Intensity of emission from intracenter 4$f$-transitions in $a$-Si:H, ZnO, and GaN films doped with rare-earth ions

It is shown that the intensity of emission from intracenter 4$f$-transitions in amorphous $a$-Si:H films and crystalline (GaN, ZnO) films doped with rare-earth ions is governed by the local environment of doping impurity ions. In the case of $a$-Si:H, a pseudo-octahedron with the $C_{4V}$ point group is present due to nanocrystallites, which provides a local environment for rare-earth ions. In the case of a hexagonal crystal lattice in crystalline GaN and ZnO films, the local symmetry of rare-earth ions introduced into the semiconductor matrix by diffusion, with a pseudo-octahedron with the $C_{4V}$ point group, is formed by stresses due to rare-earth ion-oxygen complexes with a radius exceeding that of host ions incorporated at crystal lattice sites. In contrast to GaN films, ZnO films exhibit, on being doped with Tm, Sm, and Yb, both high-intensity emission in the long-wavelength spectral region, characteristic of intracenter 4$f$ transitions in rare-earth ions, and a substantial increase in intensity in the short-wavelength spectral region ($\lambda$ = 368–370 nm). GaN films doped with rare-earth ions exhibit in this spectral range only an inhomogeneously broadened emission spectrum due to the presence of an emission band characteristic of donor-acceptor recombination.