Abstract

This paper delves into the synthesis and characterization of p-type Transparent Conducting Oxides (TCOs), with a specific focus on Gallium-doped tin oxide (SnO2:Ga)and Antimony co-doped Gallium-tin oxide thin films. The films are deposited on blue microscope glass substrates utilizing the sol-gel dip-coating method. The study tackles the challenges related to the application of p-type TCOs, highlighting advancements in electrical performance compared to their n-type counterparts. Structural characterization through X-ray diffraction indicates both crystalline and amorphous nature of the films. Sharp and narrow diffraction peaks confirm the well-defined doped phases of Ga and Sb at atomic substitutional sites of SnO2. Different phases are identified for pure SnO2and Ga-doped SnO2 films. The preferential orientation shifts with doping concentrations. Sb-co-doped SnO2 films show the highest peak intensities. The average crystallite sizes of the thin films increase with doping concentrations, ranging from 19.83 nm to 32.24 nm for Ga-doped films and 33.91 nm to 40.88 nm for Sb-co-doped films. The structural analyses suggest that SnO2:Ga and Sb-co-doped SnO2:Ga thin films are suitable p-type TCO materials for optoelectronic applications. Overall, this research contributes valuable insights into improving the performance of p-type TCOs and addresses the limitations associated with their characterization.

1. Introduction

Transparent Conductive Oxides (TCOs) constitute a unique class of materials characterized by appreciable electrical conductivity, high optical transparency in the visible range, and significant infrared reflectivity ¹. Their distinctive properties underpin advancements in technology such as touch screen displays, solid-state sensors, organic light emitting diodes (OLEDs), liquid crystal displays, and photovoltaic cells ^{2, 3}. However, unlike their n-type counterparts, the establishment of suitable p-type TCOs remains a challenge ^{4, 5, 6}. The complexity arises from the electronic configuration of oxide materials, where the Valence Band Maximum (VBM), the transport path for holes in p-type TCOs, is localized and anisotropic, leading to high hole effective mass and low hole mobility ^{7, 8}.

This difficulty impedes the formation of p-n junctions critical for various optoelectronic applications ⁹. Despite these challenges, efforts guided by chemical design principles have led to the discovery of several p-type TCOs, including copper-bearing oxides (CuMO₂, M=Al, Ga, In), binary copper oxides (CuO and Cu₂O), and zinc spinel oxides ^{10, 11}. Notably, Copper Aluminium oxide (CuAlO₂) has shown promise as a highly transparent p-type TCO, although its resistivity remains higher compared to n-type indium tin oxide ^{10, 11} Zinc, when doped or deposited under specific conditions, also exhibits p-type TCO characteristics, albeit with challenges related to rapid decay due to the self-compensation effect ¹².

The inadequacy of current p-type TCOs has repercussions, particularly in transparent electronic devices like solar cells, where stability and efficiency are compromised. Substituting p-type TCOs with semi-crystalline metallic organic materials has resulted in reduced device stability and efficiency ^{13, 14, 15}. Addressing this gap in suitable p-type TCOs could enable the fabrication of transparent p-n junctions for diverse applications in the transparent electronic industry ^{13, 14, 15}.

Tin (IV) oxide stands out among TCOs due to its unique performance and versatility in optoelectronic industry and green energy devices ^{3, 9, 16}. SnO₂, an n-type semiconductor with a direct band gap of 3.6 eV to 4.0 eV, finds applications in gas sensors, solar cells, LEDs, transistors, and photo detectors ^{17, 18, 19}. Various studies have explored the fabrication of p-type tin-based films, particularly using group (III) elements such as Al and Ga, owing to their unique structures and sizes ^{20, 21}. For instance, the smaller radius of Ga compared to Sn enhances the solubility of Ga ions into the tin site, facilitating p-type conductivity ^{21, 22, 23, 24, 25, 26, 27}.

However, high resistivity due to low intrinsic carrier density and mobility poses a challenge for SnO₂ thin films in achieving p-type conductivity. Co-doping with Sb is proposed to produce thermally and chemically stable materials, as Sb acts as an acceptor at certain doping levels ^{28, 29, 30}.

Various deposition techniques have been employed for tin-based thin films, including DC and RF magnetron sputtering, spray pyrolysis, chemical vapor deposition sputtering, thermal evaporation, pulsed laser deposition, and the sol-gel method ^{22, 31, 32, 33, 34, 35}. Among these, the sol-gel method offers advantages such as cost-effectiveness, environmental friendliness, ease of controlling underlying materials and doping concentrations, low temperature requirements for film densification, and the formation of homogeneous structures ^{35, 36, 37}.

In this study, transparent p-type conductive thin films of tin doped with Gallium and co-doped with Sb are prepared using the sol-gel dip-coating method, grown on Blue-plus microscope glass slides. Their structural properties are characterized for potential applications in transparent electronics.

2. Experimental

Tin (IV) chloride of trace metal basis (99.95 % purity) Sigma Aldrich, Gallium (III) chloride, ultra dry, 99.999 % purity (metal basis) Sigma Aldrich, Ethanol and acetone (solvents) Antimony (III) ethoxide, co-dopant, (99.999 % purity) Sigma Aldrich were used for sol preparations. The Blue plus microscope glass slides were used as the substrates for sol coatings.

GaCl₃-ultra dry, was dissolved in Acetyl acetone at room temperature. Table 1 illustrates the concentration composition values of SnO₂:Ga and Sb-co-doped SnO₂:Ga with the obtained thin films. At various concentration weights; 0.25 g, 0.5 g, 0.75 g and 1.0 g of Ga, the solution was refluxed at 70^o C using a reflux set-up for 2 hours to ensure effective homogenization. Hydrated SnCl₄ was then dissolved in ethanol in another beaker with the help of a magnetic stirrer at room temperature, cooled down and the solution added drop wise to the Ga dissolved solution. The sol was then aged in the dark room for 7 days ³⁶ as in the flow chart in Figure 1 which illustrates the dissolving, refluxing and aging of the sol.

After preparation of the gallium oxide sol, Antimony (III) Ethoxide was added in various concentrations of 0.5 ml, 1.0 ml, 1.5 ml and 2.0 ml (Table 1). The solution was then aged for 7 days to obtain the Sb-co-doped SnO₂:Ga coating sols as shown in flow chart in Figure 2 which illustrates the dissolving, refluxing, co-doping and aging of the sol.

The Blue-Plus microscope glass substrates were used for sol coatings. The substrates were treated ultrasonically in ethanol and acetone and kept in a desiccator, ready for film deposition ³⁸.

Film deposition was carried out in air at room temperature by the dip-coating method (Figure 3) using the cleaned glass substrate. The substrates were left to dry horizontally at 50^oC in the oven to remove any organic residuals. The coating-drying procedure was repeated 3-times with an approximated withdrawal speed of 4 cm/min to obtain the desired film thickness ³⁹ The films were then annealed at 150^oC (optimal temperature) for 5 minutes and then cooled down at room temperature for crystal formation.

3. Results and Discussions

XRD analysis technique was utilized to determine the crystal structure and crystallinity of the thin films using a Bruker D8 SSS X-Ray diffractometer. X-rays were generated using a Cu source with CuKα radiation (λ=1.5418) at 20kV and 15mA. The grazing incident beam angle was kept at 0.8°. Angular ranges of the patterns collected were between 10 and 70 degrees at a rate of 1 minute per step with a scanning step size of 0.05. XRD spectra were indexed using the originPro85 software. The patterns were analyzed for crystallinity and preferred orientations as shown in Figure 4 and Figure 5.

The presence of fairly sharp and narrow significant diffraction peaks of the prepared thin films indicates the crystalline nature of the films. The doped phases of Ga and Sb were well defined and successfully replaced at atomic substitutional sites of SnO₂.

For pure (undoped) SnO₂ thin film, 4 diffraction peaks were identified as (126), (151), (285) and (407) reflections of the orthorhombic structure. For the Ga doped SnO₂ thin films, the samples related to the monoclinic structure with SnO₂ phases identified due to the mixing of both Ga and SnO₂. The results indicate that the preferential orientation for pure SnO₂ (407) is shifted to (408), (407) and (409) for the Ga-doped SnO₂ and (404), (302), (115) and (408) for the Sb-co-doped SnO₂:Ga. Change of preferred orientation is attributed to the occupancy of additional tin vacancy sites by Ga and Sb sites which were initially unoccupied. The increase in preferred orientation is associated with increase of crystallite growth along that particular plane.

Increase in peak intensity with film concentration is also observed for all the prepared thin films. All the films indicate one strong peak with the Sb-co-doped thin films registering the highest intensities as compared to the Ga-doped thin films. Thus, the spectra of the films show a well-defined orientation. This can be attributed to an improvement of the films with doping and enhancement in grain sizes. A similar result of increase in peak intensities with doping concentrations and change of preferred orientations is reported by ⁴⁰ with morphological evaluation of co-precipitated ZnO nanoparticles.

The effective grain sizes of the thin films were calculated from the Full Width at Half Wavelength

(FWHW) of x-ray peak lines of plane orientation using the Debye-Scherrer formula;

(1)

where; D = mean crystallite size

λ = wavelength of radiation (incident x-ray) used=0.15418

β = peak (line) broadening at half the maximum intensity

θ = Bragg angle (diffracted angle) in radians

An observed improvement in average crystallite sizes is noted as the doping concentrations increase for both the Ga-doped and Sb-co-doped SnO₂ thin films. Specifically, the grain sizes of the thin films exhibit an increase from 19.83 nm to 32.24 nm for the Ga-doped tin oxide thin films and from 33.91 nm to 40.88 nm for the Sb-co-doped SnO₂ films with increasing concentrations of Ga and Sb. Additionally, the Full Width at Half Maximum (FWHM) of all the films decreases with an increase in precursor concentration, indicating enhanced crystallinity of the thin films.

This observed increase in crystallite sizes and decrease in FWHM is attributed to the reduced strain on the surface of the films. This reduction in strain occurs because the dopants act as nucleation sites in nanoparticle synthesis, thereby lowering the energy required for particles to initiate formation. Similar results of increased crystallite sizes with doping have been reported in studies by Rahul et al. ⁴¹ with doped ZnO and Ian E. et al. ⁴² regarding the effect of doping-induced aggregation on film morphology, as well as by Peddavarapu et al. ⁴³ with SnO₂ thin films.

4. Conclusions

In conclusion, the observed increase in crystallite sizes upon doping of SnO₂ and Sb-co-Doped SnO₂;Ga thin films signifies enhanced crystallinity, which is essential for their optimal performance.. The presence of sharp and narrow diffractions in XRD spectra indicates a combination of crystalline and amorphous nature in the prepared thin films. Moreover, the increase in preferred orientations of the thin films corresponds to enhanced crystallite growth along specific planes. The intensification of film peaks with doping concentrations is attributed to the creation of sub-energy levels within the film lattice. Notably, a strong peak observed in all XRD spectra signifies a well-defined orientation of the thin films. The homogeneous grain sizes observed in SnO₂ and Sb-co-Doped SnO₂;Ga films contribute to their optimal electrical properties. Overall, doping concentrations significantly influence the structural properties of the prepared p-type TCO thin films of SnO₂and Sb-co-Doped SnO₂, confirming their suitability for high-quality device applications, particularly in solar cell technology which still records low efficiency.

References