Quantum size effects and tunable visible photoluminescence in a-Si:H/nc-Si:H superlattices
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Quantum size effects are commonly observed in semiconductor nanocrystals and quantum dots. Here, we demonstrate unexpected quantum size effects in an unusual bulk system with multiple interfaces, consisting of alternating layers of nanocrystalline silicon (nc-Si:H) and amorphous silicon (a-Si:H) material thin films. The nc-Si:H layers consist of silicon nanocrystals embedded in an amorphous matrix, with an amorphous-crystalline interface separating the two structures. Plasma-enhanced chemical vapor deposition was utilized to grow nanocrystalline-amorphous silicon superlattices with a varying thickness of the nanocrystalline layer. Strong visible photoluminescence at room temperature was deconvoluted into individual peaks. As the nanocrystalline silicon layer thickness was increased from 5 to 20 nm, the photoluminescence spectra red-shifted with the emission wavelength varying as d2 (d is the size of the nanocrystallites), the characteristic signature underlying quantum size effects. The size d of the nanocrystals was estimated by the measured shift of the Raman peak, and could be tuned by varying the thickness of the nc-Si:H layers. High resolution transmission electron microscopy show nanocrystals with a narrow size distribution, in an amorphous matrix. We also observe long wavelength photoluminescence from interfacial states that leads to persistent photconductivity. Nanocrystalline-amorphous superlattices offer a unique pathway for synthesizing embedded nanocrystals with controlled sizes and photonic signatures.
Financial support for fabricating rf-PECVD system was received from Department of Science and Technology (DST) [Grant No. DST/TM/SERI/2K11/78(G)]; and Defence Research and Development Organization (DRDO) [Grant No. ERIP/ER/0900376/M/01/1297], New Delhi, India. The study was sponsored by the Council of Scientific and Industrial Research (CSIR), New Delhi, India [80(0082)/13/EMR(II)]. One of the authors (Asha Yadav) acknowledges CSIR, New Delhi, India for the financial support. This work was supported (in part, R.B.) by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. The research was performed at Ames Laboratory, which is operated for the U.S. DOE by Iowa State University under contract [DE-AC02-07CH11358].
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