Influence of Ga doping ratio on the saturable absorption mechanism in Ga doped ZnO thin solid films processed by sol–gel spin coating technique

Abstract

In the present study, the nonlinear optical properties of sol–gel spin coated gallium doped zinc oxide (GZO) thin solid films are explored with nanosecond laser pulses using the z-scan technique. The higher doping ratios of Ga result in a large redshift of the energy gap (0.38 eV) due to the existence of enhanced grain boundary defects in GZO films. A positive nonlinear absorption coefficient is observed in undoped 1 at.wt.% GZO and 2 at.wt.% GZO films, and a negative nonlinear absorption coefficient in 3 at.wt.% GZO film. Fewer defects in undoped 1% GZO and 2% GZO films resulted in reverse saturable absorption (RSA), whereas a saturable absorption (SA) mechanism is observed in 3% GZO films and is attributed to the enhanced defect concentration in the band structure of GZO. However, all the films showed a self-defocusing mechanism, derived by a closed aperture z-scan technique. The present work sheds light on the defect mechanism involved in the observed nonlinear properties of GZO films.

Gallium-Modified Zinc Oxide Thin Films Prepared by Chemical Solution Deposition

“…The high intensity of the (002) diffraction peak indicates a preferential crystal orientation in the c-axis, perpendicular to the glass substrate. The caxis crystal orientation has been reported for GZO thin films on glass [9,17,20]. Figure 4 shows representative SEM images of the fracture cross-section and the surfaces of the GZO thin films annealed in oxygen, air, and Ar/H 2 .…”

“…The high intensity of the (002) diffraction peak indicates a preferential crystal orientation in the c-axis, perpendicular to the glass substrate. The c-axis crystal orientation has been reported for GZO thin films on glass [9,17,20].…”

“…However, the electrical conductivity decreases above a certain amount of Ga 3+ , which is attributed to a solubility limit of Ga 3+ in the ZnO lattice [11,12]. When the solid solubility limit of Ga in ZnO is exceeded, it may segregate at grain boundaries, may form spinel phases such as GaZn 2 O 4, and reduce the mobility of charge carriers [9,11,13,14]. The literature is not consistent regarding the solubility limit of Ga 3+ in ZnO.…”

“…The ionic radius of Zn 2+ is 0.060 nm, while the ionic radii of Ga 3+ , Al 3+, and In 3+ are 0.047 nm, 0.039, and 0.079 nm, respectively [6]. Ga 3+ , whose ionic radius is most similar to that of Zn 2+ , decreases crystallinity and leads to lattice deformations and larger lattice parameters for Ga-doped ZnO thin films (GZO) [1,[7][8][9]. Ga 3+ significantly affects the intrinsic defects of ZnO.…”