Nanograting p-n Junctions with Enhanced Charge Confinement.

Recently, geometry-induced quantum effects in a new quasi-1D system, or nanograting (NG) layers, were introduced and investigated. Dramatic changes in band structure and unconventional photoluminescence effects were found in silicon quantum wells with high-energy barriers. Nanograting metal-semiconductor junctions were fabricated and investigated. Here, we report the latest results on a special type of p-n junction in which the charge confinement of the NG is enhanced. The reverse bias dark current is increased in contrast to the metal-semiconductor junctions. When such a junction works as a photovoltaic cell, NG significantly increases short-circuit current and conversion efficiency without affecting open-circuit voltage. These effects are explained by the formation of geometry-induced excitons. To distinguish exciton formation from G-doping effects, we fabricated NGs in both n-type and p-type top layers and obtained qualitatively the same results. To further verify the excitonic mechanism, we analyzed photoluminescence spectrums previously obtained from NG and other NG-like periodic structures. The collected experimental results and previous findings are well explained by the formation of geometry-induced excitons and corresponding quasi-flat bands. Geometry-induced quantum effects can be used to significantly increase the conversion efficiency of photovoltaic cells and enhance the characteristics of other optoelectronic devices.