Measurement of minority carrier lifetimes in nanocrystalline silicon devices using reverse-recovery transient method
A new technique for measuring minority carrier lifetimes in p-i-n device configurations, and is used to measure the effective hole lifetimes in nanocrystalline silicon materials. The technique is the reverse-recovery transient method. A forward current in a p-n junction is suddenly switched, and a high constant reverse current flows for a certain period of time. This time is related to the time the injected minority carriers take to recombine in the base. The technique allows for analysis of hole lifetimes in typical thin film nanocrystalline silicon device with very common testing equipment.
The samples used for testing were fabricated using VHF-PECVD reactors using silane and hydrogen as source gases. Both hydrogen profiled nanocrystalline silicon and superlattice nanocrystalline silicon devices were fabricated. The shallow and deep donor states were measured using a junction capacitance technique. The minority carrier diffusion length was determined using a combination of capacitance and quantum efficiency techniques.
The measured hole lifetimes was seen to range from 200-600 ns. Lifetimes were plotted versus inverse defect density, and a linear correlation was seen. This showed the lifetimes followed the Shockley-Read-Hall recombination model. It was also seen for devices deposited at high temperature that a final hydrogen plasma treatment or hydrogen anneal step, the lifetimes and diffusion lengths improved, possibly due to grain boundary passivation. Finally, it was shown that lifetimes in superlattice devices correlated with defect density and diffusion lengths indicating transport in these samples is the same as hydrogen profiled samples.
This technique has been demonstrated for the first time in nanocrystalline silicon devices. An advantage to measuring lifetimes with this technique is that the actual device was not modified in any way to accommodate the measurement. Also, the method only required equipment that can be found in any common electronics lab.