Abstract:
An apparatus for producing large diameter monocrystalline Group III-V, II-VI compounds that have reduced crystal defect density, improved crystal growth yield, and improved bulk material characteristics. The apparatus comprises a crucible or boat, an ampoule that contains the crucible or boat, a heating unit disposed about the ampoule, and a liner disposed between the heating unit and the ampoule. The liner is preferably composed of a quartz material. When the liner and the ampoule are made of the same material, such as quartz, the thermal expansion coefficients of the liner and ampoule are the same, which significantly increases the lifetime of the liner and the single-crystal yield.

Description:
FIELD  
         [0001]    The invention relates to the growth of semiconductor crystals. More particularly, the invention relates to an apparatus for growing Group II-VI and III-V monocrystalline compounds.  
         BACKGROUND  
         [0002]    Electronic and opto-electronic device manufacturers routinely require commercially grown, large and uniform single semiconductor crystals. These crystals can be sliced and polished to provide substrates for microelectronic device production. An extensive range of deposition and lithography techniques well known in the art is employed to build thin film layers and microcircuits on the monocrystalline substrates to produce integrated circuits, light emitting diodes, semiconductor lasers, sensors, and other microelectronic devices. In radio-frequency integrated circuit and opto-electronic integrated circuit applications, crystalline uniformity and defect density are essential characteristics of the substrates that influence device production yield, life span, and performance. Consequently, improvements in crystal growth technology constitute an ongoing pursuit in academic and industrial research.  
           [0003]    Compound semiconductor crystals are typically grown by one of four techniques: Liquid Encapsulated Czochralski (LEC), Horizontal Bridgman (HB), Horizontal Gradient Freeze (HGF), and Vertical Gradient Freeze (VGF). LEC is a commonly used technique for producing semi-insulating semiconductor crystals, such as GaAs. In the LEC process, a single crystal seed is lowered into a GaAs melt which is covered by a layer of boron oxide (B 2 O 3 ) to prevent the loss of the volatile As and maintain stoichiometry. The temperature of the melt is reduced until crystallization starts on the seed. The seed is then raised at a uniform rate, and a crystal is pulled from the melt. The seed and melt are contained inside a steel chamber at high pressure to prevent the volatile Group V and Group VI elements of the polycrystalline compound from leaving the melt.  
           [0004]    In the LEC process, because the cooling and crystallization occur above the heated melt, unstable convection in the melt and turbulence in the inert gas atmosphere in the growth system are inevitable. In addition, LEC requires a pronounced thermal gradient for success because it is necessary to cool a solidifying crystal rapidly to prevent the escape of volatile arsenic. As a consequence of this high gradient, crystals grown by LEC techniques tend to have a high intrinsic stress, and crystals grown under thermal stress are known to exhibit a relatively high defect density. The impact of this drawback is increasingly apparent in the growth of large diameter crystals. As used herein, “large diameter,” refers to crystals having a diameter on the order of several inches or greater. Large diameter crystals having exceptional substrate characteristics and uniformity are preferred by the electronics industry because such crystals significantly improve device production yield and reduce unit cost.  
           [0005]    The horizontal crystal growth techniques, including Horizontal Bridgman and Horizontal Gradient Freeze, largely reduce the turbulence associated with LEC by using a horizontal furnace. In the horizontal growth techniques, crystals are grown in horizontal boats. The boat containing the raw materials is sealed in an ampoule. Heating elements are used to generate a temperature profile. After the polycrystalline compound melts, one of the temperature gradient, the ampoule, or the heater apparatus is slowly moved so that a solid-liquid interface moves along the length of the boat. Monocrystal growth results as the charge solidifies and cools.  
           [0006]    Typically in horizontal techniques, growth is generally chosen to be in a &lt;111&gt; direction. The completed crystal has a cross-sectional shape matching the shape of the boat, most frequently a “D” shape. If the crystal is sawed perpendicular to its growth axis &lt;111&gt;, the resulting wafers are &lt;111&gt; material. However, usually (100) wafers are desired. For this reason, HB crystals are usually sawed at an angle of about 55° to the ingot axis. With this angular sawing, compositional variations along the axis of the crystal are translated into variations across individual wafers.  
           [0007]    The HB technique does not scale well to large diameters as the technique produces non-cylindrical crystals. Wafers sliced from horizontally grown crystals must be ground to a circular shape for device manufacturing. Since silicon contamination is difficult to avoid in the horizontal growth technique, HB crystals are suitable for LED manufacturers but less attractive for electronics and high-performance opto-electronic device manufacturers.  
           [0008]    The VGF technique for single crystal growth of compound semiconductors resembles the LEC technique in that the crystal is grown in a crucible in an apparatus with a high degree of vertical symmetry. Both VGF and LEC produce cylindrical crystals. The fundamental differences between LEC and VGF are the magnitude of the temperature gradient, the location of the seed crystal, and the direction of the crystal solidification. A VGF crystal growth system employs a smaller temperature gradient on the order of 10 degrees Celsius per centimeter or less, as compared with an LEC system in which the temperature gradient is typically 50-100 degrees Celsius per centimeter. Crystals grown in the relatively low temperature gradient of a VGF system incorporate less thermal stress and, consequently, are known to exhibit a lower defect density than those grown in LEC systems.  
           [0009]    The seed crystal is positioned on the bottom of the crucible in a VGF system, and the crystal cools and solidifies from the bottom up. Contrasted with LEC, the VGF temperature gradient that controls the melting and cooling of the charge is inverted with the cooler crystal situated below the hotter melt. Thus, at the solid-liquid interface in an LEC process, turbulence can be a detrimental factor. VGF, with the crystal below the melt, does not suffer this problem.  
           [0010]    VGF has been demonstrated to be highly scalable to the manufacture of large diameter single crystals. For this reason and because of the demonstrated high crystal quality, VGF is an appealing technology that produces crystals appropriate to consumer markets of compound semiconductor substrates, high-performance microelectronics and opto-electronics.  
           [0011]    The productivity and crystal quality of VGF technology is improved by the inclusion of a ceramic or refractory diffuser between the quartz ampoule and the heating coils in the apparatus. A diffuser of mullite or silicon carbide is often inserted or installed in a VGF growth apparatus to reduce hot spots and turbulence. The diffuser provides more uniform heating and better temperature gradient control. As a result, crystals grown in an apparatus with a diffuser made of mullite or silicon carbide can be grown with reduced intrinsic stress.  
           [0012]    Unfortunately, there are drawbacks associated with the use of mullite or silicon carbide diffusers in crystal growth apparatus when quartz ampoules are used. The diffusers become brittle after repeated cycles of heating and cooling. Also, the diffusers often break after a limited number of uses. An additional concern is the mismatch between the coefficients of thermal expansion of the diffuser and the ampoule. The crystal growth apparatus is often heated to temperatures in excess of 1,200 degrees Celsius. At these temperatures, the sealed quartz ampoule expands since the gas pressures inside and outside the ampoule are not balanced. During cooling, the ampoule tends to contract at a different rate than the furnace liner because quartz has a very low coefficient of thermal expansion. On the other hand, diffusers in the cooling phase tend to rapidly contract to their original dimensions. Diffusers made of mullite or silicon carbide compress the enlarged ampoule, often resulting in a break of the diffuser, ampoule or both. Ampoule breakage usually destroys the charge and thus severely reduces crystal production yield.  
           [0013]    In practice, a silicon carbide diffuser can be used for 3 to 5 crystal growth cycles, making its benefit impracticably expensive. Mullite is less expensive, but the mullite is less useful as a diffuser because of relatively poor thermal conductivity compared to silicon carbide and the difficulty in obtaining high-quality large diameter mullite cylinders. Thus, mullite is of limited benefit in improving the uniformity of the temperature gradient.  
         SUMMARY  
         [0014]    Aspects of the present invention relate to an apparatus for producing monocrystalline Group III-V, II-VI compounds. The apparatus comprises a crucible or boat, an ampoule that contains the crucible or boat, and a heating unit disposed about the ampoule. A liner is disposed between the heating unit and the ampoule. The liner is preferably composed of a quartz material. When the liner and the ampoule are made of the same material, such as quartz, the thermal conductivities of the liner and ampoule are substantially the same, as are the thermal expansion coefficients of the liner and ampoule. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 shows an apparatus for growing monocrystalline Group II-VI and III-V compounds constructed according to a first embodiment of the invention; and  
         [0016]    [0016]FIG. 2 shows an apparatus for growing monocrystalline Group II-VI and III-V compounds constructed according to a second embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0017]    As used herein, the terms “quartz,” “fused quartz,” and “fused silica” are used interchangeably, and all refer to the entire group of materials made by fusing silica (SiO 2 ). Monocrystalline Group II-VI and III-V compounds having resistivities typically within the range of approximately 10 −3  ohm-cm to 10 9  ohm-cm are referred to as “semiconductors” (SC). Group II-VI and III-V monocrystalline compounds that have a resistivity greater than about 1×10 7  ohm-cm are referred to as “semi-insulating” (SI) semiconductors. Depending on the doping level in Group II-VI and III-V compounds, the monocrystalline form may be “semi-insulating” in its “undoped” or intrinsic state, or in its “doped” state. Examples of compounds in doped states include GaAs with chromium or carbon as a dopant, and InP with iron as dopant. The terms “crucible” and “boat” are used interchangeably, as both refer to a container in which a monocrystalline compound or crystal can be grown.  
         [0018]    [0018]FIG. 1 shows an apparatus  100  for growing monocrystalline Group II-VI and III-V compounds constructed according to a first embodiment of the invention. The apparatus  100  includes a crucible  130  of generally cylindrical shape. The crucible  130  is made of pyrolytic boron nitride (PBN) The crucible  130  has a conical bottom  104  with a central region  106  that contains a solid seed crystal material  108  as shown in FIG. 1. The seed crystal  108  extends upward towards a top  110  of the seed well  106  to present a seed crystal surface  112 . This surface  112  provides a crystalline format for growth of a monocrystalline compound  114  in the crucible. The monocrystalline compound  114  grown in accordance with the present invention is preferably a Group III-V, II-VI or related compound such as GaAs, GaP, GaSb, InAS, InP, InSb, AlAs, AlP, AlSb, GaAlAs, CdS, CdSe, CdTe, PbSe, PbTe, PbSnTe, ZnO, ZnS, ZnSe or ZnTe.  
         [0019]    Large solid chunks of polycrystalline compound are initially loaded into crucible  130 . Solid pieces of an oxide of boron such as B 2 O 3  are loaded with the larger solid chunks of polycrystalline compound into the crucible  130 . Suitable dopant materials such as carbon may then be introduced directly into the crucible  130  or other parts of a sealed ampoule  120  to produce doped monocrystalline compounds  114  in accordance with techniques familiar to those skilled in the art.  
         [0020]    In FIG. 1, the loaded crucible  130  is placed in an ampoule  120  preferably made of quartz. The ampoule  120  is preferably sealed with a quartz cap after the crucible  130  is placed in the ampoule  120 . The sealed ampoule  120 , containing the crucible  130 , is then inserted into a liner  122  in a heating unit  123  having heating elements  124 . This liner  122  is preferably shaped as a cylindrical tube which is open at both ends. The liner  122  surrounds the ampoule  120  which encloses the charge  108  and crucible  130 . The relative spacing between the liner  122  and the ampoule  120  is preferably 0.1 mm or greater. The wall thickness of both the liner  122  and the ampoule  120  is greater than 1 mm and preferably in the range of 2-8 mm. The crucible  130 , ampoule  120 , and liner  122  have longitudinal axes oriented substantially vertically as is accustomed in a VGF or LEC system.  
         [0021]    After assembly, the apparatus  100  is heated by heating elements  124  such that the solid chunks of raw material are melted. Applying varying power to the heating elements  124  forms a temperature gradient and a solid-liquid interface  102 . Initially, all the raw material is a melt and the seed crystal  108  is the only solid. The solid-liquid interface is initially at the top surface  112  of the seed crystal  108 . The temperature gradient is slowly moved up through the melt such that a monocrystal  114  grows from the seed crystal  108 . The solid-liquid interface  102  gradually rises as more of the melt  116  solidifies and the monocrystal grows.  
         [0022]    In FIG. 1, the liner  122  is preferably made of quartz. Quartz has a relatively low thermal conductivity, as shown in Table 1 below. Thus, by forming the liner  122  of a quartz material, the liner  122  provides excellent temperature uniformity to the charge during the melting of the raw materials, the formation of the monocrystalline compound or crystal  114 , and the cooling of the crystal  114 . As a result, the quartz liner  122  generates a controlled, gradual, uniform temperature gradient that enables crystal growth with minimal thermal stress. Because of the presence of liner  122 , crystals  114  grown using apparatus  100  have reduced intrinsic stress and fewer crystallographic defects. Crystal growth yield is dramatically improved, and enhanced yield and performance of microelectronic devices made from these crystals  114  can also be measured.  
         [0023]    By forming both the liner  122  and the ampoule  120  of the same material, such as quartz, not only do the liner  122  and the ampoule  120  have substantially the same thermal conductivity. The liner  122  and ampoule  120  also have substantially the same thermal expansion coefficients. Thus, physical stress between the liner  122  and the ampoule  120  is averted. The propensity of the ampoule  120  to crack is reduced during crystal growth, and fewer crystals are lost. Crystal production yield is improved, and the liner  122  can be used in more growth cycles than diffusers made of other materials.  
         [0024]    Table 1 provides a comparison between coefficients of thermal expansion and thermal conductivity for the materials quartz, silicon carbide, and mullite.  
                             TABLE 1                           Comparison between Coefficients of thermal       expansion and thermal conductivity                Coefficient of thermal   Thermal conductivity       Material   expansion cm/cm ° C.   g cal/(sec) (cm −2 ) (° C./cm)               Quartz   5.5 × 10 −7     .0033       Silicon Carbide   3.8 − 4.8 × 10 −6     1.19 − 3.26       Mullite   2.3 − 5.0 × 10 −6      .09 − .143                  
 
         [0025]    Other properties make quartz an appropriate material for liner  122  in crystal growth apparatus  100 . Quartz does not react with most acids, metals, chloride, and bromide at ordinary temperatures. Quartz has good mechanical and electrical properties and is elastic. For these reasons, a quartz liner  122  is well suited for an apparatus  100  for growing monocrystalline Group II-VI and III-V compounds. The liner can be reused for several crystal growth processes.  
         [0026]    In FIG. 1, the heating unit  123  is disposed about the ampoule  120 . The liner  122  is disposed between the ampoule  120  and the heating unit  123 . The heating unit  123  includes, for example, heating coils or other suitable heating elements  124  for controllably heating the liner  122 , ampoule  120 , and crucible  130 . The heating unit  123  further includes a means for monitoring the temperature.  
         [0027]    In FIG. 1, the crystal growth apparatus  100  is acted on in a sequence of control procedures well known in the art. The crucible  130  inside the ampoule  120  is heated, melted and cooled under controlled conditions. After the crucible  130  and ampoule  120  are cooled to room temperature, the ampoule  120  can be removed from the liner  122  and opened to reveal a single crystal ingot.  
         [0028]    [0028]FIG. 2 shows an apparatus  200  for growing monocrystalline Group II-VI and III-V compounds, constructed according to a second embodiment of the invention. The apparatus  200  includes a boat  202  in which raw materials  203  are deposited. The boat  202  is contained in an ampoule  204 . The ampoule  204  is preferably made of quartz. A liner  206  made of a quartz material is provided in apparatus  200 . The liner  206  has the same tubular shape and properties as the liner  122  described above with reference to FIG. 1.  
         [0029]    In FIG. 2, the liner  206  is disposed between the ampoule  204  and a heating unit  208  surrounding the ampoule  204 . The liner  206  surrounds and encloses the ampoule  204 . The boat  202 , ampoule  204 , and liner  206  have longitudinal axes oriented substantially horizontally as is accustomed in an HB or HGF system.  
         [0030]    In FIG. 2, the apparatus  200  establishes a fixed temperature gradient that is horizontally oriented and encloses a movable deck. The boat  202  moves on the deck through the gradient under controlled conditions, and raw materials  203  within boat  202  are thus melted and converted to a monocrystalline compound. The liner  206  has substantially the same effect as liner  122  of the first embodiment described with reference to FIG. 1. That is, the liner  206  enables uniform heating and cooling and provides a uniform temperature gradient that can be carefully controlled and free from hot spots.  
         [0031]    It should be emphasized that the above-described embodiments of the invention are merely possible examples of implementations set forth for a clear understanding of the principles of the invention. Variations and modifications may be made to the above-described embodiments of the invention without departing from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the invention and protected by the following claims.