ANNEALING EFFECTS ON PHOTOLUMINESCENCE OF ZnO FILMS GROWN BY PLASMA ENHANCED ATOMIC LAYER EPITAXY

초록

ZnO is a very promising compound semiconductor which can be used for short wavelength optoelectronic devices including lightemitting diodes (LEDs) and laser diodes (LDs). ZnO has not only a direct energy gap of 3.37eV at room temperature but also a large excitonic binding energy of about 60meV implying that it has potentially higher efficiency of light emission than GaN. For the optoelectronic applications it is essential to grow high-quality ZnO thin films with high efficiency of blue or ultraviolet light emission.Many techniques have been tried to grow epitaxial ZnO films including pulsed-laser deposition (PLD), molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), reactive e-beam evaporator, high temperature sputtering. However, there are few reports on the epitaxial ZnO films grown by atomic layer epitaxy for all that it is a very powerful technique. Plasma enhanced atomic layer epitaxy (PEALE) offers high quality films over large area with excellent thickness uniformity, since it is a digital deposition technique.High quality ZnO thin films were deposited on the sapphire (0001) substrate using the PEALE technique. DEZn and H2O were used as a zinc source and an oxidant, respectively. Nitrogen was used as a purge gas. Two kinds of substrate temperature were used one of which is 170℃ in the temperature range of atomic layer deposition and the other of which is 400℃ in the temperature range of chemical vapour deposition. After deposition the ZnO thin films were annealed at 600, 800, and 1000℃ for 1 hr in oxygen atmosphere. The photoluminescent spectrometry was used to evaluate the luminescent characteristics of the films. The PL characteristics were significantly enhanced by annealing treatment. A strong peak was obtained in the green band for the ZnO thin film deposited at 400℃ due to the large amount of oxygen vacancies caused by excess Zn atoms. In contrast nearly no peak appeared for the one deposited at 170℃.