Investigation Of The Process And Properties Of Silicon-based Luminescent Thin Films By Radio Frequency Plasma Enhanced Chemical Vapor Deposition

In this paper, silicon-based luminescent thin films were deposited by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) technique with the mixture of SiH4 and Ar as reaction gases under different radio frequency power conditions. The microscopic appearance, structure, chemical composition and photoluminescence (PL) were characterized. The effect of radio frequency power on the composition and relative amount of plasma was investigated by optical emission spectrum (OES). The influence of SiH4 plasma chemical reactions, the interaction of plasma and solid phase, the composition and relative amount of plasma on the film micrograph, structure, crystallinity, chemical composition and photoluminescence was further studied.The monotube device and double-pipe device were self-designed and fabricated to yield the plasma torch for film deposition. These devices made the area and position of plasma generation adjustable and film produced conveniently. Though the double-pipe device had high discharge intensity, it was more complicated with uneven gas flow in the device. The monotube device was simple and adjustable easily with even gas flow, which was adopted in the film deposition experiments.The prepared films were characterized by Scanning Electron Microscope (SEM). It showed that the appearance of films was affected greatly by radio frequency power. When the RF power was 60 W, the film was with loose structure and consisted of clusters with size of ca.1μm. The cluster consisted of particles with size of ca.50 nm. The particles fused, the clusters radical arranged and the film became compacted with raised RF power. When the RF power was 120 W, the film was polycrystal with face-centered cubic lattice (Fcc) revealed by the X-ray Diffraction (XRD) measurement. The size of the crystal grain was calculated by Scherrer equation. The crystalline phase of the film was verified by Transmission Electron Microscope (TEM) measurement. It consisted with the result of XRD. And the real size of the crystal grain tallied closely with the calculated value. From the Fourier Transform Infrared (FTIR) results it could be found that the amount of Si-H bond in the film rose and then attained saturation with increasing RF power. The amount of Si-Si bond increased sharply and the amount of Si-O bond declined sharply when the RF power was 120 W, which was favorable to crystallinity. It was explained that the raised RF power promoted SiH4 cleavage and the produced H helped forward reforming of Si structure of the film.With the light source of 280 nm wavelength as exciting light, the Photoluminescence Spectroscopy (PL) was obtained. The PL spectroscopy consisted of the bands with the peaks around～390 and～470 nm. The band with the peak around～390 nm resulted from the boundary structure defect between the Si crystal grain and SiO2 basic phase. The band with the peak around～470 nm came from neutral oxygen vacancy of O3=Si-Si=O3 structure in the film. The intensity of these two peaks both decreased with increased RF power because the amount of luminescence center declined as a result of decreasing amount of impurity O in the film and difficult oxidization due to film densification.The composition and relative amount of reactive plasma were measured by OES. The line intensity of SiH* and H* both increased while the RF power increased, which indicated that the cleavage level of SiH4; lifted. The line intensity of Ha attained saturation and it was because of a certain amount of reactive gas. The line intensity of Hp increased sharply at 120 W RF power and it might because that the transition energy level of Hp was high. Integrated with the change law of the amount of Si-Si bond and Si-O bond in the FTIR, it showed that Hp favored the reduction of Si-O bond and it consisted with the results of SEM and XRD.