Long Wavelength Plasmonic Absorption Enhancement in Silicon Using Optical Lithography Compatible Core-Shell-Type Nanowires

Plasmonic properties of rectangular core-shell type nanowires embedded in thin film silicon solar cell structure were characterized using FDTD simulations. Plasmon resonance of these nanowires showed tunability from λ = 750 nm to λ = 2400 nmwith variation of dimensional parameters within the feature resolution specifications of Deep Ultraviolet and Laser Interference Lithography techniques. A half-shell nanowire structure was proposed for simplifying device integration which showed 10 times absorption enhancement in silicon at λ = 940 nm. However this absorption was significantly smaller than the Ohmic loss in the silver shell due to very low near-bandgap absorption properties of silicon. Prospect of improving enhanced absorption in silicon to Ohmic loss ratio by utilizing dual capability of these nanowires in boosting impurity photovoltaic effect and efficient extraction of the photogenerated carriers was discussed. Our results indicate that high volume fabrication capacity of optical lithography techniques can be utilized for plasmonic absorption enhancement in thin film silicon solar cells over the entire long wavelength range of solar radiation.


Introducti on
Thin film solar cells are expected to play pro minent role in clean energy generation.Further improvement in efficiency and reliability is needed to materialize this prospect.Silicon is one of the main materials in thin film solar cell development due to earth abundance and low cost.Co mplete absorption of solar radiation in thinner films and effective extraction of photogenerated carriers are the key challenges in improving efficiency of thin film silicon solar cells.A nu mber of methods such as light trapping, impurity band photovoltaic effect and upconversion have been suggested for increasing absorption in next generation silicon solar cells [1][2][3].Impu rity photovoltaic effect can convert sub-bandgap photons to charge carriers through transitions involving deep level impurities [1,4].Impurity photovoltaic zones can be implemented using a number of conventional doping methods [1].However reco mb ination in the impurity zone limits net gain in carrier generation.Light trapping was shown to shift the balance towards increased net carrier generation [4].Plasmonic effect has drawn strong interest as a light trapping method for enhancing absorption [1][2][3].Plas monic light trapping utilizes plas mon resonance in metal nanostructures for increasing absorption by scattering or coupling solar radiat ion into solar cell materials and/or local enhancement of electric field intensities of incident light near the surface of the nanostructures [2].A variety of nanostructures such as nanoparticles, nanowires and gratings have been proposed for plasmonic enhancement.These nanostructures are typically less than 100 nm in size and their practical incorporation in production scale solar cell devices still remains a challenge.Selforganized layers of nanoparticles can be formed by annealing thin silver or gold films [5].Throughput of this technique is limited by long annealing times.Metal nanoparticles and nanowires can be grown by a number of methods [6] and applied to solar cells by spin coating.However spin coating do not allow placement of the nanoparticles with controlled spacing.Aggregation of the nanoparticles causes large change of their plasmonic properties due to coupling and sparse distribution makes plasmonic enhancement less than optimal.Ebeam lithography, Nano Imprint Lithography (NIL) and Substrate Conformal Imprint Lithography (SCIL) can be used for forming regular arrays of sub-100 n m structures [7].These techniques are yet to meet the low cost large scale fabrication requirements of solar cell technologies.Ebeam lithography can be applied to small areas and throughput is low due to sequential nature of the process.Both NIL and SCIL needs highly clean environ ment.NIL has stringent surface flatness and roughness requirements.SCIL technique has been developed to relax the surface quality requirements of NIL so that it can become the volume production technique for nanofabrication.Still SCIL needs high resolution ebeam lithography for making master masks and special equip ment for replicating production masks wh ich have limited lifetime.On the other hand Deep Ultra Violet Lithography (DUVL) and Laser Interference Lithography (LIL) techniques can be used for large area high volume fabrication of photonic nanostructures with feature sizes larger than 100 n m [8,9].LIL is particularly suitable for making periodic grating type patterns in square meter scale areas without requiring a mask [9].In this work FDTD simulat ions were performed to explo re feasibility of using DUVL and LIL fabricated nanostructures for plasmonic absorption enhancement in silicon solar cells.Core-shell type rectangular nanowires were chosen in our studies since plasmon resonance of core-shell type structures can be tuned widely by varying core or shell dimensions [10,11] and rectangular shape is compatible with planar thin film fab rication technologies.Light trapping properties of core-shell type structures were discussed in [12,13].Analytical solutions for spherical core -shell particles were reported in [14].Ou r simulat ion results show that plasmon resonance wavelength of these nanostructures can be tuned from λ=750 n m to λ=2400 n m by adjusting dimensional parameters .These plasmonic structures may imp rove performance of impurity photovoltaic solar cells by serving the dual purpose of absorption enhancement and effective separation photogenerated carriers.) can be used for defining the width and height of the nanowires.A metallization method with good step coverage such as sputtering or ebeam evaporation with sample rotation should be used for depositing the shell layer (2(c)).For half-shell nanowire fabrication, the next step is lift-off followed by silicon deposition to embed the nanowires in silicon.For core-shell nanowires, silicon deposition should be done next fo r forming the core.A directional deposition method such as ebeam evaporation without sample rotation should be used for silic on deposition so that metal on the sidewalls (Fig. 2 (c

Simulation Set-up
Simu lations were performed in 2D using Lu merical FDTD Solutions [16].Nanowires significantly longer than the wavelength of incident light can be considered infinitely long in  the context of light scattering.The nanowires fabricated by DUVL or LIL techniques can be much longer than the optical wavelengths in solar radiation.Hence these nanowires can be assumed to be infinitely long and cross -sectional 2D simulat ion can be applied to study their localized plasmon properties.Silver and silicon models were based on data from Palik [17].Staircase appro ximation was applied at the material boundaries .Mesh size was at least 10 times smaller than the smallest feature present in the simulation.The nanowires were positioned at the center of an 800×800 n m simu lation cell with Perfectly Matched Layer (PM L) boundary.300 n m of g lass substrate and 500 n m of silicon layer were included in the simu lation cell (Fig .1).Same parameters of the Total Field Scattered Field (TFSF) source were used in all related simu lations so that power absorption data from different simu lation runs could be compared.Spatial distribution of optical power absorption was recorded using Lu merical"s built-in advanced power monitor for better accuracy.According to Lu merical documentation, electric field co mponent values are interpolated to the center of Yee cells for power absorption calculations.Advanced algorithms are used for more accurate estimation of electric field values at metal-nonmetal boundaries.Power absorption was recorded in a 200×150 n m bo x shaped region with the nanowire at the center for all simulat ions with nanowire widths smaller than 150 nm.Size of the power absorption recording box was 300×150 nm for wider nanowires.In this geometry power absorption due to enhanced local electric fields of the nanowire can be assumed to be much larger than absorption due to scattered and incident fields.Plas mon resonance causes increase in electric field intensities in the metal of the nanowire and in the semiconductor near the nanowire surface.Power absorption at a wavelength "ω" in a Yee cell with refract ive index "n(x,y, ω)" and elec tric field intensity "E(x,y, ω)" is calculated using the formu la Summation of absorption from all Yee cells in the simu lation area gives the total absorption at a particular wavelength.This process is repeated for all wavelengths.Hence plasmon resonance gives rise to peaks in the power absorption vs. wavelength plots.A computer program was used for determin ing whether a Yee cell is located in silicon or silver.Thus power absorption in silicon and silver regions were determined separately. .

PLAS MONIC ENHANCEMENT PROPERTIES OF CORE-S HELL AND HALF-SHELL NANOWIRES
Fig. 3 shows absorption in silicon and silver vs. wavelength plots for the nanowires.Core material for the core-shell nanowire was silicon and shell material was silver for both of nanowires.Keeping the shell thickness constant at 10 n m, the widths and heights of the nanowires were adjusted to bring their plasmon resonance close to λ=950 n m for simplify ing absorption comparison since plasmon resonance wavelength, enhanced electric field distribution and absorption varies with wavelength dependent refractive indices of the metal and surrounding semiconductor.Increase of width and height dimensions caused plasmon resonance to shift to longer wavelengths.Such shift was observed in spherical core -shell nanoparticles for increasing core diameters [10,11].Plasmon resonance was at λ=951 n m for a core-shell nanowire with 120 n m width and 40 n m height.A half-shell nanowire with 120 nm width and 35 nm height showed plasmon resonance at λ=946 n m.Traces (a) and (c) show absorption in silicon for the core-shell and half-shell nanowires.Traces (b) and (d) show absorption in silver downscaled by 10 times for the same nanowires.Trace (e) shows absorption in bare silicon without any nanowire scaled up by a factor 5 for comparison.These traces were scaled for improving graphical representation.Comparing the traces (a), (c) and (e), absorption enhancement in silicon for the core-shell and half-shell nanowires were 7 and 10 times respectively.5 times absorption enhancement was reported in [18] due to a solid spherical silver nanoparticle at λ=870 n m.Hence geometry of plasmonic nanostructures plays important role in determining absorption enhancement.Fig. 4(a) shows a spatial map of optical power absorption in the vicinity a core-shell nanowire in logarith mic scale.Power absorption was calculated using Eq. ( 1).This figure shows strong absorption in the core region.In co mmon ly used solar cell structures such as p-i-n or p -n type devices, effective extraction of the photogenerated carriers fro m the core region would require additional device design and fabrication steps.Hence the half-shell nanowire structure was investigated.This nanowire would resemb le U-shape for larger nanowire heights.Various truncated structures such as split ring type metamaterials are known to show interesting plasmonic properties which mot ivated us to investigate this structure.Fortuitously plasmon resonance of the halfshell nanowires was also tunable in the desired wavelength ranges and their absorption enhancement properties were better than the core-shell nanowires.Spatial distribution of optical power absorption for a half-shell nanowire is shown in Fig. 4(b ).Fro m this figure it is apparent that photogenerated carriers can drift towards the collection contacts in conventional solar cell structures without encountering obstacle.
Co mparison of the traces (c) and (d) in Fig. 3 shows that even the enhanced absorption in silicon was less than 10% of the Ohmic absorption loss in the silver shell of the half-shell nanowires.Ohmic absorption loss for solid spherical silver nanoparticles was also found to be large which prompted the authors in [14] to hold pessimistic view regarding effective plas monic absorption enhancement in silicon.We argue that our proposed plasmonic enhancement structures can potentially lead to more favorable enhanced absorption to Ohmic loss ratio by serving the dual purpose of plasmonic enhancement of impurity optical absorption in silicon and extraction of one type of the photogenerated carriers.Sin ce spherical silver nanoparticles must be isolated from each other in a solar cell structures to preserve their individual p lasmonic properties, they cannot play effective role in carrier transport and extraction.According to the material models used in our simulat ions, imaginary co mponent of refract ive index of silver was 6200 t imes larger than silicon at λ=940 n m.However Oh mic loss in silver was not 6200 times larger.Hence the spatial distribution of plasmonic electric fields is very effective in enhancing absorption in silicon.Fo rmation of narrow impurity photovoltaic zones around the nanowires may lead to better utilization of such field distribution for enhancing absorption.Impurity photovoltaic effect can be described as electron-hole pair generation by sub-bandgap photons through transitions involving deep level impurities.Deep level impurities also increase carrier reco mbination in the impurity zone which hinders effective separation of generated carriers specially for wider impurity zones.Our proposed structure can resolve this dilemma by serving the dual purpose of plasmonic enhancement and carrier collection.The silver shell can form Oh mic contact to n -type silicon and extract electrons effectively.Reduction of the impurity zone width and immed iate collection of one type of the photogenerated carriers would decrease recombination loss in the impurity zone.Since the real parts of refractive index of amorphous silicon (a -Si:H) and crystalline silicon are co mparable, we speculate that nanowires with similar d imensions embedded in a-Si:H would show plasmon resonance at nearly the same wavelengths.The poorly absorbing photons on the upper side of crystalline silicon bandgap are sub -bandgap photons for a-Si:H since the bandgap of a-Si:H is larger.Hence these nanowires may be used for enhancing impurity photovoltaic effect in a-Si:H solar cells as well.Plasmonic enhancement of impurity photovoltaic effect in a-Si:H has been reported recently [19].

PLAS MON RES ONANCE TUNING OF HALF-SHELL NANOWIRES
Solar radiation has significant power density up to approximately λ=2500 n m [20] which can be harvested by impurity photovoltaic effect discussed above.Hence tunability of plasmon resonance of the half-shell nanowires was investigated for absorption enhancement at the longer wavelengths of solar spectra.Keeping the shell thickness constant at 10 nm, widths and heights of the nanowires were varied to study their effect on plasmon resonance.Optical power absorption spectra were recorded for the bandwidth λ=750 n m to λ=1850 n m.Fig. 5 shows total power (silicon +silver) absorption spectra for nanowire heights from 20 to 80 n m.Width of these nanowires was 120 n m.Absorption spectrum fo r a nanowire with 140 n m width and 60 n m height is also shown in this figure to demonstrate the effect of width variation.Most of the absorption takes place in silver at longer wavelengths.Absorption in silicon and silver were not separated since the purpose of this plot is to show tunability of plasmon resonance.Fig. 5 shows three plasmon resonance peaks for each nanowire.
These peaks shift to longer wavelengths with increase of height.It is possible to tune plasmon resonance from λ =750 n m to λ =1850 n m by choosing an appropriate nanowire height from 20 n m to 80 n m while keep ing the nanowire width fixed at 120 n m.Increase of nanowire width also shifts plasmon resonance to longer wavelengths as can be seen by comparing the absorption traces for the 120×60 and 140×60 n m nanowires.The longest Fig. 5 Total (silicon+silver) power absorption spectra for half-shell nanowires with heights from 20 to 80 nm.Width of these nanowires was 120 nm.T he trace marked as (140w,60 nm) shows the spectrum for a (140×60) nanowire.Power absorption was normalized with respect to source power.plasmon resonance peak for a 250×60 n m nanowire was located at λ= 2400 n m.This nanowire is not shown in Fig. 5 since the simulat ion bandwidth and power absorption recording box size for this nanowire were different fro m the other nanowires.Simu lation bandwidth was fro m λ =1000 n m to λ=2500 n m for this nanowire due to valid ity limit of silver material model fro m λ = 1 µm to 10 µm.Silver model used in the previous simulat ions was valid up to λ=2 µm.Hence p lasmon resonance of half -shell nanowires can be tuned in the λ = 750 n m to λ=2400 n m range by adjusting the width and height dimensions.These nanowires may enhance impurity photovoltaic effect over the entire long wavelength range of solar radiation.

Conclusion
Nu merical investigations on the feasibility of fabricat ing plasmonic absorption enhancement structures for silicon thin film solar cells using optical lithography techniques were perfo rmed.Simu lation results indicate reliab le mass production capability of optical lithography techniques can be utilized for fabricat ing nanowires with plasmon resonance tunability fro m λ=750 n m to λ =2400 n m covering the entire long wavelength range of solar radiation.These nanowires may lead to improved absorption enhancement to Ohmic loss ratio by serving the dual purpose of plasmonic enhancement of impurity photovoltaic effect and effective ext raction of photogenerated carriers.

Fig. 1
Fig. 1 shows a core-shell nanowire in the simulation set-up discussed next.The dimensional parameters of the nanowire s uch as width, height and shell thickness are also shown in this figure.Fig 2 shows suggested fabrication sequence and structure (Fig. 2(d )) of a half-shell nanowire.Photolithography (Fig. 2(a)) and etching (Fig. 2(b)) can be used for defining the width and height of the nanowires.A metallization method with good step coverage such as sputtering or ebeam evaporation with sample rotation should be used for depositing the shell layer (2(c)).For half-shell nanowire fabrication, the next step is lift-off followed by silicon deposition to embed the nanowires in silicon.For core-shell nanowires, silicon deposition should be done next fo r forming the core.A directional deposition method such as ebeam evaporation without sample rotation should be used for silic on deposition so that metal on the sidewalls (Fig.2 (c)) do not get coated with silicon.Residual silicon fro m the sidewalls can be removed by mild etching.Then deposition of the top metal layer and liftoff would co mplete the core-shell nanowire.These fabrication steps and structures are compatible with polycrystalline silicon solar cell develop ment using ebeam evaporation[1 5].
Fig. 1 shows a core-shell nanowire in the simulation set-up discussed next.The dimensional parameters of the nanowire s uch as width, height and shell thickness are also shown in this figure.Fig 2 shows suggested fabrication sequence and structure (Fig. 2(d )) of a half-shell nanowire.Photolithography (Fig. 2(a)) and etching (Fig. 2(b)) can be used for defining the width and height of the nanowires.A metallization method with good step coverage such as sputtering or ebeam evaporation with sample rotation should be used for depositing the shell layer (2(c)).For half-shell nanowire fabrication, the next step is lift-off followed by silicon deposition to embed the nanowires in silicon.For core-shell nanowires, silicon deposition should be done next fo r forming the core.A directional deposition method such as ebeam evaporation without sample rotation should be used for silic on deposition so that metal on the sidewalls (Fig.2 (c)) do not get coated with silicon.Residual silicon fro m the sidewalls can be removed by mild etching.Then deposition of the top metal layer and liftoff would co mplete the core-shell nanowire.These fabrication steps and structures are compatible with polycrystalline silicon solar cell develop ment using ebeam evaporation[1 5].

Fig. 1
Fig. 1 Sketch of a core-shell nanowire in the simulation set-up. .

Fig. 3
Fig. 3 Power absorption spectra in silver (traces (a) and (c)) and silicon (traces (b) and (d)) for the coreshell and half-shell nanowires.Absorption in bare silicon without nanowire is shown in (trace (e)).This trace was scaled up by 5 times and absorption in silver was downscaled by 10 times.Power absorption was normalized with respect to source power.