Influence of InN and AlN concentration on the compositional inhomogeneity and formation of InN-rich regions in InxAlyGa1−x−yN

Gunnar Kusch,Tim Wernicke,Michael Kneissl,Tolga Teke,Johannes Enslin,Paul R Edwards,Robert W Martin,Lucia Spasevski

doi:10.7567/1347-4065/ab147a

Gunnar Kusch, Tim Wernicke + Show 6 more

Open Access

https://doi.org/10.7567/1347-4065/ab147a

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Abstract

The application of quaternary InxAlyGa1−x−yN active regions is a promising path towards high efficiency UVB-light emitting diodes (LEDs). For the utilization of InxAlyGa1−x−yN, detailed knowledge of the interplay between growth parameters, adatom incorporation, optical and structural properties is crucial. We investigated the influence of the trimethylaluminium (TMAl) and trimethylindium (TMIn) flux on the composition and luminescence properties of InxAlyGa1−x−yN layers by multi-mode scanning electron microscopy. We found that varying the molar TMIn flow from 0 to 17.3 μmol min−1 led to an InN concentration between 0% and 3.2% and an emission energy between 4.17 and 3.75 eV. The variation of the molar TMAl flow from 3.5 to 35.4 μmol min−1 resulted in a AlN composition between 7.8% and 30.7% with an emission energy variation between 3.6 and 4.1 eV. Cathodoluminescence hyperspectral imaging provided evidence for the formation of nanoscale InN-rich regions. Analyzing the emission properties of these InN-rich regions showed that their emission energy is inhomogeneous and varies by ≈150 meV. We provide evidence that the formation of these InN-rich regions is highly dependent on the AlN and InN composition of the layer and that their formation will strongly affect the performance of InxAlyGa1−x−yN LEDs.

Highlights

In this paper we investigate the influence of the trimethylindium (TMIn) and trimethylaluminium (TMAl) fluxes during growth of InxAlyGa1−x−yN layers on their optical, morphological and compositional properties in order to further understand the optical quality and compositional homogeneity
CL spectra acquired for all samples show a decrease in the energy of the near band edge (NBE) emission with either an increasing In flux or a decreasing Al flux (Fig. 1), following the trend observed by Ref. 17
The InN concentration increased with increasing TMIn flux from 0% (0 μmol min−1) to 2% (17.3 μmol min−1) with a maximum concentration at 3.2% (8.7 μmol min−1) while the Al concentration decreases with increasing TMIn flux from 34% (0 μmol min−1) to 17.6% (17.3 μmol min−1)

Summary

Introduction

Ultraviolet (UV) light emitting diodes (LEDs) in the UVB wavelength region show potential for applications including: UV-curing, gas-sensing and photo-therapy of, for example, psoriasis.1–3) the external quantum efficiency (EQE) of UVB emitting devices is not high enough to satisfy the demands of these commercial applications, exhibiting EQE values even lower than UV-C LEDs.3) There are indications that the internal quantum efficiency of UVB emitting quantum wells (QWs) can be increased by replacing the conventionally used ternary AlGaN/AlGaN QW/quantum barrier (QB) active region with a QW structure based on the quaternary InxAlyGa1−x−yN alloy.4,5) This was attributed to In segregation effects, creating areas with higher InN fractions, leading to enhanced carrier localization and an increased electron hole overlap, analogous to the InGaN case.6–8) Currently a lack of understanding of the incorporation, segregation and phase separation mechanisms9,10) of InN in InxAlyGa1−x−yN is a barrier to the full utilization of the beneficial properties of InxAlyGa1−x−yN for the growth and fabrication of UVB LEDs. In this paper we investigate the influence of the trimethylindium (TMIn) and trimethylaluminium (TMAl) fluxes during growth of InxAlyGa1−x−yN layers on their optical, morphological and compositional properties in order to further understand the optical quality and compositional homogeneity. We correlate these different properties by utilizing multiple signals generated by a beam of high energy electrons in an SEM, allowing us to investigate the optical properties via cathodoluminescence (CL) hyperspectral imaging,11,12) the morphology by secondary electron (SE). Imaging and the composition by wavelength dispersive X-ray (WDX) spectroscopy

Experimental

Results and discussion

Conclusion