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ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 3 2023 ISSN 0005-2531 (Print)
UDC 546.3-126:544.2
FORMATION OF NANOSTRUCTURED TIN OXIDE FILM ON POROUS SILICON KB.Kim1*, A.S.Lenshin1' 2, F.M.Chyragov3, S.I.Niftaliev1
1 Voronezh State University of Engineering Technologies, Voronezh, Russia 2 Voronezh State University, Voronezh, Russia 3 Baku State University, Baku, Azerbaijan
Received 23.12.2022 Accepted 09.02.2023
Porous silicon is actively used in the fabrication of sensors and detectors because of its large specific surface area, which is an important characteristic for gas adsorption. To improve the operating parameters of the sensors and increase the stability of operation, a film of tin oxide was deposited on the substrate of porous silicon by vacuum-thermal evaporation. The choice of tin is due to its wide forbidden zone, low cost, and high sensitivity. Porous silicon was obtained by the electrochemical anodization of single-crystalline silicon KEF (100). The data on morphology, composition and optical properties of the initial sample of porous silicon and the sample with deposited tin have been obtained by scanning electron microscopy, infrared and photoluminescence spectroscopy. It was found that the chemical tin deposition on porous silicon leads to the formation of composite structure, which significantly prevents further oxidation of the porous layer during storage, and to the shift of the luminescence band maximum.
Keywords: porous silicon, composites, tin, thin films.
doi.org/10.32737/0005-2531-2023-3-83-89
Introduction
Porous silicon (por-Si) is a low-density, high specific surface area functional material consisting of pores and a silicon-based framework. Porous silicon is obtained by a simple and inexpensive method of electrochemical etching of silicon in hydrofluoric acid solution. Por-Si has unique properties such as variable chemical surface composition, high chemical activity and specific surface area, photoluminescence, and biodegradability [1]. Because of its properties, por-Si is widely used in optoelectronics, photovoltaics, and gas sensors [2-7]. Porous silicon-based sensors are a type of sensors that utilize the porous silicon structure to detect and measure various physical values, such as pressure, temperature, humidity, and gases, as well as to perform biochemical analysis [8-9]. Porous silicon has a large surface area, due to which it is effectively used to create sensors with higher sensitivity. Its pores can be filled with various substances or special reagents that interact with the analyzed substances, causing a change in conductivity or optical properties of the material, which allows the sensor to respond to certain chemical or physical changes. Applications for porous silicon-based sensors include medical diagnostics, environ-
mental, industrial, and safety applications. They can be used in air quality monitoring, gas control and detection, detection of biomolecules and other biological substances, and in optical and electronic devices [10, 11]. However, more research and development is required to improve their robustness, sensitivity, and reliability for use in a wide range of applications.
Modern sensors require continuous improvement to achieve the required sensitivity, selectivity, and reliability of operation, as well as short response times and long lifetime. Porous silicon (por-Si), which has a high specific surface area and resistivity as well as a reduced operating temperature of the sensor device, is used as a substrate [12, 13].
In some studies [14-16] porous silicon has been effectively utilized to fabricate integrated gas sensors operating at room temperature. High sensitivity and detection limit for a large number of gases (nitrogen oxides, hydrogen sulfide, hydrocarbons, alcohols, ketones, etc.) [17-19] can be explained by the developed surface of porous silicon, which provides a stronger interaction between the sensor and gas molecules. To optimize the characteristics of porous silicon-based sensors, studies have been conducted to improve their optical and electrical properties. The au-
thors [20-22] considered the methods of oxidation, hydrosilylation, carbonization, and metallization of porous silicon, which make it possible to increase the reliability, sensitivity, and selectivity of sensors, as well as to slow down the processes of "aging" of the surface.
Porous silicon with deposited metal-locoside film is an interesting material that can be used in a variety of sensor applications. It combines the advantages of porous silicon, such as a large surface area and high sensitivity, with the unique properties of metallocosides. The deposition of metals on the porous silicon layer can significantly improve the performance of sensors [23-24]. A metallocoside film can be deposited on the porous silicon surface using various methods such as sputtering, vapor deposition, or chemical methods. This film can consist of various metallocosides, for example, metal oxides such as titanium oxides (TiO2), zirconium oxides (ZrO2), aluminum oxides (Al2O3), and others [25-26]. Coating porous silicon with a metallocoside film can improve its chemical stability, mechanical strength, and selectivity toward the analyzed substances. In addition, such a coating can modify the optical, electrical, or catalytic properties of the material, which opens new possibilities for the development of sensor devices with improved characteristics.
Applications of porous silicon with a deposited metallocoside film can vary depending on the type and properties of the metallocoside.
However, for the successful application of this material in sensor devices, further research is required to optimize and control the film deposition processes and to understand the interactions between porous silicon and metallocoside.
Semiconducting metal oxides have certain physical and chemical properties that allow them to be used in gas sensors, conductive coatings, etc. [27-29].
In [30], sensors based on porous silicon and WO3 were obtained. It was found that the tungsten oxide sensor on porous silicon has good response and recovery characteristics for NO2 at room temperature. Devices based on porous silicon with a sputtered copper (II) oxide film [31] showed high nitrogen (IV) oxide capture capability. The authors [32] obtained a composite of porous silicon with V2O5 synthe-
sized by heating a film of pure vanadium on porous silicon. The por-Si: V2O5 structure exhibits perfect reversibility, a high response value, and good selectivity toward NO2 at room temperature.
One of the materials used for the formation of nanostructures is tin dioxide (a wide bandgap n-type semiconductor) used as active layers in multisensor microsystems, UV detection systems, electrodes of lithium-ion batteries, electrocatalysts, as well as in polymer solar cells [33-35]. Tin is highly conductive to electricity; therefore, it can be used in electronic or microelectronic devices, including sensors and semiconductor components. It also has chemical resistance and can protect porous silicon from oxidation or interaction with the environment.
The method of deposition of tin dioxide on porous silicon substrate usingthe photoelec-trochemical method at different temperatures is well known [36]. Such composites are used for the fabrication of toxic gas sensors. The authors [37] obtained samples of por-Si:SnO2 nanocom-posites for use in H2S gas sensors. The fabricated sensor with a tin layer deposited on porous silicon has higher sensitivity than the initial porous silicon, mainly due to the creation of p-n hetero-junctions and the presence of broken bonds in the interfaces between Si and SnO2 [37].
Vacuum evaporation followed by thermal oxidation is a simple and effective method to fabricate smooth, dense, and controllable thin films over a large area. Depositing a tin film on porous silicon using vacuum thermal sputtering is one coating method that can be applied to create a heterostructured material. Vacuum thermal sputtering involves heating tin to a temperature at which it vaporizes and condenses on the surface of porous silicon. This method provides good adhesion between the tin film and the porous silicon surface because the materials are in contact at the atomic level. In addition, vacuum sputtering can achieve a uniform distribution of tin on the porous silicon surface and create a film with the desired thickness. In general, vacuum thermal sputtering of tin on porous silicon is an effective method for obtaining a material with unique properties and potential applications in electronics, sensors, and other fields [38-40].
Experimental part
In this work, film samples of the por-Si-Sn system were prepared by thermal vacuum sputtering through a mask on a VUP-4 unit. Porous silicon (KEF 0.2 Ohm-CM , with orientation <100>) and tin of VHF-000 grade were used for
sputtering. Sputtering was performed on a "cold" porous silicon substrate, which was obtained by electrochemical anodization (Figure 1) in an electrolyte based on hydrofluoric acid and iso-propyl alcohol [40].
Fig. 1. Scheme of electrochemical etching of porous silicon samples: 1 - cathode, 2 - anode, 3 - stainless steel electrode, 4 - electrolyte solution.
The pressure of residual gases in the chamber of the unit was of the order of 5-10-510-4 mmHg. The thickness of the metal film was ~200 nm, and the sputtering rate was 3-5 nm/s. Comparative analysis of the samples was carried out using scanning electron microscopy (microscope JSM-6380LV with microanalysis attachment). Data on chemical bonds and their possible deformations on the surface of por-Si samples were obtained by IR spectroscopy (IR Fourier spectrometer Vertex 70 (Bruker) with the ATR attachment).
Luminescence measurements are one of the most informative sources of analytical data. Photoluminescence spectroscopy is a highly sensitive and widely used nondestructive method for the diagnostics of quantum-dimensional structures, identification and control of the content of impurities and defects in semiconductors, and registration of the spectral characteristics of optical radiation sources and receivers. FL spectra were measured on an automatic spectral-luminescence complex with an MDR-4 monochromator. To excite photoluminescence, a laser with an emission wavelength of 405 nm
was used. All studies were conducted six months after the samples were obtained.
Results and discussion
Figure 2 shows SEM images of cleaved facets of the initial porous silicon and samples with chemically deposited tin. The microanaly-sis data confirmed the presence of tin on the wafer surface. The thickness of the porous layer of the samples was about 10 p,m, the thickness of the tin island film did not exceed 100-150 nm, with an average diameter of tin particles on the surface of ~200 (nm).
Transmission spectra of the samples of porous silicon and porous silicon with deposited tin are presented in Figure 3. The IR transmission spectra of porous silicon samples after 30-day atmospheric storage under laboratory exposure conditions exhibit features characteristic of this material. Analysis of the por-Si spectrum allows us to conclude that there is a main bandwidth corresponding to Si-Si vibrations (616 cm-1) and various configurations of Si- Hx bonds (625, 2084, 2200 cm-1) and bonds of the Ox-SiHy (865 cm-1) and O3-SiH (900 cm-1)
types. In the wave number range of 2500-4500 cm-1, almost no peculiarities were observed in the spectra of the samples. The absorption bands in the region of 2360 cm-1 correspond to adsorbed CO2.
The IR spectrum of the sample with deposited tin, in general, is similar to the spectrum of the substrate of the initial crystalline silicon (Figure 3b) and shows much less pronounced features in the same regions as in the initial porous silicon; there are practically no absorption band corresponding to Si-O-Si bonds and bands characteristic of Si-Hx, Oy-Si-Hy bonds.It is noteworthy that in porous silicon Si-Hx, Oy-Si-Hy type bonds during storage of the samples actively participate in oxidation processes, be-
ing replaced by Si-O-Si type bonds, and lead to changes and degradation of various functional characteristics of these structures. When studying the sorption kinetics of porous silicon in air, it was noted that after 30 days its oxidation was observed. In the case of porous silicon with precipitated tin, however, such changes occur to a much lesser extent. Therefore, their absence after 30 days of exposure of the samples to the atmosphere may indicate a significant stabilization of the composition and surface properties of the composite under the assumed further change in the functional characteristics of the initial porous silicon in the process of natural aging.
A
B
C
Fig. 2. SEM images of the surface and the porous silicon cleaved facet with deposited tin: a cleaved facet.
surface, b, c -
A B
Fig. 3. IR spectra of porous silicon and por-Si with deposited tin (a), as well as spectra of standards (b).
Fig. 4.PL spectra of the samples of the initial por-Si and the composite por-Si:Sn.
The photoluminescence (PL) spectra of por-Si samples are shown in Figure 4. The high-intensity PL peak is located in the region from 600 to 750 nm. For porous silicon samples with tin particles, the peak shifts to the region from 550 to 650 nm. Since the sample of initial por-Si was also in the chamber during the composite formation, it can be stated that the change in the PL peak position is not related to the influence of technological parameters of thermal deposition procedure.
Consequently, considering that the depth of PL excitation of the samples at Xexcit =405 nm is ~ 10-30 nm [40, 41], significant changes in the PL properties of the sample may be due to:
- partial reflection of excitation radiation by the film, local charge redistribution when metal is introduced into the pores, and exciton screening, as in the case of electrochemical deposition;
- reduction of the number of silicon nanocrystals (clusters) in the analyzed compo-
site layer as compared to the initial por-Si, and a decrease in their average size.
Besides, for silicon nanostructures possessing relatively low PL intensity, the role of radiative recombination centers of defective silicon oxides SiOx, whose PL peak also falls in the region of 500-550 nm, may be enhanced [42].
Conclusions
Nanostructured porous silicon composites with the deposited porous silicon layer were obtained by the vacuum-thermal evaporation method. When using vacuum-thermal evaporation, tin is deposited on the surface of the porous layer in the form of an island film, slowing down the oxidation process of the porous layer during atmospheric storage for up to one month. Thus, the methodology worked out within the framework of this research can be successfully applied to the creation of composite materials with improved properties.
Acknowledgements
The work was supported by the Russian Science Foundation (No 22-73-00154 of 28.07.2022).
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MOSAMOLI SILIKON ÜZORINDO NANOSTRUKTURLU QALAY OKSID TOBOQOSINNN OMOLO
GOLMOSi
K.B.Kim, A.S.Len$in, F.M.£iraqov, S.LNifoliyev
Masamali silisiumun ilkin nümunasinin va qalaylanmi§ nümunanin morfologiyasi, tarkibi va optik xassalari haqqinda malumatlar skan edilmi§ elektron mikroskopiya, infraqirmizi va fotolüminessensiya spektroskopiyasi vasitasila alda edilmiijdir. Müayyan edilmi§dir ki, masamali silisiumun üzarinda kimyavi qalay gökmasi kompozit strukturun yaranmasina gatirib gixanr ki, bu da saxlama zamani masamali tabaqanin daha da oksidla§masinin qar§isim alir va lüminessensiya zolaginin maksimum yerdayi§masina sabab olur.
Agar sözlar: masamali silisium, kompozitlar, qalay, nazikplyonkalar
ФОРМИРОВАНИЕ НАНОСТРУКТУРИРОВАННОЙ ПЛЕНКИ ОКСИДА ОЛОВА
НА ПОРИСТОМ КРЕМНИИ
К.Б.Ким, А.С.Леньшин, Ф.М.Чырагов, С.И.Нифталиев
Методами сканирующей электронной микроскопии, инфракрасной и фотолюминесцентной спектроскопии получены данные о морфологии, составе и оптических свойствах исходного образца пористого кремния и образца с осажденным оловом. Установлено, что химическое осаждение олова на пористый кремний приводит к формированию композитной структуры, существенно препятствует дальнейшему окислению пористого слоя при хранении, а приводит к сдвигу максимума полосы люминесценции.
Ключевые слова: пористый кремний, композиты, олово, тонкие пленки.
