Dopant Distribution

As noted earlier in Chapter 3, epitaxial silicon films deposited by CVD can be affected by autodoping. If diffusion of the doping species is excessive, the film is not a useful one. Therefore, quite a lot of effort has been spent to accurately measure the distribution of dopant through the film thickness. 

For less highly-doped epi films, one can use the C-V method. In this case, use is made of the fact that a Schottky semiconductor diode has a voltage-dependent capacitance. In other words, when such a diode is reverse biased, a depletion layer forms which then has a capacitance determined by the depth of this layer (w) as well as the doping (N) at its edge. The doping profile can be determined from the following relations.9 

For some doped layers, such as a low-dose ion implant for a MOS transistor, this procedure does not reveal the entire doping profile.10 In this case, a MOS structure is examined rather than a Schottky diode. Typically, a bare silicon wafer is oxidized, and then aluminum dots are sputtered on to form many MOS structures. When a MOS device is examined, Equations (8) and (9) have to be supplemented by 

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Shallow doping profiles, particularly those of As, require nanoscale information on dopant distribution. Although SIMS can be reliably applied for layers below 5 nm... 

Electronic structure of crystallites and dopant distributions by catbodoluminescence electron microscopy... 

Figure 3.34. Electronic structure and dopant distributions in catalysts revealed by cathodoluminescence in electron microscopy unreacted Sb/Sn oxide catalyst with a band-gap of 1.9 eV is shown by the spectrum B. The reacted catalyst at A shows a band-gap of 2.6 eV, demonstrating a peak shift indicative of surface segregation of Sb. (After Boyes et al 1985.)...

Gavrilov, K.L., Bennison, S.J., Mikeska, K.R., Chabala, J.M. and Levi-Setti, R., Silica and magnesia dopant distributions in alumina by high-resolution scanning secondary ion mass spectrometry ,, /. Am. Ceram. Soc., 1999 82(4) 1001-1008. 

An often debated issue is the stationarity of the ionic conductivity which also addresses the question of the stationarity of the dopants distribution.91 93 A variety of different, partly contradicting reports in this respect have been given recent studies indicate relaxation phenomena to be rather small for ca. 10% Sc doping.92 93... 

Neither the dopant distribution coefficient nor the catalytic effect on growth rate is understood in detail. It is reasonable to expect that the dissociation rate of an impurity molecule might be higher than for SiH4, enhancing the concentration in the film. When the plasma contains molecular species a and and the dissociation rate is higher for p by a factor d, then a simple model predicts a distribution coefficient of... 

A different application of chemical equilibrium leads to an explanation of how the incorporation of defects and dopants depends on the growth conditions (Winer and Street 1989). Section S.l describes the unexpected rf power and gas concentration dependence of the dopant distribution coefficient, particularly for arsenic doping. A schematic diagram of the growth process is shown in Fig. 6.19, in which three-fold and four-fold silicon and dopants are deposited from the gas phase. The deposition reactions proposed for arsenic doping are... 

The conditions for stable growth of a crystal are closely associated with segregation of dopants or impurities. Crystal growth theory includes an expression for the effective segregation coefficient K, for a crystal growing from a medium in which the dopant distribution is characterized by a boundary layer of thickness 5 ... 

Such uniform dopant distribution in the porous layer was observed in all SiC samples in which the thicknesses of porous layers varied from 2 to 10 pm in our experiments on diffusion. This can be explained by the fact that diffusion of dopant atoms in a porous layer occurs not only via diffusion through the SiC walls of porous layer but also through gas-phase transport within pore openings and surface diffusion. 

A schematic diagram of the cross section and dopant distributions in a planar npn bipolar junction transistor in Si are shown in Fig. 9.4a, b. The transistor is made by implanting donors at high concentrations, ND, into the n-type Si to form the emitter, and implanting a lower concentration, NA, of acceptors deeper into the n-type Si to form the p-type base. 

Fig. 14.8. Schematic dopant distributions in a conventional well formed by the drive-in diffusion and in a retrograde well formed by high energy implantation...

F. Lahoz, I. R. Martin, J. Mendez-Ramos, P. Nuflez, Dopant distribution in a Tm -Yb codoped silica based glass ceramic An infrared-laser induced upconversion study, J. Chem. Phys. 120, 6180-90 (2004)... 

T. Kotani, J.K. Chen, H.L. Tuller, The Dopant Distribution in Ti-, Zr- and Cr-doped Y3AI5O12 Fibers Grown by the Laser Heated Floating Zone Method. J. Electroceramics, 2, 7-20 (1998)... 

Figure 8.4 shows the changes in series resistance (RJ of the HfOj-based MOS-CAP devices before and after irradiation for various Li°+ ion fluences. It is observed that the values of the series resistance (Rj) increase as the ion fluence increases from 5 x 10 ° to 1 x 10 ° LP+ cm"° with respect to a virgin sample. The increase in series resistance of irradiated devices is attributed to the nonuniformity in the dopant distribution in the silicon bulk caused due to Li ion irradiation. Similar effects of change in dopant distribution have also been reported for other radiation sources (Wei and Ma 1984, de Vasconcelos and da Silva 1996). 

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