Effect of etching time on morphological, optical, and electronic properties of silicon nanowires
© Nafie et al.; licensee Springer. 2012
Received: 27 April 2012
Accepted: 8 June 2012
Published: 16 July 2012
Owing to their interesting electronic, mechanical, optical, and transport properties, silicon nanowires (SiNWs) have attracted much attention, giving opportunities to several potential applications in nanoscale electronic, optoelectronic devices, and silicon solar cells. For photovoltaic application, a superficial film of SiNWs could be used as an efficient antireflection coating. In this work we investigate the morphological, optical, and electronic properties of SiNWs fabricated at different etching times. Characterizations of the formed SiNWs films were performed using a scanning electron microscope, ultraviolet–visible-near-infrared spectroscopy, and light-beam-induced-current technique. The latter technique was used to determine the effective diffusion length in SiNWs films. From these investigations, we deduce that the homogeneity of the SiNWs film plays a key role on the electronic properties.
Silicon nanowires (SiNWs) have attracted much attention in the recent years due to their importance in the field of electronic devices and photovoltaic [1–4]. Hence, SiNWs could be used as an antireflection coating due to the reduction of optical loss which is an important factor to obtain efficient Si solar cells. However, when SiNWs are used as an antireflection coating, a great care should be taken to avoid degradation of the electronic properties, which in turn can increase the serial resistance of the solar cell. Different methods have been employed to fabricate SiNWs, such as chemical physical deposition , laser ablation [6, 7], thermal evaporation [8, 9], and etching. In this paper, we used the silver-assisted chemical etching technique [10–15]. We fabricate SiNWs at different durations, ranging from 10 to 90 min.
Substrates used in this study are P+ silicon wafers, boron-doped and (100) oriented, with thickness of 500 μm and resistivity of 0.01 to 0.02 Ωcm. After cleaning, silicon samples were immersed into the etching solution containing 0.05 M AgNO3, 40% HF, and H2O2 at room temperature for different etching times; 10, 20, 30, 40, 50, 60, 70, 80, and 90 min. After etching, samples were rinsed with deionized water to remove residual HF and immersed in a H2O-HNO3 (2 and 1 V) solution during several seconds to remove the silver film.
The morphology of samples was analyzed using a scanning electron microscope (SEM). We performed top and cross-section SEM images of the samples. The cross-section SEM images were used to evaluate the length of SiNWs. We measured the surface reflectivity in the 250 to 1,250 nm spectral range by a UV–vis-NIR spectrophotometer. To study the electronic properties of the formed films, we evaluate the effective diffusion lengths (L) of minority carriers in the SiNWs films. Values of L were carried out from the light-beam-induced-current (LBIC) profiles measured on metal/SiO2/SiNWs/c-Si/metal diode.
Results and discussion
In this study, we present a morphological, optical, and electronic study of SiNWs films elaborated at different durations; 10, 20, 30, 40, 50, 60, 70, 80, and 90 min. At some etching durations, a regular structure of formed SiNWs was observed. The SiNWs lengths vary from 21 to 38 μm. We notice a spectacular very low value of the total reflectivity reaching a minimum less than 1% in the 250 to 400 nm and a minimum of 1.5% in the visible domain. From LBIC investigations, we deduce that the homogeneity of the SiNWs film plays a key role on the electronic properties. Hence, we carried out that when the SiNWs film is inhomogeneous, surface recombination of photo-generated carriers can decrease the effective diffusion length.
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