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A model for the electroluminescence of porous n-silicon

: Kozlowski, F.; Steiner, P.; Lang, W.

Journal of luminescence (1993)
ISSN: 0022-2313
European Materials Research Society (Meeting) <1993, Strasbourg>
Fraunhofer IFT; 2000 dem IZM eingegliedert
Elektrolumineszenz; Modell; poröses Silizium; Strukturmodell; Strukturuntersuchung

At the present stage of the exploration of porous silicon it seems impossible to explain all experimental observations in one unified model. It is necessary to put the main emphasis on some special aspects. We present a structure model for the electroluminescence (EL) of our porous silicon devices of the first generation that is consistent with the experimental observations we made on this material. The devices are fabricated by light supported electrochemical etching of n-silicon. Semitransparent metal contacts are applied to allow electrical contacting. The photoluminescence (PL) observed from the samples seems to have its origin in small clusters of silicon with typical dimensions smaller than 50 A. The main intensity of PL is emitted from a layer located about 5 mym below the surface of the sample. In contrast to this observation we found that electroluminescence light is emitted from a 1-2 mym wide layer at the top of the sample. In this layer we find crystalline silicon structure s larger than 100 A surrounded by oxide (6). We interprete these silicon structurtes as constricted wires that allow current transport. The constrictions should have dimensions that allow an increase of the effective band gap due to quantum size effects. It seems very likely that EL is generated in the constrictions where an increased band gap can be assumed. Following a smart quantum confinement for PL (8) we think that the charges that are forced to pass the constrictions are captured by surface states and the red luminescence observed is caused by a radiative recombination at these surface states. Other groups explain the luminescence mechanism by excitonic recombination (9) in quantum structures or by special surface chemistry (10). Both alternatives are consistent with our structure model.