Full Length ArticleGrowth of high-quality AlN films on sapphire substrate by introducing voids through growth-mode modification
Graphical abstract
Introduction
The epitaxial growth of high-quality AlN films is essential for high-efficiency deep-ultraviolet (DUV) optoelectronic devices [1], [2], [3], such as light-emitting diodes (LEDs) [4], [5], [6], laser diodes (LDs) [7], and high-frequency power electronics [8], [9], [10]. Owing to the lack of economic AlN bulk substrate, AlN films are usually obtained by epitaxially grown on foreign substrates through metal-organic chemical vapor deposition (MOCVD) method [11], [12], [13]. Sapphire is the preferred substrate for AlN epitaxy because of its low cost and transparency to UV light [14], [15]. However, due to the large mismatch in lattice constants and thermal expansion coefficients (TEC) between AlN and sapphire, the epitaxial AlN films suffering from severe stresses are generally with high density of threading dislocations (109 –1010 cm−2) [16], [17], [18]. These defects can significantly degrade the device performance by acting as non-radiative recombination centers or leakage current pathway [19], [20]. Moreover, AlN cracking will be induced by the large tensile stress, which is a major concern for the device reliability issue.
Several approaches have been explored to improve the epitaxial quality of AlN on sapphire. High growth temperature (over 1300 °C) has been adopted to enhance the migration of Al adatoms for an atomically flat AlN surface [21], [22]. In addition, high temperature annealing (1700 °C) in N2 + CO ambient was demonstrated to significantly reduce the threading dislocation density (TDD) [23]. However, AlN films grown on sapphire suffer from increasing risk of cracking at such high temperature and achieving these high temperature conditions always require specially designed reactor which increases the manufacturing cost. Sapphire offcut and surface pretreatments were also investigated to improve the crystal quality [24], [25], [26]. Nonetheless, precise control of the sapphire surface is difficult and additional substrate processing steps are costly. Recently, by taking advantages of nanoscale epitaxial lateral overgrowth (ELOG), AlN grown on nano-patterned sapphire substrate (NPSS) has attracted much research interest [27], [28], [29]. The coalescence of AlN films on NPSS always embeds voids into the epilayers, which are critical to the crystal quality improvement by playing the role of dislocation filter and additional stress relief channel. However, in order to achieve nanoscale patterns on sapphire substrates, the surface patterning processes of NPSSs usually involve sophisticated and costly photolithography procedure.
In this work, we successfully brought voids into AlN films grown on flat sapphire substrate (FSS) through a growth-mode modification process, which enabled the realization of high quality, crack-free AlN films on FSS. Dislocations termination at the void sidewalls are observed, verifying the practicability of introducing voids for dislocation filtering in the epitaxial growth of AlN on FSS. Moreover, we believed that the voids were responsible for an observed drastic decrease of tensile stress from 1.34 GPa to 0.44 GPa during growth. With the introducing of voids, the 3 μm-thick AlN film grown on FSS demonstrated a low TDD of 1.7 × 108 cm−2 and was nearly free of residual stress. Compared to the patterned substrates based ELOG process, our work provides a much more cost-efficient way to achieving high-quality AlN film, which largely promotes the prospect of commercial AlN-based devices.
Section snippets
Experiments
AlN films were heteroepitaxially grown on 2-inch FSS and NPSS by using an AIXTRON Crius I close-coupled showerhead reactor MOCVD system. The NPSS used in this study is featured with hexagonally arranged holes as shown in Fig. 1a. The period, diameter, and depth of dimples are 1 μm, 900 nm, and 500 nm, respectively. AlN epilayers grown on the sapphire substrates take the epitaxial relationship of AlN(0 0 0 2)//Al2O3(0 0 0 6) and AlN(1 0 0)//Al2O3(1 1 0), which were confirmed by the
Results and discussion
First, reflectance transients during the whole AlN growth process were in-situ monitored to provide information on surface morphology evolution (as shown in Fig. 2a). In the reflectance profiles, the Fabry-Pérot oscillations are indicative of increasing AlN layer thickness and the ascent/descent of the oscillation amplitude implies surface smoothing/roughening. A lower initial reflectance was observed for the NPSS sample due to its inherent rough patterned surface. The reflectance profiles
Conclusions
In summary, the success of introducing voids into AlN film grown on FSS for dislocation filter and strain relief was demonstrated. We show that dislocation annihilation in the AlN film grown on FSS was facilitated by the embedded voids, resulting in a low TDD of 1.7 × 108 cm−2 in the 3 μm-thick AlN film. Furthermore, with the introduction of voids into the AlN film grown on FSS, a drastic drop of the in-situ tensile stress from 1.34 GPa to 0.44 GPa was observed, which reduced the risk of
CRediT authorship contribution statement
Bin Tang: Conceptualization, Investigation, Writing - original draft. Hongpo Hu: Validation, Investigation, Writing - review & editing. Hui Wan: Writing - review & editing. Jie Zhao: Visualization, Writing - review & editing. Liyan Gong: Visualization, Writing - review & editing. Yu Lei: Visualization, Writing - review & editing. Qiang Zhao: Writing - review & editing. Shengjun Zhou: Conceptualization, Supervision, Writing - review & editing.
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.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (51675386, U1501241, and 51775387), Natural Science Foundation of Hubei Province (2018CFA091), and the National Key Research and Development Program of China (2017YFB1104900).
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B. Tang, H. Hu, and H. Wan contributed equally to this work.