Experimental study and modeling of reverse-bias dark currents in PIN structures using amorphous and polymorphous silicon

Abstract

Polymorphous silicon (pm-Si:H) is a nanostructured silicon thin film, with a lower defect density of states and better electronic properties than standard amorphous silicon. We have studied the reverse-bias dark current in PIN structures using this material as the intrinsic layer and compared the results to amorphous silicon PIN devices. All the structures were grown using a standard plasma enhanced chemical vapor deposition process. For thick pm-Si:H devices, we have achieved reverse-bias dark current densities about ten times lower than those obtained using amorphous silicon as the intrinsic layer. This is consistent with the lower defect density of states in polymorphous silicon, which is about 7×1014 cm−3 against 5×1015 cm−3 for amorphous silicon. For a 2.5-μm-thick pm-Si:H diode, the current density obtained is as low as 3 pA cm−2 at −3 V. However, for thinner structures (∼0.5 μm), polymorphous and amorphous silicon show nearly the same reverse-bias leakage current. The experimental dark as well as illuminated characteristics of the diodes have been simulated using a model that incorporates field enhanced thermal generation under reverse-bias conditions (Poole–Frenkel effect). Results reveal that in pm-Si:H diodes, the P/I interface is much more defective than in standard a-Si:H PIN diodes. This fact is shown to completely mask the advantage of the lower defect density of pm-Si:H, in thin PIN diodes. However, in thick samples the electric field in the device and, therefore, also the Poole–Frenkel enhancement of thermal generation are smaller. The effect of the lower density of states in polymorphous silicon is then dominant, and we have achieved a dark current density of 3 pA cm−2 at −3 V for a 2.5-μm-thick diode, as already stated.