Doubly doped semiconductor makes a fast scintillator

At cryogenic temperatures some semiconductors can convert ionizing radiation into visible light with high efficiency and speed. In cadmium sulphide, for example, electron-hole pairs produced by ionizing radiation promptly form excitons with a radiative decay time of about 0.2 ns. However, at room temperature almost all of the holes are trapped on non-radiative centres, that is, crystal defects and impurities.

Now Stephen Derenzo, Edith Bourret-Courchesne, Mattias Klintenberg and Marvin Weber at the Lawrence Berkeley National Laboratory (LBNL) have found that if one impurity is added to trap the holes and another impurity is added to provide an abundant supply of electrons, then bright fast scintillation can occur at room temperature. They hope that their work will lead to a new class of fast, luminous scintillators based on radiative electron-hole recombination in direct-gap semiconductors.

Many inorganic scintillators such as tellurium-doped cadmium sulphide work by combining ionization electrons and holes to form an exciton, but the excited state promptly converts into a triplet (with electron spins aligned) that produces light slowly because the transition to the singlet ground state is spin forbidden. The work at LBNL shows that it is possible to overcome this speed limitation by adding additional different impurity atoms to make the material n-type, as this provides an abundant supply of electrons of both spins to combine with the ionization holes.

The team successfully applied this strategy to cadmium sulphide by codoping it with tellurium and indium. The tellurium is an efficient isoelectronic hole trap and the indium is used to make the material bulk n-type, which provides a band of donor electrons near the bottom of the conduction band. When an electron or X-ray produces holes and electrons in this material, many of the holes are promptly trapped on tellurium atoms in less than 0.05 ns. They then recombine with electrons from the donor band to produce scintillation light with a decay time of 3.5 ns. The light has a wavelength spectrum that peaks at 620 nm and is the same as that of tellurium-doped cadmium sulphide, which has a primary decay time of 3 microseconds.

Derenzo and his colleagues believe that among the vast combination of host crystals and codopants, there are many fast new scintillators that can be developed, and that band structure calculations can guide the search. They are currently exploring the possibility of using ionized acceptor impurities to trap the ionization holes, and codoping other semiconductors such as lead iodide, which has good efficiency for detecting gamma rays and produces 200,000 electron-hole pairs per MeV.