Porosity control in metal-assisted chemical etching of degenerately doped silicon nanowires

Balasundaram, Karthik; Sadhu, Jyothi; Shin, Jae Cheol; Azeredo, Bruno; Chanda, Debashis; Malik, M. Rizwan; Hsu, Kuo Chin; Li, Jinghua; Ferreira, Placid M.; Sinha, Sanjiv; Li, Xiuling

doi:10.1088/0957-4484/23/30/305304

Cited by 126 publications

(134 citation statements)

References 24 publications

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“…For P(+) and N(+)-Si, however, due to a high doping level, h + can easily tunnel through the Si-HF solution interface regardless of the dopant type, thus their current density increases faster than P-Si. 29 During MaCE of the heavily doped Si, the tunneling process not only induces sidewall etching, but also makes the Si between trenches highly porous on both substrates (Figure 6), which is consistent with results from previous results of electrochemical etching 34 and nano-MaCE [35][36][37] on heavily-doped Si. Now that CT1 and CT2 have been discussed, we can further investigate the correlation between CT1 and CT2.…”

Section: Resultssupporting

confidence: 88%

Charge Transport in Uniform Metal-Assisted Chemical Etching for 3D High-Aspect-Ratio Micro- and Nanofabrication on Silicon

Zhao

Wong

2015

ECS J. Solid State Sci. Technol.

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Recently, metal-assisted chemical etching (MaCE) has been proposed as a promising method for micro-and nanostructures fabrication on silicon (Si) with high aspect ratio, high geometric uniformity and low cost. In MaCE, electron holes (h + ) are injected into Si through catalytic reduction of H 2 O 2 on metal catalyst thin film patterns. Si beneath the metal is etched through a redox reaction where h + are involved. This work investigated a fundamental electrochemical process during MaCE: the transport of h + , and revealed its unique correlation with the 3D profile of the etching results. It is discovered that under the uniform etching condition, etching occurs both in the Si beneath the catalysts as well as on the sidewall of the etched space. On N-type Si, the sidewall etching is intrinsically depressed, and highly vertical HAR structures are formed; on P-type Si and undoped Si, the sidewall tapering becomes significant as the pattern number and density increase. The variation of the 3D profile can be explained by the CT during etching using a Schottky junction model, which show dependence on the intrinsic properties of the Si. CT involved in the two etching process can further be correlated by diffusion or drift of h + , which explains the influence of catalysts geometry. Fabrication of micro-and nanostructures on silicon (Si) is a key step in manufacturing of modern electronic and optoelectronic devices. Especially, the high-aspect-ratio (HAR) structures, of which the vertical dimension is much larger than the lateral dimension, enable orders-of-magnitude higher integration density and superior system performance compared to that of traditional planar structures by utilizing the space inside Si. HAR structures, such as deep trenches and deep holes, have been widely used in advanced Si-based devices. For example, deep trenches are the core structures in the microelectromechanical systems; 1 deep holes serve as the interconnect routes in the emerging 3D integration technology in microelectronic packaging. 2Most of these HAR structures are fabricated by selective removal of certain volume of Si from the bulk Si substrate, which is generally referred to as the etching of Si. Until now, the major applicable Si etching method for HAR structures fabrication is the deep reactive ion etching (DRIE). 3,4 In DRIE, Si is put in a gas chamber and etched by plasma. Although DRIE is able to fabricate a wide range of HAR structures, it is suffered from high cost. On the other hand, metal-assisted chemical etching (MaCE), a novel low-cost wet chemical etching method, has attracted attention from both academia and industry.

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Section: Resultssupporting

confidence: 88%

Charge Transport in Uniform Metal-Assisted Chemical Etching for 3D High-Aspect-Ratio Micro- and Nanofabrication on Silicon

Zhao

Wong

2015

ECS J. Solid State Sci. Technol.

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“…This effect plays a role in pore formation during potentiodynamic electrochemical dissolution of Si in HF electrolytes, but has not been observed for galvanic displacement or electroless etching reactions for Si NWs. Higher AgNO 3 concentrations, temperatures or etching time would be required for the p-type wires to becomes mesoporous, but increasing the p-type doping density would also increase the etch rate to allow a transition to mesoporous ptype wires, as was demonstrated by Balasundram et al 49 In highly doped electrodes, h þ diffusion would also be possible due to high carrier concentrations, allowing dissolution to occur away from the reduction site of the Ag at the interface with the Si. For the p-type and n-type Si used here, the h þ diffusion lengths are at most on the order of 10 À3 cm, and less for the lower h þ mobility n-type Si.…”

Section: -8mentioning

confidence: 95%

“…Previously reported studies show that some degree of porosity control is attained by tuning the Si resistivity (doping density), where a trend toward higher roughness with decreasing resistivity (or higher doping density) 4,46-48 is found. There are few formal investigations of the influence of doping concentration and doping type for both roughness and porosity formation 49 within NWs.…”

Section: Introductionmentioning

confidence: 99%

Doping controlled roughness and defined mesoporosity in chemically etched silicon nanowires with tunable conductivity

McSweeney

Lotty

Mogili

et al. 2013

Journal of Applied Physics

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By using Si(100) with different dopant type (n++-type (As) or p-type (B)), we show how metal-assisted chemically etched (MACE) nanowires (NWs) can form with rough outer surfaces around a solid NW core for p-type NWs, and a unique, defined mesoporous structure for highly doped n-type NWs. We used high resolution electron microscopy techniques to define the characteristic roughening and mesoporous structure within the NWs and how such structures can form due to a judicious choice of carrier concentration and dopant type. The n-type NWs have a mesoporosity that is defined by equidistant pores in all directions, and the inter-pore distance is correlated to the effective depletion region width at the reduction potential of the catalyst at the silicon surface in a HF electrolyte. Clumping in n-type MACE Si NWs is also shown to be characteristic of mesoporous NWs when etched as high density NW layers, due to low rigidity (high porosity). Electrical transport investigations show that the etched nanowires exhibit tunable conductance changes, where the largest resistance increase is found for highly mesoporous n-type Si NWs, in spite of their very high electronic carrier concentration. This understanding can be adapted to any low-dimensional semiconducting system capable of selective etching through electroless, and possibly electrochemical, means. The process points to a method of multiscale nanostructuring NWs, from surface roughening of NWs with controllable lengths to defined mesoporosity formation, and may be applicable to applications where high surface area, electrical connectivity, tunable surface structure, and internal porosity are required

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“…[38][39][40] Si NWs can developed significant roughness under these etching conditions, 41 but when an n-type semiconductor-solution interface is biased electrolessly via the potential difference between the redox even of Ag + reduction and the Si Fermi level, significant charge transfer can occur, limited by a very thin depletion region in highly doped n-type materials resulting in internal mesoporosity formation within the NWs. [42][43][44][45][46] Controlling the internal crystal density and overall structure through electrochemically formed porosity is useful for large area surface modification of silicon, 47 and can in some cases, avoid the need for complex intermediate steps to promote consistent or ordered surface assembly. These methods often require organic ligands, control of surface wetting and nanocrystal interaction forces to maximize packing to create uniform areal density deposition at device-scale areas.…”

mentioning

confidence: 99%

Quantum Confined Intense Red Luminescence from Large Area Monolithic Arrays of Mesoporous and Nanocrystal-Decorated Silicon Nanowires for Luminescent Devices

O’Dwyer

McSweeney

Collins

2015

ECS J. Solid State Sci. Technol.

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We report intense red luminescence from mesoporous n + -Si(100) nanowires (NWs) and nanocrystal-decorated p-Si NWs fabricated using electroless metal assisted chemical (MAC) etching. n + -Si NWs are composed of a labyrinthine network of silicon nanocrystals in a random mesoporous structure. p-type Si(100) NWs exhibit solid core structure, with a surface roughness that contains surfacebound nanocrystals. Both mesoporous n + -Si NWs and rough, solid p-Si NWs exhibit red luminescence at ∼1.7 and ∼1.8 eV, respectively. Time-resolved photoluminescence (PL) measurements indicated long (tens of μs) radiative recombination lifetimes. The red luminescence is visible with the naked eye and the red light is most intense from mesoporous n + -Si NWs, which exhibit a red-shift in the emission maximum to 1.76 eV at 100 K. The red PL from monolithic arrays of p-type NWs with nanocrystal-decorated rough surfaces is comparatively weak, but originates from the surface bound nanocrystals. Significant PL intensity increase is found during excitation for mesoporous NWs. X-ray photoelectron spectroscopy identifies a stoichiometric SiO 2 on the rough p-Si NWs with a SiO x species at the NW surface. No distinct oxide is found on the mesoporous NWs. The analysis confirms that long life-time PL emission arises from quantum confinement from internal nanoscale crystallites, and oxidized surface-bound crystallites, on n + -and p-Si NWs respectively. Recent advancements in the growth and formation of semiconducting nanostructures, particularly group 14 metalloid semiconductors such as silicon and germanium have shown how quantum effects can be engineered and controlled 1-3 to modify thermal and optical properties. 4,5 In particular, the nanostructuring of silicon materials mediates phonon scattering and confinement, including reduction of thermal conductivity 6-9 and Si-based nanowire heterostructures have been employed as solar cell materials and nanoelectronic power sources.10 Crystal sizes below the Bohr radius also results in light emission, and effective bandgap modification can influence absorbed thermopower in nanoscale silicon, and for photovoltaics.11 In photovoltaics, polycrystalline silicon comprises more than 90% of the PV cell production. The efficiency is limited to ∼33-35%, with most of the power converted to heat via phonon emission.12 Recent developments in nanoscale silicon crystals that provide multiple exciton generation events (at energies greater than twice the bandgap) per incident photon may improve efficiencies. 13Porous semiconductors and clusters of quantum dots, particularly of silicon, have also been the subject of intense research recently for Li-ion batteries [14][15][16] water oxidation 17 and electroluminescent devices 18 and those exhibiting optical gain, 19-21 and a range of (bio)sensors. 22,23 Since the discovery of visible light emission in porous silicon [24][25][26] that in the main, arose from quantum confinement effects, porous silicon has had some usefulness for nanoelectronic devices, 27 and field ef...

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Porosity control in metal-assisted chemical etching of degenerately doped silicon nanowires

Cited by 126 publications

References 24 publications

Charge Transport in Uniform Metal-Assisted Chemical Etching for 3D High-Aspect-Ratio Micro- and Nanofabrication on Silicon

Charge Transport in Uniform Metal-Assisted Chemical Etching for 3D High-Aspect-Ratio Micro- and Nanofabrication on Silicon

Doping controlled roughness and defined mesoporosity in chemically etched silicon nanowires with tunable conductivity

Quantum Confined Intense Red Luminescence from Large Area Monolithic Arrays of Mesoporous and Nanocrystal-Decorated Silicon Nanowires for Luminescent Devices

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