(delay in reaching both the intensity of transit EL and the subsequent recombination. Due to the combined effects of the weaker carrier localization, stronger QCSE, and the potential barrier, the poorer relaxation efficiency of the samples LED1, LED2, and LED3 needs more time to reach the intensity of transit EL.
Figure 1.35 (a), (b) and (c) show the recombination time as a function of applied pulse voltage for the samples LED1 and LED1Gs, LED2 and LED2Gs, and LED3 and LED3Gs, respectively. The recombination time can be determined by fitting the TREL decay profile with a single exponential. The recombination of the samples LED1Gs, LED2Gs, and LED3Gs are longer than those of the samples LED1, LED2, and LED3 for each applied pulse voltage.
The longer recombination of the samples LED1Gs, LED2Gs, and LED3Gs implies a slower carrier recombination. With a larger applied pulse voltage, the samples LED1Gs, LED2Gs, and LED3Gs has more carrier injection than the samples LED1, LED2, and LED3, so the samples LED1Gs, LED2Gs, and LED3Gs need more time for carrier recombination.
1.5 Discussion and summary
In summary, we have demonstrated experimental results of optical microscopy images, EL images, EL spectrum, current density, output power, EQE, and TREL measurements of
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InGaN/GaN MQW LEDs with multilayer graphene transparent electrodes on the p-layer and on the sapphire substrate. Optical and electrical performances were found to be greatly enhanced by employing graphene as transparent conductive electrodes on the p-layer and on the sapphire substrate in LEDs.
In the first part, optical and electrical performances were greatly enhanced by employing graphene as transparent conductive electrodes on the p-layer of GaN-based LEDs.
The EL intensity, current density, output power, and EQE of the GaN-based LEDs with a graphene transparent conductive electrode on the p-layer can be enhanced as high as 18, 10, 26, and 15 %, respectively. In TREL measurement, the shorter response, rise, and delay times of the GaN-based LEDs with a graphene transparent conductive electrode provide more efficient carrier injection, transport, and relaxation. The LEDs with multilayer graphene transparent electrodes on the p-layer have more carrier injection than those without multilayer graphene transparent electrodes on the p-layer, so the LEDs need more time for carrier recombination and show a longer recombination time.
In the second part, optical and electrical performances were also enhanced by employing graphene as transparent conductive electrodes on the sapphire substrate in GaN-based LEDs. The EL intensity, current density, output power, and EQE of the GaN-based LEDs with a graphene transparent conductive electrode on the sapphire substrate can be enhanced as high as 12, 10, 12, and 12 %, respectively. The shorter response, rise, and delay times of the GaN-based LEDs with a graphene transparent conductive electrode provide more efficient carrier injection, transport, and relaxation. The GaN-based LEDs with multilayer graphene transparent electrodes on the sapphire substrate have more carrier injection than those without multilayer graphene transparent electrodes on the sapphire substrate, so the GaN-based LEDs need more time for carrier recombination and show a longer recombination time.
Using multilayer graphene transparent electrode either on the p-layer or on the
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sapphire substrate of InGaN/GaN MQW LEDs can effectively enhance device performance, carrier transport, and luminous efficiency. These advantages show great potentials of graphene transparent conductive electrodes in InGaN/GaN MQW LEDs.
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Figure 1.1 The applications of graphene [1-4]
; Figure 1.2 The quality and production cost of various kinds of graphene [5].
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Figure 1.3 Schematic diagram of the G-VLED fabrication process [6].
Figure 1.4 (a) Light output vs. current for G-VLEDs and R-VLEDs. (b) EL image of G-VLEDs. (c) Current-voltage characteristics of G-VLEDs and R-VLEDs before annealing;
(d) Leakage current of G-VLEDs and R-VLEDs before annealing [6].
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Figure 1.5 (a) GaN-based UV-LED structures, and (b) fabricated device covered with GO nanosheets [10].
Figure 1.6 (a) Light output power vs. current of conventional, l-GO-passivated, and h-GO-passivated UV-LEDs. (d)I-V cures of p-GaN with and without GO nanosheets measured by (e)vertical and (f)in-plane configuration. Higher (lower) current in vertical (in-plane) configuration was observed from p-GaN with GO compared to that without GO.
Energy band diagram of p-GaN (g)without and (h)with h-GO. (i) Schematic energy band diagram of active region in UV-LED with applied forward bias [10].
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Figure 1.7 (a) UPS spectra of as-grown and doped graphene films around the secondary-electron cutoff region and the Fermi level position. (b)Energy band diagram at the graphene/p-GaN interface [12-14].
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Figure 1.8 Experimental flow chart of this chapter [15, 16].
Three conventional InGaN/GaN LEDs LED2Gp, and LED3Gp, respectively).
Optical microscope images, EL images,
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Figure 1.9 Fabrication processes of the InGaN/GaN LEDs with the graphene transparent conductive electrodes on the p-GaN layer：(a) Graphene was grown on a copper sheet by a chemical vapor deposition (CVD). (b) Poly(methyl methacrylate) (PMMA) was spin-coated on the graphene/copper sheet. (c) The copper sheet was etched in a 1 wt. % (NH4)2S2O8
solution [2]. (d) The graphene with PMMA was transferred onto the p-GaN layer of the InGaN/GaN LEDs. (e) The PMMA layer was removed by acetone solution.
Figure 1.10 Optical microscope images of the samples (a) LED1, (b) LED2, (c) LED3, (d) LED1Gp, (e) LED2Gp, and (f) LED3Gp.
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Figure 1.11 EL images for the samples (1)~(6) LED1, (7)~(12) LED1Gp, (13)~(18) LED2, (19)~(24) LED2Gp, (25)~(30) LED3, and (31)~(36) LED3Gp with the excitations of 2.5, 2.6, 2.7, 2.8, 2.9, and 3.0 Volt electron voltages under room temperature, respectively.
Figure 1.12 Experimental setup of EL measurement.
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Figure 1.13 EL spectra as a function of applied voltages for the samples (a) LED1, (b) LED2, (c) LED3, (d) LED1Gp, (e) LED2Gp, and (f) LED3Gp at room temperature.
Figure 1.14 EL peak position as a function of applied voltages for the samples LED1, LED2, LED3, LED1Gp, LED2Gp, and LED3Gp at room temperature.
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Figure 1.15 Current density (I) vs. applied voltage (V) for the samples (a) LED1 and LED1Gp, (b) LED2 and LED2Gp, and (c) LED3 and LED3Gp.
Figure 1.16 Output power vs. applied voltage (V) for the samples (a) LED1 and LED1Gp, (b) LED2 and LED2Gp, and (c) LED3 and LED3Gp.
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Figure 1.17 Output power vs. applied current (I) for the samples (a) LED1 and LED1Gp, (b) LED2 and LED2Gp, and (c) LED3 and LED3Gp.
Figure 1.18 External quantum efficiency (EQE) as functions of applied voltage (V) for the samples (a) LED1 and LED1Gp, (b) LED2 and LED2Gp, and (c) LED3 and LED3Gp.
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Figure 1.19 Experimental setup of TREL measurement.
Figure 1.20 TREL profiles of the samples (a) LED1, (b) LED2, (c) LED3, (d) LED1Gp, (e) LED2Gp, and (f) LED3Gp at room temperature.
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Figure 1.21 Response, rise, and delay times as a function of applied pulse voltage for the samples (a) LED1 and LED1Gp, (b) LED2 and LED2Gp, and (c) LED3 and LED3Gp.
Figure 1.22 Recombination time as a function of applied pulse voltage for the samples (a) LED1 and LED1Gp, (b) LED2 and LED2Gp, and (c) LED3 and LED3Gp.
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Figure 1.23 Experimental flow chart of this chapter.
Three conventional InGaN/GaN LEDs LED2Gs, and LED3Gs, respectively).
Optical microscope images, EL images,
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Figure 1.24 Fabrication processes of the InGaN/GaN LEDs with the graphene transparent conductive electrodes on the sapphire substrate：(a) Graphene was grown on a copper sheet by a chemical vapor deposition (CVD). (b) Poly(methyl methacrylate) (PMMA) was spin-coated on the graphene/copper sheet. (c) The copper sheet was etched in a 1 wt. % (NH4)2S2O8 solution. (d) The graphene with PMMA was transferred onto the sapphire substrate of the InGaN/GaN LEDs. (e) The PMMA layer was removed by acetone solution [2].
Figure 1.25 Optical microscope images of the samples (a) LED1, (b) LED2, (c) LED3, (d) LED1Gs, (e) LED2Gs, and (f) LED3Gs.
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Figure 1.26 EL images for the samples (1)~(6) LED1, (7)~(12) LED1Gs, (13)~(18) LED2, (19)~(24) LED2Gs, (25)~(30) LED3, and (31)~(36) LED3Gs with the excitations of 2.5, 2.6, 2.7, 2.8, 2.9, and 3.0 Volt electron voltages under room temperature, respectively.
Figure 1.27 EL spectra as a function of applied voltages for the samples (a) LED1, (b) LED2, (c) LED3, (d) LED1Gs, (e) LED2Gs, and (f) LED3Gs at room temperature.
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Figure 1.28 EL peak position as a function of applied voltages for the samples LED1, LED2, LED3, LED1Gs, LED2Gs, and LED3s at room temperature.
Figure 1.29 Current density (I) vs. applied voltage (V) for the samples (a) LED1 and LED1Gs, (b) LED2 and LED2Gs, and (c) LED3 and LED3Gs.
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Figure 1.30 Output power vs. applied voltage (V) for the samples (a) LED1 and LED1Gs, (b) LED2 and LED2Gs, and (c) LED3 and LED3Gs.
Figure 1.31 Output power vs. applied current (I) for the samples (a) LED1 and LED1Gs, (b) LED2 and LED2Gs, and (c) LED3 and LED3Gs.
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Figure 1.32 External quantum efficiency (EQE) as functions of applied voltage (V) for the samples (a) LED1 and LED1Gs, (b) LED2 and LED2Gs, and (c) LED3 and LED3Gs.
Figure 1.33 TREL profiles of the samples (a) LED1, (b) LED2, (c) LED3, (d) LED1Gs, (e) LED2Gs, and (f) LED3Gs at room temperature.
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Figure 1.34 Response, rise, and delay times as a function of applied pulse voltage for the samples (a) LED1 and LED1Gs, (b) LED2 and LED2Gs, and (c) LED3 and LED3Gs.
Figure 1.35 Recombination time as a function of applied pulse voltage for the samples (a) LED1 and LED1Gs, (b) LED2 and LED2Gs, and (c) LED3 and LED3Gs.
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