**4. Enhancement of spontaneous emission rate**

Fig. 2 shows that huge enhancements were observed by Ag coating especially at the shorter wavelength region. It was found that the enhancement effect became lower and lower with increasing of wavelength. This wavelength dependence of the SP enhancement effect is well correlated to the properties of the SPP. Fig. 4 shows the dispersion diagrams of SPP modes on metal/GaN surfaces calculated by Eq. (1). The SP frequency (*ω*SP) at GaN/Ag is 2.84 eV (437nm). Thus, Ag is suitable for SP coupling to blue emission, and we attribute the large increases in the PL intensity from Ag-coated samples to such resonant SP excitation. By this reason, the SP coupling becomes remarkable when the energy is near to the SP frequency described as dotted line in Fig. 2. In contrast, *ω*SP at GaN/Au is 2.462 eV (537 nm), and no measurable enhancement is observed in Au-coated InGaN emitters as the SP and QW energies do not match. In the case of Al, the *ω*SP is 5.50 eV (225 nm), and the real part of the

Plasmonics for Green Technologies: Toward High-Efficiency LEDs and Solar Cells 203

coupling rate as *k*PL\*= *k*rad+*k*non+*k*SPC. Usually, *k*SPC should be much faster than *k*rad because of

Theoretically, the SP coupling rate is given by the Fermi's Golden rule as follows (Gontijo, et

the electric field of the SPP at the location of the active layer, and *ρ* is the DOS of the SSP.

( ) ( ) ( ) *SPC rad*

We plotted *F* value against wavelength in Fig. 5. The solid line in this figure shows slope of the dispersion diagram (*dk*/*dω*), which is proportional to DOS of SPP. We found that F and *dk*/*dω* are almost same values. This suggests that the enhanced emission rates and IQEs are

> ( ) ( ) () () () *spc*

*kk k*

The *η*\*int values can be estimated separately by the temperature dependence of the PL intensities by assuming IQE~100% at low temperature (Okamoto et al., 2004). Obtained *η*\*int values were also shown in Fig. 6 (broken line). The ratios of *η*\*int and *η*ex-sp give the SPphoton coupling efficiency (*C'*ext) by Eq. (3) and were shown in Fig. 6 (solid line). Fig. 6 shows that all these efficiencies are reached to almost 100% at the shorter wavelength region. This suggests that if we can control the SP frequency and obtain the best SP coupling condition, we can develop super bright LEDs which have perfect efficiencies at any

Our proposed mechanism suggests that very high efficiency may be achievable at any wavelength if we can control the SP frequency and obtain the best matching condition. Tuning of SP coupling should be attainable by choosing the appropriate metal or controlling nanostructures. For example, we fabricate several types of metal nanostructures shown

Fig. 7(a) shows the Scanning electron microscope (SEM) image of nano grating structure created by electron beam lithography and ion milling. By using this structure, we could enhanced the green emission from InGaN/GaN QW a few times larger. Fig. 7(b) shows the nano grain structure created by control of metal vapour deposition conditions. A few tens

*k*

*rad non spc*

This equation suggests that the SP coupling rate should be proportional to the DOS. We defined the enhancement factor (*F*) of the spontaneous emission rate as follows,

> *<sup>k</sup> <sup>F</sup> k*

2 2 () () () *SPC k dE* 

(4)

(6)

 is

(5)

is the dipole moment of the electron-hole pair, *E*

the higher DOS of SPP.

al., 1999; Neogi et al., 2002),

determined only by the DOS.

**5. Tuning of the SP coupling** 

wavelength.

in Fig. 7.

Fig. 6 shows the *η*ex-sp values estimated by

*ex sp*

where is the reduced Plank constant, *d*

dielectric constant is negative over a wide wavelength region for visible light. Thus, a substantial and useful PL enhancement is observed in Al-coated samples, although the energy match is not ideal at 470 nm and a better overlap is expected at shorter wavelengths.

Fig. 4. Dispersion diagrams of the SPP at Al/GaN, Ag/GaN, and Au/GaN interfaces.

The external quantum efficiency (*η*ext) of LED is given by a product of the light extraction efficiency (*C*ext) and the internal quantum efficiency (IQE: *η*int). The original *η*int value is determined by the ratio of the radiative (*k*rad) and nonradiative (*k*non) recombination rates of excitons.

$$
\eta\_{ext}\text{(co)} = \mathcal{C}\_{ext}\text{(co)} \times \eta\_{int}\text{(co)} = \mathcal{C}\_{ext}\text{(co)} \frac{k\_{rad}\text{(co)}}{k\_{rad}\text{(co)} + k\_{non}\text{(co)}}\tag{2}
$$

Under the existence of the SP coupling, the enhanced IQE value (*η*\*int) can be described as follows,

$$
\eta\_{\rm limit}^{\*}\text{(co)} = \frac{k\_{\rm rad}\text{(co)} + \text{C}\_{\rm ext}^{\prime}\text{(co)}k\_{\rm SPC}\text{(co)}}{k\_{\rm rad}\text{(co)} + k\_{\rm non}\text{(co)} + k\_{\rm SPC}\text{(co)}} \approx \text{C}\_{\rm ext}^{\prime}\text{(co)} \times \eta\_{\rm ex-sp}\text{(co)}\tag{3}
$$

where *k*SPC is the SP coupling rate and should be very fast because the density of states (DOS) of SPP is much larger than that of the excitons in the QW. *C'*ext is the probability of photon extraction from the SPPs. *C'*ext is decided by the ratio of light scattering and dumping of the SPPs through non-radiative loss. *C'*ext should depend on the roughness and nanostructure of the metal surface. If the SP coupling is much faster (*k*SPC >>*k*rad), the *η*\*in is given by the product of *C'*ext and the exciton-SP coupling efficiency (*η*ex-sp).

The increased emission rates were observed by the TRPL measurement. Fig. 3 (inset) shows the PL decay profiles of uncoated and Ag-coated InGaN/GaN QW samples emitters at 440nm. The PL decay rate (*k*PL) is attributed to the radiative and nonradiative recombination rate of excitons as *k*PL=*k*rad+*k*non. After Ag-coating, *k*PL values were increased to *k*PL\* by the SP coupling rate as *k*PL\*= *k*rad+*k*non+*k*SPC. Usually, *k*SPC should be much faster than *k*rad because of the higher DOS of SPP.

Theoretically, the SP coupling rate is given by the Fermi's Golden rule as follows (Gontijo, et al., 1999; Neogi et al., 2002),

$$k\_{SPC}(\text{co}) = \frac{2\pi}{\hbar} \left| \vec{d} \cdot \vec{E}(\text{w}) \right|^2 \rho(\text{w})\tag{4}$$

where is the reduced Plank constant, *d* is the dipole moment of the electron-hole pair, *E* is the electric field of the SPP at the location of the active layer, and *ρ* is the DOS of the SSP. This equation suggests that the SP coupling rate should be proportional to the DOS.

We defined the enhancement factor (*F*) of the spontaneous emission rate as follows,

$$F(\text{co}) = \frac{k\_{SPC}(\text{co})}{k\_{rad}(\text{co})} \tag{5}$$

We plotted *F* value against wavelength in Fig. 5. The solid line in this figure shows slope of the dispersion diagram (*dk*/*dω*), which is proportional to DOS of SPP. We found that F and *dk*/*dω* are almost same values. This suggests that the enhanced emission rates and IQEs are determined only by the DOS.

Fig. 6 shows the *η*ex-sp values estimated by

202 Advanced Photonic Sciences

dielectric constant is negative over a wide wavelength region for visible light. Thus, a substantial and useful PL enhancement is observed in Al-coated samples, although the energy match is not ideal at 470 nm and a better overlap is expected at shorter wavelengths.

Fig. 4. Dispersion diagrams of the SPP at Al/GaN, Ag/GaN, and Au/GaN interfaces.

int

' \* '

*rad ext SPC*

*k Ck <sup>C</sup>*

 

*rad non SPC*

given by the product of *C'*ext and the exciton-SP coupling efficiency (*η*ex-sp).

*ext ext ext*

excitons.

follows,

int

The external quantum efficiency (*η*ext) of LED is given by a product of the light extraction efficiency (*C*ext) and the internal quantum efficiency (IQE: *η*int). The original *η*int value is determined by the ratio of the radiative (*k*rad) and nonradiative (*k*non) recombination rates of

( ) () () () () () ()

() () () ( ) () () () () ()

where *k*SPC is the SP coupling rate and should be very fast because the density of states (DOS) of SPP is much larger than that of the excitons in the QW. *C'*ext is the probability of photon extraction from the SPPs. *C'*ext is decided by the ratio of light scattering and dumping of the SPPs through non-radiative loss. *C'*ext should depend on the roughness and nanostructure of the metal surface. If the SP coupling is much faster (*k*SPC >>*k*rad), the *η*\*in is

The increased emission rates were observed by the TRPL measurement. Fig. 3 (inset) shows the PL decay profiles of uncoated and Ag-coated InGaN/GaN QW samples emitters at 440nm. The PL decay rate (*k*PL) is attributed to the radiative and nonradiative recombination rate of excitons as *k*PL=*k*rad+*k*non. After Ag-coating, *k*PL values were increased to *k*PL\* by the SP

*kk k*

*<sup>k</sup> C C*

Under the existence of the SP coupling, the enhanced IQE value (*η*\*int) can be described as

*rad*

(2)

(3)

*ext ex sp*

*rad non*

*k k*

$$\eta\_{\rm ex-sp}(\alpha) = \frac{k\_{\rm spc}(\alpha)}{k\_{\rm rad}(\alpha) + k\_{\rm non}(\alpha) + k\_{\rm spc}(\alpha)}\tag{6}$$

The *η*\*int values can be estimated separately by the temperature dependence of the PL intensities by assuming IQE~100% at low temperature (Okamoto et al., 2004). Obtained *η*\*int values were also shown in Fig. 6 (broken line). The ratios of *η*\*int and *η*ex-sp give the SPphoton coupling efficiency (*C'*ext) by Eq. (3) and were shown in Fig. 6 (solid line). Fig. 6 shows that all these efficiencies are reached to almost 100% at the shorter wavelength region. This suggests that if we can control the SP frequency and obtain the best SP coupling condition, we can develop super bright LEDs which have perfect efficiencies at any wavelength.

#### **5. Tuning of the SP coupling**

Our proposed mechanism suggests that very high efficiency may be achievable at any wavelength if we can control the SP frequency and obtain the best matching condition. Tuning of SP coupling should be attainable by choosing the appropriate metal or controlling nanostructures. For example, we fabricate several types of metal nanostructures shown in Fig. 7.

Fig. 7(a) shows the Scanning electron microscope (SEM) image of nano grating structure created by electron beam lithography and ion milling. By using this structure, we could enhanced the green emission from InGaN/GaN QW a few times larger. Fig. 7(b) shows the nano grain structure created by control of metal vapour deposition conditions. A few tens

Plasmonics for Green Technologies: Toward High-Efficiency LEDs and Solar Cells 205

annealing temperature, and annealing temperature. Fig. 7(d) shows the nano particle sheet structure created by the Langmuir-Blodgett (LB) technique at an air–water interface. Quite recently, we succeeded in fabrication of the beautiful periodic nanostructures by the bottomup technique without E-beam lithography and found very interesting optical properties

Fig. 7. Scanning electron microscope (SEM) image of various nanostructures of Ag. (a) Nano grating structure created by electron beam lithography and ion milling. (b) Nano grain structure created by control of metal vapour deposition conditions. (c) Nano particulate array structure created by thermal annealing under nitrogen atmosphere after metal thin film vapour deposition. (c) Nano particle sheet structure created by the Langmuir-Blodgett

By using the metal nanoparticle structures, we can use the localized surface plasmon (LSP) mode shown in Fig. 8(a). The LSP mode is tenable by the particle size and inter-particle distance. Fig. 8(b) shows the FDTD simulations of the LSP resonance spectra of Ag nanoparticle with various diameters. Inset of Fig. 8(b) show the spatial distributions of the electric field of the LSP mode generated around the Ag nano-particle with 40 nm diameter. The electric fields are strongly localized near the metal particle surface, and the penetration depth is as long as the radii of the particles regardless of the wavelength. The LSP is the non-propagated mode of the electromagnetic field and the localized area is much smaller than the wavelength. Therefore the LSP mode should be applicable to the photonics with

nanometer scale, such technology is called plasmonic nanophotonics.

(Toma et al., 2011).

(LB) technique at an air–water interface.

Fig. 5. Enhancement factor of the spontaneous emission obtained by the ratios of surface plasmon coupling rates and radiative recombination rates (*k*SPC/*k*rad). The solid line is *dk*/*dω* of the SPP at Ag/GaN interface. (Inset) PL decay profiles of uncoated and Ag coated sample at 440nm.

Fig. 6. Wavelength dependences of the exciton-SPP coupling efficiency (marks), the enhanced internal quantum efficiency (broken line), and the SSP-photon coupling efficiency (solid line).

nanometre sized metal grains ware generated under the slow deposition rate ~1 Å/s. This structure is very easy to fabricate and control, so we used this structure for the sample used in Fig. 2. Fig. 7(c) shows the nano particulate array structure created by thermal annealing under nitrogen atmosphere after metal thin film vapour deposition. We can control both the particle size and inter-particle distance independently by initial metal film thickness,

Fig. 5. Enhancement factor of the spontaneous emission obtained by the ratios of surface plasmon coupling rates and radiative recombination rates (*k*SPC/*k*rad). The solid line is *dk*/*dω* of the SPP at Ag/GaN interface. (Inset) PL decay profiles of uncoated and Ag

Fig. 6. Wavelength dependences of the exciton-SPP coupling efficiency (marks), the enhanced internal quantum efficiency (broken line), and the SSP-photon coupling

nanometre sized metal grains ware generated under the slow deposition rate ~1 Å/s. This structure is very easy to fabricate and control, so we used this structure for the sample used in Fig. 2. Fig. 7(c) shows the nano particulate array structure created by thermal annealing under nitrogen atmosphere after metal thin film vapour deposition. We can control both the particle size and inter-particle distance independently by initial metal film thickness,

coated sample at 440nm.

efficiency (solid line).

annealing temperature, and annealing temperature. Fig. 7(d) shows the nano particle sheet structure created by the Langmuir-Blodgett (LB) technique at an air–water interface. Quite recently, we succeeded in fabrication of the beautiful periodic nanostructures by the bottomup technique without E-beam lithography and found very interesting optical properties (Toma et al., 2011).

Fig. 7. Scanning electron microscope (SEM) image of various nanostructures of Ag. (a) Nano grating structure created by electron beam lithography and ion milling. (b) Nano grain structure created by control of metal vapour deposition conditions. (c) Nano particulate array structure created by thermal annealing under nitrogen atmosphere after metal thin film vapour deposition. (c) Nano particle sheet structure created by the Langmuir-Blodgett (LB) technique at an air–water interface.

By using the metal nanoparticle structures, we can use the localized surface plasmon (LSP) mode shown in Fig. 8(a). The LSP mode is tenable by the particle size and inter-particle distance. Fig. 8(b) shows the FDTD simulations of the LSP resonance spectra of Ag nanoparticle with various diameters. Inset of Fig. 8(b) show the spatial distributions of the electric field of the LSP mode generated around the Ag nano-particle with 40 nm diameter. The electric fields are strongly localized near the metal particle surface, and the penetration depth is as long as the radii of the particles regardless of the wavelength. The LSP is the non-propagated mode of the electromagnetic field and the localized area is much smaller than the wavelength. Therefore the LSP mode should be applicable to the photonics with nanometer scale, such technology is called plasmonic nanophotonics.

Plasmonics for Green Technologies: Toward High-Efficiency LEDs and Solar Cells 207

Fig. 9. (a) SPM image of the Ag nanoparticles array structure on InGaN/GaN. (b) PL enhancement ratio plotted against wavelength taken for InGaN/GaN QW with Ag

Fig. 10. Energy conversion schemes of the SP enhanced LEDs and solar cells.

depend on the pumping method. So now we try to make super bright plasmonic LEDs by

Fig. 10 shows the energy conversion scheme of the SP coupling and light emission. The SPexciton and SP-photon coupling processes provide new emission pathways. The high efficiency LEDs should be achievable if the new emission path through the SP coupling is much faster than the original emission path [ (2) + (11) > (3), (4) > (12) in Fig. 10]. By the similar way, plasmonics should be also able to improve high-efficiency solar cells, because the SP-exciton and SP-photon coupling processes are reversible. The sun-light can couple to the SP at the metal/dielectric interface and generate the excitons in the dielectric materials. This process should increase efficiencies of light absorption and photocurrent conversion if

particles and Ag thin film.

(8) > (7), (9) > (12) in Fig. 10.

electrical pumping.

Fig. 8. (a) Schematic diagram of the localized surface plasmon (LSP) mode. (b) The FDTD simulations of resonance spectra of the LSP generated around the Ag nanoparticles with various diameters. (Inset) the spatial distributions of the electric field of the LSP mode.

Fig. 8(b) suggests that the LSP resonance is tunable within whole visible wavelength region by changing the Ag size, and 100~150 nm diameter should be suitable to couple to green light which has been very difficult to enhance. Fig. 9(a) shows the Scanning Probe Microscopic (SPM) image of Ag nanoparticle arrays fabricated on InGaN/GaN by a metal deposition and a thermal annealing. We could fabricate the Ag nano-particle array structure with about 100~150 nm diameter. The wavelength dependence of the PL enhancement ratios were shown in Fig. 9(b). Remarkable enhancement was observed at 500-520 nm with Ag particles, while the ratios were almost flat with Ag film. This difference should be due to the properties of the LSP mode and the propagating SPP mode. A huge enhancement of green emission, which has been very difficult to achieve, was observed at certain wavelength and special ranges by controlling the metal nanostructures. This result suggests that high efficiency light emitters can be achievable at various wavelength regions by further optimization of nanostructures.

#### **6. Device applications using plasmonics**

One of the most important targets of this study is device application of the SP coupling. The SP coupling technique increases IQEs by increasing spontaneous emission rates. This suggests that this should be applicable for electrical pumping because the IQEs do not

Fig. 8. (a) Schematic diagram of the localized surface plasmon (LSP) mode. (b) The FDTD simulations of resonance spectra of the LSP generated around the Ag nanoparticles with various diameters. (Inset) the spatial distributions of the electric field of the LSP mode.

Fig. 8(b) suggests that the LSP resonance is tunable within whole visible wavelength region by changing the Ag size, and 100~150 nm diameter should be suitable to couple to green light which has been very difficult to enhance. Fig. 9(a) shows the Scanning Probe Microscopic (SPM) image of Ag nanoparticle arrays fabricated on InGaN/GaN by a metal deposition and a thermal annealing. We could fabricate the Ag nano-particle array structure with about 100~150 nm diameter. The wavelength dependence of the PL enhancement ratios were shown in Fig. 9(b). Remarkable enhancement was observed at 500-520 nm with Ag particles, while the ratios were almost flat with Ag film. This difference should be due to the properties of the LSP mode and the propagating SPP mode. A huge enhancement of green emission, which has been very difficult to achieve, was observed at certain wavelength and special ranges by controlling the metal nanostructures. This result suggests that high efficiency light emitters can be achievable at various wavelength regions by further

One of the most important targets of this study is device application of the SP coupling. The SP coupling technique increases IQEs by increasing spontaneous emission rates. This suggests that this should be applicable for electrical pumping because the IQEs do not

optimization of nanostructures.

**6. Device applications using plasmonics** 

Fig. 9. (a) SPM image of the Ag nanoparticles array structure on InGaN/GaN. (b) PL enhancement ratio plotted against wavelength taken for InGaN/GaN QW with Ag particles and Ag thin film.

depend on the pumping method. So now we try to make super bright plasmonic LEDs by electrical pumping.

Fig. 10 shows the energy conversion scheme of the SP coupling and light emission. The SPexciton and SP-photon coupling processes provide new emission pathways. The high efficiency LEDs should be achievable if the new emission path through the SP coupling is much faster than the original emission path [ (2) + (11) > (3), (4) > (12) in Fig. 10]. By the similar way, plasmonics should be also able to improve high-efficiency solar cells, because the SP-exciton and SP-photon coupling processes are reversible. The sun-light can couple to the SP at the metal/dielectric interface and generate the excitons in the dielectric materials. This process should increase efficiencies of light absorption and photocurrent conversion if (8) > (7), (9) > (12) in Fig. 10.

Fig. 10. Energy conversion schemes of the SP enhanced LEDs and solar cells.

Plasmonics for Green Technologies: Toward High-Efficiency LEDs and Solar Cells 209

(Atwater & Polman, 2010). However these palsmonic solar cells are still far from practical utilizations. Further optimization of the metal nanostructure and tuning of the SP coupling process are required in order to improve the plasmonic solar cell to the practical level

Fig. 11. Possible device structures of high efficient LEDs based on plasmonics with electrical pumping. (a) Metal electrode is located a few nm above the active layer. (b) Metal particles are embedded a few nm above the active layer, (c) p-GaN has 2-dimentional structures.

Prizum

Metal Film

(a) (b) (c)

SPP mode

Fig. 12. The previously reported plasmonic solar cell structures based on (a) metal nano particles disposed on the materials, (b) attenuated- total-reflection (ATR) consignation with

The previous reported plasmonic solar cells can be classified into three types shown in Fig. 12. Fig. 12(a) show the structure by using the metal nanoparticles, which were simply

Metal Nano-Grating

SPP mode

Waveguide Mode

prism, and (c) nano periodic grating structure.

Material

Metal particles

Localized Mode

Possible device structures of high efficient LEDs are shown in Fig. 11. Fig. 11(a) shows the simplest structure using a usual LED structure with a p-n junction. The metal layer can be used both as an electrical contact and for exciting plasmons. The important point of this structure is that the distance between the metal surface and the InGaN QW must be very close to get a good SP coupling. Therefore, the p-type GaN layer must be thinner than 10 nm. The PL enhancement ratios become exponentially decay with increasing of the thickness of the GaN spacer layer (Okamoto et al., 2004). This feature makes the device application of the SP coupling so difficult. We already fabricated the structure shown in Fig. 10(a) but we were not able to obtain a huge enhancement of emission. There are two reasons; first, p-doping was very difficult into 10 nm thick GaN layer. Second, we could not get a good ohmic contact because the p-GaN layer is too thin. Another possible structure of a plasmonic LED was shown in Fig. 11(b). In this structure, the metal layer for electrode and for SP coupling is different. The SP coupling should happen at the metal particles implanted just above a QW layer in a LED wafer. Fig. 11(c) shows another promising device structure which has a two-dimensional structure fabricated by the lithography and the dry etching processes. By using this structure, the electrons injection and the SP coupling can be well performed at the thick areas and the thin areas, respectively. This should enable both good ohmic contact and SP enhancement effects at the same time.

Recently a few groups reported about the SP enhanced LEDs based on our technique. Yeh et al. reported the SP coupling effect in an InGaN/GaN single-QW LED structure (Yeh et al., 2007). Their LED structure has a 10 nm p-type AlGaN current blocking layer and a 70 nm ptype GaN layer between the metal surface and the InGaN QW layer. The total distance is 80 nm, which is too far to obtain an effective SP coupling. By this reason, they obtained only 1.5 fold enhancement of the emission. Kwon, et al. also reported a plasmonic LED which has similar structure to Fig. 11(c) (Kwon et al., 2008). They put silver particles on the InGaN QW layer first, and over grew a GaN layer above the Ag particles. However, a large amount of Ag particles were gone by high temperature of the crystal growth and only 3% particles remained. Therefore, they obtained only 1.3-fold enhancement of the emission. These tiny enhancement ratios should be not good enough for device application. Therefore, a high efficient LED structure based on plasmonics is not yet achieved.
