Pulsed-Laser-Deposited AlN Films for High-Temperature SiC MIS Devices
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(CVD) and molecular beam epitaxy (MBE) are based on the equilibrium growth conditions and produce columnar structures in the III-V nitrides on SiC and sapphire. A highly non-equilibrium growth process is desirable to circumvent these growth problems. For example, energetic processes involved in the non-equilibrium growth conditions can be utilized to form stacking faults in the plane of the interface to manage the lattice mismatch induced strain rather than the formation of vertical threading dislocations, or columnar structures in the films. Such energetic and non-equilibrium growth conditions can be achieved, for example, in Pulsed Laser Deposition (PLD) [6]. Such a process can be utilized in the early stages of growth of the dielectric layer without the formation of columnar structures or vertical grain boundaries. In this paper, we highlight the growth of AlN films by PLD, their characterization, and their application for the development of high-temperature SiC based thyristors. Our major objective is to develop and fabricate thyristors based on SiC, with integration of other wide band gap materials suitable for high-temperature operation (300-500 °C). In this context, we have studied the suitability of the PLD technique for the fabrication of AlN thin films for encapsulation, passivation, and as a dielectric layer on SiC. EXPERIMENTAL The schematic of the PLD process is shown in Fig. 1. A KrF excimer laser (λ = 248 nm, τ = 25 ns) was used for the ablation of a polycrystalline, stoichiometric AlN target (99.99 purity) at an energy density of ~1 J/cm2. Upon laser absorption by the target surface, a strong plasma plume is produced as shown in Figure 1. The laser-induced plasma consists of atoms, molecules, excited atomic and molecular species, and clusters. The laser ablated species are allowed to condense on to a substrate kept at suitable temperature. The NH3 background gas pressure in our experiment was varied from 10-6 to 10-3 Torr. The deposition rate and the film thickness were controlled by the pulse repetition rate (5 to 10 Hz) and total deposition time (30 to 60 min.). The PLD films were characterized by four-circle x-ray diffraction (XRD), atomic force microscopy (AFM), UV-visible spectroscopy, Rutherford backscattering spectrometry (RBS) and ion channeling, transmission electron microscopy (TEM), and electrical transport (I-V) measurements. TEM studies were performed using JEOL 2010 system operated at 200KV. RESULTS AND DISCUSSION The crucial parameters in the PLD of AlN dielectric films are the deposition temperature and laser fluence. We have studied the dependence of these parameters on the crystalline quality, surface morphology, and the optical and electrical properties of the III-V nitride films [6]. We found that AlN grows epitaxially on sapphire, SiC, and GaN at a substrate temperature as low as 600 °C. However, the crystalline quality of these films improves with an increase in the substrate temperature. High epitaxial quality was obtained when the films were grown under a NH3 background
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