SCIENCE CHINA Physics, Mechanics & Astronomy. Study on growing thick AlGaN layer on c-plane sapphire substrate and free-standing GaN substrate

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1 SCIENCE CHINA Physics, Mechanics & Astronomy Article December 2012 Vol.55 No.12: doi: /s z Study on growing thick AlGaN layer on c-plane sapphire substrate and free-standing GaN substrate WANG DangHui 1,2*, ZHOU Hao 1, ZHANG JinCheng 1, XU ShengRui 1, ZHANG LinXia 1, MENG FanNa 1, AI Shan 1 & HAO Yue 1 1 State Key Lab. of Fundamental Science on Wide Band-Gap Semiconductor Technology; School of Microelectronics, Xidian University, Xi an , China; 2 School of Materials Science and Engineering of Xi an ShiYou University, Xi an , China Received May 23, 2012; accepted July 15, 2012; published online November 21, 2012 In this study, the thick AlGaN epilayers have been grown on the c-plane sapphire substrate and the free-standing GaN substrate using low-temperature AlN nucleation layers by low-pressure metal-organic chemical vapor deposition (LPMOCVD). High resolution X-ray diffraction (HRXRD), atom force microscopy (AFM), scanning electron microscopy (SEM), photoluminescence (PL) and Raman scattering measurements have been employed to study the crystal quality, threading dislocation density, surface morphology, optical properties and phonon properties of thick AlGaN epifilms. The results indicate that AlGaN epifilms crystal quality can be improved greatly when grown on the free-standing GaN substrate. We calculated the threading dislocation density and found that thick AlGaN epifilm grown on the free-standing GaN substrate is much lower in total threading dislocation density than that grown on the sapphire substrate, although the surface morphology is rougher than that of sapphire substrate. metal-organic chemical vapor deposition, crystal quality, surface morphology, AlN nucleation layer PACS number(s): Kk, 78.30Fs, Cr Citation: Wang D H, Zhou H, Zhang J C, et al. Study on growing thick AlGaN layer on c-plane sapphire substrate and free-standing GaN substrate. Sci China-Phys Mech Astron, 2012, 55: , doi: /s z 1 Introduction In the past decade, III-nitride (GaN, AlN and InN) and its compounds have been established as materials of extreme significance for next generation high-density power devices. They are used in microwave telecommunications in radar and mobile operations [1,2]. AlGaN alloys are important for optoelectronic devices such as LEDs and photodetectors in the UV spectral region between the range of 200 nm to 365 nm, which can be tuned according to the band gap of Al- GaN [3]. Despite the rapid commercialization of III-nitride semiconductor devices for applications in visible, ultraviolet *Corresponding author ( wdhyxp@163.com) optoelectronics, high-power and high-frequency electronics, their full potential is limited by two primary obstacles: a high defect density and biaxial strain due to the heteroepitaxial growth on foreign substrates, which result in poor performance and shortened device lifetime. In order to obtain high-quality epitaxial layers of III-nitrides, many research groups have adopted methods or technology to reduce the threading dislocation density and strain/stress due to the larger mismatch in lattice constant and thermal expansion coefficient between the epitaxial layer and substrate [4 8]. Recent advances in the research, and commercial production of native GaN substrates with low defect density and high crystal and optical quality have created opportunities to overcome these obstacles and have led to significant progress in the development of several optoelectronic and Science China Press and Springer-Verlag Berlin Heidelberg 2012 phys.scichina.com

2 2384 Wang D H, et al. Sci China-Phys Mech Astron December (2012) Vol. 55 No. 12 high-power devices. Thus, growing AlGaN epifilms on free-standing GaN substrate is a wiser choice. Ref. [9] found that the threading dislocation density should be eliminated using the GaN substrate, and compared with sapphire and SiC substrate, careful optimization of nucleation and buffer layer growth on sapphire and SiC substrate, and obtaining 10 7 cm 2 dislocation density is possible, but the cost is still too high for laser diodes with high output power, and improvements of the crystal quality of AlGaN alloys are still needed. Relying on high growth rates typically used in hydride vapor phase epitaxy (HVPE), free-standing GaN substrate attracts recently significant attention mostly because of its ability to produce high quality III-nitrides thick films. Up to now, there have been few general studies on the crystal quality and surface morphology for thick AlGaN layer grown on free-standing GaN substrate. In this paper, a comparison of the threading dislocation density and crystal quality with AlGaN thick layer grown on the c-plane sapphire substrate and GaN substrate is made; we obtain higher crystal quality thick AlGaN epitaxial films grown on the free-standing GaN substrate, although the surface morphology is rougher than that of sapphire substrate. Figure 1 Cross-section structures of sample A (a) and sample B (b). diffraction (HRXRD), atom force microscopy (AFM), scanning electron microscopy (SEM), photoluminescence (PL) and Raman scattering. For the purpose of revealing the complex defects, and unstanding thoroughly the microstructure of AlGaN and its correlation with the growth dynamics, we measured X-ray rocking curves (XRCs) for both symmetry and asymmetry diffraction planes by HRXRD. The HRXRD was performed using Bruker D8-discover system equipped with Ge (220) monochromator and channel-cut analyzer, delivering a pure Cu K line of wavelength = nm. A 325-nm line of He-Cd laser was used as an excitation source to measure Raman scattering for the two thick AlGaN layers. 2 Experimental procedure The two-step growth method or growth using nucleation layer has now become a standard method for heteroepitaxial growth of III-Nitride thin films. A 15-nm-thick low-temperature (LT) AlN nucleation layer was first deposited on (0001) sapphire at 660 C. Then a 1200-nm-thick hightemperature (HT) AlN nucleation layer epifilms was deposited on LT-AlN nucleation layer at 1100 C. At last, a 1200-nm-thick AlGaN epifilm was deposited at 1030 C denoted as sample A. Sample B has the same structure and growth processing with sample A instead of growing on the free-standing GaN substrate. Cross-section structures of sample A and sample B are shown in Figure 1. The asgrown samples were characterized by high resolution X-ray 3 Results and discussions 3.1 High resolution X-ray diffraction The 2 - scan XRD profiles for the two samples are shown in Figure 2(a) and (b). The peak located at is diffraction from the AlGaN (002) planes of sample A, obviously. As can be seen from Figure 2(b), a shoulder peak appears at about the GaN (002) peak from sample B. We separate the AlGaN (002) peak from GaN (002) peak using triple-axis X-ray diffraction, and find that the AlGaN (002) peak is located at and the GaN (002) peak is located at Using Vegard s law, the Al mole fraction x in thick Al x Ga 1 x N layer was calculated from the observed peaks shown in Figure 2 to be about 7.0% for the two samples. Figure scan XRD profiles for sample A and sample B of AlGaN (002) planes. (a) Double-crystal X-ray diffraction; (b) triple-axis crystal X-ray diffraction for sample B.

3 Wang D H, et al. Sci China-Phys Mech Astron December (2012) Vol. 55 No Figure 3 shows measured symmetrical (002) and asymmetrical (102) omega-scan HRXRD full-width at halfmaximum (FWHM) of the two AlGaN epitaxial layers. A comparison of the FWHM of sample A with sample B is made. We observed that FWHM of X-ray rocking curve (XRC) for (102)-reflecting plane of AlGaN epitaxial layers can be reduced significantly by using the free-standing GaN substrate. Previously, it has been demonstrated that the FWHM of XRC for the asymmetric (102)-reflecting plane is directly influenced by all threading dislocations, including edge dislocations [10,11], and the smaller FWHM of XRC suggests that we reduce the edge dislocation density through using the free-standing GaN substrate. Therefore, we believe that the improved crystal quality is related to the alleviation of lattice mismatch between AlGaN and the freestanding GaN substrate. Furthermore, we extracted the values of FWHM from sample A and sample B shown in Figure 3 and the calculated results are listed in Table 1. Eq. (1) is used to be described as the relationship between the dislocation density and FWHM values of XRCs [12,13]. 2 2 s e s,. 2 e (1) c 4.35a Here, s and e are the screw and edge threading dislocation densities, respectively; the quantities s and e refer to the FWHM of (002) and (102), respectively, c and a are the relevant Burgers vectors of the AlGaN epifilms. The calculated results of threading dislocation densities are also shown in Table 1. From Table 1, the X-ray rocking curve FWHM of the (102) planes for sample B (244.4 arcsec) is much smaller than that of sample A (1447 arcsec), while sample A has a smaller FWHM of the (002) planes rocking curve than sample B. We calculated the threading dislocation density from measured results and found that the edge dislocation is the major component in sample A, while the screw dislocation is the major component in sample B. In addition, the total threading dislocation density for sample B is much lower than sample A, which means that sample B has a higher crystalline quality than sample A. From the analysis results above, we can draw a conclusion that thick AlGaN layer with higher crystal quality and lower dislocation density can be obtained by growth on the free-standing GaN substrate. 3.2 AFM and SEM surfaace morphology Figure 4 shows the 5 m 5 m AFM and SEM images of the two thick AlGaN samples. The difference in surface morphology is clearly seen from Figures 4(a) (f). One can see from these images that the thick AlGaN surface morphology in sample B is much rougher and has larger fluctuation than that in sample A. Figures 4(c) and (d) show the 3D surface morphology of the two thick AlGaN layer samples. The root mean square (RMS) value is nm for sample A and 27.4 nm for sample B, which indicates that sample A has a smoother surface morphology than sample B does. We believe it is the free-standing HVPE-GaN substrate that induced these morphology differences between sample A and sample B. This result can be confirmed by the SEM analysis in Figures 4(e) and (f). As can be seen, there appear many irregular convex disks on the two samples, and the number of irregular convex disks for sample B is bigger than for sample A. 3.3 Photoluminescence and Raman scattering We carried out the PL measurement to examine the optical Figure 3 X-ray rocking curve for (002) reflection for sample A and sample B. (a) is the (002) planes and (b) is the (102) planes. Table 1 Experimental results of the φ and ω scan FWHM values and the calculated threading dislocation densities of sample A and sample B Sample HRXRD FWHM of XRC (arcsec) (002) (102) s (10 8 cm 2 ) s (10 9 cm 2 ) (10 9 cm 2 ) A B

4 2386 Wang D H, et al. Sci China-Phys Mech Astron December (2012) Vol. 55 No. 12 Figure 4 (Color online) AFM and SEM images of 5 m 5 m Surface morphology for sample A and sample B. (a) Surface morphology of sample A; (b) surface morphology of sample B; (c) 3D surface morphology of sample A (RMS=0.253 nm); (d) 3D surface morphology of sample B (RMS=27.4 nm); (e) SEM image of sample A; (f) SEM image of sample B. properties of the two AlGaN samples at room temperature. The PL spectra of sample A and sample B are shown in Figure 5 for comparison. As it is shown, a spectral peak is at nm/3.67 ev for sample A and sample B, which is the corresponding band emission peak of the two thick AlGaN samples. The Al mole fraction was calculated by compositional dependence of the optical band gap of ternary alloys: E ( x) E x E (1 x) bx(1 x). (2) g,algan g,aln g,gan Here, E g, x and b are the optical band gap, Al molar fraction and bowing parameter, respectively. E g, GaN, E g, AlN and b is 3.4, 6.2 and 0.9 ev, respectively [14]. Using eq. (2), we calculated the Al mole fraction as 13.25% for the two samples. Ref. [15] studied this difference between XRD and PL in detail, and thought it is the variable bowing parameter b for the whole composition range and larger value for smaller aluminum fraction that made this difference. Raman scattering was performed to measure the two samples recorded in the z(xx)-z backscattering configuration with 325-nm wavelength excitation at room temperature. The selection rules for Raman-active phonons mode can be determined by standard group-theoretical methods. At room temperature, the A 1 (LO) mode is the so-called one-mode Figure 5 PL spectra recorded at room temperatures for sample A and sample B. behavior [16]. According to the PL spectra shown in Figure 5, the band emission of AlGaN is at nm, which is close to the laser excitation wavelength. Thus we think that there are resonant Raman scattering phonon mode A 1 (LO) peaks in the present thick AlGaN epifilms. As is shown in Figure 6, the E 2 phonon of thick AlGaN layer is much

5 Wang D H, et al. Sci China-Phys Mech Astron December (2012) Vol. 55 No PL and Raman scattering measurements have been employed to study the crystal quality, surface morphology, optical properties and strain condition of thick AlGaN layer. The conclusion indicates that AlGaN epifilms crystal quality can be improved greatly by growth on free-standing GaN substrate. Through calculating the threading dislocation density we have found that thick AlGaN epifilms grown on the free-standing GaN substrate have much lower total threading dislocation density than grown on the sapphire substrate, although the surface morphology is rougher than that of sapphire substrate. Figure 6 Raman scattering for sample A and sample B at room temperature. weaker than the A 1 (LO) phonons under the present resonant condition and invisible on the intensity scale. According to ref. [17], about the relationship between A l (LO) resonant Raman shifts and Al mole fraction x, = x 814.7x 2, we calculated the theoretical A 1 (LO) phonon shift of cm 1 in the two samples, which is a little larger than sample A (763.9 cm 1, FWHM=28.71 cm 1 ) and a little smaller than sample B (769.4 cm 1, FWHM=21.23 cm 1 ). Ref. [18,19] indicates that this biaxial strain can be reflected using the Raman shift of the A 1 (LO) phonon peaks. In the following discussion concerning the strain/stress condition, as the lattice constant of AlGaN epilayer is smaller than sapphire and GaN, there exists tensile strain/ stress in the two thick AlGaN samples, and we predict that the tensile strain/stress in sample A is larger than in sample B because of a larger lattice mismatch in sample A. Ref.[20] shows that the AlN interlayers apply a compressive strain to the AlGaN layer and the compressive strain was found to increase with interlayer thickness. Thus total strain/stress in AlGaN layer for sample A is different from that for sample B. The A 1 (LO) phonon shifts to blueshift compared with the theoretical value cm 1 in sample B, while sample A has the opposite strain/stress condition and the A 1 (LO) phonon shifts to redshift compared with the theoretical value cm 1. We believe that it is the sapphire substrate and free-standing GaN substrate that make the different strain/stress condition change induced by lattice-mismatch and thermal-mismatch between the substrate and AlGaN thick layers during the growth process. 4 Conclusions In summary, the thick AlGaN epilayer using low-temperature AlN nucleation layers grown on c-plane sapphire substrate and free-standing GaN substrate have been investigated by LPMOCVD in this study. HRXRD, AFM, SEM, This work was supported by the National Key Science and Technology Special Project, China (Grant No. 2008ZX ), the Major Program and State Key Program of the National Natural Science Foundation of China (Grant Nos and ), and the Fundamental Research Funds for the Central Universities, China (Grant No. JY ) 1 Nuttinck S, Gebara E, Laskar J, et al. Development of GaN wide bandgap technology for microwave power applications. IEEE Microwave Magazine, 2002, 3: Kuzuhara M, Miyamoto H, Ando Y, et al. High-voltage rf operation of AlGaN/GaN heterojunction FETs. Phys Stat Sol A, 2003, 200: Iwaya M, Terao S, Sano T, et al. Suppression of phase separation of AlGaN during lateral growth and fabrication of high-efficiency UV-LED on optimized AlGaN. J Cryst Growth, 2002, 237: Hussein A S, Hassan Z, Thahab S M, et al. Structural, optical and electrical properties of undoped and Si-doped Al x Ga 1 x N thin films on Si(111) substrate grown by PA-MBE. 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