Optimization of Laser Energy Fluence in Pulsed Laser Deposition of ZnO on Al 2 O 3 (0001)
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Optimization of Laser Energy Fluence in Pulsed Laser Deposition of ZnO on Al2O3(0001) W. Yang†, R. D. Vispute*, S. Choopun, R. P. Sharma, H. Shen‡, and T. Venkatesan† CSR, Department of Physics, University of Maryland, College Park, MD 20742 † also with the Electrical and Computer Engineering Department ‡ US Army Research Lab, Sensors and Electron Devices Directorate, Adelphi, MD 20783 * for contact: Phone: (301) 405-5992, FAX:(301) 405-3779, e-mail: [email protected] ABSTRACT The effects of laser energy fluence on the growth of pulsed laser deposited ZnO thin films on c-plane sapphire substrates were systematically investigated by using x-ray diffraction, Rutherford backscattering spectrometry with ion channeling, and scanning electron microscopy techniques. Optical and electrical properties of the ZnO epilayers were characterized by using ultraviolet-visible transmission spectroscopy and Van der Pauw measurements, respectively. It was found that the laser fluence has strong effects on the crystalline, optical and electrical qualities of the ZnO films. At low laser fluence, ZnO film grows via 3D-island mode with low deposition rate, loss of Zn near the surface and particulates on top of the film. High laser fluence may also cause simultaneous multi-layer growth and the degradation of crystalline, electrical, and optical quality of the ZnO films. The optimal laser fluence window was found between 1.2J/cm2 and 2.5 J/cm2 for obtaining high quality ZnO films for optoelectronic applications. The dependence of laser fluence on the ZnO growth mode, surface morphology and electrical and optical properties is discussed. INTRODUCTION Zinc oxide (ZnO) is a wide bandgap (Eg=3.3 eV at room temperature) semiconductor that is particularly attractive for potential optoelectronic (OE) applications. The dominant reason is attributed to its large exciton binding energy (60 meV at room temperature) and the high exciton population that is promising for low threshold current injection laser devices [1]. In the context of OE material, ZnO also possesses other figures of merit including tunable bandgap between 2.8 eVand 7.8 eV by alloying with MgO, high radiation hardness, low growth temperature (100 0 C~750 0C), and the availability of lattice matched single crystal substrates [2,3]. Practically, bulk ZnO must be tailored into the form of single-crystal thin films to enable OE device fabrication. Among the various approaches, pulsed laser deposition (PLD) is the most popular and successful technique that enables high quality ZnO thin films to be deposited on various substrates. Based on PLD deposited ZnO or MgxZn1-xO films, room temperature optically pumped ZnO lasers, quantum wells, ZnO/MgxZn1-xO superlattices, and high sensitivity UV photodetectors were demonstrated in recent years [1,4,5]. After the successful growth of high-Tc oxide superconducting films by Venkatesan et al in the 1980s, PLD has been widely recognized as a powerful tool for the congruent transformation of bulk oxides into thin films for micro- and optoelectronic
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