Pulsed-laser deposition of ZnO and related compound thin films for optoelectronics

Abstract

ZnO is a wide and direct band-gap material (3.37 eV at room temperature) making this compound very suitable for UV photodetector applications as well as for UV and blue light emitting devices. As an electronic conductor, ZnO may be used as transparent and conducting electrodes for flat panel displays and solar cells. ZnO doped with various atoms can also lead to new or enhanced functional properties. For example, doping with Al, Ga, In, Si or H allows decreasing its resistivity to below 10^-4 Ω.cm, while keeping the high optical transparency. Rare-earth doped ZnO thin films have been studied for optics and optoelectronics such as visible or infrared emitting devices, planar optical waveguide amplifiers. Ferromagnetic semiconductors can be obtained by doping ZnO with transition metal atoms (Mn, Co, Ni...) that could be used as spin injectors in spintronics. These new and exciting properties of pure and doped ZnO request the use of thin films or multilayer structures. ZnO thin film growth by pulsed-laser deposition (PLD) with or without any dopants or alloyed atoms has been intensively studied. In this paper, we will review the aspects of ZnO thin films grown by PLD, in order to prepare dense, stoichiometric and crystalline epitaxied ZnO layers or to form nanocrystalline films. Then, the optical and electrical properties will be discussed with a special emphasis on the growth conditions in relation to the physical properties for applications in p-n junctions, light emission devices, spintronics and bandgap tuning.