Studies On Preparation And Structure Of Si-rich Silicon Nitride Thin Films And Silicon Quantum Dots

With the development of solar cells,the research of silicon quantum dots materials and their matrix materials embedded in silicon quantum dots is still one of the hot topics.For silicon quantum dot solar cells,the performance of matrix material silicon nitride（SiN_x）embedded silicon quantum dots,as well as the size and density change of silicon quantum dots have a great impact on the performance and photoelectric conversion efficiency of solar cells.Therefore,many researchers have been studying silicon quantum dots and their matrix material silicon nitride.In this paper,a series of silicon-rich silicon nitride thin films were prepared by plasma enhanced chemical vapor deposition（PECVD）technology,and changing nitrogen flow rate,deposition pressure and radio frequency（RF）power.In addition,in order to obtain silicon quantum dots condensed from the Si-rich maxtrix SiN_x materials,the Si-rich SiN_x thin films were annealed in a nitrogen atmosphere furnace.By changing the RF power of the parent material,the same annealing temperature and annealing time,a group of silicon nitride thin film samples with embedded silicon quantum dots were prepared.The structure and performance of these samples were characterizedandstudiedbyFouriertransforminfrared spectroscopy（FTIR）,ultraviolet-visible absorption spectum（UV-Vis）,X-ray diffraction spectroscopy（XRD）,photoluminescence（PL）spectum and Raman scattering spectra,respectively.The main results are as follows:1.Based on the PECVD system,in which the high purity silane and nitrogen were used as reaction gases and other depositin parameters remain unchanged,four gradients of N₂ flow,including 100、200、300 and400sccm,were set up to investigate the effect of nitrogen on preparation of SiN_x films.The results show that with increasing of N₂ flow,the concentration of nitrogen atom decreases in the SiN_x films,which leads to reduce the density of Si-N bond,and increase the concentration of Si-H bond.Meanwhile,the Si-Si bonds appear in the films and its concentration increases gradually.At the same time,it is found that the optical band gap and the density of defect states increases gradually,and the degree of order of microstructure decreases with the increase of N₂flow.In addition,with the increase of N₂ flow rate,Si₃N₄ crystal grains appear in the films,and the grain size firstly increases and then decreases,which further indicate that the films gradually change from amorphous SiN_x to Si-rich SiN_x containing Si₃N₄ crystal grains.This experiment proves that when the SiN_x films are prepared by PECVD,the transition from amorphous SiN_x to Si-rich SiN_x films containing Si₃N₄ crystal grains can be facilitated by controlling the flow rate of N₂.2.The Si-rich SiN_x films were deposited using radio frequency（RF）power of 50W,80W,110W,140W,170W and deposition pressure of200Pa,250Pa,300Pa,350Pa and 400Pa,based on PECVD system,in which high purity silane and nitrogen were used as reaction gases and other sedimentary parameters remain unchanged.The results show that the compactness and deposition rate of the films are related to the RF power and deposition pressure.It is important that the increase of the RF power leads to the increase of the optical band gap value,while the optical band gap value has a non-linear relationship with the deposition pressure.The optical band gap values of both films are between the optical band gap values of silicon and Si₃N₄ films.So,with the increase of the RF power,the N-N bond breaks completely in the reaction chamber,and a large number of Si-N bonds are formed by bonding with Si atoms,while the decrease of the content of Si atoms results in the increase of nitrogen content and Si₃N₄ grain size in the films,indicating that the Si-rich SiN_x films containing Si₃N₄ crystal grains are deposited under this condition.3.The Si-rich SiN_x films were deposited using radio frequency（RF）power of 80W,110W,140W and 170W,based on PECVD system,in which Silane,high-purity nitrogen and high-purity hydrogen were used as reaction gases and other sedimentary parameters remain unchanged.The films were then placed in an annealing furnace for heat treatment at700℃for 24h.The results show that the strength of Si-N bond is positively correlated with RF power for annealed Si-rich SiN_x films.The strength of Si-Si and N-H bonds absorption peak decreases with the increase of RF power,indicating that the number of H atom overflow is decreases with the increase of RF power in the films.For laser Raman spectroscopy,Raman peaks from silicon substrates appear at wavelength520 cm^-1,while small humps appear at RF power 110W and 140W,wavelength 154 cm^-11 and 476 cm^-1.The humps show the Raman peaks of LA mode of amorphous silicon,and the peak vibration decreases with the increase of RF power.PL spectra show that different RF power can increase or decrease the kinds of defect states in thin films.While there are silicon quantum dots in thin films,there are a lot of defect states in thin films,and the average size and density of silicon quantum dots decrease with the increase of RF power.So,it is inferred that higher RF power has no positive effect on the crystallization of thin films.