Reduction of oscillator strength due to piezoelectric fields inGaN/Alx

We demonstrate a dramatic reduction of the oscillator strength in ${\mathrm{G}\mathrm{a}\mathrm{N}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ quantum wells due to piezoelectric fields. Our study using time-resolved photoluminescence spectroscopy reveals a strong increase of the luminescence decay time of the dominating transition with increasing well width by several orders of magnitude in parallel to a redshift of the emission peaks. The experimental results are consistently explained by a quantitative model based on the piezoelectric fields in strained wurtzite quantum wells. We estimate the piezoelectric constant of GaN to ${d}_{31}=\ensuremath{-}0.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ cm/V.