Abstract:
An effusion source comprises a vitreous C filament and a heater to increase the temperature of the filament to produce a C vapor. Also described is a deposition method comprising (a) depositing a layer of material on a substrate, and (b) during step (a), heating a body of material that includes vitreous carbon so that carbon from the body is vaporized and incorporated into the deposited layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of application Ser. No. 11/114,828 file on Apr. 26, 2005, which is also entitled “Deposition of Carbon-Containing Layers Using Vitreous Carbon Source.” 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to the deposition of carbon-containing layers and, more particularly, to effusion cells and methods for their use in the molecular beam epitaxial (MBE) growth of such layers.  
         [0004]     2. Discussion of the Related Art  
         [0005]     Molecular beam deposition of layers of material (e.g., semiconductors, metals, insulators, or superconductors) on a heated substrate in an ultra high vacuum is well known in the art. In particular, MBE is one of the principal techniques used in the semiconductor device industry to fabricate high quality, single crystal, semiconductor layers with thickness control on the order of a monolayer. In MBE a single crystal substrate or wafer is placed in a vacuum chamber where it is heated. Effusion cells loaded with source materials in solid or liquid form are heated to vaporize the material and generate beams of constituent atoms, which are directed at the substrate. Alternatively, one or more of the effusion cells may be replaced by a gas jet coupled to a source of gaseous material to generate one or more of the requisite beams. (The latter deposition technique is known as chemical beam epitaxy, or CBE, especially if a chemical reaction occurs on the substrate surface during, or just before, incorporation of a component of the beam.) In both MBE and CBE the constituent atoms adsorb on the substrate surface and incorporate into the underlying crystal structure to form a layer. Control is so good that the layer is literally formed one monolayer at a time.  
         [0006]     Although the term molecular is used to describe the vaporized source material in this deposition process, those skilled in the art understand that the source material may be elemental (or atomic) as well as compound (or molecular).  
         [0007]     In the MBE growth of Group III-V compound semiconductor layers, for example, the crucible of one effusion cell would contain a Group III metal (e.g., liquid Ga), and the crucible of another cell would contain a Group V material (e.g., a solid source such as elemental As, or less commonly polycrystalline GaAs). On the other hand, in the CBE growth of such layers, one or more of the crucibles containing, for example, Group V material would be replaced by a gas source of, for example, arsine or phosphine. In either case, a third effusion cell or gas source would contain the source of a dopant. One consideration in the choice of a dopant is the conductivity-type of the layer to be grown. For example, to dope Group III-V compound layers n-type from a solid source tin (Sn) and silicon (Si) have been commonly used as dopants, and to dope such layers p-type from a solid source beryllium (Be) has been commonly used for many years. More recently, however, Be has been largely replaced by carbon (C).  
         [0008]     Carbon has several characteristics that make it preferable as a p-type dopant in Group III-V compound layers deposited by MBE. First, Be is toxic; C is not. Second, Be has a relatively high vapor pressure and, therefore, during the high temperatures used in an MBE deposition processes, Be contaminates the growth chamber. Third, Be diffuses in the growing layer at a much higher rate than C. Therefore, precise control of the location and concentration of Be within very thin layers is difficult.  
         [0009]     CBr 4  is currently used in the industry to provide a source of C. See, for example, page 67 of the Product Guide 2000 of the EPI MBE Product Group, St. Paul, Minn., which is incorporated herein by reference. However, Br is corrosive, and extreme care must be exercised in evacuating it from the deposition chamber. Alternatives to CBr 4  have been suggested in the prior art. For example, direct resistive heating of C filaments has been reported by R. J. Malik et al.,  J. Cryst. Growth,  Vol. 127, pp. 686-689 (1993), which is incorporated herein by reference. Various methods for producing the C filaments have been tried including machining the filaments from a block of solid graphitic C or patterning them from a sheet of graphite foil. A. Mak et al.,  J. Vac. Sci. Technol. B,  Vol. 12, No. 3, pp. 1407-1409 (1994), which is also incorporated herein by reference, describe a woven filament that comprised a bundle of 6000, 10-μm-diameter graphite fibers. The fibers were clamped at both ends to a refractory metal support attached to an ultrahigh vacuum feed-through, as shown in  FIG. 1  of the A. Mak et al. paper. The authors report a relatively short period of operation: only 15 hr at a power dissipation level corresponding to a hole concentration of 5×10 18  cm −3  at 1 μm/hr growth rate. They also predict that repeated temperature cycling will shorten the filament lifetime.  
         [0010]     We have found that, due to the relatively low resistivity of graphite filaments, they must be driven at relatively high input current levels to attain suitable doping levels. In addition, the high thermal conductivity of graphite filaments requires relatively high input power to attain requisite filament temperature. However, these high current and power levels tend to cause outgassing of the apparatus supporting the filament and of other components in the deposition system, which leads to undesirable contamination and, in turn, to decreased mobility of semiconductor layers grown in such systems.  
         [0011]     On the other hand, a paper by R. J. Malik et al [ Appl. Phys. Lett.,  Vol. 53, No. 26, pp. 2661-2663 (1988)] and the product literature of MBE Komponenten GmbH, Germany [MBE Komponenten, Dr. Karl Eberl, Products 2003, pp. 38-39] both describe a C sublimation source that utilizes a pyrolytic graphite, serpentine filament. Both of these references are incorporated herein by reference. However, pyrolytic graphite also has relatively high electrical and thermal conductivity, which means that correspondingly high power/current must be applied to generate suitable doping levels. In addition, the typical serpentine shape of the filament employed in these references suffers from hot spots at the sharp bends, which tends to decrease the filament lifetime.  
         [0012]     As pointed out in the Komponenten literature, these issues of C doping also apply to the deposition of other than Group III-V compound layers; e.g., the deposition of Si—C and Si—Ge—C alloys.  
         [0013]     Thus, a need remains in the MBE deposition art for a source of C doping that operates at lower power/current levels, and hence produces less contamination, and has a relatively longer lifetime than is currently available from graphite filaments.  
       BRIEF SUMMARY OF THE INVENTION  
       [0014]     In accordance with one aspect of our invention, an effusion source comprises a vitreous C filament and a heater to raise the temperature of the filament sufficiently to produce a C vapor. By vitreous C we mean that the C atoms are arranged in a tetrahedral structure akin to that found in amorphous diamond; i.e., each C atom is located at the center of an equilateral tetrahedron and is bonded in four directions pointing at the four vertices of the tetrahedron.  
         [0015]     In a currently preferred embodiment, the heater provides electric current to the filament. In this regard, we have found that the resistivity of vitreous C is considerably higher than that of graphite, and its thermal conductivity is considerably lower, which means that correspondingly less input power/current has to be applied to vitreous C filaments to achieve the same doping level. Accordingly, our vitreous C filaments produce considerably less contamination than graphite filaments.  
         [0016]     In accordance with another aspect of our invention, a method comprises (a) depositing a layer of carbon-containing material on a substrate, and (b) during step (a), heating a body of material that includes vitreous carbon so that carbon from the body is vaporized and incorporated into the deposited layer.  
         [0017]     By the phrase carbon-containing we mean that C is incorporated either as a dopant (e.g., C-doped GaAs) or as a primary constituent (e.g., a Si—C alloy). 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0018]     Our invention, together with its various features and advantages, can be readily understood from the following more detailed description taken in conjunction with the accompanying drawing, in which:  
         [0019]      FIG. 1  is a schematic view of a prior art MBE apparatus;  
         [0020]      FIG. 2  is a schematic, cross sectional view of a fixture for a C filament, in accordance with one embodiment of our invention;  
         [0021]      FIG. 3  is a schematic top view of the C filament of  FIG. 1 , in accordance with one embodiment of our invention; and  
         [0022]      FIG. 4  is a schematic side view of the fixture of  FIG. 1 , in accordance with one embodiment of our invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     General Molecular Beam Deposition Apparatus Before discussing our invention in detail, we first turn to  FIG. 1 , which shows a well known ultra-high vacuum apparatus  10  for the molecular beam deposition of layers of, for example, semiconductor materials sequentially on a substrate  12 . The apparatus  10 , which is typically made of stainless steel, comprises a growth chamber  14  and a pump chamber  16 . The growth chamber  14  is provided with a multiplicity of ports, which serve a variety of functions. For example, manipulator port  36  is used to position the substrate  12 , oven port  44  is used to generate molecular beams, and viewing port  45  is used to visually observe the substrate. Additional ports  46  or  47  may be used to couple ion gauges (not shown) to the growth chamber. Illustratively, one ion gauge is used to monitor the chamber pressure; another is used to measure atomic or molecular beam flux.  
         [0024]     Vacuum conditions (e.g., a base pressure of 10 −9  to 10 −12  Torr) are achieved and maintained in the growth chamber  14  by suitable pumping means, typically a Ti sublimation pump  18  coupled to a commercially available cryogenic vacuum pump (not shown) via port  20 . The sublimation pump  18  includes a Ti element  18 . 1  positioned within a cryogenically cooled (e.g., liquid nitrogen) first shroud (not shown). A multiplicity of staggered, liquid-nitrogen-cooled baffles  22  blocks line-of-sight paths between element  18 . 1  and substrate  12 .  
         [0025]     The substrate  12  is mounted on a holder  32  and is heated by means of a suitable heater  33 . Holder  32  is in turn secured to a manipulator illustrated as a rod  34  that extends through port  36  to the exterior of the apparatus. Arrows  38  and  39  indicate that the rod, and hence the substrate, may be translated or rotated, or both, into a desired position within the growth chamber. Typically the substrate is surrounded by a cryogenically cooled second shroud (not shown), which is apertured to allow access to the substrate surface by growth and test beams and for visual inspection.  
         [0026]     As shown in  FIG. 1 , the manipulator has been used to locate the substrate  12  in a growth position. In this position substrate  12  faces a multiplicity of shuttered effusion cells  40 , which are located in oven port  44  and are each surrounded by cryogenically cooled third shrouds (not shown). Cells  40  are loaded with source materials typically in a liquid or solid state, although in some cases the source materials may be gaseous. (In a liquid state, the source material is commonly referred to as a melt.) When suitably heated and the shutters  42  are opened, the solid or liquid source materials evaporate to form a multiplicity of beams of constituent materials (known as molecular beams) that are adsorbed onto the heated substrate  12  where they form, for example, a semiconductor layer. At least one of the cells  40  is a source of a dopant beam, and in particular, a filament source of generating a beam of C that is incorporated into the deposited layer, either as a dopant (e.g., in the case of p-type doping of Group III-V compounds) or as a primary constituent (e.g., in the case of Si alloys such as Si—C or Si—Ge—C).  
         [0027]     Carbon may be incorporated into a device as a dopant in either (or both) of two well-known ways: by a bulk-doping process or by a delta-doping process. In bulk-doping, deposition of device layers continues while the C beam is on, so that C is incorporated into the layer as it is being deposited. In delta-doping, deposition of a layer is interrupted while the C beam remains on, so that C is deposited as a fraction of a monolayer (typically 10 −3  of a monolayer) on the previously deposited layer.  
         [0028]     Thus, when we state that our invention is used to deposit at least one layer of a carbon-containing material, in the context of doping we mean this phrase to include at least one bulk-doped layer that includes C as a dopant or at least one delta-doped layer (or fraction of a monolayer) of C itself. Of course, in the context of depositing C-containing layers in general, the phrase also includes depositing at least one bulk layer that includes C as a primary constituent.  
         [0029]     Depending on the growth conditions and the nature of the substrate  12 , the deposited semiconductor layer may be monocrystalline (single crystal), polycrystalline or amorphous. Although our invention is primarily concerned with high quality, monocrystalline, semiconductor layers, our effusion cells may also be used to fabricate semiconductor layers that are not monocrystalline or to fabricate non-semiconductor materials such as metals, insulators or superconductors.  
         [0030]     Although modern designs of MBE apparatus have evolved considerably in the last 25 years, many of the features of a basic MBE apparatus are described by A. Y. Cho in U.S. Pat. No. 4,239,955 issued on Dec. 16, 1980, which is incorporated herein by reference.  
         [0031]     CBE apparatus is essentially identical to the MBE apparatus described above, except that one or more of the effusion cells  40  is replaced by a gas source.  
         [0000]     Other Vacuum Deposition Systems  
         [0032]     Our C source, which is described below, may be useful in other types of vacuum deposition systems or apparatus as long as the mean free path of the carbon atoms/molecules is long enough that a sufficient number of them reach the substrate and are incorporated into the carbon-containing layer deposited thereon. In this regard, the system should provide a working vacuum of at least 10 −3  Torr, and illustratively a base vacuum of 10 −9  to 10 −12  Torr, as mentioned above for MBE.  
         [0000]     Carbon Filament Design  
         [0033]     In accordance with one aspect of our invention, an effusion cell  40 , as shown in  FIG. 2 , comprises a fixture for supporting a vitreous C filament  40 . 1  within the oven port  44  of, for example, a typical MBE apparatus of the type shown in  FIG. 1 . The fixture includes a pair of refractory metal rods  40 . 2  with filament  40 . 1  mounted on the coplanar ends of the rods  40 . 2 . The refractory rods  40 . 2  are mechanically and electrically coupled to conductive metal rods  60 . 1  of a standard high vacuum feed-through  60  via a standard threaded bolt and bore arrangement (not shown). Feed-through  60  extends through a sidewall of the growth chamber  14  of  FIG. 1 , typically through a sidewall  44 . 1  of the oven port  44 , to a power source  50  (e.g., a current source).  
         [0034]     Mechanical stability is illustratively provided to the refractory rods  40 . 2  by means of an electrically insulating refractory holder  40 . 4 .  
         [0035]     Typically the refractory rods  40 . 1  comprise tantalum (Ta) or molybdenum (Mo) or alloys of either, the conductive rods  60 . 1  comprise copper (Cu), and the holder  40 . 4  comprises quartz.  
         [0036]     In a preferred embodiment of our invention, as shown in  FIG. 3 , the vitreous C filament  40 . 1  is a thin, planar member that, in top view, has the general shape of a bar bell; that is, it includes a relatively narrow central portion or neck  40 . 1   a  disposed between and integrally connected to relatively wider end portions  40 . 1   b.  The latter portions have holes  40 . 1   d  aligned with corresponding threaded holes or bores  40 . 2   a  in the refractory rods  40 . 2 , as shown in  FIG. 4 . Refractory metal bolts  40 . 7  extend through the holes  40 . 1   d  into the bores  40 . 2   a  in order to hold the filament  40 . 1  in place.  
         [0037]     The neck  40 . 1   a  serves to concentrate electric current, and thus heat, in the narrower central portion of the filament  40 . 1  away from bolts  40 . 7 , thereby decreasing the temperature of the bolts, which in turn decreases outgassing from the refractory rods  40 . 2 , decreases the power required for a particular C flux, and also decreases the likelihood that they will react with other materials in the fixture. However, care should be exercised that the neck  40 . 1   a  does not become so hot that the vitreous C undergoes a phase transition to graphitic C. This phase transition has an onset at ˜2300° C. and becomes more rapid as the temperature is raised further.  
         [0038]     In addition, the filament  40 . 1  is secured in place by a spring-loaded arrangement, which illustratively includes a spring-loaded refractory metal washer  40 . 6  disposed between the head of each bolt  40 . 7  and the top surface of the filament. Illustratively, the washers  40 . 6  have a conical shape; e.g., they are well-known Belleville washers. Illustratively, the washers  40 . 6  comprise a material that retains its resiliency at temperatures above about 1400° C. Suitable materials include Ta alloys such as 1-10% W and 99-90% Ta. The bolts  40 . 7  typically comprise Ta, but may also comprise the same type of alloys used for the washers  40 . 6 .  
         [0039]     Refractory spacers  40 . 5  are disposed between the tops of the refractory rods  40 . 2  and the underside of the vitreous C filament  40 . 1 . The spacer material should have a low vapor pressure and should have little or no reaction with either the refractory rods  40 . 2  or the vitreous C filament  40 . 1  at the operating temperature of the effusion cell  40 . Preferably the spacers  40 . 5  comprise rhenium (Re) foil, but tungsten (W) foil or alloys of either could also be used.  
         [0040]     Finally, the refractory rods  40 . 2  are each provided with a hole  40 . 2   b,  which extends radially from the exterior surface of the rod to the bore  40 . 2   a,  thereby enabling the bores  40 . 2   a  to be pumped out when the growth chamber is also pumped down to a predetermined vacuum.  
         [0041]     In operation, the power source  50  delivers about 100 W of electrical power to the vitreous C filament  40 . 1 , which resistively heats the filament  40 . 1  to a temperature in excess of 2000° C. (but below the vitreous-to-graphitic phase transition onset temperature of ˜2300° C.) in order to generate sufficient C vapor, for example, to dope a Group III-V compound layer or to grow a Si—C-based alloy layer.  
       EXAMPLE  
       [0042]     The following design parameters illustrate the construction of a C effusion cell in accordance with an illustrative embodiment of our invention. Various materials, dimensions and operating conditions are provided by way of illustration only and, unless otherwise expressly stated, are not intended to limit the scope of the invention.  
         [0043]     Filament  40 . 1 : vitreous C with approximate dimensions d 1 =12 mm; d 2 =22 mm; d 3 =28 mm; w 1 =2 mm; w 2 =6 mm, and t=0.5 mm  
         [0044]     Refractory rods  40 . 2 : made of Ta; diameter=6 mm  
         [0045]     Holes  40 . 2   b  in rods  40 . 2 : diameter=0.5 mm  
         [0046]     Conductive rods  60 . 1 : made of Cu; diameter=6 mm  
         [0047]     Washers  40 . 6 : made of 93% Ta, 7% W  
         [0048]     Bolts  40 . 7 : made of Ta  
         [0049]     Spacers  40 . 5 : made of Re foil  
         [0050]     Input power: ˜100 W (˜18.4 A at ˜6 V), which produces a filament temperature of about 2100° C. (±100° C.)  
         [0051]     We have successfully operated this type of vitreous C filament in an MBE apparatus to grow Group III-V epitaxial layers doped with C for over  100  hr without observing any significant degradation of the filament or its ability to deliver acceptable C flux.  
         [0052]     More specifically, we have fabricated a high mobility, two-dimensional hole system (2DHS) confined in GaAs/AlGaAs quantum wells grown by MBE on the [100] surface of GaAs. The quantum wells were modulation doped with C utilizing our invention. At a temperature of 0.3° K and carrier density of about p=6×10 10  cm −2 , a mobility of about 3.0×10 6  cm −2  Vs was achieved.  
         [0053]     More generally, we have achieved C doping levels of 7×10 18  cm −3  in bulk doped GaAs structures, but higher doping levels can be achieved in several ways. First, the temperature of the filament  40 . 1  may be increased while remaining below the aforementioned phase transition onset temperature. Second, the growth rate may be decreased. Third, the growth may be pulsed; i.e., one or more of the sources (e.g., a Ga source) may be turned on and off at prescribed times to effectively reduce the growth rate while the C source remains on to increase the C doping level.  
         [0054]     It is to be understood that the above-described arrangements are merely illustrative of the many possible specific embodiments that can be devised to represent application of the principles of the invention. Numerous and varied other arrangements can be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention. In particular, the C filament may be heated by well known techniques other than by passing an electric current through it. For example, the C filament may heated by electromagnetic energy; e.g., by an RF signal from a radio frequency source or by an optical signal from a high power laser, such as a CO 2  laser.