A. R. Zanatta

Nitrogen in germanium

The known properties of nitrogen as an impurity in, and as an alloy element of, the germanium network are reviewed in this article. Amorphous and crystalline germanium–nitrogen alloys are interesting materials with potential applications for protective coatings and window layers for solar conversion devices. They may also act as effective diffusion masks for III-V electronic devices. The existing data are compared with similar properties of other group IV nitrides, in particular with silicon nitride. To a certain extent, the general picture mirrors the one found in Si–N systems, as expected from the similar valence structure of both elemental semiconductors. However, important differences appear in the deposition methods and alloy composition, the optical properties of as grown films, and the electrical behavior of nitrogen-doped amorphous layers. Structural studies are reviewed, including band structure calculations and the energies of nitrogen-related defects, which are compared with experimental data. ...

Green photoluminescence from Er-containing amorphous SiN thin films

Green light emission at room temperature was achieved from nonhydrogenated amorphous silicon–nitrogen (a-SiN) thin films. The films were deposited by cosputtering a silicon target covered with metallic erbium platelets in an Ar+N2 atmosphere. According to the deposition conditions, the nitrogen concentration [N] reached ∼40 at. % rendering an optical gap of approximately 3.5 eV while the Er concentration [Er] was estimated to be ∼10 at. % in the present films. The high [Er] associated to the optical band gap allows the direct excitation of Er3+ ions. This optical excitation is more efficient at low temperatures as a consequence of the reduction in nonradiative processes, and when exciting the samples with the 488.0 nm line of an Ar+ laser which is in resonance with the 4F7/2Er3+ energy level. In addition to light emission at ∼520 and ∼545 nm, transitions in the infrared energy region could be easily verified in as-deposited samples.

Erbium luminescence from hydrogenated amorphous silicon-erbium prepared by cosputtering

Hydrogenated amorphous silicon with small amounts of erbium (Er/Si concentration ∼5 at. %) was prepared by radio frequency sputtering from a Si target partially covered by tiny metallic Er chunks. Four sets of samples were studied: nonintentionally contaminated hydrogenated and nonhydrogenated amorphous silicon-erbium (a-SiEr:H and a-SiEr); nitrogen doped a-SiEr(N):H and oxygen contaminated a-SiEr(O):H. Samples from the first two sets present only faint 1.54 μm photoluminescence characteristic from Er3+ ions even at 77 K. Samples from the other sets show this luminescence at 77 K as deposited, without any further annealing step. Thermal annealing up to 500 °C increases the photoluminescence intensity, and room temperature emissions become strong enough to be easily detected. These results indicate that in an amorphous silicon environment the chemical neighborhood of the Er3+ ions is crucial for efficient 1.54 μm emission. Raman scattering from both as-deposited and annealed samples showed that network dis...

Optical spectroscopy of Er3+ and Yb3+ co-doped fluoroindate glasses

Optical absorption, Stokes, and anti-Stokes photoluminescence were performed on Er3+–Yb3+ co-doped fluoroindate glasses. For compounds prepared with a fixed 2 mol % ErF3 concentration and YbF3 contents ranging from 0 to 8 mol %, important upconversion processes were observed as a function of temperature and photon excitation energy. Based on the experimental data, two mechanisms for the upconversion (or anti-Stokes photoluminescence) processes were identified and analyzed in detail. At high Yb contents, the upconversion mechanisms are mostly determined by the population of the 2F5/2 levels of Yb3+ ions (or 4I11/2 levels of Er3+ ions, by energy transfer) regardless of the photon excitation energy and temperature of measurement. Moreover, green and red light emission have similar intensities when a large Yb3+ content is present.

Photoluminescence quenching in Er-doped compounds

The temperature-induced quenching in the photoluminescence intensity (IPL) of Er3+ ions in different semiconductor hosts is discussed in this letter. Based on a compilation of several IPL(T) experimental curves, corresponding to either crystalline or amorphous Er-doped samples, and after a critical data analysis, it was possible to determine a temperature of quenching Tq such that IPL(Tq)=0.95 IPL (lowest T). These experimental Tq values have been analyzed in terms of certain host characteristics such as optical band gap and phonon frequency. As a result of this study it was possible to state that: (i) Tq strongly depends on the optical band gap and atomic structure of all studied semiconductor hosts; (ii) the host phonon frequency plays a minor role in the quenching of IPL; and (iii) based on the partial ionic character of each semiconductor host, both the energy and the localization of the photon-generated electron–hole pairs determine the IPL quenching. Moreover, the present phenomenological model is c...

Low-temperature al-induced crystallization of amorphous Ge

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 proc...

Aluminum-induced crystallization of hydrogenated amorphous germanium thin films

Al-induced crystallization of co sputtered hydrogenated amorphous germanium films, deposited at 220 °C, onto crystalline silicon substrates is investigated by Raman and infrared spectroscopies as a function of the Al concentration (2×10−6<[Al/Ge]<2.5×10−2). Aluminum induces partial crystallization of the films for metal concentrations smaller than ∼1.3 at. %. A sort of explosive crystallization of the films occurs within a narrow Al concentration range (∼1.3<[Al/Ge]<∼1.8 at. %). Raman spectra do not display any crystallization signal for metal concentrations above this narrow range. Data of the extended x-ray absorption fine structure of the coordination and of the local order around gallium, in Ga-doped a-Ge:H, are used to propose an overall picture of the microscopic mechanisms behind these results. A comparative analysis suggests that the crystallization seeds are fourfold-coordinated Al atoms sitting at the center of perfect tetrahedral Ge sites.

Visible luminescence from a-SiN films doped with Er and Sm

Relatively strong and narrow red and green light emission has been achieved from amorphous (a-) SiN films independently doped with Er3+ and Sm3+ ions. The films were deposited by cosputtering a Si target partially covered with small pieces of metallic Er (and Sm) in an atmosphere of pure nitrogen. As a consequence of the deposition method and conditions, the films have an amorphous structure, and contents of Er (and Sm) in the low 0.5 at. %. All characterizations were accomplished on as-deposited samples and at room temperature and included: ion-beam analysis (Rutherford backscattering spectrometry and nuclear reaction analysis) and optical techniques (light absorption, Raman scattering, and photoluminescence and cathodoluminescence). A detailed examination of the experimental results allowed the identification of all luminescence features existing in the films.

Exponential absorption edge and disorder in Column IV amorphous semiconductors

We discuss the likely origin of the exponential absorption tail, or Urbach edge, of fourfold coordinated amorphous (a-)semiconductors. The present analysis is based on a compilation of a considerable amount of experimental data originating from a great variety of samples, alloys, and authors, and obtained with quite different spectroscopic techniques. An attempt is made to correlate the measured Urbach edge with the structural and optical properties of the samples. The present analysis indicates that the Urbach edge may not only reflect the shape of the joint density of states of the valence and conduction band tails, but may also have important contributions from short-range order potential fluctuations produced by charged defects or impurities.

Metal-induced nanocrystalline structures in Ni-containing amorphous silicon thin films

The mechanisms of silicon nanocrystal structure formation in amorphous Si films have been studied for a relative Ni impurity content varying between 0.1 and 10at.%, i.e., from a Ni doping range to the Si–Ni alloy phase. The films, deposited by the cosputtering technique at 200°C, were submitted to isochronal (15min) annealing cycles up to 800°C. Four different substrates were used to deposit the studied films: crystalline (c-) quartz, c-Si, c-Ge, and glass. Both the two orders of magnitude impurity concentration range variation and the very short annealing times were selected on purpose to investigate the first steps of the mechanism leading to the appearance of crystal seeds. The conclusions of this work are the following: (a) Ni impurity induces the low-temperature crystallization of amorphous silicon; (b) the NiSi2 silicide phase mediates, at the surface or in the bulk of the film, the crystallization process; and (c) the onset of crystallization and the crystalline fraction of the samples at each temp...