Localized states and quantum effect of photo-generated carriers in photovoltaic system.

We have fabricated the multiple nanolayers impuritied on silicon pillars for Si solar cells to pick up photons in ultraviolet and infrared region of solar spectra, in which the localized states originated from nanosilicon doped with oxygen are built to avoid Auger recombination, and some interesting quantum phenomena in the localized states have been observed. The quantum effect of photo-generated carriers has been observed in I-V curve measurement on the photovoltaic sample prepared in oxygen by using nanosecond pulsed laser. More interesting, the twin states of quantum vibration are measured in the localized states originated from the impuritied nanosilicon, which provides a stable reservoir for electrons in the photovaltaic system. It should be noted that the amplitude change of the quantum vibration occurs under magnetic field with 0.33T on the sample prepared in oxygen, owing to the electron spin in the localized states. The photoluminescence (PL) spectra measured from 300 nm to 1700 nm exhibit the localized states in various regions in the photovoltaic system, in which the electrons can stand in the localized states with longer lifetime to be uneasy into Auger recombination.

Electronic States of Nanocrystal Doped with Oxygen and Visible Emission on Black Silicon Prepared by ns-Laser.

We fabricated the black silicon (BS) structures by using nanosecond pulsed laser (ns-laser) in vacuum or in oxygen environment. It is interesting that the enhanced visible emission occurs in the photoluminescence (PL) spectra measured at room temperature and at lower temperature on the BS surface after annealing, in which lasing near 600 nm is observed on the BS surface with Purcell cavity structure. It is demonstrated in the PL spectra analysis that the electronic states in the nanocrystal doped with oxygen play a main role in the visible emission on the BS surface. The origin of the visible emission near 400, 560, or 700 nm is univocally revealed in the PL spectra analysis. A visible emission is promising for the development of the white light device on the BS.

Electronic states of defect with impurity and infrared emission on black silicon prepared by an ns-laser.

It is found that the optimum annealing temperature is about 1000°C for the infrared emission of defect states at room temperature on black silicon (BS) prepared by using a nanosecond-pulsed laser. In addition, it is observed that the suitable annealing time is 6∼8 min at 1000°C for the emission on the BS. The crystallizing proceeding in annealing on the BS can be used to explain the above annealing effect. It is interesting that the emission band becomes intensive and broader on the BS prepared in oxygen atmosphere than that prepared in vacuum in the analysis of photoluminescence spectra, where the electronic states localized at the defects from D1 to D4 doped with oxygen play an important role in the emission with the broader band which are obviously enhanced in the room temperature.

Lasing with Pumping Levels of Si Nanocrystals on Silicon Wafer.

It is reported that the silicon nanocrystals (NCs) are fabricated by using self-assembly growth method with the annealing and the electron beam irradiation processes in the pulsed laser depositing, on which the visible lasing with higher gain (over 130 cm-1) and the enhanced emission in optical telecommunication window are measured in photoluminescence (PL). It is interesting that the enhanced visible electroluminescence (EL) on silicon nanocrystals (Si-NCs) is obviously observed by the naked eyes, and the light-emitting diode (LED) of the Si-NCs with external quantum efficiency of 20% is made on silicon chip in our laboratory. A four-level system is built for emission model in nanosilicon, in which the PL and EL measurement and transmission electron microscope (TEM) analysis demonstrate that the pumping levels with shorter lifetime from the rising energy of the Si quantum dots due to the quantum confinement effect occur, and the electronic localized states with longer lifetime owing to impurities bonding on Si-NCs surface are formed in the crystallized process to produce the inversion of population for lasing, where the optical gain is generated.