HWCVD-grown Silicon Nanocrystals: A study of the effect of annealing on structures evolved with varying growth rates
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HWCVD-grown Silicon Nanocrystals : A study of the effect of annealing on structures evolved with varying growth rates Prantik Mahajan1, Tarkeshwar C. Patil1 and Subhananda Chakrabarti1 1 Center for Nanoelectronics, Department of Electrical Engg, Indian Institute of Technology, Bombay, Powai, Mumbai – 400076, Maharashtra, India. ABSTRACT In this work, we investigated the evolution of Silicon Nanostructures with progressive annealing under different growth conditions. A structure – SixNy/a-Si/ SixNy/a-Si – was grown on n-type Silicon substrate using Hot Wire CVD (HWCVD) deposition technique. We report here the growth of Silicon Nanostructures by HWCVD technique with a special focus on the nature and morphology of the nanostructures with variation in growth rate and post-annealing temperature. AFM studies revealed promising results hinting at the presence of Silicon Nanostructures. With progressive annealing the morphology of the Nanostructures changed from particles to sharp pillars particularly in one of the samples elaborated in the text below. FWHM from the Confocal Raman data at room temperature was found to be 3.19 cm-1 with the a-Si peak at 520 cm-1. INTRODUCTION In the past few decades, Silicon Nanostructures fabricated by different processes such as PECVD [1 - 2], LPCVD [3], RTCVD [4], Pulsed-gas VHF plasma cell methods [5] , Micro plasma [6], MBE combined with oxidization by UV / ozone [7] have received considerable attention primarily because of their potential application in Optoelectronic devices viz. LASERs, LEDs, Photodetectors, Solar cells, etc. We study here an alternate approach where the Silicon Nanostructures evolve from a-Si films deposited at low temperatures. This can be accomplished by HWCVD process. This technique is often preferred over others, partly due to its cost-effectiveness but primarily because of its ability to ensure a controlled growth. The key behind the success of HWCVD in depositing exceptionally good films at low temperatures (< 500°C) lies in its efficiency in cracking the feedstock gases into atomic radicals at the surface of the hot (catalyst) filament (usually tungsten or tantalum) with its temperature maintained significantly above 1500 °C. The reactive species are then transported to the substrate under High Vacuum (HV) conditions (10-6 to 10-7 Torr) where they are adsorbed on the surface, thus facilitating a smooth deposition without gas-phase particle formation. EXPERIMENT In our experiments, we varied the deposition rate of Amorphous Silicon by cracking Silane for different time intervals. The thickness of the deposited Amorphous Silicon layer was kept constant in each case. The samples were then annealed at two temperatures, 800°C and
900°C respectively, in a quartz tube furnace in Ar ambient for 30 mins. The samples were characterized by AFM, Confocal Raman and Cross-sectional SEM techniques. A SixNy/a-Si/ SixNy/a-Si structure (4nm/3nm/4nm/3nm) was deposited on a RCA-cleaned oriented n-type Si-wafer (4-7 Ohm-cm resistivity, one-side polished, 20 cm2 ar
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