Research On The Optical Properties And Device Applications Of Si And Ge Nanostructures

Silicon quantum dots have potential on the field of photoelectric devices such as silicon light emitting devices and solar cells because of their different physical characteristic from bulk silicon.How to improve device performance has become a conspicuous research frontier.People usually fabricate silicon quantum dots/dielectric layers(such as SiO2 and SiNx)to control the size of silicon quantum dots.The optical properties of multilayer devices are not well because of the high refractive index of silicon and the influence of dielectric layers.People can design and fabricate silicon nanostructures to do light management,which is an effective way to improve device performance.In this paper,to research the effect of different structures on the surface reflectivity,we used Finite-Difference Time-Domain(FDTD)method to simulate the optical properties of different periodic silicon nanostructures.Then we used the nanosphere lithography technique to fabricate silicon nanostructures with different depth.We tested the optical properties of these structures and compared them with the simulation results.We have observed enhancement of photoluminescence and electroluminescence by introducing nanostructures into silicon quantum dots/silicon dioxide multilayers light emitting devices.The best structure can enhance the luminescence intensity by an order of magnitude.We also introduced the method of fabricating silicon nanostructures into germanium material and fabricated germanium nanostructures with different morphology.The primary content and conclusions shows as blow:1.FDTD Solutions software was used to simulate the optical properties of silicon nanostructures.We have found that the morphology can influence the surface reflection.By maintaining the structures' diameter unchanged and increasing the depth we have found that the reflectance will reduce.The nanostructure with diameter=300nm and depth=150nm have a reflectivity lower than 15%in the wavelength range of 300-850nm.By keeping the depth unchanged and changing the period,the reflectivity will not change a lot.And by keeping the longitudinal width ratio constant,there will be a best diameter to obtain best anti-reflection ability.We have fabricated different nanostructures on the surface of silicon substrates by using nanosphere lithology technique and characterize their optical properties.The experimental results matched the simulation results very well.We have found that the gradient index theory and Mie scattering are important reasons of the anti-reflection ability of silicon nanostructures.2.Based on the silicon nanostructures with good anti-reflection ability,we used the structures in silicon quantum dots/silicon dioxide multilayers light emitting devices.We have found that the luminescence intensity of multilayers on nanostructures is stronger than the flat ones.The luminescence intensity will increase by increasing the depth to 91nm.While further increasing the depth,the luminescence intensity will decrease.This is because the surface defects of nanostructures will increase by increasing the etching time,the carrier will recombine on the surface.Meanwhile,the electroluminescence efficiency of electroluminescence devices with nanostructures is stronger than the flat ones.The electroluminescence intensity enhanced by an order of magnitude.3.We have introduced the nanosphere lithography technique into germanium substrates.We fabricated different structures by choosing nanospheres with different diameters and controlling the etching time.There will be side-wall etching effects while etching germanium and it will damage the periodicity of nanostructures.While using nanospheres with large diameters(such as 2?m)as mask,we can achieve periodic germanium nanostructures and realize wide spectrum absorption enhancement.We have researched the optical properties of germanium nanostructures by using FDTD Solutions and analyzed with experiment results.The absorption of short-wavelength light will be confined on the surface of germanium because of its narrow bandgap.The near-infrared light can be absorbed by germanium very well.By fabricating silicon thin film solar cells on germanium substrates,we have found that there will be photoresponse when the wavelength is greater than 1200nm.There will be possibility to use germanium nanostructures to improve the near-infrared response of devices.