ISSN 0020-1685, Inorganic Materials, 2007, Vol. 43, No. 4, pp. 331–337. © Pleiades Publishing, Inc., 2007. Original Russian Text © S.A. Denisov, S.P. Svetlov, V.Yu. Chalkov, V.G. Shengurov, D.A. Pavlov, E.V. Korotkov, E.A. Pitirimova, V.N. Trushin, 2007, published in Neorganicheskie Materialy, 2007, Vol. 43, No. 4, pp. 391–398.
Structural Perfection of Heteroepitaxial Silicon Layers Grown on Sapphire by Sublimation-Source Molecular Beam Epitaxy S. A. Denisov, S. P. Svetlov, V. Yu. Chalkov, V. G. Shengurov, D. A. Pavlov, E. V. Korotkov, E. A. Pitirimova, and V. N. Trushin Research Physicotechnical Institute, Lobachevski State University, pr. Gagarina 23/3, Nizhni Novgorod, 603950 Russia e-mail:
[email protected] Received July 21, 2006
Abstract—Structurally perfect, single-crystal silicon layers have been grown on (1102 ) sapphire by sublimation-source molecular beam epitaxy. Electron and x-ray diffraction data demonstrate that silicon-on-sapphire epitaxy occurs at substrate temperatures from 550 to 850°C. As the thickness of the layers decreases from 1.0 to 0.2 µm, their structural perfection degrades. In the layers grown at 600°C, the density of nucleation sites in the initial stages of growth is 5 × 109 cm–2. DOI: 10.1134/S0020168507040012
INTRODUCTION Molecular beam epitaxial (MBE) growth of silicon on (1102 ) sapphire substrates prepared by a special procedure was described in [1, 2]. Prior to growth, the substrates were annealed in situ at ts = 1450°C for 30 min (optimized conditions). The growth temperature was 670 [1] or 750°C [2]. Submicron (100) silicon layers were grown via electron-beam evaporation of a silicon source in a vacuum of 1 × 10–7 Pa. The quality of the layers was characterized by the microtwin density and microtwin differential volume fraction (MDVF). The highest MDVF was found about 20 nm from the film–substrate interface. The maximum MDVF in the MBE-grown film was an order of magnitude lower than that in a silicon-on-sapphire (SOS) structure grown by vapor phase epitaxy. The effect of process parameters (substrate surface condition, growth temperature and rate, pressure and composition of the residual gas) on the defect structure of silicon epilayers on sapphire is an important characteristic of the epitaxy process. Such data are crucial for assessing the potential of a growth process for practical application and must be obtained for a wide range of growth conditions. Although MBE-grown SOS structures offer good crystallinity, their morphology has not yet been studied in sufficient detail [1]. Little work has been directed toward the study of the initial stages of SOS MBE. The above-mentioned high-temperature annealing was reported to notably accelerate the coalescence process and reduce the surface roughness of the resulting silicon films [3]. Silicon coalescence occurred at 50-nm film thicknesses, and 200-nm-thick layers had smooth
surfaces. Although these thicknesses are well below those reported previously, growth of smooth submicron-thick layers continues to be a challenge. In this context, an important point is to study in detail the initial stages of SOS growth. The objective of this work was to investigate the structural perfection and morphology of silicon layers grown on ( 1102 ) sapphire by sublimation-source MBE in relation to the annealing temperature of the substrate and growth temperature, and to analyze in detail the initial stages of SOS growth. EXPERIMENTAL We used ( 1102 ) sapphire substrates manufactured by Monokristall Co. (Stavropol). After cleaning by acetone and isopropanol and rinsing in H2O, the substrates were mounted in an MBE chamber. The chamber design was described elsewhere [4]. As a silicon/dopant source, we used a rectangular-shaped silicon bar 90 × 4 × 4 mm in dimensions, which was heated resistively to a temperature of 1380°C. The substrate, 35 × 35 mm in dimensions, was first heated to 1300–1450°C in 30 min by a purpose-designed radiative heater [5] and then cooled to 550–850°C. Next, the shutter was opened, and silicon layers were grown at a rate v = 1 µm/h. The microstructure of the silicon layers was studied by electron diffraction, atomic force microscopy (AFM), and x-ray diffraction (XRD). Electron backscattered diffraction patterns were taken on an EMR102 unit (Russia) at an accelerating voltage of 50 kV. Surface morphology was examined by AFM on an
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(c) Fig. 1. Electron diffraction patterns from the surface of substrates annealed at (a) 1350 and (b) 1450°C and (c) from a silicon layer.
Accurex TMX-2100 instrument. XRD patterns were collected on a two-crystal x-ray spectrometer with Cu K α1 radiation in an (n; –n) geometry, using an Ge511 monochromator (2θ511 = 90.05°). The elemental composition of the films was determined by Auger electron spectroscopy (AES) using an Omicron MultiProbe integrated ultrahigh vacuum system. The primary electron energy was 3 keV. AES depth profiles were obtained using Ar+ ion milling (ISE-10 ion gun, accelerating voltage of 5 kV, current from 10 to 30 µA). RESULTS AND DISCUSSION Effect of annealing on the structure of sapphire. In preparing the sapphire surface for epitaxy, the final step was high-temperature vacuum annealing, as in earlier studies [1–3]. Since we used a special substrate heater design, it was necessary to optimize substrate
annealing conditions. Particular attention was paid to structural and morphological changes in the near-surface region. The effect of annealing on the structure and morphology of the substrate surface was assessed in air after cooling. The annealing temperature was varied from 1200 to 1450°C. Electron diffraction patterns from the surface of substrates annealed at 1350 and 1450°C for 30 min (Fig. 1) showed only Kikuchi lines and Kikuchi bands. Both before and after annealing, the substrate surface was mirror-smooth, but AFM examination revealed some morphological changes. The effect of substrate annealing on the structure of silicon epilayers reflects whether or not the substrate surface is properly prepared for the growth of singlecrystal layers and is, therefore, the most rigorous criterion in deciding whether or not a surface preparation procedure is suitable for MBE. The growth temperature was 700°C. As will be shown below, this temperature INORGANIC MATERIALS
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Fig. 3. Effect of layer thickness on the structural perfection of the silicon grown at 700°C.
insures the growth of structurally perfect epilayers on properly prepared substrates. Heat treatment of substrates at temperatures from 1300 to 1450°C was found to have no effect on the structural perfection of silicon layers: the full width at half maximum of their rocking curves was ≤24′ and was independent of the annealing duration. Annealing leads to recrystallization (reconstruction) of the thin sapphire layer deformed by polishing. In addition, high-temperature annealing is needed to remove surface contamination, e.g., carbon. The development of steps on the sapphire surface during annealing may also have a positive effect on the epitaxial process. Even though no growth steps were detected by AFM after annealing, AFM examination of a thin silicon layer grown on sapphire after high-temperature (1450°C) annealing revealed oriented rectangular silicon islands (see below). After anneals at lower temperatures, no oriented islands were detected. Thus, given that only thermal evaporation can be used in the final step of surface preparation for epitaxial growth, the substrate annealing conditions that we selected were nearly optimal. Effect of growth temperature on the structure of silicon layers. This issue is commonly addressed by determining the lowest epitaxy temperature and the temperature at which structurally perfect layers can be grown. In our studies, the effect of substrate temperature on the structural perfection of silicon layers on sapphire was analyzed using electron diffraction and XRD data. In a series of experiments, 0.5-µm-thick silicon layers were grown at substrate temperatures from 550 to 850°C. According to electron diffraction data, all of the layers grown in this temperature range were singlecrystal. Electron diffraction patterns showed only Kikuchi lines and bands. Even though the Kikuchi lines in the diffraction patterns of the layers grown at 550°C were not very sharp, no diffraction rings from polycrystalline material were detected. With increasing growth
temperature, the Kikuchi patterns became sharper, attesting to a better structural perfection of the silicon layer.
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XRD data for SOS structures (Fig. 2) indicate that the most structurally perfect silicon layers grow at sub4
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Fig. 5. AFM images illustrating the silicon island distribution over the surface of SOS structures; the layers were grown at 600°C on a ( 1102 ) sapphire substrate annealed at 1400°C; growth time τ = (a) 20, (b) 80, and (c) 260 s.
strate temperatures from 700 to 750°C. The value of ∆ω1/2 in such structures is 16′ (Fig. 2). With decreasing growth temperature, this parameter increases. The layers grown at higher temperatures also have poorer structural perfection. Note that a nonmonotonic variation of ∆ω1/2 with ts was also observed at larger silicon thicknesses (d = 1 µm) in SOS structures. The effect of substrate temperature on silicon epitaxy on sapphire with the use of a silicon sublimation source was studied in detail by Chang [6]. According to his results, silicon layers grown on ( 1102 ) sapphire substrates were single-crystal in the range ts = 600– 850°C. Below 600°C, polycrystalline films were obtained. At substrate temperatures from about 850 to 925°C, the growing layer was faceted. Heating to above 925°C led to etching of the substrate.
Similar results were obtained in this study. Chang [6], however, did not anneal his substrates at 1450°C, which seems to account for the discrepancy between his results on low-temperature growth and ours. In our experiments, lowering the growth temperature to 550°C had an adverse effect on the structural perfection of the layers, but they remained single-crystal. At the same time, an additional orientation, (110), was found in those layers. The development of an additional orientation was also reported by Chang [6], who pointed out that the nucleation rate of (110) crystallites dominated at low temperatures. Note that, in sublimation-source MBE, the minimal growth temperature of silicon layers on sapphire is lower (ts = 550°C) than that in conventional MBE (ts = 650°C), where silicon is commonly evaporated by an INORGANIC MATERIALS
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Fig. 6. AFM images of 1-µm-thick silicon layers grown on sapphire at (a) 600, (b) 700, and (c) 800°C.
electron beam [1, 2]. The likely reason for this is that silicon sublimation produces, for the most part, atomic beams [7]. Electron-beam evaporation produces more polyatomic species, and their condensation in an incoherent position leads to defect formation. Moreover, the background doping level in sublimation-source MBE layers is lower than that in layers grown by electronbeam evaporation of silicon: ≤2 × 1013 [8] and ≤5 × 1014 cm–3 [9], respectively. Figure 3 plots ∆ω1/2 versus layer thickness (0.2 to 1.0 µm) for ts = 700°C. As seen, the structural perfection of the layers degrades with decreasing thickness. Note, however, that even thin (0.2 µm) layers are single-crystal: electron diffraction patterns of their surface show Kikuchi lines and point to (100) orientation. Surface morphology of SOS structures. To elucidate the mechanism of SOS MBE, we investigated the INORGANIC MATERIALS
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initial stages of silicon growth. Silicon layers of different thicknesses were grown at ts = 600 and 700°C on sapphire annealed at 1450°C for 30 min. To this end, during silicon deposition, parts of the substrate were masked at regular intervals. The morphology of the grown layers was examined by AFM. Using SPMLab_NT_5.0 software, we evaluated the density and average size of islands as functions of growth time (v = 1 µm/h) for layers of different thicknesses. Figure 4 plots the average size and density of silicon islands versus growth time. The average island size is seen to increase with growth time, while the island density decreases. We also find that, at a given deposition time, the average island size increases with substrate temperature, while the island density drops. For example, at a deposition time of 10 s, the island size is 0.08 µm at ts = 600°C and 0.15 µm at ts =
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Fig. 7. AES depth profiles of oxygen and aluminum in SOS structures grown by (a) vapor phase epitaxy and (b) MBE.
700°C; the island density is 3.6 × 109 and 1.2 × 109 cm–2, respectively. On the surface of samples grown at 600°C in 30 s (which corresponds to a layer thickness d 8 nm), the island density is 1 × 109 cm–2. Using linear extrapolation to zero layer thickness, we find that the density of nucleation sites is 5 × 109 cm–2, which is close to that in layers grown by vapor phase epitaxy using silane [10]. Figure 5a shows an AFM image of a thin (5.5 nm) silicon layer on sapphire. The silicon islands, rectangular in shape and uniform in area, are seen to be well aligned. The preferential orientation can be explained as follows. Before deposition, the sapphire surface is always slightly (≤0.5°) misoriented with respect to atomic planes, which leads to a terrace surface structure. As pointed out by Yoshimoto et al. [11], annealing (0001) sapphire at 1400°C gives rise to significant changes in its surface morphology, resulting in a transition from a disordered surface to monatomic steps. The edges of the steps are the most stable growth sites for migrating silicon atoms. Consequently, silicon atoms attach predominantly at the edges of steps, leading to the formation of rectangular islands.
Figure 5 shows AFM images of silicon layers of different thicknesses, which provide clear evidence for island growth of silicon on sapphire. Therefore, the initial stages of silicon growth on sapphire follow a threedimensional mechanism. A continuous silicon layer is formed at a thickness of 50 nm. The surface morphology of SOS structures was found to depend on the growth temperature (Fig. 6). The smoothest layers were obtained at low substrate temperatures. With increasing temperature, the layer surface becomes rougher because the island density rises with decreasing growth temperature (Fig. 4). The surface of the layers grown at 600°C is smoother compared to the layers grown at 700°C because of the higher density and smaller size of the islands. Self-doping. Reducing the deposition temperature in MBE notably slows down the self-doping process, as evidenced by AES data. Figure 7 presents AES data for SOS structures grown by vapor phase epitaxy and MBE. In the MBE-grown SOS structures, no aluminum was detected in the silicon layer, whereas the vapor phase deposited silicon layers contained aluminum throughout the thickness of the layer (Fig. 7). The likely reason for this is that the growth temperature in vapor phase epitaxy (1000°C) is higher than that in MBE. CONCLUSIONS Sublimation-source MBE offers the possibility of growing structurally perfect submicron-thick (100) silicon layers at temperatures from 600 to 800°C on (1102) sapphire annealed at high temperature (1450°C). At a substrate temperature of 550°C, the layers are bioriented, (100) + (110). The most structurally perfect silicon layers on sapphire were grown at substrate temperatures between 700 and 750°C. According to XRD data, reducing the thickness of silicon layers from 1.0 to 0.2 µm impairs their structural perfection, but even thin layers grow epitaxially. In the initial stages of growth, the average size of Si islands increases with increasing substrate temperature, while their density drops. The density of nucleation sites is 5 × 109 cm–2, which is close to that in layers grown by vapor phase epitaxy using silane. In the initial stages of growth, the silicon islands are aligned along the steps produced on the sapphire surface by high-temperature annealing. With increasing deposition time, the island growth of silicon gives way to layer-by-layer growth. A continuous silicon layer is formed at a thickness of 50 nm. The surface of the silicon layers grown at low temperatures (600°C) is rather smooth. With increasing growth temperature, it becomes rougher. AES data for the MBE-grown SOS structures indicate that there is no self-doping with aluminum, in contrast to structures grown by vapor phase epitaxy. INORGANIC MATERIALS
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