Zinc aluminate spinel impurity phase in Al doped ZnO ceramic target and pulsed laser ablated films: Curse or blessing?

Abstract

Al doped ZnO films have been grown by pulsed laser deposition (PLD) technique using ceramic targets sintered at different temperatures in the range 800 o C to 1400 o C. The effect of target sintering temperature on the structural, optical and electrical properties of the AZO films has been investigated. The X-ray diffraction (XRD) patterns show that besides the major hexagonal wurtzite phase of ZnO, the zinc aluminate (ZnAl 2 O 4 ) spinel impurity phase is present predominantly in the targets sintered at 900 o C onwards. The XRD peak intensity of the spinel phase increases as the sintering temperature increases. Although, no such impurity has been primarily appeared in the XRD pattern of the films deposited from these targets, the presence of a probable spinel phase has been sensed by the X-ray photoelectron spectroscopy (XPS) measurements. All the films show more than 90% transparency in the region 500 nm to 1400 nm.The electrical resistance of the target pellets also decreases with an increase in the sintering temperature up to 1100 o C and then increases. Similarly, the carrier concentration increases and the resistivity decreases with the sintering temperature attaining the values of 6.36×10 20 cm -3 and 6.38×10 -4 ohm-cm respectively for the films ablated from the targets sintered at temperatures 1000 o C (AZO1000) and 1100 o C (AZO1100) beyond which both the values deteriorate. An increase in the carrier concentration value is assigned to the modification of the grain boundaries by the spinel phase. Vacuum annealing of AZO1000 film at 350 o C results in a further increase in the carrier concentration to a value of 1×10 21 cm -3 . This film has been applied as a transparent electrode without texturing for a-Si:H solar cell which shows a conversion efficiency of ~3.93%.Our study for the first time shows that the presence of a ZnAl 2 O 4 phase to a certain extent in AZO film rather helps to achieve higher carrier concentration.

An insight into doping mechanism in Sn–F co-doped transparent conducting ZnO films by correlating structural, electrical and optical properties

“…The targets of doped and undoped ZnO were prepared by conventional ceramic method as described somewhere else [36]. To prepare the Sn-F co-doped target, high purity commercial ZnO (MERCK, 98% purity), Sn (SIGMA-ALDRICH, 99.8% purity) and NH 4 F (MERCK, 99% purity) powders were mixed proportionately in an agate mortar for several hours and pressed them in the form of a pellet with a diameter of 1.5 cm under 5 ton pressure.…”

“…8 shows the deconvoluted O signal from 10TZO film which consists of two peaks centered at 530.6 eV and 531.5 eV. The component on the lower binding energy side of the O1S 1/2 spectrum is attributed to O -2 ions within hexagonal wurtzite structure surrounded by Zn 2+ (or the substituted Sn) [14,36]. The peak located at 531.5 eV at the higher binding energy side is normally attributed to the presence of loosely bound chemisorbed O on the surface of the films [14,36].…”

“…The component on the lower binding energy side of the O1S 1/2 spectrum is attributed to O -2 ions within hexagonal wurtzite structure surrounded by Zn 2+ (or the substituted Sn) [14,36]. The peak located at 531.5 eV at the higher binding energy side is normally attributed to the presence of loosely bound chemisorbed O on the surface of the films [14,36]. The Sn3d 5/2 peak has a clear shoulder at lower energy side and thus fitted to two peaks with centre at 486.7 eV and 485.3 eV as shown in Fig.…”

A simple process step for tuning the optical emission and ultraviolet photosensing properties of sol–gel ZnO film

“…ZnO, II-VI semiconductor material belonging to the wurtzite family, has a wide range of applications, from skin care, phosphors, and varistors to transparent electrodes in displays, piezoelectric devices and the biomedical eld. [1][2][3][4][5] This oxide material has received signicant attention due to its application in optoelectronic devices, such as UV detectors, LEDs and LDs, for its direct and wide band gap of 3.37 eV at room temperature. [6][7][8][9][10] Under above band gap photoexcitation, ZnO radiates light in the UV region at around 380 nm.…”

Comparative investigation on cation-cation (Al-Sn) and cation-anion (Al-F) co-doping in RF sputtered ZnO thin films: Mechanistic insight

“…Particularly in RF magnetron sputtering, a wide variation of Al dopant concentration has been attempted wherein the variation of resistivity from 2.2×10 -3 -1.4×10 -4 Ω-cm has been reported [17][18][19][20]. Some post-deposition treatment has been adopted to enhance the conductivity of the films [20][21][22] further. There are several process parameters like gas pressure, deposition temperature, RF power which are found to affect the resistivity significantly [23][24][25][26][27].…”