Using mechanically flexible substrates for complex (opto-)electronic device architectures is of growing interest in the area of consumer electronics, [1] for wearables [2] or for applications in biology and life science. [3] Potential candidates for light-emitting devices (LEDs) on curved or flexible surfaces are organic LEDs (OLEDs) [4] or quantum dot LEDs [5,6] (QD-LEDs). Higher durability is achieved by bottom-up approaches, which combine individual inorganic micro-LEDs to flexible arrays. [1,3] Although two-dimensional (2D) materials are ideally suited for flexible device applications, [7-9] only a few flexible prototype devices such as transistors [10-12] or photodetectors [13-15] have
2D semiconductors based on transition metal dichalcogenides are highly promising for ultrathin photodetectors due to their thickness in the nanometer range and their exceptional light absorption properties. To enable efficient separation of optically generated electron–hole pairs heterostructures have to be implemented, which are usually prepared by poorly controlled mechanical steps such as exfoliation, transfer and stacking processes that prevent industrial upscaling. Here, semitransparent photodetectors in the mm2 range based on MoS2/WS2 heterostructures are presented that are realized without any transfer step by a scalable metal-organic chemical vapor deposition process on a sapphire substrate in a continuous growth run. The heterostructure device exhibits a responsivity, which is enhanced by about 5–6 orders of magnitude with respect to reference devices based on either MoS2 or WS2 monolayers only. The large gain enhancement is attributed to efficient charge carrier separation at the MoS2/WS2 heterointerface combined with hole trapping, leading to an improved electron transport in the heterostructure under illumination.
A promising strategy toward ultrathin, sensitive photodetectors is the combination of a photoactive semiconducting transition-metal dichalcogenide (TMDC) monolayer like MoS2 with highly conductive graphene. Such devices often exhibit a complex and contradictory photoresponse as incident light can trigger both photoconductivity and photoinduced desorption of molecules from the surface. Here, we use metal–organic chemical vapor deposition (MOCVD) to directly grow MoS2 on top of graphene that is deposited on a sapphire wafer via chemical vapor deposition (CVD) for realizing graphene–MoS2 photodetectors. Two-color optical pump–electrical probe experiments allow for separation of light-induced carrier transfer across the graphene–MoS2 heterointerface from adsorbate-induced effects. We demonstrate that adsorbates strongly modify both magnitude and sign of the photoconductivity. This is attributed to a change of the graphene doping from p- to n-type in case adsorbates are being desorbed, while in either case, photogenerated electrons are transferred from MoS2 to graphene. This nondestructive probing method sheds light on the charge carrier transfer mechanisms and the role of adsorbates in two-dimensional (2D) heterostructure photodetectors.
The chemical vapor deposition (CVD) growth of graphene on copper is controlled by a complex interplay of substrate preparation, substrate temperature, pressure and flow of reactive gases. A large variety of recipes have been suggested in literature, often quite specific to the reactor, which is being used. Here, we report on a relation between growth rate and quality of graphene grown in a scalable 4″ CVD reactor. The growth rate is varied by substrate pre-treatment, chamber pressure, and methane to hydrogen (CH:H) ratio, respectively. We found that at lower growth rates graphene grains become hexagonal rather than randomly shaped, which leads to a reduced defect density and a sheet resistance down to 268 Ω/sq.
Structural defects in transition metal dichalcogenide (TMDC) monolayers (ML) play a significant role in determining their (opto)electronic properties, triggering numerous efforts to control defect densities during material growth or by post-growth treatments. Various types of TMDC have been successfully deposited by MOCVD (metal-organic chemical vapor deposition), which is a wafer-scale deposition technique with excellent uniformity and controllability. However, so far there are no findings on the extent to which the incorporation of defects can be controlled by growth parameters during MOCVD processes of TMDC. In this work, we investigate the effect of growth temperature and precursor ratio during MOCVD of tungsten diselenide (WSe2) on the growth of ML domains and their impact on the density of defects. The aim is to find parameter windows that enable the deposition of WSe2 ML with high crystal quality, i.e. a low density of defects. Our findings confirm that the growth temperature has a large influence on the crystal quality of TMDC, significantly stronger than found for the W to Se precursor ratio. Raising the growth temperatures in the range of 688 °C to 791 °C leads to an increase of the number of defects, dominating photoluminescence (PL) at low temperatures (5.6 K). In contrast, an increase of the molar precursor ratio (DiPSe/WCO) from 1,000 up to 100,000 leads to less defect-related PL at low temperatures.
Thermal chemical vapor deposition (TCVD) is the current method of choice to fabricate high quality, large area graphene films on catalytic copper substrates. In order to obtain sufficiently high growth rates at reduced growth temperatures an efficient dissociation of the precursor molecules already in the gas phase is required. We used plasma enhanced chemical vapor deposition (PECVD) to fabricate high quality graphene films at various temperatures. The efficient, plasma-induced dissociation of the precursor molecules results in an activation energy of 2.2 eV for the growth rate in PECVD, which is reduced by almost a factor of 2 compared to TCVD growth in the same reactor. By varying the growth time, we demonstrate that crystalline graphene grains surrounded by amorphous carbon formed during the early stage of growth merge into an almost defect-free graphene film with growth time via a recrystallization process. Almost defect-free graphene is prepared with negligible (I /I < 0.1) contributions of the D peak in Raman spectroscopy and with a sheet resistance down to 470 Ω/sq.
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