Growth of Wide-Band Gap AlN and (SiC)x(AlN)1-x Thin Films by Reactive Magnetron Sputter Deposition
Sammanfattning: The research presented in this thesis is focused on thin film synthesis of epitaxial wurtzite structure aluminum nitride (AlN) and related alloy, (SiC)x(AlN)1-x,by ultra-high-vacuum (UHV) reactive magnetron sputter deposition, on silicon carbide (6H-SiC) substrates. The emphasis of the work is on controlling the growth and quality of the films to be able to use the materials in electronic device applications. The quality of epitaxial AlN films is significantly improved by usin glow-energy ion assistance (Ei = 17-27 eV), during growth. The ion-assisted growth gives an increased surface mobility, which promotes domain boundary annihilation and epitaxial growth. This results in lateral expansion of column width (100 nm-wide at film thickness above 100 nm). Structural characterization by high-resolution XRD and electron microscopy reveal a very good crystal quality. The measured concentrations of O, C, and Si impurities in the films are at 3.5x1018, 1.3x 10x18 andl 1.0 x1017 cm-3, respectively, which are among the purest AlN material that has been reported. The appearance of near band-edge CL emission (6.02 eV at 4K) is also an evidence of a high quality material. For metal-insulator-semiconductor devices (MIS) fabricated using AlN as the dielectric layer, electrical characterization by C-V measurement shows hysteresis with 1.2 V of flat-band voltage shift due to fixed charge and interface states, and a current leakage due to domain boundary formation in the AlN layer. Pulsed low-energy ion-assisted reactive magnetron sputtering was also applied to the growth of AlN in order to overcome the thickness limitation of DC ion-assisted growth due to a surface charging effect. AlN films with very large domain widths can thus be realized. The structural evolution resulting from increasing adatom mobility can be extended up to twice the thickness, compared to DC ionassisted deposition. The growth rate also increased by a factor of ∼4, compared to growth conditions with no ion assistance (Ei = 2 eV), and by a factor of ∼2 from the DC ion-assisted growth. Finally, solid solution (SiC)x(AlN)1-x thin films have beengrown epitaxially on 6H-SiC substrates. The composition of the films was controlled by varying the power of each magnetron during co-sputtering from A1 and SiC targets in a gas mixture of Ar and N2. Compositional investigation from AES showed a decreasing Si and C content for an increasing magnetron power ratio, (PA1/PSiC). The microstructure of the films was improved as the PA1/PSiC increases. Films grown at PA1/PSiC 3. 6 show structural evolution of domain width similar to the growth of pure AlN. High-resolution XRD shows a minimum in c-axis lattice parameter as PA1/PSiC =3.6. CL spectra show defect-related peaks corresponding to O and C impurities with some shifts due to structural defects and concentration of the impurities.
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