On the reduction of efficiency loss in polar c ‐plane and non‐polar m ‐plane InGaN light emitting diodes

Abstract

We have undertaken a series of experiments in InGaN light emitting diode (LED) structures both on polar (c‐plane) and non‐polar (m‐plane) GaN substrates with and without magnesium doped AlGaN electron blocking layers (EBLs) on the p‐side of the p‐n junction to shed the much needed light on the carrier injection and transport. The LEDs grown on c‐plane bulk GaN substrates without EBL show 40% peak electroluminescence (EL) intensity, while the EL peak intensity of the LEDs grown on m‐plane bulk GaN substrates without EBL is 30% of that with EBL. However, optical measurements for internal quantum efficiency (IQE) reveal that the IQE values of LEDs with and without EBL are comparable for both cases, which are in the range of 80‐85% for m‐plane variety and 50% for c‐plane variety. Furthermore, with varying Al composition (15%, 8%, 0%) in the EBL, the EL intensity for both m‐pane and c‐plane LEDs decrease progressively as the Al composition decreases. When highly conductive and transparent Ga doped ZnO layers are used as the current spreading layers for LEDs, the degradation of the efficiency with injection is significantly reduced to 27% compared to LEDs with semitransparent Ni/Au contacts having 50% efficiency degradation up to the same current density ∼3500 A/cm2. This can be due to presumably the reduction in current filamentation or current crowding. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparative studies of efficiency droop in polar and non-polar InGaN quantum wells

“…Several groups have linked this behaviour directly to the absence of the internal electric fields. From measurements on non-polar LEDs, Ling et al 18 and Li et al 19 assigned the reduction in droop to reduced losses associated with carrier spill over due to the improved carrier confinement. However, an alternative explanation was put forward by Li et al 20 and Chang et al 21 who suggested that droop was reduced because of the improvement in hole distribution amongst the QWs in multiple QW LEDs.…”

Suppression of electron overflow and efficiency droop in N-polar GaN green light emitting diodes

“…8,9,15 In addition, previous reports showed that EBLs mitigate efficiency droop in non-polar devices implying that electron overflow plays an important role even in non-polar InGaN LEDs. 16 Most commercial LEDs are fabricated with the p-top junction oriented along the þc or (0001) direction (p on top of n on a þc oriented substrate). Reversed polarization devices can be achieved using a p-down structure on a Ga-polar substrate (n on top of p on a þc oriented substrate) or a p-up junction on a Àc oriented or N-polar substrate (p on top of n on a Àc oriented substrate).…”

A review of non linear piezoelectricity in semiconductors

“…The need to manage piezoelectric polarization in InGaN/GaN superlattices 29 has led to attempting to synthesize devices on substrates with non-polar and semi-polar crystal orientations. 30,31,32,33 However, the output powers at high injection current of these devices currently do not outperform the best devices grown on (0001) planes (c-planes). 14,34, 35 Crutchley et al 36 showed experimentally that there appears to be a linear correlation between the change in internal field and the reduction of the optical effici-FIGURE 5.…”