Stokes shift in semi-polar (112¯2) InGaN/GaN multiple quantum wells

Abstract

The mechanism for the large Stokes Shifts of InGaN/GaN structures is under debate. Here, we report a systematic study on the Stokes shift of semi-polar (112¯2) InGaN/GaN multiple quantum wells (MQWs) with a wide spectral range from green (490 nm) to yellow (590 nm) by means of both photoluminescence excitation and time resolved PL measurements in comparison with their c-plane counterparts. The semi-polar samples exhibit a lower Stokes shift than their c-plane counterparts, although they show stronger localization effect than their c-plane counterparts. In the long wavelength region, the Stokes shift of the semi-polar MQWs shows a linear relationship with emission energy, but with a smaller gradient compared with their c-plane counterparts. The time-resolved PL measurements confirm a significant reduction in piezoelectric field of the semi-polar samples compared with the c-plane counterparts. It is suggested that the piezoelectric field induced polarization is the major mechanism for causing the large Stokes shift. The presented results contribute to better understanding of the long standing issue on the mechanism for the large Stokes shift.

Exciton localization in semipolar (112¯2) InGaN multiple quantum wells

“…It should be noted that the trend (y H ) is almost the same as a trend previously reported for polar InGaN MQW and layers. 32,33 The Stokesshift values estimated for the MQW samples (this work) and InGaN layers 28 are smaller than that of Zhang et al, 14 this might be due to differences in growth optimization. Though for the samples studied here, a different slope (y L ) has been observed compared to that of Zhang et al, 14 it somehow confirms their data whereby a "smaller" slope of Stokes-shift has been found for (11 22) MQWs compared to polar MQWs.…”

“…The Stokes-shift values estimated for the InGaN/ GaN MQW samples and GaN are shown in Fig. 5(b) together with previous results estimated for (11 22) InGaN MQW 14 and (11 22) InGaN layers. 28 Despite the scattered data points, two possible trends (after linear fittings) can be observed, whereby the Stokes-shift values show linear dependence on emission energy, and tend to a zero value around the GaN bandgap of 3.5 eV.…”

“…28,31 The strong ELOC in m-plane MQW 18 and the (11 22) MQW samples studied here can be another reason that should be taken into account with the reduced polarization fields 4,5 to explain why non-/semi-polar MQW structures have a much shorter radiative lifetime compared to polar MQW. 4,[12][13][14]20 To investigate the Stokes-shift of the samples, PL spectra together with PLE absorption edge were measured at 10 K using the Xe-lamp excitation source (P ex $ 10 -5 W/cm 2 ). The PLE measurements were performed with a detection energy fixed at each MQW peak emission energy.…”

“…16,17 It should be noted that the ELOC intrinsically exists in InGaN alloys, irrespective of growth orientation employed. 2,3,11,[13][14][15][18][19][20][21] Zhang et al 14 have reported that (11 22) QWs exhibit larger localization depths than polar QWs (estimated from TR-PL measurements at 7 K). Recently, by temperature-dependent photoluminescence (TD-PL) measurements, a strong ELOC degree has been found in (11 22) In 0.2 Ga 0.8 N QWs that causes a blue-shift of the QW exciton emission with rising temperature from $200 K to 340 K, irrespective of excitation source used.…”

“…In contrast, high efficiency green and yellow semipolar (11 22) InGaN LEDs grown on low threading dislocation density (TDD $ 5 Â 10 6 cm À2 ) high-cost (11 22) GaN bulk substrates have previously been demonstrated. 10,11 By means of time-resolved photoluminescence measurements (TR-PL), it has been experimentally reported that mplane InGaN 4 and (11 22) InGaN [12][13][14] QWs have shorter recombination lifetimes compared to polar QWs. However, it should be noted that those samples show recombination dynamics identical to those observed in polar QWs.…”