Light emitting devices based on silicon nanostructures

Sammanfattning: Although silicon is the dominant semiconductor today, lightemitting devices are currently based on compound semiconductorsdue to their direct band-gap, which promotes fast radiativerecombination. However, in nanometer-size silicon structures,carrier confinement enhances the radiative recombination,while, at the same time, suppresses diffusion to non-radiativerecombination centra, resulting in a significant increase inlight emission efficiency. Moreover, the band-gap is wideningas the crystal size is reduced (quantum confinement), enablinglight emission in the visible range. In this work, twodifferent approaches to manufacture a light emitting diode(LED) in silicon have been investigated. The first type ofsilicon LED's is based on porous silicon (PSi) and manufacturedby electrochemical etching of a previously formed pn diodestructure. After optimizing the etching process, PSi LED's wereproduced with an external quantum efficiency of ~0.2% underpulsed excitation, more than an order of magnitude higher thanpreviously reported. Tunability of the emission wavelength inthe range 1.6-2eV was demonstrated by varying the etchingparameters. The EL wavelength is determined by the band-gap ofthe nanocrystals, i. e. their size, as evidenced by a lowerthreshold for longer EL wavelengths, due to lower barriers forinjection into larger crystallites. The EL decay after the biaspulse follows a stretched exponential shape, in agreement witha model involving exciton migration in partially interconnectednanocrystals. Under constant bias, the EL and forward currentare decreasing, due to charging, caused by carrier trapping inthe porous network. After the etching the hydrogen passivatedporous silicon surface is being gradually oxidized, resultingin increased barriers, permanent conductivity reduction and ELdegradation. To improve stability, the second LED approach,based on Si nanocrystals embedded in SiO2, was studied. Nanocrystals were formed by theimplantation of Si into thermally grown SiO2and by subsequent annealing at high temperatures(mostly 1100°C). Photoluminescence investigation showedthat luminescence properties are dependent on nanocrystal sizeand similar to those of PSi. However, decay shapes and timeconstants revealed a stronger isolation of the nanocrystalsthan in PSi. For the EL, good current transport properties werenecessary. That required a thin SiO2layer and efficient injection, realized using anin-situ doped poly-Si cap layer. The Si nanocrystal LED's werestable, although the total light intensity was lower than inPSi, as a consequence of a thin active layer.Key words: Electroluminescence, photoluminescence, lightemitting diode, porous materials, nanostructured materials,silicon, etching, anodized layers, ion implantation.