Structure And Properties Of AlN And Fe-doped AlN Films

AlN films are widely used in ultraviolet light emitters/detectors and surface-acoustic-wave devices due to their large band gap, high velocity of surface acoustic wave, excellent piezoelectricity, good temperature stability and resistance to harsh environments. In theory, transition metal-doped AlN is expected to be ferromagnetic in room temperature and can be used in the spintronic devices and quantum computer. Undoped and Fe–doped AlN films were fabricated by means of DC magnetron sputtering. The effects of the N₂ partial pressure and the total sputtering pressure on structural and optical properties of the AlN films were studied. The structure and magnetism of the Fe–doped AlN films were also investigated.For the as–deposited AlN films, as N₂ partial pressure increases, the energy of deposited particles and the mobility of the impinging particles increases then decrease, which leads to the change of orientation and surface morphology of the films. Due to the change of concentration of the defects, the optical properties of the films were changed as N₂ partial pressure increases. Hermite interpolation was used to get the envelope curves of the transmission spectra and Swanepoel envelope method was used to calculate the refractive index of the films. The refractive index of the AlN films is in the range of 1.84?1.91 at wavelength of 550 nm, and increases with increasing N₂ partial pressure, which can be explained as the densification of the films. Considering that increasing N₂ partial pressure will decrease the concentration of defects, the absorption edge of the films shifts towards blue end and the optical band gap increases from 5.83 eV to 5.91 eV. In the transmission spectra of film deposited at N₂ partial pressure of 10%, an additional strong absorption was observed at 4.03 eV, which is considered to be induced by the oxygen impurity and the nitrogen vacancies.The effect of total sputtering pressure on the structural and optical properties of AlN films has been investigated. Due to the collision effect, lower sputtering pressure prefers to form the plane with higher formation energy. The deficiency of nitrogen induced by the lower sputtering pressure will lead to the smaller refractive index and optical bang gap of the films. The refractive index and optical band gap calculated from the transmission spectra are in range of 1.86?1.90 eV and 5.87 eV?5.89 eV, respectively, both increase as total sputtering pressure increases.Fe-doped AlN films with different doping concentration were fabricated by DC magnetron co?sputtering. When Fe is doped with small concentration, it will occupy the substitutional lattice sites. As Fe concentration increases, it begins to occupy the interstitial sites and the secondary phase, such as Fe-N and metal Fe, appear. The ferromagnetism was found in all doped samples at room temperature. The maximum saturated magnetization is 2.81 emu/cm³ (0.3μB/Fe), which is found to be induced by the AlFeN ternary alloy.The effects of N₂ partial pressure on the structural and optical properties of the Fe-doped AlN films have also been investigated. It was found that increasing N₂ partial pressure will make the Fe atoms entering the lattice easily and increase the formation of the crystal plane. But higher N₂ partial pressure is more likely to form the plane with higher formation energy by means of increasing the energy of deposited particles. As the N₂ partial pressure increases, the granules on the surface are refined and the roughness decreases. With increasing N₂ partial pressure the transparency of the films increases and the optical band gap shifts towards blue end.