range. High mobilities were observed in μc-Si produced by laser annealing although the material contains impurities diffused from the substrate and is doped. Hot wire and plasma enhanced chemical vapor deposited (PECVD) μc-Si films are characterized in the best case by mobilities in the order of magnitude of the mobility in a-Si:H.

Recombination at high charge carrier concentrations in a-Si:H films

Abstract The recombination at high excess charge concentrations (higher than 1018 cm−3) in a-Si:H has been studied by comparison of the excess electron concentration obtained by contactless transient photoconductivity measurements with numerical calculations. The first stage of the recombination process is described satisfactorily by a recombination between mobile excess electrons and all excess holes. In later stages this simple model predicts a too high recombination rate. A better description has been obtained if the recombination rate parameter depends on the energy of the hole.

Contactless characterization of a-Si:H films on crystalline silicon substrates

The passivating influence of a-Si:H films on the c-Si surface is shown by contactless transient photoconductivity measurements in the microwave frequency range. A relation between the signal and the performance of a-Si:H/c-Si heterojunctions for solar energy conversion was found. The injection of excess charge carriers from the a-Si:H film into the c-Si substrate was observed and shown to yield information on charge carrier transport in the a-Si:H film and in the space charge region.

Optoelectronic characterization of microcrystalline silicon films

The optoelectronic properties of μc-Si films produced in different ways are studied by contactless transient photoconductivity measurements in the microwave frequency range. A strong charge carrier trapping is observed in most μc-Si films. The effective mobility determined from these measurements appears to be an appropriate parameter to characterize these films. The films show a high absorption at 1064 nm, tentatively attributed mainly to defect absorption.

Charge carrier transport in a-Si:H/c-Si heterojunctions

Contactless transient photoconductivity measurements of a-Si:H / c-Si heterojunctions are presented. It is shown that n a-Si:H / n c-Si junctions furnish an excellent passivation of the n c-Si surface. For i a-Si:H / n c-Si junctions the passivation is better for thicker films, whereas for very thin films (10 nm or less) a deviating behaviour probably due to inhomogeneities is observed. For the p a-Si:H / n c-Si junction separation of excess charge carriers in the space charge region is observed leading to a slowly decaying tail of the signal. This separation is also observed in thick i a-Si:H / n c-Si samples where electrons are injected from the a-Si:H film in the c-Si substrate.