Deep-Ultraviolet Photodetection Using Single-Crystalline Î²

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Functional Inorganic Materials and Devices

Deep-Ultraviolet Photodetection Using SingleCrystalline #-Ga2O3/NiO Heterojunctions Kuang-Hui Li, Nasir Alfaraj, Chun Hong Kang, Laurentiu Braic, Mohamed Nejib Hedhili, Zaibing Guo, Tien Khee Ng, and Boon S. Ooi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10626 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Deep-Ultraviolet Photodetection Using Single-Crystalline

β -Ga2O3/NiO

Heterojunctions †,¶

Kuang-Hui Li,

Nasir Alfaraj,

‡

Nejib Hedhili,

†Photonics

†,¶

Chun Hong Kang,

‡

Zaibing Guo,

†

Tien Khee Ng,

‡

Laurentiu Braic,

†

and Boon S. Ooi

Mohamed

∗,†

Laboratory, Computer, Electrical and Mathematical Science and Engineering

Division (CEMSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia

‡Core

Labs, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia

¶Equally

contributing authors

E-mail: [email protected]

Abstract In recent years, β -Ga2 O3 /NiO heterojunction diodes have been studied, but reports in the literature lack an investigation of an epitaxial growth process of high-quality single-crystalline β -Ga2 O3 /NiO thin lms via electron microscopy analysis and the fabrication and characterization of an optoelectronic device based on the resulting heterojunction stack. This work investigates the thin-lm growth of a heterostructure stack comprised of n-type β -Ga2 O3 and p-type cubic NiO layers grown consecutively on c -plane sapphire using pulsed laser deposition, as well as the fabrication of solarblind ultraviolet-C photodetectors based on the resulting pn junction heterodiodes.

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Several characterization techniques were employed to investigate the heterostructure, including X-ray crystallography, ion beam analysis, and high-resolution electron microscopy imaging. X-ray diraction analysis conrmed the single-crystalline nature of the grown monoclinic and cubic (¯ 201) β -Ga2 O3 and (111) NiO lms, respectively, whereas electron microscopy analysis conrmed the sharp layer transitions and high interface qualities in the NiO/β -Ga2 O3 /sapphire double-heterostructure stack. The photodetectors exhibited a peak spectral responsivity of 415 mA/W at 7 V reverse-bias voltage for a 260-nm incident-light wavelength and 46.5-pW/µm2 illuminating power density. Furthermore, we also determined the band oset parameters at the thermodynamically stable heterointerface between NiO and β -Ga2 O3 using high-resolution X-ray photoelectron spectroscopy. The valence and conduction band osets values were found to be 1.15 ± 0.10 and 0.19 ± 0.10 eV, respectively, with a type-I energy band alignment.

Keywords Deep-ultraviolet, epitaxial growth, gallium oxide (Ga2 O3 ), nickel oxide (NiO), oxide heterojunctions, photodetectors, solar-blind, ultraviolet-C.

1

Introduction

Deep-ultraviolet photodetectors (PDs) incorporating beta-polymorph gallium oxide (β -Ga2 O3 ) thin lms have been the subject of extensive studies since the introduction of epitaxial deposition techniques for group IIIoxide materials because of the enhancements in photosensitivity and solar-blind photodetection characteristics. 16 Although β -Ga2 O3 /NiO heterostructures have been investigated by various research groups, none of these studies provided details about the epitaxial growth process supported by electron microscopy imaging or applied and comprehensive optoelectrical device fabrication and characterization. 710 With regards to optoelectronic applications, the majority of research groups have so far focused on the rel-

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atively narrow-bandgap gallium/aluminum nitride (Ga(Al)N)-based devices 1115 that exhibit varying degrees of operational stability 1620 and ultrawide-bandgap group IIIoxides-based devices grown and fabricated directly on bulk α-Al2 O3 (sapphire). 21 Han et al. have demonstrated solar-blind deep-ultraviolet Mg0.58 Zn0.42 O Schottky-type metalsemiconductormetal (MSM) photodetectors grown on (100) magnesium oxide (MgO) substrates with an MgO buer layer. 22 Their photodetectors exhibited peak spectral responsivities (R) between 2.8 and 15.8 mA/W with reverse-bias voltages (Vbias ) between 5 and 15 V for an incident-light wavelength (λin ) of around 240 nm. Xu et al. have demonstrated solar-blind deep-ultraviolet

β -Ga2 O3 MSM photodetectors grown on c -plane sapphire by mist chemical-vapor deposition. 21 Their photodetectors exhibited remarkable peak spectral responsivities of above 150 A/W for an incident-light wavelength of 254 nm under a relatively high reverse-bias of 20 V. In the former report by Han et al., the authors reported a low dark current (Idark ) value of 0.16 pA by virtue of the high crystal quality and excellent Schottky contact characteristics. 23 The MgZnO lms were grown on an MgO template using metalorganic chemical vapor deposition (MOCVD), enabling the realization of thin lms with high crystalline qualities and low surface roughnesses. However, the reported peak responsivity values were relatively low, which are typical for Schottky-type MSM photodetectors without internal gain mechanisms, such as defect-assisted tunneling, the avalanche process, and minority carrier trapping. 2430 Moreover, illuminating power densities (Pin ) were not reported, rendering the analysis of the photodetector sensitivity and the eciency of exciton transport as a function of incident illuminating power infeasible. In the latter report by Xu et al., while remarkably high photoconductive gains and peak spectral responsivities were achieved at relatively low illuminating power density of 2 pW/µm2 , their photodetectors exhibited slow photoresponse times. The photodetectors exhibited such high gains and spectral responsivities values because the minority carrier trapping caused by the existence of oxygen vacancy deep-level defects in the grown β -Ga2 O3 lm. 31,32 The application of high electric elds resulted in the release of these carriers from defect centers, causing high photoconductive gains at high operational 3



reverse-bias voltages, at the expense of slow response times, a common phenomena observed in Ga2 O3 -based photodetectors and is referred to as persistent photoconductivity (PPC). 33,34 In this work, we report on the growth of a heterostructure thin lm stack comprised of single-crystalline p-type NiO and n-type β -Ga2 O3 layers on c-plane sapphire by pulsed laser deposition (PLD). Several characterization techniques were employed to investigate the heterostructure, including X-ray diraction (XRD), Rutherford backscattering spectrometry (RBS), scanning and high-resolution transmission electron microscopy (SEM/HRTEM), and energy-dispersive X-ray (EDX) spectroscopy. EDX analysis combined with SEM imaging conrmed the chemical composition of each deposited lm, whereas we observed the sharp layer transitions and high quality of interfaces in the double-heterostructure stack of NiO/β Ga2 O3 /sapphire through HRTEM imaging. We also fabricated solar-blind deep-ultraviolet photodetectors based on the resulting pn heterojunction diodes. Our photodetectors exhibited a peak spectral responsivity of 415 mA/W at 7 V reverse-bias voltage for a 260-nm incident-light wavelength and 46.5-pW/µm2 illuminating power density. With the emergence of NiO as a highly stable photoactive material under ultraviolet illumination, 35,36 it is paramount that surface science research also considers heterojunctions involving NiO, necessitating the investigation of their interfacial energy band properties and their potential implication on the operational integrity of optoelectronic devices based on such heterojunctions. We herein employed high-resolution X-ray photoelectron spectroscopy (HRXPS) to examine the band oset parameters, namely the valence band oset (VBO) and conduction band oset (CBO), at the β -Ga2 O3 /NiO heterojunction. The VBO and CBO were determined to be 1.15 ± 0.10 and 0.19 ± 0.10 eV, respectively, with a type-I heterostructure.

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2

Experimental methods

2.1 Thin lm growth and device fabrication After cleaning a c -plane sapphire substrate using acetone and isopropyl alcohol (IPA), an n -type

β -Ga2 O3 (silicon-doped 1% wt) lm was grown at a substrate temperature of 640 °C

and an oxygen partial pressure of 5 mTorr. A p -type NiO (lithium-doped 1% wt) lm was then grown on top the n -type β -Ga2 O3 lm at a substrate temperature of 240 °C and an oxygen partial pressure of 200 mTorr. In both cases, the laser pulse frequency was set to 5 Hz with an energy per pulse of 450 mJ and a laser uence of 1 J/cm2 . The lms were deposited with a target-to-substrate distance of 80 mm and deposition rates of 86 (n -Ga2 O3 ) and 85 (p -NiO) mÅ/pulse; their thicknesses were estimated to be 260 nm (n -Ga2 O3 ) and 170 nm (p -NiO) by combining cross-sectional SEM imaging and EDX spectroscopy analyses, which provided precise elemental mapping 37 of the composition of the cross section of a p NiO/n -Ga2 O3 /sapphire sample. After the growth of the β -Ga2 O3 and NiO thin lms, NiO mesa was formed using inductively-coupled plasma dry etch (ICP-RIE, Figure 1(d), 20 sccm Ar, 5 sccm Cl2 , 700 W RF power, 300 DC voltage, and 5 mTorr chamber pressure, for three minutes) after a lithography step. Then, metal contacts were deposited on the NiO (Au) and β -Ga2 O3 (Au/Ti) lms after two lithography steps using magnetron sputter deposition, which were subsequently thermally annealed at 600 °C for one minute in an Ar ambience to form Ohmic contacts (Figure 1(e)). The photodetectors were fabricated with six, seven, and nine interconnected Au and Au/Ti parallel ngers with 50 µm, 40 µm, and 30 µm spacings, respectively, constituting our 50-µm, 40-µm, and 30-µm photodetectors.

5



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β

β

β-

β-

Figure 1: Fabrication process: (a) Sapphire substrate preparation; (b) PLD deposition of β -Ga2 O3 ; (c) PLD deposition of NiO on β -Ga2 O3 ; (d) NiO mesa formation using inductivelycoupled plasma dry etch. (e) Deposition of Au/Ti thin lms after two lithography steps using magnetron sputter deposition. The Au ngers act as an Ohmic contact electrode to the ptype NiO lm, while the Au/Ti ngers were deposited directly on the n-type β -Ga2 O3 lm through a mesa, and act as an Ohmic contact electrode to the n-type β -Ga2 O3 lm.

2.2 X-ray crystallography The crystal structure properties of a β -Ga2 O3 /NiO heterojunction on c -plane sapphire were examined by a Bruker D8 Advance X-ray diractometer using Cu Kα (λ = 1.5405 Å) radiation.

2.3 Ion beam analysis For RBS measurements and analysis, a thin β -Ga2 O3 /NiO sample with a total thickness of approximately 70 nm was prepared on a c -plane sapphire substrate. The RBS experiments were carried out using a high-resolution RBS system (Kobe Steel, Ltd. HRBS-V500). A detection angle of 107.5° and a 400 keV beam of He+ ions were used for the analysis. The sample was coated with Ir and Ag to attain electrical conductivity.

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2.4 High-resolution electron microscopy TEM images and fast Fourier transform (FFT) patterns were acquired using an FEI Titan ST microscope operating at 300 keV. A crystal model of each material was created by a

®

computer software, CrystalMaker , and the corresponding FFT pattern was simulated based on the created crystal model and matched to the FFT pattern extracted from HRTEM images. The TEM lamella of a β -Ga2 O3 /NiO heterojunction on c -plane sapphire sample was prepared through focused ion beam (FIB) milling using an FEI Helios G4 FIB-SEM. NiO/β -Ga2 O3 /sapphire sample cross-sectional images were acquired using a Zeiss Merlin scanning electron microscope with an AZtec EDX system operating under a probe current of 2 nA at 20 keV with a working distance of about 8.5 mm.

2.5 High-resolution X-ray photoelectron spectroscopy The HRXPS measurements were carried out using a Kratos Axis Supra DLD spectrometer equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) operating at 150 W, a multi-channel plate, and delay line detector under a vacuum of about 10−9 mbar. The high-resolution spectra were collected at xed analyzer pass energy of 20 eV. The samples were mounted in oating mode to avoid dierential charging. Charge neutralization was required for all samples. All binding energies were referenced to the C 1s binding energy of adventitious carbon contamination, which was taken to be 284.8 eV.

3

Results and discussion

3.1 Electrical characterization of grown thin lms and heterojunction diode Figure 2(a) shows the XRD measurements of the single-crystalline lms, manifesting (¯ 201)oriented β -Ga2 O3 and (111)-oriented cubic NiO lms on c -plane sapphire, 38,39 whereas Figure 7



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2(b) shows the measured RBS curves for thinner NiO and β -Ga2 O3 lms on sapphire, conrming the stoichiometry of each layer and the well-dened interfaces. It can be seen that the Ni and Al signals overlap with that of Ga and O, respectively; the Ir and Ag signals originate from the metallic layer sputtered onto the sample before the RBS experiment in order to avoid electrostatic charging. Linear- and semilogarithmic-scale J V curves for a 50-µm β -Ga2 O3 /NiO pn junction diode, demonstrating that the diode sustained relatively high-voltage operation (from −10 to 10 V) without a breakdown, are shown in Figure 2(c). The diode started to conduct appreciably and in an unstable manner at around 13 V reversebias voltage (breakdown point). It exhibited a clear rectifying J V characteristics with a series resistance (Rs ) of 3.04 kΩ, an ideality factor (n) of 3.43, and a forward sub-turn-on voltage (VON ) of 6.50 V. The bias-dependent rectication ratio (σ ), a commonly used efciency metric to quantitatively characterize the rectication performance of a diode, was calculated as

IVf , σ= I(Vr )

(1)

where IVf is the measured pn junction current at a xed forward-bias voltage (Vf = 10 V in our case) and I(Vr ) is the reverse-bias-dependent measured pn junction current. Figure 2(d) plots the evolution of σ values with increasing |Vr | levels up to 10 V reverse-bias. Figure 2(d) conrm that there is an intermixed layer at the β -Ga2 O3 /NiO heterointerface in the 77-nm sample prepared for RBS measurement, with a thickness of about 13 nm. The RBS sample was grown at a high temperature, so interdiusion was inevitable. However, this 77-nm thick sample was used for RBS measurements only and not for our photodetector device fabrication. Our photodetector samples are approximately 420-nm thick (170-nm thick NiO and 260-nm thick β -Ga2 O3 ). If there are intermixed layers in our photodetector devices, the layers would only account for less than 3.3% of the overall thickness. That did not signicantly aect the performance of our photodetector. In our RBS model, the stoichiometry of NiO and β -Ga2 O3 was conrmed for both materials, except within the intermixed layer, to be: Ni:O = 1:1.002 and Ga:O = 1:1.504. 8




 





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

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 







Figure 2: (a) XRD and (b) RBS measurements for a PLD-grown NiO/Ga2 O3 /sapphire stack. (c) Measured linear-scale J V curve for the 50-µm β -Ga2 O3 /NiO photodetector (Inset: semilogarithmic-scale J V curve). (d) Simulation-extracted Atomic concentration depth prole. (e) Plot of bias-dependent rectication ratios. Table 1: Fabricated β -Ga2 O3 /NiO p-n junction device performance parameters, with performance comparison with select p-n junctions characterized at room temperature reported in the literature. Ref. This work 7

7 9 9 9 16 16

Heterojunction

Sub-turn-on voltage (V)

Onset current

Ideality factor

Series resistance

β -Ga2 O3 /NiO β -Ga2 O3 /NiO β -Ga2 O3 /ZnCo2 O4

6.50 1.95 2.27

194 mA/cm2 ≈ 500 mA/cm2 ≈ 500 mA/cm2

3.43 1.961.97 2.122.27

3.04 kΩ 0.81 Ω · cm2 0.65 Ω · cm2

Cux Oy /β -Ga2 O3 Sputtered NiO/β -Ga2 O3 ALD-deposited NiO/β -Ga2 O3 Planarized Alx Ga1−x N/Aly Ga1−y N MQD nanowires As-grown Alx Ga1−x N/Aly Ga1−y N MQD nanowires

0.28 0.751 ≈ 0.25

1 mA 2 µA/cm2 2 µA/cm2

62.5 mΩ · cm2

2.5 × 104 b 105 c ≈ 2 × 105 d

7.31

52 µA/cm2

7.58

9Ω

7.61

47 µA/cm2

8.52

13 Ω

a I = 10V. I(V ) between −10 Vf r b Measured at ±2 V. c Measured at ±5 V. d Measured at ±3 V.

and

−0.01

V.

9


Rectication ratio

7.18 × 102 4.63 × 105 ≈ 109 ≈ 109

a


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3.2 Electron microscopy analysis Figure 3(a,b,c) show cross-sectional HRTEM images of a grown NiO/β -Ga2 O3 /sapphire stack (reference stack, used for TEM imaging only) for the a -plane zone axis, conrming the high interface qualities exhibited at the β -Ga2 O3 /NiO and β -Ga2 O3 /sapphire interfaces through the sharp layer transitions, whereas FFT images conrm the symmetry of the lattice fringes on the cross sectional NiO ((10¯ 1) plane FFT) and β -Ga2 O3 ((102) plane FFT) HRTEM images. 40,41 Figure 3(d) displays a cross-sectional SEM image with EDX spectra that conrm each layer composition and thicknesses, as well as the low thermally-induced interdiusion characteristics during layer growth. As the β -Ga2 O3 /NiO heterojunction was grown on c plane sapphire, detailed analysis of HRTEM images for the a -plane zone axis with FFT patterns reveal the zone axes of α-Al2 O3 , β -Ga2 O3 , and cubic NiO to be a -plane (11¯ 20), (¯ 10¯2), and (11¯2), respectively. The orientation relationships between β -Ga2 O3 /NiO and

α-Al2 O3 were determined as follows NiO (111) k β -Ga2 O3 (¯ 201) k α-Al2 O3 (0006),

(2)

NiO (110) k β -Ga2 O3 (010) k α-Al2 O3 (10¯ 10),

(3)

NiO (11¯ 2) k β -Ga2 O3 (¯10¯2) k α-Al2 O3 (11¯20).

(4)

and

These determined orientation relationships are in agreement with the out-of-plane XRD results presented in Figure 2(a) (which provide the orientation relationship along the c plane) as the plane relationship NiO (111) k β -Ga2 O3 (¯ 201) k α-Al2 O3 (0006) was resolved by FFT analysis. The rst observed spots located right above the center of each indexed FFT pattern were used to attribute the Miller indices to the observed spots because our homogeneous crystalline thin lms were aligned in the epitaxy orientation.

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b b a

(0003 (112ത 3) (112ത 0)

(101ത 0)

(101ത 0)

a-

(111)

(2ത 01) (002) (102)

(010)

(101ത 0)

(002) (11-2)

b

(11ത 0)

Figure 3: (a) Cross-sectional TEM images of a reference NiO/β -Ga2 O3 /sapphire stack with FFT patterns (a -plane sapphire zone axis). Cross-sectional views of the (b) β Ga2 O3 /sapphire and (c) β -Ga2 O3 /NiO interfaces. (d) SEM cross-sectional images of the NiO/β -Ga2 O3 /sapphire stack with EDX spectra. The plane spacings (dn , where n ∈ {1, 2, 3}) of α-Al2 O3 (0006), β -Ga2 O3 (¯ 201), and cubic NiO were estimated by the HRTEM images and found to be 2.166 Å, 4.783 Å, and 2.450 Å, respectively, which closely match the reported values. 4247 The plane spacing values for hexagonal α-Al2 O3 , cubic NiO, and monoclinic β -Ga2 O3 , respectively, were calculated using the following relations:

4 h2 + hk + k 2 1 = d21 3 a2 11

! +

l2 , c2


(5)


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1 h2 + k 2 + l2 = , d22 a2 and

1 1 = 2 d3 sin2 β

2

2

2

2

(6)

!

h k sin β l 2hl cos β , + + 2− 2 2 a b c ac

(7)

where h, k , and l are Miller indices, a, b, and c are axis-specic lattice parameters, and β is the angle between the a- and c-axes.

3.3 Heterointerface energy band oset characteristics HRXPS measurements were employed to determine the VBO at the β -Ga2 O3 /NiO heterojunction interface. In order to evaluate the VBO at the β -Ga2 O3 /NiO heterointerface, the energy dierence between the Ga 2p 3/2 and Ni 2p 3/2 core levels from a β -Ga2 O3 /NiO heterojunction sample and the energy of Ga 2p 3/2 and Ni 2p 3/2 core levels relative to the respective valence band maximum (VBM) of β -Ga2 O3 and NiO, respectively, need to be acquired. The VBO at the β -Ga2 O3 /NiO heterojunction can be calculated using the relation provided by Kraut et al., which is expressed as 48

β -Ga2 O3 /NiO β -Ga2 O3 /NiO β -Ga2 O3 β -Ga2 O3 NiO NiO ∆EV = ENi − E − E − E + E − E , (8) 2p 3/2 VBM Ga 2p 3/2 Ga 2p 3/2 Ni 2p 3/2 VBM ∆EC = Egβ -Ga2 O3 − EgNiO − ∆EV ,

(9)

where ∆EV = VBO, Egβ -Ga2 O3 and EgNiO are the optical band gaps of β -Ga2 O3 and NiO, respectively, and ∆EC = CBO. Figure 4(a) shows the Ni 2p core-level and valence band spectra obtained from a bulk NiO sample. The binding energy of Ni 2p 3/2 is equal to 854.05 eV whereas VBM is equal to 0.70 eV. Therefore, the separation between the core-level energy of Ni 2p 3/2 and VBM, i.e.

NiO NiO NiO ENi = E − E 2p 3/2 −VBM VBM , Ni 2p 3/2

12


(10)

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for NiO was calculated to be 853.35 eV. Similarly, Figure 4(b) shows the Ga 2p core-level and valence band spectra of a bulk β -Ga2 O3 sample. The binding energy of Ga 2p 3/2 is equal to 1118.65 eV whereas VBM is equal to 3.30 eV. Thus, the separation between the core-level energy of Ga 2p 3/2 and VBM, i.e.,

β -Ga2 O3 β -Ga2 O3 β -Ga2 O3 EGa = E − E , 2p 3/2 −VBM Ga 2p 3/2 VBM

(11)

for β -Ga2 O3 was determined to be 1115.35 eV.

Figure 4: (a) Ni 2p and (b) Ga 2p core-level and valence band spectra for NiO and β -Ga2 O3 , respectively. Figure 5(a) shows the Ga 2p and Ni 2p core-level spectra of a thin NiO lm grown on

β -Ga2 O3 . The binding energies of Ga 2p 3/2 and Ni 2p 3/2 are equal to 1117.35 eV and 854.20 eV, respectively. The energy dierence, i.e,

β -Ga O /NiO β -Ga O /NiO β -Ga O /NiO ∆EGa 2p23/23 −Ni 2p 3/2 = EGa 2p23/23 − ENi 2p23/23

(12)

between Ga 2p 3/2 and Ni 2p 3/2 core-levels was observed to be 263.15 eV. After substituting these experimentally obtained values into equation (8), we calculated a VBO value of ∆EV =

1.15 eV. 13



The optical band gaps of β -Ga2 O3 and NiO are Egβ -Ga2 O3 = 5.01 eV and EgNiO = 3.67 eV, respectively, as determined from absorption data obtained from ultravioletvisible spectrophotometry (UVVis) measurements and using the Tauc plot method. By substituting the obtained VBO (∆EV ) and optical bandgap values of β -Ga2 O3 and NiO into equation (9), we determined the CBO = ∆EC value at the β -Ga2 O3 /NiO heterointerface to be 0.19 eV. The determined band oset parameters at our PLD-grown β -Ga2 O3 /NiO heterointerface are presented schematically as a band alignment diagram in Figure 5(b), which establishes that this band alignment pertains to a type-I heterojunction.

Figure 5: (a) Ga 2p and Ni 2p core-levels for β -Ga2 O3 /NiO heterojunction and (b) schematic representation of band alignment at the β -Ga2 O3 /NiO heterointerface.

3.4 Photocurrent characteristics and deep-ultraviolet photodetection Given a reverse-biased photodiode under an incident-light illumination with a particular wavelength and optical power density, the spectral responsivity of the photodetector is a parameter that quantizes the internal quantum eciency (ηλVinbias ,Pin ) and the photoelectric gain of a photodetector, which are determined by the generated number of carriers per incident photon and the number of carriers that conduct current through the electrical contact per

14


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generated electronhole pair 49,50

RVλbias =q in ,Pin

IλVbias gηλVinbias ,P − Idark ,Pin λin = in in λin , hc PIL

(13)

where q is the elementary charge, gηλVinbias ,Pin is the external quantum eciency (EQE), h is λin Planck's constant, c is the speed of light, and PIL is the eective illuminating power in W λin (PIL = P · (S/A), where P is the total power of the irradiating beam, S is the eective

irradiation area of the photodetector (taken as the total device area), and A is the area of the incident-light beam). From equation (13), the EQE can be expressed as Vbias gηλVinbias ,Pin = Rλin ,Pin ×

hc . qλin

(14)

The signal-to-noise ratio (SNR) yielded by a photodetector with an eective irradiation area

S of 1 cm2 at an incident-light power of 1 W with an electrical bandwidth of 1 Hz is quantied through the specic detectivity (D) as follows

s Vbias DλVinbias ,Pin = Rλin ,Pin ×

S , 2qIdark

(15)

where herein S is expressed in cm2 , yielding the customary specic detectivity unit of √ cm· Hz/W, or Jones. Another measure of the sensitivity of a photodetector is the noiseequivalent power (NEP), which is dened as the signal power that produces a SNR of unity in a 1 Hz output bandwidth. The NEP can be expressed as

√ 2eIdark S NEP = = Vbias , Vbias Rλin ,Pin Dλin ,Pin √

(16)

where e is the elementary charge. We should note that in equations (13) and (15) we are underestimating R and D values because S was taken as the total device area without subtracting the area of nontransparent metal contacts. Figure 6(a,b,c) depicts measured

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photocurrent density vs. Vbias (Jph Vbias ) curves for the 50-µm, 40-µm, and 30-µm photodetectors at an Pin = 28.7 ± 3.0 pW/µm2 . Measured power-dependent Jph Vbias curves for the 50-µm, 40-µm, and 30-µm photodetectors at an λin = 260 nm are shown in Figure 6(d,e,f). The devices exhibited dark-current densities (Jd ) of 1.58 × 10−8 A/cm2 (50-µm), 2.82 × 10−9 A/cm2 (40-µm), and 1.76 × 10−8 A/cm2 (30-µm) at zero-bias voltage applied and 1.78 × 10−4 A/cm2 (50-µm), 1.77 × 10−5 A/cm2 (40-µm), and 1.15 × 10−4 A/cm2 (30-µm) at a Vbias = 7 V.

















 

 





















 



















Figure 6: Measured Jph Vbias curves for the (a) 50-µm, (b) 40-µm, and (c) 30-µm photodetectors at an Pin = 28.7 ± 3.0 pW/µm2 . Measured Jph Vbias curves for the (d) 50-µm, (e) 40-µm, and (f) 30-µm photodetectors at an λin = 260 nm. Figure 7(a,b,c) plots the evolution of R, NEP, and EQE values with increasing Pin levels up to 100 pW/µm2 at an λin = 260 nm. We observe that the 40-µm photodetector exhibited the best photoresponsivity performance, with a R value of 415 mA/W at an Pin = 46.5 pW/µm2 . For the 50-µm photodetector, the distance between the metal ngers is seemingly far; we conjecture that photocurrents conducted through a relatively wide separation between the metal ngers and thereby some of the electrons fell back to the valence band before reaching the other electrode. For the 30-µm photodetector, the distance between the 16


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metal ngers is shorter; however, the photodetector exhibited low photosensitivity and SNR performances, with D value of 6.75 × 1010 Jones (NEP = 7.78 × 10−13 W/Hz1/2 ) at a Vbias = 7 V for Pin = 46.5 pW/µm2 , manifesting low photocurrent generation eciency caused presumably because lower illumination area and higher metal contact coverage caused less ecient light absorption characteristics when compared to the other photodetectors. For all photodetectors, R values saturated after 46.5 pW/µm2 , indicating a limit for ultraviolet-C photons to excite photoelectrons from the valence band to conduction band in the NiO lm. In this case, we argue that our photodetectors require ultraviolet-C light with only about 46.5 pW/µm2 Pin level to operate. The 40-µm photodetector exhibited a specic detectivity value of 2.27 × 1011 Jones and an EQE value of 197.81% for an λin = 260 nm and Pin = 46.5 pW/µm2 . Without high-mobility current spreading layers, it can be expected that generated carriers would be trapped in the lower-mobility NiO and β -Ga2 O3 layers, limiting the achievable photocurrents and gains in our photodetector devices. 51 Table 2 summarizes each device design parameters and performance. Given that our main absorption layer is

β -Ga2 O3 , we believe that the response times of our photodetectors to high-frequency signals is not better than what already reported in the literature for β -Ga2 O3 -based deep-ultraviolet photodetectors. 21,52

17



Page 18 of 28

l

m m m

m

l m m m

m

l

m m m

m

Figure 7: Evolution of the calculated (a) R, (b) NEP, and (c) EQE values with Pin level for an λin = 260 nm at a Vbias = 7 V.

18


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Table 2: Summary of each fabricated photodetector device design and performance parameters, with performance comparison with select oxide-based photodetectors reported in the literature. Ref. This work This work This work 53

54 55 56 57 58 59 60 61

Heterojunction

Distance between electrode ngers (µm)

β -Ga2 O3 /NiO β -Ga2 O3 /NiO β -Ga2 O3 /NiO NiO/ZnO (Al0.28 Ga0.72 )2 O3 /Si β -Ga2 O3 /graphene β -Ga2 O3 /ZnO PEDOT:PSS/β -Ga2 O3 /Si MgZnO/graphene MgZnO/GaN Graphene/MgGaO/SiC 2D MgO

50 40 30

Total device area (cm2 )a

Electrode areal coverage 58.18% 58.77% 59.18% 7 mm2

−3

2.75 × 10 2.60 × 10−3 2.55 × 10−3 0.125 × 10−3 240 × 10−3

Exposed β -Ga2 O3 area 41.82% 41.23% 40.82%

Peak responsivity (A/W) nm)b

0.362 (7 V, 260 0.415 (7 V, 260 nm)c 0.323 (7 V, 260 nm)b 0.3 (6 V, 360 nm) 1.17 (2.5 V, 230 nm)d 12.8 (6 V, 254 nm)e 0.35 (5 V, 254 nm)f 0.029 (0 V, 255 nm)g 0.00264 (0 V, 255 nm) 0.002 (0 V, xx nm) 0.0103 (0 V, xx nm) 1.86 (4 V, 150 nm)

Corresponding specic detectivity (Jones)

6.28 × 1010 2.27 × 1011 6.75 × 1010 9.6 × 1011 1.3 × 1013 1.8 × 1010

a The parameter S reported earlier. b Observed at P = 75 pW/µm2 . in c Observed at P = 46.5 pW/µm2 . in d Observed at P = 74 µW/µm2 . in e Observed at P = 21.2 µW/cm2 . in f Observed at P = 50 µW/cm2 . in g Observed at P = 15.06 µW/cm2 . in

4

Concluding remarks

This paper has demonstrated the material growth and characterization of single-crystalline

β -Ga2 O3 /NiO heterojunctions on c -plane sapphire. X-ray diraction and fast Fourier transform analysis revealed a plane relationship of NiO (111) k β -Ga2 O3 (¯ 201) k α-Al2 O3 (0006). The resulting heterojunction stack was employed to fabricate solar-blind deep-ultraviolet photodetectors using conventional nanofabrication processes and exhibited a peak spectral responsivity of 415 mA/W for an incident-light wavelength of 260 nm at a reverse-bias voltage of 7 V. The valence and conduction band osets values were found to be 1.15 ± 0.10 and 0.19 ± 0.10 eV, respectively, with a type-I band alignment. This is the rst reported

β -Ga2 O3 /NiO heterostructure photodetector to have been grown and fabricated on sapphire, which will pave the way for the fabrication of cheap and vertically structured deep-ultraviolet solar-blind photodetection devices.

19



Acknowledgement The authors acknowledge receipt of KAUST baseline funding, BAS/1/1614-01-01.

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ν

cubic NiO

b-Ga2O3 g = (0006)


10 nm

a-Al2O3

(

)

Deep-Ultraviolet Photodetection Using Single-Crystalline Î²

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