Flexible gallium oxide electronics

Flexible Ga2O3 devices are becoming increasingly important in the world of electronic products due to their unique properties. As a semiconductor, Ga2O3 has a much higher bandgap, breakdown electric field, and dielectric constant than silicon, making it a great choice for next-generation semiconductor materials. In addition, Ga2O3 is a particularly robust material that can withstand a wide range of temperatures and pressure levels, thus is ideal for harsh environments such as space or extreme temperatures. Finally, its superior electron transport properties enable higher levels of electrical switching speed than traditional semiconducting materials. Endowing Ga2O3-based devices with good mechanical robustness and flexibility is crucial to make them suitable for use in applications such as wearable electronics, implantable electronics, and automotive electronics However, as a typical ceramic material, Ga2O3 is intrinsically brittle and requires high temperatures for its crystallization. Therefore fabricating flexible Ga2O3 devices is not a straightforward task by directly utilizing the commonly used polymer substrates. In this context, in recent years people have developed several fabrication routes, which are the transfer route, in situ room-temperature amorphous route, and in situ high-temperature epitaxy route. In this review, we discuss the advantages and limitations of each technique and evaluate the opportunities for and challenges in realizing the applications of flexible Ga2O3 devices.

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Ga 2 O 3 has an exceptionally high Baliga figure of merit of 3214.1, making it a promising candidate for various applications, including power electronics, solar-blind ultraviolet (UV) detectors, memory devices, and electronics that operate under harsh environments such as high temperature and radiation [11][12][13][14][15][16][17][18].
In addition, various electronics that have been developed in the past decade have become indispensable in our daily lives. Hence, there is considerable interest in developing portable and wearable electronics for real-time applications. Mechanically flexible electronic systems must be designed to withstand applied bending stress caused by curvilinear surfaces and movements [19][20][21][22][23][24]. The application of flexible electronics is depicted in figure 1. Organic or low-temperature-processed  [19]. Copyright 2020 Elsevier.
inorganic flexible semiconductor electronics have been extensively investigated [25][26][27][28]. Alternatively, the growth of oxide ceramic semiconductor materials usually requires high-temperature processing and epitaxial templates [29][30][31]. In addition, owing to the lack of suitable flexible substrates, the integration of these high-temperature-processed materials into flexible electronics remains challenging. Ga 2 O 3 is a typical representative of oxide semiconductor materials [1]. For instance, Ga 2 O 3 has five polymorphs: α, β, γ, δ, and ε. [32,33] Among them, β-Ga 2 O 3 has the highest thermal stability [1,32]. However, the crystallization temperature of β-Ga 2 O 3 is 1000 • C, at least for solid reactions under normal pressure, which is considerably higher than the decomposition temperatures of conventional organic flexible substrates (usually 200 • C-300 • C) [1,34]. Accordingly, the selection of the substrate, deposition technique, and Ga 2 O 3 polymorph is critical. The fabrication of Ga 2 O 3 has been extensively studied for flexible electronics recently [35,36]. In this review, we highlight key developments in Ga 2 O 3 flexible electronics and predict the direction of their future applications.

Crystal structure and deposition techniques
A detailed understanding of the polymorphs and the corresponding deposition techniques of Ga 2 O 3 is essential for designing an appropriate fabrication process for flexible electronics. Ga 2 O 3 has mainly five polymorphs, i.e. α-Ga 2 O 3 (trigonal (R3-c) corundum structure), β-Ga 2 O 3 (monoclinic (C2/m)), γ-Ga 2 O 3 (cubic (Fd3-m) spinel structure), δ-Ga 2 O 3 (orthorhombic system (Cmcm)), and ε-Ga 2 O 3 (hexagonal system (Pna21)). The structural information and unit cells  [3,33,[37][38][39][40]. Other than these crystalized structures, amorphous Ga 2 O 3 also plays an important role in flexible electronics as it does not require high temperatures for deposition [41][42][43]. Specifically, α-Ga 2 O 3 is a crystallized metastable phase that can be obtained at very low temperatures. α-Ga 2 O 3 thin films can be obtained through mist chemical vapor deposition (mist-CVD), [44][45][46] which is facile and cost-effective and does not require vacuum equipment. In addition, it allows the tailoring of the deposition precursor solution. Using aqueous halide solutions, gallium salts can be easily mixed with indium and aluminum to obtain corundum-structured III-oxide alloys with various stoichiometries, offering bandgap engineering from 3.8 eV to 8.8 eV [3,47]. Further, α-Ga 2 O 3 can be  [40]. Copyright 2022 AIP. obtained by simply heating GaOOH using the hydrothermal method and halide vapor phase epitaxy (HVPE) [48,49]. However, the hydrothermal route is difficult to scale up, whereas the HVPE route requires highly corrosive HCl gas as the precursor and is relatively more expensive compared to mist-CVD.
Finally, amorphous Ga 2 O 3 has a large number of defects and oxygen vacancies compared with its crystalline counterparts, thus making its use in electronic devices disadvantageous because of its relatively slow photoelectric response and unstable performance over time [1,3,63]. Amorphous Ga 2 O 3 thin films can be deposited on polymer substrates via sputtering at room temperature, which can be valuable for fabricating flexible electronics [64]. In addition, the aforementioned inferior properties of amorphous Ga 2 O 3 can be improved by subsequent processes after film deposition [64].

Flexible substrates
The primary property of flexible substrates is deformability, which allows coated electronics to withstand bending and twisting when placed in pockets or directly attached to the body. Further more, the good mechanical performance of flexible substrates can enable the large-scale production of electronics in a continuous reel-to-reel system, thereby decreasing the production cost [65][66][67]. In general, three types of materials, polymers, glass fibers, and metal foils, can meet these mechanical requirements, the main properties of which are summarized in table 2 [68][69][70][71][72].
[72] Few materials, such as carbon nanotubes, transition metal dichalcogenides, organic compounds, and perovskites, have been grown on polymers because they require neither high processing temperature (>300 • C) nor high crystallinity (in terms of grain size and orientation) [26,77,78]. Moreover, the elastic moduli of polymer substrates are significantly smaller than those of ceramic semiconductor materials, and thermal mismatch can cause shattering and falloff during fabrication [72].
Compared with polymers, glass has higher thermal stability and can withstand elevated temperatures (up to 1200 • C) [72]. Usually, flexible glass substrates can be obtained by thinning a thick rigid glass plate to hundreds of microns using a down draw method. As glass offers electrical resistance and high optical transparency, it has been used as the standard substrate in flexible panel displays. However, despite its high thermal stability, glass cannot serve as an epitaxial template for the growth of epitaxial semiconductor thin films owing to its amorphous nature.  As another class of flexible substrates, metal foils exhibit good mechanical performance and high thermal stability and do not react with moisture and oxygen; thus, they have been used in flexible reflective display panels and amorphous silicon solar cells [72,79,80]. For instance, stainless steel can withstand high temperatures (up to 1300 • C) and is thermally much more stable than polymers and glass [81]. Because of this property, it can be used as a substrate for the growth of ceramic semiconductor thin films [82,83]. The processing temperature ranges for the representative metal foil and polymer substrates and the required temperatures for the crystallization of representative oxide thin films are shown in figure 4 [35]. The thermal stability of metal foils meets the requirement for the crystallization of oxide thin films. In contrast, metal foils have relatively high roughness [72,[84][85][86]. Because metal foils are produced by pressing metal ingots and then elongating them in a reel system, the roughness of the product is highly dependent on the smoothness of the reel and the subsequent polishing processes and is rarely <100 nm. Therefore, an additional planarization oxide coating becomes necessary for decreasing roughness before the growth of semiconductor layers, which undoubtedly increases the complexity of the use of metal foils [35,84]. In contrast, by applying ionbeam-assisted deposition (IBAD), the coated oxide planarization layer can be crystallized along a preferred orientation [87,88]. This technique allows metal foils to serve as a high-temperature epitaxial growth template for high-quality semiconductor thin films [35,82,89]. So far, the technique has been successfully applied in the coated-conductor industry, especially for the production of superconductor tapes [87,88].
As a ceramic material crystalized in a monoclinic structure, muscovite (mica) can intrinsically serve as an epitaxial template [36]. Such structure property makes mica sheet ideal for the deposition of crystalized Ga 2 O 3 thin films. By thinning to micrometers, the obtained mica sheets can show excellent flexible performances. Moreover, as a typical ceramic material, mica sheets have thermal stability at high temperatures (up to 1600 • C), amazing dielectric strength, chemical inertness, elasticity, lightweight, and refractive properties. Therefore, mica sheets can simultaneously meet the two basic requirements, that is, thermal stability and epitaxial ability, for the direct deposition of flexible crystallized oxide materials.

Transfer technique
Monoclinic β-Ga 2 O 3 is the most stable polymorph of Ga 2 O 3 [1,33]. However, the growth of high-quality β-Ga 2 O 3 requires ideal templates and high temperatures [33,35]. Thus, it is challenging to grow them directly on conventional flexible polymer substrates. A straightforward strategy to fabricate flexible β-Ga 2 O 3 electronics is to use the transfer technique in which bulk single-crystalline β-Ga 2 O 3 is first obtained using conventional high-temperature growth routes and β-Ga 2 O 3 thin films are then exfoliated from bulk materials and pasted on flexible polymer substrates [90][91][92][93][94][95][96][97]. Single-crystalline β-Ga 2 O 3 has been commonly obtained using the Czochralski method, in which Ga 2 O 3 powders are melted in an iridium crucible at a temperature of >1820 • C in an oxygen-deficient atmosphere [98]. Upon cooling to room temperature, cylinder bulk β-Ga 2 O 3 materials with diameters and lengths of tens of millimeters are obtained. Low-dimensional β-Ga 2 O 3 can be directly exfoliated from bulk materials; otherwise, bulk materials can be first used as the homoepitaxial template to grow single-crystalline β-Ga 2 O 3 thin films using MBE; then, the thin films serve as the materials for exfoliation [90][91][92][93][94][95][96][97]. Figure 5 depicts the typical exfoliation-transfer procedure, which has been used for the fabrication of various flexible materials, including graphene, black phosphorus, GaN, and MoS 2 [99][100][101][102].
First, a scotch tape is used to cleave β-Ga 2 O 3 thin layers from the bulk/thin film materials. The layer thickness can be decreased to a nanometer scale by folding and unfolding the scotch tape multiple times. Notably, the acquired layer is usually (100)-oriented because it is the easiest cleavage plane of β-Ga 2 O 3 . Then, a SiO 2 /Si substrate is pasted on the scotch tape after cleaning with acetone and isopropanol. Upon removing the scotch tape, β-Ga 2 O 3 layers remain on top of the SiO 2 due to the adhesive force. Then, the SiO 2 /Si substrate is cleaned with acetone under ultrasonication, which removes weakly attached β-Ga 2 O 3 fragments. Finally, β-Ga 2 O 3 thin layers are transferred from the SiO 2 /Si substrate to flexible polymer substrates using an elastomeric stamp. Although the transfer technique ensures high-quality materials, it cannot be scaled up owing to uncontrollable and cumbersome manual operation. Thus, to date, the transfer technique has been applied only to basic research [90][91][92][93][94][95][96][97].
Swinnich et al [93], Hasan et al [95] and Lai et al [96] synthesized flexible β-Ga 2 O 3 nanomembrane SBDs for applications in high-power electronics using the aforementioned transfer technique [93,95]. The source single-crystalline Sndoped β-Ga 2 O 3 was obtained via MBE, and the mechanically cleaved fragmental β-Ga 2 O 3 layers were transferred to the flexible PI substrates. Because the polymer substrate cannot withstand high temperatures, an inductively coupled plasma treatment was used instead of high-temperature rapid annealing to achieve an ohmic contact between Ti/Au and β-Ga 2 O 3 , which is important for obtaining high electrical performance [103]. Swinnich et al further investigated the influence of bending-induced strain on the electrical performance of flexible devices. The fabricated device and the corresponding electrical performances together with the microstructural images before and after the bending tests are shown in figure 6 [93]. First, using Raman spectroscopy, Swinnich et al demonstrated that the exfoliation process did not induce any built-in strain; thus, the high quality of single-crystalline β-Ga 2 O 3 can be largely maintained (figure 6(a)). Correspondingly, the flexible SBD exhibited an excellent rectifying behavior similar to β-Ga 2 O 3 SBDs with conventional rigid singlecrystalline substrates (figure 6(d)). Moreover, the device had a remarkably high breakdown voltage of −119 V, corresponding to a critical breakdown field strength of 1.2 MW cm −1 (figure 6(c)). However, the electrical characteristics noticeably degraded under strain, and the forward current and the reverse current were hundreds of times lower when the device was bent (figure 6(d)). The scanning electron microscopy (SEM) images obtained using the initial flat SBD showed a smooth surface, whereas the samples after bending showed fractured surfaces with noticeable microcracks (figures 6(e) and (f)). These microcracks caused the degradation of the current by creating voids and defects, which could lengthen the shortest path of electrons from one end to the other. Hasan et al reported another reason for electrical performance degradation, i.e. the generation of nanogaps in the microstructure according to the atomic force microscopy and SEM images after the bending test, as shown in figure 7 [95]. In this model, under bending conditions, the uniaxial strain was assumed to induce disconnected points at the nanogaps of the β-Ga 2 O 3 layer, which could severely hinder the current flow and result in high resistance. Interestingly, the authors found that the bendinginduced microcracks and nanogaps can be healed through either thermal annealing or water-vapor treatment to recover the electrical performance to a great extent (figures 7(e)-(g)).
The microcracks or nanogaps present in the β-Ga 2 O 3 layers may reattach by van der Waals (vdW) interactions at high temperatures or chemical binding under a water-vapor atmosphere.
In another study, Zhang et al fabricated a similar β-Ga 2 O 3 flexible device, as shown in figure 8(a) [92]. The only difference is that the target substrate was muscovite instead of a polymer. They found that the current increased as the bias voltage increased from 0 to 1 V because of an increased probability of electron transfer from β-Ga 2 O 3 to Ni contacts owing to the reduced conduction band minimum of β-Ga 2 O 3 under the bending condition; however, such  influence was saturated when the bias voltage was >1 V, as shown in figure 8(b). The increased and unchanged current in low and high bias ranges, respectively, resulted in a degraded subthreshold swing (SS) under bending conditions (figure 8(c)). Their finding is valuable because the SS value is a key parameter for evaluating the on-off performance of transistors.
Other than SBDs, the transferred β-Ga 2 O 3 can also be used to fabricate flexible PDs. The bending effect on the photoelectrical performance of flexible β-Ga 2 O 3 PDs was systematically investigated by Lai et al as shown in figure 9 [96]. They showed a wavelength shift from 252 nm to 260 nm in the photoresponse spectrum, together with enhanced higher photoresponsivity when the PD was bent (figures 9(d)-(f)). According to a comprehensive structural analysis and density functional theory calculations, the shift was caused by the changes in the refractive index and the extinction coefficient formation of β-Ga 2 O 3 , whereas the enhanced photoresponsivity was caused by secondary light absorption at the nanogaps upon bending (figures 9(g) and (h)).
Further, Li et al explored the application of Ga 2 O 3 in flexible PDs [94]. In their study, using transferred β-Ga 2 O 3 as the channel layer and transferred BN as the dielectric layer, they fabricated a flexible phototransistor ( figure 10(a)). Here, the phototransistor can be understood as a combination of the PD and TFT; that is, the device can display excellent on/off behavior under illumination at a specific wavelength range. The phototransistor showed high responsivity, large detectivity (D * ), and ultrafast response, superseding those of conventional Ga 2 O 3 PDs (figures 10(b) and (c)). More importantly, they fabricated an array of Ga 2 O 3 phototransistors, realizing a UV detector composed of 15 × 15 pixels. By combining the detector array with an artificial neural network, the system can be used for image recognition in robots (figure 10(d)). Manual mechanical cleavage is an inevitable step for typical transfer routes. However, the thickness of the cleaved layer can be random because it depends on the habits of the operator [93,95]. Such disadvantages make the typical transfer route incompatible with scaling. To solve this problem, Wang et al used water-soluble Sr 3 Al 2 O 6 as the buffer layer to grow free-standing Ga 2 O 3 flexible membranes [91]. Using this strategy, Sr 3 Al 2 O 6 was first deposited on a conventional rigid substrate; then, Ga 2 O 3 thin films were deposited on top of Sr 3 Al 2 O 6 . Because the stacked structure was grown using an additive deposition route, the thickness of the Ga 2 O 3 thin film was controlled by tuning the deposition time, temperature, and other parameters. After deposition, Sr 3 Al 2 O 6 was easily removed by rinsing the stacked structure in water because it is water-soluble. Upon the removal of the Sr 3 Al 2 O 6 buffer layer, Ga 2 O 3 was dispatched from the rigid substrate, yielding a free-standing flexible Ga 2 O 3 membrane. The whole fabrication procedure is illustrated in figure 11(a). However, in Wang et al's research, the Sr 3 Al 2 O 6 and Ga 2 O 3 thin films were deposited via sputtering at room temperature; thus, they remained either polycrystalline or amorphous. Such poor crystal quality is against the original purpose of the transfer method, which is to ensure single-crystalline Ga 2 O 3 . Although single-crystalline Ga 2 O 3 could not be obtained, the Sr 3 Al 2 O 6 buffer layer method remains promising. Sr 3 Al 2 O 6 has a cubic structure, as shown in figure 11(b) [104]. The epitaxial relationship between β-Ga 2 O 3 (−201)-and cubic (001)-structured thin films has been demonstrated [105][106][107][108]. Moreover, Sr 3 Al 2 O 6 has a high thermal stability of up to 800 • C (figure 11(c)) [109]. Therefore, a single-crystalline β-Ga 2 O 3 (−201) thin film can be obtained on Sr 3 Al 2 O 6 (001) at high temperatures using PLD, MOCVD, and MBE.

Amorphous Ga 2 O x grown in situ
To overcome the complexity and irreproducibility of the transfer technique, the straightforward route is used to deposit Ga 2 O 3 thin films on flexible substrates in situ. However, the thermal stability of conventional polymer substrates cannot meet the requirements for the high-temperature growth of β-Ga 2 O 3 . Hence, amorphous Ga 2 O x thin films can be deposited at room temperature [110][111][112][113][114][115][116][117][118][119][120]. Compared with β-Ga 2 O 3 , amorphous Ga 2 O x has relatively low thermal stability, and its stoichiometry is variable and highly dependent on deposition parameters [110][111][112].
Liang et al [110] Cui et al [111] and Wang et al [112] varied the Ga/O ratio to improve the performance of amorphous Ga 2 O x . They grew amorphous Ga 2 O x thin films directly on PEN substrates at room temperature by controlling the oxygen flux. The samples were then fabricated into flexible PDs by depositing ITO contacts on the surface, as shown in figure 12(a) [111]. The photocurrent and the dark current decreased with the oxygen partial pressure. For instance, by increasing the oxygen partial pressure from 0 sccm (sample     S0) to 0.15 sccm (sample S4), the dark current and the photocurrent of the flexible PDs decreased from 10 −6 A to 10 −13 A and from 10 −3 to 10 −10 A, respectively, and the response time was shortened from seconds to tens of microseconds (figures 12(b)-(d)). The ultraslow response observed on S0 is known as the persistent photocurrent (PPC) and is regarded as the main drawback of amorphous Ga 2 O x PDs [33]. For a detailed understanding of the effect of oxygen on the photoelectrical performances of flexible PDs, x-ray photoelectron spectroscopy (XPS) was performed on the samples deposited using different oxygen fluxes, as shown in figures 12(f)-(h). First, the normalized XPS valence band spectra indicated that the valence band gradually shifted toward the lower binding energy with the increased oxygen flux. Using the linear extrapolation method, the valence band maximum values for the sample deposited without and with the highest oxygen flux were obtained as 2.73 eV and 2.28 eV, respectively. Hence, an upward bending value of 0.45 eV was deduced for the valence band maximum and conduction band minimum values. Using the bandgap (E g ) and electron affinity (χ) for Ga 2 O x and the work function of ITO as 5 eV, 4 eV, and 4.9 eV, respectively, the schematic energy band diagrams for S0 and S4 are shown in figure 12(e). The surface energy band for S0 bent downward by 0.23 eV, resulting in a Schottky barrier (∆E C ) of 0.67 eV, whereas for S4, the surface energy band bent upward by 0.22 eV, resulting in a Schottky barrier (∆E' C ) of 1.12 eV. High SB is responsible for the decreased photocurrent and dark current in S4 obtained by blocking the movement of photoinduced electrons from Ga 2 O 3 to ITO. Furthermore, the O 1 s and Ga 2p emissions of the samples are shown in figures 4(c) and (d). By integrating the enveloped area of the O 1 s and Ga 2p peaks, the ratio SO 1s /SGa 2p was calculated as 0.61 and 0.65 for S0 and S4, respectively, suggesting higher oxygen content and lower oxygen vacancies in S4. In oxide PDs, oxygen vacancies serve as deep-level trapping centers; hence, the decreased oxygen vacancies enhance the response speed, thereby eliminating the PPC effect.
A similar structure for flexible x-ray detectors was also demonstrated [110]. Upon x-ray illumination, high-energy electrons are generated throughout the Ga 2 O x thin film owing to the high penetrability of x-rays. The schematic energy band diagrams illustrating the conductive electrons released from the valence band, valence bond theory states, and ionization of Vo under UV and x-ray radiation are shown in figure 13. In the vicinity of the thin film surface, such high-energy electrons may escape from Ga 2 O x , inducing a surface photochemical reaction and an air ionization effect. Such photochemical reactions can change the Ga/O stoichiometry of the materials, thus affecting the long-term stability of the devices. For high-energy electrons generated far below the surface, owing to the considerably higher energy of x-rays compared with UV light, their excessive energy can be effectively transferred to Most recently, Wang et al [112] integrated these Ga 2 O x PDs with a full-wave bridge rectifier and a receiving electrode to construct a flexible self-powered DUV detection system, as shown in figure 14. The power source of the system was the human body-absorbed ambient electromagnetic radiation energy. The radiation resulted in an inductive current, which was received by the electrode, rectified by the full-wave bridge rectifier, and eventually supplied to the connected PDs. The self-powered PD had a good photoelectrical performance with responsivity and detectivity of 0.1 A W −1 and 2.24 × 10 11 Jones, respectively. However, the light-to-dark ratio was only 8.31. In contrast, the light-to-dark ratio obtained from the conventional bias-powered Ga 2 O x PD is usually >10 5 [3,57].
To utilize flexible Ga 2 O x PDs in actual applications, such as solar-blind imaging, obtaining the spatial distribution of the light source intensity, and real-time light trajectory detection, the PDs must be fabricated into arrays comprised of multiple PD diodes, which require flexible Ga 2 O x thin films with low roughness, homogeneous material quality, and low performance variation between individual diodes on a large scale. Compared with the complicated transfer technique, the in situ growth of amorphous Ga 2 O x has a great advantage in that it can ensure the large-scale deposition of homogeneous Ga 2 O x thin films on which conventional photolithography patterning can be performed to create photodetection arrays. Following this strategy, Chen et al fabricated a large-scale Ga 2 O x PD array with an area of 0.9 mm × 0.9 mm containing 7 × 7 rectangular cells on PEN substrates, as shown in figure 15 [119]. For the actual imaging applications, first, they examined the performance uniformity of the PD cells, which was statistically described using the coefficient of variation (CV), which is defined as the percentage of the standard deviation and the mean value of a set of data. As shown in figure 15(a), the dark currents of all PD cells were in the range of 15 ± 2.5 pA, yielding a CV of 6.9%, whereas the photocurrents of all PD cells were in the range of 13 ± 1.2 pA, yielding a CV of 6.9%, which indicated high uniformity of the PD array. A photomask with an E-shaped hollow pattern was covered on the PD array to test the actual imaging performance. Upon 254 nm light illumination, the PD array showed a clear E shape, suggesting good imaging ability, as shown in figure 15(b). To test the sensitivity of the array to light intensity, the photomask was removed, and only a narrow single beam was subjected to the center of the array. The response photocurrent distribution of the PD array is shown in figures 15(c) and (d), which provided a clear 2D mapping of the illuminated light intensity. To test the real-time light-tracing capability of the PD array, a single light beam was moved from cell 1 to cell 7. Correspondingly, the current of the single PD cell increased upon illumination and then decreased when the source of illumination moved away. As the beam moved, the currents of the PD cells increased and then decreased from cell 1 to cell 7 in the same order.
To further explore the application of flexible Ga 2 O x PDs, Chen et al fabricated a 3D array by employing the origami/kirigami technique, as shown in figure 16 [118]. Compared with the 2D array, the 3D counterparts offered unique advantages, such as an extremely wide detection space angle and excellent spatial recognition. The uniformity test performed on the 3D array provided a CV of only 3.5%, which is lower than that of its 2D counterpart. Further, in the actual detection examinations, the 3D array showed unique capabilities, such as multiple light signal detection, arc signal movement, and altitude change movement, as shown in figure 16.
In addition to PDs, amorphous Ga 2 O x flexible thin films can be used in flexible memory and TFTs [120]. The fabrication of flexible TFT devices by Qian et al [57] is particularly interesting because TFTs have a more sophisticated device structure than PDs and chemical solution deposition (CSD) instead of RF deposition was employed for the growth of Ga 2 O x . Compared with the RF route, the CSD route offers cost-effectiveness, high production speed, compatibility with the reel-to-reel system, and scalability [121][122][123]. The actual large-scale reel-to-reel system for producing yttrium barium copper oxide (YBCO) superconductor tapes via CSD is shown in figure 17 [124,125]. Moreover, CSD allows the adjustment of chemical composition. For instance, indium or aluminum can be easily doped in the Ga 2 O x thin film to tune the bandgap or device performance by adding In(NO 3 ) 3 or Al(NO 3 ) 3 to the Ga(NO 3 ) 3 precursor [47]. In general, the fabricated TFT device showed excellent on/off transition behaviors, including a low threshold voltage of 0.61 V, low SS of 0.5 V dec −1 , and high mobility (µ sat ) of 2.74 cm 2 V −1 s.

Epitaxial films grown in situ
As described above, the transfer technique can ensure high crystal quality at the price of scaling and reproducibility. In the in situ room-temperature deposition route, Ga 2 O x thin films are directly grown on flexible polymer substrates. However, because of the low deposition temperature limited by the low thermal stability of polymer substrates, the grown films remain amorphous, with oxygen vacancies highly dependent on the oxygen flux during deposition, resulting in unpredictable photoelectrical performance. To realize the in situ growth of single-crystalline oxide thin films, high thermal stability up to the temperature range for the crystallization of oxide ceramic materials and ideal lattice match with the oxides are required [126][127][128][129][130].
As shown in figure 4, Hastelloy flexible substrates can meet the thermal stability requirements for the crystallization of β-Ga 2 O 3 because its melting deposition is up to 1300 • C. However, Hastelloy substrates are polycrystalline and thus cannot serve as epitaxial templates for β-Ga 2 O 3 . Functionalization with an additional single-oriented ceramic buffer layer is a feasible route for endowing them with an ideal lattice constant for the epitaxial growth of β-Ga 2 O 3 [131][132][133][134][135][136]. The most commonly adopted growing facet of β-Ga 2 O 3 is (−201), the atomic distribution of which has a six-fold symmetry. Therefore, the epitaxial deposition of high-quality single-oriented β-Ga 2 O 3 (−201) thin films has been demonstrated on sapphire (0001), yttrium-stabilized zirconate (111), and 3 C-SiC (001), all of which have the symmetry of an integer multiple of three [1,3,33,105]. However, the growth of such  figure 18(a).
Based on these findings, Tang et al [105] further explored the epitaxial deposition of β-Ga 2 O 3 (201) thin films on CeO 2 (001)-functionalized Hastelloy substrates. The functionalization procedure was developed and employed in flexible coated-conductor production [87,88]. In general, the CeO 2 (001)-functionalized Hastelloy tape has a multilayer stacked structure, as shown in figure 18(b). To construct such a structure, first, the tape was coated with amorphous Al 2 O 3 (80 nm) and Y 2 O 3 (20 nm) layers successively using reactive RF sputtering. Here, the Al 2 O 3 layer serves as a barrier to minimize the diffusion of metallic elements during subsequent hightemperature treatments, whereas the Y 2 O 3 layer serves as a seed layer for the following deposition of crystallized epitaxial layers. Next, a highly crystalized MgO (001) layer (5 nm) was realized using the IBAD technique. Thereafter,  We can conclude that flexible substrates that are promising for the epitaxial growth of Ga 2 O 3 films must possess i) high material quality, ii) smooth surface, and iii) high-temperature stability. Muscovite mica (KAl 2 (Si 3 Al)O 10 (OH) 2 ), which possesses high-temperature tolerance, great flexibility, high transparency, high crystallinity, and satisfactory thermal stability, can be used for the in situ epitaxial growth of Ga 2 O 3 . Its aluminosilicate and layered structure allows for artificial exfoliation on an exceptionally large wafer scale, resulting in an atom-level flat surface without dangling bonds. Therefore, under high temperatures, the physisorption of atoms leads to deposition and crystallization on mica, resulting in a singlecrystalline epitaxial layer, i.e. a vdW epitaxy [126].
The synthesis of Ga 2 O 3 on mica was conducted at 600 • C using PLD by Tak et al [137] The polycrystalline Ga 2 O 3 film provided a 9.7 A W −1 photoresponsivity under 270-nm illumination [137]. Sui et al also demonstrated a flexible transparent Ga 2 O 3 solar-blind UV PD using thermal annealing on mica and obtained robust flexibility (10 000 cycles in a fatigue test) [138]. Nevertheless, the device performances from both these works were insufficient due to the noncrystalline or polycrystalline structure of Ga 2 O 3 . Tak et al demonstrated singleoriented β-phase Ga 2 O 3 epitaxially grown on mica, but the performance of the fabricated PD was poor owing to the low material quality under the growth temperature of ∼550 • C [139]. This is attributed to the ultrahigh synthesis temperature of the most thermodynamically stable β-phase Ga 2 O 3 , which is usually beyond the mica substrate tolerance, i.e. 750 • C, for achieving high crystallinity and low defect density [62]. Accordingly, Lu et al [140] demonstrated a metastable κphase Ga 2 O 3 (002) thin film on mica under 680 • C with high quality and high thermal stability. The epitaxy of κ-phase Ga 2 O 3 was enabled by metal-oxide-catalyzed epitaxy, i.e. SnO 2 was incorporated in Ga 2 O 3 as a metal-oxide source [141] ( figure 19(a)). It provides a new perspective in that the κ-phase Ga 2 O 3 can be used in flexible Ga 2 O 3 electronics in addition to the β-phase Ga 2 O 3 . Moreover, the demonstrated flexible PD has a record high responsivity (703 A W −1 ), on/off ratio (>10 7 ), detectivity (4.08 × 10 14 Jones), and external quantum efficiency (3.49 × 10 5 %), demonstrating the significant potential of the κ-phase Ga 2 O 3 for ultrahigh-performance wearable oxide optoelectronics (figures 19(b) and (c)). Another superior property of the Ga 2 O 3 /mica film is its highly robust flexibility. As shown in figures 19(b) and (d), the fabricated flexible PD possessed stable performance when considering bending radii of <1 cm and mechanical stability in case of 10 000 bending test cycles ( figure 19(d)). This is attributable to the film's extraordinary flexibility and interlayer sliding ability to sufficiently release the strain in the epitaxial layer when the substrate-layer composite was bent.
In general, mica and Hastelloy substrates pave the way for the in-situ growth of crystalized Ga 2 O 3 for flexible devices. Each of them has unique advantages. For mica substrate, because of its single-crystalline nature, high-quality Ga 2 O 3 thin film with both in-plane and out-of-plane orientation can be readily obtained; different from mica, the Hastelloy substrate as a metallic material, has a much higher tensile strength, hence might be more promising in actual flexibility-required applications. However, due to its polycrystalline nature with no preferred orientation, it cannot directly serve as an epitaxial template thus considerably adding more complicity to the deposition process.

Summary and future challenges
The application of β-Ga 2 O 3 in flexible devices can pave the way for the development of wearable and portable highperformance devices, such as power electronics, solar-blind UV detectors, and memory devices. As a ceramic material, the growth of high-quality β-Ga 2 O 3 thin films requires high crystallization temperature and templates with appropriate lattice constants for epitaxial growth; however, polymer substrates cannot withstand high temperatures. Accordingly, three main strategies have been developed for the fabrication of flexible β-Ga 2 O 3 devices: the transfer route, in-situ roomtemperature amorphous route, and in-situ high-temperature epitaxy route. Each method has advantages and limitations. In the transfer route, bulk single-crystalline β-Ga 2 O 3 is first obtained using conventional techniques for growth under hightemperature conditions; then, β-Ga 2 O 3 thin films are exfoliated from bulk materials and transferred to the flexible polymer substrates. Owing to the high crystal quality inherited from the bulk β-Ga 2 O 3 material, the intrinsic changes in the photoelectric performance of flexible β-Ga 2 O 3 devices caused by lattice deformation or intergranular fracture can be studied under bending conditions. However, the high crystal quality is obtained at the price of scaling and reproducibility owing to complicated manual operation. Hence, the transfer route can hardly be employed in production. To overcome complexity and irreproducibility, the straightforward route is used to deposit Ga 2 O 3 thin films on flexible substrates in-situ. However, the thermal stability of polymer substrates does not meet the requirements for the high-temperature growth of β-Ga 2 O 3 . Hence, a feasible solution is to deposit amorphous Ga 2 O x thin films at room temperature. Compared with β-Ga 2 O 3 , amorphous Ga 2 O x has relatively low thermal stability, and its stoichiometry is variable and highly dependent on deposition parameters. In this regard, oxygen flux and post-thermal treatment were employed to obtain high-performance amorphous Ga 2 O x . Such amorphous Ga 2 O x flexible thin films can be readily fabricated into PDs with good photoelectric performance. Moreover, compared with the complicated transfer route, the amorphous Ga 2 O x grown in-situ offers large-scale deposition of homogeneous Ga 2 O x thin films on which conventional photolithography patterning can be performed to create photodetection arrays. However, most applications of Ga 2 O x thin films remain limited to the photodetection area. Owing to the relatively unpredictable and unstable electrical performances of the thin films, their use in power devices is challenging. To realize the in-situ growth of single-crystalline β-Ga 2 O 3 thin films, the primary requirements for flexible substrates include high thermal stability up to the temperature range for β-Ga 2 O 3 crystallization and ideal lattice match for β-Ga 2 O 3 growth. Consequently, single-crystalline Ga 2 O 3 thin films were fabricated on mica and CeO 2 -functionalized Hastelloy flexible substrates. As a ceramic material, single-crystalline mica sheets can simultaneously meet thermal and lattice requirements.
Hastelloy can withstand high temperatures up to 1300 • C. However, because of its polycrystalline nature, additional multiple oxide coatings are necessary to obtain its epitaxy ability. Until now, the development of such ceramic/functionalized alloy substrates is still in the early stages; hence, the produced Ga 2 O 3 flexible thin films currently have limited application.
Various challenges must be overcome before the development of flexible Ga 2 O 3 devices. First, the absence of solid demonstrations of p-type conductivity and the prediction that holes are self-trapped to form polarons limit the current range of Ga 2 O 3 devices to unipolar conductivity [141,142]. Hence, efforts are required to explore p-type Ga 2 O 3 for wider electronic applications.
In view of the device configuration, nowadays, the aforementioned works are more limited to individual ones or arrays of the same type of device with the same functions. To fully explore the potential of Ga 2 O 3 in electronic applications, the devices must be implanted on a chip in a more complicated arrangement or with multiple functions. For instance, arranging the Ga 2 O 3 TFTs in series or in parallel mode can make them into AND, NAND, NOR, and OR logic gates, which can open a larger world for the flexible Ga 2 O 3 devices, such as wearable processors; by utilizing similar designs, the TFTs can be integrated into DC/DC converter, which is crucial to the adaptability of the Ga 2 O 3 photoelectric devices such as memory, PDs, sensors, and processors, to a variety of powersupply systems with different input voltages. Moreover, by modifying the TFT structures to a floating gate configuration, the TFTs can function as a logic memory, therefore largely shrinking the required circuit size for the potential wearable computing devices. Given the fact that for wearable and portable electronics, the size limitation is a crux for their commercial end, thus for all these devices, it is desired to integrate them in a monolithic, which can also considerably eliminate the parasitic effects and enhance the working efficiency. In this context, it triggers more tackles in the nanofabrication processes when using flexible substrates, such as the thickness inhomogeneity for the individual layers of the multi-layer stacked structure, and the defocus problem during the patterning procedures caused by the unavoidable tension of the flexible substrates. Another research trend is the integration of Ga 2 O 3 with other semiconductor materials to create various heterostructures, therefore providing enhanced or multiple functions of the integrated devices, such as PDs with enhanced performances, self-powered PDs, and CMOS circuits [143][144][145][146]. However, in such a regime, the epitaxial ability of the flexible template for materials with varied lattice structures becomes a new challenge.
Besides, the tests of the aforementioned in-situ deposition techniques remain limited to the laboratory scale. To realize large-scale production, techniques compatible with reelto-reel systems must be developed [147]. The establishment of a pilot production line becomes imperative to evaluate and improve the production cost and material quality.
At last, the use of wearable electronics relies heavily on power supply systems that are flexible and lightweight and provide stable and operation-matched output voltage. However, due to the lack of stable and flexible power supply systems, conventional rigid electronics powered by rigid battery pads dominate the market [148,149]. To advance the development of flexible electronics, the integration of such individual components is essential. The utmost challenge to confront is the feasibility of the power supply system. For instance, the solar cell is regarded as the most convenient and cost-effective power supply system for wearable electronics [150]. However, the light harvesting of solar cells can fluctuate dramatically depending on weather conditions, thereby hindering the allocation of stable and adjustable input voltage to electronic devices. In this regard, it is obligatory to embed a flexible circuit board to regulate and tune the output voltage in line with specific requirements. Tackling these issues and developing other electronic components (i.e. solar cells and circuit boards) can enable the development of flexible Ga 2 O 3 thin film-based integrated electronics.

Data availability statement
No new data were created or analysed in this study.