Crystallinity and microstructures of aluminum nitride films deposited on Si(111) substrates
Introduction
Hexagonal 2H–AlN is a candidate material for high power, optical, and acoustic wave device applications. This is due to its high, direct band gap energy, high acoustic wave velocity and its solubility in GaN that allows the formation of AlGaN alloys and AlGaN/GaN heterojunctions. In addition, AlN is considered as a suitable intermediate buffer layer for GaN growth on Si(111) substrates. The epitaxial growth of AlN and GaN on Si(111) is of particular interest for potential integration of AlN and/or GaN devices with Si-based devices in one chip. However, low-defect, single-crystal epitaxial AlN is difficult to grow on Si(111), since the lattice mismatch is 18.8%.
The epitaxial relationship of a hexagonal AlN film and a cubic Si(111) substrate has been observed. Chubachi et al. [1] used metal-organic chemical vapor deposition and obtained single-crystal AlN(0001) on Si(111) with the AlN[11-20]//Si[110] relationship. The same crystallographic relationship was observed by Meng et al. [2] using reactive sputtering of AlN. Bourret et al. [3] used gas source MBE to grow AlN on Si(111) and observed two epitaxial relationships in one film, namely, AlN[2-1-10]//Si[02-2] and AlN[10-10]//Si[02-2]. Sanchez-Garcia et al. [4] deposited AlN on Si(111) by plasma-assisted molecular beam epitaxy and showed a single-crystal X-ray diffraction pattern of AlN. Although single-crystal-like AlN films have been obtained when deposited directly on Si, they either contained high densities of dislocations of approximately 3×1011/cm2 [2] or lacked long range order [3].
To improve the AlN crystallographic quality, attempts have been made to find a suitable buffer layer between the AlN and the Si substrate. Barski et al. [5] and Tamura and Hiroyama [6] used a thin (40 Å) crystalline layer of 3C–SiC(100) on Si(100) on which they deposited cubic AlN and GaN. Hong et al. [7] also used a thin 3C–SiC buffer layer for the deposition of hexagonal AlN on Si(111). The use of a 3C–SiC buffer layer between AlN(0002) and Si(111) seems to be a good approach for improving crystal perfection of the AlN, since the 3C–SiC(220) spacing (1.54 Å) is much closer to the AlN(11-20) spacing (1.56 Å) than the Si(220) spacing (1.92 Å) is to AlN(11-20).
This study investigated the crystallinity and the microstructures of MOCVD AlN films deposited on Si(111) with and without a buffer layer(s). The suitability of the deposited AlN for use as a substrate for GaN growth was also evaluated.
The AlN films were deposited using an RF heated, single-wafer, vertical, low-pressure chemical vapor deposition reactor, triethylaluminum (TEA) and ammonia as the source gases and H2 as the carrier and diluent gas. The films were grown at 1100°C, and 45 torr. The gas flow rates were 0.134, 0.067, and 26×10−6 mol/min, respectively, for H2, NH3 and TEA. This yielded an AlN growth rate of approximately 11 Å/min.
The typical substrates used were Si(111) on which had been grown a thin (30–50 Å) 3C–SiC buffer layer via conversion of the surface by exposure to flowing ethylene at 970°C in an MBE reactor [8]. The two AlN films deposited on these substrates will be referred to as samples A1 and A2.
The second substrate used was bare Si(111). The film deposited on this substrate will be referred to as sample B. The third substrate was a film stack consisting of Si (on the bottom), SiC, AlN, GaN, and AlxGa1−xN alloys with x graded from 0 to 1. The layer thicknesses in the stack were 40 Å, 1000 Å, 5000 Å and 2000 Å, respectively, for SiC, AlN, GaN and the AlxGa1−xN alloys. The AlN film deposited on this film stack was referred to as sample C.
Section snippets
Results
The top surfaces of samples A1 and A2 were much rougher than those of samples B and C. The roughness values were measured by an atomic force microscope for the maximum thickness variation (Zmax) and root mean square thickness variation (rms) within an area of 1×1 μm and 5×5 μm. The results are tabulated in Table 1. The results show that sample B deposited directly on Si(111) had the smoothest surface. A comparison of surface morphologies for sample A1 and sample B is shown in Fig. 1(a) and (b).
Discussion
The microstructure in the AlN films observed in this study may have resulted from the specific deposition conditions employed or as a result of the substrate surfaces having only a thin film of converted SiC rather than a thicker (0.2–0.5 micron) deposited layer of SiC or for both reasons. Our previous study [8] showed that the thickness of converted 3C–SiC varied greatly within a film and its top surfaces were very rough. The rough surfaces likely provided a large number of low energy sites
Conclusion
The growth of highly textured 2H–AlN(0001) films on Si(111) substrates was significantly improved by the use of a 3C–SiC buffer layer. The buffer layers were formed by the conversion of Si(111) surfaces without an additional deposition of a 3C–SiC layer by CVD growth. The top surfaces of the converted SiC layers were very rough and resulted in the nucleation, growth and coalescence of the polycrystalline AlN films. The AlN grains were well aligned crystallographically. The crystallographic
Acknowledgements
The authors of NCSU acknowledge the support of the Semiconductor Research Corporation and Motorola Inc. R. Davis was supported in part by Kobe Steel, Ltd. University Professorship.
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