Observation of Size Dependent Structural Defects in Silicon Nanowires

Abstract

Summary form only given. The controlled synthesis of defect-free Si nanowires with small diameters is a prerequisite to realize Si nanowire based electronics and photonics devices. The crystallinity of vapor-liquid-solid (VLS) grown nanowires can be very good and some times termed as defect-free. Here, the authors report the size dependent defects in Si nanowires grown epitaxially in as a preferred direction. Si nanowires with small diameter were found defect-free while nanowires with large diameters have large number of multiple twin defects. Such defects can deteriorate the performance of optoelectronic nanowire devices. These investigations provide deep understanding about defect formation mechanism and strategies to control the crystallinity of Si nanowires grown by VLS-technique. Si nanowires with diameters 12 plusmn 5 nm and 20 plusmn 5 nm were grown from Au droplets 9 plusmn 5 nm and 12 plusmn 5 nm respectively prepared on H-terminated Si(lll) with resistivity 1-1.2 Omega cm. The high density growth of Si nanowires was carried out in low pressure chemical vapor deposition (LPCVD) chamber by typically exposing the substrates covered by Au droplets to 1 sccm of Si2H6 and 49 sccm of H2 at a pressure of 3 Torr at 350degC. Scanning electron microscopy (SEM) and tunneling electron microscopy (TEM) has been employed to analyze the samples. TEM image and Fast Fourier Transform revealed that is predominant growth direction of SiNWs synthesized by VLS-mechanism. Si nanowires with large diameters about 20 nm have multiple twin defects that cut across the nanowires in exactly perpendicular to the growth direction. These twin defects extending down the entire length of the nanowires induce micro-faceting or zig-zag appearance of the nanowire surface. It clearly reveals that these defects are twins and not dislocations or crystal imperfections. Si nanowires with diameter about 10 nm and 6 nm respectively are defect-free and have single crystal core. We have shown that twining in Si nanowires is size-dependent. Nanowires with small diameters grown at low temperature are defect-free and suitable for electronics and optical applications. It shows that low temperature growth process is essential to fabricate high crystal quality Si nanowires with small diameters.