Comparison between Polar (0001) and Semipolar ($11\bar{2}2$) Nitride Blue–Green Light-Emitting Diodes Grown on $c$- and $m$-Plane Sapphire Substrates

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We present a comparative study of semipolar ($11\bar{2}2$) and polar (0001) (In,Ga)N/GaN light emitting diodes grown simultaneously under equal conditions by metalorganic vapor deposition. These structures are grown on 2-in. GaN templates with two different ($11\bar{2}2$) and (0001) orientations. The templates were formerly obtained by MOCVD, on ($10\bar{1}0$) $m$-plane and (0001) $c$-plane sapphire substrates. The semipolar quantum wells exhibit a 30% higher indium content compared to the polar quantum wells. This was confirmed by complementary X-ray diffraction and energy dispersive X-ray spectroscopy measurements on thick (${\sim}60$ nm) InGaN layers grown on ($11\bar{2}2$) and (0001) GaN templates. The results of electroluminescence and photocurrent show that the emission of semipolar QWs is strongly red-shifted with respect to the band edge. This large Stokes’ shift is attributed to carrier localization at potential minima induced by indium composition fluctuations. A strong blueshift under forward DC injection is observed for the semipolar (Ga,In)N diodes. Since a reduced piezoelectric field is expected in the semipolar diodes, the observed blueshift is mainly due to carrier filling of the localized states.
2009-03-25