Elsevier

Journal of Crystal Growth

Volume 487, 1 April 2018, Pages 34-39
Journal of Crystal Growth

Control of conduction type in ferromagnetic (Zn,Sn,Mn)As2 thin films by changing Mn content and effect of annealing on thin films with n-type conduction

https://doi.org/10.1016/j.jcrysgro.2018.02.018Get rights and content

Highlights

  • MBE-grown ZnSnAs2:Mn thin films are room-temperature ferromagnetism.

  • Control of conduction type in ZnSnAs2:Mn thin films by changing Mn content.

  • The n-type ZnSnAs2 films are suitable for n-type spin-polarized injectors.

Abstract

The conduction type in (Zn,Sn,Mn)As2 thin films grown by molecular beam epitaxy (MBE) on InP substrates was found to be controllable from p-type to n-type as a function of Mn content. n-type (Zn,Sn,Mn)As2 thin films were obtained by Mn doping of more than approximately 11 cat.%. It is likely that Mn interstitials (MnI) incorporated by excess Mn doping are located at tetrahedral hollow spaces surrounded by Zn and Sn cation atoms and four As atoms, which are expected to act as donors in (Zn,Sn,Mn)As2, resulting in n-type conduction. The effect of annealing on the structural, electrical and magnetic properties of n-type (Zn,Sn,Mn)As2 thin films was investigated as functions of annealing temperature and time. It was revealed that even if the annealing temperature is considerably higher than the growth temperature of 320 °C, the magnetic properties of the thin films remain stable. This suggests that a MnI complex surrounded by Zn and Sn atoms is thermally stable during high-temperature annealing. The n-type (Zn,Sn,Mn)As2 thin films may be suitable for application as n-type spin-polarized injectors.

Introduction

One of the key issues for progress in semiconductor spintronics is the realization of ferromagnetic semiconductors at room temperature. As candidate spintronic materials, Mn-doped II-IV-V2 (II-IV-V2:Mn) ternary compounds have been studied as materials with potential room-temperature ferromagnetism suitable for semiconductor spintronics applications [1], [2], [3], [4], [5]. In recent years, ZnSnAs2:Mn thin films have been of particular interest because of their ferromagnetic behavior above room temperature and their promising compatibility with InP-based semiconductor heterostructures [6], [7], [8]. In the family of II-IV-V2 ternary compounds, ZnSnAs2, a pnictide chalcopyrite semiconductor in the bulk form with a band gap of 0.73 eV at 300 K, is a tetrahedrally bonded ternary compound with a c/a lattice parameter ratio of 2.0 that crystallizes in a form that is analogous to the zinc blende crystal structure [9]. A major advantage of this compound is that its lattice constant almost matches to that of InP (0 0 1) substrates [10], [11], allowing high-quality epitaxial thin films to be grown on InP substrates.

ZnSnAs2 thin films grown by molecular beam epitaxy (MBE) on InP substrates have usually shown p-type conductive character for a Mn content of less than 10 cat.%. Regarding the control of the conduction type in ZnSnAs2 for device applications, the growth of Ga-incorporated ZnSnAs2 thin films was carried out on InP and GaAs substrates, and n-type conductivity was achieved in heavily Ga-doped ZnSnAs2 thin films grown at Ga/Sn beam equivalent pressure ratios of 0.99 and 1.12 [12], [13]. On the other hand, it has been reported without systematic studies that ZnSnAs2 thin films doped with a relatively high Mn concentration of more than 10% exhibited n-type conductivity together with room-temperature ferromagnetism [14]. In several studies on the electrical properties of II-IV-V2:Mn compounds, n-type conductivity was observed in MnGeAs2 [15] and in CdGeAs2 doped with 6 mass% Mn [16]. Therefore, such magnetic semiconductors do not always show p-type conductivity and also occasionally show n-type conductivity. A possible explanation for this is that the n-type conductivity in MnGeAs2 may be due to As vacancies and antisite defects (GeMn), whereas the n-type conductivity in CdCeAs2:Mn was explained by the existence of interstitial Mn sites. On the basis of such previous reports, there has been renewed interest in the n-type conductivity of II-IV-V2:Mn systems, which could be effective as n-type spin injectors in semiconductor spintronic devices.

In this work, the magnetic and electrical properties of (Zn,Sn,Mn)As2 thin films were systematically investigated as a function of Mn content to reconsider the relationship between the conduction type and the Mn content in (Zn,Sn,Mn)As2 thin films. It was clearly found that the conductivity of (Zn,Sn,Mn)As2 films changes from p-type to n-type with increasing Mn content. Subsequently, the effect of annealing effect on n-type (Zn,Sn,Mn)As2 films was studied as functions of both annealing temperature and time. The results revealed that the magnetic and electrical properties of n-type (Zn,Sn,Mn)As2 thin films make them suitable for n-type spin-polarized injectors in ZnSnAs2/InP-based spintronics. We discuss these results in the context of existing theoretical and experimental data on (Zn,Sn,Mn)As2 and the related material GaMnAs.

Section snippets

Experimental

(Zn,Sn,Mn)As2 thin films were grown on semi-insulating epi-ready InP(0 0 1) substrates by conventional solid source MBE. Detailed growth procedures have been reported elsewhere [17]. Prior to the growth of (Zn,Sn,Mn)As2 thin films, the substrates were degassed at 300 °C for 30 min then exposed to As irradiation of 5.0 × 10−6 Torr at 350 °C. The substrate temperature was increased to 550 °C for thermal cleaning. After maintaining the substrates at 340 °C, they were first irradiated with Sn flux

Change in conduction type in (Zn,Sn,Mn)As2 thin films

Transport properties such as resistivity and carrier concentration were determined for the ferromagnetic samples by Hall measurements without considering the contribution of the so-called anomalous Hall effect because the contribution from this effect was not clearly observed at room temperature. Fig. 1 shows typical results obtained from Hall measurements at 300 K. Sample (a) doped with 10.1 cat.% Mn shows p-type conduction. In contrast samples (b) and (c), respectively doped with 11.2 and

Conclusions

We found that the type of conduction can be controlled type from p to n-type in (Zn,Sn,Mn)As2 thin films depending on the Mn concentration. n-type (Zn,Sn,Mn)As2 thin films were observed upon Mn doping of more than approximately 11 cat.%. It is probable that Mn interstitials (MnI) incorporated by excess Mn doping are located at tetrahedral hollow spaces surrounded by Zn and Sn cation atoms or As atoms, leading to the n-type conduction. The effect of annealing on the structural, electrical and

Acknowledgment

This work was partially supported by a JSPS Grant-in-Aid for Scientific Research on Innovative Areas “3D Active-Site Science” (Grant No. 266105001).

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