Growth of Al-doped ZnO nanostructures in low pressure background gas by pulsed laser deposition

Abstract

In this work, the deposition of nanostructures by room temperature pulsed laser deposition (PLD) in low pressure (2.6 Pa) O2, N2, He, and Ar at different substrate positions are studied. 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. Optical emission spectra and ion velocity are measured by using spectrometer and ion probe. The overall intensity of optical emission is in the order of Ar > O2/N2 > He while the ion velocity is in the reverse order. The inverse relation of optical emission and ion velocity suggest that collisions of the ablated specie possibly result in strong emission and impede the ion velocity at the same time. 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 further suggest that in N2, He or Ar, the ablated plasma plume is spatially and thermally confined; governed by the properties of the gas. 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 some large structure as compared to those obtained in He. In addition, in the presence of N2, He or Ar, Zn crystallites are also detected, which can act as catalyst for nanostructure formation.