Q2. How many nm of coalescence did the authors get on the first one?
On the second one, the authors obtained full coalescence within only 120 nm above the PhC region— corresponding to a lateral to vertical growth ratio of at least 1.
Q3. How much is the k of the PhC layer?
In practice, a value of k =10−3a−1 corresponds to an exponential extraction length Ldecay=1 /2k 100 m, which is a reasonable value for a LED.
Q4. What is the problem with the AlN layer?
growth of the AlN layer is challenging as for nitride laser diodes , and the PhC still has to be formed in the p-GaN, potentially hurting p-doping and hindering current injection.
Q5. Why is the light emitted in the PhC layer so low?
Due to the low average index in the PhC layer, 50% of the light can be emitted in this CLM, according to solid angle considerations.
Q6. How can the authors fit the dispersion of the two CLMs?
The dispersion of these two CLMs can be fitted by imposing that their vertical wavevectors obey the resonance condition kz= p /L, where L is the thickness of the cavity formed by the top layer and p is an integer p=1 and 2 for the first and second CLMs, respectively .
Q7. What is the optimum thickness of the PhC layer?
for efficient operation of their design, the PhC layer must be thick enough to confine the CLM and coalescence and growth of the pn junction must occur within an optically thin layer.
Q8. What is the optical properties of the PhC?
To analyze the optical properties of the PhC, the authors resort to angle-resolved measurements: the far field of the LED ismeasured as a function of emission angle from −90° to 90° .
Q9. How does the design of a PhC work?
efficient PhCs require full optimization of the design,9 including choices of the crystal lattice6 and of the vertical structure.
Q10. Why is the top layer less confined?
Because the top layer is 500 nm thick, it supports a second optical CLM resonance, but this one is less confined and couples to the multiple modes of the substrate as manifested by anticrossings .
Q11. What is the way to extract light from nitride?
In this case, light emission in the low-order modes of the GaN buffer is avoided and replaced by strong emission in a mode guided above the cladding layer—the so-called cap layer mode CLM —which is efficiently extracted by the PhC.
Q12. What is the potential of the proposed approach?
This approach circumvents the limitations of surface PhCs and, pending optimization and generalization to 2D coalescence, is a promising candidate for a high brightness InGaN LED.
Q13. Why is the output power twice larger for the thinner device?
Of note, however, is that the output power is twice larger for the thinner device, which the authors attribute to a more efficient light extraction.
Q14. Why is the dispersion of the top layer so low?
It is unclear whether this is due to the unoptimized growth conditions used for the LEDs on the LEO mask or to some potential detrimental effect of the mask itself potential contamination by oxygen and diffusion of Si acting as an n-dopant .
Q15. What is the difference between a simple surface and a guided mode?
In particular, Ref. 5 evidenced that a simple surface PhC on top of an as-grown structure is of limited efficiency because most of the light is emitted in low-order guided modes which are poorly extracted by the PhC.