Effect of Fluorine on the Lateral Crystallization of Amorphous Silicon Nanowires

Abstract

In this work, we investigate the effect of fluorine on the metal-induced lateral crystallization (MILC) of amorphous silicon nanowires. The MILC is characterized at temperatures in the range 550 to 450 • C using Nomarski optical microscopy and Raman spectroscopy. It is shown that fluorine significantly increases the crystallization length at temperatures of 550 and 525 • C and dramatically improves the uniformity of the crystallization length at temperatures between 550 and 500 • C. A fluorine implant improves the standard deviation of the crystallization length from 39.7% to 5.8% at a temperature of 550 • C, from 35.6% to 5.2% at 525 • C and from 30.6% to 15.9% at 500 • C. This improvement in the uniformity of the crystallization is explained by the suppression of random grain nucleation by the fluorine. A comparison is also made between MILC in α-Si nanowires and larger α-Si ribbons and sheets. The crystallization length is significantly longer in ribbons and sheets, but fluorine is again shown to give a significant improvement in the uniformity of the crystallization.Polycrystalline silicon (poly-Si) has been widely researched for application in flat-panel active-matrix liquid crystal displays, 1 activematrix organic light-emitting diode (AMOLED) displays, 2,3 thin film solar cells, 4 three-dimensional (3D) electronics, 5,6 and nanowire biosensors. 7-9 For low-cost applications such as displays, the use of low cost glass substrates requires polysilicon films to be produced at temperatures significantly lower than the typical deposition temperature of 625 • C. 10 Several techniques have been investigated including solid-phase crystallization (SPC), 11 excimer laser crystallization (ELC) 12-14 and metal-induced lateral crystallization (MILC). 15 MILC is particularly attractive for low cost applications and the typical anneal temperature for MILC is 500 • C to 550 • C. 16,17 Fluorine implantation into amorphous silicon has been shown to significantly increase the lateral crystallization length during MILC 18 and hence enables polysilicon films to be produced at lower temperatures. This behavior was explained by the action of fluorine in suppressing random crystallization in the amorphous silicon film. This mechanism also gave reduced metal contamination in the crystallized film. 18 Other benefits of fluorine have been reported in thin film devices, such as the passivation of grain boundary traps 19 and the improvement of the uniformity of device electrical characteristics. To date, no work has been reported on the effect of fluorine in Si nanowires.Our recent work 20 has demonstrated that poly-Si nanowires can be fabricated by top-down lithography, reactive ion etch and metalinduced lateral crystallization at temperatures down to 450 • C. This approach has the advantage over bottom-up self-assembly of producing nanowires in defined locations on a wafer with excellent control over the nanowire width. In this work, we investigate the effect of fluorine on the metal-induced lateral crystallization of amorphous silic...

Effect of an Oxide Cap Layer and Fluorine Implantation on the Metal-Induced Lateral Crystallization of Amorphous Silicon

“…The samples without an oxide cap are seen to give larger values of To further investigate the effect of the oxide cap layer, fluorine implanted samples are also studied as fluorine implant has been found to suppress random crystallization in amorphous silicon. 2,16 Fig. 5 shows optical Nomarski micrographs of fluorine implanted samples without (Fig.…”

“…6. Our earlier research 2,16 showed that fluorine enhances the crystallization length in amorphous silicon sheets and nanowires due to the suppression of random grain nucleation at the interface between the amorphous silicon and the underlying silicon dioxide layer. This mechanism also explains the results in Fig.…”

“…The crystallization length improvement for samples without an oxide cap layer is explained by the elimination of random grain crystallization at the interface between the amorphous silicon and the oxide cap layer. Metal-induced lateral crystallization (MILC) has been widely researched as an alternative to solid-phase crystallization (SPC) of amorphous silicon because of its lower process temperature and higher quality polysilicon (poly-Si) film with higher carrier mobility, larger grain size, and lower defect density.1 For MILC, nickel reacts with amorphous silicon (α-Si) to form nickel disilicide and the lateral transport of Ni induces crystallization of the adjacent amorphous silicon, 2,3 creating a crystallized region with low Ni contamination suitable for high performance transistors. 4 In most published work, the Ni is defined either by a lift-off process [5][6][7][8][9] or by Ni deposition in a window opened in a cap layer, which is normally deposited silicon dioxide.…”

“…1 For MILC, nickel reacts with amorphous silicon (α-Si) to form nickel disilicide and the lateral transport of Ni induces crystallization of the adjacent amorphous silicon, 2,3 creating a crystallized region with low Ni contamination suitable for high performance transistors. 4 In most published work, the Ni is defined either by a lift-off process [5][6][7][8][9] or by Ni deposition in a window opened in a cap layer, which is normally deposited silicon dioxide.…”