ZnO Single Crystalline Films Grown By P-MBE And The Investigation On Na Doping

As a novel Ⅱ-Ⅵ compound semiconductor, ZnO is considered to be a potential candidate for short wavelength optoelectronic devices, such as blue-ultraviolet light-emitting diodes (LEDs) and laser diodes (LDs), due to its wide direct band gap of3.37eV and large room-temperature exciton binding energy of60meV. The large exciton binding energy in ZnO allows for efficient UV excitonic emission at room temperature or even higher temperature. In this work, we performed a systematic study of growth and properties of c-plane, non-polar m-plane and a-plane ZnO epitaxial thin films; On the basis of high quality ZnO films, we investigate the growth and characterization of Na-doped polar and non-polar ZnO thin films, in an attempt to reveal the p-type doping mechanism.ZnO thin films were grown on2inch c-plane sapphire substrates by plasma-assisted molecular beam epitaxy (MBE). Using MgO buffer layers, the ZnO thin films are single-crystalline with high structural, electrical, and optical quality. The full width at half maximum (FWHM) of (002) plane ω-rocking curves is42.7arc sec, which is very small. Due to the MgO buffer layers, the screw dislocation density of the ZnO thin films can be significantly reduced (3.62×106cm-2). The films have a resistivity of0.51Ω·cm, an electron concentration of2.2×1017cm-3, and a mobility of55cm2V-1s-1at room temperature.ZnO thin films were grown on2inch a-plane sapphire substrates by plasma-assisted MBE. The films are single-crystalline with high structural, electrical, and optical quality. The FWHM of (002) and (302) planes ω-rocking curves are467.8and813.5arc sec. A screw dislocation density of4.35×108cm-2and an edge dislocation density of3.38×109cm-2are estimated by x-ray diffraction. The films have a resistivity of0.12Ω·cm, an electron concentration of6.9×1017cm-3, and a mobility of77cm2V-1s-1at room temperature. Low temperature photoluminescence measurements confirmed these films were of good optical properties comparable to ZnO single crystals.Non-polar ZnO thin films were grown epitaxially on m-plane and r-plane sapphire substrates using plasma-assisted MBE. For m-plane ZnO, the FWHM of (100) and (101) planes ω-rocking curves are1212.2and1780.2arc sec. For a-plane ZnO, the FWHM of (110) and (101) planes ω-rocking curves are1725.3and2148.4 arc sec. On the basis of high quality non-polar ZnO films, we investigate the growth and characterization of non-polar ZnMgO thin films, m-plane ZnMgO films with Mg content of14.8at.%and a-plane ZnMgO films with Mg content of25.1at.%have been grown. Compared to m-plane ZnMgO film grown on m-plane sapphire substrates, the a-plane ZnMgO film grown on r-plane sapphire substrates has a smoother surface and a higher Mg content under the same condition. The nonintentionally doped non-polar ZnO films are found to have p-type conductivity. Low temperature photoluminescence (PL) and transmission electron microscopy (TEM) have been carried out to investigate the origin of p-type behavior. It is inferred that stacking faults are not (at least not fully) responsible for the p-type behavior. In view of the above fact, non-polar ZnO thin films exhibit advantage to obtain p-type behavior than polar films.The Na-doped polar and non-polar ZnO thin films using NaF as the acceptor dopant source were grown on a-plane, r-plane and m-plane sapphire substrates by the plasma-assisted MBE method. With the effective incorporations of Na, weak p-type conductivity has been achieved in Na-doped polar and non-polar ZnO thin films. Compared to the polar ZnO films, the non-polar ZnO films realize p-type conduction with less Na content. So, non-polar ZnO thin films exhibit advantage to obtain p-type behavior than polar films. The first use of NaF as the acceptor dopant source and the investigation of p-type doping in non-polar ZnO films are groundbreaking work, and it will provide potential to pave the way for "useful" p-type ZnO.