N-polar AlN nucleation layers grown by hot-wall MOCVD on SiC: Effects of substrate orientation on the polarity, surface morphology and crystal quality
Introduction
Group III-nitride semiconductors have continuously attracted strong research interest due to their application in optoelectronic [1] and electronic devices [2], [3]. In particular, GaN-based high electron mobility transistor (HEMT) heterostructures have been intensively investigated for radio frequency electronic and power switching applications [4], [5]. Group III-nitrides have a wurtzite crystal structure and epitaxial layers of these materials are typically grown along the polar -axis. Most works so far have been focused on metal (Ga, Al, In)-polar epitaxy due to easier growth, and the majority of devices utilize metal-polar structures grown along the (0001) direction. Recently, much efforts have been focused on N-polar epitaxy because structures grown along direction are found to be advantageous for some device applications [6]. For example, compared with conventional metal-polar HEMTs, the N-polar counterparts exhibit improved characteristics such as feasibility to fabricate low-resistance ohmic contacts, enhanced carrier confinement with a natural back barrier, as well as better device scalability [7]. A record breakdown voltage over 2 kV and low on-resistance have been reported for N-polar HEMT grown on miscut sapphire substrates [8].
Epitaxial N-polar GaN layers have been demonstrated on different substrates such as GaN [9], sapphire [10] and SiC [11]. The presence of hexagonal hillocks on the surface was identified as a common problem for the growth on on-axis substrates including the case of homoepitaxy. It was found that the formation of hexagonal hillocks can be suppressed or eliminated by employing vicinal substrates with different misorientation angles [9], [10], [11], [12].
In general, when GaN is grown on SiC a thin AlN nucleation layer (NL) is employed in order to improve crystal quality and morphology. The main role of the AlN NL is to reduce the density of the misfit dislocations as a result of the smaller lattice mismatch of 1% between AlN and the SiC compared with the lattice mismatch of 3.5% between GaN and SiC. High-quality thin AlN NL was also demonstrated to reduce the thermal boundary resistance at the interface between the SiC substrate and the GaN buffer layer in HEMT structures [13]. The nucleation conditions have a strong impact on the polarity of AlN NL and of subsequently grown GaN layers [14]. Specifically for N-polar III-nitrides epitaxy, AlN NL significantly affects the hexagonal hillock formation and crystallization of N-polar GaN layers [12], [15].
Won et al. [12] investigated the effect of different growth conditions (e.g. V/III ratio) and the thickness of AlN NL on the surface morphology and structural properties of N-polar GaN films grown on vicinal C-face SiC substrates misoriented towards the by 3.6°and by 4°. Lemettinen et al. [16] have performed a comprehensive study of N-polar and Al-polar AlN on 4H-SiC substrates and reported on the effect of the substrate miscut angle on the quality of N-polar AlN. The substrate pre-treatment have been found to have a strong impact on the polarity of AlN and GaN layers grown on C-face SiC [17]. All these investigations proved that the SiC substrate orientation and the growth conditions play an important role for the polarity control and crystalline quality of AlN NLs, which in turn affects the polarity and the properties of GaN grown on top.
In this work, we report on the effect of the substrate orientation on the polarity, surface morphology and crystal quality of AlN NLs grown by hot-wall metalorganic chemical vapor deposition (MOCVD) on 4H-SiC. Hot-wall MOCVD has demonstrated a superior quality of group-III nitride epitaxial layers and HEMT structures [13], [18], [19]. Compared to the conventional cold-wall MOCVD in which only the substrate is heated from the back, hot-wall MOCVD employs a heated susceptor providing highly uniform temperature distribution [13], [20]. In addition, it enables better cracking efficiency of the precursors which prevents growth-limited species consumption by gas-phase adduct formation [21]. The hot-wall MOCVD allows a larger growth temperature window up to 1600 °C, while it can also achieve high quality AlN layers at reduced growth temperatures. Despite the number of advantages, the hot-wall MOCVD has not yet been explored for N-polar growth. Here, we aim at achieving N-polar AlN NLs and optimizing their quality, which is of a significant importance for subsequent growth of GaN layers in HEMT heterostructures.
Section snippets
Experimental details
Epitaxial AlN NLs with a thickness of 50 nm were grown by hot-wall MOCVD simultaneously on on-axis semi-insulating (SI) 4H-SiC (0001) and (000), as well as on n-type off-cut 4H-SiC (000) with [0001] misoriented towards the [110] by 4°. The substrates were pre-cleaned first in acetone, and then in methanol in a liquid ultrasound bath for 3 min and second in 80 °C heated ammonia solution for 5 min, followed by 5 min etching in 80 °C heated hydrochloric acid solution and rinsing by deionized
Results and discussion
Figs. 1 (a)–(i) show AFM images of the as-grown AlN NLs for each growth temperature and the tree types of substrates used. It is seen that the AlN NLs exhibit island-like morphology with a variation of grain size and density that is dependent of the growth temperature and the substrate orientation. The surfaces of the AlN NLs grown at low temperature of 850 °C [Figs. 1 (a), (b) and (c)] exhibit high densities of small islands. With increasing the growth temperature the island size and the
Conclusion
The effects of substrate orientation on the polarity, surface morphology and crystal quality of AlN NLs grown by hot-wall MOCVD on SiC were investigated. AlN NLs with N-polarity are achieved on both on-axis and off-cut C-face SiC (000), while the layers grown on Si-face SiC (0001) posses Al-polarity. It is shown that with increasing growth temperature the growth mode changes from island-like to step-flow on off-axis SiC (000) and to layer-by-layer on SiC (0001), respectively. In contrast,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work is performed within the framework of the competence center for III-Nitride technology, C3Nit — Janzén supported by the Swedish Governmental Agency for Innovation Systems (VINNOVA) under the Competence Center Program Grant No. 2016-05190, Linköping University, Chalmers University of Technology, ABB, Ericsson, Epiluvac, FMV, Gotmic, On Semiconductor, Saab, SweGaN, and UMS. We further acknowledge support from the Swedish Research Council VR under Award No. 2016-00889, Swedish Foundation
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