Photoluminescence oscillations: thickness sensibility of a film
 

Recently, our group developed a nondestructive method for the measurement of the thickness and refractive index with a high resolution based on the material's photoluminescence. We reported the results in 13 November 2006 issue of Appl. Phys. Lett. [1]. It is found out that the strong oscillations exposed on the laser-excited photoluminescence spectrum of porous alumina film show highly sensitive to the product of the film's thickness (L) and the refractive index (n), that is, optical length (nL). Detailed analysis elucidates that the phenomenon, designated as Photoluminescence Oscillations (PLO), can be ascribed to the interference within a Fabry-Pérot optical cavity where the oscillation period corresponds to the difference between two longitudinal modes.

The samples, porous alumina films, were fabricated using constant dc 40 V anodization of the annealed and polished pure Al foils in 0.3 M oxalic acid for different time. These conditions give typical parameters of porous alumina films with pore diameter of ~30 nm and a corresponding cell tip (barrier layer) of ~ 100 nm, namely, the lattice constant of the hexagonal pore lattice, the distance of the nearest neighboring pores, is 100 nm. With these parameters, the calculated porosity (p) is about 8% based on the standard structure model of porous alumina film. The PL was excited by a He–Cd laser (325 nm). Corresponding to the recorded photon energies [3.54 to 2.25 eV (350–550 nm)], the material's dispersion (dn/dλ) is nearly zero, which resulted in the equidistant PL oscillating peaks (ΔE is the separation of two neighboring oscillating peaks), thus the constant optical length [nL= hc/2ΔE, where h is Planck's constant (6.626×10^-34 Js), c is the speed of light (2.998×10⁸ m/s)]. 

Figure1.  Link to the large image: HighRes.

(b) Measurement of the film’s thickness L = hc/(2n_effΔE_PLO):

Besides the tedious method by using TEM, for a thicker film, the accuracy of the thickness measurement decreases much by using SEM due to that it is difficult to determine the exact location of the films' interface. Moreover, this SEM limitation also accompanies with the insulant or the transparent sample. Therefore, providing that we can get accurate data of the effective refractive index n_eff from the Bruggeman equation or measuring with the typical skills of spectral reflectance or transmittance, x-ray diffraction or ellipsometry, and then determine the modes' space ΔE_PLO with Photoluminescence Oscillations, the thickness can be easily and accurately predicted in a nondestructive manner.

Sometimes, the spectrum of x-ray diffraction is complicated due to the defects and the impurities. Whereas ellipsometry is not appropriate for measuring the optical thickness of porous materials. First, thickness measurement using ellipsometry is simulated together with the effective refractive index (n) and extinction coefficient (k) of a film. For common films, such as silicon dioxide film, there is a calibrated n & k database, and it is easy to measure thickness accurately. However, for the films used uncommonly, for example, porous alumina film, there has no calibrated n & k database which causes a problem. Second, the effective refractive index varies with the porosity that is so sensitive to the fabricated conditions. Therefore, it is unpractical to acquire a uniform n & k database of porous alumina films for ellipsometry. 

"Everybody takes for granted that the Photoluminescence Oscillations is nothing special but just interference, however, there is indeed a little of surprise when we note that the thickness or the refractive index can be extracted easily and accurately in such a nondestructive manner..." said the group leader Prof. Yi SHI in a group's seminar.

Moreover, we believe that this optical phenomenon is not a special case but just structure related; that is, if the film thickness is compared with N(λ/2) (N=1,2,3...), the photoluminescence oscillations will be observed. However, if dn/dλ is far beyond zero, the oscillations are somewhat complex because the spaces between neighboring oscillating peaks will be different. The detailed discussion will be published elsewhere. Considering that the optical parameters are related to band energy, luminescence center, exciton, and phonon could be extracted from the PL spectrum; therefore, this oscillation endows photoluminescence spectrum with a fascinating dimension.

As endnotes, porous alumina film with periodic nanopore lattice [3-7] is widely used as a template or mask in the fabrication of mesostructures [8-12].  It also acts as an insulator host to sustain the functional material on many occasions.

* Lin PU is in the Physics Department of Nanjing University, Nanjing 210093, CHINA.

Citation 

L. PU

Lin PU. Photoluminescence oscillations: thickness sensibility of a film. Scidea Sketch 1 (2) ss20061113a1 (2007).

♦ doi: 10.3128/ss20061113a1 | Scidea :: Abs . Full | CrossRef
♦ Scidea Sketch :: ISSN: 1992 - 8548