PACS 78.55. Et, 78.66.Hf, 81.07.St, 81.15.Hi Control of ZnO and CdO crystalline structures and p-type doping in ZnO were studied. Formation of zincblende ZnO layers as well as zincblende CdO/ZnO short-period superlattices (SPSL) are demonstrated and bright luminescence was observed even at room temperature from the CdO/ZnO SPSL. pType conductivity observed in N-doped ZnO is studied and the relation to the photoluminescence peaks is discussed. This may open the potential applications such as high-performance short-wavelength light emitters based on their excitonic properties and "integrated transparent electronics" such as transparent displays. To explore such applications with this oxide materials family, there remain two major problems to be cleared. One is the structural mismatch. ZnO is stabilized in the wurtzite structure, while MgO and CdO are predicted to be stabilized in the rocksalt structure [2]. This structural mismatch has to be cleared to construct stable heterostructures for light emitters.

Atomically flat terraced ZnO surfaces were prepared with thermal annealing in air. CdO dots were successfully formed on the ZnO substrate surfaces with in-situ post-growth thermal annealing of CdO layers grown by metalorganic molecular-beam epitaxy. Well-organized alignment of the CdO dots along surface atomic-step edges on the terraced surfaces is demonstrated.1 Introduction ZnO and related oxide materials have large exciton binding energies and the corresponding large oscillator strengths [1]. Further enhancement of excitonic properties is expected in oxide quantum structures such as quantum dots and wires, and higher-performance lasers and/or single-photonbased quantum information processing devices are expected with these distinguished features in oxidebased quantum nanostructures. Especially enhanced oscillator strengths in oxide-based nanostructures are expected to open the high possibility to realize strong coupling regime between excitons and photonic fields [2]. That has been difficult with widely used III-V semiconductors with some exceptions [3,4].Researches on oxide-based nanostructures therefore have been activated recently, and open-shaped ZnO dots [5,6] and nanowires [7][8][9] or nanorods [10] have been mainly studied. These ZnO-based nanostructures cover the ultraviolet wavelength range shorter than 350 nm. The other oxide material candidates which will cover visible wavelength range will be CdO or CdZnO. However, growth of single-crystalline CdO or CdZnO has been difficult. The growth of CdZnO faced a phase separation problem and a reproducible growth of CdZnO alloy semiconductors was difficult [11]. Very recently a growth of wurtzite (WZ) CdZnO was reported with plasma-assisted metaloragnic vapor-phase epitaxy [12]. The growth of single-crystalline rocksalt (RS) CdO was also reported with metalorganic molecularbeam epitaxy (MOMBE) [13].Following the above background, trials to grow CdO-based nanostructures are a challenging issue to open a new possibility of oxide nanostructures in visible regions. In this paper, a first successful formation of CdO dots is reported on atomically flat ZnO substrate surfaces with in-situ post-growth thermal annealing. Well-organized alignment of the CdO dots along the surface atomic step edges on terraced surfaces is also demonstrated and its formation mechanism is discussed.

T cell anergy is one of the mechanisms of immunological tolerance. We examined in this study the distinct responses of Th1 and Th2 cells to in vitro anergic stimulation using Th1 and Th2 cells from two strains of T cell receptor transgenic mice. Proliferation of the Th2 cells was difficult to suppress by anergic stimulation, while that of Th1 cells was significantly inhibited even by weak stimulation. However, IL-4 production by Th2 cells was definitely reduced by anergic stimulation, although the inhibition level of IL-4 was lower than that of IFN-gamma production by Th1 cells. We also examined the reversal of anergy in both subsets. While both the anergized Th1 and Th2 cells responded to IL-2 stimulation, only the anergy of the Th2 cells could be reversed. This result indicates that progression of the cell cycle was not sufficient for anergy reversal in Th1 cells. Our findings indicate that the induction and reversal of T cell anergy might be affected by the distinct signaling features of Th1 and Th2 cells.

Formation of ohmic contacts to p-type zinc oxide (ZnO) was studied. The p-type ZnO samples were grown by metalorganic molecular-beam epitaxy with diethylzinc, deionized water vapor and monomethylhidrazine and were annealed under oxygen gas ambient at 650 °C or 700 °C for 20 min to activate doped nitrogen acceptors. Although the current-voltage characteristics measured through gold p-contacts were not perfectly ohmic, they showed perfectly ohmic properties after rapid thermal annealing (RTA) in the temperature range of 300 to 520 °C for 2 min. The ohmic contact resistivity was measured with the transmission-line measurement method and it was decreased to approximately 1/64 with increasing the RTA temperature. The minimum contact resistivity of 3.15 × 10 -3 Ω cm 2 was observed with RTA at 520 °C for 2 min.

Nucleation and growth mode of GaN during molecular beam epitaxy on vicinal SiC surfaces, consisting of a pair of self-ordered periodic nanofacets, (0001) and (11-2n), are investigated. Well-defined surface nanostructures on SiC enable us to understand growth physics. Here, the Ga-adsorption process on SiC is noticed in particular, and its effects on initial GaN growth stages are examined using reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). With the presence of a Ga-adlayer on a SiC surface, GaN nucleation occurs at step sites on (11-2n) facets, followed by step-flow growth. In contrast, without a Ga-adlayer, GaN nucleation is predominantly observed on (0001) terraces. Interestingly, the crystal structure of the resultant film differs in each case from a typical wultzite (2H) to a 6H-polytype, without and with a Ga-adlayer, respectively.