Opto-structural studies of well-dispersed silicon nano-crystals grown by atom beam sputtering
© Saxena et al.; licensee Springer. 2012
Received: 20 July 2012
Accepted: 22 September 2012
Published: 3 October 2012
Synthesis and characterization of nano-crystalline silicon grown by atom beam sputtering technique are reported. Rapid thermal annealing of the deposited films is carried out in Ar + 5% H2 atmosphere for 5 min at different temperatures for precipitation of silicon nano-crystals. The samples are characterized for their optical and structural properties using various techniques. Structural studies are carried out by micro-Raman spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy (TEM), high resolution transmission electron microscopy, and selected area electron diffraction. The optical properties are studied by photoluminescence and UV-vis absorption spectroscopy, and bandgaps are evaluated. The bandgaps are found to decrease after rapid thermal treatment. The micro-Raman studies show the formation of nano-crystalline silicon in as-deposited as well as annealed films. The shifting and broadening in Raman peak suggest formation of nano-phase in the samples. Results of micro-Raman, photoluminescence, and TEM studies suggest the presence of a bimodal crystallite size distribution for the films annealed at higher temperatures. The results show that atom beam sputtering is a suitable technique to synthesize nearly mono-dispersed silicon nano-crystals. The size of the nano-crystals may be controlled by varying annealing parameters.
The discovery of light emission in porous silicon in past decades  stimulated the research interest in the development of silicon nano-crystals dispersed in insulating matrix preferably silicon oxide. This is due to the fact, that it is one of the promising systems for silicon-based optoelectronic devices compatible with existing technology . Silicon nano-crystals embedded in insulating matrix have various advantages like robust, stable, and luminescent. They may be utilized in photovoltaic applications , charge storage devices [4, 5], light emitting diodes , laser , and for biomedical applications . The full compatibility of this system with CMOS technology extends its possibilities for fully integrated optoelectronics, high–bandwidth intrachip and inter-chip connections , and non-volatile semiconductor memories . The light emitting properties, in particular the efficiency and the wavelength, depend on size as well as size distribution. For the fabrication of optical devices from low dimensional structures, one needs to have a precise control on the size, size distribution, and dispersion.
Many attempts have been made by researchers to synthesize luminescent nano-crystals embedded in oxide matrix [10–17]. Among these approaches, formation of non-stoichiometric silicon oxide has been investigated using various techniques [12–17] followed by some activation [18–24]. The system decomposes into pure silicon nano-crystalline phase and more stoichiometric silicon oxide during phase separation. Some groups have used electrical mobility analysis methods such as differential mobility analyzer together with pulsed laser deposition (PLD) technique to collect the classified particles of nano-size . In PLD, micron-sized particles may be ablated from the material and are deposited on the substrates as debris or droplets. Systematic studies are needed to optimize the parameters for the growth of dispersed and luminescent silicon nano-crystals using new and different methods.
Generally, rf or rf magnetron sputtering is used for the synthesis of multilayer/super-lattice  or co-sputtering of silicon and SiO2. The atom beam sputtering (ABS) has several advantages over conventional rf sputtering, viz., 2″-diameter wide source of beam, substrate rotation, and less heating of the target material during deposition results in better uniformity of the films. In rf magnetron co-sputtering process, there is higher sputtering from a narrow circular area due to the presence of magnetic field that leads to the non-uniformity in the samples for large number of samples. Warang et al.  investigated the effect of rapid thermal annealing (RTA) on silicon-rich silicon oxide films grown by ABS with two different compositions. They observed the formation of amorphous nano-clusters after RTA up to a temperature of 900°C in N2 environment for 1 min.
In this letter, we report synthesis of highly luminescent and nearly mono-dispersed silicon nano-crystals in silicon oxide matrix grown by atom beam stuttering followed by RTA. Multi-peaks are fitted in photoluminescence spectra using Gaussian function to study the shifting in emission peak and full width at half maxima (FWHM) as a function of annealing temperature. The different studies are used to optimize the conditions for highly luminescent and nearly mono-dispersed silicon nano-crystals.
The ABS set-up used for this work has been designed, developed, and installed at Inter University Accelerator Centre (IUAC), New Delhi, India . The sputtering target used here is a fused silica disk of 3″ diameter with pieces of silicon (100) glued on it, covering an area of approximately 60%. The deposition is carried out on silicon (100) wafer, optical grade quartz, and carbon-coated Cu grid for different studies. Prior to deposition, the chamber was evacuated to a pressure of about 2 × 10−6 mbar which became 1.5 × 10−3 mbar during the sputtering process. The thickness of the films on silicon and quartz substrates is kept approximately 100 nm. The thickness on TEM grids is approximately 30 nm achieved by controlling the deposition time. The films on each substrate are then subjected to RTA in Ar + 5% H2 environment for 5 min at temperatures ranging between 800°C and 950°C at a step of 50°C.
The samples are characterized for their optical and structural studies. The samples deposited on silicon substrate are investigated by Fourier transform infrared spectroscopy (FTIR) measurements taken using Thermo Nicolet NEXUS 670 FT-IR with a resolution of 4 cm−1 (Thermo Fisher Scientific, Waltham, USA). The micro-Raman spectroscopy is carried out using Renishaw Invia Ramanmicroscope (Renishaw plc, Gloucestershire, United Kingdom) with 514-nm excitation wavelength of an Ar-ion laser. Photoluminescence (PL) spectroscopy studies are carried out at room temperature using HORIBA Jobin Yvon LabRAM 800 HR (NJ, USA) with excitation wavelength at 488 nm from Ar+ ion laser. The samples on optical grade quartz substrates were analyzed by UV-vis absorption spectroscopy (Hitachi 3300 UV/visible spectrophotometer; Hitachi High-Technologies Corporation, Tokyo, Japan). The transmission electron microscopy (TEM), high resolution transmission electron microscopy, and selected area electron diffraction (SAED) studies were carried out using Tecnai G20-stwin microscope (FEI Company, Shanghai, China) operating at 200 kV equipped with LaB6 filament and a charge-coupled device camera having a point resolution of 1.44 Å and line resolution of 2.32 Å.
Results and discussion
Trwoga et al.  developed a model to study the dependence of PL peak parameters on the size distribution of silicon nano-clusters. They have used effective mass approximation model to estimate the bandgaps of silicon clusters over the range of 2 to 8 nm. They have shown that the PL peak broadens and deviates significantly from Gaussian distribution as the size distribution of the clusters increases. It was previously reported that the size of the silicon nano-crystals could be described by lognormal distribution . In view of these two observations, the sudden reduction in FWHM of PL spectrum of sample treated at 850°C indicates a narrow size distribution, which is confirmed by our lognormal size distribution analysis of TEM image showing nearly mono-dispersed particles distributed uniformly throughout the sample.
Optical energy bandgaps for different samples
Optical energy bandgapEopt(eV)
RTA at 800°C
RTA at 850°C
RTA at 900°C
RTA at 950°C
The bandgap for as-grown film is 1.76 eV, which may be due to presence of very small size of nano-crystals in the film. The bandgap decreases after rapid thermal annealing as the particles grow in size. These results are in agreement with our photoluminescence results and TEM analysis.
Thin films of silicon-rich silicon oxide are deposited by wide source ABS technique. Formation of silicon nano-crystals takes place after RTA of these films. TEM image analysis shows that at 850°C, there are isolated and nearly mono-dispersed nano-crystals embedded in the oxide matrix. The sample treated at 850°C shows intense and narrow luminescence. The results of optical bandgaps, photoluminescence, and TEM support the observation of formation of silicon nano-crystals embedded in oxide matrix.
NS is the research associate. DKab and DKan are scientists at Inter University Accelerator Centre, New Delhi, India. PK is a research scholar at the Department of Physics, Bareilly College, Bareilly, India.
The authors are thankful to Prof. B. R. Mehta, Indian Institute of Technology, Delhi, India for HRTEM studies. The assistance provided by Dr. Dinesh Agarwal, IUAC, New Delhi, India during thin film deposition is highly appreciated. The help received from Dr. Fouran Singh and Dr. Vinod Kumar, IUAC, New Delhi, India in Raman measurements is gratefully acknowledged. The authors are also thankful to Ms. Reema Gupta, University of Delhi, Delhi, India for the photoluminescence measurements.
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