氧分压对PLD制备ZnO薄膜结构和发光性质的影响

采用脉冲激光沉积(PLD)技术,在Si(100)衬底上制备出高度c轴取向的ZnO薄膜。通过X射线衍射(XRD)谱,扫描电镜(SEM)和室温光致发光(PL)光谱的测量,研究了生长气氛压强的改变对薄膜结构和光致发光的影响。实验结果表明,当氧压从10Pa升高到100Pa时ZnO(002)衍射峰的半峰全宽(FWHM)增大。可以认为这是由于较高的氧压下,到达衬底表面的离子动能减小。这样部分离子没有足够的能量迁移到生长较快的(002)面,c轴取向变差,导致(002)衍射峰的强度降低,半峰全宽增大。随着氧压增大,紫外发光强度增强。这可能是氧压变大,薄膜的化学配比升高,说明化学配比对UV发光的影响要大于薄膜微结构的影响。改变氧气压强对薄膜的表面形貌也有较大的影响。

Effect of Oxygen Pressure on Structure and Photoluminescence Properties of the ZnO Thin Film Deposited by PLD

ZnO is a wide and direct band semiconductor. It has a large fundamental band gap of 3.37 eV,which makes it a promising material for use in ultraviolet light-emitting and laser diodes. Moreover, ZnO possesses a large exciton binding energy (60 meV), which is much larger than that of GaN (25 meV) as well as the thermal excitation energy at room temperature, and can ensure an efficient exciton emission at room temperature. ZnO thin film can be fabricated by various methods such as pulse laser deposition, chemical or physical vapor deposition and molecular beam epitaxy.But pulse laser deposition has many advantage.ZnO thin films were prepared using a pulsed laser deposition (PLD) technique on Si (001) substrates.All samples were grown at the same substrate temperature and different oxygen partial pressure.After growing the samples, the influence of ambient oxygen pressure, ranging from 10 to 100 Pa, on the structural and optical properties of ZnO films was investigated systematically by XRD,SEMand PL.X-ray diffraction patterns indicate all films have (002) preferred orientation.But the experiment results indicated that as the oxygen pressure increases from 10 Pa to 100 Pa during the thin film deposition,the FWHMof (002) diffraction peaks becomg larger from 0.19° to 0.43°.We suppose this is because that at higher O₂ pressure the kinetics of Zn and Oatoms are degraded by collisions with the ambient gas. These atoms have no sufficient energy to move to the plan of (002), so the crystallinity of the samples degrades.But the photoluminescence intensity of ultra-violet (UV) luminescence increases as the oxygen pressure increases.This is probably because the stoichiometry of oxygen-deficient ZnO film is improved by increasing oxygen pressure. At high oxygen pressure atmosphere, ZnO films bnough oxygen to absorb,so the stoichiometry of oxygen-deficient ZnO film is improved. Comparing XRDresults,it is concluded that the UV luminescence intensity strongly depended on the stoichiometry in the ZnO film rather than the micro-structural quality of the crystal.SEM resultes indicate that ambient oxygen pressure has an effect on the morphology of the samples.