Elsevier

Journal of Crystal Growth

Volume 480, 15 December 2017, Pages 90-95
Journal of Crystal Growth

Characterization and optimization of sputtered AlN buffer layer on r-plane sapphire substrate to improve the crystalline quality of nonpolar a-plane GaN

https://doi.org/10.1016/j.jcrysgro.2017.10.018Get rights and content

Highlights

  • Three types of AlN buffer layer on r-plane sapphire substrate were characterized.

  • Sputtered AlN (sp-AlN) with or without annealing and epitaxial AlNs were examined.

  • Annealing caused several marked changes in the sp-AlNs depending on thickness.

  • The optimal buffer layer was 30-nm-thick annealed sp-AlN.

  • High-quality a-plane GaN epilayer was obtained by using 30-nm-thick annealed sp-AlN.

Abstract

Here we examined the use of AlN buffer layers of various thicknesses to improve the crystalline quality of nonpolar a-plane GaN (a-GaN) grown on an r-plane sapphire substrate. Three types of AlN buffer layers were used: sputtered AlN buffer layers (sp-AlNs) with or without annealing, and epitaxially grown AlN buffer layers (ep-AlNs). Buffer layer thicknesses of 30, 90, and 180 nm were used. We found that the surface morphological transitions with increasing thickness were different between the sp-AlNs and the ep-AlNs, and that the sp-AlNs had poorer crystallographic orientations than did the ep-AlNs. Annealing caused marked changes to occur in the surface morphologies and crystallographic orientations of the sp-AlNs; however, the positive effect of annealing was limited because the in-plane crystallographic orientation degraded with increasing layer thickness. The optimal buffer layer was found to be the 30-nm-thick annealed sp-AlN, which was composed of uniformly arranged oval grains with better crystallographic orientation than the other sp-AlNs and annealed sp-AlNs. The crystalline quality of the a-GaN epilayer grown on 30-nm-thick annealed sp-AlN had a narrower X-ray rocking curve–full width at half maximum for both the on- and off-axis planes compared with that grown on any other AlN buffer layers.

Introduction

White light-emitting diodes (LEDs) consisting of GaInN-based blue LEDs and yellow phosphors are used as components in displays, illumination devices, and automotive lighting. Although the performance of LEDs has been improved by many technological advances [1], [2], [3], there is a considerable interest in the problem called “efficiency droop”, which is the decline in efficiency at high operating current [4]. Reducing carrier density in the active layer by increasing its thickness is one potential means of suppressing both internal non-radiative loss (i.e., Auger recombination) and electron leakage, which are the proposed causes of efficiency droop [5]. However, polar c-plane GaN (c-GaN) grown on c-plane sapphire (c-sapphire) substrate, which is commonly used in the manufacture of commercial GaInN-based LEDs, suffers from the quantum-confined Stark effect due to its strong internal electric field [6], [7], meaning that the radiative recombination rate of the carrier notably decreases as the thickness of the active layer increases. In contrast, nonpolar GaN (a-plane [a-GaN] or m-plane GaN [m-GaN]) have no internal electric field and, therefore, may be useful for the manufacture of thick, nonpolar active layers with higher radiative recombination rates.

Nonpolar bulk GaN, which can be produced by using hydride vapor phase epitaxy or ammonothermal methods [8], is an ideal substrate to obtain nonpolar GaN. Indeed, the results of several studies using nonpolar bulk GaN (e.g., examining the growth behavior of GaN [9] or the characteristics of optical devices [10], [11]) have been reported. However, nonpolar bulk GaN is not widely used because of the cost disadvantage. To address this issue, low-cost and scale-up fabrication of nonpolar GaN by using hetero-epitaxy of GaN onto foreign substrates such as a-GaN on r-plane sapphire (r-sapphire) [12] or m-GaN on m-plane SiC [13] has been examined. In the present study, we examined the former case.

One problem associated with hetero-epitaxy is the quality of the crystals produced. Epilayer produced by using this method can have a high density of defects, such as threading dislocations and basal plane stacking faults, if there is a lattice mismatch between the epilayer (in this case a-GaN) and the substrate (in this case r-sapphire), which reduces the performance of devices. Although various growth techniques have been used to reduce the density of defects in a-GaN epilayers, almost all involve complicated, and time-consuming processes because of the regrowth on particular patterns [14], [15], [16].

One simple means of reducing the density of defects in an a-GaN epilayer is the insertion of a buffer layer between the epilayer and the substrate [1], [2]. Recently, several studies examining sputtered AlN buffer layers (sp-AlNs) have been reported [17], [18], [19], [20], [21]. Hu et al. examined the use of a thin (15 nm) sp-AlN buffer layer on a patterned c-sapphire substrate and reported the improvements not only of the crystalline quality of the c-GaN epilayer but also of the electrical and optical characteristics of the GaN-based ultraviolet LEDs produced using the thin buffer layers [22]. Also, Miyake et al. reported that they were able to improve the crystalline quality of sp-AlN by annealing thick (170 or 340 nm) sp-AlN on a c-sapphire substrate [23]. Although many studies have examined the use of sp-AlN as a buffer layer on c-sapphire substrates, few have examined the use of sp-AlN on an r-sapphire substrate [24] or the effect of annealing on crystalline quality of the a-GaN epilayer.

In the present study, we examined the effect of AlN buffer layer thicknesses on the crystalline quality of a-GaN epilayers grown on r-sapphire substrates. To find the optimal AlN buffer layer thickness, the qualities and surface morphologies of three different thicknesses of sp-AlNs or annealed sp-AlNs and epitaxially grown AlN buffer layers (ep-AlNs) were examined. We found that 30-nm-thick annealed sp-AlN had the best crystallographic orientation and crystalline quality compared with the any other AlN buffer layers. We also found that an a-GaN epilayer grown on a 30-nm-thick annealed sp-AlN had a better crystalline quality compared with that grown on any other AlN buffer layers.

Section snippets

Experimental

r-Sapphire substrate with an offcut angle of 0.5° in the [0 0 0 1] direction was prepared as reported previously [25]. All sp-AlNs were deposited by using a planar magnetron radio-frequency sputtering system. A sintered AlN target was placed approximately 85 mm from the r-sapphire substrate. After the chamber was evacuated to <5.0 × 10−5 Pa, the r-sapphire substrate was heated to 300 °C and an Ar–N2 gas mixture was introduced as the sputtering gas (10 standard cc min−1). By using a radio-frequency

Results and discussion

Fig. 1(A-1) shows the 2θ/θ profiles (out-of-plane) obtained for the AlN buffer layers on r-sapphire substrates. In all of the 2θ/θ profiles, two diffraction peaks can be seen at approximately 53 and 59°, which represent diffractions from sapphire (2 0 −2 4) and AlN (1 1 −2 0), respectively, demonstrating that all of the AlN buffer layers were stacked on the r-sapphire substrates oriented along the a-axis.

Fig. 1(A-2) shows the peak diffraction intensities of AlN (1 1 −2 0) as a function of thickness. The

Conclusions

Here we examined the use of AlN buffer layers of various thicknesses to improve the crystalline quality of nonpolar a-plane GaN (a-GaN) grown on r-plane sapphire substrates. Three types of AlN buffer layers were used: sputtered AlN buffer layers (sp-AlNs) with or without annealing, and epitaxially grown AlN buffer layers (ep-AlNs). Buffer layer thicknesses of 30, 90, or 180 nm were used. Annealing caused marked changes to occur in the surface morphologies and crystallographic orientations of

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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