Study of efficient semipolar (11-22) InGaN green micro-light-emitting diodes on high-quality (11-22) GaN/sapphire template.

We investigated the electrical and optical performances of semipolar (11-22) InGaN green µLEDs with a size ranging from 20 × 20 µm2 to 100 × 100 µm2, grown on a low defect density and large area (11-22) GaN template on patterned sapphire substrate. Atom probe tomography (APT) gave insights on quantum wells (QWs) thickness and indium composition and indicated that no indium clusters were observed in the QWs. The µLEDs showed a small wavelength blueshift of 5 nm, as the current density increased from 5 to 90 A/cm2 and exhibited a size-independent EQE of 2% by sidewall passivation using atomic-layer deposition, followed by an extremely low leakage current of ~0.1 nA at -5 V. Moreover, optical polarization behavior with a polarization ratio of 40% was observed. This work demonstrated long-wavelength µLEDs fabricated on semipolar GaN grown on foreign substrate, which are applicable for a variety of display applications at a low cost.

Effects of annealing temperature on optical, morphological, and electrical characteristics of polyfluorene-derivative thin films on ITO glass substrate.

The effects of postdeposition annealing temperature (125°C-200°C) toward optical, morphological, and electrical characteristics of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-phenylene)] end capped with dimethylphenyl group deposited on indium tin oxide glass substrates were investigated. Green and red-infrared photoluminescence emissions, originating from П-conjugation aggregates and keto-type defects did not attenuate the intensity of the blue emission peak. This suggested that the aggregates and defects might serve as local traps for radiative recombination. In samples annealed at 125°C-175°C, a decreasing optical energy gap (E(g)) that decreased barrier height as well as an increasing amount of traps have increased current conduction via thermionic emission and trap-assisted tunneling. Nonetheless, an acquisition of the largest E(g) and amount of traps testified that thermionic emission was dominating current conduction, surpassing trap-assisted tunneling in samples annealed at 200°C.

Surface phonon polariton responses of hexagonal sapphire crystals with non-polar and semi-polar crystallographic planes.

The surface phonon polariton (SPhP) characteristics of hexagonal sapphire crystals with non-polar and semi-polar crystallographic planes are investigated. A formulation that considers the effects of crystal orientation is employed to calculate the SPhP dispersion curves of the samples. The SPhP dispersion curves indicate that the SPhP responses of sapphire crystals in non-polar and semi-polar orientations are directionally sensitive. Resonance frequencies and spectral strengths of the SPhP modes can be modulated simply by tuning the angular positions of the samples. The validity of the theoretical results is confirmed by the polarized infrared attenuated total reflection measurements.

Growth and Characterization of Ternary Hf x Ta y O z Films via Nitrogen-Infused Wet Oxidation.

Nitrogen-infused wet oxidation at different temperatures (400-1000 °C) was employed to transform tantalum-hafnia to hafnium-doped tantalum oxide films. High-temperature wet oxidation at 1000 °C marked an onset of crystallization occurring in the film, accompanied with the formation of an interfacial oxide due to a reaction between the inward-diffusing hydroxide ions, which were dissociated from the water molecules during wet oxidation. The existence of nitrogen has assisted in controlling the interfacial oxide formation. However, high-temperature oxidation caused a tendency for the nitrogen to desorb and form N-H complex after reacting with the hydroxide ions. Besides, the presence of N-H complex implied a decrease in the passivation at the oxide-Si interface by hydrogen. As a consequence, defect formation would happen at the interface and influence the metal-oxide-semiconductor characteristics of the samples. In comparison, tantalum-hafnia subjected to nitrogen-infused wet oxidation at 600 °C has obtained the highest dielectric constant, the largest band gap, and the lowest slow trap density.

Determination of acceptor concentration, depletion width, donor level movement and sensitivity factor of ZnO on diamond heterojunction under UV illumination.

The concentration of acceptor carriers, depletion width, magnitude of donor level movement as well as the sensitivity factor are determined from the UV response of a heterojunction consisting of ZnO on type IIb diamond. From the comparison of the I-V measurements in dark condition and under UV illumination we show that the acceptor concentration (∼10(17) cm(-3)) can be estimated from p-n junction properties. The depletion width of the heterojunction is calculated and is shown to extend farther into the ZnO region in dark condition. Under UV illumination, the depletion width shrinks but penetrates both materials equally. The ultraviolet illumination causes the donor level to move closer to the conduction band by about 50 meV suggesting that band bending is reduced to allow more electrons to flow from the intrinsically n-type ZnO. The sensitivity factor of the device calculated from the change of threshold voltages, the ratio of dark and photocurrents and identity factor is consistent with experimental data.

Ohmic-rectifying conversion of Ni contacts on ZnO and the possible determination of ZnO thin film surface polarity.

The current-voltage characteristics of Ni contacts with the surfaces of ZnO thin films as well as single crystal (0001) ZnO substrate are investigated. The ZnO thin film shows a conversion from Ohmic to rectifying behavior when annealed at 800°C. Similar findings are also found on the Zn-polar surface of (0001) ZnO. The O-polar surface, however, only shows Ohmic behavior before and after annealing. The rectifying behavior observed on the Zn-polar and ZnO thin film surfaces is associated with the formation of nickel zinc oxide (Ni1-xZnxO, where x = 0.1, 0.2). The current-voltage characteristics suggest that a p-n junction is formed by Ni1-xZnxO (which is believed to be p-type) and ZnO (which is intrinsically n-type). The rectifying behavior for the ZnO thin film as a result of annealing suggests that its surface is Zn-terminated. Current-voltage measurements could possibly be used to determine the surface polarity of ZnO thin films.