Experimental Investigation of Si/SnOx Heterojunction for Its Tunable Optoelectronic Properties

We report growth and characterization of n-Si/p-SnO<sub>x</sub> heterojunction using RF sputtering for deposition of p-type SnO<sub>x</sub> under controlled growth oxygen pressure over n-type silicon (Si) wafer. The heterojunction properties of Si/SnO<sub>x</sub> were varied by controlling the growth oxygen pressure of SnOx. Several characterization techniques, including PL (photoluminescence), AFM (atomic force microscopy), FESEM (field emission scanning electron microscopy), XRD, I-V characteristics and Hall measurement, were conducted to analyze the structural, optical, and electrical properties of the n-Si/p-SnOx heterojunction. The knee voltage (V<sub>knee</sub>), or cut-in voltage, was calculated by analyzing the gradient of the dark current-voltage (J-V) curves when the bias was applied in the forward direction. The V<sub>knee</sub> values for type-I, type-II, and type-III n-Si/p-SnO<sub>x</sub> heterojunctions were determined to be 0.62 V, 0.84 V, and 1.0 V, respectively. The ideality factors (n<sub>1</sub> and n<sub>2</sub>) were determined to be 1.52, 2.22, 3.52, and 8.41, 9.31, 10.34, respectively, for various heterojunction types. The reverse saturation current densities, J<sub>01</sub> and J<sub>02</sub> ranging from approximately 10<sup>−7</sup> to 10<sup>−6</sup> A/cm<sup>2</sup>, and 10<sup>−5</sup> to 10<sup>−4</sup> A/cm<sup>2</sup>, respectively. The objective of this experimental work is to investigate especially, the prospect of silicon /metal-oxide (Si/SnOx) based heterojunction to be used as optical sensors with tunable optoelectronic properties of SnOx.


I. INTRODUCTION
M ETAL-OXIDES are being intensively inspected due to its abundant nature, stability, and ease of fabrication including low processing temperature and reduced cost of fabrication [1], [2] Generally, metal oxide-based semiconductor material such as TiO 2 , ZnO, AZO, NiO x , CoO x , NiCoO x , MnOx, ITO, SiO 2 , and SnO x shows it's tunable electrical and optical properties with oxygen content [3], [4].These metal-oxides have been used in several technological applications for opto-electronic devices such as TCO (transparent conducting oxide) layer [5], emitter layer [6], carrier transport layer in organic solar cells [7] and sensors [8].Moreover, Si/metal-oxide heterojunctions such as Si/TiO 2 , Si/TiO 2 , Si/ZnO, Si/AZO, Si/ FeO, Si/FTO [9], Si /NiO, Si/CoO [10] etc., have also been reported by the researchers.But the expensive and complex processes like PLD, ALD and MBE (for growth of CoO, NiO and FeO film) limit its extensive use in opto-electronic industries.Rest of the heterojunctions mentioned above, suffer either from poor interface quality or from significant amount of band discontinuity or offsets.Basically, the poor interface of the heterojunction may result from a high degree of lattice mismatch and significant differences in their electron affinities and bandgaps.It is a well-known fact that a large mismatch in electron affinities and bandgaps leads to conduction and valence offsets, which are highly undesirable for most optoelectronic devices [11], [12].
In order to overcome these to some extent, few researchers have reported Si/SnO x based heterojunction with tunable optoelctronic properties, so that heterointerface between Si/SnO x can be optimized to obtain improve their interface quality [13], [14], [15], [16].Easily tunable opto-electronic properties of SnO x by controlling the growth oxygen pressure in SnO x is an additional advantage [3], [4].Recently, some authors have experimentally observed the well-regulated light absorption property of SnO x along with its p-type conductivity and electrical property band gap in particular.They also observed that the optical bandgap of SnO x film can be tuned in the range of 1.1 eV to 3.4 eV [17], [18].Furthermore, silicon/tin oxide (Si/SnO 2 ) heterojunction presents an appealing prospect due to its low growth temperature with tunable properties, making it desirable for various applications in optoelectronic devices.Previous studies have utilized methods such as chemical vapor deposition (CVD) and electrochemical processes to fabricate this heterojunction.For instance, Badawy et al. employed a combined spray-CVD process to consistently produce high-quality Si/SnO 2 junctions [19].In a related context, Ling et al. introduced a nano litchi shell structure for ultrafast, high-detectivity, self-powered broadband photodetectors, addressing the growing demand for selfpowered photodetectors with superior performance metrics [20].However, the challenge remains in developing light-absorbing materials with efficient carrier separation.In this investigation, we present a novel SnO 2 nano litchi shell structure/n-Si heterojunction designed to enhance broadband light absorption for self-powered photodetection.Additionally, Hernandez et al. highlighted tin oxide as a protective heterojunction with silicon, demonstrating its efficacy in photoelectrochemical water oxidation under strongly acidic or alkaline conditions [21].Furthermore, Vidhya et al. conducted photoluminescent studies on porous silicon/tin oxide heterostructures, predicting their influence on interface states at the SnO 2 /porous silicon (PSi) junction.It exhibit good stability, indicating promising prospects for further research and application [22].
The most significant advantage of silicon based heterojunction in terms of optoelectronic performance is the implementation of common light management techniques, such as the incorporation of textured interfaces and optical interlayers, which reduce reflection, diminish parasitic absorption, and increase light trapping within the Si based optoelectronic devices [23].Moreover, light management is critical for enhancing silicon's photocurrent absorption.Another advantageous aspect is the existence of a band bending at the interface between silicon and its native oxide silicon dioxide which produces an electric field that effectively segregates the electron-hole pairs formed by light in silicon to enhance responsiveness, utilize low-dimensional techniques [24].In this context, nanoplates, nanorods, nanowires, and quantum dots can be integrated with silicon to create optoelectronic devices that had distinctive architectures and photoelectric characteristics.Nevertheless, the application of Si in optoelectronics is comparatively restricted owing to its inadequate optical emission and mediocre optical absorption, which are consequences of its indirect bandgap [25].This paves the way of developing Si/SnO x heterojunction with SnO x and Si as dual light absorber.The objective of this experimental work is to investigate especially, the prospect of silicon /metal-oxide (Si/SnO x ) based heterojunction to be used as solar cell, photo detector and photo electrodes, and several other optoelectronic devices and sensors with tunable optoelectronic properties of SnOx.However, to achieve the desired film properties and heterojunction characteristics, the optimization of sputtering parameters for depositing p-type SnO x films onto n-type silicon (Si) wafers requires precise adjustment of various process variables.The selection of the sputtering target material is vital in determining the composition and properties of the film that is deposited.In this scenario, it is probable that a tin (Sn) target was utilized for sputtering in order to deposit SnO x films over silicon wafer.The degree of purity and the quality of the target material have a significant impact on the stoichiometry and structural properties of the film.

II. FABRICATION PROCESS
First of all, Si wafer is dipped in piranha solution for removal of any organic or inorganic impurities, thereafter it is cleaned ultrasonically with acetone and isopropanol.Subsequently the cleaned Si wafers are dipped in de-ionized water and finally dried in nitrogen environment.A thin-film of SnO x layer are grown on Si substrates by RF magnetron sputtering process at an optimized 400 °C temperature using Sn sputtering target (Alfa Aesar, 2.00 inches Dia.× 0.125 inches thick and 99.99% purity), keeping substrate to target distance as 15 cm.Prior to the growth, the chamber is thoroughly evacuated to a base pressure of around ∼10 -7 mbar.This ensures that no contaminant particles are present inside the chamber, which might potentially impact the formation of the SnO x layer.This ensure the reproducibility and quality control of the fabrication process.To begin with, argon gas was purged into the evacuated RF sputtering chamber at a flow rate of 50 sccm (standard cubic centimeters per minute).Then, the RF sputtering power was gradually increased to 60 watts to initiate the plasma necessary for sputtering.Once the plasma stabilized, the argon gas flow rate was gradually reduced to 10 sccm.Afterwards oxygen gas (10 sccm) was purged into the growth chamber with the help of mass flow controller (MFC) and the substrate cover was opened to carry out RF sputtering process at 60 Watt (RF power) at growth rate of 30 A 0 /sec to deposit an optimized 1 µm thick SnO x on glass substrate and Si wafer to form type-I SnO x sample and type-I Si/SnO x heterojunction.Similarly, keeping argon 10 sccm, oxygen at 20 sccm, SnO x was deposited over glass substrate and Si wafer to form type-II SnOx sample and Si/SnO x heterojunction, respectively.The above process was repeated for type-III SnO x sample and Si/SnO x heterojunction keeping argon at 10 sccm and oxygen at 30 sccm.Thereafter, all three, i.e., type-I, type-II and type-III Si/SnO x heterojunction (shown in Fig. 1(a)-(c)) were vacuum annealed at 550 °C temperature for an optimized duration of 30 minutes.The RF sputtering power and chamber pressure are crucial factors that impact the rate of film deposition, microstructure, and surface morphology.Increasing the sputtering powers typically leads to higher deposition rates, but it can also cause increased film roughness or non-uniformity if not properly optimized.In addition, the pressure within the chamber has an influence on the ionization and collision processes of the sputtering gas, which in turn can affect the composition and structure of the film.The temperature of the Si wafer substrate during sputtering can impact the adhesion, crystallinity, and stress of the film.Controlling the substrate temperature optimizes the nucleation and growth kinetics of the deposited films, resulting in enhanced film quality and interface properties in the heterojunction.

III. RESULTS AND DISCUSSION
The different types of SnO x samples (Type-I, Type II and Type III) which were deposited on quartz glass substrate were characterized using Hall measurement setup to find out Hall mobility, carrier concentration and sheet resistivity.These parameters were found out to be 7.51 cm 2 /Vs, 7.62×10 18 cm -3 and 1.73×10 -3 Ω-cm, respectively for type-I SnO x , whereas for type-II SnO x , values of Hall mobility, carrier concentration and sheet resistivity are 8.72 cm 2 /Vs, 7.62×10 17 cm -3 and 1.1×10 -3 Ω-cm, respectively.Finally, the Hall mobility, carrier concentration, and sheet resistivity values for type-III SnO x samples are determined to be 9.41 cm 2 /Vs, 2.72×10 17 cm -3 , and 1.62×10 -3 Ω-cm, respectively.Obtained results are in good agreement with their respective reported values [26].It is observed that hole carrier concentration of type-I SnO x is higher as compared to other type (type-II and III) SnO x samples whereas its Hall mobility is lowest.This is primarily because the oxidation process is significantly intensified with an increase in growth-O 2 pressure/oxygen flow rate, leading to the creation of a substantially larger quantity of SnO 2 phase with reduced oxygen vacancy [27].Thus at reduced oxygen vacancy at higher growth oxygen pressure/flow rate decreases hole carrier concentration in SnO x samples.The limited amount of the SnO phase which is responsible for hole carrier concentration in the SnO x sample at increased growth-O 2 pressure is a significant factor contributing to the reduced hole concentration.In the field of semiconductors, it is common to see an inverse relationship between Hall mobility and carrier concentration.As the carrier concentration of SnO x falls, the Hall mobility increases.However, resistivity is inversely related to the Hall mobility and directly proportional to the carrier concentration.Therefore, in this work, it has been observed that, type-II Si/SnO x sample exhibits the lowest resistivity due to its optimal carrier concentration and Hall mobility.Hall measurement shows its p-type nature and further increase in growth oxygen pressure (above 50 sccm) of SnO x change its nature towards n-type.Inherently, SnO 2 is an n-type semiconductor but recent literature has highlighted the potential to modify the nature of SnO 2 films to p-tytpe SnO x by controlling Sn (tin) to O (oxygen) ratio during their growth process [28], [29], [30], such as through RF sputtering and e-beam evaporation techniques.However, by adjusting the oxygen pressure during growth, it is possible to alter Sn:O ratio, creating oxygen vacancies or reducing them, thereby tuning its nature and optoelectronic properties.This controlled alteration can produce p-type SnO x films, which exhibit p-type conductivity due to their modified oxygen content.Researchers have successfully demonstrated the formation of p-SnO x /n-SnO x heterojunctions [31], showing the feasibility of to form p-SnO x .Furthermore, it is possible to form p-SnO x /n-Si heterojunctions [32], leveraging the naturally n-type silicon to create junctions useful in electronic and optoelectronic devices.It may be noted that the SnO x phase includes Sn, Sn 2+ and Sn 4+ oxidation states.Moreover, these states can be regulated by adjusting the growth oxygen pressure of SnO x samples.At lower growth oxygen pressure/flow rate, the Sn phase is relatively more whereas at higher growth oxygen pressure/ flow rate, the amount of Sn phase decreases as Sn get oxidized to form more amount of SnO 2 phase (Sn 4+ oxidation states).But, in this work, the oxygen flow rates have been kept in lower ranges (between 10-30 sccm) in order to ensure p-type nature of SnO x sample.With an increase in growth oxygen pressure or flow (50 sccm), the Sn and Sn 2+ phases undergo oxidation, leading to the formation of Sn 4+ phase and consequently, a greater amount of SnO 2 is formed, which is known as a highly n-type transparent oxide according to the literature.To ascertain its n-type characteristics, we conducted Hall measurements characterization to confirm its n-type nature and its carrier concentration, resistivity and Hall mobility were found out to be 9.4×10 18 cm −3 , 1.2×10 −3 Ω cm and 15.2 cm 2 V −1 s −1 , respectively.Therefore, growth oxygen pressure of SnO x was restricted to 30 sccm to ensure its p-type nature.
Furthermore, XRD patterns (as shown Fig. 2) of SnO x samples (type-I, type-II, and type-III) grown under different conditions have been obtained in order to analyze its different composition of phases.These XRD patterns were compared with standard (Joint Committee on Powder Diffraction Standards-JCPDS) Sn, SnO, and SnO 2 patterns.Specifically, type-I SnOx was grown with an argon to oxygen ratio of 10 sccm : 10 sccm, type-II with a ratio of 10 sccm:20 sccm, and type-III with an argon to oxygen ratio of 10 sccm:30 sccm.The figure shows a significant SnO peak at 30°for all types of SnO x films.For the type-I SnO x sample, significant number of Sn peaks are observed at 44.5°, and 62°.However, for the type-III sample shows prominent SnO 2 peaks which occur at 50°and 64°.For type-II SnO x sample, grown at intermediate oxygen pressure, all states of SnO x (Sn, Sn 2+ , and Sn 4+ ) are present.Thus, by regulating the oxygen pressure during growth, the state of SnO x can be controlled to tune its optoelectronic properties.From the figure, it can be concluded that as the oxygen pressure during growth increases from type-I to type-III, Sn is increasingly oxidized to Sn 4+ , resulting in more SnO 2 peaks.Now, the dark current density-voltage (J-V) characteristics (at room temperature) of different types of Si/SnO x heterojunctions were obtained by using the Agilent Source Measurement Unit (B2901A) and are shown in Fig. 3. Some important parameters such as knee voltage (V knee ), ideality factor (n), and reverse saturation current density (J 0 ) have been extracted, which are based on two-diode model given by ( 1) [33].The knee voltage (V knee ), often termed as the cut-in voltage of a diode, is calculated from the slope of a dark J-V curve in forward bias condition, as shown in Fig. 2(a)-(c), which are found out be 0.62 V, 0.84 V and 1.0 V for type-I, type-II and type-III n-Si/p-SnO x hetero-junctions as shown in inset of Fig. 2, respectively.It is observed that knee voltage of type-I Si/SnO x hetero-junctions is relatively higher than the other two, i.e., type-II and type-III (n-Si/p-SnO x ) configurations.
This may be due to the significant amount of barrier potential height of SnO x layer with increase in growth oxygen pressure as it tends to more of oxide nature with increase in oxygen content [34].It is observed that slope of JV curve of type Si/SnO x heterojunction has steep slope whereas the slope of type II and type III heterojunction, which shows lower series resistance of type-I heterojunction as compared to other two heterojunction.The lower series resistance of type-I is attributed to higher carrier concentration of holes in type-I SnO x as mentioned above.In order to analyze the performance of heterointerface properties of the heterojunctions, the ideality factor (n1 and n2) have been extracted under the dark condition as per two diode model equation from the slope of the linear regions of the ln(J)-V characteristics, shown in Fig. 3 by using (2) [35].
Here, n 1 and n 2 are ideality factors respectively for low bias (0<V<1) and high bias (1<V<5) regions where respectively the depletion region and bulk (quasi-neutral) region recombination are dominating.Ideality factor n 1 is found out to be at 1.52, 2.22 and 3.52 whereas n 2 is found out to be 8.41, 9.31, and 10.34 respectively for different types (type-I, type-II and type-III) of Si/SnO x heterojunctions.The higher values (>2) of ideality factors attribute to poor interface quality and hence significant defect density at the hetero-interface and large series resistance [36].In addition, reverse saturation current densities i.e., J 01 and J 02 have been extracted by extrapolating the slope of the linear region of ln(J)-V curves to zero Volt (shown in Fig. 4), for two different bias regions (i.e., low bias (0<V< 1) and high bias (1<V<5)) [33].The J 01 and J 02 values of type-I Si/SnO x hetero-junction are relatively lower as compared to other two heterojunctions which often indicate lower probability of minority carriers tunneling via interface traps and hence signify a better interface.However, a lower current density in reverse bias indicates a smaller tunneling current, consequently, indicating lower interface defects [37].This work provides a qualitative analysis of different types of Si/SnOx heterojunction fabricated with change in growth pressure of SnO x layer.
Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.Furthermore, photoluminescence (PL) study reveals that emission is more at ∼700 nm wavelength for type-I Si/SnO x heterojunction, whereas for type-II and type-III Si/SnO x heterojunctions, the significant emission is in the range 800 nm and 900 nm respectively as shown in Fig. 5.In the near to visible region range, i.e., 700 nm-900 nm, the emission is attributed to intrinsic defects induced during the synthesis of Si/SnO x itself.It is also seen that maximum absorption corresponds 1100 nm which corresponds to Si absorption [38].Both theoretical and experimental evidence confirms that there is a noticeable alteration in the band gap and absorption coefficient of SnO x when the oxygen pressure during the sputtering process is changed.Consequently, overall absorption properties of the Si/SnO x heterojunction, where both Si and SnOx serve as the absorber layer, are altered by variations in the growth oxygen pressure [39].Moreover, oxygen pressure during the growth process significantly affects the properties of SnO x films, including their band gap, which can be controlled by ratio of argon to oxygen gas flow ratio during RF sputtering process.Several studies have reported an increase in the band gap of SnO x films with higher growth oxygen pressure [40], [41].As the oxygen pressure increases during growth techniques such as RF sputtering or e-beam evaporation, the band gap of SnOx films also increases.This is because SnO x films are composed of different phases, including Sn, SnO, and SnO 2 , and the proportion of these phases' changes with the growth oxygen pressure, influencing the overall band gap.Therefore, significant absorption peak at 940 nm is indicative of changes in the band structure for type III Si/SnO x heterojunction sample which has been developed at higher growth oxygen pressure as compared to other type-I and type-II Si/SnO x heterojunction.This peak suggests that the material has a wider band gap, possibly due to a reduction in defect states and better stoichiometry, leading to improved optoelectronic properties.Thus, by controlling growth oxygen pressure of SnO x , it is possible to obtain dual light absorber Si/SnO x heterojunction for various optoelectronic device like solar cell, photodetector, sensors, etc.Finally, field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) imaging of Si/SnO x heterojunctions were obtained in order to analyze its surface characteristics as shown in Figs. 6 and 7, respectively, which shows that surface roughness of type-I Si/SnO x heterojunction has relatively higher surface roughness as compared other two, i.e., type-II and type-III Si/SnO x heterojunctions.There is noticeable variation in the shape of the 'grains' in SnO x films based on different oxygen pressure, which consequently affects the surface roughness.This is because SnO x films are composed of different phases, including Sn, SnO, and SnO 2 , and the proportion of these phases' changes with the growth oxygen pressure, influencing its optoelectronic properties including surface roughness of SnOx film [40].The XRD spectra indicate that at lower growth oxygen pressures, Sn peaks are  [42].This is attributed to the increased availability of tin atoms at lower growth oxygen pressures, resulting in segregation and larger grain sizes compared to type-II and type-III Si/SnO x heterojunction [43], where the peak of tin atoms gradually diminishes with rising oxygen pressure, as confirmed by XRD spectra.This disparity in grain size is the reason why type-I Si/SnO x heterojunction exhibit higher surface roughness compared to type-II and type-III, as supported by AFM characterization results shown in Fig. 7.The main reason for this is the segregation property of metallic Tin (Sn) phase, which is more abundant at lower growth-O 2 pressures.A significant aggregation of metallic Sn atoms occurs, resulting in the formation of a relatively large grain when the growth-O 2 pressure is low, as depicted in Fig. 7(a).However, when the growth-O 2 pressure increases, the abundance of metallic Sn atoms is greatly diminished due to oxidation [44].As a result, the electrical resistivity and surface reflectivity of a thin film significantly increases due to scattering at the grain boundaries, surfaces, and interfaces as reported in literature [45].From AFM imaging, and FESEM imaging, it can be concluded that surface roughness decreases with increase in growth oxygen pressure of SnO x , i.e., from type-I Si/SnO x heterojunction to type-III Si/SnO x heterojunction.The surface roughness may be regulated to control the surface reflectance of Si/SnO x heterojunctions [46].The overall extracted parameters of different types of n-Si/p-SnO x heterojunctions have been summarized in Table I for ready references.

IV. CONCLUSION
This study investigates crucial junction parameters, such as the ideality factor, knee voltage, reverse saturation current density, and J-V characteristics, in Si/SnOx heterojunctions for various growth oxygen pressures of SnOx.AFM analysis demonstrates a decrease in surface roughness of SnOx samples with increasing oxygen pressure during growth.Moreover, the results indicate a shift in maximum absorption towards longer wavelengths as SnOx growth oxygen pressure increases.This control over growth enables precise tuning of maximum absorption at specific wavelengths, benefiting a wide array of optoelectronic devices.The manipulation of SnOx growth oxygen pressure alters the heterojunction characteristics of Si/SnO x .Utilizing various characterization techniques, including photoluminescence (PL), AFM, FESEM, I-V characteristics, and Hall measurement, we analyzed the structural, optical, and electrical properties of n-Si/p-SnOx heterojunctions.Vknee values for type-I, type-II, and type-III n-Si/p-SnOx heterojunctions were determined as 0.62 V, 0.84 V, and 1.0 V, respectively, exhibiting a positive correlation with increased growth oxygen pressure.Ideality factors (n1 and n2) ranged from 1.52 to 10.34 for different heterojunction types, while reverse saturation current densities (J 01 and J 02 ) fell within the ranges of approximately 10 -7 to 10 -6 A/cm 2 and 10 -5 to 10 -4 A/cm 2 , respectively.The increase in both ideality factor values and reverse saturation current density with growth oxygen pressure suggests degradation in Si/SnO x heterojunction interface properties.

Fig. 2 .
Fig. 2. XRD patterns of different types of SnO x samples grown under different conditions by RF sputtering.

Fig. 3 .
Fig. 3. Dark JV characteristics of Si/SnO x heterojunction for different types of SnOx (type-I, II and III) deposited over silicon wafer.

Fig. 4 .
Fig. 4. The ln (J) versus Voltage curve of Si/SnO x heterojunction for extracting their ideality factor and reverse saturation current density.

TABLE I EXTRACTED
PARAMETERS OF DIFFERENT TYPES OF N-SI/P-SNO x HETEROJUNCTION predominant with fewer SnO and SnO 2 peaks, whereas at higher growth oxygen pressures, SnO 2 peaks dominate.At intermediate oxygen pressures, a significant SnO peak is observed alongside reduced Sn and SnO 2 peaks.Films with more SnO 2 peaks under higher growth oxygen pressures exhibit behavior typical of SnO 2 films, while those at lower oxygen pressures exhibit characteristics akin to Sn films