We developed buffer layer methods to fabricate highly crystalline thin films of π-conjugated organic semiconductors. One is low-temperature self buffer layer method that can change 3-dimensional growth to 2-dimensional growth in the growth mode of pentacene film. Another is pentacene monolayer method that can change amorphous like films to c-axis oriented films in rubrene films. In the both cases, the wettability of the substrate surface to the organic compounds plays a great role in the thin film growth. These methods of carrying out surface treatment to raise such a growth temperature or boil a high material of wettability are applicable to other organic device material. The surface treatment of the substrate for the wettability is applicable to other organic material.

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We have investigated the variation of low-temperature (LT) GaN buffer layer on sapphire substrate for the growth of GaN film by metalorganic chemical vapor deposition, changing the deposition conditions such as substrate treatment, V/III ratio, thickness and ambient during the annealing. The dependence of the buffer layer on the conditions is qualitatively discussed for the deposition of GaN film with better quality. Another function of the buffer layer except for the nucleation site and relaxation of lattice-mismatch has been found by taking the approaches to the control and evaluation of the polarity in the buffer layer and GaN film. Decreasing the temperature down to 500-600℃ for the deposition of buffer layer after cleaning sapphire substrate in H_2 ambient at 1000℃ would suppress the unintentional fluctuation at the interface of the substrate resulting in the generation of nucleation site with N-face polarity. We have considered that the key point of LT-buffer layer would convert the polarity from N-face GaN film with hexagonal-facetted surface to Ga-face GaN film with smooth surface by lowering the temperature.

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Buffer layer is a quite useful technique for thin film growth; it can act as "glue" between substrate and film and it can relax the lattice mismatch between them. In this article, we discuss in detail the role of buffer layer to relax the lattice mismatch with taking such technologically relevant examples as ferroelectric BaTiO_3 and ultraviolet light emitting ZnO films grown on slightly lattice mismatched substrates. The interface free energy of lattice strain and relaxation by introducing dislocation is discussed from thermodynamic and kinetic view points. By taking this consideration into account, we can selectively make strained films with coherent interface and completely relaxed films. We also demonstrate a way of reducing the activation energy for introducing misfit dislocation. The use of completely relaxed films as buffer layers can mimic lattice constant tunable substrates and may open up an interesting research arena of lattice strain engineering of oxide thin films.

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We have investigated characteristics of GaN and AlN grown on nearly lattice-matched substrates such as (Mn, Zn) Fe_2O_4 and ZnO by the use of pulsed laser deposition (PLD) at low substrate temperatures. It has been found that the interfacial reactions between the nitride films and the substrates are suppressed with the reduction of the growth temperature and high quality AlN and GaN grow epitaxially at room temperature on the chemically vulnerable substrates. RHEED observations have revealed that these films grow with the layer-by-layer mode and the atomically flat surfaces with stepped and terraced structures are obtained. These results indicate that the room temperature grown GaN and AlN are promising candidates for the buffer layers of group III nitrides.

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It has become attractive for developing high performance Si-based semiconductor devices to constructively introduce strained lattice into a channel layer of the device. To realize such strained channels with large strain which effectively enhances the carrier mobility, the heteroepitaxial growth of high crystalline quality buffer layers formed on Si(001) and Silicon-On-Insulator (001) substrates is inevitable. We have recently developed a novel process to form strain-relaxed Si_<1-x>Ge_x buffer layers on Si. The process allows us to obtain strain-relaxed Si_<1-x>Ge_x layers with extremely-small mosaicity by introducing not glide (60゜) dislocations, which are conventional in the diamond-type lattice structure, but pure-edge dislocations forming a characteristic network structure at the Si_<1-x>Ge_x/Si(001) interface. In this paper, we report on our recent activities on the analyses of the dislocation structure and morphology which plays a significant role in determining the crystallinity of Si_<1-x>Ge_x buffer layers.

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