STUDY OF POINT DEFECTS IN GaN AND RELATED COMPOUND SEMICONDUCTORS BY MEANS OF POSITRON ANNIHILATION

正电子湮灭是研究材料中点缺陷的一种成熟的技术。采用单能正电子束气源分子束外延的方法，研究了在蓝宝石衬底上生长的铕和铽掺杂氮化镓薄膜的缺陷。在铕和铽掺杂样品中，我们观察到由两个或多个空位组成的空位团簇。这些缺陷是通过用稀土元素代替镓而引入的，并导致基体变形。我们研究了由Eu^<3+>的4f内跃迁引起的发光与GaN薄膜晶体质量之间的关系。在2-at掺杂的薄膜中。在% Eu中，氮化镓薄膜与蓝宝石衬底之间的界面附近空位的平均开放体积大于亚表面区域。缺陷开放体积的增大与Eu^<3+>配位对称性的降低和其4f电子跃迁速率的增加有关。研究了在ScAlMgO_4衬底上生长氧化锌（ZnO）薄膜。测量了激光分子束外延和单晶ZnO薄膜的湮灭辐射多普勒展宽光谱和光致发光光谱。虽然单晶ZnO中正电子的寿命接近于自由态湮灭的正电子寿命，但正电子的扩散长度比典型的无缺陷材料短。我们把这归因于正电子被天然缺陷散射。对于ZnO薄膜，我们观察到缺陷与束缚激子发射寿命τ_<Ex>之间存在相关性；正电子湮没检测到的缺陷主要是Zn空位或其他相关缺陷。在750 ~ 850℃等时退火过程中，薄膜中出现了空位型缺陷，但τ_<Ex>的值几乎没有变化。

Positron annihilation is an established technique to investigate point defects in materials. We probed defects in Eu-and Tb-doped GaN films grown on sapphire substrates by gas-source molecular beam epitaxy with a monoenergetic positron beam. In both Eu- and Tb-doped samples, we observed vacancy clusters consisting of two or more vacancies. These defects were introduced by replacing Ga with rare-earth elements, and resulting in distortion of the host matrix. We studied the correlation between luminescence originating from the intra-4f-transitions of Eu^<3+> and the crystal quality of GaN film. In film doped at 2-at.% Eu, the mean open volume of the vacancies near the interface between the GaN film and the sapphire substrate was found to be larger than that in the subsurface region. The increase in the open volume of the defects correlated with the lowering coordination symmetry of Eu^<3+> and the increase in the transition rate of its 4f-electrons.Zinc oxide (ZnO) thin films grown on ScAlMgO_4 substrates were also studied. We measured Doppler broadening spectra of annihilation radiation and photoluminescence spectra for the ZnO films deposited by laser molecular-beam epitaxy and single crystal ZnO. Although the lifetime of positrons in single crystal ZnO was close to the lifetime of positrons annihilated from the free state, the diffusion length of positrons was shorter than that for typical defect-free materials. We attribute this to the scattering of positrons by native defects. For the ZnO films, we observed a correlation between the defects and the lifetime of bound exciton emissions τ_<Ex> ; the main defect species detected by positron annihilation was Zn vacancies or other related defects. Isochronal annealing at 750-850℃ was found to introduce additional vacancy-type defects into the film, although the value of τ_<Ex> was scarcely changed by the annealing.