Date Published: January 30, 2017
Publisher: Public Library of Science
Author(s): Ting Su, Haifeng Zhang, Yogendra Kumar Mishra.
Charge trapping properties of electrons and holes in copper-doped zinc oxide (ZnO:Cu) films have been studied by scanning probe microscopy. We investigated the surface potential dependence on the voltage and duration applied to the copper-doped ZnO films by Kelvin probe force microscopy. It is found that the Fermi Level of the 8 at.% Cu-doped ZnO films shifted by 0.53 eV comparing to undoped ZnO films. This shift indicates significant change in the electronic structure and energy balance in Cu-doped ZnO films. The Fermi Level (work function) of zinc oxide films can be tuned by Cu doping, which are important for developing this functional material. In addition, Kelvin probe force microscopy measurements demonstrate that the nature of contact at Pt-coated tip/ZnO:Cu interface is changed from Schottky contact to Ohmic contact by increasing sufficient amount of Cu ions. The charge trapping property of the ZnO films enhance greatly by Cu doping (~10 at.%). The improved stable bipolar charge trapping properties indicate that copper-doped ZnO films are promising for nonvolatile memory applications.
Ferroelectric thin films have attracted much attention due to their potential application of nonvolatile random-access-memory devices . Among potential candidates, copper-doped ZnO is one of the most promising because its excellent versatility in electrical properties [2,3]. Mishra et al. [4,5] successfully reported direct growth of freestanding ZnO tetrapod networks for multifunctional applications. Tiwari et al.  reported the observation of room temperature ferromagnetism in Cu-doped ZnO films, which are very important findings. The electrical property of the ZnO film is dominated by doped atom incorporated during production and growth processes. An efficient way of improving the properties of ZnO films is the addition of certain dopants. Kumar et al. [7,8] reported tailoring of extrinsic dopants provide interesting enhancement in electronic transport properties of ZnO films. It is well known that copper atoms in zinc oxide are electron traps. The interesting features of ZnO:Cu have inspired us to explore its electrical properties for potential charge storage application. Understanding the charge transport mechanism in copper-doped ZnO film is important and intriguing for memory applications. Little has been reported on the charge trapping property of Copper-doped ZnO films, which governs essential physics. Therefore, further studies are still needed to discover optimum Cu doping concentration in ZnO films for better charge trapping property.
The ZnO films have been grown by using a variety of growth techniques, including pulsed laser deposition, [16,17] chemical vapor deposition and molecular beam epitaxy. Tiwari and Jin et al. [18,19] successfully produced high-quality zinc oxide thin films on silicon using pulsed laser deposition technique. In this study, the 240-nm-thick pure ZnO film (reference sample) and copper-doped ZnO films with varying Cu concentration ranging from 2 to 10 at.% (2, 8, 10 at.%) were grown at 600°C using pulsed laser deposition (PLD) on Si/SiO2/Ti/Pt and (001) quartz substrate. A KrF excimer laser operating at a wavelength of 248nm and an average energy density of 1.8 J/cm2 per pulse was used during the PLD.
Original surface potential of the undoped ZnO and copper-doped ZnO film were measured before charge injection. Fig 2 shows the topography images of undoped and copper-doped ZnO films. Fig 3(a) and 3(c) show surface potential images of the undoped ZnO and 8 at.% copper-doped ZnO film. The KPFM measurements show significant difference between the ZnO and ZnO:Cu film. The surface potential was 550mV and 18 mV for the ZnO and 8 at.% copper-doped ZnO film respectively. The difference in the Kelvin probe force microscopy data can be understood in terms of the doping-induced shift of the Fermi level. The position of the Fermi level can be changed with increasing doping level. The Fermi level , important in the physics of the semiconductor, also provides a good pictorial representation of the characteristics of the semiconductor material. Therefore, the position of the Fermi level reflects doping level of the copper-doped ZnO films.
In conclusion, Kelvin probe force microscopy is a powerful technique for measuring the nanometerscale surface potential and optimizing the design and performance of new devices based on semiconductor nanostructures. We have studied the charge trapping properties for both electron and hole in copper-doped zinc oxide films by KPFM. The KPFM measurements demonstrate that for ZnO:Cu (~10 at.%), copper doping results in important change in the Fermi level, which has potential to improve the charge trapping. In addition, increasing appropriate amount of Cu ions (~10 at.%) leads to the conversion from Schottky contact to Ohmic contact at Pt-coated tip/ZnO:Cu interface, which are proved by KPFM. Therefore, a greater amount of charge can be effectively trapped and stored in the films for a long time. The copper-doped ZnO films exhibit enhanced bipolar charge trapping properties and show great promise for nonvolatile memory applications.