Peer review of: Rational design of Al2O3/2D perovskite heterostructure dielectric for high performance MoS2 pho (2020)

RR2
 REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author):
  Although the authors have made attempts to correct several errors in the manuscript, which is an
 indicator of how poorly the manuscript was written in the first place, the scientific aspects are still
 unconvincing and inconsistent. The authors need to go back to the drawing board, perform more
 analysis and do additional experiments to justify their proposed mechanism. Some examples
 below:
 (i) The choice of MoS2 is still not clear because the device exploits neither its optical properties nor
 any specific electronic property for this device. The authors claim that improved gate control can
 lead to better short-channel effects and low power operation but this is true for logic transistor
 technologies and not necessarily phototransistors. Also, the authors have used long gate lengths
 and 5 nm thick MoS2, where short channel effects do not come into play. Which specific
 photodetector metric is being benefitted by the choice of MoS2?
 (ii) The authors have now corrected Fig. 1e, where electrons have been shown to be trapped in
 trap states in the 2D perovskite. But the authors haven’t answered the question (in rebuttal or
 revised manuscript) of how these electrons move towards the gate electrode since they are
 trapped now.
 (iii) Regarding the answer to the query on how the photogenerated holes in 2D perovskite
 recombine with supplemental electrons in MoS2 the authors have speculated “In addition, the
 carrier lifetime is most likely determined by trap-assisted recombination through the traps or
 defect levels in Al2O3 or perovskite layer27.”
 Recombination in Al2O3 is highly unlikely- the carriers are separated by a 9 nm thick dielectric, the
 conduction band offset for MoS2/Al2O3 and valence band offset for 2D perovskite/Al2O3 – both
 are large (Fig. 1e) thereby impeding tunneling into Al2O3.
 Similarly recombination in perovskite is also unlikely since electrons will have to tunnel from MoS2
 to perovskite through 9 nm Al2O3.
 (iv) The authors mention in their rebuttal letter that “it is hard to compare the hysteresis due to
 perovskite (n = 3) alone and due to Al2O3 alone.” But it is a central claim on the paper that “the
 two effects of charge trapping and ionic polarization under gate bias could be neutralized with each
 other, resulting in negligible hysteresis in MoS2 phototransistor.” A mathematical analysis of how
 this is achieved, is critical to support this claim. For instance, if it was a simple subtractive/additive
 process, can the authors show that an MoS2/peroskite (n=3)/Al2O3/gate metal stack also has zero
 hysteresis?
 (v) The authors’ analysis of series capacitance effect (response to question 5) and Vth does not
 seem to be correct.
 (a) If the 2D perovskite had a large low frequency dielectric constant then the “only n=3” should
 have better sub-threshold characteristics than “only Al2O3” instead of 220 mV/decade vs 93
 mV/decade
 Also, irrespective of how high the dielectric constant is, SS cannot improve from 93 to 82 mv/dec
 by adding Al2O3 in series.
 (b) The Vth of Al2O3 and Al2O3/PEA (n=3) are different by nearly a Volt in the figure below (see
 pdf copy being attached). How did the authors extract values of -3.7 and -3.8 V?

 (vi) The authors have tried to correct the band diagram in Fig. 4a for MoS2 based on my inputs.
 But, to stay consistent with their mechanisms, they had to keep the band bending direction the
 same in the 2D perovskite, leading to a discontinuity in electric field (hence new questions on
 interface charge etc.) at the Al2O3/perovskite interface. This has led to them to come up with
 highly speculative reasons (such as 2D perovskite phases) to justify the band bending in the
 perovskite.
 This is a glaring example of poorly understood device physics and mechanisms, where principles of
 basic semiconductor textbook physics seem to have been glossed over.
 Reviewer #2 (Remarks to the Author):
Reviewer #3 (Remarks to the Author):
The authors have addressed my comments carefully. The paper can be published now.
 The authors have addressed all my previous comments in this revision. I recommend its
 publication in Nature Communications.