US 2944875 A
Description (OCR text may contain errors)
2\\ 8 mgm 23 .frylldunnnaada/Ill W. F. LEVERTON CRYSTAL-GROWING APPARATUS AND METHODS Filed July 15, 1953 July 12, 1960 rulli/11011110 wg /f 1 United, States Patient emtsrancnownso APPARATUS AND; Mamans Walter. F. Ileverton, Waltham, Mass., assignor to Raytheon Company, a corporation of Delaware Filedluly 13, 1,953, Ser. No. 361,480
'6; Claims.- (Cl. 2li-273) This invention relates to an lapparatus and a method used? to grow single crystals of uniform electrical resis'tivity` to be employed: in the manufacture of transistors and crystal rectiers, and specifically to the crucible; used to obtain such crystals from a melt of semiconductor material.
In the/preparation of'semiconductor single crystals, the seedpulling method hasl been used with considerable success. However, inorde'r that these single crystals may be useful forfrectiers and transistors, the electrical resis-ti'vityy off the crystal should be uniform throughout its length and the crystal 'should be readily reproducible. Since1 several' importanty parameters of transistors vary moreJ or lessv linearly. with the resistivity of' the single crystal, the difficulty of making units with predictable olflaratterfisticsV isv greatly increasedI if uniform resistivity is not obtained in the crystal during the growing process.
`The changein resistivityv along the length of a single crystal may be attributed primarily to the fact that the ptypeandn1type doping agents, such as arsenic, antimony, gallium andindium, commonly added to a melt ofv pure semiconductormaterial, such as germanium or silicon, are more soluble in the liquid semiconductor material thann i'n the solid semiconductor material; Hence, in a growing crystal, the concentration of doping'a-ge'nt inA the solidi semiconductor crystal, hereinafter referred to. as (3g, is less than theconcentration` of doping agent in the adjacent liquid semiconductor material, hereinafter ref'erlredl'to asCn. Therefore, as a crystal is progressively grown by. the seed-pulling-method, a steadily increasing concentration'fofdoping agent is left in the remaining melt offsemiconductormaterial.
.For example,y thev value of CS/CL may vary from 0.001 to- 0.1f depending on the doping element used,l and for these values of Cs/CL, CL isV approximately-inversely proportional-to the Vvolume of liquidsemiconductor mate*- rialfremaininginthecrucible. Since Csp/CII remains-citeetivelyf constant throughoutv the crystal-growing process, an. increased CL leads to an increasing Cs and since theelectricalresistivityfof the crystal is inversely proportional' to Cs, theresistivity progressively decreases through- 'outfthelength of the crystal. For example, in growing a: 40G-gram crystal' of doped germanium from a SOO-gram melt, the resistivity may decrease by atactor of four oriive along the length of the crystal. Such variations necessitate detailed selection and classification ofi the'slices-'made from the single crystal material, and this. is. both wasteful of time and material. Whereas severalmethod'shave been proposed'to improve this situ'- ation, sucht` as precisely varying the crystal-pulling rate, using larger melts of semiconductor material or controlling the rate of soliditcation, these methods involved complicated procedures and equipment and the chances of growing animpure or imperfect crystal are increased.
This inventionr'elates-to means whereby the concentra'- tion 'of dopingr a gent infthe liquid semiconductorA material, CL, and-the volume. of the semiconductor material is kept substantially constant throughout-the seedspulling process so that a single crystal of. uniform resistivityy may he grown. In. one. embodiment. of' the inventionav pair of cylindrical crucibles, the second. of which is designed to t loosely into the rst,.is used. The. second or inner Crucible is' provided with a small hole drilled. through the bottom thereof. Thus,l the crucibles. are proportioned so that, when, a charge of high purity undoped. or lightly doped semiconductor material is melted. in.v the outer crucible, the; inner cruciblc may beplacedttherein so the bottom thereof rests on the. surface of. the melt. By pushing down slightly on theinner. crucible the: melt may be forced Athrough the hole until the: inner Crucible assumes a position of equilibrium so that it is floating in the melt and contains a portion-of themel-t therein.. A doping agent is. then added to the melt inthe inner Crucible. Thus, the outer crucible contains. a melt. of lightly doped or undoped semiconductor material: and the. inner Crucible holds a melt of` semiconductorl ma terial. having.. a'. substantially greater concentration of dopingV agent therein.
A. single: crystal may be; grown. fromv the.. melt described above bythe seed-pulling, method? and, as; the crystal grows, they lightly doped semiconductormaterial will ow'from the. outer. crucible into the inner Crucible to maintain the equilibrium level. Therefore, untill the inner crucible. touches. the. `bottom of the outer Crucible, the;` volumey of liquid semiconductor.' material. in. the inner. Crucible will remain exactlyf constant; Since only a small fraction of theY doping agent isiused up; inf the growing crystal CL remains; practically constant through,- out. the process, and. the resistivity of the. crystal is un'rform. The. small decrease in CL during` growth-1V due to the. fraction, of doping agent: used up byithef. crystal is corrected by lightly doping. the. entire: original melt, as previouslyy mentioned,. withl a quantityv of' doping.; agent equivalent to thatV used up by thecrystal.
With the 'easily fabricated apparatus described above, large p-type. or. n-type single crystals` having. uniform electrical resistivityV throughout: may be. grown.V This` novel device is easyto operate: and thefurnace structure used to heat: the cruciblesA and'v the operation. thereof is greatly simplified becausev the melt is kept,A at aifixedltern perature throughout. the.V ygrotv/ing process. Also, no elaborate equipment for programming the pulling speed is.Y required.
This invention` and the; features thereof'will be understood. more clearly and fully from the-following-*detailed description of one. embodiment` of thel invention with reference-to. the accompanying drawing wherein a' schematic view of the crystalfgrowing apparatus: is' shown.
Referringnow to the drawings, a pair of cr-uciblesi l1 and. 2.: made; in. accordance with this; particular: embodimentof the invention is shown mounted withinacrystalgrowing. apparatus. For purposes of illustration, it will be assumed that these crucibles are to be inductively heatediandthata.germanium singlecrystal isz-to be grown". Therefore, the crucibles 1 and 2 should be electrically conductive,y thermally responsive to the heating. means; chemically.V inert with respect to` germanium and readily heated inv a high-frequency lield. Crucibles made of high. purity graphite Afulfill these requirements. andhave been used successfully for thepurposes.V of this invention; Both of the crucibles shown are cylindrical in shape, and each is provided with acylindrical recess therein adapted to hold. a melt of semiconductor material, inthis instance germanium. The inner crucible 2 is designed to fit into the recess in the outer crucible. 1 and shouldVv be free to move up or down therein. The inner crucibleZ is provided with a small opening 3 extending through the oor thereof; This opening should be so proportioned that; when aicrystal of germanium is drawn', as explained below; the diffusion or mixing ofthe doping agent stan-tially constant as said crystal is pulled from said second melt.
5. A crystal-growing apparatus comprising a pair of movably nested containers, the outer one of which is adapted -to receive an initial melt of semiconductive material having a rst predetermined concentration of doping agent therein, and the inner one of which is adapted to float in said initial melt and to' receive a second melt of semiconductive material having a second predetermined concentration of doping agent therein, means for pulling a single crystal from said second melt, an orifice in the base of said inner container of such size as to allow said initial melt to pass slo'wly into said second melt, but substantially prevent said second melt from passing into said initial melt whereby the concentration of doping agent in said second melt is maintained substantially constant as said crystal is pulled from said second melt, and means for maintaining said initial and said second melts at substantially the same temperature during the operation of said pulling means.
6. A crystal-growing apparatus comprising a pair of movably nested containers, the outer one of which is adapted to receive an initial melt of semiconductive material having a first predetermined concentration of doping agent therein, and the inner one of which is adapted to oat in said initial melt and to receive a portion of said References Cited in the le of this patent UNITED STATES PATENTS 1,353,571 Dreibrodt Sept. 21. 1920 2,459,869 Christensen et al. Jan. 25, 1949 2,647,043 Imber July 28, 1953 2,674,520 Sobek Apr. 6, 1954 2,727,839 Sparks Dec. 20, 1955 2,754,180 Horton July 10, 1956 FOREIGN PATENTS 512,461 Belgium July 15, 1952 OTHER REFERENCES Lehovec et al.: Apparatus for Crystal Pulling in Vacu-