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(a) Schematic illustration of a multilayer structure for flexible thin-film transistors (TFTs) developed in this study, (b) In-situ reflection high-energy electron diffraction (RHEED) patterns showing the evolution of the surface texture of bare flexible glass, CeO2 by ion-beam-assisted deposition (IBAD), and Ge by plasma-enhanced chemical vapor deposition (PECVD), and (c) a flexible glass tape.
Source publication
Thin-film transistors (TFTs) grown on a flexible glass substrate using single-crystal-like germanium (Ge) channel to simultaneously achieve high carrier mobility, high performance characteristics, mechanical flexibility, and cost-effective large-area manufacturing are reported. High-crystalline-quality materials of biaxially textured CeO2 deposited...
Contexts in source publication
Context 1
... flexible glass tapes and two-step deposition process used in this study are compatible with continuous roll-to-roll manufacturing necessary for economical process and scale up of flexible electronic components. Figure 1a shows a schematic illustration of a layer structure developed in this work. Crucial are the selection of flexible substrates that are compatible with relatively high-processing temperatures and growth technique for epitaxy of thin-film materials with electrical properties approaching those of single-crystal wafers. ...
Context 2
... Flexible glass can offer flexibility, light weight, thermal stability, good barriers to moisture and oxygen, and compatibility with large scale processing of TFTs. A thin, flexible, and thermally stable (up to ≈650 °C) glass tape was used as a substrate, as shown in Figure 1c. ...
Context 3
... development eliminates the requirement of depositing multiple oxide buffer layers between the flexible substrates and Ge films. [26] The CeO 2 film was deposited by IBAD for a biaxially textured material in a favorable channeling direction (see Figure S1 in Supporting Information for a schematic configuration of IBAD). The angle of assisted ion beam was set at 54.7° from the surface normal of the substrate. ...
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Citations
... Hence, to fabricate high-performance devices, it is necessary to select appropriate active layers. According to the different active layers, we classify TFT devices into the following three categories: silicon- based TFTs [190][191][192] , organic TFTs 30,41,[193][194][195] , and metal oxide TFTs [196][197][198][199][200] . ...
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... In addition to the heat dissipation effect of the buffer layer, the device maintained its structure during the ILLO process because of the nanosecond pulsed laser beam. [24,25] The attachment material between the μLED and the PEO/ BTO/glass carrier substrate is especially important for the stable maintenance of the device structure during the ILLO process with extremely high thermal/mechanical stresses. To obtain the optimal adhesive, several materials, including the PDMS polymer, Pd-In alloy, SU8 photoresist, and optical adhesive, were investigated in the ILLO. Figure 2 shows the microscopic images of the laser-reacted material surfaces. ...
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Graphical abstract
... [29][30][31][32][33][34][35] Moreover, because of the relatively low Young's modulus of Ge, its synthesis on flexible substrates such as polyimide has also been considered. [36][37][38][39][40] Recent progress in SPC technology has significantly improved the crystallinity and electrical properties of Ge thin films. The grain size of the polycrystalline Ge layer increased by more than one order of magnitude after SPC with the densification of the amorphous Ge precursor. ...
Despite its long history, synthesizing n-type polycrystalline Ge layers with high-electron mobility on insulating substrates has been difficult. Based on our recently developed solid-phase crystallization technology, here, we have demonstrated the highest recorded electron mobility (450 cm2 V−1 s−1) for Ge-based polycrystalline thin films on insulating substrates. The underlayer type and small amount of Sn addition were the key parameters controlling both the density and barrier height of the grain boundaries in the P-doped polycrystalline Ge layers. The low growth temperature (≤400 °C) allowed us to develop a GeSn layer on a heat-resistant polyimide film, which exhibited the highest electron mobility (200 cm2 V−1 s−1), as a semiconductor thin film synthesized directly on a flexible substrate. These achievements herald the development of high-performance polycrystalline Ge-based devices on inexpensive glass and flexible plastic substrates.
... To integrate Ge-CMOS into electronic devices, including three-dimensional large-scale integrated circuits or flat panel displays, it is necessary to form a high-quality Ge layer at low temperatures that does not damage the substrate or surrounding devices. Fortunately, the crystallization temperature of Ge is lower than that of Si, and many low-temperature synthesis methods have been proposed, including solid-phase crystallization (SPC), 15−17 laser annealing, 18−23 chemical vapor deposition, 24,25 lamp annealing, 26,27 plasma irradiation, 28 seed layer technique, 29 and metal-induced crystallization. 30 −34 Although these methods produce polycrystalline Ge layers containing grain boundaries, large grain size and grain boundary control enable quasi-single-crystal channels in transistors. ...
... To date, the low-temperature syntheses of Ge layers have been achieved using several methods, including solid-phase crystallization (SPC), [11][12][13][14] laser annealing, [15][16][17][18] chemical vapor deposition, 19,20 seed layer technique, 21 lamp annealing, 22,23 and metal-induced crystallization (MIC). [24][25][26][27][28][29] Using these methods, the operation of flexible TFTs using polycrystalline (poly-) Ge has been reported, 21,29 exhibiting higher field-effect mobilities than most flexible TFTs using amorphous, oxide, and organic semiconductors. ...
... To date, the low-temperature syntheses of Ge layers have been achieved using several methods, including solid-phase crystallization (SPC), [11][12][13][14] laser annealing, [15][16][17][18] chemical vapor deposition, 19,20 seed layer technique, 21 lamp annealing, 22,23 and metal-induced crystallization (MIC). [24][25][26][27][28][29] Using these methods, the operation of flexible TFTs using polycrystalline (poly-) Ge has been reported, 21,29 exhibiting higher field-effect mobilities than most flexible TFTs using amorphous, oxide, and organic semiconductors. These results demonstrate the excellent potential of the Ge-based flexible TFTs. 3 In recent years, SPC has evolved significantly. ...
... To form a high-quality c-Ge film on an insulator, various crystallization techniques such as solid-phase crystallization (SPC), [12][13][14] rapid thermal annealing based on the seed layer technique, 15) metal-induced crystallization, [16][17][18] laser annealing, 19) and flash lamp annealing 20) have been proposed. Besides these techniques, we have proposed the application of an atmospheric-pressure DC arc discharge micro thermal plasma jet (μ-TPJ) to the rapid crystallization of amorphous group IV semiconductor films on insulators. ...
Crystalline-germanium (c-Ge) is an attractive material for a thin-film transistor (TFT) channel because of its high carrier mobility and applicability to a low-temperature process. We present the electrical characteristics of c-Ge crystallized by atmospheric pressure micro-thermal-plasma-jet (µ-TPJ). The µ-TPJ crystalized c-Ge showed the maximum Hall mobility of 1070 cm ² ·V ⁻¹ ·s ⁻¹ with its hole concentration of ~ 10 ¹⁶ cm ⁻³ , enabling us to fabricate the TFT with field-effect mobility ( μ FE ) of 196 cm ² ·V ⁻¹ ·s ⁻¹ and ON/OFF ratio ( R ON/OFF ) of 1.4 × 10 ⁴ . On the other hand, R ON/OFF s and μ FE s were dependent on the scanning speed of the TPJ, inferring different types of defects were induced in the channel regions. These findings show not only a possibility of the TPJ irradiation as a promising method to make a c-Ge TFT on insulating substrates.
... Furthermore, a low temperature growth technique will enable fabrication of semiconductor devices on flexible substrates. 6) The metal-induced crystallization (MIC) technique is extensively studied to realize high-quality crystalline Ge thin films at low temperature. [7][8][9][10][11][12][13][14][15][16] The crystallization temperature by the MIC process is much lower than that by solid phase crystallization method. ...
Au layer thickness dependence (9–34 nm) of Ge crystallization in the metal-induced layer exchange process has been investigated. It has been
found that Ge crystals are (111) oriented when the Au layer is as thin as 9 nm, whereas crystal grains are randomly oriented when the Au layer is
as thick as 34 nm. The difference is discussed in terms of the difference in the position of nucleation sites of Ge crystals.
... The performances of metal-oxide-semiconductor field-effect transistors (MOSFETs) based on a single-crystal Ge-on-insulator structure, formed using a single-crystal wafer [3][4][5] and/or high-temperature process (> 900 °C) 6,7 , surpassed those of Si MOSFETs. Low-temperature syntheses of Ge thin-film transistors (TFTs) have been achieved using conventional solid-phase crystallization (SPC) [8][9][10][11] , laser annealing 12 , seed layer technique 13 , and metal-induced crystallization [14][15][16] . Although these methods are potentially useful to expand the application of Ge, the crystallinity of the resulting polycrystalline Ge (poly-Ge) is still insufficient to increase the performances of Ge TFTs over those of Si MOSFETs. ...
Polycrystalline Ge thin films have attracted increasing attention because their hole mobilities exceed those of single-crystal Si wafers, while the process temperature is low. In this study, we investigate the strain effects on the crystal and electrical properties of polycrystalline Ge layers formed by solid-phase crystallization at 375 °C by modulating the substrate material. The strain of the Ge layers is in the range of approximately 0.5% (tensile) to -0.5% (compressive), which reflects both thermal expansion difference between Ge and substrate and phase transition of Ge from amorphous to crystalline. For both tensile and compressive strains, a large strain provides large crystal grains with sizes of approximately 10 μm owing to growth promotion. The potential barrier height of the grain boundary strongly depends on the strain and its direction. It is increased by tensile strain and decreased by compressive strain. These findings will be useful for the design of Ge-based thin-film devices on various materials for Internet-of-things technologies.
... Polycrystalline Ge (poly-Ge) thin films have been formed on insulators at low temperatures using solid-phase crystallization (SPC) [16][17][18][19][20] , laser annealing [21][22][23][24] , chemical vapor deposition 25,26 , lamp annealing 27,28 , seed layer technique 29 and metal-induced crystallization (MIC) [30][31][32][33][34] . The poly-Ge layers are naturally highly p-type because of their defect-induced acceptors 35 . ...
High-electron-mobility polycrystalline Ge (poly-Ge) thin films are difficult to form because of their poor crystallinity, defect-induced acceptors and low solid solubility of n-type dopants. Here, we found that As doping into amorphous Ge significantly influenced the subsequent solid-phase crystallization. Although excessive As doping degraded the crystallinity of the poly-Ge, the appropriate amount of As (~1020 cm−3) promoted lateral growth and increased the Ge grain size to approximately 20 μm at a growth temperature of 375 °C. Moreover, neutral As atoms in poly-Ge reduced the trap-state density and energy barrier height of the grain boundaries. These properties reduced grain boundary scattering and allowed for an electron mobility of 370 cm2/Vs at an electron concentration of 5 × 1018 cm−3 after post annealing at 500 °C. The electron mobility further exceeds that of any other n-type poly-Ge layers and even that of single-crystal Si wafers with n ≥ 1018 cm−3. The low-temperature synthesis of high-mobility Ge on insulators will provide a pathway for the monolithic integration of high-performance Ge-CMOS onto Si-LSIs and flat-panel displays.
... 27 Single-crystal-like biaxiallytextured Ge films were obtained on polycrystalline metal and amorphous glass substrates using ion-beam assisted deposition (IBAD) or oblique-angle deposition technique. [28][29][30][31][32] IBAD is a process by which a biaxially-textured film (a film aligned both in-plane and out-of-plane directions) can be achieved on essentially any substrate at room temperature, using simultaneous deposition (sputter, e-beam evaporation, etc.) and lowenergy ion-assist (typically argon) at a certain incident angle. 36 As opposed to a polycrystalline film where grains are highly misoriented (410 degrees misorientation angle) and disordered, a biaxially-textured film consists of crystalline grains which are aligned to each other both in the in-plane and out-of-plane directions, assuming a single-crystalline-like structure with grain misorientation angle less than 5 degrees. ...
In this report, we describe a unique roll-to-roll plasma-enhanced chemical vapor deposition (R2RPECVD) technique to grow high-quality single-crystalline-like Ge films on flexible metal foils, an
important advancement towards scalable processing of epitaxial Ge films at low-cost. Ion-beam assisted deposition was used to create single-crystalline-like substrate templates to enable epitaxial growth of Ge films. The Ge films were highly (004) oriented, biaxially-textured and showed remarkable crystalline quality, equivalent to single-crystal Ge wafers. Subsequently, the Ge films on metal foils were used as substrates to fabricate flexible GaAs single-junction solar cell by metaloxide chemical vapor deposition (MOCVD). The champion device showed efficiency of 11.5%, and the average efficiency of four devices was 8% at 1 Sun, the highest reported on GaAs PV directly deposited on alternative flexible substrates. Devices made on CVD-Ge film exhibited significantly improved performance compared to the ones grown on sputtered Ge films. Scalable production of inexpensive and flexible epi-Ge films will not only be useful for developing lowcost and high-performance III-V solar cells, but also for emerging flexible electronic devices
applications.
