Magnetic , electrical and structural properties of annealed ferromagnetic ( Zn , Sn ) As 2 : Mn thin films on InP substrates : comparison with undoped ZnSnAs 2

We report the magnetic and electrical properties in Mn-doped and undoped (Zn,Sn)As2 epilayers, that were annealed at slightly higher temperatures than the growth temperature. (Zn,Sn)As2:Mn were epitaxially grown on InP (001) substrates at 300°C, and showed room-temperature ferromagnetism. The hole concentration, saturation magnetization and Curie temperature were measured and evaluated as a function of annealing temperature. The Curie temperature had a tendency to slightly increase at annealing temperatures up to 340°C, and completely disappeared at 400°C. The ferromagnetism could be attributed to hole-mediated ferromagnetism resulting from Mn ion substitutions at both the II-group Zn and IV-group Sn sites, especially from the large solubility of Mn substitution at Zn sites. The disappearance of ferromagnetism may be explained by several types of mechanisms : migration of mobile interstitial Mn atoms, diffusion of substitutional Mn ions to the surface, substitution of interstitial Mn atoms on Zn vacancies, and formation of MnAs clusters. It is noteworthy that the growth of magnetic semiconductor thin films from substrate lattice matching is essential for avoiding magnetic secondary phases such as MnAs clusters.


Introduction
III-V based diluted magnetic semiconductors (Ga,Mn)As have been extensively studied from both experimental and theoretical viewpoints in attempts to achieve a higher Curie temperature [1][2][3].In particular, post-growth annealing in temperatures only slightly exceeding a growth temperature of molecular beam epitaxy (MBE) has been performed as an effective means of enhancing the Curie temperature of (Ga,Mn)As by removing Mn interstitial atoms, since Mn interstitials incorporated during low-temperature MBE growth act as double donors that compensate for holes introduced by substitutional Mn Ga [4][5].
As alternative ferromagnetic semiconductors, II-IV-V 2 based semiconductors such as CdGeP 2 ,[6] and ZnSnAs 2 [7] have been shown to become ferromagnetic by Mn doping, and have Curie temperatures higher than 300 K. Magnetic properties of several kinds of chalcopyrites including ZnSnAs 2 :Mn were also theoretically investigated by Kent and Schulthess, who found that ZnSnAs 2 :Mn has a preference for anti-ferromagnetic interaction with Mn II -Mn II pairs and for ferromagnetic interaction with Mn II -Mn IV and Mn IV -Mn IV site pairs.The magnetic ion Mn, which occupies the cation IV site in host chalcopyrite or zinc-blende (sphalerite) structures, has a local spin and at the same time acts as an acceptor [8].Very recently, ZnSnAs 2 :Mn thin films have been epitaxially grown on InP (001) without any secondary phases, and showed room-temperature ferromagnetism [9,10].Until now the effect of thermal annealing on the transport and magnetic properties of (Ga,Mn)As has been widely studied to improve its magnetic properties and crystallinity and to enhance the Curie temperature up to room temperature.(Zn,Sn)As 2 :Mn, as well as (Ga,Mn)As, is expected to offer opportunities for systematic investigations of magnetic and electrical properties, especially the influence of thermal annealing on ferromagnetism, both experimentally and theoretically.(Zn,Sn)As 2 is probably regarded as a "vertical GaAs" to some extent, consisting of two interposing zinc-blende lattices, and permits a high degree of Mn incorporation because Mn 2+ ions easily substitute on the group II Zn sites.(Zn,Sn)As 2 thin films grown by MBE tend to show zinc-blende structure rather than chalcopyrite structure, which is analogous to the GaAs zinc-blende structure with Ga atoms being randomly replaced with either a Zn or an Sn atom.
In this paper, we investigated the effect of thermal annealing on the electrical and magnetic properties of (Zn,Sn)As 2 :Mn thin films together with undoped (Zn,Sn)As 2 thin films in order to understand the relationship between thermal treatments and their magnetic properties.We compared our results with the extensive experimental and theoretical results for the case of (Z mismatch of th as initially 0. eased, the (Z rds the higher n annealing a decreased t ly with increa f the full w ed for a 40 than those -340°C, w ity above 340°S n)As 2 :Mn fil mperature in F ant decreased d the line of t n)As 2 :Mn film

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