AlN Interlayers Allow Robust Hexagonal Barium Ferrite Heteroepitaxy on 6H-SiC

Virtually all electronic and microwave components have been constantly shrinking, including the ones used in Transmit/Receive Modules. The microwave circulator is an essential part of every T/R module and has largely remained unchanged since the inception. The concept of T/R module crystalized in the 1970s and became practical in the 1980s with the emergence of GaAs monolithic integrated circuits. Circulators are not solid state components and have ferrites at their cores. The first attempts to integrate the ferrites and semiconductor platforms have been made in the 1990s, but mostly failed - GaAs substrates could not withstand high temperatures required for spinel ferrite processing. Further work focused on wide band gap semiconductors that offered better temperature stability, such as SiC or GaN. These semiconductors were used as substrates to grow crystallographycally textured hexagonal barium ferrite films (BaM) by means of pulsed laser deposition due to their lattice parameters. Several buffer layers have been explored, including MgO and Pt. A number of promising results on SiC substrates have been published in the 2000s [1–2]. Eventually, it turned out that even SiC coated with magnesium oxide (MgO) is not stable under barium hexaferrite films at elevated temperatures [3]. Z. Chen et al. concluded that this phenomenon is due to the chemical affinity of Si and Fe. In fact, the eutectic transition in the Fe–Si phase diagram occurs at about 875°C, and barium hexaferrite films are usually grown at temperatures exceeding 900°C. To address this problem, we investigated the behavior of buffer layers commonly used between SiC and BaM at elevated temperatures, and also explored novel options. The primary function of a nucleation layer is to improve the crystalline quality of the overlying hexaferrite. This is achieved by providing a surface that improves the wetting and adhesion of the PLD seed layers, adjusting the lattice constant to a closer match to the hexaferrite, mitigating interfacial strain and controlling interfacial chemical reactions between the hexaferrite seed layers and SiC. In addition to providing a seed for hexaferrite deposition, nucleation layers are needed to protect the SiC substrate during the high temperature and potentially corrosive hexaferrite growth process. As such the nucleation/protection layers must be chemically stable and robust. To this end, traditional magnesium oxide and platinum, as well as the novel (with respect to BaM) aluminum nitride interlayers were studied. Our approach to achieving the nucleation and growth of protective MgO or Pt thin films is based on Atomic Layer Deposition (ALD). The advantages of this process include the ability to conformally coat structured substrates free from pinholes and other surface defects. Novel AlN layers were deposited by means of Metal-Organic Chemical Vapor Deposition (MOCVD). To study temperature stability of these nucleation layers, coated 6H-SiC wafers were annealed at temperatures between 600 and 900°C, in air, oxygen, and nitrogen atmospheres. Further analysis included scanning electron microscopy (SEM), atomic force microscopy (AFM), X-Ray diffractometry (XRD), and surface resistivity measurements. As deposited nucleation layers had the thickness of about 30nm, surface roughness Ra of about 0.4nm, and high degrees of crystalline orientation. Pt nucleation layers mostly preserved their properties after annealing at 600°C, but experienced boil off at temperatures approaching 900°C. Although MgO interlayers on SiC substrates demonstrated much better thermal stability, BaM films grown on MgO faced a number of problems, including low density, delamination, and cracking. AlN interlayers are widely used in semiconductor manufacturing to reduce the tensile stress and cracks, but have never been used as nucleation layers for magnetic films. Our annealing study demonstrated that AlN on SiC retains its properties after annealing at elevated temperatures even in oxygen atmosphere. However, we were not able to grow high quality hexaferrite films directly on AlN due to poor epitaxy, poor adhesion, and cracking. Therefore, an additional layer of MgO has been introduced on top of AlN by means of ALD. Such heterostructures (MgO/AlN/SiC) exhibited excellent temperature stability, the surface roughness remained unchanged after the annealing at 900°C and equaled 2.4nm (Figure 1). We were able to reliably produce high quality, perpendicularly textured BaM films on these structures under varying pulsed laser deposition conditions. Optimal PLD parameters can be found elsewhere [1–3]. As deposited 1.5um thick BaM films on MgO/AlN/SiC heterostructures have been rigorously tested and demonstrated excellent functional properties. Vibrating sample magnetometery (VSM), XRD, and pole figures confirmed a high degree of crystalline orientation ((00n) texture). The surface of these films, as well as ion milled holes, were analyzed by SEM and AFM, and appeared to have a near theoretical density. This conclusion was supported by the VSM analysis that revealed a near theoretical saturation magnetization and a low magnetic coercively of about 200 Oe. The FMR linewidths were measured by a coplanar waveguide connected to a Vector Network Analyzer (VNA) and equaled 500 to 600 Oe [4–5]. As demonstrated in prior art, microwave magnetic properties of as-deposited BaM films can be further improved by subsequent annealing in oxygen atmosphere.