Research progress of out-of-plane GeSn nanowires

With the increasing integration density of silicon-based circuits, traditional electrical interconnections have shown their technological limitations. In recent years, GeSn materials have attracted great interest due to their potential direct bandgap transition and compatibility with silicon-based technologies. GeSn materials, including GeSn films, GeSn alloys, and GeSn nanowires, are adjustable, scalable, and compatible with silicon. GeSn nanowires, as one-dimensional (1D) nanomaterials, including out-of-plane GeSn nanowires and in-plane GeSn nanowires, have different properties from those of bulk materials due to their distinctive structures. However, the synthesis and potential applications of out of plane GeSn nanowires are rarely compared to highlighting their current development status and research trends in relevant review papers. In this article, we present the preparation of out-of-plane GeSn nanowires using top-down (etching and lithography) and bottom-up (vapor–liquid–solid) growth mechanism in the vapor-phase method and supercritical fluid–liquid–solid, solution-liquid–solid, and solvent vapor growth mechanisms in the liquid-phase method) methods. Specifically, the research progress on typical out of plane GeSn nanowires are discussed, while some current development bottlenecks are also been identified. Finally, it is also provided a brief description of the applications of out-of-plane GeSn nanowires with various Sn contents and morphologies.


Introduction
Moore's Law is a reasonable speculation based on the realization of the development of the integrated circuit industry.New theories and technologies have propelled the industry into the post-Moore era [1].As device sizes continue to decrease, technological bottlenecks have significantly constrained process development, and the current rate of product iteration has declined.Therefore, the development direction of the IC industry and technology needs to be revisited.Currently, the industry has proposed four paths for industry development in the post-Moore era, i.e.More Moore, More than Moore, Beyond Moore, and Much Moore.Currently, in the architecture of integrated circuits, the transfer and processing of information is based on the electron as the basic unit.From the point of view of information transfer, a single electron cannot transfer information, and a combination of multiple electrons can carry information.At the same time, there will be energy consumption and heat generation in the signal transmission process.If we can find other basic units can carry information or information transfer process will not consume energy, reduce power consumption and improve performance, break the bottleneck faced by the development of the problem, which belongs to Beyond Moore.
At present, Beyond Moore is mainly in the research stage, nanowires (NWs) based devices are one of the hot spots in the Beyond Moore.Therefore, with the development of nanomaterials and nanodevices, we are entering an era of nanotechnology.Nanomaterials exhibit more fascinating new physical and chemical properties than similar bulk materials [2].Haraguchi et al realized the first device fabricated from 1D nanostructures in 1992 [3], thus opening the revolutionary era of 1D nanomaterials.NWs have quantum confinement effects due to their diameter at the nanoscale, but the unconstrained length of NWs makes them easy to integrate into the existing devices.Therefore, the potential applications of 1D NWs in various fields (photodetectors [4], biosensors [5], solar cells [6], transistors [7], battery electrodes [8], energy storage [9,10], catalysis [11]) have attracted attention.The research on group IV semiconductor NWs has been of great interest [12,13].Among group IV semiconductor NWs, silicon NW is the most representative, which has the special properties of semiconductors.Group IV semiconductor NWs have many properties such as environmental friendliness, biocompatibility, easy surface modification, and compatibility with the semiconductor industry.Therefore, they have great potential for applications in nano-electronic devices, optoelectronic devices and new energy sources.However, the inherent properties of silicon and its lack of photoemission have limited the development of microelectronics and optoelectronics.
Although miniaturization beyond Moore's law can be pursued through the design and architecture of different samples and materials, it is limited by quantum-mechanical effects [14].Therefore, research is emerging to find other elements, such as germanium (Ge) and germanium-tin (GeSn), to be integrated into silicon-based technologies to improve their performance and increase their photoemission.GeSn materials are one of the binary materials in the family of group IV semiconductors.The Bohr radius of Ge, at 24.3 nm, is larger than that of silicon, at 4.9 nm, and Ge has a lower effective mass of electrons and holes, so that Ge exhibits quantum confinement effects in larger dimensions compared to silicon [15][16][17][18].
Interestingly, when the content of Sn in GeSn alloys is greater than 8 at%, the indirect bandgap of Ge becomes a direct bandgap.Theoretical calculations have shown that the indirect bandgap successively transforms to the direct bandgap as the Sn content increases [19,20].Thus, the energy band tuning of GeSn materials can be achieved by adjusting the Sn doping so that they exhibit excellent charge carrier properties.GeSn materials have been shown to be promising candidates for the fabrication of optoelectronic integrated circuits (OEICs) based on Si crisps [21][22][23][24], exhibiting higher hole mobility, enhanced light absorption, etc.In 2009, the first GeSn photodetector with 2% Sn content was presented [25].With the development of GeSn material preparation technology, the performance of GeSn detectors has been improved, with the dark current decreased by more than two orders of magnitude [26], the wavelength cutoff widened [27], and the sensitivity and peak specific detectivity improved [28], making it an excellent candidate for midinfrared detectors.In addition, it was found that GeSn materials have potential applications in laser devices [29][30][31][32][33][34][35][36].
In 2020, electrically pumped GeSn/SiGeSn heterostructure lasers can already operate at temperatures of up to 100 K [37,38].Recently, electrically pumped GeSn LEDs at room temperature and GeSn/Ge multi-quantum well LEDs have been reported [39][40][41][42][43][44].GeSn materials should also be ideal for transistor applications due to their extremely high carrier mobility and high electron tunneling probability [45,46].Some works have successfully demonstrated GeSn p-FinFETs [47,48], GeSn n-FinFETs [49], stacked 3-GeSnnanosheet p-type gate-all-around field effect transistors (GAAFETs) [50], and GeSn/Ge vertical nanowire p-FETs [47].Significantly, highly Sn-doped GeSn NWs (∼19 at%) also exhibit high conductivity values and semiconducting properties [51].Despite the low equilibrium solubility of Sn in Ge (∼1%) [52] and the large lattice mismatch (14%) [53], it has been demonstrated that GeSn NWs with high Sn content can be successfully fabricated [54][55][56].GeSn NWs includes in-plane GeSn NWs and out-of-plane GeSn NWs, but the out-of-plane GeSn NWs have been studied relatively little compared to in-plane GeSn NWs.Therefore, building a knowledge base on the growth of out-of-plane GeSn NWs is crucial for their future research.Figure 1 shows the development of out-of-plane GeSn NWs, which were first prepared by Regina Ragan's team in 2003 [57].By 2014, M Noroozi's group prepared GeSn NWs for the first time by the top-down method [58].Until recent years, the potential of out-of-plane GeSn NWs with different morphologies for different applications has been investigated.

'Top-down' approach
Silicon-based optoelectronic devices usually use Si as a substrate, however, epitaxial GeSn films on silicon-based substrate are characterization by lattice mismatches and easy condensation of Sn, making high-quality and high-tincomponent GeSn epitaxy difficult.Therefore, a strain-relaxed Ge buffer layer (Ge virtual substrate (VS)) needs to be deposited on the Si substrate as a template for GeSn epitaxy.GeSn NWs are typically grown using the top-down approach in three phase [59][60][61][62][63][64][65].In the first step, a strain-free Ge buffer layer and GeSn layer are deposited on Si substrate by either CVD mechanism or MBE mechanism, and the Sn content in the GeSn layer determines the Sn content in the GeSn NWs prepared by the top-down method, and thickness of the Ge-VS layer and GeSn layer almost determine the minimum length of out-of-plane vertical GeSn NWs.However, the minimum length of horizontal NWs will be a multiple of the minimum feature size for the patterning method used [66].In the second step, arrays with variable NW diameters and pitch lengths were patterned by photolithography.For the fabrication of self-assembled GeSn NWs, Sn nanodots, which are produced from the GeSn layer by rapid thermal annealing (RTA) treatment, can be employed as hard masks [62,63].In the third step, GeSn NW arrays are prepared by inductively coupled plasma reactive ion etching (ICP-RIE) with Cl-based or F-based chemistries [65,67].The RF power, gas flow, chamber pressure and temperature during the etching process can be regulated to mitigate the under-cutting, micro-trenching effect, and keep the surface clean.
In addition, periodic digital etching, which consists of selflimited O 2 plasma and HF rinse to remove oxide, can further reduce the diameter of NWs precisely [61,64] while minimizing the damage caused by RIE and fabricating GeSn NW with an anisotropic profile and smooth sidewall.The fabrication process of self-assembled GeSn vertical NWs is depicted in figures 2(a)-(c) [62].The initial GeSn samples (figure 2(a)) were through RTA process (figure 2(b)) and the ICP dry etching procedure (figure 2(c)), eventually forming the out-ofplane GeSn NWs.Figures 2(d) and (e) show the top-and tiltedview SEM images of the GeSn sample after RAT at 550 °C and subsequent Cl-based ICP dry etching, respectively.The minimum critical dimension (CD) of the photoresist written during lithography determines the minimum CD of the NW, which is linearly related to the wavelength of incident light in the lithography system and inversely related to the refractive index of the medium that separates the PR surface from the final projection lens of the lithography system.Currently, state-ofthe-art optical lithography uses deep ultraviolet (DUV) and extremely deep ultraviolet (EUV) illumination to expose the photoresist through a patterned metal mask layer to realize nanostructures with a CD of ∼10 nm [68][69][70], but the process complexity and cost are rapidly increasing as the critical size decreases.Besides, out-of-plane of GeSn NWs by top-down approach have not been fully evaluated.Therefore, the fabrication of uniform diameter, single crystal and Sn segregation free NWs is limited.

'Bottom-up' approach
In order to address the expensive as well as technically challenging, time-consuming, cumbersome, and relatively low throughput issues present in top-down methods.Bottom-up chemical approach offers an alternative technique to produce nanostructures with simple setup, high materials diversity, good control over stoichiometry and doping, and the ability to cover large homogeneous areas.In bottom-up growth mechanisms, GeSn NWs are usually prepared by vapor-and solution-based synthesis method.Vapor-based synthesis mechanism including vapor-liquid-solid (VLS) [71][72][73][74][75]. Solution-based growth includes solution-liquid-solid method (SLS) [76][77][78][79] and supercritical fluid-liquid-solid method (SFLS) [54,80].The first word, which may be 'vapor', 'solution', 'supercritical fluid' refers to the medium in which the reaction will occur as well as the source of the reaction's constituents.

Vapor-liquid-solid (VLS)
3.1.1.Solvent-vapor growth (SVG) system.High boiling point SVG system are a cost-effective method that allows the formation of Ge NWs using a variety of catalytic processes [81].In this method, NW growth has been demonstrated via the VLS mechanism from vapor-deposited catalytic layers that turn into discrete seeds upon thermal annealing, as well as by self-induced solid-phase seeding from bulk metal substrates [82,83].Depending on the boiling point, NWs can be grown by VLS (In [84] and Sn [85] seeds) or vapor-solidsolid (VSS) (using bulk Cu [73,82,86]).In the growth of GeSn NWs, the catalyst chosen is Sn, which has a low melting point (231.9°C), so the VLS mechanism is used to grow GeSn NWs in the SVG system.Figure 3(a) shows the schematic diagram of the GeSn NWs prepared by the SVG system, and figure 3(b) shows the SEM image of the GeSn NWs.The stainless steel substrate is first coated with a layer of Sn catalyst by thermal evaporation and stored in an Arfilled glove box to reduce exposure to O 2 .The NWs growth reaction was performed in the SVG system.The system encompasses a long-necked round-bottomed flask within a three-zone furnace.The reaction is carried out in the gas phase of the high boiling point solvent (HBS) squalene.At the reaction temperature of 430 °C [75], diphenylgermane (DPG) decomposed by phenyl redistribution to produce tetraphenylgermane (QPG) and germane reaction endproducts [87].Ge supply from the DPG precursor is incorporated into the Sn island until saturation, precipitating at the triple-phase point to form GeSn NWs.SVG system can fabricate high yield NWs at low temperature, however, it cannot control the morphology of NWs, such as NW growth direction, NW shape.

Molecular beam epitaxy (MBE).
There has been a lot of literature on the preparation of Ge NWs by MBE [88][89][90], but the growth of out-of-plane GeSn NWs has hardly been reported by this method.Yuekun Yang et al successfully prepared large-area, high-density and high-aspect-ratio Ge NWs using MBE, and utilized them as templates to obtain GeSn/Ge bilayer NW structures with Sn fractions up to ∼10% by secondary deposition [91].Firstly, the In/ graphene/Ge substrate was transferred into the MBE growth chamber and then the preparation of GeSn/Ge dual-NWs was realized in two steps, as shown in figure 4(a).Ge flow flux of 0.012 nm s −1 at 570 °C for 150 min served as the initial stage in the formation of Ge NW.GeSn overlayer was formed on the sidewall of Ge NW after the growth of Ge NW in the second step of the process, which subsequently started at 210 °C with Ge flux of 0.1 nm s −1 and Sn flux of 0.015 nm s −1 for 850 s.This GeSn overlayer was 100 nm thick and had a Sn concentration of 10%.Finally, massive GeSn/Ge dual-NWs with dimensions of 150∼200 nm in diameter and 8∼10 μm in length.The TEM images of Ge NW and GeSn/Ge dual-NW are shown in figures 4(b) and (e), respectively, along with the corresponding energydispersive x-ray spectroscopy (EDX) mapping shown in figures 4(c), (d), (f) and (h).Note that the conjunction of graphene and several micro-meters thick In catalytic layer is essential for the successful growth of ultra-long Ge NW.
3.1.3.Chemical vapor deposition (CVD) mechanism.In the VLS mechanism, the CVD system for growing high quality GeSn NWs is the most common and more commercially available method [92,93].Currently, there are different CVD techniques to grow GeSn NWs, including reduced-pressure CVD (RPCVD) [55,92,94], plasma-enhanced CVD (PECVD) [95], high pressure CVD (HPCVD) [96], atmospheric pressure CVD (APCVD) [56], liquid injection CVD (LICVD) [9,97,98].Currently, there has been some progress in the preparation of in-plane GeSn NWs based on PECVD equipment by solid-liquid-solid growth mechanism [99][100][101].However, Tang's group demonstrated for the first time that PA-VLS can be also used to grow out-of-plane GeSn NWs.Using Sn as the catalyst and GeH 4 as the precursor gas, conical and cylindrical GeSn NWs were prepared by PECVD equipment at low (235 °C) and high (400 °C) temperatures, respectively, and it was shown that, within a certain parameter range, the method can supress the problems of side-wall deposition and taper of NWs, resulting in ultrathin cylindrical single-crystal GeSn NWs [95]. Figure 5(a) shows the schematic diagram of out-of-plane GeSn NWs prepared by plasma-assisted VLS growth mechanism using Sn as catalyst.Figure 5(b) shows the SEM image of GeSn NWs prepared at a temperature of 235 °C, a total pressure of 600 mTorr (the corresponding partial pressure of GeH 4 is 6 mTorr), and the plasma power density of ∼40 mW cm −2 .

Solution-liquid-solid (SLS) method
SLS is a variant of the VLS mechanism and the use of SLS to fabricate semiconductor NWs has been reported [77,102].Sven Barth et al used the bottom-up method for the first time to grow GeSn NWs with high Sn content by the microwaveassisted SLS mechanism.In contrast to the two growth stages of nucleation and growth of NWs of a certain diameter formed with metallic seed particles according to the VLS and SLS mechanism, the SLS growth of GeSn NWs is divided into three stages, the first of which is the nucleation stage, in which Sn-catalysed droplets are gradually formed during the growth of the NWs.These can be either single nuclei that grow slowly as the diameter of the NWs expands, or larger preformed nuclei such as metal particles or self-assembled larger heterodimers that maintain their diameter during growth [103].
Later, Xu et al developed the SLS mechanism to efficiently synthesize homogeneous, straight GeSn NWs with high aspect ratio (∼100) via a simple one-pot liquid phase [77].6(d)-(e) show the EDX map of GeSn NWs.In contrast to VLS, SLS growth is carried out in solution (usually using organic solvents), so the boiling point of organic solvents also limits the choice of candidate metals or metal alloys [78].For the typical reaction conditions of the SLS mechanism (200 °C-350 °C), low melting point metals such as Sn (231.9 °C) [104], In (156.6 °C) [105,106], Bi (271.4 °C) [107,108], Ga (29.8 °C) [109,110] are commonly chosen, while for high melting point metals such as Au [111], Au nanoparticles need  to be confined to small sizes to keep their melting points within the boiling points of organic solvents when grown by the SLS mechanism [112].
Typically, VLS-grown NWs have adventitious oxide coatings [113].Because the surface structure, passivation and ligation can affect the luminescence properties, solubility and possibly the electrical transport behaviour of the NWs [114,115].The solution-based SLS and SFLS mechanism might be the most effective in regulating these properties.Compared to VLS growth, SLS growth may be able to fabricate NWs with lower diameters (<10 nm), low synthesis temperature, ligand passivation [116,117] and stronger quantum-confinement effects [118,119].Besides, metal nanocrystal seeds are disseminated in solution during SLS NW growth, allowing NWs to grow over the full volume of the solution-phase reaction, which increases scalability [120,121].The SLS technique for producing NWs also has the benefit of not requiring specialized apparatuses.

Supercritical-fluid-liquid-solid (SFLS) approach
The SFLS method uses a high-pressure growth process similar to VLS.The catalytic manufacture of Ge 1-x Sn x NWs with high Sn content (10% ∼ 35%) from a supercritical fluid (SCF) was described by Doherty et al [80] Adrià Garcia-Gil et al prepared GeSn NWs with a high aspect ratio (>440), extremely thin (∼9 nm), Sn concentration of 3.1 at %, and good performance as Li-ion battery anodes using the SFLS mechanism [122].The synthesis of GeSn NWs is carried out in a stainless steel reaction cell.First, catalytic nanoparticles (e.g.Au [122], AuAg [80]) were deposited on the substrate and placed into the stainless steel cell [80,122].Second, the pressure of the reaction cell was adjusted and the precursor solution was injected into the injection cell.Pressure induced or different ratios between the precursor liquids can affect the solute trapping of Sn in the GeSn NWs [54,80,122,123].
Since gas-liquid coexistence occurs when the temperature and pressure of the solvent exceed a critical point, producing a unidirectional fluid with intermediate properties, uniquely high diffusivity, density, low surface tension and low viscosity [124].The synthesis of NWs can be tuned by varying the temperature or pressure, which in turn leads to the desired liquid-or gas-like conditions.The following are some of the advantages of SFLS: (i) precursors are present at a higher concentration than usual, (ii) higher chemical flexibility, (iii) ligand passivation during growth, (iv) wide range of catalysts, (v) scalability, and (vi) high yields.The main drawback of the SFLS mechanism is the possible agglomeration and coarsening of the nanoparticle seed, which leads to an uneven distribution of nanodiameters.In addition, the sequence of precursors is subject to many constrains, which does not favour the synthesis of axial heterojunctions.Meanwhile, SFLS requires the preparation of NWs at high temperature and pressure, and its growth rate in the order of μm/min is much higher than that of VLS (nm/min), which is unfavourable for quantitative analysis of mechanics and growth kinetics [125].

Catalyst materials
In the bottom-up approach, catalysts for growing Ge NWs are generally classified into three types [126]: (1) Type A, i.e.Au, Al, Ag, with simple binary phase diagrams and high eutectic solubility (Ge solubility in eutectic alloys is greater than 10%); (2) Type B, i.e.In, Bi, and Sn, which also have simple binary phase diagrams, but with low eutectic solubility, usually Ge solubility in eutectic amalgams is less than 1%; (3) Type C, i.e.Fe, Cu, Ni, and germanide-forming catalysts have complex binary phase diagrams and high eutectic temperatures (>1000 °C), so C-type catalyst are generally in the solid-state induced VSS mechanism for growing NWs.The low toxicity and chemical stability of Au in air, as well as its low evaporation pressure at high temperatures and simplicity of the process, have led to gold being widely recognized as a standard catalyst.Since Au [127] atoms as well as binary alloys of Au (e.g.AuAg [56] and AuSn [128]) introduce deep energy levels in the NWs and other exogenous atoms affect the properties of the NWs [56,128,129], the use of Sn and SnO 2 as catalysts for the preparation of GeSn NWs has been reported [55,100,129].However, Sn with its low surface energy is prone to wetting phenomena and it is difficult to maintain stability at the top of the NWs.Therefore, it is less suitable for VLS growth and is more commonly used for VSS growth, but a small number of studies have nevertheless shown that growth of GeSn NWs with low surface energy metals is possible by a VLS mechanism [95].

Sn content
For a GeSn materials to become a direct bandgap, it requires an equilibrium solubility of Sn of more than 1 at% [52] and a content of more than 8 at% [130] in Ge.For nominally undoped Ge 1-x Sn x NWs to be used as field effect transistors (FET), the most important metric is that the Sn content in the NWs should be at least 10 at% [54].In the top-down method, the Sn content in the NWs is mainly determined by the Sn content in the GeSn films deposited in the previous stage.In the bottom-up method, due to the variety of growth methods and the relative complexity of the growth process, the amount of Sn doping in Ge is extremely dependent on the growth conditions, such as the temperature [56], the proportion of precursors [131,132], pressure [54], and the catalyst material [56].In the VLS growth mechanism, Sn doping in GeSn NWs can be explained by the kinetically relevant 'solute trapping' [96].The doping of any impurity by solute trapping increases with increasing growth kinetics of the NWs, and the increased growth kinetics will definitely lead to a faster growth rate.The growth kinetics of GeSn NWs prepared by VLS growth depends on three factors: (i) dissociation of the growth material from the precursor to its binding to the liquid-phase catalyst, (ii) diffusion of the growth material in the catalytic droplet, and (iii) crystallization of the growth material at the liquid-solid interface [133], and during supersaturation confinement, the growth rate of the VLS is mainly determined by factors (i) [134,135] and (iii) [96,133].
Supersaturation directly affects the growth kinetics of NWs, which can be controlled by using catalysts with different equilibrium concentrations of growth substances [136].Faster growth kinetics can be promoted by using higher temperatures [137] and precursors with high catalytic decomposition rates to induce the incorporation of the growing species into the catalytic liquid seed, which in turn promotes faster growth kinetics.For systems involving Sn precursors, such as allyltributylstannane (ATBS) and tetraethyl tin (TET), high temperatures may also lead to rapid decomposition of Sn precursors with low boiling points, which may dominate the uniform nucleation of Sn and lead to clustering of Sn, which in turn leads to a lower Sn content in the GeSn NWs, in addition to high temperature leading to lower NWs yields.Therefore, the selection of appropriate growth temperature and Sn precursors is crucial to improve the growth kinetics.The supersaturation dependent on the concentration of growing species can be expressed as follows [138]: where C is the concentration of growth species, C e is the equilibrium concentration of the growing material in the liquid metastable alloy, k is the Boltzmann constant, and T is the temperature.Thus, the C e of the growth material can be reduced by adding exogenous species to the metal seed.For example, the Sn content of out-of-plane GeSn NWs obtained by growth using the Au 1-x Ag x catalytic alloy is higher than that of NWs catalyzed using pure Au in the case of similar diameters [56].Therefore, the composition of liquid droplet is also one of the factors affecting the nucleation of NWs and the incorporation of Sn in GeSn NWs [139,140].Since nano-systems are characterized by high surface-to-volume ratios, the surface energy contribution to the thermodynamics is prominent, making the growth rate dependent on the diameter, and the radial dependence of the chemical potential can be expressed as follows [141][142][143]: where Δμ 0 is the effective difference between the chemical potential of growth materials in vapor phase and in the NW at a plane boundary (diameter (d)→ ∞), Ω is the atomic volume of Ge, and α is the surface energy.Therefore, the diameter decreases and the chemical potential between adatoms of growth materials in the vapor phase and the solid crystal phase increases [138], which in turn facilitates the Sn capture, i.e. it has also been shown that the Sn content in the NW is inversely proportional to the NW diameter [54].
Table 1 shows several bottom-up methods to fabricate out-of-plane GeSn NWs with different Sn content.SLS methods can realize GeSn NWs with high Sn content at lower temperatures using Sn as a catalyst, and diameter of the NWs can be less than 10 nm.SFLS and SVG mechanism prepare GeSn NWs at higher temperatures (>400 °C).The use of AuAg alloy as catalyst in SFLS can realize GeSn NWs with up to 35 at% Sn content, but also introduces exogenous atoms.Multi-element catalysts affect the crystallinity of the NWs, and twinning defects are generated in the grown NWs [128].The liquid-phase based growth mechanism can prepare GeSn NWs with large scale, but it is impossible to control the growth direction of GeSn NWs during the growth process, and even through it is reported in the literature that it is possible to control the morphology of the NWs, it is limited to controlling the diameter of the NWs.Cold-wall CVD can also achieve GeSn NWs with high Sn content at low temperatures, whereas LICVD preparation of NWs requires relatively high temperatures and uses exogenous catalysts, in addition to obtaining NWs with a Sn content of <10 at%.
Since the lattice constant of Sn is larger than that of Ge, the incorporation of Sn changes the orientation and bond strength of the original Ge, forming a compression-strained GeSn material [146,147].The strain value is related to the Sn content, and as the Sn content increases, the compressive strain increases [148,149], and the carrier mobility decreases, which in turn leads to a decrease in the photoluminescence (PL) strength.For GeSn materials, the following methods have been reported to solve the problem of increased compressive stress by increasing Sn doping: (i) RTA process; (ii) Wrapping insulating stressor layer on the GeSn surface; (iii) Successive deposition of a tensile-stressed Ge buffer layer and a tensilestressed GeSn layer [150].For out-of-plane GeSn NWs, core/ shell NW geometries were used to provide an additional degree of freedom in accommodating the effects of strain in the growth of lattice-mismatched heterostructures [56,145,151,152], where the strain relaxation of the shell increases with the increase in the thickness of the free surface of the sidewalls and the elastic compliance of the NW cores allows an increase in the strain relaxation of the shell to adapt to the lattice mismatch of the system and avoid buckling [131].In addition, the bandgap is tuned by modulating the Sn content in the GeSn material, which in turn modulates the emission properties of the GeSn components [153].The incorporation of Sn also has an effect on the absorption coefficient of GeSn compounds, as the band gap of GeSn shrinks compared to that of the Ge, so that the sensitivity increases.The higher the Sn concentration, the higher the sensitivity at the cut-off wavelength and the stronger the dark current [154,155].

Morphology of out-of-plane GeSn NWs
The morphology of NWs, including diameter, length, growth orientation, surface roughness, kinks, defects, twins and overall crystallinity, is an important factor affecting the performance of optoelectronic devices.The morphology of NWs is modulated by regulating the growth conditions such as pressure, temperature, catalyst or the sequence of the growth process.Sun et al reported the effect of catalysts containing different Sn contents on the growth direction and crystallinity of GeSn NWs [128].By using Schmidt's theoretical model, it was explained that the growth direction of GeSn NWs depends on the diameter of NWs as well as the Sn concentration in the catalyst [156].According to the Sn concentration dependent Schmidt model equation [128]: Where á ñ f uvw is free energy per unit circumference of a á ñ uvw oriented NW, ∆z is the interfacial thickness of the GeSn NW side, s á ñ s uvw is the surface energy of a á ñ uvw -oriented NW, á ñ a uvw is the geometrical parameter of the cross section of a á ñ uvw -oriented NW, s á ñ ls uvw is the interfacial tension of the liquid-solid interface, and r is the radius of the NW.If only the directions á ñ f uvw and á ñ f uvw are considered, the radius of the catalyst increases with the Sn concentration.The Sn-rich catalytic droplets are destabilized due to their low surface tension, so that the catalytic droplets experience wetting phenomena and modify the sidewalls of the NWs, which in turn changes the surface energy of the system and leads to a tendency of the NWs to grow in the á ñ 110 rather than the á ñ 111 direction.Doherty et al reported for the first time the self-catalyzed, single-step growth branched GeSn nanostructures via a VLS mechanism [98].Figure 7 shows the branched nanostructures prepared by liquid-injection CVD (LICVD) method using Au 0.80 Ag 0.20 as catalysts and DPG and tetraethyl tin (TET) as precursors.The unique structure of the branched GeSn NWs leads to a higher specific capacity compared to conventional GeSn NWs.In addition, the high surface-to-volume ratio and increased charge carrier paths of this highly ordered nanostructure make it a potential anode material for lithium-ion batteries.
In addition to GeSn NW structures with high Sn content, there are heterostructured GeSn NWs, such as core/shell Ge/ Ge 1-x Sn x NWs [131,157] or GeSn/Ge dual-NW heterostructure [158], which have unique optoelectronic characteristics [159].The core-shell NW geometry has a dual synergistic effect on the optical properties.The Ge core serves as an elastic substrate for the growth of axial lattice-matched epitaxial Ge 1-x Sn x shells, which facilitates the growth of highquality Ge 1-x Sn x single crystals with strong photoluminescence.The distribution of elastic strain at the coreshell interface and the distribution of Sn composition in the GeSn shell affect the optical properties of the core-shell GeSn NWs.The tensile mismatch strain in the Ge cores reduces the direct-gap jump energy with respect to the indirect-gap jump energy, which enhances their optical emission [160].NWs with the shell-core structure initially suffer from a high epitaxial mismatch strain, which gradually relaxes.For GeSn/Ge dual NWs, the elastic strain relaxation of the heterostructure is also better with the increase of the thickness ratio [161].By modulating the Sn content in the GeSn shell, the Ge/Ge 1-x Sn x core/shell NW devices can be realized from shortwave (SWIR) (8 at% Sn content) to mid-wave infrared (MWIR)  [97] 440 °C Au DPG, ATBS 7.1-9.7 at% SFLS [54] 405 °C AuAg DPG, ATBS 35 at% SVG [75] 420 °C Sn DPG Not known SLS [77] 300 °C Sn SnI 4 Ge(Ph) 3 Cl 10 at% RPCVD [139] 400 °C AuAl SnCl 4 , GeH 4 9 at% PECVD [95] 235 °C 400 °C Sn GeH 4 Not known (18 at% Sn content) photodetection [162].Alternatively, morphology tuning of core-shell GeSn NWs can be achieved by adjusting the precursor gas flux, which leads to bending of the NWs and Sn-rich precipitation when there is a localized imbalance between the gas precursor flux and the available surface sites for Ge or Sn incorporation into the grown GeSn shells [151].
Besides, Assali et al experimentally observed that the cross-sectional morphology of the Ge/GeSn core/shell structured NWs changes from hexagonal to dodecagonal when the Sn precursor is increased.This transition will also strongly affect the Sn distribution, with a higher Sn content measured in the (112) direction, leading to tunable photoluminescence emission from the core/shell NW system in optoelectronics at mid-infrared wavelengths [132,163].They also showed that at a lower growth temperature (300 °C), they successfully prepared Ge/GeSn core-shell heterostructured NWs arrays with up to 13 ± 1 at% Sn content, and it also avoids the precipitation of Sn in GeSn shells the low-temperature condition.The unique structure of the radial core/ shell NWs permits strain relaxation and limits the formation of structural defects, which allows the NW photoluminescence at room temperature to be centered on 0.465 eV, and the absorption rate can be increased to more than 98%, making the structured NWs a promising low-cost, high-efficiency material for use in photodetectors for shortwave infrared and thermal imaging devices [164].Braun et al have demonstrated that the change in the surface morphology of the core-shell structured GeSn NWs is related to the surface Sn precitation [165].They reported that annealing the NWs in a rough vacuum environment led to the formation of in situ GeSn oxides on the surface, which in turn prevented the segregation of Sn to the surface, resulting in the formation of NWs with relatively smooth GeSn shell surfaces, which are different from the NWs with rough surfaces obtained by annealing in a H 2 environment.

Infrared photodetectors
The optical properties of GeSn NWs are related to the Sn content and the morphology of the nanowires.As mentioned in section 4, GeSn alloys turn into a direct band gap with strongly increased band edge absorption when the Sn content in the alloys is high enough (>10 at%) [164].In addition, there are large lattice mismatches between α-Sn and Ge [146], leading to a large number of defects between the GeSn and Ge layers.These defects trap charge carriers and reduce their mobility, which in turn affects the optical properties of the GeSn NWs.Heterojunction GeSn NWs can relax the strain between Ge and GeSn due to their unique structural features, minimising the impact of defects on the performance of optoelectronic devices.
GeSn/Sn core/shell NWs (25 nm diameter Ge core, 75 nm thickness GeSn shell with Sn content above 4 at%) show strong photoluminescence with a direct band gap at room temperature [113].They found that as long as the optical generation rate in the Ge core exceeds the non-radiative recombination rate due to the defects on the outer surface of the shell, the spillover of charge carriers from the core into the GeSn layer contributes to the observation of strong photoluminescence at room temperature, which promises to enable highly sensitive submicro-footprint photodetection in the midinfrared region.Peng et al optimized the Mie resonances of individual NWs at the desired bandgap wavelength [166].They tuned the Mie resonances of GeSn/Ge structures (40 nm diameter Ge core, 110 nm thickness GeSn shell) with refractive index (n > 4) to the desired wavelength by varying the diameter and height of individual NWs.Further, they also compared randomly distributed Ge NWs (40 nm diameter) and GeSn/Ge core-shell NWs (60 nm diameter), and found that the photocurrent spectrum intensity of the latter is seven times higher than that of the former, providing further evidence that high-quality GeSn NWs can be a good platform for midinfrared detection at room temperature.
It should be noted that Yang et al prepared GeSn/Ge dual NWs by MBE and investigated their photodetection properties [91].They found that this NW has a low dark current of 40 nA at 10 mV drain voltage.In addition, the wavelength of the GeSn/Ge dual NW can be extended to 2.2 μm compared to the Ge NW.However, the device has a relatively low responsivity at 1.55 μm compared to other GeSn film devices.Therefore,  The room-temperature photoresponse is ∼1.1 A W −1 at zero bias and ∼70.8A W −1 at −1 V, which is much higher than that of the previously reported GeSn-based photodetectors, and this opens a new path for group IV semiconductor nano-infrared detectors for the optoelectronic integrated system [159].
Recently, Lin et al fabricated self-assembled vertical GeSn NWs based on RTA and ICP dry etching and investigated their optoelectronic properties [62].They found that these NWs can be used as a prototype GeSn NW photodetector with fast switching capability.Since this GeSn NW photodetector has an extremely low dark current density of ∼33 nA cm −2 , a response rate of 0.245 A W −1 , and a high specific detectivity of 2.40 × 1012 cm Hz 1/2 W −1 at 1550 nm under −1 V at 77 K, it is expected to be used in nanophotonic devices in the near-infrared or short-wavelength infrared.

Li-ion battery anodes
The main disadvantage of Ge and GeSn as anode materials for Li-ion batteries is structural cracking and pulverisation due to the large volume change during lithiation/delithium formation, which in turn leads to the problems of capacity decline and poor cycle life.The above problems can be solved  if the anode material is nanostructured.Since the unique structure of nanowires gives them the advantages of large specific surface area, one-dimensional conductive paths and shorter carrier diffusion paths (especially for thin NWs), and also has the characteristics of larger buffer space for volume changes and stronger crack resistance [176], selecting nanomaterials as the anode of lithium-ion batteries can be a good way to mitigate the above shortcomings.Adrià Garcia-Gil et al reported the growth of GeSn NWs with high aspect ratio (>440), ultra-thin diameter (∼9 nm) and well below the Bohr radius under supercritical toluene conditions at a reaction temperature of 440 °C using a simple solvothermal-like growth method directly on a current collector substrate (titanium) [122].Figures 10(a)-(c) show galvanostatic cycling of GeSn NWs with a Sn content of 3.1 at%, which allows determining the specific capacity values that the NWs can provide and checking their capacity retention ability.Figures 10(d) and (e) show the specific capacity and Coulombic efficiency value of GeSn NWs with Sn content of 3.1 at% and 7.9 at%, respectively.It was found that GeSn NWs with the highest aspect ratio and the lowest Sn content (3.1 at%) exhibited high capacity retention with values of ∼90% and ∼86% from the 10th to the 100th and 150th cycles, respectively, and specific capacity values of 1176 and 1127 mAh g −1 after the 100th and 150th cycles, respectively, making them a promising advanced anode material for lithium-ion batteries.

Nanowire SWIR laser
In addition to potential applications in optical infrared detectors and lithium-ion batteries, Kim et al found that GeSn NWs also have potential applications in infrared lasers [152,170].They performed the first experimental observation of a strong cavity resonance in a single Ge/GeSn core/shell NW fabricated by the bottom-up method.Figure 11(a) shows the experimental schematic of photoluminescence measurements on a single GeSn NWs.∼500 nm, there is a strong cavity resonance.This paves the way for the realisation of group IV nanowire lasers.

Nanowire transistor
GeSn alloys are promising candidates for the next generation of complementary metal oxide semiconductor (CMOS) circuits, low-power devices and silicon photonics [177].Liu et al fabricated vertically heterostructured GeSn/Ge nanowires by the top-down method and used the Ge 0.92 Sn 0.08 layer as the source of GeSn/Ge-NW p-EFTs to obtain high-performance vertical gate all-around NW p-type FETs (GAA NW p-FETs) with a small subthreshold slope of 72 mV dec −1 and a high I ON /I OFF ratio of 3 × 10 6 [178].The main advantages of GAA NW p-FETs are the valence band offset at the Ge 0.92 Sn 0.08 /Ge heterojunction and the low NiGeSn/ GeSn contact resistance.Further, Junk et al have also fabricated smaller diameter (20 nm) vertical GAA NW p-FETs by a top-down method based on Ge and GeSn/Ge heterostructures, with a subthreshold slope of 66 mV dec −1 , a drainbarrier reduction of 35 mV V −1 , and an I ON /I OFF ratio of 2.1 × 10 6 [61].Emmanuele Galluccio et al reported for the first time some of the key electronic FET performance metrics of nominally undoped, VLS-grown Ge 1-x Sn x such as mobility, I ON /I OFF ratio, subthreshold swing and transconductance (gm) as a function of Sn content (x = 0.03, 0.06 and 0.09) [171].Recently, Liu's team proposed vertical GeSn-based GAA NW MOSFETs (VFETs) with NW diameters as small as 25 nm and designed two epitaxial heterostructures, GeSn/ Ge/Si and Ge/GeSn/Si, for the p-and n-VFETs for joint optimization [174].A schematic of an all-GeSn n-VFET with source/drain is shown in figure 12   reference device, while the transconductance of the p-FET is increased to 850 μS μm −1 by utilising the tiny bandgap of the GeSn as a source, which results in high injection speeds.The excellent performance of GeSn-based GAA NWs beyond Si CMOS logic was demonstrated through GeSn inverters.

Conclusions
The preparation and study of GeSn NWs is one of the groves to continue Moore's law.In this paper, we found that the preparation of out-of-plane GeSn NWs can be obtained by various methods, such as top-down method and bottom-up method.The bottom-up method includes the vapor-phase growth mechanism (VLS) and the liquid-phase growth mechanism (SFLS, SLS and SVG).The vapor-phase growth mechanism is more complex compared to the liquid-phase growth mechanism, and therefore, the NWs have greater tunability through the vapor-phase growth mechanism.The type of catalyst also affects the density, crystal quality, and Sn content of the out-of-plane GeSn NWs.The morphology of the NWs can be modulated by conditions and factors such as temperature, pressure, and Sn content.By modulating the Sn content in the shell of heterostructured GeSn NWs, they can be used in different detectors.In recent years, a great deal of attention has been paid to the preparation and study of inplane GeSn NWs, but relatively few studies have been carried out on the preparation of out-of-plane GeSn NWs, especially the preparation of NWs by the gas-phase method.In the future, there are many problems of out-of-plane GeSn NWs looking forward to be solved, such as Sn wetting phenomenon, and nucleation of GeSn NWs, as well as the technical difficulties and challenges faced by out-of-plane GeSn NWs in terms of compatibility with the existing planar process, reduction of the preparation cost, and breakthrough of the NWs' precise positioning and other key issues.

Figure 2 .
Figure 2. (a) Sample structure diagram of self-assembled GeSn vertical NWs after each process: (a) as grown sample; (b) annealed sample; (c) NW sample.(d) Top-view and (f) tilted-view SEM images of the GeSn sample after RAT at 550 °C and subsequent Cl-based ICP dry etching.Reproduced from [62].CC BY 4.0.

Figure 3 .
Figure 3. (a) Schematic diagram of the SVG system for NW synthesis.(b) Tilted high magnification SEM image of high density, high aspect ratio GeSn NWs.Reproduced with permission [75].Copyright 2013, American Chemical Society.

Figure 6 (
a) shows the schematic diagram of SLS mechanism.In a three-neck round bottom flask, SnI 4 was first reduced with NaBH 4 to form Sn nanoparticles at 300 °C in Trioctylamine (TOA), and then Ge(Ph) 3 Cl and NaBH 4 were continuously and rapidly loaded and maintained at 300 °C for several hours [77].
Figure 6(b) shows the SEM image of GeSn NWs fabricated by SLS mechanism.Figures

Figure 4 .
Figure 4. Growth and characterization of GeSn/Ge dual-NWs fabricated by MBE.(a) Schematic diagram of the growth process of GeSn/Ge dual-NWs.(b) TEM image of a single Ge NW.(c) and (d) EDX mapping of Ge and Pt, respectively.(e) TEM image of a GeSn/Ge dual-NW.(f) to (h) EDX mapping of Ge, Sn and Pt, respectively.Reproduced with permission [91].Copyright 2020, American Chemical Society.

Figure 5 .
Figure 5. (a) Schematic diagram of the growth of GeSn NWs by PECVD.(b) SEM image of GeSn NWs prepared by plasma-assisted VLS method using Sn as catalyst.Reproduced from [95].© IOP Publishing Ltd.All rights reserved.

Figure 6 .
Figure 6.(a) SLS mechanism proposed by Buhro et al for analogous growth from solution.Reprinted with permission from [102].Copyright (2006) American Chemical Society.(b) A SEM image of a single GeSn NW with a Sn seed by SLS mechanism.(c) The corresponding EDX-line scanning analysis.(d) and (e) EDX elemental maps of NW.Reproduced from [77].© The Author(s).Published by IOP Publishing Ltd.CC BY 4.0.

Figure 9 (
b) shows a FESEM image of the fabricated device.The semi-logarithmic current-voltage (I-V ) characteristic of the fabricated NW device at room temperature is shown in figure 9(c), which was measured in a dark environment.The NW photodetector was excited with a light intensity of about 6.37 mW cm −2 and its temporal photoresponse was evaluated at a wavelength of 1.55 μm at zero bias voltage, as shown in figure 9(d).Such simple metal-semiconductor-metal (MSM)-based Ge/GeSn core/shell NWs exhibit low dark current (tens of nA) at an applied bias voltage of ±1.0 V.The fabricated single NW device shows a very high room temperature photoresponse at an optical communication wavelength of 1.55 μm.

Figure 8 .
Figure 8. Potential applications of out-of-plane GeSn NW in different research areas.

Figure 9 .
Figure 9. (a) Schematic diagram and (b) corresponding FESEM image of Ge-Ge 0.92 Sn 0.08 core-shell single NW photodetector device.(c) The room temperature semi-logarithmic current-voltage (I-V ) curve of the prepared single NW device measured under dark.(d) Temporal photoresponse of the fabricated single NW photodetector at a wavelength of 1.55 μm measured at zero bias under the excitation with an incident light intensity of ∼6.37 mW cm −2 .Reproduced with permission [159].Copyright 2022, AIP Publishing.
Figure 11(b)  shows the SEM image of a single GeSn NWs with a length of ∼18 μm and the simulated crosssectional optical mode profile of the nanowire as shown in figure11(c).
Figure 11(d) shows the emission spectrum of the single nanowire in figure 11(b).
Figure 11(e)  shows an enlarged view of the black dashed box in figure 11(d).It was found that two very strong cavity resonances were observed at 2251 and 2275 nm when the pump power density was increased to 120.7 kW cm −2 .Through theoretical modelling, it was shown that at a carrier injection density of ∼2.2 × 10 17 cm −3 , the net gain is maximum and the measured single nanowire approaches the transparent region, resulting in a strong resonance.Even if the diameter of the nanowire is
(a).The I D -V GS transfer characteristic curves obtained by measuring the device at 300 K and 20 K are shown in figures 12(b) and (d), respectively.By using GeSn as channel material, the mobility of the n-FET is increased by a factor of 2.5 compared to the Ge

Figure 11 .
Figure 11.(a) Schematic illustration of photoluminescence measurements on the transferred NW sample.(b) SEM image of a single NW.Scale bar, 7 μm.(c) Simulated cross-sectional optical mode profile of the transferred NW (500 nm diameter) at the wavelength of 2175 nm.(d) Power-dependent photoluminescence spectra form a single transferred NW.(e) Enlarge image of a spectra captured at 120.7 kW cm −2 in a black dashed line in panel (d).Reproduced from [170].CC BY 4.0.

Figure 12 .
Figure 12.Schematic of an all Ge 0.922 Sn 0.078 vertical n-FET (n-VFET) with a NW diameter of 50 nm.(b) I D -V GS transfer characteristics measured at 300 K and, (c) at 12 K.Reproduced from [174].CC BY 4.0.