Al2O3

Al2O3

Superlattices and Microstructures 39 (2006) 429–435 www.elsevier.com/locate/superlattices Interfaces in heterostructures of AlInGaN/GaN/Al2O3 Shengqi...

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Superlattices and Microstructures 39 (2006) 429–435 www.elsevier.com/locate/superlattices

Interfaces in heterostructures of AlInGaN/GaN/Al2O3 Shengqiang Zhou a,∗ , M.F. Wu a , S.D. Yao a,∗∗ , J.P. Liu b , H. Yang b a School of Physics, Peking University, Beijing 100871, People’s Republic of China b National Research Center for Opto-Electronic Technology, Institute of Semiconductors, Chinese Academy of Sciences,

Beijing 100083, People’s Republic of China Received 7 June 2005; received in revised form 16 September 2005; accepted 10 October 2005 Available online 11 November 2005

Abstract Rutherford backscattering/channeling (RBS/C) and X-ray diffraction (XRD) are used to comprehensively characterize a heterostructure of AlInGaN/GaN/Al2 O3 (0001). The AlInGaN quaternary layer was revealed to process a high crystalline quality with a minimum yield of 1.4% from RBS/C measurements. The channeling spectrum of 1213 exhibits higher dechanneling than that of 0001 at the interface of AlInGaN/GaN. XRD measurements prove a coherent growth of AlInGaN on the GaN template layer. Combining RBS/C and XRD measurements, we found that the interface of GaN/Al2 O3 is a nucleation layer, composed of a large amount of disorders and cubic GaN slabs, while the interface of AlInGaN/GaN is free of extra disordering (i.e. compare with the GaN layer). The conclusion is further evidenced by transmission electron microscopy (TEM). c 2005 Elsevier Ltd. All rights reserved.  Keywords: Nitride semiconductors; Interface; Rutherford backscattering/channeling; Transmission electron microscopy; X-ray diffraction

1. Introduction In the last decade, InGaN-based ternary alloys attracted a great deal of interest because of their success in blue light emission diodes, in which the InGaN layer normally bears a compressive stress due to the lattice mismatch between InGaN and GaN crystals. The use of AlInGaN quaternary alloys allows the strain and bandgap engineering by varying Al and In content [1]. ∗ Corresponding author. ∗∗ Corresponding author. Tel.: +86 10 62757534.

E-mail addresses: [email protected] (S. Zhou), [email protected] (S.D. Yao). c 2005 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter  doi:10.1016/j.spmi.2005.10.003

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Therefore, the growth and characterization of AlInGaN films play a key role for the further application of GaN-based wide bandgap semiconductors. Rutherford backscattering (RBS) is a powerful technique to directly and nondestructively characterize the composition, the thickness and the interface of films [2]. When the sample is a single-crystal solid, and if the ion beam enters the sample along a high-symmetry crystal direction, a phenomenon called “channeling” occurs. The incident ion beam will undergo a series of correlated small angle scattering along the atom chains and consequently the backscattering events are greatly reduced compared to the backscattering events when the ion beam is directed in a random direction. χmin , the ratio of the backscattered yield when aligned to a crystallographic axis (Y A ) to that to the random direction (Y R ), is a measure of the crystalline quality of the film. It is typically calculated behind the surface peak. Given its depth dependent nature, RBS combined with channeling can give the surface and interface structures. Moreover, the channeling measurement can be performed both along normal and off-normal axes, consequently distinguishing the disorders along different directions. The RBS/C method has been applied to measure the distortion in thin films, and to locate the impurity atom introduced by implantation, which are already comprehensively addressed in the handbook [3]. In this paper, we applied RBS/C to characterize AlInGaN/GaN/Al2 O3 heterostructures, especially the dechanneling behavior at interfaces. Additionally, XRD measures the lattice parameters for the AlInGaN layer both in perpendicular and parallel directions. Combining RBS/C, XRD and transmission electron microscopy (TEM) techniques, the structures at these two interfaces (AlInGaN/GaN and GaN/Al2 O3 ) were revealed. 2. Experiment The quaternary AlInGaN epilayers were grown by low pressure metal-organic chemical vapor deposition using a thick GaN layer as a template. The details of the growth process can be found in Ref. [4]. For RBS/C measurements, a collimated 1.57 or 2.00 MeV He+ beam was used. The sample was mounted on a high precision (±0.01◦) three-axis goniometer in a vacuum chamber so that the orientation of the sample relative to the He+ beam could be precisely controlled. The X-ray measurements were performed on a high resolution X-ray diffractometer equipped with a four-crystal monochromator in Ge(220) configuration and one or two 200 µm slits before the detector. Monochromatic Cu kα1 X-rays (λ = 0.15406 nm) were the incident light. 3. Results and discussion Fig. 1 shows the random and aligned backscattering spectra of the AlInGaN/GaN sample. The geometry and energy of He+ used in the backscattering measurements are shown in the inset. The arrows (labeled with In, Ga and Al) indicate, respectively, the energy for backscattering from In, Ga, and Al atoms at the surface. The 0001 aligned spectrum indicates that the χmin of the AlInGaN layer is 1.4%, which is a typical value for high-quality GaN layers grown on sapphire. From the simulation results by RUMP code [5], the In and Al contents in the AlInGaN layer are determined to be 0.013 and 0.35, respectively, and the thickness of the AlInGaN and GaN layers is 120 nm and 380 nm, respectively. 3.1. Interfaces in AlInGaN/GaN/Al2O3 Additionally, χmin can be defined at a given backscattering energy which corresponds to a specific depth. The increase of χmin at a deeper place is dechanneling and is caused by the fact

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Fig. 1. Random (), 0001 axis aligned (+), and simulated (solid line) RBS spectra of an AlInGaN/GaN/Al2 O3 heterostructure. The backscattering geometry is shown inset.

that a number of channeled ions scatter and diverge into a non-channeling direction with higher backscattering chance, or by an increasing of defects at a corresponding depth. Fig. 2 shows the random and aligned spectra along different channeling directions with 1.57 MeV He+ as the incident beam. Different regions of the specimen are indicated in the figure. Note that the sample was offset by ∼32◦ from its normal when collecting the 1213 aligned spectrum, and in that case a given depth in 1213 aligned spectrum corresponds to a lower energy than that of the 0001 aligned spectrum. Obviously at the GaN/Al2 O3 interface, both channeling spectra show a very high dechanneling, which comes from the nucleation layer with a lot of defects and cubic GaN slabs. However, at the AlInGaN/GaN interface, the 0001 aligned spectrum shows no detectable dechanneling, while drastic dechanneling was in the 1213 aligned spectrum. The different dechanneling behaviors indicate that at the AlInGaN/GaN interface there is no different Ga atom disorder along the 0001 direction, but much different along 1213. The angle between 0001 and 1213 in the AlInGaN layer is determined to be 31.79◦ by an angular scan along the AlInGaN {1010} plane (Fig. 3), while the angle between 0001 and 1213 in bulk GaN is 31.59◦. This motivated one to postulate the dechanneling along the 1213 at the AlInGaN/GaN interface to be caused by the kink at the interface of the heterostructure [6,7]. This postulation is evidenced below by XRD measurements. 3.2. Lattice parameter High resolution XRD is a powerful tool to precisely determine the lattice constants of thin crystal film. In this work, the lattice constants of the AlInGaN layer are calculated from symmetric (0004) and skew symmetric (1015) 2θ/ω scans. Fig. 4 shows the 2θ/ω scans for the AlInGaN/GaN heterostructures. The dominant peaks are from thick GaN layers, while the satellite peaks at higher angles are from AlInGaN layers. First, taking the Al2 O3 (0006) as a reference, the GaN layer is found to be fully relaxed. In the lattice spacing calculations for the AlInGaN layer, GaN peaks are taken as references. By using the method given by Wu et al. [8], the perpendicular lattice constant cepi of AlInGaN can be calculated from the 2θ/ω scan of (0002) (Fig. 4(a)), while combining a skew symmetric 2θ/ω scan of (1015) diffraction (Fig. 4(b)), the

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Fig. 2. RBS spectra along the 0001, 1213 channeling direction and a random direction at around the 1213 axis. Abnormally high dechanneling is observed along the 1213 direction at the AlInGaN/GaN interface.

Fig. 3. Angular scan along the AlInGaN {1010} plane (open circles: experimental data and solid lines: Gaussian fitting). The angle between 0001 and 1213 is determined to be 31.79◦ .

parallel lattice constant aepi can be obtained. For this specimen, the lattice parameters, cepi and ˚ and 3.191 ± 0.004 A, ˚ respectively. aepi, are 5.131 ± 0.001 A The AlInGaN layer is coherently grown on GaN template, given the a lattice parameter ˚ for GaN material. Fig. 5 schematically shows the lattice configuration at the of 3.189 A AlInGaN/GaN interface and its influence on the channeling. The angle between 0001 and 1213 varies from 31.88◦ for AlInGaN to 31.59◦ for GaN. The 0.29◦ “kink” angle sufficiently introduces the extra dechanneling in the 1213 aligned spectrum [6,7]. Additionally a further and direct confirmation is given below by TEM.

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Fig. 4. XRD 2θ/ω-scan of AlInGaN/GaN (0002) (a) and (1015) (b).

Fig. 5. Schematic drawing for the lattice strain at the AlInGaN/GaN interface, and its influence on channeling.

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Fig. 6. Bright-field cross-section image for AlInGaN/GaN/Al2 O3 heterostructure under (0002) two-beam diffraction conditions. Note the high density of defects at the GaN/Al2 O3 interface, while no extra defects are present at the AlInGaN/GaN interface.

3.3. TEM Fig. 6 is a bright-field image taken under two-beam diffraction condition with the g vector of 0002. An extremely high density of defects is observed In the GaN/Al2 O3 interface, above which threading dislocations are present with a density of 109 /cm2 below the sensitivity of RBS/C [9] and have no influence on channeling. Selected area diffraction (not shown) at the GaN/Al2 O3 interface indicates an inclusion of cubic GaN, which is quite common in the GaN growth on Al2 O3 [10,11]. At the AlInGaN/GaN interface, there is no significant contrast, i.e. no significant extra defect. This observation directly confirmed the conclusion that lattice strain induced the extra dechanneling in the 1213 direction. 4. Conclusion In this article, we have shown a comprehensive study on AlInGaN/GaN/Al2 O3 heterostructures by RBS/C, XRD and TEM. The GaN/Al2 O3 interface is composed of an extremely high density of disorders, while the AlInGaN layer is coherently grown on GaN and their interface is free of extra defects. This study again proves RBS/C as a powerful method in characterization of multi-layered thin films. Acknowledgments The authors thank Dr. B. van Daele for TEM measurement. This work was supported by the Bilateral Cooperation between China and Flanders (BIL 02-02 and BIL 04-05) and National Natural Science Foundation of China under Grant No. 10375004. References [1] M.E. Aumer, S.F. LeBoeuf, S.M. Bedair, M. Smith, J.Y. Lin, H.X. Jiang, Appl. Phys. Lett. 77 (2000) 821. [2] W.K. Chu, J.W. Mayer, M.A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1978.

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[3] J.A. Davies, Handbook of Modern Ion Beam Materials Analysis, Materials Research Society, Pittsburgh, 1995. [4] J.P. Liu, B.S. Zhang, M. Wu, D.B. Li, J.C. Zhang, R.Q. Jin, J.J. Zhu, J. Chen, J.F. Wang, Y.T. Wang, H. Yang, J. Cryst. Growth 260 (2004) 388. [5] L.R. Doolittle, Nucl. Instrum. Methods B 9 (1985) 344. [6] J.H. Barrett, Phys. Rev. B 28 (1983) 2328. [7] W.K. Chu, C.K. Pan, C.A. Chang, Phys. Rev. B 28 (1983) 4033. [8] M.F. Wu, S.Q. Zhou, S.D. Yao, Q. Zhao, A. Vantomme, B.V. Daele, E. Piscopiello, G.V. Tendeloo, Y.Z. Tong, Z.J. Yang et al., J. Vac. Sci. Technol. B 22 (2004) 920. [9] W.K. Chu, F.W. Saris, C.A. Chang, R. Ludeke, L. Esaki, Phys. Rev. B 26 (1982) 1999. [10] X.H. Wu, D. Kapolnek, E.J. Tarsa, B. Heying, S. Keller, B.P. Keller, U.K. Mishra, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 68 (1996) 1371. [11] L. Cheng, K. Zhou, Z. Zhang, G. Zhang, Z. Yang, Y. Tong, Appl. Phys. Lett. 74 (1999) 661.