The role of flash lamp annealing for the recrystallization of ion implanted ZnO

A highly doped n-type ZnO thin layer is an attractive candidate to replace the much more expensive indium-tin-oxide layer in photovoltaics and low cost electronics. The optoelectronic properties of ZnO are determined by the type of doping and carrier concentration. The n-type conductivity of ZnO is easily achieved by substitution of Zn through the group III elements (Al, Ga, In), or by doping with halogen elements (F, Cl or I) substituting into the oxygen lattice site. However, the effective p-type doping of ZnO remains challenging. The most promising p-type dopants in ZnO are group V elements. In this paper, we have investigated the influence of millisecond range flash lamp annealing (FLA) on the recrystallization mechanism and optoelectronic properties of ion implanted ZnO thin films. The 120 nm thick ZnO films were grown on Si substrates by atomic layer deposition and implanted with P and Sb ions. After ion implantation FLA was used to anneal defects created during the ion implantation process and to activate finally the dopants. Samples were annealed for 3 or 20 ms using oxygen-poor (N2 or Ar) and pure oxygen atmosphere. The influence of the annealing conditions (atmosphere, annealing time and flash energy) on the optical and electrical properties of implanted ZnO was investigated using temperature dependent photoluminescence, Raman spectroscopy and Hall Effect measurements. The microstructural properties of fabricated ZnO films were studied using cross-section TEM and X-ray diffraction spectroscopy. It will be demonstrated that via millisecond range FLA treatment not only the implanted ions can be efficiently incorporated into the lattice of ZnO but also defect engineering is possible. By a proper selection of the implanted species and annealing atmosphere the main optical emission observed from doped ZnO can be easily changed from the UV to the red. This allows the fabrication of spectrally-clean blue, green and red emitters. According to Hall Effect and PL measurements the annealing atmosphere during FLA is crucial for the realization of p-type ZnO layers. The oxygen-poor atmosphere promotes the Zn-interstitial formation enhancing the n-type conductivity of ZnO. Annealing in oxygen suppresses the formation of n-type defects and stabilizes the p-type conductivity of ZnO films. This work has been partially supported by the EU 7th Framework Programme (EAgLE) (REGPOT-CT-2013-316014).