Review Reports - The Time Response of a Uniformly Doped Transmission-Mode NEA AlGaN Photocathode Applied to a Solar-Blind Ultraviolet Detecting System

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear authors,

The paper discusses the time-resolved characteristics of AlGaN photocathodes for solar-blind ultraviolet detection systems, using a theoretical model to analyze various parameters affecting the photocathode's response time. While the study presents interesting results and shortened response time of 9.91ps, there are areas that could be improved, such as expanding the literature review, providing more details on the numerical methods used, and including experimental validation of the theoretical results.

While the study presents a systematic theoretical analysis using the photoelectron diffusion model, several issues need to be addressed:

Parameter Justification: The choice of parameters used in the modeling (e.g., specific ranges for diffusion coefficients and recombination rates) lacks clear justification. The authors should explain the rationale behind these choices and discuss their relevance to real-world applications.
Many assumptions are made and not clearly discussed such as the choice of 0.7 Al group in AlGaN. Reflectance R, F and absorption coefficient relationship are not clearly explained. The author is referring to a pulse width in equation (6) but no discussion is given for its implication in this study.
Model Limitations: The paper does not adequately address the limitations of the model, particularly regarding ideal material properties assumed versus real-world AlGaN layers that often contain defects. The description given in section 3 doesn't discuss the reasons of the response time saturation neither the k ink observed in the electron current density versus Time curve.
Numerical Methods: The description of the numerical methods used to solve the equations is insufficient for reproducibility. More details on the matrix difference technique employed should be provided.

The results presented are largely intuitive and align with basic semiconductor physics principles. However, several improvements are necessary:

Experimental Validation: The complete lack of experimental data to corroborate the theoretical findings significantly limits the paper's impact. The authors should either provide experimental comparisons or clearly state this limitation and suggest ways to validate their results in future work.
Literature Context: There is a noticeable absence of discussion on relevant literature, particularly recent developments in AlGaN photocathode technology. The authors should expand their literature review to better contextualize their findings within the current state of the field.
Result Interpretation: While the analysis is systematic, the interpretation of results, particularly the practical implications for photocathode design, needs expansion.
Comparative Analysis: The paper would benefit from comparing its results with alternative theoretical models or approaches used in the field.

The presentation of the results requires improvement:

Figures: The plots need significant enhancement. The ticks, labels, and legends are too small, making them difficult to read. The authors should ensure all graphical elements are clear and legible. A zoom on the peak area could be made as an inset.

While the paper presents an interesting theoretical study, several aspects of scientific rigor need improvement:

Methodology Details: More information on the numerical methods and their implementation is required for reproducibility.
Error Analysis: A discussion of potential sources of error in the model and calculations is missing and should be included.
Practical Implications: The authors should expand on the practical implications of their findings for the design and optimization of AlGaN photocathodes.

Comments on the Quality of English Language

The concept presented in the paper is highly relevant and of significant interest to the community; however, the clarity of the English language and some of the writing is problematic. Several paragraphs are particularly confusing and could benefit from revision.

For instance:

- The phrase "transmitted photocathode" (lines 60-63) is ambiguous. It is unclear whether the author is referring to a photocathode operating in transmission mode or something else. This ambiguity needs to be clarified for the reader.

- The explanation in lines 70-72 is also confusing. The phrase "photoelectrons quickly heat to the bottom" is unclear and should be rewritten to convey a more precise meaning.

- The content in lines 97-104 is redundant and lacks coherence. The sentences should be restructured to improve readability and logical flow.

- In the conclusion section (line 167), the term "these parameters" is vague. The specific parameters being referred to should be explicitly stated.

Overall, the paper would greatly benefit from a thorough revision to improve sentence structure and clarity.

Author Response

Reply to the Reviewer’s Comments

Dear Editor and Reviewers:

Thank you very much for your email, regarding the reviewers’ comments on our manuscript (manuscript No: photonics-3175341) entitled “The Time Response of uniformly doped transmission-mode NEA AlGaN Photocathode Applied to Solar-Blind Ultraviolet Detecting System”. Thank you very much for your recognition of our manuscript. We are grateful for the patient corrections from the editor and reviewers, which have improved the quality and significance of our manuscript. According to the reviewers’ comments, we have made appropriate explanations and modifications as presented below.

Reviewer 1

Comment 1) While the study presents a systematic theoretical analysis using the photoelectron diffusion model, several issues need to be addressed:

Parameter Justification: The choice of parameters used in the modeling (e.g., specific ranges for diffusion coefficients and recombination rates) lacks clear justification. The authors should explain the rationale behind these choices and discuss their relevance to real-world applications.
Many assumptions are made and not clearly discussed such as the choice of 0.7 Al group in AlGaN. Reflectance R, F and absorption coefficient relationship are not clearly explained. The author is referring to a pulse width in equation (6) but no discussion is given for its implication in this study.
Model Limitations: The paper does not adequately address the limitations of the model, particularly regarding ideal material properties assumed versus real-world AlGaN layers that often contain defects. The description given in section 3 doesn't discuss the reasons of the response time saturation neither the k ink observed in the electron current density versus Time curve.
Numerical Methods: The description of the numerical methods used to solve the equations is insufficient for reproducibility. More details on the matrix difference technique employed should be provided.

Reply 1)

The main research object of this letter is AlGaN photocathode. The response characteristic analysis of AlGaN NEA photocathode is generally obtained by solving the one-dimensional continuity equation of photoelectron transport, which mainly involves the diffusion coefficient and recombination rate. Among them, the diffusion coefficient is mainly used to measure the degree of diffusion of photoelectrons in the material, specifically, the unit time unit area, the speed or number of photoelectrons through the interface, in the practical application process, the change of the diffusion coefficient affects the time of photoelectrons escaping to the vacuum. The interface recombination rate refers to the physical quantity of the electron and hole recombination speed at the interface between the emission layer and the window layer in the photocathode. It can be seen that the interface recombination rate will reduce the sensitivity of the cathode. The specific demonstration has been completed in the book "Semiconductor Photocathode" edited by Jia Xinzhi.
Thanks for your suggestion. Firstly, it is very sorry that there is an error here. It should be 0.7 Ga content instead of Al content. It has been modified in the revised manuscript. The sentence is changed from “the Al group of AlGaN photoemission layer is set to 0.7” to “the Al group of AlGaN photoemission layer is set to 0.3” (line 95, page 3, in the revised manuscript). Since the main response band of the missile's tail flame is 275nm, the response wavelength of the UV detector is 275nm. According to the relationship between response wavelength, band gap width and Al component, the Al group of AlGaN can be divided into 0.3 and Ga group into 0.7.

Next, a detailed supplement to formula (6). In order to better study the response time characteristics of the photocathode, it is necessary to eliminate the influence of light time on the response performance of the cathode as much as possible. The pulse width of the pulse light source irradiating the photocathode is enough to be negligible compared with the response time of the cathode. In this way, we can think that the cathode only receives light source irradiation during the time from t=0- to t=0+, and produces photogenerated electron distribution in the material body, the formula is as follows:

Where R is the reflectance of the cathode light surface, is the photo-generated carrier generation rate, and N is the number of incident photons. A is the photocathode area, α is the absorption coefficient of AlGaN in the incident light band. These parameters jointly determine the concentration distribution of photogenerated electrons, and the formula has universality and importance in the semiconductor field. In addition, the formula in this paper cites relevant authoritative literature, specifically as follows:

[1] Cai Zhipeng, Yang Wenzheng, Tang Weidong, Hou Xun. Numerical analysis of temporal response of a large exponential-doping transmission-mode GaAs photocathode. Materials Science in Semiconductor Processing, 2013, 16(2), 238–244.

[2] Cai Zhipeng, Yang Wenzheng, Tang WeiDong, Hou Xun. Theoretical analysis for response characteristics for the large exponential-doping transmission-mode GaAs photocathodes. Acta. Phys. Sin, 2012, 61(18), 187901.

[3] Cai Zhipeng, Yao Juncai, Huang Wendeng, He Junfeng. Theoretical study of response characteristic of transmission-mode GaAs photocathode with exponential inner electric field. Chinese journal of luminescence, 2018, 39 (05): 661-667.

The research content of this letter does not consider the lattice matching between the material interface, but mainly focuses on the relationship between the photoelectron emission density and time when the cathode surface is irradiated by the light source, in which the trapping center introduced by the lattice defect is negligible for the electron emission time.
Firstly, the relationship between the outgoing photoelectron current density and time is obtained through the simultaneous formula and the difference calculation by using relevant software in this letter. The changed parameters are mainly the interface recombination rate, diffusion coefficient and thickness. When the above three parameters are fixed, only a unique curve of the outgoing photocurrent density with time can be obtained. And the corresponding response time can be obtained by fitting the half-peak width of the curve.

Secondly, for more details about the calculation, we fully understand that your journal attaches great importance to the data calculation process and related, but due to the laboratory policy, we cannot disclose the calculation process. If the reviewer has questions about specific data, we will try to provide more detailed explanations and explanations.

Comment 2) The results presented are largely intuitive and align with basic semiconductor physics principles. However, several improvements are necessary:

Experimental Validation: The complete lack of experimental data to corroborate the theoretical findings significantly limits the paper's impact. The authors should either provide experimental comparisons or clearly state this limitation and suggest ways to validate their results in future work.
Literature Context: There is a noticeable absence of discussion on relevant literature, particularly recent developments in AlGaN photocathode technology. The authors should expand their literature review to better contextualize their findings within the current state of the field.
Result Interpretation: While the analysis is systematic, the interpretation of results, particularly the practical implications for photocathode design, needs expansion.
Comparative Analysis: The paper would benefit from comparing its results with alternative theoretical models or approaches used in the field.

Reply 2)

Thank you very much for the reviewer's suggestion, which is indeed a problem in our work. Limited by the testing conditions in our laboratory, the work of analyzing the response time of the ultraviolet photocathode in this letter mainly focuses on the theoretical aspect. In the future, appropriate cooperation units will be sought to measure the response time of the ultraviolet photocathode to verify the accuracy of the calculation.
According to the comments of the reviewers, the relevant references to the latest research progress of AlGaN photocathode technology are added as follows. At the same time, these references have been added to the revised manuscript (line 193-204, page 6, in the revised manuscript), and the references have been renumbered. The modified parts have been marked in red.
Hao, G.H., Shi, F., Cheng, H.C., et al. Photoemission performance of thin graded structure AlGaN photocathode. Applied Optics, 2015, 54(10), 2572-2576.
Tang, G.H., Yan, F., Chen, X.L. Research on activation mechanism of AlGaN photocathodes in an ultra-high vacuum system. Materials Science in Semiconductor Processing, 2020, 118, 105210.
Liu, L., Zhangyang, X.Y., Lv, Z.S., et al. Enhanced photoemission of field-assisted NEA AlGaN nanoporous array photocathode. Materials Science and Engineering B-Advanced Functional Solid-state Materials, 2022, 277, 115606.
Zhangyang, X.Y., Liu, L., Lu, F.F., et al. Surface plasmon-enhanced AlGaN heterojunction nanorod array photocathode. Materials Science and Engineering B-Advanced Functional Solid-state Materials, 2023, 289, 116277.
Du, J.J., Cheng, H.C., Li, Y., et al. Enhanced photoemission performance of an AlGaN photocathode by a superlattice emission layer, AIP Advances, 2023, 13(9), 095301.
Wang, H.G., Ji, X.H., Hou, D.L., et al. Effect of electron lateral diffusion in transmission-mode varied-doping Al0.37Ga0.63N photocathode on resolution, Journal of Optoelectronics and Advanced Materials, 2022, 24(1), 28-34.
Thank you very much for your suggestions. I would like to supplement here with some practical significance of photocathode design.

This article primarily focuses on the application in solar-blind warning systems, specifically the solar-blind ultraviolet (UV) missile warning system. This system utilizes the intense UV radiation emitted from the tail flame of a missile during launch or flight to detect and determine the direction and flight time of an incoming missile. Rapid response time is a prerequisite for achieving swift recognition, making it a crucial Index parameter. As the AlGaN UV photocathode serves as the core component of UV detection, it directly determines the device's response time. Therefore, based on the photoelectric diffusion model, this article systematically analyzes the effects of the diffusion coefficient, interface recombination rate, and thickness factors of the photocathode material on the response time, providing theoretical guidance for subsequent designs.

However, the practical applications of fast-response UV photocathodes are not limited to missile warning systems alone. They also hold significant value in fields such as UV communication systems, environmental monitoring, and UV bi-color guidance. Therefore, research on the fast response mechanism of UV photocathodes is not only highly necessary but also of great significance.

Research on the response time of the cathode began after the birth of Spicer's three-step photoemission model. In 1984, C. C. Phillips et al. used a mod-locked argon ion activator pump as a light source to measure the response time of 50nm~500nm thick transmitted GaAs photocathode. The response time is several to dozens of Ps. In the following years, Guo Lihui, CAI Zhipeng and others began to conduct theoretical research on the electron motion process in GaAs photocathode based on the theory of photoelectron transport and achieved some research results. For ultraviolet photocathodes, the response time of ultraviolet cathodes has not been reported.

Comment 3) The presentation of the results requires improvement:

Figures: The plots need significant enhancement. The ticks, labels, and legends are too small, making them difficult to read. The authors should ensure all graphical elements are clear and legible. A zoom on the peak area could be made as an inset.

Reply 3)

Thanks very much for the reviewer's suggestion which increases the academic rigor of our manuscript. According to the reviewer’s suggestion, the Figure 2, Figure 3 and Figure 4 are changed as below in the revised manuscript.

Figure 2. The curve of the electron current density of AlGaN photocathode with different electron diffusion coefficient with time

Figure 3. The curve of the electron current density of AlGaN photocathode with different rear interface recombination rate with time

Figure 4. The Curve of the electron current density of AlGaN photocathode with different active layer thickness with time

Comment 4) While the paper presents an interesting theoretical study, several aspects of scientific rigor need improvement:

Methodology Details: More information on the numerical methods and their implementation is required for reproducibility.
Error Analysis: A discussion of potential sources of error in the model and calculations is missing and should be included.
Practical Implications: The authors should expand on the practical implications of their findings for the design and optimization of AlGaN photocathodes.

Reply 4)

The research idea of this paper is based on the photoelectron transport model, and the main calculation process is shown in the formula in the paper. However, due to laboratory policy, we are sorry to be unable to disclose too many calculation details. If the reviewer has questions about specific data, we will try to provide more detailed explanations and explanations.
The working process of photocathode is mainly divided into three steps, namely photon absorption, photoelectron transport and photon escape. This paper focuses on the calculation of the relationship between photoelectron transport and escape process and time, in which the collision and scattering process of electrons inside the material are not considered, mainly because the time of these two processes is much smaller than the time of electron transport and escape, so it is not considered in this paper. However, in the next stage of work arrangement, we will fully consider your suggestions and analyze the error factors in the transmission process.
Ultrafast radiation refers to the process of fast signal emission of alpha particles, protons, neutrons, and other particles with a time of less than 100 ps, involving a wide spectral range from X-rays to far infrared bands. By studying these particles or light waves, ultrafast radiation processes can be effectively restored and evaluated, so researchers have carried out a lot of work on the fast response detector of these photons and particles ultrafast phenomena. As the core device of the ultraviolet alarm system, the faster the response time of the photocathode means that it can quickly respond to ultraviolet radiation, capture more detailed information, and significantly improve the early warning ability and response speed of the system. In this paper, the effects of recombination rate, electron diffusion coefficient and activation layer thickness on the time response characteristics of AlGaN photocathode are studied, which provides theoretical basis for the structural design and material parameter design of AlGaN photocathode, and provides necessary theoretical guidance for the experimental research and application of fast response AlGaN photocathode.

In order to strengthen the research significance of this paper, the introduction is modified as follows.

The sentence is changed from “High quantum efficiency and high time resolution are the prerequisites for the application of AlGaN photocathode in the field of sun-blind UV missile warning system [12]. At present, researchers have focused on the theoretical research and experimental verification of the quantum efficiency model of AlGaN photocathode, and the results show that improving the photoelectron transport characteristics is an effective method to improve the quantum efficiency of AlGaN photocathode. The parameters that affect the photoelectron transport characteristics mainly include electron diffusion coefficient, rear interface recombination rate and photoemission layer thickness [13-16], but the mechanism of the above parameters on the response time of AlGaN photocathode is still unclear.” to “High quantum efficiency and high time resolution are the prerequisites for the application of AlGaN photocathode in the field of sun-blind UV missile warning system [12]. Among them, fast response time is a prerequisite for realizing rapid missile identification, which makes it a key indicator parameter. AlGaN UV photocathode, as the core component of UV detection, directly determines the response time of the device. However, at present, researchers mainly focus on improving the quantum efficiency of AlGaN photocathodes. A large number of theoretical studies and experimental verification results show that improving the photoelectron transport characteristics is an effective means to improve the quantum efficiency of AlGaN photocathodes. The parameters that affect the photoelectron transport characteristics mainly include electron diffusion coefficient, rear interface recombination rate and photoemission layer thickness [13-16], but the mechanism of the above parameters on the response time of AlGaN photocathode is still unclear.” (line 43-54, page 2, in the revised manuscript)

Comment 5) The concept presented in the paper is highly relevant and of significant interest to the community; however, the clarity of the English language and some of the writing is problematic. Several paragraphs are particularly confusing and could benefit from revision.

- The phrase "transmitted photocathode" (lines 60-63) is ambiguous. It is unclear whether the author is referring to a photocathode operating in transmission mode or something else. This ambiguity needs to be clarified for the reader.
- The explanation in lines 70-72 is also confusing. The phrase "photoelectrons quickly heat to the bottom" is unclear and should be rewritten to convey a more precise meaning.
- The content in lines 97-104 is redundant and lacks coherence. The sentences should be restructured to improve readability and logical flow.
- In the conclusion section (line 167), the term "these parameters" is vague. The specific parameters being referred to should be explicitly stated.
Overall, the paper would greatly benefit from a thorough revision to improve sentence structure and clarity.

Reply 5)

The phrase "transmitted photocathode" (lines 60-63) is incorrectly stated as "transmission photocathode" and it has been modified. (line 60-62, page 2, in the revised manuscript)
The sentence is changed from “After the photoelectrons quickly heat to the bottom of the conduction band, they follow the carrier transport equation.” to “The excited photoelectrons that transition to the bottom of conduction band follow the carrier transport equation.”(line 70-71, page 2, in the revised manuscript)
The sentence is changed from “Further considering the P-type heavily doped structural design of AlGaN photoemission layer and the technical difficulty of AlN/AlGaN epitaxial growth, the typical value of photoelectron diffusion coefficient of AlGaN photoemission layer is set as Dn=60 cm²/s, and the typical value of recombination rate of AlN/AlGaN interface is Sv=10⁵ cm/s. AlGaN photoemission layer thickness d=150 nm. Among the three parameters of AlGaN photoelectron diffusion coefficient, AlN/AlGaN interface recombination rate and AlGaN optical emission layer thickness, one parameter is calculated and analyzed, while the other two parameters remain unchanged.” to “Further considering the structural design of P-type heavily doped AlGaN photoemission layer and the technical difficulty of AlN/AlGaN epitaxial growth, the typical value of photoelectron diffusion coefficient of AlGaN photoemission layer is set as D_n=60 cm²/s, the AlN/AlGaN interface recombination rate is S_v=10⁵ cm/s, and the thickness of AlGaN optical emission layer is d=150nm. One of the parameters is calculated and analyzed, and the other two parameters remain unchanged.” (line 96-101, page 3, in the revised manuscript)
The sentence is changed from “In conclusion, the influence of these parameters on the outgoing photoelectron flux density and response time of AlGaN photocathode is analyzed in this study by the photoelectron diffusion model when the electron diffusion coefficient, the recombination rate at the back interface and the thickness of the photoemission layer are changed respectively.” to “In conclusion, the influence of electron diffusion coefficient, recombination rate at the back interface and thickness of optical emission layer on the outgoing photoelectron flux density and response time of AlGaN photocathode were analyzed by the photoelectron diffusion model. ” (line 164-167, page 5, in the revised manuscript).
In order to enhance the accuracy of this letter, the superscript and subscript of the full manuscript have been checked, and the modified part has been marked in red in the revised manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors studied the response time of an AlGaN photocathode as a function of the diffusion coefficient, interface recombination rate and activation layer thickness through numerical calculation. However, the method used in this manuscript is very common to the field and the obtained results are rather intuitive. Besides, the discussion is very general and does not highlight any uniqueness of the AlGaN system under investigation. Apart from the concerns about the novelty of this study, there are also a few clarifying questions that need to be addressed. Please see the comments below:

1. The focus of this work seems to be on solving the well-studied “three-step model” equations applicable to all material systems. It is important to highlight what is unique about the AlGaN system and why it warrants a dedicated study. Are any of the conclusions drawn specific to the AlGaN system?

2. The uniform doping in AlGaN does not necessarily imply the absence of built-in electric field. In fact, due to the polarization effect, there exists built-in electric field in AlGaN even without chemical doping, see e.g., Z. Zhang, Appl. Phys. Lett. 122, 212106 (2023), https://doi.org/10.1063/5.0145826. Please perform a self-consistent energy band calculation taking into account the polarization charges and show the results.

3. What is the p-type doping concentration of AlGaN? How does that impact the photocathode performance? Please also comment on the technical challenges to achieve p-doping in high Alcontent AlGaN.

4. Please explain how the initial condition in eqn. (4) is derived. What is the origin of the coordinate? It will be extremely helpful to include an example of the photoelectron density profile and clearly label the meaning of each term in eqns. (1)-(6).

5. Please add references for the typical values of diffusion coefficient and interface recombination rate used in this manuscript. Experimentally how can one control the diffusion coefficient and the recombination rate?

6. What do the 𝑡𝑚 and Δ𝑡 in Figs. 2(b), 3(b) and 4(b) represent respectively?

7. The title of section 3.2 needs to be changed. The “GaAs” in Fig. 4 caption should be “AlGaN”

Comments for author File: Comments.pdf

Author Response

Reply to the Reviewer’s Comments

Dear Editor and Reviewers:

Reviewer 2

Comment 1) The focus of this work seems to be on solving the well-studied “three-step model” equations applicable to all material systems. It is important to highlight what is unique about the AlGaN system and why it warrants a dedicated study. Are any of the conclusions drawn specific to the AlGaN system?

Reply 1) It is true that the "three-step model" is applicable to all types of photocathode materials, but the AlGaN photocathode is mainly studied in this paper, because AlGaN photocathode is the core device of UV detector, and the response speed of AlGaN photocathode directly determines the response speed of UV detector. Therefore, it is necessary to study the influencing factors. In addition, in the model parameter setting in this paper, the incident wavelength is set to 275nm and the Al component is set to 0.3. Therefore, the conclusion is drawn for the AlGaN system.

Comment 2) The uniform doping in AlGaN does not necessarily imply the absence of built-in electric field. In fact, due to the polarization effect, there exists built-in electric field in AlGaN even without chemical doping, see e.g., Z. Zhang, Appl. Phys. Lett. 122, 212106 (2023), https://doi.org/10.1063/5.0145826. Please perform a self-consistent energy band calculation taking into account the polarization charges and show the results.

Reply 2) This paper mainly considers the theoretical calculation of uniform doping, which provides a theoretical basis for the subsequent preparation of fast response photocathode. The calculation formula of electric field strength is introduced as follows.

Where, β is the coefficient factor related to the doping function. Generally speaking, when the material is variously doped, the internal electric field will play a practical role in the motion velocity of electrons. When the material is uniformly doped, the existence of weak electric field caused by the concentration difference due to the diffusion movement of electrons will not have a great impact on the motion velocity of electrons. Therefore, the built-in electric field is not taken into account in this paper. In the future, we will conduct an in-depth analysis of the materials of the variable doping system to clarify the effect of the internal electric field on the electron transport speed.

Comment 3) What is the p-type doping concentration of AlGaN? How does that impact the photocathode performance? Please also comment on the technical challenges to achieve p-doping in high Al content AlGaN.

Reply 3) The p-type doping concentration set in the model is 10¹⁸ cm^-3. Heavy p-type doping in the emission layer can make the depletion region width of the emission layer surface as narrow as possible, the ionization impurity travel distance is minimal, and the scattering of photoelectrons before emission is minimal, so as to improve the escape probability of photoelectrons and improve their quantum efficiency.

The first challenges to achieve p-doping in high Al content of AlGaN is that Mg, as a P-type doping source, has a high ionization energy E_a (125~215 meV) and is difficult to be ionized ^[1-2]. In addition, Mg also forms Mg-H complexes with H atoms introduced during material growth, causing it to lose acceptor properties. Particularly, with the increase of Al component in AlGaN material, the material band gap increases, making it more difficult to obtain p-type AlGaN with high hole concentration ^[3-4].

References:

[1] Kozodoy, P.; Xing. H. Heavy Doping Effects in Mg-doped GaN. J. Appl. Phys. 2000, 87(4): 1832-1835.

[2] Oblo, H.; Bachem, K.H.; Kaufmann, U.; Kunzer, M.; Maier, M.; Ramakrishnan, A.; Schlotter, P. Self-compensation in Mg doped p-type GaN grown by MOCVD. J. Cryst. Growth. 1998, 195(1-4): 270-273.

[3] Li, J.; Oder, T.N.; Nakarmi, M.L.; Lin, J.Y.; Jiang, H.X. Optical and electrical properties of Mg-doped p-type Al_xGa_1–xN. Appl. Phys. Lett. 2002, 80, 1210-1212.

[4] Yang, M.Z; Chang, B.K; Hao, G.H; Shi, F.; Wang, H.G; Wang, M.S. Research on electronic structure and optical properties of Mg doped Ga0.75Al0.25N. Opt. Mater. 2014, 36, 787-796.

Comment 4) Please explain how the initial condition in eqn. (4) is derived. What is the origin of the coordinate? It will be extremely helpful to include an example of the photoelectron density profile and clearly label the meaning of each term in eqns. (1)-(6).

Reply 4) As shown in the Figure R1 below, after photons of various frequencies pass through the window layer, the activation layer can absorb photons and produce photoelectrons only when the energy of the photon is greater than the band gap of the activation layer but smaller than the band gap of the window layer.

Figure R1. Energy band diagram of transmitted ultraviolet photocathode

The cathode is irradiated by a light source in the time t=0- to t=0+, and immediately produces a distribution of photogenerated electrons in the body. The formula for photogenerated electrons is as follows:

(1)

When t > 0, the resulting photoelectrons are diffused towards the surface of x=L+d, and the distribution of the time-born electrons in the material must be a function of time. According to the continuity equation of charge carriers in semiconductors, the distribution of photogenerated electrons in the cathode when t > 0 should meet the following boundary conditions:

(2)

(3)

(4)

(5)

Where, S_v is the interface recombination rate, and D_n is the diffusion coefficient of photoelectrons in the AlGaN photoemission layer; τ_n is the lifetime of photoelectrons in the AlGaN photoemission layer, △n is the distribution of photoelectrons, and d is the thickness of the activation layer. The meaning of the boundary condition (3) is that the photogenerated electrons diffused into the interface of the window layer/active layer are recombined through the interface state. The meaning of the boundary (4) is that some of the photogenerated electrons diffused to x= d are emitted into the vacuum, and the other part is recombined through the surface state, where an accumulation of photogenerated electrons is formed, and thus equal to 0. By mathematically solving the above equation (2) ~ (5), it can be obtained △n(x, t), and then substituting the value into the following equation, it can be obtained that the density of the outgoing photoelectron changes with time.

(6)

Comment 5) Please add references for the typical values of diffusion coefficient and interface recombination rate used in this manuscript. Experimentally how can one control the diffusion coefficient and the recombination rate?

Reply 5) According to the reviewer's comments, the references are added as follows: (line 215-220, page 6, in the revised manuscript)

Gutierrez, W.A., Wilson, H.L., Yee, E.M. GaAs transmission photocathode grown by hybrid epitaxy. Applied Physics Letters, 1974, 25(9): 482-483.
Hyder, S.B. The film GaAs photocathodes deposited on single crystal sapphire by a modified rf sputtering technique. The Journal of Vacuum Science and Technology, 1971, 8(1): 228-232.
Hayfuji, H., Mizuguchi, K., Ochi, S., et al. Highly uniform grown of GaAs and GaAlAs by large-capacity MOCVD reactor. Journal of Crystal Grown, 1986, 77: 281-285.

The diffusion coefficient is related to the scattering effect in the semiconductor material, and the scattering effect in the semiconductor is related to the crystal mass of the material, that is, the more lattice defects in the material, the greater the scattering effect and the smaller the diffusion coefficient. Therefore, in the experiment, it is necessary to optimize the crystal quality of the epitaxial material and reduce the internal defects of the material.

Interfacial recombination rate refers to the recombination rate of photoelectrons at the interface of a material. The higher the interface recombination rate, the more photoelectrons are lost at the interface and the fewer photoelectrons reach the surface, thus affecting the quantum efficiency of the photocathode. The interface recombination rate is related to the defects at the interface, which result from lattice mismatch and thermal mismatch between the window layer and the active layer. Therefore, it is necessary to strictly control the material epitaxial growth process, reduce mismatched dislocations and defects, improve the crystal quality of materials, and effectively reduce the interface recombination rate.

Comment 6) What do the ?? and Δ? in Figs. 2(b), 3(b) and 4(b) represent respectively?

Reply 6) In Figs. 2(b), 3(b) and 4(b), t_m refers to the response time corresponding to the peak photocurrent density and Δ? refers to the response time of the photocathode, that is, the half-height width of the photocurrent density curve.

Comment 7) The title of section 3.2 needs to be changed. The “GaAs” in Fig. 4 caption should be “AlGaN”.

Reply 7) Thanks very much for the reviewer's suggestion which increases the academic rigor of our manuscript. According to the reviewer’s suggestion, the sentence is changed from “3.2. Effect of photoelectron diffusion coefficient on response time of AlGaN photoemission layer” to “3.2. Effect of photoelectron different rear interface recombination rate on response time of AlGaN photoemission layer”(line 122-123, page 4, in the revised manuscript).

Meanwhile, the illustration of Figure 4 is changed from “Figure 4. The Curve of the electron current density of GaAs photocathode with different active layer thickness with time” to “Figure 4. The Curve of the electron current density of AlGaN photocathode with different active layer thickness with time” (line 161-162, page 5, in the revised manuscript).

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This paper discussed the factors that would have impact on response time in AlGaN photocathode with the application in solar-blin ultraviolet detecting systems. From my opinion this paper needs to address the following issues before publishing on Photonics.

1. In part 2, the authors introduced a physical model for photocathode response calculation. It would be more convincing to have some discussion or comparison with other models.

2. In part 3, the authors calculated the response time based on the model created. Is there any previous report or experimental result to support the results?

3. The importance of the work might need to be reinforced.

Author Response

Reply to the Reviewer’s Comments

Dear Editor and Reviewers:

Reviewer 3

Comment 1) In part 2, the authors introduced a physical model for photocathode response calculation. It would be more convincing to have some discussion or comparison with other models.

Reply 1) Research on the response time of the cathode began after the birth of Spicer's three-step photoemission model. In 1984, C. C. Phillips et al. used a mod-locked argon ion activator pump as a light source to measure the response time of 50nm~500nm thick transmitted GaAs photocathode. The response time is several to dozens of Ps. In the following years, Guo Lihui, CAI Zhipeng and others began to conduct theoretical research on the electron motion process in GaAs photocathode based on the theory of photoelectron transport and achieved some research results. For ultraviolet photocathodes, the ultraviolet cathodes response time has not been reported.

Comment 2) In part 3, the authors calculated the response time based on the model created. Is there any previous report or experimental result to support the results?

Reply 2) At present, the ultraviolet cathodes response time has not been reported. Limited by the testing conditions in our laboratory, the work of analyzing the response time of the ultraviolet photocathode in this letter mainly focuses on the theoretical aspect. In the future, appropriate cooperation units will be sought to measure the response time of the ultraviolet photocathode to verify the accuracy of the calculation.

Comment 3) The importance of the work might need to be reinforced.

Reply 3) Ultrafast radiation refers to the process of fast signal emission of alpha particles, protons, neutrons, and other particles with a time of less than 100 ps, involving a wide spectral range from X-rays to far infrared bands. By studying these particles or light waves, ultrafast radiation processes can be effectively restored and evaluated, so researchers have carried out a lot of work on the fast response detector of these photons and particles ultrafast phenomena. As the core device of the ultraviolet alarm system, the faster the response time of the photocathode means that it can quickly respond to ultraviolet radiation, capture more detailed information, and significantly improve the early warning ability and response speed of the system. In this paper, the effects of recombination rate, electron diffusion coefficient and activation layer thickness on the time response characteristics of AlGaN photocathode are studied, which provides theoretical basis for the structural design and material parameter design of AlGaN photocathode, and provides necessary theoretical guidance for the experimental research and application of fast response ALGAN photocathode.

In order to strengthen the research significance of this paper, the introduction is modified as follows.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have answered most of my comments and concerns. However, in their response they failed to share the formula for comment 2.

Comments on the Quality of English Language

English has been improved form the original manuscript.

Author Response

Reply to the Reviewer’s Comments

Dear Editor and Reviewers:

Reviewer 1

Comment 1) The authors have answered most of my comments and concerns. However, in their response they failed to share the formula for comment 2.

Reply 1) About the reviewer’s last review comments “Many assumptions are made and not clearly discussed such as the choice of 0.7 Al group in AlGaN. Reflectance R, F and absorption coefficient relationship are not clearly explained. The author is referring to a pulse width in equation (6) but no discussion is given for its implication in this study.”, the relationship between reflectance R, F and absorption coefficient have been explained. And the calculation process of dividing Al group into 0.3 is given below.

Since the peak corresponding wavelength of the missile’s tail flame is 275nm, it is necessary for the UV detector to have higher sensitivity and faster response speed at 275nm. Therefore, the AlGaN detector corresponds to a response wavelength of 275nm. The relationship between wavelength and bandgap is shown in formula R1. For AlGaN materials with Al group x, the relationship between Al component and band gap is shown in formula R2 [R1].

E_g=1240/λ (R1)

E_g(Al_xGa_1-xN)=x E_g(AlN)+(1-x) E_g(GaN)-bx(1-x) (R2)

Where λ is the wavelength; E_g(Al_xGa_1-xN) is band gap for the AlGaN materials with Al group x; E_g(AlN) and E_g(GaN) are bandgap for AlN and GaN, which is 6.2 eV and 3.4 eV, respectively; b is bending coefficient, set to 1 eV [R1]. By calculation, it can be obtained that when λ is 275nm, the corresponding Al group x is 0.3.

References:

[R1] Li D, Zhang X H, Zhu Z Y, et al. First-principles calculation of structural, electronic, and optical properties of zinc-blende Al_xGa_1-xN alloys [J]. Solid State Sciences, 2011, 13:1731-1734.

Comment 2) English has been improved form the original manuscript.

Reply 2) In order to increase the academic and professional nature of this letter, English of the full manuscript has been improved. Since there are many changes, I will not list them here. The changed parts have been marked in red in the revised manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Thanks for the author’s response. However, it is felt that some of the comments were addressed in a very cursory fashion. Importantly, the manuscript used parameters from a different material system for calculation so the conclusions drawn could be misleading. Moreover, the concern about the novelty of the manuscript remains. In summary, the manuscript lacks the scientific rigor and novelty for publication.

1. The “three-step model” is widely-used and routine to the field. If the highlight of this manuscript is merely the application of this well-established model by simply plugging in the material parameters for AlGaN without providing any new insights, I highly doubt that the manuscript has sufficient novelty/significance for publication in Photonics.

2. As has been mentioned in the original comment#2, unlike Si or GaAs, the AlGaN material system has broken inversion symmetry that gives rise to built-in electric field in the absence of any chemical doping. This built-in electric field is strong and is fundamental to the formation of two-dimensional electron and hole gases, see e.g., Ł. Janicki, Phys. Status Solidi RRL, 15, 2000573 (2021). I urge the authors to take this built-in electric field which is unique to the AlGaN system into serious consideration.

3. The material parameters from Refs. 17-19 are for GaAs not AlGaN. Applying the same set of parameters to AlGaN is unjustified. Since almost all the calculations of this manuscript depend on these parameters, the conclusions drawn are questionable.

Author Response

Reply to the Reviewer’s Comments

Dear Editor and Reviewers:

Reviewer 2

Comment 1) The “three-step model” is widely-used and routine to the field. If the highlight of this manuscript is merely the application of this well-established model by simply plugging in the material parameters for AlGaN without providing any new insights, I highly doubt that the manuscript has sufficient novelty/significance for publication in Photonics.

Reply 1) In fact, the three-step model has been widely used in the study of photocathodes, but it is focused on the study of its quantum efficiency. In this letter, the response time of photocathode is studied, and the calculation parameters, formulas and methods are very different. In contrast, this paper obtained the relationship between the outgoing photoelectron current density and time through the simultaneous formula and the difference calculation by using relevant software. The parameter changes in this paper mainly include interface recombination rate, diffusion coefficient and thickness. When the above three parameter variables are fixed, there is and only can be a unique curve of the outgoing photoelectron current density with time. The corresponding response time can be obtained. However, due to laboratory policy, we cannot disclose the more detailed calculation process. In particular, there is no theoretical calculation and ready-made model of the response time of AlGaN photocathodes. Therefore, this letter does not simply apply the established model by simply inserting the material parameters of AlGaN and the research in this latter has certain novelty.

Comment 2) As has been mentioned in the original comment#2, unlike Si or GaAs, the AlGaN material system has broken inversion symmetry that gives rise to built-in electric field in the absence of any chemical doping. This built-in electric field is strong and is fundamental to the formation of two-dimensional electron and hole gases, see e.g., Ł. Janicki, Phys. Status Solidi RRL, 15, 2000573 (2021). I urge the authors to take this built-in electric field which is unique to the AlGaN system into serious consideration.

Reply 2) In fact, the theoretical calculation is mainly based on uniformly doped AlGaN materials. Generally speaking, when the material is variously doped, the internal electric field will play a practical role in the electron motion velocity. The existence of weak electric field caused by the concentration difference of uniformly doped materials due to the electron diffusion motion will not have a greater impact on the electron motion velocity. In addition, the internal electric field caused by polarization effect in uniformly doped AlGaN material has relatively little influence on the motion of charge carriers in the device. Therefore, the built-in electric field is not taken into account in this letter. In the future, we will conduct an in-depth study on the influence of the built-in electric field introduced by the polarized electric field on the electron transport capacity, and revise our calculation formula to obtain more accurate calculation results.

Comment 3) The material parameters from Refs. 17-19 are for GaAs not AlGaN. Applying the same set of parameters to AlGaN is unjustified. Since almost all the calculations of this manuscript depend on these parameters, the conclusions drawn are questionable.

Reply 3) Thanks to the reviewers for their comments, which added to the professionalism of this paper. The reasons for setting the parameters of AlGaN's photoelectron diffusion coefficient, the recombination rate of AlN/AlGaN interface and the thickness of AlGaN photocathode are as follows:

The selection of the thickness of the active layer of AlGaN photocathode is related to the diffusion length. The study shows that the diffusion length of GaN is in the range of 50nm-200nm. With reference to this value, the thickness of AlGaN photocathode activation layer is selected in the range of 50nm-250nm [R1-R2]. The interface coincidence rate and electron diffusion coefficient refer to the typical parameters set in the calculation of quantum efficiency of AlGaN photocathode in the book < GaN based photocathode> [R3]. In addition, it should be noted that this paper mainly provides the calculation method and key parameters of AlGaN photocathode response time. The range of parameters is only a reference, and the variation trend of AlGaN photocathode response time and these parameters is also given. In fact, the actual effects of these parameters on the response time of AlGaN photocathodes need to be further verified by experiments, and the study in this paper only serves as a guide.

Meanwhile, the references of 17-19 are changed to “17. Bandic, Z.Z.; Bridger, P.M.; Piquette, E.C.; McGill, T.C. Electron diffusion length and lifetime in p-type GaN. Applied. Physics. Letters. 1998, 73: 3276-3281. 18. Kumakura, K; Makimoto, T; Kobayashi, N; Hashizume, T; Fukui, T; Hasegawa, H. Minority carrier diffusion length in GaN: Dislocation density and doping concentration dependence. Applied. Physics. Letters. 2005, 86: 052105. 19. Chang, B.K. GaN based photocathode. Science Press, 2018, 9787030581860.” (line 223-227, page 6 in revised manuscript)

References:

[R1] Bandic. Z. Z, Bridger. P. M, Piquette. E. C, McGill. T. C. Electron diffusion length and lifetime in p-type GaN. Applied. Physics. Letters. 1998, 73: 3276-3281.

[R2] Kumakura. K, Makimoto. T, Kobayashi. N, Hashizume. T, Fukui. T, Hasegawa. H. Minority carrier diffusion length in GaN: Dislocation density and doping concentration dependence. Applied. Physics. Letters. 2005, 86: 052105.

[R3] Chang. B. K. GaN based photocathode. 2018, Science Press, 9787030581860.

Author Response File: Author Response.pdf