Method of varying the crystalline structure of or the concentration of impurities contained in a tubular starting crystal or both using diagonal zone melting
United States Patent Keller [4 Apr. 8, 1975 [5 METHOD OF VARYING THE 2.471.437 5/1949 Lester 23/273 CRYSTALLINE STRUCTURE or OR THE 85:32? b g s an un a CONCENTRATION OF IMPURITIES 3.238.024 3/1966 Cremer et at 23/301 CONTAINED IN A TUBULAR. STARTING 3.258.314 6/1966 Redmond et al. 23/301 CRYSTAL 0R BOTH USING DIAGONAL 3.423.l89 1/1969 Pfann 23/301 ZONE MEL&#39;UNG 3.454.367 7/1969 Reuschel 23/301 3.505.032 Bennett 23/30] [75] Inventor: Wolfgang Keller, Pretzfeld. Germany [73] Assignee: Siemens Alttiengesellschaft, Berlin and Munich. Germany [22] Filed: July 3], 1972 [2]] Appl. No: 276,306  
  Related U.S. Application Data {63} Continuation of Ser. No. 872,279. Oct. 29. 1969.  
 abandoned.  
 [30] Foreign Application Priority Data Oct. 30, W68 Germany l80597l [52] U5. Cl 23/30l SP; 23/273 SP; 2l9/l0.43 [51] Int. Cl B0lj l7/l0 158] Field of Search 23/301 SP, 273 V. 273 SP; 2l9/l0.43  
 [56] References Cited UNITED STATES PATENTS l.892.lit)6 l/l933 Pederscn 23/273 FOREIGN PATENTS 0R APPLICATIONS 75.36l 7/1954 Netherlands 2l9/l0.43  
 Primary Examiner-Norman Yudkoff Assistant E.\&#39;aminerR. T. Foster Attorney, Agent, or Firm-Herbert L. Lerner [57] ABSTRACT Method of varying the crystalline structure of or the concentration of impurities contained in a tubular starting crystal includes heating the tubular starting crystal to a temperature at which an annular molten zone is formed therein and passing the annular molten zone through the tubular starting crystal in the direction of the longitudinal axis thereof.  
 7 Claims. 8 Drawing Figures METHOD OF VARYING THE CRYSTALLINE STRUCTURE OF OR THE CONCENTRATION OF IMPURITIES CONTAINED IN A TUBULAR STARTING CRYSTAL OR BOTH USING DIAGONAL ZONE MELTING This is a continuation, of application Ser. No. 872,279, filed Oct. 29, I969, and now abandoned.  
  My invention relates to method of varying the crystalline structure of a tubular starting crystal or the concentration of impurities contained therein or both.  
  Tubular monocrystals of semiconductor material, such as silicon particularly, having considerable length such as 50 cm, for example, and considerable diameter such as 25 cm, for example, as well as a relatively thin wall thickness, such as mm, for example, are required, for example, in Rontgen spectroscopes. Such tubular monocrystals can be produced by the method in copending application Ser. No. 872,278, for example, which has been simultaneously filed with the hi stant application and of which l am a coinventor. Specific concentrations of impurities is the tubular monocrystal are also desirable.  
  Such tubular monocrystals can be produced by mechanical processing from rod-shaped monocrystals previously obtained by crucible-free zone melting. A central opening must be bored through the rod-shaped monocrystal in the axial direction thereof. This operation is very costly and time-consuming especially for relatively long and thick rod-shaped monocrystals because of the attendant danger that the wall of the hollowed-out tubular monocrystal will burst or collapse due to crack-formation therein, unless great care is taken in performing the boring operation. Furthermore, the tubular monocrystal becomes contaminated by the material of the boring tool and by the coolant employed during the boring operation. In addition, the diameter of rod-shaped monocrystals produced by cruciblefree, floating-zone melting or by pulling from a melt contained in a crucible is limited to a maximum of about 7.5 cm.  
  It is an object of my invention to provide method of varying the crystalline structure of or the concentration of impurities contained in a tubular starting crystal or both which avoids the aforementioned difficulties of heretofore known methods of producing tubular monocrystals.  
  With the foregoing and other objects in view, I pro vide, in accordance with my invention, method of varying the crystalline structure of or the concentration of impurities contained in a tubular starting crystal, which comprises heating the tubular starting crystal to a temperature at which an annular molten zone is formed therein, and passing the annular molten zone through the tubular starting crystal in the direction of the longitudinal axis thereof.  
  Other features which are considered as characteristic for the invention are set forth in the appended claims.  
  Although the invention is illustrated and described herein as method of varying the crystalline structure of or the concentration of impurities contained in a tubular starting crystal or both, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalent of the claims.  
  The invention, however, together with additional ob jects and advantages thereof, will be best understood from the following description when read in connection with the accompanying drawings in which:  
  FIG. 1 is a longitudinal view, partly in section, of a tubular crystal and apparatus for passing a molten zone therethrough in accordance with the method of my invention;  
  FIG. 2 is a cross-sectional view of FIG. I taken along the line II II in the direction of the arrows;  
  FIG. 3 is another view similar to that of FIG. 1 showing modified apparatus for performing another mode of the method of the invention performed by the apparatus of FIG. 1;  
  FIG. 4 is a perspective view of the heating device forming part of the apparatus of FIG. 3;  
  FIGS. 5 and 5a are top plan and perspective views, respectively, of a special heating device for carrying out the method of my invention according to FIGS. 1 and 3&#39;, and  
  FIGS. 6 and 7 are longitudinal sectional views ofa tubular crystal and apparatus for performing another mode of the method of my invention.  
  Referring now to the drawings, and first particularly to FIGS. 1 and 2 thereof, there is shown a tubular starting crystal 101 having a circular cross section and formed of silicon. Such tubular starting crystals, which can either have varying cross sections or uniformly equal cross section over the entire length thereof. can be produced by boring through a polycrystalline rod produced by precipitation or deposition of semiconductor material as a result of a reaction of a gaseous semiconductor compound with a heated rod-shaped carrier member of the same semiconductor material as described in German Pat. No. 1,061,593. Such tubular starting crystals can also be produced by the method of the aforementioned copending application simultaneously filed with the instant application which discloses a method of precipitating a layer of semiconductor material from a gaseous compound of the semiconductor material, wherein I deposit on the outer surface ofa heated cylindrical carrier member ofa different resistant material a layer of semiconductor materal of predetermined thickness from a gaseous compound of the semiconductor material and, after the semiconductor layer has been deposited, mechanically or chemically removing the semiconductor layer without destroying or damaging the shape thereof.  
  The starting crystal 101, as shown in FIG. I, is secured in an end holder 102 by three screws 103 spaced circumferentially at equal angles from one another. In the interest of clarity, the holder 102 and the starting crystal 101 are shown partly broken away and in section in FIG. 1, and only two of the three screws I03 are visible in the figure.  
  A relatively thin monocrystalline seed crystal 107, which is, in turn, fastened at one end to a holder 108, is fused at the other end thereof to a part I06 of the lower end surface, as shown in FIG. 1, of the tubular starting crystal 101. The tubular crystal 101 is surrounded by an elliptical heating device I04, whose shape is seen more clearly in FIG. 2 which is a cross section of FIG. 1, taken at the level of the heating device 104. the heating device 104 of FIGS. I and 2 is in the form of a coil having a single winding supplied with high frequency alternating current but may also be any suitable electrically heated radiant heating device. The  
 heating device 104 heats a molten zone 105 formed in the tubular starting crystal 101 in the shape of ellipse, as shown in FIG. 2.  
  The elliptical molten zone 105 is disposed in a plane which is neither parallel nor perpendicular to the axis 109 of the tubular starting crystal 101. Starting from the location 106 at which the rod-shaped seed crystal 107 is fused to the lower surface of the tubular starting crystal 101, as shown in FIG. 1, the molten zone 105 is passed at least once through the starting crystal 101, in accordance with the method ofinvention. Ifthe concentration distribution of impurities contained in the starting crystal 101 is to be varied, several passes of the molten zone 105 through the crystal 101 may be required.  
  For this purpose, the tubular starting crystal 101 and the heating device 104 of elliptical shape. are moved relative to one another in the relative direction of the arrow 112 so that the axis of the starting crystal 101 is inciined at an angle varying between and 90 with respect to the major axis 110 of the elliptical heater winding 104, as shown in FIG. I, and is perpendicular to the minor axis 111 of the elliptical winding 104 as shown in FIG. 2. The angle of inclination of the axis 109 to the major axis 110 of the ellipse is advanta geousiy within the range of to 65 and is preferably 45.  
  The heating device can also be in the form of a flat coil of elliptical cross section, having several windings disposed in one plane or a coil with several elliptical windings in the shape of an inclined cylinder having elliptical top and bottom end surfaces. both of the coils surrounding the tubular starting crystal and being supplied with high-frequency alternating current. In the case of such coils also, the tubular starting crystal 101 and the coil are relatively displaced so that the axis 109 of the crystal 101 is inclined at an angle between 0 and 90 with respect to the major axis of the elliptical cross section of the coil and is perpendicular to the minor axis of the ellipse.  
  Due to this inclination of the axis 109 of the tubular starting crystal 101 to the major axis of the elliptical cross section of the heating device 104, assurance is provided that the material in the wall of the tubular starting crstyal 101 recrystallizes uniformly and as a monocrystal when the molten zone 105. starting from the location 106 at which the monocrystalline seed crystal 107 is fused thereto. is passed therethrough.  
  An electron beam or a plasma discharge are also suited for heating the molten zone 105. Furthermore. it may also be advantageous to dispose additional heat ing devices. such as coils or radiant heaters energized by highfrequency alternating current, above or below or both above and below the heating device 104, so as to thereby respectively preheat the tubular starting crystal I01 prior to the molten zone pass therethrough and/or postheat the recrystallized monocrystal after the molten zone pass therethrough. The dislocation density in the recrystallized monocrystalline material can thereby be especially reduced.  
  It is desirable to dispose the heating device 104 in a horizontal plane so that the molten zone 105 is also located in a horizontal plane. whereby assurance is provided that the molten semiconductor material will not drip out of the molten zone 105 during passage of the molten zone through tubular starting crystal 101.  
  To vary the wall thickness of the tubular starting crystal 101, the spacing between the ends of this crystal 101 can be varied durng the molten zone pass therethrough. To thicken the wall of the tubular starting crystal 101, the holders 102 and 108 and accordingly the ends of the starting crystal 101 are displaced in the axial direction toward one another during the zonemelting pass. The tubular wall of the crystal 101 is correspondingly made thinner by displacing the holders 102 and 108 and accordingly the ends of the tubular crystal 101 away from one another in the axial direction of the crystal 101.  
 In the apparatus embodiment of FIG. 3, wherein like elements are identified by the same reference numerals as in FIGS. 1 and 2, the tubular starting crystal 101 and an annular heating device 304 are also displaceable relative to one another. The heating device 304 is pro vided with relatively long current-supply leads 312 and is disposed within the tubular starting crystal 101 for heating the molten zone 105.  
  As shown in FIG. 4, the heating device 304 is in the form of an ellipse. The inclination of the axis 109 of the starting crystal 101 to the minor and major axes 310 and 311 of the elliptical cross section of the heating device 304 is the same as for the heating device 104 of FIG. 1.  
  The heating device 304 can also be a flat or cylindri cal coil having elliptical cross section or an electrically heated radiant heater in the form of an ellipse, all energized with high-frequency alternating current.  
  The molten zone being passed through the tubular starting crystal can also be heated by two coils supplied with high-frequency alternating current, one of the coils being disposed inside the tubular starting crystal and the other of the coils surrounding the outside of the starting crystal. Both of such coils are preferably singly or multiply-wound flat coils of elliptical cross section which lie in the same plane. Both cells are connected serially in opposite phase i.e., they are transversed by energizing current flowing in opposite direction therethrough. FIG. 5 shows in top plan view, two of such oppositely phased serially connected coils 513 and 514 having a single winding. respectively, and formed with an elliptical cross section. The coils 513 and 514 are disposed in a common plane and, as shown in FIG. 5, the wall of the tubular starting crystal 501, whose cross section is in the form of a circular ring, passes therebetween. The opposite current flow through both coils S13 and 514 is represented by the arrows 516 and 517. In the interest of clarity, only the cross section of the part of the starting crystal 501 actually located between the two coils 513 and 514 is shown in broken lines in FIG. 5. In the interspace between the two coils 513 and 514, the field strength of the electromagnetic field emanating from the coils 513 and 514 has a maximum value. The current leads 515 to the induction heating coil 514 located in the interior of the tubular starting crystal 501 are advantageously bent into an elongated U-shaped bow between the legs of which the tubular starting crystal S01 is located in the course of the relative movement thereof during the molten zone pass through the crystal 501. The spacing of this U-shaped bow from the tubular starting crystal 501 is suitably sufficiently great enough so that the bow does not act as an additional heat device. The inclination of the axis of the tubular starting crystal 501 with respect to the major and minor axes of the elliptical cross section of both coils 513 and 514 is the same as for the mode of the method of my invention illustrated in FIG. 1.  
  For a clearer understanding of the construction of the coils 513 or 514 of FIG. 5, a perspective view thereof is shown in FIG. 5a and is believed to require no further explanation.  
  FIG. 6 illustrates another mode of the method of my invention, wherein there is processed a tubular starting crystal 601 having a circular cross section and conically tapering end to which a monocrystalline seed crystal 607 is fused. The starting crystal 601 can also be produced by the method of the aforementioned copending application simultaneously filed with this application, wherein a suitably shaped carried member is employed.  
  The tubular starting crystal 601 is secured in a holder 602 by means of screws 603 so that the axis 609 of the crystal 601 extends in vertical direction. A tubular monocrystalline seed crystal 607, which is fastened at one end thereof to a holder 608, is fused at the other end thereof to the face of the lower conically tapering end of the tubular starting crystal 601 by an annular heating device 604 of circular cross section. The tubular seed crystal 607 can be produced relatively simply, for example. by partially boring a longitudinal recess in a relatively thin rod-shaped monocrystal, for example of 44 mm diameter. A molten zone 605 is passed through the tubular starting crystal 601 by relative displacement of the tubular starting crystal 601 and the heating device 604, as shown further in FIG. 7, in axial direction of the crystal 601, beginning from the location at which the starting crystal 601 and the seed crystal 607 are fused to one another. The material recrystallizing from the molten zone 605 after it has passed through the polycrystalline starting crystal 601 is monocrystalline. The holders 602 and 608 and accordingly both ends of the tubular starting crystal 601 can be set in relative rotation about the axis of the tubular crystal 601 during the molten zone pass through the crystal 601. The recrystallized tubular monocrystal that is thereby produced has a uniformly round circular cross section. The heating device 604 can also be in the form of a coil having one or more windings or an electrically heated radiant heater supplied with highfrequency alternating current.  
  The thickness of the tubular wall of the starting crystal 601 shown in FIGS. 6 and 7 can also be increased by reducing the axial spacing between the holders 602 and 608, and the wall thickness of the crystal 601 can conversely be reduced by increasing the spacing between these holders 602 and 608.  
 1 claim:  
  1. Method of producing a monocrystalline tube of semiconductor material which comprises fusing to a part of the edge of one end of a polycrystalline tube of semiconductor material a monocrystalline seed crystal of the semiconductor material having a diameter that is considerably smaller than the diameter of the tube, positioning the tube so that the axis thereof is oblique and the junction of the seed crystal and the tube is at the lowest point on the tube and sequentially exposing the tube from the bottom upwards to an essentially horizontal heat zone essentially coaxial with the tube and at a temperature at least as high as the melting point of the tube thereby first to melt the tube at and adjacent to said junction and thereafter move an essentially horizontal annular molten zone upwards along the tube.  
  2. The method of claim 1, wherein the heat zone is produced by a coil having an elliptical cross section, said coil surrounds the tube and is energized by highfrequcncy, alternating current.  
  3. The method of claim 1, wherein a radiant heater of elliptical cross section surrounding said tube is used for producing the heat zone.  
  4. The method of claim 1, wherein a radiant heater of elliptical cross section, situated in the interior of the tube, is used for producing the heat zone.  
  5. The method of claim 1, wherein the heat zone is produced by two oppositely phased series-connected coils, one of said coils being situated inside and the other of said coils being situated outside said tube.  
  6. The method of claim 1, in which the inclination of the axis of the tube from the horizontal is from 25 to 65.  
 7. The method of claim 6, in which the inclination of