なかのにっき

Many optical-based plasma diagnostic techniques require electron-impact excitation cross sections. In recent years, a considerable number of new results have become available for excitation of rare-gas atoms from both the ground state and metastable states. Using relatively simple techniques these cross sections can be combined with plasma emission measurements to extract many useful plasma parameters such as the electron temperature. Many of the limitations of simple plasma emission models such as the corona model can be overcome by using cross section measurements to select what particular emission lines to use in the analysis.

• The plasma-sheath boundary region
• Low-pressure gas discharge modelling
• Magnetron sputtering of transparent conductive zinc oxide: relation between the sputtering parameters and the electronic properties

Irradiation of high-OH containing amorphous SiO2 is known to introduce a broad absorption band at 4.8 eV and an associated sharp photoluminescence band in the visible red domain, at 1.9 eV. Both absorption and luminescence bands have been unambigously attributed to the nonbridging oxygen (NBO) center, a common defect in irradiated SiO2. Using results of first-principles calculations we show that the NBO center and the interstitial OH molecule (OHi) may both be responsible for the observed absorption and luminescence bands. Although the absorption spectra of the two defects are very similar, their luminescence spectra are different, but overlapping. We found that the NBO center has a sharp luminescence line centered around 1.8 eV in the visible red, whereas OHi molecules have a wide emission spectrum ranging from the infrared (0.8 eV) to visible red (1.8 eV). Investigation of the results show that the difference in the emission spectra is due to the different extent of atomic and electronic relaxations around the two defects: in the case of the NBO, site-independent electronic relaxations are responsible for the defect emitting a photon only at a particular energy, whereas in the case of OHi molecules an interplay between site-dependent atomic and (therefore site-dependent) electronic relaxations result in a wide emission spectrum. A further, intriguing result is that both defects show similar vibrational and polarization properties, therefore their contribution to the red luminescence band of irradiated amorphous SiO2 may not have been decoupled in previous studies.

We report high resolution simultaneous measurements of electrical conductance and force gradient between two sharp gold tips as their separation is varied from the tunneling distance to atomic-size contact. The use of atomically sharp tips minimizes van der Waals interaction, making it possible to identify the short-range metallic adhesion contribution to the total force.

We report cavity ring-down spectroscopy measurements of the gas-phase number density of titanium and molybdenum sputtered by argon ions. A neodymium: yttrium?aluminum-garnet pumped optical parametric oscillator laser system is used to probe optical absorption features of titanium and molybdenum in the 375-400 nm region. For an 18 mA ion beam current, and 750 eV ions, we find that the spatially averaged number density of sputtered atoms is 6.4±0.6×10^8 and 5.1±0.5×10^8 cm^-3, for titanium and molybdenum, respectively. The measured number densities exhibit the expected linear behavior versus beam current, and are in reasonable agreement with values found from a simple sputtering model. Our current configuration yields number density detection limits of ~9×10^6 and ~7×10^5 cm^-3, for titanium and molybdenum, respectively. The technique may ultimately provide a means to measure sputter erosion rates, which are of particular importance for the electric propulsion field.

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In a previous paper [C.-S. Jiang et al., Appl. Phys. Lett. 84, 3477 (2004)], we reported the existence of a local built-in potential on grain boundaries (GBs) of photovoltaic Cu(In,Ga)Se2 (CIGS) thin films. However, whether the built-in potential benefits photovoltaic properties of the device has not been proven. Using a scanning Kelvin probe microscope, we found that, with increasing Ga content in the CIGS film, the built-in potential on the GB drops sharply in a Ga range of 28%?38%. Comparing the changes in the built-in potential, the device efficiency, and the CIGS band gap, we conclude that the built-in potential on the GB plays a significant role in the device conversion efficiency of NREL's three-stage CIGS device.

A reactively sputtered ZrN reflector layer on top of the conventional Mo back contact yields enhanced absorber/back contact reflectance in Cu(In,Ga)Se2 thin film solar cells. Improved long wavelength quantum efficiency is demonstrated with a ZrN reflector at a Cu(In,Ga)Se2 thickness of 0.5 �μm. The optical gain with respect to a standard Mo back contact is initially offset by increased back contact recombination and contact resistance, but these electronic losses can be suppressed by Ga grading of the absorber or by inclusion of a contact layer of MoSe2. This allows for a significantly improved power conversion efficiency of devices with sub-micron Cu(In,Ga)Se2 thickness.