However, the committee believes that a breakthrough in native GaN substrate technology would allow the elimination of a vast number of crystal defects, revolutionize the materials growth process, and have profound benefits for LED efficiency, reliability, and yield. DOE workshop participants speculate that “in principle, the use of a GaN substrate, if it were available at reasonable cost, might simplify the buffer layer technology (thinner buffer layers with shorter growth times) and allow flat, uniform epiwafers to be manufactured” (DOE, 2012a, p. 35.).
FINDING: Significant improvements in LED efficiency, yield, and reliability are possible by using GaN substrates and latticed-matched epitaxial growth processes. Currently, there are no viable techniques for producing high-quality, low-cost GaN substrates. While realization of low-cost GaN substrates is not assured, the potential payoff of this research is immense.
RECOMMENDATION 3-4: The Department of Energy should make a long-term investment in the development and deployment of gallium nitride substrates.
The development of III-nitride LED technology has brought many surprises to the semiconductor community. Never before have production devices been formed in a materials system in which the light-emitting layers were produced on non-native substrates with thermal and lattice mismatch. Although GaN-based devices have worked well enough to initiate a lighting revolution, materials issues have re-emerged as defining elements in the technology. As has been discussed above, current devices suffer from a high concentration of defects and dislocations that limit the internal quantum efficiency achievable.
The DOE (2011a) roadmap goals relating to device efficiency, shown in Table 3.1, can only be achieved by substantial improvements in the control and quality of the materials growth and in the reduction of defects that arise through the growth and fabrication processes, which are aggravated by the strain between substrate and overlayers. Moreover, improvements in the basic technology that forms the starting materials of the LEDs will have a profound feed-forward effect that will influence yield, and thus cost, at every stage of the LED package formation and performance. For example, strain between the substrate and the overlayer results in the non-uniformity of LED characteristics across wafers, leading to the wasteful practice of “binning.” Fluctuations in the composition of the LED layers, and particularly in the quantum well region, compromise control over the LED emission wavelength. Defects have an impact on the electrical resistance of the LEDs, increasing power dissipation and limiting higher-temperature performance, as well as lifetime. Limitations at the device level necessitate compensating solutions (e.g., heat sinking) at the packaging level, which may increase the overall cost. For example, Krames et al. (2007) have calculated that an improvement in IQE from 2010 values (Table 3.1) to 2020 values could result in a fourfold reduction in the amount of wasteful heat generated in a 70 lm/W device. The ancillary issues of increased device lifetime and reliability will also have an impact on cost.
TABLE 3.1 Internal Quantum Efficiency Values of Light-Emitting Diodes
|Metric(s)||2010 Status||2020 Target(s)|
|IQE at 35 A/cm2||80% (blue)
||90% (blue, green, red)|
|EQE at 35 A/cm2||64% (blue)
||81% (blue, green, red)|
|Power Conversion Efficiency @ 35 A/cm2||44% (blue)
||73% (blue, green, red)|
|Relative EQE at 100 A/cm2 versus 35 A/cm2 (droop)||77%||100%|
SOURCE: DOE (2011a, Table 2.1, p. 71).
Thus, investments in improving the control and uniformity of the epitaxial growth process can have a profound effect on long-term device performance, reliability, and cost. Improvement in the cumulative manufacturing yield of the LED module, currently in the range of 50 to 70 percent to more than 95 percent, will further lower the cost and improve the quality of SSL. But although not directly shown in the projection of LED package costs (Figure 3.8), improvements in the cumulative yield will benefit enormously from improvements in the earlier part of the manufacturing process, such as improved uniformity in the epitaxial process. These improvements will exercise a “lever” effect on the cumulative yield and have a large impact on the final device cost and selling price through improved binning yield (DOE, 2011b).
FINDING: LED efficiency and performance is still limited by materials issues. Improvements in efficiency at the device level, as targeted by the DOE SSL roadmap, will have a “lever effect,” influencing design, performance, and cost of the luminaires. Improvements in efficiency and performance are linked to further fundamental investigations in core technology on emitter materials.
RECOMMENDATION 3-5: The Department of Energy should continue to make investments in light-emitting diode core technology and fundamental emitter research. Its portfolio of investments in these areas should be extensive enough to ensure that the targeted goals of device performance can indeed be met.