- Nano Express
- Open Access
Controlled morphology and optical properties of n-type porous silicon: effect of magnetic field and electrode-assisted LEF
© Antunez et al.; licensee Springer. 2014
- Received: 19 May 2014
- Accepted: 26 August 2014
- Published: 19 September 2014
Fabrication of photoluminescent n-type porous silicon (nPS), using electrode-assisted lateral electric field accompanied with a perpendicular magnetic field, is reported. The results have been compared with the porous structures fabricated by means of conventional anodization and electrode-assisted lateral electric field without magnetic field. The lateral electric field (LEF) applied across the silicon substrate leads to the formation of structural gradient in terms of density, dimension, and depth of the etched pores. Apart from the pore shape tunability, the simultaneous application of LEF and magnetic field (MF) contributes to a reduction of the dimension of the pores and promotes relatively more defined pore tips as well as a decreased side-branching in the pore walls of the macroporous structure. Additionally, when using magnetic field-assisted etching, within a certain range of LEF, an enhancement of the photoluminescence (PL) response was obtained.
- Lateral electric field
- Magnetic field
- Structural gradient
- Porous silicon
It is well known that a wide gamut of different morphologies of porous silicon (PS) can be obtained under a variety of different fabrication parameters. Generally, morphology is highly dependent on the intrinsic properties of the silicon substrate along with key fabrication parameters such as current density, hydrofluoric acid (HF) concentration, doping type, dopant concentration, and, in some cases, the illumination conditions . Moreover, PS formed using p-type silicon (p-Si) or n-type silicon (n-Si) have many differences in terms of morphological characteristics (i.e., pore size, degree of branching, and orientation) [2, 3]. Additionally, PS fabricated in the dark or under illumination exhibit different morphological properties . Although most of the PS photonic devices are produced on p-Si, for light-emitting diode technology and microelectronic applications, n-Si is preferred. On the other hand, control of the morphology is necessary when distinct structural characteristics are required on the same chip, i.e., samples with a structural gradient (in terms of density, dimension, and depth of the pores) which are widely used in biological applications as a porous media for the culturing of biological specimens [5–8]. Conventional methodologies have been well established for n-type porous silicon (nPS) fabrication, and the preferred method requires light-assisted etching (back/frontside) to photogenerate valance band holes necessary for silicon dissolution ; nevertheless, it is a depth-limited process (light cannot penetrate through to the bottom layers). Hence, some methods have been proposed to fabricate photoluminescent nPS under dark conditions (without illumination): Hall effect  and electrode-assisted lateral electric field . The former involves the application of mutually perpendicular electric and magnetic fields (within the range of 0 to 20 mT) resulting in photoluminescent nPS displaying a structural gradient in terms of thickness and light-emission properties along the lateral electric field (LEF) direction. On the other hand, the second method reports macropore formation using an electrode-assisted LEF (e-LEF) setup (30 to 50 V). Photoluminescent sample exhibiting minimum PS formation from the total effective area exposed to the HF electrolyte was obtained. However, the effects of the fabrication parameters on the resultant morphology of the sample along with the addition of a magnetic field (MF) under the e-LEF setup  have not been explored yet. In this work, we report on the resultant structural effect due to the simultaneous application of electric and magnetic field during the fabrication process and the morphologies thus obtained when one of those key parameters is varied. The corresponding changes in the photoluminescence (PL) properties have also been explored.
To study the effect of an increased lateral EF, a lateral potential of 50 V was also tested across a n-Si substrate accompanied by two different MFs (60 and 80 mT). Figure 4e, f shows the top and cross-sectional micrographs of nPS fabricated with a LEF of 50 V and a perpendicular MF of 60 mT (etching time of 3 min). Similar to the above-mentioned results, square-shaped macropores (approximately 1.5-μm width) were formed with a notably less-interconnected pore-to-pore distance (as compared with the sample prepared using the e-LEF, LEF of 50 V without MF; Figure 3c, d. Reduced square-shaped macropores of 600-nm average width were found close to the cathodic region due to the structural gradient caused by the application of LEF (50 V). However, presence of a higher MF of 80 mT during the fabrication process leads to a reduction in the width of the large square-shaped pores (approximately 1.2 μm) close to the anode location while the dimension of the pores reduces close to the cathodic region (750 nm), as shown in Figure 4g, h, respectively. Furthermore, the latter result showed a minimum pore size variation (of approximately 450 nm) in the reduction of the main width of the macropores (from 1.2 μm to 750 nm) formed along the LEF direction. Highly doped n-Si substrates under the e-LEMF configuration also resulted in a sponge-like structure (composed mostly of micro- and mesoporous features) irrespective of the lateral current and/or magnetic field (within 0 to 80 mT range) used during the fabrication process. Figure 4i shows the cross-sectional micrograph of the sample fabricated under a galvanostatic regime, a lateral current of 150 mA biased across the n++ sample accompanied with a MF of 60 mT (for 2.5 min). Sponge-like porous film of approximately 345 nm thick was obtained with the above-mentioned experimental conditions; inset of Figure 4i shows a top view image of the n++ sample. As the structural changes are governed by valence band holes available at the etching interface for the reaction, higher magnetic fields (close to 1 T) should be considered in order to observe significant structural changes using n++ substrates.
Structural gradient formation (i.e., density, dimension, and depth of the pores) is highly dependent on the LEF applied across the n-type silicon substrates. At a high lateral potential (50 V), major density of pores is obtained all over the sample as compared with the PS obtained with 30 V. As compared to the other reports  using an e-LEF setup, demonstrating the formation of PS only at the anodic region of the total effective area of the sample, our results achieved to form PS in the complete area exposed to the electrolyte, thus enhancing the optical properties of the samples. The combined effect of high LEF (50 V) and a perpendicular magnetic field is majorly observed as a relative reduction in side-branching and tapering ends of the macropores. Under particular fabrication parameters involving the joint contribution of a LEF of 30 V and a MF of 80 mT, a structural transition from square-to-round-shaped macropores appears close to the cathodic region of the sample, opening the possibility of tuning the structural properties of the PS structure. Enhancement of the PL response was achieved by using an increased MF during the fabrication process.
E.E.A. is a third year PhD student at the Research Center of Autonomous State University of Morelos, Mexico (CIICAp-UAEM). J.C. is a senior technician at the Energy Research Institute of National Autonomous University of Mexico (UNAM). M.B. is a scientist at CIICAp and UAEM. V.A. is a senior scientist working in the field of porous silicon and its applications at CIICAp and UAEM.
The work was financially supported by CONACyT project: Ciencias Basicas No.128953.
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