Forbidden energy band gap in diluted a-Ge_(1-x)Si_x:N films

摘要

By means of electron gun evaporation Ge_(1-x)Si_x:N thin films, in the entire range 0≤x≤1, were prepared on Si (100) and glass substrates. The initial vacuum reached was 6.6×10~(-4) Pa, then a pressure of 2.7×10~(-2) Pa of high purity N_2 was introduced into the chamber. The deposition time was 4 min. Crucible-substrate distance was 18 cm. X-ray diffraction patterns indicate that all the films were amorphous (a-Ge_(1-x)Si_x:N). The nitrogen concentration was of the order of 1 at% for all the films. From optical absorption spectra data and by using the Tauc method the energy band gap (E_g) was calculated. The Raman spectra only reveal the presence of Si-Si, Ge-Ge, and Si-Ge bonds. Nevertheless, infrared spectra demonstrate the existence of Si-N and Ge-N bonds. The forbidden energy band gap (E_g) as a function of x in the entire range 0≤x≤1 shows two well defined regions: 0≤x≤0.67 and 0.67≤x≤l, due to two different behaviors of the band gap, where forx>0.67 exists an abruptly change of E_g(x). In this case E_g(x) versus x is different to the variation of E_g in a-Ge_(1-x)Si_x and a-Ge_(1-x)Si_x:H. This fact can be related to the formation of Ge_3N_4 and GeSi_2N_4 when x ≤ 0.67, and to the formation of Si_3N_4 and GeSi_2N_4 for 0.67≤x.