Abstract
We report on a comparative study of the low temperature emission and polarisation properties of InGaN/GaN quantum wells grown on nonpolar () a-plane and () m-plane free-standing bulk GaN substrates where the In content varied from 0.14 to 0.28 in the m-plane series and 0.08 to 0.21 for the a-plane series. The low temperature photoluminescence spectra from both sets of samples are broad with full width at half maximum height increasing from 81 to 330 meV as the In fraction increases. Photoluminescence excitation spectroscopy indicates that the recombination mainly involves strongly localised carriers. At 10 K the degree of linear polarisation of the a-plane samples is much smaller than of the m-plane counterparts and also varies across the spectrum. From polarisation-resolved photoluminescence excitation spectroscopy we measured the energy splitting between the lowest valence sub-bands to lie in the range of 23–54 meV for the a- and m-plane samples in which we could observe distinct exciton features. Thus the thermal occupation of a higher valence sub-band cannot be responsible for the reduction of the degree of linear polarisation at 10 K. Time-resolved spectroscopy indicates that in a-plane samples there is an extra emission component which is at least partly responsible for the reduction in the degree of linear polarisation.
Highlights
Nonpolar InGaN/GaN multiple quantum wells (MQWs) are attracting a great deal of interest because of their ability to emit linearly polarised light
At the moment InGaN/GaN QWs grown on m-plane GaN repeatedly show superior polarisation properties with degree of optical linear polarisation (DLP) values over 90% at room temperature [13] compared with structures grown on a-plane GaN. [14,15] From a theoretical point of view [16] the change of nonpolar plane should make no difference to the polarisation properties of InGaN/GaN QWs
In this paper we report on a study of the optical and polarisation properties InGaN/GaN QWs grown on ammonothermal a- and m-plane GaN substrates
Summary
Nonpolar InGaN/GaN multiple quantum wells (MQWs) are attracting a great deal of interest because of their ability to emit linearly polarised light. [12] Such high densities of defects can lead to extrinsic non-radiative and radiative processes that reduce the efficiency of the intrinsic emission process For these reasons the most promising choice for high efficiency, large DLP structures is homoepitaxial growth on freestanding, ammonothermal GaN substrates with a threading dislocation density lower than 105 cm−2 and negligible basal plane stacking faults. [14,15] From a theoretical point of view [16] the change of nonpolar plane should make no difference to the polarisation properties of InGaN/GaN QWs. At the moment InGaN/GaN QWs grown on m-plane GaN repeatedly show superior polarisation properties with DLP values over 90% at room temperature [13] compared with structures grown on a-plane GaN. We have used a combination of polarised photoluminescence (PL) spectroscopy, time resolved spectroscopy and polarisation resolved photoluminescence excitation (P-PLE) spectroscopy to compare the properties of the QWs where the differences between the sample sets are entirely due to the growth on the different non-polar planes
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