In this thesis we study the LSP behaviors of metal nano-particles and the strain release of InGaN/GaN quantum well. We fabricate gold and silver nano-clusters with diameter between 40 and 100 nm with the AAO technique. This topic will be discussed in Chapter 2. In Chapter 3, we report the strain release phenomenon by fabricating nano-hole array patterns with the AAO technique on an InGaN/GaN QW structure. Finally, the conclusions are drawn in Chapter 4.
Table 1.1 Extrinsic and intrinsic size effects of the optical response of metal clusters [22].
Cluster radius R R≦10nm R≧10nm
Electrodynamics of Mie theory
Independent of R f(R)
Optical material functions ε = ε(R) Independent of R
Size effect intrinsic extrinsic
Table 1.2 Classification of clusters according to the number N of atom per cluster. In addition cluster diameter 2R for Na clusters and the ratio of surface to inner volume atoms Ns/Nv are given [22].
Very small clusters Small clusters Large clusters 2 < N ≦ 20 20 ≦ N ≦ 500 500 ≦ N ≦ 107 2 RNa ≦ 1.1nm 1.1nm≦2RNa≦3.3nm 3.3nm≦2RNa≦100nm Surface and inner
volume not separable
0.9 ≧ Ns/Nv ≧ 0.5 0.5 ≧ Ns/Nv
Fig. 1.1 Structure of Anodic alumina oxide.
Fig. 1.2 The relation of interpore distance and anodic voltage [13].
Al Cell
Pore
Barrier
(a)
(b)
Fig. 1.3 (a) is voltage-time anodization curve for current-static anodization of an Al sheet in 4% H3PO4 at a current density of 5mA/cm2, and (b) is current-time anodization curve for voltage-static anodization in 3% H2C2O4 at 40V [12].
(a)
(b)
Fig. 1.4 (a) shows the characteristic current density versus time for a constant anodization voltage [14], and (b) shows the process of crack creation [15].
Fig. 1.5 Schematic of aluminum during anodic oxidation [16].
Electrolyte/oxide interface
Metal/oxide interface
Fig. 1.6 SEM images of PAA obtained in 6 wt.% EGPA at 5 mAcm−2 and 298 K for (a) 71 s, (b) 300 s and (c) 600 s [17].
Fig. 1.7 Schematic diagram showing the electrochemical reactions and ionic paths involved during anodization of aluminum [14].
Fig. 1.8 Effect of stirring on the current density at 40 V in 3% H2C2O4 at 15℃.
Fig. 1.9 Effect of the bath temperature on the current density at 40V in 3% H2C2O4 [12].
Fig. 1.10 A scheme illustrating the excitation of the dipole surface lasmon oscillation.The electric field of an incoming light wave induces a polarization of the (free) conduction electrons with respect to the much heavier ionic core of a spherical gold nanoparticle.A net charge diOEerence is only felt at the nanoparticle boundaries (surface) which in turn acts as a restoring force. In this way a dipolar oscillation of the electrons is created with period T. This is known as the surface plasmon absorption [24].
Fig. 1.11 Sketch of a homogeneous sphere placed in an electrostatic field [23].
Fig. 1.12 Electric and magnetic fields far away from the clusters of L=1, 2, and 3 [22].
Fig. 1.13 Compressive and tensile strains.
Fig. 1.14 Misfit dislocation.
Fig. 1.15 Schematic drawings of band diagram, polarization field, and the sheet charges at the interfaces of InGaN/GaN QWs.
Chapter 2
Fabrication of Metal Nano-particles and Surface Plasmon Characteristics
2.1 Fabrication of AAO and Metal Nano-particles on GaN Substrate
The AAO technique has been developed for many years. However, the AAO arrays are irregular with the conventional one-step anodization process. Some anodic methods have been brought up, such as the two-step anodization process or employing an imprinting process for an aluminum substrate before anodization [27]. The imprinting process, as show in Fig. 2.1, was first developed by Masuda [27] and could achieve regular AAO matrices. However, the imprint molds are expensive, and they are expendables. In contrast, although the two-step anodization process cannot be used to achieve AAO arrays as regular as imprinting process, it can still be useful for most cases of application. Hence, we chose the two-step process for fabricating AAO.
To fabricate AAO on GaN substrate, we first deposit an Al thin film on GaN substrate with an electron-gun evaporator. However, if we
anodize the aluminum on GaN substrate directly, GaN substrate may be damaged. Therefore, we grow a 100nm SiO2 thin film on GaN substrate with plasma-enhanced chemical vapor deposition (PECVD) before depositing Al.
In the first anodization stage, we used 0.3 M oxalic as the electrolyte and maintained the electrolyte temperature at 15℃ to anodize alumina for 45 sec. The oxide layer on the aluminum thin film was then removed by immersing the sample into chromic acid at 60℃ for 60 min. Next, the aluminum was anodized at the same temperature and electrolyte again for 7.75 min. After anodization, AAO is immersed into 5% phosphoric acid to widen the pore diameter. We chose the anodization voltage at 40V and 60V resulting in pore difference of 120 and 150 nm, respectively.
If our goal is to fabricate a regular metal nano-particle array on GaN template, the following procedures are needed after the AAO process is finished. First, we utilize reactive ion etching (RIE) to transfer the nano-pore pattern onto SiO2. The second step is to remove the AAO film on SiO2 by immersing the sample into phosphoric acid until the alumina oxide is fully dissolved in the acid solution. Then, we use the electron-gun evaporator to deposit sliver or gold into the pores of SiO2.
Finally, we remove SiO2 to obtain the metal particle arrays. A schematic diagram of the complete procedures of the nano-metal cluster fabrication is shown as follows:
1. Growing SiO2 on GaN followed by the deposition of Al 2. AAO Fabrication
3. Transfering pore pattern from AAO onto SiO2 4. Removing AAO
5. Depositing metal by E-gun 6. Removing SiO2
With different pore-widened times, we can change the AAO pore
diameter and control the metal particle size. For each applied anodization voltage, we use different pore-widened times after anodization and obtain a series of metal particles of different sizes. Therefore, we can compare the surface plasmon properties of the metal particle arrays of different particle sizes with the same interpore distance. Then, we can study the influence of the interpore distance.
2.2 Surface Plasmon Characteristics of Nano-particles on GaN Substrate
Figure 2.3 through 2.6 show the white light transmission spectra of Ag nano-particles on GaN substrate. The anodization voltage is 60 V and the pore-widened times are 70, 80, 90, and 110 min, respectively. The spectrum of the white-light source ranges from 400 though 750 nm. For each sample, we measured the GaN substrate transmission spectrum as a reference and we use the Ag nano-particle transmission spectraum divided by the GaN transmission spectrum to obtain the ratio of transmission, as shown in Fig. 2.2. The oscillations are due to the Fabre-Perot effect between the GaN-sapphire interface and the GaN-air
interface. In order to determine the minimum spectral position of the ratio, we used the Matlab to filter out high frequency components.
In Figs. 2.3 through Fig. 2.6, one can see minimum spectral positions of the ratios at 450, 475, 490, and 500 nm for the pore diameters of 50, 70, 80, and 110 nm, respectively. The spectral minimum corresponds to the LSP absorption peak of the metal particles. This phenomenon can be explained by two reasons: First, if the particle size increases, the distance between the charges of the opposite interfaces of particle also increases, leading to a smaller restoring force and a lower resonance frequency.
Therefore, the LSP absorption peaks red shift. Second, because our metal dimensions are larger than 50 nm, we cannot just consider the dipole mode. Higher-order modes such as quadrupole can exist and even dominate the optical properties, as shown in equations (1.20) ~ (1.23) and Fig. 1.12. Energy of higher-order modes is lower than that of the dipole mode such that the absorption peaks red shift.
Figures 2.7, 2.8, and 2.9 show the white light transmission spectra of the samples of Au nano-particles on GaN substrate. The results are similar to those of the samples of Ag nano-particles. The absorption peaks are at 580, 590, and 610 nm as the pore diameters are 80, 90, and 100 ~
110 nm, respectively. Fig. 2.10 shows the SEM images of different resolutions.
Fig. 2.11 shows the transmission spectral ratios of Ag nano-particle arrays on sapphire substrate. Under the same fabrication conditions, we can see that the absorption peak positions blue shift when the GaN substrate is replaced by sapphire template. Also, if the transmission measurement is performed with light incident from the air side, absorption peak positions also blue shift, as shown in Fig. 2.12. We can see that the higher is the dielectric constant of the surrounding medium, the lower the resonance energy will be. This phenomenon can be explained by equation
2
If the dielectric constant, εm, of the surrounding medium decreases, the ε1
at the corresponding resonant frequency also increases (|ε1| decreases).
Thus, according to equation (1.6), the resonant frequency will increase to match the resonant condition.
If the transmission dip was due to the reflection of the metal particles, it will not be just a ‘dip’ at particular wavelength. Instead, it should be a broad band, as shown in Fig. 2.13. In addition, the dip positions are dependent on the medium dielectric constant, which represents another important characteristic of SP.
Figure 2.15 and 2.16 show the results of angle-dependent transmission spectral measurements. Figure 2.14 shows the optical measurement setup. We placed the sample between the light source and a fiber, then rotated the sample to measure the transmission spectrum at each direction. The x-axis in Figures 2.15 and corresponds to the incident angle. Here, the y-axis represents wavelength. The red color regions represent high transmission ratio, and blue color regions represent low transmission ratio. Figure 2.15(a) and 2.16(a) show the original data before filtering out the Fabre-Perot oscillations. Figures 2.15(b) and 2.16(b) show the ratios after filtering out the high-frequency components.
Because the length of the resonant cavity increases if the sample is rotated, the resonant peaks of the Fabre-Perot oscillation shift with incident angle. Therefore, we can see many colored stripes bending with the incident angle. However, this behavior has nothing to do with SP.
After filtering out the Fabre-Perot oscillation, it is become clear that the dips of the transmission ratios of each angle are at the same wavelength, as shown in Figures 2.16(b) and 2.17(b). This result shows that the behavior of the metal nano-particles is due to LSP, not SPP.
Although we have studied certain properties of Ag and Au nano-particles, all of the characteristics discussed above are the properties of single nano-particles. In nano-particle arrays, additional shifts are expected to occur due to the electromagnetic interactions between the localized SP modes [23]. In order to study the properties of nano-particle arrays, we change the anodization voltage into 40V to increase the particle density. We fabricated two samples with the same anodization voltage of 40 V and different pore-widened times of 70 and 90 min.
Figures 2.18 and 2.19 show the SEM images and optical transmission ratios of the two samples. From SEM images, we can see that the interpore distances are decreased to 100nm. The absorption peak is at 455 nm for particle diameter of 45 nm and 490 nm for particle diameter of 65 nm. By comparing Fig. 2.3 with Fig. 2.18, one can see that the particle sizes are similar and their electron resonant peaks are the same. However, comparing Fig. 2.5 with Fig. 2.19 we can see that they have similar
electron resonance peaks but different particles sizes. The particle size is 80nm in Fig. 2.5 but is only 65nm in Fig. 2.19. We can see that the interparticle coupling leads to a red shift as the particle separation decreases. This phenomenon can be understood by considering the Coulomb force associated with the polarization of the particle chain. As shown in Fig 2.20, if the electronic resonances of the particles belong to the transverse modes, the restoring force acting on the oscillating electrons of each particle in the chain is increased by the charge distribution of neighboring particles, which will make the resonant mode blue shifted. In contrast, plasmon resonant modes red shift in the longitude modes [23]. Therefore, we can determine that the phenomenon we observed corresponds to the longitudinal SP modes.
From equation (1.18), due to the scaling of the interaction strength with d−3, the interaction strength between metal particles decreases significantly as the particle spacing increases, as shown in Fig. 2.21. This is the reason why the surface plasmon resonances of metal nano-particles of 50 nm in diameter do not change significantly.
2.3 Discussions and Summary
We have successively fabricated large area nano metal arrays on GaN substrate and studied the LSP properties of Ag and Au nano-particles.
Although our particle sizes are too larger to use the simple dipole approximation, the trends of the SP characteristics are the same. Based on our results, we can change the metal nano-particle size and density with different AAO process conditions to adjust the LSP resonance frequency.
Thus, we can fabricate suitable metal nano-particle arrays to create LSP with resonant frequency we need. Combining this technique with different substrates, we can study other properties of LSP, such as the fabricating of nano-particles on InGaN/GaN QW to observe the coupling of LSP with QW.
FIG. 1. Process for the ordered channel array; SiC mold with hexagonally ordered array of convexes (a), molding on the Al (b), textured Al (c), nodization and growth of channel architecture (d), removal of Al and barrier layer (e) [27].
350 400 450 500 550 600 650 700 750 800 0
Fig. 2.2 Transmission spectrum of GaN substrate and Ag nano-particles on GaN substrate and their ratio
350 400 450 500 550 600 650 700 750 800 0.85
0.9 0.95 1 1.05 1.1
Wavelength (nm)
ratio
Fig. 2.3 Transmission ratio of Ag nano-particle on GaN substrate.
AAO parameters: 60V, widen pores for 70 min.
Particle distance ≈ 150 nm, particle diameter ≈ 50 nm.
350 400 450 500 550 600 650 700 750 800 0.8
0.85 0.9 0.95 1 1.05 1.1
Wavelength (nm)
ratio
Fig. 2.4 Transmission ratio of Ag nano-particle on GaN substrate.
AAO parameters: 60V, widen pores for 80 min.
Particle distance ≈ 150 nm, particle diameter ≈ 70 nm.
350 400 450 500 550 600 650 700 750 800 0.75
0.8 0.85 0.9 0.95 1
Wavelength (nm)
ratio
Fig. 2.5 Transmission ratio of Ag nano-particle on GaN substrate.
AAO parameters: 60V, widen pores for 90 min.
Particle distance ≈ 150 nm, particle diameter ≈ 80 nm.
350 400 450 500 550 600 650 700 750 800 0.75
0.8 0.85 0.9 0.95 1
Wavelength (nm)
ratio
Fig. 2.6 Transmission ratio of Ag nano-particle on GaN substrate.
AAO parameters: 60V, widen pores for 110 min.
Particle distance ≈ 150 nm, particle diameter ≈ 100nm~110 nm.
350 400 450 500 550 600 650 700 750 800 0.7
0.8 0.9 1 1.1 1.2
Wavelength (nm)
ratio
Fig. 2.7 Transmission ratio of Au nano-particle on GaN substrate.
AAO parameters: 60V, widen pores for 90 min.
Particle distance ≈ 150 nm, particle diameter ≈ 80 nm.
350 400 450 500 550 600 650 700 750 800 0.6
0.7 0.8 0.9 1 1.1
Wavelength (nm)
ratio
Fig. 2.8 Transmission ratio of Au nano-particle on GaN substrate.
AAO parameters: 60V, widen pores for 100 min.
Particle distance ≈ 150 nm, particle diameter ≈ 90 nm
350 400 450 500 550 600 650 700 750 800 0.6
0.7 0.8 0.9 1 1.1
Wavelength (nm)
ratio
Fig. 2.9 Transmission ratio of Au nano-particle on GaN substrate.
AAO parameter: 60V, widen pores for 110 min.
Particle distance ≈ 150 nm, particle diameter ≈ 100nm~110 nm.
(a)
(b)
Fig. 2.10 SEM images of Ag nano-particles with the AAO process condition as (a) pore-widen time at 70 min (b) pore-widen time at 90 min.
300 400 500 600 700 800 sapphire 60V 70min sapphire 60V 100min
Fig. 2.11 Transmission ratio of Ag nano-particles on sapphire substrate.
300 400 500 600 700 800 900 1000 1100 0.6
Fig. 2.12 Transmission ratio of silver nano-particles as the light source incident from the airside.
300 400 500 600 700 800 900 1000 1100 0
10000 20000 30000 40000 50000
GaN
Ag thin film 20nm ratio
Wavelength (nm)
Intensity
0.2 0.4 0.6 0.8 1.0 1.2
ratio
Fig 2.13 Transmission spectrum of the Ag thin film of 20nm.
Fig. 2.15 Schematic of our measurement system.
(a)
(b)
Fig 2.16 Angle dependent transmission ratio with the AAO process condition of the pore-widen time at 70 min.
(a)
(b)
Fig 2.17 Angle dependent transmission ratio with the AAO process condition of the pore-widen time at 110 min.
350 400 450 500 550 600 650 700 750 800 0.7
0.8 0.9 1 1.1
Wavelength (nm)
ratio
Fig. 2.18 Transmission ratio of Ag nano-particle on GaN substrate.
AAO parameters: 40V, widen pores for 70 min.
350 400 450 500 550 600 650 700 750 800 0.5
0.6 0.7 0.8 0.9 1 1.1
Wavelength (nm)
ratio
Fig. 2.19 Transmission ratio of Ag nano-particle on GaN substrate.
AAO parameter: 40V, widen pores for 90 min.
Particle distance ≈ 100 nm, particle diameter ≈ 65 nm.
Fig. 2.20 Schematic of the near-field coupling between metallic nanoparticles for the two different polarizations [23].
Fig. 2.21 SEM image of arrays of closely spaced gold nanoparticles (a) and dependence of the spectral position of the dipole plasmon resonance on interparticle spacing (b). The dotted lines show a fit to the d−3 dependence of the coupling expected from a point-dipole model.
Reprinted with permission from [28].
Chapter3
Strain Release of InGaN/GaN Quantum Wells through Nano-hole Fabrication
3.1 Introduction
InGaN/GaN quantum-well (QW) structures grown along the c-axis on sapphire substrate are widely used for fabricating color and white-light light-emitting diodes (LEDs). However, because of the large lattice mismatches between GaN and sapphire (36 %) and between InN and GaN (11 %), normally strong compressive strain exists in an InGaN QW layer, particularly when the indium content is high for green-yellow emission.
Such a strain distribution results in significant piezoelectric field across the QW layer, leading to the quantum-confined Stark effect (QCSE) [29-33]. The piezoelectric field tilts the QW potential such that the overlap integral of electron and hole wave functions is reduced and hence the radiative recombination rate is decreased. When carriers are supplied into the QW, the charged carriers can screen the piezoelectric field, leading to a flattened QW potential. In this situation, the radiative recombination rate can be enhanced and the emission spectrum is blue shifted. Therefore, the strain-induced piezoelectric field in an InGaN/GaN
QW not only reduces its internal quantum efficiency, but also leads to unstable output spectrum when it is used for fabricating an LED. Efforts have been made to reduce or delete the piezoelectric field in such a QW for improving the overall LED efficiency. Non-polar and semi-polar growths of such a QW represent one of the efforts to this end [44-36].
However, so far the non-polar and semi-polar crystal qualities are still not as good as those with polar growth, unless high-quality non-polar GaN free-standing substrate is available. Hence, any approach for reducing the piezoelectric field in a c-plane InGaN/GaN QW is useful for improving the emission efficiency.
In this letter, we demonstrate the reduction of the QCSE in an InGaN/GaN QW by generating nano-pores on the epitaxial structure with the anodic aluminum oxide (AAO) technique [9,4]. This electrochemical method can be used to produce pores of several tens nm in cross-section dimension, 5-25 nm in depth, and 109-1010 cm-2 in pore density on the surface of an InGaN/GaN heterostructure. Although the produced pore dimension and location are irregular, they are statistically uniform on the sample surface. This technique has been used for enhancing the light extraction efficiency in an LED [47, 48]. By producing nano-pores on an InGaN/GaN QW structure, it is found that the photoluminescence (PL) spectral feature is significantly blue-shifted. Also, the internal quantum
strain built in the QW layer is partially released through the generation of those nano-pores.
3.2 Fabrication of nano-holes array on InGaN/GaN quantum wells
The InGaN/GaN QW structure was grown on c-plane sapphire substrate with metalorganic chemical vapor deposition. First, a 30-nm GaN nucleation layer was grown at 537 oC. Then, after the growth of a 3-μm u-GaN at 1080 oC, an InGaN/GaN QW structure was deposited.
The lower GaN barrier of 20 nm in thickness was grown at 800 oC. In growing the QW structure, after the growth of a 3-nm InGaN QW layer at 670 oC, the growth temperature was ramped to 890 oC within one minute for depositing the GaN cap layer of 24 nm in thickness. The QW structure without AAO process is assigned as sample A.
The fabrication of nano-holes array is very similar of the method of nano-particles fabricating. First, a SiO2 layer of 100 nm in thickness was grown on the GaN cap layer, followed by the deposition of an Al layer of 500 nm in thickness. In the first anodization stage, the sample was
immersed in the electrolyte of 0.3 M oxalic, which was maintained at 15
oC, for 45 sec to anodize aluminum. The applied anodization voltage was 60V. Then, the oxide layer on aluminum was removed by immersing the
oC, for 45 sec to anodize aluminum. The applied anodization voltage was 60V. Then, the oxide layer on aluminum was removed by immersing the