Ferromagnetic GaMnAs for spintronic devices

A. Koeder, W. Schoch, S. Frank, R. Kling, M. Oettinger, V. Avrutin, W. Limmer, R. Sauer and A. Waag

Abt. Halbleiterphysik, Universität Ulm, Germany

Abstract. Ferromagnetic Ga_1–xMn_xAs films containing up to 5.1 at %Mn were grown by low-temperature MBE. The structural, electrical, and magnetic properties of the layers are reported. At x> 0.01, the materials show a ferromagnetic behavior. The Curie temperature reaches 80 K at 5.1 at % Mn. We propose the use of a n⁺-GaAs/p⁺-GaMnAs Esaki-diode (ferromagnetic Esaki-diode, FED) to provide injection of spin-polarized electrons via interband tunneling. Under reverse bias, spin-polarized electrons at the Fermi level in the valence band of GaMnAs tunnel to the conduction band of GaAs in contrast to the injection of spin-polarized holes used before.

Introduction

In order to implement the spin of electrons for information processing and storage, spin manipulating semiconductor devices have to be developed. These device structure should ideally be based on magnetic materials compatible with conventional silicon or GaAs technology. One approach is to use diluted magnetic semiconductors (DMS), which are lattice matched to these materials. Recently, it has been possible to create a spin-polarized current and to inject it into a GaAs LED using either BeMnZnSe [1] or GaMnAs [2]. This was a first step towards the realization of semiconductor devices based on spin control rather than charge control. Both BeMnZnSe and GaMnAs materials contain magnetic ions leading to a pronounced exchange interaction between the Mn 3d states and the conduction and valence bands of the host. BeMnZnSe is paramagnetic, with a giant Zeeman splitting of the relevant bands. This leads to a complete spin polarization (as long as the Fermi energy is smaller than the Zeeman splitting). A disadvantage of using II–VI based semimagnetic semiconductors like BeMnZnSe is the temperature sensitivity of the magnetic effects. For temperatures above 10 K, the large Zeeman splitting and hence the spin polarization decreases rapidly, so that room temperature operation is excluded. In this respect, GaMnAs is more robust. Due to the high intrinsic p-type doping (Mn is an acceptor in GaAs), the Mn spin system shows a ferromagnetic phase transition. Curie temperatures T_c depend on Mn concentration, and values of up to 110 K have been reported [3].

MBE-Growth and HRXRD

The Ga_1–xMn_xAs films were grown in a Riber 32 MBE system equipped with an As-cracking source operating in the As₄ mode. All substrates were fixed with In on the molybdenium block. First a 100 nm thick buffer layer was grown on undoped GaAs substrates at a growth temperature of T_s = 585 ^°C, then the growth was interrupted and Ts was lowered to 250 ^°C. The GaMnAs layers have been grown using a V/III BEP-ratio of 30 or 10. The ( $2{\times}4$ ) and ( $2{\times}2$ ) surface reconstructions were observed for HT- and LT-GaAs, respectively. On the onset of GaMnAs growth, the surface reconstruction changed to ( $1{\times}2$ ), the RHEED pattern became more streaky, and intensity oscillations were much more pronounced compared to LT-GaAs. All GaMnAs layers grown at 250 ^°C with Mn content up to x = 0.051 showed mirror-like surfaces. The increase in growth temperature and/or Mn flux led to the formation of MnAs clusters in the near-surface region. In this case, the wafers showed matt surface. The Mn content was determined by high resolution X-ray diffraction (HRXRD) measurements of symmetric (004) reflections using Vegard's law,

$a_{{\rm Ga}_{1-x}{\rm Mn}_{x}{\rm As}}=a_{\rm GaAs}+x \left( a_{\rm MnAs}-a_{\rm GaAs}\right)$ (1)

where a_GaAs = 0.566 nm and a_MnAs = 0.598 nm (hypothetical zincblende MnAs [3]).

All grown GaMnAs layers (up to 900 nm thickness) were fully strained as followed from measurement of asymmetric (115) reflections.

Transport properties and ferromagnetic transition in GaMnAs

The transport properties of GaMnAs layers were studied by resistivity and Hall measurements on photolithographically defined Hall bars in the temperature range (4.2–300) K and in magnetic fields up to 22 T. All samples studied showed p-type conduction. The Mn content in GaMnAs has a strong influence on the temperature dependence of the resistivity. As the Mn fraction rises to about x = 0.02, a insulator-metal transition is observed. All measured samples showed anomalous Hall effect and a negative magnetoresistance in a wide temperature range due to magnetization of the samples. The Hall resistance $\rho_{\rm H}$ in magnetic materials can be described as [3]:

$\rho_{\rm H} = R_0 B +R_s M$ (2)

where R₀ is the normal Hall coefficient, R_s is the anomalous Hall coefficient, and M is the magnetization of the sample. An estimate of the hole density in the range of magnetic fields 20–22 T, when the magnetization saturates and the variation of the magnetoresistance is within 1%, gives for metallic GaMnAs $p\sim(4{-}5){\times}10^{20} \rm cm^{-3}$ which is much smaller than the total Mn concentration.

The interaction responsible for the ferromagnetism in GaMnAs is assumed to be an indirect exchange interaction between the Mn spins mediated by holes, while in the direct exchange the Mn-Mn and the Mn-hole interactions are antiferromagnetic [3, 4]. The ferromagnetic transition in GaMnAs has a strong influence on the transport properties. Near T_c the resistivity $\rho (T)$ shows a maximum or a kink, the negative magnetoresistance is pronounced. Below T_c, the magnetoresistance is positive in the range of small magnetic fields. The Curie temperature T_c was determined from magnetotransport measurements using the method described in [5]. Annealing of the samples has a bearing on the Mn content in the Ga-sublattice, as a consequence on the carrier concentration and Curie temperature for T > T_s. The decrease of the Mn content should be taken into account even during growth.

A ferromagnetic Esaki diode (FED) as spin-injector

The first spin injection from GaMnAs into GaAs was demonstrated recenntly [2], but only a very small spin injection efficiency of 2% could be achieved. One reason is likely the fast spin dephasing of holes — in contrast to electrons [6] — caused by the pronounced spin orbit coupling of states in the valence band. In order to circumvent this problem, a concept using a ferromagnetic Esaki diode has been proposed by us [7]. The idea is based on the tunnelling of spin-polarized electrons from the valence band of the p-type magnetic semiconductor (here GaMnAs) into the conduction band of the n-type non-magnetic counterpart, assuming that this tunneling is spin-conserving. In an external magnetic field, the valence band of the GaMnAs splits into different Zeeman levels, resulting in a net polarization of the hole spin. The achievable spin polarization depends on the Fermi energy in the GaMnAs (given by the hole concentration) relative to the Zeeman splitting. The principle of the ferromagnetic Esaki concept is shown in (Fig. 1).

Fig 1. The band diagram of FED-LED structure under (a) zero- and (b) reverse bias

The concept will be analysed taking into account the different light hole and heavy hole Zeeman splitting. The resulting spin-polarisation of holes is shown in (Fig. 2) as a function of Fermi energy assuming a $N_0 \beta$ of 1.2 eV and an external magnetic field high enough for saturation of manganese moments.

Fig 2. Spinpolarization of holes in GaMnAs as a function of the Fermi energy. $\Delta E_{lh}$ , $\Delta E_{hh}$ are the Zeeman splittings for light and heavy holes

Conclusion

We studied the transport properties of GaMnAs layers grown by LT MBE. Relatively high Curie temperatures and compatibility with conventional GaAs technology make this material promising candidate for spintronic applications. The use of ferromagnetic Esaki diodes based on GaMnAs for the spin injection in device structures is proposed, and the achievable degree of spin polarization is discussed.

Acknowledgements

This work was supported by Deutsche Forschungsgemeinschaft (Project Wa 860/4-1). The authors thank Dr. D. Maude (CNRS, Grenoble) for the technical assistance in Hall measurements in high magnetic fields.

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URL: http://link.edu.ioffe.ru/nano2002/koeder
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