Enhanced Deep-Ultraviolet Responsivity in AluminumâGallium Oxide

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Functional Inorganic Materials and Devices

Enhanced deep-ultraviolet responsivity in aluminumgallium oxide photodetectors via structure deformation by high-oxygen-pressure pulsed laser deposition Shuo-Huang Yuan, Sin-Liang Ou, Shiau-Yuan Huang, and Dong-Sing Wuu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Apr 2019 Downloaded from http://pubs.acs.org on April 26, 2019

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ACS Applied Materials & Interfaces

Enhanced deep-ultraviolet responsivity in aluminum-gallium oxide photodetectors via structure deformation by high-oxygen-pressure pulsed laser deposition

Shuo-Huang Yuana, Sin-Liang Oub, Shiau-Yuan Huanga, and Dong-Sing Wuua,*

a Department

of Materials Science and Engineering, National Chung Hsing

University, No. 145, Xingda Road, Taichung, 40227, Taiwan

b

Bachelor Program for Design and Materials for Medical Equipment and Devices,

Da-Yeh University, Changhua 51591, Taiwan

*Corresponding

author. Tel.: +866 4 2284-0500#714; Fax: + 886 4 2285-5046

E-mail address: [email protected]

KEYWORDS: Pulsed laser deposition, aluminum gallium oxide, lattice deformation, photodetector, oxygen pressure.

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Abstract

Aluminum–gallium oxide (AGO) thin films with wide bandgaps of greater than 5.0 eV were grown using pulsed laser deposition. As evidenced by X-ray photoelectron spectroscopy, X-ray diffraction, and transmission electron microscopy, the oxygen chamber pressure considerably affected the lattice deformation in the AGO materials. Under high oxygen pressure, the lattice deformation reduced the d-spacing of the AGO (-201) plane. In the measured transmittance spectra of the AGO films, this narrowing of the d-spacing in the main plane manifested as a high-energy shift of the absorption edge. The AGO films were then installed as the active layers in the metal–semiconductor–metal photodetectors (PDs). The lattice deformation was observed to enhance the photocurrent and reduce the dark current of the device. The responsivity was 20.7 times higher in the lattice-deformed AGO-based PD sample than that in the non-deformed sample. It appeared that the lattice deformation induced the separation of the piezopotential, improving the efficiency of the photogenerated carrier recombination and, consequently, shortening the decay time of the photodetector.


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Introduction A wide-bandgap semiconductor, monoclinic β-gallium oxide (β-Ga2O3), is emerging as a fourth-generation semiconductor that promises to revolutionize the optoelectronic device industry.1 A recent study reported a sharp absorption edge in the spectrum of β-Ga2O3 variation, which can be attributed to the polarized light being parallel to the b- or c-axis in the crystal.2 Owing to its energy bandgap (4.8–4.9 eV)3,4, β-Ga2O3 is suitable for fabricating deep-ultraviolet (DUV) photodetectors (PDs)5,6,7. Furthermore, the energy bandgap of Ga2O3 can be modulated by incorporating other elements,8,9 forming complex oxides. Among the incorporated elements, the purely monoclinic Al2O3 with an energy bandgap of approximately 7.0 eV is the optimal selection. The similar electronic configurations of Ga and Al atoms are an additional advantage. Meanwhile, the Al3+ ions are highly soluble in the Ga2O3 crystal structure (with solubilities reaching 78%).10 Thus, when Al2O3 is incorporated into Ga2O3, the energy bandgap of the β-phase aluminum–gallium oxide (β-AGO) can be continuously tuned from 4.8 to 6.6 eV.11 These complex oxide compositions also exhibit similar crystal structures; hence, their electronic, optical, and ionic properties can be selected and varied under similar processing conditions. This advantage ensures the material development and extends the practical application range of the materials. Currently, the complex oxides of the AGO films are grown by molecular beam epitaxy12, chemical vapor deposition,13 pulsed laser deposition (PLD),14,15 and sputtering.16 The PLD -3ACS Paragon Plus Environment


technique is more suitable for multi-component film growth when compared with the other methods, and the atomic-layer thickness can be controlled by adjusting the laser frequency. Meanwhile, the high energy of the source particles generated from the pulsed lasers enhances the surface mobility of the ad-atoms. Therefore, the AGO thin films are suitably grown by PLD. According to some studies, the band gap in the PLD-grown AGO films is strongly correlated with the aluminum fraction and the crystal structure of the AGO film.11,17 The AGO film with a monoclinic crystal structure is deposited at substrate temperatures of greater than 400°C, and the crystallinity gradually decreases with increasing Al concentration.14 The PLD growth of Ga2O3 is critically dependent on the oxygen pressure.18 Under high oxygen pressure, Ga suboxides (Ga2Ox) transition to Ga2O3 as the substrate temperature increases from 450°C to 600°C.19 However, the effect of oxygen growth pressure on the characteristics of AGO films and the PDs in which they are installed has not been clarified. In this study, AGO thin films were grown by the PLD technique while modulating the oxygen pressure from 710-4 to 210-1 torr, then incorporated into DUV PDs. The compositional, optical, and structural characteristics of the AGO films, and the optoelectronic performance of the PDs, were investigated in detail.

Experimental methods -4ACS Paragon Plus Environment

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The AGO films (of thickness 100 nm) were grown by PLD (PLD/MBE-2000, PVD Products Inc.) on c-plane sapphire substrates. The PLD was powered by a KrF excimer laser with a pulse energy density of 2.4 J/cm2 and a wavelength of 248 nm. The target was an (Al0.05Ga0.95)2O3 disk (diameter 3.0 inches, purity 99.99%), and the oxygen growth pressure was ranged from 710-4 to 210-1 torr. The substrate temperature (Ts) and laser repetition frequency were optimized as 800°C and 10 Hz, respectively, in our previous study. To fabricate the metal–semiconductor–metal (MSM) PD, a Schottky contact metal [Ti/Au (40/60 nm) electrode] was prepared on the sample by thermal evaporation. The active area and finger width of the MSM-based PD were fixed at 1.05 × 1.05 mm2 and 50 μm, respectively. The optical transmittance characteristics of the PLD-grown AGO films were measured by an n&k analyzer (N&K Technology 1280). The crystalline quality and structure were analyzed by X-ray diffraction (XRD; PANalytical X’Pert Pro MRD). The chemical states of the AGO films were analyzed by X-ray photoelectron spectroscopy (XPS; ULVAC-PHI PHI 5000 VersaProbe). Prior to XPS measurements, the AGO

film surface was etched with Ar+

ions supplied at 3 keV for 1 minute. The surface morphologies of the films were measured by an atomic force microscope (AFM; Dimension 5000) with a resolution >1.0 nm in the X–Y plane and >0.1 nm in the depth (Z) direction. To assess the electrical performances of the AGO-based PDs, the current–voltage characteristics of the AGO-based PDs were measured by an HP 4156 semiconductor analyzer at room temperature. Spectral responsivity -5ACS Paragon Plus Environment


measurements were carried out in an Omni 3029i system with a 30-W deuterium lamp light source. The time-dependent photoresponsivity measurements were performed at a fixed voltage of 5.0 V.


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Results and discussion Panels (a)–(d) of Figure 1 display the XPS spectra of the O 1s core level of the PLD-AGO films deposited under various oxygen pressures. In the Gaussian fitting analysis, the O 1s core-level spectrum of each sample was resolved into two separated components centered at 530.6 eV (OI) and 531.2 eV (OII). The OI and OII components were related to the O2− ions in the oxygen-saturated and oxygen-deficient regions, respectively.20 As the peak area is proportional to the intensity of the corresponding component, the intensity ratio OII/(OI + OII) reflects the density of the oxygen vacancies. The intensity ratios OII/(OI + OII) of the PLD-AGO films deposited under oxygen pressures of 7 × 10-4, 7 × 10-3, 7 × 10-2 and 2 × 10-1 torr were determined as 52.0%, 50.3%, 50% and 49.5%, respectively. This result indicates that increasing the oxygen pressure reduced the number of oxygen-deficient defects in the grown film. Figure 2(a) shows the XRD scans of the PLD-AGO films deposited under the various oxygen pressures. In the film grown under 7×10-4 torr, the three diffraction peaks located at 18.7°, 38.2°, and 58.9° were indexed to the (-201), (-402), and (-603) planes, respectively. When the oxygen pressure increased to 2×10-1 torr, these diffraction peaks shifted to 19.1°, 38.5°, and 59.3°, respectively. The shift of diffraction peak to higher angles with increasing oxygen pressure indicates compressive strain in the AGO thin film (-201) family of planes.21



The higher oxygen pressure also improved the crystal quality of the AGO film, as shown in Figure 2(b). Panels (a)–(d) of Figure 3 show high-resolution TEM images of the PLD-AGO samples described in Figure 2. The main planes in the four samples were the AGO (-201) and AGO (400) orientations. Note that the AGO (-201) planes exhibit two d-spacings (one of 4.63, the other of 4.55 Å), whereas the AGO (400) plane possesses a single d-spacing (of 2.57 Å). Moreover, the area covered by the smaller d-spacing in the AGO (-201) plane increased proportionally to the oxygen pressure. This trend can be explained by the lattice deformations formed by interactions between the (-201) and (400) planes. The lattice deformation could be generated by the compressive strain, consistent with the XRD results in Figure 2(a). Figure 4(a) shows the light transmittances of the PLD-grown AGO films deposited under the different oxygen pressures. The optical transparency of the films in the visible region increased with increasing oxygen pressure, and the transmittances decreased at incident wavelengths smaller than 300 nm. The bandgap of the AGO film was obtained by extrapolating the linear part of (αhν)2–hν to the horizontal axis (see Figure 4(b)). Here, α is the absorption coefficient and hν is the incident photon energy. The bandgap of the AGO film increased from 4.91 to 5.20 eV as the oxygen pressure increased from 7 × 10-4 to 2 × 10-1 torr. The variable energy gap might be sourced from the varying Al contents and oxygen vacancies in the AGO films. However, the Al content should minimally affect the bandgap -8ACS Paragon Plus Environment

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because the Al/(Al+Ga) ratios varied by only 0.02 among the four samples (0.023, 0.028, 0.022, and 0.018 under oxygen pressures of 7 × 10-4, 7 × 10-3, 7 × 10-2 and 2 × 10-1 torr, respectively). Impurities created by the oxygen vacancies influenced the middle of the bandgap in the present study, and have been reported in other oxides such as Ga2O3 and ZnO.22,23 When more oxygen vacancies exist in AGO films, the impurity states become more delocalized and overlap with the valence band edge. Consequently, the band gap narrows and the d-spacing of the main plane widens. Accordingly, the higher-energy shift in the absorption edge can be attributed to the decreased d-spacing of the main (-201) plane.24 Figure 5 shows the grain sizes and growth rates of the AGO films as functions of oxygen growth pressure. The average crystalline grain size was determined by the Scherrer formula. As the oxygen pressure increased from 7 × 10-4 to 2 × 10-1 torr, the grain size of the AGO film increased and was maximized at 2 × 10-1 torr. Meanwhile, the growth rate of the AGO film increased from 4.68 to 10.63 nm/min as the oxygen pressure increased from 7 × 10-4 to 7 × 10-2 torr, but decreased to 9.92 nm/min when the oxygen pressure reached 2 × 10-1 torr. The TEM images of Figure 3 reveal another interesting phenomenon, namely, the formation of an AGO (400) phase that gradually became more obvious with increasing oxygen growth pressure. The experimental results clarify that increasing the oxygen pressure enhances the growth rate of the AGO film (the reduction in growth rate between 7 × 10-2 and 2 × 10-1 torr was slight.) As the oxygen pressure increases, more of the oxygen atoms would react with the -9ACS Paragon Plus Environment


metal atoms to grow the film, increasing the growth rate. Naturally, the higher growth rate induces the formation of other phases (such as AGO (400)) in the AGO film. Panels (a)–(d) of Figure 6 show the surface morphologies of the PLD-grown AGO films deposited under the different oxygen pressures (7 × 10-4 to 2 × 10-1 torr), obtained by AFM. The root-mean-squared surface roughnesses of the AGO films deposited at 7 × 10-4, 7 × 10-3, 7 × 10-2 and 2 × 10-1 torr were 1.34, 1.54, 1.55 and 1.57 nm, respectively. The surface roughness of the AGO film was affected by oxygen growth pressures from 7 × 10-4 to 7 × 10-3 torr, but was unaffected by further increases (above 7 × 10-3 torr). This result indicates that oxygen benefits the grain growth in the direction perpendicular to the substrate. It is worth mentioning that oxygen growth pressures above 7 × 10-2 torr limited the horizontal growth direction of the AGO grains. Figure 7 shows the current–voltage characteristics of the corresponding AGO-based PDs, measured in the dark and under an illumination wavelength () of 240 nm. The dark currents were reduced in the AGO-based PDs prepared under high oxygen pressures (above 7 × 10-2 torr). The main factor determining the dark current of an AGO-based PD is the Schottky barrier height at the semiconductor/metal interface.25 The dark-current reduction is mainly attributed to the widened bandgap of AGO, which increases the Schottky barrier height. The photocurrents in the AGO-based PDs (measured at 5 V and = 240 nm) gradually increased as the oxygen pressure increased from 7 × 10-4 to 2 × 10-1 torr. The higher photocurrent might - 10 ACS Paragon Plus Environment

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also reflect the wider bandgap, leading to more effective absorption of the photon energy (Figure 4(b)). Figure 8 shows the spectral responsivities of the AGO-based PDs. The spectral responsivity is calculated as R = ((Iph-Idark)/(PA)), where P is the power density of the incident light, Iph is the photo current, Idark is the dark current, and A is the effective illuminated area.26 The responsivity (at 5 V and  = 240 nm) of the AGO-based PD prepared under 2 × 10-1 torr reached 0.50 A/W (solar-blind rejection ratio at 240 nm and 280 nm (R240/R280) ~1.24 × 102), approximately 20.70 times greater than in the AGO-based PD prepared under 7 × 10-4 torr. In addition, the responsivity of the AGO-based PDs slowly declined as the wavelength reduced from 250 to 200 nm. This slow responsivity drop indicates that photon energies in the 4.96–6.20 eV region were absorbed efficiently and generated little thermal energy. The resistivity drop might be facilitated by the built-in electric field.27 Figure 9(a) shows the time-dependent responses (at 5 V and  = 240 nm) of the AGO-based PD samples; the corresponding rise times (from I10% to I90%) and decay times (from I90% to I10%) are shown in Figure 9(b). Increasing the oxygen pressure shortened both the rise and decay times of the AGO-based PDs. These trends are attributed to piezopotential formation via compressive strain development.28 Regarding the decay time, the positive piezopotential will enhance the efficiency of the photogenerated carrier

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recombination after turning off the light. The performances of solar-blind photodetectors reported in the literature are listed in Table 1.

Conclusion We investigated the effect of oxygen growth pressure on the characteristics of AGO films and PDs prepared by PLD. Increasing the oxygen pressure reduced the number of oxygen-deficient defects in the AGO film. Moreover, the XRD peaks of the AGO film shifted to higher angles as the oxygen pressure increased, indicating the development of compressive strain in the AGO (-201) family of planes. TEM observations revealed that the d-spacing of the (-201) plane narrows through interactions between the (-201) and (400) planes, consistent with the compressive strain formation. In turn, the reduced d-spacing of the (-201) plane will shift the absorption edge toward higher energies. The higher photocurrents in the AGO-based PDs grown under higher oxygen pressures are attributable to the wider bandgap, which enhances the absorption of photon energy. Observing the optoelectrical properties of the AGO PDs, the slow responsivity drop from 250 to 200 nm in the measured wavelength region was possibly enhanced by the built-in electric field. This field can separate the piezopotential into two components; the positive piezopotential will boost the efficiency of the photogenerated carrier recombination. Author Information - 12 ACS Paragon Plus Environment

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Corresponding Author E-mail: Dong-Sing Wuu ([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 This work was supported by the Ministry of Science and Technology of Taiwan under grant No. 105-2221-E-005-059-MY3. This work was financially supported by the “Innovation and Development Center of Sustainable Agriculture” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.



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Table 1. Summary of the solar-blind PDs with various semiconductors. Semiconductor

Eg (eV)

λpeak (nm)

Responsivity (A W−1) Rejection ratio 1.24102

Ref. This work

(Al0.018Ga0.982)2O3

5.20

240

0.5 @5 V

Ga2O3 Nanowire

4.8~4.9

250

-

Graphene-Ga2O3

4.8~4.9

254

39.3 @20 V

-

[30]

Diamond

5.5

220

~0.2 @50 V

-

[31]

Diamond

5.47

~225

21.8 @50 V

w- ZnMgO

4.59

270

0.022 @130 V

[33]

c-ZnMgO

4.86

255

0.0158 @15 V

[34]

p-i-n AlGaN

4.57

271

0.15 @0 V

[35]

h-BN

5.5

212

0.0001 @20 V

[36]


(R240/R280) 2103 (R250/R280)

2.4103 (R218/R280)

[29]

[32]


Figure.1 XPS spectra of O 1s core levels for PLD grown AGO films deposited with four different oxygen pressures: (a) 7 × 10-4, (b) 7 × 10-3, (c) 7 × 10-2, and (d) 2 × 10-1 torr.


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Figure. 2 (a) XRD diffraction patterns and (b) rocking curves of PLD grown AGO films deposited with four different oxygen pressures from 7 × 10-4 to 2 × 10-1 torr.



Figure. 3 TEM images of the AGO films prepared with various oxygen pressures of (a) 7 × 10-4, (b) 7 × 10-3, (c) 7 × 10-2, and (d) 2 × 10-1 torr.


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Figure. 4 (a) Optical transmittance spectra and (b) Tauc plots of PLD grown AGO films deposited with four different oxygen pressures from 7 × 10-4 to 2 × 10-1 torr.



Figure. 5 Variations of grain size and growth rate of the PLD grown AGO films as a function of oxygen growth pressure.


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Figure. 6 AFM images of surface morphologies for the PLD grown AGO films deposited with four different oxygen pressures of (a) 7 × 10-4, (b) 7 × 10-3, (c) 7 × 10-2, and (d) 2 × 10-1 torr.



-7

10

Current (A)

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 26 of 29

Light current

-9

10

-11

10

-13

10

Dark current

-15

10

0

5

Oxygen pressure -4 710 torr -3 710 torr

10

Voltage (V)

-2

710 torr -1 210 torr

15

20

Figure. 7 Current-voltage characteristics of the AGO PDs prepared with four different oxygen pressures ranging from 7 × 10-4 to 2 × 10-1 torr. Photocurrent was measured under 240 nm illumination.


Page 27 of 29

0.6

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


AGO MSM photodetector Bias voltage = 5V Oxygen pressure -4 710 torr -3 710 torr -2 710 torr -1 210 torr

0.5 0.4 0.3 0.2 0.1 0.0 200

250

300

350

Wavelength (nm)

Figure. 8 Spectral responses of the AGO PDs prepared with four different oxygen pressures ranging from 7 × 10-4 to 2 × 10-1 torr. Responsivity was measured at 5 V bias voltage.



Figure. 9 (a) Time-dependent responses and (b) decay times and rise times for the AGO PDs prepared with four different oxygen pressures ranging from 7 × 10-4 to 2 × 10-1 torr.


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Enhanced Deep-Ultraviolet Responsivity in AluminumâGallium Oxide

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