Anomalous Super-Linear Growth Rate Reduction with Increasing Aluminum Flux in M-Plane AlGaN Grown by PAMBE

Abstract

Abstract Body: III-Nitride materials have long been promising candidates for the development of infrared optoelectronic devices. However, design of multiple quantum well (MQW) structures on conventional c-plane (0001)-oriented GaN substrates is hindered by the spontaneous and piezoelectric polarization fields associated with lack of inversion symmetry. Low-defect density free-standing m-plane (10-10) GaN substrates have enabled the growth of non-polar MQW structures without strong built-in polarization fields. We have previously demonstrated the potential of m-plane Al0.16Ga0.84N/GaN heterostructures grown by plasma-assisted molecular-beam epitaxy (PAMBE) for terahertz devices [1]. To support higher energy near-infrared optical transitions, though, a larger conduction band offset (CBO) is required. This can be achieved through higher Al composition in the barriers. However, m-plane AlGaN layers with aluminum fractions above 0.2 grown under conventional gallium-rich PAMBE conditions exhibit severe material inhomogeneities [2]. Recently, we demonstrated that growth of m-plane AlGaN by indium surfactant assisted epitaxy (ISAE) at low temperature significantly reduces composition inhomogeneities [3]. Here, we present an in-depth comparison of the PAMBE growth of m-plane AlGaN with and without indium surfactant at 565°C. We investigated the compositions and growth rates of low temperature m-plane AlGaN layers grown by conventional gallium-rich epitaxy and by ISAE across a range of aluminum fluxes. It is expected that under nitrogen-limited growth conditions the growth rate would be independent of metal flux. We find the opposite to be true: as aluminum flux increases, the growth rate of AlGaN decreases rapidly. We attribute this phenomenon to nitrogen loss during PAMBE of m-plane AlGaN that increases super-linearly with increasing aluminum flux. These films also show a super-linear increase in aluminum content with increasing aluminum flux. Transmission Electron Microscopy (TEM) images indicate that these effects are correlated with high aluminum containing defects within the material. Our results indicate that ISAE mitigates this effect almost entirely. Films grown by ISAE with identical aluminum fluxes to the gallium-rich layers do not exhibit a significant decrease in growth rate with increasing aluminum flux, and their aluminum compositions increase linearly with aluminum flux. TEM images show the ISAE layers are thicker, and exhibit a higher degree of material uniformity up to 0.30 aluminum metal fraction. X-ray diffraction (XRD) measurements suggest minimal indium incorporation in the ISAE AlGaN layers. The CBO of MQWs can be further increased by using InGaN as the quantum well (QW) material. We have recently demonstrated the growth of high-quality m-plane InGaN with up to 0.16 indium metal fraction [4], enabling the growth of In0.055Al0.19Ga0.755N/In0.16Ga0.84N MQW structures using the ISAE technique [3]. We will also present results of structural and optical characterization of m-plane (In)Al0.3Ga0.7N/In0.16Ga0.84N MQW structures. Due to the anisotropic nature of strain in m-plane structures, AlGaN/InGaN structures cannot be perfectly strain-balanced. We will discuss the method used to control strain accumulation along the (0001) and (11-20) directions below the level for defect formation in both the InGaN QWs and AlGaN barriers. [1] C. Edmunds et al., Appl. Phys. Lett. 105, 021109 (2014). [2] M. Shirazi-HD et al., J. Appl. Phys. 123, 161581 (2018). [3] B. Dzuba et al., J. Appl. Phys. 128, 115701 (2020). [4] A. Senichev et al., APL Mater. 7, 121109 (2019).