Efficient visible luminescence of nanocrystalline silicon prepared from amorphous silicon films by thermal annealing and stain etching
© Timoshenko et al; licensee Springer. 2011
Received: 14 December 2010
Accepted: 19 April 2011
Published: 19 April 2011
Films of nanocrystalline silicon (nc-Si) were prepared from hydrogenated amorphous silicon (a-Si:H) by using rapid thermal annealing. The formed nc-Si films were subjected to stain etching in hydrofluoric acid solutions in order to passivate surfaces of nc-Si. The optical reflectance spectroscopy revealed the nc-Si formation as well as the high optical quality of the formed films. The Raman scattering spectroscopy was used to estimate the mean size and volume fraction of nc-Si in the annealed films, which were about 4 to 8 nm and 44 to 90%, respectively, depending on the annealing regime. In contrast to as-deposited a-Si:H films, the nc-Si films after stain etching exhibited efficient photoluminescence in the spectral range of 600 to 950 nm at room temperature. The photoluminescence intensity and lifetimes of the stain etched nc-Si films were similar to those for conventional porous Si formed by electrochemical etching. The obtained results indicate new possibilities to prepare luminescent thin films for Si-based optoelectronics.
Research interest in all silicon (Si)-based light-emitting device (LED) is stimulated by observation of efficient light emission in structures of nanocrystalline silicon (nc-Si) in SiO2 matrix (see for examples ref. ). Porous Si (por-Si) formed by electrochemical etching (anodization) of crystalline Si (c-Si) is a bright example of nc-Si-based material with rather high (up to 10%) quantum efficiency of the light emission under optical excitation or electrical current injection (see for example ref. ). Since por-Si based LEDs are characterized by low stability, the search of new materials, which are based on nc-Si and exhibit efficient and stable luminescence, is still belonging to actual tasks of modern technology and physics of low-dimensional structures.
Recently nc-Si films with efficient photoluminescence (PL) in the visible spectral range were formed by wet chemical etching, i.e. stain etching (SE), of microcrystalline Si deposited by plasma-enhanced chemical vapor deposition (PECVD) in silane highly diluted by hydrogen . According to ref. , both as-deposited a-Si:H films and those after SE did not exhibit remarkable PL. This fact agrees with previously reported results . While a-Si:H can be crystallized by using conventional furnace annealing (see for example ref. ) or rapid thermal annealing (RTA) , as-crystallized films are usually characterized by high density of non-radiative defects and do not demonstrate remarkable PL properties . Furthermore, the furnace annealing (FA) of a-Si:H results in formation of polycrystalline Si films with rather large sizes of crystallites, which are typically far from the quantum confinement regime . In the present article, we report the preparation of photoluminescent nc-Si films by using RTA or FA of a-Si:H films followed by SE treatments in HF-based solution.
Experimental results and discussion
Preparation parameters of the annealed samples and data on the mean size and volume fraction of nc-Si obtained from the Raman spectra analysis.
Mean size of nc-Si (nm)
Volume fraction of nc-Si to amorphous Si (%)
The initial a-Si:H films and thermally annealed ones were investigated by means of the transmission electron microscopy (TEM), the reflection optical spectroscopy in the ultraviolet-visible-near-infrared (UV-VIS-NIR) region, Raman scattering and PL methods. The TEM analysis was done with a LEO 912 AB Omega electron microscope. A Perkin-Elmer Lambda 35 spectrophotometer was used for the reflectance measurements in the spectral range from 200 to 1100 nm. The Raman scattering was investigated by using a LabRAM HR 800 micro-Raman spectrometer with an Ar-laser at wavelength of 488 nm for the excitation. The PL was excited by a N2-laser (wavelength 337 nm, pulse duration 10 ns, repetition rate 100 Hz). The PL signal was dispersed by a 50-cm monochromator and was detected by a charge-coupled-device in the spectral range from 350 to 1100 nm. The PL transients were measured by a photomultiplier with a time resolution of about 100 ns. All optical studies were carried out at room temperature in air.
According to the TEM data (non shown), as-deposited a-Si:H films were amorphous and did not include voids or incorporations. The same films subjected to RTA or FA at temperatures about 900 to 950°C were found to consist of small (2 to 10 nm) nc-Si with volume fraction depended on the annealing time.
Figure 3 shows PL spectra of the annealed films after the SE treatment in HF:HNO3 solution. Similar spectra were obtained for another solution used for SE. The PL intensity increases for the films annealed for the longer time. The PL intensity of the annealed film after the SE treatment in HF:FeCl3:H2O solution was found to be two to three times higher than that for the sample treated in HF:HNO3. However the latter was characterized by better stability of the PL properties during illumination with excitation intensity above 100 W/cm2 for several hours. Similarly to the PL of porous Si electrochemically grown in HF:HCl mixtures  this fact can be explained by better passivation of nc-Si surfaces because of the oxidizing effect of chlorine ions. The stronger PL intensity of the nc-Si films after SE in HF:FeCl3:H2O correlates with the Raman spectroscopy data, which indicate the presence of small nc-Si (see Figure 1 and Table 1). Both observations can be explained by slower rate of the chemical dissolution of nc-S/a-Si composite in HF:FeCl3-based solution in comparison with that for HF:HNO3 mixture. Also the stronger PL intensity of the samples treated in HF:FeCl3-based solution agrees with the results reported for microcrystalline Si films prepared by PECVD .
The films of nc-Si with efficient PL were formed from a-Si:H by combining RTA and stain etching procedures and were investigated by means of the optical spectroscopy. An analysis of the Raman spectroscopy data showed that the mean size and volume fraction of nc-Si were depended on the preparation conditions. The obtained results demonstrate that the fast procedure of RTA followed by wet chemical etching can be used to obtain nc-Si-films with desired optical properties.
H: hydrogenated amorphous silicon
plasma-enhanced chemical vapor deposition
rapid thermal annealing
transmission electron microscopy
Authors are grateful to Dr. S.S. Abramchuk for the TEM measurements and Prof. A.G. Kazansky for stimulating discussions. The optical measurements were done by using equipment of the Center of User's Facilities of M.V. Lomonosov Moscow State University. The work was partially supported by the Ministry of Education and Science of the Russian Federation and by the Russian Foundation for Basic Research (Grant # 11-02-01342-a).
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