Epitaxial growth of M-type Ba-hexaferrite films on MgO (111)‖SiC (0001) with low ferromagnetic resonance linewidths

Zhaohui Chen, Aria Yang, Antone Gieler, V. G. Harris, C. Vittoria, P. R. Ohodnicki, K. Y. Goh, M. E. McHenry, Zhuhua Cai, Trevor L. Goodrich, Katherine S. Ziemer
2007 Applied Physics Letters  
Barium hexaferrite ͑BaM͒ films were deposited on 10 nm MgO ͑111͒ films on 6H silicon carbide ͑0001͒ substrates by pulsed laser deposition from a homogeneous BaFe 12 O 19 target. The MgO layer, deposited by molecular beam epitaxy, alleviated lattice mismatch and interdiffusion between film and substrate. X-ray diffraction showed strong crystallographic alignment while pole figures exhibited reflections consistent with epitaxial growth. After optimized annealing, these BaM films have a
more » ... ar magnetic anisotropy field of 16 900 Oe, a magnetization ͑as 4M s ͒ of 4.4 kG, and a ferromagnetic resonance peak-to-peak derivative linewidth at 53 GHz of 96 Oe, thus demonstrating sufficient properties for microwave device applications. It has been a longstanding goal of the microwave device community to integrate nonreciprocal microwave devices ͑e.g., circulators, isolators, phase shifters, etc.͒ with semiconductor device platforms in order to meet the demands of increasing microwave power and reduced device volume. 1,2 Previous attempts to deposit ferrite materials onto silicon and gallium arsenide failed in that the high temperatures required for the processing of low microwave loss ferrite materials resulted in either alloying with the silicon substrate or chemical disassociation of the GaAs substrate. In both cases, the films' microwave performances degraded. Today, there are new options for high power microwave semiconductor materials, which include the wide bandgap materials silicon carbide ͑SiC͒ and gallium nitride ͑GaN͒. These materials are ideal candidates for ferrite-semiconductor integration because of their compatibility with high power, high frequency device operation. 2 Furthermore, 6H-SiC has the same hexagonal crystal structure as barium ferrite ͑BaFe 12 O 19 , BaM͒ with a lattice mismatch of 4.38% along ͑0001͒ plane providing a suitable substrate for the ferrite film. Our previous attempts at depositing BaM on 6H-SiC by pulsed laser deposition ͑PLD͒ showed promising results with evidence of strong crystal texture in the films as well as magnetic anisotropy fields greater than 15 000 Oe. 3 However, the films from these previous studies were unable to obtain microwave losses, as measured by ferromagnetic resonance peak-to-peak derivative linewidths ͑⌬H͒, of less than 1000 Oe. In addition to some random orientation in the film seen by x-ray diffraction ͑XRD͒, x-ray photoelectron spectroscopy ͑XPS͒ revealed evidence of silicon diffusion to the surface. It is known that lattice mismatch strain impacts both structure and magnetic properties of a BaM film deposited by PLD. 4 MgO and Pt have been deposited by PLD and used as a buffer layer between Si and PLD-deposited BaM with some improvement in both structure and magnetic properties. 5,6 Although an MgO buffer is expected to reduce the lattice mismatch between the BaM and the SiC from 4.38% to 1.3%, the use of a PLD-grown MgO buffer did not significantly improve the magnetic quality of the films owing to high porosity and less than optimal crystallite orientation. The magnetic properties of the BaM did improve, however, with the introduction of an interwoven MgO / BaM multilayer at the film-substrate interface. 7 This layered MgO / BaM buffer reduced interface diffusion and, consequently, the ⌬H decreased to less than 500 Oe. 7 Postdeposition heat treatments were found to be ineffective in further reducing the ferromagnetic resonance ͑FMR͒ linewidth. The impact of different initial surfaces on BaM morphology and magnetic hysteresis curves was also observed by Capraro et al., 8 comparing PLD-deposited BaM on silicon to sapphire, and in recent work by Heindl et al., 9 comparing bilayers of barium strontium titanate and BaM on MgO and sapphire substrates. In this letter, we report on the improved magnetic and structural properties of BaM deposited by PLD on an MgO buffer layer grown by molecular beam epitaxy ͑MBE͒. This combined use of MBE to provide a high quality buffer layer followed by the growth of a thick film of BaM by PLD is shown to be a simple and effective method to allow integration of BaM with SiC for microwave device applications. The results show high crystal quality epitaxial films without silicon diffusion and with FMR linewidths of less than 100 Oe. The 6H-SiC ͑0001͒ single crystal wafers, acquired from Cree, Inc., were cleaned in a custom-built hydrogen furnace to produce an atomically smooth stepped surface free of oxygen and carbon contamination and polishing scratches. 10 A 10 nm crystalline MgO ͑111͒ film was grown by MBE at ϳ150°C on the silicon face of 6H-SiC by using a low-a͒
doi:10.1063/1.2794011 fatcat:cxllx62r45fzxnxvbnfai76wya