Xiaofan Jiang

Size-dependent electroluminescence from Si quantum dots embedded in amorphous SiC matrix

Si quantum dots (QDs) were formed by thermal annealing the hydrogenated amorphous silicon carbide films (a-SiCx:H) with different C/Si ratio x, which were controlled by using a different gas ratio R of methane to silane during the deposition process. By adjusting x and post annealing temperature, the QD size can be changed from 1.4 to 4.2 nm accordingly, which was verified by the Raman spectra and transmission electron microscopy images. Size-dependent electroluminescence (EL) was observed, and the EL intensity was higher for the sample containing small-sized Si QDs due to the quantum confinement effect (QCE). The EL peak energy as a function of the Si QDs size was in good agreement with a modified effective mass approximation (EMA) model. The calculated finite barrier potential of the Si QDs embedded in SiC matrix is 0.4 and 0.8 eV for conduction and valence band, respectively. Moreover, the current-voltage properties and the linear relationship between the integrated EL intensity and injection current i...

a-SiNx:H-based ultra-low power resistive random access memory with tunable Si dangling bond conduction paths.

The realization of ultra-low power Si-based resistive switching memory technology will be a milestone in the development of next generation non-volatile memory. Here we show that a high performance and ultra-low power resistive random access memory (RRAM) based on an Al/a-SiNx:H/p+-Si structure can be achieved by tuning the Si dangling bond conduction paths. We reveal the intrinsic relationship between the Si dangling bonds and the N/Si ratio x for the a-SiNx:H films, which ensures that the programming current can be reduced to less than 1 μA by increasing the value of x. Theoretically calculated current-voltage (I–V ) curves combined with the temperature dependence of the I–V characteristics confirm that, for the low-resistance state (LRS), the Si dangling bond conduction paths obey the trap-assisted tunneling model. In the high-resistance state (HRS), conduction is dominated by either hopping or Poole–Frenkel (P–F) processes. Our introduction of hydrogen in the a-SiNx:H layer provides a new way to control the Si dangling bond conduction paths, and thus opens up a research field for ultra-low power Si-based RRAM.

Nanocrystalline Si pathway induced unipolar resistive switching behavior from annealed Si-rich SiNx/SiNy multilayers

Adding a resistive switching functionality to a silicon microelectronic chip is a new challenge in materials research. Here, we demonstrate that unipolar and electrode-independent resistive switching effects can be realized in the annealed Si-rich SiNx/SiNy multilayers with high on/off ratio of 109. High resolution transmission electron microscopy reveals that for the high resistance state broken pathways composed of discrete nanocrystalline silicon (nc-Si) exist in the Si nitride multilayers. While for the low resistance state the discrete nc-Si regions is connected, forming continuous nc-Si pathways. Based on the analysis of the temperature dependent I-V characteristics and HRTEM photos, we found that the break-and-bridge evolution of nc-Si pathway is the origin of resistive switching memory behavior. Our findings provide insights into the mechanism of the resistive switching behavior in nc-Si films, opening a way for it to be utilized as a material in Si-based memories.

Trap state controlled bipolar resistive switching effect and electronic transport in LaAlO3/Nb:SrTiO3 heterostructures

We studied the resistive switching (RS) effect in LaAlO3/Nb:SrTiO3 heterostructures at different temperatures with AC impedance technique in addition to the conventional I–V measurements. It was demonstrated that the bipolar RS effect originates from LaAlO3/Nb:SrTiO3 interface and the resistance and capacitance states are controlled by the filling status of traps. A model based on the variation of trap state was proposed to explain the RS effect and the thermal history dependent electronic transport behavior. This work demonstrates the key role of trap state in the RS effect and electronic transport.

Engineering island-chain silicon nanowires via a droplet mediated Plateau-Rayleigh transformation

The ability to program highly modulated morphology upon silicon nanowires (SiNWs) has been fundamental to explore new phononic and electronic functionalities. We here exploit a nanoscale locomotion of metal droplets to demonstrate a large and readily controllable morphology engineering of crystalline SiNWs, from straight ones into continuous or discrete island-chains, at temperature <350 °C. This has been accomplished via a tin (Sn) droplet mediated in-plane growth where amorphous Si thin film is consumed as precursor to produce crystalline SiNWs. Thanks to a significant interface-stretching effect, a periodic Plateau-Rayleigh instability oscillation can be stimulated in the liquid Sn droplet, and the temporal oscillation of the Sn droplets is translated faithfully, via the deformable liquid/solid deposition interface, into regular spatial modulation upon the SiNWs. Combined with a unique self-alignment and positioning capability, this new strategy could enable a rational design and single-run fabrication of a wide variety of nanowire-based optoelectronic devices.

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.

Strong blue light emission from a-SiNx:O films via localized surface plasmon enhancement

Strong and stable blue photoluminescence (PL) at room temperature has been observed from amorphous oxidized silicon nitride (a-SiNx:O) films with Ag nanoparticles inserted between a-SiNx:O films and Si substrates. The resonant excitation of localized surface plasmons (LSPs) with the emission of a-SiNx:O films has resulted in an increase in the internal quantum efficiency, from 3.9% to 8.4%. We have found that the PL efficiency ratio induced by resonant coupling is close to the enhancement of the spontaneous emission rate of a-SiNx:O, which demonstrates that a-SiNx:O films with LSP-enhanced blue emission is promising for silicon-based light-emitting applications.

Effect of the slight surface oxidation of ribbons on the dynamic magnetization of nanocrystalline soft magnetic ribbons

The effect of slight surface oxidation of ribbons of the optimal-annealed Nanoperm alloys on the dynamic properties of domain wall motion is studied. It is shown that the surface oxidation of ribbons may strengthen the domain wall pinning, make the number of domain walls unpinned from the initial sites decrease and the effective permeability measured smaller than the real value. Simultaneously, the existence of the surface oxidation layer may reduce the distance between the pinning sites of domain walls, which results in the relaxation frequency moving toward higher frequency. The condition of the ribbon surface is very important to the dynamic behavior of nanocrystalline soft magnetic ribbons.

The Change of Electronic Transport Behaviors by P and B Doping in Nano-Crystalline Silicon Films with Very High Conductivities

Nano-crystalline Si films with high conductivities are highly desired in order to develop the new generation of nano-devices. Here, we first demonstrate that the grain boundaries played an important role in the carrier transport process in un-doped nano-crystalline Si films as revealed by the temperature-dependent Hall measurements. The potential barrier height can be well estimated from the experimental results, which is in good agreement with the proposed model. Then, by introducing P and B doping, it is found that the scattering of grain boundaries can be significantly suppressed and the Hall mobility is monotonously decreased with the temperature both in P- and B-doped nano-crystalline Si films, which can be attributed to the trapping of P and B dopants in the grain boundary regions to reduce the barriers. Consequently, a room temperature conductivity as high as 1.58 × 103 S/cm and 4 × 102 S/cm is achieved for the P-doped and B-doped samples, respectively.

Hexagonal Ag nanoarrays induced enhancement of blue light emission from amorphous oxidized silicon nitride via localized surface plasmon coupling

A significant enhancement of blue light emission from amorphous oxidized silicon nitride (a-SiNx:O) films is achieved by introduction of ordered and size-controllable arrays of Ag nanoparticles between the silicon substrate and a-SiNx:O films. Using hexagonal arrays of Ag nanoparticles fabricated by nanosphere lithography, the localized surface plasmons (LSPs) resonance can effectively increase the internal quantum efficiency from 3.9% to 13.3%. Theoretical calculation confirms that the electromagnetic field-intensity enhancement is through the dipole surface plasma coupling with the excitons of a-SiNx:O films, which demonstrates a-SiNx:O films with enhanced blue emission are promising for silicon-based light-emitting applications by patterned Ag arrays.