Formation of Al-Doped ZnO Nanostructures in Low Pressure Background Gas by Pulsed Laser Deposition

Abstract

In this work, nanostructures are obtained by pulsed laser deposition (PLD) in low pressure (2.6 Pa) O₂, N₂, He, and Ar at specific substrate positions. The substrates are placed at the normal on-axis/center position and off-axis i.e. 4.5 cm above and below the center position. The ions velocity is measured by using ion probes at the corresponding substrate position for different gases. It is the highest for He>O₂/N₂>Ar while the reverse order is obtained for the optical emissions, indicating that collisions is the most intense in Ar. Amorphous films are obtained for the substrates placed at on-axis/center in all the background gases and at the bottom position for O₂. However, crystalline ZnO and Zn nanostructures are deposited at the bottom position in N₂, He, and Ar. Larger structures are deposited in Ar and the presence of nanorods are detected by TEM. The results suggest that in N₂, He or Ar, the ablated plasma plume is spatially and thermally confined; governed by the properties of the gas. The results suggest that nucleation of nanostructures occurred in the plasma plume even at low background pressure and deposited on the substrates that are placed at the bottom position. Between the two inert gases, Ar with higher atomic mass and lower thermal conductivity results in high collisions and thermal confinement that leads to larger structure as compared to He. In addition, in the absence of O₂, Zn crystallites are also detected, which can act as catalyst for nanostructure formation.

Keywords: ZnO Nanostructures, On/off - Axis Deposition, Pulsed Laser Deposition, Background Gases, Time Of Flight, Optical Emission Spectra