Abstract
Neutron transmutation doping (NTD) has been considered as a superior approach in introducing dopants in semiconductors due to its advantages of good controllability and extreme homogeneity in dopant concentration. The mechanism of NTD is based on the nucleus transmutation by neutron irradiation of semiconductors. In NTD-Si, 31P dopants can be transmuted through the（n, ）nuclear reaction of the isotope 30Si, while NTD-Ge relies on the transmutation of three isotopes of 70Ge, 74Ge, and 76Ge into three stable nuclides of 71Ga, 75As, and 77Se, respectively. However, neutron irradiation would induce lattice damage in semiconductors, and an adequate thermal annealing process is thus needed to recover the damage as well as to activate the transmuted dopants. For this reason, the purpose of this study is to establish the process of NTD such that the temperature-dependent doping properties and lattice defect behavior in NTD-Si and NTD-Ge can be investigated in depth by the electrical measurements and spectroscopic defect analyses. The specimens employed in this study were intrinsic silicon and germanium wafers. The neutron irradiation experiments of the specimens were carried out using the Tsing Hua Open-pool Reactor (THOR) operated at 1.5 MW. The specimens were irradiated with different thermal neutron fluences and neutron spectra. After neutron irradiation, the isochronal annealing was performed to anneal the irradiated specimens in N2 ambient. For silicon specimens, the annealing process were annealed at 400-800 oC for 0.5-2 hours, and for germanium ones were annealed at 300-500 oC for 1-6 hours. Four-point probe, Hall Effect analyzer, and microwave photoconductance decay (µ-PCD) were employed to determine the doping properties of the specimens, such as resistivity, carrier concentration, carrier type, mobility, and minority carrier lifetime. The lattice defects in NTD specimens were detected by electron paramagnetic resonance (EPR). Finally, the carrier density versus depth was identified by spreading resistance probe system (SRP). The results revealed that subsequent thermal annealing treatments can repair the lattice damage and activate the transmuted dopants. The transmuted 31P dopants in the NTD-Si specimens lead to the n-type electrical conduction. In NTD-Ge ones, the transmuted 71Ga, 75As, and 77Se dopants lead to the p-type electrical conduction. The NTD-Si specimens require an annealing temperature of 800 oC to well recover the lattice defects and activate the dopants, while the NTD-Ge ones require an annealing temperature of 400 oC. Due to the fact that germanium corresponds to a larger neutron absorption cross section than silicon does, a higher dopant concentration can be achieved in NTD-Ge than that in NTD-Si under the same neutron irradiation time. In addition, when the annealing temperature is lower than that for full recovery, the residual lattice defects would form defect levels and dominate the electrical properties of specimens. Also, the excellent doping homogeneity of NTD process can be verified from the results of SRP analysis.