Characterization of nonpolar a-plane GaN epi-layers grown on high-density patterned r-plane sapphire substrates
Introduction
At the present time, commercially available GaN-based light-emitting diodes (LEDs) are usually grown on c-axis–orientated GaN (c-GaN) epi-layers. Although the performance of LEDs has been improved by various technological advances [1], [2], further improvements in the performance of LEDs are hindered by intrinsic spontaneous and extrinsic piezoelectric polarizations [3], [4], which always occur in hetero-structures that use c-GaN epi-layers. To avoid these polarization effects, the growth of a-axis–orientated GaN (a-GaN) epi-layers, which are free of polarizations, has attracted attention for many years [5], [6], [7]. Although high-quality, free-standing bulk a-GaN substrates are ideal from the standpoint of lattice matching with a-GaN epi-layers, these substrates are not readily available because of their small sizes and high costs. Therefore, a-GaN epi-layers grown on r-plane sapphire substrates have been considered more suitable for mass production of LEDs. However, a-GaN epi-layers grown on flat r-plane sapphire substrates (r-FSS) usually contain numerous defects such as threading dislocations (TDs) because of the large lattice mismatch between the epi-layers and the substrates [5]. Also, a high density of structural defects, such as basal stacking faults (BSFs), prismatic stacking faults (PSFs), and partial dislocations (PDs) is a common problem in heteroepitaxially grown a-GaN epi-layers [7]. Finding an effective way to reduce the defects in a-GaN epi-layers has thus become an important issue. Although epitaxial lateral overgrowth [8] and growth based on epitaxial lateral overgrowth [9], [10], [11] are among the techniques that can most successfully reduce the defects in a-GaN epi-layers, the low-defect regions in a-GaN epi-layers are usually limited to areas several micrometers wide. In addition, these techniques require complicated and time-consuming processes because of the mask patterning process and growth interruption. In contrast, insertion of suitable buffer layers [12] between epi-layers and substrates is among the simplest and most effective techniques for obtaining epi-layers with fewer defects over a wide area. For example, our group has recently reported that the crystalline quality of a-GaN epi-layers on r-FSS can be improved by insertion of ex-situ sputtered AlN buffer layers (sp-AlNs) or annealed sp-AlNs [13]. Also, the growth of GaN epi-layers on patterned sapphire substrates (PSS) has the potential to simultaneously reduce the number of defects and enhance the light extraction efficiency of LEDs, in addition to being relatively easy and requiring little additional time. Many reports have demonstrated the effectiveness of GaN epi-layers grown on PSS by evaluating their crystalline quality or the optical characteristics of the LEDs [14], [15], [16]. Kong et al. were able to grow a-GaN epi-layers approximately 4 μm thick on hemispherical, patterned r-plane sapphire substrates (r-PSS); the patterns had 2.0 μm diameters, 2.5 μm intervals, and 1.5 μm heights [17]. These investigators showed that the number of TDs was distinctly lower on the patterned regions, whereas cross-sectional transmission electron microscopy (TEM) revealed a large number of TDs on the flat sapphire regions between the patterns.
In this study, we focused on high-density patterned r-plane sapphire substrates (r-HPSS), the patterns of which were placed at intervals of several hundred nanometers, in order to reduce the flat sapphire regions between the patterns. Because the flat sapphire regions were reduced, we hypothesized that the number of TDs in the a-GaN epi-layers could also be reduced. The effects of r-HPSS on the crystalline quality of a-GaN epi-layers were investigated. Two types of r-HPSS, the patterns of which had diameters and heights of several hundred nanometers (r-NHPSS) or several micrometers (r-MHPSS), were prepared with conventional r-FSS. The effectiveness of these r-HPSS on the a-GaN epi-layers was demonstrated through evaluation of their surface morphology and crystalline quality. The behavior of the TDs in the a-GaN epi-layers was also observed and examined in terms of the growth interruptions in the initial growth stages. We found that the use of r-NHPSS improved the crystalline quality of a-GaN epi-layers relative to that with the use of r-FSS, whereas the use of r-MHPSS did not.
Section snippets
Experimental
The r-NHPSS and r-MHPSS were fabricated by using photolithography or nano-imprinting lithography and chlorine-based inductive coupled plasma etching processes. The r-NHPSS had diameters, intervals, and pattern heights of 800, 200, and 600 nm, respectively (Fig. 1(a)). The r-MHPSS had diameters, intervals, and pattern heights of 2700, 300, and 1700 nm, respectively (Fig. 1(b)). The diameters and heights of the patterns of the r-NHPSS and r-MHPSS were quite different, but the intervals between
Results and discussion
Fig. 2 shows OM, plan-view SEM, and AFM images of the a-GaN epi-layers. The OM and plan-view SEM images confirmed that the surfaces of the a-GaN epi-layers grown on both r-FSS (Fig. 2(a), (d)) and r-NHPSS (Fig. 2(b), (e)) were pit-free, but streaks along the c-axis direction of the a-GaN epi-layer (cGaN) were commonly observed. These streaks were probably caused by the unequal growth rates of the a-GaN epi-layers, as reported previously [18], [19]. The AFM images of 20 × 20 μm2 areas also
Conclusions
Evaluation of the surface morphology and crystalline quality of a-GaN epi-layers was used to demonstrate the effectiveness of r-NHPSS and r-MHPSS in reducing the number of TDs in the a-GaN epi-layers. The a-GaN epi-layers grown on r-FSS and r-NHPSS showed pit-free surfaces, whereas the surface of an a-GaN epi-layer grown on r-MHPSS was very rough due to the large, irregular GaN islands that grew on the patterns in the initial growth stage. The crystalline quality of an a-GaN epi-layer grown on r
Acknowledgements
This work was supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) Private University Research Branding Project (2016–2020), Japan Society for the Promotion of Science (JSPS) KAKENHI for Scientific Research A [grant number 15H02019], JSPS KAKENHI for Scientific Research A [grant number 17H01055], JSPS KAKENHI for Innovative Areas [grant number 16H06416], and Japan Science and Technology CREST [grant number 16815710].
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