Control of Defect Structures in CuGaSe 2 Epitaxial Films
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ABSTRACT An in-situ annealing process, which is conducted in the growth chamber by exposing the film to a Se beam immediately after film deposition, is developed to control defect structures in CuGaSe 2 epitaxial films. TEM observations showed that the films annealed at a temperature above 450'C could completely remove anti-phase domain boundaries and effectively annihilate threading dislocations. The annealing process also changes the distributions of intrinsic point defects in the film. Photoluminescence (PL) may reflect the microstructure improvement in the films. A suppress of the donor-to-acceptor transitions caused by antisite defects and an enhancement in the PL intensity were found in a annealed film. INTRODUCTION CuGaSe 2 with 1.71 eV bandgap may have applications in solar cell and light emitting devices [1]. It is essential to develop the techniques for growing high-quality epitaxial films to realize device applications. In this work, thin films of CuGaSe 2 are grown on GaAs substrates by molecular beam epitaxy (MBE). Epitaxial growth of a film with chalcopyrite structure on a zincblendesubstrate has two structural problems need to work out, i.e. domain structure and interfacial structure [2]. We had developed an in-situ annealing process to prepare domain-free CuInSe 2 epitaxial films and a low-temperature MBE technique to modify interfacial structure [3,4]. Similar techniques can be applied to grow CuGaSe2 films. In this article, the emphasis is on the study of the annealing effects on the defect structures in CuGaSe 2 films. In-situ annealing process is conducted in the MBE chamber immediately after film deposition by exposing the film to the Se-beam flux for annealing. The reason that the annealing is carried out in the Se-containing atmosphere is to surpress thermal decomposition of the film [3]. Since the mean free path of molecular species is extremely long in a high vacuum, the exposure of the film to a Se-beam gives the direct impingement of Se molecules on the film surface. A quasi-equilibrium condition is thus established near the film surface. It has been known that the MBE-grown film is not at a equilibrium state [5]. The annealing process let the film to reach chemical equilibrium. This changes the distribution of intrinsic point defects in the films. Also, the annealing process may remove the crystalline defects, such as dislocations and anti-phase domain boundaries in the film, just as it did to the CuInSe2. The annealing condition especially the annealing temperature high enough to cause the atomic movements in CuGaSe 2 is studied and compared with that of CuInSe2 . In this work, transmission electron microscopy (TEM) is used for the observation of the microstructure changes in the films. A photoluminescence (PL) study is also carried out. Since intrinsic point defects as well as structural defects introduce energy levels in the bandgap, PL measurements may provide valuable information about the defect types and defect concentrations, which related to the peak positions and peak intensiti
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