No abstract

In this work the relationship between the characteristic energy of the Urbach edge E 0 and the parameter B 1/2 of the Tauc's representation of the absorption coefficient of a-SiN-and a-GeN-based alloys is presented and discussed. No correspondence has been experimentally found between B 1/2 and the topological disorder induced by small impurity concentrations in the network ͑less than a few at. %͒, which provokes a broadening of the Urbach tail. In the alloy regime, nevertheless, E 0 and B 1/2 present a linear correspondence. This fact is discussed in terms of the structural changes induced by atoms of different atomic coordination, i.e., on the base of the dominant bonding character ͑which changes from purely covalent to partially ionic͒ and the electronic states at the top of the valence band, as the nitrogen content is increased. The effects of hydrogen, carbon, and silicon in the a-Si and a-Ge networks are also discussed in terms of the Tauc slope B 1/2 parameter.

This work reports on the low-temperature crystallization of hydrogenated amorphous germanium (a-Ge:H) films induced by aluminum. A series of aluminum-doped a-Ge:H films ([Al∕Ge]∼10−6–10−2 range) were deposited onto crystalline silicon substrates at 220°C by the cosputtering technique under the same nominal conditions, except for the Al∕Ge concentration. Raman scattering and infrared transmission spectroscopy were used for the structural characterization. The analysis of experimental data indicates that as-deposited Al-doped a-Ge:H films having an Al relative concentration between 1 and 2at.% crystallize spontaneously. Aluminum contents below this range induce a partial crystallization of the films, whereas [Al∕Ge]>2at.% does not induce any crystallization. The mechanisms involved in the crystallization of these Al-doped a-Ge:H films were also investigated after thermal annealing treatments up to a temperature of 500°C. Since the films are hydrogenated, the influence of hydrogen in the crystallization process was considered in detail. The ensemble of the data leads us to associate the induced crystallization with the coordination of, and the local order around, aluminum atoms in the a-Ge:H network. A microscopic mechanism behind the low-temperature crystallization is proposed. The present research indicates that both fourfold coordinated aluminum atoms and hydrogen species are fundamental in the crystallization phenomenon: the former acting as crystallization seeds, and the latter determining the dynamics of the process.

This work presents the application of a recently developed numerical method to determine the thickness and the optical constants of thin films using experimental transmittance data only. This method may be applied to films not displaying a fringe pattern and is shown to work for a−Si:H (hydrogenated amorphous silicon) layers as thin as 100 nm. The performance and limitations of the method are discussed on the basis of experiments performed on a series of six a−Si:H samples grown under identical conditions, but with thickness varying from 98 nm to 1.2 µm. 2Modern electronic devices, such as thin−film transistors, solar cells, active matrix displays and image sensors, possess thin semiconductor layers of hydrogenated amorphous silicon (a−Si:H). For most electronic applications, the optical properties and the thickness t of these films play an important role, in the sense that they govern the device performance. The quality of the as−deposited material can be monitored in production lines through the in−situ determination of its optical constants (refractive index n and extinction coefficient k) and the thickness homogeneity. For that aim, ellipsometry is the most appropriate tool [1] due to the fact that it is not influenced by the adopted substrate. Alternatively, for the ex−situ analysis of samples grown on top of transparent substrates like glass, the use of optical transmittance is the most attractive method because optical transmission is a very easy, accurate and non−destructive measure.The problem of estimating the thickness and the optical constants of thin films using transmission data only represents a very ill−conditioned inverse problem with many local A set of experimental data [λ i , T meas (λ i )], λ min ≤ λ i ≤ λ i+1 ≤ λ max , for i = 1,…,N, is given, and we want to estimate t, n(λ), and k(λ). The problem seems highly underdetermined. In fact, for known t and given λ, the following must hold [5]: T meas (λ) = T theor (λ, s(λ), t, n(λ)where T theor is the calculated transmission of the film+substrate [3] and s the refractive index of the transparent substrate. This equation has two unknowns n(λ) and k(λ) and, in general, its set of solutions (n,k) is a curve in the two−dimensional (n(λ), k(λ)) space. Therefore, the set of functions (n,k) satisfying T meas = T theor for a given t is infinite and, roughly speaking, is represented by a nonlinear manifold of dimension N in R 2n . However, physical constraints (PC) drastically reduce the range of variability of the unknowns n(λ), k(λ). The optimization process looks for a thickness that, subject to the physical input of the problem, minimizes the difference between the measured and the theoretical spectra, i. e.,(1)The minimization process starts sweeping a thickness range ∆t R divided into thickness steps ∆t S and proceeds decreasing ∆t R and ∆t S until the optimized thickness t opt is found. In the examples to follow, the starting ∆t R and ∆t S were 5 µm and 100 nm, respectively.As seen, the most important issue of the present method is the ret...

We discuss a new method to estimate the absorption coefficient, the index of refraction, and the thickness of thin films using optical transmission data only. To solve the problem we used a pointwise constrained optimization approach, defining a nonlinear programming problem, the unknowns of which are the coefficients to be estimated, with linear constraints that represent prior knowledge about the physical solution. The method applies to all kinds of transmission spectra and does not rely on the existence of fringe patterns or transparency. Results on amorphous semiconductor thin films and gedanken films are reported. They show that the new method is highly reliable.