Calculation of defect densities in nano-crystalline and amorphous silicon devices using differential capacitance measurements
A technique for determining trap densities as a function of energy in
semiconductors is presented. Through differential capacitance measurements, trap states can be accurately measured and profiled within the bandgap as a function of energy. Measurements were carried out on samples made at the Microelectronic Research Center at Iowa State University. Hydrogen profiled nano-crystalline silicon samples and amorphous samples were made in a VHF-PECVD reactor. ITO was deposited using RF sputtering to serve as the top contact.
Defects for amorphous silicon were shown to be Gaussian approximately .7 eV below the conduction band, on the order of 1015 - 1016 cm-3 depending on deposition. This agrees with both external a-Si measurements and C-V defect measurements.
Defects in nano-crystalline silicon were studied as a function of oxygen present in the material. Five different depositions were carried out with varying amounts of oxygen (0, 9, 18, 27, and 36 sccm). The defect densities of each device were then measured. A large increase in defect densities corresponding to an increase in oxygen content is shown. Thus it is critical to minimize oxygen contamination during device fabrication.
This measurement technique is detailed for the first time on amorphous and nanocrystalline silicon. Through extensive de-noising procedure it produces results with greater accuracy than previous attempts. It provides an excellent non-destructive look at the defect profile of a solar cell and can easily be applied to other photovoltaic materials such as organics.