Room-temperature printing of 2D GaN semiconductor via liquid metal gallium surface conned nitridation reaction

Outstanding wide-bandgap semiconductor materials like gallium nitride (GaN) have been extensively utilized in power electronics, radiofrequency power ampliers, and harsh environment adaptability. Due to its quantum connement impact in enabling desired deep-ultraviolet emission, excitonic impact, and electronic transport features, two-dimensional (2D) GaN has been one of the most remarkable areas for the future growth of microelectronic devices. Here, for the rst time, we report a large area, wide bandgap, and room-temperature 2D GaN synthesis and printing strategy via liquid metal gallium surface-conned nitridation reaction. The developed low-temperature synthesis and printing process is consistent with various electronic device manufacturing methods and thus opens a way for the cost-effective growth of the third-generation semiconductor. In particular, the fully printed eld-effect transistors relying on the GaN show p-type switching with an on/off ratio greater than 105, maximum eld-effect hole mobility of 53 cm2 V−1 s−1, and a small sub-threshold swing at room temperature. The current study establishes a room temperature way to produce the GaN, which can be further veried, generalized, and realized for various upcoming electronic and photoelectronic applications.


Introduction
Semiconductor technology is the fundamental core of the integrated circuit industry. Since the bottleneck of the rst two generations of semiconductors, the third-generation high-temperature wide-band-gap semiconductor nanomaterials like gallium nitride (GaN) 1 , zinc oxide (ZnO), aluminum nitride (AlN) 2 , silicon carbide (SiC) 3,4 , and diamond, have been developed recently. GaN nanomaterials are novel semiconductor materials owning some bene ts like high saturated electron mobility, radiation resistance, acid and alkali corrosion resistance, high thermal conductivity, and high breakdown eld. In GaN electronic devices, the inevitable electron transfer from the valence band to the conduction band can be suppressed through the wide energy bandgap. The ambient energy from high electric elds, high temperature, and high-energy particles can activate the mentioned electron transfer 5 . Accordingly, the devices can retain their electrical features in various scenarios. The GaN is one of the most attractive semiconductor materials due to its outstanding e ciency and stability 6,7 . Various studies have been devoted to developing and analyzing the GAN materials in the literature 8- 10 .
Compared with the corresponding bulk GaN materials, various nanoscale effects of low dimensional GaN materials can show better photoelectric, mechanical, thermal stability, electrical 11,12 , and chemical features 13 . Apart from the fundamental physicochemical features of GaN, they also have the surface, small-size, and quantum-con nement impacts as one of the exciting areas for future growth of microelectronic devices. However, it is not easy to construct a low-dimensional GaN. Syed et al. reported a two-step process to synthesis two-dimensional (2D) GaN nanosheets, the process includes obtaining 2D Ga 2 O 3 by extrusion printing, and then converting Ga 2 O 3 to GaN by ammonolysis in a tubular furnace 14 .
Chen et al. reported the development of 2D GaN single crystals attained using a surface-con ned nitridation reaction (SCNR) through the chemical vapor deposition (CVD) 15 . In 2016, Al Balushi et al. employed the migration-enhanced encapsulated growth method to construct 2D GaN monolayer nanosheets 16 . However, the temperatures used in the most of process are above 500 ℃ that is inconsistent with various electronic industry operations. The long run time of the deposition process can increase the cost and feasibility. The construction and research technology of low-dimensional GaN materials and devices should be developed and enhanced to satisfy the appeals of practical applications.
The low temperature preparation of large-area, high-quality and uniform GaN lms will have a signi cant impact on high thermal stability 2D integrated circuit industry designed for power electronics applications. However, so far, there is still no report on the room temperature preparation of 2D GaN lms.
Herein, we proposed and demonstrated for the rst time to print the 2D GaN lms on SiO 2 /Si substrates through introducing a liquid metal-based synthesis and printing processes at room temperature. The process relies on using nitrogen plasma to trigger nitriding of gallium droplets at room temperature, and then transferring them to the substrate through the proposed van der Waals (vdW) printing technology.
The whole preparation process is performed at room temperature that is consistent with the current manufacturing processes. The proposed method can produce monolayer, and multilayer GaN attained via individual or multiple prints, respectively. Morever, we report excellent electronic performance of the printed 2D GaN. Fully printed side-gated eld-effect transistors (FETs) are fabricated, from which the 2D GaN-FETs exhibit outstanding performance with a large current on/off ratio (> 10 5 ), high eld-effect mobility (~53 cm 2 V −1 s −1 ), and tiny subthreshold slopes (~ 98 mV dec −1 ) with a high degree of reproducibility. This study introduces a reliable and straightforward large-scale manufacturing technique for 2D GaN and its features, which opens great practical potential for wafer-scale processes. It also paves the way for the application of GaN semiconductor in a new generation of all printed electronic devices, integrated circuits and more functional devices.

Results And Discussion
Liquid metal-based 2D GaN printing technology. The schematic in Fig. 1a illustrates the preparation of ultrathin 2D GaN layer using N 2 -plasma treatment technology at room temperature. The plasma nitriding process employs an unusual glow discharge involving high current and charge densities. A potential difference is generated by applying a DC voltage between the components (cathode) and the furnace walls (anode), which ionizes the treatment gas to produce the glow discharge. The treatment gas's positive ions speed up to the negatively connected parts and hit the surface with high kinetic energy, sputtering the surface and heating the whole parts. Nitrogen plasma is an electrically neutral aggregate composed of N + , N 2 + , e and N 2 formed after nitrogen is ionized. In the plasma nitriding of gallium, nitrogen ions and accelerated neutral nitrogen atoms lie on the surface to construct a nitrogen-rich lm that forms a nitride layer on the surface. Because the chemical reaction of nitrogen plasma is a thermodynamically stable excited state and ionic state, and the reaction activation energy is very low, it is much easier to generate GaN by the reaction of nitrogen plasma with liquid Ga than by the traditional nitriding reaction Ga (CH 3 ) 3 + NH 3 → GaN + 3CH 4 .
The schematic setup of the system to transform the liquid Ga to 2D GaN is elucidated in Fig. 1b. The formation of 2D GaN thin lms was realized by nitrogen plasma triggering surface limited nitriding reaction. Gallium droplets with plasma treated surface were placed on SiO 2 /Si substrate and moved on the surface with the help of scraper, during the scraping process of liquid Ga, the surface nitride produced by plasma bombardment in nitrogen environment sticks to the substrate through van der Waals force ( Fig. 1c) (Detailed description of the process parameters will be given in the Methods.). The nitridation reaction and printing of Ga both are conducted at room temperature that is consistent with the current electronic device production processes. The covered substrate area can be expanded by choosing a larger droplet diameter and a longer travel distance of the scraper. The thickness of the lm can be increased via frequent printing (Methods and Supplementary Fig. 2). Figure 1d revealed a large and continuous ultrathin GaN lm reaching lateral dimensions more than many centimeters. Based on atomic force microscopy (AFM), the thickness of the deposited GaN layer is ~4 nm that is moderately greater than a single GaN unit cell (Fig. 1e). AFM also indicates that the printed GaN lm's surface roughness is analogous to that of the SiO 2 substrate, demonstrating that the 2D GaN lm has minimum cracks, holes, folds, or bubbles, re ecting conformal and homogeneous attachment. The uniform thickness in broad regions and numerous samples supports the proposed Cabrera-Mott growth mechanism, where concurrent nitrides are formed among the whole metal interface, resulting in self-limiting growth to an accurate thickness. The mentioned approach is highly reproducible for growing large-area GaN sheets because the process was terminated more than 50 times and always yielded identical, continuous, laterally extensive atomically thin GaN lms with reproducible features. The GaN lm was initially printed on the SiO 2 /Si substrate, but further tests demonstrated that the formation of uniform centimeter-scale semiconductor lms on various substrates could be reproduced through the printing technology, indicating that the presented construction approach is suitable to deposit 2D GaN for several materials. Moreover, it should be pointed out that this method is also applicable to the fabrication of GaN heterostructures. In order to be compatible with silicon-based electronic technology, the 2D lm printed on SiO 2 /Si surface should be employed for further description and device construction.
Properties of the 2D GaN. Transmission electron microscopy (TEM) can be utilized to verify the crystallographic features of printed 2D GaN. The 2D GaN lms were immediately moved to a TEM grid since it was printed. A high-resolution TEM micrograph (HRTEM) and selective area electron diffraction (SAED) pattern of the GaN are presented in Fig. 2a and 2b, respectively, which con rms the crystallization of the printed GaN in the polymorph. The atomic spacing of 0.285 nm in the HRTEM image and the distance of the fringes at 0.266 nm correspond to the (100) and (002) planes of GaN, respectively, and that at 0.248 nm corresponds to the (101) plane.
The 2D GaN lm's phonon modes also change compared to those for the bulk 17 . Based on the Raman spectra presented in Fig. 2c, two peaks at 561 and 735 cm −1 appear in 2D printed GaN, respectively, corresponding to E 1 (TO) and A 1 (LO) modes in the bulk phase. Moreover, the above distinction re ects the change in GaN's phonons modes in 2D limit, while its location at 566.2 cm −1 is blue-shifted comparing to that of the bulk, re ecting a tensile-strain state in the 2D limit 18 . The peak position of A 1 (LO) mode corresponds to 731 cm −1 , and there is a blue shift relative to the peak position of 736 cm −1 in the bulk phase. Generally speaking, the parameters in uencing Raman scattering involve material size, order, internal stress and structural defects. Copwell et al. 19 believe that the reduction of nano material size will lead to the movement, broadening and asymmetric peak shape. In addition, the proposed spectrum lacks various Raman properties of Ga 2 O 3 (i.e., peaks at ~167, ~320, ~344, and ~475 cm −1 ), indicating that GaN is successfully constructed and Ga is quantitatively changed within plasma discharge nitriding reaction. It was observed that the Raman peak intensity for 456.6 cm −1 modes was notably decreased than that of the bulk counterpart, while the A 1 Raman mode is susceptible to the free charge carrier density in graphene and 2D metal chalcogenides [20][21][22][23] Fig. 3b, the measured band gap derived from the UV-Vis absorption is 3.3 eV, which agrees well with the value derived from HSE06 calculation. By comparison, the calculated bandgap using energy density PBE functional analysis is smaller than the measurement result. Figure 3c shows a periodic slab of GaN with a non-polar (110) surface sliced from the wurtzite bulk phase.
Application of 2D GaN in electronic devices. For experimental assessment of the electron transport features of printed 2D GaN and to investigate the potential for electronic devices, eld-effect transistors (FETs) were constructed to assess the 2D GaN for electronic device applications. Figure 4a shows the structure diagram of the transistor based on printed 2D GaN. We have adopted an individual side-gate design for all of the devices fabricated in this study, and detailed description of the fabrication process will be presented in the Methods section. Figure 4b presents a scanning electron microscopy (SEM) image of the device. Ag was used as gate electrodes and source-drain metal contacts, and FET channels were patterned with a width of W ch = 1000 µm and the length of L ch = 50 µm. Electrical measurements were carried out for the printed side-gated 2D GaN FET. Figure 4c presents the transfer (drain current, I ds , with respect to the gate voltage, V gs ) features of a representative 2D GaN FET. Figure 4d presents the I ds with respect to the drain-source voltage (V ds ) at various values of V gs applied to the device. As presented in Fig. 4c and d, the printed 2D GaN FET devices' p-type switching feature with an on/off ratio is more  (Fig. 4e-g), respectively. A desired was seen between various devices, considering that the mentioned devices were constructed in an academic laboratory. This uniformity yields con dence in employing the mentioned devices and methods in numerous applications like integrated circuits and active-matrix back-planes for displays. Moreover, the 2D GaN-based FETs were signi cantly stable, having a high cyclability with a stable on/off ratio and stable on currents for more than 100 switching cycles under ambient conditions ( Supplementary Fig. 4).
Notably, the observed eld-effect mobility is greater compared to that of traditional high-e ciency broadbandgap GaN-based devices 14,29 . For further broader comparison, Fig. 5 plots SS versus mobility for ptype and n-type FETs reported in the literature as well as the device from this study, the value of SS is comparable to that appeared in some of the best p-type and n-type oxide semiconductors like indium oxide, this highlights the fact that our approach offers a high current mobility while maintaining a small SS, which is mainly attributed to the high-quality electron-level GaN semiconductor. Besides, no detectable performance degradation was observed in the devices while working under ambient situations without encapsulation. This highlights that the printed GaN is of exceptional quality while indicating that additional enhancements can be achieved via the enhanced device construction. Future studies should concentrate on constructing devices that can integrate separately addressable transistors into more complicated circuits. Potential approaches can involve using vdW heterostructures with dielectrics like hexagonal boron nitride or Ga 2 O 3 combined with top gates 30,31 . The mentioned device con gurations can be employed to determine essential parameters like the pinch-off voltage that can be informative for incorporating future devices into functional circuits.

Conclusions
Liquid metals have various possible functionalities. The essential value of the mentioned materials is that they can serve as a reactive and templating media simultaneously and thus to synthesis and print a wide range of 2D semiconductor materials with extensive applications. In the current work, we presented the successful printing of the high-quality 2D GaN lms with a large scale via a plasma nitridation reaction and through transferring occurring in the liquid metal's nitride skin constructed under this speci c condition. Moreover, complete explanation and analysis have been performed on the electrical features of printed 2D GaN lms, giving essential advice for further evolution of liquid metal printed highperformance semiconductors and indicating their considerable potential for future nanoelectronics.
Notably, we effectively presented the printed FET devices using 2D GaN for the rst time, indicating signi cant eld-effect mobility and relatively small values of SS, demonstrating an extremely steep subthreshold voltage switching behavior. This high-e ciency, simple, large-size, and cheap production process provides a new way for advancing GaN transistors for power electronics applications. Moreover, the 2D semiconductor can be produced as the favored material in printing different electronic devices due to the mentioned advantages. This sets up a new standard for subsequent electronics, sensors, and more practical devices. It also presents a pathway toward employing printed 2D high-e ciency semiconductors to construct novel electronic devices using liquid metal-enabled techniques.

Methods
Materials. Gallium with a purity of >99.99% was bought from Sigma Aldrich and utilized without additional re nement. The remaining materials were bought from usual suppliers and employed without re nement.
Formation of GaN on liquid gallium by nitridation reaction. In order to obtain 2D GaN lms with high purity and low defects, the whole preparation and printing process were carried out in a glovebox in a pure N 2 environment at a pressure of 1 atm ~ 3 atm. The O 2 content in the glove box atmosphere is controlled below 3 ppm and the H 2 O content is controlled below 0.5 ppm. High purity Ga was melted and then placed in NaOH solution to remove the oxide scale on the surface. The clean liquid Ga was extracted to 10 ml by a syringe and place it on the surface of the low-voltage electrode stainless steel plate of the plasma trigger device, and the stainless steel plate is grounded. In the environment of high purity N 2 , it can be seen that the surface of liquid Ga is bright without any oxide lm. The stainless steel disk wrapped in transparent quartz of plasma high-voltage electrode is suspended vertically above liquid Ga, and the distance between the disk and the surface of liquid Ga is about 1mm. A voltage of 50 kV is applied between the two electrodes of the plasma trigger device through the voltage regulating device. At this time, the electric eld strength of N 2 breakdown is 5 × 10 7 V m −1 , the output current is 16 A, and the uniform and dense GaN skin can be obtained on the liquid Ga surface after glow discharge treatment for a certain time.
Printing process of 2D GaN lms. Wafers of 500 nm SiO 2 on Si (SiO 2 /Si) were washed by the deionized (DI) water for 1 min, and then sonicated in acetone and isopropyl alcohol (IPA) for 10 min (25°C) and blown dry with N 2 gas. The 3 min of oxygen (O 2 ) plasma (Emitech K-1050X) at 100 W was then applied to wafers under the low vacuum (0.6 Torr). The nitrided Ga droplets was placed on the SiO 2 /Si substrate and execute the complete printing program. The size of 2D nitride lm changes with the droplet diameter and the travel distance of the scraper. The nitride layer formed on the liquid Ga's surface can be transformed and printed on the substrate by gently scraping the droplets from one end of the substrate to the other end with a scraper. An extra force within the extrusion phase can damage the nitride layer. By this extrusion printing method, high-quality GaN lms with a transverse size greater than a few centimeters can be effectively printed on the substrate.
Mechanical and chemical cleaning process. In order to remove all liquid metal parts that remained on the sample, a facile mechanical ethanol cleaning method was employed. At rst, around 100 ml of ethanol were taken in a beaker, and the beaker was heated on a heating plate to 100°C. Then, the substrate with printed 2D nitride lm was immersed in hot ethanol with tweezers. In order to eliminate the metal residues, a wiping tool (swab) was utilized for wiping the substrate immersed in ethanol. Due to a strong vdW adhesion between the nitride lm and the bottom layer, the nitride lm still sticks to the silicon oxide surface within the wiping proceeding. Besides, the weak adherence between the deposited nitride layer, the liquid metal, and the lm could be quickly removed to maintain the 2D lm clean and intact. Moreover, a chemical process was employed to clean the samples for a complete elimination of the metal residue on the substrate. An Iodine/triiodide (I − /I 3− ) solution (100 mmol L −1 LiI and 5 mmol L −1 I 2 ) was constructed in ethanol and then located on a hot plate to heat up to 50 ℃. In order to eliminate metallic inclusions, a substrate printed with the 2D-GaN lm was immersed in a heated I − /I 3− solution for a time interval. At last, the residual etchant was removed by cleaning the sample in deionized water. The liquid metal particles can be successfully eliminated using the mentioned two cleaning processes.
FET fabrication. In order to construct fully printed side-gated 2D GaN FETs, rstly, part of the 2D GaN/SiO 2 region is etched with HF solution with concentration of 1mol L −1 for about 10 s. Then, the etching area was cleaned with ethanol, the clean Si of a certain area was obtained on the substrate. The Ag ink (Ag40X, UT Dots, Inc.) involved 40 wt % Ag nanoparticles, with about 20 nm particle diameters, dispersed in a solvent mixture of xylene and terpineol (9:1 by volume). The constructed ink was printed on the 2D GaN lms and SiO 2 /Si substrates. A scienti c 3B inkjet printer from Prtronic was adopted to verify the inkjet printing details. The obtained sample was located on the inkjet printer's panel at room temperature. A target image le was applied to the computer, which could be converted into a printable le through the software. Under computer control, source/drain electrodes and side-gate electrodes were subsequently patterned by printing Ag on the substrate. Finally, the printed samples were then sedimented at 120°C in the air for 30 min in a furnace (MDL 281, Fisher Scienti c Co.) to improve the conductivity. Characterization.
The AFM images were employed by a Bruker Dimension Icon with "Scanasyst-air" AFM tips. A JEOL 2100F TEM/STEM (2011) system working at a 200 kV acceleration voltage involving a bright-eld Gatan OneView 4k charge-coupled device (CCD) camera was utilized for both the low-resolution HRTEM imaging and SAED. A laser micro-Raman spectrometer (Renishaw in Via, 532 nm excitation wavelength) was adopted to accomplish the Raman spectroscopy. Moreover, the energy dispersive X-ray spectroscopic (EDS) measurements were used to collect the elemental mapping of the as-prepared samples. A thermo Scienti c K-alpha XPS spectrometer associated with monochromatic X-rays from an Al anode (hν=~1486.6 eV) was adopted to perform the XPS analysis. A UV-visible (UV-vis) absorbance spectrometer (Hitachi U3900 UV-vis spectrophotometer) was utilized to evaluate the lm's optical bandgaps. A Cascade Microtech Summit 12000 semiautomated probe station linked to a Keithley 4200 Semiconductor Device Analyzer was adopted to measure 2D GaN FETs at room temperature.

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download. GaNSI.docx