Designing of room temperature diluted ferromagnetic Fe doped diamond semiconductor

Abstract Semiconductor devices generally take advantage of the charge of electrons, whereas magnetic materials are used for recording information involving electron spin. To make use of both charge and spin of electrons in semiconductors, a high concentration of magnetic elements can be introduced in nonmagnetic III-V semiconductors to make magnetic semiconductor. In this work, Fe-Diamond was obtained with low solubility by modified microwave plasma chemical vapor deposition technique. Magnetic measurements revealed that the magnetic transition temperature from paramagnetic to ferromagnetic-like is above room temperature. The bandgap of Fe-Diamond is calculated to be 1.65 eV, which indicates that Fe-Diamond is a room temperature diluted ferromagnetic semiconductor.


Introduction
With the advancement of science and technology, people are increasingly demanding smaller and more powerful electronic devices [1]. At present, electronic devices are still centered on silicon-based chips in the semiconductor manufacturing industry [2][3][4][5][6]. However, thermal dissipation and technical difficulties such as tunneling have hindered the further development of conventional silicon-based transistors. Researchers continue to look for solutions, and one of the most promising methods is spintronics, because contrasting charge movement allows less energy to be used to control electron spin flipping [7][8][9][10]. But Schmidt et al. used a transition metal (such as Ni, Fe, Co) as a spin source to inject into the semiconductor channel [11]. Because of mismatching, the efficiency of spin injection is very low (less than 5%) and it is difficult to detect. Yet a dilute magnetic semiconductor obtained by magnetic element doping to obtain spin polarization has a high spin injection efficiency (above 90%). Therefore, the incorporation of transition metals into semiconductors has become the most widely studied object, such as magnetic element Mn doped GaAs, GaN and other dilute magnetic semiconductors and transition metal element doped ZnObased dilute magnetic semiconductors [12][13][14][15][16].
Diamond has excellent performances such as wide band gap, high breakdown field strength, high carrier mobility, low dielectric constant and good thermal conductivity. It also has stable chemical properties, high hardness, wear resistance and radiation resistance [17][18][19]. It is a promising high-temperature, high-power and wide-bandgap semiconductor material that is known as the fourth-generation semiconductor of choice [20]. Therefore, it is of great significance to explore the preparation process of Fe-doped diamond films. However, there are no literatures and patent reports of such diamonds at present, because such diamond growth conditions are complicated and very sensitive to growth parameters, so it is difficult to obtain by conventional methods.
In the current study, we tried to use C 15 H 21 FeO 6 as an Fe source to introduce it by volatilized anhydrous ethanol, and to explore the growth of such diamond by microwave plasma chemical vapor deposition. The phase, morphology, magnetic, optical and electronic properties of Fe-Diamond.

Experimental section
First, the silicon wafer (100) was ultrasonically cleaned using a diamond suspension for 60 min to increase the surface energy of the silicon wafer and facilitate diamond nucleation. After that, Fe-Diamond was deposited on the silicon wafer by microwave plasma chemical vapor deposition (MPCVD) using iron acetylacetonate as the iron source and ethanol as the carbon source. The power is 800 W, the pressure is 11 kPa, and the flow ratio of H 2 to Ar is 200:7. Finally, the sample with a growth time of 30 h was prepared under these growth conditions. X-ray diffraction (XRD) analysis was carried out using Shimadzu 6100. The magnetic properties were measured using a superconducting quantum interference device (SQUID). The field-cooled and zerofield-cooled (FC-ZFC) magnetization curves in a field of 1000 Oe were measured in the temperature range from 5 K up to 300 K. The optical absorption spectra were measured by using a UV-VISIR spectrometer (Lambda950, PerkinElmer). The PL spectra were measured by using a fluorescence spectrometer (FluoreMax, HORIBA). Raman spectra (Nanobase XperRam-200) were recorded on the spectrometer by using a laser excitation of 785 nm at room temperature. The surface morphology was measured by MFM mode in an atomic force microscope (AFM, Cypher ES, Asylum Research).

Results and discussion
As shown in Figure 1 (a), two major diffraction peaks at 44.009° and 75.364° were observed, originating from (111) and (220) lattice planes of diamond cubic structure (JCPDF06-0675). The (111) peak intensity is stronger than (220) peak, revealing a preferred (111) orientation in the as-grown diamond film. The full width at half maximum (FWHM) is 0.153, indicating the good crystallinity of the obtained diamond. The different intensity ratios of the XRD peaks can exhibit the various fracture strengths of the as-grown diamond samples. The ratio of I (111) /I (220) is calculated to be 2.368 for Fe doped diamond, revealing the high breaking strength [21]. Raman spectroscopy is measured to characterize the nature of our diamond samples, which is a useful tool to distinguish between sp 2 and sp 3 hybrid orbitals. In Figure 1 (b), there is a very sharp diamond characteristic peak at 1336 cm −1 with an extreme small FWHM of 5.3 cm −1 , indicating that Fe-doped diamond has good crystallinity and quality, which corresponds to the conclusion obtained from XRD pattern as shown in Figure 1 (a).
It can be seen from Figure 2 (a) inset that the film partially transmits light in the visible light region, and the light absorption rate in the ultraviolet region is good. The bandgap of Fe-Diamond is calculated to be 1.65 eV. The smaller value compared to the pure diamond is due to the fact that the Fe doping causes an increase in the graphite phase and defects. Figure 3 shows the magnetic properties of the Fe-Diamond. Under a magnetic field of 100 Oe, FC-ZFC curves show the splitting of the two curves starting from high temperature above our measurement limit 350 K, which shows that magnetic ordering appears in our sample at room temperature. Magnetization VS magnetic field (M-H) measurement at 300 K shows a clear hysteresis loop with a small coercive field and a saturation moment of 0.07 emu/g at 8 T. This evidence clearly shows that the nature of ferromagnetism of the obtained Fe doped diamond at room temperature.    Figure 4 (b). The contrast observed in the MFM images is caused by the interaction between MFM magnetic tip and the magnetic field from the samples. These interactions cause a shift in the phase of the oscillating probe, demonstrating an effect called dipolar contrast, with half of the phase contrast being dark, and half being light for each individual magnetic structure. This dipolar contrast for magnetic nanoparticles is typically found only when external magnetic fields are applied perpendicularly to the measurement direction as is the case for this study.

Conclusion
Fe-Diamond samples were synthesized by MPCVD technique. Magnetic measurements and MFM images revealed its room temperature ferromagnetic properties. The M-T results indicate that the magnetic transition temperature from paramagnetism to ferromagnetism is higher than room temperature. It is a promising room temperature diluted ferromagnetic semiconductor with the bandgap of 1.65 eV. This study paves a pathway to exploration of new spintronic materials and applications.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
The work was financially supported by the Basic and Applied Basic Research Foundation of Guangdong Province (2020B1515120019), the Shenzhen Science and Technology Innovation Committee (KQTD20170810160424889).