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This paper firstly reports key factors which are to be necessarily considered for the successful two-bit (four-level) cell operation in a phase-change random access memory (PRAM). They are 1) the writeand-verify (WAV) writing of four-level resistance states and 2) the moderate-quenched (MQ) writing of intermediate resistance levels, 3) the optimization of temporal resistance increase (so-called resistance drift) and 4) of resistance increase after thermal annealing. With taking into account of them, we realized a two-bit cell operation in diodeswitch phase change memory cells with 90nm technology. All of four resistance levels are highly write endurable and immune to write disturbance above 10 8 cycles, respectively. In addition, they are nondestructively readable above 10 7 read pulses at 100ns and 1uA. IntroductionPhase-change random access memory (PRAM) is most promising to realize a multi-level cell (MLC) operation because it has very wide range of resistance across two orders of magnitude or the higher, with respect to writing current. According to the PRAM road map [1], it is expected that highest memory densities of PRAM become comparable to conventional memories such as NOR Flash and DRAM in coming years when MLC operation is fully accomplished. In this paper, we systematically investigated a four-level (two-bit) cell operation in diode-switch phase change memory cells with 90nm technology and discussed its possibilities and issues as well.

A series of GaN:Mg structures were grown in molecular beam epitaxy, using either one or two rf nitrogen sources, and in metalorganic chemical vapor deposition systems with varying Mg flux. Acceptor energies were measured using the Hall effect and admittance spectroscopy techniques. The acceptor energies were found to be different for the two methods, i.e., 135–155 meV for the Hall effect measurement and 80–115 meV for the admittance spectroscopy measurement. The apparently small acceptor energies from the admittance spectroscopy measurement were explained, through a simulation process, by the combined effects of (1) high Mg acceptor concentration with no other free carrier sources, and (2) the Mg emission kinetics assisted by the Frenkel–Poole field effect in the GaN:Mg structures.

The regional distribution and relative frequency of neurohormonal peptides-producing cells were demonstrated in the gastrointestinal (GI) tract of the Korean aucha perch Coreoperca herzi, using 10 types of specific antisera raised against mammalian regulatory peptides. The GI tract was divided into four portions: stomach, gastro-intestinal junction, and small and large intestine. Most of the immunoreactive (IR) cells were in the mucosal epithelium and they were generally spindle shaped with a long cytoplasmic process. In addition, ovoid cells were found in the gastric regions. Serotonin-, somatostatin-, glucagon-, cholecystokinin-8 (CCK-8)-and pancreatic polypeptide (PP)-IR cells were observed with various relative frequencies. No chromogranin A-, secretin-, vasoactive intestinal peptide-, substance P-or bombesin-IR cells, however, were found. Serotonin-IR cells occurred throughout the GI tract and were the most numerous. Somatostatin-IR cells were restricted to the stomach and gastro-intestinal junction in numerous and moderate frequencies, respectively, but small numbers of glucagon-IR cells were restricted to the small intestine. Numerous CCK-8-IR cells were found in the small intestine but variable numbers of PP-IR cells occurred throughout the GI tract except for the large intestine. In general the distribution and relative frequency of these IR cells correspond well to previous reports in teleosts but there are some difference in this species. # 2004 The Fisheries Society of the British Isles