The built-in piezoelectric field induced by compressive stress in InGaN/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) was investigated using the electric field dependent electroreflectance (ER) spectroscopic method. InGaN/GaN MQW structures were prepared on sapphire substrates of different thicknesses. Thinning the sapphire substrate enables control of the compressive stress by changing the curvature of the wafer bowing. The wafer bowing-induced mechanical stress alters the piezoelectric field in the InGaN/GaN MQW. The flat band voltage, estimated by measuring the applied reverse bias voltage that induces a 180 degrees phase shift in the ER spectra, was decreased from -11.21 V to -10.51 V by thinning the sapphire substrate thickness from 200 to 60 mu m. To calculate the piezoelectric field (F-pz) from the compensation voltage, the depletion width was obtained from the capacitance-voltage measurement. The F-pz estimated from the energy shift in ER peak in a bias range from 0 to -12 V was changed by 110 kV/cm. (C) 2014 Elsevier B.V. All rights reserved.