Improved crystal quality of semipolar (101¯3) GaN on Si(001) substrates using AlN/GaN superlattice interlayer

doi:10.1016/j.jcrysgro.2016.09.004

Journal of Crystal Growth

Volume 454, 15 November 2016, Pages 114-120

https://doi.org/10.1016/j.jcrysgro.2016.09.004 Get rights and content

Highlights

•
Single-domain semipolar (101¯3) GaN layer was grown on Si(001) substrates.
•
Coherent AlN buffer layers were induced by directional sputtering.
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AlN/GaN interlayers were applied to improve the crystal quality of semipolar GaN.
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For both the N-face and Ga-face of (101¯3) GaN, the structural and optical properties were studied.

Abstract

The planar epitaxial growth of semipolar (10 $\bar{1}$ 3) GaN on a Si(001) substrate was performed on a directionally sputtered AlN buffer layer. Three types of interlayers, i.e., single AlN, double AlN, and a stack of AlN/GaN layers were grown by metalorganic chemical vapor deposition (MOCVD) to achieve high quality GaN films. The results for the stack of AlN/GaN layers provide highest crystal quality and optical properties for GaN. Comparing the top (Ga face) and bottom (N face) surfaces of grown semipolar (10 $\bar{1}$ 3) GaN confirms the defect density reduction that is due to the application of interlayers. Moreover, reduced inversion domain density on the bottom surface is attributed with the insertion of interlayers. Improving the quality of semipolar GaN on Si(001) substrates is expected to be useful for GaN/Si(001) integrated optoelectronics.

Introduction

III-nitride-based device applications have been extensively developed over the last few decades [1], [2]. In particular, c-plane GaN can be grown on both common c-sapphire and Si(111) because of the preferential epitaxial relationship between GaN and mentioned substrates [3], [4], [5]. However, the well-known strong internal electric field at the InGaN/GaN interface reduces the quantum efficiency of light-emitting devices, such as light-emitting diodes (LEDs) and laser diodes (LDs) [6]. Theoretically, nonpolar and/or semipolar GaN can lower the induced built-in field, suppressing the spectral shift and efficiency droop under high current injection [7], [8]. Nonpolar and/or semipolar GaN layers have been grown on a variety of specifically oriented substrates such as m-sapphire, a-sapphire, etc [9], [10], [11]. However, semipolar GaN epitaxy has also been grown on variously oriented Si substrates because of its compatibility with Si substrates. Honda et al. succeeded in growing semipolar (10 $\bar{1}$ 1) GaN on a grooved Si(001) substrate via chemical etching [12]. Similar approaches have been implemented on patterned Si(113) [13], Si(110) [14], and Si(112) [15] substrates to grow semipolar (112) GaN, nonpolar (110) GaN, and nonpolar (100) GaN, respectively. Current challenges regarding planar epitaxial growth of semipolar GaN on Si substrates includes the rough morphology and high density of defects; however, the planar epitaxy of semipolar (10 $\bar{1}$ 2) GaN on Si(210) has been used despite severe roughness [16]. Therefore, the further development of GaN-on-Si epitaxy is necessary to demonstrate the integration of GaN-based devices on mainstream Si(001) platforms [17].

The complication in realizing planar GaN on Si(001) results from the fourfold symmetry of Si(001), which leads to multiple-domain GaN crystals [18]. To obtain single-domain GaN on Si(001), we used an AlN buffer layer fabricated by directional sputtering technology, resulting in single-domain semipolar (10 $\bar{1}$ 3) GaN films on Si(001) substrates. In addition, the use of sputtered AlN (sp-AlN) can have major benefits, such as benefiting from a simple and inexpensive process compared with a chemical vapor deposition (CVD)-based system and the elimination of strain-induced defects via a weak-epitaxy growth, i.e., an almost deposition process [19]. Many research groups investigated and reported the deposition of AlN on Si substrates by sputtering method, and several parameters such as power [20] and the distance [21] between the source target and substrate have been modified to control the orientation of the sp-AlN layer.

After achieving the growth of single-domain GaN on Si(001), the next issue is the crystal quality of the grown GaN. Bläsing et al. reported improved crystal quality for semipolar (10 $\bar{1}$ 3) GaN by applying a low-temperature AlGaN interlayer, which made a key role in reducing the stacking fault (SF) density to <5×10³ cm⁻¹ [22]. Also, reduction of the defect density using specific interlayer structures has been commonly reported for structures, such as multiple AlN interlayers, and superlattice buffer layers [23], [24]. Therefore, in this study, we report the effect of several interlayer structures on semipolar (10 $\bar{1}$ 3) GaN on Si(001) substrates in an attempt to improve the crystal quality. Interlayer structures ranging from single to multiple buffer layers were formed on sp-AlN/Si(001) substrates. Their crystal quality and optical properties were investigated for Ga-face (10 $\bar{1}$ 3) GaN, and N-face (10 $\bar{1} \bar{3}$ ) GaN, which facilitates analyzing the initially generated defects.

Section snippets

Experimental details

To prepare sp-AlN templates, a 45-nm-thick AlN layer was deposited on Si(001) substrates by DC magnetron sputtering (ULVAC, ACS-4000-UHV-C3). As we previously found that a directional sp-AlN layer on Si(001) is effective for controlling the crystal orientation of GaN [25], we induced single-domain sp-AlN by directional sputtering, i.e., the Si(001) substrate was not rotated and the Al target was placed parallel to the Si[110] orientation direction. Subsequently, the sp-AlN templates were loaded

Growth of semipolar (10 $\bar{1}$ 3) GaN on Si(001) with interlayers

Fig. 1 shows a schematic of semipolar (10 $\bar{1}$ 3) GaN epitaxial structures with three different interlayers. Samples A, B, and C are consisted of a single buffer layer [HT-AlN (20 nm)], double buffer layers [HT-AlN (20 nm)/LT-AlN (10 nm)], and superlattice (SL) layers [25 pairs of AlN/GaN (5/3 nm) on HT-AlN (20 nm)], respectively. The growth temperature was maintained at 750 °C for LT-AlN and at 1050 °C for HT-AlN. The same temperature of HT-AlN was also used for the AlN/GaN SL layers. Before the growth

Conclusion

Three types of interlayers were used with the aim of improving the crystal quality of semipolar (10 $\bar{1}$ 3) GaN grown on Si(001) substrates. Compared with the simple interlayer of a single HT-AlN layer, using multiple interlayers such as HT-AlN/LT-AlN and the AlN/GaN SL resulted in significant improvements in the film's structural and optical properties. The highest crystal quality was obtained when using the AlN/GaN SL layers, where the resulting GaN exhibited the lowest RMS roughness and FWHM of

Acknowledgments

This work was partially supported by Japan Society for the Program of Science (JSPS) KAKENHI # 25000011. The author (S. Y. Bae) is an international research fellow of the JSPS. We would also like to acknowledge the use of the facilities in the Akasaki Research Center of Nagoya University.

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These unfavorable factors can lead to the uneven sheet resistance of devices, which affects the heat dissipation and further hinders the increase of power. In order to reduce the dislocations and large wafer bow, low-temperature (LT)-AlN [10], high-temperature (HT)-AlN [11], compositionally graded AlGaN layer [12], step-graded AlGaN layer [13], and various superlattice structures [14] have been proposed as buffer layer structures, and these buffers indeed reduce the wafer bow and dislocations to a certain extent. Furthermore, some researchers have put forward the idea that the main course of the uneven resistance in GaN-based HEMT devices is the wafer bow [15]; however, till date its mechanism that affects the uniformity of the sheet resistance has rarely been investigated comprehensively.
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In this work, we report a method that the Si substrate is pre-treated in the atmosphere of high temperature AlN before the depositon of Al to improve the quality of GaN-on-Si, which is studied by few people. Compared to the untreated sample, the pre-treated one exhibits smoother surface morphology with a root mean square roughness (RMS) value of 0.580 nm and the full width at half maximum (FWHM) of GaN (102) plane also decreases from 1484 arcsec to 1303 arcsec. Moreover, we give an explanation for the extra shoulder peaks in the photoluminescence (PL) spectrum. This enhancement of quality and surface morphology of GaN can be attributed to the improvement of surface flatness of Si substrate, which contributes to the conversion of AlN to two-dimensional growth mode, leading to a smoother surface of GaN. And the reason for the improvement of Si substrate is also investigated in this work.
How to obtain metal-polar untwinned high-quality (1 0 −1 3) GaN on m-plane sapphire
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Therefore, we want to realize the metal-polar (1 0 −1 3) surface orientation which has a weaker, but still positive polarization field. There are very few reports on the preparation of metal-polar (1 0 −1 3) GaN [5–8]. Single phase N-polar (1 0 −1 −3) GaN has been grown on (1 1 0) spinel [9].
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Improved crystal quality of semipolar (101¯3) GaN on Si(001) substrates using AlN/GaN superlattice interlayer

Highlights

Abstract

Introduction

Section snippets

Experimental details

Growth of semipolar (101¯3) GaN on Si(001) with interlayers

Conclusion

Acknowledgments

J. Cryst. Growth

J. Cryst. Growth

J. Cryst. Growth

J. Cryst. Growth

J. Cryst. Growth

Surf. Coat. Technol.

Thin Solid Films

J. Cryst. Growth

J. Cryst. Growth

J. Cryst. Growth

J. Cryst. Growth

Thin Solid Films

Jpn. J. Appl. Phys.

J. Appl. Phys.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Improved crystal quality of semipolar (10 $\bar{1}$ 3) GaN on Si(001) substrates using AlN/GaN superlattice interlayer

Growth of semipolar (10 $\bar{1}$ 3) GaN on Si(001) with interlayers