Nonalloyed Cr/ Au-based metal contacts to n-GaN have been demonstrated. The deposited Au/ Cr/ n-GaN contacts exhibited a specific contact resistance ͑ c ͒ of approximately 5.6 ϫ 10 −5 ⍀ cm 2. Although the nonalloyed Ti/ Al-based contacts to n-GaN can also exhibit a comparable c value, their thermal stability is inferior to the Cr/ Au-based contacts. This could be attributed to the fact that Al tends to ball up during thermal annealing. Thus, the surface morphology of most of the annealed Ti/ Al-based contacts was quite rough, and the contacts became rectified when they were annealed at a temperature below 700°C. However, the annealed Cr/ Au-based contacts exhibited an Ohmic characteristic and had a smooth surface when annealing temperatures did not exceed 700°C. In addition, the thermal stability could be further improved by inserting a Pt layer between the Cr and Au layers. This scheme could prevent the diffusion of Au into the Cr layer, thus preventing Au from reaching the Cr/ GaN interface where it could form a possible Ga-Au phase, which would degrade the Ohmic contacts.
The correlation between electron field-emission properties of diamond films prepared by the chemical vapor deposition (CVD) process and the defect structure induced by boron doping was examined. Secondary ion mass spectroscopic analysis indicates that the solubility limit of boron in diamond is (B3+)2=5×1021 cm−3, whereas the infrared absorption (IR) spectroscopic analysis reveals that the largest boron concentration that can be incorporated as substitutional dopants is only one tenth of the solubility limit, (B3+)d=5×1020 cm−3. Including boron species higher than this concentration induces large strain and atomic defects, which are inferred by the distorted Raman resonance peak, noisy IR spectra, and twinned microstructure for diamond. Presumably, the presence of atomic defects, which behave as electron traps, is the mechanism deteriorating the electron field-emission properties of CVD diamonds.
In this research, a dc thermal plasma reactor was used to produce ZnO nanorods with a diameter of about 30nm and a length of 100–200nm. In the photocatalytic study, visible light absorption of the ZnO nanorods was achieved by doping up to a few thousands ppm of nitrogen. The nitrogen-doped ZnO nanorods under the visible light radiation exhibited excellent antimicrobial ability. UV-visible spectroscopy of the N-doped ZnO nanorods annealed in a reductive atmosphere revealed a strong absorption of near-IR light starting at around 1μm, which is attributed to the effect of plasmon resonance. Room-temperature photoluminescence spectroscopy of the N-doped ZnO nanorods showed an UV emission peak at 380nm, a green emission peak at 520nm, and a weak near-IR emission peak at 760nm. The UV emission was assigned to the near band-edge emission, while the green and the near-IR emissions corresponded to the deep-level emission from different defects. In addition, we found that photoluminescent characteristic of the ZnO nanorods depends strongly on the synthesis and annealing atmospheres. Finally, discrete UV lasing modes were observed in the random-packed ZnO nanorods at room temperature. This may be attributed to recurrent light scattering that provides coherent feedback for lasing.
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