This work was partially supported by FCT foundation (Portugal) and Russian foundation for basic research (RFBR project N 01-02-16439).
1. Introduction 1–3 to 8 nm) were deposited and studied in both near IR (1.54 µm) and visible luminescence ranges. The results are Recently, Er-doped Si materials were widely studied discussed in terms of the role of Si nanocrystallites in films in context with the interest in temperature-stable light luminescent properties.
emitters for optical communication systems. The intra4f-shell transitions between two lowest spin-orbit levels of 4 2. Experimental Er3+ ions, namely the transitions I13/2 I15/2, occur at 1.54 µm, a wavelength close to that with minimum loss in Erbium doped nanocrystalline silicon thin films were silica based optical fibers. In has been reported on the regrown by r. f. magnetron sputtering in an Ar/H2 atmosphere alization if electroluminescence devices on Si:Er basis [1–3].
on the ordinary glass substrates. The procedure applyed was Nevertheless the main drawback to use a bulk Si crystal similar to that used for the preparation of undoped µc-Si:H as a host for Er3+ is the strong temperature quenching of films , only modified by the introduction of small pieces 1.54 µm luminescence. It appears that this situation may be of metallic erbium to c-Si target for Er doping. The target considerably improved by the incorporation of Er ions in used was a c-Si of high purity (99.99%). The erbium was nanocrystal containing materials. The idea is based on the placed in a low erosion area on the silicon target, in order to bandgap widening of nanometer size Si that consequently keep the moderate rate of Er impurity. The substrate–target has to result in the reducing of thermal quenching for distance was fixed at 55 mm.
Er luminescence. Moreover, Si nanocrystals that are well Samples with the different structural parameters, i. e., known to emit in the visible range (due to the recombination different crystalline fraction and grain sizes were obtained of confined excitons within the nanostructures) may act as by the varying of experimental parameters (RF power, the efficient sensitizers for rare earth ions [4,5].
temperature, Er content and gas mixture composition).
In this contribution we discuss the luminescent properties In particular, amorphous films (see Er28) were obtained of Er-doped nanocrystalline silicon thin films (nc-Si:Er) at low hydrogen dilution, i. e., low RH value (see Tabproduced by the r. f. reactive magnetron sputtering method.
le 1). Nanocrystalline samples were grown in a H2 rich The advantages of these films, when compare with the atmosphere, where the role of atomic hysrogen in to etch intensively studied nanocrystal containing SiO2 structures preferentially he amorphous phase and promote the amorand a-Si:H,O,Er films, is their relatively high conductivity phous-to-crystalline transition. The films growth conditions that makes the material attractive for device applications.
are listed in Table 1.
One can show that the presence of crystalline fraction in thin films results in the increase of films conductivity in several The chemical composition of films was determined orders of magnitude . Er-doped nc-Si:H films with the by the Rutherford backscattering spectroscopy and elastic well-defined crystallinity and nanocrystal sizes (varied from recoil detection techniques. The structural characterisation The role of microstructure in luminescent properties of Er-doped nanocrystalline Si thin films Figure 1. XRD (a) and Raman specrta (b) of nc-Si:Er thin films.
was performed by a standard micro-Raman spectroscopy 3. Results and discussion under excitation with the 514.5 nm Ar+-laser line and Fig. 1 shows the XRD and Raman spectra for the ncby X-ray diffractometry in the grazing incidence geoSi:Er samples studied. The broad band, related to the silicon metry. For the more detailed analysis of microctructure amorphous matrix, is present in spectra for all samples. The and films anatomy“, highresolution transmission electron ” sample Er28 grown at higher RF power but in Ar rich microscopy (HRTEM) and specrtoscopic ellipsometry (SE) atmosphere does not show any crystalline peak in both were applied. SE spectra of the pseudodielectric functhe XRD and Raman spectra (the same behaviour was tion, = 1 + i 2, were acquired in the 1.5–5.5 eV observed also for the sample Er33). In contrast, the (111), energy range by using a phase-modulated spectroscopic (220) and (311) diffraction peaks of c-Si are visible for all ellipsometer (UVISEL-Jobin Yvon) at an incident angle of other samples. The peaks are well evident for the Er70.5. SE spectra were analysed in terms of optical models sample grown at high H2 dilution and high temperature based on the Bruggeman effective medium approximation of 400C, both parameters promoting the amorphous-to(for more details see ). The thickness of the films crystalline transition. The diffraction peaks have a lower was evaluated from the analysis of interference pattern intensity for the Er19 and Er24 samples, indicating a in transmission spectra making use of the Swanepoel decrease of the crystallinity and/or the ctystallites grain sizes method  and from the spectroscopic ellipsometry data.
that in also confirmed by the intensity ratio of the Stokes Photoluminescence (PL) measurements in the near IR peaks at 480 cm-1 (a-Si related) and at 520 cm-1(transers range have been performed witn a Brucker 66 V Fourieroptical (TO) mode of c-Si) in Raman spectra (Fig. 1, b). The transform spectrometer. The signal was detected with diffraction peak analysis, by fitting a pseudo-Voigt function a North-Coast EO-817 liquid nitrogen cooled germanium to the (111) c-Si diffraction peak , gives the average detector. The 514.5 nm line of an Ar laser was used for crystal size for Si nanocrystals presented in Table 2. In the the excitation. PL studies in the visible spectral range same table, there are also the data of Raman spectroscopy, were carried out under excitation with a 325 nm line of To analyse the Raman spectra, TO replica of amorphous cw He-Cd laser with a Spex 1704 monochromator and structure has been approximated by a Gaussian profile, and cooled Hamamatsu R928 photomultiplier in the detection the crystalline response analysis were performed on the chain. basis of Strong Phonon Confinement model .
Neir IR photoluminescence spectra measured at 77 K in nc-Si:Er samples are shown in Fig. 2. Let consider the highly Table 1. Growth conditions for erbium doped nanocrystalline crystalline samples according to XRD and Raman data (the silicon thin films samples Er24, Er19, Er22 with the nanocrystal sizes of 5.5–7.9 nm and crystallinity CR = 23-65%). The spectra Sample Temperature, C RF power, W RH of these samples show the luminescence peak at 1.54 µm Er22 400 80 0.63 related with the itra-atomic (4I13/2 I15/2) transitions of Er19 200 80 0.Er3+ ions. Being relatively broad in low crystalline sample Er24 50 80 0.(sample Er24) with the maximum at 6500 cm-1 and a Er28 200 150 0.characteristic shoulder at around 6457 cm-1, like for Er in Er33 25 80 0.a glass-like and amorphous materials , the Er-related spectrum transforms in highly crystalline films (sample RH = pH2/(pH2 + pAr) is the hydrogen fraction.
8 Физика твердого тела, 2004, том 46, вып. 116 M.V. Stepikhova, M.F. Cerqueira, M. Losurdo, M.M. Giangregorio, E. Alves, T. Monteiro, M.J. Soares Table 2. Elements content, thickness and structural parameters of nc-Si:Er samples Sample Er, % Si, % O, % H, % d, nm DX, nm DR, nm CR, % SE data Er22 0.10 71.7 8.8 17.6 2089 7.0 7.9 65 87% µc-Si Er19 0.12 62 34 23 483 5.7 6.5 43 31% µc-Si Er24 0.17 56.5 17.6 25.4 538 3.9 5.5 23 25% µc-Si Er28 0.11 60.9 2.9 34.3 1295 - - 0 10% nc-Si Er33 0.02 73.4 < 1 25.8 1499 - - 0 38% nc-Si D — average crystal size, R — Raman spectroscopy, X — XRD analysis, CR — crystalline volume fraction determined by Raman spectroscopy, d — film thickness.
Er22) in the spectrum with a fine line structure (see the The most intense Er-related PL was observed in low inset A in Fig. 2) giving the evidence for the incorporation crystalline samples determined as „amorphous“ accordind of Er ions in regular crystalline surroundings. At the same to the Raman and XRD analysis (samples Er28 and Ertime, in the spectra of highly crystalline samples appear in Table 2). In particular, the PL intensity of the sample and increase with the crystallinity the lines at 7500 and Er33 exceeds that for highly crystalline sample Er22 by 9435 cm-1 that could be assigned by their energetic position about two orders of magnitude (see the insert in Fig. 2), as the defect-like and excitonic transitions in Si crystallites. this is despite the lower Er content. These samples show However, one can see that the increase of crystallinity in the strong luminescence at room temperature (Fig. 3). The these samples results in the strong quenching of Er related PL intensity decreases only 5-fold when going from 77 to photoluminescence. Though the samples have a similar 300 K with the deactivation energies of 154 and 170 meV.
Er atomic percentage (0.1–0.17%, as estimated by RBS), It seems that it would be difficult to explain thas dramatic PL intensity in them reduces by more than an order of increase of the luminescent efficiency arising upon the magnitude with the increasing of crystalline fraction from transition from crystalline to amorphous film structure even 23 to 65% (and crystallite sizes from 5.5 to 7.9 nm, see the because of the strong dufference in the excitation cross insert B in Fig. 2).
sections for Er ions in these two matrixes. It is known Figure 2. PL spectra of nc-Si:Er samples with the different content of crystalline fraction. Inserts show: A — enlarged Er-related region in the PL spectrum for the sample Er22; B — correlation between the PL intensity (Er-related peak) and the presence of crystalline fraction in nc-Si:Er samples. Right part of the insert (B) shows the situation for highly crystalline samples according to XRD and Raman analysis (the CR and DR values on the bottom and top axis correspond to the Raman data). On the left part of the insert (B), PL intensities for the samples with the nanocrystallite fraction with 1–3 nm crystallite grain sizes are presented, where the values for nc fraction are taken from SE data. The luminescence intensities at 1.54 µm in this insert are normalised to the films thickness.
Физика твердого тела, 2004, том 46, вып. The role of microstructure in luminescent properties of Er-doped nanocrystalline Si thin films cannot exclude the role of nonradiative recombination channels in these composite structures. So, we can image that the enlargement of crystalline fracton will enhance the nanocrystalline interactions in films and therefore the probability for excitons to propagate in crystalline network and recombine nonradiatively. But in fact we didn’t observe any direct evidence for the strong influence of nonradiative processes on the films luminescent efficiency. There is no strong correlation between the amount of hydrogen in films and their luminescent properties (as a rule, hydrogen passivates the dangling bonds in amorphous/crystalline tissue thus untensifying PL). Even opposite, we have observed the increase of films PL intensity after annealing at 500C for 5 hours, a procedure depleting the material with hydrogen. Moreover, the presence of oxygen in films, an element known as an „activator“ for Er ions, doesn’t influence also noticeably the films luminescence (see Table 2).
One can assume that the luminescence of Er ions in „amorphous“ samples is activated by the nanocrystals with very small grain sizes (< 3nm). The first indication for the presence of these nanocrystals in films has been obtained by the spectroscopic ellipsometry studies. SE data predict Рис. 3. Temperature dependencies of Er-related PL intensity the presence of small nanocrystals in samples Er28 and obtained in nc-Si:Er thin films (samples Er28 and Er33). The solid Er33 in amount of 10 and 38%, respectively (SE analysis lines are exponential fits of experimental data with the deactivation of the nc-Si thin films with the small nanocrystal grain energies of 170 and 154 meV, for the samples Er33 and Er28, respectively. The insert shows PL spectra of the samples at room sizes were discussed in details in , see also the models temperature.
in Fig. 4, b). Indeed, this prediction can be confirmed by HRTEM measurements. A cross-sectional HRTEM image obtaines for the sample Er33 is presented in Fig. 4, a). The that the excitation cross section for the direct excitation of micrograph shows that the a-Si:H films matrix contains a Er ions in amorphous matrixes is by about five orders of high density of small clusters that have been identified, on magnitude less then that for Er in crystalline Si (8 · 10-21 the basis of the electron diffraction analysis, as the silicon and 3 · 10-15 cm2, respectively [12,13]). Of course, one nanocrystals. The lattice fringes corresponding to the (111) Рис. 4. HRTEM image obtained for the sample Er33 (a) and structural models of the samples Er33 and Er22 derived from SE analysis (b).
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