Dangling bonds density

Robertson has summarized the three recent classes of models of a-Si H deposition [439]. In the first one, proposed by Ganguly and Matsuda [399, 440], the adsorbed SiHa radical reacts with the hydrogen-terminated silicon surface by abstraction or addition, which creates and removes dangling bonds. They further argue that these reactions determine the bulk dangling bond density, as the surface dangling bonds are buried by deposition of subsequent layers to become bulk defects. 

That hydrogen is responsible for the large reduction of the dangling bond density in amorphous silicon is demonstrated by studies of films grown by sputtering of silicon with an inert gas (Paul et al., 1976). When hydrogen is added to the argon carrier gas, the spin density is reduced to 1016/cm3, and the films can be doped. In contrast, sputtered amorphous... 

Fig. 19. Hydrogen diffusion coefficient, measured at 240°C, as a function of phosphine and diborane gas phase doping level, deduced from the data in Fig. 17. The dependence on dangling bond density is indicated on the top horizontal scale (Street el al., 1987b).

Figure 3.7. (a) Changes in dangling bond density per unit surface area as the size of a particle increases. When the number of units constituting the nucleus increases from four to sixteen, the dangling bond density per unit area decreases from 8 to 16/9. 

Fig. 5.16 shows the doping dependence of n obtained from sweep out, for the phosphorus, arsenic, and boron dopants, compared to the dangling bond density. The results confirm that n is about an order of magnitude less than the defect density and is even lower in the case of light boron doping. It should be noted that the value of actually depends quite sensitively on the thermal history of the sample, as is discussed in the next chapter, but it always remains less than the defect density. These data are for samples slowly cooled from the deposition temperature and stored at room temperature for an extended period and so are typical of samples that have not had any deliberate thermal treatment. 

The low defect density in compensated material is apparent from the optical data in Fig. 5.18, which show a much reduced defect absorption band. The same result is deduced from time-of-flight and ESR data. Although the drift mobility is low, the mobility-lifetime product is comparable with the best undoped material, confirming the low defect density (see Fig. 8.24). The dangling bond density in the dark ESR experiment is about 4x10 cm" , with little dependence on the doping... 

Thermal equilibration effects are also present in undoped a-Si H (Smith et al. 1986). The dangling bond density varies reversibly with temperature, as is shown in Fig. 6.8 for samples of different deposition conditions. The experiment is performed by rapidly cooling the material from different temperatures to freeze in the equilibrium configuration and measuring the ESR spin density of the g = 2.0055 resonance. The defect density increases four-fold between 200 C and 400 °C, with an activation energy of 0.15-0.2 eV. The relaxation times are slower than in the doped material at the same temperature, and the equilibration temperature of about 200 °C, for a cooling rate of... 

Fig. 7.6. Plot of the conductivity activation energy and prefactor and the dangling bond density at different stages of annealing of electron-bombarded a-Si H (Stuke 1987).

Plane Atom density (cm ) Dangling bonds Dangling bond density... 

Resonant x-ray spectra and dangling bond density Hexagonal boron nitride Resonant x-ray emission or resonance x-ray Raman scattering has recently been often studied using synchrotron radiation facilities. A successful example of the application of the DV-ATa method to the x-ray spectra is this resonance x-ray emission spectra of hexagonal boron nitride (A-BN). 

Observed desorption of hydrogen from Psi layer at 400 °C or lower causes the strain variation Dangling bond density starting to increase at 350 ° up to 500 °C and plays a significant role in the structural changes ... 

In the first demonstration of HWCVD PTFE [18], the predominance of the -CF2-bonding environment in the films was shown by X-ray photoelectron spectroscopy (XPS). The FTIR spectra of HWCVD PTFE films are dominated by the CF2 symmetric (1155 cm ) and asymmetric stretches (1215 cm" ), which are also observed for conventionally synthesized bulk PTFE. Additionally, electron spin resonance (ESR) results found the dangling bond density 10 spins/cm for HWCVD PTFE, which is... 

Table 9.4 Number of dangling bonds, dangling bond density, and energy gain relative to the (1 x 1) bulk-terminated structure for the various possible structures on a 51(111) surface.

Figure 7.14 Shows the effect of a dislocation on local Fermi level in a semiconduetor. The dangling bonds associated with the dislocation give rise to states in the energy gap. When the dangling bond density exceeds the local doping density (whieh is virtually assured within some distance) the Fermi energy is pinned at or among the energies of the dangling bond states.

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