Phosphorus doping of Si nanocrystals: Interface defects and charge compensation

Using electron paramagnetic resonance (EPR), Fourier-transform infrared absorption (FTIR) and temperature programmed desorption (TPD), we have investigated the doping of silicon nanocrystals (Si-ncs) and the interaction between intrinsic defects and dopants. Si-ncs were produced in a low-pressure microwave plasma reactor using silane as precursor gas. Phosphorus doping was achieved by addition of phosphine to the precursor gas. The low temperature EPR spectra of all P-doped samples display a line at g = 1.998, which is the fingerprint of substitutional P in crystalline silicon for [P] > 10¹⁸ cm^{- 3}. In addition, the characteristic hyperfine signature of P in Si is also observed for samples with nominal P doping levels below 10¹⁹ cm^{- 3}. Two more features are observed in our EPR spectra: a broad band located at g = 2.0056, due to isotropic Si dangling bonds (Si-dbs), and an axially symmetric powder pattern (g_⊥ = 2.0087, g_∥ = 2.0020) arising from Si-dbs at the interface between the crystalline Si core and a native oxide shell. The formation of this oxide layer and the presence of different H-termination configurations are revealed by FTIR spectroscopy. The density of Si-dbs is reduced in P-doped samples, indicating a sizable compensation of the doping by Si-dbs. This compensation effect was verified by H desorption, which enhances the density of Si-dbs, in combination with TPD and FTIR.