Investigation of Position Sensitive Photoelectric Effect in Nanostructures of Metal and Textured Silicon

Author:Mei Chun Lian

Supervisor:wang hui liu shi yong


Degree Year:2018





Due to the prominent optical and electrical properties,nano materials have broad application prospects in the field of photoelectric detection,photovoltaic,biomedical instrument and so on.Silicon has been suggested as one of the most promising and widely used semiconductors to realize excellent performances.However,there is a limitation that absorption of near-bandgap light is small for indirect-bandgap Si,resulting in a hindrance to the convertions from optical to electric signal and light to electrical energy.The limitation directly astricts the sensitivity and efficiency of photoelectric devices.According to the theoretical and experimental results,the large specific surface area of nanotextured silicon can effectively promote the light absorption and antireflection in broad wavelength.At present nanotextured silicons are widely applied in solar cells,and it is also significant to study their value in photoelectric detections.Acctually,the superior capacity of light absorption facilitates the generation of photo-induced carriers.Therefore,in this thesis,we combine the nanotextured silicon with metallic nanoparticles to investigate the generation,separation and transport mechanism of the carriers in nanostructures of metal and textured silicon.On the basis of lateral photovoltaic effect(LPE),we focus on extending the application prospects of silicon nanotextured materials in the field of position sensitive detection(PSD).Firstly,LPE with a prominent sensitivity of 74.0 mV/mm has been observed on the surface of copper covered silicon nanopyramids(Cu@Si-pyramid)in a linear range of 3 mm.The sensitivity increases to 157.9 mV/mm when the linear range decreases to 1 mm.Random and aperiodic Si nanopyramid textured surface is fabricated by anisotropic etching in a mixed alkali solution with silver nanoparticles.Compared to photoetching,this is a cost-effective and simple method based on an all-solution process,which provides the possibility for the industrialization.The unique morphology effectively increases the surface and provides large attachment area for Cu nanoparticles.The properties of antireflection and light trapping are large improved because of the combination of Cu and Si nanopyramids,leading to remarkable photoelectric responses in broadband from 405 nm to 780 nm.Secondly,a mechanism based on localized surface plasmons(LSPs)is innovatively proposed to induce a high-performance LPE.Both the theoretical and experimental results demonstrate that the structure modification with silicon nanowires(SiNWs)and the LSPs excited in noble silver nanoparticles are two practicable approaches to improve light absorption and carrier generation.A simple,cost-effective and repeatable process based on metal assisted chemical etching(MACE)and magnetron sputtering is applied to prepare Ag/SiNWs/Si structures with good uniformity.In the range from visible to near infrared(NIR),the antireflection is ideal and strong LSPs can be induced.When Ag/SiNWs/Si is served as surface enhanced raman scattering(SERS)active substrate,obvious Raman signal is observed in the detection of rhodamine 6G(R6G)in low concentration(10-8 M).A large increased LPE is detected with a maximum positional sensitivity of 65.35 mV/mm,which is approximately 53-fold compared to the conventional Ag/Si(1.24 mV/mm).This is significant for PSD technique in visible and NIR regions since it contributes to photoelectric responses with good linearity and high sensitivity from 405 nm to 980 nm.Besides,a non-equipotential surface photovoltaic effect is observed in nano metal-semiconductor under uniform illumination in this thesis.As known to all,metals are always equipotential even if they are placed in some external electri fields.However,a surface photovoltage is detected to be as high as 53 mV in uniformly illuminated Ti/Si nanostructures.Electronic behavior in low dimensional nanostructures is substantially different from that of bulk materials,resulting in a central-high metallic potential on the surface.Based on the experimental results and theoretical analyses,the parameters which may relate to the surface photovoltage are revealed,such as the size of the samples,the thickness of metal films and the light power.In this work,an electron diffusion model in low dimensional structures is proposed to understand the physical mechanism of the surface photovoltage.This finding may suggest some novel applications based on the nanoscale metal-semiconductors.