Abstract
We present a comprehensive study of the emission spectra and electrical characteristics of InGaN/GaN multi-quantum well light-emitting diode (LED) structures under resonant optical pumping and varying electrical bias. A 5 quantum well LED with a thin well (1.5 nm) and a relatively thick barrier (6.6 nm) shows strong bias-dependent properties in the emission spectra, poor photovoltaic carrier escape under forward bias and an increase in effective resistance when compared with a 10 quantum well LED with a thin (4 nm) barrier. These properties are due to a strong piezoelectric field in the well and associated reduced field in the thicker barrier. We compare the voltage ideality factors for the LEDs under electrical injection, light emission with current, photovoltaic mode (PV) and photoluminescence (PL) emission. The PV and PL methods provide similar values for the ideality which are lower than for the resistance-limited electrical method. Under optical pumping the presence of an n-type InGaN underlayer in a commercial LED sample is shown to act as a second photovoltaic source reducing the photovoltage and the extracted ideality factor to less than 1. The use of photovoltaic measurements together with bias-dependent spectrally resolved luminescence is a powerful method to provide valuable insights into the dynamics of GaN LEDs.
Highlights
InGaN-based light-emitting diodes (LEDs) have reached wall-plug efficiencies in excess of 80%1 making these devices strong candidates for general illumination
We present a comprehensive study of the emission spectra and electrical characteristics of InGaN/GaN multi-quantum well light-emitting diode (LED) structures under resonant optical pumping and varying electrical bias
We have investigated the effect of a change in the thickness of the quantum barriers (QBs) from 4 nm to 6.6 nm in the active region of InGaN/GaN LEDs when the quantum wells (QWs) thickness is 1.5 nm
Summary
InGaN-based light-emitting diodes (LEDs) have reached wall-plug efficiencies in excess of 80%1 making these devices strong candidates for general illumination. There are several challenges still to overcome such as the sub-linear increase of the light output at high current densities, known as ‘droop’[2,3,4] and the lack of efficient devices with wavelengths in the 530 nm – 630 nm range, known as the ‘green gap’.5,6. Addressing these challenges requires unambiguous identification of the different underlying mechanisms of current transport and recombination in LEDs which are grown under different conditions and can result in varying alloy fluctuations, interface qualities and doping profiles for nominally identical structures.
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