NITROGEN IN THE AMORPHOUS-GERMANIUM NETWORK - FROM HIGH DILUTION TO THE ALLOY PHASE

In this work experimental data referring to the structural and optoelectronic characteristics of amorphous-germanium-nitrogen thin films are presented and discussed. The nitrogen content of the a-Ge: N samples, deposited by the rf-sputtering technique in an Ar+N2 atmosphere, was allowed to vary from typical impurity levels (less than 0.5 at. %) up to around 35 at. %. The material properties change depending on the nitrogen concentration, determined from a deuteron-induced nuclear reaction. The likely mechanisms of nitrogen incorporation into the solid phase are discussed, as well as the influence of the nitrogen content on the transport and optical properties of the films. A proportionality constant relating the total nitrogen concentration in the solid phase and the integrated absorption of the in-plane stretching vibration mode of the Ge-N dipole has been determined. It has been found that a close analogy exists between the general properties of a-Ge:N alloys and those measured in amorphous-silicon-nitrogen alloys.4874560457

ON THE DOPING EFFICIENCY OF NITROGEN IN HYDROGENATED AMORPHOUS-GERMANIUM

This letter reports on the doping efficiency of nitrogen in a-Ge:H films of electronic quality. It has been found that nitrogen is an effective dopant in the a-Ge:H network, its doping efficiency being similar to the one corresponding to phosphorus in a-Si:H. The concentration of active nitrogen atoms decreases with impurity content following a square root dependence on total nitrogen. This behavior is similar to the one determined for column V dopants in a-Si:H films of electronic quality.621586

TRANSPORT-PROPERTIES OF NITROGEN-DOPED HYDROGENATED AMORPHOUS-GERMANIUM FILMS

This paper discusses experimental data referring to the electronic properties of N-doped hydrogenated amorphous germanium films (a-Ge:H). The a-Ge:H films, prepared by rf sputtering, possess a low density of electronic states in the pseudogap (in the low 10(16) cm-3 range) and exhibit a temperature-activated dark conductivity down to below 200 K. It is shown that N atoms incorporated into the a-Ge:H films produce large changes of both the room-temperature dark conductivity and the activation energy. The results of the present work are consistent with the overall picture of the active doping mechanism of group-V elements in tetrahedrally coordinated amorphous semiconductors. The donor level introduced by fourfold-coordinated nitrogen in a-Ge:H is found to be around 50 meV below the conduction-band edge.4642119212

Absorption edge, band tails, and disorder of amorphous semiconductors

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 ionic) 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.5373833383

Erbium luminescence from hydrogenated amorphous silicon-erbium prepared by cosputtering

Hydrogenated amorphous silicon with small amounts of erbium (Er/Si concentration similar to 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 mu m 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 degrees 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 mu m emission. Raman scattering from both as-deposited and annealed samples showed that network disorder relaxation by annealing is not determinant for efficient Er3+ luminescence. (C) 1997 American Institute of Physics.70451151

Aluminum-induced crystallization of hydrogenated amorphous germanium thin films

Al-induced crystallization of co sputtered hydrogenated amorphous germanium films, deposited at 220 degreesC, onto crystalline silicon substrates is investigated by Raman and infrared spectroscopies as a function of the Al concentration (2x10(-6)< [Al/Ge]<2.5x10(-2)). Aluminum induces partial crystallization of the films for metal concentrations smaller than similar to1.3 at. %. A sort of explosive crystallization of the films occurs within a narrow Al concentration range (similar to1.3 < [Al/Ge]< similar to1.8 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. (C) 2001 American Institute of Physics.79203233323

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 sample, 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. (C) 1998 American Institute of Physics. [S0021-8979(98)02921-1].8495184519

Study of structural changes in amorphous germanium-nitrogen alloys by optical techniques

Thin films of amorphous germanium-nitrogen (alpha-GeN) alloys prepared by the rf sputtering deposition technique have been studied by Raman scattering spectroscopy. The nitrogen content of the samples varies between zero and approximate to 3 x 10(22) atoms cm(-3), as determined from nuclear reaction analysis data. Modifications of the structural characteristics of the Ge-N alloys, probed through their phonon density, were investigated as a function of the nitrogen concentration. In addition to an increase of the network's topological disorder, nitrogen is responsible, at relatively high concentrations, for a structural transition in the alpha-Ge host. The optical and electronic characterization of the samples confirm these changes which are highly dependent on the nitrogen concentration. (C) 1996 American Institute of Physics.79143343

Optical diffraction gratings produced by laser interference structuring of amorphous germanium-nitrogen alloys

We use the interference of two pulsed laser beams (wavelength=355 nm) to produce an optical diffraction grating in amorphous germanium-nitrogen alloy (a-GeN). At the constructive maxima of the interference pattern, the absorption of light leads to crystallization. The crystallized region results of pure microcrystalline germanium (muc-Ge). An indication that Ge-N bonds have broken and nitrogen outdiffused of the film is obtained from infrared spectroscopy and confirmed by Raman spectra. A pattern of alternating a-GeN and muc-Ge lines with a period of about 4 mum acts as an optical diffraction grating due to the difference in optical properties between the two materials, and the three dimensional surface profile, caused by N-2 effusion, that is formed on the sample. (C) 2002 American Institute of Physics.81152731273

Epitaxial pulsed laser crystallization of amorphous germanium on GaAs

We have investigated the crystallization of amorphous germanium films on GaAs crystals using nanosecond laser pulses. The structure and composition of the crystallized layers is dominated by nonequilibrium effects induced by the fast cooling process following laser irradiation. Perfect epitaxial films are obtained for fluencies that completely melt the Ge film, but not the substrate. For higher fluencies, partial melting of the substrate leads to the formation of a (GaAs)(1-x)Ge-2x epitaxial alloy with a graded composition profile at the interface with the substrate. Since Ge and GaAs are thermodynamically immiscible in the solid phase, the formation of the alloy is attributed to the suppression of phase separation during the fast cooling process. Lower laser fluencies lead to polycrystalline layers with a patterned surface structure. The latter is attributed to the freeze-in of instabilities in the melt during the fast solidification process. (C) 2001 American Institute of Physics.9052575258