Yi-Hao Pai

Comparison on the electroluminescence of Si-rich SiNx and SiOx based light-emitting diodes

Electroluminescence (EL) of the metal-insulator-semiconductor light-emitting diodes (MISLEDs) made by Si-rich SiNx and SiOx films with buried Si nanocrystals are compared. The SiNx facilitates carrier transport and EL from MISLED with turn-on current and voltage of 4u2002μA and 12 V by reducing barrier heights at indium tin oxide /SiNx and SiNx/Si-nc interfaces. The SiNx MISLED exhibits larger charge loss rate of 12% within 200 s and shorter delay time of 3.86×10−4u2002sec than SiOx one, which limit its external EL quantum efficiency by strong carrier escaping effect due to the insufficient carrier confinement in Si nanocrystals with low interfacial barriers.

Microwatt MOSLED Using

Microwatt light emission from a metal-oxide-semiconductor light-emitting diode (MOSLED) made by using SiOx film with buried Si nanocrystals on Si nano-pillar array is demonstrated. The Si nano-pillar array obtained by drying the rapidly self-aggregated Ni nano-dot-masked Si substrate exhibit size, aspect ratio, and density of 30 nm, 10, and 2.8times1010 cm-2, respectively. These high-aspect-ratio Si nano-pillar array helps to enhance the Fowler-Nordheim tunneling-based carrier injection and to facilitate the complete relaxation on total internal reflection, thus increasing the quantum efficiency by one order of magnitude and improving the light extraction from the nano-roughened device surface by three times at least. The light-emission intensity, turn-on current and power-current slope of the MOSLED are 0.2 mW/cm2 , 20-30 muA, and 3plusmn0.5 mW/A, respectively. At a biased current of 400 muA, the highest external quantum efficiency is over 0.2% to obtain the maximum EL power of > 1 muW. Compared with the same device made on smooth Si substrate under a power conversion ratio of 1 times 10-4 , such an output power performance is enhanced by at least one order of magnitude.

{\hbox {SiO}}_{\rm x}

The synthesis of few-layer graphene sheets on an ultra-thin nickel film-coated SiO2/Si substrate using hydrogen-free plasma-enhanced chemical vapor deposition (PECVD) with in situ low-temperature carbon dissolution is preliminarily demonstrated. The deposited carbon atoms are initially dissolved into the nickel matrix and subsequently precipitate out onto the nickel film surface. The threshold carbon dissolution temperature for synthesizing few-layer graphene is observed to be as low as 475 °C, and the critical thickness of the host nickel film is at least 30 nm. Due to the ultra-low solubility of the carbon atoms in the nickel film at a threshold temperature of 475 °C, the layer number in few-layer graphene can be precisely controlled. Raman scattering analysis indicates almost identical D and 2D peak intensities for nickel films with different thicknesses, whereas the G peak is enhanced with an increasing layer number of graphene which precipitates from thicker nickel films. Saturation of the G peak for the 50 nm thick nickel film is observed, due to the finite carbon dissolution within a limited deposition time, and results in a stabilized, high quality precipitated few-layer graphene. The linear transmittance of few-layer graphene at 550 nm increases from 83 to 93% when the deposition time is shortened from 600 to 100 s, which corresponds to a decrease of the graphene layer number from 8 to 3 layers. The Raman scattering peak ratio of ID/IG decreases from 1.8 to 0.2 and the G-band linewidth shrinks from 67 to 37.2 cm−1, providing strong evidence for the improved quality of few-layer graphene synthesized by hydrogen-free PECVD at the threshold temperature on an ultra-thin nickel host.

With Buried Si Nanocrystals on Si Nano-Pillar Array

Carrier retention and recombination in Si quantum dots (Si-QDs) embedded SiNx light emitting diode with dark-yellow emission of 93 nW and linear P-I slope of 1.2 mW/A at 80 μA bias are compared. The capacitance-voltage hysteresis of <0.5 V reveals low carrier density per Si-QD, and the 1 ms charge retention concludes only 4% carrier retention within ms time. The fast radiative recombination with electroluminescent (EL) response is shorter than the charge retention within Si-QDs, elucidating that the low Si-QD/Si3N4 interfacial barrier is not dominant for small EL quantum efficiency. Few injected carriers and Auger effect in Si-QDs fail to promote the external quantum efficiency.

Hydrogen-free PECVD growth of few-layer graphene on an ultra-thin nickel film at the threshold dissolution temperature

A 24-pair Si-rich SiNx/SiOx-based distributed Bragg reflector (DBR) architecture, in situ doped with Si nanocrystals (Si-ncs), is studied to show self-photoluminescence (PL) with narrow-linewidth green-color emission pattern. By cascaded depositing, the broadband luminescent SiNx/SiOx pairs with SiNx and SiOx layer thickness of 45 and 86 nm and corresponding refractive indices of 1.96 and 1.62, respectively, and the transmitted PL linewidth of the in situ Si-nc-doped DBR emitter/filter centered at a wavelength of 533 nm greatly reduces from 150 to 10 nm, which is achieved by blocking the UV and blue luminescence at 400-510 nm with the DBR filter bandwidth up to 95 nm. A multilayer DBR modeling is established to simulate the transmitted PL from the summation of each emissive SiNx/SiOx pair, providing a coincident PL shape with a spectral linewidth of 15 nm.

Comparing retention and recombination of electrically injected carriers in Si quantum dots embedded in Si-rich SiNx films

Plasma power controlled PECVD of SiO(x) under SiH(4)/N(2)O gas mixture with manipulated Si quantum dot (Si-QD) size for tailoring photoluminescent (PL) wavelength is demonstrated. The incomplete decomposition of N(2)O at high plasma power facilitates Si-rich SiO(x) deposition to enlarge O/Si composition ratio and to shrink Si-QD size. As RF plasma power increases from 20 to 70 W, the O/Si ratio is increased from 1 to 1.6 and the average Si-QD size is reduced from 4.5 to 1.7, which increases Si-QD density from 3.2 x 10(17) to 3.02 x 10(18) cm(-3) and blue-shifts PL wavelength from 780 to 380 nm.

A 533-nm self-luminescent Si-rich SiN x /SiO x distributed Bragg reflector

This work investigates enhancing UV transmittance and resistivity of indium tin oxide (ITO) film via nanograin crystalline transformation. After annealing, the ITO transforms its crystallinity from amorphous to columnar nanograins, and enriches the Sn–O bonds with absorption at 572u2002cm−1 to greatly reduce its resistivity to 1.2×10−4u2002Ωu2009cm. Long-term annealing beyond 475u2009°C transforms ITO crystallinity to equiaxed nanograins, promoting high UV transmittance of 50%–76% at 325–405 nm but increasing resistivity up to 1.6×10−3u2002Ωu2009cm. A compromised UV transmittance of >50% and resistivity of <2×10−4u2002Ωu2009cm can be simultaneously achieved in annealed ITO with hybrid crystallinity of columnar and equiaxed grains.

Plasma power controlled deposition of SiO x with manipulated Si Quantum Dot size for photoluminescent wavelength tailoring

Enhanced Stoke Raman scattering of large-area vertically aligned Si nanorod surface etched by metal-particle-catalytic is investigated. By enlarging the surface area with lengthening Si nanorods, the linear enhancement on Stoke Raman scattering intensity at 520 cm(-1) is modeled to show well correlation with increasing quantity of surface Si dangling bonds. With Si nanorod length increasing from 0.19 to 2.73 μm, the Raman peaks of the as-etched and oxidized samples gradually shift from -4 cm(-1) and from -4.5 cm(-1) associated with their linewidth broadening from 3 to 9 cm(-1) and from 7 to 18 cm(-1), respectively. The peak intensity of Raman scattering signal from Si nanorod could be enhanced with the increase of interaction area as the number of phonon mode directly corresponds to the tetrahedrally coordinated Si vibrations in the bulk crystal lattice. The asymmetric linewidth broadening and corresponding Raman peak shift is affected by the strained Si nanorod surface caused by etching and the crystal quality. Fourier transform infrared spectroscopy corroborates the dependency between nanorod length and Si-O-Si stretching mode absorption (at 1097 cm(-1)) on oxidized Si nanorod surface, elucidating the increased transformation of surface dangling bonds to Si-O-Si bonds for passivating Si nanorods and attenuating Stoke Raman scattering after oxidation.

Nanograin crystalline transformation enhanced UV transparency of annealing refined indium tin oxide film

Depolarization of sub-mum-high Si nano-pillar/nano-rod surface reflectance with morphologically controlled anti-reflection spectrum is demonstrated. Extremely small reflectance dip of 1.5% at 400-450 nm for Si nano-pillars is extraordinary when comparing with Si nano-rods, in which the reflectance vs. L/lambda for Si nano-pillars coincides well with the graded-index multilayer based modeling spectrum. Alternatively, Si nano-rods preserve its flattened reflectance spectrum up to 1700 nm, whereas the Si nano-pillar surface reflectance monotonically increases to approach that of bulk Si. The destructive interference is only induced on Si nano-pillar surface with larger aspect-ratio > or =15 and small sidewall slope <7 to suppress surface reflectance at blue-green wavelength region. Anomalous depolarization observed from disordered Si nano-pillar/nano-rod surface reflection indicates that TM-mode incidence interacts with more bound electrons than TE-mode to preserve its effective dielectric permittivity less deviated from the bulk Si. The degraded depolarization ratio observed under TE-mode incidence which correlates well with a simplified bounded-electron resonance model is elucidated.

Si nanorod length dependent surface Raman scattering linewidth broadening and peak shift

The Si nanopillars with high aspect ratio were fabricated by dry-etching the thin SiO(2)-covered Si substrate with a rapidly self-assembled Ni nanodot patterned mask. Aspect-ratio-dependent ultra-low reflection and anomalous luminescence of Si nanopillars are analyzed for applications in all-Si based lighting and energy transferring systems. The Si nanopillars induce an ultra-low reflectance and refractive index of 0.88% and 1.12, respectively, at 435 nm due to the air/Si mixed structure and highly roughened surface. The reflectance can be <10% with a corresponding refractive index of<1.80 between 190 and 670 nm. Lengthening the Si nanopillars from 150 +/- 15 to 230 +/- 20 nm further results in a decreasing reflectance, corresponding to a reduction in refractive index by Delta n/n = 18% in the visible and near-infrared wavelength region. After dry-etching an Si wafer into Si nanopillars, the weak blue-green luminescence with double consecutive peaks at 418-451 nm is attributed to the oxygen defect (O(2-))-induced radiation, which reveals less relevance with the ultra-low-reflective Si nanopillar surface.