Observation of Quantum Size Dependent Blue Shift in the Luminescence of Recrystallized Si/SiN x Superlattices
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BSTRACT Light emitting Si nanostructures were fabricated by recrystallizing Si/SiN1 superlattices grown by plasma enhanced chemical vapor deposition. After recrystallisation by a laser annealing or by rapid thermal annealing the Raman spectra show clearly a confinement effect. A substantial photoluminescence around 470 to 550 nm and at 620 nm is observed after a hydrogen passivation step. The photoluminescence at 620 nm is assigned to carrier recombination via efficient surface states, whereas the blue emission at 470 to 550 nm shows a behaviour expected for quantum confinement effects. INTRODUCTION Porous Si has created excitement with the discovery by Lehmann and G6sele ' and by Canham 2 that it exhibits efficient room temperature photoluminescence (PL) in the visible. Today there are basicly 3 concepts to interpret the origin of the luminescence: 1) the quantum confinement effect 3, 2) a surface effect of Si-O-H complexes 4and 3) extended surface states 5. The most accepted model might be a combination of a quantum effect and of surface states, which might explain the shift in wavelength comparing PL and electroluminescence 6.The role of Si-O-H might be unimportant 7.The difficulty in dealing with porous Si is the structure itself, it is a highly disordered structure containing Si quantum wires and dots of different sizes and orientation. In addition the chemical structure of the interfacial layer between the Si and the surrounding oxide is widely unknown, making porous Si a difficult object to deal with in exact science. Numerous ways to overcome at least part of the problem are today studied. Reactive ion etching to achieve an ordered array of nanowires and size exclusion chromatography and also reversible size selective precipitation to partially separate the size of Si nanoparticles fabricated by the aerosol method has been used 8. Some indications for a quantum effect have been found
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supported by model calculations proposing light emitting states in Si quantum wires and quantum dots '. Luminescence was also observed for a recrystallized amorphous Si/SiN1 multiple quan'tum well structure ". The sample had rather wide (6nm) Si wells and a systematic study was not done. However, the idea of using quantum wells consisting of micro- or nanoctystalline Si separated by amorphous SiNK barriers offers several advantages. By controlling the Si well width the size appears to be controllabe in the direction of growth and, moreover, the Si/SiNx interfaces are oriented along the plane of growth and the chemical composition is known. In this paper we report on a more detailed study of recrystallized and subsequently hydrogenated Si/SiNx superlattice and multiple quantum well structures in order to gain further understanding of the luminescence observed in Si nanostructures. 833 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
EXPERIMENTAL The amorphous Si:H/SiN,:H quantum well structures were deposited by a plasma enhanced chemical vapor deposition technique. Si"H (5% in He), NH3 and N2 were us
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