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High-Responsivity Deep-Ultraviolet-Selective Photodetectors Using Ultrathin Gallium Oxide Films Seung Hyun Lee, Soo Bin Kim, Yoon-Jong Moon, Sung Min Kim, Hae Jun Jung, Myung Su Seo, Kang Min Lee, Sun-Kyung Kim, and Sang Woon Lee ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01054 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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High-Responsivity Deep-Ultraviolet-Selective Photodetectors Using Ultrathin Gallium Oxide Films Seung Hyun Lee†, Soo Bin Kim†, Yoon-Jong Moon‡, Sung Min Kim†, Hae Jun Jung†, Myung Su Seo†, Kang Min Lee†, Sun-Kyung Kim*‡, and Sang Woon Lee*† †

Department of Energy Systems Research and Department of Physics, Ajou University,

Gyeonggi-do 16499, Republic of Korea,

‡

Department of Applied Physics, Kyung Hee

University, Gyeonggi-do 17104, Republic of Korea KEYWORDS photodetectors, amorphous gallium oxide, deep-ultraviolet wavelengths, flexible optoelectronic devices, wide bandgap materials, atomic layer deposition ABSTRACT Wavelength-selective photodetectors responding to deep-ultraviolet (DUV) wavelengths (λ = 200 – 300 nm) are drawing significant interest in diverse sensing applications, ranging from micron biological molecules to massive military missiles. However, most DUV photodetectors developed thus far have suffered from long response times, low sensitivity, and high processing temperatures, impeding their practical use. Here, we report fast, highresponsivity, and general-substrate-compatible DUV photodetectors based on ultrathin (3 – 50 nm) amorphous gallium oxide (GaOX) films grown by low-temperature (~< 250 °C) atomic layer deposition (ALD) for the first time. ALD grown GaOX films on glass substrates display a typical amorphous nature, which is identified by electron beam diffraction and X-ray diffraction

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measurements, while their bandgap is sharply featured at ~4.8 eV. Metal–semiconductor–metal photodetectors (active area of 30 × 30 µm2) using the 30-nm-thick GaOX films work reliably only for DUV wavelengths; the responsivity is maximized to 45.11 A/W at λ = 253 nm, which dropped off at λ ~ 300 nm (i.e., a cut-off wavelength). The dark current measured at 10 V is as low as 200 pA and the signal-to-noise ratio reaches up to ~104, underpinning the pristine material quality of the ALD grown GaOX films. In addition, the rise time (i.e., the time interval for photocurrent to increase from 10% to 90%) is as quick as 2.97 µs at λ = 266 nm. Such reliable and fast photoresponse is achieved for even atomically thin (~3 nm) devices. The substratecompatible and low-temperature ALD growth permits the demonstration of flexible DUV photodetectors using amorphous GaOX films grown on polyimide substrates, suggesting their facile integration into other curved optoelectronic systems. We believe that photodetectors developed herein will provide an economically viable solution for high-performance DUV detection and create a variety of sensing applications.

Sunlight in the solar-blind wavelength range (λ = 200 – 300 nm) cannot penetrate the atmosphere to reach the earth’s surface because of the strong absorption by ozone and atmospheric vapors.1 Thus, solar-blind photodetectors are appropriate for missile tracking and space communication in order to avoid sky background interference.2,3 Recently, the detection of such deep-ultraviolet (DUV) wavelengths has drawn significant interest for use in water and air purification systems4 because the high energy (> 4 eV) of DUV radiation is fatal to human cells. DUV detection is also important for sensing biological molecules (e. g., food-borne fungi) because their absorption and florescence spectrum is within the range of DUV wavelengths.5,6


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Therefore, the development of DUV photodetectors that operate reliably under various environmental conditions is necessary for the safety of human life. DUV photodetectors must respond selectively to λ < 300 nm while avoiding absorbing light with other longer wavelengths including visible spectrum, which negates the use of Si materials. In order to utilize DUV photodetectors in various applications, a fast response speed, high signalto-noise ratio, low operation power, and large wavelength selectivity need to be considered. However, the DUV detectors reported thus far have exhibited slow progress in improving the response speed; the time interval necessary for the current to increase from 10% to 90% under illumination (i.e., rise time) often remains on the order of seconds.7-15 In general, AlxGa1-xN, ZnxMg1-xO, and diamond are appropriate candidates for DUV absorbing materials.16-18 Among them, AlxGa1-xN with a controlled Al composition (typically, x ~ 0.4) has been intensively studied for DUV detection; however, AlxGa1-xN must be grown epitaxially and thus, complex buffer layers are inevitably grown to reduce the lattice mismatch, causing high production costs and skills. In addition, the total thickness of AlxGa1-xN and its buffer layers is readily larger than 500 nm, and the growth temperature is as high as ~1000 oC, which prevents AlxGa1-xN based devices from merging into other functional devices as a system on chip scheme.12,19 Besides, AlxGa1-xN based metal–semiconductor–metal (MSM) DUV photodetectors suffer from long response times, hindering their practical use.16 The degraded device performance is attributed to a high level of threading dislocation density within AlxGa1xN.

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Gallium oxide material has recently received much interest because it also possesses a wide bandgap of approximately 5 eV. For most gallium oxide based DUV photodetectors, monoclinic β-Ga2O3 films (200 – 300 nm in thickness) are epitaxially grown on identical thick β-Ga2O3


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substrates by molecular beam epitaxy (MBE), vapor phase epitaxy (VPE), or chemical vapor deposition (CVD) process.8,20,21 However, β-Ga2O3 based photodetectors still suffer from slow response speeds and low sensitivity.7,8,11,22 Additionally, top multi-finger electrodes should be adopted because of the short carrier diffusion length, which further increases the complexity of the devices.8 In the meantime, nanostructured photodetectors using β-Ga2O3 nanowires (NWs) have been investigated for enhancing light absorption at various wavelengths.10,15,21,23 However, the detection speed was extremely low at a few seconds, which limits their commercial use as sensors. Although it was reported that the rise time of β-Ga2O3 NW based DUV photodetectors dramatically decreased to the 1 – 100 µs range21,24, the processing temperature still exceeded 1000 °C, causing a radical increase of the thermal budget. In this work, we, for the first time, demonstrate fast, high-responsivity, and general-substratecompatible DUV photodetectors using extremely thin (3 – 50 nm) amorphous GaOX films. The GaOX films were grown by atomic layer deposition (ALD), for which the processing temperature throughout growth was kept below 250 °C. Crystallinity of the GaOx films grown on general substrates such as glass and polyimide was investigated by means of X-ray diffraction (XRD) measurement and high-resolution transmission electron microscopy (HRTEM) analysis. Measurements of the absorption spectra determined a bandgap feature of GaOX films. We fabricated GaOX film-based MSM photodetectors (an active area of 30 × 30 µm2) on a glass substrate and evaluated all of the metrics (response time, responsivity, wavelength selectivity, and dark current) relevant to photodetector performances for various film thicknesses (3 – 50 nm). More interestingly, we successfully achieved the growth of GaOX films on a polyimide substrate, and examined the possibility of flexible DUV photodetectors.


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RESULTS AND DISCUSSION Material Characterization of the GaOX Thin Films Grown on Glass Substrates. To implement high-performance DUV photodetector devices, we synthesized amorphous GaOX thin films by conducting an ALD process (Methods). In general, the ALD process is beneficial for depositing high-quality thin films at low temperatures (< 300 °C), while its slow growth rate (typically < 0.1 nm/cycle) may limit the application of ALD for depositing thicker (> 100 nm) films.25 For previous DUV photodetector applications, relatively thick (200 – 300 nm) crystalline Ga2O3 films were typically deposited by MBE or CVD process to collect a large photocurrent.8,21 Here, we examine whether extremely thin (50 nm or less) ALD grown amorphous GaOX films serve reliably as DUV photodetectors, which provides an economically viable strategy for DUV sensing applications. First, we synthesized GaOX thin films with various thicknesses (3, 5, 10, 30, and 50 nm) on glass substrates by an ALD process. The growth rate of the GaOX films was 0.58 Å/cycle followed by the typical ALD sequence, consisting of trimethyl gallium (TMG) exposure (1.5 sec.)/purge (5 sec.)/ozone exposure (4 sec.)/purge (5 sec.) steps at the processing temperature of 250 °C. The thickness of GaOX was readily controllable by simply adjusting the growth time, which is confirmed by HRTEM analysis (Figure 1a). To identify the crystallinity of the synthesized GaOX, selected-area electron diffraction (SAED) pattern was acquired (Figure 1b); the SAED pattern of the GaOX/glass interface region is featureless without a clean diffraction spot. In addition, the measured XRD spectrum (theta–2theta mode) of the same GaOX film also shows a typical amorphous nature without a pronounced peak (Figure 1c). To delineate the feasibility of DUV sensitive detection, we obtained the absorption spectrum of the as-grown GaOX film by using a spectrophotometer system (Methods); a clear threshold (i.e., bandgap) is


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observed at ~4.8 eV with a direct allowed transition assumed (Figure 1d). Despite of the inaccuracy in the extraction of the bandgap due to the amorphous nature of the GaOX films using the Tauc plot, the estimated bandgap of the amorphous GaOX film was consistent with the wavelength-dependent responsivity result as shown in Figure 2. Taken together, low-temperature (~< 250 °C) ALD growth developed herein yields amorphous GaOX thin films with nanometerscale thickness control, inducing strong light absorption only for DUV wavelengths.

Figure 1. (a) HRTEM images of the GaOX films with various thicknesses (3, 10, and 30 nm) grown on glass substrates. (b) Selected-area electron diffraction (SAED) pattern acquired from the interface of the GaOX/glass substrate. (c) XRD pattern (theta–2theta mode) of an as-grown 30-nm-thick GaOX thin film on a glass substrate. (d) Tauc plot for the bandgap extraction of the GaOX film obtained by spectrophotometry measurement.

Photoresponsivity of the GaOX Thin-Film Photodetectors. In order to probe the ability of DUV detection, an MSM-type photodetector was fabricated using amorphous GaOX films (Figure 2a and Methods). Two square-patterned Pt electrodes were deposited on opposite edges of the GaOX film surface after a 30-nm-thick GaOX film was grown on a glass substrate. We note


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a simple configuration of our device structure compared to previously reported ones adopting complex finger electrodes.8 The current-voltage (I–V) measurement of the fabricated GaOX photodetector shows a dark current as low as 200 pA at 10 V (Figure 2b), and the photocurrent is enhanced by a factor of ~104 at 10 V under DUV illumination (UV lamp with a center wavelength of 253 nm and an intensity level of 0.7 mW/cm2). Repetitive measurements on photocurrents under pulse laser illumination (λ = 266 nm with a period of 270 µs and a pulse width of 5 ns) ensured the reliable operation of the fabricated photodetector (Figure 2c). A clear and stable photoresponse to the DUV illumination is observed for other GaOX devices thinner or thicker than 30 nm (Figure S1). Notably, even an atomically thin (3 nm) GaOX film yields quite a reliable photoresponse. The wavelength-selective photoresponse was revealed by obtaining its responsivity as a function of the wavelength of the illuminated light at 20 V (Figure 2d and Methods). The GaOX photodetector responds selectively to a specific range of DUV wavelengths (λ = 200 – 300 nm) with a maximum responsivity of 45.11 A/W achieved at λ = 253 nm, whereas the photocurrent with λ > 300 nm is negligible. As a result, the DUV/visible rejection ratio (i.e., the ratio of responsivity at λ = 253 and 400 nm) is as high as 558 at 20 V, consistent with the absorption measurement of the synthesized GaOX film (Figure 1d). The maximum responsivity obtained herein is much higher than any other previously reported DUV photodetectors using β-Ga2O3 and AlXGa1-XN thin films, after accounting for the film thickness (30 nm) of our device.7,8,21,24,26 These experimental findings demonstrate that ALD grown amorphous GaOX films provide a facile route to realize reliable and high-responsivity DUV photodetectors.


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Figure 2. (a) Schematic of the fabricated MSM-type photodetector. (b) Current–voltage curve of the photodetector depending on DUV illumination (UV lamp with a center wavelength of 253 nm and intensity of 0.7 mW/cm2). (c) Repetitive detection performances of the photodetector under pulse laser (λ = 266 nm and pulse width 5 ns) illumination with a period of 270 µs at 10 V. (d) Responsivity as a function of wavelength of illuminated light at 20 V.

Detection Speed of the GaOX Thin-Film Photodetectors. Response time is another important metric to assess the capability of photodetectors, however, most β-Ga2O3 or AlxGa1-xN based DUV photodetectors reported thus far exhibit a few milliseconds to the order of seconds (Table S1),7,8,11,22 which limits their practical use for high-speed sensing and imaging applications. To accurately evaluate the rise time of fabricated DUV photodetectors, a highspeed oscilloscope connected to a current sourcemeter was used to measure their transient photocurrent under pulse DUV laser illumination (Figure S2 and Methods). A pulse DUV laser


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(λ = 266 nm) with a pulse width of 5 ns was used as the illumination source. To investigate the response time, we fabricated DUV photodetectors based on GaOX thin films with various thicknesses (3 – 50 nm) and measured the time-resolved photocurrents (Figure 2c and Figure S1). For the 30-nm-thick amorphous GaOX device, the rise time (i.e., the time interval for photocurrent to increase from 10% to 90%) is typically ~3 µs (Figure 3a), which rather surpasses previously reported crystalline thin-film devices.21,24 Even for other amorphous GaOX film devices (3, 5, 10, 50 nm in film thickness), the rise time is in the range between 1 – 6 µs, while the level of photocurrent tends to exponentially decrease with decreasing the film thickness, except for the 3-nm-thick GaOX film device (Figure 3b).

Figure 3. (a) Photocurrent as a function of the time in order to measure the rise time of the photodetector using a 30 nm-thick amorphous GaOX thin film. (b) Measured rise times and photocurrents of GaOX thin-film photodetectors with various film thicknesses. (c) Comparison of


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our DUV GaOX thin-film photodetector devices with previously reported ones (the reference numbers are shown in the bracket) for rise time and responsivity. (d) Ga 2p XPS result of GaOX film before and after annealing in air at 500 °C for 1 h.

It should be noted that the rise time and responsivity need to be considered at the same time for DUV photodetector applications. Thus, the performances (rise time and responsivity) of our amorphous GaOX DUV photodetectors are quantitatively compared to those of previously reported devices (Figure 3c). This comparison work clearly reveals that photodetectors developed herein outperform other MSM devices using β-Ga2O3-, AlxGa1-xN-, and MgZnObased thin films. The amorphous GaOX device represents a rise time of ~3 µs, whereas other compound (e.g., AlxGa1-xN) thin-film devices display a rise time of a few milliseconds to even a few seconds, which is mostly ascribed to their poor crystallinity and material defects. Remarkably, vertically oriented β-Ga2O3 NWs following a vertical Schottky photodiode configuration exhibit a decent rise time due to their short carrier collection length.24 For responsivity, the amorphous GaOX device using an extremely thin (30 nm) film surpasses other thin-film devices, except for laterally-oriented β-Ga2O3 NWs.15,23 However, for laterally-oriented NWs, it is a nontrivial issue to uniformly assemble them for the implementation of large-area devices. In particular, to support the fast photoresponse of GaOX photodetectors developed herein, we carried out X-ray photoelectron spectroscopy (XPS) for Ga 2p binding energy values of the asgrown 30-nm-thick amorphous GaOX film, before and after annealing in air at 500 °C for 1 h (Figure 3d). In the measured spectra, the Ga 2p spectra are identical before and after annealing in the ambient atmosphere, indicating that the as-grown GaOX has negligible oxygen vacancies by


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the highly oxidative ozone used as the oxygen source for the growth of the GaOX films by the ALD process. The O 1s binding energy spectra were also preserved well before and after the annealing (Figure S3). It is well known that oxygen vacancies can form trap states in the bandgap, thereby increasing the transit and the rise time constants.8,27 Therefore, negligible oxygen vacancies presenting in the as-grown GaOX thin film account for the fast photoresponse of the fabricated devices. It should be noted that any carbon or hydrogen impurity marginally exists in the GaOX films grown by ALD using trimethylgallium and ozone, indicating a clean ALD reaction.30 The absence of the oxygen vacancies together with the pristine material quality in the ALD grown amorphous GaOX thin film also lead to the improvement of carrier transport properties, thereby providing the high responsivity of the fabricated devices.

Flexible Photodetectors Using the GaOX Thin Films Grown on Polyimide Substrates. The use of inexpensive, easy-to-handle, and easily processed substrate materials is necessary for the extension of DUV photodetector applications. Rigid substrates such as glass and sapphire are hard and brittle, and thus, devices fabricated on them need a sophisticated technique (e.g., laser scribing) for chip breaking. Therefore, the development of photodetectors using amorphous GaOX thin films with low (< 250 °C) growth temperature is crucial because it shows promise for the use of general substrate materials such as soft polymers. In addition, the success of amorphous-film photodetectors suggest that the entire processing scheme can be utilized in flexible optoelectronic devices because of the low growth temperature (< 250 °C), the extreme thinness (< 30 nm) of GaOX films, and the use of soft growth substrates. As a proof-of-concept experiment, we demonstrated MSM photodetectors using a 30-nmthick GaOX film grown on a polyimide substrate (Figure 4a), maintaining the ALD growth


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temperature ~< 250 °C. The lower camera image in Figure 4a depicts a photodetector device fabricated on a polyimide substrate, exhibiting its mechanical flexibility. The XRD spectra show that an as-grown amorphous GaOX film on a polyimide substrate together with the used polyimide substrate exhibit no clear peak, indicative of an amorphous disorder (Figure 4b). The flexible GaOX photodetector also yields a high level of signal-to-noise ratio under DUV illumination (Figure 4c); the dark current is as low as 0.7 pA at -10 V, and the photocurrent increases significantly with enhancement factors (I/I0) of approximately 8 × 103 (at -10 V) and 3 × 103 (at +10 V) under the DUV illumination, of which I/I0 is similar to that of the photodetector using an amorphous GaOX film grown on a glass substrate (Figure 2b). We hypothesize that the slightly asymmetrical difference of I/I0 between the negative and positive voltages originated from a contact problem caused by a physical instability of the Pt electrode on GaOX film/flexible substrate. A delamination of Pt electrodes from GaOX film/polyimide substrate was observed due to a weak adhesion strength between Pt and GaOX, which might be improved by using an adhesion layer. The flexible GaOX photodetector with the high I/I0 indicates that our ALD grown GaOX films operate as high-responsivity DUV photodetectors on any growth substrate.


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Figure 4. (a) Schematic of the flexible solar-blind photodetector (upper panel) and photograph of the flexible DUV photodetector (lower panel). (c) Current–voltage curve of the Pt/amorphous GaOX film/Pt photodetector depending on DUV illuminations (UV lamp with a center wavelength of 253 nm and intensity = 0.7 mW/cm2). (d) Repetitive detection performances of the photodetector under the same DUV illuminations before and after bending the photodetector with a bending radius of 14 mm. To examine the performance of the fabricated amorphous GaOX photodetector under a bending strain, on/off repetitive measurements were carried out while its bending radius was changed from a flat state to a bending radius of 14 mm (Figure 4d and Methods). A bending strain was illustrated as shown in Figure 4a, indicating a strain perpendicular to the GaOx gap. The photodetector using a 30-nm-thick amorphous GaOX film exhibits a stable operation even after several bent-flat steps. Notably, the same trend was observed with a smaller bending radius of 5 mm (data was not shown here). Although the photocurrent is slightly degraded via the


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repeated bending cycles due to delamination of the Pt electrodes, we believe that the problem can be readily solved by using a proper adhesion layer between GaOX film and Pt electrodes, which requires further investigations. Taken together, flexible DUV photodetectors were successfully realized by extremely thin amorphous GaOX films grown on polyimide substrates, which had not yet been accomplished by using other materials because of thick (> 100 nm) films and high growth temperature (> 1000 °C).8,21,24

CONCLUSIONS In this work, fast, general-substrate-compatible, and flexible DUV photodetectors were developed using extremely thin amorphous GaOX films. A rise time of 1~6 µs was realized for a 30-nm-thick GaOX thin film with a high responsivity (45.11 A/W). The fast detection speed was kept even for an atomically thin (3 nm) GaOX thin film. Finally, our low-temperature (< 250 °C) ALD growth enabled polyimide-substrate based GaOX photodetectors, showing its feasibility for use in flexible DUV photodetector applications. We believe that ALD grown amorphous GaOX films studied herein will provide an economically viable and cost-effective solution for the development of diverse DUV photodetector applications.

METHODS ALD Growth and Device Fabrication. A traveling-wave type ALD reactor (150 mmscale wafer, CN-1 Co., Atomic Classic ALD system) was used for the deposition of GaOX thin films on glass or polyimide substrates at 250 °C. Trimethyl gallium (TMG) and ozone were used as the Ga precursor and oxygen source, respectively. The TMG precursor was kept in a canister at 5 oC. The ALD process was performed at a working pressure of ~1 torr with an ozone


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concentration of 200 g/cm3. For device fabrication, two 50-nm-thick Pt top electrodes were deposited by electron-beam evaporation, followed by defining square shapes of 30 × 30 µm2 by photolithography. The distance between neighboring Pt electrodes was 30 µm. Material Characterization. The crystallinity of the GaOX film was analyzed by XRD (D8 discover, Bruker) in the theta-2theta mode. HRTEM (JEOL, JEM-2100F) was used with SAED to investigate the crystallinity at the atomic scale. The specimen for TEM analysis was prepared by focused ion beam (FEI, NOVA 600 Nanolab). The chemical nature of the GaOX film was analyzed by XPS (ThermoVG SIGMAPROBE) using monochromatic Al Kα radiation. The film transmittance and absorbance were measured by spectrophotometry (Cary 5000, Varian). Device Characteristics. Current-voltage (I-V) curves were obtained by a HP4155 parameter analyzer, and a UV lamp with a center wavelength of 253 nm was used as a DUV illumination source (intensity level of 0.7 mW/cm2 on the sample surface). A DUV laser (a 266nm pulse laser with a pulse width of 5 ns and a frequency of 4 kHz) was used to irradiate the devices to investigate the rise and decay times for the detection of DUV, and the photocurrent was measured as a function of time using a sourcemeter (Keithley 2400) and an oscilloscope (Agilent, 54832D) as shown in Figure S2. Responsivity (R) was defined as R = (Ip − Id)/P·S, where Ip is the photocurrent, Id is the dark current, P is the light intensity of the DUV source, and S is the illuminated area. The wavelength-dependent responsivity of developed photodetectors was acquired over deep UV to visible wavelengths (λ = 200 – 600 nm) using a broadband Xe light source. A monochromator was used to resolve the wavelength of the incident Xe light and the wavelength step was set to 1 nm. For the sensing performance under the bending strain (i.e., the bending state), the photodetector was fixed on the cylindrical surface of a plastic bottle with bending radii of 14 and 5 mm by using a Kapton tape. And the photodetector was separated from


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the plastic bottle and fixed on a flat glass substrate to represent the flat state of the photodetector. The I-V curves were obtained by the HP4155 parameter analyzer for both states under the repetitive turning on/off the DUV using a UV lamp with a center wavelength of 253 nm (intensity level of 0.7 mW/cm2 on the sample surface).

ASSOCIATED CONTENT Supporting Information Additional information and ﬁgures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S.W.L.); [email protected] (S.-K.K.) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT S.W.L. was supported by the Basic Science Research Program through the National Research Foundation

of

Korea

(NRF)

funded

by

the

Ministry

of

Education

(No.

NRF-

2017R1D1A1B03031729), and “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), with a grant of financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (No.


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20164030201380). S.-K.K. acknowledges the Basic Science Research Program through the National Research Foundation of Korea (NRF), which is funded by the Ministry of Science, ICT and Future Planning (NRF- 2017R1A2B4005480).

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For Table of Contents Use Only

High-Responsivity Deep-Ultraviolet-Selective Photodetectors Using Ultrathin Gallium Oxide Films Seung Hyun Lee†, Soo Bin Kim†, Yoon-Jong Moon‡, Sung Min Kim†, Hae Jun Jung†, Myung Su Seo†, Kang Min Lee†, Sun-Kyung Kim*‡, and Sang Woon Lee*†

Schematic of deep-ultraviolet-selective photodetectors using gallium oxide films (left; planar photodetector, right; flexible photodetector)


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High-Responsivity Deep-Ultraviolet-Selective ... - ACS Publications

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