Doped silicon films forming

Figure 5.18 shows the relationship between the resistivity and phosphorus concentration of the initial solution for the film formed from various solutions and heated under the same polycrystallizing RTA conditions (750 °C for 5min). As the initial phosphorus concentration increases, the resistivity decreases down to 2.1mQcm. The film formed from a l-wt% postpolymerization addition solution and the film formed from a 0.01-wt% copolymerized solution exhibit almost the same resistivity, which is reasonable since the two films have almost the same amount of phosphorus atoms, as shown in Fig. 5.16. To apply these doped-silicon films to the source and drain regions of poly-Si TFTs, the initial concentration of 0.1-1 wt% will be sufficient in the case of the copolymerized solution for this heating condition.

According to the macropore formation mechanisms, as discussed in Section 9.1, the pore wall thickness of PS films formed on p-type substrates is always less than twice the SCR width. The conductivity of such a macroporous silicon film is therefore sensitive to the width of the surface depletion layer, which itself depends on the type and density of the surface charges present. For n-type substrates the pore spacing may become much more than twice the SCR width. In the latter case and for macro PS films that have been heavily doped after electrochemical formation, the effect of the surface depletion layer becomes negligible and the conductivity is determined by the geometry of the sample only. The conductivity parallel to the pores is then the bulk conductivity of the substrate times 1 -p, where p is the porosity. 

In addition to silicon and metals, a third important element being deposited as thin films is diamond (Celii and Butler, 1991 May, 2000). For many years, diamonds were synthesized by a high pressure/high temperature technique that produced bulk diamonds. More recently, the interest in diamonds has expanded to thin films. Diamond has a slew of properties that make it a desired material in thin-film form hardness, thermal conductivity, optical transparency, chemical resistance, electrical insulation, and susceptibility to doping. Thin film diamond is prepared using chemical vapor deposition, and we examine the process in some detail as a prototypical chemical vapor example. Despite its importance and the intensity of research focused on diamond chemical vapor deposition, there remains uncertainty about the exact mechanism. 

Fig. 6.2 Raman spectrum of boron-doped diamond film on silicon (a) silicon, (b) boron atoms, (c) diamond (sp3 carbon), and (d) other carbon forms (amorphous)...

Arie et al. [116] investigated the electrochemical characteristics of phosphorus-and boron-doped silicon thin-film (n-type and p-type silicon) anodes integrated with a solid polymer electrolyte in lithium-polymer batteries. The doped silicon electrodes showed enhanced discharge capacity and coulombic efficiency over the un-doped silicon electrode, and the phosphorus-doped, n-type silicon electrode showed the most stable cyclic performance after 40 cycles with a reversible specific capacity of about 2,500 mAh/g. The improved electrochemical performance of the doped silicon electrode was mainly due to enhancement of its electrical and lithium-ion conductivities and stable SEI layer formation on the surface of the electrode. In the case of the un-doped silicon electrode, an unstable surface layer formed on the electrode surface, and the interfacial impedance was relatively high, resulting in high electrode polarization and poor cycling performance. 

Electrografting onto the native oxide layer of doped silicon is also feasible," because this layer is thin enough to allow for the tunneling electron transfer from silicon to the adsorbed monomer. However, the Si-O-C bond that is formed is hydrolytically unstable, and the polymer film can be dissolved by extensive washing of the surface with a good solvent (from the shelf) for the polymer. 

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