Pulsed Laser Deposition of ZnO: Energetic Rydberg State Atoms and Their Impact on Film Growth
- PDF / 1,295,009 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 73 Downloads / 195 Views
INTRODUCTION Zinc oxide is an important optoelectronic material: it is a transparent conductor and of use for diverse applications such as flat panel displays [1], and solar ccll technology [2]. In this study, pulsed laser deposition (PLD) was used to fabricate cand a-axis oriented thin films of ZnO using either a Zn metal or ZnO target under deposition conditions that allowed important insights to be gained about film growth. Many of the fundamental aspects of the PLD technique are not well understood [3,4,5]. The mechanisms driving particle ejection with kinetic energies sometimes exceeding 100 eV [6,7] and the complex spatial distributions [8,9] of the ejected material are just two examples of areas that require further study. Of present interest are the properties of the laser-produced vapor in PLD that enable oriented film growth to proceed at relatively low substrate temperatures. A prerequisite for oriented film growth is that adequate surface mobility occurs such that the condensing atoms find appropriate lattice sites [101. In PLD, the average kinetic energy of the ejected particles can be large enough to enhance surface diffusion. However, films are often prepared under background gas pressures high enough to thermalize the ablated material through gas phase collisions [11,12]. Alternatively, electronically excited state particles formed in the laser ablation process, such as Rydberg state atoms [13], have a large amount of internal energy, essentially equivalent to their ionization potential. If they survive transport to the substrate, they can deposit all of their internal energy at the surface. It is hypothesized that the condensation of these excited state species, as well as ions, provides additional energy, sufficient to augment surface mobility at lower temperatures. In this paper, we report on experiments that examine the electron-ion recombination process, the first step in the formation of long-lived Rydbergs. Then, by depositing films under various deposition pressures and temperatures, the relative importance of electronic excitation and kinetic (ballistic) impact on film growth could be examined.
EXPERIMENTAL ZnO films were grown in a PLD deposition system with a target of either pure Zn metal, or a pellet pressed from ZnO powder (both -99.995% purity). The ablation laser beam (248 nm KrF excimer laser, -30nsec FWHM) was focussed at 450 onto the target held inside a vacuum chamber. The laser fluence was kept constant at -1.25 157 Mat. Res. Soc. Symp. Proc. Vol. 397 01996 Materials Research Society
J/cm 2 (focal spot size: 0.059 cm 2 ). The target-substrate distance was 5.5 cm, and the background gas pressure (oxygen) was adjusted using mass flow controllers, and held constant at either 0.01 or 0.1 torr. The base pressure was 2x10- 7 torr. The substrates were either lxO.5 cm 2 -MgO or 2.54x2.54 cm 2 glass (SiO2), attached with silver paint to a substrate holder, and heated to either 50, 300, 400, or 500 0 C. These temperatures were chosen to cover a morphological transition known to occur in
Data Loading...
