Author’s Accepted Manuscript Ge distribution in Si0.9Ge0.1 alloy ingot grown from thin melt layer Michael A Gonik, Arne Cröll, Amalia Ch Wagner www.elsevier.com/locate/moem PII: DOI: Reference: S2452-1779(16)30084-6 http://dx.doi.org/10.1016/j.moem.2016.12.001 MOEM40 To appear in: Modern Electronic Materials Received date: 21 November 2016 Accepted date: December 2016 Cite this article as: Michael A Gonik, Arne Cröll and Amalia Ch Wagner, Ge distribution in Si0.9Ge0.1 alloy ingot grown from thin melt layer, Modern Electronic Materials, http://dx.doi.org/10.1016/j.moem.2016.12.001 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain Ge distribution in Si0.9Ge0.1 alloy ingot grown from thin melt layer Michael A Gonik1*, Arne Cröll2, Amalia Ch Wagner3 Centre for Material Researches “Photon”, 10 Cheska Lipa Str., Aleksandrov, Vladimir Region 601655, Russia Institute för Geosciences of University of Freiburg, Hermann-Herde-Straβe, Freiburg 79104, Germany Institute för Inorganic and Analytical Chemistry, 21 Albertstraβe, Freiburg D-79104, Germany * Corresponding author michael.a.gonik@gmail.com Abstract We have studied experimentally and theoretically the possibility to obtain a uniform single crystal of SiGe alloy enriched at the Si side The content of the second component in a crystal 15 mm in diameter and 40 mm in length grown by the modified floating zone technique from the charge of 79.8 at.% Si and 20 at.% Ge composition with 0.2% B admixture has been investigated using selected area X-ray analysis in different points and in line scanning mode along and across the crystal axis The longitudinal changes in the germanium concentration proved to be well described by the analytical equation previously derived for conditions of Sb (Ga) doped Ge growth from a thin melt layer in the presence of a heater submerged into the melt For a more accurate description of the experimental data we have made allowance for the change in the melt layer thickness between the growing crystal and the bottom of the submerged heater The lateral distribution of the second component not exceeding 5% over the crystal diameter can be significantly improved by reducing the curvature of the phase interface during the growth Keywords Setup and modified floating zone technique of crystal growth, Submerged into the melt heater, Silicon, Alloy with germanium, X-ray microanalysis, Crystal homogeneity Introduction Si0.9Ge0.1 solid solution crystals find broad application e.g as high-temperature thermoelectric in photodetectors and solar cells Homogeneous composition Si0.9Ge0.1 crystals can be used as references for the characterization of e.g Seebeck’s contact potential difference or heat conductivity Furthermore they can be used as reference specimens in X-ray microanalysis (EDX) The technology of acceptable quality SiGe single crystals is complex Si1-xGex solid solutions at this quality level have already been grown using Czochralsky, Bridgman and floating zone techniques [1-7] However, these crystals have large inhomogeneous regions containing structural defects Quite promising results were reported [8, 9] for homogeneous SiGe crystal growth under microgravity conditions with the zone melting method It is however not likely to expect similar results for large crystals, especially on Earth Since recently we have been developing a new approach to the growth of SiGe crystals combining the advantages of using a submergible melt heater [10, 11] and the vertical zone melting technique This method [12] which we called the modified floating zone method is of special interest for obtaining SiGe melt enriched at the silicon side because in that case crucible cannot be used For the axial heat processing (AHP) method, installing an additional heater near the crystallization front close to the growing crystal dramatically changes the convection pattern In the thin melt layer between the heater and the phase boundary, natural convection is almost completely suppressed, the only type of convection being slight laminar convection originating from crystal pulling to the cold zone of the growth chamber On the other hand, the presence of the heater affects the shape of the crystallization front making it more flat due to the far lower radial temperature gradient compared to the axial one This allows developing favorable conditions for the growth of binary and ternary compounds [13, 14] the crystalline structure of which is sensitive to the magnitude of thermal stress A typical feature of crystallization from a thin melt layer is a relatively short time of transient processes [15] This feature allows one to bring the effective segregation coefficient to in a relatively short time and, given a specific concentration ratio of the feeding material and the material in the layer in front of the growing layer, to achieve a constant impurity concentration [16] Based on the results of Si0.05Ge0.95 growth simulation [17], prerequisites for the implementation of the AHP method were specified and heat conditions were determined providing for a constant concentration of the second component along the phase boundary and hence a homogeneous second component distribution in the bulk of the growing crystal [18] Later [19] capillary stability conditions were studied for the growth of silicon, germanium and their Si0.8Ge0.2 alloy with the AHP zone melting method, and relationships were determined between the main crystallization parameters: growth rate, crystal diameter, growth layer thickness and feeding material supply rate The composition of the SiGe crystal grown with the modified floating zone method [12] and its structural perfection were studied in detail elsewhere [20] Below we will analyze the longitudinal and lateral germanium distribution patterns Crystal Growth and Characterization SiGe crystals were grown by zone melting with radiation heating (Fig a) The lamp radiation is focused on the graphite casing of the AHP heater (Fig b) and is partially incident on the feeding charge rod and the pulled crystal located above and below the AHP heater, respectively These rods melt to form the melt feeding zone and the melt layer from which the crystal grows, these zones being held at the AHP heater by surface tension The crystal was grown from the charge of 79.8 at.% Si and 20 at.% Ge composition with 0.2% B admixture at pulling rates of 1.2 to 2.4 mm/h without rotation of the crystal and the seed The seed was a oriented single crystal silicon rod The method, growth setup and the experiment were described in greater detail elsewhere [12, 20] The growth experiment stages are traced in Fig After the feeding charge rod melted and the melt filed the gap between the AHP heater and the seed (Fig a) we kept that temperature of the system at a constant level in order to melt the seed (Fig b) We increased the volume of the molten zone under the heater and started the growth (Fig c and d) trying to keep the melt layer height h constant To finish crystallization by separating the crystal from the AHP heater surface we reduced the crystal diameter and the melt layer height (Fig e) To this end we had to increase the pulling rate and hence the growth rate Table shows the variation of h in the course of the experiment Fig (a) Radiation furnace and (b) modified floating zone technique growth experiment setup Fig Growth experiment stages Table Melt layer thickness h during crystal growth Growth Stage as per Fig Parameter b c d Crystal Length, 1.5 3.0 7.0 mm h, mm 5.5 7.0 7.5 e 14.0 4.0 The SiGe ingot appearance was polycrystalline (Fig a) However, an optical micrograph (IC) of the crystal cut along the growth direction (Fig b and c) shows that the ingot bulk is actually single crystal The initial part of the single crystal contains a large agglomeration of etch pits However, polycrystalline regions are only present on the ingot surface where they formed as a result of melt solidification at the side of the cylindrical silicon seed (Fig a) that did not melt The melt could not dissolve the surface of the seed (Fig c) because the temperature there was far lower than in the center A fully polycrystalline region formed near the end of the crystal (Fig c) after the morphological stability of crystal growth was violated as a result of growth rate increase by several times Fig (a) Si—Ge crystal, (b) EDX linear scanning (gray line) along the As−grown crystal and (c) IC micrograph near the Si deed: Black line: Ge measurements across the crystal for z = 1.5 mm, points at the edge and on the crystal axis correspond to the same growth band; LS are measurement points The composition and structural perfection of the crystal were studied using Raman spectroscopy and photoluminescence; the latter technique and its results were analyzed in detail earlier [20] The germanium distribution along the specimen was studied using X-ray microanalysis (EDX) in several points and in linear scanning mode Results and Discussion The data on Ge concentration in several points on the longitudinal cut of the specimen are summarized in Table and graphically presented in Fig b along the crystal axis (gray line) and across it (black line) at 1.5 mm from the seed/crystal interface Table Selected area analysis data on Ge content in SiGe crystal Distance from seed, mm Point number Ge concentration, % Specimen edge 0.1 LS 4(1) 5.28 0.5 LS 4(2) 5.42 0.9 LS 4(3) 5.56 1.3 LS 4(4) 5.73 1.5 LS 4(5) 5.71 Specimen center 0.5 LS 3(5) 5.44 7.3 LS 7(1) 7.96 7.6 LS 7(2) 8.19 7.9 LS 7(3) 8.26 8.2 LS 7(4) 8.56 8.5 LS 7(5) 8.73 8.8 LS 7(6) 8.63 9.1 LS 7(7) 9.15 9.4 LS 7(8) 9.54 9.7 LS 7(9) 8.81 10.0 LS 7(10) 10.07 Longitudinal Segregation To keep germanium concentration constant along the crystal axis one should keep the effective segregation coefficient equal to That is, germanium concentration in the melt layer under the AHP heater from which the crystal grows (C1) should correlate with germanium concentration in the growing crystal (CS) as follows: CS = C1k0 The germanium content in the feeding charge rod C2 was 20 at.% Thus, in order for the crystal to be homogenous in length the germanium concentration in the melt layer under the AHP heater should be C1 = 47 at.% which can be achieved e.g by placing a tablet with the abovementioned germanium concentration in the gap between the seed and the AHP heater Without it the gap is filled with the feeding melt where the germanium concentration is the same as in the feeding charge rod (C2) Finally, except the small transition zone near the pure silicon seed the initial germanium concentration (Fig b) in the crystal CS(t0) was 7% This result is in a good agreement with the theoretical one for k0(Ge) = 0.25 in this region To describe the lateral distribution of the second component for growth from thin melt layer one can use the relationship derived earlier [21] for Sb and Ga impurities in Ge single crystal grown with a submergible heater: 1 exp k z z0 Cz (z) Cs (t ) C2 k0C1(t ) h (1) According to this relationship one can find the CS(z) concentration at an arbitrary point z in the crystal knowing the impurity concentration in the crystal at the point z0 at the initial moment of time t0 and the thickness h of the melt layer under the AHP heater Curve in Fig for constant h = mm describes the Ge distribution in the specimen adequately well However, one can achieve a better fit by allowing for h variation during crystallization, i.e by using the actual melt layer thickness data (Table 2), since h increased in the first 10 and then was constant for 30 This resulted in slight stagnation in the growth of germanium concentration in the 3-10 mm crystal section The theoretical germanium concentration curve (Fig 4, curve 3) compared with curve exhibits a clear Ge concentration growth in the ingot paralleled with a rapid decrease in the melt layer thickness that started close to the end of crystallization Analysis was restricted to Ge concentration distribution in the first 15 mm of the crystal Later the crystallization rate was higher (by several orders of magnitude at the end of growth) so Eq (1) is not applicable under these conditions Fig Longitudinal Ge distribution in the Si-Ge crystal: (1) Experimental data, (2) Calculation using Eq (1), (3) Calculation with allowance for change in melt layer thickness h during crystallization Curve in Fig for the bulk beyond that region shows that the transition process ends at z ~ 35 mm Thus, for this experimental technique, one should cut the first 30-40 mm of the crystal to obtain a longitudinally homogeneous SiGe alloy plate Otherwise the Ge concentration in the melt under the AHP heater should be chosen depending the desired solid solution composition following the relationship C1 = C2/k0 Lateral Segregation The radial Ge segregation in the ingot is not high As can be seen from Fig 5, germanium concentration measured across the crystal at two points z = 1.5 and mm is within 3-6% except the leftmost point A in curve falling out visibly from the data for z = 1.5 mm Ignoring this point is quite justified because it is beyond the single crystal section of the ingot where segregation pattern may be different Furthermore, linking the data to a curve replicating the phase boundary shape rather than to the crystal radius (for a constant coordinate z) one can obtain a good agreement between the Ge concentration data These curves are well decorated by the growth striations originating from boron admixture to the charge These striations can be well seen in Fig b For example, one of them beginning at the crystal edge pertains to the transverse green striation at x = 1.5 mm passing through the point This striation crosses the crystal axis at the point 3, i.e at x = mm The concentrations at the points B and C in curves and corresponding to x = 1.5 and mm in Fig 5, respectively, are almost equal, this being in agreement with an earlier theoretical study [17] This is further confirmed by X-ray microanalysis (Table 2) The Ge concentrations at the points LS 3(5) and LS 4(2) in Fig c pertaining to different radii but located at the same distance from the seed melting front differ by only 0.02% Fig Transverse Ge distribution in the Si-Ge crystal: (1) 1.5 mm from growth start point, (2) 5.0 mm from growth start point, (3) crystallization front curvature allowance, (A) outlier in curve 1, (B) and (C) points belonging to the same growth band at the edge and in the center of the crystal, respectively (shown as points and in Fig 3) The reconstructed curve in Fig is plotted so that the peripheral data begin from the point at z = 1.5 mm, following which the data are taken from the crystallization front (along the growth band) and finally from the center at z = mm Thus one can assume that if the phase boundary shape was sufficiently flat (at least not worse than at the left-hand side) the crystal would be quite homogeneous in the radial direction Summary In this work we made the first attempt to grow a homogeneous SiGe crystal using the floating zone method in a radiation heated zone melting furnace Based on the visual and optical studies and the data on the physical parameters of the specimen we conclude that the method is suitable for single crystal growth The polycrystalline regions form in the crystal at an early stage of crystal growth when the side part of the silicon seeding rod is not melted The germanium distributions along the crystal axis and in the radial direction are in agreement with the heat and mass transfer regularities 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Role and Degrees Michael A Gonik1: Cand Sci (Eng.), Director (michael.a.gonik@gmail.com); Arne Cröll2: Prof Dr rer nat., Dipl.-Min., Director of Institute; Amalia Christina Wagner3: PhD, student .. .Ge distribution in Si0. 9Ge0 .1 alloy ingot grown from thin melt layer Michael A Gonik1*, Arne Cröll2, Amalia Ch Wagner3 Centre for Material Researches “Photon”, 10 Cheska Lipa Str.,... Azhdarov G Kh Distribution of aluminum and indium impurities in crystals of Ge- Si solid solutions grown from the melt Crystallography Reports 2006, vol 51, pp S192-S195 DOI: 10 .11 34/S1063774506070273... a Si0. 5 0Ge0 .50 crystal by the traveling liquidus-zone (TLZ) method in microgravity J Cryst Growth 2 014 , vol 388, pp 12 -16 http://dx.doi.org /10 .10 16/j.jcrysgro.2 013 .11 .020 10 Ostrogorsky A G Single-crystal