The results of persistent photoconductivity (PPC) and photoluminescence measurements made on radio-frequency plasma assisted molecular beam epitaxy grown, undoped, GaN are reported in this work. Hexagonal GaN (h-GaN) epilayers grown on sapphire and cubic GaN (c-GaN) epilayers grown on GaAs and cubic SiC substrates, are employed in this study. Three clear experimental evidences are reported to claim that the commonly seen persistent photoconductivity and yellow luminescence (YL) are related to each other through the same defect. First, PPC is observed only in those samples which show YL. Second, the threshold (the minimum photon energy required) to observe PPC is determined as 1.6±0.2 eV, which is almost at the same energy at which the YL band starts raising. Third, the photocurrent increases monotonically from 1.8 to 2.2 eV, which is consistent with the broad nature of YL band.

Paraffin sections of livers obtained at autopsy from 50 cases of hepatocellular carcinoma (HCC), 58 cases of cirrhosis and 54 cases of other miscellaneous liver disorders (controls) were stained for both surface (HBsAg) and core (HBcAg) components of hepatitis B virus (HBV) by immunoperoxidase and immunofluorescence techniques and rigidly controlled for antigen specificity, and in addition stained by orcein for HBsAg. The material was collected from different regions of India and adequate amounts of tissue were examined in most specimens t o overcome possible sampling error caused by random distributions of the antigens i n liver. HBsAg was detected i n 94% of HCC, 71% of cirrhosis and only 2% of control livers, while HBcAg was found i n 22%, 31% and none respectively. Antigen positivity seems t o be directly related t o the amount of tissue examined. Peroxidase staining detected smaller amounts of HBcAg than fluorescence and was also much more convenient for identifying the antigen. Both antigens were present i n 9 of 41 HCC cases, 12 of 39 cirrhosis and none of 25 controls. Most of these livers contained 1+ HBsAg and I+ t o 2+ HBcAg, an antigen expression pattern suggestive of a carrier state or, rarely, of mild chronic liver disease. Among all livers tested, HBsAg alone was present i n 48, both antigens were found in 21, and HBcAg alone i n none. HBsAg was seen inside tumour cells i n four cases, but no tumour showed HBcAg. Most HCC was associated with cirrhosis (92%) and antigen-positive cirrhosis had a higher chance of harbouring HCCthan antigennegative disease. HBsAg was detected in all four non-cirrhotic livers associated with HCC. while two of these also had HBcAg. Active cirrhosis was very frequently associated with HBsAg. These results and the overwhelming evidence of serological and epidemiological studies from various parts of the world suggest a strong association of the hepatitis B virus with HCC. The possible ways in which the two could be related are discussed.

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