Study On The Fabrication And Optical Properties Of ZnCdO Alloy Films And Heterostructure

Zn O is a direct transition II-VI group compound semiconductor with a wide band gap of 3.37 e V and a high exciton binding energy of 60 me V at room temperature. It is considered to be an ideal material to realize short wavelength light emitting diodes and semiconductor lasers. According to the energy band engineering, Zn O based alloy thin films which can achieve the lattice match with Zn O are applied to the preparation of the superlattice or quantum-well structures. This can effectively improve the performance of the Zn O-based optoelectronic devices. Zn Cd O alloy films which were fabricated by doping moderate cadium(Cd) in Zn O have narrower band-gap than Zn O, thus it can realize that the near band edge emission may be adjusted from ultraliolet to blue or green light regions. However, because the ionic radius of Cd2+ is larger than Zn2+, the most important problem is how to dope Cd element as much as possible into Zn O lattice in the case of no phase separation. This is related to many factors such as target composition, preparation method, experimental conditions and so on.In this paper, main research is focus on optimizing the growth conditions of the prepared of Zn Cd O alloy films. To further study the preparation of Zn O/Zn Cd O heterostructure can lay the experimental and theoretical foundations. The main contents are as follows:(1) ZnCdO thin films were grown on c-Al2O3(006) substrates by pulsed laser deposition method(PLD) with a ceramic target Zn O/Cd O(Zn O:Cd O=4:1at%). A series of samples were obtained by using different growth conditions, such as substrate temperatures(200-600℃), oxygen pressure(0.2-8Pa) and target-substrate dictances(50-75mm). The influence of the growth conditions on the crystal strucrures, morphology and luminescence properties of Zn Cd O thin films were studied, and then we can obtain the optimum experimental parameter for the preparation of Zn Cd O thin films. The results show that Zn Cd O thin films prepared in the substrate temperature of 300℃, oxygen pressure of 1Pa and target-substrate dictances of 60 mm have a good crystal quality andexcellent luminescence properties.(2) The influence of oxygen pressure on the structure and luminescence properties of the prepared Zn Cd O films was studied. To further confirm the intrinsic luminescence mechanism of Zn Cd O thin films, the temperature-dependent PL, X-ray energy dispersive spectroscopy(EDS), X-ray photoelectron spectroscopy(XPS) and raman spectrum were measured. With the increase of oxygen pressure, room temperature PL spectrum shows that the emission peak shfts to low energy side and the full width at half maximum(FWHM) becomes broad. This is considered that the content of Cd in the grown Zn Cd O films increases gradually as the oxygen pressure increase. The results measured from EDS, XPS and Raman spectra further support this conclusion. The luminescence mechanism of Zn Cd O thin films prepared under different oxygen pressures were investigated by the temperature dependences of the PL intensity and peak position. Because the fluctuation of Cd atom concentration will form different isoelectronic trap centers, this results in the radiative recombination originating from excitons localized at these trap centers composed of a few Cd atoms. When the oxygen pressure is 0.6Pa, the content of Cd in the thin film is relatively low, the exciton are mainly bound to the isolated Cd atom(X/Cd). With increasing the oxygen pressure, the content of Cd in the films generally increases, the emission originating from the excitons bound to a few isolated Cd atoms(X/Cdn, n≥2) become the main in the PL spectra. When the oxygen pressure enhance to 8Pa, Cd cluster will play a dominant role in the formed isoelectronic trap centers, many discriminated bands corresponding to exciton localized at the clusters of different types(X/Cd cluster) were observed in the PL spectra. Because inter-cluster exists a strong interaction,giving rise to the inhomogeneous broadening of the PL spectra.(3) The research on fabrication and characterization of the Zn O/Zn Cd O heterostructures was carried out. In contrast to single-layer structure, the heterostructures have taken place significant changes in the surface morphology, crystal quality and optical properties. In particular, the content of Cd in alloy thin films will decrease obviously due to the interface effect. The influence of high temperature Zn O buffer layer on the heterostructure interface was studied. The results show that the surface roughness and alloy composition can be obviously improved by introducing a high temperature Zn O buffer layer. This will beconducive to realize a high quality heterostructure and obtain a high efficient light emitting device. On the basis, we prepared the Zn O/Zn Cd O/Zn O double heterostructures with different thicknesses of Zn Cd O layer. PL spectra show that the luminescence from the Zn O cap layer and Zn Cd O layer can be seen clearly when the Zn Cd O layer is enough thick. As the thickness of Zn Cd O layer decreases, the near band edge emission peak from Zn O layer gradually weakened, the luminescence peak from Zn Cd O layer has a blue shifts, and finally only one emission peak was observed.