Coherent magnetic semiconductor nanodot arrays
© Wang et al; licensee Springer. 2011
Received: 24 November 2010
Accepted: 11 February 2011
Published: 11 February 2011
In searching appropriate candidates of magnetic semiconductors compatible with mainstream Si technology for future spintronic devices, extensive attention has been focused on Mn-doped Ge magnetic semiconductors. Up to now, lack of reliable methods to obtain high-quality MnGe nanostructures with a desired shape and a good controllability has been a barrier to make these materials practically applicable for spintronic devices. Here, we report, for the first time, an innovative growth approach to produce self-assembled and coherent magnetic MnGe nanodot arrays with an excellent reproducibility. Magnetotransport experiments reveal that the nanodot arrays possess giant magneto-resistance associated with geometrical effects. The discovery of the MnGe nanodot arrays paves the way towards next-generation high-density magnetic memories and spintronic devices with low-power dissipation.
Ferromagnet/semiconductor hybrid structures attract great attention as artificial materials for semiconductor spintronics since they have magnetic and spin-related functions and excellent compatibility with semiconductor device structures . By embedding magnetic nanocrystals into conventional semiconductors, a unique hybrid system can be developed, allowing not only utilizing the charge properties but also the spin of carriers, which immediately promises next-generation non-volatile magnetic memories and sensors [2, 3]. On the other hand, spin-injections into the semiconductor can be dramatically enhanced via coherent nanostructures, which considerably reduce undesired spin scatterings . Although magnetic hybrid systems, such as MnAs/GaAs, have been extensively studied over several decades, the control (over the spatial location, shape and geometrical configuration) of the magnetic nanostructures (for instance MnAs) still remains a major challenge to further improve the performance of the related magnetic tunnel junctions (MTJs) and spin valves . Here, we report a general and innovative growth approach to produce coherent and defect-free self-assembled magnetic nanodot arrays with an excellent reproducibility in the MnGe system, which reveals a geometry-enhanced giant and positive magnetoresistance (MR). The discovery of the controllable MnGe nanodots with excellent magnetotransport property paves the way towards future magnetoelectronic and spintronic devices with novel device functionalities and low power dissipation. Remarkably, this innovative method can be possibly extended to other similar systems, such as (Ga,Mn)As, (Ga,Mn)N , and (Zn,Cr)Te [2, 3].
Magnetic semiconductors, making use of both the charge and the spin of electrons, have been studied extensively in the past few years because of their promising applications in spintronic devices [1–11]. Examples of such devices include ferromagnetic heterojunction bipolar transistors, MTJs, magnetically tunable resonant tunneling diodes, magneto-optical modulators, and spin field effect transistors (Spin FETs) . However, the realization of these devices relies significantly on the ability to coherently integrate ferromagnetic materials with semiconductors and effectively control the shape or/and geometrical configuration of the integrated magnetic semiconductors, avoiding undesired spin scatterings, which is extremely crucial for the injection and detection of spin-polarized currents [1, 4, 12, 13]. In pursuit of coherent magnetic/semiconductor systems, previous efforts were predominately devoted to the developments of hybrid ferromagnet/semiconductors, in which epitaxial ferromagnet layers grown on lattice matched semiconductors are desirable to reduce detrimental spin scatterings [1, 4, 12, 13]. As a consequence, a hexagonal (H)-structured MnAs ferromagnet, epitaxially grown on or embedded into the zinc-blende (ZB)-structured GaAs, becomes a promising candidate for spin injection devices. Unfortunately, the difficulty to fabricate a coherent MnAs-based MTJ makes it a challenging task to probe spin injection  and also the dislocations or distorted lattices at the H-MnAs/ZB-GaAs interfaces would inevitably degrade the spin-polarization [13, 14]. To overcome these problems, a feasible solution is to find a coherent MnAs/GaAs system where the lattices of ZB-MnAs nanocrystals match with the ZB-GaAs matrix [15–17]. Indeed, the coherent ZB-MnAs/ZB-GaAs system can be technically achieved through spinodal decomposition in Mn-doped GaAs. Interestingly, the magnetic and magneto-optical properties of this coherent hybrid ZB-(Ga,Mn)As system are quite different from the H-MnAs/ZB-GaAs system and some exciting phenomena have been observed [16, 17]. For instance, the Curie temperature (T c) has been increased from 313 K (H phase) to 360 K (ZB phase)  and a striking memory effect was observed in the system . However, the coherent ZB-MnAs nanocrystals produced by the spinodal decomposition in ZB-(Ga,Mn)As are difficult to control their locations, shapes and geometrical configurations, which has been a major barrier to integrate these hybrid materials in order to make use of their full potentials in spintronic applications and to discover new collective properties from these unique systems [15–17].
Similar to the (Ga,Mn)As system, coherent dopant-rich nanocrystals induced by the spinodal decomposition also exist in most magnetic impurity-doped semiconductor systems, such as MnGe [18–24], (Ga,Mn)N , and (Zn,Cr)Te . The common disadvantage of these current available coherent magnetic nanocrystals, as mentioned above, is their random distribution, in terms of size and location, and low controllability. For instance, in the MnGe system, although Jamet et al.  recently employed the spinodal decomposition method to fabricate self-organized MnGe nanocolumns with high ferromagnetism, the growth window is narrow and difficult to reproduce. On the other hand, strain fields generated at strained interfaces of two materials with different lattice parameters have been successfully employed to grow quantum dots for several decades [9, 10, 25–28], underpinning a promising development of high-density three dimensional memories and spatial light modulators for advanced photonic applications . Here, we uniquely combine these two growth strengths (spinodal decomposition and strain field) and, for the first time, demonstrate a general and well-repeatable method to produce coherent and self-organized magnetic nanostructures with superior magnetoresistance in the MnGe system. More strikingly, this innovative method can be easily employed to other diluted magnetic semiconductor systems with spinodal decomposition , such as (Ga,Mn)As, (Ga,Mn)N, and (Zn,Cr)Te. Indeed, it is expected to be applicable in any systems where the spinodal decomposition exists.
A physical property measurement system and superconducting quantum interference device was used to measure the magnetotransport and magnetic properties, respectively. Both equipments were manufactured from Quantum Design.
Results and discussions
To determine structural characteristics of the MnGe nanodots at the atomic level, high-resolution TEM (HRTEM) was used and an example is shown in Figure 3b, where the HRTEM image was taken from the dashed rectangle area in Figure 3a. Interestingly, a careful examination of the HRTEM image shows that the MnGe nanodots have an identical single-crystalline structure to the Ge matrix (the diamond structure) with no observed lattice defects, consistent with other reports (with irregular shape of MnGe clusters) [18, 19]. As mentioned above, this type of MnGe nanodots is lattice coherent. This is substantially different from other Mn-rich precipitates such as hexagonal Mn5Ge3 and Mn11Ge8 which have a different phase, other than a diamond structure as Ge matrix. This is also verified by our selected area diffraction patterns (refer to Figure S2 in Additional file 1), where no extra diffraction spots or diffused ring(s) can be observed. To further determine the possible lattice distortion of the MnGe nanodots with respect to the Ge matrix, the inversed Fourier transform (Bragg filtering) technique  was used where two sets of nano atomic planes are shown in Figure 3c and 3d. As can be observed, the interfaces between the MnGe nanodots (the dark areas) and the Ge matrix are perfectly coherent without noticeable lattice distortion or bending of the atomic planes. In fact, using the (111) atomic spacings away the Ge matrix as a reference, the MnGe spacing of (111) atomic planes are determined to be identical to that of the Ge matrix. A quantitative EDS analysis suggests that the dots have a Mn concentration as high as 11% (Figure 2f), which can be further adjusted by altering the Mn flux during the growth. The high Mn doping is comparable to the reported Mn concentration of 15% in Ref. . Since the atomic radius of Mn (140 pm) is larger than that of Ge (125 pm) , it is expected that these Mn-rich dots experience a compressive stress caused by the surrounding Mn-poor Ge matrix. In fact, such a stored stress can be visualized from the strong contrast of the nanodots shown in Figure 3. Therefore, the successful vertical alignment of stacked nanodots can be attributed to the strain fields induced by the underlying Ge spacer layers, which is consistent with the growth mechanism of stacked quantum dot systems.
Since the nanodot array samples are ferromagnetic below 300 K (Figure S3 in Additional file 1), it is of great interest to study their magnetotransport properties. To do this, the samples were then fabricated into standard Hall bars with a typical channel width of 500 μm. For all measurements, the external magnetic field (H) was applied perpendicular to the sample surface. In order to completely avoid the substrate (Ge) conducting effect (Figure S4 in Additional file 1) , we have also successfully grown the same nanostructures on GaAs substrates under the same growth conditions as GaAs has the almost identical lattice parameter as Ge.
where the magnetic length λ equals to (cħ/eH)1/2 and C is a field and temperature independent constant. Note that the Equation 2 is only valid in a strong-field limit [37–39]. The inset in Figure 4b shows the best fitting results, in which a linear behavior of MR versus T -1/3 is obtained, further confirming the hopping conduction mechanisms (T ≤ 8 K). Note that the absolute values of MRs were used for the fitting purpose. These fitting results are reasonably close to the obtained hopping regions determined from the zero-magnetic-field resistivity measurements (T ≤ 10 K, Figure 4a).
Here, β = μH. At zero magnetic field, β vanishes. The conductivity tensor is diagonal when lacking of the magnetic field; and the current density can be simply described by j = σE. Since the electric field is normal to the surface of a metallic inclusion and j || σE , the current flowing through the material is concentrated into the metallic region which behaves like a "short circuit" (Figure S5 in Additional file 1) . As a result, the inclusion of metallic clusters can lead to a higher conduction than that of a homogeneous semiconductor [19, 40, 41]. However, at high magnetic fields (β>>1), the off-diagonal terms of dominate. Equivalently, the Hall angle between j and E approaches 90°(j⊥E); and the current becomes tangent to the nanodots. This further indicates that the current is deflected to flow around the nanodots, resembling an "open circuit" state (Figure S5b in Additional file 1) . The transition from the "short circuit" at the zero field to the "open circuit" at high fields produces an increase of resistance, i.e., a positive geometrically-enhanced MR . The above explanation has been successfully applied to several material systems, including Au/InSb  and MnAs/MnGaAs . Similarly, the geometrically-enhanced MR (ca. 200% at 10 Tesla, 300 K) was identified in MnGe2 nanostructures with a high Mn concentration of approximately 33% .
In conclusion, we have successfully developed a novel approach to fabricate extraordinarily coherent and self-organized MnGe nanodot arrays embedded in the Ge and GaAs matrixes by low-temperature MBE. A high yield of such aligned nanodot arrays was confirmed on different substrates, showing an ideal controllability and reproducibility. More importantly, giant positive magneto-resistances were obtained due to the geometrically-enhanced effect. We anticipate that our studies will advance the development of MnGe magnetic semiconductors and/or other similar systems. The obtained coherent and self-assembled nanostructures could be potentially used as the building blocks in the high-density magnetic memories, sensors and spintronic devices, enabling a new generation of low-dissipation magnetoelectronic devices.
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We acknowledge the Australia Research Council, the Focus Center Research Program-Center on Functional Engineered Nano Architectonics, Western Institution of Nanoelectronics and Intel (the Spin-Gain FET project) for their financial supports of this project. Y.W. thanks the Queensland International Fellowship. The authors thank G. Auchterlonie and H. Y. Xu at UQ for his assistance on this study. The authors also acknowledge Drs. A. Jacob and N. Dmitri of Intel Incorporation for their advices on the experiments.
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