Yunqing Cao

Enhanced photovoltaic property by forming p-i-n structures containing Si quantum dots/SiC multilayers.

Si quantum dots (Si QDs)/SiC multilayers were fabricated by annealing hydrogenated amorphous Si/SiC multilayers prepared in a plasma-enhanced chemical vapor deposition system. The thickness of amorphous Si layer was designed to be 4 nm, and the thickness of amorphous SiC layer was kept at 2 nm. Transmission electron microscopy observation revealed the formation of Si QDs after 900°C annealing. The optical properties of the Si QDs/SiC multilayers were studied, and the optical band gap deduced from the optical absorption coefficient result is 1.48 eV. Moreover, the p-i-n structure with n-a-Si/i-(Si QDs/SiC multilayers)/p-Si was fabricated, and the carrier transportation mechanism was investigated. The p-i-n structure was used in a solar cell device. The cell had the open circuit voltage of 532 mV and the power conversion efficiency (PCE) of 6.28%.PACS81.07.Ta; 78.67.Pt; 88.40.jj

Microscopic and macroscopic characterization of the charging effects in SiC/Si nanocrystals/SiC sandwiched structures

Microscopic charge injection into the SiC/Si nanocrystals/SiC sandwiched structures through a biased conductive AFM tip is subsequently characterized by both electrostatic force microscopy and Kelvin probe force microscopy (KPFM). The charge injection and retention characteristics are found to be affected by not only the band offset at the Si nanocrystals/SiC interface but also the doping type of the Si substrate. On the other hand, capacitance-voltage (C-V) measurements investigate the macroscopic charging effect of the sandwiched structures with a thicker SiC capping layer, where the charges are injected from the Si substrates. The calculated macroscopic charging density is 3-4 times that of the microscopic one, and the possible reason is the underestimation of the microscopic charging density caused by the averaging effect and detection delay in the KPFM measurements.

Enhanced broadband spectral response and energy conversion efficiency for hetero-junction solar cells with graded-sized Si quantum dots/SiC multilayers

In order to circumvent the narrow spectral response of Si quantum dots (Si QDs)-based solar cells, a novel hetero-junction cell structure containing graded-sized QDs-based multilayers was proposed. The size of Si QDs varies from 8 nm to 2 nm which corresponds with the bandgap from 1.2 eV to 2.1 eV. The graded-sized Si QDs-based hetero-junction cell exhibits an enhanced spectral response in a wavelength range from 400 nm to 1200 nm, which is obviously improved compared with that of conventional Si QDs-based cells. Furthermore, by combining the graded-sized Si QDs multilayers with Si nanowire arrays, a Si QDs/Si NWs hetero-junction solar cell was fabricated and the corresponding power conversion efficiency can be as high as 12.80%, due to the significant spectral loss suppression and optical absorption enhancement by forming nano-patterned light trapping structures.

Electronic properties and charge storage effect of amorphous SiN passivated nanocrystalline silicon

Nanocrystalline Si (nc-Si) with mean size of about 4 nm embedded in amorphous SiN film was prepared by annealing Si-rich amorphous SiN film. The film compositions and microstructures were revealed by x-ray photoelectron spectroscopy, Raman spectroscopy, and transmission electron microscopy. It was found the room temperature conductivity is increased from 7  × 10−9 to 1  × 10−5 S/cm due to the formation of nc-Si. The carrier transport process of nc-Si embedded in amorphous SiN matrix is dominated by trap-assisted tunneling mechanism. Moreover, by forming a-SiN0.81/nc-Si(SiN)/a-SiN0.81 sandwiched floating gate structures, both electron and hole can be injected and stored in nc-Si by controlling the applied bias polarity. A large memory window up to about 7 V was observed, and the stored carrier density was about 1012 cm−2. Our experimental results suggested that the interface states of nc-Si can be well passivated by the amorphous SiN matrix, which results in the good charge storage effect.

Electroluminescence Devices Based on Si Quantum Dots/SiC Multilayers Embedded in PN Junction

We deposited a p-i-n structure device with alternative amorphous Si (a-Si) and amorphous SiC (a-SiC) multilayers as an intrinsic layer in a plasma-enhanced chemical vapor deposition (PECVD) system. A KrF pulsed excimer laser-induced crystallization of a-Si/a-SiC stacked structures was used to prepare Si quantum dots (Si QDs)/SiC multilayers. The formation of Si QDs with an average size of 4 nm was confirmed by Raman spectra, whereas the layered structures were revealed by cross-sectional transmission electron microscopy. Electroluminescence (EL) devices containing Si QDs/SiC multilayers embedded in a p-n junction were fabricated, and the device performance was studied and compared with the reference device without the p-i-n structure. It was found that the turn-on voltage was reduced and that luminescence efficiency was significantly enhanced by using the p-i-n device structure. The recombination mechanism of carriers in a Si-QD-based EL device was also discussed, and the improved device performance can be attributed to the enhanced radiative recombination probability in a p-i-n EL device.

Si nanocrystals-based multilayers for luminescent and photovoltaic device applications

Low dimensional Si materials have attracted much attention because they can be developed in many kinds of new-generation nano-electronic and optoelectronic devices, among which Si nanocrystals-based multilayered material is one of the most promising candidates and has been extensively studied. By using multilayered structures, the size and distribution of nanocrystals as well as the barrier thickness between two adjacent Si nanocrystal layers can be well controlled, which is beneficial to the device applications. This paper presents an overview of the fabrication and device applications of Si nanocrystals, especially in luminescent and photovoltaic devices. We first introduce the fabrication methods of Si nanocrystals-based multilayers. Then, we systematically review the utilization of Si nanocrystals in luminescent and photovoltaic devices. Finally, some expectations for further development of the Si nanocrystals-based photonic and photovoltaic devices are proposed.

Enhanced Electroluminescence From Si Quantum Dots-Based Light-Emitting Devices With Si Nanowire Structures and Hydrogen Passivation

Enhanced electroluminescence (EL) has been achieved from the light-emitting devices containing Si quantum dots/SiO2 multilayers deposited on Si nanowire arrays because of the good antireflection characteristics of Si nanowire structures, which improves the light extraction efficiency. However, it is found that the EL is first enhanced with increasing the depth of the Si nanowires and then reduced to further increase the depth, though it exhibits the lowest reflectance (~3%), which may be due to the increased surface defect states after long time etching, as revealed by electron spin resonance measurements. It is demonstrated that the posthydrogen plasma annealing treatments can passivate the surface defect states which, in turn, improve device performance. The best device shows the turn-on voltage as low as 3.5 V, and the EL intensity of devices on Si nanowire arrays is enhanced 16-fold, compared with that of flat one.