Ultrahigh Deep-UV Sensitivity in Graphene-Gated Î²-Ga2O3

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Ultrahigh Deep-UV Sensitivity in Graphene-Gated #-GaO Phototransistors Suhyun Kim, Sooyeoun Oh, and Jihyun Kim ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.9b00032 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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Ultrahigh Deep-UV Sensitivity in Graphene-Gated β-Ga2O3 Phototransistors Suhyun Kim‡, Sooyeoun Oh‡, and Jihyun Kim* Department of Chemical and Biological Engineering, Korea University, Seoul 02841, Korea

ABSTRACT

Deep-ultraviolet (UV) photodetectors based on ultra-wide bandgap β-Ga2O3 have a great potential in civil or military applications especially due to its inherent solar-blindness. Metalsemiconductor phototransistors based on exfoliated β-Ga2O3 were integrated with graphene as a highly transparent gate electrode. Controlling the potential barrier at the metal-semiconductor junction through the UV-transparent graphene gate expanded the difference between the UVilluminated current and the dark current. Therefore, the photo-to-dark current ratio (PDCR) was raised by six orders of magnitude under the optimal gate bias. The performances of βGa2O3 phototransistors were exceptionally superior among the deep-UV photodetectors based on wide bandgap semiconductor materials; PDCR of 6.0 × 108 and rejection ratio of 5.3 × 106 could be achieved. The synergetic combination of an ultra-wide bandgap semiconductor and two-dimensional UV-transparent graphene provides a new opportunity for high performance deep-UV photodetectors.

KEYWORDS: graphene, gallium oxide, deep-ultraviolet, phototransistor

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-Ga2O3 with an ultra-wide bandgap energy of 4.8~4.9 eV at room temperature has been regarded as a promising material for next-generation power electronic devices and optoelectronic devices. Due to its outstanding critical electric field (theoretical value: ~8 MV/cm), Baliga’s figure of merit (~3214), chemical robustness, and radiation hardness, it has been

intensively

investigated

to

metal-oxide-semiconductor-field-effect

transistors

(MOSFET), metal-semiconductor field-effect transistors (MESFET), Schottky diodes, PNdiodes, etc.1–5 -Ga2O3 is also suitable for solar-blind photodetectors (PDs) that detect the deepultraviolet (UV) light with wavelength below 280 nm because it has inherent solarblindness.2,6,7 This eliminates unnecessary interference with visible and infrared wavelength under solar radiation and makes it efficient for detecting deep-UV light with weak intensity.8 Highly sensitive UV-C PDs are also of interest for numerous civil and military applications such as fire (flame) detection, missile tracking, and non-line-of-sight or secure optical communication.8,9 The PDs based on -Ga2O3 with different structures of materials have been developed including nanowire, bulk, or thin film owing to various growth methods ranging from epitaxial growth to melt-growth techniques; molecular beam epitaxy, halide vapor phase epitaxy, metal organic chemical vapor deposition, edge-defined film fed growth, and floating zone.2,10–14 Additionally, their photoresponse properties were enhanced through the application of diverse types including photoconductive type, metal-semiconductor-metal (MSM) type, Schottky diode, and PN diode.10,11,14–17 Among different types of devices, phototransistors have a distinguished characteristic, which is the current modulation.18-20 The ability to control the channel current using external bias is significantly important for reducing the dark current (noise) of PDs and thus improving the signal-to-noise ratio. MOSFETs and MESFETs are typical types of phototransistors. MOSFETs suffer from the problems related to the oxide-semiconductor interface such as the 2 ACS Paragon Plus Environment

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interface traps, which can result in slow switching characteristics. A MESFET consists of a Schottky junction between metal and semiconductor, which determines the current path through the external bias. According to the previous reports, graphene is a Schottky contact electrode for -Ga2O3 because it has a larger work function (graphene ~4.6 eV) than that of the unintentionally n-doped -Ga2O3 (-Ga2O3 ~4.0 eV).21,22 An outstanding rectifying behaviors were observed in the devices based graphene/-Ga2O3 junction.10,23–25 Graphene is especially suitable for transparent conductive electrodes in the solar-blind PDs due to its high UV transmittance. ~8 nm thick graphene exhibited transmittance of around 80% at the wavelength of ~250 nm.26 Unlike the opaque metal electrodes with high reflectivity that leads to low photon absorption, graphene electrodes allow the incident light to be absorbed into the active layer with the minimal shading, resulting in an improved responsivity. Ai et al. used graphene as transparent Schottky contact electrodes in vertical-MSM type PDs in order to improve their responsivity and response speed.23 Oh et al. reported that the width of the depletion region between graphene and -Ga2O3 depended on the photon energy of the incident light.27 In this work, the synergetic effect of the gate-tunable switching characteristics and the high UV-C transparency of the graphene gate in the -Ga2O3 MESFETs led to a significantly enhanced photosensitivity. Mechanically exfoliated -Ga2O3 flakes with high crystal quality are used as the channel material (absorber layer), and graphene was employed as deep-UV transparent Schottky contact electrodes. The fabricated phototransistor showed extremely low dark current, excellent photosensitivity (photo-to-dark current ratio), and UV-C selectivity via control of potential of the deep-UV transparent graphene gate. The detailed characteristics of the graphene-gated -Ga2O3 metal-semiconductor phototransistors are investigated, and the possible current transport mechanism are analyzed.


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RESULTS AND DISCUSSION The optical microscope image (Figure 2a) shows that the graphene flake at the very top of the device has a high optical transparency and is uniformly stacked on to the layers despite the height difference, consequently revealing the shape of the layers below including β-Ga2O3 channel and Ti/Au source and drain electrodes. Micro-Raman spectra (graphene (bottom), β-Ga2O3 (top), Figure 2b) were obtained from the overlapped graphene/β-Ga2O3 junction area. The phonon positions of the exfoliated β-Ga2O3 are consistent with those measured for highly crystalline β-Ga2O3 in previous reports, indicating that the deformations formed during the fabrication process are negligible.28 The absence of D peak along with the strong G and 2D peaks from graphene also shows the high quality of graphene.29 Atomic force microscope (AFM) was also employed to analyze the surface morphology and the thickness of the materials forming the heterostructure (Figure 2c). The exfoliated β-Ga2O3 maintains smooth surface and graphene seems to be stacked uniformly on the layers despite the height difference. β-Ga2O3 has a thickness of ~280 nm (red line in Figure 2d) and graphene is ~6.4 nm thick (blue line in the inset of Figure 2d). The performances of the graphene-gated β-Ga2O3 phototransistor were investigated by electrical characterizations (Figure 3). Figure 3a shows the source-to-drain current (IDS)-drainto-source voltage (VDS) output curves at a gate bias varied from 0 to −5 V. A typical n-type behavior was obtained in the output curves, which is attributed to the oxygen vacancies or the impurities unintentionally introduced during the growth of β-Ga2O3 crystal. A sharp pinch-off phenomena was observed at a low VDS and it clearly divided the output curves into the linear region and the current saturation region, demonstrating good device characteristics of the graphene-gated β-Ga2O3 MESFETs. The IDS versus gate-to-source voltage (VGS) curves (forward sweep: black line, reverse sweep: blue line) and the corresponding transconductance (gm) curve (red line) are plotted in Figure 3b. The MESFET operates in a depletion mode, where 4 ACS Paragon Plus Environment

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the device is normally on at zero VGS, with barely any hysteresis. The maximum gm was 121.16 μS/mm and the current on/off ratio was estimated to be 7.5 × 106, which are comparable to the previous reports.30–32 Figure 4 shows the photoresponse properties of the β-Ga2O3 phototransistor with UVtransparent graphene gate. From the IDS-VDS curves of the device under dark condition and UVC illumination (Figure 4a), a high response to 254 nm (UV-C) light was observed. When the device was exposed to 254 nm wavelength, a significant photocurrent was generated due to the band-to-band excitation. Furthermore, the highly transparent graphene allows the incident UVC light to penetrate through graphene gate and reach the underlying β-Ga2O3.33 As a result, the photogenerated carriers below the graphene/β-Ga2O3 junction would narrow the depletion region and allow more carriers to flow through the channel at the same gate bias.27,34 The IDSVGS curves were obtained at VGS ranging from 0 to −15 V and a fixed VDS of 10 V (Figure 4b). The increased carriers under UV-C illumination would require more negative gate bias (VGS) in order to turn the device to the off state, therefore shifting the threshold voltage (VT) to the negative side of VGS. The VT of −4.6 and −12 V under dark and illuminated conditions, respectively, were extracted from the lines fitted to the (IDS)1/2 vs. VGS curves. The current on/off ratio decreased from ~8 × 106 (dark) to ~36 (UV-C exposure) due to the significantly increased off currents. The photogenerated carriers reduce the thickness of the depletion layer and increases the off currents of the phototransistor. The operations of our solar-blind phototransistor were divided into three regions by controlling VGS as marked in Figure 4b. Regions I, II and III refer to VGS lower than the VT when 254 nm-illuminated, VGS lower than the VT without illumination but higher than the VT with illumination, and VGS higher than the VT without illumination, respectively. In other words, the device is at the off-state in region I and at the on-state in region III whether or not UV-C light is illuminated, and in region II, the device is at the on-state only when exposed to UV-C 5 ACS Paragon Plus Environment

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light. The various solar-blind PD parameters at each region labelled in Figure 4b were analyzed in Figure 4c and the calculated values are summarized in table I. As shown in the equation, 𝑃𝐷𝐶𝑅 =

𝐼𝑖𝑙𝑙𝑢𝑚 ― 𝐼𝑑𝑎𝑟𝑘 𝐼𝑑𝑎𝑟𝑘

× 100

, where Iillum is the illuminated current and Idark is the dark current, an increase or a decrease in the photo-to-dark current ratio (PDCR) can either indicate an enhancement or a deterioration, respectively, in the sensitivity of the solar-blind PD. Compared to the lowest PDCR at zero gate bias (region III), the PDCR was six orders of magnitude higher at the VGS of −8 V (region II). The huge enhancement of the PDCR is attributed to the greatly reduced dark current (5.1 × 10-14 mA/mm, table I) as our device operates at the off-state under dark condition in both regions I and II. Especially, region II has the advantage of higher illuminated current and the PDCR could be maximized to 6.0 × 108% as the device is at the on-state when exposed to UV-C light. The responsivity of a solar-blind PD is the measure of the output current per incident light power and thus given by10,35 𝑅=

𝐼𝑝ℎ𝑜𝑡𝑜 𝐴𝑃𝑖𝑛𝑐

, where R is the responsivity, Iphoto is the photocurrent defined as Iillum - Idark, A is the effective area of the PD channel and Pinc is the irradiation power density. The responsivity is the lowest in region I and the highest in region II due to the gate-controlled photocurrent; for the photocurrent of 3.0 × 10-7 mA/mm (table I), the responsivity is calculated to be 2.6 × 103 A/W at VGS = −8 V (region II). In other words, the difference in the illuminated and the dark current was expanded when the gate bias was modulated to region II since the dark current decreased substantially to the off-state while the illuminated current was still high at the on-state. Varying the VGS particularly enhanced the rejection ratio to 5.3 × 106 (region II) by six orders of magnitude from 4.8 (region III). The rejection ratio is used to describe the ratio of the signal 6 ACS Paragon Plus Environment

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from the detecting light to the noise from the specific background radiation. Another PD parameter, the detectivity (D*) is given by10,35 𝑅𝐴0.5

∗

𝐷 =

(2𝑒𝐼𝑑𝑎𝑟𝑘)0.5

The high responsivity and the low dark current in region II again gives the best D* of 9.7 × 1013 Jones. Compared to the device parameters of the previously reported PDs based on β-Ga2O3, the performance of the fabricated phototransistor in this experiment is superior; while the dark current is exceptionally low, its PDCR and rejection ratio are among the best (table II).10,27,30,36–42 The time-dependent photoresponse to 254 nm light at a VDS of +10 V and a VGS of −8 V is shown in Figure 5. The device exhibited a stable photoresponse over repeated measurements. To further analyze the rise and decay edges, the obtained curve was enlarged to display only one rise and one decay curves (Figure 5b). The rise edge showed a biexponential growth behavior and thus the curve was fitted with43 𝐼(𝑡) = 𝐼0 + 𝐴1(1 ― 𝑒

―𝑡 𝜏 𝑟1

) + 𝐴2(1 ― 𝑒 ―𝑡 𝜏 ) 𝑟2

, where I0 is the dark current, A1 and A2 are the positive constants, and тr1 and тr2 are the rise time constants. The rise time constants, тr1 and тr2, are estimated to be 1.0 and 8.3 s, respectively. The relaxation behavior followed bi-exponential decay function,43 𝐼(𝑡) = 𝐼0 + 𝐴3𝑒

―𝑡 𝜏 𝑑1

+ 𝐴4𝑒

―𝑡 𝜏 𝑑2

, where A3 and A4 are the positive constants, and тd1 and тd2 are the decay time constants. The calculated decay time constants, тd1 and тd2, are 0.6 and 9.7 s, respectively. Our device generally displayed a comparable response and recovery time to UV-C light. With all the excellent photoresponse properties, the graphene-gated metal-semiconductor phototransistors based on the exfoliated β-Ga2O3 realize the potential of high performance solar-blind deep-UV 7 ACS Paragon Plus Environment

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PDs. The integration of an ultra-wide bandgap β-Ga2O3 with two-dimensional materials also shows another possibility of multi-functional semiconductor (opto)electronics based on heterostructures.

EXPERIMENTAL SECTION Device Fabrication β-Ga2O3 microflakes were mechanically exfoliated using the conventional Scotch-tape method from unintentionally n-doped single crystalline β-Ga2O3 substrate (carrier concentration: ∼ 4.0 × 1017 cm−3, Tamura corp.). The exfoliated flakes were then drytransferred onto SiO2/Si substrates. (Fig. 1(a)) Along with source and drain electrodes, the metal pads for the gate electrodes were patterned by using electron-beam lithography and the subsequent electron-beam evaporation of Ti/Au (50 nm/100 nm). (Fig.1 (b)) Rapid thermal annealing (RTA) was performed at 500 °C for 1 min under Ar ambient (~1 Torr) to improve the Ohmic property of the contacts between source and drain electrodes and β-Ga2O3 flake. The Scotch-tape method was also used to prepare graphene flakes from bulk highly oriented pyrolytic graphite crystal. Then selected graphene flakes were stacked accurately onto β-Ga2O3 flakes and the pre-patterned gate pad through dry-transfer technique (Gel-Pak gel-film). (Fig.1 (c)) RTA process was performed again at 200 °C for 2 min under Ar ambient (~1 Torr) to improve the contacts between graphene and Ti/Au by removing any voids that may have been formed during dry-transfer process and providing better adhesion. The fabrication steps are schematically shown in Figure 1.

Characterization and Measurement The optical properties of graphene/β-Ga2O3 heterojunction were analyzed using microRaman spectroscopy. A diode-pumped solid-state laser beam with emission wavelength of 532 8 ACS Paragon Plus Environment

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nm was used as an excitation source under a back-scattering configuration. The surface morphology and the thickness were measured by AFM (Bruker) in a tapping mode. For photoresponse measurements, the devices were characterized while illuminated using a UV lamp (UVTECH Ltd.) that emits light with wavelength of 254 nm. The timedependent photoresponse was recorded as UV lamp was repeatedly turned on and off with a time interval of 30 sec. All the electrical characterization was performed using a semiconductor analyzer (4155C, Agilent) connected to a low vacuum (~10 mTorr) probe station to avoid the effects of the UV-induced ambient molecules. The optical power of the illuminated UV light was measured with a power meter (FieldMax II-TO, Coherent).

CONCLUSION High-performance solar-blind PDs were demonstrated through metal-semiconductor phototransistors based on mechanically exfoliated -Ga2O3 flakes with high crystallinity. Their photoresponse properties were significantly improved by employing UV-transparent graphene electrodes. By modulating the channel by the graphene gate, the dark current of this structure could be reduced to 5.1  10-14 A. Moreover, the high optical transparency of graphene gate prevents the shadowing effect under UV-C illumination and allows carriers to be generated at the graphene/-Ga2O3 junction. The threshold voltage of the fabricated phototransistors was consequently shifted further towards the negative side of VGS. The gate bias could be chosen where the current level of the phototransistor is the lowest (off-state) in dark condition while maintained at a high level (on-state) under UV-C illumination. This optimal gate bias (VGS = 8 V) maximized the PDCR (photosensitivity) to 6.0  108%, which was 6 orders of magnitude higher than that without the gate bias. The fabricated device also exhibited outstanding UV-C selectivity (rejection ratio, R254nm/R365nm 5.3  106), responsivity (2.5  103), and detectivity 9 ACS Paragon Plus Environment

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(2.7  1014) under the optimal gate bias. These results show a great potential for highly sensitive and selective solar-blind PDs through the integration of ultra-wide bandgap semiconductors with van der Waals materials.

Acknowledgements This research was supported by the New & Renewable Energy Core Technology Program of Korea Institute of Energy Technology Evaluation and Planning (KETEP), which was granted financial resources from the Ministry of Trade, Industry & Energy, Korea (20172010104830), the Technology Development Program to Solve Climate Changes funded by National Research Foundation of Korea (2017M1A2A2087351) from the Ministry of Science and ICT, and LG Innotek-Korea University Nano-Photonics Program.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author contributions ‡These authors contributed equally.

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Figure captions

Figure 1. Schematic of the fabrication process for graphene-gated metal-semiconductor phototransistor based on β-Ga2O3 flake Figure 2. (a) Optical microscope image of the fabricated phototransistor (b) Raman spectra for β-Ga2O3 (top) and graphene (bottom) in the junction region. (c) AFM image and (d) the corresponding height profile of β-Ga2O3 (red) and graphene (blue) used in the device shown in Figure 2a. Figure 3. (a) IDS vs. VDS, (b) IDS vs. VGS and the corresponding gm characteristics of the fabricated MESFET shown in optical microscope and AFM images in Figure 2 without UV illumination. Figure 4. (a) IDS vs. VDS curves at VGS = 0 V and (b) IDS vs. VGS curves at VDS = +10 V obtained from the identical phototransistor in Figure 3 under dark and illumination conditions. (c) PD parameters in region I (VGS = −14 V), II (VGS = −8 V) and III (VGS = 0 V) labelled in Figure 4b. Figure 5. (a) Time-dependent photoresponse curve to UV-C light at VGS = −8 V and VDS = +10 V. (b) Enlarged view of the rise and decay edges of the curve in Figure 5a and the corresponding fitted rise (red) and decay (blue) functions.

Table captions

Table I. The calculated values for the PD parameters with varying gate voltage. Table II. Comparison of the device parameters of the reported deep-UV PDs based on βGa2O3. 17 ACS Paragon Plus Environment

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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Table I


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Table II


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For Table of Contents Use Only Title: Ultrahigh Deep-UV Sensitivity in Graphene-Gated β-Ga2O3 Phototransistors Authors: Suhyun Kim, Sooyeoun Oh, and Jihyun Kim

Synopsis: The use of gate-tunable switching characteristics and the high UV-C transparency of the graphene gate in the -Ga2O3 MESFETs led to a significantly enhanced photosensitivity.


Ultrahigh Deep-UV Sensitivity in Graphene-Gated Î²-Ga2O3

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