Korea has recently experienced an increasing number of acute hepatitis A cases. We investigated the dynamics of hepatitis A and changes in the mean age of patients in a hospital in Seoul, Korea. Mean age increased consistently from 19 years in 1996 to 30 years in 2009 (P < 0·0001). Between two acute hepatitis A outbreaks in 1998–1999 and in 2008–2009, mean age increased from 23 to 30 years (P < 0·001). However, the hepatitis A clinical outcomes were similar between the outbreaks. Duration of hospital stay, creatinine level and prothrombin time did not differ. Throughout the study period, individuals born in the 1970s and 1980s comprised the largest proportion (84%) of patients. As this susceptible generation ages, the mean age of hepatitis A patients in Korea will increase consistently. However, at present, the impact of increasing age on clinical outcomes is not apparent.

We screened tuberculosis (TB) contacts as an outbreak investigation with tuberculin skin test (TST) and interferon-gamma release assay (IGRA). We evaluated adverse events and TB incidence in all persons screened after rifampicin (RFP) prophylaxis, and specifically assessed the new TB cases in relation to initial TST and IGRA results. The 180 contacts were divided into four groups: TST+/IGRA+ (n=101), TST+/IGRA− (n=22), TST−/IGRA+ (n=16), and TST–/IGRA– (n=41). RFP treatment (4 months) was prescribed only to the TST+/IGRA+ group. Of 87 contacts who initiated prophylaxis, adverse events occurred in 21 contacts (24·1%) including hepatotoxicity (11·5%), flu-like syndrome (5·7%), and thrombocytopenia (3·4%). TB developed in two TST+/IGRA+ subjects after completion of prophylaxis, including one multidrug-resistant (MDR)-TB case during 21·8 months of follow-up. Adverse events were frequent, and development of TB including MDR-TB occurred after RFP prophylaxis.

Structural and optical properties of In0.2Ga0.8N/GaN heterostructures grown by plasma-assisted molecular beam epitaxy have been investigated as a function of rf plasma power. Indium incorporation resulted in the higher rf power level suppressing 3D island growth with reduced introduction of defects in In0.2Ga0 8N in comparison with GaN. Sharp morphology at interfaces and strong transitions in photoluminescence reveal the optimum rf power around 400 W in our experimental set up for the growth of In0.2Ga0.8N/GaN heterostructures. Our experimental observations suggest that the presence of indium on surface modulates the rate of plasma stimulated desorption and diffusion, and reduces the formation of damaged subsurface.

AlGaN/GaN heterostructures with multiple quantum wells were grown by plasmaassisted molecular beam epitaxy (PAMBE). Structural and optical properties of the heterostructures were analyzed using x-ray diffraction, cathodoldminescence, and photoluminescence. Interband transitions were clearly observed in the GaN quantum wells at both room- and liquid-helium temperatures. The efficiency of the interband recombination due to the confinement effect was greatly enhanced in the thinner quantum wells. The functional dependence of the interband peaks on the well thickness is shown to be in good agreement with the calculated positions of the quantized levels in the wells.

The field emission characteristics of the Si emitters and the diamond coated Si emitters are investigated. The Fowler-Nordheim plots of the two types of Si emitters show linear slopes. It means that the I-V characteristics follow the Fowler-Nordheim relation. Field emission for the two types of diamond coated Si emitters exhibits significant enhancement both in turn-on voltage and total emission current. The Raman spectrum shows that the high intensity graphite peak is observed with diamond peak and thereby large amounts of graphite may be included in the diamond grain boundary. It seems to be thought that the graphite participates in the low field emission. However, further investigations are needed to understand whether the graphite may enhance the emission characteristics of diamond or not.

Ohmic contacts on p-type GaN have been investigated. High quality GaN epilayers on cplane sapphire were prepared using plasma-assisted molecular beam epitaxy that utilized an inductively coupled rf nitrogen plasma source and solid source beams. The resulting film thickness and the doping concentration of the grown samples were in the range of 0.7–1.35 μm and 1018 – 1020/cm3, respectively. The metallization consisted of high work function metal bilayers which included a combinations of 25 nm-thick Ni, Ti, Pt and/or Cr and 200 nm-thick Au on the highly p-doped GaN in a transmission line model pattern. Ohmic contacts were formed by alloying the bi-layers using rapid thermal annealing (RTA) at temperatures in the range of 300–700 °C for 1 min under nitrogen ambient. Current-voltage measurements showed that the specific contact resistance was as low as 1.2 × 10 −4 Ω–cm2 for the sample having 1.4 × 1020/cm3p-type doping concentration with a Cr/Au contact annealed at 500 °C for 1 min by RTA. Judging from the scanning Auger microscopy results and the glancing angle x-ray diffraction analysis, this resistance is attributed to Cr diffusion into the GaN layer.

p-type GaN films were grown on a (0001) sapphire substrate by the plasma-assisted molecular beam epitaxy. A low-contamination, high-power efficiency inductively coupled radio frequency plasma source was used, which was developed at the University of Illinois. Using an MBE system equipped with this plasma source, high-quality p-type GaN films were grown without post-growth treatment. X-ray rocking curve measurements for (0002) diffraction showed a full width at half maximum of less than 7 arcmin. The highest room-temperature hole concentration obtained was 1.4× 1020 cm−3, and for the same sample, the mobility was 2.5 cm2/Vs It is believed that the Mott-Anderson transition occurred in this sample resulting in a metallictype conductivity in the impurity subband. Low-temperature photoluminescence had a blue emission band and no deep-level transitions, indicating the high quality of the grown films. Uniformity of the Mg doping was confirmed by secondary ion mass spectrometry.