1. Introduction

Light emission and amplification in wavelengths around 1.53 µm are important since this is the wavelength band with minimum propagation loss in optical fibers and thus used heavily in communication systems. The Er emission lines fall exactly in this wavelength window and can be incorporated into silicon-compatible form to serve as efficient emitters, potentially integrated with electronics. This has been the main driving force for developing all types of Er-based materials such as Er-doped Si, SiO₂, and Si-rich SiO₂ [1–9]. Besides, Er-containing materials are also of great importance for other applications such as metamaterials [10], quantum information [11], and solid state lasers [12]. However the Er-concentration in Er doped materials is usually below 10¹⁹ cm⁻³. Such a low Er concentration is not enough to produce enough optical gain [13,14] for many applications, especially for chip-scale integrated systems. Typically, higher concentration Er doping leads to saturation or sublinear increase of 1.53 µm emission with increase of pumping [15–18], preventing such materials from being used for on-chip applications which requires high optical gain. Erbium compounds such as Erbium silicate (ES) are shown to be superior to Er-doped materials in terms of high Er-density [19–27]. ES material is typically produced by magnetron sputtering and has to be annealed at high temperature to re-crystallize. Here we report on the synthesis of a new Er-compound, the erbium chloride silicate (ECS, Er₃Cl(SiO₄)₂) single crystal nanowires. Light emission results show that the linewidth of 1.53 µm line is narrower compared to that of most other Er-materials. We believe that the ECS nanostructures provide an important alternative Si-compatible material platform for 1.53 µm emission and many other photonic applications. The nanowire form maybe especially suited for future nanophotonic systems.

2. Experimental

2.1 Nanowire synthesis

The single-crystal ECS nanowires were grown using Au-catalyzed chemical vapor deposition in a tube reactor. In a typical procedure, silicon powder (Alfa Aesar, 99.99%) in a ceramic boat was placed at the center of a 2-inch quartz tube, which was inserted into a horizontal furnace. The silicon substrate pre-sputtered with Au film was positioned downstream at a distance of 17 cm from the center of the furnace for the deposition of sample. Anhydrous ErCl₃ micro beads (Alfa Aesar, 99.9%, diameter ~1 mm) in another ceramic boat were loaded upstream and close to the substrate. The tube chamber was evacuated to a pressure below 100 mT and a constant flow of 50 SCCM Ar-H₂ 5% mixed gas was introduced as a carrier gas through the quartz tube. The pressure inside the quartz tube was adjusted to 400 mT with a valve. The center of the furnace was then heated to 1080 °C, and maintained at this temperature for 180 min. The measured temperature was ~600 °C for the growth substrate and ~650 °C for the ErCl₃ micro beads. After the growth, the furnace was naturally cooled to room temperature.

2.2 Characterization methods

The scanning electron microscopy (SEM) images and in situ energy dispersive X-ray spectroscopy (EDS) analysis were performed using a Philips XL-30 field-emission SEM equipped with an energy-dispersive X-ray detector. The transmission electron microscopy (TEM) images were collected with a JEOL JEM-2010 Hi-Resolution Transmission Electron Microscopy at 200 kV, equipped with a Link EDS detector. X-ray diffraction (XRD) data were collected on the PANalytical X’Pert Pro Materials Research X-ray Diffractometer equipped with a CuK_a radiation ( = 1.54178 A). Photoluminescence (PL) was conducted using a home build Near Infrared PL system. A Ti: Sapphire laser at 800nm was focused to the sample for optical excitation. The PL signal was focused to the entrance slit of a 0.3 m monochromator and detected by a liquid nitrogen cooled InGaAs array detector. Low temperature PL was carried out in a cryostat (Janis ST500).

3. Results and discussion

3.1 Structure characterization

The SEM image of the as-grown nanowires is shown in Fig. 1(a) . The wires have diameters from several tens of nanometers up to 400 nm, with their length from several to more than ten micrometer. The wires are composed of elements Si, Er, O and Cl, which can be seen from the corresponding EDS of the sample (inset of Fig. 1(a)). Figure 1(b) shows the XRD spectrum of the nanowire sample. The peaks marked with stars are well indexed with an orthorhombic crystal with lattice parameters of a = 0.682 nm, b = 1.765 nm, and c = 0.616 nm, and matched with data (JCPDS card: No. 00-042-0365) for Er₃Cl(SiO₄)₂. A unit cell of this crystal structure is also illustrated in the inset of Fig. 1(b), showing the atomic planes in [060] direction. The remaining peaks of the XRD spectrum are identified to those of cubic Silicon (JCPDS card: No. 00-026-1481). Thus XRD results show that the nanowires simultaneously contain ECS crystal and silicon.

 

Fig. 1 (a) SEM image of the as-grown Si-ECS nanowires and the corresponding EDS (inset), (b) XRD pattern of the Si-ECS nanowires and the crystal structure of ECS (inset).

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The microstructure of an individual nanowire was characterized by TEM. Figure 2(a) shows a typical low-magnification TEM image of a single wire with a uniform diameter of ~80 nm. The large bright-dark contrast variation between the middle and outer sections indicates that the wires consist of materials with different masses. The TEM-EDS spectra collected from the darker outer and lighter interior regions are shown in the inset of Fig. 2(a), respectively. Both spectra show peaks of elements Er, O, Cl, Si and Cu (from the copper grid), but the Si concentration in the interior is much higher than that in outer region. Figures 2(b)–2(e) show the respective two-dimensional element mapping of this wire for the detected elements O, Cl, Si and Er, respectively. A higher Si concentration was found in the inner core region than that in the outer stripes, whereas Cl, O, and Er elements show complimentary distributions. The combination of TEM image, EDS spectral mapping, and XRD indicates that the investigated nanowires are Si-ECS core-shell heterostructure, with the lighter Si in the core and the heavier elements of ECS compound in the shell.

 

Fig. 2 Si-ECS core-shell nanowire analysis: (a) TEM image of a representative Si-ECS core-shell nanowire. Insets: EDS collected at the shell and core region, respectively; (b)-(e) Two-dimensional element mapping of O, Cl, Si and Er, respectively; (f) HRTEM image at the core-shell interface of the core-shell wire; (g) The correspongding FFT pattern converted from the interface region as well as from a selected shell region (inset).

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This structure is further substantiated by the high-resolution TEM (HRTEM) investigations. Figure 2(f) shows the HRTEM image taken from the interface region between the core and shell with their interface sharp at the atomic scale. The core has a lattice spacing of 0.31 nm, consistent with the interplanar spacing of the Si {111} lattice planes. The measured lattice spacing of the shell is 0.29 nm which is in good agreement with the {060} interplanar distance of orthorhombic structure of ECS, corresponding to the strongest peak in the XRD pattern (see Fig. 1(b)). The fast Fourier transform (FFT) analysis in Fig. 2(g) further demonstrates both core and shell are of high-quality single crystalline materials. Furthermore, the relative orientations of the two materials can be determined from this FFT analysis. The inset of Fig. 2(g) is the FFT of the ECS shell which confirms the HRTEM image taken along the zone axis. The determined orientation of Si core is along the $[1\overline{1}0]$zone axis. Based on the FFT pattern, the parallel lattice planes and the lattice directions of the two crystal structures can be determined as follows:$[001]Si//[1\overline{1}2]ECS$,$[111]Si//[0.6441.28]ECS$, $[1\overline{1}0]Si//[20\overline{1}]ECS$. The relationship between the unit cell bases of the Si and the ECS can be deduced from the above orientations as follows:

3.2 Growth mechanism

After careful examination of TEM images of the as-grown sample, we found that the thickness of the shells differs for some wires with the same diameter. Especially interesting is that some Si wires are completely converted into pure ECS wires without Si core. Figures 3(a) and 3(b) show TEM images of such a nanowire. HRTEM images reveals the lattice spacing of 0.87 nm, corresponding to the {020} planes of ECS. Notice that this growth direction is the same as the [060] direction observed in the core-shell ECS structure, indicating the consistency of the growth. The existence of wires with different shell sizes and even pure ECS wires reflects the different growth stages of the ECS structures.

 

Fig. 3 (a) TEM image and the FFT (inset) of a pure ECS nanowrire, (b) HRTEM image of such a wire.

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Based on these experimental evidences, a two-step growth mechanism is suggested: Si wires are first grown via the Au catalyzed Vapor-liquid-solid (VLS) mechanism, which serve as templates for the subsequent growth of the ECS shells. Subsequently, the active surface of the pre-grown Si wires react with oxygen and the Er³⁺/Cl^- ions from the slow decomposition of the ErCl₃ micro beads, to form the ECS compound of the shells. Thus the outer layers of pre-grown Si wires convert into the Si-ECS core/shell heterostructures with the proceeding of the surface reaction.

To verify the proposed growth mechanism, a contrasting experiment was conducted. The experiment setup is shown in Fig. 4(a) . Comparing with the typical setup which was described in the experimental section, another Au coated quartz substrate was positioned between the Silicon powder and the anhydrous ErCl₃ micro beads. SEM image (Fig. 4(b)) shows that this substrate also has NWs growth besides the typical Si-ECS NWs on the second substrate. In situ EDS analysis (Fig. 4(c)) of these NWs confirmed that they are pure Si NWs without ECS shell. This result confirms that Si NW is formed for the given growth condition in the absence of the ErCl₃ source.

 

Fig. 4 (a) growth setup for the contrasting experiment (b) SEM image and the corresponding EDS spectrum (c) of the contrasting sample.

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We would like to emphasize that the proposed growth process is different from the reported Si-based heterostructures [28], where the pre-grown Si wires only serve as a physical template for the subsequent shell deposition, and the Si wires themselves are not chemically involved in the formation of the shells. Here the slow decomposition of the ErCl₃ micro beads into the Er³⁺/Cl^- ions is very important for achieving the ECS structure, leading to a step by step formation of ECS at the surface of the pre-grown Si nanowires. Comparative growth experiments with the ErCl₃ micro beads at a high temperature position or with very fine ErCl₃ powder as the erbium source, can only obtain Er doped Si nanowires but not the ECS compound, as reported by Huang et. al. [5]. There are reports of many semiconductor nanowires with multiple compositions including many of our own research work [29–31]. These nanowires are made of alloys of binary compounds. Similar to those wires, the basic growth mechanism is based on VLS processes. Unlike those nanowires, our ECS nanowires and the erbium silicate core-shell structure of [21] are not semiconductor wires and there have been very few of such wires reported. As we mentioned, the likely scenario of core-shell formation is the two-step process, but details remain to be further studied.

3.3 Photoluminescence

Light emission property is the most important aspect of Er-containing materials. Figure 5(a) shows the representative photoluminescence (PL) spectrum of the Si-ECS nanowires collected at 77 K, under the 800 nm excitation from a Ti-sapphire laser. The 1.53 μm band emission comes from the ⁴I_13/2 → ⁴I_15/2 intra-4f states transition of Er³⁺ ions. A series of sharp lines within this band is due to the Stark splitting of both the first and the ground state under the presence of crystal field. These lines are broadened but still well separated at room temperature (see the inset). Figure 5(b) shows the linewidth (black squares) of the 1.53 μm main peak of ECS at different temperatures by fitting the spectra with a multi-Lorentz function. It is noticed that the linewidth at 77K is only 0.8 nm. To our knowledge, this is the narrowest linewidth from erbium compounds. Such a narrow linewidth indicates high crystal quality of the material and large absorption and large emission cross sections, although within a narrower wavelength window. This is especially critical for getting a high optical gain [32]. Well separated sharp emission lines have been observed in erbium doped materials with very low erbium concentration [33]. However, the low Er concentration prevents these materials from achieving enough optical gain for actual application. With increase in Er doping concentration, the linewidth becomes broadened because different Er ions have more chance to feel different crystal fields in the hosting material. For example, the typical linewidth of erbium doped material with a concentration higher than 10¹⁹ cm⁻³ is more than 10 nm [18]. Due to the crystal nature of ECS, ideally Er compound should have very narrow linewidth at a very high erbium concentration. However, the poor crystal quality related to the existing erbium compounds, such as erbium silicate and erbium oxide [19–26,34], results in a much broader linewidth. The red dots in Fig. 3(b) represent FWHM data of other Er compounds taken from the literatures. The much narrower linewidth of our ECS material indicates very high crystalline quality of the as-grown ECS NWs, consistent with the HRTEM results.

 

Fig. 5 (a) Normalized near infrared PL band of the Si-ECS nanowires at 77K and at the room temperature (inset). (b) FWHM of the 1.53 μm peak at different temperatures (black squares); Red dots: the FWHM reported in the literature [19–26,34].

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4. Conclusion

In summary, a new Si based Er-containing compound in single crystal ECS nanowire form was realized in both core-shell and in solid wire forms using VLS growth mechanism. Structural characterizations demonstrated the high-quality single crystal nature of the as-grown ECS compound. PL spectra of the ECS nanowires exhibit well separated sharp emission lines at both low temperature and room temperature, with the 1.53 µm linewidth smaller than that of other Er-compounds. Such narrow linewidth is an indication of the high crystallinity of the materials. This new nanomaterial of high crystal quality with high Er-concentration is expected to find many applications in optical communication and nanophotonic devices.