Abstract
Precise integration of two-dimensional (2D) semiconductors and high-dielectric-constant (k) gate oxides into three-dimensional (3D) vertical-architecture arrays holds promise for developing ultrascaled transistors1,2,3,4,5, but has proved challenging. Here we report the epitaxial synthesis of vertically aligned arrays of 2D fin-oxide heterostructures, a new class of 3D architecture in which high-mobility 2D semiconductor fin Bi2O2Se and single-crystal high-k gate oxide Bi2SeO5 are epitaxially integrated. These 2D fin-oxide epitaxial heterostructures have atomically flat interfaces and ultrathin fin thickness down to one unit cell (1.2 nm), achieving wafer-scale, site-specific and high-density growth of mono-oriented arrays. The as-fabricated 2D fin field-effect transistors (FinFETs) based on Bi2O2Se/Bi2SeO5 epitaxial heterostructures exhibit high electron mobility (μ) up to 270 cm2 V−1 s−1, ultralow off-state current (IOFF) down to about 1 pA μm−1, high on/off current ratios (ION/IOFF) up to 108 and high on-state current (ION) up to 830 μA μm−1 at 400-nm channel length, which meet the low-power specifications projected by the International Roadmap for Devices and Systems (IRDS)6. The 2D fin-oxide epitaxial heterostructures open up new avenues for the further extension of Moore’s law.
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Data availability
The data that support the findings of this study are available from the corresponding author on reasonable request.
Code availability
All computational data are presented in the manuscript. All DFT calculations were performed using VASP, which is commercially available at https://www.vasp.at/.
Change history
03 May 2023
A Correction to this paper has been published: https://doi.org/10.1038/s41586-023-06093-6
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Acknowledgements
We thank C. Qiu and J. Jiang for helping with the device fabrication and providing useful discussion. We acknowledge the Molecular Materials and Nanofabrication Laboratory (MMNL) at the College of Chemistry and Molecular Engineering at Peking University for the use of instruments. This work was supported by the National Natural Science Foundation of China (21733001, 21920102004, 52021006, 22205011, 92164205 and 22105009), National Key Research & Development Program (2021YFA1202901), Beijing National Laboratory for Molecular Sciences (BNLMS-CXTD-202001) and the Tencent Foundation (The XPLORER PRIZE). C.T. acknowledges the support from the China Postdoctoral Science Foundation and Boya Postdoctoral Fellowship. F.D. and Y.Y. acknowledge the Institute for Basic Science (IBS-R019-D1) of the Republic of Korea.
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H.P. and C.T. conceived the project and designed the experiments. C.T. and M.Y. carried out the synthesis of the 2D fins and 2D fin-oxide heterostructures. C.T., Y.Z. and M.Y. prepared the ultrathin 2D fins. Xiaoyin Gao and C.T. conducted the STEM and energy-dispersive spectroscopy characterizations and analysed the results. Xinyu Gao, K.J. and C.T. performed the high-resolution SEM characterizations. C.T., J.T. and J.W. were involved in device fabrication and electrical characterization. F.D. and Y.Y. performed the theoretical calculations. C.Z., X.Z., L.Z. and H.L. provided the data analysis and suggestions. C.T. and H.P. cowrote the manuscript. H.P. supervised this research. All authors contributed to discussions.
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Extended data figures and tables
Extended Data Fig. 1 Schematics of the crystal structures of the 2D Bi2O2Se semiconductor, Bi2SeO5 insulator, LaAlO3 and MgO substrates.
a–c, 3D representation (a), top view (b) and lateral view (c) of the Bi2O2Se crystal structure. d–f, 3D representation (d), top view (e) and lateral view (f) of the Bi2SeO5 crystal structure. Note that the lattice parameters and the positions of the Bi and Se atoms in the layered Bi2SeO5 crystal structure are determined experimentally. However, the O positions in the [SeO3]2− layers are inferred from the combination of lattice parameters, spatial relations and coordination of Se–O, because the light atom O needs to be confirmed precisely by more advanced experimental measurements22. g,h, 3D representation (g) and (100) facet lattice (h) of the LaAlO3 crystal structure. i,j, 3D representation (i) and (110) facet lattice (j) of the MgO crystal structure.
Extended Data Fig. 2 Schematic illustration for dual epitaxy of vertical 2D Bi2O2Se/Bi2SeO5 fin-oxide heterostructures on insulated substrates.
The high-mobility 2D layered Bi2O2Se fins are first epitaxially prepared as a backbone by chemical vapour synthesis, because the \({[{{\rm{Bi}}}_{2}{{\rm{O}}}_{2}]}_{{n}}^{2{n}+}\) layers of Bi2O2Se crystal have many dangling bonds at two side edges, which can easily incorporate with active atoms coming directly from the substrate surfaces and form the strong edge-bonding interfaces. The epitaxy of 2D Bi2O2Se fins was triggered from the vertically oriented nuclei and anisotropic growth. Furthermore, Bi2O2Se crystals were facially oxidized into high-k Bi2SeO5 dielectric by means of a low-temperature intercalation chemistry. Bi2SeO5 epitaxially encapsulates over 2D layered Bi2O2Se fins to form the 2D fin-oxide heterostructures on insulated substrates.
Extended Data Fig. 3 Different growth approaches for vertical 2D Bi2O2Se fin arrays.
a, Photograph of the homemade CVD system used for vertical 2D Bi2O2Se fin arrays growth. b, Temperature gradient profile in the centre of a quartz tube when the furnace temperature is set to 640 °C. c–e, Schematic and SEM images of the vertical 2D Bi2O2Se fin arrays synthesized by the vertical co-evaporation method. f,g, Schematic and SEM image of the vertical 2D Bi2O2Se fin arrays synthesized by the gas transport method. h,i, Schematic and SEM image of the vertical 2D Bi2O2Se fin arrays synthesized by the oxidation method.
Extended Data Fig. 4 Anisotropic epitaxial growth of vertical 2D Bi2O2Se fins.
a, Schematic for anisotropic growth of vertical 2D Bi2O2Se fins. b,c, Cross-sectional-view crystallographic modelling of the interfacial atomic arrangement between the Bi2O2Se fin and the LaAlO3 (100) substrate. d, In-plane lattice matching between the Bi2O2Se fin and the LaAlO3 (100) substrate. e–h, Schematics (e), AFM (f) and tilted SEM (g,h) images of a vertical 2D Bi2O2Se fin synthesized with different growth times of 10 s, 1 min and 5 min. Note that the sample and the insulating substrate were coated with a transparent and conducting carbon nanotube film to eliminate the charging effect during SEM imaging57. i,j, Tilted SEM images of fins with different aspect ratio obtained with 0 and 40 ppm O2, respectively. The growth time is approximately 10 s. k, Statistics for the aspect ratio of fins as a function of oxygen concentration. l, Statistics for fin height and fin thickness as a function of oxygen concentration. m,n, The AFM image and corresponding profile of a 10-nm-thick 2D fin with an aspect ratio of about 10 transferred onto mica substrates. The high aspect ratio (fin height/thickness) is induced by anisotropic growth of layered 2D fins, which can be further modified by tuning the oxygen concentration during growth. As the oxygen concentration was changed from 0 ppm to about 40 ppm, the aspect ratio of 2D fins decreased from about 50 to about 8. The possible reason for the above phenomenon is probably related to the oxygen absorption on the substrate surface during the nucleation process of 2D fins. When the oxygen concentration is relatively high, the absorbing rate of the oxygen is relatively high on the substrate surface, so the absorbed precursors would accumulate and nucleate on the substrate with greater probability, then crystallize into relatively thick 2D Bi2O2Se fins, resulting in smaller aspect ratio. Also, after further shortening the growth time, the aspect ratio of the 10-nm-thick 2D fin is about 10, which is comparable with the state-of-the-art Si fin (also about 10)10.
Extended Data Fig. 5 DFT calculations of the binding energies between vertical/horizontal 2D Bi2O2Se islands on LaAlO3 (100) and MgO (110) substrates.
a,b, Optimized structures of 2D layered Bi2O2Se islands on the LaAlO3 (100) surface, in which the difference of interfacial interactions is also shown. The calculations showed direct edge bonding of the unsaturated \({[{{\rm{Bi}}}_{2}{{\rm{O}}}_{2}]}_{{n}}^{2{n}+}\) layers to substrate in the vertical nucleation process of 2D Bi2O2Se fins. c, DFT calculations of the binding energies of a Bi2O2Se island with different nucleation types on the LaAlO3 (100) and MgO (110) surfaces. The results clearly showed that, on the LaAlO3 (100) and MgO (110) surfaces, the vertically aligned 2D Bi2O2Se is much more stable than the horizontally aligned one by direct bonding to the epitaxial surface through the \({[{{\rm{Bi}}}_{2}{{\rm{O}}}_{2}]}_{{n}}^{2{n}+}\) layer edge. We conclude that the nucleation of vertical 2D Bi2O2Se is governed by an edge-bonding-guided mechanism.
Extended Data Fig. 6 Dual epitaxy of vertical 2D fin-oxide heterostructure arrays on diverse insulating substrates.
Using chemical vapour synthesis, the 2D layered Bi2O2Se fins are first epitaxially prepared as backbones and then partially and intercalatively oxidized into 2D layered Bi2O2Se/Bi2SeO5 fin-oxide heterostructure arrays on LaAlO3 (100) (a,b), MgO (110) (c,d), CaF2 (110) (e,f), LaAlO3 (110) (g,h), SrTiO3 (110) (i,j) and KTaO3 (110) (k,l) substrates.
Extended Data Fig. 7 Interface structure of an epitaxial 2D Bi2O2Se/Bi2SeO5 fin-oxide heterostructure on the LaAlO3 (100) substrate.
a, Cross-sectional high-resolution STEM micrograph of the interface structures between the heterostructure and the LaAlO3 (100) substrate. b,c, Experimental (b) and simulated (c) FFT pattern of a, showing the epitaxial relationship of Bi2O2Se, Bi2SeO5 and LaAlO3. d, Cross-sectional high-resolution STEM micrograph of the interface structures between the Bi2O2Se fin and the LaAlO3 (100) substrate. e, Strain mapping (ɛxx) estimated from a filtered version of panel d. f, High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the Bi2O2Se/LaAlO3 interface with atomic model.
Extended Data Fig. 8 Contact resistance for 2D Bi2O2Se/Bi2SeO5/HfO2 FinFETs.
a–n, Transfer and output curves for channel lengths ranging from 400 nm to 3,280 nm. o, Transfer length model plot of total resistance (Rtot) versus channel length (Lch) from 2D Bi2O2Se/Bi2SeO5/HfO2 FinFETs. The lines represent linear fits to data and the intercept is used to extract contact resistance (RC) by means of the equation Rtot = Rch + 2RC, in which Rch is channel resistance.
Extended Data Fig. 9 Electrical performance of 2D FinFETs fabricated with Bi2SeO5 dielectric.
a, Schematic diagram of a 2D FinFET fabricated with Bi2SeO5 dielectric solely. b, Tilted-view SEM image of as-fabricated FinFET with a channel length (Lch) of 3 μm. c,d, Transfer (c) and output (d) curves of the FinFET in b.
Extended Data Fig. 10 Field-effect mobility of 2D Bi2O2Se/Bi2SeO5/HfO2 and Bi2O2Se/HfO2 FinFETs.
a,b, Schematic diagram of 2D FinFETs fabricated on 2D Bi2O2Se/Bi2SeO5 fin-oxide heterostructure (a) and 2D Bi2O2Se fin (b). c,d, Transfer curves obtained from fabricated 2D Bi2O2Se/Bi2SeO5/HfO2 FinFETs (c) and 2D Bi2O2Se/HfO2 FinFETs (d) with 1.5-μm channel length. e, Transconductance (gm) as a function of gate voltages for 2D Bi2O2Se/Bi2SeO5/HfO2 and Bi2O2Se/HfO2 FinFETs. f, Field-effect mobility (μ) as a function of gate voltages for 2D Bi2O2Se/Bi2SeO5/HfO2 and Bi2O2Se/HfO2 FinFETs.
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Tan, C., Yu, M., Tang, J. et al. 2D fin field-effect transistors integrated with epitaxial high-k gate oxide. Nature 616, 66–72 (2023). https://doi.org/10.1038/s41586-023-05797-z
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DOI: https://doi.org/10.1038/s41586-023-05797-z
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