LED(Light Emitting Diode) 적용을 위한 Si 나노 결정의 합성과 광학적 특성에 미치는 질소 플라즈마의 영향

Abstract

Silicon is a prevalent material in the microelectronics, not only because of its abundance and low cost, but also because it has the excellent electrical property and electrochemically stable oxide, which acts a great passivation layer. However, due to its indirect band gap, silicon is little for light emitting devices. Ever since efficient photoluminescence (PL) at visible ranges from porous silicon structures was discovered by Canham, silicon nanostructures has been investigated to apply for silicon based light source. In order to fabricate the silicon nanostructures, especially silicon nanocrystals(Si - ncs)*, various methods have been attempted such as laser ablation, implantation of Si to the SiO2 and deposition of SRSO(silicon rich silicon oxide) by sputter or PECVD(plasma enhanced chemical vapor deposition) These methods, however, have some disadvantages such as the formation of defects on the surface of nanocrystals, the difficulty of mono-dispersed nanocrystals synthesis and the limitation of high temperature process(> 1100℃). Therefore, it is worthy to find the easy and reliable method in fabricating the high quality and mono-dispersed silicon nanocrystals without high temperature process.In present study, direct plasma synthesis was applied to synthesis the high quality silicon nanocrystals of various sizes. Extensive TEM (transmission electron microscope) analysis revealed that mono-dispersed silicon nanocrystals with a size ranging from 2 to 30nm have been successfully synthesized by changing the flow rates of Ar and SiH4 gases. Slightly oxidized silicon nanocrystals with a size of 2 ~ 6 nm showed the photoluminescence of 648 ~ 729nm wavelengths. To apply the Si - ncs* for LEDs, excellent emission efficiency and color (wavelength) control are required properties. In order to enhance the quantum yield for photo-emission from silicon nanocrystals, nitrogen plasma treatment has been applied with the plasma power from 80 to 270W. The results revealed that the optimum nitrogen plasma treatment (210W) enhanced the quantum yield of photoemission from silicon nanocrystals, due to the sufficient passivation of non-radiative recombination center on the surface of slicon nanocrystals. However, quantum yield from silicon nanocrystals was decreased with the increase of plasma exposure time. Therefore, effects of nitrogen plasma on compositional, electrical and optical properties of silicon nanocrystals have been investigated by high resolution electron microscopy (HREM), secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). The result showed that increased electrical property by nitrogen plasma treatment made the quantum yield from silicon nanocrystals low. This relation between electrical and optical properties of silicon nanocrystals could be explained by Onsager length which is the minimum distance between electron and hole for radiative recombination.