Study On The Electronic Transition Mechanisms Of Silicon Nanoporous Pillar Array

Silicon nanoporous pillar array (Si-NPA) is a typical porous silicon material anda special type of the nc-Si/SiOxnanostructures. Study on the electronic transitionmachanisms during the optical processes can afford the detailed principles of thephysical properties of Si-NPA, and can provide the fundamental theorical surpportsfor designing and constructing the Si-NPA based nanocomposite devices with thedifferent physical properties. In this dissertation, the optical and electric properties ofSi-NPA samples (prepared by hydrothermal method and stain method) and thepoly(N-vinylcarbazole)(PVK)/Si-NPA nanocomposite are investigated by usingsurface photovoltage (SPV) spectroscopy, photoluminescence (PL) spectroscopy andcurrent-voltage (I-V) characteristics. The main achieved results in this dissertation arelisted as the followings:1. Electronic transition machanisms of the porous layer in Si-NPABy comparing the SPV spectra of single crystal silicon (sc-Si) with that ofSi-NPA, the silicon nano-crystallites (nc-Si)/SiOxnanostructure in the Si-NPA couldproduce SPV in the wavelength range of580–300nm. And580nm (~2.14eV) wasconsidered as the absorption edge of the nc-Si/SiOxnanostructure. And the SPV forthe sc-Si layer and the nc-Si/SiOxnanostructure show some contrary characters in thedifferent external electric field. Through analysis, some kind of localized states at henc-Si/SiOxnanostructure interface are believed having dominated the SPV for thenc-Si/SiOxnanostructure.After the sample was annealed and oxidized in air, and when the annealingtemperature was increased from100to500°C, both the SPV in the wavelength rangeof580–300nm and the PL emission band around690nm for the nc-Si/SiOxnanostructure were weakened and disappeared at high temperatures. But both thered–infrared PL band in the wavelength range from710nm to higher wavelength andthe violet-blue PL band were enhanced by increasing the annealing temperature. After2years of natural oxidation in air, the SPV features for sc-Si disappeared completely, and the SPV characteristics were clearly observed in the nc-Si/SiOxnanostructures.After analysis, the Si-O structure related localized states at the nc-Si/SiOxinterfacedominated the electronic transitions during the red PL emission and the SPV for thenc-Si/SiOxnanostructure in Si-NPA, the red–infrared PL was due to the Si=Ostructure related electronic transitions, and the violet-blue PL emission could attributeto the oxygen-related defect related recombination of the photoinduced carriers.2. Evolutions of the electronic transition properties of Si-NPA during thedifferent preparing methodsScanning electron microscopy (SEM) images of Si-NPA samples, which areprepared by hydrothermal method with the different etching time, show that theregular nanoporous silicon pillars only can be well formed when the etching time islonger than25min. By comparing SPV spectra of the prepared samples, the changesof the photon absorption features of the nc-Si/SiOxnanostructure in the porous siliconlayer can be found with the etching time increasing, and this result can attribute to theformation of lots of nc-Si with larger diameter. When the etching time increasescontinuously, the SPV for the nc-Si/SiOxnanostructure in the wavelength range from~580nm to~300nm was weakened obviously, and the PL emission band in thewavelength range from~550nm to~800nm was also been weakened. These changesof SPV and PL properties all can attribute to decreasing of the native oxidation degreeat the nc-Si/SiOxinterface when the etching time increases from10min to45min.SEM images of Si-NPA samples which are prepared by stain method with thedifferent etching time show that regular nanoporous silicon pillars only can be wellformed when the etching time is longer than30h, but the pillars are much bigger thanthose in the hydrothermal prepared Si-NPA samples. By comparing SPV spectra ofthe stain etched Si-NPA samples and the annealed stain etched samples to those ofsc-Si and hydrothermal prepared Si-NPA sample, the photon absorption features ofthe nc-Si/SiOxnanostructure in the porous silicon layer also show great differences tothose of the hydrothermal prepared Si-NPA sample. This result can attribute to theformation of a lot of bigger nc-Si in the nc-Si/SiOxnanostructure of stain etchedSi-NPA samples. All of30h,40h and50h stain etched Si-NPA samples show the strong PL emission in the wavelength range from~550nm to~800nm and the strongSPV in the wavelength range from~500nm to~300nm. But the PL and SPV intensitydo not show regular relationship with the stain etching time.3. PVK/Si-NPA compositeBy comparing to the photon absorption and PL emission properties of PVK andSi-NPA, the PVK/Si-NPA composite shows the obvious differences, and thesedifferences may come from the interactions between PVK molecules and nc-Si/SiOxnanostructure in Si-NPA. And I-V characteristic of the Ag/c-Si/PS/PVK/Ag devicewhich is based on the PVK/Si-NPA composite also shows the possibility of theinteractions between PVK molecules and nc-Si/SiOxnanostructure in Si-NPA.