ISOTOPE HETEROSTRUCTURES SELECTIVELY DOPED BY NEUTRON TRANSMUTATION

The author presents a novel family of semiconductor multilayer structures, isotope heterostructures. The simplest isotope heterostructure is created by epitaxial growth of chemically pure layers of pure or deliberately mixed isotopes of one chemical element that can be doped after growth by exposure to thermal neutrons (neutron transmutation doping (NTD)). He shows that isotope multilayer structures offer selective doping using NTD. The advantages of isotope multilayers include but are not limited to the complete absence of autodoping and dopant impurity induced intermixing, choice of dopant concentration through the choice of the neutron fluence after growth, perfect homogeneity of doping, fixed dopant concentration ratios in the various layers given by the isotope abundances and possibly doping to concentrations above solubility limits. The element germanium is used as an example for isotope multilayer formation because NTD can produce shallow acceptors, shallow donors or no doping depending only on the choice of the appropriate Ge isotopes. This allows an almost limitless number of possible combinations of selectively doped and pure layers. Isotope heterostructures, especially isotope homo and hetero superlattices of Ge, Si and compound semiconductors, offer an abundance of novel properties enabling new physics and new devices.