Uni-traveling-carrier photodiode based on MoS_2/GaN van der Waals heterojunction for high-speed visible-light detection

Domain: physics
Published: 2025-11-25
Editorial outcome: accept

Abstract
Uni-traveling-carrier photodiodes (UTC-PDs), which utilize only electrons as the active carriers, have become indispensable in high-speed optoelectronics due to their unique capabilities, such as high saturation power and broad bandwidth. However, extending the operating wavelengths into the visible region for wider applications is challenging due to the lack of suitable wide-bandgap III-V semiconductor combinations with the necessary band alignment and lattice matching. Here, we show that a UTC-PD based on a van der Waals heterojunction composed of a 2D transition metal dichalcogenide, molybdenum disulfide (MoS2), as a photoabsorption layer and a gallium nitride (GaN) film as a carrier collection layer, offers a solution to this challenge. The fast vertical carrier transport across the heterointerface is enabled by the direct epitaxial growth of a MoS2 layer on a GaN film. Our device demonstrates a frequency response in the several-GHz range with a quantum efficiency on the order of 1% throughout the entire visible spectrum, highlighting the promise for high-speed visible optoelectronics.

Introduction
Wide-bandgap semiconductors offer intrinsically favorable electronic properties such as high-power operation, high breakdown voltage, and high-temperature stability1,2. In particular, gallium nitride (GaN) has attracted attention owing to its high electron mobility, large electron saturation velocity3,4. GaN-based transistors have already become key elements in high-frequency electronic devices and wireless communications5,6. It is thus natural to anticipate that GaN-based photodiodes can exhibit excellent high-speed high-power characteristics in the visible wavelength range. Such high-speed wide-bandgap photodiodes can become key components in emerging visible-light technologies, since visible light offers higher directivity, lower absorption in water, and higher spatial resolution compared to near-infrared light. Applications include intersatellite communication7, visible-light communication (Li-Fi), particularly for underwater communication8,9, and high-resolution distance sensing technologies such as the Light Detection And Ranging (LiDAR) and Optical Coherence Tomography (OCT)10,11.

Uni-traveling carrier photodiodes (UTC-PDs)12,13 are one of the most successful examples of high-speed photonic devices in the telecommunication industry. They utilize the type-II band-alignment of the photoabsorption layer and diffusion block layer, which allows only electrons to transit in the carrier collection layer, to maximize response bandwidth and suppress space-charge saturation14. Adapting this concept to the visible range is nonetheless challenging, as it requires semiconductor combinations with specific band alignment and good lattice matching15, which, to our knowledge, have not yet been realized for GaN-based materials. Moreover, InGaN ternary alloys need to be inserted to shift the absorption edge of GaN (360 nm, corresponding to the bandgap ~ 3.4 eV) to the visible range. The InGaN/GaN quantum-well structure, however, reduces bandwidth due to slow carrier escape dynamics, resulting in the bandwidth typically in several hundred MHz to a few GHz16,17.

Recent advances have shown that two-dimensional (2D) transition metal dichalcogenides, such as molybdenum disulfide (MoS2), offer unique electrical and photonic properties. The combination of MoS2 with GaN is highly promising for high-performance photoabsorption layers, owing to the material system’s small lattice mismatch18–20, type-II band alignment21–23, and strong light-matter interaction in the visible region24–26. However, several groups have explored MoS2/GaN heterojunction photodetectors27–31, finding relatively slow responses on the order of microseconds or longer32–35. Such slow responses might be attributed to the imperfect interface of the heterojunction34. Although the intrinsic speed capabilities of MoS2/GaN interfaces remain unclear, we note that the ultrafast (less than 100 fs) carrier transport across heterointerfaces of 2D materials has been reported36,37, suggesting the intrinsic speed capabilities of the MoS2/GaN system have yet to be fully realized.

In this work, we report the successful realization of a UTC-PD based on a MoS2/GaN van der Waals heterojunction, which exhibits frequency response exceeding several GHz for visible light. Here, a few-layer MoS2 absorption layer is epitaxially grown directly on a GaN film using metal-organic chemical vapor deposition (MOCVD), as opposed to the stamp transfer method. The demonstrated response of our device is currently constrained by an RC limit rather than fundamental carrier transport limits, suggesting that further structural optimization can push the bandwidth to tens of GHz or higher.

Results
Formation of MoS2/GaN Type-II heterojunction
The structure and operation principle of the MoS2/GaN UTC-PD developed in this study is schematically illustrated in Fig. 1a. The device is composed of an anode electrode, an absorption layer of MoS2, an i-GaN carrier collection layer, an n-GaN contact layer, and a cathode electrode. Incident photons absorbed within the MoS2 layer generate electron-hole pairs. The holes rapidly diffuse to the anode electrode adjacent to the MoS2 layer, whereas the electrons traverse the type-II aligned conduction band interface into the i-GaN layer. Subsequently, the electrons alone travel through the externally biased i-GaN layer and eventually reach the cathode through the n-GaN contact layer. Similar to standard UTC-PDs, high-speed operation is anticipated because of the smaller effective mass and the larger saturation velocity of electrons (~3×10^7 cm/s) than those of holes (~0.6×10^7 cm/s)38.

To achieve optimal interface conditions, we employed epitaxial growth of the MoS2 layers directly onto a GaN film on a c-plane sapphire substrate using metal-organic chemical vapor deposition (MOCVD). More detailed information on the growth conditions is described in the Method section.

Raman spectroscopy analysis reveals a frequency difference Δω of 22 cm−1 between the E1 2g and A1g peaks (Fig. 1b), indicating that the MoS2 layers consist of bilayer or trilayer39. The atomic alignment of MoS2 layers on GaN is confirmed by cross-sectional high-resolution scanning transmission electron microscopy (STEM), shown in Fig. 1c. Remarkably, the MOCVD-deposited MoS2 layers are epitaxially grown on the GaN film, likely due to the small lattice mismatch (0.8%) between MoS2 and GaN crystals18. The absence of unintended interfacial layers, such as native oxide, is also evidenced by the small discrepancy between measured and theoretical interlayer atomic spacing (Supplementary Fig. 1).

Figure 1d shows the photoabsorption properties of the MoS2/GaN heterostructure. Absorption peaks are attributed to the excitonic peaks A, B, and C of the bare MoS2 absorption40,41 (inset in Fig. 1d).

In order to examine the band alignment of the MoS2/GaN van der Waals heterostructure, Kelvin Probe Force Microscopy (KPFM) measurements were conducted. This technique is capable of probing the surface potential of materials, making it a widely used method for determining the band offsets in heterostructures involving 2D materials42–44. Figure 2a shows the mapping image of the MoS2 on GaN/sapphire substrate, showing change in surface potential between MoS2 and GaN, and a line-scan of the surface potential along the indicated line in this image is reported in Fig. 2b. The results indicate the type-II band alignment of the conduction band and the offset ΔEc of 0.26 eV, as shown in Fig. 2c, which is consistent with previous reports22,23,45, thereby verifying that MoS2 layers effectively function as the active absorption layer for UTC-PD applications.

Device fabrication
Figure 3a provides a three-dimensional image of the fabricated device, whose photosensitive area is 60 μm in diameter. The device is designed to operate with illumination from the substrate side, as indicated in Fig. 3b. Photolithography and reactive ion etching (RIE) processes are utilized to form the mesa pattern of the device. The stacking structure comprises an n-GaN contact layer, an i-GaN carrier collection layer with a thickness of 0.3 μm and the carrier density ~10^16 cm−3, MoS2 photoabsorption layers. A passivation layer of SiO2 is deposited around the mesa structure. Subsequently, Cr/Au is deposited as the anode electrode, and Ti/Al/Ti/Au is deposited as the cathode electrode using sputtering techniques. Finally, Ti/Pt/Au is deposited as the pad electrode. The device exhibits high stability due to the MoS2 being encapsulated by the anode metal and the SiO2 passivation layer, which minimizes susceptibility to atmospheric moisture and oxygen, as shown in Supplementary Fig. 7.

To evaluate the basic electrical properties and demonstrate the rectifying behavior of the fabricated device, we measured the I–V characteristics shown in Figure 3c. The solid lines represent measurements under dark conditions, while the dotted lines correspond to measurements under illumination with a 405 nm laser diode at a power density of 75 W/cm2. I–V characteristics of the device measured under dark conditions clearly exhibit diode-like rectifying behavior. The summary of Hall effect measurements, sheet resistivities, mobilities, and carrier densities of MoS2 and GaN layers, are found in Supplementary Table 1. Notably, the Hall measurements of the MoS2 layers indicate n-type conduction, which may be attributed to the sulfur atom vacancies46,47. Transmission line model (TLM) measurements (described in Supplementary Fig. 3) confirms the ohmic contact between the anode electrode and MoS2 layers, indicating the rectification originates from the MoS2/GaN heterojunction rather than Schottky barrier at the contact. The physics behind the rectifying behavior of the device is discussed in Supplementary Section 3, where the two factors are considered: the crystal-momentum mismatch between the conduction electrons in MoS2 and GaN48, and the polarization charge of GaN49.

Experimental Verification
We evaluate the large-signal response of the device by measuring the time-domain photocurrent signal when illuminating the light with a wavelength of 445 nm and a pulse width of 100 μs. In Fig. 4a, the black line represents the pulse waveform of the incident light, which is taken by a Si-PIN photodiode (Hamamatsu Photonics; S9055), as a reference, while the blue line represents the photocurrent signal of the device. The photocurrent signal closely follows the pulse waveform of the incident light, where the rise and fall times are on the order of microseconds, which are limited by the time-response of the light source itself. Note that the slight distortions of the baseline and the top of the pulse in the photocurrent signal are mainly due to the peripheral circuits, as discussed in Section 6 of Supplementary Information. The photocurrent response is thus three to six orders of magnitude faster than previously reported MoS2/GaN photodetectors22,29,34,50,51. Such improvements can be attributed to the high-quality epitaxial MoS2/GaN van der Waals heterointerface, which effectively reduces carrier trapping effects at the heterojunction. Figure 4b shows the dependence of the photocurrent signal on the incident light power density. Based on the peak photocurrent values in Fig. 4b, we examine the linearity between the optical input power and the output current. Figure 4c presents the peak photocurrents as a function of the optical input power, showing excellent linearity between them and no saturation up to the maximum optical input power of approximately 100 W/cm2.

To investigate the characteristics of the device in much higher frequency regions, we examine the frequency response of the device with intensity-modulated light (Fig. 5a). Figure 5b demonstrates that the MoS2/GaN PD achieves a −3 dB bandwidth of approximately 5 GHz at a reverse bias of 4 V. This response is well explained by an RC low-pass filter model, where R corresponds to the external impedance of Rload = 50 Ω, and C corresponds to the i-GaN depletion capacitance of Cj = 0.8 pF. The equivalent circuit of the photodiode system at a reverse bias of 4 V is presented in Section 3 of Supplementary Information. Here, the device capacitance Cj is identified by capacitance-voltage (C–V) measurement as described in Supplementary Fig. 4. The cutoff frequency of 5 GHz is thus neither due to carrier escape delay in the MoS2 layer nor carrier transit time in the i-GaN layer. We can anticipate a faster response by minimizing the capacitance by, for instance, optimizing the thickness of the electron collection layer, and/or miniaturizing the size of the photosensitive area, as discussed in Section 12 of Supplementary Information.

Next, we evaluate the dependence of sensitivity and frequency response on wavelength and reverse bias voltage. Figure 6a–d show the results for the wavelengths of the light at 405 nm, 465 nm, 530 nm, and 620 nm, respectively. For measurements at 405 nm, 530 nm, and 620 nm, the intensity modulation is performed by direct modulation of the laser diodes rather than by using the separate phase modulator described in Fig. 5a. Even after smoothing, non-ignorable noise remains above 2 GHz (shaded region) for those results, which is mainly due to the larger electrical crosstalk caused by the direct modulation circuit. The broad sensitivity across the visible wavelength suggests that electron-hole pair generation occurs in the MoS2 layer rather than GaN layers. Thus, the photocurrent signal corresponds to electron transit in the i-GaN layer, in accordance with the UTC-PD model shown in Fig. 1a. Highest quantum efficiency is obtained at 405 nm, reaching approximately 1.5%, while the efficiencies are lower around 0.5% for other wavelengths (465 nm, 530 nm, and 620 nm), which is consistent with the photoabsorption spectrum (Fig. 1d). Additionally, Supplementary Fig. 6 provides further details on other photoresponse characteristics, such as responsivity, detectivity (D*), and noise equivalent power (NEP).

Figure 6e shows the internal quantum efficiencies for varying wavelengths and reverse biases. Here, the internal quantum efficiency is defined as the quantum efficiency divided by the absorption depicted in Fig. 1d, expressing what portion of electrons generated in the MoS2 layer turns into the photocurrent. The fabricated devices exhibit relatively low internal quantum efficiencies, ranging approximately from 1% to 5%, as shown in Fig. 6e. Note that, as the reverse bias increases, the internal quantum efficiency also increases for all wavelengths. Furthermore, the reverse bias dependence of the internal quantum efficiency is more significant for longer wavelengths.

Discussion
The internal quantum efficiency of the fabricated devices is significantly lower than unity that the conventional PIN photodiodes would attain, indicating that some of the photo-generated carriers do not contribute to the photocurrent. Several observations are consistent with the low internal quantum efficiency arising primarily from the crystal-momentum mismatch between electrons in MoS2 and GaN. A fuller discussion is provided in Section 10 of the Supplementary Information; we summarize a brief overview here. The band struture of bilayer MoS2 (2H phase), calculated using density functional theory (DFT), is shown in Fig. 6f. We focus on the C-exciton transition (~3 eV) lies on the Γ-point and the B-exciton transition (~2 eV) on the K-point (purple and red arrows in Fig. 6f) whose transitions can be excited by 405 nm and 620 nm illumination, respectively. For the B-exciton transition, photo-excited electrons in the K-point of MoS2 cannot efficiently transfer to GaN (whose conduction minimum lies at Γ) because of the crystal-momentum mismatch, so they eventually recombine within MoS2. Under reverse bias, thermally assisted tunneling enables carrier transfer, thereby improving internal quantum efficiencies. For the C-exciton transition (405 nm), on the other hand, part of photo-excited elctrons around the Γ-point of MoS2 can directly transfer to GaN before relaxing to the K valley, leading to a comparatively higher internal quantum efficiency. Trap-assisted tunneling might also be a carrier escape path. However, we infer that such a slow process would not significantly contribute to the observed wideband photocurrent response. Taken together, these considerations indicate that momentum-space matching is an important design guideline for 2D-material/GaN photodetectors, which has received little attention in previous studies.

In conclusion, we successfully fabricated a UTC-PD based on a MoS2/GaN van der Waals heterojunction, where the type-II aligned MoS2 layer selectively inject photoelectrons to the i-GaN collection layer. The fabricated UTC-PD worked at entire visible wavelengths with the quantum efficiency on the order of 1%. Our device exhibits an excellent frequency response, which corresponds to three to six orders of magnitude faster than those reported in earlier studies, thanks to the high-quality epitaxial van der Waals heterointerface between MoS2/GaN. Supplementary Table 2 provides a comparison between our device and previously reported 2D materials/GaN photodetectors as well as other high-speed photodetectors.

Through this study, we proved the feasibility of UTC-PDs for visible light by incorporating a 2D semiconductor layer as a photo-absorption layer and a wide-bandgap GaN film as an electron collection layer. Further material exploration and structural optimization are expected to improve the device performance and provide additional solutions in the field of high-speed visible optoelectronics.

Methods
Growth of MoS2
The MoS2 layers were directly grown onto a GaN film on a 4-inch c-plane sapphire substrate using an MOCVD system (Oxford Instruments; Nanofab 1000 Agile). The precursors used for the MoS2 growth are molybdenum hexacarbonyl, Mo(CO)6 (Japan Advanced Chemicals) for metal, and hydrogen sulfide, H2S for chalcogen. MoS2 was grown on a 4-inch GaN/sapphire wafer using the following process. Initially, the sample was transferred from the load lock chamber to the process chamber of the MOCVD system, and heated from room temperature to 800 °C under vacuum conditions (2.8×10−6 Torr). Next, to sulfurize the GaN surface, we treated it with H2S at a flow rate of 20 sccm for 1 minute at 800 °C. Following this, the main deposition process was conducted with Mo(CO)6 at a flow rate of 5 sccm and H2S at 20 sccm, also at 800 °C for 1 minute. In this step, the precursor bottle and piping system were heated to 60 °C to increase the vapor pressure of Mo(CO)6, with Ar used as the carrier gas. For the post-deposition process, cooling was performed at first under an H2S atmosphere at 20 sccm for 5 minutes, followed by cooling under an Ar atmosphere at 100 sccm for 10 minutes, which brings the stage temperature down to 650 °C. Finally, the sample was transferred back to the load lock chamber, cooled to room temperature under vacuum, and then removed. The MoS2 grown on the 4-inch GaN/sapphire wafer exhibits an in-plane distribution of 2 to 4 layers, as shown in Supplementary Fig. 2.

STEM analysis
To obtain atomically resolved information on the MoS2/GaN van der Waals heterojunction, a STEM analysis has been performed as shown in Fig. 1c, Supplementary Fig. 1, and Supplementary Fig. 2. The sample was prepared by thinning using a micro-sampling method with a Focused Ion Beam (FIB). The observations were made along the a-plane direction [1120] of GaN using STEM (JEOL; JEM-ARM200F).

Fabrication of the device
Photolithography and reactive ion etching (RIE) processes were used to make the mesa patterns of the device. The photosensitive area is 60 μm in diameter. The anode electrode is composed of Cr/Au (10 nm/200 nm), while the cathode consists of Ti/Al/Ti/Au (10 nm/300 nm/150 nm/50 nm) deposited by sputtering.

Experimental setup for the frequency response measurement
The device’s photoelectric properties were characterized using three distinct methods: high-frequency response analysis, wavelength-dependent measurements, and large-signal pulsed measurements. For the high-frequency response analysis (Fig. 5), we generated phase-synchronous intensity-modulated light at 465 nm. The output of a single-mode laser diode (NICHIA; NDB4916) was guided into a Mach-Zehnder interferometer (MZI) constructed with free-space beam splitters. A waveguide phase modulator (AdvR; WPM-K0450) in one arm of the MZI was driven by a vector network analyzer (VNA, ROHDE & SCHWARZ; ZNA67). The MZI was actively stabilized using a phase-lock loop that controlled the path length via a piezo-actuated mirror, where the error signal for this loop was derived from the optical intensity at the interferometer’s unused output port. An AC photocurrent signal is amplified by a 50 Ω-matched wideband amplifier (Atlanta Micro Inc.; AM1102) and fed into the VNA through a transmission line probe (TECHNOPROBE; TP26-GSG-350-A). Non-flat frequency response of the measurement system is calibrated by a commercially available GaAs PIN-photodiode (TRUMPF; ULMPIN-25-TT). A wavelength dependence of the frequency response, measured using directly-modulated LDs at 405 nm (NICHIA; NDV4316), 465 nm (NICHIA; NDB4916), 530 nm (NICHIA; NDG4716), and 620 nm (Hangzhou Brand New Technology; custom-made), as shown in Fig. 6. Q.E. is calibrated by a commercially available Si PIN-photodiode (Hamamatsu Photonics; S9055). The launch power of the laser and the insertion loss within the setup are primarily attributed to coupling losses into the optical fiber and insertion losses from components such as beam splitters, as shown in Fig. 5a. The laser power was measured using a power meter (Thorlabs; S120C) placed immediately before the device, ensuring that any power loss due to branching or other components in the setup does not affect the evaluation of the photoresponse and responsivity of the device. The pulsed light, as shown in Fig. 4, was generated by directly modulating a 445 nm LD to a pulse width of 100 μs using a variable pulse width driver (PicoLAS GmbH; LDP-V03-100).

DFT calculations
Band structure of bilayer MoS2 (2H phase), as shown in Fig. 6f, was calculated using density functional theory (DFT). All calculations were performed using the Vienna Ab initio Simulation Package (VASP). The local density approximation of Ceperley and Alder (CA) was used for the exchange-correlation functional. The projector augmented-wave (PAW) potentials were used to describe the core-valence interaction. A plane-wave energy cutoff was set to 600 eV and an energy convergence criteria of 10−7 eV was used. The atomic positions were relaxed until the residual forces on all atoms became less than 5×10−3 eV/Å. For the initial structure of the bilayer MoS2, we adopted a- and b-axes lattice constants and atomic positions of the bulk 2H-MoS2, and the vacuum layer of about 25 Å. The atomic positions of bilayer MoS2 model were relaxed, while the cell parameters were fixed. For the K-point sampling of Brillouin-zone integrals, we used a 8×8×2 for the bulk 2H-MoS2, and a 8×8×1 for the bilayer MoS2.

Characterizations
The 3D image of the device (Fig. 3a) was acquired using a laser microscope (Keyence; VK-X3000). The Raman spectra (Fig. 1b) were measured with a Raman spectrometer (Renishaw; inVia reflex) using 532 nm excitation. The absorption spectra (Fig. 1d) were measured by a spectrophotometer (Hitachi High-Tech; U-4100). The atomic resolution cross-sectional STEM images (Fig. 1c) were measured using STEM (JEOL; JEM-ARM200F). The I–V characteristics of the device (Fig. 3c) measured using a device analyzer (Keysight; B1505A). The surface potential of the materials was measured using a KPFM (SHIMADZU; SPM-9700HT).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a novel UTC-PD design using a MoS₂/GaN van der Waals heterojunction, targeting high-speed visible-light detection. The work demonstrates a frequency response in the several-GHz range across the visible spectrum. The concept is innovative, experimentally rigorous. Moreover, there are several issues that the authors need to address.\n1. The reported QE is orders of magnitude lower than commercial visible-light photodiodes. The internal QE suggests severe carrier loss, yet the manuscript only vaguely attributes this to \"momentum-space misalignment\".\n2. The 5-GHz bandwidth is attributed solely to RC limitations, but no evidence excludes carrier transit delay. The assertion that \"carrier escape/transit is ultrafast\" relies solely on indirect RC modeling.\n3. Additional key device parameters such as detectivity (D*), responsivity (R), and performance metrics across different wavelength bands should be provided.\n4. The epitaxial MoS₂/GaN interface is critical for GHz performance, but the Methods section omits key growth parameters. Reproducibility is unverified.\n5. The consistency and stability of the device should be provided.", "reviewer_2": "Kadowaki et al. present a UTC PD operating across the visible spectrum based on a MoS2 / GaN structure. The device has a quantum efficiency of around 1% and achieves a 3 dB bandwidth of 5 GHz. The achieved bandwidth is orders of magnitude higher than for other reported MoS2/GaN devices. The device is enabled by their developed MOCVD growth directly on GaN. While being able to have good growth of a 2D material directly on the layer stack of the device is an intriguing demonstration, the manuscript for me still lacks clear novelty and advancement. The paper lacks a strong motivation, several characteristics of the device are missing for me, as well as a clear comparison to state-of-the-art. Therefore, I cannot recommend publication in Nature Communications.\nPlease find below more detailed comments:\nOn the motivation:\n- line 33: The development of UTCs operating in the visible is, among others, motivated with inter satellite communication and LiDAR. It is not clear from the text why it would be advantageous to move from the NIR to the VIS for these applications.\nWith respect to the output of the PD:\n- In the abstract the importance of UTCs is given by their high saturation power. However, throughout the paper there seems to be not a single mention on which laser powers have been used. In addition to adding the missing information, a characterization on the output current of the PD as function of optical input power is needed to discuss if the PD can generate useful signal levels.\n- All applications mentioned in the introduction do not simply require high bandwidth, but also sufficient signal-to-noise ratio. The noise (and dark current) of the here developed UTCs is not discussed. The above mentioned power sweeps could give information such as the noise-equivalent power.\n- Figure 3c: why is the IV characteristic here only measured in a range from -1 to +1V, when the PD is later on only operated outside of this range from -1V to -4V? It is important to show the dark current characteristics also for the operation conditions. Also, as typically done, the same IV characteristics under illumination should be recorded.\n- line 139 and Figure 4: what is the evidence that these distortions or drifts are not occurring from the UTC itself? Is the device stable over time? Could you show the trace over several pulses? Or the output of the PD over time?\n- Could the authors state the responsivity of the device in addition to the Q.E. and I.Q.E. to make comparison to other technologies easier?\n- Is there a route to further increase the Q.E., I.Q.E, responsivity of the device?\nOn the speed of the device:\n- line 54: the discussion on the slower speed of previous state of the art demonstrations of MoS2/GaN photodetectors is not clear. The authors state microseconds have been reported, which is contradicting to the 20 femtosecond transport dynamics in the 2D layer. However, the time constant within the MoS2 layer will not be the limiting factor in the response of the MoS2/GaN structure. The sentence therefore seems out of place.\n- line 65: the authors state that the device is limited by an external electronic cut-off. Later on in the paper they state that an RC limit matches their measurements. However, an RC limit is a device characteristic and not an external electronic cut-off.\n- line 153: the device could be optimized for higher speed, but what are the trade-offs? And what is the limit in speed that is achievable based on the expected carrier dynamics?\n- Figure 5b: Considering the extreme height of the noise on the measurement (+/- 15 dB!), please provide the noise floor of the measurement system when performing the same sweep with the LD turned off.\n- Figure 5b: the y-axis label states “Gain”. Gain with respect to what?\n- Figure 5b: the authors state that the noise is the lower frequency regime is in part due to 1/f noise. Is there any evidence for this statement? 1/f noise in the 10-100 MHz regime is rather rare.\nWith respect to state of the art:\nThe authors essentially only compare their MoS2 / GaN UTC PD to other MoS2 / GaN PDs. Also, the only metric that they’re comparing is the speed of the device. While the improvement over the other demonstrations is indeed orders of magnitude higher, a useful PD should have several metrics that are advantageous.\nSpeed, responsivity, saturation power, dark current, noise-equivalent power, operational wavelength, size, required bias, operational stability are a few properties that should be considered.\nAt the moment, the paper only states the speed of 5 GHz at 465nm with a Q.E. of 0.5% (corresponding to roughly <5 mA/W). How does this compare to other PDs operating in the visible regime? For example, commercial silicon PDs are available from Thorlabs with 2 GHz and 150 mA/W at 465nm. Schottky InGaAs with operation into the visible also exist with even higher bandwidths (>20 GHz) and higher responsivities. How about state-of-the-art research on other 2D materials such as graphene, MoTe2, PdSe2, carbon nanotubes, etc. ?\nI would recommend to the authors to make a clear comparison to the state of the art of high-speed visible PDs such that the advantages of the here presented work can be discussed.\nOther points:\n- line 85: the authors state that Raman spectra reveal that the structure consists of 2 to 3 layers. In the figure description they state that STEM reveal 2 to 4 layers. However, the STEM images shown throughout the paper only seem to show 2 layers. Which is it?\nAdditionally, what is the reason for choosing 2 layers?\n- line 106: I do not see how the results in Fig. 2 lead to this conclusion. Is this intended to be with reference to Fig. 1d?\n- Figure 1a: The p-contact layer should also be illustrated.\n- Figure 5a: It would be good to label the remaining components such as the beam splitters and the reference PD. Also, the element controlling one of the mirrors in the interferometer arm is not discussed at all or how the feedback is performed.", "reviewer_3": "The authors present an UTC-PD based on the heterojunction between MoS2 and GaN vor visible light detection. Photodetectors based on this heterojuction have been already reported in literature but with limited optoelectronic bandwidth and with a different doping profile. Relying on the direct growth of MoS2 films on the GaN on sapphire substrate the authors improve the electrical interface of the heterojunction reducing trap states that are responsible for limited speed of operation. The paper is well written and the results and methods are novel and convincing. However, I would consider this work for pubblication on Nature Comm only after addressing the following points:\n1) the device photoresponse is reported in terms of internal quantum effciency as a function of the frequency of the signal. There is no mention of the device responsivity nor sensistivity. It is highly recommended to show a plot of the photoresponse in terms of photocurrent and responsivity as a function of voltage and wavelength.\n2) the rf response is attributed mainly to the device capacitance and external 50Ohm impedance. I suggest to introduce the equivalent circuit of the PD highlighting all the contribution, even if negligible. E.g. there is no comment about the contact resistance on MoS2 which is typically high.\n3) the rf response shown in fig. 5b starts from 10MHz and is significantly higher tha 0dB from 10 to 20MHz. The authors should describe what happens in that spectral region and possibly extend the test below 10MHz to better understand the behavior of the device at low frequency. E.g., in a data trasmission at 10Gb/s it is relevant to go down at least to 100kHz.\n4) testing of the photoresponse is not properly described since the launch power of the laser and the insertion loss of the setup is not reported. These details must be listed at least in the methods section.\n5) the discussion of the rectification and photodetection mechanisms should be summarized in the main text to help comprehension\n5) a table with comparison to SOTA is highly recommended to have a clear picture of the advancement in the field"}, "author_response": "Point-by-point response to the Reviewers’ comments\n<<Reviewer 1>>\n[Comment]\nThis manuscript presents a novel UTC-PD design using a MoS₂/GaN van der Waals\nheterojunction, targeting high-speed visible-light detection. The work demonstrates a\nfrequency response in the several-GHz range across the visible spectrum. The concept is\ninnovative, experimentally rigorous. Moreover, there are several issues that the authors\nneed to address.\nWe sincerely thank the Reviewer for the positive evaluation on our work by commenting\n“The concept is innovative, experimentally rigorous”. We also appreciate the Reviewer’s\ninsightful and constructive comments for improvement.\n[Comment 1]\nThe reported QE is orders of magnitude lower than commercial visible-light photodiodes.\nThe internal QE suggests severe carrier loss, yet the manuscript only vaguely attributes\nthis to \"momentum-space misalignment\".\n[Response to Comment 1]\nAs the Reviewer commented, the low internal Q.E. indicates significant carrier loss. We\nrevised the Discussion to present following arguments rather than simply mentioning\n\"momentum-space misalignment\": (i) spectral selectivity (405 nm≫620 nm) consistent\nwith Γ‑assisted vs K‑assisted transfer; (ii) bias‑selective enhancement stronger at longer\nwavelengths indicative of field‑assisted K→Γ transfer; (iii) RC‑limited flat response\ndominating the total photocurrent, which indicates trap‑dominated loss is small.\nAdditionally, here we discuss alternative possibilities unlikely to be primary under our\nconditions: (a) MoS₂ bulk recombination: the vertical transport path is atomically short\nand carrier escape is faster than recombination lifetime (under reverse bias without the\nmomentum mismatch). (b) Interface traps: contradicted by the flat, GHz‑range response\nand by the saturation of the slow parasitic component. (c) Lack of an electron‑blocking\nlayer (which is usually used to prevent electrons from diffusing to the anode in the typical\nUTC devices): the thin MoS absorption layer operates very close to the reverse-biased\n2\ncarrier transport layer. Electrons need little diffusion to reach the depletion field and are\n1\nefficiently collected even without blocking barrier on the other side, as long as there is no\nmomentum-space misalignment.\nWe thus believe that the crystal-momentum mismatch, which will be discussed in revised\nmanuscript and supplementary information, is the primary contributor to the observed\ncarrier loss.\nBased on our response above, the following sentences were added in the revised\nmanuscript and supplementary information:\n(Manuscript, page 7, lines 155) The internal quantum efficiency of the fabricated devices\nis significantly lower than unity that the conventional PIN photodiodes would attain,\nindicating that some of the photo-generated carriers do not contribute to the photocurrent.\n…\n(Supplementary Information, page 9, lines 154) Several potential causes for the observed\ncarrier loss can be considered. We consider first the carrier recombination within MoS .\n2\n…\n[Related reviewers’ comments] Reviewer 2 Comment 8\n[Comment 2]\nThe 5-GHz bandwidth is attributed solely to RC limitations, but no evidence excludes\ncarrier transit delay. The assertion that \"carrier escape/transit is ultrafast\" relies solely on\nindirect RC modeling.\n[Response to Comment 2]\nThe capacitance in the RC circuit model shown in Fig. 5b is derived from actual device\nmeasurements obtained through C-V measurements, rather than fitting. The cutoff\nfrequency determined by the RC time can be given by\n1\n𝑓 = .\n𝐶𝑅 2𝜋𝐶𝑅\nWe can, on the other hand, estimate the cutoff frequency due to the carrier transit delay\nfrom the electron drift velocity (assumed to be the electron saturation velocity\n𝑣 ~2.7×107 cm/s) in GaN with the thickness of the carrier collection layer 𝑑 = 0.3 𝜇m\n𝑒\nas\n3.5𝑣\n𝑒\n𝑓 = ,\n𝑡 2𝜋𝑑\n2\nwhich is approximately 500 GHz and far beyond the frequency we observed in Fig. 5b.\nFigure R1 illustrates the relationship between the 3-dB bandwidth 𝑓 and the\n3𝑑𝐵\nthickness of the carrier collection layer d, where 𝑓 can be obtained by\n3𝑑𝐵\n1 1 1\n= + ,\n𝑓2 𝑓2 𝑓2\n3𝑑𝐵 𝐶𝑅 𝑡\nwhere 𝑓 and 𝑓 are the cutoff frequencies determined by the RC time and by the\n𝐶𝑅 𝑡\ncarrier transit delay, respectively.\nGiven the good agreement between the RC circuit model curve and the measured response,\nwe conclude that the cutoff frequency is determined by the RC time rather than carrier\ntransit delay.\nFig. R1: 3-dB bandwidth versus carrier collection layer of a UTC-PD with different\nphotosensitive area.\nBased on our response above, the following sentences and figure were added in the\nrevised manuscript and supplementary information:\n(Manuscript, page 5, lines 134) as discussed in Section 12 of Supplementary Information.\n(Supplementary Information, page 13, lines 207) Section 12, “Prospects for the\nbandwidth and quantum efficiency of MoS /GaN UTC-PD”\n2\n(Supplementary Information, page 14) Supplementary Fig.13: Relationship between 3-\ndB bandwidth and thickness of the carrier collection layer in UTC-PDs with varying\nphotosensitive area S.\n[Related reviewers’ comments] Reviewer 2 Comment 11\n3\n[Comment 3]\nAdditional key device parameters such as detectivity (D*), responsivity (R), and\nperformance metrics across different wavelength bands should be provided.\n[Response to Comment 3]\nWe hereby illustrate the responsivity (R), detectivity (D*), and noise equivalent power\n(NEP) as functions of wavelength in Fig. R2. Here, the detectivity (D*) and noise\nequivalent power (NEP) were calculated using the following equations:\n𝐴1/2𝑅\n𝐷∗ =\n(2𝑒𝐼 )1/2\n𝑑\n(2𝑒𝐼 )1/2\n𝑑\n𝑁𝐸𝑃 =\n𝑅\nHere, 𝑅 is the responsivity, 𝐼 is the dark current of the device, 𝐴 is the\n𝑑\nphotosensitive area, and 𝑒 is the elementary charge.\nFig. R2: Plot of (a) responsivity, (b) noise equivalent power (NEP), and detectivity (D*)\nagainst various wavelength at -4 V bias.\nBased on our response above, the following sentences and figures were added in the\nrevised manuscript and supplementary information:\n(Manuscript, page 7, lines 145) Additionally, Supplementary Fig. 6 provides further\ndetails on other photoresponse characteristics, such as responsivity, detectivity (D*), and\nnoise equivalent power (NEP).\n4\n(Supplementary Information, page 5, lines 66) Section 4, “Other photoresponse\ncharacteristics (responsivity, detectivity, and noise equivalent power)”\n(Supplementary Information, page 6) Supplementary Fig. 6: a, Responsivity of the device\nas function of voltage. …\n[Related reviewers’ comments] Reviewer 2 Comment 3, 7, Reviewer 3 Comment 1\n[Comment 4]\nThe epitaxial MoS /GaN interface is critical for GHz performance, but the Methods\n2\nsection omits key growth parameters. Reproducibility is unverified.\n[Response to Comment 4]\nWe added the key growth parameters related to the growth of MoS₂ using the MOCVD\nsystem in the Methods section, as detailed below.\nThis process has been conducted on multiple 4-inch wafers, and reproducibility has been\nconfirmed.\nBased on our response above, the following sentences were added in Method section of\nthe revised manuscript:\n(Manuscript, page 8, lines 187) MoS was grown on a 4-inch GaN/sapphire wafer using\n2\nthe following process. Initially, the sample was transferred from the load lock chamber to\nthe process chamber of the MOCVD system, and heated from room temperature to 800℃\nunder vacuum conditions (2.8×10-6 Torr). …\n[Comment 5]\nThe consistency and stability of the device should be provided.\n[Response to Comment 5]\nAs evidence of consistency, we evaluated the response characteristics across multiple\ndevices using a 405 nm laser diode, and the results are shown in Fig. R3. There are no\nappreciable differences in the response among the different devices.\nAdditionally, to assess stability, we reexamined the device described in the manuscript\napproximately 8 months later and observe no degradation in performance. Since the\nMoS layer is covered by the anode metal and the SiO passivation layer, the device might\n2 2\n5\nbe less susceptible to moisture and oxygen in the atmosphere, leading to the good stability\nand reliability of the device.\n-20\n405 nm\n-40\n)\nB\nd\n(\nn -60\nia\nG\n#1\n#1 (8 months later)\n-80\n#2\n#3\nDark\n-100\n107 108 109 1010\nFrequency (Hz)\nFig. R3: Frequency response of the device measured using a 405 nm laser diode\nto confirm consistency and stability.\nBased on our response above, the following sentences and figures were added in the\nrevised manuscript and supplementary information:\n(Manuscript, page 4, lines 94) The device exhibits high stability due to the MoS being\n2\nencapsulated by the anode metal and the SiO passivation layer, which minimizes\n2\nsusceptibility to atmospheric moisture and oxygen, as shown in Supplementary Fig. 7.\n(Supplementary Information, page 5, lines 81) Section 5, “Sample-to-sample variability\nand long-term stability of the devices”\n(Supplementary Information, page 6) Supplementary Fig. 7: Frequency response of the\ndevice measured using a 405 nm laser diode to confirm consistency and stability.\n[Related reviewers’ comments] Reviewer 2 Comment 6\n6\n<<Reviewer 2>>\n[Comment]\nKadowaki et al. present a UTC PD operating across the visible spectrum based on a MoS2\n/ GaN structure. The device has a quantum efficiency of around 1% and achieves a 3 dB\nbandwidth of 5 GHz. The achieved bandwidth is orders of magnitude higher than for other\nreported MoS2/GaN devices. The device is enabled by their developed MOCVD growth\ndirectly on GaN. While being able to have good growth of a 2D material directly on the\nlayer stack of the device is an intriguing demonstration, the manuscript for me still lacks\nclear novelty and advancement. The paper lacks a strong motivation, several\ncharacteristics of the device are missing for me, as well as a clear comparison to state-of-\nthe-art. The paper lacks a strong motivation, several characteristics of the device are missing for me, as well as a clear comparison to state-of-the-art.\nWe sincerely thank the Reviewer for providing in-depth and valuable comments and\nsuggestions. Below, we aim to address all the questions and critiques raised by the\nReviewer in a point-by-point manner.\nOn the motivation:\n[Comment 1]\nline 33: The development of UTCs operating in the visible is, among others, motivated\nwith inter satellite communication and LiDAR. It is not clear from the text why it would\nbe advantageous to move from the NIR to the VIS for these applications.\n[Response to Comment 1]\nRegarding the advantage of moving from the NIR to the VIS for applications such as inter\nsatellite communication and LiDAR, we provide the following points:\ni) In space, scattering due to the atmosphere is almost negligible, and generally, shorter\nwavelength light tends to spread less than longer wavelength light. Therefore, we believe\nthat visible light can propagate further than near-infrared light.\nii) Recent studies have shown that visible light exhibits low absorption in water, making\nit a promising option for underwater communication applications.\niii) Compared to the conventional Si photodiodes, GaN-based photodiodes, with their\nwide bandgap, are expected to exhibit better temperature characteristics and high-\ntemperature operation in general.\nmanuscript:\n(Manuscript, page 1, lines 24) Such high-speed wide-bandgap photodiodes can become\nkey components in emerging visible-light technologies, since visible light offers higher\ndirectivity, lower absorption …\nWith respect to the output of the PD:\n[Comment 2]\nIn the abstract the importance of UTCs is given by their high saturation power. However,\nthroughout the paper there seems to be not a single mention on which laser powers have\nbeen used. In addition to adding the missing information, a characterization on the output\ncurrent of the PD as function of optical input power is needed to discuss if the PD can\ngenerate useful signal levels.\n[Response to Comment 2]\nWe added in the manuscript the power density of the laser used in each experiment. The\npower density was calculated under the assumption that all incident light enters the\nphotosensitive area of the device.\nFirst, the RF bandwidth of our devices shows no detectable space‑charge‑induced roll‑off\nunder our conditions (up to 10 W/cm2, in the RC‑limited regime).\nIn addition, a characterization of the output current of the photodiode (PD) as a function\nof the optical input power is shown in Fig. R4. The graph was derived from the pulse\nresponse measurement shown in Fig. R9b, which were conducted in response to\nComment 6 bellow. The excellent linearity between the optical input power and the\noutput current is hereby demonstrated, with no saturation observed up to the maximum\noptical input power of approximately 100 W/cm2.\npower.\nFig. R9b shows the dependence of the photocurrent on the incident light power density.\nBased on this result, Fig. R4 illustrates the output current of the photodiode as a function\nof optical input power.\nFig. R9: a, Time dependence of the photocurrent signal of the device. b, Dependence of\nthe photocurrent on the incident light power density.\nBased on our response above, the following sentences and figures were added in the\nrevised manuscript:\n(Manuscript, page 5, lines 119) Figure 4b shows the dependence of the photocurrent\nsignal on the incident light power density. …\n(Manuscript, page 5) Figure 4: a, Time dependence of the photocurrent …\n9\n(Manuscript, page 6) Figure 5b, ~10 W/cm2\n(Manuscript, page 7) Figure 6 caption: with power densities of 50 W/cm2, 10 W/cm2, 70 W/cm2, and 70 W/cm2\n[Related reviewers’ comments] Reviewer 2 Comment 6\n[Comment 3]\nAll applications mentioned in the introduction do not simply require high bandwidth, but\nalso sufficient signal-to-noise ratio. The noise (and dark current) of the here developed\nUTCs is not discussed. The above mentioned power sweeps could give information such\nas the noise-equivalent power.\n[Response to Comment 3]\nThe dark current of the device is shown in Fig. R5.\n10-1\n10\n10-2\n10-3\n8\n)\nA\n10-4\n)\nm\n6\n10-5 A\n(\n( t 10-6 t n\nn e\ne r r 4 10-7 r r u\nu 10-8 C\nC\n2 10-9\n10-10\n0 10-11\n-4 -3 -2 -1 0 1\nVoltage (V)\nFig. R5: I-V characteristics of the device measured under dark condition.\nAdditionally, we include a figure of the noise-equivalent power (NEP) in Fig. R6 (the\nsame figure as Fig. R2b). As mentioned in our response to Comment 2, our device\nmaintains linearity over a power density range of 0.1–100 W/cm2, so the NEP remains\nconstant irrespective of the input optical power.\n10\n8x10-10 4x107\nVr =-4 V\n7x10-10\n6x10-10 3x107\n)\n) W\n2\n/1 z\nH\n5x10-10 2 /1\nz\n/\n/W\n(\n4x10-10 2x107\nH\nm\nP c\nE\nN\n3x10-10 (\n* D\n2x10-10 1x107\n1x10-10\n400 450 500 550 600 650\nWavelength (nm)\nFig. R6: Noise equivalent power (NEP), and detectivity (D*) against various wavelength\nat -4 V bias. (The same figure as Fig. R2b.)\nBased on our response above, the following sentences and figure were added in the\nrevised manuscript and supplementary information:\n(Manuscript, page 7, lines 145) Additionally, Supplementary Fig. 6 provides further\ndetails on other photoresponse characteristics, such as responsivity, detectivity (D*), and\nnoise equivalent power (NEP).\n(Supplementary Information, page 5, lines 66) Section 4, “Other photoresponse\ncharacteristics (responsivity, detectivity, and noise equivalent power)”\n(Supplementary Information, page 6) Supplementary Fig. 6: a, Responsivity of the device\nas function of voltage. …\n[Related reviewers’ comments] Reviewer 1 Comment 3, Reviewer 2 Comment 2, 4, 11\n[Comment 4]\nFigure 3c: why is the IV characteristic here only measured in a range from -1 to +1V,\nwhen the PD is later on only operated outside of this range from -1V to -4V? It is\nimportant to show the dark current characteristics also for the operation conditions.\nAlso, as typically done, the same IV characteristics under illumination should be\nrecorded.\n[Response to Comment 4]\nR7. In addition to the I-V characteristics in the dark, we also provide the I-V\ncharacteristics under illumination using a 405 nm laser diode with the power density of\n10-1\nUnder illumination\n8 (75 W/cm2 @405 nm)\n10-3\n)\nA\n10-4\n)\nA\nm\n6\n10-5\n(\n(\nt\nn\n10-6\nt\nn\ne\ne r\nr\n4 10-7 r r\nu\nu 10-8 C\nC\n2 10-9\n10-10\n0 10-11\n-4 -3 -2 -1 0 1\nVoltage (V)\nFig. R7: I-V characteristics of the device. The solid lines represent measurements under\ndark conditions, while the dotted lines correspond to measurements under illumination\nwith a 405 nm laser diode at a power density of 75 W/cm2.\nBased on our response above, the following sentences and figure were added in the\nrevised manuscript:\n(Manuscript, page 4) Figure 3c: The solid lines represent measurements under dark\nconditions, while the dotted lines correspond to measurements under illumination with a\n405 nm laser diode at a power density of 75 W/cm2.\n(Manuscript, page 4, lines 96) To evaluate the basic electrical properties and demonstrate\nthe rectifying behavior of the fabricated device, we measured the I–V characteristics\nshown in Figure 3c. …\n[Comment 5]\nline 139 and Figure 4: what is the evidence that these distortions or drifts are not occurring\nfrom the UTC itself?\n[Response to Comment 5]\nWe confirmed that the pulse distortions are circuit‑induced; the circuit‑only waveform\n(Fig. R8a) overlays the UTC response at high power (Fig. R8b), while subtracting it\nreveals a residual waveform with a fixed shape (Fig. R8c), attributable to trap-induced\n12\ncurrent response which easily saturates and becomes negligible at higher powers. Bellow\nwe provide the detail of our analysis.\nIn Fig. R8b, we present the photocurrent responses shown in Fig. 4b in the main text,\nwith the appropriately-scaled electrical pulse shown in Fig. R8a overlaid on each of them.\nThe figure reveals that these signal distortions are similar to those seen in the pulse\nresponse results for the device, indicating that these distortions are mainly caused by the\nperipheral circuit. Based on this overlay, we illustrate the difference between the device’s\npulse response and the electrical pulse signal in Fig. R8c. The pulse response with low\ninput optical power is indeed affected by parasitic slow carrier dynamics, which could\npresumably be due to trapped carriers at the heterointerface and/or defects. The black line\nin Fig. R8c indicates the result of fitting the averaged difference data to exponentially\nrising and decaying functions with a time constant τ ~ 0.1 ms. However, as the input\noptical power increases, these slow carrier dynamics saturate and become negligible, as\nshown in Fig. R8b.\nFig. R8: Analysis of the pulse response of the device. a, Electrical output signal for the\nevaluation system. The black line represents the input signal, while the red line depicts\nthe output signal when the signal is fed into the pulse response evaluation system without\npassing through the device. b, Dependence of the photocurrent on the incident light power\ndensity. The red dotted lines represent the appropriately-scaled electrical pulse shown in\na overlaid on each of the former photocurrent signals. c, The difference between the\nphotocurrent responses of the device and the electrical output signal without passing\nthrough the device. The black line indicates the result of fitting the averaged difference\ndata to exponentially rising and decaying functions with a time constant τ ~ 0.1 ms.\nBased on our response above, the following sentences and figure were added in the\nrevised Supplementary information:\n(Manuscript, page 5, lines 115) as discussed in Section 6 of Supplementary Information.\n(Supplementary Information, page 7, lines 89) Section 6, “Analysis of the pulse response”\n13\n(Supplementary Information, page 7) Supplementary Fig. 8: Analysis of the pulse\nresponse of the device. …\n[Comment 6]\nIs the device stable over time? Could you show the trace over several pulses? Or the\noutput of the PD over time?\n[Response to Comment 6]\nRegarding the device's stability over time, we present the trace over several pulses in Fig.\nR9a.\nAdditionally, Fig. R9b shows the dependence of the photocurrent on the incident light\npower density. Based on this result, Fig. R4 illustrates the output current of the photodiode\nas a function of optical input power.\nFig. R9: a, Time dependence of the photocurrent signal of the device. b, Dependence of\nthe photocurrent on the incident light power density.\nBased on our response above, the following sentences and figures were added in the\nrevised manuscript:\n(Manuscript, page 5) Figure 4: a, Time dependence of the photocurrent …\n(Manuscript, page 4, lines 112) blue\n[Related reviewers’ comments] Reviewer 1 Comment 5\n[Comment 7]\nCould the authors state the responsivity of the device in addition to the Q.E. and I.Q.E. to\nmake comparison to other technologies easier?\n14\n[Response to Comment 7]\nWe have included the responsivity on the right axis of Fig. R10a-d, alongside the Q.E.\ngraphs, for clarity. We have also added graphs of responsivity as a function of voltage\nand wavelength, as shown in Fig. R10e-f.\nFig. R10: a-d, Wavelength dependence of the frequency response, measured using laser\ndiodes (LD) at 405 nm (a), 465 nm (b), 530 nm (c), and 620 nm (d), with power densities\nof 50 W/cm2, 10 W/cm2, 70 W/cm2, and 70 W/cm2. Each response is represented in terms\nof the quantum efficiency (Q.E.) and the responsivity calibrated by a commercially\navailable Si PIN-PD, whose quantum efficiency is specified by the manufacturer. We\nonly show the results with smoothing to eliminate the low and high-frequency noise. In\n(a), (c), (d), frequency region above 2 GHz is grayed-out to indicate the signal is\ndominated by crosstalk. The lines in each figure correspond to different reverse bias\nvoltage applied to the device. e, Responsivity of the device as function of voltage. f,\nResponsivity of the device as function of wavelength.\nBased on our response above, the following sentences and figures were added in the\nrevised manuscript:\n(Manuscript, page 7) Figure 6a\n(Manuscript, page 7) Figure 6 caption: and the responsivity\n(Supplementary Information, page 6) Supplementary Fig. 6: a, Responsivity of the device\nas function of voltage. …\n[Related reviewers’ comments] Reviewer 1 Comment 3, Reviewer 3 Comment 1\n[Comment 8]\nIs there a route to further increase the Q.E., I.Q.E, responsivity of the device?\n[Response to Comment 8]\nImprovement of Q.E.:\n• Increase the number of MoS 2 layers to enhance the absorption of incident light.\n• Utilize a waveguide or resonator structure to extend the interaction length.\nImprovement of I.Q.E.:\n• Utilize TMDC materials where the Γ point is the minimum of the conduction band\nto avoid the crystal-momentum mismatch.\n• Add an electron blocking layer between the anode electrode and MoS 2 to reduce\nlosses caused by electrons flowing towards the anode.\nBased on our response above, the following sentences were added in the supplementary\ninformation:\n(Supplementary Information, page 13, lines 225) Regarding quantum efficiency, it is\nnecessary to consider two aspects: internal quantum efficiency and the quantum\nefficiency. For internal quantum efficiency, as described in Section 10 of the\nSupplementary Information, possible approaches include, for example, using other\ntransition metal dichalcogenides…\n[Related reviewers’ comments] Reviewer 1 Comment 1\nOn the speed of the device:\n[Comment 9]\nline 54: the discussion on the slower speed of previous state of the art demonstrations of\nMoS /GaN photodetectors is not clear. The authors state microseconds have been\n2\nreported, which is contradicting to the 20 femtosecond transport dynamics in the 2D layer.\nHowever, the time constant within the MoS layer will not be the limiting factor in the\n2\nresponse of the MoS /GaN structure. The sentence therefore seems out of place.\n2\n[Response to Comment 9]\nThank you for pointing this out. We agree with the Reviewer’s point that the “20 fs\ntransport dynamics” should not be read as the observed timescale of our MoS₂/GaN\ndevice. Our intent was to cite sub‑100‑fs interlayer transfer reported in other 2D van der\nWaals heterostructures as background motivation, hypothesizing the fundamental\nlimitation of the MoS₂/GaN system itself is much faster than the previous MoS₂/GaN PD\nreports. We revised the section to clarify the distinction between the \"20 femtosecond\ntransport dynamics\" and \"the slower speed of previous state-of-the-art demonstrations of\n2\nthe meaning of the sentence should be clearer.\nBased on our response above, the following sentences were added in the revised\nmanuscript:\n(Manuscript, page 2, lines 39) Recent advances have shown that two-dimensional (2D)\ntransition metal dichalcogenides, such as molybdenum disulfide (MoS ), offer unique\n2\nelectrical and photonic properties. The combination of MoS with GaN is highly\n2\npromising for high-performance photoabsorption layers, owing to the material system’s\nsmall lattice mismatch [18, 19, 56 20], type-II band alignment [21, 23, 24], and strong\nlight-matter interaction in the visible region [25, 26, 27]. However, several groups have\nexplored MoS /GaN heterojunction photodetectors [28, 29, 30, 31, 32], finding relatively\n2\nslow responses on the order of microseconds or longer [33, 34, 35, 36]. Such slow\nresponses might be attributed to the imperfect interface of the heterojunction [35].\nAlthough the intrinsic speed capabilities of MoS /GaN interfaces remains unclear, we\n2\nnote that the ultrafast (less than 100 fs) carrier transport across heterointerfaces of 2D\nmaterials has been reported [37, 38], suggesting the intrinsic speed capabilities of the\nMoS /GaN system have yet to be fully realized.\n2\n[Comment 10]\nline 65: the authors state that the device is limited by an external electronic cut-off. Later\non in the paper they state that an RC limit matches their measurements. However, an\nRC limit is a device characteristic and not an external electronic cut-off.\n[Response to Comment 10]\nThe RC limit is indeed a device characteristic and not an external electronic cut-off. We\nrevised the text to clarify \"an external electronic cut-off\" as \"an RC limit.\"\nBased on our response above, the following sentences were added in the revised\nsupplementary information:\n(Manuscript, page 2, lines 52) an RC limit\n[Comment 11]\nline 153: the device could be optimized for higher speed, but what are the trade-offs? And\nwhat is the limit in speed that is achievable based on the expected carrier dynamics?\n17\n[Response to Comment 11]\nAssuming there is no delay in carrier transport from MoS to GaN through the\n2\nheterojunction, the trade-offs involve the transit time of electrons traveling through the\ncarrier collection layer composed of GaN, and the RC limit of the device. Figure R1\nillustrates the relationship between the 3-dB bandwidth 𝑓 and the thickness of the\n3𝑑𝐵\ncarrier collection layer d, where 𝑓 can be obtained by\n3𝑑𝐵\n1 1 1\n= + ,\n𝑓2 𝑓2 𝑓2\n3𝑑𝐵 𝐶𝑅 𝑡\nwhere 𝑓 and 𝑓 are given by\n𝐶𝑅 𝑡\n3.5𝑣\n𝑒\n𝑓 =\n𝑡 2𝜋𝑑\n1\n𝑓 = .\n𝐶𝑅 2𝜋𝐶𝑅\nHere, 𝑓 is the bandwidth determined by the transit time, 𝑑 represents the thickness of\n𝑡\nthe carrier collection layer, and 𝑣 indicates the electron saturation velocity in GaN\n𝑒\n(~2.7×107 cm/s), while 𝑓 is the bandwidth determined by the RC time constant.\n𝐶𝑅\nFrom this figure we can expect that by reducing the photosensitive area of the device, say\ndown to 100 μm2 (red line), a high-speed response exceeding 150 GHz can be achieved\nwith the thickness of the carrier collection layer being optimized.\nFig. R1: 3-dB bandwidth versus carrier collection layer of a UTC-PD with different\nphotosensitive area.\nsupplementary information:\n(Manuscript, page 5, lines 134) as discussed in Section 12 of Supplementary Information.\n(Supplementary Information, page 13, lines 207) Section 12, “Prospects for the\nbandwidth and quantum efficiency of MoS /GaN UTC-PD”\n2\n(Supplementary Information, page 14) Supplementary Fig.13: Relationship between 3-\ndB bandwidth and thickness of the carrier collection layer in UTC-PDs with varying\nphotosensitive area S.\n[Related reviewers’ comments] Reviewer 2 Comment 11\n[Comment 12]\nFigure 5b: Considering the extreme height of the noise on the measurement (+/- 15 dB!),\nplease provide the noise floor of the measurement system when performing the same\nsweep with the LD turned off.\n[Response to Comment 12]\nWe here provide the raw data for the noise floor of the measurement system in Fig. R11.\nThe gray line in the graph indicates the noise floor with the LD turned off. The low-\nfrequency noise and high-frequency noise are attributed to 1/f noise and RF crosstalk\nnoise of the network analyzer, respectively. Additionally, as addressed in our response to\nReviewer 3 Comment 3, we have also evaluated the response characteristics for\nfrequencies below 10 MHz. We confirmed a flat response in the range from 10 kHz to 50\nMHz, as shown in Fig. R12.\n0\n465 nm\n-20\n-40\n)B\nd\n(\nn ia -60\nG\n-80\nMoS2/GaN@-4V\nGaAs PIN\n-100 Si PIN\nDark\n107 108 109 1010\nFrequency (Hz)\nFig. R11: Frequency response of the device measured using a 465 nm laser diode at a -4\nV bias, without calibration or normalization against a reference GaAs PIN photodiode.\n19\n50\nNoise floor\n30\n20\n)\nB\nd 10\n(\nn 0\nia\nG -10\n-20\n-30\n-40\n-50\n104 105 106 107\nFrequency (Hz)\nFig. R12: Frequency response of the device measured using a 465 nm LD at a -4 V\nbias, with normalization against a reference GaAs PIN photodiode.\nBased on our response above, the following sentences were added in the revised\nsupplementary information:\n(Manuscript, page 6) Figure 5 caption: The response is calibrated using a reference GaAs\nPIN-photodiode (flat up to 20 GHz; see Supplementary Fig. 9 for raw data and the noise\nfloor of the measurement system.\n(Supplementary Information, page 7, lines 104) Section 7, “Raw data for noise floor of\nthe frequency measurement system”\n(Supplementary Information, page 8) Supplementary Fig. 9: Frequency response of the\ndevice …\n[Related reviewers’ comments] Reviewer 2 Comment 12, Reviewer 3 Comment 3\n[Comment 13]\nFigure 5b: the y-axis label states “Gain”. Gain with respect to what?\n[Response to Comment 13]\nThe graph represents the “Gain”, where the flat region of the frequency response is set to\n0 dB.\n20\n[Comment 14]\nFigure 5b: the authors state that the noise is the lower frequency regime is in part due to\n1/f noise. Is there any evidence for this statement? 1/f noise in the 10-100 MHz regime is\nrather rare.\n[Response to Comment 14]\nAs addressed in our response to Comment 12, we have verified the noise floor of the\nmeasurement system. Additionally, the vector network analyzer we used for the\nevaluation (ROHDE & SCHWARZ; ZNA67) is a high-frequency model with a\nmeasurement range of 10 MHz to 67 GHz, which does include 1/f noise in the 10-100\nMHz range.\nBased on our response above, the following sentences were added in the revised\nsupplementary information:\n(Manuscript, page 6) Figure 5 caption: The response is calibrated using a reference GaAs\nPIN-photodiode (flat up to 20 GHz; see Supplementary Fig. 9 for raw data and the noise\nfloor of the measurement system.\n(Supplementary Information, page 7, lines 104) Section 7, “Raw data for noise floor of\nthe frequency measurement system”\n(Supplementary Information, page 8) Supplementary Fig. 9: Frequency response of the\ndevice …\n[Related reviewers’ comments] Reviewer 2 Comment 12, Reviewer 3 Comment 3\nWith respect to state-of-the-art:\n[Comment 15]\nThe authors essentially only compare their MoS2 / GaN UTC PD to other MoS2 / GaN\nPDs. Also, the only metric that they’re comparing is the speed of the device. While the\nimprovement over the other demonstrations is indeed orders of magnitude higher, a useful\nPD should have several metrics that are advantageous.\nSpeed, responsivity, saturation power, dark current, noise-equivalent power, operational\nwavelength, size, required bias, operational stability are a few properties that should be\nconsidered.\nAt the moment, the paper only states the speed of 5 GHz at 465nm with a Q.E. of 0.5%\n(corresponding to roughly <5 mA/W). How does this compare to other PDs operating in\nthe visible regime? For example, commercial silicon PDs are available from Thorlabs\n21\nwith 2 GHz and 150 mA/W at 465nm. Schottky InGaAs with operation into the visible\nalso exist with even higher bandwidths (>20 GHz) and higher responsivities. How about\nstate-of-the-art research on other 2D materials such as graphene, MoTe2, PdSe2, carbon\nnanotubes, etc. ?\nI would recommend to the authors to make a clear comparison to the state of the art of\nhigh-speed visible PDs such that the advantages of the here presented work can be\ndiscussed.\n[Response to Comment 15]\nThank you for your detailed comments regarding the state-of-the-art. We added a table,\npresented as Table R1, which compares various properties of our work with state-of-the-\nart 2D materials/GaN and high-speed photodetectors.\nBased on our response above, the following sentences and figure were added in the\nrevised manuscript and supplementary information:\n(Manuscript, page 8, lines 177) Supplementary Table 2 provides a comparison between\nour device and previously reported 2D materials/GaN photodetectors as well as other\nhigh-speed photodetectors.\n(Supplementary Information, page 11, lines 194) Section 11, “Comparison to state-of-\nthe-art of 2D materials/GaN and high-speed photodetectors”\n(Supplementary Information, page 12) Supplementary Table 2: Comparison to …\n[Related reviewers’ comments] Reviewer 3 Comment 6\n22\n.s\nD\nP\ne lb\nis\\iv\nd\ne\ne\np\ns - h\ng\nih\nd\nna\ns r o\ntc\ne\nte\nd\no\nto\nh p\nN\na G\n/s\nla\nir\ne ta m\nD\n2\nf\no\ntr\na\n-\ne h\n-t\nf o\n-\ne\nta\nts\ne\nh\nt o\nt\nn\no\ns\nir\na\np\nm\no C\n:1\nR e lb\na T\ntnerruc\nkraD\nemit\nllaf/esiR\n)Bd\n3(htdiwdnaB\nytivisnopseR\nytisned\nrewop\ntupnI\nsaiB\naera\nevitisnesotohP\nhtgnelevaW\nfeR\nseciveD\nnoitazirogetaC\n)2/1zH/W(\n)A(\n)sm(\n)zHG(\n)W/A(\n)2mc/W(\n)V(\n)2mc(\n)mn(\nNaG-n/NaG-i/SoM\nkrow\nsihT\nmn\n564@01-01×6.5\n6-01×3.6\nmn\n564@\n5\nmn\n564@\n300.0\nmn\n564@\n01\n4-\n5-01×8.2\n026-504\n2\nelbisiV\nDP-CTU\nteehs\nataD\nAN\n21-01×1\nAN\nmn007@\n5.1\nmn007@\n52.0\nAN\n2-\n4-01×1.3\n0001-023\n)5509S\n;scinotohP\nustamamaH(\nDP-NIP\niS\nelbisiV\n5\nmn\n563@\n71-01×5.5\n11-01×1\n841/89\nAN\nmn\n563@\n81.82\nmn\n563@\n20.0\n3-\n3-01×5.2\n009-563\nNaG-n/\n)VU\nylniam(\nelbisiV\n6\nmn\n563@\n81-01×2.7\n4-01×1\n952/181\nAN\nmn\n563@\n008\nmn\n563@\n10.0\n5-\n5-01×69.1\n0501-082\nNaG-n/\nx-1 eR\nx oM-p\n)VU\nylniam(\nelbisiV\n01-01×1\n81/02\nAN\nmn\n073@\nAN\nAN\n073-053\nNaG-i/\nVU\n8\n11-01×8.3\n4-01×4\n001～\nAN\n6.813\n6-01×5\n2-\nAN\nVU\n9\n41-01×5.2\n31-01×1\n321.0/820.0\nAN\n52.0\n60.0\n0\n2-01×4\n2 eSdP\nVU\nAN\nAN\nAN\n5.1-\n7-01×89.1\nPnI/sAaGnI\nRIN\n6-01×9.8\nAN\n02\n21.0\nAN\n3-\n6-01×5.1\neG/iS\nRIN\nteehs\nataD\nmn\n0551@\n11-01×4.2\n8-01×1\nAN\n04\nmn\n0551@\n7.0\nWm\n01\n6-\nAN\n0561-0521\n)FA04XR\n;balrohT(\nDP\nsAaGnI\nRIN\nOther points:\n[Comment 16]\nline 85: the authors state that Raman spectra reveal that the structure consists of 2 to 3\nlayers. In the figure description they state that STEM reveal 2 to 4 layers. However, the\nSTEM images shown throughout the paper only seem to show 2 layers. Which is it?\nAdditionally, what is the reason for choosing 2 layers?\n[Response to Comment 16]\nThe STEM image in Fig. R13 shows a distribution of 2 to 4 layers in-plane for the MoS\n2\ngrown on the 4-inch GaN/sapphire wafer. Raman spectroscopy, with its spot size of\napproximately 10 μm in diameter, presents challenges in distinguishing the number of\nlayers due to the in-plane distribution. Given these circumstances, we believe the results\nfrom STEM and Raman spectroscopy are generally consistent. Additionally, if MoS\n2\nwere grown as a monolayer, it might not fully cover the entire GaN/sapphire wafer,\npotentially resulting in island-like growth. Therefore, we aimed to grow MoS under\n2\nconditions that result in 2 or more layers.\nFig. R13: Cross-sectional STEM image for distoribution of 2 to 4 layers in-plane.\nBased on our response above, the following sentences were added in the revised\nsupplementary information:\n(Manuscript, page 8, lines 197) The MoS grown on the 4-inch GaN/sapphire wafer\n2\nexhibits an in-plane distribution of 2 to 4 layers, as shown in Supplementary Fig. 2.\n24\n(Supplementary Information, page 2, lines 32) Supplementary Fig. 2 shows a cross-\nsectional STEM image depicting MoS grown on a 4-inch GaN/sapphire wafer, covering\n2\na larger area than Fig. 1c, and Supplementary Fig. 1. The STEM image indicates that\nthere are 2 to 4 layers of MoS in the in-plane orientation.\n2\n(Supplementary Information, page 3) Supplementary Fig. 2: Cross-sectional STEM …\n[Comment 17]\nline 106: I do not see how the results in Fig. 2 lead to this conclusion. Is this intended to\nbe with reference to Fig. 1d?\n[Response to Comment 17]\nIn line 106, \"These results\" refers to the findings presented in Fig. 1b-d and Fig. 2. The\nresults in Fig. 1b-d indicate the high quality of the MoS layers, while the results in Fig.\n2\n2 show that MoS effectively functions as the active absorption region for UTC-PD\n2\napplications due to its type-II band alignment with GaN. To prevent potential\nmisunderstandings for readers, we revised the manuscript:\n(Manuscript, page 2, lines 84) thereby verifying that MoS layers effectively function as\n2\nthe active absorption region for UTC-PD applications.\n[Comment 18]\nFigure 1a: The p-contact layer should also be illustrated.\n[Response to Comment 18]\nUnlike the typical InGaAs/InP-based UTC-PDs, our UTC-PD employs MoS to serve\n2\nboth as the absorption layer and the p-contact layer. To clarify the device configuration,\nwe have revised the figure to include the electrodes, as shown in Fig. R14.\n(Manuscript, page 3) Figure 1a\n(Manuscript, page 2, lines 58) The device is composed of an anode electrode, an\nabsorption layer of MoS , an i-GaN carrier transport layer, an n-GaN contact layer, and a\n2\ncathode electrode.\n(Manuscript, page 2, lines 62) through the n-GaN contact layer\n[Comment 19]\nFigure 5a: It would be good to label the remaining components such as the beam splitters\nand the reference PD. Also, the element controlling one of the mirrors in the\ninterferometer arm is not discussed at all or how the feedback is performed.\n[Response to Comment 19]\nAs the Reviewer commented, we have revised the figure to add labels for the remaining\ncomponents, such as the beam splitters and the reference PD, as shown in Fig. R15.\nAdditionally, we have provided detailed information in the Method section regarding the\nelement controlling one of the mirrors in the interferometer arm and how the feedback is\nperformed.\nMoS /GaN UTC-PD.\nrevised manuscript:\n(Manuscript, page 6) Figure 5a\n(Manuscript, page 6) Figure 5 caption: A single-mode laser diode (465 nm) is intensity-\nmodulated via a Mach Zehnder interferometer (see Methods).\n(Manuscript, page 9, lines 209) Method section: The device's photoelectric properties\nwere characterized…\n27\n<<Reviewer 3>>\n[Comment]\nThe authors present an UTC-PD based on the heterojunction between MoS2 and GaN for\nvisible light detection. Photodetectors based on this heterojuction have been already\nreported in literature but with limited optoelectronic bandwidth and with a different\ndoping profile. Relying on the direct growth of MoS2 films on the GaN on sapphire\nsubstrate the authors improve the electrical interface of the heterojunction reducing trap\nstates that are responsible for limited speed of operation.\nThe paper is well written and the results and methods are novel and convincing. However,\nI would consider this work for publication on Nature Comm only after addressing the\nfollowing points:\nWe sincerely thank the Reviewer for the positive evaluation on our work “The paper is\nwell written and the results and methods are novel and convincing”. We also appreciate\nthe Reviewer’s insightful and constructive comments for improvement.\n[Comment 1]\nthe device photoresponse is reported in terms of internal quantum efficiency as a function\nof the frequency of the signal. There is no mention of the device responsivity nor\nsensitivity. It is highly recommended to show a plot of the photoresponse in terms of\nphotocurrent and responsivity as a function of voltage and wavelength.\n[Response to Comment 1]\nAs the Reviewer commented, to make comparison with other technologies easier, we\nhave included the responsivity on the right axis of Fig. R10a-d, alongside the Q.E.\ngraphs, for clarity. We have also added graphs of responsivity as a function of voltage\nand wavelength, as shown in Fig. R10e-f.\ndiodes (LD) at 405 nm (a), 465 nm (b), 530 nm (c), and 620 nm (d), with power densities\nof 50 W/cm2, 10 W/cm2, 70 W/cm2, and 70 W/cm2. Each response is represented in terms\nof the quantum efficiency (Q.E.) and the responsivity calibrated by a commercially\navailable Si PIN-PD, whose quantum efficiency is specified by the manufacturer. We\nonly show the results with smoothing to eliminate the low and high-frequency noise. In\n(a), (c), (d), frequency region above 2 GHz is grayed-out to indicate the signal is\ndominated by crosstalk. The lines in each figure correspond to different reverse bias\nvoltage applied to the device. e, Responsivity of the device as function of voltage. f,\nResponsivity of the device as function of wavelength.\nBased on our response above, the following sentences and figure were added in the\nrevised manuscript and supplementary information:\n(Manuscript, page 5, lines 145) Additionally, Supplementary Fig. 6 provides further\ndetails on other photoresponse characteristics, such as responsivity, detectivity (D*), and\nnoise equivalent power (NEP).\n(Supplementary Information, page 5, lines 66) Section 4, “Other photoresponse\ncharacteristics (responsivity, detectivity, and noise equivalent power)”\n(Supplementary Information, page 6) Supplementary Fig. 6: a, Responsivity of the device\nas function of voltage. …\n[Related reviewers’ comments] Reviewer 1 Comment 3, Reviewer 2 Comment 3\n[Comment 2]\nthe rf response is attributed mainly to the device capacitance and external 50Ohm\nimpedance. I suggest to introduce the equivalent circuit of the PD highlighting all the\ncontribution, even if negligible. E.g. there is no comment about the contact resistance on\nMoS2 which is typically high.\n29\n[Response to Comment 2]\nAs the Reviewer commented, we presented high-frequency equivalent circuit model of\nthe photodiode in Fig. R16, highlighting all contributions. Here, C represents the\nj\ncapacitance of the device, R the shunt resistance due to the junction leakage, R the\nsh s\nseries resistance primarily the contact resistance between MoS and the anode metal, C\n2 s\nthe parasitic capacitance at the same interface, and R the load resistance of the probing\nload\nsystem’s transmission line.\nAccurately determining the contact resistance R between MoS and the anode metal for\ns 2\nthe vertical structure device in this study is challenging. We preliminarily estimated the\ncontact resistance from the forward I-V characteristics, as shown in Fig. R5 (Reviewer 2\nComment 3). From the slope of the forward bias, we found that the contact resistance R\ns\nis below 10 Ω, which is not extremely small compared to the 50 Ω charactaristic resistance\nof the external circuit. Here we must also consider the parasitic capacitance C in parallel\ns\nwith Rs. Given the device geometry, where the interface resistance between anode metal\nand MoS₂ layer is separated by a very small distance, the parallel-plate capacitance Cs\ncan be assumed to be significantly large, becoming negligible at GHz regime. We believe\nthat the RF response is simply attributed to the device capacitance and an impedance of\napproximately 50 Ω, justifying the RC low-pass filter model (composed of C and 50 Ω)\nj\npresented in the main text.\nFig. R16: Equivalent circuit of the photodiode system.\nBased on our response above, the following sentences and figure were added in the\nrevised manuscript and supplementary information:\n(Manuscript, page 5, lines 128) The equivalent circuit of the photodiode system at a\nreverse bias of 4 V is presented in Section 3 of Supplementary Information.\n30\n(Supplementary Information, page 4, lines 53) Section 3, “Equivalent circuit of the\nphotodiode measurement system”\n(Supplementary Information, page 5) Supplementary Fig. 5: Equivalent circuit …\n[Comment 3]\nthe rf response shown in fig. 5b starts from 10 MHz and is significantly higher the 0 dB\nfrom 10 to 20 MHz. The authors should describe what happens in that spectral region and\npossibly extend the test below 10 MHz to better understand the behavior of the device at\nlow frequency. E.g., in a data trasmission at 10 Gb/s it is relevant to go down at least to\n100 kHz.\n[Response to Comment 3]\nThe region from 10 to 20 MHz in the RF response shown in Fig. 5b, where the level\nexceeds 0 dB, is influenced by 1/f noise. We have provided the raw data for the noise\nfloor of the measurement system in Fig. R11. The gray line in the graph indicates the\nnoise floor with the LD turned off. The low-frequency noise and high-frequency noise\nare attributed to 1/f noise and RF crosstalk noise, respectively.\n0\n465 nm\n-20\n-40\n)B\nd\n(\nn ia -60\nG\n-80\nMoS2/GaN@-4V\nGaAs PIN\n-100 Si PIN\nDark\n107 108 109 1010\nFrequency (Hz)\nFig. R11: Frequency response of the device measured using a 465 nm laser diode at a -4\nV bias, without calibration or normalization against a reference GaAs PIN photodiode.\nFurthermore, as illustrated in Fig. R12, we measured frequency response below 50 MHz\nusing a different Vector Network Analyzer (LibreVNA). The elevated level in the region\nof 10 kHz to 100 kHz is influenced by 1/f noise. In conclusion, we confirmed a flat\nresponse in the range from 10 kHz to several GHz, as shown in Fig. R12 and Fig. 5b.\n31\n50\nNoise floor\n30\n20\n)\nB\nd 10\n(\nn 0\nia\nG -10\n-20\n-30\n-40\n-50\n104 105 106 107\nFrequency (Hz)\nFig. R12: Frequency response of the device measured using a 465 nm LD at a -4 V\nbias, with normalization against a reference GaAs PIN photodiode.\nBased on our response above, the following sentences and figure were added in the\nrevised supplementary information:\n(Manuscript, page 6) Figure 5 caption: See supplementary Section 8 for …\n(Supplementary Information, page 8, lines 113) Section 8, “Low-frequency response of\nthe device”\n(Supplementary Information, page 9) Supplementary Fig. 10: Low-frequency response\n…\n[Related reviewers’ comments] Reviewer 2 comment 12\n[Comment 4]\ntesting of the photoresponse is not properly described since the launch power of the laser\nand the insertion loss of the setup is not reported. These details must be listed at least in\nthe methods section.\n[Response to Comment 4]\nThe launch power of the laser and the insertion loss of the setup primarily involve\ncoupling loss into the optical fiber and insertion loss from components such as beam\nsplitters. We have included a diagram of the experimental setup, featuring components\nlike beam splitters, in Fig. R15. Regarding the incident laser power, we have addressed\nthis in Reviewer 2 Comment 2. We measured the laser power using a power meter\ndirectly before the device, ensuring that any power loss from branching or other factors\n32\nin the setup does not affect the evaluation results of the device's photoresponse and\nresponsivity.\nFig. R15: Experimental setup for measuring the frequency response of the fabricated\nMoS /GaN UTC-PD.\nMethod section of the revised manuscript:\n(Manuscript, page 9, lines 223) The launch power of the laser and the insertion loss within\nthe setup are primarily attributed to coupling losses into the optical fiber and insertion\nlosses from components such as beam splitters, as shown in Fig. 5a. …\n[Related reviewers’ comments] Reviewer 2 Comment 2, 19\n[Comment 5]\nthe discussion of the rectification and photodetection mechanisms should be summarized\nin the main text to help comprehension.\n[Response to Comment 5]\nAs the Reviewer commented, to enhance the comprehension for readers, we revised the\nmanuscript by summarizing the discussion of the photodetection mechanisms from the\nSupplementary Information into the discussion section of the main text.\nBased on our response above, the following sentences were added in the revised\nmanuscript:\n(Manuscript, page 7, lines 155) The internal quantum efficiency of the fabricated devices\nis significantly lower than unity that the conventional PIN photodiodes would attain,\nindicating that some of the photo-generated carriers do not contribute to the photocurrent.\n…\n(Manuscript, page 7) Figure 6f\n33\n(Manuscript, page 7) Figure 6 caption: f, Band structure of bilayer MoS (2H phase),\n2\ncalculated using density functional theory (DFT). …\n[Comment 6]\na table with comparison to SOTA is highly recommended to have a clear picture of the\nadvancement in the field.\n[Response to Comment 6]\nWe added a table, presented as Table R1, which compares various properties of our work\nwith state-of-the-art 2D materials/GaN and high-speed photodetectors.\nBased on our response above, the following sentences and figure were added in the\nrevised manuscript and supplementary information:\n(Manuscript, page 8, lines 177) Supplementary Table 2 provides a comparison between\nour device and previously reported 2D materials/GaN photodetectors as well as other\nhigh-speed photodetectors.\n(Supplementary Information, page 11, lines 194) Section 11, “Comparison to state-of-\nthe-art of 2D materials/GaN and high-speed photodetectors”\n(Supplementary Information, page 12) Supplementary Table 2: Comparison to …\n[Related reviewers’ comments] Reviewer 2 Comment 15\n34\nfe R\nk\nro\nw sih\nT\nte\neh\ns ata\nD\n5 6 7 8 9 0 1 1 1\nte\neh\ns ata\nD\n.s\nP E N 2/1 ) z H /W\n(\n01- m n 5 6 4 @\n0 1 × 6 .5\nA N 71- m n 5 6 3 @\n0 1 × 5 .5\n81- m n 5 6 3 @\n0 1 × 2 .7\n81-0 1 × 8\n.3\n41-0 1 × 5\n.2\nA N A N 11- m n 0 5 5 1 @\n0 1 × 4 .2\nD tn\nP\ne lb\nis\ne rru\nc k ra\nD\n)A (\n6-0\n1 × 3 .6\n21-0\n1 × 1\n11-0\n1 × 1\n4-0\n1 × 1\n01-0\n1 × 1\n4-0\n1 × 4\n31-0\n1 × 1 A N\n6-0\n1 × 9 .8\n8-0\n1 × 1\niv\nh\nd e e p s - em it lla f/e siR )sm ( A N 8 4 1 /8 9 9 5 2 /1 8 1 8 1 /0 2 ～ 0 0 1 3 2 1 .0 /8 2 0 .0 A N A N A N\ng\nih\nd n a s r\no\ntc\n)B\nd 3 (h tdiw d\nn\na B\n)z H G ( m n 5 6 4 @\n5\nm n 0 0 7 @ 5\n.1\nA N A N A N A N A N 0 5 1 0 2 0 4\ne\nte d o to h p N y tivisn o p se R )W /A ( m n 5 6 4 @ 3 0 0 .0 m n 0 0 7 @ 5 2 .0 m n 5 6 3 @ 8 1 .8 2 m n 5 6 3 @ 0 0 8 m n 0 7 3 @ 0 1 6 6 .8 1 3 5 2 .0 A N 2 1 .0 m n 0 5 5 1 @ 7 .0\na\nG\n/s la ir e ta m\ny tisn\ned re w o p tu p 2) m c/W ( m n 5 6 4 @ 0 1 A N m n 5 6 3 @ 2 0 .0 m n 5 6 3 @ 1 0 .0 A N 6-0 1 × 5 6 0 .0 A N A N W m 0 1\nD\nn\nI\n2 f saiB )V ( 4 - 2 - 3 - 5 - A N 2 - 0 5 .1 - 3 - 6 -\no\ntr a\ne\ne f h o\na\n- - t\nra\nevitisn e so 2) m c ( 5-0 1 × 8 .2 4-0 1 × 1 .3 3-0 1 × 5 .2 5-0 1 × 6 9 .1 A N A N 2-0 1 × 4 7-0 1 × 8 9 .1 6-0 1 × 5 .1 A N\ne\n- to\nh\nta P\nts e h t o t\nn\nh tg n elev aW )m n ( 0 2 6 -5 0 4 0 0 0 1 -0 2 3 0 0 9 -5 6 3 0 5 0 1 -0 8 2 0 7 3 -0 5 3 5 2 3 0 6 3 0 5 5 1 0 5 5 1 0 5 6 1 -0 5 2 1\no\ns )5\nir\na p\nm\no C :1 R e lb a T se civ e D N aG -n /N aG -i/ S o M 2 D P -C T U\n5 0\n;scin\no to h P u stam am a H (\nD\nP\nN aG -n / S o M 2 N aG -n / S e R o M -p 2 x-1 x N aG -i/ S o M 2 N aG /en eh p a rG N aG / eS d P 2 D P -C T U P n I/sA aG n\nI\nD P -C T U eG /iS\n)F\nA\n0 4 X R ;b alro h T ( D P sA\naG\n-N n\nI\nIP\niS\n)V )V\nn U U\noitaziro\ng eta\nC\nelbisiV elbisiV\nylniam\n( elbisiV\nylniam\n( elbisiV\nV\nU\nV\nU\nV\nU\nR\nIN\nR\nIN\nR\nIN\n<<Reviewer 2>>\n[Comment]\nI thank the authors for the very detailed revisions and addressing all of my comments.\nThe PD characteristics are now well defined and presented. However, there are still three\npoints that I don't think are presented correctly.\nWe will address the remaining three points below.\n[Comment 1]\nWith respect to Comment 13 on the gain. \"Gain\" where the flat region of the frequency\nresponse is set to 0 dB. This is still confusing as a reader.\n- The same \"gain\" label is now used in Fig. R11 which in my opinion makes no sense.\n- The \"gain\" label is also used in Fig. R12 where the caption reads \"normalization against\na reference GaAs PIN photodiode\". When reading this, the Gain indicates that this is the\nrelative response to the GaAs PIN photodiode. However, the device has roughly 20 dB\nless output according to Fig. R11.\nThis is a RF response (dBm), a calibrated RF response (dB), a normalized RF response\n(dBm), a calibrated, normalized RF response (dB), S21 (dB), or something else, where it\nis clearly described what is the (i) calibration and (ii) what is the normalization. It can\nalso be described as gain, but then it needs to be clearly explained to what the gain is\nreferring to. \"Gain\" with respect to the flat region of the measurement seems completely\narbitrary and is not described in the manuscript.\n[Response to Comment 1]\nTo resolve the reviewer’s concern regarding the use of the “Gain” labels and to improve\nclarity for the readers, we changed the labels as follows:\n• Fig. 5b, Supplementary Fig. 10: Gain (dB) → Cal./Norm. RF response (dB)\n• Supplementary Fig. 7, 9: Gain (dB) → S21 (dB)\n[Comment 2]\nWith respect to Comment 12: the added measurements in R11 and R12 are very helpful,\nyet, Fig. 5b remains unchanged.\n- From R11 it is clear that anything lower than 20 MHz is just noise and anything above\n1\nthe figure if the authors know that it is the noise floor.\n- From R12: This nicely extends the previous measurements by showing a clear\nresponse above the noise floor from 200 kHz to 50 MHz.\n- I highly recommend adapting Fig. 5b by showing both measurements in the same plot\n(labelled as two different measurements) and only in the regions where the\nmeasurements are valid, i.e., where you are clearly above the noise floor.\n[Response to Comment 2]\nRegarding Fig. 5b, we added the noise floor to the graph. While we appreciate the\nsuggestion to include both measurements from Figs. R11 and R12 in Fig. 5b, doing so\nwould reduce the size and clarity of the main high-frequency response, which is the\nprimary focus of this figure. Therefore, we retained only the high-frequency data in Fig.\n5b, and the extended low-frequency measurement remains in Supplementary Fig. R12,\nas referenced in the manuscript (“See supplementary Section 8 for extended low-\nfrequency data.”). We believe this approach maintains the clarity of the main results\nwhile providing readers with information about the noise floor.\nBased on our response above, the following figure was revised:\n(Manuscript, page 6) Fig. 5b\n[Comment 3]\nWith respect to Comment 15: the author's addressed my comment by adding a SOTA\ncomparison table. The table shows that the here developed technology has the highest\nbandwidth for any visible photodetector. However, I find this table incomplete. I\nmentioned in my original comment two examples of commercial diodes (2 GHz Si) and\n(>20 GHz InGaAs), which have been ignored.\nFor example:\n- InGaAs, Newport: Model: 1414. Wavelength range: 500-1630nm, 200 mA/W @\n500nm, BW 25 GHz, Rise time: 14 ps, NEP 3e-11 W/Hz^1/2\n- GaAs, Newport: Model: 818-BB-45. Wavelength range: 500-890nm, ~80 mA/W @\n500nm, BW 12.5 GHz, Rise time: 30 ps, NEP 3.5e-11 W/Hz^1/2 @830nm\nThe 2D material comparison also only includes 2D materials on GaN, which to me is not\nclear why one would limit oneself to this material platform. I also don't think the chosen\nNIR examples (InGaAs PD, Si/Ge) represent the SOTA of NIR PDs, but it anyway seems\n2\nmisplaced considering the table is titled \"Comparison to the SOTA of 2D materials/GaN\nphotodetectors and high-speed VISIBLE PDs.\"\nRealistically, the table puts this first demonstration of a high-speed, direct growth\nMoS2/GaN diode into perspective to established and other developing technologies. I\ndon't think the current representation of it as \"the fastest VIS PD\" is true.\n[Response to Comment 3]\nIn response to the reviewer’s comments regarding the comparison to state-of-the-art\nphotodetectors, we made the following revisions to improve the completeness and clarity\nof the comparison:\n• We updated Supplementary Table 2 to include additional commercial high-speed\nphotodetectors, specifically GaAs PIN and InGaAs PIN photodiodes, as well as\nphotodetectors based on other material platforms such as MoS /Si and MoS\n2 2\nMSM Schottky photodiodes. This provides a more comprehensive comparison of\nour device with both established technologies and other emerging 2D material-\nbased photodetectors beyond the GaN platform.\n• We revised the corresponding description in the Supplementary Information to\npresent our results in a more moderate and precise manner. In particular, we now\nclarify that while several commercial III-V photodiodes exceed our device in\nbandwidth, our work represents the first demonstration of a direct-grown\nMoS /GaN UTC-PD exhibiting GHz-range frequency response across the visible\n2\nspectrum.\nWe believe these changes provide readers with a fair and transparent perspective on the\nposition of our technology relative to both commercial and research-grade photodetectors,\nand fully address the concerns raised by the reviewer.\nBased on our response above, the following table and sentences were revised in the\nsupplementary information:\n(Supplementary Information, page 14) Supplementary Table 2\n(Supplementary Information, page 13) Supplementary Table 2 provides a comparison\nof our device with state-of-the-art photodetectors, including 2D materials/GaN devices,\nother 2D material-based photodetectors, and commercial high-speed photodiodes. …\n3"}]

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