Erbium-doped polycrystalline ZnO fibers were prepared by electrospinning. Organo-metallic Zn and Er (0.5, 1.0 and 2.0 % Er/Zn ratio) precursors dissolved in methanol, DI water and acetic acid were forced through a syringe needle. An electric field of 20 kV over 16 cm was used to promote the growth of fibers from the needle to a rotating drum. The resulting material consisted of entangled fibers with 0.5 to 1 μm typical diameter. The as-prepared samples present faint Er3+ luminescence at 1.5 μm when excited by the 488 nm line of an Ar+ laser. This luminescence intensity increases up to 25 times when the samples are annealed for 30 min at 500 °C in air. The effect of annealing over the Er environment was determined by EXAFS measurements at the Er L3 edge. In all as-prepared samples Er is coordinated to 8 ± 0.5 oxygen atoms at up to 2.36 ± 0.02 Å interatomic separation. After annealing the coordination is reduced to 5.9 ± 0.5 and the atomic separation becomes 2.30 ± 0.02 Å. This is comparable to Er2O3 where the coordination is 6 and the first neighbor distance is 2.26 ± 0.02 Å. The overcoordination in the as-prepared samples is consistent with the Er L3 edge at 8361.7 ± 0.5 eV compared to 8360.2 ± 0.5 eV in the annealed samples and 8359.9 ± 0.5 eV in Er2O3. In conclusion, the annealing of electrospun ZnO:Er fibers makes the Er environment more similar to that of Er2O3 increasing the luminescence efficiency.

We apply combinatorial approaches to thin-film Si materials and device research. Our hot-wire chemical vapor deposition chamber is fitted with substrate xyz translation, a motorized shutter, and interchangable shadow masks to implement various combinatorial methods. For example, we have explored, in detail, the transition region through which thin Si changes from amorphous to microcrystalline silicon. This transition is very sensitive to deposition parameters such as hydrogen-to-silane dilution of the source gas, chamber pressure, and substrate temperature. A material library, on just a few substrates, led to a three-dimensional map of the transition as it occurs in our deposition system. This map guides our scientific studies and enables us to use several distinct transition materials in our solar-cell optimization research. We also grew thickness-graded wedge samples spanning the amorphous-to-microcrystalline Si transition. These single stripes map the temporal change of the thin silicon phase onto a single spatial dimension. Therefore, the structural, optical, and electrical properties can easily be studied through the phase transition. We have examined the nature of the phase change on the wedges with Raman spectroscopy, atomic force microscopy, extended x-ray absorption fine structure (EXAFS), x-ray absorption near-edge spectroscopy (XANES), ultraviolet reflectivity, and other techniques. Combinatorial techniques also accelerate our device research. In solar cells, for example, the combinatorial approach has significantly accelerated the optimization process of p-, i-, n-, and buffer layers through wide exploration of the complex space of growth parameters and layer thicknesses. Again, only a few deposition runs are needed. It has also been useful to correlate the materials properties of single layers in a device to their performance in the device. We achieve this by depositing layers that extend beyond the device dimensions to permit independent characterization of the layers. Not only has the combinatorial approach greatly increased the rate of materials and device experimentation in our laboratory, it has also been a powerful tool leading to a better understanding of structure-property relationships in thin-film Si.

One very important problem concerning erbium-doped silicon is the electronic structure of the Er3+ impurities. In particular, it is still not clear if the 4f levels can be treated as frozen core levels or their overlap with s and p states of their neighbors must be considered explicitly. For crystalline Si, the 4f levels have been supposed to be anywhere between 20 eV below the valence band and within the energy gap. In this paper we report on the first ultraviolet photoemission spectroscopy (UPS) measurements on Er-doped a-Si:H. Samples of a-Si:H<Er> with different Er contents (up to 1 at. % Er) were prepared by co-sputtering from a Si target partially covered with metallic Er platelets. In order to enhance the Er states relative to the Si and H states, the excitation energy was tuned between 40 and 140 eV with a synchrotron light source. At 140 eV excitation energy the cross-section of the Er 4f and 5p states is more than an order of magnitude higher than the cross section of the Si 3s or 3p states. As the Er concentration increases, a shoulder and then a peak appears at 10.0±0.5 eV binding energy. The intensity and width of this peak is well correlated with the Er concentration, and with the Er 5p and 5p½ levels at 26 and 32 eV binding energy, respectively. We attribute the peak at 10.0±0.5 eV binding energy to the Er 4f level. These are the only occupied states that can be related to the presence of Er, indicating that these levels are not valence states and consequently can be treated as frozen core levels.

The Er local environment of a-Si:H<Er> prepared by PECVD using a metalorganic precursor was determined by EXAFS. We found that in as-deposited samples Er is coordinated to 6 oxygen atoms at 2.28±0.01Å, very similar to Er2O3. Annealing at 420°C hardly affects the Er neighborhood, but higher annealing temperatures (starting at 600°C up to 1033°C) decrease the Er-O separation as much as 0.05Å, maintaining the Er average coordination around 6. This is interpreted as due to the formation of a carbon second neighbor shell. Our results show that the Er local environment is not related with the luminescence enhancement for annealing at moderate temperatures.

Trivalent erbium (Er3+) presents a characteristic intra 4f optical transition 4I13/2 → 4I15/2 at 1.54 μm when incorporated in several solid hosts. Hydrogenated amorphous silicon (a-Si:H) is a good candidate as a host for applications in optical communications and photonic integration. We have studied Er3+ photo and electroluminescence in a-Si:H prepared by co-sputtering from a silicon target partially covered with metallic erbium chunks. Since the presence of oxygen impurities enhances the luminescence intensity, we studied the influence of oxygen added to the sputtering gas on the material properties. We found that oxygen reduces the erbium incorporation into the films. We obtained samples presenting 1.54 μm photoluminescence as deposited for a wide range of erbium concentrations. Maximum room-temperature photoluminescence efficiency is obtained for samples that contain ∼ 1% [O]/[Si] concentrations. The temperature quenching is small, mainly due to the temperature dependence of the luminescence lifetime.

Room temperature electroluminescence at 1.54 νm was observed in reverse biased Si/a-Si:Er:O:H/Al structures.

We have measured extended x-ray absorption fine structure (EXAFS) of the Er LIII edge in a-Si:Er:O:H with different concentrations of Er and O. The samples were prepared by reactive RF co-sputtering from a silicon target partially covered with metallic erbium platelets. They present the characteristic Er 3+ photoluminescence at 1.54 μm as deposited. The FFT of the Er EXAFS provides two well separated peaks. The characteristics of the first peak resemble those of Er2O3. We associate the first neighbor shell with oxygen atoms, even in non intentionally oxygenated samples. The average coordination and Er-O separation are significantly smaller than in Er2O3. This may be the reason why Er3+ luminescence in a-Si:H presents small temperature quenching. The second shell is interpreted as being composed of silicon atoms.

The dependence of the photoluminescence (PL) decay on temperature and composition of a series of amorphous silicon-carbon alloys (a-Si1-xCx:H) with 0 <x <0.4 was studied. The samples were prepared by “low power” PECVD from SiH4/CH4 gas mixtures. The decay curves are non-exponential and can be described as lifetime distributions (LTD). For pure a-Si:H at 17 K the peak of the LTD is of the order of 5×10-4 sec. It shifts to shorter lifetimes as x or the temperature increases. Samples with x > 0.3 present in addition a faster peak with maximum at 10-8 sec. The fast peak is almost temperature independent. The slow component shifts to shorter lifetimes as the temperature increases, and vanishes at room temperature. The presence of the fast peak is interpreted as due to a change in the radiative recombination mechanism. We associate this peak to the recombination of trapped “excitons”.

We have prepared a series of hydrogenated Amorphous silicon-carbon alloys a-Si1-xCx:H in conditions where the power delivered to the plasma is below the threshold of primary decomposition of CH4 (“low-power regime”). Carbon is therefore incorporated into the growing film in the form of methyl groups -CH3, thus preserving the sp3 hybridization in the solid. Since the carbon is deposited only by chemical reaction with the radicals produced by the decomposition of SiH4, its concentration in the alloys cannot exceed the value x=0.5.

The variations of the optical gap and of the “average gap” with x, as measured by optical transmission, indicate as expected the predominantly tetrahedral coordination of the carbon, with a record value of EQ4=3.6 eV for x=0.42 (3.95 eV if corrected for the Urbach-tail absorption). Since the Si-C network does not relax to a more stable lower sp2 coordination, the material is highly strained, with an Urbach tail larger than that of the standard Material.

We report a photoluminescence study on amorphous hydrogenated silicon carbon (a-Si1-xCx:H) alloys with carbon concentration in the range O < x < 0.5, prepared by PECVD in the “low-power” regime, that preserves the tetrahedral coordination of the carbon atoms. These samples have optical gaps higher than conventional “high power” alloys with the same carbon content. For carbon concentrations below x = 0.2 the photoluminescence behaves essentially as in pure a-Si:H with increased gap, Urbach energy and DOS. For higher carbon concentrations there is a change in the recombination process, that we attribute to a change in the dominating diffusion process of the photogenerated carriers. The integrated photoluminescence intensity for carbon-rich samples is very weakly dependent on the temperature, and at room temperature it approaches that of pure a-Si:H at 77K. For all samples, the photoluminescence bandwidth can be well described by a zero-phonon model.