Mong-Kai Wu

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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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.

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

In this study, high-quality ZnO thin films were grown on sapphire substrates by atomic layer deposition (ALD), followed by high-temperature postdeposition annealing. A thin Al2O3 layer was subsequently deposited by ALD on the ZnO surface to reduce detrimental surface states. Photoluminescence measurements conducted in a backscattering configuration at room temperature show that the ZnO film exhibited stimulated emission with a low threshold intensity of 35.1 kW/ cm2. This may be attributed to the high-quality ZnO film and Al2O3 surface passivation layer grown by ALD, as well as the Al doping effect caused by the thermal diffusion of Al from the sapphire into the ZnO. Results show that ZnO films grown by the ALD technique are applicable to next-generation short-wavelength photonic devices.

UV Electroluminescence and Structure of n-ZnO/p-GaN Heterojunction LEDs Grown by Atomic Layer Deposition

Thin Al2O3 surface-passivating layers grown by atomic layer deposition at 100°C were demonstrated to be instrumental in producing efficient light emission from silicon. External quantum efficiency up to 1.3×10−4 was observed from silicon metal-insulator-semiconductor light-emitting diodes with a 5nm Al2O3 surface-passivating layer as the insulator, which is more than tenfold that from similar devices with a 5nm SiO2 insulator layer thermally oxidized at 1000°C. Anomalous temperature dependences of the photoluminescence intensities and spectra at low temperatures indicate the presence of bound excitonic traps at the Al2O3∕Si interface. The enhanced light emission may be attributed to the temporary capture of excitons by the interfacial bound excitonic traps, which effectively reduces nonradiative recombination.

Low-Threshold Stimulated Emission in ZnO Thin Films Grown by Atomic Layer Deposition

Si nanocrystals embedded in a SiO2 matrix and an n-type Al-doped ZnO (ZnO:Al) layer were applied to improve the external quantum efficiency from Si in n- ZnO/SiO2-Si nanocrystals-SiO2/p-Si heterojunction light-emitting diodes (LEDs). The Si nanocrystals were grown by low pressure chemical vapor deposition and the ZnO:Al layer was prepared by atomic layer deposition. The n-type ZnO:Al layer acts as an electron injection layer, a transparent conductive window, and an anti-reflection coating to increase the light extraction efficiency. Owing to the spatial confinement of carriers and surface passivation by the surrounding SiO2, the Si nanocrystals embedded in the SiO2 matrix lead to a significant enhancement of the light emission efficiency from Si. An external quantum efficiency up to 4.3 x 10(-4) at the wavelength corresponding to the indirect bandgap of Si was achieved at room temperature.

Enhancement in the efficiency of light emission from silicon by a thin Al2O3 surface-passivating layer grown by atomic layer deposition at low temperature

We fabricated and characterized ultraviolet (UV) light-emitting diodes (LEDs) composed of <i>n</i>-ZnO/SiO₂-ZnO nanocomposite/<i>p</i>-GaN heterostructures. Significant UV electroluminescence at 387 nm from the <i>n</i>-ZnO layer in this heterostructure LED was observed at a forward-bias current of as low as 1.8 mA. This is ascribed to the high quality of the <i>n</i>-ZnO layer and the effective function of the SiO₂-ZnO nanocomposite layer. The SiO₂-ZnO nanocomposite layer accomplishes the role of current blocking by forming the larger energy barrier for electron injection from <i>n</i>-ZnO into <i>p</i> -GaN and also contributes to, due to its low refractive index, higher light extraction efficiency from the <i>n</i>-ZnO layer.

An efficient Si light-emitting diode based on an n- ZnO/SiO2–Si nanocrystals-SiO2/p-Si heterostructure

A model calculation on optical gain and co-stimulated emission of photons and phonons in indirect bandgap semiconductors such as silicon is presented. An analytical expression for optical gain via phonon-assisted optical transitions in indirect bandgap semiconductors is presented. Population inversion can occur when the difference between the quasi-Fermi levels for electrons and holes is greater than the photon energy. The rate equations and their steady state solutions for electron, photon, and phonon involved in the phonon-assisted optical transitions are presented. It is shown that co-stimulated emissions of photons and phonons will occur when the threshold condition for laser oscillation is satisfied. The magnitude of optical gain in bulk crystalline silicon is calculated and shown to be smaller than the free carrier absorption at room temperature. However, it is shown, for the first time, that the optical gain is greater than the free carrier absorption in bulk crystalline silicon at the temperature below 23 K. Thus, the calculation predicts that the co-stimulated emissions of photons and phonons could take place in bulk crystalline silicon at the low temperature.

Structure and Ultraviolet Electroluminescence of

ZnO thin films were grown on sapphire substrates by atomic layer deposition (ALD) followed with high-temperature post-annealing. A low threshold for the onset of stimulated emission was observed at excitation intensity of 49.2 kW/cm2.