Growth of Thin-Film WS₂ for Solar Cell and Photodetector Applications

Tungsten disulfide, a transition metal dichalcogenide (TMDs) semiconductor, has a bulk phase bandgap of 1.35 eV, very close to the Shockley-Queisser limit for two-level systems at AM1.5G solar spectrum, making it a compelling material for solar photovoltaic applications[1]. Its applications in photovoltaics are still in their inception due to the difficulty of obtaining a uniform, pin-hole-free WS2 thin film with good adhesion [2,3]. This work reports on a two-step process involving sulfurization to grow a WS₂ thin film on a SiO₂ substrate (University Wafers) using a custom-built 12-zone horizontal split furnace (Quazar Technologies) for possible solar cell and photodetector applications. A thin film (~10 nm) of tungsten was deposited on a SiO₂ substrate at a rate of ~0.6A using DC sputtering system (Angstrom Engineering). This thin film was subsequently heated to 800-1100°C under 0-300 sccm of Ar flow in the furnace in presence of elemental sulfur for sulfurization. X-ray diffraction (Rigaku Ultima IV, Copper Kα = 1.54Å) scans of the film revealed a sharp and intense peak at 14.12° (2H-WS₂ nanosheet: JCPDS 08-0237), corresponding to (002) growth with a = 3.154 Å and c = 12.362 Å. The other peaks at 28.12° and 43.20° are inferred to correspond to (004) and (006) crystal planes. Further, Raman spectra (Renishaw inVia confocal microscope-based Raman spectrometer) of the sample reveal in-plane (E¹_2g) and out-of-plane (A_1g) vibrational modes at wavenumbers 348.25/cm and 412.90/cm, confirming the WS₂ growth. The wavenumber difference and intensity ratio of E¹_2g and A_1g vibrational mode peaks are estimated to be 64.65 and 1.03, respectively, confirming the multilayer growth. Raman mapping over an area of 5.1 x5.1 μm with a step size of 300 nm suggests consistent growth free of directional bias in the sulfurization process. The peak differences range from 64.65/cm to 70.72/cm, with an average of 68.34/cm[4], while the intensity ratio ranges from 0.88 to 1.13, with an average of1.03, again confirming the uniform multilayer growth. Field-emission scanning electron microscope (FESEM, JEOL JSM-7800F Prime) scans indicate formation of nanoflakes. Surface photovoltage spectroscopy (KP Technology KP020+SPS040) was used to measure the work function (-4.613 eV). Atomic force microscopy scans (Asylum Research MFP3D-BIO) were used to estimate the surface roughness of the grown WS2 layer to be 62.33 nm, while the underlying tungsten layer had a surface roughness of 72.99 pm. This suggests that the process of bulk incorporation of sulfur leads to significant increase in surface roughness. As we expected to fabricate heterojunctions of WS₂ with a variety of solution-phase p-type materials in ongoing work, the resulting increase in surface contact area of the heterojunction is expected to provide a larger number of exciton dissociation sites expected to increase the short circuit current, and to reduce optical reflection of the device[5,6].<br/><br/>References :<br/>1. Physica status solidi (a) 214, no. 12 (2017): 1700218<br/>2. Scientific reports 10, no. 1 (2020): 1-11<br/>3. Materials Today Advances 8 (2020): 100098<br/>4. Scientific Reports 3 (2013): 1755<br/>5. Micromachines 10, no. 9 (2019): 619<br/>6. Progress in Photovoltaics: Research and Applications 30, no. 6<br/>(2022): 622-631