We report on the effect of GaN buffer threading dislocation (TD) optimization and InGaN/GaN quantum well (QW) hydrogen (H-2) treatment on the efficiency of GaN light emitting diodes (LEDs) operating in the spectral range from 400 to 500 nm. A tenfold reduction of the TD density in the GaN buffer increased the efficiency of blue LEDs operating at high current density, while in green LEDs it had very little effect. The reduced TD density also increased the compressive strain in the InGaN QWs, and caused blue shift to the electroluminescence (EL) peak wavelength. The H-2 treatment of the QWs increased strain inside the MQW stack. It was possible to apply the H-2 treatment only to UV LEDs, as the increased strain in blue and green LEDs caused relaxation of the MQW stack. Although this resulted in smooth surface morphology of the MQW stack, it did not lead to any increase in the efficiency of the UV LEDs. (c) 2006 Published by Elsevier B.V.