Controlled fabrication and optical properties of one-dimensional SiGe nanostructures

Zilong Wu, Hui Lei, Zhenyang Zhong

Introduction Controlled Si and SiGe nanopillars/nanowires were fabricated on Si (001) substrate in a large area. By combining self-assembled nanosphere lithography, reactive ion etching (RIE) and metal assisted chemical etching (MACE), periodic Si nanopillars and nanowires with desired period, radius and height in large scales can be readily obtained. Defect-free SiGe coaxial quantum wells (CQW) were grown by molecular beam epitaxy (MBE) around Si nanopillars (Fig.1). The morphology of these nanostructures were characterized by scanning electron microscope (SEM), atomic force microscope (AFM) and transmission electron microscope (TEM), as shown in Fig.2-6.The photoluminescence (PL) of the SiGe CQW nanopillars was measure. Normally, the PL of SiGe quantum

State Key Laboratory of Surface Physics and Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education) and Department of PhysicsFudan University, Shanghai 200433, China

wells on flat substrates has only one peak. For our SiGe CQW nanopillars, an anomalous PL was observed, which is composed of four peaks (Fig. 7). Moreover, an abnormal excitation-power dependence of these four peaks was observed (Fig. 8). The light extractions from the nanopillar simulated using finite-difference time-domain (FDTD) simulations matches well with those of experiment, as shown by green curve in Fig. 7. These unique PL results are attributed to the coupling between the spontaneous emission of the SiGe CWQs and the Mie resonance (Fig. 9) in the nanopillars. By designing the radius of the SiGe CQWs nanopillars, the light emission can be easily modulated by the strong coupling between the spontaneous emissions and the Mie resonant. In addition, the light emission can be further managed by the photonic crystal band gap for two-dimensionally (2D) ordered pillars. Such unique features of the ordered SiGe CQW nanopillars make them have potential applications in optoelectronic devices.

The solar energy harnessing ability of the ordered Si nanowires was also investigated. The total reflectance spectra of ordered Si nanowire array with different geometrical morphologies were measured (Fig.10). Due to the little transmission of Si substrates with large optical thickness, the incident solar energy loss of the samples can be approximated by the reflection loss. The morphology influence on the total solar energy loss at AM1.5 was analyzed (Fig.11). For further understanding the experimental results, a plane-wave-based transfer-matric method (TMM) was performed to theoretically simulate the light harnessing ability of ordered Si nanowires. The optimal diameter of the ordered Si nanowires for best light harnessing ability was found (Fig.11). Our results also indicate that maintaining the straightness of nanowires is more crucial than increasing the length for better light harnessing ability (Fig. 11, Fig.12). The relation between solar energy loss and the diameter of the Si nanowire array is mainly dominated by the effective antireflection layer, of which the solar energy loss was calculated (Fig. 13).

Figure 10. (a) The reflectivity spectra of the ordered Si nanowires with various diameter. (b) The reflectivity spectra of the ordered Si nanowires with various length.

Figure 1. Schematic illustration of the fabrication of ordered Si nanopillars, (a) a close-packed monolayer of PS nanospheres on a clean Si substrate, (b) after RIE, (c) after Au deposition, (d) after the MACE, (e) after the removal of the Au and the remained PS nanospheres. The bold arrow in (e) denotes the general incident fluxes of Si and Ge atoms during the MBE growth.

Figure 2. (a) SEM image of PS nanospheres template after RIE. (b) SEM image of a typical Si nanopillar array after MACE.

Figure 3. (a) 2D AFM image of typical Si nanopillar array. (b) 3D AFM image of a typical Si nanopillar array.

Figure 5. (a) XTEM image of a single SiGe coaxial quantum well nanopillar. (b) HRTEM image of the region marked by a rectangle in (a).

Figure 11. (a) Dependence of solar energy loss caused by reflection of the ordered Si nanowires on diameter (black squares). TMM simulation results are also plotted (red circles). (b) Dependence of solar energy loss caused by reflection of ordered Si nanowires on length (black squares). TMM simulation results for straight nanowires (blue stars) and slightly bending nanowires (red circles) are also plotted.

Figure 8. (a) the Wavelengths, (b) the FWHMs of the four decomposed peaks as a function of excitation power for the SiGe coaxial quantum well nanopillars.

Figure 9. Electric field (|E|) distribution based on FDTD simulations of electromagnetic waves coupled with a SiGe coaxial quantum well nanopillar composed of a cylinder and a sphere on the top. The radius and the height of the cylinder are 160 nm and 1370 nm, respectively. The radius of the sphere is 200 nm with its center at 50 nm above the top of the cylinder.. Light is incident from left-hand side with an amplitude of 1 V m -1 for all wavelengths with the electric field perpendicular to the paper plane.

Figure 12. (a) Schematic of straight Si nanowires and slightly bending Si nanowire. (b) TMM simulated reflectivity spectra of straight Si nanowires and slightly bending Si nanowires, respectively.

Figure 7. The PL spectrum (red curve) of the SiGe CQW nanopillars at 16 K fitted by four Gaussian peaks (dashed blue curves) with the labels of 1,2,3,4, and the simulated intensity vs the wavelength of light extracted from the nanopillars by FDTD (green curve). The inset shows the schematic illustration of a nanopillar used in the numerical simulation.

Figure 4. (a) SEM image of a typical Si nanopillar array after MACE. (b) SEM image of the SiGe CQW nanopillar array after the growth of Si and SiGe alloy layer. The inset of (b) shows a side-view SEM image of a SiGe CQW nanopillar.

Figure 6. SEM images of ordered Si NW array with different height. (a) 0.54μm, (b) 2μm, (c) 3.35μm, (d) 4.59μm.

Figure 13. The solar energy of the effective antireflection layer of the ordered Si nanowire arrays with different fill factors, that is different diameters.

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