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Epitaxial cascading of nitride LEDs overcomes efficiency droop

As the number of the cascaded LEDs increases, efficiency droop is greatly reduced
Researchers at the Ohio State University have experimentally demonstrated epitaxial cascading of multiple p-n junction diodes with low series resistance.

They have shown that cascading multiple LEDs circumvents nitride LED efficiency droop and reduces overall joule heating.

Efficiency droop in GaN LEDs is one of the major roadblocks to widespread adoption of solid state lighting. In the last decade there has been extensive work on identifying and overcoming the nitride LED efficiency droop. But the underlying reason is still under debate and no designs has been completely successful in solving the problem. It is possible that material limitations of III-nitride material system may preclude complete elimination in traditional single active region LED structures.

Cascading multiple LEDs pushes the input power of the peak efficiency to higher values by exploiting each injected electron for multiple emissions rather than a single emission process as in conventional LEDs.

Therefore, higher radiative output power can be obtained at lower current levels and efficiency loss due to droop can be minimised. The design can be applied to all existing nitride emitters regardless of energy of the emission.

To show the feasibility of cascading GaN emitters, the authors demonstrate devices using multiple (1, 2 and 4) epitaxially cascaded p-n junctions with gadolinium nitride (GdN) visible wavelength transparent tunnel junctions by plasma assisted MBE.

All of the devices have n-type GaN on top and bottom layers device since tunnel junctions eliminates the need for a p-type contact, as shown in Figure 1.

Figure 1: Epitaxial design of the cascaded p-n junctions

As the p-n junctions forward biased, tunnel junctions get reverse biased. Electrons from valance band of the p-type layer tunnel into conduction band of n-type layer, leaving a hole behind, in p-type layer. The carriers generated in this process get injected into p-n junction diode regions, thus tunnel junctions work as carrier regeneration centres, supplying majority carriers to device active regions. 

The cascaded diode structures showed rectifying behaviour. Diode turn-on voltage increased with N-repeats of the device sections, as expected. Analysis of series resistances of the 100µm2 devices leads to a very low resistance ~ 5x10-4 Ω-cm2 per tunnel junction.   

Using the performance parameters of these tunnel junctions, the authors calculated the characteristics of LEDs designed with multiple cascaded junctions, with each junction simulating the characteristics of a commercial LED.

The calculation shows that as the number of the cascaded LEDs increases, efficiency droop is greatly reduced, and the wall plug efficiency of a conventional LED can be boosted at elevated powers, as depicted in Figure 2 below.

Figure 2: The change in wall plug efficiency of the modeled commercial LED (N=1) and cascaded LEDs with N=5, N=20, and N=50

The enhancement is not only due to superior external quantum efficiency, but also suppression of joule heating. Since the LED is operated at higher voltage and lower current, resistive losses are lower.

This work is described in detail in the paper, "Tunneling-based carrier regeneration in cascaded GaN light emitting diodes to overcome efficiency droop," by Fatih Akyol in Applied Physics Letters, 103, 081107 (2013). http://dx.doi.org/10.1063/1.4819737


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