Oxides and nitrides as alternative plasmonic materials in the optical range [Invited]

TLDR: In this paper, the authors provide details of fabrication and characterization of these new materials and discuss their suitability for a number of metamaterial and plasmonic applications, as well as their properties.

Abstract: As alternatives to conventional metals, new plasmonic materials offer many advantages in the rapidly growing fields of plasmonics and metamaterials. These advantages include low intrinsic loss, semiconductor-based design, compatibility with standard nanofabrication processes, tunability, and others. Transparent conducting oxides such as Al:ZnO, Ga:ZnO and indium-tin-oxide (ITO) enable many high-performance metamaterial devices operating in the near-IR. Transition-metal nitrides such as TiN or ZrN can be substitutes for conventional metals in the visible frequencies. In this paper we provide the details of fabrication and characterization of these new materials and discuss their suitability for a number of metamaterial and plasmonic applications.

Figures

Fig. 2. Optical properties of GZO thin films with different thicknesses deposited on glass substrates. The films were deposited under identical conditions except for the duration of deposition.

Fig. 3. Comparison of the optical properties of pulsed laser deposited TCO films with the smallest cross-over wavelengths. The films were deposited onto glass substrates at 100 ◦C (AZO and ITO) and 50 ◦C (GZO) with oxygen partial pressures of 0.4 mTorr.

Fig. 6. Comparison of optical properties of alternative plasmonic materials with those of conventional metals. Optical constants of low loss thin films of TiN, ZrN and TCOs (AZO, GZO and ITO) are plotted along with those of gold and silver taken from [30]. The arrows show the wavelength ranges in which nitrides and TCOs are respectively metallic. Panel a) shows that TCOs and nitrides have smaller negative permittivity values than those of metals, while b) compares losses and shows that losses in TCOs are many times smaller than those in either gold or silver. The losses in nitrides are slightly higher than metals due to inter-band transitions at the cross-over.

Fig. 4. Dielectric function retrieved from spectroscopic ellipsometry measurements on thin films of TiN, TaN, HfN and ZrN deposited on c-sapphire. The substrate temperature during deposition was 800 ◦C, and the chamber pressure was 5 mTorr. The flow ratios of N2:Ar were 6 sccm: 4sccm for TaN and TiN films and 2 sccm: 8 sccm for the rest. The sputter power was 150 W for Ta and Hf targets and 200 W for the rest.

Fig. 5. Optical constants for ZrN films deposited on glass at different N2:Ar flow ratios (sccm/sccm). The sputtering pressure was held at 5 mTorr and the substrate temperature during deposition was 350 ◦C. While the ZrN film deposited in a nitrogen-rich ambient shows dielectric properties, the film deposited in a nitrogen-poor ambient shows metallic properties.

Fig. 1. Left panel: Cross-over frequency (frequency at which real permittivity crosses zero) of Al:ZnO, ITO and Ga:ZnO films as a function of dopant concentration. Right panel: Drude-damping coefficient (γ) vs. dopant concentration. The films were deposited at 100 ◦C (AZO and ITO) and 50 ◦C (GZO) with oxygen partial pressures of 0.4 mTorr. The ablation energy was about 2 J/cm2.

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Oxides and nitrides as alternative plasmonic materials in the optical range
[Invited]
Naik, Gururaj V.; Kim, Jongbum; Boltasseva, Alexandra
Published in:
Optical Materials Express
Link to article, DOI:
10.1364/OME.1.001090
Publication date:
2011
Document Version
Publisher's PDF, also known as Version of record
Link back to DTU Orbit
Citation (APA):
Naik, G. V., Kim, J., & Boltasseva, A. (2011). Oxides and nitrides as alternative plasmonic materials in the
optical range: [Invited]. Optical Materials Express, 1(6), 1090-1099. https://doi.org/10.1364/OME.1.001090

Oxides and nitrides as alternative
plasmonic materials in the optical range
[Invited]
Gururaj V. Naik,
1
Jongbum Kim,
1
and Alexandra Boltasseva
1,2,3,∗
1
Birck Nanotechnology Center and School of Electrical & Computer Engineering,
Purdue University, West Lafayette, Indiana 47906, USA
2
DTU Fotonik, Technical University of Denmark, Lyngby 2800, Denmark
3
Erlangen Graduate School in Advanced Optical Technologies (SAOT),
Friedrich-Alexander-Universit
¨
at Erlangen-N
¨
urnberg, 91052 Erlangen, Germany
∗
aeb@purdue.edu
Abstract: As alternatives to conventional metals, new plasmonic materials
offer many advantages in the rapidly growing fields of plasmonics and meta-
materials. These advantages include low intrinsic loss, semiconductor-based
design, compatibility with standard nanofabrication processes, tunability,
and others. Transparent conducting oxides such as Al:ZnO, Ga:ZnO
and indium-tin-oxide (ITO) enable many high-performance metamaterial
devices operating in the near-IR. Transition-metal nitrides such as TiN or
ZrN can be substitutes for conventional metals in the visible frequencies. In
this paper we provide the details of fabrication and characterization of these
new materials and discuss their suitability for a number of metamaterial and
plasmonic applications.
© 2011 Optical Society of America
OCIS codes: (160.3918) Metamaterials; (160.4236) Nanomaterials; (250.5403) Plasmonics;
(310.6860) Thin films, optical properties.
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1 October 2011 / Vol. 1, No. 6 / OPTICAL MATERIALS EXPRESS 1090

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(C) 2011 OSA
1 October 2011 / Vol. 1, No. 6 / OPTICAL MATERIALS EXPRESS 1091

1. Introduction
The introduction of new materials into the realm of plasmonics and metamaterials (MMs) is ex-
panding the application domain of feasible devices [1]. Recent demonstrations of superlensing
in the mid-IR [2], semiconductor plasmonic quantum dots [3] and an epsilon-near-zero (ENZ)
light funnel [4] are but a few examples. The integration of new materials not only opens up pos-
sibilities for new devices, but it also significantly improves the performance of many existing
MM and plasmonic devices. One of the most important challenges in the fields of plasmon-
ics and MMs is the high loss in the metallic components of a device. New plasmonic materials
have the promise of overcoming this major bottle-neck and enabling high-performance devices.
Also, new plasmonic materials allow greater flexibility in the design of a device owing to the
moderate magnitude of the real part of permittivity in such materials. On the contrary, metals
such as gold and silver have very large negative real permittivities in the near-IR and visible
ranges, which is a major obstacle in the design and fabrication of efficient devices. Alternative
plasmonic materials have two other major advantages: they can exhibit tunable optical proper-
ties [5], and they can be compatible with standard fabrication and integration procedures [6].
Clearly, alternative plasmonic materials have significant advantages over conventional metals
for plasmonic and metamaterial designs.
Alternative plasmonic materials in the near-IR and visible ranges can be classified into cat-
egories such as semiconductor-based [7], intermetallics [8], ceramics [9] and organic materi-
als [10]. In this article, we show that inorganic ceramic materials: semiconductor-based ox-
ides and transition-metal nitrides can be alternative plasmonic materials in the near-IR and
visible ranges respecively. These materials have advantages over the other types, since ox-
ides enable low-loss all-semiconductor based plasmonic and MM devices in the near-IR, while
metal-nitrides are CMOS compatible and provide alternatives to gold and silver in the visible
frequencies. Here, we describe our fabrication methods for these materials and study their op-
tical properties in the context of plasmonic and MM applications. A brief discussion on the
suitability of these new materials for various plasmonic and MM devices is also presented.
2. Processing and characterization
Whether we realize it or not, oxide plasmonic materials are a familiar element in everyday
life since transparent conducting oxides (TCOs) are regularly used in liquid-crystal displays.
These materials can exhibit metallic properties in the near-IR when heavily doped [11]. Doping
produces high carrier concentration (N > 10
20
cm
−3
) which results in large plasma frequency
(
ω
p
∝
√
N). A large plasma frequency results in Drude-metal-like optical properties [12]. The
following equation describes the Drude response of such degenerately doped semiconductors:
ε
=
ε

+ i
ε

=
ε
∞
−
ω
2
p
ω
(
ω
+ i
γ
)
. (1)
ε
∞
is due to the screening effect of bound electrons in the material and can be considered
as a constant in the frequency range of interest.
γ
is the Drude-relaxation rate or damping co-
efficient of free carriers. This term signifies the optical losses incurred in the material. In order
to have negative
ε

in the optical range,
ω
p
must be large and
ε
∞
must be small. Lower losses
require smaller
γ
. Heavy doping of 10
21
cm
−3
poses problems due to solid-solubility limits in
many semiconductors. However, oxide semiconductors such as zinc oxide and indium oxide
overcome this problem [13] and can be heavily doped to be metallic substitutes in the near-IR.
Further increases of the carrier concentration to about 10
22
cm
−3
are required to make metal
substitutes in the visible range; at those concentrations, oxides also suffer from solid-solubility
limits. However, transition-metal nitrides can posses such high carrier concentrations and there-
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Received 15 Jul 2011; revised 29 Aug 2011; accepted 30 Aug 2011; published 6 Sep 2011
(C) 2011 OSA
1 October 2011 / Vol. 1, No. 6 / OPTICAL MATERIALS EXPRESS 1092

1 3 5 7 9 11 13
0.3
0.4
0.5
0.6
0.7
0.8
0.9
cross−over frequency ω
c
(eV)
dopant concentration (weight %)
Al:ZnO
Ga:ZnO
Sn:In
2
O
3
1 3 5 7 9 11 13
0
0.1
0.2
0.3
Drude−damping coefficient (eV)
dopant concentration (weight %)
Al:ZnO
Ga:ZnO
Sn:In
2
O
3
Fig. 1. Left panel: Cross-over frequency (frequency at which real permittivity crosses zero)
of Al:ZnO, ITO and Ga:ZnO films as a function of dopant concentration. Right panel:
Drude-damping coefficient (
γ
) vs. dopant concentration. The films were deposited at 100
◦
C (AZO and ITO) and 50
◦
C (GZO) with oxygen partial pressures of 0.4 mTorr. The
ablation energy was about 2 J/cm
2
.
fore can exhibit metallic properties at visible frequencies. In this work, we have studied the
optical properties of aluminum-doped zinc oxide (AZO), gallium-doped zinc oxide (GZO),
indium-tin-oxide (ITO) and nitrides of titanium, tantalum, hafnium and zirconium.
2.1. Transparent conducting oxides
Thin films of TCOs can be deposited by many physical-vapor and chemical-vapor deposition
techniques. Highly conductive TCO films can be produced by techniques such as sputtering
and pulsed-laser-deposition (PLD). In our oxide-film studies, we have employed PLD (PVD
Products, Inc.) with a KrF excimer laser (Lambda Physik GmbH) at a wavelength of 248 nm
for source material ablation. The chosen ablation targets were Ga
2
O
3
and ZnO for GZO, Al
2
O
3
and ZnO for AZO, and In
2
O
3
and SnO
2
for ITO. The targets were purchased from the Kurt J.
Lesker Corp. with purities of 99.99% or higher. The required composition of the deposited film
was achieved by alternating the laser ablation over two different targets with an appropriate
number of pulses on each target. A single cycle consisting of a few laser pulses on each target
was repeated many times until the desired film thickness was achieved. The number of pulses
in each cycle was designed to be small enough so that the effective layer thickness deposited
in a single cycle would be less than a few atomic layers. This ensured a homogeneous mixture
of the constituent materials in the final film. All the films were grown in an oxygen ambient
with an oxygen partial pressure of 0.4 mTorr (0.053 Pa) or lower. The substrate was heated to
temperatures around 50-100
◦
C during deposition. The deposition conditions were optimized
to achieve highest possible carrier concentration and lowest possible losses. High carrier con-
centration produces metal-like properties for larger frequency-range in the near-IR and low loss
would enable high performance devices. The optimization curves for GZO, ITO and AZO are
shown in Fig. 1. The optical characterization of the thin films was performed using a spectro-
scopic ellipsometer (V-VASE, J. A. Woollam). The dielectric function was retrieved by fitting
#151151 - $15.00 USD
Received 15 Jul 2011; revised 29 Aug 2011; accepted 30 Aug 2011; published 6 Sep 2011
(C) 2011 OSA
1 October 2011 / Vol. 1, No. 6 / OPTICAL MATERIALS EXPRESS 1093

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Oxides and nitrides as alternative plasmonic materials in the optical range [invited]" ?

In this paper the authors provide the details of fabrication and characterization of these new materials and discuss their suitability for a number of metamaterial and plasmonic applications. 31. G. Naik and A. Boltasseva, “ A comparative study of semiconductor-based plasmonic metamaterials, ” Metamaterials 5, 1–7 ( 2011 ). 

Q2. How many pulses were used in each cycle?

The number of pulses in each cycle was designed to be small enough so that the effective layer thickness deposited in a single cycle would be less than a few atomic layers. 

Q3. What are the main categories of alternative plasmonic materials?

Alternative plasmonic materials in the near-IR and visible ranges can be classified into categories such as semiconductor-based [7], intermetallics [8], ceramics [9] and organic materials [10]. 

Q4. What are the advantages of alternative plasmonic materials?

Alternative plasmonic materials have two other major advantages: they can exhibit tunable optical properties [5], and they can be compatible with standard fabrication and integration procedures [6]. 

Q5. How thick were the films deposited on the substrates?

Films about 30-50 nm thick were deposited on the substrates, and their optical characterization was performed using a spectroscopic ellipsometer. 

Q6. How was the composition of the deposited film achieved?

The required composition of the deposited film was achieved by alternating the laser ablation over two different targets with an appropriate number of pulses on each target. 

Q7. What is the main challenge in plasmonics and MMs?

One of the most important challenges in the fields of plasmonics and MMs is the high loss in the metallic components of a device. 

Q8. What is the effect of thickness on the optical properties of TCO thin films?

In many nanoplasmonic devices, thin film structures are used as building blocks, and therefore it is necessary to understand how the optical properties of TCO thin films depend on their thickness. 

Q9. What is the effect of reactive sputtering on nitrides?

In their study of nitrides for plasmonic applications, the authors have employed DC reactive sputtering to deposit thin films of metal nitrides. 

Q10. How many times was the laser pulsed on each target repeated?

A single cycle consisting of a few laser pulses on each target was repeated many times until the desired film thickness was achieved. 

Q11. What are the advantages of plasmonic materials?

These materials have advantages over the other types, since oxides enable low-loss all-semiconductor based plasmonic and MM devices in the near-IR, while metal-nitrides are CMOS compatible and provide alternatives to gold and silver in the visible frequencies. 

Q12. What is the optimum wavelength for the deposited film?

In their oxide-film studies, the authors have employed PLD (PVD Products, Inc.) with a KrF excimer laser (Lambda Physik GmbH) at a wavelength of 248 nm for source material ablation. 

Q13. Why is due to the screening effect of bound electrons in the material?

(1)ε∞ is due to the screening effect of bound electrons in the material and can be considered as a constant in the frequency range of interest. 

Q14. What is the effect of the change in film properties on the optical properties of nitri?

This drastic change in film properties can be useful in building plasmonic devices where the properties of the material components need to be tuned or graded.