Transition metal dichalcogenides (TMDC) have become attractive candidates for 2D electronics and optoelectronics. While several concepts for light emitting devices have been reported, many of them realized using exfoliated TMDC flakes of micrometer size, only few approaches tackle the challenge of upscaling to relevant device sizes. We demonstrate a light emitting diode based on WS2 monolayers in a scalable design. The devices are fabricated by combining two industrially relevant deposition processes in a vertical p-n architecture: Metal organic CVD (MOCVD) is used to realize the optically active WS2 monolayers, while ZnO deposited by spatial atomic layer deposition (sALD) is employed as an electron injection layer on the cathode side. Organic layers spin-coated on an ITO covered glass substrate provide hole injection and transport. The resulting devices exhibit rectifying behavior and red electroluminescence from an area of 6 mm2.
The 2D transition metal dichalcogenides MoS2 and WS2 have attracted great interest due to their unique (opto)electronic properties. For their fabrication on an industrial scale, high-productivity MOCVD systems are most suitable because of precisely controlled precursor fluxes, advanced temperature control and superior homogeneity. Here, we report on the development of an MOCVD process for 2D WS2 and 2D MoS2 on sapphire (0001) substrates in a commercial AIXTRON planetary hot-wall reactor in 10 × 2" configuration. Molybdenum hexacarbonyl (MCO), tungsten hexacarbonyl (WCO) and di-tert-butyl sulfide (DTBS) are used as sources, respectively. A one-step process was developed to control nucleation and lateral growth of both 2D materials at 30 hPa total pressure in an N2 atmosphere. It was found that a fine-tuned S-to-metal ratio can inhibit the parasitic deposition of carbon contaminations. Investigations of the influence of deposition temperature on lateral growth of WS2 confirm previous findings for MoS2. The optimum growth temperature for MoS2 and WS2 is 845 °C. WS2 deposition experiments show that in order to achieve a fully coalesced 2D film, it is necessary to supply a sufficient amount of WCO, the limiting species during WS2 growth. Increasing the WCO flow from 1 nmol/min to 20 nmol/min raises the total substrate coverage from 2.5 % to >50 % in 10 h processes. Extending gradually the growth time to 20 h results in deposition of fully coalesced WS2 samples without carbon contaminations and only sparse bilayer nucleation. Moreover, the fully coalesced WS2 samples exhibit strong PL signals.