MICROSCOPY & IMAGING
instrument – the Park NX-Hivac AFM. While the topography images in air and high vacuum appeared largely similar (Fig. 3a and b), C-AFM data diff ered signifi cantly (Fig. 3c and d). Here, the average current increased by three orders of magnitude in high vacuum compared to the measurement in air. In air, the average current was 1.4nA at 5V sample bias. In high vacuum on the other hand the average current increased to 1.1 μA with the same sample bias. T e increase in average current originates from the removal of the thin layer of water that forms on the surface of materials in air due to the humidity level in ambient conditions. T is layer of water
is particularly problematic for MoS2 because water p-dopes the material and therefore becomes insulating. T e C-AFM results agree with the results of previous studies, in which researchers showed that the on-state current and charge-carrier mobility in MoS2
the average current, C-AFM improved in sensitivity for the measurements in a high vacuum environment. While the current image recorded in air appeared largely homogeneous without defi ned features, C-AFM in high vacuum was able to detect fi ne structures in the current channel. As such, C-AFM revealed that the current at grain boundaries and step edges of the MoS2
islands decreases. Fig. 4 shows semiconductor devices dropped
signifi cantly after contact with deionised water. In addition to the overall increase in
commercial semiconductor devices. T is correlative current-topography behaviour may have been missed if not for the increased sensitivity of C-AFM in high vacuum. T is study examined the morphological and electrical properties of the 2D material MoS2
the reduced current fl ow at step edges on a 1-2-layer sample that could not be resolved in air (Fig. 3c). T e decrease in the detected current at the position of step edges and grain boundaries translates into a local reduction in conductivity. T us, these topographic features act as electrostatic barriers on the charge- carrier transport and are detrimental for the implementation of MoS2
in . T e direct comparison of
cAFM measurements in air and in a vacuum <1·10-5
Torr demonstrated that
the water layer on the sample surface in air considerably lowered the reliability of C-AFM. Removal of this water layer in high vacuum increased the current by three orders of magnitude. Furthermore, C-AFM in high vacuum allowed imaging of fi ner structures, thus enabling researchers to resolve the reduced current fl ow at the MoS2
step edges.
References 1. Chiappe, D. et al. Layer-controlled epitaxy of 2D semiconductors: bridging nanoscale phenomena to wafer-scale uniformity. Nanotechnology 29, 425602 (2018).
2. Ludwig, J. et al. Eff ects of buried grain boundaries in multilayer MoS2. Nanotechnology 30, 285705 (2019).
Jonathan Ludwig and Kristof Paredis are with IMEC. For more information visit
www.parksystems.com/hivac
www.scientistlive.com 69
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72
