GaN/AlGaN DEVICES
NANOPHOTONIC LIGHT SOURCES AT UV WAVELENGTHS
The main goal of this work was to implement metal plasmonic nanoparticles arrays at the surface of LED emitting in the UV-visible frequency range to assist the output emission.For this purpose, the overlapping between the Localized Surface Plasmon (LSP) band of the metal nanostructures and the emission band of the LED is required. Under these conditions, we expect the emission light to be efficiently coupled to the LSP modes of the particles and to be radiated toward the far-field according to specific directions defined by the grating properties.LED structure was grown in Metal-Organic Chemical Vapor Deposition (MOCVD) system in BilkentUniversity Nanotechnology Research Center in Ankara, Turkey. LEDs were then fabricated at the same center using a 7 step-microfabrication process. On the basis of simulations based on the Fourier Modal Method from The institute Fresnel (CLARTE) in Marseille-France, metal nanoparticles were fabricated and characterized in The Institute of Photonic Sciences – ICFO in Barcelona Spain.
LED Fabrication
LED structures were grown on a c-plane (0001) sapphire surface by MOCVD. InGaN/GaN quantum well-barrier pairs were stacked between GaN layers. C-plane GaN heterostructures suffer from strong strain-induced polarization effects. Therefore the emission wavelength is not apparent beforehand like in AlGaAs/GaAs quantum wells, and emission wavelengths are generally determined after wafer growth and also after fabrication. The wavelength of emission is being adjusted by following parameters:
-Well thickness
-Barrier thickness
-Growth temperature of well/barrier pairs.
Temperature generally affects the crystal quality induced strains and that in turn affects the polarization fields, so emission wavelengths are being shifted for appropriate wavelengths of nanoparticles.
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Figure 1: Layer structure of LEDs |
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The large bandgap AlGaN block layer is for confining charges close to InGaN/GaN pairs so that more charges are recombined to generate photons, and it certainly enhanced LED output.
LEDs were fabricated in Class-100 clean room environment. Mask sequence consisted of 7 steps. The device areas in mask range from 160µm x 160 µm to 480µm x 480 µm. First step was to etch all samples down to n+ layer, and then n+ contact with Ti/Al metals is deposited. Then from n+ down to mesa (where n+ contact layer finished) all samples except the devices were etched. After that, p contact Ni/Au or ITO with thicknesses total of 800 Å and 750 Å respectively evaporated onto the lithographed samples. This metal covers all top surface of LED so the metal must be transparent and thin, later it requires one more metal: touch pad with Ti/Au metals on the p contact. Contacts made ohmic by annealing at 550 ºC for 5 min for Ni/Au and at 650 ºC for 2 min. for ITO contacts. After touch metal, dielectric coated on the samples and openings for interconnects are etched. The choice of the dielectric material where will rely the metal nanostructures is determinant since it strongly influences their plasmon resonance. Considering the short LED emission wavelength, we chose to work with SiO2 rather than SiNx for its lower refraction index. The last fabrication step consists of depositing Ti/Au metal for electrical measurements.
Then electrical and optical measurements for LEDs are performed. We fabricated two different wafers namely B374 and B377 with Ni/Au and ITO contacts. As can be seen from Figure 2, emission wavelengths were 420 nm and 425 nm respectively which is seen as blue as in Figure 3.
Figure 2: LED emissions with electrical current injection.
Figure 3: LED Emission under microscope illumination and dark.
Current-Voltage (IV) measurement revealed that turn on voltages are around 2-3 V and diode currents reaching mill amperes about 5 V. The blue emissions can be seen with naked eyes around these voltage and current levels. Samples from two different wafers emit close output light power with B377 slightly higher, as seen in Figure 4. There were no structural differences between two wafers, and the difference at emitting power can be attributed to MOCVD growth conditions.
Figure 4: Relative emissions of two wafers with ITO as top p-metal.
Metal nanoparticle fabrication
Exploiting plasmon properties at very short wavelengths, typically shorter than 500 nm is very challenging since this is well below the Localized Surface Plasmon (LSP) band of usual noble metals used in plasmonics. Silver (Ag) has been chosen rather than gold for its shorter wavelength plasmon resonance.
In order to design the silver particles and optimize their resonant properties, simulations were performed using the Fourier Modal Method code from Fresnel-CLARTE. The Fourier Modal Method has been long plagued when metals are considered in optics. For these parameters, the LSP band is centered on 490 nm, still far away from the diode emission wavelength. Because, smaller dimensions can not be reasonably considered from the fabrication point of view, we opted for the use of nano-ellipsoidal particles. By using the elongated particles, we expect to exploit the quadrupolar LSP resonance rather than the dipolar one. We ran the simulations for nano-ellipsoidal (“cylinder with an ellipse base”) gratings lying onto a SiO2 substrate. Those ellipses are 20nm high with 80nm long short axis and 128nm (=1.6´80) for the long axis. These parameters can be achieved with a good reproducibility with our lithography system.
In a first step, a 120 nm PMMA layer is spin coated on top of the LED and baked during 1h30 at 170 ºC. In order to permit charges evacuation during the e-beam writing a 30 nm layer of a conductive and water-soluble polymer (AQUASAVE) is added. From the original position list used to design the LEDs, arrays of dots (100 microns ´ 100 microns, period 300 nm) are exposed on top of the emissive areas. Before depositing the silver layer (40 nm-thermal evaporation), a thin Ti layer (2 nm-e-beam evaporation) is used to increase its adherence on SiO2. The dimensions of the ellipses obtained after lift-of rang from 140 nm to 170 nm for the long axis and from 70 nm to 80 nm for the short one (see figure 5).
Figure 5: SEM images of deposited nanoparticles on LEDs
Figure 6: Evolution of the particles reflection with the incident polarization
The evolution of the light reflected by one nanoparticle matrix as a function of the polarization of the incident white light is shown in Figure 6. These results illustrate the dependence of the LSP resonance of the nano-elliposoïdal particles with regards of the incident field polarization. This is corroborated by reflection spectroscopy measurements performed for two samples exposed with different doses (figure 7) where the dip is the signature of the resonance band. Note that the quadrupolar resonance seems to be too weak in reflection to be resolved (field polarization along the long axis of the ellipsoids-0º).
Figure 7: Evolution of the reflection spectrum of LED surfaces coated with silver nanoparticles as a function of the incident white light polarization.
We have investigated the possibility of integrating periodic arrays of plasmon nanoparticles on top of LED emitting in the UV-visible range. The project has been quite effective in bringing together different R&D capabilities of 3 participating institutions. By combining the skills of the different partners (LED fabrication, modeling and plasmon optics) we have been able to identify the different conditions to fulfill in order to assist “plasmon-assist” the LEDs emission.
Nanophotonic Light Sources at UV
Metamaterials - Photonic Crystals - GaN/AlGaN Devices - Other Semiconductor Devices and Fabrication Techniques
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