The structure, optical and magnetic properties of arsenic implanted ZnO films prepared by molecular beam epitaxy
Introduction
ZnO based diluted magnetic semiconductors are considered to be promising materials for potential application in spintronic device [1]. Many authors report that the inclusion of magnetic ions in ZnO leads to ferromagnetism the disorder-active LO phonons above room temperature (RT) [2], [3], however the origin of the magnetism remains controversial. It has been found that defects play an important role in establishing the long-range ferromagnetism. RT ferromagnetism has been observed in non-TM or un-doped ZnO and other wide-band gap semiconductors [4], [5]. For un-doped ZnO, various intrinsic defects, such as O vacancy [6], Zn vacancy [7], Zn interstitial [8] and other crystal defects, such as grain boundaries [9], lattice distortion [10], have been reported to induce ferromagnetism.
Arsenic is a type five element that has been considered as a candidate for hole doping in ZnO. It has been confirmed that As prefers to substitute on the Zn rather than the O site due to the large difference in radius and electronegativity between O and As ions and it forms a (AsZn-2Znv) complex, which will introduce holes to the host [11]. Recently, it was predicted that local magnetic moments will also be introduced by the (AsZn-2Znv) complex [12]. In our previous work, As ions have been implanted into high quality O polar and Zn polar ZnO films and substantial temperature-independent ferromagnetism has been observed for both films, with the O-polar film having approximately twice the magnetization of the Zn-polar film [13]. In this paper we present results on the effect of different concentrations of As and also the effects of annealing on O polar ZnAsO films. Although there are many reports of the realization of p type ZnO by As doping, we have failed to find any other experimental studies of the magnetism of As doped ZnO.
Ion implantation is a nonequilibrium and reproducible approach to introduce doped elements and defects in crystalline materials. Accelerated ions will leave a trail of atoms displaced from their equilibrium lattice sites, thus creating vacancies, interstitials or antisites [14] before they finally come to rest. We use ion implantation to introduce As ions into ZnO films. The evolution of the structure, optical and magnetic properties of the doped films with doping concentration has been studied and RT ferromagnetism has been observed with As concentration higher than 6×1018 cm−3. The origin of the ferromagnetism is attributed to defects introduced by the ion implantation rather than the As related (AsZn-2VZn) complex.
Section snippets
Experimental details
A rf plasma-assisted molecular beam epitaxy (MBE) system was used to grow single crystal O polar ZnO films of 400 nm on sapphire substrates. Arsenic ions were introduced into ZnO by using ion implantation. An even distribution of As ions was obtained using an overlapping energy implantation procedure [13]. Three samples were prepared. Sample 1 has an As concentration of 6×1017 cm−3 while Sample 2 and Sample 3 of about 6×1018 cm−3and 6×1019 cm−3, respectively. The As-doped films should in principle
Results and discussion
Fig. 1(a) shows the XRD patterns obtained from the as-grown and As implanted ZnO samples in θ−2θ geometry. All the films exhibit a single phase with the typical wurtzite structure, showing a c-axis out-of-plane orientation with only ZnO (0002) and (0004) diffraction peaks and the diffraction peaks from the sapphire substrate. No secondary phases were detected within the detection limit. The enlarged ZnO (0002) peaks are shown in Fig. 1(b), which illustrates that the diffraction peaks shift
Conclusions
Arsenic ions with different concentrations have been implanted into high quality ZnO films prepared by rf-plasma assisted MBE on sapphire substrates. RT ferromagnetism has been observed for the films implanted with As concentration higher than 6×1018 cm−3. XRD, Raman, RBS/Channeling and optical absorption measurements confirm that the lattice disorder is increased by implantation. Ion implantation is a distinct method for producing defects from films grown in either an oxygen deficient or rich
Acknowledgments
This work is financially supported by the Fundamental Research Funds for the Central Universities (310421103), the Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics (KF201404) and the National Natural Science Foundation of China under Grant nos. 51171028 and 11305009. The measurements of optical absorption and MCD were taken on apparatus initially funded by the UK Engineering and Physical Sciences Research EP/D070406/1.
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