Past exam of the chemistry A course of the University of Cambridge / 2025 / Paper 2 / A10 / d

Using $E_{\gamma }=hc/\lambda$ and an ideal band-edge photon energy equal to the quoted gap,At low temperature an electron and hole can form a bound exciton. Its recombination releases approximately $E_g-E_B$, so its wavelength is longer than $hc/E_g$ evaluated using the gap at that same temperature. GaN excitation and emission measurements resolve such excitonic transitions below the fundamental gap.^[ref]

For the quoted cubic-phase gap,The same exciton binding argument can shift the actual emission to longer wavelength; impurity-bound transitions can also lie below the fundamental gap. The comparison must use $E_g$ at the operating temperature. Ordinary temperature-dependent gap changes can shift the band edge in the opposite direction, so the numerical values based on fixed supplied gaps are not universal lower bounds for every cooled device.