Highly Narrowband Polarization-sensitive Solar-blind Photodetectors

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Functional Inorganic Materials and Devices

Highly Narrowband Polarization-sensitive Solar-blind Photodetectors Based on #-Ga2O3 Single Crystals Xuanhu Chen, Wenxiang Mu, Yang Xu, Bo Fu, Zhitai Jia, Fang-Fang Ren, Shulin Gu, Rong Zhang, Youdou Zheng, Xutang Tao, and Jiandong Ye ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19524 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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ACS Applied Materials & Interfaces

Highly Narrowband Polarization-sensitive Solar-blind Photodetectors Based on β-Ga2O3 Single Crystals

Xuanhu Chen1,3, Wenxiang Mu2, Yang Xu1, Bo Fu2, Zhitai Jia2,*, Fangfang Ren1, Shulin Gu1, Rong Zhang1, Youdou Zheng1, Xutang Tao2, Jiandong Ye1,3,*

1

School of Electronic Science and Engineering, Nanjing University, Nanjing 210023, China

2

State Key Laboratory of Crystal Materials and the Key Laboratory of Functional Crystal

Materials and Device, Shandong University, Jinan 250100, China 3 Research

Institute of Shenzhen, Nanjing University, Shenzhen, China

* E-mail: Zhitai Jia ([email protected]) and Jiandong Ye ([email protected]) Xuanhu Chen and Wenxiang Mu contributed equally to this work.

Keywords: solar-blind ultraviolet, anisotropic absorption, narrowband photodetector, gallium oxide, ultra-wide bandgap semiconductors

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Abstract To suppress noise from full daylight background or environmental radiation, spectrally selective solar-bind photodetector is widely required in many applications that need to detect only the light within specific spectral range. Here, we present highly narrowband solar-blind photodetectors by light polarization engineering of the anisotropic transitions in β-Ga2O3 single crystals. The polarized transmittance characteristics reveal that direct transitions from valance sub-bands to the conduction band minimum are tuned between 4.53 and 4.76 eV for the light polarized E//c and E//b. The polarization-dependent photoresponsivity verifies that the order of fundamental band-to-band transitions well obey the selection rules in terms of the valence-band splitting in the β-Ga2O3 monoclinic crystal band structure. By combining an identical β-Ga2O3 (100) single crystal with an orthogonal alignment to the planar detector, measured at a chopper frequency of 17 Hz, a highly narrowband detection is produced with a peak responsivity of 0.23 A/W at 262 nm, an EQE of 110%, a bandwidth of 10 nm, a light rejection ratio over 800 and a response time of 0.86 ms. This provides a new paradigm for a narrowband solar-blind photodetector with broad applications where background noise emission needs to be suppressed.


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Introduction Spectrally selective photodetection is broadly applied in various civil and defense applications where light within only a specific wavelength needs to be detected while the remainder, usually background or environmental radiation, needs to be rejected.1-3 Especially, high efficient narrowband photodetection in solar-blind spectral region enables reliable sensitivity to weak UV signals and avoid false alarms in monitoring ozone holes, detecting flame, space communicating, missile guidance, biochemical detection, and inspection of ultraviolet leakage.4-7 The common approach to achieve narrowband detection is combining broadband photodetectors with bandpass filters.1, 8 However, for commercial bandpass filters, solar-blind deep-ultraviolet spectral range remains not covered and the bandwidth is limited to only 40-80 nm.8 A particular challenge also lies in the efficient manipulation of light polarization states, which is promising for data storage, sensing/imaging applications, and biophotonics. For semiconductor optoelectronic devices, light polarization engineering is usually obtained in terms of selection rules in low-dimensional structures with low-symmetric band structures, and the polarized light-emitting diodes, tunable polarization converters, polarization switches and polarization sensitive narrowband photodetectors have been reported by utilizing the anisotropic optical properties of non-polar III nitrides and ZnO materials.9-15 For instance, GaN-based visible-blind narrowband detectors have been developed by the modification of the asymmetry of valance band structures and their interband transition orders.9, 15-17 Whereas, due to limited bandgap of GaN and ZnO, narrowband photodetectors based on the modification of the asymmetry of valence band structures and their interband transition orders can only be operated at visible-blind region 3 ACS Paragon Plus Environment


and solar-blind narrowband photodetectors are still absent. For solar-blind optoelectronic devices, the emerging gallium oxide (Ga2O3) material is one of promising candidates owing to its desirable bandgap of about 4.8 eV, superior physical properties and its availability of large-size single crystalline wafers.18-19 Beneficial from the development of advanced epitaxial techniques, high performance solar-blind photodetectors base on β-Ga2O3 materials in various forms of bulk crystal, thin films, heterostructures and nanostructures have been reported.20-26 In spite of their rapid progresses, solar-blind narrowband photodetectors have not been reported and the trade-off between the photoresponsivity and response speed remains a bottleneck problem. Owing to the highly anisotropic monoclinic structure of β-Ga2O3 with its space group symmetry of C32h,27 the fundamental band-to-band transitions, their polarization rules and energetic order, and the resulting anisotropy in the dielectric response and optical properties allows the effective manipulation of the light polarization states to design photodetectors with novel functionalities.28-32 Fundamental valence band ordering, selection rules, effective mass and excitonic contributions in β-Ga2O3 have been investigated and identified by density functional theory (DFT) calculations, polarized optical absorption/reflectance, angle resolved photoemission spectroscopy (ARPES), and ellipsometry.32-37 For instance, theoretical and experimental results have accurately quantified that the anisotropy in β-Ga2O3 (100) with the onsets of absorption occurring at 4.52 eV and 4.79 eV for the polarized light parallel to c- and b-axis, respectively.28 The energy difference of about 0.2 eV for two interband transitions in the vicinity of the energy gap provides a desirable spectral window for the narrowband detection of weak ultraviolet signals with low false alarm rates. 4 ACS Paragon Plus Environment

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In this article, we design and demonstrate highly narrowband solar-blind photodetectors by using the anisotropy of absorption onset of β-Ga2O3 (100) single crystal. The photoresponse cut-off edge of the resulting devices are polarization-dependent, well consistent with the predicted energetic order of band-to-band transitions. Combining an identical β-Ga2O3 (100) filter with an orthogonal alignment to the detector, a highly narrowband detection is produced with a peak responsivity of 0.23 A/W at 260 nm, an EQE of 110%, a bandwidth of 10 nm, a rejection ratio over 800 and a response time of 0.86 ms.

Results and Discussion The polarized transmittance spectra of a 200-μm-thick β-Ga2O3 (100) single crystal are shown in Figure 1a. The sharp absorption edges are located at 4.76 and 4.53 eV for the polarized light parallel to b and c axis, respectively. Distinct shoulders in the spectral window of 4.5-4.7 eV are observed for the incident light with both polarization components, which is associated with the band mixing effects.32 As the absorption is proportional to the incident power, the angular dependence of transmittance magnitudes at 4.6 eV can be expressed as T    TE / / b sin 2    TE / / c cos 2   , where the in-plane polarization angle φ is defined as the

angle between the b axis and the electric field vector E, TE//b and TE//c represent the transmittance magnitudes for the cases of φ = 0o and φ = 90o at 4.6 eV, respectively.9 Figure 1b shows that the experimental results are well described by the above relationship. Consequently, the absorption in the spectral range of 4.5-4.7 eV for any arbitrary polarization could be characterized by its TE//b and TE//c components. The origin of the dichroism of the absorption edge have been well understood in terms of polarization selection rules and 5 ACS Paragon Plus Environment


energetic order of the valence band structure of β-Ga2O3.31-32 Theoretical calculations suggest that the conduction band minimum (CBM) is isotropic and mainly contributed by Ga 4s orbitals with its symmetry denoted as Γ1+ , while the valence band maximum (VBM) at the center of Brillouin zone consists of oxygen 2p states with six splitting sub-bands.29-30, 32 For the b-c in-plane of (100) β-Ga2O3, the schematic valence band ordering is shown in the inset of Figure 1a. According to the selection rules, only the direct transition of 4.76 eV from the Γ1- band of the VBM to the Γ1+ of the CBM is dipole allowed for E//b, while the transition of Γ2- (1)→Γ1+ with an energy gap of 4.53 eV is dipole allowed for E//c, which is consistent with previous theoretical and experimental results.31 Consequently, the spectral shift of photodetection edges exhibit similar angular dependence on the in-plane polarization as shown in Figure 2 for the interdigital photodetectors under a bias of 5 V. As shown in Supporting Information Figure S2, the almost linear current-voltage characteristics in dark condition reveals that interdigital fingers form Ohmic contacts to the underneath β-Ga2O3 and the photodetector is operated in a photoconductive mode. The rejection ratio of responsivity at 214 nm in the ultraviolet C (UVC) region to the one at 338 nm in the ultraviolet A (UVA) region is higher than 103, indicative of its strong capability in solar-blind detection. The photoresponse cutoff edge is blue-shifted as the polarization of light changed from E//c to E//b. The peak responsivity R for E//c located at 4.82 eV is 0.48 A/W, which is lower than the peak value of 1.20 A/W at 5.29 eV for E//b. To determine the polarization sensitivity in vicinity of energy gap, the spectral dependence of the response contrast (RE//c/RE//b) is shown in the inset of Figure 2. A maximum response contrast of 1.9 is yield at E1 = 4.68 eV with a narrow bandwidth of 20 nm, 6 ACS Paragon Plus Environment

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which is consistent with the spectral window of absorption onset and is limited by the oscillation strength ratio for E//c and E//b.32 A spectral dip located at E2 = 5.29 eV for the response contrast is consistent with the energy positions of dips in the polarized reflectance spectra reported by Onuma et al,31 as a consequence of band-to-band or discrete excitonic transitions.31-32 Based on the theoretically calculated valence band ordering, the excitonic absorption at 4.68 eV is a result of the transition of Γ2- (1)→Γ1+ for E//c while the 5.29 eV peak observed for E//b is contributed by the excitonic transition from the deeper valence subbands Γ1- or 2 to the CBM Γ1+ , as marked by arrows in Figure 2.31 It indicates that the absorption of near-band edge excitons should be taken into account when the internal field is not enough to dissociate the excitons due to its large binding energy.38-40 It is noted that, the products of external quantum efficiency and internal gain are 2.31 for E//c at 4.82 eV and 6.35 for E//b at 5.29 eV, respectively. The latter high responsivity is associated with the high joint density of states in Γ1- or 2 bands supported by the theoretical calculations.32 The modulation of response contrast by light polarization control allows the design of solar-blind narrowband photodetectors. Figure 3a shows that an orthogonal alignment of the b-axis of the photodetector and the filter (an identical β-Ga2O3 (100) single crystal), i.e. bfilt⊥ bdet. This configured design produces a narrowband detection with a peak responsivity of 0.23A/W at 4.73 eV (λ = 262 nm) and a bandwidth of about 10 nm for the light polarized perpendicular to b axis, which ensures a lower spurious background signal. For E//bdet, photons with E ≥ 4.76 eV are absorbed by the filter and consequently not able to reach the photodetector. However, photons with 4.53 eV ≤ E ≤ 4.76 eV are transmitted through the filter and absorbed by the detector. For E//cdet, the incident light with E ≥ 4.53 eV will be 7 ACS Paragon Plus Environment


almost absorbed by the filter so that no light will be detected. The angular dependence of bandpass photoresponse shown in Figure 3b indicates that the bandwidth and the energy position of narrowband detection remains unchanged, which are consistent with the response contrast spectrum in the inset of Figure 2 and the spectral window of anisotropic absorption onset in Figure 1. The maximum response is sensitive to the polarization angle of incident light, which can be well described by the relation of R    RE //c sin 2    RE //b cos 2   as shown in the inset of Figure 3b. Therefore, the constructed narrowband photodetector demonstrates a strong capability to detect weak signals of DUV light with a specific wavelength and its polarization state. Figure 3c shows the characteristics of narrowband detection of the photodetector with an orthogonally aligned filter upon the unpolarized light at different biases. Even for the unpolarized light, the narrowband photoresponse with the same peak position at 4.73 eV and a bandwidth of 10 nm is yield, which is coincided with the absorption onset window between E//c and E//b, as shown in Figure 3c. The maximum responsivity shows a linear dependence on the bias, as summarized in the inset of Figure 3c, which confirms that the photodetector is operated under photoconductor mode. If assuming the quantum efficiency is maximized to be unity and ignoring the surface recombination, the internal gain of a photoconductor is determined as g   eff tt = eff

l

2

eVb  , where τeff is the effective lifetime of the photon-

generated excess carriers, tt = l 2 eVb is transition time of electrons across the finger spacing (l) with the mobility (μe) under bias (Vb).41-42 Therefore the photoconductive responsivity is proportional to the bias and high internal gain is expected by decreasing the transition time across the fingers under higher biases. 8 ACS Paragon Plus Environment

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To determine the excess carrier lifetime and the response speed of the narrowband photodetector, frequent-dependent responsivity spectra are shown in Figure 3d. At a chopper frequency of 8 Hz, a maximum responsivity of 0.32 A/W at 4.73 eV with a high UVC-toUVA rejection ratio over 300 is achieved, where the UVC and UVA represent photon energies of 4.73 eV and 3.55 eV, respectively, as shown in the inset of Figure 3d. With increasing the modulation frequency of incident light, the narrowband has the unchanged bandwidth of ΔE = 0.19 eV (Δλ ≈ 10 nm) while its peak responsivity gradually decreases. The similar frequency dependence of photoresponsivity is also observed for the single interdigital photodetector without filter upon illumination of unpolarized light, as shown in Figure 4a. The responsivity at 5.0 eV and the UVC-to-UVA rejection ratio (RE=5.0 eV/RE=3.5 eV) as a function of the light modulation frequency are plotted in the inset of Figure 4a. The effective lifetime of excess carriers can be determined by the frequency-dependent responsivity in terms of R  R0 1   2 eff2



1/2

, where ω is the angular frequency, R0 is the

absolute value of stationary voltage responsivity, which is the function of effective carrier lifetime, geometry of the photodetector, intrinsic carrier concentration, quantum efficiency, detection wavelength and applied bias.41, 43 The good fitting to the solid experimental points in the inset of Figure 4a give rise to a carrier lifetime of 2.0 ms. As the resistance-capacitance constant of the device is in the ns level, and thus response time of photodetector is predominantly limited by the excess carrier lifetime. Incidentally, despite of responsivity decreasing, the UVC-to-UVA rejection ratio increases and become saturated with a value over 2500 when the chopper frequency increases. It is due to the faster decrease of photoresponse to the illumination at 3.5 eV, which is below the lowest energy gap of β-Ga2O3. The observed 9 ACS Paragon Plus Environment


weak response is attributed to the presence of deep recombination centers, which can be verified by the low photocurrent component in circumstance of the 365 light illumination in Figure S2 and the observation of distinct photoluminescence emission band at 3.1 eV in Figure S3. Due to the slow re-emission processes involved in these traps, carriers cannot be swept out to the ohmic contact and less contribution to the photocurrent at higher frequency. Finally, the transient photoresponse was measured with 5 V bias under illumination of a 266 nm pulsed laser with light intensity of 1.5 mW/cm2. The response waveform at a modulation frequency of 150 Hz is shown in Figure 4b. By fitting the rise and fall edges of response waveform with a single exponential function, the rise time and decay time is estimated to be 0.48 and 0.38 ms, respectively. As a result, the total response time of 0.86 ms, which is shorter than the response time determined by chopper-modulated Xe-lamp light due to its rather lower incident light power than that of pulsed laser. The excitation-dependent response time is also observed in the photoconductive detectors fabricated from wide bandgap materials and can be attributed to the redistribution of the excess carriers with increased excitation level.41 For better comparison, a benchmark of state-of-the-art Ga2O3-based bulk and film-type solar-blind photodetectors and narrowband photodetectors based on other widebandgap semiconductors with critical parameters has been performed in Table 1. Indeed, for the photovoltaic photodetector based on Schottky diode or p-n junction, the photoresponse speed can be improved due to the fast separation and transport processes of photo-generated carriers driven by the built-in electric field. To break the trade-off between the photoresponsivity and response speed, the avalanche photodetector (APD) is an ideal candidate, in which, high multiplication gain is achieved through impact ionization processes 10 ACS Paragon Plus Environment

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R7.

In APDs, high electric field is necessary for the avalanche process, which is often

delivered by the formation of p-n junction. However, as an intrinsic n-type semiconductor, ptype conduction of Ga2O3 material remains challenging. Nevertheless, it is worthy to notice that, the narrowband photodetectors reported in this work are working in photoconductive modes and the overall performance are comparable to other state-of-the-art Ga2O3-based bulk and film-type photodetectors. The response speed of this reported narrowband detector is among the best reported Ga2O3 based photoconductive detectors. By increasing the bias or decreasing the spacing of interdigital fingers, the response speed can be further improved to meet the practical applications. Furthermore, the narrowband detection capability of the photodetector reported herein is better than those based on other wide-bandgap semiconductors (ZnMgO or AlGaN) in the aspect of response wavelength, responsivity, rejection ratio and band width. The result of this work provides a new paradigm for a narrowband solar-blind photodetector used in specific wavelength.

Conclusion In summary, we report the photoresponse modulation of the solar blind photoconductive detector by manipulation of anisotropic absorption onset of β-Ga2O3 (100) single crystal. The excitonic absorptions contributing to the peak responsivities are attributed to the direct bandto-band transitions with different energetic order of valence sub-bands. A design of photodetector combined with an orthogonal aligned β-Ga2O3 (100) filter produces a narrowband detection with a peak responsivity of 0.23 A/W at 262 nm, an EQE of 110%, a highly narrow bandwidth of 10 nm, a high UVC-to-UVA rejection ratio over 800 and a 11 ACS Paragon Plus Environment


response time of 0.86 ms. The light polarization controlled responsivity with established relationship in this work endows the designed highly narrowband detector with efficient sensing of weak signals having their specific wavelengths and polarization states.

Experimental Methods The 200-μm-thick unintentionally doped (UID) β-Ga2O3 (100) single crystals were grown by the edge defined film-fed growth (EFG) method with a background electron concentration of about 1017 cm-3.51 Polarized transmittance spectra were measured at room temperature using a PerkinElmer Lambda 950 system with a Galan-laser calcite polarizer. The incident light is normal to the (100) surface of specimens, and the in-plane polarization angle φ is defined as the angle between the b axis and the electric field vector E, i.e. φ = 0o for E//b and φ = 90o for E//c. Following standard lithographic process, Pt (10 nm)/Au (100 nm) stack was deposited on β-Ga2O3 (100) surface using an e-beam evaporator. As a result, interdigital metalsemiconductor-metal (MSM) photodetectors were made with the finger spacing of 5 μm and a photosensitive area of 260 μm × 240 μm. The spectral photoresponse was measured using a monochromator (Horiba, iHR320) equipped with a 300 W xenon-arc lamp as the optical excitation source and a low noise current preamplifier (Stanford Research System, SR570) with the lock-in measurement technique (Stanford Research System, SR830). The incident power density was calibrated by a Si reference photodiode. A 266 nm pulsed laser was employed as the excitation source for transient photoresponse measurements, and a digital oscilloscope (Tektronix, TBS 1102) for data collection. 12 ACS Paragon Plus Environment

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Associated Content The manuscript is accompanied by Supporting Information containing -

X-ray diffraction patterns of the β-Ga2O3 (100) single crystal

-

Current−Voltage (I−V) characteristics of the photodetectors in the dark and under illumination with 254 and 365 nm light, respectively.

-

Photoluminescence (PL) of β-Ga2O3 (100) single crystal.

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Author Information Corresponding Author E-mail: Zhitai Jia ([email protected]) and Jiandong Ye ([email protected]) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgements Project supported by the National Key R&D Project of China (No. 2018YFB0406502 and 2017YFB0403003), the National Natural Science Foundation of China (Nos. 61774081 and 61322403), Training Program of the Major Research Plan of the National Natural Science Foundation of China (91850112), the State Key R&D project of Jiangsu (BE2018115), the key Research and Development Program of Shandong Province (Grant No: 2018CXGC0410), Shenzhen Fundamental Research Project (Nos. 201773239 and 201888588), State Key 13 ACS Paragon Plus Environment


Laboratory of Wide–Bandgap Semiconductor Power Electric Devices (No. 2017KF001), the Young Scholars Program of Shandong University (Grant No. 2015WLJH36), the Fundamental Research Funds for the Central Universities (021014380093 and 021014380085), Postgraduate Research & Practice Innovation Program of Jiangsu Province.


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Page 22 of 27

Tables Table 1. Benchmark of the state-of-the-art Ga2O3 Photodetectors and narrowband UV photodetectors based on other wide-bandgap materials. Material

Structure

Responsivity (A/W)

β–Ga2O3 Bulk β–Ga2O3 Bulk β–Ga2O3 Bulk β–Ga2O3 Thin film a–Ga2O3

MSM

0.23 @ 262 nm 10 nm (BW)a 3.7 × 10–2 @ 250 nm 103 @ 240 nm

β–Ga2O3 nanosheet M-plane GaN

Vertical– Schottky Vertical– Schottky Vertical– Schottky Film–based MSM MSM

Rejection Reference ratio 800 This work

9

1.5 × 104

20

–

~106

44

4 × 10–3 @ 254 nm

~100

>103

45

1.9 × 10–1 @ 254 nm 19.31 @ 254 nm

0.1

–

46

23

–

47

–

< 250

15

–

–

48

–

> 100

49

< 40

–

50

~0.023 @ 360 nm 6 nm (BW) Mg0.07Zn0.93O Heterojunction 0.192 @ 340 nm 30 nm (BW) AlGaN p–i–n 0.136 @ 282 nm 12.5 nm (BW) p-NiO/n-ZnO Heterojunction 1.4 × 10-3 @ 380 NW nm < 30 nm (BW) aThe

Response time (ms) 0.86

Schottky

value with the unit nm in front of (BW) represents the bandwidth of the semiconductor-

based narrowband photodetectors, where BW means the bandwidth.


Page 23 of 27

Figures Wavelength (nm)

(a)

310 90

300

290

280

270

260

80

(b) 1.0



60

E//b 40

250

E//c

Energy CBM

1+

E//b

E//c E//b

20

0 4.0

VBM

4.2

0 15 30 45 60 75 

0.8

T()/T(E//b)

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 0.4 0.2

 -2 1

 1   2 1 1

Exp. Fitting

0.0

4.4

4.6

4.8

5.0

0

60

120

180

240

 (deg.)

Photon Energy (eV)

Figure 1. (a) The polarized transmittance spectra of an β-Ga2O3 (100) single crystal, and (b) transmittance magnitude with respect to that for E//b as a function of in-plane polarization angle (φ) between electric field vector E and b axis. The inset shows band structure and irreducible representations of β-Ga2O3 with energetic order of valence band.31 Reprinted with permission from Onuma et al., Jpn. J. Appl. Phys. 2015, 54, 112601. Copyright © 2015 The Japan Society of Applied Physics.



Wavelength (nm) 300

275

250

225 1 or 2+1

10

-1

10-2

f = 17 Hz

2(1)+1

 0 15 30 45 60 75 90

2.0

R(E//c)/R(E//b)

100

Responsivity (A/W)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

E1

1.5 1.0

E2

0.5 0.0 3.5

4.0

4.5

5.0

5.5

6.0

h (eV)

10-3 4.0

4.5

5.0

Photon Energy (eV)

5.5

Figure 2. Photoresponsivity spectra for different in-plane polarization angle (φ) under normal incidence at room temperature. The inset shows the contrast of the responsivities for polarization E//b and E//c.


Wavelength (nm)

(c)

Ephoton: ~4.53-4.76 eV

340

0.25

320

300

b

0.10 0.05

β-Ga2O3 (100) photodetector

@17 Hz 0 (E//bdet)

0.15 0.10 0.05 0.00

Responsivity (A/W)

0.25

0.15

0

30

60

90 120

4.2

1

5

4.0

4.5

0.3 10-1

0.2 -2

10

4.8

f = 8 Hz

4 FWHM=0.19 eV f (Hz) 8 17 68 171 341 731

0.1

0.0 3.5

5.1

6

E//b

4.0

4.5

5.0

Photon energy (eV)

4.5

0

5.0

Photon energy (eV)

10-3 3.5

 (deg.) 3.9

4 Bias (V)

E//c

Exp Fitting

0.05 -30

3

(d) 0.4

0.10

3.6

0.05

0.00 3.5

15 30 45 60 75 90 (E//cdet)

0.20

5 2.0 V 4 2.5 V 3.0 V 3.5 V 3 4.0 V 4.5 V 2 5.0 V

Responsivity (A/W)

c

(b) 0.25 0.20

0.10

6

Bias

2

Responsivity (A/W)

b

0.15

E//b

Responsivity (A/W)

β-Ga2O3 (100) single crystal

0.20

240

Exp. Fitting

0.20

Responsivity (A/W)

c

260

E//c 0.25

0.15

280

(h)2 (104 cm-2 eV2)

(a)

Responsivity (A/W)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.0

4.5

5.0

2

(h)2 (104 cm-2 eV2)

Page 25 of 27

0 5.5

Photon energy (eV)

Photon energy (eV)

Figure 3. (a) The configuration of narrowband detector combined with an orthogonal aligned filter, (b)-(d) the photoresponsivity spectra as a function of in-plane polarization angle, applied biases and chopper modulation frequencies. The inset of (b) and (c) summarize the dependence of peak responsivity on in-plane polarization angle and biases with fitted curves, respectively. The inset of (d) shows the responsivity of the narrowband photodetector at a chopper frequency at 8 Hz in exponential scale.



Wavelength (nm) 360 330

10-1

10-2

270

240

Vb = 5 V Unpolarized Chopper frequency 17 Hz 67 Hz 172 Hz 341 Hz 511 Hz 638 Hz R (A/W)

Responsivity (A/W)

100

300

10-3

210

1.5

3000

1.2

2500

0.9

Exp. Fitting

0.6

2000 1500

0.3 1000 0.0

10-4

(b)

0

200

400

600

Rejection ratio (R5.0 eV/R3.5 eV)

(a)

f (Hz)

3.5

4.0

4.5

5.0

5.5

6.0

Photon energy (eV) 6

 = 266 nm @RT Exp. Fitting Fitting

5 4

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.38 ms

0.48 ms

3 2 1 0 -4

-2

0

2

4

6

8

10

Time (ms)

Figure 4. (a) The photoresponsivity spectra upon illumination of unpolarized light recorded at different chopper modulation frequencies. Inset: The dependence of responsivity at 5.0 eV and the UVC-to-UVA rejection ration on the modulation frequencies. (b) The fitting of waveform curves of transient photoresponse measured at 150 Hz.


Page 26 of 27

Page 27 of 27

Table of Contents Wavelength (nm) 310 90

300

290

280

270

260

Transmittance (%)

80

250

Ephoton: ~4.53-4.76 eV



60

E//b E//c

Energy

40

CBM

1+

E//b

E//c E//b

20

4.0

0 15 30 45 60 75 

β-Ga2O3 (100) single crystal

c

b

 -2 1

1- 1 1-  2 

VBM

0

4.2

4.4

4.6

4.8

b

5.0

Photon Energy (eV) 0.4

E//c

6

6

E//b

FWHM=0.19 eV f (Hz) 8 17 68 171 341 731

10-3

0.1 3.5

4.0

4.5

5.0

Photon energy (eV)

0.0 3.5

4.0

4.5

5.0

2

β-Ga2O3 (100) photodetector

Exp. Fitting Fitting

4

Voltage (V)

10-2

4

(h)2 (104 cm-2 eV2)

0.2

10-1

Responsivity (A/W)

0.3

f = 17 Hz

c

 = 266 nm @RT

5

Responsivity (A/W)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.38 ms

0.48 ms

3 2 1 0

0 5.5

-4

-2

0

2

4

Time (ms)

Photon energy (eV)


6

8

10

Highly Narrowband Polarization-sensitive Solar-blind Photodetectors

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