Structural and optical properties of magnetron-sputtered Er-doped AIN films grown under negative substrate bias

In seeking suitable host-microstructures enhancing photoluminescent (PL) efficiency of reactively sputtered Er-doped AIN films deposited under varying negative substrate bias, we observed that the films' Er-emitted PL intensity at the wavelength of 1.54 pin builds up with bias up 50 V, before falling sharply and collapsing for U above 90 V. We studied for further insight the bias-induced effects on the film microstructural and optical properties. The transmission electron microscope images of these films biased up to 50 V showed improved film crystallographic quality with wide and long columnar microstructures. The columnar crystallized volumes therein are maximized around a particular bias value around 90V, above which they have finely grained nano-granular microstructures. This film-microstructural evolution concurs well with the film refractive index n and extinction coefficient k obtained from ellipsometry. Both surge for films biased from 50 V to 90V. These correlating evidences point out that adequate adatom mobility gained under low-energy ion bombardment with bias up to 50 V properly configures the AIN Wurzite planes while leaving behind fewer sites for PL-quenching defects. Meanwhile, Urbach tail states associated with other visible-light absorbing defects created by some material disorder in appropriate locations within the columns should have absorb the exciting photon energy and transfer it efficiently to the emitting Er3' ions. These evidences further indicate that the newer defects created under bias above 100 V act as non-radiative centers in putting out the PL. We suggest that these latter defects would be located with a high density, at the numerous grain boundaries of the nano-crystallized volumes. (C) 2014 Elsevier B.V. All rights reserved.