Microstructure And Properties Of A-Si_1-xC_x:H Thin Films Prepared By PECVDs

Hydrogenated amorphous silicon carbide (a-Si_1-xC_x:H) is a wide band gap amorphous semiconductor material. Its optical band gap changes with varying the carbon content in the thin films. So the a-Si_1-xC_x:H thin films with different optical band gaps could be deposited according the needs. But when a-Si_1-xC_x:H thin films are used in the application of solar cells and LEDs, its structural disorder will influence the stability of the optical and electric properties of the thin films, and then decrease the change efficiency of solar cells or decrease the luminous efficienty of LED. Therefore it is necessary and important to study the influence of different deposition methods and conditions on the microstructure of the a-Si_1-xC_x:H thin films and discuss the influence mechanism. It will develop a high quality a-Si_1-xC_x:H thin films to research the structural disorder of the thin films. Furthermore, when a-Si_1-xC_x:H thin films are used as the window layer of the amorphous silicon thin film solar cells, it will effect the decrease of change efficiency due to its low conductivity. A-Si_1-xC_x:H thin films with microcrystal Si phase embedded in the amorphous network will meet the needs of both the wide optical band and high conductivity.The main of our work is to study the structure and properties of a-Si_1-xC_x:H thin films deposited by capacitive coupled plasma enhanced chemical vapor deposition (CCP-CVD) and inductively coupled plasma enhanced chemical vapor deposition (ICP-CVD). After discussing the mechanism of the formation of a-Si_1-xC_x:H thin films, found out the essential effect factor on the structure disorder of the thin films. Develop a new method on depositing an a-Si_1-xC_x:H thin films with crystal phase especially oriented crystal at low temperature and matching with the semiconductor integration tecnnics. And then compared the microstructure and properties of a-Si_1-xC_x:H deposited by different PECVDs. Designed a spectial one-chamber deposition apparatus. Deposite the Al/a-Si_1-xC_x:H bilayer films by thermal evaporation and ICP-CVD in one chamber, and obtain a-Si_1-xC_x:H/n-Si composite structure by Al induced crystallization at low temperature. Some interesting results and couclusions obtained in our work are as follows:(1) The formation of a-Si_1-xC_x:H thin films on glass substrated deposited by CCP-CVD has been studied. The thin film deposited by CCP-CVD on the glass substrate by using the H₂ diluted CH₄ and SiH₄ as the reactive gases is a-Si_1-xC_x:H thin films. The Si-C bond formation at different carbon contents is effected by the proportion of Si and C atoms in the thin films. When more Si atoms with large radiussurround the C atom with small radius, the Si atoms with positive electricity are closer and repulsive force between the Si atoms increase, resulting instability of Si-C bond and against the formation of Si-C bond. When more C atoms with small radius surround the Si atom with large radius, the C atoms with negative electricity are far away from each other and repulsive force between the C atoms could be neglected, the Si-C bonds formed are stable. And the formation of Si-C bond in the a-Si_1-xC_x:H thin films deposited at different deposition powers actually is influenced by the gases decomposition in the chamber. The CH₄ and SiH₄ gases decompose more when the RF power is high, increasing the Si, C plasmas and then more Si-C bond forms. When the the RF power is low, the gases could not decompose drastically, resulting much intermediate bond deposited on the substrate and then decreasing the formation of Si-C bond.(2) The formation of a-Si_1-xC_x:H thin films on Si substrate deposited by CCP-CVD has been researched. The thin films depoited by CCP-CVD at the substrate temperature of 300 °C is a-Si_1-xC_x:H thin films. SiH₄ and CH₄ decompose at the RF glow discharge, depositing Si-C bond on Si substrate. When change the deposition conditions, viz. gas proportion or deposition power, the gas resolution rate changes accordingly. More CH4 gas or higher deposition power result in high gas resolution rate, then more Si-C bonds form in the thin films. Whereas less CH4 gas or lower deposition power lead to less Si-C bonds in the thin films. Furthermore, a multiphase microstructrue of the a-Si_1-xC_x:H thin films is suggested, which reveals that the high oriented (100) polycrystalline silicon embeds in the Si-C-H matrix in which crystalline SiC phase tends to form. The resistivity of the high oriented a-Si_1-xC_x:H thin films actually is related with the structure of the thin films. It is evidently that the resistivity is controlled by the silicon crystalline phase formed in the thin films. As silicon crystalline phase increases, the structure disorder extent is small, then low resistivity is obtained.(3) The formation of a-Si_1-xC_x:H thin films deposited by ICP-CVD has been investigated. The thin films depoited by ICP-CVD is a-Si_1-xC_x:H thin films. The formation ability of Si-C and Si-CH_n bond, both of them contain Si and C atom, shows almost the same change with increasing the carbon content. At Si-rich region, the collision of CH_n plasma surrounded by more Si atom is more sufficient, then more atom state C plasma generates. The formation ability of Si-C bond therefore is higher than Si- CH_n bond at the carbon content lower than the stoichiometric ratio. At the carbon content a little higher than the stoichiometric ratio, Si plasma is consumed to form the Si-CH₃ bond in advance. Then Si-C bond is hard to increases due to the absence of Si. The formation of Si-CH_n restrains the formation of Si-C bond at about the stoichiometric ratio of C/Si. Both the two bonds formation ability decrease as the Si plasma content decreases sharply when continue increasing the carbon content.(4) The optical band gap and bond formation of a-Si_1-xC_x:H thin films at different deposition conditions by CCP-CVD and ICP-CVD has been investigated. The bond configuration in the a-Si_1-xC_x:H thin films will affect the optical properties. The maximum in the optical band gap observed in a-Si_1-xC_x:H thin films at around the stoichiometric ratio of Si / C is due to a change in band edge character, from Si-Si bond states from x<0.55 to C sp² states for x>0.55. The optical band gap opens up as Si-Si bonds replaced by stronger Si-C bonds with increasing the carbon content to the stoichiometric ratio of Si / C. At high C-rich region, more three-fold coordination C atoms (graphite-like structure) instead of tetrahedral bonding structure exist in the films. Then the optical band gap E_opt decreases with increasing the carbon content. The a-Si_1-xC_x: H thin films with near the stoichiometric ratio of Si / C had a wider optical band gap than that shifted away from that ratio. The disorder extent and conductivity of the thin films is related with the radius of the two atoms. When Si atoms with large radius dominate in the films, the C atom filling into the Si-dominated network will hardly influence the microstructure distortion. Therefore the thin films are more ordered and the thin films have higher conductivity. But when C atoms with relative small radius dominate in the films, the Si atom filling into the C-dominated network will distort the microstructure. Therefore the thin films are more disordered and the thin films have lower conductivity.(5) The microstructue and optical properties of the a-Si_1-xC_x:H thin films deposited by CCP-CVD and ICP-CVD has been compared. Under ICP-CVD, more active carbon plasmas generate comparing with that in CCP-CVD, bringing more carbon into the a-Si_1-xC_x:H thin films deposited by ICP-CVD under the same gas composition. The high capability of the gas decomposition in ICP-CVD promotes the formation of the chemical bonds Si-C significantly in the deposited a-Si_1-xC_x:H thin films. That the electrons in plasma seldom strikes the deposited surface in ICP-CVD contributes the formation of perfect Si-C bond configuration and stronger Si-H bonding force in the thin films compared with that deposited by CCP-CVD. The as deposited molecules keep a long time to move together and form the microstructure with large clusters in the thin films deposited by ICP-CVD. In the thin films deposited by CCP-CVD, while the plasma bombs the thin film surface perpendicularly, the short moving time of as deposited molecules but large amount of active sites contribute their microstructure with much smaller droplets. The thin films deposited by ICP-CVD have much narrower optical band gap, exhibiting a nature approaching toward crystalline phase structure with more ordered bond configuration than that by CCP-CVD.(6) Al/a-Si_1-xC_x:H bilayer films were deposited on glass substrate by thermal evaporation and ICP-CVD in a specially designed one-chamber deposition apparatus. The morphology, the Al induced crystallization theory and the crystallization of a-Si_1-xC_x:H thin films has been studied. The roughness of the bilayer films is relatedwith the deposition process and the substrate. The shadow effect is evident as the substrate is rough, resulting the film deposited on the substrate asymmetric. Furthermore the shadow effect leads to the decrease of deposition rate. Before annealing, Al/ a-Si_1-xC_x:H bilayer films appear two clear layers. After annealing, the layer structure disappears and the intermixed layer of Al-Si-C presents. When Al diffusing into the a-Si_1-xC_x:H layer, the covalent bond in the films will be changed to metal bond, decreasing the bond energy of Si-C bond. The Si-C bonds are broken up and the crystalline Si is formed after annealing. The content of crystalline Si could be controlled by controlling the diffusion of Al. and then the conductance of the thin films could be controlled. The effects of annealing temperature, annealing time and Al layer thickness on the crystallization of Al/ a-Si_1-xC_x:H bilayer films has been investigated. Due to the diffusion of Al, the bond in a-Si_1-xC_x:H layer are broken up and crystalline silicon are formed. The content of crystalline Si increases with increasing the amount of Al into the a-Si_1-xC_x:H layer.