ZnO is a wide band gap, naturally n-type semiconductor with great promise for optoelectronic applications; the main obstacle yet to be overcome is p-type doping. Nitrogen, the most promising candidate currently being pursued as a dopant, has been predicted to preferentially incorporate into the ZnO lattice in the form of a N-2 molecule at an O site when a plasma source is used, leading to compensation rather than p-type doping. We demonstrate this to be incorrect by using N K-edge x-ray absorption spectra and comparing them with first-principles calculations showing that nitrogen, in fact, incorporates substitutionally at O sites where it is expected to act as an acceptor. We also detect the formation of molecular nitrogen upon annealing. These results suggest that effective p-type doping of ZnO with N may be possible only for low-temperature growth processes.

We have applied superconducting stripline detectors (SSLDs) for time-of-flight mass spectrometry (TOF-MS) as molecule detectors. Two SSLDs, which consist of 7-nm-thick niobium nitride (NbN) striplines with different linewidths of 200 or 300 nm on a MgO substrate, were fabricated to investigate the effects of kinetic inductance on time resolution. We have observed ultrafast ion detection signals with risetimes of 360–640 ps, and successfully obtained mass spectra for a peptide, Angiotensin I, and a protein, bovine serum albumin (BSA) at an energy of 17.5 keV. It has been confirmed that the response time is governed by the kinetic inductance of the nano-striplines.

Superconducting nano-stripline detectors (SSLDs) are promising for realizing ideal ion detection in mass spectrometry. We have investigated the ion detection efficiency of a niobium nitride (NbN) SSLD, measuring the bias current dependence of the detection efficiency for Ar þ , Ar 2þ , and Ar 3þ ions accelerated by static voltages between 0.5 and 3 kV. The bias current dependence exhibited a distinct plateau in a high bias region and an abrupt reduction at a certain bias current (threshold bias current) that decreased with increasing kinetic energy. It was found that the kinetic energy dependence of the threshold bias current is consistent with a hot-spot model. #

A large-area (200x200 um2) superconducting stripline detector based on a parallel configuration of superconducting Nb nanowires is presented. We show that the parallel configuration provides a smart way to control the physical nonequilibrium state induced by the molecular impacts, which allows realizing large sensitive area and subnanosecond response at the same time. The experiments were carried out with molecular ions radiation in a keV energy range. The observed rise time was below 400 ps and the relaxation time was 500 ps, the best in this class of superconducting molecular detectors

Fluorescence-yield X-ray absorption fine structure (FY-XAFS) is extensively used for investigating atomic-scale local structures around specific elements in functional materials. However, conventional FY-XAFS instruments frequently cannot cover trace light elements, for example dopants in wide gap semiconductors, because of insufficient energy resolution of semiconductor X-ray detectors. Here we introduce a superconducting XAFS (SC-XAFS) apparatus to measure X-ray absorption near-edge structure (XANES) of n-type dopant N atoms (4 ×1019 cm−3) implanted at 500°C into 4H-SiC substrates annealed subsequently. The XANES spectra and ab initio multiple scattering calculations indicate that the N atoms almost completely substitute for the C sites, associated with a possible existence of local CN regions, in the as-implanted state. This is a reason why hot implantation is necessary for dopant activation in ion implantation. The SC-XAFS apparatus may play an important role in improving doping processes for energy-saving wide-gap semiconductors and other functional materials.