Neutron transmutation doping

Neutron transmutation doping (NTD, abbreviated n-transmutation doping or “neutron doping”) is a process to achieve a highly homogeneous doping with phosphorus in silicon . The doping of gallium with arsenic is also possible with this method. The substrate to be doped is bombarded with thermal neutrons from a neutron source . The NTD process is described below using silicon.

Some of the existing silicon in the natural stable ³⁰ Si isotopes absorb in the context of a Neutronenanlagerung a neutron and become the emission of gamma radiation to ³¹ Si:

${\displaystyle \mathrm {{}^{30}Si\ +n\longrightarrow {}^{31}Si+\gamma } }$

The unstable ³¹ Si isotope decays to ³¹ P with a half-life of 2.62 hours. Electrons are released ( beta radiation ):

${\displaystyle \mathrm {{}^{31}Si\ {\stackrel {2,62\,\mathrm {h} }{\longrightarrow }}\ {}^{31}P+\beta ^{-}} }$

The phosphorus atoms formed during the neutron irradiation are also exposed to the neutron irradiation, which is why further reactions take place in some of these atoms that make the transmuted material radioactive for a while:

${\displaystyle \mathrm {{}^{31}P\ +n\longrightarrow {}^{32}P+\gamma } }$

The conversion of ³² P into ³² S takes place with a half-life of 14.3 days:

${\displaystyle \mathrm {{}^{32}P\ {\stackrel {14,3\,\mathrm {d} }{\longrightarrow }}\ {}^{32}S+\beta ^{-}} }$

The crystal lattice is severely disturbed by the radiation damage, which is why it is healed in a subsequent tempering step at 700 to 800 ° C.

The number of phosphorus atoms produced is proportional to the irradiation time and the neutron flux. Together with the low absorption of thermal neutrons in silicon, very homogeneously doped samples can be obtained. The method is therefore used for the basic doping of substrates, especially for components in power electronics.

The stable isotope ³⁰ Si has a share of about 3.1% in silicon, the atomic density of silicon is 5 · 10 ²²  cm ⁻³ . With the NTD process, a high phosphorus doping of up to 10 ²⁰  cm ⁻³ can therefore be achieved and thus, in addition to the homogeneity of the doping, has a further advantage over other doping processes.

literature

• Rolf Sauer: Semiconductor physics: textbook for physicists and engineers . Oldenbourg, 2009, ISBN 3-486-58863-X .
• Wilhelm T. Hering: Applied nuclear physics . Teubner, Stuttgart 1999, ISBN 3-519-03244-9 .
• Josef Lutz: Semiconductor power components . Springer, Berlin 2006, ISBN 3-540-34206-0 .
• Adolph Blicher: Field-Effect and Bipolar Power Transistor Physics . Academic Press, Inc., New York 1981, ISBN 0-12-105850-6 .
• Roland Schindler, Wolfgang R. Fahrner: Course 02175 Semiconductor Technology I . FernUniversität in Hagen, Hagen 1997.