This result accompanied by the observed regular diffraction patterns further verified the single-crystalline nature of the GaN grown on the rGO pattern.
From the average profile of intensity analysis Fig. These results indicate the feasibility of using metal-organic chemical vapour deposition system for the effective reduction of graphene oxide and the simultaneous direct growth of GaN onto rGO in a single sequence. The crystal plane direction and lattice spacing are marked in b.
This study has a particular spotlight on addressing the effective heat dissipation in graphene oxide-embedding LED chips. First, the infrared imaging technique was used to monitor the temperature profile on the surface of the LED chips. To achieve a reliable data, the images were taken after waiting for a sufficiently long time to reach the thermal steady state.
The peak temperature of the chip surface rose up to 58 and In addition, the average temperature distribution on the chip surface is decreased for the rGO-embedded LED The scale bar located at the right of b gives an estimation of the temperature associated with different colours. In LED packaging, T j and R th are the two physical quantities generally considered for the evaluation of heat dissipation.
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Therefore, to explore the extent of heat dissipation that the rGO can give, the above two parameters were studied using transient thermal analysis 40 , 41 see Methods for more details. Here each curve represents time dependence of T j rise of a chip under a constant injection current.
Figure 6b elucidates the measured T j as a function of injection current for the three LED packages under study. It should be noted that the T j increases with the injection current in all cases, a behaviour common to standard LED packages. The differential structure functions that are derived by direct mathematical transformations from the experimental cooling curves are illustrated in Fig. Considering the chip as a sandwich structure, where the layers have the same area, the local peaks and valleys in the structure function denote reaching new materials in the heat-flow path.
In Fig. In the structure functions shown in Fig.
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The reliability of the data was further verified by conducting measurements on a number of LED chips for each case under same experimental conditions. These results corroborate the infrared imaging observations and the T j results that the embedded rGO layer in the LED chip significantly improves the heat dissipation. The embedded rGO had no influence on the current-voltage characteristics of the device see Supplementary Fig.
Figure 6d shows the variations in the light output power as a function of injection current. It is observed that the light output power of the rGO-embedded LED is slightly enhanced over that of conventional LED, which may be likely due to the improved internal quantum efficiency of the devices see Supplementary Fig.
Interestingly, the observed enhancement in the thermal and optical characteristics of the rGO-embedded LED is analogous to the results reported for vertical LEDs on graphite or Cu substrates Therefore, the present findings promise that graphene-based materials will soon find their way into high-power LEDs for thermal management.
In general, heat generated in an LED flows along different paths primarily by conduction and convection processes. For conventional LED on sapphire, when the chip is mounted on a heat-sink, most of the heat flows from the active region through n-type cladding layer and the sapphire substrate to the heat-sink by conduction, and only a fraction of heat flows by convection means at surface-air boundaries 41 , This situation is different if there is a high-thermal-conductivity layer between the n-type cladding layer and sapphire substrate.
Before addressing this later situation, it is necessary to clarify the issues associated with the thermal conductivity of graphene oxide. It is known that graphene oxide has different phonon dispersion and reduction never recovers graphene properties completely. The accurate thermal conductivity of the rGO is presently unknown. However, recent studies on the graphene-based thermal interface materials give strong evidences that chemically derived somewhat disordered graphene-multilayer graphene or graphene-few-layer graphene composites in flakes form could offer and preserve high thermal conductivity 28 , These benefits are known to originate from the disorder structure in disordered materials, the thermal conductivity is found to increase with increasing temperature as a result of better phonon transmission through the interface and decreasing Kapitza resistance at high temperature and the geometrical shape of the flake well-ordered flakes of different sizes increase the thermal conductivity by the formation of better percolation network.
Furthermore, the heat propagation along the in-plane direction of graphene being much larger than its perpendicular direction 23 , the embedded rGO is likely to ease out the heat from the chip by acting as a heat spreader. To understand the nature of the real mechanism by which heat dissipation occurs, a systematic study considering several other factors has been carried out.
However, the cross-sectional scanning electron microscopy images evidenced no major thickness difference between the samples grown simultaneously on planar sapphire and rGO buffer layer. From the AFM analyses, the density values of the etch pits for the GaN epilayers grown on planar sapphire and rGO buffer layer are found to be 1. This result supports the HR X-ray diffraction measurements that the GaN crystal quality is significantly improved with the embedded rGO pattern, which certainly acted as a buffer layer to reduce the number of lattice defects that would propagate up to the critical GaN layers.
Therefore, an 8. To comprehend the prime mechanism at play, a similar LED epilayer structure was grown on a low-thermal-conductivity amorphous SiO 2 buffer layer with patterns analogous to that of the rGO patterns used in our experiments. The benefit of this device design is that it can help to understand the impact of thermal conductivity of the buffer layer on the heat spreading.
All the factors studied above were taken into consideration. Both the GaN thickness and the etch pit density 4. However, a clue for the explication was from the transient thermal measurements, which indicated surprisingly very high R th If crystal quality improvement was the reason then devices grown on either of the buffer layers would have comparable T j and R th values.
We surmise that this difference in interface thermal properties comes because of the difference in thermal conductivity of the buffer layers. In these structures, the poor crystal quality of the AlN and associated low thermal conductivity were shown to restrict heat transfer from the GaN into the substrate, leading to high TBR values.
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A numerical study on the self-heating effects of oxide-confined vertical-cavity surface emitting lasers gives further evidence that the oxide layer, due to its low thermal conductivity, strongly suppresses the heat flow Even higher TBR can be expected in practical devices depending on the quality of the nucleation layer presently low-temperature grown AlN or GaN is used as a nucleation layer and the extent of interface disorder.
In order for this to happen, the GaN crystal quality should be sufficiently high with low number of dislocations. Although the lateral overgrowth of GaN on SiO 2 micropatterns can result in high crystal quality, the poor thermal conductivity of the SiO 2 restricts the heat flow across the interface, as evidenced from the results. In light of the above, it is quite plain that the relatively high thermal conductivity of rGO and its associated rapid heat-spreading ability 23 , 29 has a key role in reducing the T j and R th for the devices grown on rGO pattern.
In other words, the heat spreading within the GaN as a result of enhanced crystal quality and by the embedded rGO pattern, as well as the improved heat transfer to the sapphire through the rGO due to low TBR, should account for the observed heat dissipation in the GaN LEDs. In summary, we have demonstrated the experimental realization of GaN-based LEDs with embedded graphene oxide pattern for effective heat removal. Scalable graphene oxide microscale pattern has been generated by using a combination of facile lithography and spray-coating methods. The appealing versatility is that the generated graphene oxide pattern acts as a buffer layer for the epitaxial lateral overgrowth of single-crystal high-quality GaN in the fabrication of graphene oxide-embedded devices, which offered excellent heat dissipation while maintaining electrical and optical properties superior to that of its conventional counterpart.
This study comprising of facile approaches to unravel the heat dissipation issues in LED imparts the potential of graphene in optoelectronic devices for a better thermal management. Our findings may, therefore, instigate yet new discoveries that eventually make roads to the development of durable high-power LEDs for the next generation of solid-state lighting. The graphene oxide film with circular hole arrays was obtained as follows.
The graphene oxide dispersion was sprayed onto the substrate using an airbrush system with nitrogen as a carrier gas. The deposition of SiO 2 was carried out in a plasma-enhanced chemical vapour-deposition system on a similarly photoresist-patterned sapphire substrate. The photoresist was subsequently removed by using acetone, and the resultant graphene oxide and SiO 2 buffer-layer-covered substrates were cleaned with methanol and deionized water, before being set into an AIXTRON RF horizontal metal-organic chemical vapour deposition reactor.
Trimethylgallium and ammonia were used as the gallium and nitrogen sources, respectively. Then the growth of undoped GaN layer was conducted at a high temperature in two successive steps. Mesa etching was carried out using standard photolithography and inductively coupled plasma reactive ion etching. The thermal sensitivity of the camera was better than 0. The thermal transient measurements were carried out in a thermal transient tester T3ster, MicRed.
Silver paste was used for mounting the chips on an aluminium heat-sink. The theoretical framework of the evaluation of the T3ster is based on a representation of the distributed RC networks. After a calibration process, which determines the ratio between the forward voltage drop as a temperature-sensitive parameter and temperature, transient cooling curve was measured for different injection currents. The structure functions were obtained by direct mathematical transformations from the transient cooling curves. Current-voltage and light-output-current measurements were carried out using a probe station system.
How to cite this article: Han, N. Improved heat dissipation in gallium nitride light-emitting diodes with embedded graphene oxide pattern. Ponce F. Nitride-based semiconductors for blue and green light-emitting devices. Nature , — Schubert E. Solid-state lighting—a benevolent technology. Krames M. Status and future of high-power light-emitting diodes for solid-state lighting. Crawford M.
LEDs for solid-state lighting: Performance challenges and recent advances. IEEE J.
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