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║Advanced Laser and Optics Division, Raja Ramanna Center for Advanced Technology, Indore,. 452 013, India, and Homi Bhabha National Institute, Train...
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Investigation of dual-ion beam sputter-instigated plasmon generation in TCOs: A case study of GZO Vivek Garg, Brajendra S. Sengar, Vishnu Awasthi, amitesh kumar, Rohit Singh, Shailendra Kumar, C. Mukherjee, Victor V. Atuchin, and Shaibal Mukherjee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15103 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

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Investigation of dual-ion beam sputter-instigated plasmon generation in TCOs: A case study of GZO Vivek Garg†, Brajendra S. Sengar†, Vishnu Awasthi†, Amitesh Kumar†, Rohit Singh†, Shailendra Kumar §, C. Mukherjee║, V. V. Atuchin£, Shaibal Mukherjee*,† †

Hybrid Nanodevice Research Group (HNRG), Electrical Engineering, Indian Institute of Technology (IIT) Indore, Madhya Pradesh-453552, India § Raja Ramanna Center for Advanced Technology, Indore-452013, India ║ Advanced Laser and Optics Division, Raja Ramanna Center for Advanced Technology, Indore, 452 013, India, and Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai, 400 094, India £ Laboratory of Optical Materials and Structures, Institute of Semiconductor Physics, SB RAS, Novosibirsk 630090, Russia and Functional Electronics Laboratory, Tomsk State University, Tomsk 634050, Russia, and Laboratory of Single Crystal Growth, South Ural State University, Chelyabinsk 454080, Russia. *

E-mail: [email protected]

ABSTRACT The use of the high free-electron concentration in heavily doped semiconductor enables the realization of plasmons. We report a novel approach to generate plasmons in Ga: ZnO (GZO) thin films in the wide spectral range of ~1.87-10.04 eV. In this, dual-ion beam sputtering (DIBS) instigated plasmon is observed because of the formation of different metallic nanoclusters are reported. Moreover, formation of the nanoclusters and generation of plasmons are verified by field emission scanning electron microscope, electron energy loss spectra obtained by ultraviolet photoelectron spectroscopy, and spectroscopic ellipsometry analysis. Moreover, the calculation of valence bulk, valence surface, and particle plasmon resonance energies are performed, and indexing of each plasmon peaks with corresponding plasmon energy peak of the different nanoclusters is carried out. Further, the use of DIBS instigated plasmon enhanced GZO can be a novel mean to improve the performance of photovoltaic, photodetector, and sensing devices.

Keywords: Plasmons, TCO, Solar Cells, Photovoltaics, CIGSe, DIBS, GZO.

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INTRODUCTION Ultrathin metal layers and nanoparticles (noble metals, e.g., Au, Ag, Cu, etc.), support collective oscillations of free electrons (plasmonics) at optical frequencies1-4. Such plasmonic features of metal nanostructures, having the ability of electromagnetic field coupling and confinement, due to sustained electronic oscillations, are appreciable for light manipulation. Electromagnetic field confinement in metals made plasmonics an area of interest for light harvesting in solar cells, photodetectors, optical sensing, imaging, and drug-delivery5-12. However, few challenges related to technical and fabrication processes are the main obstacles to complete realization of the plasmonic-based commercial grade devices. The major fabrication issue is the unavailability of economically feasible techniques for precise nanostructure patterning12. The plasmonic resonances in the metals are highly reliant on the physical parameters such as size, shape, and geometry of the nanostructure, indicating the requirement for precisely controlled design parameters during the fabrication process2, 12. Moreover, the significant challenge associated with metal plasmonics are parasitic optical absorption and high carrier density near the metalsemiconductor junction. Several studies are going on to investigate the optical behavior of different alternative plasmonic materials, and various reports have been published worldwide such as metal alloys13, transition metal nitrides14, and heavily doped semiconductors15-19. Recent studies on different transparent conducting oxides (TCOs), such as ZnO:Al (AZO), ZnO:Ga (GZO) have reported metallic behavior, and small losses as compared to those for noble metals make TCOs appropriate candidates for plasmonic materials3. Among these heavily doped semiconductors, a high solubility limit and doping efficiency of Ga in the ZnO lattice without severe lattice distortion makes GZO the best candidate as a plasmonic material in NIR region3, 20. Few demonstrations of using TCO as a plasmonic material in semiconductor plasmonic modulators, plasmonic quantum dots and epsilon near zero (|ɛ|/√2

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where VBPE is the energy of VBP. Now, the PPR energy (ℏωBB ) for different nanoclusters is calculated using equation 36-37, 70.

Figure 4: Plasmonic peaks in the UPS spectra of a) GZO200, b) GZO300. Expanded plasmon peaks are depicted in c) GZO200 and d) GZO300, (plasmon peaks indexed by α, β, γ, and # with subscripts of corresponding peak numbers). FESEM images of sample e) GZO200 and f) GZO300 are shown. (inset: magnified image and processed image parameters plot of Counts vs. Particle Size) 12 ACS Paragon Plus Environment

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ω =

CED

(6)

FG HFI

where J is the high-frequency dielectric constant, 3 (for ZnO ~8.59 and air ~1), is the dielectric constant of surrounding medium of a nanoclusters25. The values of different constants required for the calculation of equations (3)-(6) are given in table 1. Due to unavailability of J value for Zn and Ga in the literature, the value of J is taken to be 3 for Zn and Ga nanoclusters, for plasmon energy calculation25, 37. Here, VBP, VSP and PPR energies of the nanoclusters embedded in GZO, are calculated and the data are summarized in table 2. The plasmonic peak locations, their distance from the Ga 3d peak position, and their corresponding energy values (VBPs and VSPs of Ga, Zn, GaO, and ZnO nanoclusters in the GZO thin films) are presented in table 3. Table 2: Different plasmon energies for metal and metal oxide nanoclusters. Thin film

GZO

Clusters of elements and compounds

VBPE (eV)

VSPE (eV)

PPRE (eV) (in ZnO medium)

PPRE (eV) (in air medium)

Ga

14.53

10.27

3.82

6.49

Zn

13.46

9.52

3.54

6.02

GaO

23.69

16.75

6.10

10.04

ZnO

21.60

5.17

1.87

3.04

As seen from table 3, for GZO200, broad plasmonic peaks Pa1 and Pa2, observed at 23.2 and 41 eV, are located at the distance of 3.14 and 20.94 eV, respectively, from the Ga-3d core level peak position. Moreover, for GZO300, the plasmon peaks Pb1-Pb4 are observed, and their respective peak positions and the plasmon peak energy difference with the core level peak Ga-3d are tabulated in table 3. The tabulated energy difference is the plasmon energy, which can be attributed to the collective contribution of surface plasmon resonance energy, and particle plasmons energy of Ga, GaO, Zn and ZnO nanoclusters in ZnO, and in an air medium. For further quantification of the contribution from individual nanocluster peaks, deconvolution of the broad plasmon peaks is performed. The deconvoluted plasmon peaks are plotted in Figures 4(c)(d). As followed from Figure 4(c), the peak Pa1 is originated from the particle plasmon generation and surface plasmon resonance at the air/GZO interface. Additionally, the contribution of particle plasmon resonance energy in ZnO is also possible within few nanometers of the GZO film. Moreover, for sample GZO300, the broad plasmon peaks Pb1-Pb4, are deconvoluted, as shown in Figure 4(d), which gives clarity to the contribution of different metallic nanoclusters and their combinations to the broad plasmonic peak generation. The plasmon generation in GZO300 is governed by the contribution from VSP and PPR in air medium at air/GZO interface, and particle 13 ACS Paragon Plus Environment

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plasmon resonance in ZnO medium, due to different metallic and metal oxide nanoclusters generation 25, 37. Table 3: Calculation of plasmon energies of different peaks and their indexing with different plasmonic nanoclusters. Thin film

Peak

Plasmon peak position (eV)

Distance of plasmon peak from Ga 3d (at 20.06 eV) position (eV)

Nanoclusters with corresponding VBPE

Nanocluste rs with correspondi -ng VSPE

Nanoclusters with corresponding PPRE (in ZnO medium)

Nanoclusters with corresponding PPRE (in air medium)

Pa1

23.2

3.14

-

-

Ga, Zn

ZnO

Pa2

41

20.94

GaO, ZnO

-

Pb1

27.94

7.88

-

(Ga + Zn), (GaO + ZnO) Zn, ZnO

(GaO + ZnO + Zn), (ZnO + Ga + GaO) Ga

Pb2

31.36

11.3

Zn

Ga, Zn

Pb3

38.40

18.34

ZnO

GaO

Ga + Zn + GaO -

Pb4

46.72

26.66

GaO

Ga + GaO

-

GZO200

GZO300

Ga + Zn

GaO or Ga + Zn (Zn + GaO + ZnO), (GaO + ZnO + Zn) Ga + Zn + GaO + ZnO

Table 4: Possible nanoclusters contribution in broad plasmon peaks indexed by α, β, γ, and # with the subscripts of corresponding peak numbers. Sample

Peak

GZO200

Pa1 Pa2

GZO300

Pb1 Pb2 Pb3 Pb4

At GZO/Air interface

PPR (Air Medium) ZnO (GaO + ZnO + Zn) (γ2), (ZnO + Ga + GaO) (#2) Ga (α1) GaO(α2) or [Ga + Zn] (γ2) [Zn + GaO + ZnO] (β3), [GaO + ZnO + Ga] (γ3) Ga + Zn + GaO + ZnO

Within GZO

VSP [Ga + Zn] (β2), [GaO + ZnO] (α2) Zn(β1), ZnO(α1) Ga(β2), Zn(α2) GaO (α3)

PPR (ZnO Medium) Ga (β1), Zn (α1) [Ga + Zn] (β1), [Ga + Zn + GaO](γ2) -

Ga + GaO

-

-

The probable locations of plasmon generation in GZO are at a) GZO/air interface, and b) GZO bulk. At air/GZO interface, the possibility of VSP occurrence is high due to low dielectric constant (air)/high dielectric constant (GZO) interface, in comparison with VBP. Meanwhile, one more 14 ACS Paragon Plus Environment

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possibility of PPR in the air medium is also as high as VSP at the air/GZO interface. Additionally, in GZO bulk, there is a high probability of particle plasmon resonance in GZO medium within few nm of the film because of the presence of metal and metal oxide nanoclusters. Considering abovementioned hypothesis, the possible nanocluster contribution in broad plasmon peaks are indexed and presented in table 4 and Figures 4(b) and (c). Field Emission Scanning Electron Microscope (FESEM) Further, verification of metal and metal oxide nanocluster formation the FESEM analysis is performed. The FESEM micrograph recorded from GZO200 and GZO300 are shown in Figures 4(e) and (f), respectively, and they confirm the formation of nanoclusters in GZO thin films. These nanoclusters are further investigated by processing of FESEM images using the ImageJ software. From image processing, the calculated average sizes of different nanoclusters present in GZO200 and GZO300 thin films are in the range of 20-35 and 40-60 nm, respectively. Moreover, the clusters formed in GZO200 are small, and the surface coverage is less, as compared to that of GZO300. When analyzing the carrier concentration increase, the effect of larger cluster dimension, lesser interparticle distance, and greater surface coverage are more prominent, leading to the origin of slight red shift (0.36 eV) of the plasmon energy in GZO300 sample, as observed in spectroscopic ellipsometry measurement. The particle size increase leads to weaker surface-to-bulk charge carrier ratio resulting in a smaller restoring force, and, hence, the loss in the plasmon energy and red shift in plasmon peaks. Moreover, the increased particle size also leads to larger surface coverage and reduced inter-particle distance, resulting in inter-particle coupling effects and loss in the plasmon energy.2 The particle plasmon resonance energy is in the range of 1.87-10.04 eV for different nanoclusters in both ZnO and air mediums. The energy range of nanoclusters fall in the ultraviolet-visible-infrared range of solar spectrum and can certainly improve the absorption cross-section by scattering mechanism, where the effective optical path length in the photo-active medium increases and hence ultimately enhances the solar cell performance 71. Currently, high-efficiency and ultra-thin solar cells, precious metals like Au and Ag are used with particle plasmon energy of 2.8 and 4.5 eV, respectively, which adds to the cost of large-area device fabrication70. Nevertheless, these precious and noble metals can be replaced by the DIBS grown GZO thin films for realizing economical and high-performance photovoltaics. Such plasmons can enhance the performance of heterojunction solar cells as plasmon enhanced n-type layer. Additionally, TCOs are the essential part of thin film solar cells, which can be used as plasmon enhanced back contact, and the top layer of a buffer and buffer-less solar cells. Additionally, these layers possess enormous potential to enhance the performance of Silicon, thin film, and tandem solar cells. Moreover, besides Ga, the effect of other dopant materials, i.e., Al, Fe, etc. on the plasmonic properties of DIBS grown TCO thin films is another area of interest for the future work.

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CONCLUSIONS In this study, DIBS grown high-quality GZO thin films grown on Si substrates at varying growth temperatures ranging from 200-600 ℃ are reported. The structural and electrical characteristics of the films confirm the suitability of samples grown at 200 and 300 oC for plasmonics applications, having high carrier concentration (~1020) and low resistivity. Further, the successful realization of plasmon generation in GZO thin film with the application of the secondary ion source is demonstrated. Moreover, the formation of different nanoclusters of GaO, Ga, ZnO, and Zn due to different sputtering rates for different materials leads to the generation of valence bulk, valence surface and particle plasmons in GZO thin films. The plasmons generations are confirmed through electron energy loss analysis from UPS spectra, cluster formations by FESEM and collective plasmon resonance energy calculation using spectroscopic ellipsometry analysis. In conclusion, a novel approach to realize plasmons in GZO thin films by utilizing DIBS deposition technique is demonstrated. ACKNOWLEDGMENT This work is partially supported by the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy (DAE), Government of India (No. 37(3)/14/20/2014-BRNS) and Clean Energy Research Initiative (CERI), Department of Science and Technology (DST), Government of India (DST/TM/CERI/C51(G)), DST-RFBR Project under India-Russia Programme of Cooperation in Science and Technology (No. DST/INT/RFBR/IDIR/P-17/2016). We are thankful to DIBS, FESEM, EDX, and XRD facility equipped at Sophisticated Instrument Centre (SIC) at IIT Indore. The authors Vivek Garg acknowledge UGC and, Brajendra S. Sengar and Amitesh Kumar acknowledge CSIR India for their fellowships. Prof. Shaibal Mukherjee is thankful to Ministry of Electronics and Information Technology (MeitY) Young Faculty Research Fellowship (YFRF) award under Visvesvaraya Ph.D. Scheme for Electronics and Information Technology and DST and IUSSTF for BASE Fellowship award. We are thankful to Dr. D. M. Phase, Dr. R. J. Chaudhary and Mr. A. Wadikar for using AIPES beam line Indus facility at RRCAT. This research is supported by Grant no. 8.2.18.2017 of the Tomsk State University Academician D.I. Mendeleev Fund Program. The reported study is funded by RFBR according to research project 16-52-48010. The work was supported by Act 211 Government of the Russian Federation, contract № 02.A03.21.0011. Additionally, the work was partially supported by the Ministry of Education and Science of the Russian Federation (10.9639.2017/8.9).

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