Photocurrent spectroscopy as a tool for determining piezoelectric fields in ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ multiple quantum well light emitting diodes

We discuss photocurrent spectroscopy as a powerful tool for the determination of piezoelectric fields in ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ multiple-quantum well light emitting diodes. The observed photocurrent spectra can be interpretted in terms of the Franz-Keldysh effect, and allow for the simultaneous determination of band gap and piezoelectric field in the quantum wells. As an illustration of the method we use results obtained on a structure with an 8% indium content in the quantum well. We find a band gap of 3.10 eV at 300 K and a piezoelectric field of $1.1\pm 0.2\mathrm{M}\mathrm{V}/\mathrm{c}\mathrm{m}.$ The observed photocurrent spectra can be accurately accounted for by invoking the presence of indium fluctuations, which cause the formation of an exponential band tail in the band gap. This band tail is seen to increase with increasing electric field. With these band tail extensions, a good agreement is obtained between the experimental data and simulations based on the Franz-Keldysh effect.