Growth temperature and N 2 ambient pressure-dependent crystalline orientations and band gaps of pulsed laser-deposited AlN/(0 0 0 1) sapphire thin films

Abstract

We report a study on the role of substrate temperature ( T s), ranging from 850 to 1150 °C and the effect of ambient nitrogen pressure (5×10 −4 and 5×10 −5 Torr) on the structural and optical properties of pulsed laser deposited AlN thin films on c-axis Al 2O 3 substrates. It is found that along with the band gap of the pulsed laser-deposited AlN thin films, the crystallographic orientation turns from ( 1 0 1 ¯ 0 ) to (0 0 0 2) depending on the substrate temperature ( T s) and the ambient nitrogen pressure. At T s=850 °C and above-mentioned nitrogen pressures, the deposited thin films showed preferential orientation along ( 1 0 1 ¯ 0 ) and a band gap of about 5.4 eV. At higher T s of 950 °C, both the orientation i.e. ( 1 0 1 ¯ 0 ) and (0 0 0 2) were observed. The variation in intensity of these orientations depends on the ambient nitrogen pressure. At 5×10 −4 Torr, both the orientations were observed with almost equal proportions and the band gap of the corresponding thin film was about 5.4 eV. Interestingly, at lower pressure of 5×10 −5 Torr the film, predominantly, showed orientation along (0 0 0 2) and the band gap of the corresponding film was about 5.9 eV. At even higher T s=1050 °C, even though the films were oriented mainly along (0 0 0 2), the crystalline quality of the films deposited at 5×10 −5 Torr nitrogen pressure was much superior when compared to those deposited at 5×10 −4 Torr nitrogen pressure. Investigations of the thin films using atomic force microscopy and Raman spectroscopy support the above observations. Attempts have been made to explain the observations in terms of the temperature and pressure-dependent growth modes.