Elsevier

Applied Surface Science

Volume 362, 30 January 2016, Pages 434-440
Applied Surface Science

Controlling the stress of growing GaN on 150-mm Si (111) in an AlN/GaN strained layer superlattice

https://doi.org/10.1016/j.apsusc.2015.11.226Get rights and content

Highlights

  • The interface of the 80-paired AlN/GaN SLS was periodical.

  • Adjusting the thickness of the GaN layer in the SLS controls stress of epilayer.

  • The compressive stress in the SLS exists during growth and after cool down.

  • Unintended AlGaN played a critical role in reducing the mismatch in SLS.

  • The AlGaN efficiently accumulated stress without causing relaxation in the SLS.

Abstract

For growing a thicker GaN epilayer on a Si substrate, generally, a larger wafer bowing with tensile stress caused by the mismatch of thermal expansion coefficients between GaN and Si easily generates a cracked surface during cool down. In this work, wafer bowing was investigated to control stress by changing the thickness of a GaN layer from 18.6 to 27.8 nm in a 80-paired AlN/GaN strained layer superlattice (SLS) grown on a 150-mm Si (111) substrate. The results indicated that wafer bowing was inversely proportional to the total thickness of epilayer and the thickness of the GaN layer in the AlN/GaN SLS, since higher compressive stress caused by a thicker GaN layer during SLS growth could compensate for the tensile stress generated during cool down. After returning to room temperature, the stress of the AlN/GaN SLS was still compressive and strained in the a-axis. This is due to an unintended AlGaN grading layer was formed in the AlN/GaN SLS. This AlGaN layer reduced the lattice mismatch between AlN and GaN and efficiently accumulated stress without causing relaxation.

Graphical abstract

The strain state in the AlN/GaN SLS was caused by the diffusion of Al from AlN into GaN in the SLS. The unintended AlGaN played a critical role in reducing the mismatch between the AlN and GaN layers, and efficiently accumulated stress without causing relaxation in the AlN/GaN SLS.

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Introduction

In recent years, considerable attention has increased on GaN-based high electron mobility transistors (HEMTs) formed using AlGaN/GaN. Because the GaN-based HEMT shows a lower on-resistance, higher power efficiency, and faster switching speed compared to the conventional silicon power devices, it is expected to improve power conversion efficiency and contribute to module miniaturization [1]. In view of mass production, suitable substrates for growing a GaN epilayer are sapphire, silicon carbide, and Si substrates. Recently, an increasing number of studies are focusing on a GaN epilayer grown on a Si substrate, which has the advantages of lower cost, a larger wafer size, and favorable thermal conductivity. However, the mismatch of in-plane lattice constants and thermal expansion coefficients (TECs) between GaN and Si are two major obstacles that must be overcome during epilayer growth. The in-plane lattice mismatch of −16.94% can easily cause defects to affect the crystalline quality of the GaN epilayer grown on a Si (111) substrate and the performance of the GaN HEMT device. For growing a thicker GaN epilayer on a Si substrate, tensile stress caused by a 55% mismatch of the TEC generates a cracked surface during cool down process. To solve this, a thinner GaN epilayer, a thicker Si substrate, or a smaller Si substrate can be used. However, these solutions do not have any advantage in device manufacturing and performance.

Various researchers have been working on growing thick GaN epilayers on Si substrates for the applications of high breakdown voltage devices. Each group has developed its own stress controlling technology. In 1999, Ishikawa et al. grew a GaN epilayer on a Si (111) substrate by using AlGaN/AlN transition layer [2]. Dadgar et al. demonstrated strain relaxation by inserting a low-temperature AlN layer [3]. Because the relaxed AlN layer had smaller lattice constant, compressive stress was induced in the subsequently grown GaN layer. The induction of compressive stress is required for the growth of thick and crack-free GaN on Si (111) where thermal stress must be compensated for. Feltin et al. found that the insertion of an AlN/GaN strained layer superlattice (SLS) could reduce tensile stress for avoiding crack formation [4]. Selvaraj et al. demonstrated that an AlGaN/GaN HEMT on Si (111) with an AlN/GaN SLS exhibited a breakdown voltage as high as 1.4 kV in 2012 [5]. Ubukata et al. obtained abrupt interfaces in SLS and observed the strained stress of an AlN/GaN SLS by using asymmetrical reciprocal-lattice space mapping [6]. Ni et al. determined that compressive stress in GaN was introduced by the relatively thicker AlN layer in an AlN/GaN SLS, which compensated for the tensile stress in the top GaN during cooling [7]. The compressive strain of the GaN in the SLS was linearly dependent on the relative thickness and partially relaxed AlN in the SLS. The aforementioned literatures indicate that SLS plays a major role in strain control to grow the thick GaN on a Si (111) substrate.

Although many studies have examined the stress in an AlN/GaN SLS, the stress-control mechanism in an AlN/GaN SLS has not yet been ascertained. Therefore, this paper is aimed to (a) investigate a controllable stress by adjusting the thickness of the GaN layer in an AlN/GaN SLS, and to examine the stress in an AlN/GaN SLS during its growth and after cooling; and (b) explore the mechanism for stress accumulation in the AlN/GaN SLS interface without relaxation.

Section snippets

Experimental procedure

A GaN epilayer with an AlN/GaN SLS was grown on a 150-mm Si (111) substrate by using a metal-organic chemical vapor deposition system produced by Hermes-Epitek. The thickness of the 150-mm Si substrate was 675 μm. Trimethylgallium, trimethylaluminum, and ammonia were used as a precursor for gallium, aluminum, and nitrogen, respectively. Hydrogen was used as the carrier gas. The sequences for growing the epi-structure are described as follows: An AlN buffer layer of 150 nm was grown on the Si

Results and discussion

The BF-STEM image in Fig. 1(b) reveals a distinct interface, especially the interfaces in the 80-paired AlN/GaN SLS. A ω–2θ scan of the GaN (002) plane observed through X-ray diffraction is shown in Fig. 2(a). The scan indicates clear fringes, demonstrating that the interfaces of the AlN/GaN SLS were periodic. Growing a 100-nm GaN layer on an AlN buffer layer can smooth the rough AlN buffer layer to obtain flat surface for subsequent SLS growth. During hetero-epitaxial AlN layer growth, limited

Conclusion

In conclusion, the 3873-nm GaN epilayer grown on a 150-mm Si (111) substrate without crack was investigated using 80 pairs of AlN/GaN in the SLS. The interface of the AlN/GaN SLS was periodic. The compressive stress from −0.298 to −0.534 GPa generated during the growth of SLS was controllable by adjusting the thickness of the GaN layer from 18.6 to 27.8 nm in the AlN/GaN SLS. The thicker layer of 27.8 nm could compensate for the tensile stress caused by mismatch in TECs between the GaN epilayer

Acknowledgments

We would like to thank Mr. Chih Sheng Wu of Hermes-Epitek Corporation (Taiwan, R.O.C.) for his encouragement and discussions for MOCVD growths. This work was supported by the Ministry of Science and Technology (Taiwan, R.O.C.) under contract nos. 101-2221-E-005-023-MY3 and 104-2622-E-005-005-CC2.

References (14)

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