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

Reviewers' comments: 
Reviewer #1 (Remarks to the Author): 
 
Lei Liao et al. report high optoelectronic performance MoS2 transistors using a hybrid Al2O3/2D 
perovskite gate dielectric approach. Specifically, they attempt to show how net hysteresis close to 
zero can be achieved by choosing the perovskite layer such that the anti-clockwise hysteresis due 
to ionic movement in the perovskite cancels out the clockwise hysteresis due to negative charge 
trapping in the Al2O3 dielectric. They also show stable dark current with the hybrid dielectric, high 
photoresponse over visible-near IR range, good LDR and detectivity, and switching speed of the 
order of 20-30 ms. 
 
Although the photoresponsivity values, LDR and broad spectral response metrics are impressive, 
not as much the switching speed, there are substantial concerns regarding the motivation, 
proposed mechanism and supporting data. The claims are novel but not scientifically convincing as 
outlined in the questions below. 
 
(i) Accumulation of photogenerated holes at the Al2O3/perovskite interface leads to an electron 
current in the MoS2 through an electrostatic gating effect. If this is the correct understanding, 
then what is the motivation for choosing MoS2, why not Si or any other conventional 
semiconductor with much higher electron mobility than MoS2? 
 
(ii) The photoresponse at 914 nm is beyond the absorption edge of the perovskite- the authors 
attribute this to excitation from valence band to trap states (lines 136-140). 
(a) Isn’t this inconsistent with Fig. 1e where electrons are shown in the conduction band? Where 
are the trap states in this picture? 
(b) If photogenerated electrons are captured by traps from the valence band leading to 
photogenerated holes in the valence band- how do these electrons move towards the gate 
electrode under external gate bias as shown in Fig. 1e? 
 
(iii) What are the mechanisms behind recombination of photogenerated holes in 2D perovskite and 
electrostatically induced “supplemental” electrons in MoS2 – in their respective layers? 
 
(iv) The authors claim that the hysteresis due to Al2O3 cancels that due to the 2D perovskite. 
However, voltage shifts due to dielectric charge need not be simply additive/subtractive, since the 
shift does not depend on charge density but the moment of charge density. Hence the contribution 
to hysteresis due to positive ionic charges in the perovskite will be less once it is placed on top of 
Al2O3. Comparing the hysteresis in Fig. S5 due to perovskite (n=3) alone and due to Al2O3 alone 
(Fig. 2a), it seems unlikely that these will cancel out in the composite stack. Please clarify. 
 
(v) Comparing the dark transfer curves (Ids-Vgs) in Fig. 2a for Al2O3/n=3, Al2O3 only and only 
n=3 in Fig. S5 one comes up with the following inconsistencies/questions: 
(i) The threshold voltage of n=3 is nearly 0 V, that of only Al2O3 is ~-2 V and that of Al2O3/n=3 is 
nearly -3 V. This seems to indicate net fixed positive charge in Al2O3. How did it increase with n=3 
that seems to have no net fixed charge (VT ~ Vflatband)? 
(ii) n=3 has poor sub-threshold slope (visually) and on-current. How did these improve with the 
addition of a 9 nm dielectric in series? Please specify quantitative values of carrier mobility, 
subthreshold slope, VT, hysteresis, Ion and Ioff for all three key transistors- Al2O3/n=3, Al2O3 
only and only n=3, maybe in a table in the supplementary information. 
 
(vi) What could be the reason for a high gamma value of 2.3? 
 
(vii) The energy band diagram in Fig. 4a under Vg < VRc does not seem to be correct. As per Fig. 
3a the transistor is below threshold (close to being off) at large negative Vg. In this case the band 
bending should be in the opposite direction than what is shown, and MoS2 is depleted (not 
accumulated as shown). The electric field in the perovskite will be in the opposite direction leading 
to electron accumulation at Al2O3/perovskite interface. 
 
(viii) The explanation for VRC crossover is not clear- if it is due to increasing electric field in the 
perovskite, what determines the critical electrical field- corresponding to VRC, in terms of 
perovskite material parameters, where the light intensity dependence of R flips over? Further, if a 
built-in electric field due to hole accumulation opposes further hole drift to the interface (Fig. 4f), 
shouldn’t the R value saturate for increasing light intensity? 
 
(ix) Can the authors show stability of optical photoresponse over a period of time? 
 
(x) There are a large number of writing errors, the manuscript needs thorough proof reading. A 
few examples: 
(a) line 206 – “stack” should be “stacked” 
(b) line 168- “charges” should “charge” 
(c) line 92 – “consisted” should be “consisting” 

 
 
Reviewer #2 (Remarks to the Author): 
 
In this work, the authors present a simple yet universal Al2O3/2D perovskite heterostructure 
dielectric for high-performance phototransistors. The type-II energy band alignment in 2D 
perovskite facilitates the effective spatial separation of photoexcited carriers, thus inducing the 
outstanding photoresponse properties. By adding a high-k Al2O3 layer between the 2D perovskite 
layer and the semiconductor layer, the two effects of charge trapping and remnant polarization 
under gate bias are found to be neutralized with each other, resulting in low-voltage transistors of 
negligible hysteresis and improved reliability. The devices also exhibit excellent linear dynamic 
characteristics. 
 
Overall, I think this paper is sufficiently innovative and the experiments are nicely executed. I 
think the work will be of interest to diverse researchers working on perovskite materials and 2D-
material phototransistors and I would recommend its publication in Nature Communications after 
the following questions are clarified. 
 
(1) Although there has been significant progress on the development of perovskite materials in 
recent years, the poor stability is still a grand challenge yet to be tackled. How’s the stability of the 
devices reported in this paper and what are the possible approaches to improve the lifetime of 
perovskite-based devices? Some discussions would be helpful. 
 
(2) The device is capable of broadband photodetection ranging from 457 nm to 1064 nm. 
However, the absorption edge of the upper perovskite layer is located at around 800 nm. Is the 
MoS2 layer responsible for the near-infrared response? 
 
(3) Page 12, Line 5, the authors estimated the Ilight/Idark ratio to be 2.6 × 10^6 at Vgs = -3.5 V 
where the incident power density (Plight) is ~1 mW/cm2. The electrical measurements were 
carried out by varying the intensity from ~0.01 - ~1 mW/cm2. In this case how can the author say 
that Plight is as low as 1 mW/cm2? 
 
(4) What are the channel length and width of the phototransistors? 
 (5) First of all, Fig. 2b has not been mentioned in the manuscript. Furthermore, I assumed the 
arrows in Fig. 2b is for double sweep to show hysteresis (or lack thereof) in the Ids-Vds 
characteristics of the phototransistors. Usually people present hysteresis on transfer Ids-Vgs 
characteristics of the devices (e.g. Fig. 2a of the paper). It is not clear to me why it is necessary to 
also present it for the Ids-Vds characteristics. 
 
 
 
Reviewer #3 (Remarks to the Author): 
 
Jiang et al. have demonstrated the fabrication of a high performance MoS2 phototransistor 
employing a novel Al2O3/2D perovskite hybrid dielectric configuration. The results highlight the 
achievement of both ultrahigh responsivity and excellent linear dynamic range in one device. In 
addition, this work on Al2O3/2D perovskite dielectric to achieve greatly improved operational 
stability is very crucial for the technology to be applied in practical fields. I appreciate the 
experimental work which is systematically performed and discussed. Considering that the overall 
quality and significance of this work, I would like to recommend this work for publication after the 
authors address the following questions: 
1. In the Al2O3/2D perovskite hybrid dielectric, the thickness of Al2O3 layer is 9 nm. In my 
understanding, the Al2O3 layer is employed as a high-k dielectric, and the thinner thickness could 
help to enhance the gate capacitance. Does the Al2O3 thickness influence the performance of the 
device? 
2. In Figure 3d, the photoresponsivity remains almost constant within the light power density 
ranging from 7.2 to 1232.2 μW/cm2. What is the R-squared (coefficient of determination) of the 
linear fitting for the device? 
3. Why can the devices detect near-infrared light? The perovskites are usually employed for visible 
light detection, which is in accordance with the absorption spectrum shown in Figure S1. 
4. Line 8, Page 3. the sentence “...is also become more eminent.” should be corrected; Line 7, 
Page 10. “...and S denotes the active area of photodetector, ...” Compared to the illumination 
area, which is larger in the experiment setup? The authors should specify this value in the 
manuscript. 
5. Line 16-18, Page 12. “In general, the γ value is in the range of 0 < γ <1 for photogating 
devices, ...photoconductive effect31. ” The author should elaborate more on this point, as this 
value depends also on the surface defects and quality of film. 
6. In Figure 2a and Figure 5a, what is the source-drain voltage used in the measurements?