A study of the structural and magnetic properties of ZnO implanted by Gd ions

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Abstract

The structural and magnetic properties of ZnO (0 0 0 1) single crystals implanted with 200 keV Gd ions up to a fluence of 5 × 1015 cm−2 and subsequently annealed at 800 °C in various atmospheres were studied. The chemical composition and concentration depth profiles of ion-implanted layers were characterised by Rutherford Back-Scattering spectrometry (RBS) and compared to SRIM simulations. The as-implanted Gd depth profiles were found to be broader than those simulated by SRIM, but the projected range coincided well with that simulated. After annealing at 800 °C, the depth profiles became narrower. The structural changes in the layers modified by ion implantation and subsequent annealing were characterised by RBS channelling. The annealing led to partial recrystallisation and a decrease in the number of Gd atoms situated in substitutional positions. Raman spectroscopy showed that the point defects in Zn and O vacancies had been created by implantation and that these defects are most effectively cured after annealing in oxygen atmosphere. AFM analysis was used to determine the surface-morphology changes after the implantation and annealing procedures. The as-implanted samples exhibited ferromagnetism persisting up to room temperature. The annealing procedure led to paramagnetic behaviour, probably caused by the formation of gadolinium clusters.

Introduction

The current generation of semiconductor electronic and photonic devices is based on charge transport by electrons and holes. Semiconductor spintronics seeks to exploit the spin of charge carriers in a new generation of transistors and lasers, which are going to be employed in ultra-low power high-speed memories and photonic devices. The practical development of these novel spin-based devices depends on the availability of materials with the magnetic ordering temperature (TC) above room temperature [1]. Diluted magnetic semiconductors (DMS) are materials that simultaneously exhibit ferromagnetic and semiconducting properties [2], [3]. The ferromagnetism in DMS is driven by free charge carriers and can be controlled by an electrical field. DMS materials could fundamentally change the functionality of standard semiconductor devices. Zinc oxide (ZnO) doped with transition-metal (TM) atoms has been predicted to demonstrate ferromagnetic ordering at room temperature [4], [5]. The observation of room-temperature ferromagnetism in a system with poor conductivity or in a highly resistive material suggests that also other mechanisms might be involved, such as the inclusion of magnetic ions in metallic clusters [6]. Recently, a high magnetic moment has been observed in GaN [7] and ZnO doped with rare-earth (RE) elements [8].

The main aim of this paper is to investigate the structural changes of ZnO induced by Gd-ion implantation and the effect of these structural modifications on the magnetic properties of ZnO. Also the influence of subsequent annealing in O2 and Ar atmosphere is used to follow the influence of the structural reconstruction ZnO on the magnetic properties.

Section snippets

Experimental details

ZnO substrates of (0 0 0 1) orientation prepared by the hydrothermal method were obtained from MaTecK GmbH (Germany). The samples were implanted with 200 keV Gd+ ions up to a fluence of 5 × 1015 cm−2. The implantation angle was 7° to prevent channelling effects. All implantations were carried out at room temperature. Subsequent annealing was performed at 800 °C in a rapid thermal annealing (RTA) furnace. The annealing procedure was performed in argon (99.9999% purity) and oxygen (99.9999% purity) for 5

Results

RBS was used to determine the elemental composition of the implanted ZnO surface layer and to determine the depth profiles of the implanted Gd atoms. The RBS results have been summarised in Table 1, comprising the measured range Rp and the range straggling ΔRp. The measured profiles are compared with those simulated by the SRIM code employing the real binary-collision approximation. The measured range is in good agreement with that of the SRIM simulated profile of 200 keV Gd in ZnO see Fig. 1a.

Conclusions

The ZnO crystals with (0 0 0 1) orientation were implanted with 200 keV Gd ions to a fluence of 5 × 1015 cm−2. The implanted layers were subsequently annealed in argon and oxygen atmosphere. A strong influence of the annealing atmosphere on the structure of the annealed layer was observed. A significant reduction of the point defects was observed by Raman spectroscopy after the annealing in oxygen atmosphere and it is accompanied by Gd atoms mostly occurring in interstitial positions. RBS channelling

Acknowledgements

The research has been conducted at the CANAM (Center of Accelerators and Nuclear Analytical Methods LM2011019) infrastructure and has been supported by projects CSF 106/09/0125, 13-20507S, MSMT No. 20/2013 and UJEP 5322215000301. This work has been supported by the European Community as an Integrating Activity SPIRIT under EC contract no. 227012.

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