technology LEDs
excitation power. Again, we compare the yellow and blue-emitting structures: A blue shift of 29 meV occurs in the blue MQW when the excitation power is increased from 1 to 10 mW, indicating the presence of the piezoelectric field; in comparison, the blue shift in the yellow quantum well is negligible.
To reveal whether this strongly suppressed piezoelectric field in our yellow emitting pyramids stems from the semi-polar plane or results from strain relaxation in these nanostructures we scaled this structure, building equivalent pyramids with a bottom diameter of about 2 µm. In this case the blue shift was 47 meV.
Figure
3.Integrated PL intensity as a function of power density for yellow InGaN MQWs on nano- pyramids and blue InGaN MQW on a c-plane
substrate.Light intensity (L) is proportional to I p
,
where I is the carrier density and P is power index.If P =1,radiative recombination is dominant and if P >1,Shockley-Read-Hall recombination is dominant
Figure 1 (b). This dislocation filtering that occurs when carrying out selective-area growth through nano-scale openings arises due to the thermal mismatch between GaN and the dielectric mask.
We are able to produce a wide range of colours with efficient emission using our nano-pyramid structure. Photoluminescence (PL) measurements reveal green, yellow and red emission (see Figure 2 a), with corresponding internal quantum efficiencies of 61 percent, 45 percent and 29 percent, respectively, according to Arrhenius plots of the normalized integrated PL intensity over a 10 to 300 K temperature range (see Figure 2b).
To identify the origin of this high efficiency, we excite the yellow MQW on nano-pyramids and compare its emission with that produced by another structure – a blue MQW grown on the c-plane of another wafer. PL spectra generated by pumping both structures at a range of energies uncovers a linear relationship between excitation power and PL intensity that kicks in at lower incident powers in the yellow-emitting structure, indicating that this one has fewer defects than the blue MQW (see Figure 3).
We have determined the strength of the piezoelectric field through low-temperature measurements of the shift in the emission peak as a function of excitation power density (see Figure 4). Photo-generated carriers screen the piezoelectric field, so it is possible to estimate the field strength from blue shifts in the emission peak with
36
www.compoundsemiconductor.net November/December 2011
However, the blue shift associated with the micron-sized pyramids is still far, far larger than that occurring in its nano-scale cousin. Our conclusion: Growth of MQWs on {1122} facets of nano-size pyramids effectively suppresses the piezoelectric field via the semi-polar growth plane and strain relaxation.
Building a white source
One of the benefits of using selectively grown InGaN is that its composition can be varied through changes in both the growth condition and the type of selective growth employed. This has very important implications: The wavelength of an InGaN layer on nano-pyramids is different from that on a planar substrate, and it is
This is a relatively small shift for emission centred around 570 nm, indicating significant influence from the semi-polar growth plane (In comparison, variations in excitation power of one order of magnitude have been reported to produce a 143 meV blue shift in MQW structures emitting at 500 nm).
Figure 4.Emission peak shifts for three different structures as a function of injection power.Due to their difference in emission wavelength,relative peak shifts are compared
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