Measurement of internal electrical field across InGaN quantum wells in GaN LEDs

An approach has been developed to measure the internal electrical field (E_internal) in InGaN quantum wells of GaN LEDs, which is vital for the internal quantum efficiency of GaN LEDs. Firstly, the initial forward voltages, V_f0, of the studied GaN LEDs were measured to be 3.1579 V with pulse mode at a current input of 350 mA. A linear relation between the forward voltage change and the stressing voltage was established in this work. Voltage-stressing GaN LED with a large reverse bias, −7 V, we found that the initial compressive stress in MQWs was converted to be in tensile state. A stress-free condition (i.e., flat-band condition) occurs at the exact transition point of the compressive-to-tensile stress conversion. The stress-free condition implies the maximum V_f.max (flat-band condition) of the studied GaN LEDs, which can be defined to be 3.1597 V with the forward voltage measurement against relaxation time. The difference between V_f0 and V_f,max (flat-band condition) is approximately 2 mV. Plugging 2 mV in the above obtained relation between the forward voltage change and the stressing voltage, the flat-band voltage is determined to be a reverse bias of 1.18 V. With Renner's equation, the initial E_internal across the In_0.15Ga_0.85N quantum wells can be calculated to be 1.03 MV/cm. The magnitude of the present calculated E_internal agrees well with the reported E_internal, which is in the range of −0.92 to 2.04 MV/cm.