Mengshi Yu ,Congwei Tan ,Xiaoyin Gao ,Junchuan Tang,Hailin Peng

Center for Nanochemistry,Beijing Science and Engineering Center for Nanocarbons,Beijing National Laboratory for Molecular Sciences,College of Chemistry and Molecular Engineering,Peking University,Beijing 100871,China.

Abstract: Two-dimensional(2D)semiconductors offer an atomic thickness that facilitates superior gate field penetration and enables transistors to maintain shrinking with suppressed short-channel effects,thereby being considered as channel materials for future transistors in the post-Moore era.As a member of highmobility 2D semiconductors,the air-stable Bi2O2Se with a moderate bandgap has drawn significant attention.Distinguished from other 2D materials,Bi2O2Se can be oxidized layer-by-layer to form a high-k native-oxide dielectric,Bi2SeO5,with an atomically sharp interface,similar to Si/SiO2 in the semiconductor industry.These characteristics make Bi2O2Se an ideal material platform for fabricating various devices with excellent performance,such as transistors,thermoelectrics,optoelectronics,sensors,flexible devices and memory devices.To realize advanced applications of 2D Bi2O2Se,it is essential to develop scalable and high-quality preparation methods with relatively low cost.Chemical vapor deposition(CVD)has shown promise in meeting these requirements.Over the past years,CVD has been widely used to synthesize 2D Bi2O2Se despite some remaining challenges.In this review,we summarize the recent progress in the controlled growth of 2D Bi2O2Se viathe CVD method.We begin by introducing the crystal structure and properties of Bi2O2Se.Next,we focus on the morphology control of 2D Bi2O2Se,including various nucleation modes and different dimensionalities by carefully manipulating the CVD process.In terms of nucleation modes,in-plane and vertical epitaxial growth of Bi2O2Se,achieved by controlling the interaction between epitaxial layer and substrate,are reviewed.Wafer-scale continuous Bi2O2Se film facilitates the device integration while vertical 2D fins pave the way for fabricating high-performance fin field-effect-transistors(FinFET).As for the dimensionality control,the transition from 2D nanoplates to 1D nanoribbons is investigated.Parameters such as precursor ratio,growth temperature and types of catalyst play a key role in such transition.We then discuss the construction of ordered arrays of Bi2O2Se with the above morphology by selective growth and post treatment for potential device integration.In addition,we highlight the electrical quality improvement of the grown material viadefect control and strain release.For example,both the Se poor growth condition and the out-of-plane strain-free growth contribute to higher mobility of Bi2O2Se.Lastly,we propose potential strategies for precise control of Bi2O2Se structures and quality.In order to meet the demands of advanced electronic applications,more efforts are expected to made to achieve uniform,transferable and site-specific preparation of highquality single-crystal Bi2O2Se on a large scale.

Key Words: Bi2O2Se;Chemical vapor deposition;Nucleation mode;Dimensionality;Array;Electrical quality

1 Introduction

The progress of modern information technology relies heavily on the development of integrated circuits based on silicon semiconductor.The complementary metal-oxide semiconductor(CMOS)transistor,which is based on traditional silicon semiconductor/oxide(such as Si/SiO2and Si/HfO2),has been the core technology for maintaining chip iteration under Moore’s law through continuous miniaturization of device sizes and optimization of device structures1.Currently,silicon-based integrated circuits have successfully iterated to the 3-nm technology nodeviathe fin field-effect transistor(FinFET)structure2.As transistor sizes continue to shrink,silicon-based semiconductor devices gradually expose difficulties that were not anticipated in their early development,such as increased surface defects,decreased mobility,and short-channel effect3,4.In order to overcome these challenges,high-mobility 2D semiconductors have emerged as potential candidates for channel materials in post-silicon era5.These materials possess unique properties,including a dangling-bond-free surface and atomic-scale thickness.Moreover,they offer distinct advantages such as superior gate control and high driving current,making them highly promising for next-generation electronics.

As an emerging member of 2D semiconductor family,the novel air-stable high-mobility 2D semiconductor Bi2O2Se has attracted much attention.It has an ultra-high carrier mobility(～2.8×105cm2·V-1·s-1at 2 K),a suitable bandgap(～0.8 eV)and small effective mass6,which is comparable to that of Si.Additionally,like Si/SiO2in the semiconductor industry,2D Bi2O2Se allows for the epitaxial integration of 2D semiconductor and high-κgate oxide(Bi2O2Se/Bi2SeO5)7,8.The layered nativeoxide Bi2SeO5is formed by controlled oxidation of Bi2O2Se,and the Bi2O2Se/Bi2SeO5interface intrinsically exhibits atomically flat.In particular,the epitaxial integration of Bi2O2Se/Bi2SeO5heterostructure is applicable to both planar and vertical fin structures simultaneously.Aforementioned characteristics have aroused researchers to employe Bi2O2Se as a material platform to explore high-performance 2D devices,such as electronics6,8-12,thermoelectrics13,optoelectronics14-16,sensors17,flexible devices18and memory devices19-21,etc.

Material preparation is the foundation for its advanced applications with increasing requirements.For 2D Bi2O2Se,the bottom-up strategy,such as chemical vapor deposition(CVD)6,22,23,molecular beam epitaxy(MBE)24,pulsed laser deposition(PLD)25,metal organic chemical vapor deposition(MOCVD)26,solution-assisted methods18,and hydrothermal methods27,28,plays a key role in synthesizing high-quality nanostructures.Among all these methods,CVD has attracted much attention,due to its potential of scalable and low-cost preparation of high-quality 2D Bi2O2Se.In addition,various combinations of raw reactants enable diverse synthesis methodologies of 2D Bi2O2Se.During CVD process,the complicated vapor-phase growth is susceptible to the influence of factors like growth temperature,growth rate and substrates.In recent years,numerous efforts have been made to manipulate the above conditions in order to obtain high-quality Bi2O2Se with desired structures.

Therefore,in this review,we summarize the recent progress of CVD synthesis of 2D Bi2O2Se,which exercises precise control over the structure and quality of Bi2O2Se crystal through manipulation of key parameters during the growth process.We start with a brief introduction of Bi2O2Se’s crystal structure and electrical properties.Then,we focus on the tunable nucleation modes of Bi2O2Se which lead to diverse structures and dimensionalities,including in-plane 2D nanoplates and waferscale continuous films,out-of-plane vertical 2D fins,and 1D nanoribbons.We also discuss the progress of constructing Bi2O2Se arraysviapost treatment and pre-designed selective growth.In addition,we highlight the defect and strain control of 2D Bi2O2Se to improve its electrical properties.Finally,based on the current achievement of CVD growth of 2D Bi2O2Se,we propose a few potential challenges and demand of growth for further advanced electronic applications in industry.

2 Structure and properties of Bi2O2Se

2.1 Crystal structure of Bi2O2Se

Fig.1 Superior properties of 2D semiconductor Bi2O2Se.

2.2 Electronic band structure and electric properties of Bi2O2Se

Bi2O2Se can be regarded as a derivative of the topological insulator Bi2Se3with substitution of selenium by oxygen.The presence of Bi element contributes to a strong spin-orbit coupling(SOC)in Bi2O2Se like other low-dimensional Birelated materials31.In experiments,Menget al.observed weak anti-localization in 2D Bi2O2Se nanosheets through lowtemperature magnetotransport measurements,demonstrating the existence of strong SOC in Bi2O2Se for the first time32.Strong SOC lift the degeneracy of electron states for electron moving in 2D geometries without inversion symmetry,i.e.Rashba effect.Eremeevet al.used ab initio calculation to obtain the surface structure of Bi2O2Se,and found that there is a large Rashba spin splitting on the polar surface of Bi2O2Se31.

The band structure of Bi2O2Se can be calculated using firstprinciples theory and characterized by angle-resolved photoemission spectroscopy(ARPES).The results from both methods are in good agreement6,showing that Bi2O2Se is an indirect bandgap semiconductor with a bandgap of approximately 0.8 eV(Fig.1a).The conduction band minimum(CBM)is located at theΓpoint and mainly consists of Biporbital electrons,while the valence band maximum(VBM)is located at theΧpoint and mainly consists of Sep-orbital holes(Fig.1a).For typical digital applications,the band gap of 2D semiconductors plays a crucial role in determining the on/off current ratio of transistors.Specifically,to achieve a high on/off current ratio exceeding 104,a minimum band gap of 0.4 eV is required.Therefore,the moderate bandgap of Bi2O2Se ensures the realization of advanced electronics with a significant high on/off ratio.Additionally,similar to other 2D materials,2D Bi2O2Se processes the remarkably potential to adjust its bandgap by varying thickness(Fig.1b),thereby exhibiting distinguished properties.

ARPES characterization also unveils the electron effective mass of Bi2O2Se,which measures approximately 0.14m0(Fig.1a).This value is lower than that of silicon(0.26m0)and comparable to other 2D semiconductor materials like MoS2(0.4-0.6m0)and black phosphorus(0.15m0)(Fig.1b).The low effective mass implies that Bi2O2Se has a high electron mobility.Wuet al.experimentally measured the electron mobility of 2D Bi2O2Se crystals at low temperature(2 K)to be as high as 28900 cm2·V-1·s-16.Remarkably,unlike 3D semiconductor materials,2D Bi2O2Se can maintain high mobility without significant attenuation as its thickness decreases(Fig.1c).More importantly,sub-10-nm-thick Bi2O2Se exhibits apparent fieldeffect mobility higher than 1000 cm2·V-1·s-1(Fig.1c)6.Furthermore,the mobility of Bi2O2Se at sub-5 nm is still comparable to that of Si and surpasses most other 2D semiconductor materials(Fig.1c)33,indicating its immense potential for fabricating ultra-scaled transistors free from shortchannel effect.

3 CVD Synthesis of 2D Bi2O2Se

3.1 Brief introduction of controlled CVD synthesis of 2D Bi2O2Se

Large-scale synthesis of high-quality materials is the foundation for the advanced functional applications.In order to obtain high-quality 2D Bi2O2Se,various material properties ought to be taken into consideration,such as the size,thickness,dimensionality,defects,strain,etc.For CVD method,the quality control of 2D Bi2O2Se relies on manipulation of key parameters throughout the growth process,such as synthesis methodology,growth temperature,growth rate and substrate engineering.The above factors influence processes including mass and heat transport,precursor adsorption,nucleation,diffusion and desorption.

Expanding synthesis methodologies enable new pathways to obtain 2D Bi2O2Se with different structures,properties,and applications.According to the Bi-O-Se ternary phase diagram(Fig.2),sources containing the elements Bi,O,and Se can all be utilized to synthesize Bi2O2Se.Therefore,several synthesis methodologies have been proven feasible to obtain Bi2O2Se,such as Bi2Se3and Bi2O36,7,23,77,78,Se + Bi2O379,Bi2Se3+ O280and Bi2O2Se22.Meanwhile,the growth temperature and rate play a key role in controlling the nucleation quality and structure of Bi2O2Se.As for the substrate engineering,symmetry and surface modification affect the orientation and electrical quality of Bi2O2Se.All these efforts contribute to the synthesis of 2D Bi2O2Se with diverse nucleation modes,dimensionality and quality.For example,as is shown in Fig.2,the nucleation modes contribute to different morphologies of Bi2O2Se,including inplane epitaxy of nanoplate and film,vertical epitaxy of 2D fins and out-of-plane free-standing 1D and 2D nanostructures.

Fig.2 Systematic design and control of chemical vapor deposition(CVD)synthesis of 2D Bi2O2Se.

3.2 Controlled nucleation modes of 2D Bi2O2Se

Van der Waals(vdW)epitaxial growth is widely employed for the synthesis of 2D materials.By carefully controlling the interaction between the epitaxial layer and the substrate,it is possible to manipulate the nucleation mode and achieve a transition between in-plane epitaxy and vertical epitaxy.

Using the CVD method,Wuet al.first synthesized in-plane 2D Bi2O2Se nanosheets on freshly exfoliated mica using Bi2Se3and Bi2O3as co-evaporation sources6.Mica,which is a typical non-neutral layered material,is composed of alternating layers of positively charged K+and negatively charged[Mg3(AlSi3O10)F2]-.The freshly cleaved surface of mica consists of atomically smooth K+layers,which establish robust electrostatic interaction with the epitaxial layer of Bi2O2Se(Fig.3a)80.This strong interaction plays a crucial role in promoting the lateral growth of Bi2O2Se,enabling the synthesis of 2D nanosheets.The as-grown in-plane 2D Bi2O2Se exhibited a square shape(Fig.3b)and its single-crystalline nature was further confirmed by transmission electron microscopy.Via precisely control the growth parameters including growth temperature,flow rate and growth time,it is available to synthesize 2D Bi2O2Se nanosheets with diverse thickness,even down to single-layer thickness(Fig.3c).

Fig.3 Controlled CVD synthesis of 2D Bi2O2Se on mica.

The synthesis of wafer-scale single crystal holds great significance because it enables large-scale integration of highperformance devices.One of the main challenges to synthesize large-size single crystal is grain boundaries caused by merging domains with different orientations.The strong scattering effect at grain boundaries severely degrades carrier mobility.Therefore,in recent years,numerous efforts have been devoted to achieve growth of large-size 2D Bi2O2Se without grain boundaries,which can be classified into two types.

The first approach involves the continuous growth of a single nucleus,leading to the formation of millimeter-sized single crystals.Khanet al.synthesized single-crystalline 2D Bi2O2Se nanosheet with domain size up to 2 mm,by utilizing bulk-Bi2O2Se as the precursor(Fig.3e)22.In this particular condition,the growth rate surpasses 300 μm·min-1,which notably exceeds the typical values reported in other studies,which are usually no more than 200 μm·min-1(Fig.3d).Another strategy is to suppresses the random nucleation and promote the lateral size of 2D Bi2O2Se crystal by introduce of a reverse Ar flow during the initial temperature ramping stage(Fig.3f)81.The largest domain size of as-synthesized 2D Bi2O2Se crystal is up to 750 μm(Fig.3g).The successful growth of millimeter-sized 2D Bi2O2Se is primarily attributed to its bonding anisotropy with mica substrate.This anisotropy facilitates significantly faster lateral growth compared to vertical growth.In addition,salt was introduced into CVD system to further lower the growth temperature for the synthesis of large-size single crystal82.The addition of NaCl plays a crucial role in effectively reducing the melting point of Bi2Se3,ensuring a high mass flux of the metallic precursor.Notably,through careful optimization of the salt-to-Bi2Se3ratio,2D Bi2O2Se nanosheet with domain size exceeding 3 mm can be obtained at a low temperature of 500 °C.

The second approach relies on the careful choice of a symmetrically matched substrate,allowing for the preparation of wafer-scale single-crystalline thin films through the seamless stitching of numerous mono-oriented nuclei.The symmetry relationship between epitaxy layer and substrate is a key factor that affected the orientation of epitaxy layer.As a widely employed substrate for growing 2D Bi2O2Se nanosheets,mica has a 6-fold symmetry.Therefore,when the 4-fold symmetry Bi2O2Se is epitaxially grown on mica,it exhibits three distinct orientations differing by 30°.In order to achieve mono-oriented nucleation,Tanet al.chose 4-fold symmetry perovskite oxides[LaAlO3,SrTiO3,(La,Sr)(Al,Ta)O3]as the epitaxial substrate,which show perfect symmetry/lattice match with Bi2O2Se(Fig.4a,b)23.These nuclei seamlessly stich together during subsequent growing process to form wafer-scale thin film with a maximum size of 2-inch(Fig.4c,d).Atomic-resolution crosssectional STEM-HAADF image and the corresponding FFT image demonstrate the perfect epitaxy and single crystallinity of Bi2O2Se and perovskite oxides(Fig.4e,f).

Fig.4 Wafer-scale CVD synthesis of 2D Bi2O2Se on perovskite oxides.

Fig.5 Controlled CVD synthesis of vertical Bi2O2Se fins.

3.3 Dimensionality control of Bi2O2Se

Different application scenarios require materials with different dimensionalities.Except for 2D structure,1D nanowire or nanoribbon structure have attracted significant attention due to its novel quantum confinement effects and edge-related properties.Normally,2D Bi2O2Se nanoplates undergo isotropic growth,while 1D Bi2O2Se nanoribbons go through anisotropic growth(Fig.6a).To achieve anisotropic growth with controlled length and width,there are two distinct approaches.One approach involves adjusting the key parameters in traditional Vapor-solid-solid(VSS)growth to achieve in-plane 1D growth,while the other focus on utilizing the Vapor-liquid-solid(VLS)growth mode to achieve out-of-plane 1D growth.

Fig.6 Dimensionality control of Bi2O2Se viaCVD synthesis.

For VSS growth mode,Khanet al.successfully achieved the transition from 2D Bi2O2Se nanoplates to 1D Bi2O2Se nanoribbons by precisely controlling the precursor ratio of Bi2O3and Bi2Se3,as well as the growth temperature(Fig.6b).Specifically,high ratio of Bi2O3/Bi2Se3(＞3)and relatively low growth temperature(600-670 °C)enable the synthesis of 1D Bi2O2Se nanoribbons.It is noteworthy that this method is effective in producing 1D Bi2O2Se nanoribbons with a monolayer thickness(Fig.6f-g)77.In addition,Liet al.developed a space-confined CVD method by placing the substrate in an inner tube,realizing the oriented epitaxial growth of 1D Bi2O2Se nanowires(Fig.6c)84.The epitaxial relationship between Bi2O2Se(110)and mica(00n)planes lead to three distinct orientations differing by 60°.As for VLS growth mode,Yinget al.reported the synthesis of 1D Bi2O2Se nanowires with the assistance of Au catalyst(Fig.6d)85.There is evident Auball on the top of nanowire,which further confirms the VLS growth mode.Tanet al.also proposed the facile growth of outof-plane 1D Bi2O2Se nanoribbonsviametal Bi catalyzed VLS mechanism86.The as-synthesized Bi2O2Se nanoribbons can be easily broken from the root under slight external perturbation,such as low-voltage SEM irradiation.As the temperature increases,the length of as-grown Bi2O2Se nanoribbons initially increases and then decreases gradually,exhibiting a peak value of approximately 280 μm at 610 °C(Fig.6e).This phenomenon may attribute to the competition between catalyzed effect and chemical reaction of Bi.The out-of-plane nanoribbons can be easily transferred to Si/SiO2substrates,facilitating the fabrication of high-performance electronic devices.

3.4 Precise preparation of 2D Bi2O2Se ordered arrays

Patterned and highly ordered arrays of Bi2O2Se are essential for constructing complex logic circuits,therefore attract the attention of many researchers87.In 2017,Wuet al.reported a method for preparing an ordered array of Bi2O2Se crystals by chemical etching(Fig.7a)88.They used photolithography to pattern the continuous film,and selectively etched the exposed areas using a mixed solution of H2SO4and H2O2to obtain a regulated array of Bi2O2Se crystals(Fig.7b).Based on patterned arrays of Bi2O2Se crystals,high-performance integrated optoelectronic detectors,logic circuits and sensors can be constructed.

Fig.7 Approaches for preparing ordered 2D Bi2O2Se arrays.

In addition to post-processing,selective growth such as template-assisted growth is also widely used to prepare ordered arrays.Wuet al.used a stamp made of polydimethylsiloxane(PDMS)with a specific pattern to leave PDMS oligomers as a mask on the growth substrate,thereby achieving selectivegrowth of 2D Bi2O2Se films(Fig.7c-d)80.For the patterned growth of vertical Bi2O2Se fins,it is hard to create such small confinement spaces to achieve selective-growth due to the extremely small contact area between the fins and the substrate.Therefore,Tanet al.developed a defect-induced method to synthesize patterned 2D Bi2O2Se fins(Fig.7e)7.The arrays of pre-designed defect sites on the substrate surface act as the preferential nucleation sites of Bi2O2Se fins,resulting in the precise preparation of 2D Bi2O2Se fin arrays(Fig.7f).

3.5 Electrical quality control of 2D Bi2O2Se

The preparation of high-quality materials helps to improve and expand their application performance.High-speed electronic devices require semiconductor channel materials with high mobility to increase device operation speed.Low-power devices require channel materials with low residual carrier concentration to turn off the device at lower operating voltages,thereby achieving lower power consumption.Therefore,for semiconductors aimed at electronic device applications,carrier concentration and mobility are two basic parameters that measure their quality.

Defect is an important factor affecting the carrier concentration and mobility.Therefore,we can control the quality of the material by changing the type and concentration of defects during the growth process.As shown in Fig.8a,there are five possible types of defects in Bi2O2Se,namely Se vacancies(VSe),Se antisites at Bi positions(SeBi),oxygen vacancies(VO),Bi vacancies(VBi),and Bi antisites at Se positions(BiSe)89.VSeand SeBidefects are the dominant electron donors,which can donate 2 and 1 electron,respectively.Unlike ordinary semiconductors whose defect states are located in the band gap,VSeand SeBidefects make the Fermi energy up into the conduction band(Fig.8b,c).Calculations indicate that under Serich conditions,SeBihas the lowest formation energy(Fig.8d)and is the major defect in Bi2O2Se,while VSeis the major defect under Se-poor conditions(Fig.8d)89.SeBiis located in the electron transport layer(Bi—O layer),which affects electron transport and significantly reduces the electron mobility.VSeis located far away from the electron transport layer and has little scattering effect on electrons.Therefore,under Se-poor growth conditions,SeBiin Bi2O2Se can be suppressed to achieve high mobility.The low-temperature mobility of the Bi2O2Se grown under Se-poor conditions can reach as high as 10000 cm2·V-1·s-1,while that of the 2D Bi2O2Se grown under Se-rich conditions is only 1000 cm2·V-1·s-1(Fig.8e)89.Growth conditions not only affect the type of defects but also the defect concentration.When Se and Bi2O3are used as sources to grow Bi2O2Se,VSeis the major defect and the VSeconcentration is low.Therefore,compared with the growth method using Bi2Se3and Bi2O3as sources,the carrier concentration of the material is reduced by 2-3 orders of magnitude,while the mobility remains high(Fig.8f)79.In the case of bulk Bi2O2Se used as the source,the carrier concentration falls between those of the aforementioned two types.However,the mobility appears to be higher,which can be attributed to the presence of fewer defects.The as-synthesized Bi2O2Se with low carrier concentration has great advantages in constructing low-power devices.Further studies are required to clarify the relationship between residual carrier concentration and defects for controllable preparation of Bi2O2Se with low carrier concentration.

Fig.8 Defect control and self-modulation doping effect of 2D Bi2O2Se viaCVD synthesis.

The magnitude of the material's inherent strain also affects the mobility.2D Bi2O2Se nanosheets grown in-plane may form wrinkles and introduce internal strain during the cooling process due to the different thermal expansion coefficients with the substrate(Fig.9a,b).In contrast,2D Bi2O2Se grown out-of-plane has minimal contact with the substrate surface and is not affected by the contraction of the substrate lattice during cooling,thereby avoiding the structural strain(Fig.9c).Recently,Tanet al.achieved the preparation of strain-free 2D Bi2O2Se nanosheets based on self-catalyzed VLS growth without any external catalyst(Fig.9d)51.The Bi2Se3source was premelted to deposit Bi spheres on the substrate,which acted as the catalyst of growth.The strainfree synthesis results in a high-quality 2D Bi2O2Se single crystal with ultra-high carrier mobility compared with in-plane strained nanoplate(Fig.9e,f),which has unique advantages in building ultra-fast,low-power electronic and optoelectronic devices.Moreover,the free-standing 2D Bi2O2Se nanosheets can be directly transferred to any functional substrate without polymer assistance.It could further potentially overcome the material bottleneck issue in exploring new multifunctional and highperformance electronic and optoelectronic devices.

Fig.9 Improved electrical properties of 2D Bi2O2Se viastrain-free out-of-plane CVD synthesis.

4 Conclusions and outlook

In this review,we systematically discuss the synthesis of 2D Bi2O2Se semiconductorviaCVD approach.We first introduce the crystalline structure and electrical properties of 2D Bi2O2Se,emphasizing its unique non-van der Waals layered structure,moderate bandgap,and exceptional mobility.Next,we delve into the control of morphology and dimensionality of Bi2O2Se including planar 2D nanoplates/films,vertical 2D fins and 1D nanoribbons,determined by various nucleation types.By employing various synthesis methodologies and adjusting parameters like substrates,temperatures,growth rate and so on,precise control over the anisotropic growth and shape of Bi2O2Se can be achieved,leading to tailored properties suitable for specific purposes.We then discuss the precise preparation of ordered arrays of Bi2O2Seviapost treatment and selective growth.Finally,we focus on methods for improving the electrical quality of 2D Bi2O2Se,including the manipulation of defect types and the adjustment of strain magnitude.Through this comprehensive discussion,we aim to provide valuable insights into the synthesis,control,and quality improvement of 2D Bi2O2Se,contributing to the advancement of this promising semiconductor material.

Despite the significant progress that has been made in synthesizing high-quality Bi2O2Se through CVD for highperformance devices,there are still several obstacles that need to be addressed for its broader applications in the industry.It is still challenging to achieve uniform,transferable,and site-specific preparation of high-quality single crystals on a large scale.In the following section,we will discuss the synthesis requirements of Bi2O2Se film and fin arrays for further industrial applications.

Currently,industrial logic devices are mainly fabricated on 8-inch wafers,while the largest reported size of Bi2O2Se thin film is only 2 inches.Therefore,additional efforts are required to achieve large-area preparation of thin films comparable to the industry.One of the main challenges in larger-scale synthesis is the control of film uniformity,including thickness and surface roughness.As for fin arrays,the current fabrication has been limited to 1-inch wafers,which is constrained by the size of the growth substrates.To achieve larger-scale fabrication of fin arrays,two approaches can be considered.The first one involves obtaining larger single crystal substrates that have been proven suitable for vertical growth.The other one involves selecting commercially available single crystal substrates with larger scale that have the potential for vertical growth.Similar to in-plane growth,uniformity control of fin arrays is also a challenge.The variation in fin thickness and height makes it difficult for the integration of ultra-thin fin/oxide heterostructures.To address the issues above,novel CVD systems can be designed and constructed.

To further integrate Bi2O2Se into semiconductor industry,especially the future 3D integration with multiple stacking,direct growth on commercial substrates like SiO2/Si as well as a clean and non-destructive transfer method for large-scale films/fin arrays are highly desirable.For 3D integrated circuits,it is imperative to enhance the Bi2O2Se’s thermal stability and heat dissipation capability of Bi2O2Se-based devices.Furthermore,as complementary components of future Si chips,back-end-of-line integration of 2D Bi2O2Se crystals on the top of Si-CMOS circuits through wafer-bonding is also expected.

In addition to large-scale synthesis and transfer,precise site control of 2D Bi2O2Se is essential for its integration into the semiconductor industry.Several methods have been proposed to achieve ordered arrays of Bi2O2Se,including top-down etching,selective-area epitaxy,and defect-induced growth.These methods allow for the fabrication of 2D nanoplates with sizes ranging from several micrometers to tens of micrometers,and with spacing in the micrometer to tens of micrometer range.Similarly,the spacing of 2D fin arrays can also be achieved at the micron scale.However,the integration of advanced FinFET and gate-all-around devices require dimensions and spacing in the tens of nanometers range.Therefore,it is necessary to employ higher-resolution lithography equipment to achieve higher-density and miniaturized nanoplate/fin arrays on the wafer-scale substrate.On the one hand,advanced lithography is beneficial to create high-density small-sized patterned masks,enabling synthesis high-density 2D nanoplate arraysviaselective-area etching or selective-area epitaxy.On the other hand,it is possible to construct high-density nanoscale defect arrays to induce vertical growth.In this way,the density and regularity of the fin arrays can be further improved.The synthesis of high-density ordered arrays will facilitate the fabrication of integrated circuits based on 2D Bi2O2Se with various structures.

The high quality of CVD grown 2D Bi2O2Se crystals plays a crucial role in advanced applications.As is discussed in our text,defects and strain greatly influence the material quality.By selecting appropriate growth methodology,we can control the volatile precursor composition,thereby probably influencing the types and concentrations of defects in 2D Bi2O2Se.On the other hand,more strain-free growth approaches and substrates should be developed.

To conclude,Bi2O2Se is a promising 2D semiconductor material for next-generation electronics,which serve as a complementary channel material to Si and enable highperformance devices.However,integration of 2D Bi2O2Se into semiconductor industry still requires significant effort in terms of scalable synthesis of high-quality and uniform 2D single crystals.

Author Contributions:Investigation,Mengshi Yu,Xiaoyin Gao,Junchuan Tang;Writing - Original Draft Preparation,Mengshi Yu,Xiaoyin Gao;Writing - Review &Editing,Mengshi Yu,Congwei Tan,Xiaoyin Gao;Supervision,Hailin Peng.