Abstract:
A method for growing high-resistivity single crystals includes placing a raw material in a vacuum-sealable ampoule, heating the raw material in the vacuum-sealable ampoule to vaporize the moisture in the raw material, exhausting the vaporized moisture from the vacuum-sealable ampoule, vacuum-sealing the vacuum-sealable ampoule, heating the raw material in the vacuum-sealable ampoule to vaporize the oxide compounds in the raw material, cooling a bulb in a cap on the vacuum-sealable ampoule to produce condensed oxide compounds on an inner surface of the bulb, removing the bulb and the condensed oxide compounds from the vacuum-sealable ampoule, wherein the raw material in the vacuum-sealable ampoule comprises carbon as an impurity, and placing the vacuum-sealable ampoule comprising the raw material in a crystal growth apparatus to grow a high-resistivity crystal from the raw material.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to technologies for growing single crystals, and in particular, to technologies for growing single crystals with desirable properties. 
         [0002]    The fabrication of electronic devices often involve the formation of integrated circuitry on large and uniform single semiconductor crystals followed by slicing and polishing to form individual device chips. For example, power amplifiers in wireless devices such as mobile phones are first formed in a batch on a large GaAs substrate and then cut into separate dies for different wireless devices. 
         [0003]    Typical industrial methods for growing GaAs crystals includes pulling method, horizontal boat method, horizontal gradient freeze method, vertical boat method, and vertical gradient freeze method. In a crystal growth process, a raw material (e.g. a polycrystalline GaAs material) is first heated by a heater (not shown) to above its melting point. The melt is brought into contact with a seed crystal (e.g. made of GaAs), allowing the melt to crystallize from the seed crystal. An exemplified commercial crystal growth apparatus  100 , shown in  FIG. 1 , includes a ampoule  110  that provides vacuum for crystal growth, a crucible  120  that holds the raw material, and heaters (not shown) around the ampoule  110  configured to melt the raw material in the crucible  120  to form a material melt  130 . The crucible  120  has a seed well  140  that holds a seed crystal to start the crystal growth. A dopant source  170  can be placed in the ampoule  110  to dope substance in the crystal during its growth. 
         [0004]    Many communication devices require integrated circuitry to be constructed on semiconductor substrates with high intrinsic resistivity in order to suppress noises in wireless communications. Traditionally, high resistivity group III-V compounds can be made by helium bombardment of low-resistivity GaAs single-crystal substrate, or doping during the growth of the crystals. 
         [0005]    Despite the progresses made in industrial processes of growing signal crystals, there continues to be a need to simplify the manufacturing process and reduce costs, especially for growing high quality high-resistance single crystals. 
       SUMMARY OF THE INVENTION 
       [0006]    The presently application discloses improved methods for growing high-resistance single crystals. The raw materials for growing the single crystal are selectively purified, which can beneficially produce high resistivity in the single crystal product. The presently disclosed crystal growth apparatus and methods are applicable to crystal growths of a range of materials such as Group III-V, Group II-VI, and Group IV materials. 
         [0007]    Electronic devices often require high purity in the single crystal substrate to ensure low losses, fast response, and high signal-to-noise ratios. On the other hand, for some applications, the single crystals are also required to have high resistivity, which is commonly achieved by doping “impurities” in the crystal. An advantageous feature of the presently disclosed methods and systems is that high resistivity can be achieved in single crystals without intentionally doping crystal material with impurities. 
         [0008]    In a general aspect, the present invention relates to a method of growing high-resistivity single crystals. The method includes placing a raw material in a vacuum-sealable ampoule, wherein the raw material comprises polycrystalline Group III-V material, Group II-VI material, or Group IV material, wherein the raw material comprises moisture and minute amount of impurities that include oxide compounds and carbon; capping an open end of the vacuum-sealable ampoule while leaving an air channel between the cap and the rim of the open end of the vacuum-sealable ampoule, wherein the cap includes a bulb connected to the opening of the cap with a neck; heating the raw material in the vacuum-sealable ampoule to vaporize the moisture in the raw material; exhausting the vaporized moisture from the vacuum-sealable ampoule; fusing the cap to the rim of the open end of the vacuum-sealable ampoule to vacuum seal the vacuum-sealable ampoule; heating the raw material in the vacuum-sealable ampoule to vaporize the oxide compounds in the raw material; cooling at least a portion of the bulb to below the vaporization temperature of the oxide compounds to produce condensed oxide compounds on an inner surface of the bulb; fusing the neck of the bulb to isolate the condensed oxide compounds from the vacuum-sealable ampoule; breaking the neck to remove the bulb and the condensed oxide compounds from the vacuum-sealable ampoule, wherein the raw material in the vacuum-sealable ampoule comprises carbon as an impurity; and placing the vacuum-sealable ampoule comprising the raw material in a crystal growth apparatus to grow a high-resistivity crystal from the raw material. 
         [0009]    Implementations of the system may include one or more of the following. The raw material can be placed in a crucible positioned in the vacuum-sealable ampoule. The vacuum-sealable ampoule can be formed by quartz or glass. The impurities can include at least one of silicon, carbon, or germanium. The high-resistivity crystal can have an electric resistivity above 10 7  Ω-cm at room temperature. The impurities can include arsenide or arsenic compounds. The method can further include after the step of fusing, heating the raw material in the vacuum-sealable ampoule to vaporize the arsenide and arsenic compounds in the raw material; and cooling a portion of the vacuum-sealable ampoule to produce condensed arsenide and arsenic compounds on an inner surface of the portion of the vacuum-sealable ampoule to remove the arsenide and arsenic from the raw material. The arsenide or the arsenic compounds in the impurities can be selected from the group consisting of AlAs, InAs, YAs, B 12 As 2 , Ca 3 As 2 , and Zn 3 As 2 . The polycrystalline material can include GaAs, AlAs, GaN, CdTe, InAs, GaSb, Si, or Ge. A seed crystal can be placed in contact with the raw material in the vacuum-sealable ampoule. The method can further include heating the raw material comprising carbon as an impurity to form a melt in contact with a seed crystal in the crystal growth apparatus; and growing the high-resistivity crystal from the melt. 
         [0010]    In another general aspect, the present invention relates to a method of growing high-resistivity single crystals. The method includes placing a raw material in a vacuum-sealable ampoule, wherein the raw material comprises polycrystalline Group III-V material, Group II-VI material, or Group IV material, wherein the raw material comprises moisture and minute amount of impurities that include oxide compounds and carbon; heating the raw material in the vacuum-sealable ampoule to a first temperature to vaporize the moisture in the raw material; removing the vaporized moisture from the vacuum-sealable ampoule; heating the raw material in the vacuum-sealable ampoule to a second temperature to vaporize the oxide compounds in the raw material, wherein the second temperature is higher than the first temperature; removing the condensed oxide compounds from the vacuum-sealable ampoule; and heating the raw material comprising carbon as an impurity to form a melt; and growing a high-resistivity crystal from the melt, wherein the high-resistivity crystal has an electric resistivity above 10 7  Ω-cm at room temperature. 
         [0011]    Implementations of the system may include one or more of the following. The first temperature can be above 100° C., and wherein the second temperature is above 120° C. The steps of heating the raw material can include carbon as an impurity to form a melt and growing a high-resistivity crystal from the melt are performed in a crystal growth apparatus. 
         [0012]    In another general aspect, the present invention relates to a method of growing high-resistivity single crystals. The method includes placing a raw material in a vacuum-sealable ampoule through an open end of the vacuum-sealable ampoule, wherein the raw material comprises polycrystalline GaAs, moisture, and minute amount of impurities, wherein the impurities comprise arsenide or arsenic compounds and carbon; heating the raw material in the vacuum-sealable ampoule to vaporize the moisture in the raw material; exhausting the vaporized moisture from the vacuum-sealable ampoule; sealing the vacuum-sealable ampoule; heating the raw material in the vacuum-sealable ampoule to vaporize the arsenide and arsenic compounds in the raw material; cooling a portion of the vacuum-sealable ampoule to produce condensed arsenide and arsenic compounds on an inner surface of the portion of the vacuum-sealable ampoule; and placing the vacuum-sealable ampoule comprising the raw material comprising carbon as an impurity in a crystal growth apparatus, wherein a high-resistivity GaAs crystal is grown. 
         [0013]    Implementations of the system may include one or more of the following. The impurities can include oxide compounds, wherein the step of heating the raw material in the vacuum-sealable ampoule to vaporize the arsenide and arsenic compounds in the raw material vaporizes the oxide compounds in the raw material, wherein the method further comprising: after the step of cooling a portion of the vacuum-sealable ampoule, lowering the temperature of the raw material in the vacuum-sealable ampoule to below the vaporization temperature of the arsenide and arsenic compounds but above the vaporization temperature of the oxide compounds; cooling a surface in connection with the vacuum-sealable ampoule to below the vaporization temperature of the oxide compounds to produce condensed oxide compounds on the surface; and removing the condensed oxide compounds from the vacuum-sealable ampoule. The raw material can be placed in a crucible positioned in the vacuum-sealable ampoule, wherein the condensed arsenide or arsenic compounds are formed outside of the crucible, wherein the high-resistivity crystal is grown in the crucible. The vacuum-sealable ampoule can be formed by quartz or glass. The method can further include capping the open end of the vacuum-sealable ampoule after the step of exhausting the vaporized moisture, wherein the step of sealing comprises fusing the cap to the rim of the open end of the vacuum-sealable ampoule to vacuum seal the vacuum-sealable ampoule. The impurities comprise oxide compounds, wherein the step of heating the raw material in the vacuum-sealable ampoule to vaporize the arsenide and arsenic compounds in the raw material vaporizes the oxide compounds in the raw material, wherein the cap includes a bulb connected to the opening of the cap with a neck, the method further comprising: after the step of sealing the vacuum-sealable amp, lowering the temperature of the raw material in the vacuum-sealable ampoule to below the vaporization temperature of the arsenide and arsenic compounds but above the vaporization temperature of the oxide compounds; cooling at least a portion of the bulb in the cap to below the vaporization temperature of the oxide compounds to produce condensed oxide compounds on an inner surface of the bulb; fusing the neck of bulb to isolate the condensed oxide compounds from the vacuum-sealable ampoule; and breaking the neck of the bulb to separate the bulb and the condensed oxide compounds from the vacuum-sealable ampoule. The high-resistivity GaAS crystal can have an electric resistivity above 10 8  Ω-cm at room temperature. The method can further include heating the raw material to form a melt in the crystal growth apparatus; and growing the high-resistivity GaAs crystal from the melt. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The following drawings, which are incorporated in and from a part of the specification, illustrate embodiments of the present specification and, together with the description, serve to explain the principles of the specification. 
           [0015]      FIG. 1  is a schematic diagram for a conventional crystal growth apparatus. 
           [0016]      FIG. 2  is a block diagram of a crystal growth system for growing high-resistance single crystals in accordance with the present invention. 
           [0017]      FIG. 3  shows an exemplified flowchart for growing high-resistance single crystals in accordance with the present invention. 
           [0018]      FIGS. 4A-4D  are cross-sectional views of a material-preparation apparatus at different steps of a process for preparing raw material to be used for growing high-resistance single crystals in accordance with the present invention. 
           [0019]      FIG. 5  shows the ampoule containing the crucible holding the raw material prepared by the material-preparation apparatus. 
           [0020]      FIGS. 6A-6C  are cross-sectional views of a crystal growth apparatus for growing high-resistance single crystals using the raw material prepared by the material-preparation apparatus. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Referring to  FIG. 2 , a crystal growth system  200  for growing high-resistance single crystals includes a material-preparation apparatus  210  and a crystal growth apparatus  220 . A vacuum sealable ampoule  250 , as described in more detail below, is configured to contain a crucible which in turn holds a raw material for crystal growth. The raw material in the vacuum sealable ampoule  250  is selectively purified by the material-preparation apparatus  210 . Afterwards, the vacuum sealable ampoule  250  is transferred to the crystal growth apparatus  220 . The raw material is melted and the crystal is grown in the crucible. 
         [0022]    Referring to  FIGS. 3 and 4A , the material-preparation apparatus  210  includes a heating chamber  410  which defines a cavity  415  configured to receive the vacuum sealable ampoule  250 . The vacuum sealable ampoule  250  has an upper rim  251  and can be made of quartz. The material-preparation apparatus  210  includes a lower heater  420  at the bottom of the heating chamber  410 . The lower heater  420  can be moved upward into the cavity  415  to come into contact with a bottom surface  470  of the vacuum sealable ampoule  250  (shown in  FIG. 4B ). A raw material  425  for crystal growth is placed in a crucible  430  through an opening on the top of the crucible  430  (step  305 ,  FIG. 3 ). The raw material is typically in a polycrystalline phase of the same material for the single crystal to be formed. For example, the raw material can be a polycrystalline GaAs material for forming GaAs single crystals. The crucible  430  can be made of pyrolytic boron nitride (pBN). The crucible  430  has a narrow portion in the bottom where a seed crystal  435  is placed in contact with the raw material  425 . The crucible  430  containing the raw material  425  and the seed crystal  435  is placed in the vacuum sealable ampoule  250  (step  310 ,  FIG. 3 ). The vacuum sealable ampoule  250  is capped by a cap  440  (step  315 ,  FIG. 3 ). Initially, the cap  440  does not seal vacuum sealable ampoule  250 . A gap separates the cap  440  from the upper rim  251  which defines an air channel  252 . The cap  440  can be made of glass or quartz. The cap  440  includes a neck  447  and a bulb  445  connected to the neck  447 . 
         [0023]    The vacuum sealable ampoule  250  is inserted into the cavity  415  while leaving the rim  251  above the heating chamber  410 . The outer surface of the rim  251  of the vacuum sealable ampoule  250  is sealed by a sealing device  450  that includes an outlet  455 . The heating chamber  410  is heated by a heater (not shown) to above 100° C., such as about 200° C. to vaporize the moisture in the raw material  425  (step  320 ,  FIG. 3 ). The water vapor from the raw material  425  is exhausted out of the vacuum sealable ampoule  250  through the outlet  455  by a vacuum pump (step  325 ,  FIG. 3 ). 
         [0024]    Raw materials for crystal growth commonly include impurities such as arsenide compounds (other than GaAs) such as AlAs, InAs, YAs, B 12 As 2 , Ca 3 As 2 , Zn 3 As 2 , etc. The impurities can also include oxide compounds, carbon, silicon, germanium, etc. Electronic devices often require high purity to ensure low losses, fast response, and high signal-to-noise ratios. For this reason, impurities are to be removed from the raw material materials before they are melted and recrystallized. On the other hand, for some applications, the single crystals are also required to have high resistivity, which is commonly achieved by doping “impurities” in the simple crystals. 
         [0025]    An advantageous feature of the presently disclosed methods and systems is to remove the need for doping to achieve high resistivity. Undesirable impurities are selectively removed whereas certain impurities are not removed but are used to beneficially increase resistivity in the crystal product. In other words, certain impurities are intentionally kept in the raw material to provide desired property to the crystal product. 
         [0026]    After the water vapor is exhausted from the raw material  425 , as shown in  FIG. 4B , the cap  440  is heated by a flame and fused to the upper rim  251  of the vacuum sealable ampoule  250 , which vacuum seals the crucible  430  and the raw material  425  in the vacuum sealable ampoule  250  (step  330 ,  FIG. 3 ). The vacuum sealable ampoule  250  is moved down inside the heating chamber  410  (step  335 ,  FIG. 3 ). The heating chamber  410  is capped by a cover  460  mounted with a moveable upper temperature control device  465 . 
         [0027]    The heating chamber  410  is next heated to about 250° C. to vaporizing arsenide or arsenic compounds as well as oxide compounds in the raw material  425  (step  340 ,  FIG. 3 ). The lower heater  420  is raised to contact the bottom of the vacuum sealable ampoule  250 . The lower heater  420 , and thus the bottom of the vacuum sealable ampoule  250 , is kept at about 200° C., which is lower than the condensation temperature for arsenide and arsenic compounds. Thus, the vaporized arsenide and arsenic compounds condense at the bottom surface  470  of the vacuum sealable ampoule  250  to form condensed arsenide and arsenic compounds  475  (step  345 ,  FIG. 3 ). 
         [0028]    The vacuum sealable ampoule  250  is then cooled by reducing heating to the heating camber  410  to a temperature above 120° C., such as about 220° C. (step  350 ,  FIG. 3 ). At this temperature, the vaporizations of arsenide and arsenic compounds are stopped while the oxide compounds remain in a vaporized state. The upper temperature control device  465  is lowered to come into contact with the bulb  445 . The upper temperature control device  465  keeps the top surface of the bulb  445  at about 20° C., on which condensed oxide compounds  480  are formed (step  355 ,  FIG. 3 ). 
         [0029]    Next, the cover  460  and the upper temperature control device  465  are removed from the heating chamber  410  in the material-preparation apparatus  210 . The vacuum sealable ampoule  250  is lifted position the bulb  445  and the neck  447  above the heating chamber  410 , as shown in  FIG. 4D . The neck  447  is then melted by a flame to form a pinch point  448  which seals the bulb  445  off the rest of from the crucible  430  and the vacuum sealable ampoule  250  (step  360 ,  FIG. 3 ). 
         [0030]    The vacuum sealable ampoule  250  is then removed from the heating chamber  410 . The pinch point  448  is then broken by force to separate the bulb  447  from the vacuum sealable ampoule  250  (step  365 ,  FIG. 3 ) as shown in  FIG. 5 , removing the condensed oxide compounds  480  from the vacuum sealable ampoule  250  (and the raw material  425 ). 
         [0031]    Referring to  FIG. 6A , the crystal growth apparatus  220  can include a lower heater  620  and a cylindrical shaped furnace  610  that defines a cavity  615  in the center. The vacuum sealable ampoule  250  containing the crucible  430  holding the purified raw material  425  is placed in the cavity  615  through an opening on the top of the furnace  610  (step  370 ,  FIG. 3 ). The furnace  610  is then covered by an upper heater  615 , as shown in  FIG. 6A . 
         [0032]    The furnace  610  is then heated to about 1238° C. (within 20° C. of 1238° C.) to melt the raw material  425  to form a melted material  630  in the crucible  250 , as shown in  FIG. 6B  (step  375 ,  FIG. 3 ). The seed crystal  435  provides the initial crystal surface from which a single crystal  640  can grow vertically out of the melted material  630  (step  375 ,  FIG. 3 ), as shown in  FIG. 6C . 
         [0033]    Due to the selective impurity removals in the raw materials, the single crystal  640  can have certain un-removed impurities such as carbon, which results in electrical resistivity higher than 10 7  Ω-cm at room temperature. In some embodiments, the resulting electrical resistivity is higher than 10 8  Ω-cm at room temperature. 
         [0034]    Details about a crystal growth apparatus is also disclosed in the commonly assigned U.S. patent application Ser. No. 12/908,157, filed Oct. 20, 2010 by the same inventor, the content of which is incorporated herein by reference. 
         [0035]    It is understood the presently disclosed crystal growth system and methods are compatible with other variations without deviating from the spirit of the present invention. For example, other methods of crystal growth can be used in conjunction with the disclosed material preparation apparatus. The crystals can be grown from melted material vertically, as described above, or horizontally. For example, the crucible in the crystal growth apparatus can have the shape of a horizontal boat and the seed crystal is positioned at one side of the crucible. Additionally, crystal growth materials are not limited to the examples given above. Suitable materials can include other semiconductor materials and AlAs, GaN, CdTe, InAs, GaSb, Si, Ge, and other Group III-V, Group II-VI, Group IV materials.