Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2022 Oct 14;14(42):47922–47930. doi: 10.1021/acsami.2c14661

Transferable Ga2O3 Membrane for Vertical and Flexible Electronics via One-Step Exfoliation

Yi Lu †, Shibin Krishna †, Che-Hao Liao †, Ziqiang Yang ‡, Mritunjay Kumar †, Zhiyuan Liu †, Xiao Tang †, Na Xiao †, Mohamed Ben Hassine §, Sigurdur T Thoroddsen ‡, Xiaohang Li †,*
PMCID: PMC9614724  PMID: 36241169

Abstract

graphic file with name am2c14661_0006.jpg

Transferable Ga2O3 thin film membrane is desirable for vertical and flexible solar-blind photonics and high-power electronics applications. However, Ga2O3 epitaxially grown on rigid substrates such as sapphire, Si, and SiC hinders its exfoliation due to the strong covalent bond between Ga2O3 and substrates, determining its lateral device configuration and also hardly reaching the ever-increasing demand for wearable and foldable applications. Mica substrate, which has an atomic-level flat surface and high-temperature tolerance, could be a good candidate for the van der Waals (vdW) epitaxy of crystalline Ga2O3 membrane. Beyond that, benefiting from the weak vdW bond between Ga2O3 and mica substrate, in this work, the Ga2O3 membrane is exfoliated and transferred to arbitrary flexible and adhesive tape, allowing for the vertical and flexible electronic configuration. This straightforward exfoliation method is verified to be consistent and reproducible by the transfer and characterization of thick (∼380 nm)/thin (∼95 nm) κ-phase Ga2O3 and conductive n-type β-Ga2O3. Vertical photodetectors are fabricated based on the exfoliated Ga2O3 membrane, denoting the peak response at ∼250 nm. Through the integration of Ti/Au Ohmic contact and Ni/Ag Schottky contact electrode, the vertical photodetector exhibits self-powered photodetection behavior with a responsivity of 17 mA/W under zero bias. The vdW-bond-assisted exfoliation of the Ga2O3 membrane demonstrated here could provide enormous opportunities in the pursuit of vertical and flexible Ga2O3 electronics.

Keywords: Ga2O3 membrane, mica, van der Waals exfoliation, solar-blind photodetector, vertical electronics

1. Introduction

The advances in semiconductor membrane technology offer incredible opportunities and feasibilities to develop unprecedented devices in the fields of optoelectronics and power electronics. For example, flexible, lateral, or vertical configuration devices could be developed based on a single-layer semiconductor membrane.1−3 Moreover, through purposely transferring different types of membranes onto one substrate, i.e., heterogeneous integration, multifunctional devices can be created.4−6 Despite these benefits, the progress of membrane-driven devices is limited by the challenges of acquiring and transferring a monolayer membrane, which is usually strongly bonded with rigid substrate such as sapphire, Si, and SiC. This is due to the grown three-dimensional (3D) membranes typically having high covalent bonding energy (200–6000 meV) with the 3D substrate, preventing their spontaneous or artificial separation.7

To exfoliate the 3D epitaxial membrane from the 3D substrate (so-called 3D/3D exfoliation), methods including laser-induced liftoff8−10 and substrate removal (wet etching11,12 and dry etching13,14) have been used. However, these techniques suffer from either high cost, complexity, or low efficiency. In recent years, two-dimensional (2D) materials such as h-BN or graphene have been employed as a buffer layer on bulk substrates for 3D membrane growth with weak van der Waals (vdW) force.15−17 As such, the weak vdW bonding energy (40–70 meV)7 at the 3D/2D interface allows for the separation of 3D membrane from 2D material by artificial exfoliation (so-called 3D/2D exfoliation). Nonetheless, the precovered 2D materials on bulk substrates introduce further complexities, as well as potential issues such as nonuniformity and high cost.18,19

Muscovite mica (KAl2(Si3Al)O10(OH)2), a 3D substrate with a 2D-like layered framework, could compensate for the limitations of both 3D/3D and 3D/2D exfoliation. Its neighboring layers of the unit cells are weakly bonded by vdW force connections. The unique property of a strong intralayer but weak interlayer provides scope for artificial exfoliation and forms an atomically flat surface.20 Moreover, mica has high flexibility, high transparency, and high thermal stability, which could represent an ideal platform for vdW epitaxial growth of semiconductor membranes requiring a high crystalline temperature.21−23 The weak vdW bond induced by the physical absorption of atoms on the surface can be easily broken; therefore, following the 3D/2D exfoliation strategy, it is plausible that semiconductor membranes could be exfoliated by directly peeling them from the mica substrate using the adhesive tape.

In this work, to verify this straightforward exfoliation method, vdW epitaxy of Ga2O3 membranes are conducted on the mica substrate, followed by vdW-bond-assisted exfoliation to transfer the Ga2O3 membrane from the mica substrate to adhesive tape. Ultrawide band gap Ga2O3 is examined specifically, owing to its superior properties, including large bandgap (≈5 eV), high critical breakdown field (≈ 8 MV/cm), and relatively high electron mobility,24−26 which has great potential in the applications of solar-blind photonics and high-power electronics. Moreover, except for the thermodynamically stable β-phase, Ga2O3 has many other polymorphs, namely α, γ, δ, ε, and κ.27−31 For instance, the metastable κ-phase Ga2O3 could have a high application potential owing to its unique spontaneous polarization property.32 From our previous study, the lower crystalline temperature of κ-phase Ga2O3 compared with β-phase also favors the high-quality Ga2O3 growth on mica substrate.33

In earlier reports, Ga2O3 thin films have been grown on bulk substrates such as sapphire, Si, and SiC.34,35 These bulk substrates determine the lateral configuration of Ga2O3-based devices, such as the metal–semiconductor–metal (MSM) solar-blind photodetector (PD)36−38 and lateral field-effect transistor.39,40 However, they also constrain the merits of Ga2O3 owing to the long transition path of carriers in the lateral device configuration, which is generally a few micrometers to dozens of micrometers based on the photolithography resolution.26,41 Instead, in this work, the proposed vdW-bond-assisted exfoliation provides the feasibility of vertical device configuration in Ga2O3 membrane, allowing for applications such as vertical PD with short transition path of carriers and quick response of the devices. This electrical transition path is determined by the thickness of the Ga2O3 membrane, which could be tens to hundreds of nanometers based on the growth design. Given the high quality of the Ga2O3 membrane, the short electrical path in vertical devices could lead to a fast response. Notably, single-oriented κ-phase Ga2O3 and β-phase n-type Ga2O3 were epitaxially grown on the mica substrate, followed by their exfoliation and transfer to adhesive tape. Cross-sectional SEM and AFM characterization confirm the consistency of the Ga2O3 thickness from the exfoliated boundary depth. The exfoliation method is further verified by exfoliating n-type Ga2O3 and measuring film conductivity. Subsequently, two types of vertical PDs with different metal contacts were demonstrated to display the photoconductive performance of the exfoliated Ga2O3 membrane.

2. Experimental Section

2.1. Material Deposition

The Ga2O3 films were deposited on freshly cleaved and atomically flat mica (001) substrate through pulsed laser deposition (PLD), with a target-substrate distance of approximately 80 mm. During the deposition, the 248 nm KrF excimer laser energy was set at 300 mJ with a 5 Hz frequency. For the κ-Ga2O3 thin film, a Ga2O3:SnO2:SiO2 (98.4%:1.5%:0.1%, wt %) target was ablated by laser under the substrate temperature of 680 °C. The chamber was maintained at a high vacuum (<10–6 Torr). A set of 45 000 and 15 000 laser pulses were performed, producing a ∼ 380 nm and ∼95 nm Ga2O3 thin film on the mica substrate, respectively. The slight growth rate difference was caused by the coating-induced transmittance difference of optical windows which is common for PLD processes. For the conductive n-type β-Ga2O3 film, a Ga2O3:SiO2 (99.9%: 0.01%, wt %) target was ablated with substrate temperature 650 °C under the ambient O2 pressure of 4 mTorr. The film thickness was ∼700 nm with 40 000 laser pulses.

2.2. Membrane Characterization and Exfoliation

The exfoliation and transfer of the Ga2O3 membrane from the mica substrate were accomplished by the adhesive tape. Due to the weak vdW bond between mica and Ga2O3 film and the strong bond of the lateral Ga2O3 connection, the slablike Ga2O3 membrane could be completely transferred to the tape. For the n-type β-Ga2O3 membrane, a resistive Kapton tape was used for exfoliation to avoid the conductivity effect. The details of the exfoliation process are presented in Figure 1. This peeling process was observed using a high-speed video camera (Phantom V2511) operating at 5000 fps and magnification with a Leica long-distance microscope at magnification up to 2.56. A 45° mirror was underneath the transparent glass plate approximately 5 cm, and illumination from above was produced by a 350 W Sumita metal halide lamp and a diffuser.

Figure 1.

Figure 1

Open in a new tab

(a) Process flow of Ga2O3 deposition, membrane exfoliation, and vertical PD-A and PD-B fabrication. (b) Exfoliation process under a high-speed camera.

Before the exfoliation and after the exfoliation, the crystal structure properties of the orthorhombic κ-Ga2O3 and monoclinic β-Ga2O3 on the mica substrate were examined using a Bruker D2 PHASER X-ray diffraction (XRD) system with a Cu tube (λ = 1.54184 Å) source at 30 kV. For the XRD 2θ–ω scan, the exfoliated Ga2O3/tape samples were attached on a flat holder (e.g., sapphire and Si). Atomic force microscopy (AFM; Bruker Dimension Icon scanning probe microscope) was performed in the tapping mode to analyze the surface morphology and exfoliation profile between the substrate and thin film. Cross-sectional profiles of the Ga2O3 membrane were obtained using a Zeiss Merlin Scanning Electron Microscope (SEM) at an acceleration voltage of 5 kV. X-ray photoelectron spectroscopy (XPS) was performed in a high vacuum using a Kratos Amicus XPS system equipped with a monochromatic Al Kα X-ray source operating at 10 kV. The lamella for transmission electron microscopy (TEM) was prepared by focused ion beam (FIB) in the FEI Helios G4 SEM system. The TEM imaging and fast Fourier transforms (FFT) were acquired with the FEI Titan ST microscope system with an acceleration voltage of 300 kV. The images were processed using the Velox software.

2.3. Device Fabrication and Characterization

Two types of vertical Ga2O3 PDs with different electrodes were fabricated based on the exfoliated Ga2O3 membrane obtained in this work. PD-A was developed from the direct exfoliation of the Ga2O3 membrane on conductive Cu tape. For PD-B, before exfoliation, a Ti/Au (20 nm/50 nm) stack was deposited on top of the Ga2O3 as the electrode by reactive sputtering, followed by rapid thermal annealing at 500 °C for 1 min in Ar ambient to form an Ohmic contact between the electrode and Ga2O3 membrane. Subsequently, both samples were etched by an Inductively Coupled Plasma–Reactive Ion Etching (ICP-RIE) system under Ar ambient with RF power 100 W and Ar flow 20 sccm for 30 s to remove the ultrathin residual mica. The samples were then patterned by a physical shallow mask. The patterned electrode had five pairs of interconnected parallel fingers with 30 μm in width, 90 μm in the spacing gap, and 500 μm in length. And the effective exposure area (S) was 0.162 mm2. A Ni/Ag (200 nm/100 nm) electrode was deposited on the film through the electron beam evaporator to form the top Schottky contact for both PD-A and PD-B. Both PD-A and B were fabricated based on the above-mentioned 380 nm κ-phase Ga2O3 membrane. The photoelectrical characteristics were measured using a Zolix DSR600-X150-200-UV automated spectra radiometric measurement system. All the measurements were performed under ambient conditions in an optically tight enclosure to prevent interference from the surrounding light.

3. Results and Discussion

The overall exfoliation process involves the following steps (Figure 1a): (i) epitaxial growth of Ga2O3 layers on the fresh-cleaved mica substrate using PLD (Ga2O3 layers can be κ-phase or β-phase); (ii) coverage of Cu tape and directly exfoliate Ga2O3 (i.e., PD-A), or coating Ti/Au, rapidly annealing it, and then using adhesive tape to exfoliate Ga2O3/Ti/Au (i.e., PD-B); and (iii) metalization and fabrication of vertical PD. Moreover, the mica substrate is reusable for deposition after releasing the Ga2O3 layers. The details of the Ga2O3 deposition and device fabrication are included in the Experimental Section. The exfoliation process is revealed using a high-speed video camera (Figure 1b). Benefiting from the weak vdW bond between Ga2O3 and mica substrate, the Ga2O3 membrane can be transferred by peeling off the adhesive tape. After exfoliation, a clear boundary was observed between the Ga2O3 film and the remaining mica, as indicated by the arrows in Figure 1b. The size of the exfoliated membrane is determined by the tape size, thereby improving the flexibility of the exfoliation process.

The cross-sectional TEM and FFT images of the κ-Ga2O3/mica film are presented in Figure 2a. The κ-Ga2O3 thin film has a distinct interface with the mica substrate from the middle image. From the high-resolution TEM (left image), the mica substrate has a 2D-like layered framework along (001) direction under high-resolution TEM. Ga2O3 (right image) is also single-orientated from FFT, which fits the orthorhombic structure, confirming the κ-phase Ga2O3 growth on mica substrate. No extra spots could be observed except those from the film, indicating that the film is free of the secondary phase. The difference between κ- and β-phase Ga2O3 could be found in our previous reports.33,42 Moreover, compared with the FFT of mica substrate, the right image in Figure 2a shows that the epitaxial relationship is ⟨001⟩ κ-Ga2O3 || ⟨001⟩ mica.

Figure 2.

Figure 2

Open in a new tab

(a) Large-scale cross-sectional TEM image of as-grown Ga2O3/mica (middle). High-resolution TEM images of mica substrate (left) and epitaxial κ-Ga2O3 membrane (right). Insets are the FFT images of mica and κ-Ga2O3. (b) Boundary and energy dispersive X-ray analysis (EDX) distribution (Ga/Al) between Ga2O3 and mica substrate after exfoliation, showing the slablike Ga2O3 grown on mica. (c) Exfoliated Ga2O3 membrane transferred to Cu tape and the EDX distribution (Ga/C). The thicknesses of the Ga2O3 membrane in (b) and (c) are both ≈380 nm.

Figure 2b,c represent the SEM images of the Ga2O3 membrane when grown on the mica substrate and transferred to a Cu tape, respectively. In Figure 2b, the slablike Ga2O3 membrane, which has a weak vdW bond with mica, could be easily separated from the mica substrate, indicating the feasibility of exfoliation. After the exfoliation, a clear boundary between the unexfoliated Ga2O3 and mica could be observed, which is confirmed by the EDX distribution of gallium (Ga) and aluminum (Al) in Ga2O3 and mica, respectively. After being exfoliated and transferred to the Cu tape, the Ga2O3 membrane maintains its slablike structure, and the membrane thickness is similar to the as-grown membrane (≈380 nm). This exfoliation and transfer process is also confirmed by the EDX distribution of gallium (Ga) and carbon (C) in Ga2O3 and tape, respectively. The SEM images of exfoliated Ga2O3/tape under different magnifications are also shown in Figure S1a,b.

Figure 3a shows the XRD 2θ–ω scan of the κ-Ga2O3 membrane before and after the exfoliation. Before exfoliation (as-grown κ-Ga2O3/mica), clear diffraction peaks are observed at 19.15, 38.80, and 59.75°, belonging to κ-Ga2O3 (002), (004), and (006), respectively, confirming that single-oriented Ga2O3 is grown on the mica substrate. The remaining diffraction peaks are attributed to the mica substrate (001), (002), and higher-order diffractions. The epitaxial relationship ⟨001⟩ κ-Ga2O3 || ⟨001⟩ mica, is also consistent with the FFT images in Figure 2. After the exfoliation of the Ga2O3 membrane, i.e., transfer to Cu tape, the tape is attached to a holder (e.g., sapphire in Figure 3a) for the 2θ–ω scan. In Figure 3a, only Ga2O3 diffraction peaks located at the same positions are observed except for the sapphire holder and Cu tape peaks, indicating the successful transfer of Ga2O3 from the mica substrate.

Figure 3.

Figure 3

Open in a new tab

(a) XRD 2θ–ω scan of the as-grown κ-Ga2O3/mica and exfoliated κ-Ga2O3/Cu tape. (b) AFM profile of the boundary between the exfoliated and unexfoliated part in a 10 μm × 10 μm range. (c) Depth profile of the boundary extracted from (b). (d) SEM surface of the boundary between the exfoliated and unexfoliated part and its (e) EDX distribution of Ga, O, Al, and K. (f) XPS spectra of mica substrate (black curve), exfoliated κ-Ga2O3 surface (red curve), and exfoliated κ-Ga2O3 with dry etching (blue curve).

AFM was employed to verify the surface roughness and exfoliation depth. The surface morphology and roughness of mica substrate and as-grown κ-Ga2O3/mica are shown in Figure S2. From the microscope image under the AFM tip in Figure 3b, after exfoliation, there is a clear contrast difference separated by one boundary between the as-grown κ-Ga2O3/mica and exfoliated part. Subsequently, AFM was performed in a 10 μm × 10 μm region near the boundary to assess the exfoliation depth. In Figure 3b, an obvious step with a depth of ∼380 nm was observed, and the depth profile of the boundary shows a similar thickness to Ga2O3 epitaxial film in Figure 3c. This exfoliation boundary is further confirmed by the top-view SEM image shown in Figure 3d. The clear exfoliation boundary between the unexfoliated Ga2O3 and mica could be observed after exfoliation. The EDX distributions which feature Ga in Ga2O3 and Al/K in mica, respectively, are shown in Figure 3e.

vdW-bond-assisted exfoliation usually results in only a small amount of residual material after peeling off.43 This residual material is ultrathin, which cannot be viewed from the XRD diffraction pattern. To assess the residual mica status in our exfoliated Ga2O3 membrane, XPS was performed on the mica substrate, Ga2O3 membrane after exfoliation, and exfoliated Ga2O3 membrane with dry etching. The whole XPS spectra of mica substrate, exfoliated Ga2O3 membrane, and exfoliated Ga2O3 membrane with dry etching are shown in Figure S3. Mica substrate (KAl2(Si3Al)O10(OH)2) features K 2p and Al 2p core levels (black curve), while pure Ga2O3 has Ga 2p and Ga 3d as the material evidence. The C 1s peak at 284.8 eV is used to calibrate the correct positions of the XPS signals.

In Figure 3f, the exfoliated κ-Ga2O3 membrane exhibits both mica (Al 2p and K 2p) and Ga2O3 (Ga 2p and Ga 3d) signals, indicating the small amount of residual mica on the surface of exfoliated Ga2O3 membrane (red curve). Considering the excitation limitation in the XPS system, the signal from only a few nanometers depth of film could be detected. Since the Ga2O3 signals were observed even penetrating through residual mica from the XPS spectra, the residual mica is estimated to be ultrathin (approximately ≤5 nm). This is also consistent with the invisibility of mica signals from the XRD pattern after exfoliation, since the weak diffraction signal from ultrathin mica would be automatically eliminated by the system. To remove the potential ultrathin residual mica, a dry etching process as described in the Experimental Section was performed on the exfoliated κ-Ga2O3 membrane. The XPS spectrum after this process is shown in Figure 3f (blue curve). The signals of Al 2p and K 2p from the residual mica are removed after dry etching, and the Ga 2p and Ga 3d peaks have higher intensity due to more excitation of Ga2O3 after mica removal. This membrane exfoliation and subsequent dry etching ensure the pure-Ga2O3 transfer from mica to the adhesive tape. To check the repeatability of the proposed exfoliation method, another thin Ga2O3 film (∼95 nm) was deposited by PLD on the mica substrate for exfoliation. Figure S4 shows the XRD pattern and AFM boundary profile after exfoliation, revealing the consistency of the proposed exfoliation method.

To further verify the consistency of the proposed exfoliation method with the subsequent dry etching, n-type β-phase Ga2O3 was deposited on the mica substrate for exfoliation. Resistive Kapton tape as shown in Figure 4c (dark yellow color), was used to exfoliate the conductive n-Ga2O3. As-grown n-Ga2O3 shows a single-oriented β-phase with diffraction peaks (2̅01), (4̅02), and (6̅03), respectively, on the mica substrate. With the Kapton tape exfoliation, the β-Ga2O3 membrane is transferred to the adhesive tape while the mica signal is absent, as shown in Figure 4a. The XRD pattern difference between the orthorhombic κ-Ga2O3 and monoclinic β-Ga2O3 is shown in Figure 4b.33 κ-Ga2O3(002), (004), and (006) have shifts of around +0.26, +0.52, and +0.79° relative to β-Ga2O3(2̅01), (4̅02), and (6̅03), respectively.

Figure 4.

Figure 4

Open in a new tab

(a) XRD patterns of n-type β-Ga2O3 grown on mica and exfoliated n-type β-Ga2O3 membrane. (b) Comparison of XRD patterns of β-Ga2O3 and κ-Ga2O3 grown on a mica substrate; the mica (003) peak at ∼36° was used for the peak alignment. (c) Pictures of as-grown n-type Ga2O3/mica and exfoliated n-Ga2O3/Kapton tape with interdigital patterns. (d) Linear and log scale IV curves of as-grown n-type Ga2O3/mica, exfoliated n-Ga2O3/Kapton tape, and exfoliated n-Ga2O3/Kapton tape with dry etching.

Meanwhile, another exfoliated n-Ga2O3 membrane was etched in the ICP and RIE system, with the ambient described in the Experimental Section, to remove the ultrathin residual mica. Subsequently, three samples containing the as-grown n-Ga2O3/mica, exfoliated n-Ga2O3/Kapton tape, and exfoliated n-Ga2O3/Kapton+etching, were patterned and deposited with Ti/Au (20 nm/200 nm) as the electrode simultaneously by the reactive sputtering system. Figure 4c shows the interdigital pattern of the electrode during measurement under the probe station. For the IV curve of the as-grown n-Ga2O3 in Figure 4d, a resistance of ≈300 Ω from −5 to 5 V is shown. For the exfoliated n-Ga2O3, the current is in the pA range from −5 to 5 V, denoting the existence of residual mica that blocks the carrier transport.44,45 However, with dry etching, the residual mica is eliminated, and the conductivity is similar to the as-grown n-Ga2O3 in Figure 4d. We observed slightly higher resistance and Schottky behavior of exfoliated n-Ga2O3+etching sample compared with the as-grown n-Ga2O3/mica. This phenomenon could be attributed to (i) material quality difference between the top n-Ga2O3 layer and exfoliated bottom n-Ga2O3, which may contain polycrystal interfacial layer,46,47 and (ii) plasma damages after Ar dry etching.48,49

The vdW-bond-assisted exfoliation demonstrated in this work could be employed to fabricate vertical configuration Ga2O3 devices, including PDs, Schottky barrier diodes (SBD), PN diode, and bottom-gate transistors. Here, we fabricate two vertical PDs with different metal electrodes based on the 380 nm κ-Ga2O3 membrane described in Figures 2 and 3. PD-A uses Cu (from Cu tape) as the bottom electrode. The top electrode is Ni/Ag deposited with an interdigital finger pattern (Figure 5a). Both top and bottom electrodes form the Schottky contact with the Ga2O3 membrane. From the photoresponse spectra of this PD-A in Figure 5b, a peak response is observed at ∼250 nm, corresponding to the bandgap of Ga2O3 (Figure S5). For the log scale of the spectra shown in the Figure 5b inset, a long tail can be observed after the peak response, which could be attributed to the persistent photoconductivity (PPC) effect that commonly exists in Ga2O3 PDs.35,50,51 In Figure 5c, due to the high Schottky barrier height from metal/semiconductor contact, this PD has a weak photoresponse from 1 to 7.5 V, while the photocurrent increases abruptly after bias reaching 10 V. The responsivity is ∼11 A/W under 250 nm illumination at 15 V. This responsivity is calculated by the following equation:

3. 1

where Iphoto, Idark, D, and S are the photocurrent, dark current, illumination power density, and exposure area, respectively. The power density of 250 nm deep UV light is about 21.8 μW/cm2, and the exposure area S is 0.162 mm2.

Figure 5.

Figure 5

Open in a new tab

(a) Fabrication flow of vertical PD-A. (b) Photoresponse spectra of vertical PD-A under different biases (inset is the log scale of the spectra). (c) Responsivity of vertical PD-A under different biases. (d) Time-dependent response of PD-A under 15 V bias and 250 nm illumination condition. (e) Fabrication flow of Ti/Au back-coated vertical PD-B. (f) IV curves (log scale and linear scale) of the self-powered vertical PD-B under −10 to 10 V. (h) Photoresponse spectra of self-powered PD under 0 V bias (inset is the log scale of the spectra). (i) Responsivity under reverse bias from 0 to −5 V. (j) Time-dependent response of self-powered PD under 0 V bias and 250 nm illumination condition. The details of the PD fabrication process of (a) and (e) are shown in the Experimental Section.

Figure 5d shows the time-dependent photoresponse of the PD-A under 15 V bias with 250 nm incident light opening or closing in a single on–off cycle. To extract the response (rise) and recovery (decay) time, the below exponential relaxation equation is used for the fitting:

3. 2

where I is the photocurrent, I0 is the steady-state current, t is the time, and τ represents the response time.52 Based on the fitting, the response edge and recovery edge have response times of 1.79 and 3.30 s, respectively. This slow response is related to the carrier trapping/releasing processes associated with defects in the Ga2O3 film, which is also consistent with the pronounced PPC effect in the device. Moreover, the Schottky barrier on both anode and cathode would increase the carrier transition time through the Schottky barrier.

To boost the PD performance, a Ti/Au metal stack which can form an Ohmic contact with Ga2O3 was employed to replace the bottom Cu electrode (Figure 5e), while the top metal was maintained as a Schottky contact (Ni/Ag), forming a vertical Schottky barrier PD. This process also denotes the capability of exfoliating Ga2O3 membrane together with a metal electrode (i.e., Ti/Au in this work), leading to a great flexibility of device architecture design. Hence, as shown in Figure 5f, this vertical PD-B with a top Schottky contact and bottom Ohmic contact, exhibits apparent unilateral conductivity from −10 to 10 V. This phenomenon may be explained by the band diagrams in Figure S6a. Under forward bias, the electric field could push the intrinsic carriers moving toward the electrode after crossing the Schottky barrier. While under reverse bias, this high Schottky barrier on the Ni/Ga2O3 interface could block the electrons transporting from Ni to Ga2O3, leading to a low leakage current.

Benefiting from this Schottky behavior, the fabricated Schottky barrier PD is self-powered, as shown in Figure 5h. The peak response appears with a responsivity of 0.017 A/W under 250 nm illumination with 0 V bias. This is attributed to the fact that the photogenerated electrons and holes in Ga2O3 could drift toward the electrode caused by the built-in electrical field near the Schottky barrier even without applying bias. The sudden bump at ∼580 nm could be attributed to the system noise at such a low current. Adding external bias during illumination would improve the photocurrent due to the enhanced electrical field. Hence, the responsivity would increase with the enlarged bias as shown in Figure 5i. Under −5 V bias, the responsivity could reach 36.1 A/W under 250 nm illumination, which is much higher than PD-A under 5 V. Figure 5j represents the time-dependent photoresponse of the self-powered PD under 0 V bias with 250 nm incident light opening or closing in a single on–off cycle. The time-dependent response in 400 s with an on/off interval of 20 s is shown in Figure S6b. The distinguished photoswitching response curve, suggests the reproducible and stable photoresponse of the demonstrated PD. From the fitting, the response edge and recovery edge have response times of 1.02 and 1.36 s, respectively. The improved response time compared with PD-A could be attributed to the shorter transition time through the Ohmic contact, and the less severe PPC effect under 0 V bias.53 This self-powered photoconductance and short response time in the solar-blind region could have great potential in applications in the military and civilian fields.

Notably, this vdW-bond-assisted exfoliation method could be extended to all vertically configured electronics. For example, by growing a n–-Ga2O3/n+-Ga2O3 stack on a mica substrate and exfoliating it with the metallization, the vertically configured SBD could be demonstrated, getting rid of the indispensably expensive n+-Ga2O3 substrate in Ga2O3 SBD.39,54 The heat dissipation in these high power devices could also be easily addressed by adopting metal tape or foreign substrates with high thermal conductivity. Moreover, vertical PN junctions such as n-Ga2O3/p-NiO could be also acquired after the predeposition and exfoliation from the mica substrate, which may be applied to the PN junction diode and PN junction PD. After transferring semiconductor membrane to flexible and cheap tape via the proposed one-step exfoliation method, the fabricated devices could have an enormous impact on future vertical optoelectronics and power electronics, especially in wearable and foldable applications.

Conclusion

In conclusion, in this work, a novel and straightforward one-step method to exfoliate and transfer Ga2O3 membrane is demonstrated along with the vertically configured self-powered PD for UV light detection. This exfoliation process is accomplished by intentionally breaking the weak vdW bond between the Ga2O3 membrane and mica substrate with adhesive tape. First, the exfoliation consistency was examined using single-oriented κ-Ga2O3 and n-type Ga2O3 epitaxially grown on mica under high temperature. For the κ-Ga2O3 membranes, two different thicknesses (380 and 95 nm) were successfully exfoliated and transferred to tape, confirmed by the cross-sectional SEM and exfoliation boundary profile. For n-type Ga2O3, the exfoliated membrane maintained a similar conductivity to the as-grown Ga2O3 after drily etching the residual ultrathin mica. To reveal the Ga2O3 membrane properties, two vertical UV PDs with different bottom electrodes were demonstrated. PD-A which has Cu as the bottom electrode exhibits a responsivity of ∼11 A/W under 250 nm illumination at 15 V. PD-B instead has a bottom Ti/Au Ohmic contact and behaves as a self-powered PD whose responsivity can reach 17 mA/W with 250 nm illumination under 0 V bias.

Overall, the proposed technique which exfoliates Ga2O3 from mica substrate has tremendous benefits. For example, the high-temperature tolerance and atomic-level flat surface properties of mica substrate provide a superior scope of high-quality Ga2O3 epitaxy. The exfoliation strategy is simple and feasible and does not need additional procedures or equipment. In addition to that, the primal mica substrate could be recycled for further deposition after exfoliation. Moreover, with the transfer of flexible and cheap tape, the Ga2O3 membrane and its-related devices could be applied in variable environments, including wearable and foldable applications.

Acknowledgments

The authors would like to acknowledge the support of KAUST Baseline BAS/1/1664-01-01, KAUST Competitive Research Grant URF/1/3437-01-01, URF/1/3771-01-01, and GCC Research Council REP/1/3189-01-01. The authors would also acknowledge the support from Dr. Peng Zhang in Interfacial Lab, KAUST for providing the mica substrate.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.2c14661.

  • SEM image of the exfoliated membrane and exfoliation boundary; surface morphology of mica and grown Ga2O3; XPS spectra of wide scan; 95 nm Ga2O3 exfoliation; UV–vis results; band diagram of vertical PD and time-dependent response (PDF)

Author Contributions

Y.L. conceived the idea, fabricated the device, performed the tests, and analyzed the data. S.K. helped with the PLD growth. C.-H.L. helped with the SEM imaging and analysis. Z.Y. performed the high-speed camera imaging. M.K. ran the TCAD simulation for the band diagram. Z.L. helped with the data analysis and dry etching. X.T. helped with the PD measurement and analysis. N.X. performed the AFM measurement. M.B.H. performed the TEM measurement. S.T.T. and X.L. supervised the research and corrected the paper. All authors participated in the discussion and interpretation of the results and commented on the manuscript.

The authors declare no competing financial interest.

Supplementary Material

am2c14661_si_001.pdf (2MB, pdf)

References

  1. Shahrjerdi D.; Bedell S. W.; Bayram C.; Lubguban C. C.; Fogel K.; Lauro P.; Ott J. A.; Hopstaken M.; Gayness M.; Sadana D. Ultralight High–Efficiency Flexible InGaP/(In) GaAs Tandem Solar Cells on Plastic. Adv. Energy Mater. 2013, 3 (5), 566–571. 10.1002/aenm.201200827. [DOI] [Google Scholar]
  2. Kang J.-H.; Jeong D. K.; Ryu S.-W. Transparent, Flexible Piezoelectric Nanogenerator Based on GaN Membrane Using Electrochemical Lift-off. ACS Appl. Mater. Interfaces 2017, 9 (12), 10637–10642. 10.1021/acsami.6b15587. [DOI] [PubMed] [Google Scholar]
  3. Peng M.; Liu Y.; Yu A.; Zhang Y.; Liu C.; Liu J.; Wu W.; Zhang K.; Shi X.; Kou J.; et al. Flexible Self-powered GaN Ultraviolet Photoswitch with Piezo-phototronic Effect Enhanced on/off Ratio. ACS Nano 2016, 10 (1), 1572–1579. 10.1021/acsnano.5b07217. [DOI] [PubMed] [Google Scholar]
  4. Chun J.; Lee K. J.; Leem Y.-C.; Kang W.-M.; Jeong T.; Baek J. H.; Lee H. J.; Kim B.-J.; Park S.-J. Vertically Stacked Color Tunable Light-emitting Diodes Fabricated Using Wafer Bonding and Transfer Printing. ACS Appl. Mater. Interfaces 2014, 6 (22), 19482–19487. 10.1021/am505415q. [DOI] [PubMed] [Google Scholar]
  5. Kum H.; Lee D.; Kong W.; Kim H.; Park Y.; Kim Y.; Baek Y.; Bae S.-H.; Lee K.; Kim J. Epitaxial Growth and Layer-transfer Techniques for Heterogeneous Integration of Materials for Electronic and Photonic Devices. Nat. Electron 2019, 2 (10), 439–450. 10.1038/s41928-019-0314-2. [DOI] [Google Scholar]
  6. Kum H. S.; Lee H.; Kim S.; Lindemann S.; Kong W.; Qiao K.; Chen P.; Irwin J.; Lee J. H.; Xie S.; et al. Heterogeneous Integration of Single-crystalline Complex-oxide Membranes. Nature 2020, 578 (7793), 75–81. 10.1038/s41586-020-1939-z. [DOI] [PubMed] [Google Scholar]
  7. Butler S. Z.; Hollen S. M.; Cao L.; Cui Y.; Gupta J. A.; Gutiérrez H. R.; Heinz T. F.; Hong S. S.; Huang J.; Ismach A. F.; et al. Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene. ACS Nano 2013, 7 (4), 2898–2926. 10.1021/nn400280c. [DOI] [PubMed] [Google Scholar]
  8. Liu W.; Turkani V. S.; Akhavan V.; Korgel B. A. Photonic Lift-off Process to Fabricate Ultrathin Flexible Solar Cells. ACS Appl. Mater. Interfaces 2021, 13 (37), 44549–44555. 10.1021/acsami.1c12382. [DOI] [PubMed] [Google Scholar]
  9. Zhu C.; Guo D.; Ye D.; Jiang S.; Huang Y. Flexible PZT-integrated, Bilateral Sensors via Transfer-Free Laser Lift-off for Multimodal Measurements. ACS Appl. Mater. Interfaces 2020, 12 (33), 37354–37362. 10.1021/acsami.0c10083. [DOI] [PubMed] [Google Scholar]
  10. Tao Y.; Deng C.; Long J.; Liu J.; Wang X.; Song X.; Lu C.; Yang J.; Hao H.; Wang C. Multiprocess Laser Lifting–Off for Nanostructured Semiconductive Hydrogels. Adv. Mater. Interfaces 2022, 9 (1), 2101250. 10.1002/admi.202101250. [DOI] [Google Scholar]
  11. Youtsey C.; McCarthy R.; Reddy R.; Forghani K.; Xie A.; Beam E.; Wang J.; Fay P.; Ciarkowski T.; Carlson E.; et al. Wafer-scale Epitaxial Lift–off of GaN Using Bandgap-Selective Photoenhanced Wet Etching. Phys. Status Solidi B 2017, 254 (8), 1600774. 10.1002/pssb.201600774. [DOI] [Google Scholar]
  12. Fay P.; Wang J.; Cao L.; Xie J.; Beam E.; McCarthy R.; Reddy R.; Youtsey C. Epitaxial Lift-Off of GaN and Related Materials for Device Applications. ECS Trans 2019, 92 (4), 97. 10.1149/09204.0097ecst. [DOI] [Google Scholar]
  13. Egawa T.; Shuhaimi B. A. B. A. High Performance InGaN LEDs on Si (1 1 1) Substrates Grown by MOCVD. J. Phys. D: Appl. Phys. 2010, 43 (35), 354008. 10.1088/0022-3727/43/35/354008. [DOI] [Google Scholar]
  14. Tang X.; Ma Z.; Han L.; Deng Z.; Jiang Y.; Wang W.; Chen H.; Du C.; Jia H. Stripping GaN/InGaN Epitaxial Films and Fabricating Vertical GaN-based Light-emitting Diodes. Vacuum 2021, 187, 110160. 10.1016/j.vacuum.2021.110160. [DOI] [Google Scholar]
  15. Kim Y.; Cruz S. S.; Lee K.; Alawode B. O.; Choi C.; Song Y.; Johnson J. M.; Heidelberger C.; Kong W.; Choi S.; et al. Remote Epitaxy Through Graphene Enables Two-dimensional Material-based Layer Transfer. Nature 2017, 544 (7650), 340–343. 10.1038/nature22053. [DOI] [PubMed] [Google Scholar]
  16. Chung K.; Lee C.-H.; Yi G.-C. Transferable GaN layers Grown on ZnO-coated Graphene Layers for Optoelectronic Devices. Science 2010, 330 (6004), 655–657. 10.1126/science.1195403. [DOI] [PubMed] [Google Scholar]
  17. Min J.-H.; Li K.-H.; Kim Y.-H.; Min J.-W.; Kang C. H.; Kim K.-H.; Lee J.-S.; Lee K. J.; Jeong S.-M.; Lee D.-S.; et al. Toward Large-Scale Ga2O3Membranes via Quasi-Van Der Waals Epitaxy on Epitaxial Graphene Layers. ACS Appl. Mater. Interfaces 2021, 13 (11), 13410–13418. 10.1021/acsami.1c01042. [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Jia Y.; Ning J.; Zhang J.; Yan C.; Wang B.; Zhang Y.; Zhu J.; Shen X.; Dong J.; Wang D.; et al. Transferable GaN Enabled by Selective Nucleation of AlN on Graphene for High-brightness Violet Light-Emitting Diodes. Adv. Opt. Mater. 2020, 8 (2), 1901632. 10.1002/adom.201901632. [DOI] [Google Scholar]
  19. Yi M.; Shen Z. A Review on Mechanical Exfoliation for the Scalable Production of Graphene. J. Mater. Chem. A 2015, 3 (22), 11700–11715. 10.1039/C5TA00252D. [DOI] [Google Scholar]
  20. Zhang Z.; Niu J.; Yang P.; Gong Y.; Ji Q.; Shi J.; Fang Q.; Jiang S.; Li H.; Zhou X.; et al. Van der Waals Epitaxial Growth of 2D Metallic Vanadium Diselenide Single Crystals and Their Extra-High Electrical Conductivity. Adv. Mater. 2017, 29 (37), 1702359. 10.1002/adma.201702359. [DOI] [PubMed] [Google Scholar]
  21. Zhong G.; Li J. Muscovite Mica as a Universal Platform for Flexible Electronics. J. Materiomics 2020, 6 (2), 455–457. 10.1016/j.jmat.2019.12.004. [DOI] [Google Scholar]
  22. Bitla Y.; Chu Y.-H. MICAtronics: A New Platform for Flexible X-tronics. FlatChem. 2017, 3, 26–42. 10.1016/j.flatc.2017.06.003. [DOI] [Google Scholar]
  23. Yen M.; Bitla Y.; Chu Y.-H. van der Waals Heteroepitaxy on Muscovite. Mater. Chem. Phys. 2019, 234, 185–195. 10.1016/j.matchemphys.2019.05.053. [DOI] [Google Scholar]
  24. Lu Y.; Yao H.-H.; Li J.; Yan J.; Wang J.; Li J.; Li X.. AlN/beta-Ga2O3 Based HEMT: a Potential Pathway to Ultimate High Power Device. arXiv (Applied Physics), Jan 16, 2019, arXiv:1901.05111, ver. 1. https://doi.org/10.48550/arXiv.1901.05111.
  25. Pearton S.; Yang J.; Cary IV P. H.; Ren F.; Kim J.; Tadjer M. J.; Mastro M. A. A Review of Ga2O3Materials, Processing, and Devices. Appl. Phys. Rev. 2018, 5 (1), 011301. 10.1063/1.5006941. [DOI] [Google Scholar]
  26. Tang X.; Li K.-H.; Zhao Y.; Sui Y.; Liang H.; Liu Z.; Liao C.-H.; Babatain W.; Lin R.; Wang C.; et al. Quasi-Epitaxial Growth of β-Ga2O3-Coated Wide Band Gap Semiconductor Tape for Flexible UV Photodetectors. ACS Appl. Mater. Interfaces 2022, 14 (1), 1304–1314. 10.1021/acsami.1c15560. [DOI] [PubMed] [Google Scholar]
  27. Von Wenckstern H. Group–III Sesquioxides: Growth, Physical Properties and Devices. Adv. Electron. Mater. 2017, 3 (9), 1600350. 10.1002/aelm.201600350. [DOI] [Google Scholar]
  28. Playford H. Y.; Hannon A. C.; Barney E. R.; Walton R. I. Structures of Uncharacterised Polymorphs of Gallium Oxide from Total Neutron Diffraction. Chem.: Eur. J. 2013, 19 (8), 2803–2813. 10.1002/chem.201203359. [DOI] [PubMed] [Google Scholar]
  29. Kroll P.; Dronskowski R.; Martin M. Formation of Spinel-type Gallium Oxynitrides: a Density-functional Study of Binary and Ternary Phases in the System Ga–O–N. J. Mater. Chem. 2005, 15 (32), 3296–3302. 10.1039/b506687e. [DOI] [Google Scholar]
  30. He H.; Blanco M. A.; Pandey R. Electronic and Thermodynamic Properties of β-Ga2O3. Appl. Phys. Lett. 2006, 88 (26), 261904. 10.1063/1.2218046. [DOI] [Google Scholar]
  31. Stepanov S.; Nikolaev V.; Bougrov V.; Romanov A. Gallium Oxide: Properties and Applicaion-a Review. Rev. Adv. Mater. Sci. 2016, 44, 63–86. [Google Scholar]
  32. Mezzadri F.; Calestani G.; Boschi F.; Delmonte D.; Bosi M.; Fornari R. Crystal structure and Ferroelectric Properties of ε-Ga2O3 Films Grown on (0001)-Sapphire. Inorg. Chem. 2016, 55 (22), 12079–12084. 10.1021/acs.inorgchem.6b02244. [DOI] [PubMed] [Google Scholar]
  33. Lu Y.; Krishna S.; Tang X.; Babatain W.; Ben Hassine M.; Liao C.-H.; Xiao N.; Liu Z.; Li X. Ultrasensitive Flexible κ-Phase Ga2O3 Solar-Blind Photodetector. ACS Appl. Mater. Interfaces 2022, 14 (30), 34844–34854. 10.1021/acsami.2c06550. [DOI] [PMC free article] [PubMed] [Google Scholar]
  34. Xu C.; Liu H.; Pan X.; Ye Z. Growth and Characterization of Si-doped β-Ga2O3 Films by Pulsed Laser Deposition. Opt. Mater. 2020, 108, 110145. 10.1016/j.optmat.2020.110145. [DOI] [Google Scholar]
  35. Hao S.; Hetzl M.; Schuster F.; Danielewicz K.; Bergmaier A.; Dollinger G.; Sai Q.; Xia C.; Hoffmann T.; Wiesinger M.; et al. Growth and Characterization of β-Ga2O3 Thin Films on Different Substrates. J. Appl. Phys. 2019, 125 (10), 105701. 10.1063/1.5061794. [DOI] [Google Scholar]
  36. Bae J.; Jeon D.-W.; Park J.-H.; Kim J. High Responsivity Solar-blind Metal-semiconductor-metal Photodetector Based on α-Ga2O3. J. Vac. Sci. Technol., A 2021, 39 (3), 033410. 10.1116/6.0000940. [DOI] [Google Scholar]
  37. Tak B.; Garg M.; Kumar A.; Gupta V.; Singh R. Gamma Irradiation Effect on Performance of β-Ga2O3Metal-semiconductor-metal Solar-blind Photodetectors for Space Applications. Ecs J. Solid State Sc 2019, 8 (7), Q3149. 10.1149/2.0291907jss. [DOI] [Google Scholar]
  38. Chen Y.; Lu Y.; Liao M.; Tian Y.; Liu Q.; Gao C.; Yang X.; Shan C. 3D Solar–blind Ga2O3 Photodetector Array Realized via Origami Method. Adv. Funct. Mater. 2019, 29 (50), 1906040. 10.1002/adfm.201906040. [DOI] [Google Scholar]
  39. Xue H.; He Q.; Jian G.; Long S.; Pang T.; Liu M. An Overview of the Ultrawide Bandgap Ga2O3 Semiconductor-based Schottky Barrier Diode for Power Electronics Application. Nanoscale Res. Lett. 2018, 13 (1), 290. 10.1186/s11671-018-2712-1. [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Razeghi M.; Park J.-H.; McClintock R.; Pavlidis D.; Teherani F. H.; Rogers D. J.; Magill B. A.; Khodaparast G. A.; Xu Y.; Wu J. A Review of the Growth, Doping, and Applications of Beta-Ga2O3 Thin Films. Proc. SPIE 2018, 10533, 105330R. 10.1117/12.2302471. [DOI] [Google Scholar]
  41. Chabak K. D.; Leedy K. D.; Green A. J.; Mou S.; Neal A. T.; Asel T.; Heller E. R.; Hendricks N. S.; Liddy K.; Crespo A.; et al. Lateral β-Ga2O3 Field Effect Transistors. Semicond. Sci. Technol. 2020, 35 (1), 013002. 10.1088/1361-6641/ab55fe. [DOI] [Google Scholar]
  42. Krishna S.; Lu Y.; Liao C.-H.; Khandelwal V.; Li X. Band Alignment of Orthorhombic Ga2O3 with GaN and AlN Semiconductors. Appl. Surf. Sci. 2022, 599, 153901. 10.1016/j.apsusc.2022.153901. [DOI] [Google Scholar]
  43. Min J.-H.; Li K.-H.; Kim Y.-H.; Min J.-W.; Kang C. H.; Kim K.-H.; Lee J.-S.; Lee K. J.; Jeong S.-M.; Lee D.-S.; et al. Toward Large-Scale Ga2O3Membranes via Quasi-Van Der Waals Epitaxy on Epitaxial Graphene Layers. ACS Appl. Mater. Interfaces 2021, 13 (11), 13410–13418. 10.1021/acsami.1c01042. [DOI] [PMC free article] [PubMed] [Google Scholar]
  44. Matsumoto A.; Onoki R.; Ikeda S.; Saiki K.; Ueno K. Effect of Organic Buffer Layer on Performance of Pentacene Field-effect Transistor Fabricated on Natural Mica Gate Dielectric. Jpn. J. Appl. Phys. 2007, 46 (10L), L913. 10.1143/JJAP.46.L913. [DOI] [Google Scholar]
  45. Zou X.; Xu J.; Liu L.; Wang H.; Tang W. M. Long-term Stability of Multilayer MoS2 Transistors with Mica Gate Dielectric. Nanotechnology 2020, 31 (18), 185202. 10.1088/1361-6528/ab6ab2. [DOI] [PubMed] [Google Scholar]
  46. Arata Y.; Nishinaka H.; Tahara D.; Yoshimoto M. van der Waals Epitaxy of Ferroelectric ε-Gallium Oxide Thin Film on Flexible Synthetic Mica. Jpn. J. Appl. Phys. 2020, 59 (2), 025503. 10.35848/1347-4065/ab6b70. [DOI] [Google Scholar]
  47. Tak B. R.; Yang M.-M.; Lai Y.-H.; Chu Y.-H.; Alexe M.; Singh R. Photovoltaic and Flexible Deep Ultraviolet Wavelength Detector based on Novel β-Ga2O3/muscovite Heteroepitaxy. Sci. Rep. 2020, 10 (1), 16098. 10.1038/s41598-020-73112-1. [DOI] [PMC free article] [PubMed] [Google Scholar]
  48. Polyakov A.; Lee I.-H.; Smirnov N.; Yakimov E.; Shchemerov I.; Chernykh A.; Kochkova A.; Vasilev A.; Carey P.; Ren F.; et al. Defects at the Surface of β-Ga2O3 Produced by Ar Plasma Exposure. APL Mater. 2019, 7 (6), 061102. 10.1063/1.5109025. [DOI] [Google Scholar]
  49. Yang J.; Ahn S.; Ren F.; Khanna R.; Bevlin K.; Geerpuram D.; Pearton S.; Kuramata A. Inductively Coupled Plasma Etch Damage in (−201) Ga2O3 Schottky Diodes. Appl. Phys. Lett. 2017, 110 (14), 142101. 10.1063/1.4979592. [DOI] [Google Scholar]
  50. Zhou H.; Cong L.; Ma J.; Li B.; Xu H.; Liu Y. Suppression of Persistent Photoconductivity in High Gain Ga2O3 Schottky Photodetectors. Chin. Phys. B 2021, 30 (12), 126104. 10.1088/1674-1056/ac2d1b. [DOI] [Google Scholar]
  51. Ahn J.; Ma J.; Lee D.; Lin Q.; Park Y.; Lee O.; Sim S.; Lee K.; Yoo G.; Heo J. Ultrahigh Deep-ultraviolet Responsivity of a β-Ga2O3/MgO Heterostructure-based Phototransistor. ACS Photonics 2021, 8 (2), 557–566. 10.1021/acsphotonics.0c01579. [DOI] [Google Scholar]
  52. Guo D.; Wu Z.; An Y.; Guo X.; Chu X.; Sun C.; Li L.; Li P.; Tang W. Oxygen Vacancy Tuned Ohmic-Schottky Conversion for Enhanced Performance in β-Ga2O3 Solar-blind Ultraviolet Photodetectors. Appl. Phys. Lett. 2014, 105 (2), 023507. 10.1063/1.4890524. [DOI] [Google Scholar]
  53. Guo D.; Wu Z.; Li P.; An Y.; Liu H.; Guo X.; Yan H.; Wang G.; Sun C.; Li L.; et al. Fabrication of β-Ga 2 O 3 Thin Films and Solar-blind Photodetectors by Laser MBE Technology. Opt. Mater. Express 2014, 4 (5), 1067–1076. 10.1364/OME.4.001067. [DOI] [Google Scholar]
  54. Higashiwaki M.; Sasaki K.; Murakami H.; Kumagai Y.; Koukitu A.; Kuramata A.; Masui T.; Yamakoshi S. Recent Progress in Ga2O3 Power Devices. Semicond. Sci. Technol. 2016, 31 (3), 034001. 10.1088/0268-1242/31/3/034001. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

am2c14661_si_001.pdf (2MB, pdf)

Articles from ACS Applied Materials & Interfaces are provided here courtesy of American Chemical Society

RESOURCES