Current Drops in CF₄ Plasma-Treated AlGaN/GaN Heterojunction in Polar Gas Ambient

Round 1

Reviewer 1 Report

In this paper, the author reported the polar gas sensing characteristics of the AlGaN/GaN heterojunction sensor and enhanced sensing characteristics after CF₄ plasma treatment. Overall paper was presented in a good way and I recommend the publication of this work before some issues are resolved.

Q1) The current drop of the AlGaN/GaN sensor device is ethanol < acetonitrile< acetic acid however it changes to ethanol >acetonitrile< acetic acid after CF₄ plasma treatment, it will be better to discuss the changed pattern of the current drop after CF₄ plasma treatment.

Q2) Use the same name of the device in the figure and the text (either Type-a or Type-A).

Q3) It is recommended to add a reference on AlGaN/GaN photodetector to expand the introduction for attracting the physics community’s interest.

-          Sci. Rep. 10, 22059 (2020) (10.1038/s41598-020-79135-y).

Q4) The appropriate reference should provide for this statement “People always found that the current of the fluorine plasma treated AlGaN/GaN heterojunction is not very stable even in air atmosphere. It possibly attributes to the polar gases exiting in air atmosphere, which remove the “ambient dopants” from the surface.”

 

Q5) There are many typing mistakes such as the use of appropriate superscripts and subscripts in the abstract and experimental parts, some references are not complete, and please use either the abbreviated or full name of the reference article according to the author's guidelines. 

Author Response

Q1) The current drop of the AlGaN/GaN sensor device is ethanol < acetonitrile< acetic acid however it changes to ethanol >acetonitrile< acetic acid after CF4 plasma treatment, it will be better to discuss the changed pattern of the current drop after CF4 plasma treatment.

Our response:

  Thanks for reviewer’s comments. I am so sorry for the misunderstanding caused by the bad labeling order in Figure 3b, I changed the order in the legend as shown in Figure 3b.

Figure 3. (b) Normalized I - t curves affected by polar liquids.

 

The currents of AL without F plasma treatment were reduced about 1.9 mA (16 %), 2.2 mA (17 %) and 2.7 mA (19 %) by ethanol, acetic acid and acetonitrile liquids, (ethanol<acetic acid <acetonitrile). The currents of BL with F plasma treatment were reduced 2.6 mA (54 %), 4.1 mA (61 %), and 4.17 mA (63 %) by ethanol, acetic acid and acetonitrile liquids, (ethanol<acetic acid <acetonitrile).

Corresponding change in manuscript: Yes

Location of Change:

                         Section:  2. Experiment

                         Page 4, Fig.3b. line 3.

 

Q2) Use the same name of the device in the figure and the text (either Type-a or Type-A).

Our response:

Thanks for reviewer’s comments. We used the same name (Type-A) of the device in the figure and the text.

 

 

Figure 1. Schematic diagram of the device structures of Type-A without plasma treatment and Type-B with CF₄ plasma treatment.

 

Corresponding change in manuscript: Yes

Location of Change:

                         Section:  2. Experiment

                         Page 2, Fig.1.

 

Q3) It is recommended to add a reference on AlGaN/GaN photodetector to expand the introduction for attracting the physics community’s interest.

Our response:

Thanks for reviewer’s comments. I have added the application of GaN in UV detection in the paper, as shown below: showing greater application potential in high frequency [1-3], high power [4-6], ultraviolet [UV] photodetector[7-9] and sensor areas[10-15].

7. Pandit B, Schubert EF, Cho J. Dual-functional ultraviolet photodetector with graphene electrodes on AlGaN/GaN heterostructure. Sci Rep. 2020;10(1):22059.

Corresponding change in manuscript: Yes

Location of Change:

                         Section:  1. Introduction

                         Page 1, line 37.

 

Q4) The appropriate reference should provide for this statement “People always found that the current of the fluorine plasma treated AlGaN/GaN heterojunction is not very stable even in air atmosphere. It possibly attributes to the polar gases exiting in air atmosphere, which remove the “ambient dopants” from the surface.”

Our response:

Thanks for reviewer’s comments. We have added references in the appropriate places, respectively, as shown below: People always found that the current of the fluorine plasma treated AlGaN/GaN heterojunction is not very stable even in air atmosphere [27, 28]. It possibly attributes to the polar gases exiting in air atmosphere [19], which remove the “ambient dopants” from the surface [23].

19. Neuberger R, Muller G, Ambacher O, Stutzmann M. High-electron-mobility AlGaN/GaN transistors (HEMTs) for fluid monitoring applications. Phys Status Solidi A. 2001;185(1):85-9.
20. Ridley BK, Ambacher O, Eastman LF. The polarization-induced electron gas in a heterostructure. Semicond Sci Tech. 2000;15(3):270-1.
21. Huang S, Chen H, Chen KJ, editors. Surface properties of AlxGa1-xN/GaN heterostructures treated by fluorine plasma: an XPS study. International Workshop on Nitride Semiconductors (IWN)/Fall Meeting of the European-Materials-Research-Society (E-MRS)/Symposium N/Symposium H; 2010 2011 Sep 19-24; Tampa, FL2011.
22. Wang MJ, Yuan L, Cheng CC, Beling CD, Chen KJ. Defect formation and annealing behaviors of fluorine-implanted GaN layers revealed by positron annihilation spectroscopy. Applied Physics Letters. 2009;94(6).

Corresponding change in manuscript: Yes

Location of Change:

                         Section:  4. Discussion

                         Page 6, line 49

 

Q5) There are many typing mistakes such as the use of appropriate superscripts and subscripts in the abstract and experimental parts, some references are not complete, and please use either the abbreviated or full name of the reference article according to the author's guidelines.

Our response:

Thanks for reviewer’s comments. We have checked the spelling and added references.

Corresponding change in manuscript: Yes

Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript presents the current drops observed in CF₄ plasma treated AlGaN/GaN heterojunction in a polar gas ambient, which can possibly be explained by the proposed model based on “ambient doping”. Please find the comments below.

1) The “L-device” and “S-device” have different dimensions. The reviewer recommends using the normalized current I_ds/(W/L) in Fig. 2.

2) The manuscript needs significant improvement in terms of writing quality and proofreading. For example:

Page 1: “In this letter, we report the current drops of CF4 plasma treated AlGaN/GaN heterojunction in po-lar gas ambient.”

Page 4: “Comparisons between AS and BS shows the same features as that between AL and BL.”

Page 5: “The original charge couplings are broken, new charge couplings are established.”

Page 5: “not only some of the couplings of surface negative polarization charges with surface positive ions are broken, but also some of the couplings of F- charges with surface positive ions are broken either.”

Page 6: “When surface positive ions were removed, the coupling of F- charge and surface positive ions was broken, those F- charges couple with the polarization positive charges at AlGaN/GaN interface.”

Page 6: “The initial current is a most important issue that affects the current sensitivity.”

 

 

 

 

Author Response

1) The “L-device” and “S-device” have different dimensions. The reviewer recommends using the normalized current I_ds/(W/L) in Fig. 2.

Our response:

Thanks for reviewer’s constructive comment. We accepted this suggestion and have refined the figure 2 in a normalized format.

Figure 2. The I-V curves of AL, AS, BL, and BS, respectively.

Corresponding change in manuscript: Yes

Location of Change:

                         Section:  2. Experiment

                         Page 3, Fig.2a.        Page 2, line 39.

 

2) The manuscript needs significant improvement in terms of writing quality and proofreading. For example:

 

Page 1: “This letter reported the phenomenon of current drops in AlGaN/GaN heterojunction with CF₄ plasma treated in polar gas ambient.”

 

Page 4: “Comparisons between AS and BS shows the same features as between AL and BL.”

 

Page 5: “The original charge couplings are broken, and new charge couplings are established.”

 

Page 5: “not only some of the couplings of surface negative polarization charges with surface positive ions are broken, but also some of the couplings of F- charges with surface positive ions are broken either.”

 

Page 6: “When surface positive ions were removed, the coupling of F- charge and surface positive ions was broken, those F- charges couple with the polarization positive charges at AlGaN/GaN interface.”

 

Page 6: “The initial current is a most important issue that affects the current sensitivity.”

Our response:

Thanks for reviewer’s suggestion. We corrected the grammars that reviewer pointed out. And tried our best to improve writing quality and proofreading in the manuscript.

Such as

Page 1: This letter reported the phenomenon of current drops in AlGaN/GaN heterojunction with CF₄ plasma treated in polar gas ambient.

Page 4: Compared with AS and BS, AL and BL show the same differences.

Page 5: The original charge couplings was broken, and the new charge couplings are established.

Page 5: The couplings of surface negative polarization charges with surface positive ions are broken, and the couplings of F- charges with surface positive ions are broken too.

Page 6: When surface positive ions were removed, the coupling of F^- charge and surface positive ions was broken, then F^- charges couple with the polarization positive charges at AlGaN/GaN interface.

Page 6: The initial current is an important factor affecting the current sensitivity.

Corresponding change in manuscript: Yes

Location of Change:                        

1. Page 1, line 21
2. Page 4, line 17
3. Page 6, line 18
4. Page 6, line 24
5. Page 6, line 33
6. Page 7, line 6

Author Response File: Author Response.pdf

Reviewer 3 Report

It is an interesting letter targeting the influence of various gases on GaN-based HEMTs, which is important for their usage as gas sensors and to understand current behaviour under specific ambient conditions. Overall, the article is clear, although it might benefit for an English grammar review and a less colloquial style, e.g., I would suggest to rephrase  the question at the beginning of page 2. 

What is the breakdown voltage for these devices? Can the authors comment on device passivation, if any is used in this case?

It is known that the drain current transients of GaN HEMT suffer from multiples effects, such as lag, due to iter-layer trapping. These effects can be rather long, in the same order of magnitude as the tests provided here (e.g., seconds), see: Angelotti, et al. Trapping Dynamics in GaN HEMTs for Millimeter-Wave Applications: Measurement-Based Characterization and Technology Comparison. Electronics 2021. Could the authors comment if these effects can play a role in the performed experiments?

Could the authors describe and comment on the measurement setup? Indeed, it seems to be important to have high accuracy in the transient currents in the mA range.

 

Author Response

Q1) What is the breakdown voltage for these devices? Can the authors comment on device passivation, if any is used in this case?

Our response:

Thanks for reviewer’s comments. The devices are ungated-AlGaN/GaN HEMTs that include the source and drain electrodes without gate electrodes. Those devices can’t be pinched-off. The off-state breakdown voltage cannot be measured. If those devices are passivated by electrics such as SiO₂ and SiN_x, they do not show sensitivities to polar liquids and gases. We didn’t passivate those devices.

 

Corresponding change in manuscript: No

 

Q2) It is known that the drain current transients of GaN HEMT suffer from multiples effects, such as lag, due to interlayer trapping. These effects can be rather long, in the same order of magnitude as the tests provided here (e.g., seconds), see: Angelotti, et al. Trapping Dynamics in GaN HEMTs for Millimeter-Wave Applications: Measurement-Based Characterization and Technology Comparison. Electronics 2021. Could the authors comment if these effects can play a role in the performed experiments?

Our response:

Thanks for reviewer’s comments. In actually, there are trapping effects in our fabricated devices. As shown in Figure 2b below. At the beginning of voltage bias, the current is a little higher, as the time ellipses, the current drops, for a time (~1000s), the current stabilizes. In our experiments, the initial currents shown in the I-t curves are the stable currents. The trapping effects were excluded in the experiments. We revised the manuscript to specify this procedure.

Figure 2b, I_ds-t curves for Types of A and B devices at V_ds=5V.

Corresponding change in manuscript: Yes

Location of Change:

                         Section:  2. Experiment

                         Page 3, Fig.2b. Page 3, line 4.

 

Q3) Could the authors describe and comment on the measurement setup? Indeed, it seems to be important to have high accuracy in the transient currents in the mA range.

Our response:

Thanks for reviewer’s comments. In our experiment, a voltage of V_ds=5V was applied on the samples with a sampling rate of 1 time per second. The equipment used in the test is Keithly 2400, and current accuracy is in the nA range, however, the change of current is in our test is in the mA range. We think this accuracy is sufficient to qualitatively specify the effect of current drop. In the future, this effect be applied to fabricate gas sensors, it is necessary to design new testing system with higher accuracy.

Corresponding change in manuscript: No

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Thanks for replying to the comments.

Author Response

Dear Reviewer:

  Thank you for taking the time to review my manuscript. Our manuscript has been revised as required. Have a nice day.

 

Best regards.

Yours sincerely,

Ying Ma