Miin-Jang Chen

Room-temperature electroluminescence from electron-hole plasmas in the metal–oxide–silicon tunneling diodes

An electron-hole plasma recombination model is used to fit the room-temperature electroluminescence from metal–oxide–silicon tunneling diodes. The relatively narrow line shape in the emission spectra can be understood by the quasi-Fermi level positions of electrons and holes, which both lie in the band gap. This model also gives a narrower band gap than that of bulk silicon. The surface band bending in the Si/oxide interface is responsible for this energy gap reduction.

Surface Passivation of Efficient Nanotextured Black Silicon Solar Cells Using Thermal Atomic Layer Deposition

Efficient nanotextured black silicon solar cells passivated by an Al2O3 layer are demonstrated. The broadband antireflection of the nanotextured black silicon solar cells was provided by fabricating vertically aligned silicon nanowire (SiNW) arrays on the n(+) emitter. A highly conformal Al2O3 layer was deposited upon the SiNW arrays by the thermal atomic layer deposition (ALD) based on the multiple pulses scheme. The nanotextured black silicon wafer covered with the Al2O3 layer exhibited a low total reflectance of ∼1.5% in a broad spectrum from 400 to 800 nm. The Al2O3 passivation layer also contributes to the suppressed surface recombination, which was explored in terms of the chemical and field-effect passivation effects. An 8% increment of short-circuit current density and 10.3% enhancement of efficiency were achieved due to the ALD Al2O3 surface passivation and forming gas annealing. A high efficiency up to 18.2% was realized in the ALD Al2O3-passivated nanotextured black silicon solar cells.

ZnO/Al2O3 core-shell nanorod arrays : growth, structural characterization, and luminescent properties

We reported the aqueous chemical method to fabricate the well-aligned ZnO/Al<inf>2</inf>O<inf>3</inf> nanocrystal (NC) core-shell nanorod arrays (NRAs). Structural characterization showed that the shell layers are composed of α-Al<inf>2</inf>O<inf>3</inf> nanocrystals. Photoluminescence measurements showed the enhancement of near-band-edge (NBE) emission of ZnO NRAs due to the presence of Al<inf>2</inf>O<inf>3</inf> NC shells. The Al<inf>2</inf>O<inf>3</inf> NC shell layer resulting in flatband effect near ZnO surface leads to a stronger overlap of the wavefunctions of electrons and holes in the ZnO core, further enhancing the NBE emission. This approach should be very useful in designing many other core-shell NRAs for creating varieties of high-efficiency optoelectronic devices.

Electroluminescence from metal/oxide/strained-Si tunneling diodes

The metal-oxide-silicon light-emitting diode under biaxial tensile mechanical strain is studied. The emission line shape of the device can be fitted by the electron-hole-plasma recombination model. The energy gap of strained Si extracted by the light emission spectra at the temperature of 120 K is reduced by 15 meV under 0.13% biaxial tensile strain. The light intensity of the device under 0.13% biaxial tensile strain increases 9% as compared to the relaxed-Si device. The upshift of valence band edge under mechanical strain to increase the majority hole concentration at the oxide∕Si interface may be responsible for this light emission enhancement. The mechanical strain is measured by Raman spectroscopy, strain gauge, and analyzed by the finite element method.

Amplified spontaneous emission from ZnO in n-ZnO/ZnO nanodots–SiO2 composite/p-AlGaN heterojunction light-emitting diodes

This study demonstrates amplified spontaneous emission (ASE) of the ultraviolet (UV) electroluminescence (EL) from ZnO at lambda~380 nm in the n-ZnO/ZnO nanodots-SiO(2) composite/p- Al(0.12)Ga(0.88)N heterojunction light-emitting diode. A SiO(2) layer embedded with ZnO nanodots was prepared on the p-type Al(0.12)Ga(0.88)N using spin-on coating of SiO(2) nanoparticles followed by atomic layer deposition (ALD) of ZnO. An n-type Al-doped ZnO layer was deposited upon the ZnO nanodots-SiO(2) composite layer also by the ALD technique. High-resolution transmission electron microscopy (HRTEM) reveals that the ZnO nanodots embedded in the SiO(2) matrix have diameters of 3-8 nm and the wurtzite crystal structure, which allows the transport of carriers through the thick ZnO nanodots-SiO(2) composite layer. The high quality of the n-ZnO layer was manifested by the well crystallized lattice image in the HRTEM picture and the low-threshold optically pumped stimulated emission. The low refractive index of the ZnO nanodots-SiO(2) composite layer results in the increase in the light extraction efficiency from n-ZnO and the internal optical feedback of UV EL into n-ZnO layer. Consequently, significant enhancement of the UV EL intensity and super-linear increase in the EL intensity, as well as the spectral narrowing, with injection current were observed owing to ASE in the n-ZnO layer.

Roughness-enhanced electroluminescence from metal oxide silicon tunneling diodes

An approximate two-order increase in magnitude in electroluminescence was observed for the metal-oxide-silicon tunneling diodes with oxide grown at 900/spl deg/C, as compared to 1000/spl deg/C. The X-ray reflectivity revealed that the oxide grown at 900/spl deg/C has rougher interface than that grown at 1000/spl deg/C. The role of interface roughness can be understood in a model composed of phonons and interface roughness. An external quantum efficiency of /spl sim/10/sup -6/ was obtained using Al electrodes.

Ultraviolet Electroluminescence From n-ZnO–SiO

Ultraviolet (UV) light-emitting diodes composed of n-ZnO:Al-SiO2-ZnO nanocomposite/p-GaN:Mg heterojunction were fabricated on the (0002) Al2O3 substrate. A SiO2 layer embedded with ZnO nanodots was prepared on the p-type GaN using spin-on coating of SiO2 nanoparticles together with atomic layer deposition (ALD). An n-type Al-doped ZnO layer was deposited also by ALD. The SiO2-ZnO nanocomposite layer accomplishes a role of the current blocking layer and also causes, by its low refractive index, the increase in the light extraction efficiency from n-ZnO. Significant UV electroluminescence from n-ZnO was achieved at a low forward-bias current of 1.8 mA. Strong UV emission arising from impact ionization in GaN, ZnO, and GaN:Mg states was also observed at reverse breakdown bias.

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ZnO nanowire (NW) UV photodetectors (PDs) have high sensitivity, while their long recovery time is an important limitation for practical applications. We demonstrated that the recovery time of nanostructured ZnO PDs can be significantly improved using the nanobelt (NB) network. The NB-network PDs are fabricated by only one step without tedious and costly lithography processes. As compared with a recovery time of 32.95 s in the single NB-based PD, a fast recovery time of 0.53 s observed in the NB-network PDs is achieved due to the existence of the NB-NB junction barriers. As the junction barriers accounting for the poor conductivity of NB networks hinder the electron transport, the dark current of the NB-network PDs is two orders of magnitude lower than that of the single NB-based PDs. The NB networks can be applicable to the building structures for nanostructured ZnO-based light-sensing applications with wafer-scale uniformity without compromising the unique photodetection properties exclusively provided by high surface-to-volume ratio and reduced dimensionality of an individual NW/NB.

–ZnO Nanocomposite/p-GaN Heterojunction Light-Emitting Diodes at Forward and Reverse Bias

The temperature performance of metal–oxide–silicon tunneling light-emitting diodes was studied. An electron–hole-plasma model can be used to fit all the emission spectra from room temperature to 98 K. At constant voltage bias in the accumulation region, the normalized integral emission intensity slightly increases at low temperature with activation energy as low as 12 meV. From room temperature down to 98 K, the extracted band gaps are ∼80 meV lower than the value of Varshni equation, and the linewidth drops from 65 to 30 meV. The transverse optical and longitudinal optical phonons are involved in the light-emission process due to the reduction of extracted band gaps and the resemblance between electroluminescence and photoluminescence spectra at similar temperature.

Enhanced Recovery Speed of Nanostructured ZnO Photodetectors Using Nanobelt Networks

Atomic layer deposition technique and subsequent rapid thermal annealing (RTA) were implemented to grow high-quality ZnO epilayers for the fabrication of n-ZnO/p-GaN heterojunction LEDs. The X-ray diffraction measurement reveals that the ZnO epilayer has high crystallinity with c axis orientation. Transmission electron microscopy images present that the ZnO layer is a single crystal, including only a few survivals of threading dislocations, which were generated in the GaN layer deposited by metal-organic chemical vapor deposition on the c-Al2O3 substrate and most of which were eliminated at the n-ZnO/p-GaN interface. An interfacial layer 4-5 nm thick caused by the RTA treatment was observed between the n-ZnO and p-GaN layers. Room temperature UV electroluminescence (EL) at 391 nm from ZnO was achieved at a low injection current about 10 mA. It is concluded that the competition between the ELs from the n-ZnO and p-GaN (around 425 nm) may be ascribed to the ZnO/GaN interface states coupled with the differences between the n-ZnO and p-GaN in carrier concentration and light emission efficiency.