High Quality Growth of Cobalt Doped GaN Nanowires with Enhanced Ferromagnetic and Optical Response

Group III–V semiconductors with direct band gaps have become crucial for optoelectronic and microelectronic applications. Exploring these materials for spintronic applications is an important direction for many research groups. In this study, pure and cobalt doped GaN nanowires were grown on the Si substrate by the chemical vapor deposition (CVD) method. Sophisticated characterization techniques such as X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), Transmission Electron Microscopy (TEM), High-Resolution Transmission Electron Microscopy (HRTEM) and photoluminescence (PL) were used to characterize the structure, morphology, composition and optical properties of the nanowires. The doped nanowires have diameters ranging from 60–200 nm and lengths were found to be in microns. By optimizing the synthesis process, pure, smooth, single crystalline and highly dense nanowires have been grown on the Si substrate which possess better magnetic and optical properties. No any secondary phases were observed even with 8% cobalt doping. The magnetic properties of cobalt doped GaN showed a ferromagnetic response at room temperature. The value of saturation magnetization is found to be increased with increasing doping concentration and magnetic saturation was found to be 792.4 µemu for 8% cobalt doping. It was also depicted that the Co atoms are substituted at Ga sites in the GaN lattice. Furthermore N vacancies are also observed in the Co-doped GaN nanowires which was confirmed by the PL graph exhibiting nitrogen vacancy defects and strain related peaks at 455 nm (blue emission). PL and magnetic properties show their potential applications in spintronics.


Introduction
The spintronics industry aims to develop devices that can manipulate the spin of the electron as an additional degree of freedom, thereby providing a novel kind of electronic device which is replacing the traditional electronic sensors, Light-emitting diode (LED) displays, memory devices etc. This industry is progressing by exploring different research directions, such as the incorporation of with precursors materials in such a way that its shining side was downward. The distance between precursors and the Si substrate was about 0.5 cm. Both the pretreating and preheating techniques were employed simultaneously before the synthesis of pure and Co-doped GaN nanowires. In the pretreating technique, the precursors were pretreated five times with aqueous NH 3 in a heating oven at 100 • C for 10 min each time. After each heating, a few drops of aqueous NH 3 were added and agitated with a glass rod for complete mixing. Subsequently, a preheating technique was used as mentioned elsewhere [47].
nanowires. In the pretreating technique, the precursors were pretreated five times with aqueous NH3 in a heating oven at 100 °C for 10 min each time. After each heating, a few drops of aqueous NH3 were added and agitated with a glass rod for complete mixing. Subsequently, a preheating technique was used as mentioned elsewhere [47].
The alumina semicircle tube loaded with pretreated/preheated precursors and Si substrates was shifted into the horizontal tube furnace (HTF). After that, the HTF was sealed from both ends and the environmental residual contents were evacuated by a mechanical evacuating pump. Moreover, intensive flushing with high purity (99.99% pure) NH3 gas was also performed. Subsequently, the flow rate of NH3 gas was adjusted to 50 sccm and the furnace was gradually heated at a ramp rate of 10 °C/min to obtain the required reaction temperature of 1200 °C, which was maintained for 120 minutes. After the completion of the reaction, the furnace was allowed to cool down naturally to room temperature in the presence of NH3 gas flow to avoid oxidation. The light yellow products gathered on the substrate were analyzed for further characterizations and physical properties studies. The structure of the synthesized samples was characterized by the X-ray diffraction technique (Philips X'Pert Pro MPD, Malvern Panalytical's Ltd, Malvern, UK) with a Cu-Kα radiation source (λ = 0.15418 nm), while the morphology was studied by field emission scanning electron microscopy (FESEM, S-4800, Hitachi, Tarrytown, NY, USA) equipped with an energy dispersive X-ray spectrometer (EDX), transmission electronic microscope and high-resolution transmission electronic microscope (FES TECNAI F20, FEI, Hillsborough, OR, USA). The optical properties (photoluminescence) at room temperature were studied using a PL spectrum from a LabRAM HR Evolution (HORIBA, Paris, France). The ferromagnetic response was investigated using magnetic hysteresis (MH) loops obtained through a vibrating sample magnetometer (Lake Shore 7400, Lake Shore Cryotronics Inc., Westville, OH, USA).

Structural Characterization
The crystal structure and phases of pure and Co-doped GaN nanowires (GNNWs) were analyzed using an X-ray diffraction (XRD) technique and the results are depicted in Figure 2. The crystal structures of the synthesized samples are presented for the 2θ range 10-80°. To identify the crystalline quality as a result of cobalt doping, the doped samples were also compared with pure GaN XRD peak positions and intensity variation. The alumina semicircle tube loaded with pretreated/preheated precursors and Si substrates was shifted into the horizontal tube furnace (HTF). After that, the HTF was sealed from both ends and the environmental residual contents were evacuated by a mechanical evacuating pump. Moreover, intensive flushing with high purity (99.99% pure) NH 3 gas was also performed. Subsequently, the flow rate of NH 3 gas was adjusted to 50 sccm and the furnace was gradually heated at a ramp rate of 10 • C/min to obtain the required reaction temperature of 1200 • C, which was maintained for 120 min. After the completion of the reaction, the furnace was allowed to cool down naturally to room temperature in the presence of NH 3 gas flow to avoid oxidation. The light yellow products gathered on the substrate were analyzed for further characterizations and physical properties studies.
The structure of the synthesized samples was characterized by the X-ray diffraction technique (Philips X'Pert Pro MPD, Malvern Panalytical's Ltd., Malvern, UK) with a Cu-Kα radiation source (λ = 0.15418 nm), while the morphology was studied by field emission scanning electron microscopy (FESEM, S-4800, Hitachi, Tarrytown, NY, USA) equipped with an energy dispersive X-ray spectrometer (EDX), transmission electronic microscope and high-resolution transmission electronic microscope (FES TECNAI F20, FEI, Hillsborough, OR, USA). The optical properties (photoluminescence) at room temperature were studied using a PL spectrum from a LabRAM HR Evolution (HORIBA, Paris, France). The ferromagnetic response was investigated using magnetic hysteresis (MH) loops obtained through a vibrating sample magnetometer (Lake Shore 7400, Lake Shore Cryotronics Inc., Westville, OH, USA).

Structural Characterization
The crystal structure and phases of pure and Co-doped GaN nanowires (GNNWs) were analyzed using an X-ray diffraction (XRD) technique and the results are depicted in Figure 2. The crystal structures of the synthesized samples are presented for the 2θ range 10-80 • . To identify the crystalline quality as a result of cobalt doping, the doped samples were also compared with pure GaN XRD peak positions and intensity variation. These XRD patterns of pure GaN and the gradual increase of 2%, 4%, 6% and 8% Co doped GNNWs were matched with JCPDS Card No.076-0703. Figure 2 shows sharp peaks with higher intensity to confirm the better crystallinity of the pure and doped GaN samples. It was observed that the crystal structure of GaN was hexagonal in nature and their lattice parameter values (a = 3.162 Å , c = 5.189 Å ) are in good agreement with the literature [55]. The XRD spectra showed pure and single phases of the nanowires and no impurity peak was found in the analysis. Miller indices of the hexagonal GNNWs are also marked in the graphs relevant to the peaks. It is noted that the doping of cobalt in GaN resulted in the shifting of peaks to the smaller angles which caused an increase in lattice parameters, as reported in the literature [24]. Furthermore, the doping with cobalt did not modify the diffraction profile of GaN. The good dispersion of the cobalt ions in the crystalline lattice of the GaN or the small fraction of dopant addition prevented the observation of signals referable to the cobalt incorporation in the crystalline phases. This experimental evidence suggests that, independent of the synthesis method, cobalt doping does not modify typical wurtzite structure of GaN [56,57].

SEM and EDS
Morphological and elemental analyses of pure and Co-doped GaN samples were carried out using Field Emission scanning Electron Microscope (FESEM), equipped with EDS, as depicted in Figure 3. Highly concentrated pure GaN nanowires are shown in Figure 3a while Figure 3b shows its magnified image. These GaN nanowires were found to be of a diameter in the range of 60-200 nm, whereas the lengths were in the microns. These XRD patterns of pure GaN and the gradual increase of 2%, 4%, 6% and 8% Co doped GNNWs were matched with JCPDS Card No.076-0703. Figure 2 shows sharp peaks with higher intensity to confirm the better crystallinity of the pure and doped GaN samples. It was observed that the crystal structure of GaN was hexagonal in nature and their lattice parameter values (a = 3.162 Å, c = 5.189 Å) are in good agreement with the literature [55]. The XRD spectra showed pure and single phases of the nanowires and no impurity peak was found in the analysis. Miller indices of the hexagonal GNNWs are also marked in the graphs relevant to the peaks. It is noted that the doping of cobalt in GaN resulted in the shifting of peaks to the smaller angles which caused an increase in lattice parameters, as reported in the literature [24]. Furthermore, the doping with cobalt did not modify the diffraction profile of GaN. The good dispersion of the cobalt ions in the crystalline lattice of the GaN or the small fraction of dopant addition prevented the observation of signals referable to the cobalt incorporation in the crystalline phases. This experimental evidence suggests that, independent of the synthesis method, cobalt doping does not modify typical wurtzite structure of GaN [56,57].

SEM and EDS
Morphological and elemental analyses of pure and Co-doped GaN samples were carried out using Field Emission scanning Electron Microscope (FESEM), equipped with EDS, as depicted in Figure 3. Highly concentrated pure GaN nanowires are shown in Figure 3a while Figure 3b shows its magnified image. These GaN nanowires were found to be of a diameter in the range of 60-200 nm, whereas the lengths were in the microns.
Their corresponding EDS analysis is shown in Figure 3e confirming the distribution of gallium nitride on the Si substrate. The percentage ratio of the Ga and N in the GaN are depicted in the inset of Figure 3e. The cobalt doped GaN nanowires are shown in Figure 3c, and the magnified image of 6% cobalt doped nanowires is shown in Figure 3d. The size of the doped nanowires was also found to be almost same as that of pure GaN nanowires. Figure 3d confirms that the Co-doped GaN nanowires were also smooth and there was no tube-like or belt-like structures in the doped samples. Their corresponding EDS analysis is shown in Figure 3e confirming the distribution of gallium nitride on the Si substrate. The percentage ratio of the Ga and N in the GaN are depicted in the inset of Figure 3e. The cobalt doped GaN nanowires are shown in Figure 3c, and the magnified image of 6% cobalt doped nanowires is shown in Figure 3d. The size of the doped nanowires was also found to be almost same as that of pure GaN nanowires.  The elemental analysis of the doped nanowires was also verified by the EDS mapping which is presented in Figure 4. A highly magnified portion of a single nanowire was chosen for this mapping, as shown in the base diagram of Figure 4 along with the mapping of individual constituents-Si, Ga, N and cobalt. This figure shows the uniform dispersion of the elements during the synthesis process and confirms the good quality of the grown samples. smooth, homogenous and free of domain boundaries. The SEM micrographs exhibit that most of the Co:GNNWs had a relatively smooth surface and indicated the high yield of NWs.
The elemental analysis of the doped nanowires was also verified by the EDS mapping which is presented in Figure 4. A highly magnified portion of a single nanowire was chosen for this mapping, as shown in the base diagram of Figure 4 along with the mapping of individual constituents-Si, Ga, N and cobalt. This figure shows the uniform dispersion of the elements during the synthesis process and confirms the good quality of the grown samples.  Figure 5 shows typical TEM and HRTEM images of pure and cobalt dope GNNWs. The TEM images of pure GaN nanowire is shown in Figure 5a which reveals that NWs were single crystalline with uniform thicknesses and smooth surfaces along their entire length. Figure 5b shows HRTEM of pure GaN nanowires and observations showed that the interplanar distance was 5.17 Å and this value was consistent with the XRD results.

TEM Analysis
The TEM image of the 6% doped cobalt is shown in Figure 5c. These doped nanowires were found to be free of any secondary phases along their entire lengths. These observations were consistent with previous findings for Co-doped GaN NWs, while Figure 5d represents HRTEM of doped nanowires. The HRTEM lattice image shows negligible defects in the lattice planes. The interval between the two fringes was 5.20 Å which was little bit larger than the plane distance of the pure GaN nanowires. The HRTEM images in parallel to SEM images further confirmed the smooth and controlled the synthesis of the Co-doped GaN nanowires. The high resolution TEM images, shown in Figure 5b,d, indicate that the lattice fringes were well-separated without forming any pairing structure.  Figure 5 shows typical TEM and HRTEM images of pure and cobalt dope GNNWs. The TEM images of pure GaN nanowire is shown in Figure 5a which reveals that NWs were single crystalline with uniform thicknesses and smooth surfaces along their entire length. Figure 5b shows HRTEM of pure GaN nanowires and observations showed that the interplanar distance was 5.17 Å and this value was consistent with the XRD results.

Magnetic Properties
The magnetic properties of Co-doped GaN nanowires with 2%, 4%, 6% and 8% were determined by vibrating sample magnetometer (VSM) at room temperature (RT). Figure 6 shows their magnetic hysteresis loops, indicating that the Co-doped GaN nanowires were ferromagnetic at room temperature. As a comparison, the hysteresis loop of pure GaN nanowires is also depicted which showed diamagnetic behavior at RT. In all magnetic data presented here, the contributions from the silicon substrate were removed. The ferromagnetism at 300 K was clearly shown by the remanence The TEM image of the 6% doped cobalt is shown in Figure 5c. These doped nanowires were found to be free of any secondary phases along their entire lengths. These observations were consistent with previous findings for Co-doped GaN NWs, while Figure 5d represents HRTEM of doped nanowires. The HRTEM lattice image shows negligible defects in the lattice planes. The interval between the two fringes was 5.20 Å which was little bit larger than the plane distance of the pure GaN nanowires. The HRTEM images in parallel to SEM images further confirmed the smooth and controlled the synthesis of the Co-doped GaN nanowires. The high resolution TEM images, shown in Figure 5b,d, indicate that the lattice fringes were well-separated without forming any pairing structure.

Magnetic Properties
The magnetic properties of Co-doped GaN nanowires with 2%, 4%, 6% and 8% were determined by vibrating sample magnetometer (VSM) at room temperature (RT). Figure 6 shows their magnetic hysteresis loops, indicating that the Co-doped GaN nanowires were ferromagnetic at room temperature. As a comparison, the hysteresis loop of pure GaN nanowires is also depicted which showed diamagnetic behavior at RT. In all magnetic data presented here, the contributions from the silicon substrate were removed. The ferromagnetism at 300 K was clearly shown by the remanence and coercivity suggesting that the Curie temperature (Tc) was at least 300 K. It is clear that the saturation magnetization increased with the increase in the concentration of cobalt doping. Their corresponding saturation magnetization, retentivity and coercivity are shown in Table 1. The observed magnetic behavior may be attributed to the exchange interaction between the localized magnetic dipole moments of the magnetic ions and the free delocalized charge of current carriers [32]. The comprehensive comparison of magnetic properties of different dopants incorporation in GaN nanowires is presented in Table 2. We saw that saturation magnetization was almost three times greater than those reported in [32] and the calculated values were sufficient to be exploited for spintronics applications [53,54]. The reported work was mainly focused on the enhanced morphology of the synthesized nanowires, which was far better than the already reported cobalt doped  The comprehensive comparison of magnetic properties of different dopants incorporation in GaN nanowires is presented in Table 2. We saw that saturation magnetization was almost three times greater than those reported in [32] and the calculated values were sufficient to be exploited for spintronics applications [53,54]. The reported work was mainly focused on the enhanced morphology of the synthesized nanowires, which was far better than the already reported cobalt doped nanowires.

Photoluminescence (Optical Properties)
Ultraviolet (UV) light used to excite the product is obtained from xenon lamp and its excitation wavelength has been set at 325nm. The room-temperature photoluminescence (PL) emission spectrum of The PL technique provided better information about the localized defects and impurity incorporation effects within the nanostructures. Therefore, to study the optical response of these nanowires, PL analysis of pure GaNNWs and Co-doped GaNNWs were studied at room temperature and the resulting spectrum is shown in Figure 7. The pure GaN, 6% and 8% Co-doped GaN NWs were excited with 325 nm He-Cd laser with a power of 0.0016 mW. Pure GaN NWs exhibited a single emission peak at 367 nm corresponding to a band edge transition of GaN agreeing with the literature [60]. The 6% and 8% Co-doped GaN NW excited with same laser caused band-edge (BE) emission at 368.85 nm (3.361 eV) for 6% and 370.26 nm (3.348) for 8% cobalt doping, causing a small red shift due to the inclusion of cobalt in GaN. This type of minor red shifting in band edge peak was mostly attributed to the strains occurring because of Co and other impurity (defects) in the GaN structures. Besides this, some weaker peaks were also present in the spectra, specifically at 455 nm (blue emission) for 6% and 8% cobalt doping. These extra peaks in GaN provided the confirmation of Co dopant-related energy levels in the Co-doped GaN samples. Besides this, some weaker peaks were also present in the spectra, specifically at 455 nm (blue emission) for 6% and 8% cobalt doping. These extra peaks in GaN provided the confirmation of Co dopant-related energy levels in the Co-doped GaN samples. Generally, the peak at 455 nm was ascribed to the nitrogen vacancies in GaN that may also arise due to strain and defects in the GaN structures [61]. Such kind of blue emission was also observed in the Fe doped GaN nanowires and is consistent with the literature [47]. Another important observation in the PL studies was that, with increasing the contents of cobalt in GNNWs, the defect related peak at 455nm was more prominent as compared to the less doped Cobalt in GaN. This also explains that, with the addition of Co, more defects could be created, and more blue emission could be attained. The photoluminescence response of different dopants in GaN nanowires is presented in Table 3. In this study, the Co-doped GaN nanowires showed nitrogen vacancy defects and strains at Generally, the peak at 455 nm was ascribed to the nitrogen vacancies in GaN that may also arise due to strain and defects in the GaN structures [61]. Such kind of blue emission was also observed in the Fe doped GaN nanowires and is consistent with the literature [47]. Another important observation in the PL studies was that, with increasing the contents of cobalt in GNNWs, the defect related peak at 455 nm was more prominent as compared to the less doped Cobalt in GaN. This also explains that, with the addition of Co, more defects could be created, and more blue emission could be attained. The photoluminescence response of different dopants in GaN nanowires is presented in Table 3. In this study, the Co-doped GaN nanowires showed nitrogen vacancy defects and strains at 455 nm (blue emission), which may be used for blue LEDs.

Conclusions
We have successfully synthesized Co-doped GaN nanowires by a chemical vapor deposition technique, using pretreated Ga 2 O 3 and Co(OH) 2 as precursors. The structural and morphological analyses show that pure and Co-doped GaN nanowires are single crystalline and have a size in the range of 60-200 nm diameter and without any secondary phases (Ga 2 O 3 or Co 2 O 3 ), as confirmed by XRD. The small shift in the XRD peaks proved the incorporation of Co into the GaN lattice which were in good agreement with the PL results. The EDS analysis displayed only Ga, N and Co peaks in the respective samples with no notable impurities. The room temperature magnetic properties exhibited that Co doped GNMSs are ferromagnetic in nature and an increasing trend in magnetism was observed with increasing Co contents. In PL, along with the near band edge emission at 367 nm, a relatively weaker peak in the blue emission region is ascribed to the nitrogen vacancies. A smaller shift in the peak position is attributed to the presence of Co, giving rise to the strain shifting of peaks. It is believed that Co-doped GNNWs may serve as a promising candidate for optoelectronics (blue LEDs) and spintronic devices. We assertively conclude that these nanowires are good candidates for the fabrication of next generation smart opto-electronic devices.