Oxygen Absorption in Free-Standing Porous Silicon: A Structural, Optical and Kinetic Analysis
Porous silicon (PSi) is a nanostructured material possessing a huge surface area per unit volume. In consequence, the adsorption and diffusion of oxygen in PSi are particularly important phenomena and frequently cause significant changes in its properties. In this paper, we study the thermal oxidation of p+-type free-standing PSi fabricated by anodic electrochemical etching. These free-standing samples were characterized by nitrogen adsorption, thermogravimetry, atomic force microscopy and powder X-ray diffraction. The results show a structural phase transition from crystalline silicon to a combination of cristobalite and quartz, passing through amorphous silicon and amorphous silicon-oxide structures, when the thermal oxidation temperature increases from 400 to 900 °C. Moreover, we observe some evidence of a sinterization at 400 °C and an optimal oxygen-absorption temperature about 700 °C. Finally, the UV/Visible spectrophotometry reveals a red and a blue shift of the optical transmittance spectra for samples with oxidation temperatures lower and higher than 700 °C, respectively.
KeywordsPorous silicon Thermal oxidation Kinetic analysis Structural transition Optical properties
Silicon, being the second most abundant element in the Earth’s crust after oxygen, is the base of nowadays microelectronics. However, all-silicon optoelectronic devices are still a challenger, since the bulk crystalline silicon (c-Si) has an indirect band gap of 1.1 eV, which inhibits efficient optical interband transitions. In 1990, L.T. Canham reported the first observation of a surprising visible photoluminescence at room temperature in porous silicon (PSi) , originated from its nanostructured c-Si skeleton. In consequence, as many other porous nanomaterials [2, 3, 4], PSi is expected to have a wide range of photonic and biologic applications, mainly arising from its tunable refractive index , biocompatibility  and large surface/volume ratio . Nevertheless, this large surface area makes PSi chemically unstable, and its oxidation is a common natural process. In fact, the storage of PSi samples in ambient air makes important changes in its electrical and optical properties [8, 9]. Porous materials can be thermally oxidized through different methods . Specifically, the thermal oxidation of PSi under controlled temperature and oxygen atmosphere shows an increase in the surface roughness at 200–400 °C and a decrease at 600–800 °C . Furthermore, the ac electrical conductivity of oxidized PSi reveals two power-law behaviors at mid and high temperatures, suggesting a two-stage oxidation picture . Also, PSi structural modifications during its formation have been investigated in situ by X-ray diffractions (XRD) revealing a lattice expansion , which is consistent with quantum mechanical calculations . In general, the PSi samples present narrow diffraction peaks and constitute a rare example of a porous structure inside a nearly perfect single crystal. In this paper, we report a multi-approach study of the thermal oxidation in PSi by correlating the oxygen-absorption process with their structural and optical properties. The results suggest the existence of an unusual temperature about 700 °C, where both an optimal oxygen absorption and a structural phase transition occur. In fact, the measured optical transmittance spectra obtained from thermally oxidized free-standing PSi samples seem to confirm this structural phase transition.
The PSi samples used in this study were fabricated by means of an anodic electrochemical dissolution of boron-doped p+-type (100)-oriented c-Si wafers with an electrical resistivity of 0.01–0.03 Ω cm in an electrolyte consisting of a mixture of HF (49%) and ethanol (99.9%) with a volume ratio of 1:2 . A gold film of 30 nm was deposited on the backside of c-Si wafers to ensure electrical conduction during the anodization. In the course of etching process, a dc electrical current of 25 mA/cm2 for 200 s was applied between the platinum electrode and the wafer backside contact. To remove the PSi layer from the wafer, an additional dc current of 376 mA/cm2 for 10 s was following applied, producing a free-standing PSi film with a thickness of 7.5–8.5 μm measured by optical and scanning electron microscopies. The electrolyte was recycled by a mechanical pump to remove air bubbles generated by the electrochemical reaction and to improve the homogeneity of PSi thin films, where the etched area was approximately 2.9 cm2. After etching, the samples were rinsed with ethanol, and those samples for optical measurements were put on slides of quartz.
The surface area of the samples was determined by the nitrogen-adsorption technique . These samples were fragmented into grains with an average diameter of 3 mm. The N2 adsorption–desorption isotherms were measured at 77 K using a Belsorp-Mini II gas adsorption analyzer from BEL Japan. Before the N2 adsorption process, samples were outgassed with a vacuum of 0.1 mbar at room temperature for 12 h. The specific surface area was calculated by means of the Langmuir and BET equations , while the pore diameters were estimated through the BJH model .
Later, these fragmented PSi samples were characterized by the thermogravimetric analysis (TGA) by means of a Q500HR thermobalance from TA Instruments. In particular, one of the samples was dynamically analyzed from the room temperature to 900 °C at a speed of 3 °C/min, within an O2 flux of 60 ml/min. Other 10 samples were isothermically studied at different temperatures with the same O2 flux. These samples were subsequently analyzed by the powder XRD, whose patterns were obtained from a Bruker AXS D8 Advance X-ray diffractometer using a Kα1 wavelength of 1.5406 Å. The microstructures were identified using the Joint Compounds Powder Diffraction Standards (JCPDS) database. In addition, the surface nanoscale morphology of the samples was examined by the Atomic Force Microscopy (AFM) using a scanning probe microscope (JEOL JSPM-4210) with a NSC12/50 silicon tip in the tapping mode.
On the other hand, the quartz-slide-mounted free-standing PSi samples were thermally oxidized at temperatures of 100, 300, 500, 700, 800 and 900 °C in an ultra-dry oxygen atmosphere for 30 min using a Carbolite horizontal electric tube furnace. The optical transmission spectra of these samples were recorded with a Varian Cary 100 UV–Vis double-beam spectrophotometer at room temperature and atmospheric pressure.
Results and Discussion
Conversely, the adsorption process would be understood from the same dynamic TGA, specifically, from the cooling process. In this part of the thermogram, the sample loosed around of 10 wt% between 800 and 30 °C, which may be associated with some kind of oxygen adsorption. Hence, the final weight increment was 44 wt%. However, considering that the PSi surface area was 285.7 m2/g and O2 ionic diameter was 2.8 Å, the final weight increase must be equal to 24.6 wt% if single monolayer absorption of O2 molecules was produced. Therefore, it should be assumed that oxygen is not only absorbed over the PSi surface, but also in its bulk as well. This result implies the existence of a diffusion process.
Coming back to the isothermal analysis, the different variations observed in the isotherms can be explained as follows. First, at short times, the oxidation process occurs over the PSi surface, and the sintering effect and the diffusion process do not interfere. It explains why at short times the isotherms present a uniform trend. However, the sample treated at 400 °C presented an atypical behavior at long times due to the sintering process. Around that temperature, the surface has reacted but the diffusion of oxygen is still a less important process. In other words, the sample treated at 400 °C has a smaller amount of surface area than that at 300 °C, decreasing the O2 absorbed. For higher temperatures, sintering must be enhanced. However, in these cases, sintering does not affect so importantly, since the diffusion process is becoming dominant. In the region of very high temperatures around 850–900 °C, a new equilibrium between surface absorption and desorption should be considered, since the oxygen desorption process is exponentially enhanced with the temperature, as stated in the Langmuir equation . Therefore, an optimal oxygen-absorption temperature at 700 °C is found in Fig. 2.
We have presented a multi-approach study of the oxygen absorption in p+-type free-standing PSi. By means of nitrogen adsorption, we were able to determine the average pore radius and surface area of the samples. These samples were also analyzed by the TGA technique, and both dynamic and isotherm analysis reveals the existence of an optimal oxygen-absorption temperature around 700 °C. Furthermore, the TGA analysis suggests a possible sintering process at 400 °C, which was elucidated by AFM structural observations. In fact, the red shift of optical transmittance spectra obtained from samples heated below 700 °C is in agreement with such sinterization, since growth of nanoclusters due to the sinterization leads to a diminish of the quantum confinement, in consequence a smaller optical band gap. It is important to emphasize that the temperatures found in this study could depend on the pore morphology and heating environment, for example, a sintering temperature of 1,000 °C is found in macroporous silicon  or 400 °C for mesopores as in this work.
Based on the XRD results, it could be assumed that O2 absorption on PSi occurs throughout different mechanisms. While the PSi samples formed by interconnected c-Si nanostructures are heated, part of these convert into amorphous and polycrystalline Si, if temperature is about 500 °C. At higher temperatures, the diffusion of oxygen in PSi becomes important, and these three Si structures (crystalline, polycrystalline and amorphous) absorb it, producing SiO2. The formation of crystalline SiO2 proposed by XRD data is consistent with the blue shift of optical transmittance spectra at 800 and 900 °C, since the optical band gap of crystalline SiO2 is about 8.4 eV. Finally, the results suggest that O2 absorption at low and high temperatures is respectively determined by sintering and diffusion processes.
We gratefully acknowledge the technical assistance of Adriana Tejeda and Carlos Flores, as well as many stimulating discussions with Adriana Cázares. This work was partially supported by ICyT-DF-179/2009, CONACyT-58938 and PAPIIT-UNAM (IN114008 and IN100609).
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