As an interesting phenomenon in semiconductor electrochemistry and now a powerful Mano-/Nano-fabrication technique, Metal-Assisted Chemical Etching (MacEtch) or Metal-Catalyzed Electroless Etching (MCEE) of silicon in aqueous hydrofluoric acid (HF) solution has aroused great interest in the last decades. Such low-cost electroless etching technique offers exceptional simplicity, flexibility, and scalability for the fabrication of three-dimensional silicon nanostructures such as silicon nanowire (SiNW) array with considerable lengths. In contrast to chemical etching of silicon, the MacEtch of silicon proceeds via the concurrent action of electrochemical dissolution of Si and reduction of oxidizing agents (typically hydrogen peroxide, silver nitrate) both occurred in the vicinity of noble metal catalysts such as Ag and Au, thus featuring highly site-selective etching of silicon exclusively at the metal-silicon interface. Commonly oxidizing agents used in MacEtch of silicon are typically strong oxidants such as hydrogen peroxide and silver nitrate, which are expensive and hazardous. It is well-known that silicon is thermodynamically unstable and readily oxidized by oxygen, which is nontoxic and ubiquitous in air or water. However, it remains unclear how cathodic oxygen reduction affects MacEtch of silicon in the presence of HF due to its sluggish kinetics arising from inherent difficulty of splitting oxygen molecule. Recently, we demonstrated two strategies for MacEtch of silicon with ubiquitous oxygen as the one and only oxidizing agent (1) macroscopic galvanic cell driven MacEtch of silicon in aqueous HF solution; (2) enhanced MacEtch of silicon in aerated HF/H₂O vapor.

Metal assisted etching (MAE) is an electrochemical process that exhibits great versatility for restructuring semiconductors. However, little work has been performed that quantitatively addresses the reaction mechanism. We use V₂O₅ as the oxidant in order to perform quantitative measurements [1; 2]. We have used such measurements to elucidate the mechanism of stain etching of Si [3]. We apply these methods here to MAE and attempt similar measurements for HOOH + HF. Ag, Au, Pd and Pt were deposited from solution onto H-terminated Si at coverages of only ~5% of the surface initially. The metals all catalyzed electron transfer to the oxidant and enhanced the etch rate of Si as compared to stain etching. Assuming all electron transfer happens at the surface of the metal, the rate was accelerated by roughly a factor of 100 compared to stain etching and is diffusion limited. The stoichiometry of MAE in V₂O₅ + HF depended on the chemical identity of the metal. The stoichiometry and rate of etching in V₂O₅ + HF solutions were well behaved and gave consistently reproducible kinetic results. The behavior is much different when HOOH is added instead of V₂O₅. In contrast to V₂O₅, we were unable to obtain well-behaved stoichiometric results for HOOH + HF solutions. This is related to heightened sensitivity on reaction conditions compared to the V₂O₅ system as well as nonlinearities introduced by side reactions. The results for etching with V₂O₅ + HF are summarized as follows. The mechanism of Si etching changes based on the presence of a metal catalyst during metal assisted etching and depends on the chemical identity of the metal. A valence 2 path dominates the formation of photoluminescent nanoporous Si in stain etching as well as MAE with Ag and Au. A valence 4 path dominates the formation of photoluminescent nanoporous Si in MAE with Pt. However for MAE with Pd, no nanoporous Si is formed initially and a mixture of valence 4 and valence 2 processes is observed. The nature of the electron transfer process and its dependence on the electronic structure of the metal/Si interface will be discussed. It has recently been shown [4] that the electronic structure of the metal/Si interface, i.e. band bending, is not conducive to diffusion of the injected hole away from the metal in the case of Ag, or away from the metal/Si interface in the cases of Au, Pd and Pt. Since holes do not diffuse away from the metals, the electric field resulting from charging of the metal after hole injection must instead be the cause of metal assisted etching.
[1] K.W. Kolasinski, W.B. Barclay, ECS Trans. 50 (2013) 25-30.
[2] K.W. Kolasinski, J.W. Gogola, W.B. Barclay, J. Phys. Chem. C 116 (2012) 21472-21481.
[3] K.W. Kolasinski, W.B. Barclay, Angew. Chem., Int. Ed. Engl. 52 (2013) 6731-6734.
[4] K.W. Kolasinski, Nanoscale Res. Lett. 9 (2014) 432.

The combined use of Metal Assisted Chemical Etching (MACE) of silicon with patterned metal films has enabled fabrication of a wide range of structures, ranging from arrays of nanowires with diameters down to 20nm to complexly sculpted 3D structures. However, there are still limits on the use of MACE and the mechanisms leading to these limitations are poorly understood. MACE is most successful when used in combination with continuously connected patterned films, such as the films patterned with arrays of holes to make nanowire arrays. Etching with discontinuous metal films often leads to reduced uniformity in the direction of etching. Also, etching of heavily doped silicon often leads to the formation of porous and pitted silicon features. Pitting and etch directionality are also affected by the pattern density. These phenomena are known to be strongly affected by the chemistry of the etching solution. Porosity and pitting are associated with a high concentration of oxidant (e.g. H₂O₂) relative to that of HF, and a corresponding generation of excess holes that are not consumed in etching processes occurring at the metal-silicon interface. The directionality of etching can also be affected by generation of hydrogen bubbles when the ratios of the oxidant to HF are too low. Both phenomena are also related to the attractive force that holds the metal in near contact with silicon during etching. This force is also a function of the etch chemistry, and in some cases, of the mechanical properties of the patterned metal structures as well. Recent work on the interplay of etch chemistry and etching characteristics, as probed using mechanical constraints and electric fields, will be reviewed.

Complex 3D geometry can be etched in silicon using Metal-assisted Chemical Etching (MacEtch) by properly controlling catalyst shape. Understanding the mechanism driving catalyst motion is required when trying to etch specific or complicated geometry. This seminar provides experimental evidences shown that Derjaguin and Landau, Verwey and Overbeek (DLVO) encompassed forces drive catalyst motion in MacEtch.
In MacEtch a metal catalyst is used to generate a local galvanic cell across the catalyst that locally increase the dissolution rate of silicon in an etchant solution of hydrofluoric (HF) acid and hydrogen peroxide (H₂O₂). Unlike other etching techniques were a pattern of material remains on the top surface acting as a mask, in MaCE the metal catalyst moves into the substrate at the silicon around and beneath the catalyst dissolves. Because the catalyst can travel in 3 dimensions while continuing to etch it is possible to create 3D patterns in the silicon with straight, curved, helical, and random, etching paths reported for Pt, Au and Ag nanoparticles and colloids. More recently, our group has reported on the effects of catalyst shape on etching direction and showed that cycloids, spirals, sloping channels, “S” shaped channels and more can be fabricated by controlling catalyst shape to create complex, 2D and 3D nanostructures with extremely smooth walls^.
DLVO forces drive catalyst motion. The experimental evidence presented in this study include path and deflection analysis of complex catalyst systems and distance-displacement measurements from in-situ Atomic Force Microscopy (AFM). These forces were found to exert between 11 MPa and 18 MPa of attractive pressure differentials across the catalyst and operate over extremely short distance of ~4 nm. These values were within theoretical predictions. The highly non-linear nature of these forces means that MacEtch is extremely sensitive to local etching conditions and parameter optimization is necessary to control etch path.

Combining the high surface materials generated from metal-assisted chemical etching of
GaN with Pt produced Schottky barriers that are exquisitely sensitive to the nature and
density of molecular adsorbates and, thus, make excellent chemical sensors. The sensing
response of porous GaN (PGaN)-Pt to H2 and CO has been investigated under a variety
of conditions. Sensitivity of Pt-PGaN to H2 was measured to be better than an order of
magnitude better than planar GaN. PGaN was also found to be a suitable substrate for
direct laser desorption-ionization mass spectrometry and also for surface-enhanced
Raman scattering, thus making it a versatile substrate across a wide spectrum of different
chemical sensing approaches.

Chemical routes to large-scale production of functional architectures are highly researched to improve light enhancement in photovoltaic devices. Among these techniques, catalyzed wet chemical etching is widely considered to achieve high quality nanostructured patterns with controllable dimensionality by a one-step process.
Metal-assisted chemical etching (MACE) has proven an effective method for the fabrication of silicon antireflective coatings [1], already finding application in industrial production of photovoltaic devices. Following the general trend of the photovoltaic industry to employ new low band-gap semiconductors to enhance solar conversion efficiency, extension of MACE to new material systems, such as Ge or III#8209;V compound semiconductors, is highly desirable.
In this work we report on a new simple and scalable process for the fabrication of antireflective layers on GaAs, an excellent photovoltaic material for its large absorption coefficient and high electron mobility, which enable thin, high-efficiency devices [2]
Selective etching of <111> and <311> GaAs crystallographic orientations is catalyzed by metal nanoparticles deposited by electroless process and results in randomly oriented features which enhance geometrical light trapping within the semiconductor surface. Microstructured “black GaAs” shows antireflective properties comparable to those of mainstream black Silicon, with surface reflectance as low as 10% throughout the visible to near-infrared spectral range.
Sphere-integrated photoluminescence of black GaAs shows a doubled light absorption and emission compared to unstructured GaAs, and the appearance of spectral features related to defects. Ultrafast studies of microstructured GaAs are used to elucidate the exciton dynamics and demonstrate highly efficient reabsorption of emitted light.
References:
[1] L. T. Canham, "Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers," Applied Physics Letters, vol. 57, pp. 1046-1048, 1990.
[2] O. D. Miller and E. Yablonovitch, "Photon extraction: the key physics for approaching solar cell efficiency limits," in SPIE NanoScience+ Engineering, 2013, pp. 880807-880807-10.

Germanium (Ge) is a promising channel material for future CMOS devices because of its higher carrier mobility than silicon (Si). And germanium oxide (GeO₂) is a key material in Ge-based devices. However, GeO₂ has chemical properties such as solubility and permeability in water, unlike the more familiar silicon dioxide (SiO₂). We should take account of, or even utilize, these chemical properties to develop a wet process for a Ge surface. In this study, we show the results of pitting, patterning and flattening of Ge surfaces by metal-assisted chemical etching in water mediated by dissolved oxygen molecules (O₂).
First, we show the formation of etch pits in the shape of inverted pyramids on a Ge(100) surface in water when metallic particles such as Ag and Pt were loaded on the Ge surface [1]. When we used a Ge(111) surface, triangular etch pits were formed instead. The mechanism of this anisotropic etching is proposed to be the enhanced formation of soluble oxide (GeO₂) around metals by the catalytic activity of metallic particles, reducing dissolved O₂ in water to H₂O molecules.
Secondly, we apply this metal-assisted chemical etching to the nanoscale patterning of Ge in water using a cantilever probe in an atomic force microscopy (AFM) setup [2]. We find that the enhanced etching of Ge surfaces occurs only when both a metal-coated probe and saturated-dissolved-oxygen water are used. This presents the possibility of a novel lithography method for Ge in which neither chemical solutions nor resist resins are needed.
Thirdly, we demonstrate metal-assisted chemical flattening of a Ge surface in saturated-dissolved-oxygen water. In this experiment, Pt atoms were sputtered on a rubber plate to form a thin film with the thickness of approximately 100 nm. A p-type Ge(111) wafer (1-5 Omega;#65381;cm) was placed on the Pt surface with a controlled pressure of 0.01 MPa. Both the Pt plate and the Ge sample were rotated independently in the same plane in water for 30 min. AFM observations of a 1×1 mu;m² area exhibited that the root mean square (RMS) roughness of the processed surface is 0.10 nm, which is much smaller than that (0.24 nm) of an as-received Ge surface. We imagine that only the protrusions or microbumps on an initial Ge(111) surface were in contact with the Pt surface, inducing the preferential oxidation of the protrusions. It is likely that the Ge surface was flattened because the oxidized protrusions were dissolved in water. This result indicates that metal-assisted chemical etching can be used not only to fabricate 3D structures but also for surface flattening.
[1] T. Kawase, A. Mura, K. Nishitani, Y. Kawai, K. Kawai, J. Uchikoshi, M. Morita and K. Arima, J. Appl. Phys. 111, 126102 (2012).
[2] T. Kawase, A. Mura, K. Dei, K. Nishitani, K. Kawai, J. Uchikoshi, M. Morita and K. Arima, Nanoscale Res. Lett.8, 151 (2013).

Here, we report metal-assisted chemical etching (MaCE) chemistry to fabricate nano- and micro-scale textured gallium phosphide (GaP) with catalysts size-dependent etching behaviors.
GaP is one of promising materials for solar fuel production by photoelectrochemical (PEC) water splitting and/or CO₂ reduction because of the band gap of 2.27 eV for visible light absorption and appropriate conduction band edge positions [1]. For enhanced solar energy conversion, GaP nano- and micro-wires are desired due to excellent light management (i.e., anti-reflection and light trapping) and facile charge separation and collection [2, 3]. The vapor-liquid-solid (VLS) mechanism can be used to grow GaP nanowires at elevated temperatures [4]. The VLS grown wires, however, often contain defects such as metal impurities and stacking faults that can deteriorate electrical properties of the wires, leading to poor conversion efficiency. Metal-assisted chemical etching (MaCE) is an effective method to produce defect-free nano- or micro-structures via a wet chemistry at room temperature [5]. While MaCE has been extensively used to fabricate Si nano- or micro-structures, however, there are few studies for fabrication of nano- or micro-structured III-V semiconductors [6].
Here, we present fabrication of GaP nanocones and micro-boxes made by MaCE in mixed solutions of HF and H₂O₂. Various patterns of metal catalysts, i.e., meshes and patches, with length scale ranging from 200 nm to ~ 6 mu;m were prepared by nanosphere lithography and photolithography and used for MaCE of GaP. With catalysts meshes with 200 nm periodicity, the catalysts meshes were sunk due to enhanced dissolution of GaP under the mesh and, as a result, GaP nanocones are formed with aspect ratio up to 1:1 during MaCE. Surprisingly, when the size of metal catalysts is increased more than ~ 6 mu;m, inverse metal-assisted etching occurs: GaP outside the metal catalysts was etched and the metal catalysts behave as an etching mask. In this presentation, we will report the roles of the catalysts and its size, and etching additives as well as etching kinetics during GaP MaCE in more detail.
References
[1] Jianwei Sun, Chong Liu, and Peidong Yang, J. Am. Chem. Soc., 2011, 133, 19306-19309.
[2] Howard M. Branz et al., Appl. Phys. Lett., 2009, 94, 231121.
[3] Brendan M. Kayes, Harry A. Atwater, and Nathan S. Lewis, J. Appl. Phys., 2005, 97, 114302.
[4] Rienk E. Algra et al., Nano Lett., 2010, 10, 2349-2356.
[5] Z. Huang et al., Adv. Mater., 2011, 23, 285-308
[6] Matt DeJarld et al., Nano Lett., 2011, 11, 5259-5263

Silicon nanostructures, such as nanowires and nanopores, attract intensive attention for use in highly efficient and low cost solar cells, due to excellent optical and electrical properties [1-3]. For example, silicon nanostructures show broadband anti-reflection properties for useful solar spectrum and therefore can eliminate the need for a conventional Si₃N₄ anti-reflection film [1-2]. Despite theses promises, however, silicon nanostructures often show a poor solar energy conversion efficiency due to increased recombination in the nanostructures [4]. Here, we present optical and electrical design principles for efficient nanostructured silicon solar cells. We fabricated nanoporous silicon by metal-assisted etching and controlled pore morphology to obtain optimum anti-reflection for solar cell applications. Finally, we investigated photocarrier recombination mechanisms in a nanostructured silicon solar cell and demonstrate high efficiency nanostructured silicon solar cells made by metal-assisted etching by controlling the recombination mechanisms.
References
[1] R.B. Stephens and G.D. Cody, “Optical Reflectance and Transmission of a Textured Surface,” Thin Solid Films 45, 19-29 (1977).
[2] H.M. Branz et al. “Nanostructured black silicon and the optical reflectance of graded-density surfaces,” Appl. Phys. Lett. 94, 231121 (2009).
[3] E. Garnett and P.D. Yang, “Light Trapping in Silicon Nanowire Solar Cells,” Nano Letters 10, 1082-1087 (2010).
[4] J. Oh, H.-C. Yuan, and H.M. Branz, “An 18.2%-efficient black silicon solar cell achieved through control of carrier recombination,” Nature Nanotechnology 7, 743(2012).

Metal assisted chemical etching (MaCE) is a promising technology for next generation micro- and nano- semiconductor fabrications, where noble metals are used as catalyst to anisotropically etch into bulk materials in solution. In this talk, I will report a simple yet ultra- fine etching feature size on a silicon substrate for 3D integrated device(IC) interconnects and the first reported 3D etching of silicon for potential photonic crystal, 3D micro and nano MEMS and other structural fabrications. Also, this novel MaCE approach marks the significance in functional filling of semiconductor for nano-photonic devices as well as template-based synthesis of functional nanomaterials.

It is favorable for silicon based photovoltaic device industry to reduce silicon thickness for higher profitabality. Thickness reduction also has the further advantages of short collection lengths which could lead to improved open circuit voltages and relaxed material constraints in cells with very low surface recombination velocities. However, the problem is that thin films suffer from poor light absorption. Therefore, to achieve competitive thin cells with enhanced efficiencies, advanced light trapping strategies have been sought by many.
Here, we fabricated ultrathin silicon via potassium hydroxide etching the silicon wafers to the desired thickness (30 to 50 microns). The effect of solution concentration and temperature on etching rate and roughness of the silicon wafers were optimized. Subsequent to this, vertically aligned silicon nanowire arrays were formed on the ultrathin silicon surface via metal assisted etching (MAE) method to retrieve poor light absorption of the wafers. MAE can be conducted at room temperature without the need for expensive equipment. Nanowire decorated ultrathin single crystalline silicon wafers were characterized using optical techniques such as UV-VIS spectrometer and ellipsometry. UV-VIS spectra showed that an enhancement in optical absorption can be obtained with nanowires on silicon surface. The advantage of silicon nanowires is more significant in thinner wafers as expected. We also showed that the surface can be modeled by ellipsometry, yielding an effective refractive index for the surface covered with nanowires. The dependence of the optical properties on the process parameters such as etching duration, temperature and chemical composition of the etchant were determined through a series of parametric experiments.

Self-assembly can be used to created complex 3D nano and microscale objects from 2D patterns in a variety of ways. In these small-scale origami techniques the folding action can be driven by a range of mechanical, chemical, and thermal phenomena. However, in all cases the folding action takes place above the surface of the substrate. Here we discuss a process in which patterned metal catalyst structures fold into the substrate during Metal-assisted Chemical Etching (MaCE) of silicon. In the MaCE process etching of silicon is confined to a small region near the catalyst, with the etch rate increasing with decreasing characteristic dimension of the metal pattern. We demonstrate that the folding action of the catalyst during etching can be controlled by adjusting the geometrical design of the 2D pattern prior to etching. In particular, we show that folding can be achieved by dividing the metal film into templates with mismatched etching rates separated by mechanically weakened hinge points. We explore the dynamics of the folding process of the hinged templates, demonstrating that the folding action combines rotational and translational motion of the catalyst template, which yields topologically complex 3D nanostructures with intimately integrated metal and silicon features.

Metal-Assisted chemical Etching (MacEtch) has come a long way in the past 15 years. From a techinque to make porous and photoluminescent silicon, it is now used to fabricate high efficiency photovolatics, nanowires, 3D structures, and has expanded from just silicon to other IV semiconductor and III-V semiconductor compounds. This session will bring together some of the world's leading experts in MacEtch to discuss the current outlook and areas for future work. It will serve as a forum to bring the MacEtch community together.

In the past several decades, the optical properties of low-dimensional silicon (Si) structures have attracted worldwide interest for developing the next generation Si optoelectronics. This was particularly stimulated by the observation of room-temperature photoluminescence (PL) of Si with porous features. Although many potential fabrication techniques have been presented to prepare porous Si films with visible-light emissions, the controllable preparation of one-dimensional porous Si nanostructures are still challenging until now. To prepare such intriguing structures, together with providing better controllability of nanostructure formation, metal-assisted chemical etching (MaCE), an inexpensive, simple, and rapid wet-chemical process has been investigated in recent years. Nevertheless, it has been reported that the doping level in Si crystals substantially influences their PL characteristics and therefore, the light emission derived from radiative decay and quantum confinement of carriers in various doped Si nanocrystals should be well explored. In this study, we have systematically investigated both the light emitting and antireflection properties of Si nanowire arrays with various doping levels. Interestingly, the single-band (blue light) and dual-bands (blue and red lights) PL emissions can be well controlled with the fabrication procedures of MaCE method and the doping levels of Si wafers. Moreover, the incorporation of photoactive polymers with prepared Si nanowires was found to significantly modulate the PL intensities of Si over three times of magnitude. These systematic investigations, along with fabrication strategy are anticipated to benefit versatile Si-based optical, optoelectronic and energy devices.

Metal-assisted chemical etching (MacEtch) is a scalable, low cost and powerful fabrication technique for producing well-defined and high quality 1D, 2D and 3D semiconductor nanostructures, for a variety of applications in areas, such as, photovoltaics, energy storage, transistor and sensors.
The currently accepted MacEtch mechanism is basically a two-step redox reaction. The first redox reaction generates hole carriers when the oxidant is reduced at electrolyte-metal interface. The second redox reaction involves silicon oxidation via the injection of the generated holes into semiconductor, and/or the recombination of the holes from the extraction of electrons from silicon. Currently, most or all studies involving the MacEtch mechanism focus on the whole redox reaction process. Separation of the two-step process can provide a better understanding of the etching mechanism for the effective design and selection of the metal catalyst. In this presentation, we report on the first part of this investigation whereby the two redox reaction is decoupled. In this approach, Au is adopted as the top layer in bilayer metal structures with metals such as Ti, Cr, Ni, Al and Ag as the bottom layer in contact with silicon. In doing so, the Au-electrolyte interface is kept constant while the oxidation interface can be carefully examined. We report on the blocking ability of selected metals and explain how these metals can result in the inhibition of catalytic etching. This result paves the way for fabrication of nanostructures using low cost metallic blocking metal electrodes that will be important for adoption of MacEtch in various processes.

One of the major problems with current Si wafering techniques is kerf loss. In addition, it&’s been shown that the raw material usage for Si solar cells can be substantially lowered without sacrificing much efficiency. Metal-assisted chemical etching (MACE) of Si is explored as a potential solution to both of these problems. The end goal is a highly scalable, high yield, low cost method to produce thin Si cells via MACE. MACE is currently an available option to create high-aspect-ratio patterned structures on the micron- or nano-scale. Although MACE has been around for decades, the literature is decidedly disparate on fundamental issues such as etching mechanisms and failure modes, effects from varying thicknesses or dissolution of the catalyst, and solution concentrations and their effect on different doping levels of Si.
This work focuses particularly on the development and future applications of very high-aspect-ratio silicon microstructures which can be useful in the solar and light-trapping fields. Almost none of the literature deals with very long, localized etches; this work investigates the roadblocks and failure modes within a large parameter space involved with a gold catalyst on Si, seeking to etch as far as possible into and subsequently through a Si wafer. We have already demonstrated the ability to slice through a 350 mu;m thick wafer with 5 µm wide lines, and we expect to increase that aspect ratio to over 100 for further reduction of raw Si material loss. These strips of silicon can be used to make thin Si solar cells. Previous work from this group has shown the viability of the manipulation and manufacturability of thin Si, all the way down to a 10 mu;m thick cell. The challenge now persists in the field to discover and work towards a process to produce this thin Si with most of the ideal properties listed above.

Ge nanowires (NW) due to their enhanced mobility can improve the electrical and optical properties of electronics [1] and photovoltaics [2] devices, while being compatible with CMOS technology. Several techniques are currently used to fabricate Ge NWs, such as molecular beam epitaxy [3] or chemical vapor deposition. However, homogeneous Ge NWs production by these methods, on a large area of the substrate, is still an open issue. For instance, using a catalyst such as gold introduces impurity levels inside the semiconductor band gap, which can alter the electronic transport properties of the wires. Metal assisted chemical etching (MAcE) process is a simple and economically favored method currently used to fabricate Si NWs, offering a large variety of controllability over the Si wires parameters. However, MAcE is not effective for Ge NWs fabrication. This fact can be attributed to several factors such as dangling bonds on the Ge surface; water solubility of the germanium oxide, which ceases the mass transfer process and to the disruption of the self-generated field in the metal catalyst. In the present work, we have succeeded in the fabrication of Ge NWs by developing a novel method we called anodic metal assisted catalytic etching (AMAcE), which is a combination of the anodic catalytic etching, and MAcE process. By using this method, we have achieved Ge wires with diameters ranging from 10 to 300 nm and with a length up to 10 microns. In the AMAcE, the current density in the etching process is controlled by the metal catalyst, as well as by an external bias source. The fabrication rate of Ge wires is fast compared to typical rates reported for the MAcE process of Si. Hence, Ge wires were found detached from the wafer and dispersed on its surface. Several Ge wafers, with different dopant types and resistivity values have been tested with the AMAcE process, which requires several step of preparation of the substrate before to start the fabrication of Ge wires. The morphology of the Ge wires (SEM) revealed a dense distribution of Ge wires, randomly dispersed on the Ge substrate. The structural investigation by high transmission electron microscopy (HRTEM) is still in progress while larger diameter Ge NWs have been selected for fabrication of metal contacts by Pt, in order to investigate their electronic transport properties. Either focused ion beam or electron beam lithography have been used for contacts deposition. The surface structures of these wires will play an important role in the optical and electronic transport properties of the wires such as the case of Si NWs. Hence, further studies on the parameters involved in the process still seems necessary. We believe that this method can open a new horizon in fabrication of economically favored Ge NWs.
[1] Nathaniel J., Nano lett., 8 (2008) 4410.
[2] Bozhi Tian, Nature, 449 (2007) 885.
[3] S.J. Rezvani, Nanoscale, 6 (2014) 7469.

Due to the unique physical properties, various GaAs micro- and nano-structures have attracted increasing research attentions for many technical applications such as solar cells, light emitting diodes and field-effect transistors. In this regard, numerous fabrication techniques have been explored and among all, metal-assisted chemical etching is successfully applied to GaAs in order to achieve cost-effective, large-scale and complex structures. However, the detail explanations as well as the corresponding etching mechanism have not been reported till now or simply relied on the hole injection model of Si in order to explain the phenomenon. In this work, we perform a more systematic study to further explore and assess the etching phenomenon of GaAs employing the Au catalyst and the [KMnO₄/H₂SO₄] etch system. It is revealed that the anisotropic etching behavior of GaAs is predominantly due to the Au induced surface defects at the Au/GaAs interface, which makes the particular area more prone to oxidation and thus results in the simple directional wet etching; for that reason, more anisotropic etch is obtained for the Au pattern with higher edge-to-surface-area ratio. All these findings not only offer additional insight into the MacEtch process of GaAs, but also provide essential information of different etching parameters in manipulating this anisotropic wet etching to achieve the fabrication of complex GaAs structures for technological applications.

It is well known that Si nanowire arrays along the [100] direction can be formed by the preferential movement of Ag nanoparticles in the presence of oxidizing HF/H2O2 environment via an electroless process. Most of the works in the literature deal with metal nanoparticles that were deposited on Si either by a galvanic displacement process, or by evaporation or sputtering at a thickness below the percolation threshold of Ag. Both the aforementioned processes result in metal nanoparticles of a relatively wide unimodal size distribution with the mean diameter of the formed nanowires being correlated with the average size of Ag nanoparticles. We show that sputtered Ag thin films with thickness above the percolation threshold (5-10 nm) can be transformed to nanoparticles by UV laser annealing (UVLA). UVLA results in very narrow and well controlled size distributions of Ag nanoparticles; in particular, by varying the fluence and the thickness of the sputtered Ag layer, it is possible to select either unimodal (in the range of 30-50 nm mean size) or bimodal (in the ranges of 30-50 nm and 300-500 nm mean size) particle size distributions. In this work we use UVLA-produced templates of Ag/Si[100] of various size distributions to produce Si nanowire arrays of equivalent diameter distributions. The etching process is performed by introducing the UVLA Ag/Si[100] templates in aqueous solutions of HF/H₂O₂; the effects of HF/H2O2 concentrations and of the etching time are examined thoroughly for all the templates in terms of the Si nanowires&’ morphology (by Atomic Force Microscopy -AFM- and Scanning Electron Microscopy-SEM), crystal structure and orientation (by X-ray Diffraction-XRD), as well as the specular and integrated spectral reflectivity and the hydrophobicity (by contact angle measurements). In addition, the produced bimodal and unimodal Si nanowire arrays were decorated by Ag nanoparticles deposited by sputter deposition. The presence of the pores challenges the integrity of the deposited Ag layer resulting in the self assembly of nanoparticles. The morphological characteristics of these nanoparticles are also evaluated by AFM, SEM and XRD. Finally, their plasmonic response is investigated by surface enhanced Raman scattering experiments of a standard test solution of Rhodamine 6G. Based on the optical and plasmonic performance of the produced Si nanowires, we critically assess their potential for applications in Si-based solar cells.

Hierarchically arranged nanostructures, configured in both nanopillars and nanoholes, have been fabricated via a low-cost approach that combines metal-assisted chemical etching (MaCE), nanosphere and conventional lithography. By manipulating the catalyst morphology as well as the deposition method, different interesting nanostructures like nanowalls and nanograsses were fabricated at the galleries among the nanopillar blocks. Using the similar strategy, hierarchical negative structures (nanoholes) have also been successfully demonstrated. The successful constructions of these diversified hierarchical nanostructures illustrates that MaCE could be deployed as a feasible, low-cost method for multi-scale silicon micro/nano machining, which is highly desirable for widespread applications including tissue engineering, optoelectronics, photonic devices and lab-on-chip systems.

X-ray diffractive optics provide one of the most versatile ways to shape and manipulate an x-ray beam. This is especially important in areas such as nanoscale x-ray imaging. One of the barriers to practical use of x-ray diffractive optics, especially for the hard x-ray region, is the low efficiency of these optics, stemming from the difficulty in fabricating very high aspect ratio, high resolution dense features. We will describe the use of metal assisted chemical etching schemes to address and solve problems in the area of x-ray optics.

The ability to fabricate micro ^[1] and nanostructures ^[2] on silicon (Si) with the possibility to dope the final porous structures with noble metal NPs (e.g. gold, silver) using Metal-Assisted Etching (MAE)^[3] is a unique advantage of MAE as to sensing applications. Over the standard fabrication technique such as anodic etching ^[4], MAE represents a low-cost room-temperature method for the synthesis of Si-based nanomaterials with peculiar sensing features, in terms of sensitivity and selectivity, towards specific gases, by bringing the catalytic properties and distinctive selectivity of the metals nanoparticles^[5] and the widely tunable bandgap of the porous silicon into play.
Here the prospect of using composite silicon/gold nanostructures (cSiAuN) prepared by MAE, gold-assisted, as highly sensitive material for adsorption of Nitrogen Dioxide (NO₂) is proposed, examining the controllable high-yield integration of the material into solid-state devices.
The controllable fabrication of the final nanostructures achieved by MAE approach leads to the fabrication of cSiAuN with high degree of control in terms of morphology of the pores and depth of the matrix, with enhanced sensing capabilities, which justifies their successful application in the preparation of chemi-transistor sensors, such as field-effect transistors, FETs, to be employed for gas sensing applications. As a case-of-study, we investigate the effective method for controllable integration of composite cSiAuN between electrodes of junction-field-effect transistors (JFET), aimed at the detection of NO₂ down to 100 parts-per-billion (ppb). The resulting chemi-transistor sensor, cSiAuJFET (Composite Silicon Gold JFET), consists of a p-channel JFET in which the cSiAuN material is placed on top of the p-channel and acts as an extra floating gate and are responsible for the sensing capability of the JFET device. The cSiAuJFET sensors operate at room temperature and shows fast and reliable response to NO₂ in the range 100-500 ppb without significant aging effects, in terms of baseline drift, response times, and sensitivity value, up to two days of continuous operation. The achieved approach presented in this work represent a guide for the possibility of employing MAE for gas sensing applications.
[1] A. G. F. Owen J. Hildreth, Ching Ping Wong, ACSNano 2012, 6, 9.
[2] L. Boarino, D. Imbraguglio, E. Enrico, N. De Leo, F. Celegato, P. Tiberto, N. Pugno, G. Amato, physica status solidi (a) 2011, 208, 1412.
[3] Peng K. Q. et al. Advanced Materials2002, 14, 1164.
[4] G. M. Lazzerini, L. M. Strambini, G. Barillaro, Sci Rep 2013, 3, 1161.
[5] L. C. Nicola Cioffi, Eliana Ieva, Rosa Pilolli, Nicoletta Ditaranto, Maria Daniela Angione, Serafina Cotrone, Kristina Buchholt, Anita Lloyd Spetz, Luigia Sabbatini, Luisa Torsi, Electrochimica Acta 2011, 56.

When combined with large-area and non-lithographic thin-film patterning processes - such as solid-state superionic stamping or thin-film dewetting - metal-assisted chemical etching (MacEtch) is a scalable solution to manufacturing functional nanomaterials such as high-emissivity surfaces, solar cells, and battery electrodes. However, MacEtch is still limited in its throughput rate as it requires a new 2-D catalytic thin-film pattern for each substrate to be etched. In this talk, a high-throughput and direct silicon stamping process is demonstrated with the use of MacEtch. A polymeric stamp is reused multiple times to imprint 3D Si patterns and it is capable of centimeter-scale parallel 3-D patterning with sub-hundred nanometer resolution. Porous silicon develops in the surroundings of the contact region between stamp facilitating mass-transport of reactants and products, and the overall etch rate is limited by local depletion of reactants. Through the developed semiconductor direct stamping technique, a number of porous Si curve-linear 3D features are demonstrated which have implications in the design and fabrication of optical and Si photonic devices.

Nanotube structures may have unique advantages compared to a widely-used nanowire structure due to their high surface-to-volume ratio, low thermal conductivity, and high mechanical strength enabling reliable structure with sub-100nm dimension essential for quantum effects. [1, 2] Despite the remarkable characteristics of the nanotube structure, development of a device with nanotube arrays has been relatively slow. One of the main challenges is that it is difficult to fabricate nanotube arrays using patterning and etching processes due to the required small size of the nanotubes. In this work, we fabricate vertical Si nanotube arrays with Si thicknesses under 50-nm using metal-assisted chemical etching (MaCE) process. In the proposed scheme, Au layers are used as the catalyst metal, and HF and H₂O₂ aqueous solutions are used as the oxidant. In order to realize the vertical nanotube arrays with nano-size thicknesses, the so-called sidewall spacer technique is adopted during the patterning of the catalytic metal in the MaCE process. Successful formation of the vertical Si nanotube arrays are confirmed with scanning electron microscope and transmission electron microscope images. In this way, nanotubes as long as 10 mu;m are demonstrated with the wall thickness smaller than 50nm. The results shown here indicate that the vertical Si nanotube arrays with a high aspect ratio can be easily fabricated by combining MaCE and pattern transfer with sidewall spacers. Being easy to implement and compatible with wafer-scale processing, this technique is expected to provide a facile way to form nanotube arrays, greatly extending their use in applications.
[1] Nano Lett., J. Chen et al., 10, 3978-3983 (2010)
[2] Nano Lett., H. M. Fahad et al., 11, 4393-4399 (2011)