Surface textured zinc oxide films
- PDF / 505,309 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 27 Downloads / 282 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Surface textured zinc oxide films P. M. Verghese and D. R. Clarke Materials Department, College of Engineering, University of California, Santa Barbara, California 93106-5050 (Received 4 June 1998; accepted 18 August 1998)
Both epitaxial and crystallographically fiber-textured ZnO films can exhibit a surface texturing (“cratered”) morphology when grown by pulsed laser deposition at temperatures in the range of 350–750 ±C in a background pressure of oxygen. The surface texturing is a consequence of the nucleation of oriented c axis grains that grow geometrically and impinge laterally. It is concluded that the surface texturing is due to nonequilibrium growth, being the result of a competition between the arriving flux, diffusive flux along the surface, and, possibly, concurrent ion etching from the laser-ablated plasma plume. At higher temperatures, no surface texturing occurs, presumably because of concurrent grain growth and more rapid surface smoothing by diffusion.
I. INTRODUCTION
In common with other wurtzite compounds, zinc oxide can exhibit a bewildering array of growth morphologies, ranging from flat, uniform films to needles to platelets to porous tubes, as examination of the crystal growth literature attests. In many instances, the morphology formed is sensitive to the presence of minor impurities and contaminants, and the role of several, such as water vapor, is ambiguous. One growth morphology, the surface textured or “cratered” morphology shown in Fig. 1, has potential application for enhanced light trapping in optical devices such as solar cells.1 While other compounds, such as AlN, can be grown to exhibit such a textured morphology, the advantage of ZnO is that it can also be made electrically conductive and hence serve as both an electrode and as a light-trapping layer. In this contribution we describe the growth conditions under which such textured films of ZnO can be formed by pulsed laser deposition and present a geometrical model to describe the evolution of the textured morphology that also pertains to the textured growth of other wurtzite crystal structures.
gas influx. Under these conditions the growth rate was of the order of 0.15 nmys. A range of ZnO target compositions were investigated, including nominally pure ZnO, ZnO 1 1 at.%Al, ZnO 1 1 at.%Co, and ZnO 1 0.1 at.%Al 1 1 at.%Co. A variety of substrates were also used, including (0001) sapphire, (001) silicon, fused silica, silica coated (0001) sapphire, and platinized (001) silicon. In each case, the substrates were ultrasonicated in acetone, followed by
II. EXPERIMENTAL
The films investigated in this work were all deposited by pulsed laser ablation onto a substrate clamped in a heated sample holder. The full details of the growth chamber and deposition process have been described previously.2 Briefly, the zinc oxide was ablated from sintered targets using a Q-switched, Nd-YAG laser operating at either 266 or 355 nm producing 2–5 ns pulses at a repetition rate of 10 Hz. Prior to depositio
Data Loading...
