Zinc oxide (ZnO) is a wide band gap (3.37 eV) material and significantly interesting for many applications. Recently, many studies have been directed toward the fabrication of p-type ZnO using the group V elements (N, As, P, Sb). We have fabricated ZnO thin films in nitrogen background gas by the pulsed-laser deposition (PLD), because nitrogen is the most promising dopant. The nitrogen incorporation into the films was confirmed by X-ray Photoelectron Spectroscopy (XPS) analyses for the films grown under the high nitrogen pressures. However, the nitrogen doped films do show the disordered hexagonal microstructures which induce the defects into the crystal resulting from strains and stresses. Therefore, we have introduced the ZnO low-temperature buffer layers (LTBLs) between ZnO thin films and sapphire substrates to reduce the defects. The growth conditions of the ZnO LTBL were experimentally optimized for the first time. Characteristics of ZnO thin films with and without a ZnO LTBL were determined by x-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FE-SEM), and Atomic Force Microscopy (AFM). The electrical properties of the ZnO thin films were measured by the van der Pauw method. As a result, epitaxial lateral overgrowth (ELO) and hexagonally assembled ZnO have been successfully confirmed using LTBL. Nevertheless, the films still show the n-type conductivity, our results clearly demonstrate the advantages of the ZnO LTBL.
ZnO is inherently a strong n-type semiconductor due to its intrinsic defects. Among the group V elements (N, As, P, Sb), nitrogen is considered as teh most hopeful dopant for p-type ZnO, because substitute N (N0) is a relatively shallow acceptor. However, technical issues of the low solubility for the desirable defect and compensations from undesirable donor-like defects are imposed on the development of high mobility and low resistivity p-type ZnO. Breaking through these issues is accompanied by the optimization of dopant concentration and reduction of intrinsic defects. In this study, we have investigated the dependence of the nitrogen concentration on its electrical properties. Home-made ZnO1-xNx targets were prepared and used for KrF excimer pulsed-laser deposition (PLD) at precisely controlled growth conditions. Thin films were deposited on c-cut sapphire substrates. The nitrogen concentration was tuned by adjusting the amount of nitrogen in the ablation targets. The film properties were characterized by x-ray diffraction (XRD) and x-ray photoemission spectroscopy (XPS). The electrical properties were measured by van der Pauw method. The as-grown ZnO:N films showed n-type conductivity, however, they were converted to p-type upon post-deposition thermal treatment. Further improvement was demonstrated by introducing a ZnO low-temperature buffer layer which realized the lattice mismatch relaxation.
Silicon substrate is very important for integrated opto-electronic devices applications; because it is widely used the ULSI industries and allowing technically matured processing. However, the large refractive index of Si does not allow for an optical waveguide structure on it by direct growth. Nd:KGW waveguide laser is also a very promising device, which has not only a high stimulated emission cross-section as the laser crystal, but also a high 3rd nonlinear susceptibility. Here we will present our recent results of pulsed laser deposition (PLD) of Nd:KGW thin films on (100) Si substrate by introducing (100) CeO2 buffer layer. The waveguide structure is achieved by the lower refractive index of CeO2. It is well known that films containing alkali metal fabricated with single crystal targets have lower alkali metal concentrations than the stoichiometric target. One solution is the use of K-rich ceramic targets instead to prevent lack of the K during deposition. In this paper, we demonstrate the K compensation and high quality films growth from the viewpoints of crystallinity and optical properties by use of K-rich target. Moreover, we will investigate the required thickness of CeO2 buffer layer and prove the validity of results using a numerical analysis.