AlGaInP light-emitting diodes with SACNTs as current-spreading layer
© Guo et al.; licensee Springer. 2014
Received: 23 December 2013
Accepted: 26 March 2014
Published: 8 April 2014
Transparent conductive current-spreading layer is important for quantum efficiency and thermal performance of light-emitting diodes (LEDs). The increasing demand for tin-doped indium oxide (ITO) caused the price to greatly increase. Super-aligned carbon nanotubes (SACNTs) and Au-coated SACNTs as current-spreading layer were applied on AlGaInP LEDs. The LEDs with Au-coated SACNTs showed good current spreading effect. The voltage bias at 20 mA dropped about 0.15 V, and the optical power increased about 10% compared with the LEDs without SACNTs.
The current-spreading effect is one of the most important factors limiting the external quantum efficiency of light-emitting diodes (LEDs) [1, 2]. Limited by the mobility and thickness of the current-spreading layer, most carriers crowd under the electrode, which resulted in most photons from radiation recombination being blocked or absorbed by opaque electrode and large joule heating under the electrode [3, 4]. Indium tin oxide (ITO) was widely used in case of visible wavelength LEDs as the current-spreading layer owing to its good physical properties of high optical transmittance and low sheet resistance . However, due to the scarcity of the element indium on earth and consequently the soaring prices, the advantages in nanomaterials were recently investigated for the current-spreading layer, such as graphene, metal nanowires, and carbon nanotubes (CNTs) [6–8].
Graphene has high mobility and high optical transmittance . However, large work function of graphene caused the large turn-on voltage with inefficient current spreading, which resulted in light emission occurring only near the p-metal regions, especially on p-GaN due to high sheet and contact resistance . Also, the obvious degradation of graphene layer under 20 mW of input power restricted its actual application . Ag nanowire is the strong competitor of graphene due to its intrinsically high conductivity and favorable optical transparency. However, except for the easy oxidation at ambient environment, the electromigration of silver ions under bias could pose a long-term stability issue .
Recently, the optical output power of LEDs was first improved by using the combination of graphene film and Ag nanowires as current-spreading layer. The sheet resistance decreases from 500 Ω of bare graphene to about 30 Ω because the silver nanowires bridged the grain boundaries of graphene and increased the conduction channels .
Among these three nanomaterials, CNTs have the most mature fabrication technology. In this work, AlGaInP LEDs with CNTs only and 2-nm-thick Au-coated CNTs as current-spreading layers were fabricated. The LEDs with Au-coated CNTs showed good current spreading effect.
The AlGaInP LEDs were grown on n-GaAs substrate by metal-organic chemical vapor deposition. Fifteen pairs of Al0.6Ga0.4As/AlAs with distributed Bragg reflectors (DBRs) were grown on 100-nm-thick GaAs buffer layer. The active region was composed of 800-nm-thick 60-period (Al0.5Ga0.5)0.5In0.5P/(Al0.1Ga0.9)0.5In0.5P multiquantum wells, which were sandwiched in p- and n-(Al0.7Ga0.3)0.5In0.5P cladding layer for electron and hole confinement. In order to study the current-spreading effect of CNTs, only 500-nm-thick Mg-doped p-GaP window layer with the doping density of 5 × 1018 cm−3 was grown on top.
Results and discussion
The inset of Figure 6 showed the measured peak wavelength shift with the current injection. The peak wavelength for LEDs with SACNT, Au-coated SACNT, and without SACNT was 634, 633.8, and 633.2 nm at 20 mA, respectively. Correspondingly, the wavelength red shift was 7.8, 7, and 7.8 nm from 10 to 100 mA, respectively, which indicated better thermal performance for LEDs with Au-coated SACNT due to the relatively effective current spreading.
The improvement of optical output power for LEDs with Au-coated SACNT thin film was due to the sheet resistance competition with the p-GaP, although there existed about 20% optical transmittance loss. According to the estimation, the sheet resistance of p-GaP in this experiment is about 300 to 500 Ω. When the Au-coated SACNT thin film was put on the p-GaP, lots of carriers could spread outside the opaque metal electrode, which could have the possibility to contribute to the optical output power. The 2-nm-thick Au coating on the SACNTs could form the Au nanowire which may induce an interacting electromagnetic field with multiple quantum wells (MQWs). However, this interaction is a near-field effect. Considering the distance between of Au nanowire and quantum wells in this experiment, output enhancement due to the surface plasmon resonance can be ignored. So further decreasing the sheet resistance and improvement the optical transmittance of the current-spreading layer of SACNT thin film could increase the optical output power.
The SACNT as current-spreading layer on AlGaInP LEDs was demonstrated. The voltage bias at 20 mA decreased at 0.15 V for LEDs with Au-coated SACNT, and the optical power increased about 10% compared with LEDs without SACNT due to the relatively effective current spreading. Based on the mature SACNT fabrication technique and optical transmittance performance, it is expected that SACNT could be utilized as a current-spreading layer for AlGaInP LEDs with wavelength regions from 560 to 650 nm.
This work was supported by National Natural Science Foundation of China (61222501 and 61335004). And thanks to Dr. Y. Lu and Miss L. Ma for the useful discussion and technique help.
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