K. J. Chen

Electroluminescent devices based on amorphous SiN/Si quantum dots/amorphous SiN sandwiched structures.

A single layer of dense Si quantum dots with average size of 4 nm sandwiched in amorphous SiN layers was prepared by laser crystallization of ultrathin amorphous Si film followed by subsequently thermal annealing. The electroluminescent diodes were fabricated by evaporating Al electrodes on back sides of p-Si substrates and the top surface of samples. Room temperature electroluminescence can be detected with applying the negative voltage around 10V on the top gate electrode and the luminescent intensity is increased with increasing the applied voltage. It was found that the integrated luminescent intensity is linearly proportional to the injection current which suggested the intensity depends on the concentrations of injected carriers after Fowler-Nordheim tunneling through amorphous SiN barriers. The influence of the amorphous SiN with different band gap on the device performance was also discussed briefly.

Broadband antireflection and absorption enhancement by forming nano-patterned Si structures for solar cells

In this letter, we report the antireflection and light absorption enhancement by forming sub-wavelength nano-patterned Si structures via nano-sphere lithography technique. It is found that the surface reflection can be significantly suppressed in a wide spectral range (400-1000 nm) and the weighted mean reflection is less than 5%. Meanwhile, the broad band optical absorption enhancement is achieved consequently. Heterojunction solar cells are prepared by depositing ultrathin amorphous Si film on the nano-patterned Si structures, the short circuit current density increases to 37.2 mA/cm(2)and the power conversion efficiency is obviously improved compared to the reference cell on flat Si substrate.

Enhancement of electroluminescence in p–i–n structures with nano-crystalline Si/SiO2 multilayers

Nano-crystalline Si/SiO2 multilayers were prepared by alternately changing the ultra-thin amorphous Si film deposition and the in situ plasma oxidation process followed by the post-annealing treatments. Well-defined periodic structures can be achieved with 2.5 nm thick SiO2 sublayers. It is shown that the size of formed nano-crystalline Si is about 3 nm. Room temperature electroluminescence can be observed and the spectrum contains two luminescence bands located at 650 nm and 520 nm. In order to improve the hole injection probability, p-i-n structures containing a nanocrystalline Si/SiO2 luminescent layer were designed and fabricated on different p-type substrates. It is found that the turn-on voltage of p-i-n structures is obviously reduced and the luminescence intensity increases by 50 times. It is demonstrated that the use of a heavy-doped p-type substrate can increase the luminescence intensity more efficiently compared with the light-doped p-type substrate due to the enhanced hole injection.

In-plane epitaxial growth of silicon nanowires and junction formation on Si(100) substrates.

Growing self-assembled silicon nanowires (SiNWs) into precise locations represents a critical capability to scale up SiNW-based functionalities. We here report a novel epitaxy growth phenomenon and strategy to fabricate orderly arrays of self-aligned in-plane SiNWs on Si(100) substrates following exactly the underlying crystallographic orientations. We observe also a rich set of distinctive growth dynamics/modes that lead to remarkably different morphologies of epitaxially grown SiNWs/or grains under variant growth balance conditions. High-resolution transmission electron microscopy cross-section analysis confirms a coherent epitaxy (or partial epitaxy) interface between the in-plane SiNWs and the Si(100) substrate, while conductive atomic force microscopy characterization reveals that electrically rectifying p-n junctions are formed between the p-type doped in-plane SiNWs and the n-type c-Si(100) substrate. This in-plane epitaxy growth could provide an effective means to define nanoscale junction and doping profiles, providing a basis for exploring novel nanoelectronics.

Enhanced electroluminescence from SiN-based multilayer structure by laser crystallization of ultrathin amorphous Si-rich SiN layers.

Luminescent SiN-based multilayers were prepared in a plasma enhanced chemical vapor deposition system followed by subsequently laser crystallization of ultrathin amorphous Si-rich SiN sublayers. The cross-sectional TEM analysis reveals that grain size of Si nanocrystals embedded in the Si-rich SiN sublayers is independent of the laser fluence, while the grain density can be well controlled by the laser fluence. The devices containing the laser crystallized multilayers show a low turn-on voltage of 5 V and exhibit strong green light emission under both optical and electrical excitations. Moreover, the device after laser-irradiated at 554 mJ/cm(2) shows a significantly enhanced EL intensity as well as external quantum efficiency compared with the device without laser irradiation. The EL mechanism is suggested from the bipolar recombination of electron-hole pairs at Si nanocrystals. The improved performance of the devices was discussed.

Modeling and simulation for the enhancement of electron storage in a stacked multilayer nanocrystallite silicon floating gate memory

In this article, we investigate the storage enhancement mechanism of stacked multilayer nanocrystallite silicon (nc-Si) structures in a master-equation-based equivalent circuit model. As a theoretical extension from our previous experimental works, we reveal the detail injection sequence of electrons into the multilayer nc-Si structure via a direct tunneling process, and how the retention property is enhanced by the stacked structures. Seeking a further improvement in the multilayer nc-Si-based nonvolatile memory structure, we compare two major approaches for that purpose, i.e. (1) by further increasing the number of stacked layers or (2) by adopting an asymmetric double-layer structure. It is shown that the latter is more promising for achieving better nonvolatile storage property and shows a more effective threshold shifting, while retaining the virtues of direct tunneling process like fast write/erase and low operation voltage. We suggest that these results provide important guides for practical design...

Intermediate phase silicon structure induced enhancement of photoluminescence from thermal annealed a-Si/SiO2 multilayers

a-Si/SiO2 multilayers with different a-Si sublayer thicknesses were prepared by plasma enhanced chemical vapour deposition (PECVD). An intermediate phase silicon structure (IPSS), which is intermediate in order between the continuous random network amorphous phase and the well ordered crystalline phase, was discovered in the a-Si sublayers near the crystallization onset temperatures through Raman scattering and cross-section high resolution transmission electron microscopy (HRTEM). A strong broad photoluminescence (PL) band, consisting of two peaks centred at 773 nm and 863 nm respectively, was observed with the formation of the IPSS. Based on the analysis of the temperature dependence of PL, the strong PL emission bands centred at 863 and 773 nm are ascribed to the structural defects inside the IPSS and Si = O at the surface of the IPSS, respectively.

How tilting and cavity-mode-resonant absorption contribute to light harvesting in 3D radial junction solar cells

Radial junction (RJ) architecture has proven beneficial in boosting light harvesting and fast carrier separation in thin film solar cells. While a comprehensive understanding of the detailed absorption distribution and light incoupling mechanism within such a 3D RJ configuration remains largely unexplored. Taking hydrogenated amorphous Si (a-Si:H) RJ solar cells as an example, we here address in both experimental and theoretical manners the impacts of tilting and spacing configuration on the light absorption and external quantum efficiency (EQE) responses. A nice agreement between the calculated and experimental EQE responses indicates that the light harvesting realized within RJ thin film solar cells is quite robust against geometric variations and shadowing effects. Following the concepts of optical fiber injection, we have been able to single out the contribution arising solely from a resonant-mode-incoupling into the RJ cavities against a sidewall scattering incidence scenario. These results provide insightful viewpoints as well as practical guides in developing a new generation of high performance RJ thin film solar cells.

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.

Coupling induced subband structures and collective single electron behavior in a single layer Si quantum dot array

We report a study on the coupling induced subband structures and the collective single electron behavior in a single layer Si quantum dot (Si-QD) array, which is fabricated by a layer-by-layer technique using hydrogen diluted silane gas in plasma enhanced chemical vapor deposition system. Unique peak structures are observed in both the I-V and the capacitance-voltage (C-V) characteristics. The total number of electrons charged into the Si-QD array is found to be the same as the number of coupled quantum dots under the electrode. This phenomenon originates from a collective charging behavior of electrons into the subband structures in the Si-QD array, which evolved from the discrete energy levels in the individual Si QDs due to the weak interdot coupling. The different coupling and retention properties for the s-state and p-state subbands as well as the mechanisms for the charging and redistribution of electrons among the subbands are analyzed and discussed.