Enhanced Ferromagnetic Interaction in Modulation-Doped GaMnN Nanorods
© The Author(s). 2017
Received: 15 December 2016
Accepted: 7 April 2017
Published: 20 April 2017
In this report, ferromagnetic interactions in modulation-doped GaMnN nanorods grown on Si (111) substrate by plasma-assisted molecular beam epitaxy are investigated with the prospect of achieving a room temperature ferromagnetic semiconductor. Our results indicate the thickness of GaN layer in each GaN/MnN pair, as well as Mn-doping levels, are essential for suppressing secondary phases as well as enhancing the magnetic moment. For these optimized samples, structural analysis by high-resolution X-ray diffractometry and Raman spectroscopy verifies single-crystalline modulation-doped GaMnN nanorods with Ga sites substituted by Mn atoms. Energy dispersive X-ray spectrometry shows that the average Mn concentration can be raised from 0.4 to 1.8% by increasing Mn fluxes without formation of secondary phases resulted in a notable enhancement of the saturation magnetization as well as coercive force in these nanorods.
KeywordsA1. Characterization A1. Doping A3. Molecular beam epitaxy B1. Nanomaterials B1. Nitrides B2. Magnetic materials
Combining spin and charge functionalities is expected to bring a renaissance in compound semiconductors. Diluted magnetic semiconductors (DMSs) are one group of compounds where such possibilities are feasible . Among potential candidates, there are group III nitrides that are widely used in commercial optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes (LDs). By enabling spin functionality, one could envision the creation of new devices or enhanced functionalities; hence, obtaining above-room-temperature ferromagnetism in nitride semiconductors is an important topic of scientific research . Besides this, III nitride can be grown into nanostructures as nanocolumn LEDs and nanowire photodetector and quantum dot lasers as potential next-generation devices [3–5]. Thus, studying the possibility of incorporating spin functionality in such nanostructures has been a major goal in the scientific community [6–12]. Typically, growth of one-dimensional nanostructures such as nanowires is readily achieved by chemical vapor deposition (CVD) [11, 13, 14], but a proper ordering of nanostructures, necessary for device fabrication, is lacking unless some extra fabrication steps are used . An alternative to CVD growth is the self-assembly methodology of nanorods using plasma-assisted molecular beam epitaxy (PAMBE, Veeco) wherein no additional fabrication steps are required [16–18]. Our previous results show that it is indeed possible to incorporate Mn into GaN for creating nanostructures that exhibit room temperature ferromagnetism by incorporating a modulation-doping approach wherein a thin Mn layer is sandwiched between a thicker GaN spacer .
One major bottleneck for superior devices is the need for stronger magnetic signals such as saturation magnetization, remnant magnetization, and coercive force. Since the ferromagnetic properties of semiconductors depend on the amount of magnetic dopant, it is challenging to prepare thin films without the formation of any secondary phases such as ferromagnetic nanoclusters that might result in a spurious signal in magnetic measurements. Furthermore, the itinerary carriers in the host semiconductor have to couple with the localized magnetic moment and enhance the carrier-mediated ferromagnetic properties based on double exchange mechanism. The ferromagnetism of GaN-based DMSs has been reported to relate closely with the type and concentration of activated carriers [20, 21]. Considering all above, modulation-doping technique is adopted to obtain a high dopant concentration and carrier concentration in one or few monolayers . The magnetic properties of GaN-based DMSs have been reported to be enhanced in GaMnN/GaN multilayers and GaGdN/GaN superlattice structures as compared to a single layer with normal doping method [20, 23]. In this report, we have fabricated self-aligned phase pure vertical nanorods which exhibit superior ferromagnetic properties beyond room temperature. Our investigations reveal that the amount of Mn dopant and GaN spacer thickness are crucial for the enhancement of the ferromagnetic properties as well as avoiding secondary phase formation.
GaN nanorods were grown by plasma-assisted molecular beam epitaxy (Veeco 930) on Si (111) substrate without a buffer layer. The Si wafer was chemically cleaned before loading into the chamber and thermally cleaned after loading into the chamber as usual . The growth sequence was to grow undoped GaN nanorod section first as a template, and then, modulation-doped GaMnN was grown on top of the undoped nanorod section to complete the nanorod growth. It retained the shape and vertical directions of the bottom undoped nanorod template. The modulation-doping technique utilized the metal modulation method, that is, the Mn-flux and Ga-flux modulations were achieved by controlling the open and close of the shutters of Mn and Ga while keeping the nitrogen flow uninterrupted. The metallic sources (Mn, Ga) were provided through Knudsen effusion cells and controlled by fast-action pneumatically driven shutters in front of the cells. The active nitrogen species were supplied by the radio-frequency UNI-Bulb plasma source at a fixed power of 450 W.
Sample growth parameters and Mn concentration
Growth time per pair of GaN/MnN (s)
Mn flux (10−8 Torr)
EDS percentage of Mn (atomic%)
The morphology and structure of nanorods were inspected by field emission scanning electron microscopy (FESEM, JEOL JSM-7000F) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20). The concentration of Mn in GaMnN nanorods was examined by the energy dispersive X-ray spectrometry (EDS, Oxford INCA Penta FETX3) installed in the FESEM system. The phase purity of Mn modulation-doped GaN nanorods and the local structure of Mn atoms in GaN host lattice were studied by high-resolution X-ray diffractometry (HRXRD, Bede D1) and Raman spectroscopy (Jobin Yvon T64000), respectively. Furthermore, magnetic properties of GaMnN nanorods at above room temperature were carried out by using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-XL7). X-ray absorption near edge spectra (XANES) measurements of N K-edge, Ga L-edge, and Mn L-edge was performed at BL08B1 and BL20A1 beamlines of the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. All N K-edge, Ga L-edge, and Mn L-edge spectra were recorded at the incident beam with E-field polarization parallel to the surface of the sample at room temperature by total electron yield (TEY) mode. XANES of Mn L-edge from a MnO single crystal was taken simultaneously for the energy calibration and the comparison of different electronic valence states.
Results and Discussion
The MBE growth and characteristics of modulation-doped GaMnN nanorods have been systematically investigated. The thickness of GaN in each pair of GaN/MnN was varied by the growth time of GaN. A thick GaN spacer with the 40-s growth time between Mn modulation-doped layers reduces the possibility of excess Mn atom aggregation and suppresses the formation of secondary phases effectively. Modulation-doped GaMnN nanorods with varied Mn fluxes were also investigated to achieve single-crystalline GaMnN nanorods with higher Mn content. Samples grown under Mn fluxes of 1.7 × 10−8 and 1.9 × 10−8 Torr have a Mn concentration of 0.7 and 1.8%, respectively, measured by EDS, and no secondary phases were observed by XRD. Raman scattering indicates the substitution of Ga by Mn atoms. Zero-field-cooled and field-cooled measurements reveal no clusters or secondary phases. Hysteresis loops in magnetization versus magnetic field measurements of GaMnN nanorods show above-room-temperature ferromagnetism, and the saturation magnetizations are enhanced with increasing Mn content which shows a positive correlation. This opens a possible route to applications in nanostructural spintronics.
Chemical vapor deposition
Diluted magnetic semiconductors
Energy dispersive X-ray spectrometry
Field emission scanning electron microscopy
High-resolution transmission electron microscopy
High-resolution X-ray diffractometry
- M vs. H:
Magnetization versus magnetic field
National Synchrotron Radiation Research Center
Plasma-assisted molecular beam epitaxy
Superconducting quantum interference device
Total electron yield
X-ray absorption near edge spectra
Zero-field-cooled and field-cooled
We would like to acknowledge and thank Dr. H. D. Yang for providing access to the SQUID facilities for magnetometry studies.
This work is supported by the Ministry of Science and Technology (MOST) of Taiwan, Republic of China, under grant numbers 99-2119-M-110-005-MY3, 102-2112-M-110-004-MY3, and 105-2112-M-110-005. We would also like to acknowledge the additional funding support from Ministry of Education (MOE) of Taiwan for the 5Y50B program.
YTL, YJZ, and HSK grew the samples and did the XRD, Raman, and SEM measurements. HC and NJ helped with the TEM measurements. CMC did the XANES measurements. YTL and PW wrote and analyzed the data and wrote the manuscript. LWT revised the manuscript. LWT and QYC supervised the work and offered suggestions. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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