Silicon transmutation doping

NTD has some attractiveness to reactors because it is a potential income generator. The demand for doped silicon is about 100 tons per year and a large facility can produce 20-30 tons per year. At the time of writing (early 2000), it appears that the production is greater than the demand. However, old reactors will probably produce less and less per year, but this is offset by the fact that several newly constructed and designed reactors have dedicated production facilities built into them. 

A facility wanting to break into this market needs to be able to produce the NTD silicon either with greater uniformity, in larger ingots or much cheaper. To this end, a research reactor should start with small quantity production using a simple irradiating rig and should not expand without a reasonable expectation of demand from the industry, because of the current market glut. 

Many of the characteristics of a facility that make it useful for NTD (good flux and fluence homogeneity) also make it an attractive tool for other purposes (e.g., INAA). 

A low power reactor (e.g., 250 kW) with a neutron flux of 2 produce useful quantities of doped silicon. However, for the production of commercial quantities of doped silicon, the flux should be greater than 10 n cm s but not more than about 2 X 10 n cm s This lower neutron flux limit comes about because of the desire to achieve an economic payoff The upper neutron flux limit comes about for several reasons such as keeping the irradiation time greater than just a few minutes and because the highest resistivity silicon demanded is about 1000 ohm cm. 

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IAEA-TECDOC-456 Silicon Transmutation Doping Techniques and Practices 1988... 

J. Guldberg, ed., Neutron-Transmutation-Doped Silicon, Proceedings of the Third International Conference on Transmutation Doping of Silicon, Copenhagen, Denmark, Plenum Press, Inc., New York, 1981. 

Neutron transmutation doping is specific to silicon. In this process, phosphorus-doped silicon can be obtained by using a stream of thermal neutrons to transmute silicon atoms to phosphorus. The nuclear reaction provides an even doping distribution throughout the silicon. 

Suitable sample sizes for irradiation are usually a gram or less. There is no Inherent lower limit, and the upper limit is set by the finite transparency of thick samples to neutrons and by the practical problems of inserting a sample into a neutron field. Very large samples, such as entire silicon ingots for neutron-transmutation doping (23) and even oil paintings (24.25). have been studied by NAA with special facilities. 

Guldberg, J., Ed. "Neutron-Transmutation-Doped Silicon" Plenum New York, 1 981. 

Different techniques have been developed to produce CZ doped crystals with a good longitudinal homogeneity [65], Radial fluctuations of the dopant concentration in the melt are also related to the convection current in the molten phase and to the speed of rotation of the crystal, and this can be troublesome for some technological applications. This problem has been solved in the n-type silicon by neutron transmutation doping (NTD), as shown in... 

B. Pajot, D. Debarre, in Neutron-Transmutation-Doped Silicon, ed. by J. Guldberg (Plenum, 1981), pp. 423-435... 

The next step was the introduction of ion implantation to dope Si for thermometers. Downey et al. [66] used micromachining to realize a Si bolometer with an implanted thermometer. This bolometer had very little low-frequency noise. The use of thermometers doped by neutron transmutation instead of melt doping is described by Lange et al. [67], The evolution of bolometers sees the replacement of the nylon wires to make the conductance to the bath, used by Lange et al. with a micromachined silicon nitride membrane with a definite reduction in the heat capacity associated to the conductance G [68],... 

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