The Effect of Crystallographic Imperfections on the Photoluminescence of ZnO Thin Films
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The Effect of Crystallographic Imperfections on the Photoluminescence of ZnO Thin Films Matthew W. Kelly1, Tom N. Oder2, and C. Virgil Solomon3 1
Materials Science & Engineering Program, Youngstown State University, Youngstown, Ohio 44555, USA. 2 Physics & Astronomy, Youngstown State University, Youngstown, Ohio 44555, USA. 3 Mechanical & Industrial Engineering, Youngstown State University, Youngstown, Ohio 44555, USA. ABSTRACT ZnO thin films were synthesized by radio-frequency (RF) magnetron sputtering of high purity ZnO solid targets on sapphire substrates. Depositions were carried out at selected temperatures between 293 K and 1173 K, and post-deposition annealing was performed at 1173 K for 3 min. in an O2 atmosphere. Samples for electron microscopy investigations were prepared by lift-out technique in a multi-beam FIB/SEM instrument. The ZnO thin films show generally uniform thickness (about 1µm), determined by transmission electron microscopy (TEM) imaging. Irrespective of the deposition temperature, the ZnO thin films are polycrystalline, with individual grains exhibiting columnar morphology with the long axis oriented perpendicular to the ZnO/sapphire interface. The grain size varies with the deposition temperature, and a direct correlation between grain size and photoluminescence has been observed. Analyses performed using low-temperature photoluminescence spectroscopy measurements at 12 K revealed luminescence peaks at 3.361, 3.317, 3.218 and 3.115 eV. The intensity of the luminescence peak at 3.317 eV decreased with increasing deposition temperature. The films deposited at lower temperatures also exhibited a higher density of stacking faults as observed from the atomic resolution TEM. The crystallographic imperfections/photoluminescence relationship is not clear. The purpose of this study is to quantify the observed crystallographic imperfections and understand their effect on the photoluminescence of undoped ZnO thin films deposited on sapphire substrates. INTRODUCTION ZnO has become a popular area of interest within the semiconductor research community, due to the fact that it is a wide bandgap semiconductor exhibiting a direct energy bandgap of 3.37 eV at room temperature. In this sense, it is similar to GaN, which is widely used in electronic and optical devices. One major difference, however, is the exciton binding energy of ZnO compared with that of GaN. Whereas GaN has an exciton binding energy of 24 meV, ZnO has an exceptionally high exciton binding energy of 60 meV at room temperature, making it a favorable candidate for efficient room temperature devices such as LEDs and laser diodes [1]– [3]. In its pure form, ZnO exhibits an intrinsic n-type conductivity, and many research efforts are being put into uncovering ways to dope ZnO such that stable p-type ZnO may be realized [4]. Heteroepitaxial ZnO thin films have been synthesized by different techniques which include: pulsed laser deposition (PLD), laser molecular beam epitaxy (LMBE) [5], plasma assisted MBE, and metal-organic chemical vapor dep
