UV modification of zinc oxide

Abstract

Surface nanoparticles of zinc metal and bulk zinc and oxygen vacancies are produced by irradiation of single-crystal zinc oxide with 193nm excimer-laser light.In its hexagonal, wurtzite form, crystalline zinc oxide (ZnO) is a semiconductor with a direct, wide bandgap of 3.3eV and a range of actual and potential applications. It is often used for laser and light-emitting diodes in the blue to UV and its healthy quantum efficiency makes it a strong candidate for energyefficient lighting applications. In addition, ZnO can be used as a transparent conducting oxide coating and has potential for use in solar cells, LCDs, and flat-panel displays. When doped with manganese and cobalt, ZnO has possible applications in spintronics, a new technology that exploits the spin of electrons. However, a number of potential applications require robust p-type doping (which creates an abundance of positive-chargecarrying 'holes' in the semiconductor). This has yet to be achieved. 1-3 As a result, there is intense interest in understanding ZnO. 1 In principle, zinc vacancies can serve as p-type acceptors. In many transparent, wide-bandgap materials such as ZnO, nanosecond UV-laser radiation creates excitations that decay to yield defects, such as chlorine vacancies in sodium chloride (table salt), easily seen by visible coloration of the crystal. Yet similar treatments with nanosecond pulses do not normally produce lattice defects in ZnO. Rather, any resulting defects are soon annihilated by the rapid diffusion of zinc and oxygen atoms back to their original lattice positions. This accounts for the very high resistance of ZnO to radiation damage. 2,4 However, using a bigger hammer-6.3eV photons from an argon fluoride excimer laser-we have generated gray to black spots on ZnO 5 (see Figure 1). If this were a new type of color center, it would have to be called the 'smudge' defect. We used a variety of spectroscopic methods for further investigation and found that irradiation produces metallic zinc nanoparticles on the surface as well as zinc and oxygen vacancies in the bulk.We found that UV-visible absorption spectra of the surface material are remarkably similar to that of metal zinc films. Both x-ray and Auger photoelectron spectra indicate the presence of metallic zinc and selected-area x-ray diffraction showed peaks due to crystalline zinc metal. We used time-resolved quadrupole mass spectroscopy to identify intense emissions of atomic and molecular oxygen, O and O 2 , during irradiation in vacuum. Weaker emissions of atomic zinc were observed, consistent with metallic zinc being left behind. Most intriguingly, this surface zinc takes the form of nanoparticles, typically 10-20nm in diameter (see Figure 2). Careful transmission-electron-microscopy studies indicate that this zinc is confined strictly to the irradiated surface and, therefore, that the underlying bulk is free of nanoparticles. Metallic nanoparticles display interesting electronic and optical properties and usual production methods are complex compared to this example....