Electrical, structural, and optical properties of sulfurized Sn-doped In2O3 nanowires
© Zervos et al. 2015
Received: 4 March 2015
Accepted: 30 June 2015
Published: 1 August 2015
Sn-doped In2O3 nanowires have been grown on Si via the vapor-liquid-solid mechanism at 800 °C and then exposed to H2S between 300 to 600 °C. We observe the existence of cubic bixbyite In2O3 and hexagonal SnS2 after processing the Sn:In2O3 nanowires to H2S at 300 °C but also cubic bixbyite In2O3, which remains dominant, and the emergence of rhombohedral In2(SO4)3 at 400 °C. The resultant nanowires maintain their metallic-like conductivity, and exhibit photoluminescence at 3.4 eV corresponding to band edge emission from In2O3. In contrast, Sn:In2O3 nanowires grown on glass at 500 °C can be treated under H2S only below 200 °C which is important for the fabrication of Cu2S/Sn:In2O3 core-shell p-n junctions on low-cost transparent substrates such as glass suitable for quantum dot-sensitized solar cells.
KeywordsSulfur Indium tin oxide Nanowires
Semiconductor nanowires (NWs) are attractive for the fabrication of nanoscale devices such as nanowire solar cells (NWSCs) and sensors, but some of the main issues pertaining to the realization of high performance devices is doping and control or modification of the surface properties which are important in view of the large surface to volume ratio . For instance the surface modification of III–V NWs such as InAs and GaAs NWs using sulfur has been shown to improve their electrical and optical properties [2–6], but the effect of sulfur on the structural, electrical and optical properties of metal oxide (MO) NWs has not been considered previously, despite the fact that it leads to a suppression of surface recombination and improvement of the photoluminescence (PL) in bulk ZnO [7, 8]. Controlling the surface properties of MO NWs is also necessary in order to suppress the adsorption and desorption of oxygen which is responsible for charge fluctuations and has been achieved so far by using polyimide and polymethyl methacrylate on ZnO and SnO2 NWs, respectively [9, 10].
Recently, we carried out a systematic investigation into the growth and properties of Sn-doped In2O3 or indium tin oxide (ITO) NWs grown on Si by the vapor-liquid-solid (VLS) mechanism  which may be converted to metal oxysulfide (MOxS) NWs with different properties by post growth processing under H2S that is useful for NWSCs. It has been shown that bulk β-In2S3-3xO3x has an optical band gap that varies from 2.1 eV in pure β-In2S3 to 2.9 eV when it contains 8.5 at.% of oxygen and has been proposed as an alternative to CdS buffer layers in CuInxGa1-xSe2 solar cells [12, 13]. In addition, the post growth processing of Sn-doped In2O3 NWs under H2S at different temperatures is important in understanding their properties and limitations as gas sensors which so far has been considered only up to 250 °C [14–16].
Hence, we carried out a systematic investigation into the structural, electrical, and optical properties of Sn-doped In2O3 NWs following post growth processing under H2S between 300 to 600 °C. We observe the existence of cubic bixbyite In2O3 and the formation of hexagonal SnS2 after processing the Sn-doped In2O3 NWs under H2S at 300 °C but also cubic bixbyite In2O3, which remains dominant, and the emergence of rhombohedral In2(SO4)3 at 400 °C. The Sn-doped In2O3 NWs maintain their metallic-like conductivities after exposure to H2S while we observed the emergence of PL at 3.4 eV corresponding to band edge emission from In2O3 in addition to the emission at 2.5 eV which is related to oxygen vacancies and states lying energetically in the upper half of the energy band gap of the as-grown Sn-doped In2O3 NWs. Besides the above, we have also grown Sn-doped In2O3 NWs on soda lime glass (SLG) at 500 °C, but we find that a significant deterioration in their conductivity occurs after exposure to H2S above 200 °C which might be related to Na ion diffusion as it was not observed in the case of the Sn-doped In2O3 NWs grown on Si. We discuss the importance of these findings for the fabrication of quantum dot-sensitized solar cells (QDSSCs) consisting of n-type MO NWs like SnO2, Sn:In2O3, and p-type chalcogenide semiconductors such as Cu2S or CuSnS3 .
Sn-doped In2O3 NWs were grown on Si(001) and fused silica by low-pressure chemical vapor deposition (LPCVD) described in detail elsewhere . Square samples of Si and fused silica ≈7 × 7 mm were cleaned sequentially in trichloroethylene, methanol, acetone, isopropanol, rinsed with de-ionized water, dried with nitrogen, and coated with ≈1 nm of Au. For the growth of Sn-doped In2O3 NWs, Sn (Aldrich, 2–14 Mesh, 99.9 %), and In (Aldrich, Mesh 99.9 %) were weighed with an accuracy of ± 1 mg. About 0.2 g of In containing ≈1 to 5 % wt Sn was used for the growth of the Sn-doped In2O3 NWs. Initially the LPCVD tube was pumped down to 10−4 mBar and purged with 600 sccm of Ar for 10 min at 1 mBar. Then the temperature was ramped up to 800 °C at 30 °C/min using the same flow of Ar. Upon reaching 800 °C, a flow of 10 sccm O2 was added in order to grow the Sn: In2O3 NWs over 60 min after which cool down took place over 30 min without O2. Note that the Sn-doped In2O3 NWs were grown on fused silica for 10 min in order to maintain transparency. The morphology of the Sn-doped In2O3 NWs was determined by scanning electron microscopy (SEM) while their crystal structure was determined by grazing incidence x-ray diffraction (GIXD) using a Rigaku Smart Lab diffractometer (9-kW rotating Cu-anode) with Cu-Kα1 radiation. The Sn-doped In2O3 NWs were subsequently treated under a constant gas flow of 20 sccm Ar:50 sccm H2S between 300 to 600 °C for 60 min using a ramp rate of 10 °C/min but also at 400 °C for 30 min. All of the Sn-doped In2O3 NWs were inspected by SEM after post growth processing under H2S in order to determine changes in morphology while their crystal structure and phase purity was determined by GIXD. Transmission electron microscopy (TEM) was carried out using a FEI CM20 microscope operating at 200 kV while the constituent elements were identified by energy dispersive x-ray analysis (EDX) using a FEI SEM Inspect S equipped with a Si(Li) detector from EDAX Inc. The steady state absorption-transmission spectra were obtained with a Perkin-Elmer UV-vis spectrophotometer and PL using an excitation of 266 nm while ultrafast absorption-transmission spectroscopy was carried out using a Ti:Al2O3 ultrafast amplifier generating 100 fs pulses at 800 nm and repetition rate of 1 kHz. Non-linear crystals were used to generate 266/400 nm for the purpose of exciting the Sn-doped In2O3 NWs whereas part of the fundamental pulse was used to generate a super continuum light for probing different energy states. Finally, Sn-doped In2O3 NWs were also grown at 500 °C on 10 × 20 mm soda lime glass (SLG) slides which were cleaned as described above followed by the deposition of 1 nm Au after which they were treated under H2S at 100, 200, 300, 400, and 500 °C and their resistance measured with a Kiethley 2635 A in accordance with O’Dwyer et al. .
Results and discussion
Similarly we observe the cubic bixbyite In2O3 but also the emergence of rhombohedral In2(SO4)3, identified using ICDD 00-027-1163, in the GIXD of the Sn-doped In2O3 NWs containing 2–4 % Sn after post growth processing under H2S at 400 °C as shown in Fig. 2b. A high magnification TEM image is shown in Fig. 1c from which one may observe the formation of the In2(SO4)3 crystals on the surface of the Sn-doped In2O3 NWs with a lattice spacing of 6.1 Å determined from the HRTEM of Fig. 1d and identified using PDF 83–217. The Sn-doped In2O3 NWs did not remain one dimensional above 400 °C probably due to their rapid reduction by the H2 evolving from the decomposition of H2S which requires high temperatures in the range 750 to 1250 K. Hence, we consider further the properties of the Sn-doped In2O3 NWs processed under H2S below 500 °C.
In addition to the optical properties, we find that the Sn-doped In2O3 NWs had resistances of ≈100 Ω, determined from the I–V characteristics shown as an inset in Fig. 3 and maintain their metallic-like conductivity after exposure to H2S at 300 °C with resistances of ≈20 Ω.
These findings are important for the realization of p-n junction devices between Sn-doped In2O3 NWs in contact with p-type chalcogenide semiconductors like Cu2S or core-shell Cu2S/Sn-doped In2O3 NWs via the deposition of Cu over Sn-doped In2O3 NWs followed by processing under H2S  Similar core-shell Cu2S/Sn-doped In2O3 NWs have been used as sensors or in QDSSCs by Jiang et al.  who decorated n-type Sn-doped In2O3 NWs with p-type Cu2S quantum dots (QDs) using solution-processing methods. In such devices the Sn-doped In2O3 NWs that are not covered with Cu2S QDs come into direct contact with polysulfide liquid electrolytes containing S, Na2S etc. It is well known that electron-hole recombination at the transparent conducting oxide-liquid electrolyte interface may reduce the overall efficiency. Consequently the deposition of Cu over Sn-doped In2O3 NWs followed by its conversion into p-type Cu2S under H2S will result into the formation of a core-shell p-n junction but the surface not covered by Cu will be passivated by sulfur which is compatible with polysulfide electrolytes of QDSSCs. Nevertheless one of the challenges in the fabrication of QDSSCs is to grow the Sn-doped In2O3 NWs on low-cost transparent substrates such as soda lime glass in order to maintain transparency.
Hence we have grown Sn-doped In2O3 NWs on 10 × 20 mm glass slides via the VLS mechanism at temperatures below 600 °C in order to prevent bending and melting. We obtained a high-yield, uniform distribution of Sn-doped In2O3 NWs over the 15 × 20 mm glass slide similar to that shown in Fig. 1a. The Sn-doped In2O3 NWs had metallic-like conductivities and resistances less than 100 Ω, but the conductivity changed from being metallic to insulator-like after processing under H2S above 200 °C which we also observed in the case of ITO films on glass. This deterioration in the electrical resistance and conductivity may be related to Na ion diffusion as it was not observed in the case of the Sn-doped In2O3 NWs grown on Si. Fortunately the Sn-doped In2O3 NWs on glass maintain their metallic-like conductivity by processing under H2S between 100–200 °C which is sufficient for the conversion of Cu into p-type Cu2S and the realization of p-n junctions.
We have investigated the effect of post growth-processing Sn-doped In2O3 NWs under H2S on their structural, electrical, and optical properties. We observe the existence of hexagonal SnS2 and cubic bixbyite In2O3 which is dominant after exposing the Sn-doped In2O3 NWs to H2S at 300 °C while we also observe the cubic bixbyite In2O3 and emergence of orthorhombic In2(SO4)3 at 400 °C. All of the Sn-doped In2O3 NWs maintain their metallic-like conductivity and have resistances between 10 to 100 Ω after processing under H2S while we also observed the emergence of PL at 3.4 eV corresponding to band edge emission from In2O3. Finally, we have grown Sn-doped In2O3 NWs on glass at 500 °C which may be processed under H2S only between 100 to 200 °C which allows the deposition of Cu over the Sn-doped In2O3 NWs and their subsequent conversion into Cu2S/Sn:In2O3 core-shell p-n junctions for use in QDSSCs.
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